Fuel Cell Handbook 

(Seventh Edition)

By 

EG&G Technical Services, Inc. 

Under Contract No. DE-AM26-99FT40575 

U.S. Department of Energy 

Office of Fossil Energy 

National Energy Technology Laboratory 

P.O. Box 880 

Morgantown, West Virginia 26507-0880

November 2004 
DISCLAIMER

This report was prepared as an account of work sponsored by an agency of the United States 

Government. Neither the United States Government nor any agency thereof, nor any of their 

employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or 

process disclosed, or represents that its use would not infringe privately owned rights. Reference 

herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, 

or favoring by the United States Government or any agency thereof. The views and opinions of 

authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof. 

Available to DOE and DOE contractors from the Office of Scientific and Technical Information, 

P.O. Box 62, 175 Oak Ridge Turnpike, Oak Ridge, TN 37831; prices available at 

(423) 576-8401, fax: (423) 576-5725, E-mail: reports@adonis.osti.gov 

Available to the public from the National Technical Information Service, U.S. Department of 

Commerce, 5285 Port Royal Road, Springfield, VA 22161; phone orders accepted at 

(703) 487-4650. 
iii 

TABLE OF CONTENTS 

Section Title Page 

1. TECHNOLOGY OVERVIEW ................................................................................................. 1-1

1.1 INTRODUCTION................................................................................................................ 1-1

1.2 UNIT CELLS ..................................................................................................................... 1-2

1.2.1 Basic Structure ................................................................................................... 1-2

1.2.2 Critical Functions of Cell Components.............................................................. 1-3

1.3 FUEL CELL STACKING ..................................................................................................... 1-4

1.3.1 Planar-Bipolar Stacking ..................................................................................... 1-4

1.3.2 Stacks with Tubular Cells .................................................................................. 1-5

1.4 FUEL CELL SYSTEMS....................................................................................................... 1-5

1.5 FUEL CELL TYPES............................................................................................................ 1-7

1.5.1 Polymer Electrolyte Fuel Cell (PEFC)............................................................... 1-9

1.5.2 Alkaline Fuel Cell (AFC)................................................................................. 1-10

1.5.3 Phosphoric Acid Fuel Cell (PAFC).................................................................. 1-10

1.5.4 Molten Carbonate Fuel Cell (MCFC) .............................................................. 1-11

1.5.5 Solid Oxide Fuel Cell (SOFC) ......................................................................... 1-12

1.6 CHARACTERISTICS......................................................................................................... 1-12

1.7 ADVANTAGES/DISADVANTAGES................................................................................... 1-14

1.8 APPLICATIONS, DEMONSTRATIONS, AND STATUS ........................................................ 1-15

1.8.1 Stationary Electric Power................................................................................. 1-15

1.8.2 Distributed Generation ..................................................................................... 1-20

1.8.3 Vehicle Motive Power...................................................................................... 1-22

1.8.4 Space and Other Closed Environment Power .................................................. 1-23

1.8.5 Auxiliary Power Systems................................................................................. 1-23

1.8.6 Derivative Applications.................................................................................... 1-32

1.9 REFERENCES.................................................................................................................. 1-32

2. FUEL CELL PERFORMANCE............................................................................................... 2-1

2.1 THE ROLE OF GIBBS FREE ENERGY AND NERNST POTENTIAL........................................ 2-1

2.2 IDEAL PERFORMANCE ..................................................................................................... 2-4

2.3 CELL ENERGY BALANCE................................................................................................. 2-7

2.4 CELL EFFICIENCY ............................................................................................................ 2-7

2.5 ACTUAL PERFORMANCE................................................................................................ 2-10

2.6 FUEL CELL PERFORMANCE VARIABLES........................................................................ 2-18

2.7 MATHEMATICAL MODELS............................................................................................. 2-24

2.7.1 Value-in-Use Models ....................................................................................... 2-26

2.7.2 Application Models.......................................................................................... 2-27

2.7.3 Thermodynamic System Models...................................................................... 2-27

2.7.4 3-D Cell / Stack Models................................................................................... 2-29

2.7.5 1-D Cell Models............................................................................................... 2-31

2.7.6 Electrode Models.............................................................................................. 2-32

2.8 REFERENCES.................................................................................................................. 2-33

3. POLYMER ELECTROLYTE FUEL CELLS ........................................................................ 3-1

3.1 CELL COMPONENTS......................................................................................................... 3-1

3.1.1 State-of-the-Art Components ............................................................................. 3-2

3.1.2 Component Development................................................................................. 3-11

3.2 PERFORMANCE .............................................................................................................. 3-14
iv 

3.3 PEFC SYSTEMS.............................................................................................................. 3-16

3.3.1 Direct Hydrogen PEFC Systems ...................................................................... 3-16

3.3.2 Reformer-Based PEFC Systems....................................................................... 3-17

3.3.3 Direct Methanol Fuel Cell Systems ................................................................. 3-19

3.4 PEFC APPLICATIONS..................................................................................................... 3-21

3.4.1 Transportation Applications............................................................................. 3-21

3.4.2 Stationary Applications.................................................................................... 3-22

3.5 REFERENCES.................................................................................................................. 3-22

4. ALKALINE FUEL CELL......................................................................................................... 4-1

4.1 CELL COMPONENTS......................................................................................................... 4-5

4.1.1 State-of-the-Art Components ............................................................................. 4-5

4.1.2 Development Components ................................................................................. 4-6

4.2 PERFORMANCE ................................................................................................................ 4-7

4.2.1 Effect of Pressure ............................................................................................... 4-8

4.2.2 Effect of Temperature ........................................................................................ 4-9

4.2.3 Effect of Impurities .......................................................................................... 4-11

4.2.4 Effects of Current Density................................................................................ 4-12

4.2.5 Effects of Cell Life........................................................................................... 4-14

4.3 SUMMARY OF EQUATIONS FOR AFC............................................................................. 4-14

4.4 REFERENCES.................................................................................................................. 4-16

5. PHOSPHORIC ACID FUEL CELL ........................................................................................ 5-1

5.1 CELL COMPONENTS......................................................................................................... 5-2

5.1.1 State-of-the-Art Components ............................................................................. 5-2

5.1.2 Development Components ................................................................................. 5-6

5.2 PERFORMANCE .............................................................................................................. 5-11

5.2.1 Effect of Pressure ............................................................................................. 5-12

5.2.2 Effect of Temperature ...................................................................................... 5-13

5.2.3 Effect of Reactant Gas Composition and Utilization ....................................... 5-14

5.2.4 Effect of Impurities .......................................................................................... 5-16

5.2.5 Effects of Current Density................................................................................ 5-19

5.2.6 Effects of Cell Life........................................................................................... 5-20

5.3 SUMMARY OF EQUATIONS FOR PAFC........................................................................... 5-21

5.4 REFERENCES.................................................................................................................. 5-22

6. MOLTEN CARBONATE FUEL CELL.................................................................................. 6-1

6.1 CELL COMPONENTS......................................................................................................... 6-4

6.1.1 State-of-the-Art Componments.......................................................................... 6-4

6.1.2 Development Components ................................................................................. 6-9

6.2 PERFORMANCE .............................................................................................................. 6-13

6.2.1 Effect of Pressure ............................................................................................. 6-15

6.2.2 Effect of Temperature ...................................................................................... 6-19

6.2.3 Effect of Reactant Gas Composition and Utilization ....................................... 6-21

6.2.4 Effect of Impurities .......................................................................................... 6-25

6.2.5 Effects of Current Density................................................................................ 6-30

6.2.6 Effects of Cell Life........................................................................................... 6-30

6.2.7 Internal Reforming ........................................................................................... 6-30

6.3 SUMMARY OF EQUATIONS FOR MCFC.......................................................................... 6-34

6.4 REFERENCES.................................................................................................................. 6-38
v 

7. SOLID OXIDE FUEL CELLS.................................................................................................. 7-1

7.1 CELL COMPONENTS......................................................................................................... 7-2

7.1.1 Electrolyte Materials .......................................................................................... 7-2

7.1.2 Anode Materials................................................................................................. 7-3

7.1.3 Cathode Materials .............................................................................................. 7-5

7.1.4 Interconnect Materials........................................................................................ 7-6

7.1.5 Seal Materials..................................................................................................... 7-9

7.2 CELL AND STACK DESIGNS ........................................................................................... 7-13

7.2.1 Tubular SOFC .................................................................................................. 7-13

 7.2.1.1 Performance ........................................................................................ 7-20 

7.2.2 Planar SOFC..................................................................................................... 7-31

 7.2.2.1 Single Cell Performance...................................................................... 7-35 

 7.2.2.2 Stack Performance............................................................................... 7-39 

7.2.3 Stack Scale-Up ................................................................................................. 7-41

7.3 SYSTEM CONSIDERATIONS ............................................................................................ 7-45

7.4 REFERENCES.................................................................................................................. 7-45

8. FUEL CELL SYSTEMS............................................................................................................ 8-1

8.1 SYSTEM PROCESSES ........................................................................................................ 8-2

8.1.1 Fuel Processing .................................................................................................. 8-2

8.2 POWER CONDITIONING.................................................................................................. 8-27

8.2.1 Introduction to Fuel Cell Power Conditioning Systems................................... 8-28

8.2.2 Fuel Cell Power Conversion for Supplying a Dedicated Load [2,3,4]............. 8-29

8.2.3 Fuel Cell Power Conversion for Supplying Backup Power to a Load 

 Connected to a Local Utility ............................................................................ 8-34

8.2.4 Fuel Cell Power Conversion for Supplying a Load Operating in Parallel 

 With the Local Utility (Utility Interactive) ...................................................... 8-37

8.2.5 Fuel Cell Power Conversion for Connecting Directly to the Local Utility...... 8-37

8.2.6 Power Conditioners for Automotive Fuel Cells............................................... 8-39

8.2.7 Power Conversion Architecture for a Fuel Cell Turbine Hybrid Interfaced 

 With a Local Utility.......................................................................................... 8-41

8.2.8 Fuel Cell Ripple Current .................................................................................. 8-43

8.2.9 System Issues: Power Conversion Cost and Size............................................. 8-44 

 8.2.10 REFERENCES (Sections 8.1 and 8.2) ................................................................. 8-45 

8.3 SYSTEM OPTIMIZATION................................................................................................. 8-46

8.3.1 Pressure ............................................................................................................ 8-46

8.3.2 Temperature ..................................................................................................... 8-48

8.3.3 Utilization......................................................................................................... 8-49

8.3.4 Heat Recovery.................................................................................................. 8-50

8.3.5 Miscellaneous................................................................................................... 8-51

8.3.6 Concluding Remarks on System Optimization ................................................ 8-51

8.4 FUEL CELL SYSTEM DESIGNS........................................................................................ 8-52

8.4.1 Natural Gas Fueled PEFC System ................................................................... 8-52

8.4.2 Natural Gas Fueled PAFC System................................................................... 8-53

8.4.3 Natural Gas Fueled Internally Reformed MCFC System................................. 8-56

8.4.4 Natural Gas Fueled Pressurized SOFC System................................................ 8-58

8.4.5 Natural Gas Fueled Multi-Stage Solid State Power Plant System ................... 8-62

8.4.6 Coal Fueled SOFC System............................................................................... 8-66

8.4.7 Power Generation by Combined Fuel Cell and Gas Turbine System .............. 8-70

8.4.8 Heat and Fuel Recovery Cycles ....................................................................... 8-70
vi 

8.5 FUEL CELL NETWORKS ................................................................................................. 8-82

8.5.1 Molten Carbonate Fuel Cell Networks: Principles, Analysis and 

 Performance ..................................................................................................... 8-82

8.5.2 MCFC Network................................................................................................ 8-86

8.5.3 Recycle Scheme ............................................................................................... 8-86

8.5.4 Reactant Conditioning Between Stacks in Series............................................. 8-86

8.5.5 Higher Total Reactant Utilization .................................................................... 8-87

8.5.6 Disadvantages of MCFC Networks.................................................................. 8-88

8.5.7 Comparison of Performance............................................................................. 8-88

8.5.8 Conclusions...................................................................................................... 8-89

8.6 HYBRIDS ........................................................................................................................ 8-89

8.6.1 Technology....................................................................................................... 8-89

8.6.2 Projects............................................................................................................. 8-92

8.6.3 World’s First Hybrid Project............................................................................ 8-93

8.6.4 Hybrid Electric Vehicles (HEV) ...................................................................... 8-93

8.7 FUEL CELL AUXILIARY POWER SYSTEMS..................................................................... 8-96

8.7.1 System Performance Requirements.................................................................. 8-97

8.7.2 Technology Status............................................................................................ 8-98

8.7.3 System Configuration and Technology Issues ................................................. 8-99

8.7.4 System Cost Considerations........................................................................... 8-102

8.7.5 SOFC System Cost Structure ......................................................................... 8-103

8.7.6 Outlook and Conclusions ............................................................................... 8-104

8.8 REFERENCES................................................................................................................ 8-104

9. SAMPLE CALCULATIONS .................................................................................................... 9-1

9.1 UNIT OPERATIONS........................................................................................................... 9-1

9.1.1 Fuel Cell Calculations ........................................................................................ 9-1

9.1.2 Fuel Processing Calculations ........................................................................... 9-13

9.1.3 Power Conditioners.......................................................................................... 9-16

9.1.4 Others ............................................................................................................... 9-16

9.2 SYSTEM ISSUES.............................................................................................................. 9-16

9.2.1 Efficiency Calculations .................................................................................... 9-17

9.2.2 Thermodynamic Considerations....................................................................... 9-19

9.3 SUPPORTING CALCULATIONS........................................................................................ 9-22

9.4 COST CALCULATIONS.................................................................................................... 9-25

9.4.1 Cost of Electricity............................................................................................. 9-25

9.4.2 Capital Cost Development ............................................................................... 9-26

9.5 COMMON CONVERSION FACTORS ................................................................................. 9-27

9.6 AUTOMOTIVE DESIGN CALCULATIONS......................................................................... 9-28

9.7 REFERENCES.................................................................................................................. 9-29

10. APPENDIX ............................................................................................................................... 10-1

10.1 EQUILIBRIUM CONSTANTS ............................................................................................ 10-1

10.2 CONTAMINANTS FROM COAL GASIFICATION................................................................ 10-2

10.3 SELECTED MAJOR FUEL CELL REFERENCES, 1993 TO PRESENT................................... 10-4

10.4 LIST OF SYMBOLS........................................................................................................ 10-10

10.5 FUEL CELL RELATED CODES AND STANDARDS .......................................................... 10-14

10.5.1 Introduction.................................................................................................... 10-14

10.5.2 Organizations ................................................................................................. 10-15

10.5.3 Codes & Standards......................................................................................... 10-16

10.5.4 Codes and Standards for Fuel Cell Manufacturers......................................... 10-17
vii 

10.5.5 Codes and Standards for the Installation of Fuel Cells .................................. 10-19

10.5.6 Codes and Standards for Fuel Cell Vehicles.................................................. 10-19

10.5.7 Application Permits........................................................................................ 10-19

10.5.8 References...................................................................................................... 10-21

10.6 FUEL CELL FIELD SITE DATA...................................................................................... 10-21

10.6.1 Worldwide Sites............................................................................................. 10-21

10.6.2 DoD Field Sites.............................................................................................. 10-24

10.6.3 IFC Field Units............................................................................................... 10-24

10.6.4 FuelCell Energy.............................................................................................. 10-24

10.6.5 Siemens Westinghouse................................................................................... 10-24

10.7 HYDROGEN.................................................................................................................. 10-31

10.7.1 Introduction.................................................................................................... 10-31

10.7.2 Hydrogen Production ..................................................................................... 10-32

10.7.3 DOE’s Hydrogen Research ............................................................................ 10-34

10.7.4 Hydrogen Storage........................................................................................... 10-35

10.7.5 Barriers........................................................................................................... 10-36

10.8 THE OFFICE OF ENERGY EFFICIENCY AND RENEWABLE ENERGY WORK IN FUEL 

 CELLS .......................................................................................................................... 10-36

10.9 RARE EARTH MINERALS ............................................................................................. 10-38

10.9.1 Introduction.................................................................................................... 10-38

10.9.2 Outlook........................................................................................................... 10-40

10.10 REFERENCES................................................................................................................ 10-41

11. INDEX....................................................................................................................................... 11-1
viii 

LIST OF FIGURES 

Figure Title Page 

Figure 1-1 Schematic of an Individual Fuel Cell................................................................... 1-2 

Figure 1-2 Expanded View of a Basic Fuel Cell Unit in a Fuel Cell Stack (1)..................... 1-4 

Figure 1-3 Fuel Cell Power Plant Major Processes ................................................................ 1-7 

Figure 1-4 Relative Emissions of PAFC Fuel Cell Power Plants Compared to Stringent 

 Los Angeles Basin Requirements ...................................................................... 1-13 

Figure 1-5 PC-25 Fuel Cell.................................................................................................. 1-16 

Figure 1-6 Combining the SOFC with a Gas Turbine Engine to Improve Efficiency ........ 1-19 

Figure 1-7 Overview of Fuel Cell Activities Aimed at APU Applications......................... 1-24 

Figure 1-8 Overview of APU Applications......................................................................... 1-24 

Figure 1-9 Overview of typical system requirements.......................................................... 1-25 

Figure 1-10 Stage of development for fuel cells for APU applications ................................ 1-26 

Figure 1-11 Overview of subsystems and components for SOFC and PEFC systems ......... 1-28 

Figure 1-12 Simplified process flow diagram of pre-reformer/SOFC system ...................... 1-29 

Figure 1-13 Multilevel system modeling approach ............................................................... 1-30 

Figure 1-14 Projected Cost Structure of a 5kWnet APU SOFC System. ............................. 1-32 

Figure 2-1 H2/O2 Fuel Cell Ideal Potential as a Function of Temperature............................ 2-5 

Figure 2-2 Effect of fuel utilization on voltage efficiency and overall cell efficiency 

 for typical SOFC operating conditions (800 °C, 50% initial hydrogen 

 concentration). ................................................................................................... 2-10 

Figure 2-3 Ideal and Actual Fuel Cell Voltage/Current Characteristic ............................... 2-11 

Figure 2-4 Example of a Tafel Plot ..................................................................................... 2-13 

Figure 2-5 Example of impedance spectrum of anode-supported SOFC operated at 

 850 °C. ............................................................................................................... 2-14 

Figure 2-6 Contribution to Polarization of Anode and Cathode.......................................... 2-17 

Figure 2-7 Voltage/Power Relationship .............................................................................. 2-19 

Figure 2-8 The Variation in the Reversible Cell Voltage as a Function of Reactant 

 Utilization .......................................................................................................... 2-23 

Figure 2-9 Overview of Levels of Fuel Cell Models........................................................... 2-26 

Figure 2-10 Conours of Current Density on Electrolyte ....................................................... 2-31 

Figure 2-11 Typical Phenomena Considered in a 1-D Model (17) ....................................... 2-32 

Figure 2-12 Overview of types of electrode models (9)........................................................ 2-33 

Figure 3-1 (a) Schematic of Representative PEFC (b) Single Cell Structure of 

 Representative PEFC ........................................................................................... 3-2 

Figure 3-2 PEFC Schematic (4, 5)......................................................................................... 3-3 

Figure 3-3 Polarization Curves for 3M 7 Layer MEA (12)................................................... 3-7 

Figure 3-4 Endurance Test Results for Gore Primea 56 MEA at Three Current 

 Densities............................................................................................................. 3-10 

Figure 3-5 Multi-Cell Stack Performance on Dow Membrane (9)...................................... 3-12 

Figure 3-6 Effect on PEFC Performance of Bleeding Oxygen into the Anode 

 Compartment (1)................................................................................................ 3-13 

Figure 3-7 Evolutionary Changes in PEFCs Performance [(a) H2/O2, (b) H2/Air, 

 (c) Reformate Fuel/Air, (d) H2/unkown)] [24, 10, 12, , ] .................................. 3-14 
ix 

Figure 3-8 Influence of O2 Pressure on PEFC Performance (93°C, Electrode Loadings 

 of 2 mg/cm2 Pt, H2 Fuel at 3 Atmospheres) [(56) Figure 29, p. 49]................... 3-15 

Figure 3-9 Cell Performance with Carbon Monoxide in Reformed Fuel (56) .................... 3-16 

Figure 3-10 Typical Process Flow Diagram Showing Major Components of Direct 

 Hydrogen PEFC System .................................................................................... 3-17 

Figure 3-11 Schematic of Major Unit Operations Typical of Reformer-Based PEFC 

 Systems. ............................................................................................................. 3-18 

Figure 3-12 Comparison of State-of-the-Art Single Cell Direct Methanol Fuel Cell 

 Data (58) ............................................................................................................ 3-21 

Figure 4-1 Principles of Operation of H2/O2 Alkaline Fuel Cell, Immobilized 

 Electrolyte (8) ...................................................................................................... 4-4 

Figure 4-2 Principles of Operation of H2/Air Alkaline Fuel Cell, Circulating 

 Electrolyte (9) ...................................................................................................... 4-4 

Figure 4-3 Evolutionary Changes in the Performance of AFCs (8, 12, & 16) ...................... 4-8 

Figure 4-4 Reversible Voltage of the Hydrogen-Oxygen Cell (14) ...................................... 4-9 

Figure 4-5 Influence of Temperature on O2, (air) Reduction in 12 N KOH. ...................... 4-10 

Figure 4-6 Influence of Temperature on the AFC Cell Voltage.......................................... 4-11 

Figure 4-7 Degradation in AFC Electrode Potential with CO2 Containing and CO2 

 Free Air .............................................................................................................. 4-12 

Figure 4-8 iR-Free Electrode Performance with O2 and Air in 9 N KOH at 55 to 60°C. 

 Catalyzed (0.5 mg Pt/cm2

 Cathode, 0.5 mg Pt-Rh/cm2 Anode) Carbon-based 

 Porous Electrodes (22)....................................................................................... 4-13 

Figure 4-9 iR Free Electrode Performance with O2 and Air in 12N KOH at 65 °C............ 4-14 

Figure 4-10 Reference for Alkaline Cell Performance.......................................................... 4-15 

Figure 5-1 Principles of Operation of Phosphoric Acid Fuel Cell (Courtesy of UTC 

 Fuel Cells)............................................................................................................ 5-2 

Figure 5-2 Improvement in the Performance of H2-Rich Fuel/Air PAFCs........................... 5-6 

Figure 5-3 Advanced Water-Cooled PAFC Performance (16).............................................. 5-8 

Figure 5-4 Effect of Temperature: Ultra-High Surface Area Pt Catalyst. Fuel: H2, 

 H2 + 200 ppm H2S and Simulated Coal Gas (37) .............................................. 5-14 

Figure 5-5 Polarization at Cathode (0.52 mg Pt/cm2

) as a Function of O2 Utilization, 

 which is Increased by Decreasing the Flow Rate of the Oxidant at 

 Atmospheric Pressure 100 percent H3PO4, 191°C, 300 mA/cm2

, 1 atm. (38)... 5-15 

Figure 5-6 Influence of CO and Fuel Gas Composition on the Performance of Pt 

 Anodes in 100 percent H3PO4 at 180°C. 10 percent Pt Supported on Vulcan 

 XC-72, 0.5 mg Pt/cm2. Dew Point, 57°. Curve 1, 100 percent H2; Curves 

 2-6, 70 percent H2 and CO2/CO Contents (mol percent) Specified (21) ........... 5-18 

Figure 5-7 Effect of H2S Concentration: Ultra-High Surface Area Pt Catalyst (37).......... 5-19 

Figure 5-8 Reference Performances at 8.2 atm and Ambient Pressure. Cells from Full 

 Size Power Plant (16)......................................................................................... 5-22 

Figure 6-1 Principles of Operation of Molten Carbonate Fuel Cells (FuelCell Energy)....... 6-2 

Figure 6-2 Dynamic Equilibrium in Porous MCFC Cell Elements (Porous electrodes 

 are depicted with pores covered by a thin film of electrolyte) ............................ 6-4 

Figure 6-3 Progress in the Generic Performance of MCFCs on Reformate Gas and 

 Air (12, 13).......................................................................................................... 6-6 
x 

Figure 6-4 Effect of Oxidant Gas Composition on MCFC Cathode Performance at 

 650°C, (Curve 1, 12.6 percent O2/18.4 percent CO2/69.0 percent N2; 

 Curve 2, 33 percent O2/67 percent CO2) (49, Figure 3, Pg. 2711) .................... 6-14 

Figure 6-5 Voltage and Power Output of a 1.0/m2 19 cell MCFC Stack after 960 Hours 

 at 965 °C and 1 atm, Fuel Utilization, 75 percent (50)...................................... 6-15 

Figure 6-6 Influence of Cell Pressure on the Performance of a 70.5 cm2 MCFC at 

 650 °C (anode gas, not specified; cathode gases, 23.2 percent O2/3.2 percent 

 CO2/66.3 percent N2/7.3 percent H2O and 9.2 percent O2/18.2 percent 

 CO2/65.3 percent N2/7.3 percent H2O; 50 percent CO2, utilization at 

 215 mA/cm2) (53, Figure 4, Pg. 395) ................................................................. 6-18 

Figure 6-7 Influence of Pressure on Voltage Gain (55) ...................................................... 6-19 

Figure 6-8 Effect of CO2/O2 Ratio on Cathode Performance in an MCFC, Oxygen 

 Pressure is 0.15 atm (22, Figure 5-10, Pgs. 5-20)............................................... 6-22 

Figure 6-9 Influence of Reactant Gas Utilization on the Average Cell Voltage of an 

 MCFC Stack (67, Figure 4-21, Pgs. 4-24)......................................................... 6-23 

Figure 6-10 Dependence of Cell Voltage on Fuel Utilization (69) ....................................... 6-25 

Figure 6-11 Influence of 5 ppm H2S on the Performance of a Bench Scale MCFC 

 (10 cm x 10 cm) at 650 °C, Fuel Gas (10 percent H2/5 percent CO2/ 

 10 percent H2O/75 percent He) at 25 percent H2 Utilization (78, Figure 4, 

 Pg. 443).............................................................................................................. 6-29 

Figure 6-12 IIR/DIR Operating Concept, Molten Carbonate Fuel Cell Design (29) ............ 6-31 

Figure 6-13 CH4 Conversion as a Function of Fuel Utilization in a DIR Fuel Cell 

 (MCFC at 650 oC and 1 atm, steam/carbon ratio = 2.0, >99 percent methane 

 conversion achieved with fuel utilization > 65 percent (93).............................. 6-33 

Figure 6-14 Voltage Current Characteristics of a 3kW, Five Cell DIR Stack with 

 5,016 cm2

 Cells Operating on 80/20 percent H2/CO2 and Methane (85)........... 6-33 

Figure 6-15 Performance Data of a 0.37m2 2 kW Internally Reformed MCFC Stack at 

 650 °C and 1 atm (13)........................................................................................ 6-34 

Figure 6-16 Average Cell Voltage of a 0.37m2

 2 kW Internally Reformed MCFC Stack 

 at 650 °C and 1 atm. Fuel, 100 percent CH4, Oxidant, 12 percent CO2/9 

 percent O2/77 percent N2 .................................................................................. 6-35 

Figure 6-17 Model Predicted and Constant Flow Polarization Data Comparison (98)......... 6-37 

Figure 7-1 Electrolyte Conductivity as a Function of Temperature (4, 5, 6) ........................ 7-3 

Figure 7-2 (a) Sulfur Tolerance of Ni-YSZ Anodes (16, 17) and (b) Relationship 

 between Fuel Sulfur and Anode Sulfur Concentration........................................ 7-5 

Figure 7-3 Impact of Chromia Poisoning on the Performance of Cells with Different 

 Electrolytes (From (21)) ...................................................................................... 7-6 

Figure 7-4 Stability of Metal Oxides in Stainless Steels (26,27) .......................................... 7-8 

Figure 7-5 Impact of LSCM Contact Layer on Contact Resistance in Cell with Metal 

 Interconnect (from (28)). ..................................................................................... 7-8 

Figure 7-6 Possible Seal Types in a Planar SOFC (from (29)) ........................................... 7-10 

Figure 7-7 Expansion of Typical Cell Components in a 10 cm x 10 cm Planar SOFC 

 with Ni-YSZ anode, YSZ Electrolyte, LSM Cathode, and Ferritic Steel 

 Interconnect........................................................................................................ 7-11 

Figure 7-8 Structure of Mica and Mica-Glass Hybrid Seals and Performance of 

 Hybrid Seals (29) ............................................................................................... 7-13 
xi 

Figure 7-9 Three Types of Tubular SOFC: (a) Conduction around the Tube (e.g. 

 Siemens Westinghouse and Toto (31)); (b) Conduction along the Tube 

 (e.g. Acumentrics (32)); (c) Segmented in Series (e.g. Mitsubishi Heavy 

 Industries, Rolls Royce (33,34)). ....................................................................... 7-14 

Figure 7-10 Cell Performance and Dimensions of Accumentrics Technology (32). ............ 7-15 

Figure 7-11 Schematic cross-section of cylindrical Siemens Westinghouse SOFC Tube. ... 7-16 

Figure 7-12 Gas Manifold Design for a Tubular SOFC and Cell-to-Cell Connections in 

 a Tubular SOFC (41) ......................................................................................... 7-19 

Figure 7-13 Performance Advantage of Sealless Planar (HPD5) over Conventional 

 Siemens Westinghouse Technology (42.).......................................................... 7-21 

Figure 7-14 Effect of Pressure on AES Cell Performance at 1,000 °C (2.2 cm diameter, 

 150 cm active length)......................................................................................... 7-22 

Figure 7-15 Two-Cell Stack Performance with 67 percent H2 + 22 percent CO + 11 

 percent H2O/Air ................................................................................................. 7-23 

Figure 7-16 Two Cell Stack Performance with 97% H2 and 3% H2O/Air (43) .................... 7-25 

Figure 7-17 Cell Performance at 1,000 °C with Pure Oxygen (o) and Air (∆) Both at 25 

 percent Utilization (Fuel (67 percent H2/22 percent CO/11 percent H2O) 

 Utilization is 85 percent).................................................................................... 7-26 

Figure 7-18 Influence of Gas Composition of the Theoretical Open-Circuit Potential 

 of SOFC at 1,000 °C .......................................................................................... 7-27 

Figure 7-19 Variation in Cell Voltage as a Function of Fuel Utilization and Temperature 

 (Oxidant (o - Pure O2; ∆ - Air) Utilization is 25 percent. Current Density is 

 160 mA/cm2 at 800, 900 and 1,000 °C and 79 mA/cm2 at 700 °C)................... 7-28 

Figure 7-20 SOFC Performance at 1,000 °C and 350 mA/cm2, 85 percent Fuel 

 Utilization and 25 percent Air Utilization (Fuel = Simulated Air-Blown 

 Coal Gas Containing 5,000 ppm NH3, 1 ppm HCl and 1 ppm H2S) ................. 7-29 

Figure 7-21 Voltage-Current Characteristics of an AES Cell (1.56 cm Diameter, 

 50 cm Active Length) ........................................................................................ 7-30 

Figure 7-22 Overview of Types of Planar SOFC: (a) Planar Anode-Supported SOFC 

 with Metal Interconnects(68); (b) Electrolyte-Supported Planar SOFC 

 Technology with Metal Interconnect (57,58,68); (c) Electrolyte-Supported 

 Design with “egg-crate” electrolyte shape and ceramic interconnect 

 (62,63,64,65)...................................................................................................... 7-33 

Figure 7-23 Representative State-of-the-Art Button Cell Performance of Anode- 

 Supported SOFC (1) ......................................................................................... 7-37 

Figure 7-24 Single Cell Performance of LSGM Electrolyte (50 μm thick) .......................... 7-38 

Figure 7-25 Effect of Oxidant Composition on a High Performance Anode-Supported 

 Cell..................................................................................................................... 7-39 

Figure 7-26 Examples of State-of-the-Art Planar Anode-Supported SOFC Stacks and 

 Their Performance Characteristics (69,79,78) ................................................... 7-40 

Figure 7-27 Trend in Cell and Single-Cell-Stack Performance in Planar SOFC (69)........... 7-41 

Figure 7-28 Siemens Westinghouse 250 kW Tubular SOFC Installation (31) ..................... 7-42 

Figure 7-29 Example of Window-Pane-Style Stack Scale-Up of Planar Anode-Supported 

 SOFC to 250 kW................................................................................................ 7-43 

Figure 8-1 A Rudimentary Fuel Cell Power System Schematic............................................. 8-1 

Figure 8-2 Representative Fuel Processing Steps & Temperatures....................................... 8-3 
xii 

Figure 8-3 “Well-To-Wheel” Efficiency for Various Vehicle Scenarios (9) ........................ 8-9 

Figure 8-4 Carbon Deposition Mapping of Methane (CH4)................................................ 8-24 

Figure 8-5 Carbon Deposition Mapping of Octane (C8H18)................................................ 8-24 

Figure 8-6 Block diagram of a fuel cell power system........................................................ 8-27 

Figure 8-7a Typical fuel cell voltage / current characteristics .............................................. 8-28 

Figure 8-7b Fuel cell power vs. current curve....................................................................... 8-28 

Figure 8-8 Block diagram of a typical fuel cell powered unit for supplying a load 

 (120V/240V)...................................................................................................... 8-30 

Figure 8-9a Block diagram of the power conditioning unit with line frequency 

 transformer......................................................................................................... 8-31 

Figure 8-9b Circuit topology of the power conditioning unit with line frequency 

 transformer......................................................................................................... 8-31 

Figure 8-10a Block diagram of the power conditioning unit with high frequency isolation 

 transformer within the DC-DC converter stage ................................................. 8-32 

Figure 8-10b Circuit topology of the power conditioning unit with high frequency 

 isolation transformer within the DC-DC converter stage .................................. 8-32 

Figure 8-11a Block diagram of the power conditioning unit with fewer power conversion 

 stages in series path of the power flow .............................................................. 8-33 

Figure 8-11b Circuit topology of the power conditioning unit with fewer power 

conversion stages in series path of the power flow............................................ 8-33 

Figure 8-12 Fuel cell power conditioner control system for powering dedicated loads ....... 8-33 

Figure 8-13 Diagram of a modular fuel cell power conversion unit for supplying backup 

 power to a load connected to a local utility [10,11]........................................... 8-34 

Figure 8-14 Modular power conditioning circuit topology employing two fuel cells to 

 supply a load via a line frequency isolation transformer [10,11] ...................... 8-36 

Figure 8-15 Modular power conditioning circuit topology employing two fuel cells 

 using a higher voltage (400V) dc-link [10,11]................................................... 8-36 

Figure 8-16 Fuel cell supplying a load in parallel with the utility......................................... 8-37 

Figure 8-17 Fuel cell power conditioner control system for supplying power to the 

 utility (utility interface)...................................................................................... 8-38 

Figure 8-18 A typical fuel cell vehicle system [16] .............................................................. 8-39 

Figure 8-19 Power conditioning unit for fuel cell hybrid vehicle ......................................... 8-40 

Figure 8-20 Fuel cell power conditioner control system [16] ............................................... 8-40 

Figure 8-21 Power conditioning unit for the 250kW fuel cell turbine hybrid system........... 8-41 

Figure 8-22 Alternative power conditioning unit for the fuel cell turbine hybrid system 

 with shared dc-link [19]..................................................................................... 8-42 

Figure 8-23 Possible medium voltage power conditioning topology for megawatt range 

 hybrid fuel cell systems [19].............................................................................. 8-43 

Figure 8-24 Representative cost of power conditioning as a function of power and 

 dc-link voltage ................................................................................................... 8-44 

Figure 8-25 Optimization Flexibility in a Fuel Cell Power System ...................................... 8-47 

Figure 8-26 Natural Gas Fueled PEFC Power Plant ............................................................. 8-52 

Figure 8-27 Natural Gas fueled PAFC Power System .......................................................... 8-54 

Figure 8-28 Natural Gas Fueled MCFC Power System ........................................................ 8-56 

Figure 8-29 Schematic for a 4.5 MW Pressurized SOFC...................................................... 8-58 

Figure 8-30 Schematic for a 4 MW Solid State Fuel Cell System ....................................... 8-63 
xiii 

Figure 8-31 Schematic for a 500 MW Class Coal Fueled Pressurized SOFC....................... 8-66 

Figure 8-32 Regenerative Brayton Cycle Fuel Cell Power System ...................................... 8-71 

Figure 8-33 Combined Brayton-Rankine Cycle Fuel Cell Power Generation System ......... 8-74 

Figure 8-34 Combined Brayton-Rankine Cycle Thermodynamics....................................... 8-75 

Figure 8-35 T-Q Plot for Heat Recovery Steam Generator (Brayton-Rankine).................... 8-76 

Figure 8-36 Fuel Cell Rankine Cycle Arrangement.............................................................. 8-77 

Figure 8-37 T-Q Plot of Heat Recovery from Hot Exhaust Gas ........................................... 8-78 

Figure 8-38 MCFC System Designs...................................................................................... 8-83 

Figure 8-39 Stacks in Series Approach Reversibility............................................................ 8-84 

Figure 8-40 MCFC Network ................................................................................................. 8-87 

Figure 8-41 Estimated performance of Power Generation Systems...................................... 8-91

Figure 8-42 Diagram of a Proposed Siemens-Westinghouse Hybrid System....................... 8-91 

Figure 8-43 Overview of Fuel Cell Activities Aimed at APU Applications......................... 8-96 

Figure 8-44 Overview of APU Applications......................................................................... 8-96 

Figure 8-45 Overview of typical system requirements.......................................................... 8-97 

Figure 8-46 Stage of development for fuel cells for APU applications ................................ 8-98 

Figure 8-47 Overview of subsystems and components for SOFC and PEFC systems ....... 8-100 

Figure 8-48 Simplified System process flow diagram of pre-reformer/SOFC system ....... 8-101 

Figure 8-49 Multilevel system modeling approach. ............................................................ 8-102 

Figure 8-50 Projected cost structure of a 5kWnet APU SOFC system. Gasoline fueled 

 POX reformer, Fuel cell operating at 300mW/cm2

, 0.7 V, 90 percent fuel 

 utilization, 500,000 units per year production volume. ................................... 8-104 

Figure 10-1 Equilibrium Constants (Partial Pressures in MPa) for (a) Water Gas Shift, 

 (b) Methane Formation, (c) Carbon Deposition (Boudouard Reaction), and 

 (d) Methane Decomposition (J.R. Rostrup-Nielsen, in Catalysis Science and 

 Technology, Edited by J.R. Anderson and M. Boudart, Springer-Verlag, 

 Berlin GDR, p.1, 1984.)...................................................................................... 10-2 
xiv 

LIST OF TABLES AND EXAMPLES 

Table Title Page

Table 1-1 Summary of Major Differences of the Fuel Cell Types ...................................... 1-8 

Table 1-2 Summary of Major Fuel Constituents Impact on PEFC, AFC, PAFC, 

 MCFC, and SOFC.............................................................................................. 1-14 

Table 1-3 Attributes of Selected Distributed Generation Systems..................................... 1-20 

Table 2-1 Electrochemical Reactions in Fuel Cells ............................................................. 2-4 

Table 2-2 Fuel Cell Reactions and the Corresponding Nernst Equations............................ 2-5 

Table 2-3 Ideal Voltage as a Function of Cell Temperature ................................................ 2-6 

Table 2-4 Outlet Gas Composition as a Function of Utilization in MCFC at 650°C ........ 2-24 

Table 5-1 Evolution of Cell Component Technology for Phosphoric Acid Fuel Cells ....... 5-4 

Table 5-2 Advanced PAFC Performance............................................................................. 5-8 

Table 5-3 Dependence of k(T) on Temperature.................................................................. 5-17 

Table 6-1 Evolution of Cell Component Technology for Molten Carbonate Fuel Cells..... 6-5 

Table 6-2 Amount in Mol percent of Additives to Provide Optimum Performance (39).. 6-11 

Table 6-3 Qualitative Tolerance Levels for Individual Contaminants in Isothermal 

 Bench-Scale Carbonate Fuel Cells (46, 47, and 48) .......................................... 6-13 

Table 6-4 Equilibrium Composition of Fuel Gas and Reversible Cell Potential as a 

 Function of Temperature.................................................................................... 6-20 

Table 6-5 Influence of Fuel Gas Composition on Reversible Anode Potential at 650 °C 

 (68, Table 1, Pg. 385) ......................................................................................... 6-24 

Table 6-6 Contaminants from Coal-Derived Fuel Gas and Their Potential Effect on 

 MCFCs (70, Table 1, Pg. 299) ........................................................................... 6-26 

Table 6-7 Gas Composition and Contaminants from Air-Blown Coal Gasifier After 

 Hot Gas Cleanup, and Tolerance Limit of MCFCs to Contaminants................ 6-27 

Table 7-1 Evolution of Cell Component Technology for Tubular Solid Oxide Fuel 

 Cells ................................................................................................................... 7-17 

Table 7-2 K Values for ∆VT............................................................................................... 7-24 

Table 7-3 SECA Program Goals for SOFC Stacks (71) .................................................... 7-34 

Table 7-4 Recent Technology Advances on Planar Cells and Potential Benefits.............. 7-36 

Table 7-5 SOFC Manufacturers and Status of Their Technology...................................... 7-44 

Table 8-1 Calculated Thermoneutral Oxygen-to-Fuel Molar Ratios (xo) and Maximum 

 Theoretical Efficiencies (at xo) for Common Fuels (23).................................... 8-16 

Table 8-2 Typical Steam Reformed Natural Gas Reformate ............................................. 8-17 

Table 8-3 Typical Partial Oxidation Reformed Fuel Oil Reformate (24) .......................... 8-19 

Table 8-4 Typical Coal Gas Compositions for Selected Oxygen-Blown Gasifiers........... 8-21 

Table 8-5 Specifications of a typical fuel cell power conditioning unit for stand-alone 

 domestic (U.S.) loads......................................................................................... 8-29 

Table 8-6 Example specifications for the 1kW fuel cell powered backup power 

 (UPS) unit [10,11].............................................................................................. 8-35 

Table 8-7 Specifications of 500W PEFC fuel cell stack (available from Avista 

 Labs [1])............................................................................................................. 8-36 

Table 8-8 Stream Properties for the Natural Gas Fueled Pressurized PAFC..................... 8-54 

Table 8-9 Operating/Design Parameters for the NG fueled PAFC.................................... 8-55 

Table 8-10 Performance Summary for the NG fueled PAFC .............................................. 8-55 
xv 

Table 8-11 Operating/Design Parameters for the NG Fueled IR-MCFC............................. 8-57 

Table 8-12 Overall Performance Summary for the NG Fueled IR-MCFC.......................... 8-57 

Table 8-13 Stream Properties for the Natural Gas Fueled Pressurized SOFC..................... 8-59 

Table 8-14 Operating/Design Parameters for the NG Fueled Pressurized SOFC................ 8-60 

Table 8-15 Overall Performance Summary for the NG Fueled Pressurized SOFC............. 8-61 

Table 8-16 Heron Gas Turbine Parameters.......................................................................... 8-61 

Table 8-17 Example Fuel Utilization in a Multi-Stage Fuel Cell Module........................... 8-62 

Table 8-18 Stream Properties for the Natural Gas Fueled Solid State Fuel Cell Power 

 Plant System....................................................................................................... 8-63 

Table 8-19 Operating/Design Parameters for the NG fueled Multi-Stage Fuel Cell 

 System................................................................................................................ 8-65 

Table 8-20 Overall Performance Summary for the NG fueled Multi-StageFuel Cell 

 System................................................................................................................ 8-65 

Table 8-21 Stream Properties for the 500 MW Class Coal Gas Fueled Cascaded SOFC ... 8-67 

Table 8-22 Coal Analysis..................................................................................................... 8-68 

Table 8-23 Operating/Design Parameters for the Coal Fueled Pressurized SOFC.............. 8-69 

Table 8-24 Overall Performance Summary for the Coal Fueled Pressurized SOFC ........... 8-69 

Table 8-25 Performance Calculations for a Pressurized, High Temperature Fuel Cell 

 (SOFC) with a Regenerative Brayton Bottoming Cycle; Approach Delta 

 T=30 o

F .............................................................................................................. 8-72 

Table 8-26 Performance Computations for Various High Temperature Fuel Cell 

 (SOFC) Heat Recovery Arrangements .............................................................. 8-73 

Table 9-1 HHV Contribution of Common Gas Constituents............................................. 9-23 

Table 9-2 Distributive Estimating Factors ......................................................................... 9-26 

Table10-1 Typical Contaminant Levels Obtained from Selected Coal Gasification 

 Processes............................................................................................................ 10-3 

Table 10-2 Summary of Related Codes and Standards...................................................... 10-17 

Table 10-3 DoD Field Site ................................................................................................. 10-25 

Table 10-4 IFC Field Units ................................................................................................ 10-27 

Table 10-5 FuelCell Energy Field Sites (mid-year 2000) .................................................. 10-30 

Table 10-6 Siemens Westinghouse SOFC Field Units (mid-year 2002) ........................... 10-30 

Table 10-7 Hydrogen Producers3 ....................................................................................... 10-33 

Table 10-8 World Mine Production and Reserves ............................................................. 10-39 

Table 10-9 Rhodia Rare Earth Oxide Prices in 2002 ......................................................... 10-39 
xvi 

FORWARD

Fuel cells are one of the cleanest and most efficient technologies for generating electricity. Since 

there is no combustion, there are none of the pollutants commonly produced by boilers and 

furnaces. For systems designed to consume hydrogen directly, the only products are electricity, 

water and heat. Fuel cells are an important technology for a potentially wide variety of 

applications including on-site electric power for households and commercial buildings; 

supplemental or auxiliary power to support car, truck and aircraft systems; power for personal, 

mass and commercial transportation; and the modular addition by utilities of new power 

generation closely tailored to meet growth in power consumption. These applications will be in 

a large number of industries worldwide. 

In this Seventh Edition of the Fuel Cell Handbook, we have discussed the Solid State Energy 

Conversion Alliance Program (SECA) activities. In addition, individual fuel cell technologies 

and other supporting materials have been updated. Finally, an updated index assists the reader in 

locating specific information quickly. 

It is an important task that NETL undertakes to provide you with this handbook. We realize it is 

an important educational and informational tool for a wide audience. We welcome suggestions 

to improve the handbook. 

Mark C. Williams 

Strategic Center for Natural Gas 

National Energy Technology Laboratory 
xvii 

PREFACE

The last edition of the Fuel Cell Handbook was published in November, 2002. Since that time, 

the Solid State Energy Conversion Alliance (SECA-www.seca.doe.gov) has funded activities to 

bring about dramatic reductions in fuel cell costs, and rates as the most important event to report 

on since the 2000 edition. SECA industry teams’ have continued to evaluate and test fuel cell 

designs, candidate materials, manufacturing methods, and balance-of-plant subsystems. SECA’s 

goal is to cut costs to as low as $400 per kilowatt by the end of this decade, which would make 

fuel cells competitive for virtually every type of power application. The initiative signifies the 

Department's objective of developing a modular, all-solid-state fuel cell that could be massproduced for different uses much the way electronic components are manufactured and sold 

today. 

SECA has six industry teams working on competing designs for the distributed generation and 

auxiliary power applications. These teams are headed by: FuelCell Energy, Delphi Battelle, 

General Electric Company, Siemens Westinghouse, Acumentrics, and Cummins Power 

Generation and SOFCo. The SECA industry teams receive core technology support from 

leading researchers at small businesses, universities and national laboratories. Over 30 SECA 

R&D projects are generating new scientific and engineering knowledge, creating technology 

breakthroughs by addressing technical risks and barriers that currently limit achieving SECA 

performance and cost goals. 

U.S. Department of Energy’s (DOE’s) SECA program, have considerably advanced the 

knowledge and development of thin-electrolyte planar SOFC. As a consequence of the 

performance improvements, SOFC are now considered for a wide range of applications, including 

stationary power generation, mobile power, auxiliary power for vehicles, and specialty 

applications. A new generation of intermediate temperature (650-800 o

C) SOFCs is being 

developed under the U.S. DOE’s SECA program. Fuel processing by an autothermal, steam, or 

partial oxidation reformer that operates between 500-800 °C enables fuel cell operation on 

gasoline, diesel fuel, and other hydrocarbon fuels. 

This Handbook provides a foundation in fuel cells for persons wanting a better understanding of 

the technology, its benefits, and the systems issues that influence its application. Trends in 

technology are discussed, including next-generation concepts that promise ultra-high efficiency 

and low cost, while providing exceptionally clean power plant systems. Section 1 summarizes 

fuel cell progress since the last edition, and includes existing power plant nameplate data. 

Section 2 addresses the thermodynamics of fuel cells to provide an understanding of fuel cell 

operation. Sections 3 through 7 describe the five major fuel cell types and their performance. 
xviii 

Polymer electrolyte, alkaline, phosphoric acid, molten carbonate, and solid oxide fuel cell 

technology descriptions have been updated from the previous edition. Manufacturers are 

focusing on reducing fuel cell life cycle costs. In this edition, we have included over 5,000 fuel 

cell patent abstracts and their claims. In addition, the handbook features a new fuel cell power 

conditioning section, and overviews on the hydrogen industry and rare earth minerals market. 
xix 

ACKNOWLEDGEMENTS

The authors of this edition of the Fuel Cell Handbook acknowledge the cooperation of the fuel cell 

community for their contributions to this Handbook. Many colleagues provided data, information, 

references, valuable suggestions, and constructive comments that were incorporated into the 

Handbook. In particular, we would like to acknowledge the contributions J. Thijssen. 

The authors wish to thank M. Williams, and H. Quedenfeld of the U.S. Department of Energy, 

National Energy Technology Laboratory, for their support and encouragement, and for providing 

the opportunity to enhance the quality of this Handbook. 

This work was supported by the U.S. Department of Energy, National Energy Technology 

Laboratory, under Contract DE-AM21-94MC31166.
1-1 

1. TECHNOLOGY OVERVIEW

This chapter provides an overview of fuel cell technology. First it discusses the basic workings 

of fuel cells and basic fuel cell system components. Then, an overview of the main fuel cell 

types, their characteristics, and their development status is provided. Finally, this chapter reviews 

potential fuel cell applications. 

1.1 Introduction 

Fuel cells are electrochemical devices that convert chemical energy in fuels into electrical energy 

directly, promising power generation with high efficiency and low environmental impact. 

Because the intermediate steps of producing heat and mechanical work typical of most 

conventional power generation methods are avoided, fuel cells are not limited by thermodynamic 

limitations of heat engines such as the Carnot efficiency. In addition, because combustion is 

avoided, fuel cells produce power with minimal pollutant. However, unlike batteries the 

reductant and oxidant in fuel cells must be continuously replenished to allow continuous 

operation. Fuel cells bear significant resemblance to electrolyzers. In fact, some fuel cells operate 

in reverse as electrolyzers, yielding a reversible fuel cell that can be used for energy storage. 

Though fuel cells could, in principle, process a wide variety of fuels and oxidants, of most 

interest today are those fuel cells that use common fuels (or their derivatives) or hydrogen as a 

reductant, and ambient air as the oxidant. 

Most fuel cell power systems comprise a number of components: 

• Unit cells, in which the electrochemical reactions take place 

• Stacks, in which individual cells are modularly combined by electrically connecting the cells 

to form units with the desired output capacity 

• Balance of plant which comprises components that provide feedstream conditioning 

(including a fuel processor if needed), thermal management, and electric power conditioning 

among other ancillary and interface functions 

In the following, an overview of fuel cell technology is given according to each of these 

categories, followed by a brief review of key potential applications of fuel cells. 
1-2 

1.2 Unit Cells 

1.2.1 Basic Structure 

Unit cells form the core of a fuel cell. These devices convert the chemical energy contained in a 

fuel electrochemically into electrical energy. The basic physical structure, or building block, of a 

fuel cell consists of an electrolyte layer in contact with an anode and a cathode on either side. A 

schematic representation of a unit cell with the reactant/product gases and the ion conduction flow 

directions through the cell is shown in Figure 1-1. 

Load 2eFuel In Oxidant In

Positive Ion

or

Negative Ion

Depleted Oxidant and

Product Gases Out

Depleted Fuel and

Product Gases Out

Anode Cathode

Electrolyte

(Ion Conductor)

H2

H2O

H2O

1⁄2O2

Figure 1-1 Schematic of an Individual Fuel Cell 

In a typical fuel cell, fuel is fed continuously to the anode (negative electrode) and an oxidant 

(often oxygen from air) is fed continuously to the cathode (positive electrode). The 

electrochemical reactions take place at the electrodes to produce an electric current through the 

electrolyte, while driving a complementary electric current that performs work on the load. 

Although a fuel cell is similar to a typical battery in many ways, it differs in several respects. 

The battery is an energy storage device in which all the energy available is stored within the 

battery itself (at least the reductant). The battery will cease to produce electrical energy when 

the chemical reactants are consumed (i.e., discharged). A fuel cell, on the other hand, is an 

energy conversion device to which fuel and oxidant are supplied continuously. In principle, the 

fuel cell produces power for as long as fuel is supplied. 

 

Fuel cells are classified according to the choice of electrolyte and fuel, which in turn determine 

the electrode reactions and the type of ions that carry the current across the electrolyte. Appleby 

and Foulkes (1) have noted that, in theory, any substance capable of chemical oxidation that can 

be supplied continuously (as a fluid) can be burned galvanically as fuel at the anode of a fuel 

cell. Similarly, the oxidant can be any fluid that can be reduced at a sufficient rate. Though the 

direct use of conventional fuels in fuel cells would be desirable, most fuel cells under 

development today use gaseous hydrogen, or a synthesis gas rich in hydrogen, as a fuel. 

Hydrogen has a high reactivity for anode reactions, and can be produced chemically from a wide 

range of fossil and renewable fuels, as well as via electrolysis. For similar practical reasons, the 

most common oxidant is gaseous oxygen, which is readily available from air. For space 
1-3 

applications, both hydrogen and oxygen can be stored compactly in cryogenic form, while the 

reaction product is only water. 

1.2.2 Critical Functions of Cell Components 

A critical portion of most unit cells is often referred to as the three-phase interface. These mostly 

microscopic regions, in which the actual electrochemical reactions take place, are found where 

either electrode meets the electrolyte. For a site or area to be active, it must be exposed to the 

reactant, be in electrical contact with the electrode, be in ionic contact with the electrolyte, and 

contain sufficient electro-catalyst for the reaction to proceed at the desired rate. The density of 

these regions and the nature of these interfaces play a critical role in the electrochemical 

performance of both liquid and solid electrolyte fuel cells: 

• In liquid electrolyte fuel cells, the reactant gases diffuse through a thin electrolyte film that 

wets portions of the porous electrode and react electrochemically on their respective 

electrode surface. If the porous electrode contains an excessive amount of electrolyte, the 

electrode may "flood" and restrict the transport of gaseous species in the electrolyte phase to 

the reaction sites. The consequence is a reduction in electrochemical performance of the 

porous electrode. Thus, a delicate balance must be maintained among the electrode, 

electrolyte, and gaseous phases in the porous electrode structure. 

• In solid electrolyte fuel cells, the challenge is to engineer a large number of catalyst sites into 

the interface that are electrically and ionically connected to the electrode and the electrolyte, 

respectively, and that is efficiently exposed to the reactant gases. In most successful solid 

electrolyte fuel cells, a high-performance interface requires the use of an electrode which, in 

the zone near the catalyst, has mixed conductivity (i.e. it conducts both electrons and ions). 

Over the past twenty years, the unit cell performance of at least some of the fuel cell 

technologies has been dramatically improved. These developments resulted from improvements 

in the three-phase boundary, reducing the thickness of the electrolyte, and developing improved 

electrode and electrolyte materials which broaden the temperature range over which the cells can 

be operated. 

In addition to facilitating electrochemical reactions, each of the unit cell components have other 

critical functions. The electrolyte not only transports dissolved reactants to the electrode, but also 

conducts ionic charge between the electrodes, and thereby completes the cell electric circuit as 

illustrated in Figure 1-1. It also provides a physical barrier to prevent the fuel and oxidant gas 

streams from directly mixing. 

The functions of porous electrodes in fuel cells, in addition to providing a surface for 

electrochemical reactions to take place, are to: 

1) conduct electrons away from or into the three-phase interface once they are formed (so an 

electrode must be made of materials that have good electrical conductance) and provide 

current collection and connection with either other cells or the load 

2) ensure that reactant gases are equally distributed over the cell 

3) ensure that reaction products are efficiently led away to the bulk gas phase 
1-4 

As a consequence, the electrodes are typically porous and made of an electrically conductive 

material. At low temperatures, only a few relatively rare and expensive materials provide sufficient 

electro-catalytic activity, and so such catalysts are deposited in small quantities at the interface 

where they are needed. In high-temperature fuel cells, the electro-catalytic activity of the bulk 

electrode material is often sufficient. 

Though a wide range of fuel cell geometries has been considered, most fuel cells under 

development now are either planar (rectangular or circular) or tubular (either single- or doubleended and cylindrical or flattened). 

1.3 Fuel Cell Stacking 

For most practical fuel cell applications, unit cells must be combined in a modular fashion into a 

cell stack to achieve the voltage and power output level required for the application. Generally, the 

stacking involves connecting multiple unit cells in series via electrically conductive interconnects. 

Different stacking arrangements have been developed, which are described below. 

1.3.1 Planar-Bipolar Stacking 

The most common fuel cell stack design is the so-called planar-bipolar arrangement (Figure 1-2 

depicts a PAFC). Individual unit cells are electrically connected with interconnects. Because of the 

configuration of a flat plate cell, the interconnect becomes a separator plate with two functions: 

1) to provide an electrical series connection between adjacent cells, specifically for flat plate 

cells, and 

2) to provide a gas barrier that separates the fuel and oxidant of adjacent cells. 

In many planar-bipolar designs, the interconnect also includes channels that distribute the gas flow 

over the cells. The planar-bipolar design is electrically simple and leads to short electronic current 

paths (which helps to minimize cell resistance). 

Figure 1-2 Expanded View of a Basic Fuel Cell Unit in a Fuel Cell Stack (1) 
1-5 

Planar-bipolar stacks can be further characterized according to arrangement of the gas flow: 

• Cross-flow. Air and fuel flow perpendicular to each other 

• Co-flow. Air and fuel flow parallel and in the same direction. In the case of circular 

cells, this means the gases flow radially outward 

• Counter-flow. Air and fuel flow parallel but in opposite directions. Again, in the case 

of circular cells this means radial flow 

• Serpentine flow. Air or fuel follow a zig-zag path 

• Spiral flow. Applies to circular cells 

The choice of gas-flow arrangement depends on the type of fuel cell, the application, and other 

considerations. Finally, the manifolding of gas streams to the cells in bipolar stacks can be 

achieved in various ways: 

• Internal: the manifolds run through the unit cells 

• Integrated: the manifolds do not penetrate the unit cells but are integrated in the 

interconnects 

• External: the manifold is completely external to the cell, much like a wind-box 

1.3.2 Stacks with Tubular Cells 

Especially for high-temperature fuel cells, stacks with tubular cells have been developed. 

Tubular cells have significant advantages in sealing and in the structural integrity of the cells. 

However, they represent a special geometric challenge to the stack designer when it comes to 

achieving high power density and short current paths. In one of the earliest tubular designs the 

current is conducted tangentially around the tube. Interconnects between the tubes are used to 

form rectangular arrays of tubes. Alternatively, the current can be conducted along the axis of the 

tube, in which case interconnection is done at the end of the tubes. To minimize the length of 

electronic conduction paths for individual cells, sequential series connected cells are being 

developed. The cell arrays can be connected in series or in parallel. For a more detailed 

description of the different stack types and pictorial descriptions, the reader is referred to Chapter 

7 on SOFC (SOFC is the fuel cell type for which the widest range of cell and stack geometries is 

pursued). 

To avoid the packing density limitations associated with cylindrical cells, some tubular stack 

designs use flattened tubes. 

1.4 Fuel Cell Systems 

In addition to the stack, practical fuel cell systems require several other sub-systems and 

components; the so-called balance of plant (BoP). Together with the stack, the BoP forms the 

fuel cell system. The precise arrangement of the BoP depends heavily on the fuel cell type, the 

fuel choice, and the application. In addition, specific operating conditions and requirements of 

individual cell and stack designs determine the characteristics of the BoP. Still, most fuel cell 

systems contain: 
1-6 

• Fuel preparation. Except when pure fuels (such as pure hydrogen) are used, some fuel 

preparation is required, usually involving the removal of impurities and thermal conditioning. 

In addition, many fuel cells that use fuels other than pure hydrogen require some fuel 

processing, such as reforming, in which the fuel is reacted with some oxidant (usually steam 

or air) to form a hydrogen-rich anode feed mixture. 

• Air supply. In most practical fuel cell systems, this includes air compressors or blowers as 

well as air filters. 

• Thermal management. All fuel cell systems require careful management of the fuel cell stack 

temperature. 

• Water management. Water is needed in some parts of the fuel cell, while overall water is a 

reaction product. To avoid having to feed water in addition to fuel, and to ensure smooth 

operation, water management systems are required in most fuel cell systems. 

• Electric power conditioning equipment. Since fuel cell stacks provide a variable DC voltage 

output that is typically not directly usable for the load, electric power conditioning is 

typically required. 

While perhaps not the focus of most development effort, the BoP represents a significant fraction 

of the weight, volume, and cost of most fuel cell systems. 

Figure 1-3 shows a simple rendition of a fuel cell power plant. Beginning with fuel processing, a 

conventional fuel (natural gas, other gaseous hydrocarbons, methanol, naphtha, or coal) is 

cleaned, then converted into a gas containing hydrogen. Energy conversion occurs when dc 

electricity is generated by means of individual fuel cells combined in stacks or bundles. A 

varying number of cells or stacks can be matched to a particular power application. Finally, 

power conditioning converts the electric power from dc into regulated dc or ac for consumer use. 

Section 8.1 describes the processes of a fuel cell power plant system. 
1-7 

Clean

E xhaust

Fuel

Processor

Power

Section

Power

Conditioner

Air

AC

Power

H2-Rich

Gas

DC

Power

Usable

Heat

Natural

Gas

Steam

Clean

Exhaust

Figure 1-3 Fuel Cell Power Plant Major Processes 

1.5 Fuel Cell Types 

A variety of fuel cells are in different stages of development. The most common classification of 

fuel cells is by the type of electrolyte used in the cells and includes 1) polymer electrolyte fuel cell 

(PEFC), 2) alkaline fuel cell (AFC), 3) phosphoric acid fuel cell (PAFC), 4) molten carbonate 

fuel cell (MCFC), and 5) solid oxide fuel cell (SOFC). Broadly, the choice of electrolyte dictates 

the operating temperature range of the fuel cell. The operating temperature and useful life of a fuel 

cell dictate the physicochemical and thermomechanical properties of materials used in the cell 

components (i.e., electrodes, electrolyte, interconnect, current collector, etc.). Aqueous electrolytes 

are limited to temperatures of about 200 °C or lower because of their high vapor pressure and rapid 

degradation at higher temperatures. The operating temperature also plays an important role in 

dictating the degree of fuel processing required. In low-temperature fuel cells, all the fuel must be 

converted to hydrogen prior to entering the fuel cell. In addition, the anode catalyst in lowtemperature fuel cells (mainly platinum) is strongly poisoned by CO. In high-temperature fuel 

cells, CO and even CH4 can be internally converted to hydrogen or even directly oxidized 

electrochemically. Table 1-1 provides an overview of the key characteristics of the main fuel cell 

types. 
1-8 

Table 1-1 Summary of Major Differences of the Fuel Cell Types 

PEFC AFC PAFC MCFC SOFC 

Electrolyte Hydrated 

Polymeric Ion 

Exchange 

Membranes 

Mobilized or 

Immobilized 

Potassium 

Hydroxide in 

asbestos 

matrix 

Immobilized 

Liquid 

Phosphoric 

Acid in SiC 

Immobilized 

Liquid 

Molten 

Carbonate in 

LiAlO2 

Perovskites 

(Ceramics) 

Electrodes 

Carbon Transition 

metals Carbon Nickel and Nickel Oxide 

Perovskite 

and 

perovskite / 

metal cermet 

Catalyst Platinum Platinum Platinum Electrode 

material 

Electrode 

material 

Interconnect Carbon or 

metal Metal Graphite Stainless steel or Nickel 

Nickel, 

ceramic, or 

steel 

Operating 

Temperature 40 – 80 °C 65°C – 220 °C 205 °C 650 °C 600-1000 °C 

Charge 

Carrier H+

 OH- H+ CO3= O=

External 

Reformer for 

hydrocarbon 

fuels 

Yes Yes Yes No, for some 

fuels 

No, for some 

fuels and 

cell designs 

External 

shift 

conversion 

of CO to 

hydrogen 

Yes, plus 

purification to 

remove trace 

CO 

Yes, plus 

purification to 

remove CO 

and CO2 

Yes No No 

Prime Cell 

Components Carbon-based Carbon-based Graphite-based Stainlessbased Ceramic 

Product 

Water 

Management 

Evaporative Evaporative Evaporative Gaseous 

Product 

Gaseous 

Product 

Product Heat 

Management 

Process Gas + 

Liquid 

Cooling 

Medium 

Process Gas + 

Electrolyte 

Circulation 

Process Gas + 

Liquid cooling 

medium or 

steam 

generation 

Internal 

Reforming + 

Process Gas 

Internal 

Reforming + 

Process Gas 

In parallel with the classification by electrolyte, some fuel cells are classified by the type of fuel 

used: 

• Direct Alcohol Fuel Cells (DAFC). DAFC (or, more commonly, direct methanol fuel cells or 

DMFC) use alcohol without reforming. Mostly, this refers to a PEFC-type fuel cell in which 

methanol or another alcohol is used directly, mainly for portable applications. A more 

detailed description of the DMFC or DAFC is provided in Chapter 3; 
1-9 

• Direct Carbon Fuel Cells (DCFC). In direct carbon fuel cells, solid carbon (presumably a fuel 

derived from coal, pet-coke or biomass) is used directly in the anode, without an intermediate 

gasification step. Concepts with solid oxide, molten carbonate, and alkaline electrolytes are 

all under development. The thermodynamics of the reactions in a DCFC allow very high 

efficiency conversion. Therefore, if the technology can be developed into practical systems, 

it could ultimately have a significant impact on coal-based power generation. 

A brief description of various electrolyte cells of interest follows. Detailed descriptions of these 

fuel cells may be found in References (1) and (2). 

1.5.1 Polymer Electrolyte Fuel Cell (PEFC) 

The electrolyte in this fuel cell is an ion exchange membrane (fluorinated sulfonic acid polymer 

or other similar polymer) that is an excellent proton conductor. The only liquid in this fuel cell is 

water; thus, corrosion problems are minimal. Typically, carbon electrodes with platinum electrocatalyst are used for both anode and cathode, and with either carbon or metal interconnects. 

Water management in the membrane is critical for efficient performance; the fuel cell must 

operate under conditions where the by-product water does not evaporate faster than it is 

produced because the membrane must be hydrated. Because of the limitation on the operating 

temperature imposed by the polymer, usually less than 100 °C, but more typically around 60 to 

80 °C. , and because of problems with water balance, a H2-rich gas with minimal or no CO (a 

poison at low temperature) is used. Higher catalyst loading (Pt in most cases) than that used in 

PAFCs is required for both the anode and cathode. Extensive fuel processing is required with 

other fuels, as the anode is easily poisoned by even trace levels of CO, sulfur species, and 

halogens. 

PEFCs are being pursued for a wide variety of applications, especially for prime power for fuel 

cell vehicles (FCVs). As a consequence of the high interest in FCVs and hydrogen, the 

investment in PEFC over the past decade easily surpasses all other types of fuel cells combined. 

Although significant development of PEFC for stationary applications has taken place, many 

developers now focus on automotive and portable applications. 

Advantages: The PEFC has a solid electrolyte which provides excellent resistance to gas 

crossover. The PEFC’s low operating temperature allows rapid start-up and, with the absence of 

corrosive cell constituents, the use of the exotic materials required in other fuel cell types, both in 

stack construction and in the BoP is not required. Test results have demonstrated that PEFCs are 

capable of high current densities of over 2 kW/l and 2 W/cm2

. The PEFC lends itself particularly 

to situations where pure hydrogen can be used as a fuel. 

Disadvantages: The low and narrow operating temperature range makes thermal management 

difficult, especially at very high current densities, and makes it difficult to use the rejected heat 

for cogeneration or in bottoming cycles. Water management is another significant challenge in 

PEFC design, as engineers must balance ensuring sufficient hydration of the electrolyte against 

flooding the electrolyte. In addition, PEFCs are quite sensitive to poisoning by trace levels of 

contaminants including CO, sulfur species, and ammonia. To some extent, some of these 

disadvantages can be counteracted by lowering operating current density and increasing 
1-10 

electrode catalyst loading, but both increase cost of the system. If hydrocarbon fuels are used, the 

extensive fuel processing required negatively impacts system size, complexity, efficiency 

(typically in the mid thirties), and system cost. Finally, for hydrogen PEFC the need for a 

hydrogen infrastructure to be developed poses a barrier to commercialization. 

1.5.2 Alkaline Fuel Cell (AFC) 

The electrolyte in this fuel cell is concentrated (85 wt percent) KOH in fuel cells operated at high 

temperature (~250 °C), or less concentrated (35 to 50 wt percent) KOH for lower temperature 

(<120 °C) operation. The electrolyte is retained in a matrix (usually asbestos), and a wide range 

of electro-catalysts can be used (e.g., Ni, Ag, metal oxides, spinels, and noble metals). The fuel 

supply is limited to non-reactive constituents except for hydrogen. CO is a poison, and CO2 will 

react with the KOH to form K2CO3, thus altering the electrolyte. Even the small amount of CO2

in air must be considered a potential poison for the alkaline cell. Generally, hydrogen is 

considered as the preferred fuel for AFC, although some direct carbon fuel cells use (different) 

alkaline electrolytes. 

The AFC was one of the first modern fuel cells to be developed, beginning in 1960. The 

application at that time was to provide on-board electric power for the Apollo space vehicle. The 

AFC has enjoyed considerable success in space applications, but its terrestrial application has 

been challenged by its sensitivity to CO2. Still, some developers in the U.S. and Europe pursue 

AFC for mobile and closed-system (reversible fuel cell) applications. 

Advantages: Desirable attributes of the AFC include its excellent performance on hydrogen (H2) 

and oxygen (O2) compared to other candidate fuel cells due to its active O2 electrode kinetics and 

its flexibility to use a wide range of electro-catalysts. 

Disadvantages: The sensitivity of the electrolyte to CO2 requires the use of highly pure H2 as a 

fuel. As a consequence, the use of a reformer would require a highly effective CO and CO2

removal system. In addition, if ambient air is used as the oxidant, the CO2 in the air must be 

removed. While this is technically not challenging, it has a significant impact on the size and cost 

of the system. 

1.5.3 Phosphoric Acid Fuel Cell (PAFC) 

Phosphoric acid, concentrated to 100 percent, is used as the electrolyte in this fuel cell, which 

typically operates at 150 to 220 °C. At lower temperatures, phosphoric acid is a poor ionic 

conductor, and CO poisoning of the Pt electro-catalyst in the anode becomes severe. The 

relative stability of concentrated phosphoric acid is high compared to other common acids; 

consequently the PAFC is capable of operating at the high end of the acid temperature range 

(100 to 220 °C). In addition, the use of concentrated acid (100 percent) minimizes the water 

vapor pressure so water management in the cell is not difficult. The matrix most commonly used 

to retain the acid is silicon carbide (1), and the electro-catalyst in both the anode and cathode is 

Pt. 

PAFCs are mostly developed for stationary applications. Both in the U.S. and Japan, hundreds of 

PAFC systems were produced, sold, and used in field tests and demonstrations. It is still one of 

the few fuel cell systems that are available for purchase. Development of PAFC had slowed 
1-11 

down in the past ten years, in favor of PEFCs that were thought to have better cost potential. 

However, PAFC development continues. 

Advantages: PAFCs are much less sensitive to CO than PEFCs and AFCs: PAFCs tolerate 

about one percent of CO as a diluent. The operating temperature is still low enough to allow the 

use of common construction materials, at least in the BoP components. The operating 

temperature also provides considerable design flexibility for thermal management. PAFCs have 

demonstrated system efficiencies of 37 to 42 percent (based on LHV of natural gas fuel), which 

is higher than most PEFC systems could achieve (but lower than many of the SOFC and MCFC 

systems). In addition, the waste heat from PAFC can be readily used in most commercial and 

industrial cogeneration applications, and would technically allow the use of a bottoming cycle. 

Disadvantages: Cathode-side oxygen reduction is slower than in AFC, and requires the use of a 

Platinum catalyst. Although less complex than for PEFC, PAFCs still require extensive fuel 

processing, including typically a water gas shift reactor to achieve good performance. Finally, 

the highly corrosive nature of phosphoric acid requires the use of expensive materials in the 

stack (especially the graphite separator plates). 

1.5.4 Molten Carbonate Fuel Cell (MCFC) 

The electrolyte in this fuel cell is usually a combination of alkali carbonates, which is retained in 

a ceramic matrix of LiAlO2. The fuel cell operates at 600 to 700 °C where the alkali carbonates 

form a highly conductive molten salt, with carbonate ions providing ionic conduction. At the 

high operating temperatures in MCFCs, Ni (anode) and nickel oxide (cathode) are adequate to 

promote reaction. Noble metals are not required for operation, and many common hydrocarbon 

fuels can be reformed internally. 

The focus of MCFC development has been larger stationary and marine applications, where the 

relatively large size and weight of MCFC and slow start-up time are not an issue. MCFCs are 

under development for use with a wide range of conventional and renewable fuels. MCFC-like 

technology is also considered for DCFC. After the PAFC, MCFCs have been demonstrated most 

extensively in stationary applications, with dozens of demonstration projects either under way or 

completed. While the number of MCFC developers and the investment level are reduced 

compared to a decade ago, development and demonstrations continue. 

Advantages: The relatively high operating temperature of the MCFC (650 °C) results in several 

benefits: no expensive electro-catalysts are needed as the nickel electrodes provide sufficient 

activity, and both CO and certain hydrocarbons are fuels for the MCFC, as they are converted to 

hydrogen within the stack (on special reformer plates) simplifying the BoP and improving 

system efficiency to the high forties to low fifties. In addition, the high temperature waste heat 

allows the use of a bottoming cycle to further boost the system efficiency to the high fifties to 

low sixties. 

Disadvantages: The main challenge for MCFC developers stems from the very corrosive and 

mobile electrolyte, which requires use of nickel and high-grade stainless steel as the cell 

hardware (cheaper than graphite, but more expensive than ferritic steels). The higher 

temperatures promote material problems, impacting mechanical stability and stack life. 
1-12 

Also, a source of CO2 is required at the cathode (usually recycled from anode exhaust) to form 

the carbonate ion, representing additional BoP components. High contact resistances and cathode 

resistance limit power densities to around 100 – 200 mW/cm2 at practical operating voltages. 

1.5.5 Solid Oxide Fuel Cell (SOFC) 

The electrolyte in this fuel cell is a solid, nonporous metal oxide, usually Y2O3-stabilized ZrO2. 

The cell operates at 600-1000 °C where ionic conduction by oxygen ions takes place. Typically, 

the anode is Co-ZrO2 or Ni-ZrO2 cermet, and the cathode is Sr-doped LaMnO3. 

Early on, the limited conductivity of solid electrolytes required cell operation at around 1000 °C, 

but more recently thin-electrolyte cells with improved cathodes have allowed a reduction in 

operating temperature to 650 – 850 °C. Some developers are attempting to push SOFC operating 

temperatures even lower. Over the past decade, this has allowed the development of compact and 

high-performance SOFC which utilized relatively low-cost construction materials. 

Concerted stack development efforts, especially through the U.S. DOE’s SECA program, have 

considerably advanced the knowledge and development of thin-electrolyte planar SOFC. As a 

consequence of the performance improvements, SOFCs are now considered for a wide range of 

applications, including stationary power generation, mobile power, auxiliary power for vehicles, 

and specialty applications. 

Advantages: The SOFC is the fuel cell with the longest continuous development period, starting 

in the late 1950s, several years before the AFC. Because the electrolyte is solid, the cell can be 

cast into various shapes, such as tubular, planar, or monolithic. The solid ceramic construction 

of the unit cell alleviates any corrosion problems in the cell. The solid electrolyte also allows 

precise engineering of the three-phase boundary and avoids electrolyte movement or flooding in 

the electrodes. The kinetics of the cell are relatively fast, and CO is a directly useable fuel as it is 

in the MCFC. There is no requirement for CO2 at the cathode as with the MCFC. The materials 

used in SOFC are modest in cost. Thin-electrolyte planar SOFC unit cells have been 

demonstrated to be cable of power densities close to those achieved with PEFC. As with the 

MCFC, the high operating temperature allows use of most of the waste heat for cogeneration or 

in bottoming cycles. Efficiencies ranging from around 40 percent (simple cycle small systems) to 

over 50 percent (hybrid systems) have been demonstrated, and the potential for 60 percent+ 

efficiency exists as it does for MCFC. 

Disadvantages: The high temperature of the SOFC has its drawbacks. There are thermal 

expansion mismatches among materials, and sealing between cells is difficult in the flat plate 

configurations. The high operating temperature places severe constraints on materials selection 

and results in difficult fabrication processes. Corrosion of metal stack components (such as the 

interconnects in some designs) is a challenge. These factors limit stack-level power density 

(though significantly higher than in PAFC and MCFC), and thermal cycling and stack life 

(though the latter is better than for MCFC and PEFC). 

1.6 Characteristics 

The interest in terrestrial applications of fuel cells is driven primarily by their potential for high 

efficiency and very low environmental impact (virtually no acid gas or solid emissions). 
1-13 

Efficiencies of present fuel cell plants are in the range of 30 to 55 percent based on the lower 

heating value (LHV) of the fuel. Hybrid fuel cell/reheat gas turbine cycles that offer efficiencies 

greater than 70 percent LHV, using demonstrated cell performance, have been proposed. 

Figure 1-4 illustrates demonstrated low emissions of installed PAFC units compared to the Los 

Angeles Basin (South Coast Air Quality Management District) requirements, the strictest 

requirements in the U.S. Measured emissions from the PAFC unit are < 1 ppm of NOX, 4 ppm 

of CO, and <1 ppm of reactive organic gases (non-methane) (5). In addition, fuel cells operate at 

a constant temperature, and the heat from the electrochemical reaction is available for 

cogeneration applications. Table summarizes the impact of the major constituents within fuel 

gases on the various fuel cells. The reader is referred to Sections 3 through 7 for detail on trace 

contaminants. 

Another key feature of fuel cells is that their performance and cost are less dependent on scale 

than other power technologies. Small fuel cell plants operate nearly as efficiently as large ones, 

with equally low emissions, and comparable cost.1

 This opens up applications for fuel cells 

where conventional power technologies are not practical. In addition, fuel cell systems can be 

relatively quiet generators. 

To date, the major impediments to fuel cell commercialization have been insufficient longevity 

and reliability, unacceptably high cost, and lack of familiarity of markets with fuel cells. For fuel 

cells that require special fuels (such as hydrogen) the lack of a fuel infrastructure also limits 

commercialization. 

NOx CO

L.A. Basin

Stand

Fuel

Cell

Power

Plant

Reactive Organic Gases

Figure 1-4 Relative Emissions of PAFC Fuel Cell Power Plants 

Compared to Stringent Los Angeles Basin Requirements 

 

1

. The fuel processor efficiency is size dependent; therefore, small fuel cell power plants using externally 

reformed hydrocarbon fuels would have a lower overall system efficiency. 
1-14 

Other characteristics that fuel cells and fuel cell plants offer are: 

• Direct energy conversion (no combustion) 

• No moving parts in the energy converter 

• Quiet 

• Demonstrated high availability of lower temperature units 

• Siting ability 

• Fuel flexibility 

• Demonstrated endurance/reliability of lower temperature units 

• Good performance at off-design load operation 

• Modular installations to match load and increase reliability 

• Remote/unattended operation 

• Size flexibility 

• Rapid load following capability 

General negative features of fuel cells include 

• Market entry cost high; Nth cost goals not demonstrated. 

• Endurance/reliability of higher temperature units not demonstrated. 

• Unfamiliar technology to the power industry. 

• No infrastructure. 

Table 1-2 Summary of Major Fuel Constituents Impact on PEFC, AFC, 

PAFC, MCFC, and SOFC 

Gas 

Species PEFC AFC PAFC MCFC SOFC 

H2 Fuel Fuel Fuel Fuel Fuel 

CO 

Poison 

(reversible) 

(50 ppm per 

stack) 

Poison Poison 

(<0.5%) Fuela

 Fuel 

CH4 Diluent Poison Diluent Diluentb Fuela

CO2 & H2O Diluent Poison Diluent Diluent Diluent 

S as (H2S & 

COS) 

No Studies to 

date (11) Poison Poison (<50 ppm) 

Poison 

(<0.5 ppm) 

Poison 

(<1.0 ppm) 

a

 In reality, CO, with H2O, shifts to H2 and CO2, and CH4, with H2O, reforms to H2 and CO faster than reacting as 

a fuel at the electrode. b

 A fuel in the internal reforming MCFC. 

1.7 Advantages/Disadvantages 

The fuel cell types addressed in this handbook have significantly different operating regimes. As 

a result, their materials of construction, fabrication techniques, and system requirements differ. 

These distinctions result in individual advantages and disadvantages that govern the potential of 
1-15 

the various cells to be used for different applications. Developers use the advantages of fuel 

cells to identify early applications and address research and development issues to expand 

applications (see Sections 3 through 7). 

1.8 Applications, Demonstrations, and Status 

The characteristics, advantages, and disadvantages summarized in the previous section form the 

basis for selection of the candidate fuel cell types to respond to a variety of application needs. 

The major applications for fuel cells are as stationary electric power plants, including cogeneration units; as motive power for vehicles, and as on-board electric power for space vehicles or 

other closed environments. Derivative applications will be summarized. 

1.8.1 Stationary Electric Power 

One characteristic of fuel cell systems is that their efficiency is nearly unaffected by size. This 

means that small, relatively high efficient power plants can be developed, thus avoiding the 

higher cost exposure associated with large plant development. As a result, initial stationary plant 

development has been focused on several hundred kW to low MW capacity plants. Smaller 

plants (several hundred kW to 1 to 2 MW) can be sited at the user’s facility and are suited for 

cogeneration operation, that is, the plants produce electricity and thermal energy. Larger, dispersed plants (1 to 10 MW) are likely to be used for distributed generation. The plants are fueled 

primarily with natural gas. Once these plants are commercialized and price improvements materialize, fuel cells will be considered for large base-load plants because of their high efficiency. 

The base-load plants could be fueled by natural gas or coal. The fuel product from a coal gasifier, once cleaned, is compatible for use with fuel cells. Systems integration studies show that 

high temperature fuel cells closely match coal gasifier operation. 

Operation of complete, self-contained, stationary plants continues to be demonstrated using 

PEFC, AFC, PAFC, MCFC, and SOFC technology. Demonstrations of these technologies that 

occurred before 2000 were addressed in previous editions of the Fuel Cell Handbook and in the 

literature of the period. U.S. manufacturer experience with these various fuel cell technologies 

has produced timely information. A case in point is the 200 kW PAFC on-site plant, the PC-25, 

that was the first to enter the commercial market (see Figure 1-5). 
1-16 

Figure 1-5 PC-25 Fuel Cell 

The plant was developed by UTC Fuel Cells, a division of United Technologies Corporation 

(UTC). The plants are built by UTC Fuel Cells. The Toshiba Corporation of Japan and Ansaldo 

SpA of Italy are partners with UTC Fuel Cells. The on-site plant is proving to be an economic 

and beneficial addition to the operating systems of commercial buildings and industrial facilities 

because it is superior to conventional technologies in reliability, efficiency, environmental 

impact, and ease of siting. Because the PC-25 is the first available commercial unit, it serves as 

a model for fuel cell application. Because of its attributes, the PC-25 is being installed in various 

applications, such as hospitals, hotels, large office buildings, manufacturing sites, wastewater 

treatment plants, and institutions to meet the following requirements: 

• On-site energy 

• Continuous power – backup 

• Uninterrupted power supply 

• Premium power quality 

• Independent power source 

Characteristics of the plant are as follows: 

• Power Capacity 0 to 200 kW with natural gas fuel (-30 to 45 °C, up to 1500 m) 

• Voltage and Phasing 480/277 volts at 60 Hz ; 400/230 volts at 50 Hz 
1-17 

• Thermal Energy 740,000 kJ/hour at 60°C (700,000 Btu/hour heat at 140 °F); 

(Cogeneration) module provides 369,000 kJ/hour at 120°C (350,000Btu/hour 

 at 250 °F) and 369,000 kJ/hour at 60 °C 

• Electric Connection Grid-connected for on-line service and grid-independent for 

 on-site premium service 

• Power Factor Adjustable between 0.85 to 1.0 

• Transient Overload None 

• Grid Voltage Unbalance 1 percent 

• Grid Frequency Range +/-3 percent 

• Voltage Harmonic Limits <3 percent 

• Plant Dimensions 3 m (10 ft) wide by 3 m (10 ft) high by 5.5 m (18 ft) long, not 

 including a small fan cooling module (5) 

• Plant Weight 17,230 kg (38,000 lb) 

UTC Fuel Cells: Results from the operating units as of August, 2002 are as follows: total fleet 

operation stands at more than 5.3 million hours. The plants achieve 40 percent LHV electric 

efficiency, and overall use of the fuel energy approaches 80 percent for cogeneration applications 

(6). Operations confirm that rejected heat from the initial PAFC plants can be used for heating 

water, space heating, and low pressure steam. One plant has completed over 50,000 hours of 

operation, and a number of plants have operated over 40,000 hours (6). Fourteen additional 

plants have operated over 35,000 hours. The longest continuous run stands at 9,500 hours for a 

unit purchased by Tokyo Gas for use in a Japanese office building (9). This plant ended its 

duration record because it had to be shut down because of mandated maintenance. It is estimated 

at this time that cell stacks can achieve a life of 5 to 7 years. The fleet has attained an average of 

over 95 percent availability. The latest model, the PC-25C, is expected to achieve over 96 

percent. The plants have operated on natural gas, propane, butane, landfill gas (10,11), hydrogen 

(12), and gas from anaerobic digestors (13). Emissions are so low (see Figure 1-4) that the plant 

is exempt from air permitting in the South Coast and Bay Area (California) Air Quality 

Management Districts, which have the most stringent limits in the U.S. The sound pressure level 

is 62 dBA at 9 meters (30 feet) from the unit. The PC-25 has been subjected to ambient 

conditions varying from -32 °C to +49 °C and altitudes from sea level to 1600 meters (~1 mile). 

Impressive ramp rates result from the solid state electronics. The PC-25 can be ramped at 10 

kW/sec up or down in the grid connected mode. The ramp rate for the grid independent mode is 

idle to full power in ~one cycle or essentially one-step instantaneous from idle to 200 kW. 

Following the initial ramp to full power, the unit can adjust at an 80 kW/sec ramp up or down in 

one cycle. 

The fuel cell stacks are made and assembled into units at an 80,000 ft2

 facility located in South 

Windsor, Connecticut, U.S. Low cost/high volume production depends on directly insertable 

sub-assemblies as complete units and highly automatic processes such as robotic component 

handling and assembly. The stack assembly is grouped in a modified spoke arrangement to 

allow for individual manufacturing requirements of each of the cell components while bringing 

them in a continuous flow to a central stacking elevator (14). 
1-18 

Ballard Generation Systems: Ballard Generation Systems, a subsidiary of Ballard Power 

Systems, produces a PEFC stationary on-site plant. It has these characteristics: 

• Power Capacity 250 kW with natural gas fuel 

• Electric Efficiency 40% LHV 

• Thermal Energy 854,600 kJ/hour at 74 °C (810,000 Btu/hour at 165 °F) 

• Plant Dimensions 2.4 m (8 ft) wide by 2.4 m (8 ft) high by 5.7 m (18.5 ft) long 

• Plant Weight 12,100 kg (26,700 lb) 

Ballard completed 10- and 60-kW engineering prototype stationary fuel cell power generators in 

2001. Ballard, Shell Hydrogen, and Westcoast Energy established a private capital joint venture 

to help build early stage fuel cell systems. Ballard launched the NexaTM, a portable 1.2 kW 

power module, in September 2001. Ballard is also selling carbon fiber products for gas diffusion 

layers for proton exchange membrane fuel cells. Highlights of Ballard’s fuel cell sales are 

shown below. 

FuelCell Energy (FCE): FCE reached 50 MW manufacturing capacity and plans to expand its 

manufacturing capacity to 400 MW in 2004. The focus of the utility demonstrations and FCE’s 

fuel cell development program is the commercialization of 300 kilowatt, 1.5 megawatt, and 3 

megawatt MCFC plants. 

• Power Capacity 3.0 MW net AC 

• Electric efficiency 57% (LHV) on natural gas 

• Voltage and Phasing Voltage is site dependent, 3 phase 60 Hz 

• Thermal energy ~4.2 million kJ/hour (~4 million Btu/hour) 

• Availability 95% 

Siemens Westinghouse Power Corporation (SWPC): The Siemens Westinghouse SOFC is 

planning two major product lines with a series of product designs in each line. The first product 

will be a 250 kW cogeneration system operating at atmospheric pressure. This will be followed 

by a pressurized SOFC/gas turbine hybrid of approximately 0.5 MW. After the initial 

production, larger systems are expected as well. Also, a system capable of separating CO2 from 

the exhaust is planned as an eventual option to other products. 

The commercialization plan is focused on an initial offering of a hybrid fuel cell/gas turbine 

plant. The fuel cell module replaces the combustion chamber of the gas turbine engine. 

Figure 1-6 shows the benefit behind this combined plant approach. Additional details are 

provided in Section 7. As a result of the hybrid approach, the 1 MW early commercial unit is 

expected to attain ~60% efficiency LHV when operating on natural gas. 
1-19 

100

90

80

70

60

50

40

30

20

10

0

Advanced

Gas Turbine

System

High Temperature

Fuel Cell

Gas Turbine/

Fuel Cell

Combined Cycle

EFFICIENCY (%)

Figure 1-6 Combining the SOFC with a Gas Turbine Engine to Improve Efficiency 

Siemens Westinghouse is planning a number of tests on power plants that are prototypes of 

future products. All systems employ the tubular SOFC concept and most are combined with gas 

turbines in a hybrid configuration. Capacities of these systems are 250 kilowatts atmospheric, 

300 kilowatt class hybrid, and 1 megawatt class hybrid. They are to operate at various sites in 

the U.S., Canada, and Europe. 

An eventual market for fuel cells is the large (100 to 300 MW), base-loaded, stationary plants 

operating on coal or natural gas. Another related, early opportunity may be in re-powering older, 

existing plants with high-temperature fuel cells (19). MCFCs and SOFCs coupled with coal 

gasifiers have the best attributes to compete for the large, base load market. The rejected heat 

from the fuel cell system can be used to produce steam for the existing plant's turbines. Studies 

showing the potential of high-temperature fuel cells for plants of this size have been performed 

(see Section 8). These plants are expected to attain from 50 to 60% efficiency based on the HHV 

of the fuel. Coal gasifiers produce a fuel gas product requiring cleaning to the stringent requirements of the fuel cells’ electrochemical environment, a costly process. The trend of environmental regulations has also been towards more stringent cleanup. If this trend continues, coal-fired 

technologies will be subject to increased cleanup costs that may worsen process economics. This 

will improve the competitive position of plants based on the fuel cell approach. Fuel cell systems will emit less than target emissions limits. U.S. developers have begun investigating the 

viability of coal gas fuel to MCFCs and SOFCs (20,21,22). An FCE 20 kW MCFC stack was 

tested for a total of 4,000 hours, of which 3,900 hours was conducted at the Plaquemine, LA, site 

on coal gas as well as pipeline gas. The test included 1,500 hours of operation using 9,142 kJ/m3

syngas from a slip stream of a 2,180 tonne/day Destec entrained gasifier. The fuel processing 

system incorporated cold gas cleanup for bulk removal of H2S and other contaminants, allowing 

the 21 kW MCFC stack to demonstrate that the FCE technology can operate on either natural gas 

or coal gas. 

A series of standards is being developed to facilitate the application of stationary fuel cell 

technology power plants. Standard development activities presently underway are 

• Fuel Cell Power Systems ANSI/CSA America FC1-2004 (published) 
1-20 

• Stationary Fuel Cell Power Systems 

 -Safety IEC TC 105 Working Group #3 

• Stationary Fuel Cell Power Systems 

-Installation IEC TC 105 Working Group #5 

• Interconnecting Distributed Resources IEEE P1547.1, P1547.2, P1547.3, P1547.4 

• Test Method for the Performance of 

Stationary Fuel Cell Power Plants IEC TC 105 Working Group #4 

1.8.2 Distributed Generation 

Distributed generation involves small, modular power systems that are sited at or near their point 

of use. The typical system is less than 30 MW, used for generation or storage, and extremely 

clean. Examples of technologies used in distributed generation include gas turbines and 

reciprocating engines, biomass-based generators, solar power and photovoltaic systems, fuel 

cells, wind turbines, micro-turbines, and flywheel storage devices. See Table 1-3 for size and 

efficiencies of selected systems. 

Table 1-3 Attributes of Selected Distributed Generation Systems 

Type Size Efficiency, % 

Reciprocating Engines 50 kW – 6 MW 33 – 37 

Micro turbines 10 kW – 300 kW 20 – 30 

Phosphoric Acid Fuel Cell (PAFC) 50 kW – 1 MW 40 

Solid Oxide Fuel Cell (SOFC) 5 kW – 3 MW 45 – 65 

Proton Exchange Membrane Fuel Cell 

(PEM) 

<1 kW – 1 MW 34 – 36 

Photovoltaics (PV) 1 kW – 1 MW NA 

Wind Turbines 150 kW – 500 kW NA 

Hybrid Renewable <1 kW – 1 MW 40 – 50 

The market for distributed generation is aimed at customers dependent on reliable energy, such 

as hospitals, manufacturing plants, grocery stores, restaurants, and banking facilities. There is 

currently over 15 GW of distributed power generation operating in the U.S. Over the next 

decade, the domestic market for distributed generation, in terms of installed capacity to meet the 

demand, is estimated to be 5-6 GW per year. The projected global market capacity increases are 

estimated to be 20 GW per year (23). Several factors have played a role in the rise in demand for 

distributed generation. Utility restructuring is one of the factors. Energy suppliers must now 

take on the financial risk of capacity additions. This leads to less capital-intensive projects and 

shorter construction periods. Also, energy suppliers are increasing capacity factors on existing 

plants rather than installing new capacity, which places pressure on reserve margins. This 

increases the possibility of forced outages, thereby increasing the concern for reliable service. 

There is also a demand for capacity additions that offer high efficiency and use of renewables as 

the pressure for enhanced environmental performance increases (23). 
1-21 

There are many applications for distributed generation systems. They include: 

• Peak shaving - Power costs fluctuate hour by hour depending upon demand and generation, 

therefore customers would select to use distributed generation during relatively high-cost, onpeak periods. 

• Combined heat and power (CHP) (Cogeneration) –The thermal energy created while 

converting fuel to electricity would be utilized for heat in addition to electricity in remote 

areas, and electricity and heat for sites that have a 24 hour thermal/electric demand. 

• Grid support – Strategic placement of distributed generation can provide system benefits and 

preclude the need for expensive upgrades and provide electricity in regions where small 

increments of new baseload capacity is needed. 

• Standby power – Power during system outages is provided by a distributed generation system 

until service can be restored. This is used for customers that require reliable back-up power 

for health or safety reasons, companies with voltage-sensitive equipment, or where outage 

costs are unacceptably high. 

• Remote/Standalone – The user is isolated from the grid either by choice or circumstance. 

The purpose is for remote applications and mobile units to supply electricity where needed. 

Distributed generation systems have small footprints, are modular and mobile making them very 

flexible in use. The systems provide benefits at the customer level and the supplier level, as well 

as the national level. Benefits to the customer include high power quality, improved reliability, 

and flexibility to react to electricity price spikes. Supplier benefits include avoiding investments 

in transmission and distribution (T&D) capacity upgrades by locating power where it is most 

needed and opening new markets in remote areas. At the national level, the market for distributed generation establishes a new industry, boosting the economy. The improved efficiencies 

also reduce greenhouse gas emissions. 

However, a number of barriers and obstacles must be overcome before distributed generation can 

become a mainstream service. These barriers include technical, economic, institutional, and 

regulatory issues. Many of the proposed technologies have not yet entered the market, and will 

need to meet performance and pricing targets before entry. Questions have also risen on 

requirements for connection to the grid. Lack of standardized procedures creates delays and 

discourages customer-owned projects. Siting, permitting, and environmental regulations can 

also delay and increase the costs of distributed generation projects. 

In 1998, the Department of Energy created a Distributed Power Program to focus on market 

barriers and other issues that have prohibited the growth of distributed generation systems. 

Under the leadership of the National Renewable Energy Laboratory (NREL), a collaboration of 

national laboratories and industry partners have been creating new standards and are identifying 

and removing regulatory barriers. The goals of the program include 1) strategic research, 2) 

system integration, and 3) mitigation of regulatory and institutional barriers (24). 

Fuel cells, one of the emerging technologies in distributed generation, have been hindered by 

high initial costs. However, costs are expected to decline as manufacturing capacity and 

capability increase and designs and integration improve. The fuel cell systems offer many 

potential benefits as a distributed generation system. They are small and modular, and capital 
1-22 

costs are relatively insensitive to scale. This makes them ideal candidates for diverse 

applications where they can be matched to meet specific load requirements. The systems are 

unobtrusive, with very low noise levels and negligible air emissions. These qualities enable 

them to be placed close to the source of power demand. Fuel cells also offer higher efficiencies 

than conventional plants. The efficiencies can be enhanced by using the quality waste heat 

derived from the fuel cell reactions for combined heat and power and combined-cycle 

applications. 

Phosphoric acid fuel cells have successfully been commercialized. Second generation fuel cells 

include solid oxide fuel cells and molten carbonate fuel cells. Research is ongoing in areas such 

as fuel options and new ceramic materials. Different manufacturing techniques are also being 

sought to help reduce capital costs. Proton exchange membrane fuel cells are still in the 

development and testing phase. 

1.8.3 Vehicle Motive Power 

Since the late 1980s, there has been a strong push to develop fuel cells for use in light-duty and 

heavy-duty vehicle propulsion. A major drive for this development is the need for clean, efficient cars, trucks, and buses that operate on conventional fuels (gasoline, diesel), as well as 

renewable and alternative fuels (hydrogen, methanol, ethanol, natural gas, and other hydrocarbons). With hydrogen as the on-board fuel, these would be zero-emission vehicles. With onboard fuels other than hydrogen, the fuel cell systems would use an appropriate fuel processor to 

convert the fuel to hydrogen, yielding vehicle power trains with very low acid gas emissions and 

high efficiencies. Further, such vehicles offer the advantages of electric drive and low 

maintenance because of few moving parts. This development is being sponsored by various 

governments in North America, Europe, and Japan, as well as by major automobile 

manufacturers worldwide. As of May 1998, several fuel cell-powered cars, vans, and buses 

operating on hydrogen and methanol have been demonstrated. 

In the early 1970s, K. Kordesch modified a 1961 Austin A-40 two-door, four-passenger sedan to 

an air-hydrogen fuel cell/battery hybrid car (23). This vehicle used a 6-kW alkaline fuel cell in 

conjunction with lead acid batteries, and operated on hydrogen carried in compressed gas 

cylinders mounted on the roof. The car was operated on public roads for three years and about 

21,000 km. 

In 1994 and 1995, H-Power (Belleville, New Jersey) headed a team that built three PAFC/battery 

hybrid transit buses (24,25). These 9 meter (30 foot), 25 seat (with space for two wheel chairs) 

buses used a 50 kW fuel cell and a 100 kW, 180 amp-hour nickel cadmium battery. 

The major activity in transportation fuel cell development has focused on the polymer electrolyte 

fuel cell (PEFC). In 1993, Ballard Power Systems (Burnaby, British Columbia, Canada) 

demonstrated a 10 m (32 foot) light-duty transit bus with a 120 kW fuel cell system, followed by 

a 200 kW, 12 meter (40 foot) heavy-duty transit bus in 1995 (26). These buses use no traction 

batteries. They operate on compressed hydrogen as the on-board fuel. In 1997, Ballard provided 

205 kW (275 HP) PEFC units for a small fleet of hydrogen-fueled, full-size transit buses for 

demonstrations in Chicago, Illinois, and Vancouver, British Columbia. Working in collaboration 

with Ballard, Daimler-Benz built a series of PEFC-powered vehicles, ranging from passenger 
1-23 

cars to buses (27). The first such vehicles were hydrogen-fueled. A methanol-fueled PEFC Aclass car unveiled by Daimler-Benz in 1997 had a 640 km (400 mile) range. Plans were to offer 

a commercial vehicle by 2004. A hydrogen-fueled (metal hydride for hydrogen storage), fuel 

cell/battery hybrid passenger car was built by Toyota in 1996, followed in 1997 by a methanolfueled car built on the same (RAV4) platform (28). 

In February 2002, UTC Fuel Cells and Nissan signed an agreement to develop fuel cells and fuel 

cell components for vehicles. Renault, Nissan’s alliance partner, is also participating in the 

development projects. UTC Fuel Cells will provide proprietary ambient-pressure proton 

exchange membrane fuel cell technology. 

Ballard’s fuel cell engine powered DaimlerChrysler’s NECAR 5 fuel cell vehicle in a 13-day, 

3,000-mile endurance test across the United States. The drive provided Ballard and 

DaimlerChrysler with testing experience in a variety of conditions. 

Toyota Motor Corp. and Honda Motor Co. announced they would advance their initial vehicle 

introduction plans for fuel cell vehicles to late in 2002 from 2003. Honda achieved a significant 

milestone for its product launch by receiving both CARB and EPA certification of its zero 

emission FCX-V4 automobile. This was the first vehicle to receive such certification. Ballard’s 

fuel cell powered this Honda vehicle. 

Other major automobile manufacturers, including General Motors, Volkswagen, Volvo, 

Chrysler, Nissan, and Ford, have also announced plans to build prototype polymer electrolyte 

fuel cell vehicles operating on hydrogen, methanol, or gasoline (29). IFC and Plug Power in the 

U.S., and Ballard Power Systems of Canada (15), are involved in separate programs to build 50 

to 100 kW fuel cell systems for vehicle motive power. Other fuel cell manufacturers are 

involved in similar vehicle programs. Some are developing fuel cell-powered utility vehicles, 

golf carts, etc. (30,31). 

1.8.4 Space and Other Closed Environment Power 

The application of fuel cells in the space program (1 kW PEFC in the Gemini program and 

1.5 kW AFC in the Apollo program) was demonstrated in the 1960s. More recently, three 

12 kW AFC units were used for at least 87 missions with 65,000 hours flight time in the Space 

Shuttle Orbiter. In these space applications, the fuel cells used pure reactant gases. IFC 

produced a H2/O2 30 kW unit for the Navy’s Lockheed Deep Quest vehicle. It operates at depths 

of 1500 meters (5000 feet). Ballard Power Systems has produced an 80 kW PEFC fuel cell unit 

for submarine use (methanol fueled) and for portable power systems. 

1.8.5 Auxiliary Power Systems 

In addition to high-profile fuel cell applications such as automotive propulsion and distributed 

power generation, the use of fuel cells as auxiliary power units (APUs) for vehicles has received 

considerable attention (see Figure 1-7). APU applications may be an attractive market because 

they offer a true mass-market opportunity that does not require the challenging performance and 

low cost required for propulsion systems for vehicles. In this section, a discussion of the 

technical performance requirements for such fuel cell APUs, as well as the status of technology 

and implications for fuel cell system configuration and cost is given. 
1-24 

Participants Application Size range Fuel /Fuel 

Cell type 

Nature of 

Activity 

BMW, International 

Fuel Cells (a) 

passenger car, BMW 

7-series 5kW net 

Hydrogen, 

Atmospheric 

PEM 

Demonstration 

Ballard, DaimlerChrysler (b) 

Class 8 Freightliner 

heavy-duty Century 

Class S/T truck cab 

1.4 kW net for 

8000 BTU/h A/C 

unit 

Hydrogen, 

PEM Demonstration 

BMW, Delphi, 

Global 

Thermoelectric (c) 

passenger car 1-5kW net Gasoline, 

SOFC 

Technology 

development 

program 

(a) “Fuel Cell Auxiliary Power Unit – Innovation for the Electric Supply of Passenger Cars?” J. Tachtler et al. BMW Group, 

 SAE 2000-01-0374, Society of Automotive Engineers, 2000. 

(b) “Freightliner unveils prototype fuel cell to power cab amenities”, O. B. Patten, Roadstaronline.com news, July 20, 2000. 

(c) Company press releases, 1999. 

Figure 1-7 Overview of Fuel Cell Activities Aimed at APU Applications 

Auxiliary power units are devices that provide all or part of the non-propulsion power for 

vehicles. Such units are already in widespread use in a range of vehicle types and for a variety of 

applications, in which they provide a number of potential benefits (see Figure 1-8). Although 

each of these applications could provide attractive future markets for fuel cells, this section will 

focus on application to on-road vehicles (specifically trucks). 

Vehicles Types Loads Serviced Potential Benefits 

• Heavy-duty & utility trucks 

• Airplanes 

• Trains 

• Yachts & Ships 

• Recreational vehicles 

• Automobiles & light trucks 

(not commercial yet) 

• Space conditioning 

• Refrigeration 

• Lighting and other cabin 

amenities 

• Communication and 

information equipment 

• Entertainment (TV, radio) 

• Can operate when main 

engine unavailable 

• Reduce emissions and noise 

while parked 

• Extend life of main engine 

• Improve power generation 

efficiency when parked 

Figure 1-8 Overview of APU Applications 

In 1997, the Office of Naval Research initiated an advanced development program to 

demonstrate a ship service fuel cell power generation module. The ship service generator 

supplies the electrical power requirements of the ship. This program would provide the basis for 

a new fuel cell-based design as an attractive option for future Navy surface ships. This program 

would provide the Navy with a ship service that is more efficient, and incorporates a distributed 

power system that would remain operating even if the ship’s engine is destroyed.

Fuel cells can serve as a generator, battery charger, battery replacements and heat supply. They 

can adapt to most environments, even locations in Arctic and Antarctic regions. One effort, in 

collaboration with the Army Research Office, has demonstrated a prototype fuel cell designed to 

replace a popular military standard battery. The target application is the Army's BA-5590 

primary (i.e., use-once-and-dispose) lithium battery. The Army purchases approximately 350,000 

of these batteries every year at a cost of approximately $100 per battery, including almost $30 
1-25 

per battery for disposal. Fuel cells, on the other hand, are not thrown away after each use but can 

be re-used hundreds of times. Mission weight savings of factors of 10 or more are projected. The 

prototype fuel cell, which has the same size and delivers the same power as a battery, has been 

tested in all orientations and under simulated adverse weather conditions, and was 

enthusiastically received by Army senior management. 

System Performance Requirements 

A key reason for interest in fuel cell APU applications is that there may be a good fit between 

APU requirements and fuel cell system characteristics. Fuel cells are efficient and quiet, and 

APUs do not have the load following requirements and physical size and weight constraints 

associated with propulsion applications. However, in order to understand the system 

requirements for fuel cell APUs, it is critical to understand the required functionality (refer to 

Figure 1-8) as well as competing technologies. To provide the functionality of interest, and to be 

competitive with internal combustion engine (ICE) driven APUs, fuel cell APUs must meet 

various requirements; an overview is provided in Figure 1-9. 

Key Parameter Typical Requirements Expected fuel cell 

performance 

Power output 12 – 42 V DC is acceptable for 

most applications, 110 / 220 V 

AC may be desirable for 

powering power tools etc. 

DC power output simplifies the 

power conditioning and control 

for fuel cells 

System Capacity 1 – 5 kW for light duty vehicles 

and truck cabins 

up to 15 kW for truck refrigeration 

Fits expected range for PEFCs 

and probably also advanced 

SOFCs 

System Efficiency More than 15-25% based on 

LHV 

Efficiency target should be 

achievable, even in smallest 

capacity range 

Operating life and reliability Greater than about 5,000 hours 

stack life, with regular service 

intervals less than once every 

1,000 hours 

Insufficient data available to 

assess whether this is a 

challenge or not 

Figure 1-9 Overview of typical system requirements 

Fuel cell APUs will likely have to operate on gasoline, and for trucks preferably on diesel fuel, in 

order to match the infrastructure available, and preferably to be able to share on-board storage 

tanks with the main engine. The small amount of fuel involved in fueling APUs would likely not 

justify the establishment of a specialized infrastructure (e.g. a hydrogen infrastructure) for APUs 

alone. Similarly, fuel cell APUs should be water self-sufficient, as the need to carry water for 

the APU would be a major inconvenience to the operator, and would require additional space and 

associated equipment. 

In addition to the requirement for stationary operation, fuel cell APUs must be able to provide 

power rapidly after start-up, and must be able to follow loads. While the use of batteries to 
1-26 

accomplish this is almost a given, a system start-up time of about ten minutes or less will likely 

be required to arrive at a reasonable overall package. 

Finally, fuel cell APUs are quiet and clean. These attributes may well be the key competitive 

advantages that fuel cell APUs have over conventional APUs, and hence their performance may 

more than match that of internal combustion engines’ APUs. 

Technology Status 

Active technology development efforts in both PEFC and planar SOFC technology, driven 

primarily by interest in distributed generation and automotive propulsion markets, have achieved 

significant progress. For distributed power applications, refined and even early commercial 

prototypes are being constructed. However, in the case of planar SOFC a distinction must be 

made between different types of SOFC technologies. Neither the tubular nor the electrolytesupported SOFC technology is suitable for APU applications due to their very high operating 

temperature, large size and heavy weight. Only the electrode-supported planar SOFC technology 

may be applicable to APU applications. Since it has only been developed over the past decade, as 

opposed to several decades for PEFC and other SOFC technologies, it is not developed as far, 

although it appears to be catching up quickly (See Figure 1-10). 

Research &

Development Production MarketEntry

Demonstration

Initial System

Prototypes

Refined 

Prototypes

Commercial 

Prototypes

Planar SOFC

(Residential)

PEM

(Residential)

Planar SOFC

(APU)

PEM

(APU)

Figure 1-10 Stage of development for fuel cells for APU applications 

Fuel cell APU applications could benefit significantly from the development of distributed 

generation systems, especially from residential-scale systems, because of the similarity in size 

and duty cycle. However, distributed generation systems are designed mostly for operation on 

natural gas, and do not face as stringent weight and volume requirements as APU applications. 

As a result, fuel cell APUs are in the early system prototype stage. 

Several developers, including Nuvera, Honeywell, and Plug Power are actively developing 

residential PEFC power systems. Most of the PEFC system technology can be adapted for APU 

application, except that a fuel processor capable of handling transportation fuels is required. 

However, most of the players in the residential PEFC field are also engaged in developing PEFC 
1-27 

systems for automotive propulsion applications, and are targeting the ability to use transportation 

fuels for PEFC systems. 

Relatively few developers of SOFC technology have paid attention to non-stationary markets. 

All are focused on small-to medium-sized distributed generation and on-site generation markets. 

Only Global Thermoelectric (Calgary, Canada) has been active in the application of its 

technology to APUs. A detailed conceptual design and cost estimate of a 5-kW SOFC-based 

truck APU concluded that, provided continued improvement in several technology areas, planar 

SOFCs could ultimately become a realistic option for this mass-market application. 

System Configuration and Technology Issues 

Based on system requirements discussed above, fuel cell APUs will consist of a fuel processor, a 

stack system and the balance of plant. Figure 1-11 lists the components required in SOFC and 

PEFC systems. The components needed in a PEFC system for APU applications are similar to 

those needed in residential power. The main issue for components of PEFC systems is to 

minimize or eliminate the use of external supplied water. For both PEFC and SOFC systems, 

start-up batteries (either existing or dedicated units) will be needed, since external electric power 

is not available. 

Detailed cost and design studies for both PEFC and SOFC systems at sizes ranging from 5kW to 

1 MW point to the fundamental differences between PEFC and SOFC technology that impact the 

system design and, by implication, the cost structure. These differences will be discussed in the 

following paragraphs. 

The main components in a SOFC APU are the fuel cell stack, the fuel processor, and the thermal 

management system. In addition, there are several balance of plant components, which are listed 

in Figure 11. The relatively simple reformer design is possible because the SOFC stack operates 

at high temperatures (around 800°C) and is capable of both carbon monoxide and certain 

hydrocarbons as fuel. Since both the anode and cathode exhaust at temperatures of 600-850°C, 

high temperature recuperators are required to maintain system efficiency. A recuperator consists 

of expensive materials (high temperature reducing and oxidizing atmosphere), making it an 

expensive component in the system. However, if hydrocarbons are converted inside the stack, 

this leads to a less exothermic overall reaction so that the stack cooling requirements are reduced. 

Further system simplification would occur if a sulfur-free fuel was used or if the fuel cell were 

sulfur tolerant; in that case, the fuel could be provided directly from the reformer to the fuel cell. 

In order to minimize system volume, (and minimize the associated system weight and start-up 

time) integration of the system components is a key design issue. By recycling the entire anode 

tailgas to provide steam, a water management system can be avoided, though a hot gas 

recirculation system is required. 
1-28 

SOFC-Based System

Fuel

Preparation

Fuel preprocessing

Reformate

Conditioning

Sulfur

removal

Fuel Cell

Stack

Air

Preparation

Balance of Plant:

• Compressor/Expander

• Pumps

• Controls

• Insulation & Packaging

• Safety system

• Start-up battery

PEM-Based System

Fuel

Preparation

Sulfur

Removal

Water-Gas

Shift CO clean-up Fuel CellStack

Steam

Generation

Air

Preparation

Balance of Plant:

• Compressor/Expander

• Pumps

• Controls

• Water management

• Packaging

• Safety system

• Start-up battery

Reforming

Figure 1-11. Overview of subsystems and components for SOFC and PEFC systems 

Figure 1-12 shows a simplified layout for an SOFC-based APU. The air for reformer operation 

and cathode requirements is compressed and then split between the unit operations. The external 

water supply shown in Figure 1-12 will most likely not be needed; the anode recycle stream 

provides water. Unreacted anode tail gas is recuperated in a tail gas burner. Additional energy is 

available in a SOFC system from enthalpy recovery from tail gas effluent streams that are 

typically 400-600 °C. Current thinking is that reformers for transportation fuel based SOFC 

APUs will be of the exothermic type (i.e. partial oxidation or autothermal reforming), as no 

viable steam reformers are available for such fuels. 
1-29 

Fuel Cell

Reformer

Mixer

Gasoline or Diesel

Preheat

Air

water

Anode

Cathode

Burner

Anode

Recycle

Sulfur

removal

Q

Flow 

Splitter

Figure 1-12. Simplified process flow diagram of pre-reformer/SOFC system 

Due to the operating requirements of PEFC stack technology, shift reactors and a carbon 

monoxide removal step are required to produce reformate of sufficient quality. Similarly, the 

stack operating temperature and its humidity requirements require a water management system 

as well as radiators for heat rejection. Some developers use pressurized systems to benefit from 

higher reactant partial pressures on both anode and cathode. Fuel processing for PEFC APU 

systems is identical to that needed in residential power or propulsion applications. The additional 

issue for PEFC is the minimization of steam needed for the fuel processor system. Since an APU 

is a mobile and/or remote unit, the need for external sources of water should be minimized. The 

reformate stream is further diluted by additional steam, if that water is not removed prior to the 

fuel cell stack. 

Another design integration issue in PEFC systems is water management to hydrate the 

electrolyte and provide the necessary steam for reforming and water-gas shift operations. 

Additional steam may be required for the CO clean-up device. Some reformate-based PEFC 

systems are run under pressure to increase the partial pressure of reactants for the PEFC anode 

and cathode, increasing efficiency. Pressure operation also aids in heat integration for the 

internal generation of steam at pressures greater than atmospheric (i.e. steam generated at 

temperatures greater than 100°C). PEFC system integration involves combining a reformer 

(either exothermic or endothermic at ~850-1000 °C), shift reactors (exothermic, 150-500 °C), 

CO-cleanup (primarily exothermic, 50-200 °C), and the fuel cell stack (exothermic, 80 °C). 

Each reaction zone operates at a significantly different temperature, thus providing a challenge 

for system integration and heat rejection. To alleviate some of these drawbacks and further 

reduce the cost of the PEFC systems, developers are investigating the possibility of using higher 

temperature membranes (e.g. operating slightly above 100 °C). This would increase the carbon 

monoxide tolerance, potentially simplifying the fuel processor design, and simplify the heat 

rejection. 
1-30 

The load requirements for auxiliary power applications require smaller fuel cell stacks. The heat 

losses for a SOFC stack operating at a smaller power duty are a larger proportion of the gross 

rating than in a stationary power application. Insulation required for specified skin temperature 

requirements could conceivably result in a large fraction of the total system volume. Integration 

of the high temperature components is important in order to reduce the system volume and 

insulation requirements. SOFC APU systems will require inexpensive, high performance 

insulation materials to decrease both system volume and cost. 

Cost Considerations 

As for any new class of product, total cost of ownership and operation of fuel cells will be a 

critical factor in their commercialization, along with the offered functionality and performance. 

This total cost of ownership typically has several components for power systems such as fuel 

cells. These components include fuel cost, other operating costs such as maintenance cost, and 

the first cost of the equipment. This first cost has a significant impact on fuel cells’ 

competitiveness. 

The main component of a fuel cell’s first cost is the manufacturing cost, which is strongly related 

to the physical configuration and embodiment of the system, as well as to the manufacturing 

methods used. System configuration and design, in turn, are directly related to the desired 

system functionality and performance, while the manufacturing methods are strongly linked to 

the anticipated production volume. 

Arthur D. Little carried out cost structure studies for a variety of fuel cell technologies for a wide 

range of applications, including SOFC tubular, planar, and PEFC technologies. Because 

phenomena at many levels of abstraction have a significant impact on performance and cost, they 

developed a multi-level system performance and cost modeling approach (see Figure 1-13). At 

the most elementary level, it includes fundamental chemical reaction/reactor models for the fuel 

processor and fuel cell as one-dimensional systems. 

Reformer model

M

O

M

O

C3H7 H

C3H8

Manufacturing

Cost Model

Fuel Cell Model

0.4 

0.5 

0.6 

0.7 

0.8 

0.9 

1 

cell potential (V)

0 0.2 0.4 0.6 0.8 1 1.2 1.4 

current density (A/cm2)

NG2000 H2 NG3000 H2 NG2000 ref NG3000 ref

Thermodynamic

 System Model

Conceptual

Design and

Configuration

46" 60"

53"

1 2

4

5

6 7

8

9

10

11

12

13

15

14

Illustrative

Tape Cast

Anode

Powder Prep

Vacuum

Plasma

Spray

Electrolyte

Small Powder

Prep

Screen

Print

Cathode

Small Powder

Prep

Sinter in Air

1400C Sinter in Air

Forming

of

Interconnect

Shear

Interconnect

Vacuum

Plasma

Spray

Slurry

Spray

Screen

Print

Slurry Spray

Slip Cast

Finish Edges

Note: Alternative production processes appear in gray to the

bottom of actual production processes assumed

Braze

Paint Braze

onto

Interconnect

Blanking /

Slicing

QC Leak

Check

Interconnect

Fabrication

Anode Electrolyte Cathode

Stack Assembly

$/kW

Figure 1-13 Multilevel system modeling approach 

Each detailed sub-model feeds into the thermodynamic system model, and provides sizing 

information directly to the conceptual design. The thermodynamic system model provides a 
1-31 

technical hub for the multi-level approach. It provides inputs on the required flow rates and heat 

duties in the system. Sizing information, together with information from the thermodynamic 

model, then flows to the conceptual design. 

SOFC Cost Structure 

The main difference in SOFC stack cost compared to PEFC cost relates to the simpler system 

configuration of the SOFC system. This is mainly due to the fact that SOFC stacks do not 

contain the high-cost precious metals that PEFCs contain. This is off-set in part by the relatively 

complex manufacturing process required for the SOFC electrode/electrolyte plates and by the 

somewhat lower power density in SOFC systems. Low-temperature operation (enabled with 

electrode-supported planar configuration) enables the use of low-cost metallic interconnects that 

can be manufactured with conventional metal forming operations. 

The balance of plant contains all the direct stack support systems, reformer, compressors, pumps, 

and recuperating heat exchangers. Its cost is low by comparison to the PEFC because of the 

simplicity of the reformer. However, the cost of the recuperating heat exchangers partially 

offsets that. 

To provide some perspective on the viability of SOFCs in APU applications from a cost 

perspective, NETL sponsored a cost estimate of a small-scale (5 kW), simple-cycle SOFC 

anode-supported system, operated on gasoline. The estimated manufacturing cost (see Figure 1-

14) could well be close to that estimated for comparable PEFC systems, while providing 

somewhat higher system efficiency. 

While the stack, insulation, and stack balance in this simple-cycle system is a key component; 

the balance of plant is also an important factor. The stack cost mainly depends on the achievable 

power density. Small systems like these will likely not be operated under high pressure. While 

this simplifies the design and reduces cost for compressors and expanders (which are not readily 

available at low cost for this size range in any case), it might also negatively affect the power 

density achievable. 

A key challenge with small-scale SOFC systems is to overcome heat loss. The higher the heat 

loss the more recuperation is required to maintain the fuel cell within an acceptable temperature 

range, and hence to ensure good performance. 

The large fraction of cost related to balance of plant issues is mainly due to the very small scale 

of this system, which results in a significant reverse economy of scale. While design work is still 

ongoing, it is anticipated that the cost structure of this system will reduce the cost of balance of 

plant further, and further improve the competitiveness of these systems. 
1-32 

Controls/ 

Piping/ 

Batte ry

Indirect, 

Labor, 

Depr.

Balance of 

Plant

Stack, 

insulation 

and stack 

balance

SOFC System 

Cost Structure:

Manufacturing Costs:

 $350-550/kW

Figure 1-14. Projected cost structure of a 5kWnet APU SOFC system. Gasoline fueled 

POX reformer, Fuel cell operating at 300mW/cm2, 0.7 V, 90 % fuel utilization, 500,000 

units per year production volume. 

Outlook and Conclusions 

In conclusion, both PEFC and SOFC have the potential to meet allowable cost targets, provided 

successful demonstrations prove the technology. It is critical however, that for these technologies 

to be commercially successful, especially in small-capacity markets, high production volumes 

will have to be reached. APU applications might provide such markets. It is similarly critical that 

the technologies be demonstrated to perform and achieve the projected performance targets and 

demonstrate long life. These are the challenges ahead for the fuel cell industry in the APU 

market segment. 

1.8.6 Derivative Applications 

Because of the modular nature of fuel cells, they are attractive for use in small portable units, 

ranging in size from 5 W or smaller to 100 W power levels. Examples of uses include the 

Ballard fuel cell, demonstrating 20 hour operation of a portable power unit (32), and an IFC 

military backpack. There has also been technology transfer from fuel cell system components. 

The best example is a joint IFC and Praxair, Inc., venture to develop a unit that converts natural 

gas to 99.999% pure hydrogen based on using fuel cell reformer technology and pressure swing 

adsorption process. 

1.9 References 

1. A.J. Appleby, F.R. Foulkes, Fuel Cell Handbook, Van Nostrand Reinhold, New York, NY, 

1989. 
1-33 

2. Report of the DOE Advanced Fuel-Cell Commercialization Working Group, Edited by S.S. 

Penner, DOE/ER/0643, prepared by the DOE Advanced Fuel Cell Working Group 

(AFC2WG) or the United States Department of Energy under Contract 

No. DEFG03-93ER30213, March 1995. 

3. K. Kordesch, J. Gsellmann, S. Jahangir, M. Schautz, in Proceedings of the Symposium on 

Porous Electrodes: Theory and Practice, Edited by H.C. Maru, T. Katan, M.G. Klein, The 

Electrochemical Society, Inc., Pennington, NJ, p. 163, 1984. 

4. A. Pigeaud, H.C. Maru, L. Paetsch, J. Doyon, R. Bernard, in Proceedings of the Symposium 

on Porous Electrodes: Theory and Practice, Edited by H.C. Maru, T. Katan, M.G. Klein, 

The Electrochemical Society, Inc., Pennington, NJ, p. 234, 1984. 

5. J.M. King, N. Ishikawa, "Phosphoric Acid Fuel Cell Power Plant Improvements and 

Commercial Fleet Experience," Nov. 96 Fuel Cell Seminar. 

6. www.utcfuelcells.com. 

7. Communications with IFC, August 24, 2000. 

8. K. Yokota, et al., "GOI 11 MW FC Plant Operation Interim Report," in Fuel Cell Program 

and Abstracts, 1992 Fuel Cell Seminar, Tucson, AZ, November 29-December 2, 1992. 

9. ONSI Press Release, "Fuel Cell Sets World Record; Runs 9,500 Hours Nonstop," May 20, 

1997. 

10. Northeast Utilities System Press Release, "Converting Landfill Gas into Electricity is an 

Environmental Plus," June 24, 1996. 

11. "Groton’s Tidy Machine," Public Power, March-April 1997. 

12. ONSI Press Release, "World’s First Hydrogen Fueled Fuel Cell Begins Operation in 

Hamburg, Germany," November 7, 1997. 

13. "Anaerobic Gas Fuel Cell Shows Promise," Modern Power Systems, June 1997. 

14. E.W. Hall, W.C. Riley, G.J. Sandelli, "PC25TM Product and Manufacturing Experience," 

IFC, Fuel Cell Seminar, November 1996. 

15. www.ballard.com, 1998. 

16. www.fuelcellenergy.com. 

17. Information supplied by ERC for the Fuel Cell Handbook. 

18. M.M. Piwetz, J.S. Larsen, T.S. Christensen, "Hydrodesulfurization and Pre-reforming of 

Logistic Fuels for Use in Fuel Cell Applications," Fuel Cell Seminar Program and 

Abstracts, Courtesy Associates, Inc., November 1996. 

19. Westinghouse Electric Corporation, Bechtel Group, Inc., "Solid Oxide Fuel Cell 

Repowering of Highgrove Station Unit 1, Final Report," prepared for Southern California 

Edison Research Center, March 1992. 

20. ERC, "Effects of Coal-Derived Trace Species on the Performance of Molten Carbonate Fuel 

Cells," topical report prepared for U.S. DOE/METC, DOE/MC/25009-T26, October 1991. 

21. N. Maskalick, "Contaminant Effects in Solid Oxide Fuel Cells," in Agenda and Abstracts, 

Joint Contractors Meeting, Fuel Cells and Coal-Fired Heat Engines Conference, 

U.S. DOE/METC, August 3-5, 1993. 

22. D.M. Rastler, C. Keeler, C.V. Chang, "Demonstration of a Carbonate on Coal Derived Gas," 

Report 15, in An EPRI/GRI Fuel Cell Workshop on Technology Research and Development. 

Stonehart Associates, Madison, CT, 1993. 

23. Distributed Generation, Securing America’s Future with Reliable, Flexible Power,” U.S. 

Department of Energy, Office of Fossil Energy, National Energy Technology Center, 

October 1999. 
1-34 

24. U. S. Department of Energy’s Office of Energy Efficiency and Renewable Energy webpage, 

http://www.eren.doe.gov/distributedpower Giovando, CarolAnn, “Distributed resources 

carve out a niche in competitive markets,” Power, July/August 2000, pp. 46 – 57. 

25. K.V. Kordesch, "City Car with H2-Air Fuel Cell and Lead Battery," 6th Intersociety Energy 

Conversion Engineering Conference, SAE Paper No. 719015, 1971. 

26. A. Kaufman, "Phosphoric Acid Fuel Cell Bus Development," Proceedings of the Annual 

Automotive Technology Development Contractors' Coordination Meeting, Dearborn, MI, 

October 24-27, 1994, SAE Proceedings Volume P-289, pp. 289-293, 1995. 

27. R.R. Wimmer, "Fuel Cell Transit Bus Testing & Development at Georgetown University," 

Proceedings of the Thirty Second Intersociety Energy Conversion Engineering Conference, 

July 27-August 1, 1997, Honolulu, HI, pp. 825-830, 1997. 

28. N.C. Otto, P.F. Howard, "Transportation Engine Commercialization at Ballard Power 

Systems," Program and Abstracts 1996 Fuel Cell Seminar, November 17-20, 1996, 

Orlando, FL, pp. 559-562. 

29. F. Panik, "Fuel Cells for Vehicle Application in Cars - Bringing the Future Closer," J. Power 

Sources, 71, 36-38, 1998. 

30. S. Kawatsu, "Advanced PEFC Development for Fuel Cell Powered Vehicles," J. Power 

Sources, 71, 150-155, 1998. 

31. Fuel-Cell Technology: Powering the Future, Electric Line, November/December 1996. 

32. M. Graham, F. Barbir, F. Marken, M. Nadal, "Fuel Cell Power System for Utility Vehicle," 

Program and Abstracts 1996 Fuel Cell Seminar, November 17-20, 1996, Orlando, FL, pp. 

571-574. 

33. P.A. Lehman, C.E. Chamberlin, "Design and Performance of a Prototype Fuel Cell Powered 

Vehicle," Program and Abstracts 1996 Fuel Cell Seminar, November 17-20, 1996, Orlando, 

FL, pp. 567-570. 

34. J. Leslie, "Dawn of the Hydrogen Age," Wired (magazine), October 1997. 
2-1 

2. FUEL CELL PERFORMANCE

The purpose of this section is to describe the chemical and thermodynamic relations governing 

fuel cells and how operating conditions affect their performance. Understanding the impacts of 

variables such as temperature, pressure, and gas constituents on performance allows fuel cell 

developers to optimize their design of the modular units and it allows process engineers to 

maximize the performance of systems applications. 

A logical first step in understanding the operation of a fuel cell is to define its ideal performance. 

Once the ideal performance is determined, losses arising from non-ideal behavior can be 

calculated and then deducted from the ideal performance to describe the actual operation. 

2.1 The Role of Gibbs Free Energy and Nernst Potential 

The maximum electrical work (Wel) obtainable in a fuel cell operating at constant temperature 

and pressure is given by the change in Gibbs free energy (∆G) of the electrochemical reaction: 

el W = G = n E ∆ − F (2-1)

where n is the number of electrons participating in the reaction, F is Faraday's constant 

(96,487 coulombs/g-mole electron), and E is the ideal potential of the cell. 

The Gibbs free energy change is also given by the following state function: 

∆ =∆ − ∆ G H T S (2-2)

where ∆H is the enthalpy change and ∆S is the entropy change. The total thermal energy 

available is ∆H. The available free energy is equal to the enthalpy change less the quantity T S∆

which represents the unavailable energy resulting from the entropy change within the system. 

The amount of heat that is produced by a fuel cell operating reversibly is T∆S. Reactions in fuel 

cells that have negative entropy change generate heat (such as hydrogen oxidation), while those 

with positive entropy change (such as direct solid carbon oxidation) may extract heat from their 
2-2 

surroundings if the irreversible generation of heat is smaller than the reversible absorption of 

heat. 

For the general cell reaction, 

αA B CD + →+ β c δ (2-3)

the standard state Gibbs free energy change of reaction is given by: 

G = G G G G CD AB c δ °° °° ∆ ° + −α − β (2-4)

where Gi

° is the partial molar Gibbs free energy for species i at temperature T. This potential 

can be computed from the heat capacities (Cp) of the species involved as a function of T and 

from values of both ∆S° and ∆H° at a reference temperature, usually 298K. Empirically, the heat 

capacity of a species, as a function of T, can be expressed as 

2 C = a + bT + cT p (2-5)

where a, b, and c are empirical constants. The specific enthalpy for any species present during 

the reaction is given by 

H i i = H o + 

T

pi

298

C dT ∫ (2-6)

and, at constant pressure the specific entropy at temperature T is given by 

S S i i = +o T

pi

298

C dT ∫ T (2-7)

It then follows that 

∆ = ∑ −∑

i

i out i i

i

H ni H n H in 

(2-8)
2-3 

and 

∆ = ∑ −∑

i

i out i i

i

S niS n S in (2-9)

The coefficients a, b, and c, as well as H° and S°, are available from standard reference tables, 

and may be used to calculate ∆H and ∆S. From these values it is then possible to calculate ∆G 

and E at temperature T. 

Instead of using the coefficients a, b, and c, it is modern practice to rely on tables, such as 

JANAF Thermochemical Tables (1) to provide Cp, ∆H, ∆S, and ∆G over a range of temperatures 

for all species present in the reaction. 

The Gibbs free energy change of reaction can be expressed by the equation: 

C D

A B

f f G = G + RT ln

f f

c δ

α β ∆ ∆° (2-10)

where ∆ ° G is the Gibbs free energy change of reaction at the standard state pressure (1 atm) 

and at temperature T, and fi is the fugacity of species i. Substituting Equation (2-1) in 

Equation (2-10) gives the relation 

C D

A B

RT f f E = E + ln

n ff

c δ

α β ° F (2-11)

or more generally, 

RT [reactant fugacity] E = E + ln 

n [product fugacity]

Π ° F Π (2-12)

which is the general form of the Nernst equation. The reversible potential of a fuel cell at 

temperature T, E° , is calculated from ∆ ° G for the cell reaction at that temperature. 

Fuel cells generally operate at pressures low enough that the fugacity can be approximated by the 

partial pressure. 
2-4 

2.2 Ideal Performance 

The Nernst potential, E, gives the ideal open circuit cell potential. This potential sets the upper 

limit or maximum performance achievable by a fuel cell. 

The overall reactions for various types of fuel cells are presented in Table 2-1. The corresponding 

Nernst equations for those reactions are provided in Table 2-2. 

Table 2-1 Electrochemical Reactions in Fuel Cells 

Fuel Cell Anode Reaction Cathode Reaction 

Polymer Electrolyte 

and Phosphoric Acid H2 → 2H+

 + 2e- 1⁄2 O2 + 2H+ + 2e- → H2O 

Alkaline H2 + 2(OH)- → 2H2O + 2e-

 1⁄2 O2 + H2O + 2e- → 2(OH)-

Molten Carbonate 

H2 + CO3

= → H2O + CO2 + 2eCO + CO3

= → 2CO2 + 2e- 1⁄2 O2 + CO2 + 2e- → CO3

=

Solid Oxide 

H2 + O= → H2O + 2eCO + O= → CO2 + 2eCH4 + 4O= → 2H2O + CO2 + 8e-

1⁄2 O2 + 2e- → O=

CO - carbon monoxide e- - electron H2O - water 

CO2 - carbon dioxide H+ - hydrogen ion O2 - oxygen 

CO3

= - carbonate ion H2 - hydrogen OH- - hydroxyl ion 

 

The Nernst equation provides a relationship between the ideal standard potential (E°) for the cell 

reaction and the ideal equilibrium potential (E) at other partial pressures of reactants and products. 

For the overall cell reaction, the cell potential increases with an increase in the partial pressure 

(concentration) of reactants and a decrease in the partial pressure of products. For example, for 

the hydrogen reaction, the ideal cell potential at a given temperature can be increased by operating 

at higher reactant pressures, and improvements in fuel cell performance have, in fact, been 

observed at higher pressures. This will be further demonstrated in Chapters 3 through 7 for the 

various types of fuel cells. 

The reaction of H2 and O2 produces H2O. When a carbon-containing fuel is involved in the anode 

reaction, CO2 is also produced. For MCFCs, CO2 is required in the cathode reaction to maintain an 

invariant carbonate concentration in the electrolyte. Because CO2 is produced at the anode and 

consumed at the cathode in MCFCs, and because the concentrations in the anode and cathode feed 

streams are not necessarily equal, the CO2 partial pressures for both electrode reactions are present 

in the second Nernst equation shown in Table 2-2. 
2-5 

Table 2-2 Fuel Cell Reactions and the Corresponding Nernst Equations 

Cell Reactions* Nernst Equation 

H222 + 1⁄2O → H O E = E + (RT/ 2 ) ln [P / P ] + (RT/ 2 ) ln [P ] H H 2 2 O O2 ° F F 1⁄2

2 22 (c)

2 2 (a)

H + O + CO 

H O + CO

1⁄2 → E = E + (RT/ 2 ) ln [P / P (P ) ] + 

(RT/ 2 ) ln [P (P ) ]

22 2

2 2

H H O CO (a)

O CO ( )

° F

F 1⁄2

c

CO + 1⁄2O2 2 → CO E = E + (RT/ 2 ) ln [PCO / PCO ] + (RT/ 2 ) ln [PO ] 2 2 ° F F 1⁄2

42 2

2

CH + 2O 2 H O +

CO

→

 

E = E + (RT/ 8 ) ln [P / P P ] + (RT/ 8 ) ln [P ] CH H 42 2 2 O

2

CO O

2 ° F F

(a) - anode P - gas pressure 

(c) - cathode R - universal gas constant 

E - equilibrium potential T - temperature (absolute) 

F - Faraday's constant 

* The cell reactions are obtained from the anode and cathode reactions listed in Table 2-1. 

The ideal standard potential (Eo

) at 298K for a fuel cell in which H2 and O2 react is 1.229 volts 

with liquid water product, or 1.18 volts with gaseous water product. This value is shown in 

numerous chemistry texts (2) as the oxidation potential of H2. The potential is the change in 

Gibbs free energy resulting from the reaction between hydrogen and oxygen. The difference 

between 1.229 volts and 1.18 volts represents the Gibbs free energy change of vaporization of 

water at standard conditions. 

Figure 2-1 shows the relation of E to cell temperature. Because the figure shows the potential of 

higher temperature cells, the ideal potential corresponds to a reaction where the water product is 

in a gaseous state (i.e., Eo is 1.18 volts). 

1.0

1.1

1.2

Reversible potential (V)

300 400 500 600 700 800 900 1000 1100

Temperature (K)

H2 + 1

2 O2

H2O(g)

Figure 2-1 H2/O2 Fuel Cell Ideal Potential as a Function of Temperature 
2-6 

The impact of temperature on the ideal voltage, E, for the oxidation of hydrogen is also shown in 

Table 2-3 for the various types of fuel cells. Each case assumes gaseous products as its basis. 

Table 2-3 Ideal Voltage as a Function of Cell Temperature 

Temperature 25°C 

(298K) 

80°C 

(353K) 

100°C 

(373K) 

205°C 

(478K) 

650°C 

(923K) 

800°C 

(1073K) 

1100°C 

(1373K) 

Cell Type PEFC AFC PAFC MCFC ITSOFC TSOFC 

Ideal Voltage 1.18 1.17 1.16 1.14 1.03 0.99 0.91 

The open circuit voltage of a fuel cell is also strongly influenced by the reactant concentrations. 

The maximum ideal potential occurs when the reactants at the anode and cathode are pure. In an 

air-fed system or if the feed to the anode is other than pure dry hydrogen, the cell potential will 

be reduced. Similarly, the concentration of reactants at the exit of the cell will be lower than at 

the entrance. This reduction in partial pressure leads to a Nernst correction that reduces the open 

circuit voltage locally, often by as much as 250 mV in higher-temperature cells. Because the 

electrodes should be highly conductive and the electrode within one cell consequently has close 

to uniform voltage, depressed open circuit voltage affects the operation of the entire cell. This 

significantly impacts the achievable cell operating voltage and consequently system efficiency of 

especially the higher-temperature fuel cells. 

The ideal performance of a fuel cell depends on the electrochemical reactions that occur between 

different fuels and oxygen as summarized in Tables 2-1 and 2-2. Low-temperature fuel cells 

(PEFC, AFC, and PAFC) require noble metal electro-catalysts to achieve practical reaction rates at 

the anode and cathode, and H2 is the only acceptable fuel. With high-temperature fuel cells 

(MCFC, ITSOFC, and TSOFC), the requirements for catalysis are relaxed, and the number of 

potential fuels expands. While carbon monoxide severely poisons noble metal anode catalysts 

such as platinum (Pt) in low-temperature fuel cells, it is a reactant in high-temperature fuel cells 

(operating temperatures of 300 °C and higher) where non-noble metal catalysts such as nickel (Ni) 

can be used. 

Note that H2, CO, and CH4 are shown in Table 2-1 as potentially undergoing direct anodic 

oxidation. In actuality, direct electrochemical oxidation of the CO and CH4 usually represents only 

a minor pathway to oxidation of these species. It is common systems analysis practice to assume 

that H2, the more readily oxidized fuel, is produced by CO and CH4 reacting, at equilibrium, with 

H2O through the water gas shift and steam reforming reactions, respectively. A simple reaction 

pathway analysis explains why direct oxidation is rarely the major reaction pathway under most 

fuel cell operating conditions: 

• The driving force for anodic oxidation of CO and CH4 is lower than that for the oxidation of 

hydrogen, as reflected in the higher open circuit voltage of the hydrogen oxidation. 

• The kinetics of hydrogen oxidation on the anode are significantly faster than that of CO or 

CH4 oxidation. 
2-7 

• There is vastly more surface area available for catalytic reforming and shift reaction 

throughout the anode of a practical fuel cell than there is surface area in the three-phaseboundary for electrochemical oxidation. 

• Mass-transfer of CO, CH4, and even more so of higher hydrocarbons, to the three-phase 

boundary and through the porous anode is more than ten times slower than that of hydrogen, 

leading to a more significant impact of concentration polarization. 

Nevertheless, direct oxidation can be important under certain conditions, such as at the entrance of 

a cell. The degree to which an anode supports direct oxidation will then impact the degree of prereforming of the fuel that is required, which in turn typically impacts balance of plant complexity 

and cost. This is why there remains strong interest in the development of direct oxidation anodes. 

The H2 that can be produced from CO and CH4, along with any H2 in the fuel supply stream, is 

referred to as equivalent H2. The temperature and catalyst of state-of-the-art SOFCs and MCFCs 

provide the proper environment for the water gas shift reaction to produce H2 and CO2 from CO 

and H2O. If only H2 and CO are fed to the fuel cell, it is known as an external reforming (ER) cell. 

In an internal reforming (IR) fuel cell, the reforming reaction to produce H2 and CO2 from CH4 and 

H2O occurs inside the stack. In some IR fuel cells, reforming takes place on the anode (on-anode 

reforming) while in others a reforming catalyst is placed in proximity to the anode to promote the 

reaction (in-cell reforming). 

2.3 Cell Energy Balance 

The discussion above can be used to formulate a mass and energy balance around a fuel cell to 

describe its electrical performance. The energy balance around the fuel cell is based on the 

energy absorbing/releasing processes (e.g., power produced, reactions, heat loss) that occur in 

the cell. As a result, the energy balance varies for the different types of cells because of the 

differences in reactions that occur according to cell type. 

In general, the cell energy balance states that the enthalpy flow of the reactants entering the cell 

will equal the enthalpy flow of the products leaving the cell plus the sum of three terms: (1) the 

net heat generated by physical and chemical processes within the cell, (2) the dc power output 

from the cell, and (3) the heat loss from the cell to its surroundings. 

Component enthalpies are readily available on a per mass basis from data tables such as JANAF 

(1). Product enthalpy usually includes the heat of formation in published tables. A typical 

energy balance determines the cell exit temperature knowing the reactant composition, the feed 

stream temperatures, H2 and O2 utilization, the expected power produced, and a percent heat loss. 

The exit constituents are calculated from the fuel cell reactions as illustrated in Example 9-3, 

Chapter 9. 

2.4 Cell Efficiency 

The thermal efficiency of a fuel conversion device is defined as the amount of useful energy 

produced relative to the change in enthalpy, ∆H, between the product and feed streams. 
2-8 

η = 

H

Useful Energy

∆ (2-13)

Conventionally, chemical (fuel) energy is first converted to heat, which is then converted to 

mechanical energy, which can then be converted to electrical energy. For the thermal to 

mechanical conversion, a heat engine is conventionally used. Carnot showed that the maximum 

efficiency of such an engine is limited by the ratio of the absolute temperatures at which heat is 

rejected and absorbed, respectively (3). 

Fuel cells convert chemical energy directly into electrical energy. In the ideal case of an 

electrochemical converter, such as a fuel cell, the change in Gibbs free energy, ∆G, of the 

reaction is available as useful electric energy at the temperature of the conversion. The ideal 

efficiency of a fuel cell, operating reversibly, is then 

ηideal = 

∆

∆

G

H (2-14)

The most widely used efficiency of a fuel cell is based on the change in the standard free energy 

for the cell reaction 

H2 + 1⁄2 O2 → H2O(1) (2-15)

given by 

2 22 ( )

1

2

∆= − − GG G G r H H ο ο

o o oo

l (2-16)

where the product water is in liquid form. At standard conditions of 25°C (298°K) and 

1 atmosphere, the thermal energy ( ∆H ) in the hydrogen/oxygen reaction is 285.8 kJ/mole, and 

the free energy available for useful work is 237.1 kJ/mole. Thus, the thermal efficiency of an 

ideal fuel cell operating reversibly on pure hydrogen and oxygen at standard conditions is: 

(2-17) 0.83

285.8

237.1

ηideal = =
2-9 

For other electrochemical reactions, different ideal efficiencies apply. Curiously, for direct 

electrochemical oxidation of carbon ∆G is larger than ∆H, and consequently the ideal efficiency 

is slightly greater than 100% when using this definition of ideal efficiency. 

For convenience, the efficiency of an actual fuel cell is often expressed in terms of the ratio of 

the operating cell voltage to the ideal cell voltage. As will be described in greater detail in the 

sections following, the actual cell voltage is less than the ideal cell voltage because of losses 

associated with cell polarization and ohmic losses. The thermal efficiency of a hydrogen/oxygen 

fuel cell can then be written in terms of the actual cell voltage: 

actual actual

ideal ideal

Useful Energy Useful Power Volts x Current (0.83)(V )

H ( G 0 83) Volts x Current/0.83 E / . η= = = = ∆ ∆ (2-18) 

As mentioned previously, the ideal voltage of a cell operating reversibly on pure hydrogen and 

oxygen at 1 atm pressure and 25oC is 1.229 V. Thus, the thermal efficiency of an actual fuel cell 

operating at a voltage of Vcell, based on the higher heating value of hydrogen, is given by 

η == = 0.83 x V / E 0.83 x V /1.229 0.675 x V cell ideal cell cell (2-19)

The foregoing has assumed that the fuel is completely converted in the fuel cell, as is common in 

most types of heat engines. This efficiency is also referred to as the voltage efficiency. However, 

in fuel cells, the fuel is typically not completely converted. To arrive at the net cell efficiency, 

the voltage efficiency must be multiplied by the fuel utilization. An excellent review of the 

impact of this phenomenon is provided by Winkler (4). 

Because the reactant activities in gas-fueled fuel cells drop as the utilization rises, and because 

the cell voltage cannot be higher than the lowest local potential in the cell, utilization 

considerations further limit the efficiency. Figure 2-2 shows the impact of fuel utilization on the 

Nernst voltage, voltage efficiency, and maximum overall cell efficiency for operating conditions 

typical for an SOFC (800 °C, 50% initial hydrogen concentration). Figure 2-2 shows that to 

achieve 90% fuel utilization, the Nernst voltage drops by over 200 mV. As a consequence, the 

maximum cell efficiency (on a higher heating value basis) is not 62%, as predicted based on the 

ideal potential, but 54%. Of course, practical cell operating effects and cell non-idealities further 

reduce this efficiency in real life. 

These effects are somewhat less profound at lower operating temperatures, such as those found 

in lower temperature SOFC, MCFC, or in low-temperature fuel cells. 
2-10 

0%

10%

20%

30%

40%

50%

60%

70%

80%

0% 20% 40% 60% 80% 100%

Fuel Utilization

Efficiency (based on HHV)

0.00

0.20

0.40

0.60

0.80

1.00

1.20

Nernst Voltage (V)

cell efficiency 0% Voltage Efficiency / Nerst Voltage (V)

Figure 2-2 Effect of fuel utilization on voltage efficiency and overall cell efficiency for 

typical SOFC operating conditions (800 °C, 50% initial hydrogen concentration). 

2.5 Actual Performance 

The actual cell potential is decreased from its ideal potential because of several types of 

irreversible losses, as shown in Figure 2-32

. These losses are often referred to as polarization, 

overpotential or overvoltage, though only the ohmic losses actually behave as a resistance. 

Multiple phenomena contribute to irreversible losses in an actual fuel cell: 

• Activation-related losses. These stem from the activation energy of the electrochemical 

reactions at the electrodes. These losses depend on the reactions at hand, the electro-catalyst 

material and microstructure, reactant activities (and hence utilization), and weakly on current 

density. 

• Ohmic losses. Ohmic losses are caused by ionic resistance in the electrolyte and electrodes, 

electronic resistance in the electrodes, current collectors and interconnects, and contact 

resistances. Ohmic losses are proportional to the current density, depend on materials 

selection and stack geometry, and on temperature. 

• Mass-transport-related losses. These are a result of finite mass transport limitations rates of 

the reactants and depend strongly on the current density, reactant activity, and electrode 

structure. 

In the V-I diagram, especially for low-temperature fuel cells, the effects of the three loss 

categories are often easy to distinguish, as illustrated in Figure 2-3. 

 

2

 Activation region and concentration region are more representative of low-temperature fuel cells. 
2-11 

Theoretical EMF or Ideal Voltage

Region of Activation Polarization (Reaction Rate Loss)

Total Loss

Current Density (mA/cm2)

0.5

0

1.0

Cell Voltage

Region of Ohmic Polarization

(Resistance Loss)

Region of Concentration Polarization

(Gas Transport Loss)

Operation Voltage, V, Curve

Figure 2-3 Ideal and Actual Fuel Cell Voltage/Current Characteristic 

In high-temperature fuel cells, the activation-related losses are often much less significant, and 

hence the characteristic concave portion of the V-I curve is hard to distinguish. In addition, as 

transport-related losses play a more important role, the convex portion of the curve often extends 

further to the left. 

Although it is tempting to characterize all losses in the cell as an equivalent resistance, only the 

ohmic losses actually behave that way, by definition. The ohmic loss depends only on cell 

geometry, the materials used, and the operating temperature. The other losses depend strongly on 

reactant concentrations (and hence fuel utilization) and thus they change within cells operated at 

finite fuel utilization. Attempts to include these types of polarization into the cell resistance more 

often than not lead to confusion and misinterpretation. This consideration has several 

ramifications for fuel cell engineers attempting to utilize single-cell data for stack or system 

design: 

• Activation and concentration polarization data presented are generally only valid for that 

particular cell and operating geometry. 

• A mathematical model will generally be required to interpret activation and concentration 

polarization data and translate it into data useful for stack engineers. 

• Detailed reactant concentration information (including utilization) is essential for 

interpretation of activation and concentration polarization data. In practice, sound 

interpretation for translation to practical cell designs, sizes, and operating conditions is only 

possible when data is acquired with very low utilization (typically less than 5%), and for 

many reactant inlet partial pressures. 

• Much of the single-cell data presented and published is taken at finite utilization. While 

useful for qualitative comparisons between cells, this data is generally not usable for further 

stack engineering. 
2-12 

Below the three types of losses are discussed in greater detail. 

Activation Losses: Activation losses are caused by sluggish electrode kinetics. There is a close 

similarity between electrochemical and chemical reactions in that both involve an activation 

energy that must be overcome by the reacting species. In reality, activation losses are the result 

of complex surface electrochemical reaction steps, each of which have their own reaction rate 

and activation energy. Usually, the rate parameters and activation energy of one or more ratelimiting reaction steps controls the voltage drop caused by activation losses on a particular 

electrode under specific conditions. However, in the case of electrochemical reactions with 

ηact > 50-100 mV, it is possible to approximate the voltage drop due to activation polarization by 

a semi-empirical equation, called the Tafel equation (5). The equation for activation polarization 

is shown by Equation (2-20): 

act

o

 = 

RT

n

 ln 

i

i η

α F (2-20)

where α is the electron transfer coefficient of the reaction at the electrode being addressed, and io

is the exchange current density. Tafel plots, such as in Figure 2-4, provide a visual 

understanding of the activation polarization of a fuel cell. They are used to measure the 

exchange current density, given by the extrapolated intercept at ηact = 0 which is a measure of the 

maximum current that can be extracted at negligible polarization (3), and the transfer coefficient 

(from the slope). 

The usual form of the Tafel equation that can be easily expressed by a Tafel Plot is 

ηact = a + b ln i (2-21)

where a = (-RT/αnF) ln io and b = RT/αnF. The term b is called the Tafel slope, and is obtained 

from the slope of a plot of ηact as a function of ln i. There exists a strong incentive to develop 

electro-catalysts that yield a lower Tafel slope for electrochemical reactions so that increases in 

current density result only in nominal increases in activation polarization. 
2-13 

5.0

4.0

3.0

2.0

1.0

0.0

0 100 200 300

(mV)

Log i (mA/cm2)

η

Exchange

Current

Figure 2-4 Example of a Tafel Plot 

The simplified description presented here did not consider processes that give rise to activation 

polarization, except for attributing it to sluggish electrode kinetics. Processes involving 

absorption of reactant species, transfer of electrons across the double layer, desorption of product 

species, and the nature of the electrode surface all contribute to activation polarization. 

Ohmic Polarization: Ohmic losses occur because of resistance to the flow of ions in the 

electrolyte and resistance to flow of electrons through the electrode. The dominant ohmic losses 

through the electrolyte are reduced by decreasing the electrode separation and enhancing the 

ionic conductivity of the electrolyte. Because both the electrolyte and fuel cell electrodes obey 

Ohm's law, the ohmic losses can be expressed by the equation 

ηohm = iR (2-22)

where i is the current flowing through the cell, and R is the total cell resistance, which includes 

electronic, ionic, and contact resistance: 

R = Relectronic + Rionic + Rcontact

Any of these components can dominate the ohmic resistance, depending on the cell type. For 

example, in planar electrolyte-supported SOFC the ionic resistance usually dominates; in tubular 

SOFC the electronic bulk resistance usually dominates, and in planar thin-electrolyte SOFC 

contact resistances often dominate. 

The ohmic resistance normalized by the active cell area is the Area Specific Resistance (ASR). 

ASR has the units Ωcm2

. The ASR is a function of the cell design, material choice, 

manufacturing technique, and, because material properties change with temperature, operating 

conditions. The ASR is a key performance parameter, especially in high-temperature fuel cells, 

where the ohmic losses often dominate the overall polarization of the cell. 
2-14 

Experimentally, there are several ways to determine the ohmic cell resistance. If the V-I curve 

has a substantial linear portion (in the center), the slope of this curve usually closely 

approximates the ASR of the cell. Only in such a linear portion of the V-I curve the ohmic 

resistance is dominant, and hence the determination of the ASR valid. Sometimes, a more 

accurate way to determine the ohmic resistance is from impedance spectroscopy. In an 

impedance spectrum of a fuel cell, the ohmic resistance is the real value of the impedance of the 

point for which the imaginary impedance is zero (Figure 2-5). As can be seen in the example, the 

ohmic resistance is invariant with gas concentration. The part of the impedance that is related to 

mass transport and kinetics, however, changes markedly with anode feed composition. 

Figure 2-5 Example of impedance spectrum of anode-supported SOFC operated at 

850 °C (6). Rs is Ohmic resistance. Two measurements were with hydrogen/water 

vapor mixtures, and the other in diluted hydrogen. 

Finally, the electronic portions of the ohmic resistance could also be measured directly using a 

four-point probe or with a through-measurement. 

Given a certain cell design and operating temperature, the bulk material contributions to R (and 

hence the ASR) can also be calculated. Based on the detailed cell geometry, the length of both 

the ionic and electronic current paths and cross-sectional area for current conduction can be 

measured. Together with the resistivities of the materials used, they yield the bulk ASR. The 

contact resistance cannot be calculated from fundamental data, and is usually determined by 

difference between the measured total resistance and the computed bulk resistance. 

When using literature data for ASR, it is critical to verify the definition of ASR. Some 

researchers have defined “ASR”s to include the activation and concentration polarization as well 

as the ohmic polarization. 

Mass Transport-Related Losses: As a reactant is consumed at the electrode by electrochemical 

reaction, it is often diluted by the products, when finite mass transport rates limit the supply of 

fresh reactant and the evacuation of products. As a consequence, a concentration gradient is 
2-15 

formed which drives the mass transport process. In a fuel cell with purely gas-phase reactants 

and products (such as an SOFC), gas diffusion processes control mass transfer. In other cells, 

multi-phase flow in the porous electrodes can have a significant impact (e.g. in PEFC). In 

hydrogen fuel cells, the evacuation of product is often more limiting than the supply of fuel, 

given the difference between the diffusivities of hydrogen and water (vapor). 

While at low current densities and high bulk reactant concentrations mass-transport losses are 

not significant, under practical conditions (high current densities, low fuel and air 

concentrations), they often contribute significantly to loss of cell potential. 

For gas-phase fuel cells, the rate of mass transport to an electrode surface in many cases can be 

described by Fick's first law of diffusion: 

i = n D ( F CB S − C )

δ (2-23)

where D is the diffusion coefficient of the reacting species, CB is its bulk concentration, CS is its 

surface concentration, and δ is the thickness of the diffusion layer. The limiting current (iL) is a 

measure of the maximum rate at which a reactant can be supplied to an electrode, and it occurs 

when CS = 0, i.e., 

L

B

i = 

n DC F

δ (2-24)

By appropriate manipulation of Equations (2-23) and (2-24), 

S

B L

C

C

 = 1 

i

i − (2-25)

The Nernst equation for the reactant species at equilibrium conditions, or when no current is 

flowing, is 

E = E + 

RT

n i 0= ° ln CB F (2-26)

When current is flowing, the surface concentration becomes less than the bulk concentration, and 

the Nernst equation becomes 
2-16 

E = E + 

RT

n ° ln CS F (2-27)

The potential difference (∆E) produced by a concentration change at the electrode is called the 

concentration polarization: 

∆ E = = 

RT

n

 ln 

C

C conc

S

B

η F (2-28)

Upon substituting Equation (2-25) in (2-28), the concentration polarization is given by the 

equation 

conc

L

 = 

RT

n

 ln 1 

i

i η F − ⎛

⎝

⎜ ⎞

⎠

⎟ (2-29)

In this analysis of concentration polarization, the activation polarization is assumed to be 

negligible. The charge transfer reaction has such a high exchange current density that the 

activation polarization is negligible in comparison with the concentration polarization (most 

appropriate for the high temperature cells). 

Cumulative Effect of the Losses: The combined effect of the losses for a given cell and given 

operating conditions can be expressed as polarizations. The total polarization at the electrodes is 

the sum of ηact and ηconc, or 

ηanode = ηact,a + ηconc,a (2-30)

and 

ηcathode = ηact,c + ηconc,c (2-31)

The effect of polarization is to shift the potential of the electrode (Eelectrode) to a new value 

(Velectrode): 
2-17 

Velectrode = Eelectrode + ⏐ηelectrode⏐ (2-32)

For the anode, 

Vanode = Eanode + ⏐ηanode⏐ (2-33)

and for the cathode, 

Vcathode = Ecathode – ⏐ηcathode⏐ (2-34)

The net result of current flow in a fuel cell is to increase the anode potential and to decrease the 

cathode potential, thereby reducing the cell voltage. Figure 2-6 illustrates the contribution to 

polarization of the two half cells for a PAFC. The reference point (zero polarization) is 

hydrogen. These shapes of the polarization curves are typical of other types of fuel cells as well. 

700

600

500

400

300

200

100

0

0 200 400 600 800

Cathode Loss (Air)

Electolyte IR Loss

Cathode Loss (O2)

Anode Loss (H2)

Polarization (mV)

Current density (mA/cm2)

Figure 2-6 Contribution to Polarization of Anode and Cathode 

Summing of Cell Voltage: The cell voltage includes the contribution of the anode and cathode 

potentials and ohmic polarization: 

Vcell = Vcathode – Vanode – iR (2-35)
2-18 

When Equations (2-33) and (2-34) are substituted in Equation (2-35) 

Vcell = Ecathode – ⏐ηcathode⏐ – (Eanode + ⏐ηanode⏐) – iR (2-36)

or 

Vcell = ∆Ee – ⏐ηcathode⏐ – ⏐ηanode⏐ – iR (2-37)

where ∆Ee = Ecathode – Eanode. Equation (2-37) shows that current flow in a fuel cell results in a 

decrease in cell voltage because of losses by electrode and ohmic polarizations. The goal of fuel 

cell developers is to minimize the polarization so that Vcell approaches ∆Ee. This goal is 

approached by modifications to fuel cell design (improvement in electrode structures, better 

electro-catalysts, more conductive electrolyte, thinner cell components, etc.). For a given cell 

design, it is possible to improve the cell performance by modifying the operating conditions 

(e.g., higher gas pressure, higher temperature, change in gas composition to lower the gas 

impurity concentration). However, for any fuel cell, compromises exist between achieving 

higher performance by operating at higher temperature or pressure and the problems associated 

with the stability/durability of cell components encountered at the more severe conditions. 

2.6 Fuel Cell Performance Variables 

The performance of fuel cells is affected by operating variables (e.g., temperature, pressure, gas 

composition, reactant utilization, current density), cell design and other factors (impurities, cell 

life) that influence the ideal cell potential and the magnitude of the voltage losses described 

above. The equations describing performance variables, which will be developed in Chapters 3 

through 7, address changes in cell performance as a function of major operating conditions to 

allow the reader to perform quantitative parametric analysis. The following discussion provides 

basic insight into the effects of some operating parameters. 

Current Density: The effects on performance of increasing current density were addressed in the 

previous section that described how activation, ohmic, and concentration losses occur as the 

current is changed. Figure 2-7 is a simplified depiction of how these losses affect the shape of the 

cell voltage-current characteristic. As current is initially drawn, sluggish kinetics (activation 

losses) cause a decrease in cell voltage. At high current densities, there is an inability to diffuse 

enough reactants to the reaction sites (concentration losses) so the cell experiences a sharp 

performance decrease through reactant starvation. There also may be an associated problem of 

diffusing the reaction products from the cell. 

Ohmic losses predominate in normal fuel cell operation. These losses can be expressed as iR 

losses where i is the current and R is the summation of internal resistances within the cell, 

Equation (2-22). As is readily evident from the equation, the ohmic loss and hence voltage change 

is a direct function of current (current density multiplied by cell area). 
2-19 

Figure 2-7 presents the most important trade-off in choice of the operating point. It would seem 

logical to design the cell to operate at the maximum power density that peaks at a higher current 

density (right of the figure). However, operation at the higher power densities will mean 

operation at lower cell voltages or lower cell efficiency. Setting 

operation near the peak power density can cause instability in control because the system will 

have a tendency to oscillate between higher and lower current densities around the peak. It is 

usual practice to operate the cell to the left side of the power density peak and at a point that 

yields a compromise between low operating cost (high cell efficiency that occurs at high 

voltage/low current density) and low capital cost (less cell area that occurs at low voltage/high 

current density). In reality, the precise choice of the operating point depends on complex system 

trade-offs, usually aided by system studies that allow the designer to take into account effects of 

operating voltage and current density on parasitic power consumption, sizing of balance of plant 

components, heat rejection requirements, and other system design considerations. 

0.00

0.20

0.40

0.60

0.80

0 100 200 300 400 500 600

Current Density, mA/cm2

Cell Voltage, volts

0

100

200

300

400

Power Density, mW/cm2

Figure 2-7 Voltage/Power Relationship 

It is interesting to observe that the resulting characteristic provides the fuel cell with a benefit that 

is unique among other energy conversion technologies: the fuel cell efficiency increases at part 

load conditions.3

 Even though other components within the fuel cell system operate at lower 

component efficiencies as the system's load is reduced, the combination of increased fuel cell 

efficiency and lower supporting component efficiencies can result in a rather flat trace of total 

system efficiency as the load is reduced. This is in contrast with many heat engine-based energy 

conversion technologies that typically experience a significant drop-off in efficiency at part-load. 

This gives the fuel cell system a fuel cost advantage for applications where a significant amount of 

part-load operation is required. 

 

3

. Constraints can limit the degree of part load operation of a fuel cell. For example, a PAFC is limited to 

operation below approximately 0.85 volts because of entering into a corrosion region. 
2-20 

Temperature and Pressure: The effect of temperature and pressure on the ideal potential (E) of a 

fuel cell can be analyzed on the basis of changes in the Gibbs free energy with temperature and 

pressure. 

P

E S

 = 

T n

⎛ ⎞ ∂ ∆ ⎜ ⎟ ⎝ ⎠ ∂ F (2-38)

or 

T

E V

 = 

P n

⎛ ⎞ ∂ −∆ ⎜ ⎟ ⎝ ⎠ ∂ F (2-39)

Because the entropy change for the H2/O2 reaction is negative, the reversible potential of the H2/O2

fuel cell decreases with an increase in temperature (by 0.84 mV/°C, assuming reaction product is 

liquid water). For the same reaction, the volume change is negative; therefore, the reversible 

potential increases with an increase in pressure (with the square root of the pressure, assuming 

pressure is equal on both electrodes). 

However, temperature has a strong impact on a number of other factors: 

• Electrode reaction rates. Typically, electrode reactions follow Arrhenius behavior. As a 

consequence, these losses decline exponentially with increasing temperature, usually more 

than off-setting the reduction in ideal potential. The higher the activation energy (and hence 

usually the losses) the greater the impact of temperature. The impact of total pressure 

depends on the pressure dependence of rate-limiting reaction steps. 

• Ohmic losses. The impact of temperature on cell resistance is different for different 

materials. For metals, the resistance usually increases with temperature, while for 

electronically and ionically conductive ceramics it decreases exponentially (Arrhenius-form). 

For aqueous electrolytes, the impact is limited though high temperatures can lead to 

dehydration of the electrolyte (e.g. PEFC) and loss of conductivity. As a rule of thumb, for 

high-temperature fuel cells, the net effect is a significant reduction in resistance, while for 

low-temperature fuel cells the impact over the operating range is limited. 

Mass transport processes are not strongly affected by temperature changes within the typical 

operating temperature and pressure ranges of most fuel cell types. 

An increase in operating pressure has several beneficial effects on fuel cell performance because 

the reactant partial pressure, gas solubility, and mass transfer rates are higher. In addition, 

electrolyte loss by evaporation is reduced at higher operating pressures. Increased pressure also 

tends to increase system efficiencies. However, there are compromises such as thicker piping and 

additional expense for pressurization. Section 8.1.1 addresses system aspects of pressurization. 

The benefits of increased pressure must be balanced against hardware and materials problems, as 

well as parasitic power costs. In particular, higher pressures increase material problems in MCFCs 

(see Section 6.1), pressure differentials must be minimized to prevent reactant gas leakage through 
2-21 

the electrolyte and seals, and high pressure favors carbon deposition and methane formation in the 

fuel gas. 

Reactant Utilization and Gas Composition: Reactant utilization and gas composition have 

major impacts on fuel cell efficiency. It is apparent from the Nernst equations in Table 2-2 that 

fuel and oxidant gases containing higher partial pressures of electrochemical reactants produce a 

higher cell voltage. Utilization (U) refers to the fraction of the total fuel or oxidant introduced into 

a fuel cell that reacts electrochemically. In low-temperature fuel cells, determining the fuel 

utilization is relatively straightforward when H2 is the fuel, because it is the only reactant involved 

in the electrochemical reaction,4

 i.e. 

f

2,in 2,out

2,in

2, consumed

2,in

U = 

H H

H

 = 

H

H

− (2-40)

where H2,in and H2,out are the flow rates of H2 at the inlet and outlet of the fuel cell, respectively. 

However, hydrogen can be consumed by various other pathways, such as by chemical reaction 

(i.e., with O2 and cell components) and loss via leakage out of the cell. These pathways increase 

the apparent utilization of hydrogen without contributing to the electrical energy produced by the 

fuel cell. A similar type of calculation is used to determine the oxidant utilization. For the cathode 

in MCFCs, two reactant gases, O2 and CO2, are utilized in the electrochemical reaction. The 

oxidant utilization should be based on the limiting reactant. Frequently O2, which is readily 

available from make-up air, is present in excess, and CO2 is the limiting reactant. 

A significant advantage of high-temperature fuel cells such as MCFCs is their ability to use CO as 

a fuel. The anodic oxidation of CO in an operating MCFC is slow compared to the anodic 

oxidation of H2; thus, the direct oxidation of CO is not favored. However, the water gas shift 

reaction 

CO + H2O o H2 + CO2 (2-41)

reaches equilibrium rapidly in MCFCs at temperatures as low as 650°C (1200°F) to produce H2.

5

 

As H2 is consumed, the reaction is driven to the right because both H2O and CO2 are produced in 

equal quantities in the anodic reaction. Because of the shift reaction, fuel utilization in MCFCs can 

exceed the value for H2 utilization, based on the inlet H2 concentration. For example, for an anode 

gas composition of 34% H2, 22% H2O, 13% CO, 18% CO2, and 12% N2, a fuel utilization of 80% 

(i.e., equivalent to 110% H2 utilization) can be achieved even though this would require 10% more 

H2 (total of 37.6%) than is available in the original fuel. The high fuel utilization is possible 

because the shift reaction provides the necessary additional H2 that is oxidized at the anode. In this 

case, the fuel utilization is defined by 

 

4

. Assumes no gas cross-over or leakage out of the cell. 

5

. Example 9-5 in Section 9 illustrates how to determine the amount of H2 produced by the shift reaction. 
2-22 

f

2, consumed

2,in in

U = 

H

H + CO (2-42)

where the H2 consumed originates from the H2 present at the fuel cell inlet (H2,in) and any H2

produced in the cell by the water gas shift reaction (COin). 

Gas composition changes between the inlet and outlet of a fuel cell, caused by the electrochemical 

reaction, lead to reduced cell voltages. This voltage reduction arises because the cell voltage 

adjusts to the lowest electrode potential given by the Nernst equation for the various gas 

compositions at the exit of the anode and cathode chambers. Because electrodes are usually good 

electronic conductors and isopotential surfaces, the cell voltage can not exceed the minimum 

(local) value of the Nernst potential. In the case of a fuel cell with the flow of fuel and oxidant in 

the same direction (i.e., co-flow), the minimum Nernst potential occurs at the cell outlet. When the 

gas flows are counterflow or crossflow, determining the location of the minimum potential is not 

straightforward. 

The MCFC provides a good example to illustrate the influence of the extent of reactant utilization 

on the electrode potential. An analysis of the gas composition at the fuel cell outlet as a function of 

utilization at the anode and cathode is presented in Example 9-5. The Nernst equation can be 

expressed in terms of the mole fraction of the gases (Xi) at the fuel cell outlet: 

22 2

2 2

1⁄2 1⁄2

o H O CO ,cathode

H O,anode ,anode CO

RT XXX P E + ln E 2 X X = F (2-43)

where P is the cell gas pressure. The second term on the right side of Equation (2-43), the 

so-called Nernst term, reflects the change in the reversible potential as a function of reactant 

utilization, gas composition, and pressure. Figure 2-8 illustrates the change in reversible cell 

potential as a function of utilization using Equation (2-43). 
2-23 

Figure 2-8 The Variation in the Reversible Cell Voltage as a Function of 

Reactant Utilization 

(Fuel and oxidant utilizations equal) in a MCFC at 650°C and 1 atm. Fuel gas: 80% H2/20% CO2

saturated with H2O at 25°C; oxidant gas: 60% CO2/30% O2/10% inert) 

The reversible potential at 650°C (1200°F) and 1 atmosphere pressure is plotted as a function of 

reactant utilization (fuel and oxidant utilizations are equal) for inlet gas compositions of 80% 

H2/20% CO2 saturated with H2O at 25°C (77°F) (fuel gas6

) and 60% CO2/30% O2/10% inerts 

(oxidant gas); gas compositions and utilizations are listed in Table 2-4. Note that the oxidant 

composition is based on a gas of 2/1 CO2 to O2. The gas is not representative of the cathode inlet 

gas of a modern system, but is used for illustrative purposes only. The mole fractions of H2 and 

CO in the fuel gas decrease as the utilization increases, and the mole fractions of H2O and CO2

show the opposite trend. At the cathode, the mole fractions of O2 and CO2 decrease with an 

increase in utilization because they are both consumed in the electrochemical reaction. The 

reversible cell potential plotted in Figure 2-8 is calculated from the equilibrium compositions for 

the water gas shift reaction at the cell outlet. An analysis of the data in the figure indicates that a 

change in utilization from 20% to 80% will cause a decrease in the reversible potential of about 

0.158 V. These results show that MCFCs operating at high utilization will suffer a large voltage 

loss because of the magnitude of the Nernst term. 

An analysis by Cairns and Liebhafsky (7) for a H2/air fuel cell shows that a change in the gas 

composition that produces a 60 mV change in the reversible cell potential near room temperature 

corresponds to a 300 mV change at 1200°C (2192°F). Thus, gas composition changes are more 

significant in high temperature fuel cells. 

 

6

. Anode inlet composition is 64.5% H2/6.4% CO2/13% CO/16.1% H2O after equilibration by water gas shift reaction. 
2-24 

Table 2-4 Outlet Gas Composition as a Function of Utilization in MCFC at 650°C 

Gas Utilizationa (%) 

 0 25 50 75 90 

Anodeb 

X H2 0.645 0.410 0.216 0.089 0.033 

XCO2 0.064 0.139 0.262 0.375 0.436 

XCO 0.130 0.078 0.063 0.033 0.013 

XH2O 0.161 0.378 0.458 0.502 0.519 

Cathodec 

X CO2 0.600 0.581 0.545 0.461 0.316 

XO2 0.300 0.290 0.273 0.231 0.158 

a - Same utilization for fuel and oxidant. Gas compositions are given in mole fractions. 

b - 80% H2/20% CO2 saturated with H2O at 25°C. Fuel gas compositions are based on 

compositions for water gas shift equilibrium. 

c - 30% O2/60% CO2/10% inert gas. Gas is not representative of a modern system cathode inlet 

gas, but used for illustrative purposes only. 

2.7 Mathematical Models 

Mathematical models are critical for fuel cell scientists and developers as they can help elucidate 

the processes within the cells, allow optimization of materials, cells, stacks, and systems, and 

support control systems. Mathematical models are perhaps more important for fuel cell 

development than for many other power technologies because of the complexity of fuel cells and 

fuel cell systems, and because of the difficulty in experimentally characterizing the inner 

workings of fuel cells. Some of the most important uses of mathematical fuel cell models are: 

• To help understand the internal physics and chemistry of fuel cells. Because experimental 

characterization is often difficult (because of physical access limitations and difficulty in 

controlling test parameters independently), models can help understand the critical processes 

in cells. 

• To focus experimental development efforts. Mathematical models can be used to guide 

experiments and to improve interpolations and extrapolations of data. The rigor of modeling 

often forces the explicit position of a scientific hypothesis and provides a framework for 

testing the hypothesis. 

• To support system design and optimization. Fuel cell systems have so many unit operations 

and components that system models are critical for effective system design. 

• To support or form the basis of control algorithms. Because of the complexity of fuel cell 

systems, several developers have used fully dynamic models of fuel cell systems as the basis 

for their control algorithms. 
2-25 

• To evaluate the technical and economic suitability of fuel cells in applications. Models can 

be used to determine whether a fuel cell’s unique characteristics will match the requirements 

of a given application and evaluate its cost-effectiveness. 

Each of these applications for fuel cell models has a specific requirement with respect to the 

level of detail and rigor in the model and its predictive capability. In many higher level 

applications, the predictive requirements are modest. In some cases, the operational 

characteristics of the fuel cell are not even a degree a freedom. In such cases, relatively simple 

models are satisfactory and appropriate. It is possible to encapsulate the mass and energy 

balances and performance equations for a fuel cell within a spreadsheet application. Such 

spreadsheet models are often useful for quick trade-off considerations. 

On the other end of the spectrum, models intended to improve understanding of complex 

physical and chemical phenomena or to optimize cell geometries and flow patterns are 

necessarily very sophisticated, and usually have intensive computational requirements. 

As expected, given this wide range of potential uses and the variety of fuel cell types, an equally 

wide variety of fuel cell models has been developed. While fundamentally the constitutive 

equations such as those described earlier in this chapter underlie all models, their level of detail, 

level of aggregation, and numerical implementation method vary widely. A useful categorization 

of fuel cell models is made by level of aggregation, as shown in Figure 2-9. 

As implied in the figure, the outputs of the more detailed fundamental models can be used in 

lower-order models. This flow of information is, in fact, a critical application for high fidelity 

models. Recently, much work has been done in the development of algorithms to integrate or 

embed high-fidelity models into system analysis simulation tools. 

Despite the availability of quite sophisticated fuel cell models with well-written code and 

convenient user interfaces, the fuel cell developer or engineer must be a critical user. As 

mentioned above, obtaining experimental data on the behavior of fuel cells (especially internally 

and at the micro-level) can be difficult, time-consuming, and expensive. Unfortunately this has 

lead to a dearth of accurate and detailed data of sufficient quality and quantity to allow thorough 

validation of the mathematical models. Much of the data on fuel cell performance reported in the 

literature is, while phenomenologically often interesting, insufficiently accurate and 

accompanied by far too little detail on the test conditions to be usable for model validation. In 

particular, with much of the cell and stack taken at modest utilization, it is almost impossible to 

infer kinetic data without spatially resolved data on current density, temperature and species 

concentrations. As a consequence, the validity of fuel cell models must be critically considered 

for each use. The user of the model must be thoroughly familiar with the assumptions and 

limitations embedded in the models. 
2-26 

Figure 2-9 Overview of Levels of Fuel Cell Models. 

The sub-sections following describe examples of each type of model and provide some insight 

into their uses. Khaleel (8) and Fleig (9) provide useful overviews of the active developers in 

fuel cell modeling at different levels of aggregation, in particular for SOFC applications. 

2.7.1 Value-in-Use Models 

Value-in-use models are mathematical models that allow the user to predict how the unique 

features of fuel cells will create value or benefits in a given application. Since such models are 

usually highly application-specific, two examples are provided rather than an exhaustive review. 

A typical model of this type would be an economic model that helps the user to predict the cost 

savings resulting from the installation of a fuel cell CHP system in a building. Inputs usually 

include building specifications and use, climate information, performance and cost 

characteristics of the fuel cell CHP system, and applicable utility rate structures. Generally, only 

a high-level description of the fuel cell system is embedded, representing the efficiency and 

emissions versus load curves. The models are then used, for example, to evaluate the costeffectiveness of a fuel cell CHP system or compare it with other CHP options. DOE has 

Electrode 

Models

1-D Cell Models 

3-D Cell / Stack 

Models

Thermodynamic 

System Models

Application 

Models

M

O

M

O

C3H7

H

C3H8

M

O

M

O

C3H7

H

C3H8

0.4 

0.5 

0.6 

0.7 

0.8 

0.9 

1 

ce ll p o te n tia l (V )

0 0.2 0.4 0.6 0.8 1 1.2 1.4 

current density (A/cm2)

NG2000 H2 NG3000 H2 NG2000 ref NG3000 ref

0.4 

0.5 

0.6 

0.7 

0.8 

0.9 

1 

ce ll p o te n tia l (V )

0 0.2 0.4 0.6 0.8 1 1.2 1.4 

current density (A/cm2)

NG2000 H2 NG3000 H2 NG2000 ref NG3000 ref

Value-in-Use 

Models Building Energy Cost Model Building Energy Cost Model

Building 

characteristics

Electric

SH

WH

Hour-by-hour

loads

Weather data

Building Energy Cost Model Building Energy Cost Model

Building 

characteristics

Electric

SH

WH

Hour-by-hour

loads

Weather data

46" 60"

53"

1 2

4

5

6 7

8

9

10

11

12

13

15

14

Illustrative

• Continuum electrode models

• Multi-particle models

• Local current density 

distribution models

• Micro-kinetic models

• Spreadsheet 1-D models

• Finite volume / difference 1-D cell 

models

• CFD-based models

• Structural models

• PFD models

• Spreadsheet models

• Building energy models

• Vehicle drive-cycle models

• Well-to-wheels / busbar models

• Economic models
2-27 

supported the development of a number of models of this kind (10), while national laboratories 

and private companies have developed their own versions of this type of software. 

Another well-known type of value-in-use model is the well-to-wheels analysis, in which the 

energy consumption, environmental impact, and sometimes cost of different transportation 

options are compared considering all steps from the primary resource to the vehicle. This type of 

model is commonly used to evaluate hydrogen PEFC vehicles. Argonne National Laboratories’ 

GREET model (11) is the most widely used of these models. 

A critical subset of value-in-use models is that used to help establish the manufacturing cost of 

fuel cells. Several developers have created detailed manufacturing cost models for PEFC and 

SOFC over the past years (12, 13, 14), the results of which are widely used both in value-in-use 

models and for business planning. These models typically consider the individual processing 

steps required to produce particular cell and stack geometries at a given production volume 

(usually high production volumes). Based on estimates of the material costs, capital cost, and 

labor requirements for each process step, an estimate of the stack cost is developed. Costs of 

other components and sub-systems are determined based on a combination of vendor quotes and 

other manufacturing sub-models. 

2.7.2 Application Models 

Fuel cell application models are used to assess the interactions between the fuel cell power 

system and the application environment. The most common use is in vehicle applications where 

the dynamic interactions between the power system and the vehicle are too complex to analyze 

without the help of a mathematical model. Several commercial providers of dynamic vehicle 

modeling software have developed Fuel Cell modules (e.g. Gamma Technologies’ GT Power, 

MSC Software’s MSC.EASY5 and others). The best-published vehicle simulator of this type is 

ADVISOR (Advanced VehIcle SimulatOR) developed by the National Renewable Energy 

Laboratory and now commercialized by AVL (15). The model assesses the performance and 

fuel economy of conventional, electric, hybrid, and fuel cell vehicles. The user can evaluate 

component and vehicle specifications such as electric motors, batteries, engines, and fuel cells. 

ADVISOR simulates the vehicle's performance under different driving conditions. Industry 

partnerships contributed state-of-the-art algorithms to ensure the accuracy of the model. For 

example, detailed electrical analysis is made possible by co-simulation links to Avant's Saber 

and Ansoft's SIMPLORER. Transient air conditioning analysis is possible by co-simulation with 

C&R Technologies' SINDA/FLUINT. Michelin provided data for a tire rolling resistance model, 

and Maxwell provided data for an ultracapacitor energy storage model. 

2.7.3 Thermodynamic System Models 

Fuel cell system models have been developed to help understand the interactions between 

various unit operations within a fuel cell system. Most fuel cell system models are based on 

thermodynamic process flow simulators used by the process industry (power industry, petroleum 

industry, or chemical industry) such as Aspen Plus, HYSIS, and ChemCAD. Most of these codes 

are commercially distributed, and over the past years they have offered specific unit operations 

to assist modeling fuel cell stacks (or at least a guide for putting together existing unit operations 

to represent a fuel cell stack) and reformers. Others (16) have developed more sophisticated 2-D 
2-28 

models to help with dynamic or quasi-dynamic simulations. The balance of plant components 

usually can be readily modeled using existing unit operations included in the packages. 

These types of models are used routinely by fuel cell developers, and have become an 

indispensable tool for system engineers. The accuracy of the basic thermodynamic models is 

quite good, but because the fuel cell sub-models are typically lumped parameter models or 

simply look-up tables, their accuracy depends heavily on model parameters that have been 

developed and validated for relevant situations. Aspen Plus is described below as an example, 

followed by a description of GCTools, an Argonne National Laboratory modeling set that offers 

an alternative to codes from the commercial software industry. 

Unit Operations Models for Process Analysis using ASPEN 

DOE's National Energy Technology Laboratory has been engaged in the development of systems 

models for fuel cells for over 15 years. The models were originally intended for use in 

applications of stationary power generation designs to optimize process performance and to 

evaluate process alternatives. Hence, the models were designed to work within DOE’s ASPEN 

process simulator and later ported to the commercial version of this product, ASPEN Plus. 

ASPEN is a sophisticated software application developed to model a wide variety of chemical 

processes. It contains a library of unit operations models that simulate process equipment and 

processing steps, and it has a chemical component data bank that contains physical property 

parameters that are used to compute thermodynamic properties, including phase and chemical 

equilibrium. 

The first general purpose fuel cell model was a Nernst-limited model designed to compute the 

maximum attainable fuel cell voltage as a function of the cell operating conditions, inlet stream 

compositions, and desired fuel utilization. Subsequently, customized unit operations models 

were developed to simulate the operation of solid oxide (internal reforming), molten carbonate 

(both external and internal reforming), phosphoric acid, and polymer electrolyte fuel cells 

(PEFC). These fuel cell models are lumped parameter models based on empirical performance 

equations. As operation deviates from the setpoint conditions at a "reference" state, a voltage 

adjustment is applied to account for perturbations. Separate voltage adjustments are applied for 

current density, temperature, pressure, fuel utilization, fuel composition, oxidant utilization, 

oxidant composition, cell lifetime, and production year. These models were developed in a 

collaborative effort by DOE's National Energy Technology Laboratory and the National 

Renewable Energy Laboratory. 

In recent years, participants in the SECA core program have developed a stack sub-model for 

ASPEN that adequately represents intermediate temperature SOFC. 

Stand-alone fuel cell power systems have been investigated, as well as hybrid systems using a 

wide variety of fuels and process configurations. Some of the systems analyses studies that have 

been conducted using these fuel cell models are described in Chapter 8. 

Argonne's GCTool 

Argonne National Laboratory developed the General Computational Toolkit (GCTool) 

specifically for designing, analyzing, and comparing fuel cell systems and other power plant 
2-29 

configurations, including automotive, space-based, and stationary power systems. A library of 

models for subcomponents and physical property tables is available, and users can add empirical 

models of subcomponents as needed. Four different types of fuel cell models are included: 

polymer electrolyte, molten carbonate, phosphoric acid, and solid oxide. Other process 

equipment models include heat exchangers, reactors (including reformers), and vehicle systems. 

The physical property models include multiphase chemical equilibrium. Mathematical utilities 

include a nonlinear equation solver, a constrained nonlinear optimizer, an integrator, and an 

ordinary differential equation solver. 

GCTool has been used to analyze a variety of PEFC systems using different fuels, fuel storage 

methods, and fuel processing techniques. Examples include compressed hydrogen, metal 

hydride, glass microsphere, and sponge-iron hydrogen storage systems. Fuel processing 

alternatives have included reformers for methanol, natural gas, and gasoline using either partial 

oxidation or steam reforming. 

Researchers have examined atmospheric and pressurized PEFC automotive systems. These 

analyses included the identification of key constraints and operational analysis for off-design 

operation, system dynamic and transient performance, and the effects of operation at extreme 

temperatures. 

2.7.4 3-D Cell / Stack Models 

Fuel cell stack models are used to evaluate different cell and stack geometries and to help 

understand the impact of stack operating conditions on fuel cell stack performance. Given the 

wide range of possible stack geometries and the wide range of operating parameters that 

influence stack operation, optimization of stack design under specific application requirements is 

difficult without the help of a model that represents the key physico-chemical characteristics of 

stacks. A number of three-dimensional stack models has been developed for this purpose. In all 

of these models, the stack geometry is discretized into finite elements, or volumes, that can be 

assigned the properties of the various stack components and sub-components. At a minimum, the 

models must represent electrochemical reactions, ionic and electronic conduction, and heat and 

mass transfer within the cell. As with system models, most of these models rely on existing 

modeling platforms although in the case of stack models, an advanced 3-D modeling platform is 

generally required. 

• Computational Fluid Dynamics (CFC) – based Fuel Cell Codes. These are based on 

commercial CFD codes (e.g. StarCD, Fluent, AEA Technologies’ CFX) that have been 

augmented to represent electrochemical reactions and electronic and ionic conduction. In 

many cases, refinements in the treatment of catalytic chemical reactions and flow through 

porous media are also incorporated to represent various electrode processes. In addition to 

evaluating basic fuel cell performance (current density, temperature and species 

concentration profiles) these models can help understand the impact of different manifolding 

arrangements. 

• Computational Structural Analysis – based codes. These are based on publicly or 

commercially available 3-dimensional structural analysis codes (e.g. ANSYS, Nastran, 

Abacus). Typically, these must be augmented to represent ionic conduction, fluid flow, and 

electrochemical and chemical reactions. While these codes do not provide as much insight 
2-30 

into the impact of complex flows as the CFD-based codes, they are usually more efficient 

(run faster) than CFD-based codes and can be used to assess mechanical stresses in the stack; 

a key issue in some of the high-temperature fuel cell technologies. 

Because many of the basic elements describing the core cell performance in all of these 

approaches is similar, approaches developed for one type of stack model can be ported to 

another. Below the approach taken by NETL and Fluent is described, which is similar to the 

approach taken for PEFC cells developed by Arthur D. Little (17), which also applied that 

approach to SOFC using a structural code (ABACUS (18, 19)). Pacific Northwest National 

Laboratory (PNNL) has developed several 3-D stack models based on a CFD code (StarCD) and 

structural codes (MARC). In Europe, Forschungs-Zentrum Julich has developed its own 3-D 

codes. These models have been applied to a range of cell geometries, though in recent years the 

focus has been on planar cells. 

NETL's 3-D Analysis 

The National Energy Technology Laboratory (NETL) developed a 3-dimensional computational 

fluid dynamics (CFD) model to allow stack developers to reduce time-consuming build-and-test 

efforts. As opposed to systems models, 3-dimensional CFD models can address critical issues 

such as temperature profiles and fuel utilization; important considerations in fuel cell 

development. 

CFD analysis computes local fluid velocity, pressure, and temperature throughout the region of 

interest for problems with complex geometries and boundary conditions. By coupling the CFDpredicted fluid flow behavior with the electrochemistry and accompanying thermodynamics, 

detailed predictions are possible. Improved knowledge of temperature and flow conditions at all 

points in the fuel cell lead to improved design and performance of the unit. 

In this code, a 1-dimensional electrochemical element is defined, which represents a finite 

volume of active unit cell. This 1-D sub-model can be validated with appropriate single-cell data 

and established 1-D codes. This 1-D element is then used in FLUENT, a commercially available 

product, to carry out 3-D similations of realistic fuel cell geometries. One configuration studied 

was a single tubular solid oxide fuel cell (TSOFC) including a support tube on the cathode side 

of the cell. Six chemical species were tracked in the simulation: H2, CO2, CO, O2, H2O, and N2. 

Fluid dynamics, heat transfer, electrochemistry, and the potential field in electrode and 

interconnect regions were all simulated. Voltage losses due to chemical kinetics, ohmic 

conduction, and diffusion were accounted for in the model. Because of a lack of accurate and 

detailed in situ characterization of the SOFC modeled, a direct validation of the model results 

was not possible. However, the results are consistent with input-output observations on 

experimental cells of this type. 
2-31 

Figure 2-10 Conours of Current Density on Electrolyte 

Current density is shown on the electrolyte and air-flow velocity vectors are shown for the capend of the tubular fuel cell. Cathode and support tube layers have been removed for clarity. 

Results indicate that current density and fuel consumption vary significantly along the electrolyte 

surface as hydrogen fuel is consumed and current flows around the electrodes between 

interconnect regions. Peak temperature occurs about one-third of the axial distance along the 

tube from the cap end. 

NETL’s CFD research has demonstrated that CFD-based codes can provide detailed temperature 

and chemical species information needed to develop improved fuel cell designs. The output of 

the FLUENT-based fuel cell model has been ported to finite element-based stress analysis 

software to model thermal stresses in the porous and solid regions of the cell. In principle, this 

approach can be used for other types of fuel cells as well, as demonstrated by Arthur D. Little 

and NETL (16,18) 

Further enhancement of the design tool is continuing. The next steps are to validate the model 

with experimental data and then extend the model to stack module and stack analysis. NETL 

now operates SOFC test facilities to generate detailed model validation data using wellcharacterized SOFC test specimens. These steps should make it possible to create a model that 

accurately predicts the performance of cells and stacks so that critical design information, such 

as the distribution of cell and stack stresses, can be provided to the fuel cell design engineer. 

2.7.5 1-D Cell Models 

1-D cell models are critical for constructing 3-D models, but they are also highly useful in 

interpreting and planning button cell experiments. In 1-D models, all of the critical phenomena in 

a cell are considered in a 1-D fashion. Generally they incorporate the following elements: 

 
2-32 

• Transport phenomena: 

• Convective mass transport of reactants and products to/from the surface of the electrodes 

• Mass transport of reactants and products through the porous electrodes 

• Conduction of electronic current through the electrodes and current collectors 

• Conduction of ions through the electrolyte and electrodes (where applicable) 

• Conduction, convection, and radiation of heat throughout the cell 

• Chemical reactions: 

• Electrochemical reactions at or near the triple phase boundary (TPB) 

• Internal reforming and shift reactions taking place inside the anode 

Figure 2-11 shows an example for a PEFC cell. 

Coolant 

Flow Plate Flow Plates

Gas 

Diffusion 

Layers

Electrolyte Catalyst

Layers

Heat Mass Electrons Protons

Flow Module 1-d Membrane Catalyst (MC) module

Figure 2-11 Typical Phenomena Considered in a 1-D Model (17) 

A large number of 1-D models have been developed. Some are based on numerical discretization 

methods (e.g. finite element or finite difference methods), while others are analytical in nature. 

An example of the former was given in the description of the NETL 3-D model. An example of 

an analytical approach is provided by Chick and Stevenson (20). 

2.7.6 Electrode Models 

Given the importance of electrode polarization in overall cell performance, electrode sub-models 

are critical in the development of all other fuel cell models. As described in an excellent review 

by Fleig ((9), Figure 2-12), one can distinguish four levels of electrode models: 

• Continuum electrode approach. In this approach the electrode is represented as a 

homogeneous zone for diffusion, electrochemical reaction, and ion- and electron-conduction. 

Because this approach ignores the specific processes occurring at the TPB and the impact of 

the microstructure of the electrode, this approach yields models that must be calibrated for 

each specific electrode design and for each set of operating conditions. With this approach it 

is impossible to distinguish between rate-determining steps in the electrochemically active 

zone, though the relative importance of mass transfer versus kinetic processes can be 

expressed crudely. 

• Multi-particle approach. This approach recognizes that electrodes are typically made up of 

many particles that have different (at least two) phases with different characteristics. Issues 

of connectivity, percolation, and other mass-transfer-related factors can be addressed with 

this approach, but the details of the electrochemical reaction steps at the TPB are lumped 
2-33 

together. From a numerical perspective, one or more resistor networks are added to the 

continuum model. 

• Local current density distribution approach. A refinement on the multi-particle approach, 

this approach considers that current-densities are not necessarily homogeneous within the 

particles, which can strongly impact electrode resistances. Often this approach is executed 

using a finite element method. 

• Micro-kinetics approach. In this approach, the individual reaction steps at or near the TPB 

are considered. Although analytical solutions (in Buttler-Volmer form) can be found if a 

single rate-determining step is considered, generally a numerical solution is necessary for 

multi-step reactions. This approach can be embedded in the multi-particle or local-current 

density approaches, or directly used in a 1-D model with simpler assumptions for the 

transport phenomena. This is the only approach that can give insight into the ratedetermining electrochemical processes that take place in the cell. When optimizing electrocatalysts or studying direct oxidation of hydrocarbons, this type of model can be very 

enlightening. 

 

Figure 2-12 Overview of types of electrode models (9) 

2.8 References 

1 M.W. Chase, et al., “JANAF Thermochemical Tables,” Third Edition, American Chemical 

Society and the American Institute of Physics for the National Bureau of Standards (now 

National Institute of Standards and Technology), 1985. 

2 P.W. Atkins, “Physical Chemistry,” 3rd Edition, W.H. Freeman and Company, New York, 

NY, 1986. 

3 “Fuel Cell Handbook,” J. Appleby and F. Foulkes, Texas A&M University, Van Nostrand 

Reinhold, New York (out of print), republished by Krieger Publishing Co., Melbourne, FL, 

1989. 
2-34 

4 Winkler, W., Thermodynamics, in High Temperature Solid Oxide Fuel Cells: Fundamentals, 

Design and Applications, S.C. Singhal and K. Kendall, Editors. 2003, Elsevier Ltd.: Oxford, 

UK. p. 53 - 82. 

5 S.N. Simons, R.B. King and P.R. Prokopius, in Symposium Proceedings Fuel Cells Technology 

Status and Applications, Figure 1, p. 46, Edited by E.H. Camara, Institute of Gas Technology, 

Chicago, IL, 45, 1982. 

6 P. V. Hendriksen, S. Koch, M. Mogensen, Y. L. Liu, and P. H. Larsen, in Solid Oxide Fuel 

Cells VIII, eds S. C. Singhal and M. Dokiya, The Electrochemical Society Proceedings, 

Pennington, NJ, PV2003-07, 2003, p. 1147 

7 E.J. Cairns and H.A. Liebhafsky, Energy Conversion, p. 9, 63, 1969. 

8 Khaheel, M.A. Modeling and Simulation SECA Core Program. in Modeling and Simulation 

Team Integration Meeting. 2003: US DOE. http://www.seca.doe.gov/events/2002/ 

model_simulation/pnnl_m_khaleel.pdf 

9 Fleig, J., Solid Oxide Fuel Cell Cathodes: Polarization Mechanisms and Modeling of the 

Electrochemical Performance. Annual Review of Materials Research, 2003. 33: p. 361 - 382. 

10 U.S. Department of Energy, Office of Building Technology, State and Community Programs, 

Tools Directory web site, current URL: http://www.eere.energy.gov/buildings/tools_directory/ 

11 Development and Use of GREET 1.6 Fuel-Cycle Model for Transportation Fuels and 

Vehicle Technologies, Center for Transportation Research, Energy Systems Division, 

Argonne National Laboratory, Report ANL/ESD/TM-163, June 2001. 

12 Carlson, E., Assessment of Planar Solid Oxide Fuel Cell Technology. 1999, US Department 

of Energy: Cambridge, MA, USA. 

13 Koslowske, M., A Process Based Cost Model for Multi-Layer Ceramic Manufacturing of 

Solid Oxide Fuel Cells, in Materials Science. 2003, Worcester Polytechnic Institute: 

Worcester, MA, USA. p. 42. 

14 Carlson, E. and S. Mariano, Cost Analysis of Fuel Cell System for Transportation. 2000, 

Arthur D. Little for US DOE OTT: Cambridge, MA, USA. 

15 ADVISOR: A Systems Analysis Tool for Advanced Vehicle Modeling, Markel, T., et al., 

Journal of Power Sources, 2002, available from the following URL: http://www.ctts.nrel.gov/ 

analysis/advisor.html

16 J. Pålsson, A. Selimovic, and L. Sjunnesson, J. Power Sources, 86, (2000), 442 - 448 

17 Sriramulu, S. and J. Thijssen. in Fuel Cell Seminar. 2000. Portland, OR: US DOE. 

18 Thijssen, J. and S. Sriramulu, Structural Limitations in the Scale-Up of Anode-Supported 

SOFCs. 2002, Arthur D. Little for US DOE: Cambridge, MA, USA. 

19 Fulton, C., et al. Structural Limitations in the Scale-Up of Anode-Supported SOFCs. in Fuel 

Cell Seminar. 2002. Palm Springs, CA: US Department of Energy. 

20 Chick, L.A., J.W. Stevenson, and R. Williford. Spreadsheet Model of SOFC Electrochemical 

Performance. in SECA Training Workshop. 2003. Morgantown, WV: US DOE NETL. 

http://www.netl.doe.gov/publications/proceedings/03/seca-model/Chick8-29-03.pdf
3-1 

3. POLYMER ELECTROLYTE FUEL CELLS

Polymer electrolyte membrane fuel cells (PEFC)7

 are able to efficiently generate high power 

densities, thereby making the technology potentially attractive for certain mobile and portable 

applications. Especially the possible application of PEFC as a prime mover for automobiles has 

captured the imagination of many. PEFC technology differentiates itself from other fuel cell 

technologies in that a solid phase polymer membrane is used as the cell separator/electrolyte. 

Because the cell separator is a polymer film and the cell operates at relatively low temperatures, 

issues such as sealing, assembly, and handling are less complex than most other fuel cells. The 

need to handle corrosive acids or bases is eliminated in this system. PEFCs typically operate at 

low temperatures (60o

 to 80 oC), allowing for potentially faster startup than higher temperature 

fuel cells. The PEFC is seen as the main fuel cell candidate technology for light-duty 

transportation applications. While PEFC are particularly suitable for operation on pure 

hydrogen, fuel processors have been developed that will allow the use of conventional fuels such 

as natural gas or gasoline. A unique implementation of the PEFC allows the direct use of 

methanol without a fuel processor; it is the direct methanol fuel cell (DMFC). The DMFC is seen 

as the leading candidate technology for the application of fuel cells to cameras, notebook 

computers, and other portable electronic applications. 

3.1 Cell Components 

Typical cell components within a PEFC stack include: 

• the ion exchange membrane 

• an electrically conductive porous backing layer 

• an electro-catalyst (the electrodes) at the interface between the backing layer and the 

membrane 

• cell interconnects and flowplates that deliver the fuel and oxidant to reactive sites via flow 

channels and electrically connect the cells (Figure 3-1). 

PEFC stacks are almost universally of the planar bipolar type. Typically, the electrodes are cast 

as thin films that are either transferred to the membrane or applied directly to the membrane. 

Alternatively, the catalyst-electrode layer may be deposited onto the backing layer, then bonded 

to the membrane. 

 

7. Polymer electrolyte membrane fuel cells are referred to by several acronyms; a common one is PEM, which 

stands for Proton Exchange Membrane. 
3-2 

(a) 

(b) 

Figure 3-1 (a) Schematic of Representative PEFC (b) Single Cell Structure of 

Representative PEFC(1) 

3.1.1 State-of-the-Art Components 

Membrane 

Organic-based cation exchange membranes in fuel cells were originally conceived by 

William T. Grubb (2) in 1959. That initial effort eventually led to development of the 

perfluorosulfonic acid polymer used in today’s systems. The function of the ion exchange 

membrane is to provide a conductive path, while at the same time separating the reactant gases. 

The material is an electrical insulator. As a result, ion conduction takes place via ionic groups 

within the polymer structure. Ion transport at such sites is highly dependent on the bound and 

free water associated with those sites. 

An accelerated interest in polymer electrolyte fuel cells has led to improvements in both cost and 

performance. Development has reached the point where both motive and stationary power 
3-3 

applications are nearing an acceptable cost for commercial markets. Operation of PEFC 

membrane electrode assemblies (MEAs) and single cells under laboratory conditions similar to 

transportation or stationary applications have operated for over 20,000 hrs continuously with 

degradation rates of 4 to 6 μV/hr (or about 0.67 to 1.0 percent per 1000 hrs), which approaches 

the degradation rates needed for stationary applications (about 0.1 percent per 1000 hrs is used as 

a rule of thumb). Complete fuel cell systems have been demonstrated for a number of 

transportation applications including public transit buses and passenger automobiles. For 

stationary applications, a number of demonstration systems have been developed and numerous 

systems have been installed, mostly in the 2 to 10 kW range. However, although these systems 

have collectively logged millions of kWhrs (3), developers have not yet demonstrated system or 

stack life of more than 8,000 hours with realistic catalyst loadings and realistic operating 

conditions, and then with degradation rates of several percent per 1000 hrs. Consequently, PEFC 

developers and researchers are focused on achieving critical improvements in extending stack 

life, simpler system integration, and reduction of system cost. This is true both for stationary and 

mobile applications. 

Manufacturing details of Plug Power’s cell and stack design are proprietary, but the literature 

provides some information on the cell and stack design. Example schematics for the crosssection and a current collecting plate are shown in Figure 3-2 (4, 5). An approach for sealing the 

cell with flat gaskets is shown (Label 402) but there are many alternatives with gaskets and plates 

having different shapes and grooves, respectively. The plate shows the flow path for one of the 

reactants from the inlet to the outlet manifold. The other side of the plate (not shown) would have 

channels either for coolant flow or the other reactant. 

Figure 3-2 PEFC Schematic (4, 5) 

The standard electrolyte material in PEFCs belongs to the fully fluorinated Teflon®-based family 

similar to that produced by E.I. DuPont de Nemours for space application in the mid-1960s. The 

membrane is characterized by its equivalent weight (inversely proportional to the ion exchange 

capacity). A typical equivalent weight range is 800 to 1100 milliequivalents per dry gram of 

polymer. The type used most often in the past was a melt-extruded membrane manufactured by 

DuPont and sold under the label Nafion® No. 117. The perfluorosulfonic acid family of 
3-4 

membranes exhibits exceptionally high chemical and thermal stability, and is stable against 

chemical attack in strong bases, strong oxidizing and reducing acids, Cl2, H2, and O2 at 

temperatures up to 125°C (6). Nafion consists of a fluoropolymer backbone, similar to Teflon®, 

upon which sulfonic acid groups are chemically bonded (7,29). Nafion membranes have 

exhibited long life in selected applications, operating conditions, and electrochemical 

applications. In selected fuel cell tests and water electrolysis systems, lifetimes of over 50,000 

hours have been demonstrated. The Dow Chemical Company produced an electrolyte 

membrane, the XUS 13204.10, that contained a polymeric structure similar to that of Nafion, 

except that the side chain length was shortened (8). As a result, the membrane properties were 

significantly impacted, including a higher degree of water interactions within the membrane. 

This translated to lower electrical resistance and permited higher current densities than the 

Nafion membrane, particularly when used in thinner form (9). These short side-chain 

membranes exhibited good performance and stability, but are no longer supplied by Dow. 

Furthermore, due to Nafion’s expense and other engineering issues, new alternative membranes 

are being developed by a number of different companies. 

Progress in manufacturing techniques has been made. Although melt-extruded films were the 

norm, the industry is moving to a solution-cast film process to reduce costs and improve 

manufacturing throughput efficiency. In this process, the ionic form of the polymer is 

solubilized in alcoholic solution, such as propanol, and then fabricated into a film of desired 

thickness. The conversion of the non-ionic polymer to an ionic phase, ready for use in a fuel 

cell, is carried out prior to the solubilization step.

Another advancement in membrane technology is that of using an internal support layer to 

enhance the mechanical strength of the membrane film, especially as the membrane thickness is 

decreased. The Primea 55 and 56 series membranes manufactured by W.L. Gore are examples 

of such internally-supported membranes. 

Porous Backing Layer 

The polymer membrane is sandwiched between two sheets of porous backing media (also 

referred to as gas diffusion layers or current collectors). The functions of the backing layer8

 are 

to: (1) act as a gas diffuser; (2) provide mechanical support, (3) provide an electrical pathway 

for electrons, and (4) channel product water away from the electrodes. The backing layer is 

typically carbon-based, and may be in cloth form, a non-woven pressed carbon fiber 

configuration, or simply a felt-like material. The layer incorporates a hydrophobic material, such 

as polytetrafluoroethylene. The function of polytetrafluoroethylene is to prevent water from 

“pooling” within the pore volume of the backing layer so that gases freely contact the catalyst 

sites. Furthermore, it facilitates product water removal on the cathode as it creates a non-wetting 

surface within the passages of the backing material.

One PEFC developer (10) devised an alternative plate structure that provides passive water 

control. Product water is removed by two mechanisms: (1) transport of liquid water through the 

porous bipolar plate into the coolant, and (2) evaporation into the reactant gas streams. The cell 

is similar in basic design to other PEFCs with membrane, catalysts, substrates, and bipolar plate 

components. However, there is a difference in construction and composition of the bipolar plate: 

 

8. Commonly referred to as the gas diffusion layer (GDL) even though it has additional functions. 
3-5 

it is made of porous graphite. During operation, the pores are filled with liquid water that 

communicates directly with the coolant stream. Product water flows from the cathode through 

the pores into the coolant stream (a small pressure gradient between reactant and the coolant 

stream is needed). The water in the coolant stream is then routed to a reservoir. Removal of 

water by the porous membrane results in the reactant flow stream being free of any obstructions 

(liquid water). The flooded pores serve a second purpose of supplying water to the incoming 

reactant gases and humidifying those gases. This prevents drying of the membrane, a common 

failure mode, particularly at the anode. Control of the amount of area used to humidify the inlet 

gases has eliminated the need to pre-humidify the reactant gases. 

Reasons for removing the water through the porous plate are: (1) there is less water in the spent 

reactant streams; (2) this approach reduces parasitic power needs of the oxidant exhaust 

condenser; (3) the cell can operate at high utilizations that further reduce water in the reactant 

streams; (4) higher temperatures can be used with higher utilizations so that the radiator can be 

smaller,9

 and (5) the control system is simplified. In fact, in-stack water conservation is even 

more important in arid climates, where there may exist a significant challenge to achieve water 

balance at the system level without supplying water or refrigerating the exhaust stream. 

Hand-in-hand with water management goes the thermal management of the stack. Temperatures 

within the stack must be kept within a narrow range in order to avoid local dehydration and hotspots as well as local dead zones. This is particularly challenging when one recognizes the 

narrow temperature zone and the relatively small temperature difference between the cell 

operating temperature and the ambient temperature.

Electrode-Catalyst Layer 

In intimate contact with the membrane and the backing layer is the catalyst layer. This catalyst 

layer, integral with its binder, forms the electrode. The catalyst and binder electrode structure is 

applied either to the membrane or to the backing layer. In either case, the degree of intimacy of 

the catalyst particles and the membrane is critical for optimal proton mobility. The binder 

performs multiple functions. In one case, it “fixes” the catalyst particles within a layered 

structure, while a second function is to contribute to the overall architecture of the electrode. 

This architecture has a direct bearing on performance. 

There are two schools of thought on the electrode composition, in particular, the binder. In the 

original hydrophobic, porous, gaseous electrodes developed by Union Carbide and later 

advanced by General Electric, the Dow Chemical Company, and others, the binder was 

polytetrafluoroethylene: a non-wetting component within the electrode itself. The second 

school of electrode science developed a hydrophyllic electrode in which the binder was 

perfluorosulfonic acid. The driver for this development was to enhance the membrane/catalyst 

contact to minimize the platinum loading requirements (11). In most state-of-the-art PEFC 

membrane electrode assemblies (MEAs), the catalyst is largely embedded in a solution of 

electrolyte monomer, which provides high solubility for protons as well as oxygen, and thus 

effective use of the platinum catalyst surface. 

 

9. Higher average temperature operaton is possible because of the reduction of hot spots within the cell. Water 

will evaporate through the porous plate in the vicinity of a hot spot. Conversely, a local cool spot can produce a 

concentration of water. This water is quickly removed through the porous plate. 
3-6 

The catalyst is platinum-based for both the anode and cathode. To promote hydrogen oxidation, 

the anode uses either pure platinum metal catalyst or, as is common in most modern PEFC 

catalysts, a supported platinum catalyst, typically on carbon or graphite for pure hydrogen feed 

streams. For other fuels, such as reformate (containing H2, CO2, CO, and N2), the desired 

catalyst is an alloy of platinum containing ruthenium. Oxygen reduction at the cathode may use 

either the platinum metal or the supported catalyst. 

Because of the the expense of the platinum catalyst, there have been numerous efforts to 

minimize the use of platinum in the catalyst layer. The platinum particle size has been 

extensively optimized, and general agreement is that a ~3.5 nm particle size on suitable carbon 

support is close to optimal: the activity per unit mass of platinum is near optimal under these 

conditions. In parallel, there have been numerous efforts to substitute other materials for 

platinum. Most of these attempts focused either on gold or on platinum alloys (usually with 

transition metals). So far, these efforts have not demonstrated a decisive cost advantage over 

pure platinum catalysts. 

Typically, electrodes can be cast as thin films and transferred to the membrane or applied 

directly to the membrane. Alternatively, the catalyst-electrode layer may be deposited onto the 

gas diffusion layer (GDL), then bonded to the membrane. Low platinum loading electrodes (≤

1.0 mg Pt/cm2 total on the anode and the cathode) are regularly used, and have performed as well 

as earlier, higher platinum loading electrodes (2.0 to 4.0 mg Pt/cm2). These electrodes, which 

have been produced using a high-volume manufacturing process, have achieved nearly 

600 mA/cm2

 at 0.7 V on reformate. A number of companies globally are developing such 

electrodes. An example of electrode performance is shown in Figure 3-3. The figure depicts the 

performance of a standard 100 cm2

 7-layer membrane electrode assembly (MEA) manufactured 

by the 3M Corporation operating on hydrogen and reformate at 70 °C (12). Recent advances in 

MEA performance and durability have led to tests with reformate in excess of 10,000 hours with 

the 3M 7-layer MEA. This MEA is produced using high-speed, continuous, automated assembly 

equipment. 
3-7 

Figure 3-3 Polarization Curves for 3M 7 Layer MEA (12) 

The electrochemical reactions of the PEFC are similar to those of the PAFC10: molecular 

hydrogen at the anode is oxidized to provide protons, while at the same time freeing two 

electrons that pass through an external “electrical” circuit to reach the cathode. The voltages at 

each electrode, due to the hydrogen oxidation potential and the oxygen reduction potential, form 

a voltage gradient of approximately 1 volt (depending on conditions) at open circuit, i.e., zero 

current draw. It is this potential that drives the proton through the membrane. As the proton is 

“pulled” through the membrane, it drags with it a certain number of water molecules. The proton 

reacts with oxygen to form water at the catalyst sites on the cathode. 

Because of the intrinsic nature of the materials used, the PEFC operates at temperatures between 

0 °C to 90 °C, typically in the 60 °C to 80 °C range. When compared to other fuel cells, PEFC 

technology has been capable of very high current densities: while most technologies can operate 

up to approximately 1 amp/cm2

, polymer electrolyte membrane fuel cells have operated at up to 

4 amps/cm2 (13). Stack level power densities under pra 2 ctical operating conditions (cathode 

stoichiometry less than 3, anode utilization more than 85%, pressure less than 3 bar, and catalyst 

loadings less than 1 mg/cm2

) with reformate of around 50 mW/cm2 at 0.7 V and of around 400 – 

600 mW/cm2

 when operating with hydrogen are feasible (14, 15, 16, 17). This performance is 

due primarily to the impressive ionic conductivity of PEFC membranes and the high electrical 

conductivity of the materials used in the gas diffusion layers and bipolar plates (mostly carbon or 

metals). Other desirable attributes include fast start capability and rapid response to load 

changes. Because of the high power density capability, smaller, lighter-weight stacks are 

possible (18). Other beneficial attributes of the cell include no corrosive fluid hazard and lower 

sensitivity to orientation. As a result, the PEFC is thought to be best suited for vehicular power 

applications. 

The low operating temperature of a PEFC has both advantages and disadvantages. Low 

temperature operation is advantageous because the cell can start from ambient conditions 

 

10. Equations 5-1, 5-2, and 5-3 for the PAFC apply as well to the PEFC. 

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1

Amps / cm2

Voltage

H2/air, Ambient Pressure

H2/air, 30 psig

Reformate (50 ppm CO) / Air, Ambient Pressure

These 3 polarization curves were generated using a 70oC cell, 

saturated anode and cathode, 

and the flow set to a constant stoich of 1.5 anode / 2.5 cathode.
3-8 

quickly, especially when pure hydrogen fuel is available. It is a disadvantage in carbon 

monoxide-containing fuel streams, because carbon will attack the platinum catalyst sites, 

masking the catalytic activity and reducing cell performance.11 The effect is reversible by 

flowing a CO-free gas over the electrode. To minimize CO poisoning, operating temperatures 

must be greater than 120 °C, at which point there is a reduction in chemisorption and electrooxidation. Due to CO affecting the anode, only a few ppm of CO can be tolerated at 80 °C. 

Because reformed and shifted hydrocarbons contain about one percent CO, a mechanism to 

eliminate CO in the fuel gas is needed. This can be accomplished with preferential oxidation 

(PROX) that selectively oxidizes CO over H2 using a precious metal catalyst. The low operating 

temperature also means that little, if any, heat is available from the fuel cell for endothermic 

reforming (19, 20). 

As this discussion suggests, there is a considerable advantage at the stack level to the use of pure 

hydrogen rather than reformate, but in most PEFC applications this must be traded off against the 

challenges in storing hydrogen and the limited availability of hydrogen. Although considerable 

effort has been expended to develop liquid-fueled PEFC for transportation applications, most 

believe that on-board storage of hydrogen will be necessary for practical vehicles (21). 

To overcome the challenges of operating on reformate, attempts have been made to develop socalled high-temperature PEFC, which would operate in the 120 °C to 160°C range. New or 

modified ion exchange membranes would be needed to allow this, because Nafion dehydrates 

rapidly at such temperatures unless high (greater than 10 bar) pressures are applied. One 

candidate material is polybenzimidizole (PBI) (22). The higher operating temperature eliminates 

CO poisoning by eliminating CO occlusion of the platinum sites. Also, this operating regime 

provides higher quality heat for possible use in stationary combined heat/power (CHP) 

applications. Because PBI requires significantly lower water content to facilitate proton 

transport, an additional benefit is that water management is dramatically simplified (23, 24). 

However, to achieve acceptable ionic conductivity, the membrane must be impregnated with 

phosphoric acid, which is apparently not very tightly bound to the polymer backbone. As a 

result, similar precautions are necessary as in a PAFC (avoiding liquid water, corrosion 

protection). The conductivity of PBI can approach the target of 10 S/cm set for high temperature 

membranes. 

Other approaches to high-temperature membranes are based on the modification of Nafion. 

Reports indicate that some of the modified materials achieve conductivities close to that of 

Nafion 112, while allowing operation up to 120 °C at low hydration levels (25,14 ,16 ,17) 

Both temperature and pressure significantly influence cell performance. Present cells operate at 

80 °C over a range of 0.0010 to 1.0 MPa (~0.1 to 150 psig). Nominally, 0.285 MPa (25 psig) 

(18) is used for some transportation applications although some developers (26) pursue ambientpressure technology. Using appropriate current collectors and supporting structure, polymer 

electrolyte fuel cells and electrolysis cells should be capable of operating at pressures up to 

3000 psi and differential pressures up to 500 psi (27). 

 

11. Referred to as poisoning in catalysis literature. 
3-9 

Water and Thermal Management 

Due to operation at less than 100 °C and atmospheric pressure, water is produced as a liquid. A critical 

requirement is to maintain high water content in the electrolyte to ensure high ionic conductivity. 

Maintaining high water content is particularly critical when operating at high current densities 

(approximately 1 A/cm2

) because mass transport issues associated with water formation and distribution 

limit cell output. The ionic conductivity of the electrolyte is higher when the membrane is fully 

saturated: this impacts the overall efficiency of the fuel cell. Without adequate water management, an 

imbalance will occur between water production and water removal from the cell. 

Water content is determined by balance of water12 during operation. Contributing factors to water 

transport are the water drag through the cell, back-diffusion from the cathode, and the diffusion of water 

in the fuel stream through the anode. Water transport is not only a function of the operating conditions12

but also the characteristics of the membrane and the electrodes. Water drag refers to the amount of 

water that is pulled by osmotic action along with the proton (28). One estimate is that between 1 to 2.5 

molecules are dragged with each proton (29). As a result, transported water can be envisioned as a 

hydrated proton, H(H2O)n. During operation, a concentration gradient may form whereby the anode is 

drier than the cathode. Under these conditions, there is back-diffusion of water from the cathode to the 

anode. Membrane thickness is also a factor in that the thinner the membrane, the greater the transport of 

water back to the anode. The objective of the stack engineer is to ensure that all parts of the cell are 

sufficiently hydrated, and that no excessive flooding occurs (29, 30, 31, 32). Adherence of the 

membrane to the electrode will be adversely affected if dehydration occurs. Intimate contact between 

the electrodes and the electrolyte membrane is important because there is no free liquid electrolyte to 

form a conducting bridge. Because this type of degradation is largely irreversible, operation under dry 

conditions will severely impact membrane lifetime (33). 

Reliable forms of water management have been developed based on continuous flow field design 

and appropriate operating adjustments. For this reason, flow field designs often feature 

serpentine channels or unstructured flow passages. The flow-plates (which also serve as bipolar 

plates) are typically made of graphite, an injection-molded and cured carbon material, or a metal. 

If more water is exhausted than produced, then humidification of the incoming anode gas 

becomes important (31). If there is too much humidification, however, the electrode floods, 

which causes problems with gas diffusion to the electrode. A temperature rise between the inlet 

and outlet of the flow field increases evaporation to maintain water content in the cell. There 

also have been attempts to control the water in the cell using external wicking connected to the 

membrane to either drain or supply water by capillary action.

Much progress has been made towards PEFC commercialization. Figure 3-4, from Gore Fuel 

Cell Technologies, demonstrates the company’s newest commercial offering, PRIMEA® Series 

56 MEA that has demonstrated over 15,000 hours of cell operation (34). 

 

12. A smaller current, larger reactant flow, lower humidity, higher temperature, or lower pressure will result in a 

water deficit. A higher current, smaller reactant flow, higher humidity, lower temperature, or higher pressure 

will lead to a water surplus.
3-10 

Figure 3-4 Endurance Test Results for Gore Primea 56 MEA at Three Current Densities 

To improve effectiveness of the platinum catalyst, a soluble form of the polymer is incorporated 

into the pores of the carbon support structure. This increases the interface between the 

electrocatalyst and the solid polymer electrolyte. Two methods are used to incorporate the 

polymer solution within the catalyst. In Type A, the polymer is introduced after fabrication of 

the electrode; in Type B, it is introduced before fabrication. 

Most PEFCs presently use cast carbon composite plates for current collection and distribution, 

gas distribution, and thermal management. Cooling is accomplished using a circulating fluid, 

usually water that is pumped through integrated coolers within the stack. The temperature rise 

across the cell is kept to less than 10 °C. In one configuration, water-cooling and humidification 

are in series, which results in the need for high quality water. The cooling unit of a cell can be 

integrated to supply reactants to the MEA, remove reaction products from the cell, and seal off 

the various media against each other and the outside. Metal (usually coated) plates are used as an 

alternative by some developers.

The primary contaminants of a PEFC are carbon monoxide (CO) and sulfur (S). Carbon dioxide 

(CO2) and unreacted hydrocarbon fuel act as diluents. Reformed hydrocarbon fuels typically 

contain at least 1 percent CO. Even small amounts of CO in the gas stream, however, will 

preferentially adsorb on the platinum catalyst and block hydrogen from the catalyst sites. Tests 

indicate that as little as 10 ppm of CO in the gas stream impacts cell performance (35, 36). Fuel 

processing can reduce CO content to several ppm, but there are system costs associated with 

increased fuel purification. Platinum/ruthenium catalysts with intrinsic tolerance to CO have 

been developed. These electrodes have been shown to tolerate CO up to 200 ppm (37). 

Although much less significant than the catalyst poisoning by CO, anode performance is 

adversely affected by the reaction of CO2 with adsorbed hydrides on platinum. This reaction is 

the electrochemical equivalent of the water gas shift reaction. 

y = -2E-06x + 0.7763 at 200 mA cm-2

y = -4E-06x + 0.6897 at 600 mA cm-2

y = -6E-06x + 0.6526 at 800 mA cm-2

0.5

0.55

0.6

0.65

0.7

0.75

0.8

0 2000 4000 6000 8000 10000 12000

Time / hours

Cell Voltage / V
3-11 

Other contaminants of concern include ammonia (membrane deterioration), alkali metals 

(catalyst poisoning, membrane degradation), particles, and heavy hydrocarbons (catalyst 

poisoning and plugging). Both the anode and cathode flows must be carefully filtered for these 

contaminants, as even ppb-level concentration can lead to premature cell and stack failure. 

A number of technical and cost issues face polymer electrolyte fuel cells at the present stage of 

development (35, 38, 39, 40, 41). These concern the cell membrane, cathode performance, and 

cell heating limits. The membranes used in present cells are expensive, and available only in 

limited ranges of thickness and specific ionic conductivity. Lower-cost membranes that exhibit 

low resistivity are needed. This is particularly important for transportation applications 

characterized by high current density operation. Less expensive membranes promote lower-cost 

PEFCs, and thinner membranes with lower resistivities could contribute to power density 

improvement (41). It is estimated that the present cost of membranes could fall (by a factor of 5) 

if market demand increased significantly (to millions of square meters per year) (33). 

The DOE has set platinum loading targets at 0.4 mg/cm2

 total, a maximum to allow achieving the 

automotive cost targets. This will require a significantly higher catalyst effectiveness (present 

loadings are on the order of 1 mg/cm2

 total) while achieving the other improvements in 

performance required. 

Improved cathode performance, when operating on air at high current densities, is needed. At 

high current densities, there is a limiting gas permeability and ionic conductivity within the 

catalyst layer. A nitrogen blanket forming on the gas side of the cathode is suspected of creating 

additional limitations (1). There is a need to develop a cathode that lessens the impact of the 

nitrogen blanket, allows an increase in cell pressure, and increases ionic conductivity. 

Local heat dissipation limits stack operation with air at a current density of approximately 2 A/cm2

. 

Single cells have shown the capability to operate at higher current densities on pure oxygen. It 

may be possible to increase current density and power density through better cooling schemes. 

3.1.2 Component Development 

The primary focus of ongoing research has been to improve performance and reduce cost. The 

principal areas of development are improved cell membranes, CO removal from the fuel stream, 

and improved electrode design. There has been a move toward operation with zero 

humidification at ambient pressure, increased cell temperature, and direct fuel use. DuPont now 

produces a membrane of 2 mils or less thickness that performs (at lower current densities) 

similar to the Dow Chemical Company membrane, the XUS 13204.10 depicted in the top curve 

of Figure 3-5 (42). There is ongoing work to investigate alternative membranes and MEAs that 

not only exhibit durability and high performance, but also can be manufactured inexpensively in 

high volume. 
3-12 

Figure 3-5 Multi-Cell Stack Performance on Dow Membrane (9) 

PEFCs were originally made with an unimpregnated electrode/Nafion electrolyte interface. This 

was later replaced by a proton conductor that was impregnated into the active layer of the 

electrode. This allowed reduced catalyst loading to 0.4 mg/cm2

 while obtaining high power 

density (27). The standard "Prototech" electrodes contained 10 percent Pt on carbon supports. 

Using higher surface area carbon-supported catalysts, researchers have tested electrodes with 

even lower platinum loading, but having performance comparable to conventional electrodes. 

Los Alamos National Laboratory has tested a cathode with 0.12 mg Pt/cm2

 loading, and Texas 

A&M University has tested a cathode with 0.05 mg Pt/cm2

 loading. Another example of low 

catalyst loadings is the work carried out at DLR (43) in which loadings as low as 0.07 mg/cm2

were applied to the membrane using a dry process. The binder was a Teflon-like material. 

Another approach has been developed to fabricate electrodes with loading as low as 0.1 mg Pt/ 

cm2

 (44). The electrode structure was improved by increasing the contact area between the 

electrolyte and the platinum clusters. The advantages of this approach were that a thinner 

catalyst layer of 2 to 3 microns and a uniform mix of catalyst and polymer were produced. For 

example, a cell with a Pt loading of 0.07 to 0.13 mg/cm2

 was fabricated. The cell generated 

3 A/cm2

 at > 0.4V on pressurized O2, and 0.65 V at 1 A/cm2 on pressurized air (44, 45). 

Stable performance was demonstrated over 4,000 hours with Nafion membrane cells having 

0.13 mg Pt/cm2

 catalyst loading and cell conditions of 2.4 atmospheres H2, 5.1 atmospheres air, 

and 80 °C (4,000 hour performance was 0.5 V at 600 mA/cm2

). Water management was stable, 

particularly after thinner membranes of somewhat lower equivalent weight became available. 

Some performance losses may have been caused by slow anode catalyst deactivation, but the 

platinum catalyst "ripening" phenomenon was not considered to contribute significantly to the 

long-term performance losses observed in PEFCs (1). 
3-13 

Other research has focused on developing low-cost, lightweight, graphite carbon-based materials 

that can be used in place of expensive, high-purity graphite bipolar plates. Plated metals, such as 

aluminum and stainless steel, are also under consideration for this application, despite contact 

resistance and durability concerns. Conductive plastic and composite bipolar plates have met 

with significant success in the laboratory, and have even reached commercial production. The 

time line for development of a vinyl ester configuration is shown in Reference (46) for a material 

that has reached almost 100 S/cm. 

Selective oxidation is able to decrease CO in a methanol reformed gas (anode fuel supply 

stream) from 1% to approximately 10 ppm using a platinum/alumina catalyst. The resulting 

performance of the anode catalyst, though satisfactory, is impacted even by this low amount of 

CO. Research at Los Alamos National Laboratory has demonstrated an approach to remedy this 

problem by bleeding a small amount of air or oxygen into the anode compartment. 

Figure 3-6 shows that performance equivalent to that obtained on pure hydrogen can be 

achieved using this approach. It is assumed that this approach would also apply to reformed 

natural gas that incorporate water gas shift to obtain CO levels of 1% entering the fuel cell. This 

approach results in a loss of fuel, that should not exceed 4 percent provided the reformed fuel gas 

can be limited to 1 percent CO(1). Another approach is to develop a CO-tolerant anode catalyst 

such as the platinum/ruthenium electrodes currently under consideration. Platinum/ruthenium 

anodes have allowed cells to operate, with a low-level air bleed, for over 3,000 continuous hours 

on reformate fuel containing 10 ppm CO (27). 

There is considerable interest in extending PEFC technology to direct methanol and 

formaldehyde electro-oxidation (47, 48) using Pt-based bi-metallic catalyst. Tests have been 

conducted with gas diffusion-type Vulcan XC-72/Toray support electrodes with Pt/Sn 

(0.5 mg/cm2

, 8 percent Sn) and Pt/Ru (0.5 mg/cm2, 50 percent Ru). The electrodes have Teflon 

content of 20 percent in the catalyst layer. 

Figure 3-6 Effect on PEFC Performance of Bleeding Oxygen into the Anode 

Compartment (1) 
3-14 

3.2 Performance 

A summary of the performance levels achieved with PEFCs since the mid-1960s is presented in 

Figure 3-7. Because of changes in operating conditions involving pressure, temperature, reactant 

gases, and other parameters, a wide range of performance levels can be obtained. The 

performance of the PEFC in the U.S. Gemini Space Program was 37 mA/cm2

 at 0.78 V in a 32- 

cell stack that typically operated at 50 °C and 2 atmospheres (49). Current technology yields 

performance levels that are vastly superior. Results from Los Alamos National Laboratory show 

that 0.78 V at about 200 mA/cm2

 (3 atmospheres H2 and 5 atmospheres air) can be obtained at 80 

°C in PEFCs containing a Nafion membrane and electrodes with a platinum loading of 

0.4 mg/cm2. Further details on PEFC performance with Nafion membranes are presented by 

Watkins, et al. (50). In recent years, the development effort has been focused on maintaining 

power density while reducing platinum loading, broadening temperature and humidity operating 

envelopes, and other improvements that will reduce cost (25,51,14 ,16 ,11). 

Operating temperature has a significant influence on PEFC performance. An increase in 

temperature decreases the ohmic resistance of the electrolyte and accelerates the kinetics of the 

electrode reactions. In addition, mass transport limitations are reduced at higher temperatures. 

The overall result is an improvement in cell performance. Experimental data (55, 52, 53) suggest 

a voltage gain in the range of 1.1 - 2.5 mV for each degree (°C) of temperature increase. 

Operating at higher temperatures also reduces the chemisorption of CO. Improving the cell 

performance through an increase in temperature, however, is limited by the vapor pressure of 

water in the ion exchange membrane due to the membrane’s susceptibility to dehydration and the 

subsequent loss of ionic conductivity. 

Figure 3-7 Evolutionary Changes in PEFCs Performance [(a) H2/O2, (b) H2/Air, 

(c) Reformate Fuel/Air, (d) H2/unkown)] [24, 10, 12, 54, 55] 

0.5

0.6

0.7

0.8

0.9

1

0 0.2 0.4 0.6 0.8 1 1.2 1.4

Current Density (A/cm2)

Cell Voltage (V)

‘91, d, 5 atm, 95°C 

‘01, b, 3 atm, 70°C 

‘01, b, 1 atm, 65°C 

’’88, d, 4.4 atm, 70°C 

’01, b, 1 atm, 70°C 

’01, c, 1 atm, 70C 

’75, a, 2 atm, 90°C ’86, b, 4 atm, 75°C 

’84, c, 

1 atm, 70°C 

’86, b, 4 atm, 75°C 
3-15 

Operating pressure also impacts cell performance. The influence of oxygen pressure on the 

performance of a PEFC at 93 °C is illustrated in Figure 3-8 (56). An increase in oxygen pressure 

from 30 to 135 psig (3 to 10.2 atmospheres) produces an increase of 42 mV in the cell voltage at 

215 mA/cm2. According to the Nernst equation, the increase in the reversible cathode potential 

that is expected for this increase in oxygen pressure is about 12 mV, which is considerably less 

than the measured value. When the temperature of the cell is increased to 104 °C, the cell 

voltage increases by 0.054 V for the same increase in oxygen pressure. Additional data suggest 

an even greater pressure effect. A PEFC at 50 °C and 500 mA/cm2

 (55) exhibited a voltage gain 

of 83 mV for an increase in pressure from 1 to 5 atmospheres. Another PEFC at 80 °C and 

431 mA/cm2

 (52) showed a voltage gain of 22 mV for a small pressure increase from 2.4 to 

3.4 atmospheres. These results demonstrate that an increase in the pressure of oxygen results in 

a significant reduction in polarization at the cathode. Performance improvements due to 

increased pressure must be balanced against the energy required to pressurize the reactant gases. 

The overall system must be optimized according to output, efficiency, cost, and size. 

Figure 3-8 Influence of O2 Pressure on PEFC Performance (93°C, Electrode Loadings of 

2 mg/cm2

 Pt, H2 Fuel at 3 Atmospheres) [(56) Figure 29, p. 49] 

Lifetime performance degradation is a key performance parameter in a fuel cell, but the causes of 

degradation are not fully understood. The sources of voltage decay are kinetic (or activation) 

loss, ohmic (or resistive) loss, loss of mass transport, and loss of reformate tolerance (42). 

Presently, the major focus of R&D on PEFC technology is to develop a fuel cell for terrestrial 

transportation, which requires the development of low-cost cell components. Hydrogen is 

considered the primary fuel for transportation applications, while reformed natural gas is the 

prime candidate for stationary applications. For automotive applications, the focus has shifted to 
3-16 

improving durability under realistic conditions, relaxing temperature and humidity requirements, 

and reducing cost, all while maintaining power densities. For reformate-fueled stacks, achieving 

better tolerance to CO and sulfur are critical factors. Because the operating temperature of 

PEFCs is much lower than PAFCs, poisoning of the anode electro-catalyst by CO from steam 

reformed methanol is a concern. The performance achieved with a proprietary anode in a PEFC 

with four different concentrations of CO in the fuel gas is shown in Figure 3-9. The graph shows 

that at higher current densities, the poisoning effect of CO is increased. At these higher current 

densities, the presence of CO in the fuel causes the cell voltage to become unstable and cycle 

over a wide range. Additional data (36) have suggested that the CO tolerance of a platinum 

electro-catalyst can be enhanced by increasing either temperature or pressure, which is one of the 

main reasons for pursuing high temperature PEFC membranes. 

Figure 3-9 Cell Performance with Carbon Monoxide in Reformed Fuel (56) 

3.3 PEFC Systems 

PEFC stacks require tight control of fuel and air feed quality, humidity level, and temperature for 

sustained high-performance operation. To provide this, PEFC stacks must be incorporated in a 

sophisticated system. Naturally, the architecture of these systems depends strongly on whether 

they are fueled by hydrogen or by a hydrocarbon fuel. 

 

3.3.1 Direct Hydrogen PEFC Systems 

Direct hydrogen PEFC systems require extensive thermal and water management to ensure that 

the PEFC stack operates under the desired design conditions (Figure 3-10). Key components are 
3-17 

heat exchangers, humidifiers, and condensers. To understand the challenge of designing such a 

system, contrast the operating conditions of a PEFC stack (60 °C to 80 °C and 40 to 100 percent 

RH) with the environment such systems must work in. Automotive design standards require that 

engines to operate at temperatures up to 60 °C (start in a sunny spot). Thus, very little driving 

temperature difference will be available between the PEFC cooling medium and the ambient, 

requiring a large radiator surface area. If such conditions occur in an arid region, significant 

amounts of water are lost from the exhaust unless sophisticated water recovery systems (such as 

sorbent wheels or refrigerated systems) are used. 

Fuel Cell Stack Preheat &

Humidity Control

Water and Heat 

Recovery

air

Radiator

Fuel

Thermal and Water 

Management 

Preheat &

Humidification

Fuel

Air

Waste Heat

Figure 3-10 Typical Process Flow Diagram Showing Major Components of Direct 

Hydrogen PEFC System. 

A key part of the direct hydrogen PEFC system is the hydrogen storage tank. A wide range of 

hydrogen storage methods is being considered (compressed hydrogen storage, liquid storage, 

storage in metal hydrides, and chemical storage). Each of these options offers distinct 

advantages, but also represents a compromise between energy density, weight, impact on energy 

efficiency, and cost. Special safety considerations must be made in all cases. As a consequence, 

the size and weight of the balance of plant components for these systems are important factors in 

the overall power system weight and volume. Automotive fuel cell developers have made 

tremendous strides in reducing the volume of direct hydrogen PEFC systems. Nevertheless, 

significant additional volume and weight reduction are required to match the power density of 

internal combustion engines. 

3.3.2 Reformer-Based PEFC Systems 

Reformer-based PEFC systems avoid the complexities and compromises of hydrogen storage, 

but instead the system must be designed to handle hydrocarbon fuels (similar considerations 

apply for alcohol fuels). This requires four major additional unit operations (Figure 3-11), 

collectively referred to as fuel processing: 

• Fuel preheat and vaporization. Necessary to prepare the fuel to meet the reformer’s feed 

requirements. Often, this unit operation is physically integrated with the reformer. 
3-18 

Figure 3-11 Schematic of Major Unit Operations Typical of Reformer-Based 

PEFC Systems. 

• Reformer. This unit chemically converts hydrocarbon or alcohol to synthesis gas (a mixture 

of hydrogen and carbon monoxide). The two most practical oxidants are steam and air. If air 

is used, the reformer is referred to as a partial oxidation (POX) reformer; if steam is used, a 

steam reformer (SR), and if a mix of air and steam is used, an autothermal reformer (ATR). 

The choice of reformer type depends on a number of factors. Typically, POX reformers are 

smaller, cheaper, respond faster, and are suitable for a wide range of fuels. Steam reformers 

enable a higher system efficiency. ATRs and catalytic POX reformers (CPOX) share some of 

the advantages of each type: 

• Water Gas Shift Reactor (WGSR). The WGSR reacts carbon monoxide with water vapor 

to form hydrogen and carbon dioxide. This reactor is critical in PEFC systems (as well as 

PAFC), since the stack is unable to convert carbon monoxide. 

• Reformate purification. This is necessary because the PEFC stacks are sensitive to even 

trace concentrations of contaminants. Especially CO and sulfur are problematic species, 

and must be reduced to levels of around 10 and 1 ppm or less, respectively. Sulfur 

removal is, in actuality, done upstream in the process (just before or just after the 

reformer), but CO removal must be done just prior to stack entry. 

A number of approaches can be used to purify reformate fuel (including pressure swing 

adsorption, membrane separation, methanation, and selective oxidation). Selective or 

preferential oxidation (PROX) is usually the preferred method for CO removal in the relatively 

small fuel cell systems because of the parasitic system loads and energy required by other 

methods. In selective oxidation, the reformed fuel is mixed with air or oxygen either before the 

Preheat or 

Evaporization Reformer Water Gas Shift Reactor

Reformate

Purification

Fuel Cell Stack Preheat &

Humidity Control

Water and Heat 

Recovery

air

Radiator

Fuel

Thermal and Water 

Management 

Preheat &

Humidification

Fuel

Air

Waste Heat
3-19 

fuel is fed to the cell or within the stack itself. Current selective oxidation technology can reduce 

CO levels to <10 ppm. Another approach involves the use of a selective oxidation catalyst that is 

placed between the fuel stream inlet and the anode catalyst. Since the stack cannot tolerate even 

10 ppm, air is usually bled into the anode directly to manage CO. Research to find approaches 

and materials that better tolerate impurities in the fuel continues today. 

These unit operations add weight and volume, and reduce the efficiency of the system (fuel 

processor “efficiencies” typically range from 75 to 90 percent, but similar losses occur in the 

production of hydrogen from fossil fuels). In addition to the unit operations however, it is 

important to realize that their presence also impacts the size, performance, and cost of the fuel 

cell stack: 

• The hydrogen in the anode feed of reformate-based systems is typically diluted with CO2 and 

(in case of POX or ATR) nitrogen. As a consequence, the hydrogen mole fraction at the 

anode inlet is rarely higher than 0.3 (vs. 75 percent in the case of a direct hydrogen system). 

This decreases the ideal potential of the cells and increases the concentration-related losses. 

• The presence of trace CO and sulfur and large quantities of CO2 affects the performance of 

the anode electro-catalyst. As a consequence, more platinum must be used (typically 0.4 to 1 

mg/cm2

 more), and even then the power density is typically 30 to 40 percent lower than with 

hydrogen-based systems. 

The choice between a direct hydrogen and a reformate-based system depend on the application. 

For light duty vehicles, most experts now prefer direct hydrogen systems (hence the focus of the 

U.S. DOE program), while for stationary applications natural gas reformer-based PEFC systems 

are favored. 

3.3.3 Direct Methanol Fuel Cell Systems 

Specially optimized PEFCs can be fed with methanol (or fuels with similar chemical structure), 

creating a so-called direct methanol fuel cell (DMFC). Conceptually, this could lead to a very 

simple system with a fuel that has a relatively high energy density and is a liquid under ambient 

conditions. Performance levels achieved with a DMFC using air is now in the range of 180 to 

250 mA/cm2

 (29) but because cell voltages typically range between 0.25 to 0.4 V, the power 

density ranges between 40 to 100 mW/cm2

. This low cell voltage is caused by a few common 

problems with the DMFC, several of which result from the cross-over of neutral methanol from 

the anode to the cathode side: 

• High anode overpotential has been shown to be caused by absorption of partial decomposition products of methanol (e.g. CO) 

• High cathode overpotential, caused by poisoning of the cathode electro-catalyst by cross-over 

methanol and its decomposition products 

This performance still requires platinum loadings that are almost ten times higher (around 3 to 5 

mg/cm2

) than needed in high-performance direct hydrogen PEFC. When feeding concentrated 

methanol directly, the cross-over can be as high as 30 to 50 percent compared with the amount 

oxidized electrochemically. If the concentration is reduced, the cross-over is reduced but so is the 

current density (due to reduced activity of the reactants). Obviously, the methanol crossed over is 
3-20 

lost, affecting efficiency and hence the heat generation. Research has focused on finding more 

advanced electrolyte materials to combat fuel crossover and more active anode catalysts to 

promote methanol oxidation. Significant progress has been made over the past few years in both 

of these key areas. Gottesfeld provides a good overview of the recent advances in DMFC 

technology (1). 

Other developers have focused on miniaturizing the balance of plant components necessary to 

control water balance and minimize methanol loss or even developing reformer-based portable 

systems (57). 

Another, less-well-reported disadvantage is that a large amount of water is transported across the 

membrane (has an aqueous methanol solution on one side and air on the other). This transport 

must be mitigated by sometimes complex water recovery systems that detract significantly from 

the conceptual simplicity of the DMFC. These limitations bar DMFCs from application in 

automobiles or stationary aplications until the cross-over is reduced by at least an order of 

magnitude. Some developers are focusing on membranes and MEAs that reduce water crossover (58). Despite the challenges mentioned, there is significant interest in DMFCs for portable 

power applications in the 1 W to 1 kW capacity range. 

Improvements in solid polymer electrolyte materials have extended the operating temperature of 

direct methanol PEFCs from 60 °C to almost 100 o

C. Electro-catalyst developments have 

focused on materials with higher activity. Researchers at the University of Newcastle upon Tyne 

have reported over 200 mA/cm2

 at 0.3 V at 80 °C with platinum/ruthenium electrodes having 

platinum loading of 3.0 mg/cm2

. The Jet Propulsion Laboratory in the U.S. has reported over 

100 mA/cm2 at 0.4 V at 60 oC with platinum loading of 0.5 mg/cm2. Recent work at Johnson 

Matthey has clearly shown that platinum/ruthenium materials possess substantially higher 

activity than platinum alone (59). 

All fuel cells exhibit kinetic losses that cause the electrode reactions to deviate from their 

theoretical ideal. This is particularly true for a direct methanol PEFC. Eliminating the need for a 

fuel reformer, however, makes methanol and air PEFCs an attractive alternative to PEFCs that 

require pure hydrogen as a fuel. The minimum performance goal for direct methanol PEFC 

commercialization is approximately 200 mW/cm2

 at 0.5 to 0.6 V. 

Figure 3-12 shows examples of performance typically achievable by developers. 
3-21 

Figure 3-12 Comparison of State-of-the-Art Single Cell Direct Methanol Fuel 

Cell Data (58) 

Developers in the U.S., Japan, and Europe have developed impressively integrated DMFC 

systems. Although energy density must still improve to broadly compete with state-of-the-art 

lithium-ion batteries in consumer applications, several developers have announced products for 

niche consumer or industrial applications within the next few years. If successful, this could 

represent the earliest commercialization of fuel cells beyond space applications. 

3.4 PEFC Applications 

3.4.1 Transportation Applications 

The focus for PEFC applications of PEFC today is on prime power for cars and light trucks. 

PEFC is the only type of fuel cell considered for prime motive power in on-road vehicles (as 

opposed to APU power, for which SOFC is also being developed). PEFC systems fueled by 

hydrogen, methanol, and gasoline have been integrated into light duty vehicles by at least twelve 

different carmakers. Early prototypes of fuel cell vehicles (Honda and Toyota) have been 

released to controlled customer groups in Japan and the U.S. However, all automakers agree that 

the widespread application of PEFC to transportation will not occur until well into the next 

decade: 

• Volume and weight of fuel cell systems must be further reduced 

• Life and reliability of PEFC systems must be improved 

• PEFC systems must be made more robust in order to be operable under the entire range of 

environmental conditions expected of vehicles 

• Additional technology development is required to achieve the necessary cost reductions 

• A hydrogen infrastructure, and the accompanying safety codes and standards must be 

developed. 
3-22 

3.4.2 Stationary Applications 

Several developers are also developing PEFC systems for stationary applications. These efforts 

are aimed at very small-scale distributed generation (~1 to 10 kW AC). The vast majority of 

systems are designed for operation on natural gas or propane. Hundreds of demonstation units 

have been sited in programs in the U.S., Europe, and Japan. Typical performance characteristics 

are given by Plug Power (60). Considerable progress has been made in system integration and in 

achieving stand-alone operation. System efficiency typically ranges from 25 to 32 percent (based 

on LHV). By recovering the waste heat from the cooling water, the overall thermal efficiency 

can be raised to about 80 percent, but the water temperature (about 50 to 70 °C) is rather modest 

for many CHP applications. System operating life has been extended to about 8,000 hrs for a 

single system with a single stack, with degradation of about 5 percent per 1,000 hours. 

3.5 References 

1. S. Gottesfeld, “The Polymer Electrolyte Fuel Cell: Materials Issues in a Hydrogen Fueled 

Power Source,” LANL, undated. 

2. W.T. Grubb, Proceedings of the 11th Annual Battery Research and Development 

Conference, PSC Publications Committee, Red Bank, NJ, p. 5, 1957; U.S. Patent 

No. 2,913,511, 1959. 

3. Communication with Plug Power, August 2002. 

4. W.D. Ernst, Patent No. 5,912,088, Plug Power Inc., June 15, 1999. 

5. G.S. Eisman, et al., Patent No. 6,280,865, Plug Power Inc., August 28, 2001. 

6. W.G.F. Grot, G.E. Munn, P.N. Walmsley, paper presented at the 141st National Meeting of 

the Electrochemical Society, Inc., Abstract No. 154, Houston, TX, May 7-11, 1972. 

7. T. Ralph, "Proton Exchange Membrane Fuel Cells: Progress in Cost Reduction of the Key 

Components," Platinum Metals Review, 41, pp. 102-113, 1997. 

8. B.R. Ezzell, B. Carl, and W. Mod, Ion Exchange Membranes for the Chlor Alkali Industry, 

AIChE Symp. Series, Houston, TX, March 1985, Pg. 49 

9. K. Prater, "The Renaissance of the Solid Polymer Fuel Cell," Ballard Power Systems, Inc., 

Journal of Power Sources, p. 29, 1990. 

10. D.J. Wheeler, J.S. Yi, R. Fredley, D. Yang, T. Patterson Jr., L. VanDine, “Advacements in 

Fuel cell Stack Technology at International Fuel Cells,” International Fuel Cells (now UTC 

Fuel Cells), Journal of New Materials for Electrochemical Systems, 4, 2001. 

11. Peter M. Schutz, A Preliminary Investigation of Radiation Catalysts in Fuel Cells, Master 

of Science Thesis, Virginia Polytechnic University, Blacksburg, Va., August, 1979 Pg. 59. 

12. 3M Product Bulletin, date unknown. 

13. D. S. Watkins, et al., Abstracts 37th International Power Sources Symposium (The 

Electrochemical Society) p. 782, 1988. 

14. Lousenberg, D., et al. Diferentiated Membranes and Dispersions for Commercial PEM 

Fuel Cell and Electrolysis Systems. in 2003 Fuel Cell Seminar. 2003. Miami Beach, FL, 

USA: Department of Energy. 

15. Teather, E. and J. Staser. MEA Improvements for Sub-humidified Fuel Cell Operation. In 

2003 Fuel Cell Seminar. 2003. Miami Beach, FL, USA: Department of Energy. 

16. Cleghorn, S., et al. New MEAs for Low Cost System Design. in 2003 Fuel Cell Seminar. 

2003. Miami Beach, FL, USA: Department of Energy. 

17. Peter M. Schutz, A Preliminary Investigation of Radiation Catalysts in Fuel Cells, Master 

of Science Thesis, Virginia Polytechnic University, Blacksburg, Va., August, 1979 Pg. 59. 
3-23 

18. J. C. Amphlett, M. Farahani, R. F. Mann, B. A. Peppley, P. R. Roberge, in Proceedings of 

the 26th Intersociety Energy Conversion Engineering Conference, August 4-9, 1991, 

Volume 3, Conversion Technologies/Electrochemical Conversion, American Nuclear 

Society, La Grange, Illinois, p. 624, 1991. 

19. M. Krumpelt, K. M. Myles, No. 8, An EPRI/GRI Fuel Cell Workshop on Technology 

Research and Development, April 13-14, 1993, Stonehart Associates, Madison, Connecticut, 

1993. 

20. M. Krumpelt, R. Kumar, J. Miller, C. Christianson, in 1992 Fuel Cell Seminar Program and 

Abstracts, The Fuel Cell Seminar Organizing Committee, November 29 - December 2, 1992, 

Tucson, Arizona, p. 35, 1992. 

21. W. Teagan, J. Bentley, "Status: ADL/EPYX Fuel Processing Technology," Joint 

DOE/EPRI/GRI Workshop on Fuel Cell Technology, San Francisco, CA, May 18-20, 

1998. 

22. R. F. Savinell, J. S. Wainright, M. Litt, “High Temperature Polymer Electrolyte Fuel 

Cells,” The Electrochemical Society, Journal 98-27, p.81. 

23. W.D. Ernst, ”PEM Technology Development at Plug Power,” 2000 Fuel Cell Seminar 

Program and Abstracts, Portland Oregon, sponsored by Fuel Cell Seminar Organizing 

Committee, October 30 – November 2, 2000. 

24. Gordon Research Conference, Rhode Island, July 29 - August 3, 2001.

25. Debe, M., et al. Activities of Low Pt Loading, Carbon-Less, Ultra-Thin Nanostructured 

Filmbased Electrodes for PEM Fuel Cells and Roll-Good Fabricated MEA Performances 

in Single Cells and Stacks. in 2003 Fuel Cell Seminar. 2003. Miami Beach, FL, USA: 

Department of Energy. 

26. Wheeler, D., T. Clark, and S. Motupally. PEM Fuel CEll Programs at UTC Fuel Cells. in 

2003 Fuel Cell Seminar. 2003. Miami Beach, FL, USA: Department of Energy. 

27. T.G. Coker, A.B. LaConti, L.J. Nuttall, in Proceedings of the Symposium on Membranes and 

Ionic and Electronic Conducting Polymers, edited by E.G. Yeager, B. Schumm, K. Mauritz, 

K. Abbey, D. Blankenship, J. Akridge, The Electrochemical Society, Inc., Pennington, NJ, 

p. 191, 1983. 

28. N. Giordano, et al., in Proceedings 26th Intersociety Energy Conversion Engineering 

Conference, August 4-9, 1991, Volume 3, Conversion Technologies/Electrochemical 

Conversion, American Nuclear Society, La Grange, Illinois, p. 624, 1991. 

29. T.A. Zawodzinski, et al., Journal of Electrochemical Society, p. 140, 1042, 1993. 

30. T.E. Springer, et al., Journal of Electrochemical Society, p. 138, 2335, 1991. 

31. D.M. Bernardi, Journal of Electrochemical Society, p. 137, 3344, 1990. 

32. T.E. Springer, M. S. Wilson, S. Gottesfeld, "Modeling and Experimental Diagnostics in 

Polymer Electrolyte Fuel Cells," submitted to J. ElectroChem. Soc., LA-UR-93-1469 Los 

Alamos National Laboratory, New Mexico, 1993. 

33. R. Baldwin, M. Pham, A. Leonida, J. McElroy, and T. Nalette, Hydrogen-Oxygen Proton 

Exchange Membrane Fuel Cells and Electrolyzers, Proceedings of the Space 

Electrochemical Research and Technology (SERT), NASA LEWIS Research Center, 

Cleveland, Ohio, April, 1989. 

34. S. Cleghorn, J. Kolde, W. Liu, “Catalyst Coated Composite Membranes,” Chapter 49, Fuel 

Cell Reference Book, to be published, Wiley-VCH, 2002. 
3-24 

35. S. Gottesfeld, “Polymer Electrolyte Fuel Cells: Transportation and Stationary Application,” 

No. 10, in An EPRI/GRI Fuel Cell Workshop on TechnologyResearch and Development, 

April 13-14, 1993, Stonehart Associates, Madison, Connecticut, 1993. 

36. "Investigation of Design and Manufacturing Methods for Low-Cost Fabrication of High 

Efficiency, High Power Density PEM Fuel Cell Power Plant," prepared by International Fuel 

Cells, Final Report FCR-11320A, June 10, 1991. 

37. D. Wilkinson, D. Thompsett, "Materials and Approaches for CO and CO2 Tolerance for 

Polymer Electrolyte Membrane Fuel Cells," Proceedings of the Second International 

Symposium on New Materials for Fuel Cell and Modern Battery Systems, pp. 266-285, 

(Montreal, Quebec, Canada, July 6-10, 1997). 

38. “Information Sheet,” prepared by the U.S. Department of Energy, Office of Propulsion 

Systems, Proton Exchange Membrane Fuel Cell Program, DE93000009, November 1992. 

39. K. Sikairi, K. Tsurumi, S. Kawaguchi, M. Watanabe, P. Stonehart, in 1992 Fuel Cell Seminar 

Program and Abstracts, Tucson, AZ, November 29 - December 2, 1992, sponsored by Fuel 

Cell Organizing Committee, p. 153, 1992. 

40. S. Srinivasan, O.A. Velev, A. Parthasarathy, A.C. Ferriera, S. Mukerjee, M. Wakizoe, 

Y.W. Rho, Y.T. Kho, A.J. Appleby, in 1992 Fuel Cell Seminar Program and Abstracts, 

Tucson, Arizona November 29 - December 2, 1992, sponsored by Fuel Cell Organizing 

Committee, p. 619, 1992. 

41. F.N. Buchi, B. Gupta, M. Rouilly, P.C. Hauser, A. Chapiro, G.G. Scherer, in 

27th Intersociety Energy Conversion Engineering Conference Proceedings, Volume 3, 

Conversion Technologies/Electrochemical Conversions, San Diego, CA, August 3-7, 1992, 

published by Society of Automotive Engineers, Inc., Warrendale, PA, 419, 1992. 

42. T.A. Zawodzinski, T.A. Springer, F. Uribe, S. Gottesfeld, "Characterization of Polymer 

Electrolytes for Fuel Cell Applications," Solid State Tonics 60, pp. 199-211, North-Holland, 

1993. 

43. E. Gulzow, M. Schulze, N. Wagner, et al., J. of Power Sources Vol. 86 pages 352-362, 

2000. 

44. M.S. Wilson, T.E. Springer, T.A. Zawodzinski, S. Gottesfeld, in 26th Intersociety Energy 

Conversion Engineering Conference Proceedings, Volume 3, Conversion 

Technologies/Electrochemical Conversion, Boston, Massachusetts, August 4-9, 1991, 

published by Society of Automotive Engineers, Inc., Warrendale, PA, 1991. 

45. C. Derouin, T. Springer, F. Uribe, J. Valerio, M. Wilson, T. Zawodzinski, S. Gottesfeld, in 

1992 Fuel Cell Seminar Program and Abstracts, Tucson AZ, November 29 - December 2, 

1992, sponsored by Fuel Cell Organizing Committee, p. 615, 1992. 

46. J.G. Clulow, F.E. Zappitelli, C.M. Carlstrom, J.L. Zemsky, D.M. Buskirk, M.S. Wilson, 

“Development of Vinyl Ester/ Graphite Composite Bipolar Plates,” Fuel Cell Technology: 

Opportunites and Challenges, 2002 AIChE Spring National Meeting, March 10-14, 2002. 

47. P.D. Naylor, P.J. Mitchell, P.L. Adcock, in 1992 Fuel Cell Seminar Program and Abstracts, 

Tucson, AZ, November 29 - December 2, 1992, sponsored by Fuel Cell Organizing 

Committee, 575, 1992. 

48. S.R. Narayanan, E. Vamos, H. Frank, S. Surampudi, G. Halpert, in 1992 Fuel Cell Seminar 

Program and Abstracts, Tucson, AZ, November 29 - December 2, 1992, sponsored by Fuel 

Cell Organizing Committee, p. 233, 1992. 

49. A.J. Appleby, E.B. Yeager, Energy, pg. 11, 137, 1986. 
3-25 

50. D. Watkins, K. Dircks, E. Epp, A. Harkness, Proceedings of the 32nd International Power 

Sources Symposium, The Electrochemical Society, Inc., Pennington, NJ, p. 590, 1986. 

51. Koehler, J., et al. Advanced MEA Technology for Hydrogen and Reformate Application. in 

2003 Fuel Cell Seminar. 2003. Miami Beach, FL, USA: Department of Energy. 

52. J.C. Amphlett, et al., "The Operation of a Solid Polymer Fuel Cell: A Parametric Model," 

Royal Military College of Canada, no date. 

53. K. Ledjeff, et al., "Low Cost Membrane Fuel Cell for Low Power Applications," 

Fraunhofer-Institute for Solar Energy Systems, Program and Abstracts, 1992 Fuel Cell 

Seminar. 

54. J. R. Huff, "Status of Fuel Cell Technologies," Fuel Cell Seminar Abstracts, Fuel Cell 

Seminar, October 26-29, 1986, Tucson, AZ. 

55. J. Srmivason, et al., "High Energy Efficiency and High Power Density Proton Exchange 

Membrane Fuel Cells - Electrode Kinetics and Mass Transport," Journal of Power Sources, 

p. 36, 1991 

56. A. LaConti, G. Smarz, F. Sribnik, "New Membrane-Catalyst for Solid Polymer Electrolyte 

Systems," Final Report prepared by Electro-Chem Products, Hamilton Standard for Los 

Alamos National Laboratory under Contract No. 9-X53-D6272-1, 1986. 

57. Pavio, J. Performance and Design of a Reformed Hydrogen Fuel Cell System. in 2003 Fuel 

Cell Seminar. 2003. Miami Beach, FL, USA: Department of Energy. 

58. Cox, P., S.-Y. Cha, and A. Attia. PolyFuel's Z1 Membrane and Catalyst Coatings to 

Improve the Fuel Cell Performance in Portable Power Applications. in 2003 Fuel Cell 

Seminar. 2003. Miami Beach, FL, USA: Department of Energy. 

59. M. Hogarth, G. Hards, "Direct Methanol Fuel Cells: Technological Advances and Further 

Requirements," Platinum Metals Review, 40, pp. 150-159, 1996. 

60. Feitelberg, A. On the Efficiency of PEM Fuel Cell Systems and Fuel Processors. in 2003 

Fuel Cell Seminar. 2003. Miami Beach, FL, USA: Department of Energy. 
4-1 

4. ALKALINE FUEL CELL

The Alkaline Fuel Cell (AFC) was one of the first modern fuel cells to be developed, beginning 

in 1960. The application at that time was to provide on-board electric power for the Apollo 

space vehicle. Desirable attributes of the AFC include excellent performance compared to other 

candidate fuel cells due to its active O2 electrode kinetics and flexibility to use a wide range of 

electro-catalysts. The AFC continues to be used: it now provides on-board power for the Space 

Shuttle Orbiter with cells manufactured by UTC Fuel Cells. 

The AFC developed for space application was based, in large part, on work initiated by F.T. 

Bacon (1) in the 1930s. By 1952, construction and performance testing of a 5-kW alkaline fuel 

cell, operating on H2 and O2, was completed. The fuel cell developed by Bacon operated at 200 

to 240 o

C with 45 percent KOH electrolyte. Pressure was maintained at 40 to 55 atm to prevent 

the electrolyte from boiling. At this relatively high temperature and pressure, performance of the 

cell was quite good (0.78 volts at 800 mA/cm2

). The anode consisted of a dual-porosity Ni 

electrode (two-layer structure with porous Ni of 16 μm maximum pore diameter on the 

electrolyte side and 30 μm pore diameter on the gas side). The cathode consisted of a porous 

structure of lithiated NiO. The three-phase boundary in the porous electrodes was maintained by 

a differential gas pressure across the electrode, since a wetproofing agent was not available at 

that time, i.e., PTFE (polytetrafluoroethylene) as a wetproofing material did not exist, and it 

would not have been stable in the high temperature alkaline solution (2). 

The kinetics of O2 reduction in alkaline electrolytes are more favorable than in phosphoric acid 

electrolyte. Consider a Pt cathode (0.25 mg/cm2

) in 30 percent KOH at 70 °C and in 96 percent 

phosphoric acid at 165 °C. The cathode potentials (vs. RHE - Reversible Hydrogen Electrode) at 

100 mA/cm2

 in these two electrolytes are 0.868 and 0.730 V, respectively, according to data 

reported by Appleby (Figure 2.15-1 in Reference 3). Various explanations have been advanced 

for the higher O2 reduction rates in alkaline electrolytes (4). The practical consequence of the 

higher performance of Pt cathodes in alkaline electrolytes is that AFCs are capable of higher 

efficiencies than PAFCs at a given current density, or higher power densities at the same 

efficiency. Bockris (2) estimates that the efficiency of AFCs fueled by pure H2 is about 60 

percent HHV, and that of PAFCs is about 50 percent HHV. 

The high performance of the alkaline cell relative to phosphoric acid and other cells leads to the 

plausibility of developing the technology for terrestrial application. The leading developer of 

alkaline technology for space application, UTC Fuel Cells, investigated adaptating the 
4-2 

technology to terrestrial, stationary power applications using air as an oxidant in the early 1970s. 

The predominant drawback with terrestrial applications is that CO2 in any hydrocarbon fuel or in 

the air reacts with the ion carrier in the electrolyte. During the 1970s, a high pressure drop 

platinum/palladium separator was used in the fuel processor to obtain a pure stream of H2 from 

reformed hydrocarbon fuels (primarily natural gas for stationary power plants). Similarly, a 

soda-lime scrubber treated the inlet ambient air stream to minimize CO2 entering the cell. The 

expense of the separator and scrubber was deemed uneconomical for commercial development of 

stationary power plants. Augmenting the issue was a slow build-up of K2CO3 due to the 

minuscule amount of CO2 escaping the soda-lime scrubber. There was also an issue of 

component life for stationary power applications. Alkaline cell life (now 2,600 hours on H2/O2, 

but 5,000 hour R&D underway) is suitable for space missions, but too brief for terrestrial, 

stationary power plants. As a result of the CO2 issue, UTC Fuel Cells, which uses an 

immobilized electrolyte, now focuses their alkaline program completely toward space 

applications with H2/O2 as fuel and oxidant. 

Union Carbide Corp. (UCC) developed AFCs for terrestrial mobile applications starting in the 

late 1950s, lasting until the early 1970s. UCC systems used liquid caustic electrolytes; the 

electrodes were either pitch-bonded carbon plates or plastic-bonded carbon electrodes with a 

nickel current collector. UCC also built fuel cell systems for the U.S. Army and the U.S. Navy, 

an alkaline direct hydrazine powered motorcycle, and the “Electrovan” of General Motors. 

Finally, Professor Karl V. Kordesch built his Austin A-40 car, fitted with UCC fuel cells with 

lead acid batteries as hybrid. It was demonstrated on public roads for three years. The years of 

research and development are very well summarized in reference (5) Brennstoffbatterien. 

Based on the UCC technology, other developers are now pursuing terrestrial applications of 

alkaline technology due to its high performance, particularly for motive power. The majority of 

these developers use circulating electrolytes with an external, commercial type soda-lime 

absorber that promises to resolve the problem of CO2 in the air stream. The quantity of CO2 can 

be limited to a small amount with a circulating electrolyte, versus a continual build-up with an 

immobilized electrolyte. Life expectancy increases (~5,000 hour life is ample for personal 

automobile engine life) because the cell is nearly inactive when switched off. Hence, only the 

true operating hours count for the total lifetime. During normal operation, the electrolyte 

circulates continuously, which has several advantages over an immobilized system: 1) no 

drying-out of the cell occurs because the water content of the caustic electrolyte remains quite 

constant everywhere inside the stack; 2) heat management by dedicated heat exchanger 

compartments in the stack becomes unnecessary - the electrolyte itself works as a cooling liquid 

inside each cell; 3) accumulated impurities, such as carbonates, are concentrated in the 

circulating stream and can easily be removed (comparable to a function of oil in today’s gasoline 

engines); 4) the OH–

 concentration gradient is highly diminished, and 5) the electrolyte prevents 

the build-up of gas bubbles between electrodes and electrolyte as they are washed away. 

Other attributes are that the alkaline cell could have high reactivity without the need for noble 

metal catalysts on the cell electrodes; this represents a cost savings (6). Additionally, the 

radiator of the alkaline cell system should be smaller than the radiator in the competitive PEFC 

system because of higher alkaline cell temperature and its higher performance. 
4-3 

In stacks using circulating electrolytes, parasitic currents might occur. All cells are connected via 

the electrolyte stream to all other cells, producing high voltages between the electrodes. Parasitic 

current not only lowers the stack performance, but can also harm the electrodes. Fortunately, this 

issue can be resolved easily by using a special electrode frame design with long, narrow 

electrolyte channels. 

Some developers have investigated a direct methanol alkaline cell to circumnavigate 

hydrocarbon fuel separator issues. These cells exhibit a reduced performance, and have not been 

as thoroughly investigated as the hydrogen-fueled cells. 

The unusual economics for remote power applications (i.e., space, undersea, and military 

applications) result in the cell itself not being strongly constrained by cost. The consumer and 

industrial markets, however, require the development of low-cost components if the AFC is to 

successfully compete with alternative technologies. Much of the recent interest in AFCs for 

mobile and stationary terrestrial applications has addressed the development of low-cost cell 

components. In this regard, carbon-based porous electrodes play a prominent role (6). It 

remains to be demonstrated whether alkaline cells will prove commercially viable for the 

transportation sector. Reference (7) provides an in-depth view of the development history and 

the potential of alkaline technology for terrestrial application. 

Figures 4-1 and 4-2 depict the operating configuration of the H2/O2 alkaline fuel cell (8) and a 

H2/air cell (9). In both, the half-cell reactions are: 

 H2 + 2OH ̄ → 2H2O + 2e ̄ (Anode) (4-1) 

 1⁄2O2 + H2O + 2e ̄ → 2OH ̄ (Cathode) (4-2) 

Hydroxyl ions, OH ̄ , are the conducting species in the electrolyte. The equivalent overall cell 

reaction is: 

 H2 + 1⁄2O2 → H2O + electric energy + heat (4-3) 

Since KOH has the highest conductance among the alkaline hydroxides, it is the preferred 

electrolyte. 
4-4 

ANODE

MATRIX

CATHODE

O2 in

H2 recycle carrying

product water

H2 FEED INLETS

Humidified H2 in

ELECTROLYTE RESERVOIR PLATE - ERP

(POROUS NICKEL-SINTER)

ELECTRODE

CATALYST

Figure 4-1 Principles of Operation of H2/O2 Alkaline Fuel Cell, Immobilized Electrolyte (8) 

Figure 4-2 Principles of Operation of H2/Air Alkaline Fuel Cell, Circulating Electrolyte (9) 
4-5 

4.1 Cell Components 

4.1.1 State-of-the-Art Components 

The concentration of KOH in an immobilized electrolyte typically used in the space program 

varies from 35 to 50 wt percent KOH for low temperature (<120 °C) operation to 85 wt percent 

KOH in cells designed for operation at high temperature (~260 °C). The electrolyte is retained 

in a matrix (usually asbestos), and a wide range of electro-catalysts can be used (e.g., Ni, Ag, 

metal oxides, spinels, and noble metals) to promote reaction. 

The cylindrical AFC modules used in the U.S. Apollo Space Program had a 57 cm diameter, a 

112 cm height, weighed about 110 kg, produced a peak power of 1.42 kW at 27 to 31 V, and 

operated at an average power of 0.6 kW. These cells operated on pure H2 and O2 and 

concentrated electrolyte (85 percent KOH) at a moderate pressure (4 atmospheres reactant gas 

pressure) without electrolyte boiling. With this concentrated electrolyte, cell performance was 

not as high as in the less-concentrated electrolyte; consequently, the operating temperature was 

increased to 260 o

C. The typical performance of this AFC cell was 0.85 V at 150 mA/cm2, 

comparing favorably to the performance of the Bacon cell operating at about 10 times higher 

pressure. 

The state-of-the-art alkaline fuel cell stacks in the Space Shuttle Orbiter are rectangular with a 

width of 38 cm, a length of 114 cm, and a height of 35 cm. They weigh 118 kg, produce a peak 

power of 12 kW at a minimum of 27.5 V (end of life), and operate at an average power of 7 kW. 

They operate in the same pressure range as the Apollo cells (4 atmospheres), but at a lower 

temperature (85 to 95 °C) and higher current density (0.88 V at 470 mA/cm2

; UTC Fuel Cells 

has demonstrated 3.4 W/cm2

 at 0.8 V and 4,300 mA/cm2, Reference (8)). The electrodes contain 

high loadings of noble metals: 80 percent Pt – 20 percent Pd anodes are loaded at 10 mg/cm2

 on 

Ag-plated Ni screen; 90 percent Au – 10 percent Pt cathodes are loaded at 20 mg/cm2 on Agplated Ni screen. Both are bonded with PTFE to achieve high performance at the lower 

temperature of 85 ot 95 o

C. A wide variety of materials (e.g., potassium titanate, ceria, asbestos, 

zirconium phosphate gel) have been used in the micro-porous separators for AFCs. The 

electrolyte is 35 percent KOH and is replenished via a reservoir on the anode side. Gold-plated 

magnesium is used for the bipolar plates. Sheibley and Martin (10) provide a brief survey of the 

advanced technology components in AFCs for space applications. 

An advanced cell configuration for underwater application was developed using high surface 

area Raney nickel anodes loaded at 120 mg/cm2

 (1 to 2 percent Ti) and Raney silver cathodes 

loaded at 60 mg/cm2

 containing small amounts of Ni, Bi, and Ti (11). 

The efforts of Union Carbide Corporation have formed the basis for most of today’s terrestrial 

applications of AFCs with circulating liquid electrolytes. Companies like Da Capo Fuel Cell 

Ltd. (which bought ZeTek Power (formerly Zevco and Elenco)), Astris Energy, and Apollo 

Energy System Inc. are developing circulating electrolyte cells for motive and backup power 

primarily based on that technology. A typical configuration (Apollo, Figure 4-2) uses carbonbased plastic-bonded gas diffusion electrodes with a current collector (nickel) inside. Due to the 

ease of preparation, the electrodes in present stacks use noble metals loaded to less than 

0.5mg/cm2

. The 0.3 cm thick cells are stacked in a monopolar order and are commonly 
4-6 

connected in series via edge connectors. Neither membranes nor bipolar plates are needed. The 

stacks operate at 75 °C, using a 9N KOH electrolyte. The gases are fed at ambient pressure; 

either pure hydrogen or cracked ammonia is used. Lifetime testing (12) has not been finished, 

but is >1,000 hours at intermittent operation (a few hours per day). 

Several types of catalysts are used or are being considered for the electrodes: 1) noble metals 

(expensive but simple, and acceptable for low volume stack preparation); 2) “classic” non-noble 

metals (silver for the cathode and Raney nickel for the anode), and 3) spinels and perovskites 

(often referred to as alternative catalysts, these are being developed because they cost less than 

the noble metal catalysts). 

4.1.2 Development Components 

Immobilized electrolyte AFCs, used mostly in space or closed environments, and circulating 

electrolyte AFCs, used for terrestrial application, face separate and unique development 

challenges. 

H2/O2 alkaline technology using immobilized electrolytes is considered to be fully developed. 

Confidence in the present cell technology is best represented by the fact that there is no back-up 

electric power on the Space Shuttle Orbiter. Further improvement of the present H2/O2 design is 

not considered to be cost effective with one exception: maintenance cost can be decreased 

directly by increasing the cell stack life of the Orbiter power plant. 

The life-limiting event in the present Orbiter cell is KOH corrosion of the cell frame (cell 

support). Present stack life is 2,600 hours. The cell stacks have demonstrated capability to reach 

this life in 110 flights and a total of ~87,000 hours in the Orbiter (July 2002). Present practice is 

to refurbish the power unit at 2,600 hours by installing a new stack, and cleaning and inspecting 

the balance of equipment. The stack life is being improved to 5,000 hours by elongating the path 

length associated with KOH-induced corrosion of the cell frame. A 10 cell short stack has 

demonstrated the new 5,000 hours concept. The concept is now being qualified in a complete 

power plant, presently being tested (13). 

Electrode development in circulating electrolyte AFCs has concentrated on 1) multi-layered 

structures with porosity characteristics optimized for flow of liquid electrolytes and gases (H2

and air), and 2) catalyst development. Another area for concern is the instability of PTFE, which 

causes weeping of the electrodes. Most developers use noble metal catalysts; some use nonnoble catalysts. Spinels and perovskites are being developed in an attempt to lower the cost of 

the electrodes. Development of low-cost manufacturing processes includes powder mixing and 

pressing of carbon-based electrodes, sedimentation and spraying, and high-temperature sintering. 

AFC electrolyte development has been restricted to KOH water solutions with concentrations 

ranging from 6 to12N. Still, use of less expensive NaOH has been considered. Minimal cost 

advantages appear to be far outweighed by performance reductions due to wetting angle and 

lower conductivity. However, NaOH as an electrolyte increases the lifetime of electrodes when 

CO2 is present, because sodium carbonate, although less soluble than potassium carbonate, forms 

much smaller crystals, which do not harm the carbon pores. 
4-7 

Other approaches to increasing life and reducing weight and cost include investigating epoxy 

resins, polysulfone and ABS (acrylonitrile-butadiene-styrene). Framing techniques under 

development include injection molding, filter pressing, and welding (14, 15). 

Immobilized electrolyte AFCs are highly sensitive to carbon dioxide (CO2). Non-hydrocarbon 

hydrogen fuel or pure H2 can be fed directly to the anode. For example, a carbon-free fuel gas 

such as cracked ammonia (25 percent N2, 75 percent H2, and residual NH3) can be fed directly to 

the cell. Due to the high diffusion rate of hydrogen compared to nitrogen, only a very small 

decrease in potential is observed with hydrogen content greater than 25 percent (at medium 

current densities). Gas purification is necessary when H2 is produced from carbon-containing 

fuel sources (e.g., methanol, gasoline, propane and others). There are many approaches to 

separate CO2 from gaseous or liquid streams. Physical separation and chemical separation are 

the most common methods used. However, CO2 removal by these methods requires more than 

one process step to reduce the CO2 to the limits required by the fuel cell. Two additional 

methods include cryogenic separation and biological fixation. If liquid hydrogen is used as the 

fuel for the alkaline fuel cell, a system of heat exchangers can be used to condense the CO2 out 

of the air for the oxidant stream. This technique has a potential weight advantage over the sodalime scrubber. Low-temperature distillation is commonly used for the liquefaction of CO2 from 

high purity sources. A new, potentially efficient technique that is being investigated uses 

capillary condensation to separate gases by selective wicking. Biological separation is 

promising, but must overcome the challenge of reactivation after shutdown periods. 

Another promising CO2 separation method is membrane separation. This has the advantages of 

being compact, no moving parts, and the potential for high energy efficiency. Polymer 

membranes transport gases by solution diffusion, and typically have a low gas flux and are 

subject to degradation. These membranes are relatively expensive. The main drawbacks of 

membrane separation are the significant pressure differential that may be required across the 

membrane and its high cost. The need for a high pressure gradient can be eliminated by using a 

membrane in which a potential is applied over the membrane. This approach is sometimes 

referred to as the “sacrificial cell” technique. Another approach is to use a membrane with steam 

reforming of liquid fuels. Little additional energy is needed to pressurize the liquid fuel and 

water to the pressure required for separation. 

Alkaline cell developers continue to investigate CO2 separation methods that show economic 

promise. However, circulating electrolyte is the technology of choice for terrestrial applications. 

4.2 Performance 

Performance of AFCs since 1960 has undergone many changes, as evident in the performance 

data in Figure 4-3. H2/air performance is shown as solid lines, and H2/O2 performance is shown 

as dashed lines. The early AFCs operated at relatively high temperature and pressure to meet the 

requirements for space applications. More recently, a major focus of the technology is for 

terrestrial applications in which low-cost components operating at near-ambient temperature and 

pressure with air as the oxidant are desirable. This shift in fuel cell operating conditions resulted 

in the lower performance shown in Figure 4-3. The figure shows, using dotted lines, H2/O2

performance for: 1) the Orbiter with immobilized electrolyte (8), and 2) a circulating electrolyte 

cell (12). 
4-8 

0.6

0.7

0.8

0.9

1

1.1

0 100 200 300 400 500 600 700

Current Density (mA/cm2

)

Cell Voltage (V)

1969 (220oC, 4 atm.)

1984 (65oC, 1 atm.)

1978 (110oC, 10 atm.)

2002 (95oC, 4 atm., immobilized)

2001 (75oC, 1 atm.)

2001 (75oC, 1 atm.)

Figure 4-3 Evolutionary Changes in the Performance of AFCs (8, 12, & 16) 

The data described in the following paragraphs pertains to the H2/air cell. Unfortunately, H2/air 

performance data is rather dated; there has been a noticeable lack of recent H2/air data. 

4.2.1 Effect of Pressure 

AFCs experience the typical enhanced performance with an increase in cell operating pressure. 

Figure 4-4 plots the increase in reversible e.m.f. (electromotive force) of alkaline cells with 

pressure over a wide range of temperatures (17). The actual increase in cell open circuit voltage 

is somewhat less than shown because of the greater gas solubility with increasing pressure that 

produces higher parasitic current. 

At an operating temperature (T), the change in voltage (∆VP) as a function of pressure (P) can be 

expressed fairly accurately using the expression: 

∆VP (mV) = 0.15T (°K) log(P2/P1) (4-4) 

 

over the entire range of pressures and temperatures shown in Fig. 4-4. In this expression, P2 is 

the desired performance pressure and P1 is the reference pressure at which performance is 

known. 
4-9 

Figure 4-4 Reversible Voltage of the Hydrogen-Oxygen Cell (14) 

To achieve faster kinetics, operating temperatures greater than 100 °C, accompanied by higher 

pressures, are used. Spacecraft fuel cells have operated for over 5,000 hours at 200 °C at 5 atm 

achieving HHV efficiencies exceeding 60 percent (18, 19). It should be noted that a pressure 

increase beyond about 5 atm produces improvements that are usually outweighed by a significant 

weight increase required to sustain the higher operating pressure. For space applications, weight 

is critical. Also, this increase in performance can only be realized in applications where 

compressed gases are available (such as in space vehicles or submarines). In all other cases, 

compressors are needed. Compressors are not only noisy, but incur parasitic power that lowers 

the system efficiency (20). An increase of overall efficiency when using compressors in simple 

cycles is very unlikely.

4.2.2 Effect of Temperature 

Section 2.1 describes that the reversible cell potential for a fuel cell consuming H2 and O2

decreases by 49 mV under standard conditions in which the reaction product is water vapor. 

However, as is the case in PAFCs, an increase in temperature improves cell performance because 

activation polarization, mass transfer polarization, and ohmic losses are reduced. 

The improvement in performance with cell temperature of catalyzed carbon-based (0.5 mg 

Pt/cm2

) porous cathodes is illustrated in Figure 4-5 (21). As expected, the electrode potential at a 

given current density decreases at lower temperatures, and the decrease is more significant at 

higher current densities. In the temperature range of 60 to 90 °C, the cathode performance 

increases by about 0.5 mV/°C at 50 to 150 mA/cm2

. 
4-10 

Figure 4-5 Influence of Temperature on O2, (air) Reduction in 12 N KOH. 

Source: Fig. 10, p. 324, Reference (21). 

Early data by Clark, et al. (22) indicated a temperature coefficient for AFCs operating between 

50 to 70 °C of about 3 mV/°C at 50 mA/cm2

, and cells with higher polarization had higher 

temperature coefficients under load. Later measurements by McBreen, et al. (23) on H2/air 

single cells (289 cm2

 active area, carbon-based Pd anode and Pt cathode) with 50 percent KOH 

showed that the temperature coefficient above 60 °C was considerably lower than that obtained 

at lower temperatures, as shown in Figure 4-6. The McBreen data suggest the following 

expressions for evaluating the change in voltage (∆VT) as a function of temperature (T) at 100 

mA/cm2

: 

∆Vt (mV) = 4.0 (T2-T1) for T < 63 °C (4-5) 

or 

∆Vt (mV) = 0.7 (T2-T1) for T > 63 °C (4-6) 

Alkaline cells exhibit reasonable performance when operating at low temperatures (room 

temperature up to about 70 °C). This is because the conductivity of KOH solutions is relatively 

high at low temperatures. For instance, an alkaline fuel cell designed to operate at 70 °C will 

reduce to only half power level when its operating temperature is reduced to room temperature 

(24). 
4-11 

Figure 4-6 Influence of Temperature on the AFC Cell Voltage 

Source: Figure 6, p. 889, reference (23). 

4.2.3 Effect of Impurities 

Carbon dioxide was the only impurity of concern in the data surveyed. AFCs with immobilized 

electrolytes suffer a considerable performance loss with reformed fuels containing CO2 and from 

the presence of CO2 in air (typically ~350 ppm CO2 in ambient air). The negative impact of CO2

arises from its reaction with OH ̄ 

 CO2 + 2OH ̄ → CO3

=

 + H2O (4-7) 

producing the following effects: 1) reduced OH ̄ concentration, interfering with kinetics; 

2) electrolyte viscosity increase, resulting in lower diffusion rate and lower limiting currents; 

3) precipitation of carbonate salts in the porous electrode, reducing mass transport; 4) reduced 

oxygen solubility, and 5) reduced electrolyte conductivity. 

In the case of circulating liquid electrolytes, the situation is not as critical, but is still significant. 

The influence of CO2 on air cathodes (0.2 mg Pt/cm2

 supported on carbon black) in 6N KOH at 

50 °C can be ascertained by analysis of the performance data presented in Figure 4-7 (25). To 

obtain these data, the electrodes were operated continuously at 32 mA/cm2

, and current-voltage 

performance curves were periodically measured. Performance in both CO2-free air and CO2-

containing air showed evidence of degradation with time. However, with CO2-free air the 

performance remained much more constant after 2,000 to 3,000 hours of operation. Later tests, 

however, showed that this drop in performance was caused purely by mechanical destruction of 
4-12 

the carbon pores by carbonate crystals. Improved electrodes can withstand even high amounts of 

CO2 (5 percent) over many thousands of hours, as proven recently by DLR (Deutsches Zentrum 

fuer Luft- und Raumfahrt) (26). 

Figure 4-7 Degradation in AFC Electrode Potential with CO2 Containing and CO2 

Free Air Source: Figure 2, p. 381, Reference (25) 

High concentrations of KOH are also detrimental to the life of O2 electrodes operating with CO2-

containing air, but operating the electrode at higher temperature is beneficial because it increases 

the solubility of CO2 in the electrolyte. Hence, modifying the operating conditions can prolong 

electrode life. Extensive studies by Kordesch, et al. (25) indicate that the operational life of air 

electrodes (PTFE-bonded carbon electrodes on porous nickel substrates) with CO2-containing air 

in 9N KOH at 65 °C ranges from 1,600 to 3,400 hours at a current density of 65 mA/cm2

. The 

life of these electrodes with CO2-free air tested under similar conditions ranged from 4,000 to 

5,500 hours. It was reported (2) that a lifetime of 15,000 hours was achieved with AFCs, with 

failure caused at that time by corrosion of the cell frames. 

4.2.4 Effects of Current Density 

As in the case with PAFCs, voltage obtained from an AFC is affected by ohmic, activation, and 

concentration losses. Figure 4-8 presents data obtained in the 1960s (22) that summarizes these 

effects, excluding electrolyte ohmic (iR) losses, for a catalyzed reaction (0.5 to 2.0 mg noble 

metal/cm2

) with carbon-based porous electrodes for H2 oxidation and O2 reduction in 9N KOH at 

55 to 60 °C. The electrode technology was similar to that employed in the fabrication of PAFC 

electrodes. 
4-13 

Figure 4-8 iR-Free Electrode Performance with O2 and Air in 9 N KOH at 55 

to 60°C. Catalyzed (0.5 mg Pt/cm2 Cathode, 0.5 mg Pt-Rh/cm2 Anode) 

Carbon-based Porous Electrodes (22) 

The results in Figure 4-8 yield the following current density equations for cells operating in 9N 

KOH at 55 to 60 °C: 

∆VJ (mV) = -0.18∆J for J = 40 to 100 mA/cm2 operating in O2 (4-8) 

or 

∆VJ (mV) = -0.31∆J for J = 40 to 100 mA/cm2 operating in air (4-9) 

where J is in mA/cm2

. The performance of a single cell with supported noble metal electrocatalyst (0.5 mg Pt-Rh/cm2

 anode, 0.5 mg Pt/cm2 cathode) in 12N KOH at 65 oC is shown in 

Figure 4-9 (21). These results, reported in 1986, are comparable to those obtained in 1965. The 

iR-free electrode potentials (vs. RHE) at 100 mA/cm2

 in Figure 4-9 are 0.9 V with O2 and 0.85 V 

with air. One major difference between the early cathodes and the cathodes in current use is that 

the limiting current for O2 reduction from air has been improved (i.e., 100 to 200 mA/cm2

improved to >250 mA/cm2

). 

These results yield the following equations for cells operating in 12N KOH at 65 o

C: 

∆VJ (mV) = -0.25∆J for J = 50 to 200 mA/cm2 operating in O2 (4-10) 

or 

∆VJ (mV) = -0.47∆J for J = 50 to 200 mA/cm2 operating in air. (4-11) 
4-14 

Figure 4-9 iR Free Electrode Performance with O2 and Air in 12N KOH at 65 °C. 

Catalyzed (0.5 mg Pt/cm2 Cathode, 0.5 mg Pt-Rh/cm2 Anode), Carbon-based Porous 

Electrodes (21). 

4.2.5 Effects of Cell Life 

The UTC Fuel Cells H2/O2 alkaline technology exhibits a degradation of ~25 mV/1,000 hours 

(13). AFC cell stacks have demonstrated sufficiently stable operation for at least 5,000 hours, 

with degradation rates of 20 mV per 1,000 hours or less (24). Siemens reported a total of >8,000 

operating hours with approximately 20 units (27). For large scale utility applications, economics 

demand operating times exceeding 40,000 hours, which presents perhaps the most significant 

obstacle to commercialization of AFC devices for stationary electric power generation. 

4.3 Summary of Equations for AFC 

The preceding sections described parametric performance based on various referenced data at 

different cell conditions. The following set of equations can be used to predict performance only 

if no better data or basis for estimate is available. Unfortunately, a noticeable lack of recent, 

published H2/air data is available to predict performance trends. The equations presented below 

can be used in conjunction with the measured H2/air performance shown in Figure 4-10 (12) as a 

basis for predicting performance at various operating conditions. The Space Shuttle Orbiter 

performance is included in Figure 4-10 as a reference point for H2/O2 performance (8); however, 

the trend equations should not be used for H2/O2 cells to predict operation at other conditions. 
4-15 

Parameter Equation Comments

Pressure ∆VP (mV) = 0.15 T (o

K) log (P2/P1) 1 atm < P < 100 atm (4-4) 

 100 o

C < T < 300 °C 

Temperature ∆VT (mV) = 4.0 (T2-T1) for T < 63 °C, at 100 mA/cm2

 (4-5) 

∆VT (mV) = 0.7 (T2-T1) for T > 63 °C, at 100 mA/cm2 (4-6) 

Current Density ∆VJ (mV) = -0.18∆J for J = 40 to 100 mA/cm2

 operating in O2 (4-8) 

 with 9N KOH at 55-60 °C. 

 

∆VJ (mV) = -0.31∆J for J = 40 ti 100 mA/cm2 operating in air (4-9) 

 with 9N KOH at 55-60 °C. 

∆VJ (mV) = -0.25∆J for J = 50 to 200 mA/cm2 operating in O2 (4-10) 

 with 12N KOH at 65 °C. 

∆VJ (mV) = -0.047∆J for J = 50 to 200 mA/cm2 operating in air (4-11) 

 with 12N KOH at 65 °C. 

Life Effects ∆VLifetime (mV) = 20 μV per 1,000 hours or less (4-12) 

0.6

0.7

0.8

0.9

1

1.1

0 100 200 300 400 500 600 700

Current Density (mA/cm2

)

Cell Voltage (V)

2002 (H2/O2, 95oC, 4 atm., immobilized, 35% KOH/65% H2O)

2001 (H2/air, 75oC, 1 atm.

circulating, 12N KOH, 10%

Figure 4-10 Reference for Alkaline Cell Performance
4-16 

4.4 References 

1. F.T. Bacon, Electrochim. Acta, 14, 569, 1969. 

2. J.O’M Bockris and A.J. Appleby, Energy, 11, 95, 1986. 

3. A.J. Appleby, Energy, 11, 13, 1986. 

4. K.F. Blurton and E. McMullin, Energy Conversion, 9, 141, 1969. 

5. K.V. Kordesch, Brennstoffbatterien, Springer-Verlag, New York, 1984. 

6. V. Hacker, P. Enzinger, M. Muhr, K. Kordesh, J. Gsellman, M. Cifrain, P. Prenninger, K. 

Meitz, R. Aronsson, “Advantages of Alkaline Fuel Cell Systems for Mobile Applications,” 

2000 Fuel Cell Seminar Program and Abstracts, Portland, OR; sponsored by the Fuel Cell 

Seminar Organizing Committee, October 30 – November 2, 2000. 

7. G.F. McLean, T. Niet, S. Prince-Richard, N.Djilali, “An assessment of alkaline fuel cell 

technology,” University of Victoria, in International Journal of Hydrogen Energy, p. 507 –

526, 27 (2002). 

8. Communication with UTC Fuel Cells, July, 2002. 

9. K. V. Kordesch, G. Simader, “Fuel Cells and Their Applications,“ Wiley, Weinheim – New 

York – Tokyo 1996. 

10. D.W. Sheibley and R.A. Martin, Prog. Batteries Solar Cells, 6, 155, 1987. 

11. K. Strasser, J. Power Sources 29, 152-153, 1990. 

12. M. Cifrain, K. Kordesch, J. Gsellmann, T. Hejze, R. R. Aronson, “Alkaline Fuel Cells with 

Circulating Electrolytes,” Poster presented at Fuel Cell Seminar 2000, Portland, Oregon, 

October 30-November 2, 2000. 

13. Meeting with UTC Fuel Cells, June 2002. 

14. K. Strasser, J. Electrochem. Soc. 127, 2173-2177, 1980. 

15. H. van den Broeck, G. van Bogaert, G. Vennekens, L. Vermeeren, F. Vlasselaer, J. 

Lichtenberg, W. Schlösser, A. Blanchart, Proc. 22nd IECEC Meeting, 1005, Philadelphia, 

1987. 

16. J. Huff, paper presented at the 1986 "Status of Fuel Cell Technologies," Fuel Cell Seminar 

Abstracts, Fuel Cell Seminar, Tucson, AZ, October 26-29, 1986. 

17. A.M. Adams, F.T. Bacon and R.G.H. Watson, Fuel Cells (W. Mitchell, ed.), Academic 

Press, New York, 138,1963. 

18. S.S. Penner, ed., Assessment of Research Needs for Advanced Fuel Cells, DOE/ER/300.60-

T1, US DOE, 1985. 

19. Fuel Cell Seminar Abstracts, Long Beach, CA; sponsored by the National Fuel Cell 

Coordinating Group, October 23-26, 1988. 

20. J. Larminie, A. Dicks, "Fuel Cell Systems Explained," Wiley, Chichester (GB) 2000. 

21. K. Tomantschger, F. McClusky, L. Oporto, A. Reid and K. Kordesch, J. Power Sources, 18, 

317, 1986. 

22. M.B. Clark, W.G. Darland and K.V. Kordesch, Electrochem. Tech., 3, 166, 1965. 

23. J. McBreen, G. Kissel, K.V. Kordesch, F. Kulesa, E.J. Taylor, E. Gannon and S. Srinivasan, 

in Proceedings of the 15th Intersociety Energy Conversion Engineering Conference, Volume 

2, American Institute of Aeronautics and Astronautics, New York, NY, 886, 1980. 

24. J.M.J. Blomen and M.N. Mugerwa ed., Fuel Cell Systems, Plenum Press, New York and 

London, 251, 1993. 

25. K. Kordesch, J. Gsellmann and B. Kraetschmer, in Power Sources, 9, Edited by J. 

Thompson, Academic Press, New York, NY, 379, 1983. 
4-17 

26. E. Guelzow, M. Schulze, “Long Time Operation of AFC Electrodes with CO2 Containing 

Gases,” 8th Ulm ElectroChemical Talks, Neu-Ulm, Germany, Abstracts p. 68, June 20-21, 

2002. 

27. K. Strasser, L. Blume and W. Stuhler, Fuel Cell Seminar Program and Abstracts, Long 

Beach, CA; sponsored by the National Fuel Cell Coordinating Group, October 23-26, 1988. 
5-1 

5. PHOSPHORIC ACID FUEL CELL

The phosphoric acid fuel cell (PAFC) was the first fuel cell technology to be commercialized. The 

number of units built exceeds any other fuel cell technology, with over 85 MW of demonstrators 

that have been tested, are being tested, or are being fabricated worldwide. Most of the plants are in 

the 50 to 200 kW capacity range, but large plants of 1 MW and 5 MW have been built. The largest 

plant operated to date achieved 11 MW of grid quality ac power (1, 2). Major efforts in the U.S. 

are concentrated on the improvement of PAFCs for stationary, dispersed power plants and on-site 

cogeneration power plants. The major industrial participants are UTC Fuel Cells in the U.S. and 

Fuji Electric Corporation, Toshiba Corporation, and Mitsubishi Electric Corporation in Japan. 

Figure 5-1 depicts the operating configuration of the phosphoric acid cell. The electrochemical 

reactions occurring in PAFCs are 

H 2H + 2e + 2 → − (5-1)

at the anode, and 

1

2 O2 + 2H + 2e → H2O + − (5-2)

at the cathode. The overall cell reaction is 

1

2O2 + H2 → H2O (5-3)

The electrochemical reactions occur on highly dispersed electro-catalyst particles supported on 

carbon black. Platinum (Pt) or Pt alloys are used as the catalyst at both electrodes. 
5-2 

Hydrogen

Flow

Field

Air (oxygen)

Flow Field

Water

 and Air

Cathode

Current

Collector

Anode

Current

Collector

Anode

Backing MEA

Hydrogen

Outlet

Cathode

Backing

eeElectrons Protons

Figure 5-1 Principles of Operation of Phosphoric Acid Fuel Cell 

(Courtesy of UTC Fuel Cells) 

5.1 Cell Components 

5.1.1 State-of-the-Art Components 

There have been only minor changes in cell design in recent years. The major U.S. manufacturer, 

UTC Fuel Cells, has concentrated on improving cell stability and life, and in improving the 

reliability of system components at reduced cost. 

The evolution of cell components from 1965 to the present day for PAFCs is summarized in 

Table 5-1. In the mid-1960s, the conventional porous electrodes were polytetrafluoroethylene 

(PTFE) - bonded Pt black, and the loadings were about 9 mg Pt/cm2

. During the past two 

decades, Pt supported on carbon black has replaced Pt black in porous PTFE-bonded electrode 

structures as the electro-catalyst. A dramatic reduction in Pt loading has also occurred; the 

loadings13 are currently about 0.10 mg Pt/cm2

 in the anode and about 0.50 mg Pt/cm2 in the 

cathode. 

The operating temperatures and acid concentrations of PAFCs have increased to achieve higher 

cell performance; temperatures of about 200 °C (392 °F) and acid concentrations of 100 percent 

H3PO4 are commonly used today. Although the present practice is to operate at atmospheric 

pressure, the operating pressure of PAFCs surpassed 8 atm in the 11 MW electric utility 

demonstration plant, confirming an increase in power plant efficiency. However, a number of 

 

13. Assuming a cell voltage of 750 mV at 205 mA/cm2

 (approximate 11 MW design, 8 atmospheres) and the current Pt 

loadings at the anode and cathode, ~54 g Pt is required per kilowatt of power generated. 
5-3 

issues remain whether to design and operate PAFC units at atmospheric vs. pressurized 

conditions. 

Primarily, small, multi-kW PAFC power units that were the focus of initial commercial 

applications led to atmospheric pressure operation. Although pressurization increased efficiency 

(lower fuel cost), it complicated the power unit - resulting in higher capital cost. The economic 

trade-off favored simpler, atmospheric operation for early commercial units. 

Another important issue, independent of power unit size, is that pressure promotes corrosion. 

Phosphoric acid electrolyte (H3PO4) produces a vapor. This vapor, which forms over the 

electrolyte, is corrosive to cell locations other than the active cell area. These cell locations are 

at a mixed voltage (open circuit and cell voltage), that can be over ~0.8V/cell. That is the limit 

above which corrosion occurs (active area limited to operation under ~0.8 V/cell). An increase 

in cell total pressure causes the partial pressure of the H3PO4 vapor to increase, causing increased 

corrosion in the cell. Cell temperature must also be increased with pressurized conditions to 

produce steam for the steam reformer (3). 

A major breakthrough in PAFC technology that occurred in the late 1960s was the development of 

carbon black and graphite for cell construction materials; this and other developments are reviewed 

by Appleby (4) and Kordesch (5). Carbon black and graphite were sufficiently stable to replace 

the more expensive gold-plated tantalum cell hardware used at the time. The use of high-surfacearea graphite to support Pt permitted a dramatic reduction in Pt loading without sacrificing 

electrode performance. It was reported (4) that "without graphite, a reasonably inexpensive acid 

fuel cell would be impossible, since no other material combines the necessary properties of 

electronic conductivity, good corrosion resistance, low density, surface properties (especially in 

high area form) and, above all, low cost." However, carbon corrosion and Pt dissolution become 

an issue at cell voltages above ~0.8 V. Consequently, low current densities at cell voltage above 

0.8 V and hot idle at open circuit potential should be avoided. 

 

The porous electrodes used in PAFCs have been described extensively in patent literature (6); see 

also the review by Kordesch (5). These electrodes contain a mixture of electro-catalyst supported 

on carbon black and a polymeric binder, usually PTFE (30 to 50 wt percent). The PTFE binds the 

carbon black particles together to form an integral, but porous, structure that is supported on a 

porous graphite substrate. The graphite structure serves as a support for the electro-catalyst layer, 

as well as the current collector. A typical graphite structure used in PAFCs has an initial porosity 

of about 90 percent, which is reduced to about 60 percent by impregnation with 40 wt percent 

PTFE. This wet-proof graphite structure contains macropores of 3 to 50 μm diameter (median 

pore diameter of about 12.5 μm) and micropores with a median pore diameter of about 34 Å for 

gas permeability. The composite structure, consisting of a carbon black/PTFE layer on the 

graphite substrate, forms a stable, three-phase interface in the fuel cell, with H3PO4 electrolyte on 

one side (electro-catalyst side) and the reactant gas environment on the other. 
5-4 

Table 5-1 Evolution of Cell Component Technology for Phosphoric Acid Fuel Cells 

Component ca. 1965 ca. 1975 Current Statusa

Anode PTFE-bonded Pt black PTFE-bonded Pt/C PTFE-bonded Pt/C 

 Vulcan XC-72a 

9 mg/cm2 0.25 mg Pt/cm2 0.25 mg Pt/cm2

Cathode PTFE-bonded Pt black PTFE-bonded Pt/C PTFE-bonded Pt/C 

 Vulcan XC-72a 

9 mg/cm2 0.5 mg Pt/cm2 0.5 mg Pt/cm2

Electrode 

Support 

Ta mesh screen Graphite Structure Graphite Structure 

Electrolyte 

Support 

Glass fiber paper PTFE-bonded SiC PTFE-bonded SiC 

Electrolyte 85 percent H3PO4 95 percent H3PO4 100 percent H3PO4 

Electrolyte 

Reservoir 

 Porous graphite plate. Porous graphite plate.

Cooler 1 per ~7 cells; 

imbedded (SS) tubes 

in graphite plate 

a. - Over 40,000 hour component life demonstrated in commercial power plants. 

A bipolar plate separates the individual cells and electrically connects them in series in a fuel cell 

stack. In some designs, the bipolar plate also contains gas channels that feed the reactant gases to 

the porous electrodes and remove the reaction products and inerts. Bipolar plates made from 

graphite resin mixtures that are carbonized at low temperature (~900 °C/1,652 °F) are not suitable 

because of their rapid degradation in PAFC operating environments (7, 8). However, corrosion 

stability is improved by heat treatment to 2,700 °C (4,892 °F) (8), i.e., the corrosion current is 

reduced by two orders of magnitude at 0.8 V in 97 percent H3PO4 at 190°C (374 °F) and 4.8 atm 

(70.5 psi). The all-graphite bipolar plates are sufficiently corrosion-resistant for a projected life of 

40,000 hours in PAFCs, but they are still relatively costly to produce. 

Several designs for the bipolar plate and ancillary stack components are used by fuel cell 

developers, and these are described in detail (9, 10, 11, 12). A typical PAFC stack contains cells 

connected in series to obtain the practical voltage level desired for the load. In such an 

arrangement, individual cells are stacked with bipolar plates between the cells. The bipolar plates 

used in early PAFCs consisted of a single piece of graphite with gas channels machined on either 

side to direct the flow of fuel and oxidant. Currently, both bipolar plates of the previous design 

and new designs consisting of several components are being considered. In the multi-component 

bipolar plates, a thin impervious plate separates the reactant gases in adjacent cells in the stack, and 
5-5 

separate porous plates with ribbed channels are used to direct gas flow. In a cell stack, the 

impervious plate is subdivided into two parts, and each joins one of the porous plates. The 

electrolyte vaporizes so that a portion of H3PO4 escapes from the cell in the air stream over time. 

An electrolyte reservoir plate (ERP), made of porous graphite, provides enough electrolyte to 

achieve a 40,000-hour cell life goal (there is no electrolyte replacement). The ERP also 

accommodates increases in electrolyte volume due to an increase in H2O, so the porous graphite 

electrodes don’t flood. These fluctuations in electrolyte volume occur during start-up and during 

transient operation. The porous structure, which allows rapid gas transport, is also used to store 

additional acid to replenish the supply lost by evaporation during the cell operating life. 

In PAFC stacks, provisions must be included to remove heat generated during cell operation. In 

practice, heat has been removed by either liquid (two-phase water or a dielectric fluid) or gas (air) 

coolants that are routed through cooling channels located (usually about every fifth cell) in the cell 

stack. Liquid cooling requires complex manifolds and connections, but better heat removal is 

achieved than with air-cooling. The advantage of gas cooling is its simplicity, reliability, and 

relatively low cost. However, the size of the cell is limited, and the air-cooling passages must be 

much larger than the liquid- cooling passages. 

Improvements in state-of-the-art phosphoric acid cells are illustrated by Figure 5-2. Performance 

by the ~1 m2

 (10 ft2) short stack, (f), results in a power density of nearly 0.31 W/cm2. 
5-6 

Figure 5-2 Improvement in the Performance of H2-Rich Fuel/Air PAFCs 

a - 1977: 190 °C, 3 atm, Pt loading of 0.75 mg/cm2 on each electrode (13) 

b - 1981: 190 °C, 3.4 atm, cathode Pt loading of 0.5 mg/cm2

 (14) 

c - 1981: 205 °C, 6.3 atm, cathode Pt loading of 0.5 mg/cm2

 (14) 

d - 1984: 205 °C, 8 atm, electro-catalyst loading was not specified (15) 

e - 1992: 205 °C, 8 atm, 10 ft2

 short stack, 200 hrs, electro-catalyst loading not specified (16) 

f - 1992: 205 °C, 8 atm, subscale cells, electro-catalyst loading not specified (16) 

5.1.2 Development Components 

Phosphoric acid electrode/electrolyte technology has reached a level of maturity at which 

developers commit resources for commercial capacity, multi-unit demonstrations and preprototype installations. UTC Fuel Cells has 25 (200 kW) atmospheric pressure power plants that 

have operated between 30,000 to 40,000 hours. Most cell parts are graphite, and there has been no 

electrolyte replacement over the cell life of 40,000 hours. Grid-independent units undergo 

extensive cycling. Cell components are manufactured at scale and in large quantities, 

demonstrating confidence that predicted performance will be met (3). However, further increases 

in power density and reduced cost are needed to achieve economic competitiveness with other 

energy technologies, as expressed in the early 1990s (17, 18). Fuel cell developers continue to 

address these issues. 

In 1992, UTC Fuel Cells' predecessor, International Fuel Cells, completed a governmentsponsored, advanced water-cooled PAFC development project to improve the performance and 

reduce the cost of both its atmospheric and pressurized technology for both on-site and utility 
5-7 

applications (16). The project focused on five major activities: 1) produce a conceptual design of 

a large stack with a goal of 175 W/ft2

 (0.188 W/cm2), 40,000 hour useful life, and a stack cost of 

less than $400/kW; 2) test pressurized Configuration "B" single cells developed in a previous 

program, but improved with proprietary design advances in substrates, electrolyte reservoir plates, 

catalysts, seals, and electrolyte matrix to demonstrate the 175 W/ft2

 (0.188 W/cm2) power density 

goal; 3) test a pressurized short stack with subscale size, improved component cells, and additional 

improvements in the integral separators and coolers to confirm the stack design; 4) test a 

pressurized short stack of improved full-size cell components, nominal 10 ft2

 size (approximately 

1 m2

), to demonstrate the 175 W/ft2 (0.188 W/cm2) power density goal, and 5) test an advanced 

atmospheric "on-site" power unit stack with the improved components. 

A conceptual design of an improved technology stack, operating at 120 psi (8.2 atm) and 405 °F 

(207 °C), was produced based on cell and stack development tests. The stack was designed for 

355 10 ft2

 (approximately 1 m2) cells to produce over 1 MW dc power in the same physical 

envelope as the 670 kW stack used in the 11 MW PAFC plant built for Tokyo Electric Power. The 

improvements made to the design were tested in single cells and in subscale and full size short 

stacks. 

Table 5-2 summarizes the results. Single cells achieved an initial performance of 0.75 volts/cell 

at a current density of 400 A/ft2

 (431 mA/cm2) at 8.2 atm and 207 °C. The power density, 

300 W/ft2

 (0.323 W/cm2), was well above the project goal. Several cells were operated to 

600 A/ft2

 (645 mA/cm2), achieving up to 0.66 volts/cell. The flat plate component designs were 

verified in a subscale stack prior to fabricating the full size short stack. The pressurized short 

stack of 10 ft2

 cells achieved a performance of 285 W/ft2 (0.307 W/cm2). Although the average 

cell performance, 0.71 volts/cell at 400 A/ft2 (431 mA/cm2), was not as high as the single cell 

tests, the performance was 65 percent higher than the project goal. Figure 5-3 presents single 

cell and stack performance data for pressurized operation. The stack was tested for over 

3,000 hours. For reference purposes, Tokyo Electric Power Company's 11 MW power plant, 

operational in 1991, had an average cell performance of approximately 0.75 volts/cell at 

190 mA/cm2

 or 0.142 W/cm2 (19). 
5-8 

Table 5-2 Advanced PAFC Performance 

 Average Cell 

Voltage, V 

Current Density 

mA/cm2

Power Density 

W/cm2

IFC Pressurized: 

 Project Goal 

 Single Cells 

 Full Size Short Stack 

 11 MW Reference 

0.75 to 0.66 

0.71 

0.75 

431 to 645 

431 

190 

0.188 

0.323 

0.307 

0.142 

IFC Atmospheric: 

 Single Cells 

 Full Size Short Stack 

0.75 

0.65 

242 

215 

0.182 

0.139 

Mitsubishi Electric Atmospheric 

 Single Cells 0.65 300 0.195 

Figure 5-3 Advanced Water-Cooled PAFC Performance (16) 

The atmospheric pressure short stack, consisting of 32 cells, obtained an initial performance of 

0.65 volts/cell at 200 A/ft2

 (215 mA/cm2) or 0.139 W/cm2. The performance degradation rate was 

less than 4 mV/1,000 hours during the 4,500 hour test. Single cells, tested at atmospheric 

conditions, achieved a 500 hour performance of approximately 0.75 volts/cell at 225 A/ft2

(242 mA/cm2

) or 0.182 W/cm2. 

Mitsubishi Electric Corporation investigated alloyed catalysts, processes to produce thinner 

electrolytes, and increased utilization of the catalyst layer (20). These improvements resulted in an 

initial atmospheric performance of 0.65 mV at 300 mA/cm2

 or 0.195 W/cm2, which was higher 

than the UTC Fuel Cells' performance mentioned above (presented in Table 5-2 for comparison). 

Note that this performance was obtained using small 100 cm2

 cells and may not yet have been 
5-9 

demonstrated with full-scale cells in stacks. Approaches to increase life are to use series fuel gas 

flow in the stack to alleviate corrosion, provide well-balanced micropore size reservoirs to avoid 

electrolyte flooding, and use a high corrosion resistant carbon support for the cathode catalyst. 

These improvements resulted in the lowest PAFC degradation rate publicly acknowledged: 

2 mV/1,000 hours for 10,000 hours at 200 to 250 mA/cm2

 in a short stack with 3,600 cm2 area 

cells. UTC Fuel Cells reported a similar degradation rate in 2002 for power units operating up to 

40,000 hours (3). 

Several important technology development efforts for which details have been published include 

catalyst improvements, advanced gas diffusion electrode development, and tests on materials that 

offer better carbon corrosion protection. Transition metal (e.g., iron, cobalt) organic macrocycles14

from the families of tetramethoxyphenylporphyrins (TMPP), phthalocyanines (PC), 

tetraazaannulenes (TAA) and tetraphenylporphyrins (TPP) have been evaluated as O2-reduction 

electro-catalysts in PAFCs. One major problem with these organic macrocycles is their limited 

chemical stability in hot concentrated phosphoric acid. However, after heat treatment of the 

organic macrocycle (i.e., CoTAA, CoPC, CoTMPP, FePC, FeTMPP) on carbon at about 500 to 

800 °C (932 to1,472 °F), the pyrolyzed residue exhibits electro-catalytic activity that, in some 

instances, is comparable to that of Pt and has promising stability, at least up to about 100 °C/212 

°F (21). Another successful approach for enhancing the electro-catalysis of O2 reduction is to alloy 

Pt with transition metals such as Ti (22), Cr (23), V (24), Zr, and Ta (24). The enhancement in 

electro-catalytic activity has been explained by a correlation between the optimum 

nearest-neighbor distance of the elements in the alloy and the bond length in O2 (25). 

Conventional cathode catalysts comprise either platinum or platinum alloys supported on 

conducting carbon black at 10 wt percent platinum. Present platinum loadings on the anode and 

cathode are 0.1 mg/cm2

 and 0.5 mg/cm2, respectively (12, 16). It has been suggested by Ito, et al., 

that the amount of platinum may have been reduced to the extent that it might be cost effective to 

increase the amount of platinum loading on the cathode (26). However, a problem exists in that 

fuel cell stack developers have not experienced satisfactory performance improvements when 

increasing the platinum loading. Johnson Matthey Technology Centre (J-M) presented data that 

resulted in improved performance nearly in direct proportion to that expected based on the increase 

in platinum (27). Initial tests by J-M confirmed previous results - that using platinum alloy catalyst 

with a 10 wt percent net platinum loading improves performance. Platinum/nickel alloy catalysts 

yielded a 49 wt percent increase in specific activity over pure platinum. This translated into a 

39 mV improvement in the air electrode performance at 200 mA/cm2

. 

Johnson Matthey then determined that the platinum loading in the alloyed catalyst could be 

increased up to 30 wt percent while retaining the same amount of platinum without any decrease in 

specific activity or performance; the amount of nickel, hence the total amount of alloyed catalyst, 

decreased. Next, J-M researchers increased the amount of platinum from 10 to 30 wt percent while 

keeping the same nickel catalyst loading. The total amount of alloyed catalyst increased in this 

case. Results showed an additional 36 wt percent increase in specific activity, which provided 

another 41 mV increase at 200 mA/cm2

. The ideal voltage increase would have been 46 mV for 

this increase in platinum. Thus, the performance increase obtained experimentally was nearly in 

 

14. See Reference 21 for literature survey. 
5-10 

direct proportion to the theoretical amount expected. The type of carbon support did not seem to 

be a major factor, based on using several typical supports during the tests. 

The anode of a phosphoric acid fuel cell is subject to a reduction in performance when even low 

amounts of contaminants are preferentially absorbed on the noble catalysts. Yet, hydrogen-rich 

fuel gases, other than pure hydrogen, are produced with contaminant levels well in excess of the 

anode's tolerance limit. Of particular concern are CO, COS, and H2S in the fuel gas. The fuel 

stream in a state-of-the-art PAFC anode, operating at approximately 200 °C (392 °F), must contain 

1 vol percent or less of CO (12), less than 50 ppmv of COS plus H2S, and less than 20 ppmv of 

H2S (28). Current practice is to place COS and H2S cleanup systems and CO shift converters prior 

to the cell (normally in the fuel processor before reforming) to reduce the fuel stream contaminant 

levels to the required amounts. Giner, Inc. performed experiments to develop a contaminanttolerant anode catalyst in order to reduce or eliminate the cleanup equipment (29). An anode 

catalyst, G87A-17-2, was identified that resulted in only a 24 mV loss from reference when 

exposed to a 75 percent H2, 1 percent CO, 24 percent CO2, 80 ppm H2S gas mixture at 190 °C (374 

°F), 85 percent fuel utilization, and 200 mA/cm2. A baseline anode experienced a 36 mV loss from 

the reference at the same conditions. At 9.2 atm (120 psi) pressure, the anode loss was only 19 mV 

at 190 °C (374 °F) and 17 mV at 210 °C (410 °F) (compared with pure H2) with a gas of 71 

percent H2, 5 percent CO, 24 percent CO2, and 200 ppm H2S. Economic studies comparing the 

tradeoff between decreased cell performance with increased savings in plant cost showed no 

advantage when the new anode catalyst was used with gas containing 1 percent CO/200 ppm H2S. 

A $7/kW increase resulted with the 5 percent CO gas (compared to a 1 percent CO gas) at a 

50 MW size. Some savings would result by eliminating the low temperature shift converter. The 

real value of the catalyst may be its ability to tolerate excessive CO and H2S concentrations during 

fuel processor upsets, and to simplify the system by eliminating equipment. 

As previously mentioned, state-of-the-art gas diffusion electrodes are configured to provide an 

electrolyte network and a gas network formed with the mixture of carbon black and PTFE. In the 

electrodes, carbon black agglomerates, consisting of small primary particles 0.02 to 0.04 μm, are 

mixed with much larger PTFE particles of ~0.3 μm. The carbon black surface may not be covered 

completely by the PTFE because of the large size of conventional PTFE particles. The space in the 

agglomerates or the space between the agglomerates and PTFE may act as gas networks at the 

initial stage of operation, but fill with electrolyte eventually because of the small contact angle of 

carbon black, uncovered with PTFE, to electrolyte (<90°), resulting in the degradation of cell 

performance. Attempts to solve this flooding problem by increasing the PTFE content have not 

been successful because of the offset in performance resulting from the reduction of catalyst 

utilization. Higher performance and longer lifetime of electrodes are intrinsically at odds, and 

there is a limit to the improvement in performance over life by optimizing PTFE content in the 

state-of-the-art electrode structures. Watanabe, et al. (30) proposed preparing an electrode utilizing 

100 percent of catalyst clusters, where the functions of gas diffusion electrodes were allotted 

completely to a hydrophilic, catalyzed carbon black and a wet-proofed carbon black. The former 

worked as a fine electrolyte network, and the latter worked as a gas-supplying network in a 

reaction layer. Higher utilization of catalyst clusters and longer life at the reaction layer were 

expected, compared to state-of-the-art electrodes consisting of the uniform mixture of catalyzed 

carbon black and PTFE particles. The iR-free electrode potentials for the reduction of oxygen and 
5-11 

air at 200 mA/cm2

 on the advanced electrode were 10 mV higher than those of the conventional 

electrode. 

There is a trade-off between high power density and cell life performance. One of the major 

causes of declining cell performance over its life is that electrode flooding and drying, caused by 

migration of phosphoric acid between the matrix and the electrodes, occurs during cell load 

cycling. Researchers at Fuji Electric addressed two approaches to improve cell life performance 

while keeping power density high (31). In one, the wettability of the cathode and anode were 

optimized, and in the other a heat treatment was applied to the carbon support for the cathode 

catalyst. During tests, it was observed that a cell with low cathode wettability and high anode 

wettability was more than 50 mV higher than a cell with the reverse wetting conditions after 40 

start/stop cycles. 

The use of carbon black with large surface area to improve platinum dispersion on supports was 

investigated as a method to increase the power density of a cell (32). However, some large surface 

area carbon blacks are fairly corrosive in hot potassium acid, resulting in a loss of catalytic activity. 

The corrosivity of the carbon support affects both the rate of catalyst loss and electrode flooding 

and, in turn, the life performance of a cell. Furnace black has been heat treated at high temperature 

by Fuji Electric to increase its resistance to corrosion. It was found that corrosion could be reduced 

and cell life performance improved by heat treating carbon supports at high temperature, at least to 

around 3,000 °C (5,432 °F). 

More recently, UTC Fuel Cells cites improvements to achieve 40,000 hour cell life through better 

cell temperature control, increasing H3PO4 inventory, and incorporating electrolyte reservoir plates 

in the cell stack (3). 

5.2 Performance 

There have been only minor changes in documented cell performance since the mid-1980s - mostly 

due to the operating conditions of the cell. The changes are reported in performance trends shown 

in this section that were primarily gained from contracts that UTC Fuel Cells had with the 

Department of Energy or outside institutions. New, proprietary PAFC performance data may 

likely have been observed by the manufacturer (3). 

Cell performance for any fuel cell is a function of pressure, temperature, reactant gas composition, 

and fuel utilization. In addition, performance can be adversely affected by impurities in both the 

fuel and oxidant gases.

The sources of polarization in PAFCs (with cathode and anode Pt loadings of 0.5 mg Pt/cm2, 180 

°C, 1 atm, 100 percent H3PO4) were discussed in Section 2 and were illustrated as half cell 

performance in Figure 2-4. From Figure 2-4 it is clear that the major polarization occurs at the 

cathode, and furthermore, the polarization is greater with air (560 mV at 300 mA/cm2

) than with 

pure oxygen (480 mV at 300 mA/cm2

) because of dilution of the reactant. The anode exhibits very 

low polarization (-4 mV/100 mA/cm2

) on pure H2, and increases when CO is present in the fuel 

gas. The ohmic (iR) loss in PAFCs is also relatively small, amounting to about 12 mV at 100 

mA/cm2

. 
5-12 

Typical PAFCs will generally operate in the range of 100 to 400 mA/cm2

 at 600 to 800 mV/cell. 

Voltage and power constraints arise from increased corrosion of platinum and carbon components 

at cell potentials above approximately 800 mV. 

5.2.1 Effect of Pressure 

Even though pressure operation is not being pursued, it is still of interest for possible future 

development. It is well known that an increase in the cell operating pressure enhances the 

performance of PAFCs (11, 33, 34). The theoretical change in voltage (∆VP) as a function of 

pressure (P) is expressed as

2

P

1

(3)(2.3RT) P V (mV) = log 2 P

∆ F (5-4)

where 3(2.3 ) 138

2

RT = mV F at 190°C (374 °F). Experimental data (35) reported that the effect of pressure 

on cell performance at 190°C (374 °F) and 323 mA/cm2

 is correlated by the equation: 

2

P

1

P V (mV) = 146 log 

P

∆ (5-5)

where P1 and P2 are different cell pressures. The experimental data (35) also suggest that 

Equation (5-5) is a reasonable approximation for a temperature range of 177 °C < T < 218 °C (351 

°F < T < 424 °F) and a pressure range of 1 atm < P < 10 atm (14.7 psi < P < 147.0 psi). Data from 

Appleby (14) in Figure 5-2 indicate that the voltage gain observed by increasing the pressure from 

3.4 atm (190 °C) to 6.3 atm (205 °C) is about 44 mV. According to Equation (5-5), the voltage 

gain calculated for this increase in pressure at 190 °C (374 °F) is 39 mV15, which is in reasonable 

agreement with experimental data in Figure 5-2. Measurements (33) of ∆VP for an increase in 

pressure from 4.7 to 9.2 atm (69.1 to 135.2 psia) in a cell at 190 °C (374 °F) show that ∆VP is a 

function of current density, increasing from 35 mV at 100 mA/cm2

 to 42 mV at 400 mA/cm2 (50 

percent O2 utilization with air oxidant, 85 percent H2 utilization with pure H2 fuel). From 

Equation (5-4), ∆Vp is 43 mV for an increase in pressure from 4.7 to 9.2 atm (69.1 to 135.2 psia) at 

190 °C (374 °F), which is very close to the experimental value obtained at 400 mA/cm2

. Other 

measurements (36) for the same increase in pressure from 4.7 to 9.2 atm (69.1 to 135.2 psia), but at 

a temperature of 210 °C (410 °F) show less agreement between the experimental data and 

Equation (5-4). 

The improvement in cell performance at higher pressure and high current density can be attributed 

to a lower diffusion polarization at the cathode and an increase in the reversible cell potential. In 

addition, pressurization decreases activation polarization at the cathode because of the increased 

oxygen and water partial pressures. If the partial pressure of water is allowed to increase, a lower 

acid concentration will result. This will increase ionic conductivity and bring about a higher 

exchange current density. The net outcome is a reduction in ohmic losses. It was reported (33) 

that an increase in cell pressure (100 percent H3PO4, 169 °C (336 °F)) from 1 to 4.4 atm (14.7 to 

 

15. The difference in temperature between 190 and 205 °C is disregarded so Equation (5-5) is assumed to be valid

 at both temperatures. 
5-13 

64.7 psia) produces a reduction in acid concentration to 97 percent, and a decrease of about 

0.001 ohm in the resistance of a small six cell stack (350 cm2

 electrode area). 

5.2.2 Effect of Temperature 

Figure 2-1 shows that the reversible cell potential for PAFCs consuming H2 and O2 decreases as 

the temperature increases by 0.27 mV/°C under standard conditions (product is water vapor). 

However, as discussed in Section 2, an increase in temperature has a beneficial effect on cell 

performance because activation polarization, mass transfer polarization, and ohmic losses are 

reduced. 

The kinetics for the reduction of oxygen on Pt improves16 as the cell temperature increases. At a 

mid-range operating load (~250 mA/cm2

), the voltage gain (∆VT) with increasing temperature of 

pure H2 and air is correlated by 

∆VT (mV) = 1.15 (T2 - T1) (°C) (5-6)

Data suggest that Equation (5-6) is reasonably valid for a temperature range of 180 °C < T <

250 °C (356 °F < T < 482 °F). It is apparent from this equation that each degree increase in cell 

temperature should increase performance by 1.15 mV. Other data indicate that the coefficient for 

Equation (5-6) may be in the range of 0.55 to 0.75, rather than 1.15. Although temperature has 

only a minimal effect on the H2 oxidation reaction at the anode, it is important in terms of the 

amount of CO that can be absorbed by the anode. Figure 5-4 shows that increasing the cell 

temperature results in increased anode tolerance to CO absorption. A strong temperature effect 

was also observed using simulated coal gas. Below 200 °C (392 °F), the cell voltage drop was 

significant. Experimental data suggest that the effect of contaminants is not additive, indicating 

that there is an interaction between CO and H2S (37). Increasing temperature increases 

performance, but elevated temperature also increases catalyst sintering, component corrosion, 

electrolyte degradation, and evaporation. UTC Fuel Cells operates its phosphoric acid cells at 

207 °C (405 °F), which is a compromise that allows reasonable performance at a life of 40,000 

hours (3). 

 

16. The anode shows no significant performance improvement from 140 to 180° on pure H2, but in the presence of CO, 

increasing the temperature results in a marked improvement in performance (see discussion in Section 5.2.4). 
5-14 

Figure 5-4 Effect of Temperature: Ultra-High Surface Area Pt Catalyst. Fuel: H2, H2 + 

200 ppm H2S and Simulated Coal Gas (37) 

5.2.3 Effect of Reactant Gas Composition and Utilization 

Increasing reactant gas utilization or decreasing inlet concentration results in decreased cell 

performance due to increased concentration polarization and Nernst losses. These effects are 

related to the partial pressures of reactant gases and are discussed below. 

Oxidant: The oxidant composition and utilization are parameters that affect the cathode 

performance, as evident in Figure 2-5. Air, which contains ~21 percent O2, is the obvious oxidant 

for terrestrial application PAFCs. The use of air with ~21 percent O2 instead of pure O2 results in a 

decrease in the current density of about a factor of three at constant electrode potential. The 

polarization at the cathode increases with an increase in O2 utilization. Experimental 

measurements (38) of the change in overpotential (∆ηc) at a PTFE-bonded porous electrode in 100 

percent H3PO4 (191 °C, atmospheric pressure) as a function of O2 utilization is plotted in Figure 5-

5 in accordance with Equation (5-7): 

∆ηc = ηc - ηc,∞ (5-7)

where ηc and ηc,∞ are the cathode polarizations at finite and infinite (i.e., high flow rate, close to 0 

percent utilization) flow rates, respectively. The additional polarization attributed to O2 utilization 

is reflected in the results, and the magnitude of this loss increases rapidly as the utilization 

increases. At a nominal O2 utilization of 50 percent for prototype PAFC power plants, the 

additional polarization estimated from the results in Figure 5-5 is 19 mV. Based on experimental 

data (16, 38, 39), the voltage loss due to a change in oxidant utilization can be described by 

Equations (5-8) and (5-9): 
5-15 

Figure 5-5 Polarization at Cathode (0.52 mg Pt/cm2) as a Function of O2 Utilization, which 

is Increased by Decreasing the Flow Rate of the Oxidant at Atmospheric Pressure 100 

percent H3PO4, 191°C, 300 mA/cm2, 1 atm. (38) 

0.20

P

P 0.04 (P )

(P )

V (mV) =148 log

Total

0

0 1

0 2

Cathode

2

2

2

∆ ≤ ≤ (5-8)

 <1.00

P

P 0.20 < (P )

(P )

V (mV) = 96 log

Total

0

0 1

0 2

Cathode 2

2

2 ∆ (5-9)

where Ρ O2 is the average partial pressure of O2. The use of two equations over the concentration 

range more accurately correlates actual fuel cell operation. Equation (5-8) will generally apply to 

fuel cells using air as the oxidant and Equation (5-9) for fuel cells using an O2-enriched oxidant. 

Fuel: Hydrogen for PAFC power plants will typically be produced from conversion of a wide 

variety of primary fuels such as CH4 (e.g., natural gas), petroleum products (e.g., naphtha), coal 

liquids (e.g., CH3OH), or coal gases. Besides H2, CO and CO2 are also produced during 

conversion of these fuels (unreacted hydrocarbons are also present). These reformed fuels contain 

low levels of CO (after steam reforming and shift conversion reactions in the fuel processor) that 

cause anode CO absorption in PAFCs. The CO2 and unreacted hydrocarbons (e.g., CH4) are 

electrochemically inert and act as diluents. Because the anode reaction is nearly reversible, the fuel 

composition and hydrogen utilization generally do not strongly influence cell performance. The 

voltage change due to a change in the partial pressure of hydrogen (which can result from a change 

in either the fuel composition or utilization) can be described by Equation (5-10) (16, 36, 37): 
5-16 

(P )

(P )

V (mV) = 55 log

H 1

H 2

Anode

2

2 ∆ (5-10)

where H2 P is the average partial pressure of H2. At 190 °C (374 °F), the presence of 10 percent 

CO2 in H2 should cause a voltage loss of about 2 mV. Thus, diluents in low concentrations are not 

expected to have a major effect on electrode performance; however, relative to the total anode 

polarization (i.e., 3 mV/100 mA/cm2

), the effects are large. It has been reported (16) that with pure 

H2, the cell voltage at 215 mA/cm2

 remains nearly constant at H2 utilizations up to 90 percent, and 

then it decreases sharply at H2 utilizations above this value. 

Low utilizations, particularly oxygen utilization, yield high performance. Low utilizations, 

however, result in poor fuel use. Optimization of this parameter is required. State-of-the-art 

utilizations are on the order of 85 percent and 50 percent for the fuel and oxidant, respectively. 

5.2.4 Effect of Impurities 

The concentrations of impurities entering the PAFC are very low relative to diluents and reactant 

gases, but their impact on performance is significant. Some impurities (e.g., sulfur compounds) 

originate from fuel gas entering the fuel processor and are carried into the fuel cell with the 

reformed fuel, whereas others (e.g., CO) are produced in the fuel processor. 

Carbon Monoxide: The presence of CO in a H2-rich fuel has a significant effect on anode 

performance because CO affects Pt electrode catalysts. CO absorption is reported to arise from the 

dual site replacement of one H2 molecule by two CO molecules on the Pt surface (40, 41). 

According to this model, the anodic oxidation current at a fixed overpotential, with (iCO) and 

without (iH2) CO present, is given as a function of CO coverage (θCO) by Equation (5-11): 

CO

H

2 i

i = (1 )

2

- θCO (5-11)

For [CO]/[H2] = 0.025, θCO = 0.31 at 190°C (35); therefore, iCO is about 50 percent of iH2. 

Both temperature and CO concentration have a major influence on the oxidation of H2 on Pt in CO 

containing fuel gases. Benjamin, et al. (35) derived Equation (5-12) for the voltage loss resulting 

from CO absorption as a function of temperature 

∆VCO = k(T) ([CO]2 - [CO]1) (5-12)

where k(T) is a function of temperature, and [CO]1 and [CO]2 are the mole fractions CO in the fuel 

gas. The values of k(T) at various temperatures are listed in Table 5-3. Using Equation (5-12) and 

the data in Table 5-3, it is apparent that for a given change in CO content, ∆VCO is about 8.5 times 

larger at 163 °C (325 °F) than at 218 °C (424 °F). The correlation provided by Equation (5-12) 

was obtained at 269 mA/cm2

; thus, its use at significantly different current densities may not be 
5-17 

appropriate. In addition, other more recent data (37) suggest a value for k(T) of -2.12 at a 

temperature of 190 °C (374 °F) rather than -3.54. 

Table 5-3 Dependence of k(T) on Temperature 

T T k(T)

a

(°C) (°F) (mV/ percent)

163 325 -11.1

177 351 -6.14

190 374 -3.54

204 399 -2.05

218 424 -1.30

a - Based on electrode with 0.35 mg Pt/cm2, and at 269 mA/cm2 (35) 

The data in Figure 5-6 illustrate the influence of H2 partial pressure and CO content on the 

performance of Pt anodes (10 percent Pt supported on Vulcan XC-72, 0.5 mg Pt/cm2) in 100 

percent H3PO4 at 180 °C (356 °F) (11). Diluting the H2 fuel gas with 30 percent CO2 produces an 

additional polarization of about 11 mV at 300 mA/cm2. The results show that the anode 

polarization with fuel gases of composition 70 percent H2/(30-x) percent CO2/x percent CO (x =0, 

0.3, 1, 3 and 5) increases considerably as the CO content increases to 5 percent. 

Sulfur Containing Compounds: Hydrogen sulfide and carbonyl sulfide (COS) impurities17 in fuel 

gases from fuel processors and coal gasifiers can reduce the effectiveness of fuel cell catalysts. 

Concentrations of these compounds must also be limited in a power plant's fuel processing section, 

because the fuel reformer too has catalysts. As a result, sulfur must be removed prior to fuel 

reforming with the non-sulfur tolerant catalysts now in use in PAFC power plants. It is prudent to 

be concerned about sulfur effects in the cell, however, because the fuel processor catalyst's 

tolerance limits may be less than the fuel cell catalyst's or there could be an upset of the fuel 

processor sulfur guard with sulfur passing through to the cell. The concentration levels of H2S in 

an operating PAFC (190 to 210 °C (374 to 410 °F), 9.2 atm (120 psig), 80 percent H2 utilization, 

<325 mA/cm2

) that can be tolerated by Pt anodes without suffering a destructive loss in 

performance are <50 ppm (H2S + COS) or <20 ppm (H2S) (42). Rapid cell failure occurs with fuel 

gas containing more than 50 ppm H2S. Sulfur does not affect the cathode, and the impact of sulfur 

on the anodes can be re-activated by polarization at high potentials (i.e., operating cathode 

potentials). A synergistic effect between H2S and CO negatively impacts cell performance. Figure 

5-7 (37) shows the effect of H2S concentration on ∆V with and without 10 percent CO present in 

H2. The ∆V is referenced to performance on pure H2 in the case of H2S alone and to performance 

on H2 with 10 percent CO for H2S and CO. In both cases, at higher H2S concentrations, the ∆V 

rises abruptly. This drop in performance occurs above 240 ppm for H2S alone and above 160 ppm 

for H2S with 10 percent CO. 

 

17. Anode gases from coal gasifiers may contain total sulfur of 100 to 200 ppm. 
5-18 

Experimental studies by Chin and Howard (43) indicate that H2S adsorbs on Pt and blocks the 

active sites for H2 oxidation. The following electrochemical reactions, Equations (5-13), (5-14), 

and (5-15) involving H2S are postulated to occur on Pt electrodes: 

Pt + HS-

→ Pt - HSads + e- (5-13)

Pt - H2Sads → Pt - HSads + H+

 + e- (5-14)

Pt – HSads → Pt - Sads + H+

 + e- (5-15)

Figure 5-6 Influence of CO and Fuel Gas Composition on the Performance of Pt Anodes in 

100 percent H3PO4 at 180°C. 10 percent Pt Supported on Vulcan XC-72, 0.5 mg Pt/cm2. 

Dew Point, 57°. Curve 1, 100 percent H2; Curves 2-6, 70 percent H2 and CO2/CO Contents 

(mol percent) Specified (21) 
5-19 

Figure 5-7 Effect of H2S Concentration: Ultra-High Surface Area Pt Catalyst (37) 

Elemental sulfur (in Equation (5-15) is expected on Pt electrodes only at high anodic potentials; at 

sufficiently high potentials, sulfur is oxidized to SO2. The extent of catalyst masking by H2S 

increases with increasing H2S concentration, electrode potential, and exposure time. The effect of 

H2S, however, decreases with increasing cell temperature. 

Other Compounds: The effects of other compounds (such as those containing nitrogen) on PAFC 

performance has been adequately reviewed by Benjamin, et al. (35). Molecular nitrogen acts as a 

diluent but other nitrogen compounds (e.g., NH3, HCN, NOX) may not be as innocuous. NH3 in 

the fuel or oxidant gases reacts with H3PO4 to form a phosphate salt, (NH4)H2PO4, 

H3PO4 + NH3 → (NH4)H2PO4 (5-16)

which decreases the rate of O2 reduction. A concentration of less than 0.2 mol percent 

(NH4)H2PO4 must be maintained to avoid unacceptable performance losses (44). Consequently, 

the amount of molecular nitrogen must be limited to 4 percent because it will react with hydrogen 

to form NH3 (3). The effects of HCN and NOX on fuel cell performance have not been clearly 

established. 

5.2.5 Effects of Current Density 

The voltage that can be obtained from a PAFC is reduced by ohmic, activation, and concentration 

losses that increase with increasing current density. The magnitude of this loss can be 

approximated by the following equations: 

∆VJ (mV) = -0.53 ∆J for J= 100 to 200 mA/cm2 (5-17)

∆VJ (mV) = -0.39 ∆J for J= 200 to 650 mA/cm2 (5-18)
5-20 

The coefficients in these equations were correlated from performance data for cells (45) operating 

at 120 psia (8.2 atm), 405 °F (207 °C) (16) with fuel and oxidant utilizations of 85 percent and 70 

percent, respectively18, an air fed cathode, and an anode inlet composition of 75 percent H2, and 

0.5 percent CO. Similarly, at atmospheric conditions, the magnitude of this loss can be 

approximated by 

∆VJ (mV) = -0.74 ∆J for J= 50 to 120 mA/cm2 (5-19)

∆VJ (mV) = -0.45 ∆J for J= 120 to 215 mA/cm2 (5-20)

The coefficients in the atmospheric condition equations were derived from performance data for 

cells (45) operating at 14.7 psia (1 atm) and 400 °F (204 °C), fuel and oxidant utilizations of 80 

percent and 60 percent, respectively18, an air fed cathode, and an anode inlet composition of 75 

percent H2 and 0.5 percent CO. 

5.2.6 Effects of Cell Life 

One of the primary areas of research is in extending cell life. The goal is to maintain the 

performance of the cell stack during a standard utility application (~40,000 hours). Previous 

state-of-the-art PAFCs (46, 47, 48) showed the following degradation over time: 

∆Vlifetime (mV) = -3 mV/1,000 hours (5-21)

UTC Fuel Cells reports that the efficiency of its latest power plants at the beginning of life is 40 

percent LHV. The infant life loss reduces the efficiency quickly to 38 percent, but then there is a 

small decrease in efficiency over the next 40,000 hours (expected cell life) resulting in an 

average efficiency over life of 37 percent (3). Assuming that the loss in efficiency is due solely 

to cell voltage loss, the maximum degradation rate can be determined as: 

∆Vlifetime (mV) = -2 mV/1,000 hours (5-22)

 

18. Assumes graph operating conditions (not provided) are the same as associated text of Ref.15. 
5-21 

5.3 Summary of Equations for PAFC 

The preceding sections provide parametric performance based on various referenced data at 

differing cell conditions. It is suggested that the following set of equations be used unless the 

reader prefers other data or rationale. Figure 5-8 is provided as reference PAFC performances at 

ambient pressure and 8.2 atm. 

Parameter Equation Comments

Pressure P

P V (mV) =146 log

1

2 ∆ P

1 atm ≤ P ≤ 10 atm 

177 °C ≤ T ≤ 218 °C 

(5-5) 

Temperature ∆VT (mV) = 1.15 (T2 - T1) 180 °C ≤ T ≤ 250 °C (5-6) 

Oxidant ∆ Cathode

0 2

0 1

V (mV) = 148 log 

(P )

(P ) 2

2

0.04 ≤

0

Total

P 2

P

≤ 0.20 

∆ cathode

02 2

02 1

V (mV) = 96 log (P )

(P ) 0.20 ≤

0

Total

P 2

P

 < 1.0 

(5-9) 

Fuel ∆ anode

H2 2

H2 1

V (mV) = 55 log (P )

(P ) (5-10) 

CO 

Absorption 

Impact 

∆VCO (mV) = -11.1 ([CO]2 - [CO]1) 163 °C 

∆VCO (mV) = --6.14 ([CO]2 - [CO]1) 177 °C 

∆VCO (mV) = -3.54 ([CO]2 - [CO]1) 190 °C 

∆VCO (mV) = -2.05 ([CO]2 - [CO]1) 204 °C 

∆VCO (mV) = -1.30 ([CO]2 - [CO]1) 218 °C

(5-12)

Current 

Density 

∆VJ (mV) = -0.53 )∆J for J = 100 to 200 mA/cm2, P = 8.2 atm 

∆VJ (mV) = -0.39 )∆J for J = 200 to 650 mA/cm2, P = 8.2 atm 

∆VJ (mV) = -0.74 )∆J for J = 50 to 120 mA/cm2, P = 1 atm 

∆VJ (mV) = -0.45 )∆J for J = 120 to 215 mA/cm2, P = 1 atm 

(5-17) 

(5-18) 

(5-19) 

(5-20)

Life Effects ∆Vlifetime (mV) = -2mV/1,000 hrs. (5-22) 
5-22 

Cell Voltage

0.90

0.80

0.70

0.60

0.50

0 100 200 300 400 500 600 700 800

8.2 atm 

(120 psia)

1.0 atm 

(14.7 psia)

U = 80% U = 60% f

200 C (390 F) 0

207 C (405 F) 0 0

U = 85% U = 70% f

Current Density (mAmps/cm 2

O

O

 0

Assumed Gas Composition:

 Fuel: 75% H , 0.5% CO, bal. H O & CO 

 Oxidant: Dry Air

2 2 2

Figure 5-8 Reference Performances at 8.2 atm and Ambient Pressure. 

Cells from Full Size Power Plant (16) 

 

5.4 References 

1. J. Hirschenhofer, “Latest Progress in Fuel Cell Technology,” IEEE-Aerospace and 

Electronic Systems Magazine, 7, November 1992. 

2. J. Hirschenhofer, “Status of Fuel Cell Commercialization Efforts,” American Power 

Conference, Chicago, IL, April 1993. 

3. Meeting with UTC Fuel Cells, June 2002. 

4. J. Appleby, in Proceedings of the Workshop on the Electrochemistry of Carbon, Edited by 

S. Sarangapani, J.R. Akridge and B. Schumm, The Electrochemical Society, Inc., 

Pennington, NJ, p. 251, 1984. 

5. K.V. Kordesch, “Survey of Carbon and Its Role in Phosphoric Acid Fuel Cells,” 

BNL 51418, prepared for Brookhaven National Laboratory, December 1979. 

6. K. Kinoshita, Carbon: Electrochemical and Physicochemical Properties, Wiley 

Interscience, New York, NY, 1988. 

7. L. Christner, J. Ahmad, M. Farooque, in Proceedings of the Symposium on Corrosion in 

Batteries and Fuel Cells and Corrosion in Solar Energy Systems, Edited by C. J. Johnson 

and S.L. Pohlman, The Electrochemical Society, Inc., Pennington, NJ, p. 140, 1983. 

8. P.W.T. Lu, L.L. France, in Extended Abstracts, Fall Meeting of The Electrochemical 

Society, Inc., Volume 84-2, Abstract No. 573, The Electrochemical Society, Inc., 

Pennington, NJ, p. 837, 1984. 
5-23 

9. M. Warshay, in The Science and Technology of Coal and Coal Utilization, Edited by 

B.R. Cooper, W.A. Ellingson, Plenum Press, New York, NY, p. 339, 1984. 

10. P.R. Prokopius, M. Warshay, S.N. Simons, R.B. King, in Proceedings of the 14th 

Intersociety Energy Conversion Engineering Conference, Volume 2, American Chemical 

Society, Washington, D. C., p. 538, 1979. 

11. S.N. Simons, R.B. King, P.R. Prokopius, in Symposium Proceedings Fuel Cells Technology 

Status and Applications, Edited by E. H. Camara, Institute of Gas Technology, Chicago, IL, 

p. 45, 1982. 

12. Communications with IFC, September 2000. 

13. A.P. Fickett, in Proceedings of the Symposium on Electrode Materials and Processes for 

Energy Conversion and Storage, Edited by J.D.E. McIntyre, S. Srinivasan and F.G. Will, 

The Electrochemical Society, Inc. Pennington, NJ, p. 546, 1977. 

14. A.J. Appleby, J. Electroanal, Chem., 118, 31, 1981. 

15. J. Huff, “Status of Fuel Cell Technologies,” in Fuel Cell Seminar Abstracts, 1986 National 

Fuel Cell Seminar, Tucson, AZ, October 1986. 

16. “Advanced Water-Cooled Phosphoric Acid Fuel Cell Development, Final Report,” Report 

No. DE/MC/24221-3130, International Fuel Cells Corporation for U.S. DOE under 

Contract DE-AC21-88MC24221, South Windsor, CT, September 1992. 

17. N. Giordano, E. Passalacqua, L. Pino, V. Alderucci, P.L. Antonucci, “Catalyst and 

Electrochemistry in PAFC: A Unifying Approach,” in The International Fuel Cell 

Conference Proceedings, NEDO/MITI, Tokyo, Japan, 1992. 

18. B. Roland, J. Scholta, H. Wendt, “Phosphoric Acid Fuel Cells - Materials Problems, Process 

Techniques and Limits of the Technology,” in The International Fuel Cell Conference 

Proceedings, NEDO/MITI, Tokyo, Japan, 1992. 

19. “Overview of 11 MW Fuel Cell Power Plant,” Non-published information from Tokyo 

Electric Power Company, September 1989. 

20. M. Matsumoto, K. Usami, “PAFC Commercialization and Recent Progress of Technology 

in Mitsubishi Electric,” in The International Fuel Cell Conference Proceedings, 

NEDO/MITI, Tokyo, Japan, 1992. 

21. J.A.S. Bett, H.R. Kunz, S.W. Smith and L.L. Van Dine, “Investigation of Alloy Catalysts 

and Redox Catalysts for Phosphoric Acid Electrochemical Systems,” FCR-7157F, prepared 

by International Fuel Cells for Los Alamos National Laboratory under Contract 

No. 9-X13-D6271-1, 1985. 

22. B.C. Beard, P.N. Ross, J. Electrochem. Soc., 133, 1839, 1986. 

23. J.T. Glass, G.L. Cahen, G.E. Stoner, E.J. Taylor, J. Electrochem. Soc., 134, 58, 1987. 

24. P.N. Ross, “Oxygen Reduction on Supported Pt Alloys and Intermetallic Compounds in 

Phosphoric Acid,” Final Report, EM-1553, prepared under Contract 1200-5 for the Electric 

Power Research Institute, Palo Alto, CA, September 1980. 

25. V. Jalan, J. Giner, in DECHEMA Monographs, Volume 102, Edited by J.W. Schultze, VCH 

Verlagsgesellschaft, Weinheim, West Germany, p. 315, 1986. 

26. T. Ito, K. Kato, S. Kamitomai, M. Kamiya, “Organization of Platinum Loading Amount of 

Carbon-Supported Alloy Cathode for Advanced Phosphoric Acid Fuel Cell,” in 

Fuel Cell Seminar Abstracts, 1990 Fuel Cell Seminar, Phoenix, AZ, November 25-28, 1990. 

27. J.S. Buchanan, G.A. Hards, L. Keck, R.J. Potter, “Investigation into the Superior Oxygen 

Reduction Activity of Platinum Alloy Phosphoric Acid Fuel Cell Catalysts,” in Fuel Cell 

Seminar Abstracts, Tucson, AZ, November 29-December 2, 1992. 
5-24 

28. K. Kinoshita, F.R. McLarnon, E.J. Cairns, Fuel Cells, A Handbook, prepared by Lawrence 

Berkeley Laboratory for the U.S. Department of Energy under 

Contract DE-AC03-76F00098, May 1988. 

29. N.D. Kackley, S.A. McCatty, J.A. Kosek, “Improved Anode Catalysts for Coal Gas-Fueled 

Phosphoric Acid Fuel Cells,” Final Report DOE/MC/25170-2861, prepared for 

U.S. Department of Energy under Contract DE-AC21-88MC25170, July 1990. 

30. M. Watanabe, C. Shirmura, N. Hara, K. Tsurumi, “An Advanced Gas-Diffusion Electrode 

for Long-Life and High Performance PAFC,” in The International Fuel Cell Conference 

Proceedings, NEDO/MITI, Tokyo, Japan, 1992. 

31. M. Aoki, Y. Ueki, H. Enomoto, K. Harashima, “Some Approaches to Improve the Life 

Performance of Phosphoric Acid Fuel Cell,” paper provided to the authors by Fuji Electric 

Corporate Research and Development, 1992, date of preparation unknown. 

32. M. Watanabe, H. Sei, P. Stonehart, Journal of Electroanalytical Chemistry. 261, 375, 1989. 

33. M. Farooque, “Evaluation of Gas-Cooled Pressurized Phosphoric Acid Fuel Cells for 

Electric Utility Power Generation,” Final Technical Report, NASA CR-168298 prepared by 

Energy Research Corp. under Contract No. DEN 3-201 for NASA Lewis Research Center, 

September 1983. 

34. J. McBreen, W.E. O'Grady, R. Richter, J. Electrochem. Soc., 131, 1215, 1984. 

35. T.G. Benjamin, E.H. Camara, L.G. Marianowski, Handbook of Fuel Cell Performance,

prepared by the Institute of Gas Technology for the United States Department of Energy 

under Contract No. EC-77-C-03-1545, May 1980. 

36. J.M. Feret, “Gas Cooled Fuel Cell Systems Technology Development,” Final Report, NASA 

CR-175047, prepared by Westinghouse Electric Corp. under Contract No. DEN 3-290 for 

NASA Lewis Research Center, August 1985. 

37. V. Jalan, J. Poirier, M. Desai, B. Morrisean, “Development of CO and H2S Tolerant PAFC 

Anode Catalysts,” in Proceedings of the Second Annual Fuel Cell Contractors Review 

Meeting, 1990. 

38. P.W.T. Lu and L. L. France, in Proceedings of the Symposium on Transport Processes in 

Electrochemical Systems, R. S. Yeo, K. Katan and D. T. Chin, The Electrochemical 

Society, Inc., Pennington, NJ, p. 77, 1982. 

39. P.N. Ross, “Anomalous Current Ratios in Phosphoric Acid Fuel Cell Cathodes,” 

LBL-13955; submitted to J. Electrochem. Soc., March 1986. 

40. P.Ross, P. Stonehart, Electrochim. Acta, 21, 441, 1976. 

41. W. Vogel, J. Lundquist, P. Ross, P. Stonehart, Electrochim. Acta, 20, 79, 1975. 

42. H.R. Kunz, in Proceedings of the Symposium on Electrode Materials and Processes for 

Energy Conversion and Storage, Edited by J. D. E. McIntyre, S. Srinivasan and F. G. Will, 

The Electrochemical Society, Inc., Pennington, NJ, p. 607, 1977. 

43. D.T. Chin, P.D. Howard, J. Electrochem. Soc., 133, 2447, 1986. 

44. S.T. Szymanski, G.A. Gruver, M. Katz, H.R. Kunz, J. Electrochem. Soc., 127, 1440, 1980. 

45. F.S. Kemp, IFC, “Status of Development of Water - Cooled Phosphoric Acid Fuel Cells,” in

Proceedings of the Second Annual Fuel Cell Contractors Review Meeting, U.S. DOE/METC, 1990.

46. N. Giordano, “Fuel Cells Activity at CNR, TAE Institute,” CNR/TAE, Italy, 1992. 

47. “Gas Cooled Fuel Cell Systems Technology Development,” Westinghouse/DOE, 

WAES-TR-92-001, March 1992. 

48. K. Harasawa, I. Kanno, I. Masuda, “Fuel Cell R&D and Demonstration Programs at Electric 

Utilities in Japan,” in Fuel Cell Seminar Abstracts, Tucson, AZ, November 29-December 2, 

1992. 
6-1 

6. MOLTEN CARBONATE FUEL CELL

The molten carbonate fuel cell operates at approximately 650 °C (1200 °F). The high operating 

temperature is needed to achieve sufficient conductivity of the carbonate electrolyte, yet allow the 

use of low-cost metal cell components. A benefit associated with this high temperature is that 

noble metal catalysts are not required for the cell electrochemical oxidation and reduction 

processes. Molten carbonate fuel cells are being developed for natural gas and coal-based power 

plants for industrial, electrical utility, and military applications19. Currently, one industrial 

corporation is actively pursuing the commercialization of MCFCs in the U.S.: FuelCell Energy 

(FCE). Europe and Japan each have at least one developer pursuing the technology: MTU 

Friedrichshafen, Ansaldo (Italy), and Ishikawajima-Harima Heavy Industries (Japan). 

Figure 6-1 depicts the operating configuration of the molten carbonate fuel cell. The half cell 

electrochemical reactions are 

H2 + CO3

= → H2O + CO2 + 2e- (6-1)

at the anode, and 

1⁄2O2 + CO2 + 2e-

→ CO3

= (6-2)

at the cathode. The overall cell reaction20 is 

H2 + 1⁄2O2 + CO2 (cathode) → H2O + CO2 (anode) (6-3)

 

19. MCFCs operate more efficiently with CO2 containing bio-fuel derived gases. Performance loss on the anode 

due to fuel dilution is compensated by cathode side performance enhancement resulting from CO2 enrichment. 

20. CO is not directly used by electrochemical oxidation, but produces additional H2 when combined with water in the 

water gas shift reaction. 
6-2 

Figure 6-1 Principles of Operation of Molten Carbonate Fuel Cells (FuelCell Energy) 

Besides the reaction involving H2 and O2 to produce H2O, Equation 6-3 shows a transfer of CO2

from the cathode gas stream to the anode gas stream via the CO3

=

 ion, with 1 mole CO2 transferred 

along with two Faradays of charge, or 2 gram moles of electrons. The reversible potential for an 

MCFC, taking into account the transfer of CO2, is given by the equation 

1

2

2 2 2,c

2 2,a

H O CO

H O CO

RT RT P P P E = E + ln + ln 

2 2 P P ° F F (6-4)

where the subscripts a and c refer to the anode and cathode gas compartments, respectively. When 

the partial pressures of CO2 are identical at the anode and cathode, and the electrolyte is invariant, 

the cell potential depends only on the partial pressures of H2, O2, and H2O. Typically, the CO2

partial pressures are different in the two electrode compartments and the cell potential is affected 

accordingly. 

The need for CO2 at the cathode requires some schemes that will either 1) transfer the CO2 from 

the anode exit gas to the cathode inlet gas ("CO2 transfer device"), 2) produce CO2 by combusting 

the anode exhaust gas, which is mixed directly with the cathode inlet gas, or 3) supply CO2 from an 

ANODE 

e

e 

FCE39c

81401

H2 + CO3

= → H2O + CO2 + 2e

ANODE CATALYST 

CATHODE 

CATHOD

ELECTROLYTE 

1/2O2 + CO2 + 2e- →CO3

=

HYDROGEN AND CO

CONTAINING FUEL

FCE39c

81401

ANODE 

CATA YST

CATHODE CATALYST 

CATHODE

1⁄2O2 + CO2 + 2e → CO3

=

CO + H2O → CO2 + H2 + Heat

 CO2 + Air
6-3 

alternate source. It is usual practice in an MCFC system that the CO2 generated at the anode (right 

side of Equation 6-1) be routed (external to the cell) to the cathode (left side of Equation 6-2). 

MCFCs differ in many respects from PAFCs because of their higher operating temperature (650 

vs. 200 °C) and the nature of the electrolyte. The higher operating temperature of MCFCs 

provides the opportunity to achieve higher overall system efficiencies (potential for heat rates 

below 7,500 Btu/kWh) and greater flexibility in the use of available fuels.21 On the other hand, the 

higher operating temperature places severe demands on the corrosion stability and life of cell 

components, particularly in the aggressive environment of the molten carbonate electrolyte. 

Another difference between PAFCs and MCFCs lies in the method used for electrolyte 

management in the respective cells. In a PAFC, PTFE serves as a binder and wet-proofing agent to 

maintain the integrity of the electrode structure and to establish a stable electrolyte/gas interface in 

the porous electrode. The phosphoric acid is retained in a matrix of PTFE and SiC between the 

anode and cathode. There are no high temperature, wetproofing materials available for use in 

MCFCs that are comparable to PTFE. Thus, a different approach is required to establish a stable 

electrolyte/gas interface in MCFC porous electrodes, and this is illustrated schematically in Figure 

6-2. The MCFC relies on a balance in capillary pressures to establish the electrolyte interfacial 

boundaries in the porous electrodes (1, 2, 3). At thermodynamic equilibrium, the diameters of the 

largest flooded pores in the porous components are related by the equation 

D

cos = Dcos = Dcos

a

a a

e

e e

c

γc θc γ θ γ θ (6-5)

where γ is the interfacial surface tension, θ is the contact angle of the electrolyte, D is the pore 

diameter, and the subscripts a, c, and e refer to the anode, cathode and electrolyte matrix, 

respectively. By properly coordinating the pore diameters in the electrodes with those of the 

electrolyte matrix, which contains the smallest pores, the electrolyte distribution depicted in Figure 

6-2 is established. This arrangement permits the electrolyte matrix to remain completely filled 

with molten carbonate, while the porous electrodes are partially filled, depending on their pore size 

distributions. According to the model illustrated in Figure 6-2 and described by Equation (6-5), the 

electrolyte content in each of the porous components will be determined by the equilibrium pore 

size (<D>) in that component; pores smaller than <D> will be filled with electrolyte, and pores 

larger than <D> will remain empty. A reasonable estimate of the volume distribution of electrolyte 

in the various cell components is obtained from the measured pore-volume-distribution curves and 

the above relationship for D (1, 2). 

Electrolyte management, that is, control over the optimum distribution of molten carbonate 

electrolyte in the different cell components, is critical for achieving high performance and 

endurance with MCFCs. Various processes (i.e., consumption by corrosion reactions, potentialdriven migration, creepage of salt and salt vaporization) occur, all of which contribute to the 

redistribution of molten carbonate in MCFCs; these aspects are discussed by Maru, et al. (4) and 

Kunz (5). 

 

21. In situ reforming of fuels in MCFCs is possible as discussed later in the section. 
6-4 

Porous

Ni anode

Porous

NiO cathode

Molten Carbonate/LiAlO2

electrolyte structure

CO3

= H2 + CO3

= CO2 + H2O + 2e- CO3 1 = /2O2 + CO2 + 2eFuel gas Oxidant gas

Figure 6-2 Dynamic Equilibrium in Porous MCFC Cell Elements 

(Porous electrodes are depicted with pores covered by a thin film of electrolyte) 

6.1 Cell Components 

6.1.1 State-of-the-Art Componments 

The data in Table 6-1 provide a chronology of the evolution in MCFC component technology. In 

the mid-1960s, electrode materials were, in many cases, precious metals, but the technology soon 

evolved to use Ni-based alloys at the anode and oxides at the cathode. Since the mid-1970s, the 

materials for the electrodes and electrolyte (molten carbonate/LiAlO2) have remained essentially 

unchanged. A major development in the 1980s was the evolution in fabrication of electrolyte 

structures. Developments in cell components for MCFCs have been reviewed by Maru, et al. (6, 

7), Petri and Benjamin (8), and Selman (9). Over the past 28 years, the performance of single cells 

has improved from about 10 mW/cm2

 to >150 mW/cm2. During the 1980s, both the performance 

and endurance of MCFC stacks dramatically improved. The data in Figure 6-3 illustrate the 

progress that has been made in the performance of single cells, and in the cell voltage of small 

stacks at 650 °C. Several MCFC stack developers have produced cell stacks with cell areas up to 

1 m2

. Tall, full-scale U.S. stacks fabricated to date include several FCE-300 plus cell stacks with 

~9000 cm2

 cell area producing >250 kW. 
6-5 

Table 6-1 Evolution of Cell Component Technology for Molten Carbonate Fuel Cells 

Component Ca. 1965 Ca. 1975 Current Status 

Anode • Pt, Pd, or Ni • Ni-10 Cr • Ni-Cr/Ni-Al/Ni-Al-Cr 

• 3-6 μm pore size 

• 45 to 70 percent initial 

porosity 

• 0.20 to .5 mm thickness 

• 0.1 to1 m2/g 

Cathode • Ag2O or lithiated NiO • lithiated NiO • lithiated NiO-MgO 

• 7 to15 μm pore size 

• 70 to 80 percent initial 

porosity 

• 60 to 65 percent after 

lithiation and oxidation 

• 0.5 to 1 mm thickness 

• 0.5 m2/g 

Electrolyte 

Support 

• MgO • mixture of α-, β-, 

and γ-LiAlO2

• 10 to 20 m2/g 

• 1.8 mm thickness 

• γ-LiAlO2, α-LiAlO2

• 0.1 to12 m2/g 

• 0.5 to1 mm thickness 

Electrolytea

(wt percent) 

• 52 Li-48 Na 

• 43.5 Li-31.5 Na-25 K 

• "paste" 

• 62 Li-38 K 

• hot press "tile" 

• 1.8 mm thickness 

• 62 Li-38 K 

• 60 Li-40 Na 

 51 Li-48 Na 

• tape cast 

• 0.5 to1 mm thickness 

a - Mole percent of alkali carbonate salt 

Specifications for the anode and cathode were obtained from References (6), (10), and (11). 
6-6 

0.5

0.6

0.7

0.8

0.9

1

0 50 100 150 200 250 300 350

Current Density, mA/cm2

Cell Voltage, V

1967 (1 Atm)

1976 (10 Atm) 1984 (1 Atm)

1984 (10 Atm)

2002 (1 

Figure 6-3 Progress in the Generic Performance of MCFCs on Reformate 

Gas and Air (12, 13) 

The conventional process to fabricate electrolyte structures until about 1980 involved hot pressing 

(about 5,000 psi) mixtures of LiAlO2 and alkali carbonates (typically >50 vol percent in liquid 

state) at temperatures slightly below the melting point of the carbonate salts (e.g., 490°C for 

electrolyte containing 62 mol Li2CO3-38 mol K2CO3). These electrolyte structures (also called 

"electrolyte tiles") were relatively thick (1 to 2 mm) and difficult to produce in large sizes22

because large tooling and presses were required. The electrolyte structures produced by hot 

pressing are often characterized by 1) void spaces (<5 porosity), 2) poor uniformity of 

microstructure, 3) generally poor mechanical strength, and 4) high iR drop. To overcome these 

shortcomings of hot pressed electrolyte structures, alternative processes such as tape casting 

(7) and electrophoretic deposition (14) for fabricating thin electrolyte structures were developed. 

The greatest success to date with an alternative process has been reported with tape casting, which 

is a common processing technique used by the ceramics industry. This process involves dispersing 

the ceramic powder in a solvent23 that contains dissolved binders (usually an organic compound), 

plasticizers, and additives to yield the proper slip rheology. The slip is cast over a moving smooth 

substrate, and the desired thickness is established with a doctor blade device. After drying the slip, 

the "green" structure is assembled into the fuel cell where the organic binder is removed by thermal 

decomposition, and the absorption of alkali carbonate into the ceramic structure occurs during cell 

startup. 

 

22. The largest electrolyte tile produced by hot pressing was about 1.5 m2

 in area (7). 

23. An organic solvent is used because LiAlO2 in the slip reacts with H2O. 
6-7 

The tape casting and electrophoretic deposition processes are amenable to scale-up, and thin 

electrolyte structures (0.25-0.5 mm) can be produced. The ohmic resistance of an electrolyte 

structure24 and the resulting ohmic polarization have a large influence on the operating voltage of 

MCFCs (15). FCE has stated that the electrolyte matrix encompasses 70 of the ohmic loss (16) of 

the cell. At a current density of 160 mA/cm2

, the voltage drop (∆Vohm) of an 0.18 cm thick 

electrolyte structure, with a specific conductivity of ~0.3/ohm-cm at 650 °C, was found to obey the 

relationship (14), 

∆Vohm (V) = 0.5t (6-6)

where ∆Vohm is in volts and t is the thickness in cm. Later data confirm this result (16). With this 

equation, it is apparent that a fuel cell with an electrolyte structure of 0.25 cm thickness would 

operate at a cell voltage that is 35 mV higher than that of an identical cell with an electrolyte 

structure of 0.18 cm thickness because of the lower ohmic loss. Thus, there is a strong incentive 

for making thinner electrolyte structures to improve cell performance. 

The electrolyte composition affects the performance and endurance of MCFCs in several ways. 

Higher ionic conductivities, and hence lower ohmic polarization, are achieved with Li-rich 

electrolytes because of the relative high ionic conductivity of Li2CO3 compared to that of Na2CO3

and K2CO3. However, gas solubility and diffusivity are lower, and corrosion is more rapid in 

Li2CO3. 

The major considerations with Ni-based anodes and NiO cathodes are structural stability and NiO 

dissolution, respectively (9). Sintering and mechanical deformation of the porous Ni-based anode 

under compressive load lead to performance decay by redistribution of electrolyte in a MCFC 

stack. The dissolution of NiO in molten carbonate electrolyte became evident when thin 

electrolyte structures were used. Despite the low solubility of NiO in carbonate electrolytes 

(~10 ppm), Ni ions diffuse in the electrolyte towards the anode, and metallic Ni can precipitate in 

regions where a H2 reducing environment is encountered. The precipitation of Ni provides a sink 

for Ni ions, and thus promotes the diffusion of dissolved Ni from the cathode. This phenomenon 

becomes worse at high CO2 partial pressures (17, 18) because dissolution may involve the 

following mechanism: 

NiO + CO2 → Ni2+ + CO=

3 (6-7)

The dissolution of NiO has been correlated to the acid/base properties of the molten carbonate. 

The basicity of the molten carbonate is defined as equal to -log (activity of O=

) or -log aM2O, where 

a is the activity of the alkali metal oxide M2O. Based on this definition, acidic oxides are 

associated with carbonates (e.g., K2CO3) that do not dissociate to M2O, and basic oxides are 

formed with highly dissociated carbonate salts (e.g., Li2CO3). The solubility of NiO in binary 

 

24. Electrolyte structures containing 45 wt% LiAlO2 and 55 wt% molten carbonate (62 mol% Li2CO3-38 mol% K2CO3) 

have a specific conductivity at 650°C of about 1/3 that of the pure carbonate phase (15). 
6-8 

carbonate melts shows a clear dependence on the acidity/basicity of the melt (19, 20). In relatively 

acidic melts, NiO dissolution can be expressed by 

NiO → Ni2+ + O= (6-8)

In basic melts, NiO reacts with O=

 to produce one of two forms of nickelate ions: 

NiO + O= → NiO=

2 (6-9)

2NiO + O=

 + 1⁄2O2 → 2NiO2 (6-10)

A distinct minimum in NiO solubility is observed in plots of log (NiO solubility) versus basicity 

(-log aM2O), which can be demarcated into two branches corresponding to acidic and basic 

dissolution. Acidic dissolution is represented by a straight line with a slope of +1, and a NiO 

solubility that decreases with an increase in aM2O. Basic dissolution is represented by a straight line 

with a slope of either -1 or -1⁄2, corresponding to Equations (6-9) and (6-10), respectively. The CO2

partial pressure is an important parameter in the dissolution of NiO in carbonate melts because the 

basicity is directly proportional to log PCO2. An MCFC usually operates with a molten carbonate 

electrolyte that is acidic. 

Based on a 12,000-hour full-size stack tests as well as post-test results, FCE believes that Ni 

dissolution and subsequent precipitation will not be an issue for the desired 40,000-hour (5-yr) life 

(21) at atmospheric pressure. But at 10 atm cell pressure, only about 5,000 to 10,000 hours may be 

possible with currently available NiO cathodes (22). The solubility of NiO in molten carbonates is 

complicated by its dependence on several parameters: carbonate composition, H2O partial 

pressure, CO2 partial pressure, and temperature. For example, measurements of NiO dissolution 

by Kaun (23) indicate that solubility is affected by changing the electrolyte composition; a lower 

solubility is obtained in a Li2CO3-K2CO3 electrolyte that contains less Li2CO3 (i.e., lower solubility 

in 38 mol Li2CO3-62 mol K2CO3 than in 62 mol Li2CO3-38 mol K2CO3 at 650 °C). However, 

the solubility of Ni increases in the electrolyte with 38 mol Li2CO3 when the temperature 

decreases, whereas the opposite trend is observed in the electrolyte with 62 mol Li2CO3. Another 

study reported by Appleby (24) indicates that the solubility of Ni decreases from 9 to 2 ppm by 

increasing the Li concentration in Li2CO3-K3CO3 from 62 to 75 wt percent, and a lower solubility 

is obtained in 60 mol percent Li2CO3-40 mol percent Na2CO3 at 650 °C. The compaction of 

cathodes became evident in MCFC stacks once the anode creep was eliminated when strengthened 

by oxide dispersion [i.e., oxide dispersion strengthened (ODS) anode]. 

The bipolar plates used in MCFC stacks are usually fabricated from thin (~15 mil) sheets of an 

alloy (e.g., Incoloy 825, 310S or 316L stainless steel) that are coated on one side (i.e., the side 
6-9 

exposed to fuel gases in the anode compartment) with a Ni layer. The Ni layer is stable in the 

reducing gas environment of the anode compartment, and it provides a conductive surface coating 

with low contact resistance. Pigeaud, et al. describe approaches to circumvent the problems 

associated with gas leaks and corrosion of bipolar plates (25). Corrosion is largely overcome by 

applying a coating (about 50 μm thickness) at the vulnerable locations on the bipolar plate. For 

example, the wet-seal25 area on the anode side is subject to a high chemical potential gradient 

because of the fuel gas inside the cell and the ambient environment (usually air) on the outside of 

the cell, which promotes corrosion (about two orders of magnitude greater than in the cathode 

wet-seal area (26)). Donado, et al. present a general discussion on corrosion in the wet-seal area of 

MCFCs (27). A thin aluminum coating in the wet-seal area of a bipolar plate provides corrosion 

protection by forming a protective layer of LiAlO2 after reaction of Al with Li2CO3 (28). Such a 

protective layer would not be useful in areas of the bipolar plate that must permit electronic 

conduction because LiAlO2 is an insulating material. 

A dense and electronically insulating layer of LiAlO2 is not suitable for providing corrosion 

resistance to the cell current collectors because these components must remain electrically 

conductive. The typical materials used for this application are 316 stainless steel and Ni plated 

stainless steels. However, materials with better corrosion resistance are required for long-term 

operation of MCFCs. Research is continuing to understand the corrosion processes of hightemperature alloys in molten carbonate salts under both fuel gas and oxidizing gas environments 

(29, 28) and to identify improved alloys (30) for MCFCs. Stainless steels such as Type 310 and 

446 have demonstrated better corrosion resistance than Type 316 in corrosion tests (30). 

6.1.2 Development Components 

MCFC components are limited by several technical considerations (31), particularly those 

described in Section 6.1.1. Even though present approaches function properly in full size cells at 

atmospheric pressure, research is addressing alternate cathode materials and electrolytes, 

performance improvement, life extension beyond the commercialization goal of five years, and 

cost reduction (32). The studies described in recent literature provide updated information on 

promising development of the electrodes, the electrolyte matrix, and the capability of the cell to 

tolerate trace contaminants in the fuel supply. Descriptions of some of this work follow. 

Anode: As stated in Section 6.1.1 and Reference (33), state-of-the-art anodes are made of a NiCr/Ni-Al alloy. The Cr was added to eliminate the problem of anode sintering. However, Ni-Cr 

anodes are susceptible to creep when placed under the torque load required in the stack to 

minimize contact resistance between components. The Cr in the anode is also lithiated by the 

electrolyte; then it consumes carbonate. Developers are trying lesser amounts of Cr (8 percent) to 

reduce the loss of electrolyte, but some have found that reducing the Cr by 2 percentage points 

increased creep (34). Several developers have tested Ni-Al alloy anodes that provide creep 

resistance with minimum electrolyte loss (34, 35, 36). The low creep rate with this alloy is 

attributed to the formation of LiAlO2 dispersed in Ni (35). 

 

25. The area of contact between the outer edge of the bipolar plate and the electrolyte structure prevents gas from 

leaking out of the anode and cathode compartments. The gas seal is formed by compressing the contact area 

between the electrolyte structure and the bipolar plate so that the liquid film of molten carbonate at operating 

temperature does not allow gas to permeate through. 
6-10 

Even though alloys of chromium or aluminum strengthened nickel provides a stable, 

non-sintering, creep-resistant anode, electrodes made with Ni are relatively high in cost. Alloys, 

such as Cu-Al and LiFeO2, have not demonstrated sufficient creep strength or performance. 

Because of this, present research is focused on reducing the manufacturing cost of the nickel 

alloy anodes (37). 

There is a need for better sulfur tolerance in MCFCs, especially when considering coal operation. 

The potential benefit for sulfur tolerant cells is to eliminate cleanup equipment that impacts system 

efficiency. This is especially true if low temperature cleanup is required, because the system 

efficiency and capital cost suffer when the fuel gas temperature is first reduced, then increased to 

the cell temperature level. Tests are being conducted on ceramic anodes to alleviate the problems, 

including sulfur poisoning, being experienced with anodes (31). Anodes are being tested with 

undoped LiFeO2 and LiFeO2 doped with Mn and Nb. Preliminary testing, where several 

parameters were not strictly controlled, showed that the alternative electrodes exhibited poor 

performance and would not operate over 80 mA/cm2

. At the present time, no alternative anodes 

have been identified. Instead, future work will focus on tests to better understand material 

behavior and to develop alternative materials with emphasis on sulfur tolerance. 

Cathode: An acceptable material for cathodes must have adequate electrical conductivity, 

structural strength, and low dissolution rate in molten alkali carbonates to avoid precipitation of 

metal in the electrolyte structure. State-of-the art cathodes are made of lithiated NiO (33, 38) that 

have acceptable conductivity and structural strength. However, in early testing, a predecessor of 

UTC Fuel Cells found that the nickel dissolved, then precipitated and reformed as dendrites across 

the electrolyte matrix. This decreased performance and eventual short-circuting of the cell. 

Dissolution of the cathode has turned out to be the primary life-limiting constraint of MCFCs, 

particularly in pressurized operation (35). Developers are investigating approaches to resolve the 

NiO dissolution issue. For atmospheric cells, developers are looking at increasing the basicity of 

the electrolyte (using a more basic melt such as Li/NaCO3). Another approach is to lower CO2

(acidic) partial pressure. To operate at higher pressures (higher CO2 partial pressure), developers 

are investigating alternative materials for the cathodes and using additives in the electrolyte to 

increase its basicity (37). 

Initial work on LiFeO2 cathodes showed that electrodes made with this material were very stable 

chemically under the cathode environment; there was essentially no dissolution (31). However, 

these electrodes perform poorly compared to the state-of-the-art NiO cathode at atmospheric 

pressure because of slow kinetics. The electrode shows promise at pressurized operation, so it is 

still being investigated. Higher performance improvements are expected with Co-doped LiFeO2. 

It also has been shown that 5 mol lithium-doped NiO with a thickness of 0.02 cm provided a 

43 mV overpotential (higher performance) at 160 mA/cm2

 compared to the state-of-the-art NiO 

cathode. It is assumed that reconfiguring the structure, such as decreasing the agglomerate size, 

could improve performance. 

Another idea for resolving the cathode dissolution problem is to formulate a milder cell 

environment. This leads to the approach of using additives in the electrolyte to increase its 

basicity. Small amounts of additives provide similar voltages to those without additives, but larger 

amounts adversely affect performance (39). Table 6-2 quantifies the limiting amounts of additives. 
6-11 

Table 6-2 Amount in Mol percent of Additives to Provide Optimum Performance (39) 

62 MOL percent 

Li2CO3/K2CO2

52 MOL percent 

Li2CO3/NA2CO3

CaCO3 0 to 15 0 to 5 

SrCO3 0 to 5 0 to 5 

BaCO3 0 to 10 0 to 5 

Another approach to a milder cell environment is to increase the fraction of Li in the baseline 

electrolyte or change the electrolyte to Li/Na rather than the baseline 62/38 Li/K melt (29, 39, 40). 

Within the past 10 years, a lower cost stabilized cathode was developed with a base material cost 

comparable to the unstabilized cathode (41). A 100 cm2

 cell test of the lower-cost stabilized 

cathode with a Li/Na electrolyte system completed 10,000 hours of operation. 

Electrolyte Matrix: The present electrolyte structure materials are tightly packed, fine α- or γLiAlO2 with fiber or particulate reinforcement. Long-term cell testing reveals significant particle 

growth and γ to α phase transformation, leading to detrimental changes in the pore structure. 

The particles grow faster at higher temperatures, in low CO2 gas atmospheres, and in strongly 

basic melts. The γ phase is stable at > 700 °C, whereas the α phase is stable at 600 to 650 °C. 

Such particle growth and phase transformations can be explained by a dissolution - precipitation 

mechanism. The matrix must also be strong enough to counter operating mechanical and 

thermal stresses, and still maintain the gas seal. Thermal cycling below the carbonate freezing 

temperature can induce cracking due to thermo-mechanical stress. Ceramic fiber reinforcement 

is most effective for crack deflection, followed by platelet and sphere forms. However, strong, 

cost effective, and stable ceramic fibers are not yet commercially available. Long-term, intense 

material research may be needed to develop such ceramic fibers. If particle sizes are markedly 

different, the phase transformation is more controlled by the particle sizes, according to Ostwald 

ripening where small particles preferentially dissolve and re-precipitate onto larger particles. 

Therefore, a more uniform particle size distribution is needed to maintain a desired pore 

structure. The industry trend is to switch from γ-LiAlO2 to α-LiAlO2 for better long-term phase 

and particle-size stabilities. FCE is developing a low-cost LiAlO2, aqueous-base manufacturing 

system, but must resolve slow drying rate of LiAlO2 and its instability in water (42). 

Electrolyte: Present electrolytes have the following chemistry: lithium potassium carbonate, 

Li2CO3/K2CO3 (62:38 mol percent) for atmospheric pressure operation and lithium sodium 

carbonate, LiCO3/NaCO3 (52:48 o 60:40 mol percent) that is better for improved cathode 

stability under pressurized operation and life extension. The electrolyte composition affects 

electrochemical activity, corrosion, and electrolyte loss rate. Evaporation of the electrolyte is a 

life-limiting issue for the molten carbonate fuel cell. Li/Na electrolyte is better for higherpressure operation than Li/K because it gives higher performance. This allows the electrolyte 

matrix to be made thicker for the same performance relative to the Li/K electrolyte. Thicker 

electrolytes result in a longer time to shorting by internal precipitation. Li/Na also provides 
6-12 

better corrosion resistance to mitigate acidic cathode dissolution. However, it has lower 

wettability and greater temperature sensitivity. Additives are being investigated to minimize the 

temperature sensitivity of Li/Na. The electrolyte has a low vapor pressure at operating 

temperature, and may slowly evaporate. Stack testing has shown that the electrolyte vapor loss 

is significantly slower than expected. The evaporation loss is projected to have minimal impact 

on stack life. 

Electrolyte Structure: Ohmic losses contribute about a 65 mV loss at the beginning of life, and 

may increase to as much as 145 mV by 40,000 hours (16). The majority of the voltage loss is in 

the electrolyte and the cathode components. The electrolyte offers the highest potential for 

reduction because 70 percent of the total cell ohmic loss occurs there. FCE investigated increasing 

the porosity of the electrolyte 5 percent to reduce the matrix resistance by 15 percent, and change 

the melt to Li/Na from Li/K to reduce the matrix resistivity by 40 percent. Work is continuing on 

the interaction of the electrolyte with the cathode components. At the present time, an electrolyte 

loss of 25 percent of the initial inventory can be projected with a low surface area cathode current 

collector and with the proper selection of material. 

Another area for electrolyte improvement is the ability to prevent gas crossover from one electrode 

to the other. FCE produced an improved matrix fabrication process providing low temperature 

binder burnout. FCE reported in 1997 that it had developed a high performance rugged matrix that 

increases the gas sealing efficiency by approximately a factor of ten better than the design goal 

(43). 

Electrolyte Migration: There is a tendency for the electrolyte to migrate from the positive end of 

the stack to the negative end of the stack. This may cause the end cells to lose performance 

compared to the central cells. The electrolyte loss is through the gasket used to couple the external 

manifolds to the cell stack. The standard gasket material is porous and provides a conduit for 

electrolyte transfer. A new gasket design incorporating electrolyte flow barriers inside the gasket 

(US Patent 5,110,692) plus end cell inventory capability offers the potential for reaching 

40,000 hours, if only this mode of failure is considered. Stacks with internal manifolding do not 

require a gasket, and may not experience this problem (44). 

Bipolar Plate: The present bipolar plate consists of a separator, current collectors, and the wet 

seal. The separator and current collector is Ni-coated 310S/316L and the wet seal is formed by 

aluminization of the metal. The plate is exposed to the anode environment of one side and the 

cathode environment on the other. Low oxygen partial pressure on the anode side of the bipolar 

plate prevents the formation of a protective oxide coating. After reaction with the thin, creeping 

electrolyte, heat-resistant alloys form a multi-layered corrosion scale. This condition may be 

accelerated by carbonization, higher temperature, and higher moisture gas environment. On the 

cathode side, contact electrical resistance increases as an oxide scale builds up. Electrolyte loss 

due to corrosion and electrolyte creep also contributes to power decay. Single alloy bipolar 

current collector materials that function well in both anode and cathode environments need to be 

developed. Although such development has been attempted, high cost and high ohmic resistance 

prevent it from being successful. Presently, stainless steels, particularly austenitic stainless 

steels, are the primary construction materials. More expensive nickel-based alloys resist 

corrosion as well as or only slightly better than austenitic stainless steels. A thermodynamically 
6-13 

stable nickel coating is needed to protect the anode side. Unfortunately, electroless nickel 

coatings, although dense or uniform in thickness, are expensive and contain detrimental 

impurities; electrolytic nickel coatings are not sufficiently dense or uniform in thickness. FCE 

and others have found that cladding with nickel provides excellent corrosion protection. A 

nickel cladding of 50 μm thickness is projected for >40,000 hours of life (42).

Coal Gas Trace Species: MCFCs to date have been operated on reformed or simulated natural gas 

and simulated coal gas. Testing conducted with simulated coal gas has involved the expected 

individual and multi-trace constituents to better understand coal operation (45). 

Table 6-3 shows the contaminants and their impact on MCFC operation. The table denotes the 

species of concern and what cleanup of the fuel gas is required to operate on coal gas. Confidence 

in operation with coal will require the use of an actual gasifier product. An FCE MCFC stack was 

installed (fall of 1993) using a slipstream of an actual coal gasifier to further clarify the issues of 

operation with trace gases (46). 

Table 6-3 Qualitative Tolerance Levels for Individual Contaminants in Isothermal 

Bench-Scale Carbonate Fuel Cells (46, 47, and 48) 

CONTAMINANTS 

(typical ppm in 

raw coal gas) 

REACTION MECHANISM QUALITATIVE 

TOLERANCES 

CONCLUSIONS 

NO NOTICEABLE EFFECTS 

NH3 (10,000) 

Cd (5) 

Hg (1) 

Sn (3) 

2NH3→N2+3H2

Cd+H2O→CdO(s)+H2

(Hg Vapor Not Reactive) 

(Sn(l) Not Volatile) 

~1 vol percent NH3

~30 ppm Cd 

35 ppm Hg 

No Vapor @ 650°C 

No Effects 

No Cell Deposits 

No TGA Effects 

No Cell Deposits 

MINOR EFFECTS 

Zn (100) 

Pb (15) 

Zn+H2O→ZnO(s)+H2

Pb+H2O→PbS(s)+H2

<15 ppm Zn 

1.0 ppm Pb 

sat'd vapor 

No Cell Deposits at 75 percent 

Utilization 

Cell Deposits Possible in 

Presence of High H2Se 

SIGNIFICANT EFFECTS 

H2S (15,000) 

HCl (500) 

H2Se (5) 

As (10) 

xH2S+Ni→NiSx+xH2

2HCl+K2CO3→2KCl(v)+H2O/CO2

xH2Se+Ni→NiSex+xH2

AsH3+Ni→NiAs(s)+3/2H2

<0.5 ppm H2S 

<0.1 ppm HCl 

<0.2 ppm H2Se 

<0.1 ppm As 

Recoverable Effect 

Long Term Effects Possible 

Recoverable Effect 

Cumulative Long Term Effect 

6.2 Performance 

Factors affecting the selection of operating conditions are stack size, heat transfer rate, voltage 

level, load requirement, and cost. The performance curve is defined by cell pressure, temperature, 

gas composition, and utilization. Typical MCFCs will generally operate in the range of 100 to 

200 mA/cm2

 at 750 to 900 mV/cell. 

Typical cathode performance curves obtained at 650 °C with an oxidant composition (12.6 percent 

O2/18.4 percent CO2/69 percent N2) that is anticipated for use in MCFCs, and a common baseline 

composition (33 percent O2/67 percent CO2) are presented in Figure 6-4 (22, 49). The baseline 
6-14 

composition contains O2 and CO2 in the stoichiometric ratio that is needed in the electrochemical 

reaction at the cathode (Equation (6-2)). With this gas composition, little or no diffusion 

limitations occur in the cathode because the reactants are provided primarily by bulk flow. The 

other gas composition, which contains a substantial fraction of N2, yields a cathode performance 

that is limited dilution by an inert gas. 

Figure 6-4 Effect of Oxidant Gas Composition on MCFC Cathode Performance 

at 650°C, (Curve 1, 12.6 percent O2/18.4 percent CO2/69.0 percent N2; 

Curve 2, 33 percent O2/67 percent CO2) 

In the 1980s, the performance of MCFC stacks increased dramatically. During the 1990s, cells as 

large as 1.0 m2

 are being tested in stacks. Most recently, the focus has been on achieving 

performance in a stack equivalent to single cell performance. Cells with an electrode area of 

0.3 m2

 were routinely tested at ambient and above ambient pressures with improved electrolyte 

structures made by tape-casting processes (22). Several stacks underwent endurance testing in the 

range of 7,000 to 10,000 hours. The voltage and power as a function of current density after 

960 hours for a 1.0 m2

 stack consisting of 19 cells are shown in Figure 6-5. The data were 

obtained with the cell stack at 650 °C and 1 atmosphere. 
6-15 

Figure 6-5 Voltage and Power Output of a 1.0/m2 19 cell MCFC Stack after 960 Hours at 

965 °C and 1 atm, Fuel Utilization, 75 percent (50) 

The remainder of this section will review operating parameters that affect MCFC performance. 

Supporting data will be presented, as well as equations derived from empirical analysis. 

6.2.1 Effect of Pressure 

The dependence of reversible cell potential on pressure is evident from the Nernst equation. For a 

change in pressure from P1 to P2, the change in reversible potential (∆Vp) is given by 

∆Vp = 

3/2

1,a 2,c

3/2

2,a 1,c

RT RT P P ln + ln 

2 2 F F P P (6-11)

where the subscripts a and c refer to the anode and cathode, respectively. In an MCFC with the 

anode and cathode compartments at the same pressure (i.e., P1=P1,a=P1,c and P2=P2,a=P2,c): 

∆Vp =

3/2

12 2

3/2

21 1

RT RT RT PP P ln + ln = ln 

22 4 FF F PP P (6-12)

At 650 °C 

∆Vp (mV) = 20 ln

P

P = 46 log 

P

P

2

1

2

1

⎛

⎝

⎜ ⎞

⎠

⎟ (6-13)
6-16 

Thus, a tenfold increase in cell pressure corresponds to an increase of 46 mV in the reversible cell 

potential at 650 °C. 

Increasing the operating pressure of MCFCs results in enhanced cell voltages because of the 

increase in the partial pressure of the reactants, increase in gas solubilities, and increase in mass 

transport rates. Opposing the benefits of increased pressure are the effects of pressure on 

undesirable side reactions such as carbon deposition (Boudouard reaction): 

2CO → C + CO2 (6-14)

and methane formation (methanation) 

CO + 3H2 → CH4 + H2O (6-15)

In addition, decomposition of CH4 to carbon and H2 is possible 

CH4 → C + 2H2 (6-16)

but this reaction is suppressed at higher pressure. According to Le Chatelier’s principle, an 

increase in pressure will favor carbon deposition by Equation (6-14)26 and methane formation by 

Equations (6-15) and (6-16) (51). The water-gas shift reaction (52)27

CO2 + H2 ↔ CO + H2O (6-17)

is not affected by an increase in pressure because the number of moles of gaseous reactants and 

products in the reaction is identical. Carbon deposition in an MCFC is to be avoided because it 

can lead to plugging of the gas passages in the anode. Methane formation is detrimental to cell 

performance because the formation of each mole consumes three moles of H2, which represents a 

considerable loss of reactant and would reduce power plant efficiency. 

The addition of H2O and CO2 to the fuel gas modifies the equilibrium gas composition so that the 

formation of CH4 is not favored. Increasing the partial pressure of H2O in the gas stream can 

 

26. Data from translation of Russian literature (51) indicate the equilibrium constant is almost independent of pressure. 

27. Data from translation of Russian literature (52) indicate the equilibrium constant K is a function of pressure. In 

relative terms, if K (627 °C) = 1 at 1 atm, it decreases to 0.74K at 500 atm and 0.60K at 1000 atmospheres. At the 

operating pressures of the MCFC, the equilibrium constant can be considered invariant with pressure. 
6-17 

reduce carbon deposition. Measurements (22) on 10 cm x 10 cm cells at 650 °C using simulated 

gasified coal GF-1 (38 percent H2/56 percent CO/6 percent CO2) at 10 atm showed that only a 

small amount of CH4 is formed. At open circuit, 1.4 vol percent CH4 (dry gas basis) was detected, 

and at fuel utilizations of 50 to 85 percent, 1.2 to 0.5 percent CH4 was measured. The experiments 

with a high CO fuel gas (GF-1) at 10 atmospheres and humidified at 163 °C showed no indication 

of carbon deposition in a subscale MCFC. These studies indicated that CH4 formation and carbon 

deposition at the anodes in an MCFC operating on coal-derived fuels can be controlled, and under 

these conditions, the side reactions would have little influence on power plant efficiency. 

Figure 6-6 shows the effect of pressure (3, 5, and 10 atmospheres) and oxidant composition (3.2 

percent CO2/23.2 percent O2/66.3 percent N2/7.3 percent H2O and 18.2 percent CO2/9.2 percent 

O2/65.3 percent N2/7.3 percent H2O) on the performance of 70.5 cm2

 MCFCs at 650 °C (53). The 

major difference as the CO2 pressure changes is the change in open circuit potential, which 

increases with cell pressure and CO2 content (see Equation (6-11)). At 160 mA/cm2

, ∆Vp is 

-44 mV for a pressure change from 3 to 10 atmospheres for both oxidant compositions. 

Because ∆Vp is a function of the total gas pressure, the gas compositions in Figure 6-6 have little 

influence on ∆Vp. Based on these results, the effect of cell voltage from a change in pressure can 

be expressed by the equation 

∆Vp (mV) = 84 log 2

1

P

P (6-18)

where P1 and P2 are different cell pressures. Another analysis by Benjamin, et al. (54) suggests 

that a coefficient less than 84 may be more applicable. The change in voltage as a function of 

pressure change was expressed as 

∆Vp (mV) = 76.5 log 2

1

P

P (6-19)
6-18 

Figure 6-6 Influence of Cell Pressure on the Performance of a 70.5 cm2

 MCFC at 650 °C 

(anode gas, not specified; cathode gases, 23.2 percent O2/3.2 percent CO2/66.3 percent 

N2/7.3 percent H2O and 9.2 percent O2/18.2 percent CO2/65.3 percent N2/7.3 percent H2O; 

50 percent CO2, utilization at 215 mA/cm2

) (53, Figure 4, Pg. 395) 

Equation (6-19) was based on a load of 160 mA/cm2

 at a temperature of 650 °C. It was also found 

to be valid for a wide range of fuels and for a pressure range of 1 atmosphere ≤ P ≤

10 atmospheres. Other results (55) support this coefficient. Figure 6-7 shows the influence of 

pressure change on voltage gain for three different stack sizes. These values are for a temperature 

of 650 °C and a constant current density of 150 mA/cm2

 at a fuel utilization of 70 percent. The 

line that corresponds to a coefficient of 76.5 falls approximately in the middle of these values. 

Further improvements in cell performance will lead to changes in the logarithmic coefficient. 

Additional data (56, 57, 58) indicate that the coefficient may indeed be less than 76.5, but Equation 

(6-19) appears to represent the effect of pressure change on performance. 
6-19 

Figure 6-7 Influence of Pressure on Voltage Gain (55) 

6.2.2 Effect of Temperature 

The influence of temperature on the reversible potential of MCFCs depends on several factors, 

one of which involves the equilibrium composition of the fuel gas (22, 59, 60, 61).28 The water 

gas shift reaction achieves rapid equilibrium29 at the anode in MCFCs, and consequently 

CO serves as an indirect source of H2. The equilibrium constant (K) 

K CO= 

P P

P P

2

2 2

H O

H CO

(6-20)

increases with temperature (see Table 6-4 and Appendix 10.1), and the equilibrium composition 

changes with temperature and utilization to affect the cell voltage. 

The influence of temperature on the voltage of MCFCs is illustrated by the following example. 

Consider a cell with an oxidant gas mixture of 30 percent O2/60 percent CO2/10 percent N2, and a 

fuel gas mixture of 80 percent H2/20 percent CO2. When the fuel gas is saturated with 

H2O vapor at 25 °C, its composition becomes 77.5 percent H2/19.4 percent CO2/3.1 percent H2O. 

 

28. For a fixed gas composition of H2, H2O, CO, CO2, and CH4 there is a temperature, Tb, below which the exothermic 

Boudouard reaction is thermodynamically favored, and a temperature, Tm, above which carbon formation by the 

endothermic decomposition of CH4 is thermodynamically favored; more extensive details on carbon deposition are 

found elsewhere (22, 59, 60, 61). 

29. The dependence of equilibrium constant on temperature for carbon deposition, methanation, and water gas shift 

reactions is presented in Appendix 10.1. 
6-20 

After considering the equilibrium established by the water gas shift reaction, the equilibrium 

concentrations can be calculated (see Example 9-5 in Section 9) using Equation (6-20) and the 

equilibrium constant; see for instance, Broers and Treijtel (62). The equilibrium concentrations 

are substituted into Equation (6-4) to determine E as a function of T. 

Table 6-4 Equilibrium Composition of Fuel Gas and Reversible Cell Potential as a 

Function of Temperature 

Parametera Temperature (°K) 

 800 900 1000 

PH2 0.669 0.649 0.643

PCO2 0.088 0.068 0.053

PCO 0.106 0.126 0.141

PH2O 0.137 0.157 0.172

Eb (V) 1.155 1.143 1.133

Kc 0.2474 0.4538 0.7273

a - P is the partial pressure computed from the water gas shift equilibrium of inlet gas with 

composition 77.5 percent H2/19.4 percent CO2/3.1 percent H2O at 1 atmosphere. 

b - Cell potential calculated using Nernst equation and cathode gas composition of 30 percent 

O2/60 percent CO2/10 percent N2. 

c - Equilibrium constant for water gas shift reaction from Reference (59). 

The results of these calculations are presented in Table 6-4. Inspection of the results shows a 

change in the equilibrium gas composition with temperature. The partial pressures of CO and H2O 

increase at higher T because of the dependence of K on T. The result of the change in gas 

composition, and the decrease in E° with increasing T, is that E decreases with an increase in T. In 

an operating cell, the polarization is lower at higher temperatures, and the net result is that a higher 

cell voltage is obtained at elevated temperatures. The electrode potential measurements (9) in a 

3 cm2

 cell30 show that the polarization at the cathode is greater than at the anode, and that the 

polarization is reduced more significantly at the cathode with an increase in temperature. At a 

current density of 160 mA/cm2

, cathode polarization is reduced by about 160 mV when the 

temperature increases from 550 to 650 °C, whereas the corresponding reduction in anode 

polarization is only about 9 mV (between 600 and 650 °C, no significant difference in polarization 

is observed at the anode). 

Baker, et al. (63) investigated the effect of temperature (575 to 650 °C) on the initial 

performance of small cells (8.5 cm2

). With steam-reformed natural gas as the fuel and 30 percent 

 

30. Electrolyte is 55 wt% carbonate eutectic (57 wt% Li2CO3, 31 wt% Na2CO3, 12 wt% K2CO3) and 45 wt% LiA1O2, 

anode is Co + 10% Cr, cathode is NiO, fuel is 80% H2/20% CO2 and oxidant is 30% CO2/70% air. 
6-21 

CO2/70 percent air as the oxidant, the cell voltage31 at 200 mA/cm2

 decreased by 1.4 mV/° for a 

reduction in temperature from 650 to 600 °C, and 2.16 mV/°C for a decrease from 600 to 575 °C. 

In the temperature range 650 to 700 °C, data analysis (58) indicates a relationship of 0.25 mV/°

C. The following equations summarize these results. 

∆VT (mV) = 2.16 (T2 – T1) 575°C < T < 600 °C (6-21)

∆VT (mV) = 1.40 (T2 – T1) 600°C < T < 650 °C (6-22)

∆VT (mV) = 0.25 (T2 – T1) 650°C < T < 700 °C (6-23)

The two major contributors responsible for the change in cell voltage with temperature are the 

ohmic polarization and electrode polarization. It appears that in the temperature range of 575 to 

650 °C, about 1/3 of the total change in cell voltage with decreasing temperature is due to an 

increase in ohmic polarization, and the remainder from electrode polarization at the anode and 

cathode. Most MCFC stacks currently operate at an average temperature of 650 °C. Most 

carbonates do not remain molten below 520 °C, and as seen by the previous equations, increasing 

temperature enhances cell performance. Beyond 650 °C, however, there are diminishing gains 

with increased temperature. In addition, there is increased electrolyte loss from evaporation and 

increased material corrosion. An operating temperature of 650 °C thus offers a compromise 

between high performance and stack life. 

6.2.3 Effect of Reactant Gas Composition and Utilization 

The voltage of MCFCs varies with the composition of the reactant gases. The effect of reactant 

gas partial pressure, however, is somewhat difficult to analyze. One reason involves the water gas 

shift reaction at the anode due to the presence of CO. The other reason is related to the 

consumption of both CO2 and O2 at the cathode. Data (55, 64, 65, 66) show that increasing the 

reactant gas utilization generally decreases cell performance. 

As reactant gases are consumed in an operating cell, the cell voltage decreases in response to the 

polarization (i.e., activation, concentration) and to the changing gas composition. These effects are 

related to the partial pressures of the reactant gases. 

Oxidant: The electrochemical reaction at the cathode involves the consumption of two moles 

CO2 per mole O2 (see Equation (6-2)), and this ratio provides the optimum cathode performance. 

The influence of the [CO2]/[O2] ratio on cathode performance is illustrated in Figure 6-8 (22). As 

this ratio decreases, the cathode performance decreases, and a limiting current is discernible. In the 

 

31. Cell was operated at constant flow rate; thus, the utilization changes with current density. 
6-22 

limit where no CO2 is present in the oxidant feed, the equilibrium involving the dissociation of 

carbonate ions becomes important. 

3

= 2 = CO ↔ CO + O (6-24)

Current density (mA/cm )2

Figure 6-8 Effect of CO2/O2 Ratio on Cathode Performance in an MCFC, 

Oxygen Pressure is 0.15 atm (22, Figure 5-10, Pgs. 5-20) 

Under these conditions, the cathode performance shows the greatest polarization because of the 

composition changes that occur in the electrolyte. The change in the average cell voltage of a 

ten-cell stack as a function of oxidant utilization is illustrated in Figure 6-9. In this stack, the 

average cell voltage at 172 mA/cm2

 decreases by about 30 mV for a 30 percentage point increase 

in oxidant (20 to 50 percent) utilization. Based on this additional data (55, 64, 65), the voltage 

loss due to a change in oxidant utilization can be described by the following equations: 

∆Vcathode (mV) = 250 log 2

CO O

1

CO O

2 2

1

2

2 2

1

2

P P

P P

⎛

⎝

⎜ ⎞

⎠

⎟

⎛

⎝

⎜ ⎞

⎠

⎟

 for 0.04 < 2 2

1

2

PCO O P

⎛

⎝

⎜ ⎞

⎠

⎟ < 0.11 (6-25)

Vcathode (mV) = 99 log 2

CO O

1

CO O

2 2

1

2

2 2

1

2

P P

P P

⎛

⎝

⎜ ⎞

⎠

⎟

⎛

⎝

⎜ ⎞

⎠

⎟

 for 0.11 < 2 2

1

2

PCO O P

⎛

⎝

⎜ ⎞

⎠

⎟ < 0.38 (6-26)
6-23 

where PCO2 and PO2 are the average partial pressures of CO2 and O2 in the system. 

Figure 6-9 Influence of Reactant Gas Utilization on the Average Cell Voltage of an 

MCFC Stack (67, Figure 4-21, Pgs. 4-24) 

Fuel: The data in Table 6-5 from Lu and Selman (68) illustrate the dependence of the anode 

potential on the composition of five typical fuel gases and two chemical equilibria occurring in the 

anode compartment.32 The calculations show the gas compositions and open circuit anode 

potentials obtained after equilibria by the water gas shift and CH4 steam reforming reactions are 

considered. The open circuit anode potential calculated for the gas compositions after 

equilibration, and experimentally measured, is presented in Table 6-5. The equilibrium gas 

compositions obtained by the shift and steam reforming reactions clearly show that, in general, the 

H2 and CO2 contents in the dry gas decrease, and CH4 and CO are present in the equilibrated gases. 

The anode potential varies as a function of the [H2]/[H2O][CO2] ratio; a higher potential is obtained 

when this ratio is higher. The results show that the measured potentials agree with the values 

calculated, assuming that simultaneous equilibria of the shift and the steam reforming reactions 

reach equilibrium rapidly in the anode compartments of MCFCs. 

 

32. No gas phase equilibrium exists between O2 and CO2 in the oxidant gas that could alter the composition or cathode 

potential. 
6-24 

Table 6-5 Influence of Fuel Gas Composition on Reversible Anode Potential at 650 °C 

(68, Table 1, Pg. 385) 

Typical Gas Composition (mole fraction) -Eb

Fuel Gasa H2 H2O CO CO2 CH4 N2 (mV) 

Dry gas 

High Btu (53 °C) 0.80 - - 0.20 - - 1116±3c

Intermed. Btu (71 °C) 0.74 - - 0.26 - - 1071±2c

Low Btu 1 (71 °C) 0.213 - 0.193 0.104 0.011 0.479 1062±3c

Low Btu 2 (60 °C) 0.402 - - 0.399 - 0.199 1030±c

Very low Btu (60 °C) 0.202 - - 0.196 - 0.602 1040±c

 

Shift equilibrium 

High Btu (53 °C) 0.591 0.237 0.096 0.076 - - 1122d

Intermed. Btu (71 °C) 0.439 0.385 0.065 0.112 - - 1075d

Low Btu 1 (71 °C) 0.215 0.250 0.062 0.141 0.008 0.326 1054d

Low Btu 2 (60 °C) 0.231 0.288 0.093 0.228 - 0.160 1032d

Very low Btu (60 °C) 0.128 0.230 0.035 0.123 - 0.484 1042d

 

Shift and Steam-reforming 

High Btu (53 °C) 0.555 0.267 0.082 0.077 0.020 - 1113d

Intermed. Btu (71 °C) 0.428 0.394 0.062 0.112 0.005 - 1073d

Low Btu 1 (71 °C) 0.230 0.241 0.067 0.138 0.001 0.322 1059d

Low Btu 2 (60 °C) 0.227 0.290 0.092 0.229 0.001 0.161 1031d

Very low Btu (60 °C) 0.127 0.230 0.035 0.123 0.0001 0.485 1042d

a - Temperature in parentheses is the humidification temperature 

b - Anode potential with respect to 33 percent O2/67 percent CO2 reference electrode 

c - Measured anode potential 

d - Calculated anode potential, taking into account the equilibrated gas composition

Further considering the Nernst equation, an analysis shows that the maximum cell potential for a 

given fuel gas composition is obtained when [CO2]/[O2] = 2. Furthermore, the addition of inert 

gases to the cathode, for a given [CO2]/[O2] ratio, causes a decrease in the reversible potential. On 

the other hand, the addition of inert gases to the anode increases the reversible potential for a given 

[H2]/[H2O][CO2] ratio and oxidant composition. This latter result occurs because two moles of 

product are diluted for every mole of H2 reactant. However, the addition of inert gases to either 

gas stream in an operating cell can lead to an increase in concentration polarization. 
6-25 

Figure 6-10 depicts an average voltage loss for the stack of about 30 mV for a 30 

percent increase in fuel utilization (30 to 60 percent). This and other data (66) suggest that the 

voltage loss due to a change in fuel utilization can be described by the following equation: 

∆Vanode (mV) = 173 log ( )

( )

2 H CO H O

1 H CO H O

2 22

2 22

P / P P

P / P P

(6-27)

where PH CO H 22 2 O , P , and P are the average partial pressures of H2, CO2, and O2 in the system. 

The above discussion implies that MCFCs should be operated at low reactant gas utilizations to 

maintain voltage levels, but doing this means inefficient fuel use. As with other fuel cell types, a 

compromise must be made to optimize overall performance. Typical utilizations are 75 to 85 

percent of the fuel. 

Figure 6-10 Dependence of Cell Voltage on Fuel Utilization (69) 

6.2.4 Effect of Impurities 

Gasified coal is expected to be the major source of fuel gas for MCFCs, but because coal contains 

many contaminants in a wide range of concentrations, fuel derived from this source also contains a 

considerable number of contaminants.33 A critical concern with these contaminants is the 

concentration levels that can be tolerated by MCFCs without significant degradation in 

 

33. See Table 11.1 for contaminant levels found in fuel gases from various coal gasification processes. 
6-26 

performance or reduction in cell life. A list of possible effects of contaminants from coal-derived 

fuel gases on MCFCs is summarized in Table 6-6 (70). 

Table 6-6 Contaminants from Coal-Derived Fuel Gas and Their Potential Effect on 

MCFCs (70, Table 1, Pg. 299) 

Class Contaminant Potential Effect 

Particulates Coal fines, ash • Plugging of gas passages 

Sulfur compounds H2S, COS, CS2, C4H4S • Voltage losses 

• Reaction with electrolyte 

via SO2

Halides HCl, HF, HBr, SnCl2 • Corrosion 

• Reaction with electrolyte 

Nitrogen compounds NH3, HCN, N2 • Reaction with electrolyte 

via NOX

Trace metals As, Pb, Hg, Cd, Sn 

Zn, H2Se, H2Te, AsH3

• Deposits on electrode 

• Reaction with electrolyte 

Hydrocarbons C6H6, C10H8, C14H10 • Carbon deposition 

The typical fuel gas composition and contaminants from an air-blown gasifier that enter the MCFC 

at 650 °C after hot gas cleanup, and the tolerance level of MCFCs to these contaminants are listed 

in Table 6-7 (79, 71, 72). It is apparent from this example that a wide spectrum of contaminants is 

present in coal-derived fuel gas. The removal of these contaminants can add considerably to the 

efficiency. A review of various options for gas cleanup is presented by Anderson and Garrigan 

(70) and Jalan, et al. (73). 

Sulfur: It is well established that sulfur compounds in low parts per million concentrations in fuel 

gas are detrimental to MCFCs (74, 75, 76, 77, 78). The tolerance of MCFCs to sulfur compounds 

(74) is strongly dependent on temperature, pressure, gas composition, cell components, and system 

operation (i.e., recycle, venting, gas cleanup). The principal sulfur compound that has an adverse 

effect on cell performance is H2S. At atmospheric pressure and high gas utilization (~75 percent), 

<10 ppm H2S in the fuel can be tolerated at the anode (tolerance level depends on anode gas 

composition and partial pressure of H2), and <1 ppm SO2 is acceptable in the oxidant (74). These 

concentration limits increase when the temperature increases, but they decrease at increasing 

pressures. 
6-27 

Table 6-7 Gas Composition and Contaminants from Air-Blown Coal Gasifier After 

Hot Gas Cleanup, and Tolerance Limit of MCFCs to Contaminants 

Fuel Gasa

(mol percent) 

Contaminantsb,c Contentb,c Remarksb Tolerancec,d

Limit 

19.2 CO Particulates <0.5 mg/l Also includes ZnO from 

H2S cleanup stage 

<0.1 g/l for 

large 

particulates 

>0.3 :m 

13.3 H2 NH3 2600 ppm <10,000 

ppm 

2.6 CH4 AsH3 <5 ppm < 1 ppm 

6.1 CO2 H2S <10 ppm After first-stage cleanup <0.5 ppm 

12.9 H2O HCl 500 ppm Also includes other 

halides 

<10 ppm 

45.8 N2 Trace Metals <2 ppm 

<2 ppm 

<2 ppm 

<2 ppm 

Pb 

Cd 

Hg 

Sn 

<1 ppm 

30+ ppm 

35+ ppm 

NA 

 Zn <50 ppm From H2S hot cleanup <20 ppm 

 Tar 4000 ppm Formed during 

desulfurization cleanup 

stage 

<2000 ppme

a - Humidified fuel gas enters MCFC at 650 °C 

b - (71, Table 1, Pg. 177) 

c - (79) 

d - (72) 

e - Benzene 

The mechanisms by which H2S affects cell performance have been investigated extensively (75, 

76, 77, 78). The adverse effects of H2S occur because of: 

• Chemisorption on Ni surfaces to block active electrochemical sites, 

• Poisoning of catalytic reaction sites for the water gas shift reaction, and 

• Oxidation to SO2 in a combustion reaction, and subsequent reaction with carbonate ions in the 

electrolyte. 
6-28 

The adverse effect of H2S on the performance of MCFCs is illustrated in Figure 6-11. The cell 

voltage of a 10 cm x 10 cm cell at 650 °C decreases when 5 ppm H2S is added to the fuel gas (10 

percent H2/5 percent CO2/10 percent H2O/75 percent He), and current is drawn from the cell. The 

measurements indicate that low concentrations of H2S do not affect the open circuit potential, but 

they have a major impact on the cell voltage as current density is progressively increased. The 

decrease in cell voltage is not permanent;34 when fuel gas without H2S is introduced into the cell, 

the cell voltage returns to the level for a cell with clean fuel. These results can be explained by the 

chemical and electrochemical reactions that occur involving H2S and S=

. A nickel anode at anodic 

potentials reacts with H2S to form nickel sulfide: 

H2S + CO3

= → H2O + CO2 + S= (6-28)

followed by 

Ni + xS= → NiSx + 2xe- (6-29)

When the sulfided anode returns to open circuit, the NiSx is reduced by H2: 

NiSx + xH2 → Ni + xH2S (6-30)

Similarly, when a fuel gas without H2S is introduced to a sulfided anode, reduction of NiSx to Ni 

can also occur. Detailed discussions on the effect of H2S on cell performance are presented by 

Vogel and co-workers (75, 76) and Remick (77, 78). 

The rapid equilibration of the water gas shift reaction in the anode compartment provides an 

indirect source of H2 by the reaction of CO and H2O. If H2S poisons the active sites for the shift 

reaction, this equilibrium might not be established in the cell, and a lower H2 content than 

predicted would be expected. Fortunately, evidence (77, 78) indicates that the shift reaction is not 

significantly poisoned by H2S. In fact, Cr used in stabilized-Ni anodes appears to act as a sulfur 

tolerant catalyst for the water gas shift reaction (78). 

The CO2 required for the cathode reaction is expected to be supplied by recycling the anode gas 

exhaust (after combustion of the residual H2) to the cathode. Therefore, any sulfur in the anode 

effluent will be present at the cathode inlet unless provisions are made for sulfur removal. In the 

absence of sulfur removal, sulfur enters the cathode inlet as SO2, which reacts quantitatively 

(equilibrium constant is 1015 to 1017) with carbonate ions to produce alkali sulfates. These sulfate 

 

34. The effects of H2S on cell voltage are reversible if H2S concentrations are present at levels below that required to 

form nickel sulfide. 
6-29 

ions are transported through the electrolyte structure to the anode during cell operation. At the 

anode, SO4

=

 is reduced to S=, thus increasing the concentration of S= there. 

Figure 6-11 Influence of 5 ppm H2S on the Performance of a Bench Scale MCFC 

(10 cm x 10 cm) at 650 °C, Fuel Gas (10 percent H2/5 percent CO2/10 percent H2O/75 

percent He) at 25 percent H2 Utilization (78, Figure 4, Pg. 443) 

Based on the present understanding of the effect of sulfur on MCFCs, and with the available cell 

components, it is projected that long-term operation (40,000 hr) of MCFCs may require fuel gases 

with sulfur35 levels of the order 0.01 ppm or less, unless the system is purged of sulfur at periodic 

intervals or sulfur is scrubbed from the cell burner loop (76). Sulfur tolerance would be 

approximately 0.5 ppm (see Table 6-3) in the latter case. Considerable effort has been devoted to 

develop low-cost techniques for sulfur removal, and research and development are continuing (80, 

81). The effects of H2S on cell voltage are reversible if H2S concentrations are present at levels 

below which nickel sulfide forms. 

Halides: Halogen-containing compounds are destructive to MCFCs because they can lead to 

severe corrosion of cathode hardware. Thermodynamic calculations (82) show that HCl and HF 

react with molten carbonates (Li2CO3 and K2CO3) to form CO2, H2O, and the respective alkali 

halides. Furthermore, the rate of electrolyte loss in the cell is expected to increase because of the 

high vapor pressure of LiCl and KCl. The concentration of Cl-

 species in coal-derived fuels is 

typically in the range 1 to 500 ppm. It has been suggested (83) that the level of HCl should be kept 

below 1 ppm in the fuel gas, perhaps below 0.5 ppm (47), but the tolerable level for long-term 

operation has not been established. 

Nitrogen Compounds: Compounds such as NH3 and HCN do not appear to harm MCFCs (70, 79) 

in small amounts. However, if NOX is produced by combustion of the anode effluent in the 

cell burner loop, it could react irreversibly with the electrolyte in the cathode compartment to form 

nitrate salts. The projection by Gillis (84) for NH3 tolerance of MCFCs was 0.1 ppm, but Table 6-

3 indicates that the level could be 1 vol percent (47). 

 

35. Both COS and CS2 appear to be equivalent to H2S in their effect on MCFCs (76). 
6-30 

Solid Particulates: These contaminants can originate from a variety of sources, and their presence 

is a major concern because they can block gas passages and/or the anode surface. Carbon 

deposition and conditions that can be used to control its formation have been discussed earlier in 

this section. Solid particles such as ZnO, which is used for sulfur removal, can be entrained in the 

fuel gas leaving the desulfurizer. The results by Pigeaud (72) indicate that the tolerance limit of 

MCFCs to particulates larger than 3 μm diameter is <0.1 g/l. 

Other Compounds: Experimental studies indicate that 1 ppm As from gaseous AsH3 in fuel gas 

does not affect cell performance, but when the level is increased to 9 ppm As, the cell voltage 

drops rapidly by about 120 mV at 160 mA/cm2

 (71). Trace metals, such as Pb, Cd, Hg, and Sn in 

the fuel gas, are of concern because they can deposit on the electrode surface or react with the 

electrolyte (16). Table 6-3 addresses limits of these trace metals. 

6.2.5 Effects of Current Density 

The voltage output from an MCFC is reduced by ohmic, activation, and concentration losses that 

increase with increasing current density. The major loss over the range of current densities of 

interest is the linear iR loss. The magnitude of this loss (iR) can be described by the following 

equations (64, 85, 86): 

∆VJ(mV) = -1.21∆J for 50 < J < 150 (6-31)

∆VJ(mV) = -1.76∆J for 150 < J < 200 (6-32)

where J is the current density (mA/cm2

) at which the cell is operating. 

6.2.6 Effects of Cell Life 

Endurance of the cell stack is a critical issue in the commercialization of MCFCs. Adequate cell 

performance must be maintained over the desired length of service, quoted by one MCFC 

developer as being an average potential degradation no greater than 2mV/1,000 hours over a cell 

stack lifetime of 40,000 hours (29). State-of-the-art MCFCs (55, 64, 66, 87, 88) depict an average 

degradation over time of 

∆Vlifetime(mV) = -5mV/1000 hours (6-33)

6.2.7 Internal Reforming 

In a conventional fuel cell system, a carbonaceous fuel is fed to a fuel processor where it is steam 

reformed to produce H2 (as well as other products, CO and CO2, for example), which is then 

introduced into the fuel cell and electrochemically oxidized. The internal reforming molten 

carbonate fuel cell, however, eliminates the need for a separate fuel processor for reforming 
6-31 

carbonaceous fuels. This concept is practical in high-temperature fuel cells where the steam 

reforming reaction36 can be sustained with catalysts. By closely coupling the reforming reaction 

and the electrochemical oxidation reaction within the fuel cell, the concept of the internal 

reforming MCFC is realized. The internal reforming MCFC eliminates the need for the external 

fuel processor. It was recognized early that the internal reforming MCFC approach provides a 

highly efficient, simple, reliable, and cost effective alternative to the conventional MCFC system 

(89). Development to date in the U.S. and Japan continues to support this expectation (85, 90). 

There are two alternate approaches to internal reforming molten carbonate cells: indirect internal 

reforming (IIR) and direct internal reforming (DIR). In the first approach, the reformer section is 

separate, but adjacent to the fuel cell anode. This cell takes advantage of the close-coupled thermal 

benefit where the exothermic heat of the cell reaction can be used for the endothermic reforming 

reaction. Another advantage is that the reformer and the cell environments do not have a direct 

physical effect on each other. A disadvantage is that the conversion of methane to hydrogen is not 

promoted as well as in the direct approach. In the DIR cell, hydrogen consumption reduces its 

partial pressure, thus driving the methane reforming reaction, Equation (6-34), to the right. 

Figure 6-12 depicts one developer's approach where IIR and DIR have been combined. 

 

Figure 6-12 IIR/DIR Operating Concept, Molten Carbonate Fuel Cell Design (29) 

 

36. Steam reforming of CH4 is typically performed at 750 to 900 °C; thus, at the lower operating temperature of 

MCFCs, a high activity catalyst is required. Methanol is also a suitable fuel for internal reforming. It does not 

require an additional catalyst because the Ni-based anode is sufficiently active. 
6-32 

Methane is a common fuel in internal reforming MCFCs, where the steam reforming reaction 

CH4 + H2O → CO + 3H2 (6-34)

occurs simultaneously with the electrochemical oxidation of hydrogen in the anode compartment. 

The steam reforming reaction is endothermic, with ∆H650°C = 53.87 kcal/mol (89), whereas the 

overall fuel cell reaction is exothermic. In an internal reforming MCFC, the heat required for the 

reaction in Equation (6-34) is supplied by heat from the fuel cell reaction, thus eliminating the need 

for external heat exchange that is required by a conventional fuel processor. In addition, the 

product steam from the reaction in Equation (6-1) can be used to enhance the reforming reaction 

and the water gas shift reaction to produce additional H2. The forward direction of the reforming 

reaction (Equation (6-34)) is favored by high temperature and low pressure; thus, an internal 

reforming MCFC is best suited to operate near atmospheric pressure. 

A supported Ni catalyst (e.g., Ni supported on MgO or LiAlO2) sustains the steam reforming 

reaction at 650 °C to produce sufficient H2 to meet the needs of the fuel cell. The interrelationship 

between the conversion of CH4 to H2 and its utilization in an internal reforming MCFC at 650 °C is 

illustrated in Figure 6-13. At open circuit, about 83 percent of the CH4 was converted to H2, which 

corresponds closely to the equilibrium concentration at 650°C. When current is drawn from the 

cell, H2 is consumed and H2O is produced, and the conversion of CH4 increases and approaches 

100 percent at fuel utilizations greater than about 65 percent. Thus, by appropriate thermal 

management and adjustment of H2 utilization with the rate of CH4 reforming, a similar 

performance can be obtained in internal reforming MCFC stacks with natural gas and with 

synthesized reformate gas containing H2 and CO2, Figure 6-14. The concept of internal reforming 

has been successfully demonstrated for more than 15,000 hours in a 5 kW stack (91 and more than 

10,000 hours in a 250 kW stack (92) The performance of the 2 kW stack over time can be seen in 

Figure 6-15 (13). 
6-33 

70

75

80

85

90

95

100

105

0 10 20 30 40 50 60 70 80 90 100

Fuel Utilization (%)

Methane Conversion (%)

Figure 6-13 CH4 Conversion as a Function of Fuel Utilization in a DIR Fuel Cell 

(MCFC at 650 oC and 1 atm, steam/carbon ratio = 2.0, >99 percent methane conversion 

achieved with fuel utilization > 65 percent (93) 

Current Density (mA/cm2) 

Figure 6-14 Voltage Current Characteristics of a 3kW, Five Cell DIR Stack 

with 5,016 cm2

 Cells Operating on 80/20 percent H2/CO2 and Methane (85) 
6-34 

Figure 6-15 Performance Data of a 0.37m2 2 kW Internally Reformed MCFC Stack at 

650 °C and 1 atm (13) 

Direct Internal Reforming Catalysts: The anode catalyst is deactivated by the alkali carbonate’s 

electrolyte-containing environment. Making hardware of a non-wetting metal such as nickel has 

mitigated electrolyte creepage over the hardware surface towards the catalyst. Presently DIR 

catalyst deactivation is mainly by the vapor phase alkali species. The deactivation mechanism 

includes electrolyte-accelerated sintering, pore filling/plugging, and surface coverage. Making 

hardware of a non-wetting metal such as nickel has mitigated electrolyte creepage over the 

hardware surface towards the catalyst. Alkali-resistant supports such as magnesium oxide, 

calcium aluminate, and α-alumina have been investigated to reduce vapor phase alkali species 

effects. Results show that these supports undergo different degrees of decay. Ruthenium and 

rhodium-based catalysts are more stable, but are too costly (95, 96) FCE has identified a more 

active and stable DIR catalyst (high activity supported Ni), projecting a catalyst life exceeding 

40,000 hours and pursuing further enhancement of catalyst life. Another approach is to apply a 

getter-type barrier to trap the volatile alkali species before they reach the catalysts. A porous Ni 

or a SiC membrane was placed between the cell internal catalyst and the electrolyte-containing 

components. (37) 

6.3 Summary of Equations for MCFC 

The preceding sections provide parametric performance based on various referenced data at 

different operating conditions. It is suggested that the following set of equations could be used for 

performance adjustments unless the reader prefers other data or correlations. Figure 6-16 is 

provided as reference MCFC performance. 
6-35 

Parameter Equation Comments

Pressure ∆Vp(mV) = 76.5 log 2

1

P

P

1 atm < P < 10 atm (6-19) 

Temperature ∆VT(mV) = 2.16(T2 - T1) 

∆VT(mV) = 1.40(T2 - T1) 

∆VT(mV) = 0.25(T2 - T1) 

575°C < T < 600 °C (6-21) 

600°C < T < 650 °C (6-22) 

650°C < T < 700 °C (6-23) 

Oxidant ∆Vcathode(mV) = 250 log (P P )

(P P )

2 2

2 2

CO O

1/2

2

CO O

1/2

1

0.04 (P P ) 0.11 CO O 2 2

1/2 ≤ ≤ (6-25) 

∆Vcathode(mV) = 99 log (P P )

(P P )

2 2

2 2

CO O

1/2

2

CO O

1/2

1

0.11 (P P ) 0.38 CO O 2 2

1/2 ≤ ≤ (6-26) 

Fuel ∆Vanode(mV) = 173 log (P / P P )

(P / P P )

2 22

2 22

H CO H O

1/2

2

H CO O

1/2

1

 (6-27) 

Current 

Density 

∆VJ(mV) = -1.21 ∆J 

∆VJ(mV) = -1.76 ∆J 

50 < J < 150mA/cm2

 (6-31) 

150 < J < 200mA/cm2

 (6-32) 

Life Effects ∆Vlifetime(mV) = -5mV/1000 hours (6-33) 

Figure 6-16 Average Cell Voltage of a 0.37m2 2 kW Internally Reformed MCFC Stack at 

650 °C and 1 atm. Fuel, 100 percent CH4, Oxidant, 12 percent CO2/9 percent O2/77 

percent N2 

FuelCell Energy presented a computer model for predicting carbonate fuel cell performance at 

different operating conditions. The model was described in detail at the Fourth International 

Symposium on Carbonate Fuel Cell Technology, Montreal, Canada, 1997 (97). The model 

equations are as follows: 
6-36 

The general voltage versus current density relation is: 

V E ( ) iz = −+− − Nernst ηη η a c conc r (6-41)

where 

0 0

RT V E ln

2

= + F ( ,

P

P ,P

H ,a

CO a H O

2

2 2 a

PCO , c 2 P0 ,c

1/2

2 ) (6-42)

At low current density (i<0.04 A/cm2

) 

iRT

2 η =a F

1

Ka

0 eEa/ T pH

0.5

2

β− pCO2−β pH O2−β (6-43)

c

iRT

2 η = F

1

Ka

0 eEc/ T pCO

b

2

1

' − pO

b

2

2

' − (6-44)

At high current density (i < 0.04A/cm2

) 

a 0 1 H 2 CO ,a 3 H O 4 5 ( ) 222

RT a a lnp a lnp a lnp a /T a ln (i) 2 η = + + + ++ F (6-45)

c 0 1 CO ,c 2 o 3 4 ( ) 2 2

RT b b lnp b lnp b /T b lni 2 η= + + + + F (6-46)

and 

c ln(1 i/i ) η = 6 − L (6-47)

cell resistance 
6-37 

Z Z exp[c(

1

T

1

T

)] r 0

0

= − (6-48)

A description of the parameters in the model follows: 

 V = Cell voltage, V 

 E° = Standard E.M.F., V 

 R = Universal gas constant (8.314 joule/deg-mole) 

 T = Temperature, K 

 P = Partial pressure of gas compositions at anode (a) or cathode (c), atm. 

η = Polarization, V 

 i = Current density, A/cm2

 z = Cell impedance, Ω-cm2

F = Faraday’s Constant (96,487 joule/volt - gram equivalent) 

 a,b,c = Parameters determined for experiments 

The parameters in the above equations were calibrated from 400 sets of FCE’s laboratory-scale 

test data and were further verified by several large-scale stack experiments. These parameter 

values may depend on the FCE cell design and characteristics, and may not be directly applicable 

to other carbonate technologies. Figure 6-17 is a comparison of the measured data match with 

the model prediction. 

600

650

700

750

800

850

900

950

1000

1050

1100

0 20 40 60 80 100 120 140 160 180 200

Current Density (mA/cm2

)

Cell Voltage (mV)

75% Fuel/75% CO2 Util at 160 mA/cm2 with 

Dilute Oxidant (18%CO2 and 12% O2 

 83% Pre-reformed CH4 (IIR-DIR)

 Simulated Pre-reformed CH4 

 (External Reforming)

 EXP. MODEL

Figure 6-17 Model Predicted and Constant Flow Polarization Data Comparison (98) 
6-38 

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in the Proceedings of the 27th Intersociety Energy Conversion Engineering Conference, 

Vol. 3, San Diego, CA, Aug. 3-7, 1992, Pg. 290, 1992. 

89. H.C. Maru, B.S. Baker, Prog. Batteries & Solar Cells, 5, 264, 1984. 

90. M. Faroogue, G. Steinfield, H. Maru, "Comparative Assessment of Coal-Fueled Carbonate 

Fuel Cell and Competing Technologies," in Proceedings of the 25th IECEC, Vol. 3, 

American Institute of Chemical Engineers, New York, NY, 1990. 

91. H.C. Maru and M. Farooque, FCE, “Molten Carbonate Fuel Cell Product Design 

Improvement,” Prepared for U.S. DOE/DARPA, Annual Report, DE-FC21-95MC31184, 

Pages 5-9, December 1999. 

92. H.C. Maru and M. Farooque, FCE, “Molten Carbonate Fuel Cell Product Design 

Improvement,” Prepared for U.S. DOE/DARPA, Annual Report, DE-FC21-95MC31184, 

Pages 4-10, December 2000. 
6-43 

93. ERC correspondence, laboratory data, March 1998. 

94. M. Farooque, data from ERC testing, 1992 

95. S. Katikaneni, C. Yuh, S. Abens and M. Farooque, “Catalysis Today, 2002”. 

96. S. Katikaneni, C. Yuh and M. Farooque, “The Direct Carbonate Fuel Cell Technology: 

Advances in Fuel Processing and Internal Reforming,” ACS Fuel Chemistry, 46(2) 685-688, 

Preprints, 2001. 

97. J. Ding, et al., “A Computer Model for Direct Carbonate Fuel Cells,” Proceedings of the 

Fourth International Symposium on Carbonate Fuel Cells, 191st Electrochemical Society 

Meeting, Montreal, May 1997. 

98. J. Ding, P. S. Patel, M. Farooque, H. C. Maru, in Carbonate Fuel Cell Technology IV, 

(eds. J. R. Selman, et al.), The Electrochemical Society, Inc., New Jersey, Pg. 127-138, 1997. 
7-1 

7. SOLID OXIDE FUEL CELLS

Solid oxide fuel cells (SOFCs) have an electrolyte that is a solid, non-porous metal oxide, 

usually Y2O3-stablilized ZrO2. The cell operates at 600-1000 o

C where ionic conduction by 

oxygen ions takes place. Typically, the anode is a Ni-ZrO2 cermet and the cathode is Sr-doped 

LaMnO3. There is no liquid electrolyte with its attendant material corrosion or electrolyte 

management problems. The high temperature of the SOFC, however, places stringent 

requirements on its materials. The development of suitable low cost materials and the low-cost 

fabrication of ceramic structures are presently the key technical challenges facing SOFCs. 

The cell is constructed with two porous electrodes that sandwich an electrolyte. Air flows along 

the cathode. When an oxygen molecule contacts the cathode/electrolyte interface, it acquires 

electrons from the cathode. The oxygen ions diffuse into the electrolyte material and migrate to 

the other side of the cell where they contact the anode. The oxygen ions encounter the fuel at the 

anode/electrolyte interface and react catalytically, giving off water, carbon dioxide, heat, and 

electrons. The electrons transport through the external circuit, providing electrical energy. 

Solid oxide fuel cells (SOFC) allow conversion of a wide range of fuels, including various 

hydrocarbon fuels. The relatively high operating temperature allows for highly efficient 

conversion to power, internal reforming, and high quality by-product heat for cogeneration or for 

use in a bottoming cycle. Indeed, both simple-cycle and hybrid SOFC systems have 

demonstrated among the highest efficiencies of any power generation system, combined with 

minimal air pollutant emissions and low greenhouse gas emissions. These capabilities have 

made SOFC an attractive emerging technology for stationary power generation in the 2 kW to 

100s MW capacity range. 

More recently, (planar) SOFC systems with high power densities operating at lower temperatures 

(700 to 850 °C instead of 900 to 1000 °C as was previously the norm) have been developed. 

Combined with the ability of SOFC to use conventional fossil fuels, this could help reduce the 

cost of the fuel cell because less-expensive materials of construction could be used at lower 

temperatures. This would improve the economy of applications ranging from small-scale 

stationary power (down to ~2 kW) to auxiliary power units for vehicles and mobile generators 

for civilian as well as military applications. There is even the possibility that SOFC could 

eventually be used for part of the prime power in vehicles. The present challenge for developers 

is to produce robust, high-performance stack technologies based on suitable low-cost materials 

and fabrication methods. Derivatives from SOFC technology, such as oxygen sensors used in 

automobiles, are already in widespread commercial use. 
7-2 

This chapter provides an overview of the key features and characteristics of SOFC, along with 

descriptions of the main types of SOFC and their performance. Those readers interested in 

greater detail, as well as an excellent history of SOFC development, are referred to Singhal and 

Kendall (1), and other references listed at the end of this chapter. 

7.1 Cell Components 

The major components of an individual SOFC cell include the electrolyte, the cathode, and the 

anode. Fuel cell stacks contain an electrical interconnect, which links individual cells together in 

series or parallel. The electrolyte is made from a ceramic such as yttria-stabilized zirconia (YSZ) 

and functions as a conductor of oxide ions. Oxygen atoms are reduced into oxide ions on the 

porous cathode surface by electrons, and then flow through the ceramic electrolyte to the fuelrich porous anode where the oxide ions react with fuel (hydrogen), giving up electrons. The 

interconnect serves to conduct the electrons through an external circuit. 

7.1.1 Electrolyte Materials 

As indicated by their name, SOFCs use solid oxide ceramics, typically perovskites, as the 

electrolyte. Nernst (2) realized in the 1890s that certain perovskites, stabilized zirconias, 

conducted ions in a certain temperature range. Baur and Preis (3) demonstrated in 1943 that such 

electrolytes could be used as (oxygen) ion conductors in fuel cells. Currently, yttrium stablilized 

zirconia (3, 8, or 10 percent yttria, abbreviated to YSZ) is the most commonly used electrolyte 

for SOFC. YSZ provides high conductivity at temperatures above 700 °C (Figure 7-1, (4, 5, 6)), 

while exhibiting negligible electronic conductivity at these temperatures (above 1500 °C it 

becomes an electronic conductor). In a fuel cell operating with a current density of 250 mA/cm2

at 1000 °C and an electrolyte of 200 μm thickness, the resistance loss in the electrolyte would be 

50 mV. However, for mechanical reasons it is desirable to operate the SOFC at lower 

temperatures. To operate at 800 °C, the electrolyte thickness would have to be reduced by about 

an order of magnitude to maintain a similar ohmic loss in the electrolyte. 

Colloidal fabrication and co-sintering processes have emerged, whereby YSZ membranes are 

produced as thin films (~10 μm) on porous electrode structures. These thin-film membranes 

improve performance and reduce operating temperatures of SOFCs. To enable these colloidal 

processes to be successful, finer YSZ powders are needed. These applications require nano-scale 

powders with BET surface areas of 100 to 120 m2

/g and the use of suspensions ranging from 10 

to 40 percent solid content (7, 8). 

Alternative electrolytes have been considered and are being developed. As shown in Figure 7-1, 

scandium-doped zirconia (SDZ) is more conductive than YSZ, permitting a further reduction of 

the operating temperature by 50 to 100 °C. Gadolinium-doped ceria is even more conductive, 

but is partially reduced in hydrogen at temperatures above 600 °C; formation of Ce+3 ions 

generates electron holes that make ceria electronically conductive, thus short-circuiting the cell. 

A substantially more conductive material that is stable in air and hydrogen was discovered by 

Goodenough (9). Lanthanum gallate with strontium doping on the A-site of the perovskite and 

magnesium on the B-site could be used at temperatures as low as 600 °C even on a thick 

electrolyte. Laboratory fuel cells with this electrolyte have been tested, but the typical 
7-3 

challenges of matching the thermal expansion coefficients, mechanical strength, and chemical 

compatibilities need further development. 

100

10-1

10-5

10-4

10-3

10-2

1500

0.015

0.15

1.5

15

150

0.8 1.6 1.0 1.2 1.4

1000 400 900 800 700 600 500

Ce0.9Gd0.1O2-d

in H2/H2O(Po2=10-20atm)

(Dokiya(5))

La0.8Sr0.2Ga0.83Mg0.2O3-d

(Goodenough(6))

8mol%Y2O3-ZrO2

(Yamamoto(7))

9mol%Sc2O3-ZrO2

(Yamamoto(7))

Ce0.9Gd0.1O2-d

(Dokiya(5))

1000/T(K)

T(oC)

Electrolyte Thickness for 0.15

Ωcm2(μm)

Electrical Conductivity (

Ω -1 cm-1)

Figure 7-1 Electrolyte Conductivity as a Function of Temperature (4, 5, 6) 

All of the above-mentioned solid electrolytes are oxygen conductors. An automatic consequence 

of this is that, as in molten carbonate fuel cells, the products of electrochemical reactions all end 

up on the anode side. While is beneficial for internal reforming and water gas shift reaction 

(which utilizes the water produced as a reactant), it dilutes the fuel, and at high utilization it can 

significantly reduce the Nernst potential. 

It has been shown that solid electrolytes can be made to conduct protons (10, 11, 12, 13). While 

these electrolytes are still in a very early stage of development, such proton conductors might 

eventually overcome some of the limitations of cells as oxygen ion conductors. 

7.1.2 Anode Materials 

Although a wide range of materials has been considered as anode materials for SOFC (14), most 

developers today use a cermet of nickel and YSZ. Early on in the development of SOFC, 

precious metals such as platinum and gold were used, as well as pure transition metals such as 

nickel and iron. Because of the physical and chemical instability of these materials, other 

materials such as nickel aluminide were tested. 

Finally, in 1970, Spacil (15) recognized that a composite of nickel and YSZ particles could 

provide a stable and highly active anode. The composition of the anode, particle sizes of the 

powders, and the manufacturing method are key to achieving high electrical conductivity, 

adequate ionic conductivity, and high activity for electrochemical reactions and reforming and 

shift reactions. Reduction of the NiO powder in the virgin anode mixture to Ni results in the 

desired porosity. For the more recent anode-supported cells, it also achieves good mechanical 

properties and maintains geometric stability during manufacture and operation. For example, by 
7-4 

using a combination of coarse and fine YSZ powder, mechanical strength can be ensured while 

also achieving the desired contact between the Ni phase and the YSZ phase. In some modern cell 

designs, a graded anode is used to achieve coarse porosity and high mechanical strength in most 

of the anode, and fine micro-porosity in the anode zone immediately adjacent to the electrolyte. 

Despite the relative success of the Ni-YSZ anode, it has drawbacks: 

• Sensitivity to sulfur and other contaminants. Strong reversible poisoning of the anode occurs 

at feed concentrations ranging from about 1 ppm H2S when operating at 1000 °C down to 

less than 50 ppb when operating at 750 °C (See Figure 7-2a (16, 17)). These concentrations 

require desulfurization of the anode feed, even if it is produced from low-sulfur fuels such as 

natural gas or ultra-low sulfur diesel or gasoline (See Figure 7-2b). No data is available 

publicly on the impact of other species (water or hydrocarbons) or different sulfur species on 

sulfur tolerance, or on the effect after long periods of time (e.g. 40,000 hours or more). 

Another strong anode poison reported is HCl. Poisoning by these species is reversible after 

exposure at low concentrations, but irreversible after exposure at concentrations above about 

200 ppm. 

• Oxidation reduction intolerance. Ni-YSZ anodes are made by mixing NiO with YSZ and then 

reducing the NiO to Ni. However, if the anode is subsequently exposed to air, especially at 

elevated temperatures, the Ni re-oxidizes readily. Because of the large volume change during 

the reduction/oxidation of the anode, the anode’s structure and strength are severely 

compromised. Effectively, the anodes must be kept under reducing conditions at all times. 

• The thermal expansion coefficient of the anode is substantially higher than the electrolyte 

and cathode. In anode-supported designs, this can lead to mechanical and dimensional 

stability problems, especially during thermal cycling. 

• Poor activity for direct oxidation of hydrocarbons and propensity for carbon formation when 

exposed to hydrocarbons. To improve the activity for direct oxidation and reduce the anode’s 

propensity for carbon formation, copper – ceria anodes are being developed. 

Even though these drawbacks can typically be mitigated by appropriate system design, many 

consider that better anodes will be needed. To improve the sulfur tolerance and reduction 

oxidation tolerance of the anodes, several groups are working on oxide-based anodes. 

Researchers at PNNL have demonstrated sulfur tolerance up to 100 ppm, at least for short times. 

In addition, as expected, the oxide-based anodes provide excellent oxidation/reduction stability. 

However, activity for hydrogen oxidation must still be improved to be competitive with Ni-YSZ 

anodes. In addition, though little experimental data exists, one would expect that these anodes 

must be modified to provide adequate activity for reforming and water gas shift reactions. 
7-5 

0.01

0.1

1

1000 1100 1200 1300

Tokyo Gas

Siemens

Westinhouse

Anode Sulfur Tolerance (ppmv)

Cell Operating Temperature (K)

Anode Feed Sulfur Concentration (ppmv)

Fuel Sulfur Concentration (ppmm)

anode sulfur 

with pipeline 

natural gas 

and SMR

JP-8

Current Diesel 

Standards

2010 Diesel 

Standards

0.01

0.1

1

10

100

1000

1 10 100 1000 10000

POX reformate

SMR reformate

(a) Sulfur Tolerance of Ni-YSZ Anodes (b) Relation between Fuel Sulfur Mass 

Concentratrion and Anode Gas Concentration

Figure 7-2 (a) Sulfur Tolerance of Ni-YSZ Anodes (16, 17) and (b) Relationship between 

Fuel Sulfur and Anode Sulfur Concentration. 

7.1.3 Cathode Materials 

Most cathode materials used in SOFC today are lanthanum-based perovskite materials (structure 

ABO3). During early development, platinum and other noble metals, and even magnetite (14), 

were used as cathode materials for SOFC. They are no longer pursued actively because of 

chemical and physical instability, incompatibility with most electrolytes, and, in the case of 

platinum, cost. Currently, most cathodes are based on doped lanthanum manganites. In hightemperature SOFC (operating temperature ~1000 °C), strontium-doped LaMnO3 (LSM) is used. 

The choice of this material is a compromise between a number of factors: 

• Chemical stability and relatively low interactions with electrolyte. With YSZ electrodes, 

many La-based compounds form the insulating La2Zr2O7. With ceria-based electrolytes, this 

issue is not a concern and other cathode materials are considered (e.g. (La,Sr)(Co,Fe)O3 or 

LSCF). 

• Adequate electronic and ionic conductivity. Though the conductivities are adequate, the ionic 

conductivity of LSM is significantly lower than YSZ, and its electronic conductivity is a 

fraction of any of the metals or even of lanthanum chromite. Consequently, ionic and 

electronic resistance can become a significant factor, especially in cell designs that 

incorporate long current paths through the cathode. For lower-temperature cells, conductivity 

of LSM is inadequate, and other materials, such as strontium-doped lanthanum ferrite (LSF) 

are considered. 

• Relatively high activity. 

• Manageable interactions with ceramic interconnects (notably lanthanum chromite). Though 

some interdiffusion occurs, this does not represent a major problem. 

• Thermal expansion coefficients that closely match those of YSZ. 
7-6 

Accordingly, the good compatibility with YSZ and the high electro-catalytic activity make LSM 

the cathode material of choice of SOFCs operating around 1000 °C. 

For intermediate-temperature operation (700 to 800 °C), a composite layer (typically 20 to 40 

μm thick) of YSZ and LSM is often used to overcome the modest ion conductivity at lower 

temperatures (18, 19, 20). Alternatively, LSCF or LSF are also pursued for such applications. 

A serious challenge in the use of LSM as a cathode material in intermediate temperature SOFC 

stems from the use of metallic interconnects. Many of these metals contain chromium, which 

forms a stable protective oxide (chromia) layer with reasonable conductivity (see Section 7.1.4 

on interconnects for more details). However, chromia vapors can lead to serious poisoning of the 

cathode (21, 22). Although one might attribute this problem more to the interconnect material 

than to the cathode, the poisoning effect was found to depend strongly on the electrolyte/cathode 

material combination. 

Figure 7-3 Impact of Chromia Poisoning on the Performance of Cells with Different 

Electrolytes (From (21)) 

For low-temperature operation (below 700 °C), the use of LSM as the cathode material 

represents significant potential loss, and other materials are being pursued. 

7.1.4 Interconnect Materials 

Broadly, interconnect materials for SOFC fall into two categories: conductive ceramic 

(perovskite) materials for operation at high temperature (900 to 1000 °C) and metallic alloys for 

lower temperature operation. Though the shape of SOFC interconnects depends heavily on the 

cell and stack design, the materials choice is almost entirely determined by physical and 

chemical stability under operating conditions. 

The ceramic interconnects used in higher temperature SOFCs are primarily doped lanthanum and 

yttrium chromites (dopants typically include Mg, Sr, Ca, Ca/Co). These perovskites are unique in 

that they exhibit high electronic conductivity and resist reduction under exposure to syngas at 

high temperatures. Electronic conductivity of these materials increases with temperature (making 

them unsuitable for use at low temperatures). At 1,000 °C the conductivities of these materials 
7-7 

range from 1 to around 30 S/cm, with an activation energy of 12 to 19 kJ/mol, depending on 

dopant and dopant level. The dopant levels also control thermo-mechanical properties and 

compatibility with electrode or electrolyte materials. Lanthanum chromite-based interconnects 

have shown to be stable in cells for as much as 69,000 hrs (23). However, one problem with 

ceramic interconnects is that they are rigid and weak, similar to the ceramic cells: there is no 

flexibility in any of the components to ensure good contact pressure. In some designs that use 

ceramic interconnects, a contact felt (23) or conductive contact paste is used. Unfortunately, the 

reliability of this component is not as good as the interconnect. 

In the past ten years, with the development of thin-electrolyte anode-supported SOFC operation 

at lower temperatures (lower than 800 °C), the prospect of using metallic interconnects arose. 

However, even at temperatures ranging from 650 to 800 °C, typical state-of-the-art anodesupported SOFC operating conditions and design requirements for metallic interconnects are 

challenging. For example: 

• High operating temperature in excess of the drop-off in creep strength for many common 

metals and thermal cycling. At the same time, the interconnect must maintain uniform 

contact (usually requiring some pressure) with the electrodes. 

• Exposure (at least on one side) to strongly oxidizing environment, while at the same time 

requiring low contact resistance with the electrodes. This is a challenge because many of the 

stable oxides that protect high-temperature alloys from corrosion (see Figure 7-4) such as 

alumina and silica) have very low conductivities. The most commonly-used stable oxide that 

does have some electronic conductivity (chromia) leads to evaporation and electrode 

poisoning. 

Early on, metallic interconnects for cells operating at around 900 °C included high-chrome alloys 

(notably the Cr5Fe1Y2O3 developed by Plansee A.G. and Siemens (24, 25). Aside from potential 

for electrode poisoning, the high chrome content results in a high materials cost. Because these 

alloys are typically formed using powder metallurgy followed by machining, processing results 

in an expensive interconnect. 

Lower operating temperatures would allow the use of ferritic steels, that could reduce the 

materials cost, and ferritic steels are typically easier to process with low-cost processing 

techniques. The corrosion resistance of steel depends on the formation of stable oxide layers on 

the surface (Figure 7-5). After extensive testing of commercial compositions, it was concluded 

that none possessed the corrosion resistance required, especially to withstand the thermal cycling 

requirements while still providing adequate contact resistance. Efforts were undertaken to 

develop more suitable compositions, which led to the development of several special alloys. 

Many developers now use the Krupp formulation Crofer22 APU. 
7-8 

Figure 7-4 Stability of Metal Oxides in Stainless Steels (26, 27) 

To ensure good contact resistance (primarily with the cathode) and minimize evaporation of 

chromia, many developers use interconnect coatings of strontium-doped lanthanum cobaltite or 

manganite, which have proven effective for at least several thousand hours. 

Figure 7-5 Impact of LSCM Contact Layer on Contact Resistance in Cell with Metal 

Interconnect (from (28)). 
7-9 

With these improvements, interconnects can be made that function in intermediate temperature 

SOFCs, although several additional improvements may still require attention to allow the 

construction of commercially viable products: 

• Further improvement in contact resistance, especially after long exposure and thermal 

cycling 

• Further improvements in corrosion resistance, especially after long exposure and thermal 

cycling 

• Improved performance and mechanical stability of the coatings 

• Low-cost manufacturing methods for materials, shapes, and coatings 

• Improved creep strength to increase design flexibility for cells 

SOFC anodes are fabricated from composite powdered mixtures of electrolyte materials (YSZ, 

GDC, or SDC) and nickel oxide. The nickel oxide is subsequently reduced to nickel metal prior 

to operation. The NiO/YSZ anode material is suited for applications with YSZ material, whereas 

NiO/SDC and NiO/GDC anode materials are best used with ceria-based electrolyte materials. 

Typical anode materials have nickel content of approximately 40 volume percent after reduction 

of the nickel oxide to nickel. Depending upon the application, powders have surface areas of 15 

to 20 m2

/g for screen-printing and 5 to 10 m2/g for tape casting. 

7.1.5 Seal Materials 

The challenges of sealing the oxidant from fuel in planar SOFC stacks is significant, hence a 

sub-section is devoted to potential seal materials here. The function of SOFC seals includes: 

• Prevent mixing of fuel and oxidant 

• In some configurations, prevent mixing of reactants with the ambient environment 

• In some configurations, provide mechanical bonding of components 

• In some designs, provide electrical insulation between stack components 

Seal materials must be chemically and physically stable at operating conditions. In some 

applications (e.g. in on-road vehicles), the seal must also be able to withstand acceleration forces 

associated with vibration and shock. Finally, seal materials must be low in cost and amenable to 

low-cost stack manufacturing methods. 

These requirements are tough to meet simultaneously. For example, the chemical stability of a 

material may be acceptable under either oxidizing or reducing environments. However, 

mechanistic characterizations have shown that when relatively thin pieces of material are 

exposed to both atmospheres, rapid deterioration occurs.(29). Seal designs are highly specific to 

particular cell and stack designs and, consequently, seal designs are often proprietary. Some 

tubular and monolithic designs require no seals at all. Planar designs typically require multiple 

seals per repeat unit, and even in planar designs the length of the seals can vary by two or three 

orders of magnitude for a given area cell depending on design. A number of possible seal types is 

shown in Figure 7-6 for a rectangular planar cell with metal interconnects. 
7-10 

Figure 7-6 Possible Seal Types in a Planar SOFC (from (29)) 

The requirements, material choices, and general sealing concepts are common to most planar 

SOFC stack designs. Fundamentally, two different types of seals are being developed for SOFC: 

bonded and compressive seals. 

Bonded Seals 

Bonded seals can be rigid or compliant. A hermetic seal is achieved through adhesive forces 

between the seal material and both surfaces against which the seal is to work. Naturally, the seal 

material must have good adhesive properties (good wettability of the material to be sealed). 

Some are designed to remain flexible over the operating range of the cell, while others are meant 

to be rigid. To use the rigid type of seal, the thermal expansion coefficient of the seal material 

and all other components must be closely matched. If the seal is compliant, the thermal 

expansion coefficient matching requirements are somewhat relaxed. The bonding temperature 

for this type of seal should lie between the operating temperature and the stability limit for the 

other cell materials. There are several common sub-types of bonded seals currently under 

consideration for SOFC applications. Glass and glass-ceramic seals are perhaps the most 

common. This type of seal is attractive because: 

• Viscous/wetting behavior of glass facilitates hermetic sealing 

• They are inexpensive and easy to manufacture and apply 

• Wide range of compositions of glass and ceramics allows tailoring some of the key properties 

(e.g. thermal expansion coefficient glass transition temperature) 

• Glass-ceramics can be designed to avoid viscous flow and uncontrolled progressive 

crystallization during operation 

However, glass-ceramic seals also exhibit disadvantages: 

• They are brittle, leading to seal and even cell failures during cool-down; 

• Despite control, few glass systems allow a match of thermal expansion coefficient to other 

important cell materials (typically alkaline earth-alumina-silica glasses). In any case, the cell 

materials don’t match each other close enough to allow a rigid seal in larger cells 

• Many glasses interact with adjacent cell components, especially with the interconnects 

• Some of the constituents of glass volatilize during operation (e.g. silica, borate, and alkali 

metals). These constituents will likely foul or poison the electrode catalyst or interact in an 

undesirable manner with other cell components 
7-11 

Metal brazes, which use a molten metal filler to ensure sealing, provide some attractive features: 

• Molten metal facilitates hermetic sealing 

• Easy to fabricate 

• Properties can be tailored by judicious choice of composition 

However, several factors limit their application in SOFC: 

• Brazes are electrically conductive, making them unsuitable of most seal types 

• Few braze materials are compatible with SOFC operating conditions. Noble metals are 

considered too expensive in most SOFC stack designs. Silver is less expensive, but its use in 

a dual (oxidizing and reducing) environment can lead to chemical instability 

In addition to the benefits listed above, bonded seals result in compact structures, as no loadframe or other means to apply pressure is required. However, in cells with metal interconnects, 

the mismatch in thermal expansion may be too great for the use of rigid seals. For example, 

Figure 7-7 shows that in a typical cell 10 cm across, the relative movement of the edges of the 

interconnect with respect to the edges of the anode is almost 100 μm. Compare that with the 

typical thickness of the seal (around 200 μm) and consider that the shear stresses on the seal 

would build up to around 17 MPa (30): far too much for the rigid glass or glass-ceramic seals to 

withstand. To date, no compliant bonded seals have been identified or developed. 

0

1

2

3

4

5

6

7

8

9

10

200 400 600 800

Temperature (C)

1000 * 

∆L/L

Electrolyte

Cathode

Ferritic Stainless Steel

Anode

Figure 7-7 Expansion of Typical Cell Components in a 10 cm x 10 cm Planar SOFC with 

Ni-YSZ anode, YSZ Electrolyte, LSM Cathode, and Ferritic Steel Interconnect. 
7-12 

Compressive Seals 

A hermetic seal is achieved by pressing the seal material between the surfaces to be sealed. The 

seal material must be elastic over the operating temperature range, and sufficiently soft to fill the 

micro-roughness on the surfaces to be sealed. Compressive seals offer several advantages (29): 

• Mechanically “de-couple” adjacent stack components, thus reducing thermal stress during 

cycling 

• Thermal expansion matching requirements between cell components may be somewhat 

relaxed (though electrical contact considerations may still require this) 

• Some are easy and inexpensive to fabricate 

However, there are also barriers to overcome (29): 

• Difficult to achieve a hermetic seal with some materials unless “soft seat” interlayer is 

provided 

• Few materials and structures are compliant and provide a hermetic seal at the operating 

temperatures 

• A load frame is required to provide compression to all seals. This type of hardware is 

potentially bulky and expensive. If (portions of the) load frame must be kept at lower 

temperatures than the stack itself, packaging and insulation is significantly complicated, 

especially if multiple stacks are to be combined for larger-capacity systems 

• Other stack components must be designed to withstand prolonged pressure. This can be a 

challenge, given that creep strength of the metals used in the interconnect is typically very 

low (in the 700 to 800 °C operating temperature range typical for state-of-the-art planar cells) 

• To the extent that electrical contact between cell components depends on controlled pressure, 

balancing these pressure requirements with those of the seal can be a challenge for the cell 

designer 

Recently, mica and hybrid mica seals have been developed as a viable technology. Mica seals 

were found to have many desirable characteristics, such as the ability to withstand thermal 

cycling, but exhibited unacceptable leak rates. When a thin layer of glass is inserted on either 

side of the seal to fill the voids between the seal and the other stack components, the leak rate 

was substantially reduced while other desirable properties were retained. 

Figure 7-8 shows the leak rate can be reduced to about 0.05 to 0.2 sccm/cm (which translates 

into less than 1 percent of the fuel for typical 10 cm x 10 cm cells) for at least several dozen 

cycles. 

While this progress is encouraging, the long-term physical and chemical stability of all seal types 

considered for SOFC still require additional improvement. 
7-13 

Figure 7-8 Structure of Mica and Mica-Glass Hybrid Seals and Performance 

of Hybrid Seals (29) 

7.2 Cell and Stack Designs 

Two types of cell designs are being pursued for SOFC: tubular cells and planar cells. The interest 

in tubular cells is unique to SOFC: all other types of fuel cells focus exclusively on planar 

designs. In SOFC, the benefit of a simple sealing arrangement potentially outweighs the 

disadvantages of low volumetric power density and long current path that are inherent in tubular 

cell geometries. 

7.2.1 Tubular SOFC 

Although the Siemens Westinghouse design of tubular SOFC is by far the best-known and most 

developed, two other types of tubular SOFCs, shown in Figure 7-9 illustrate ways in which the 

cells are interconnected. Numerous other designs have been proposed, but are no longer pursued 

(14). 
7-14 

(a) Conduction Around the Tube

Interconnect

Cathode

Electrolyte

Anode

Air-feed tube

Current flow

Current flow

(b) Conduction Along the Tube

Interconnect

Anode

Electrolyte

Cathode

Current 

Collector

Current flow

(c) Segmented-in-Series

Cathode

Electrolyte

Interconnect

Anode

Support Tube

Current flow

Figure 7-9 Three Types of Tubular SOFC: (a) Conduction around the Tube (e.g. Siemens 

Westinghouse and Toto (31)); (b) Conduction along the Tube (e.g. Acumentrics (32)); 

(c) Segmented in Series (e.g. Mitsubishi Heavy Industries, Rolls Royce (33, 34)). 

Inevitable in tubular designs is conduction of the current in the plane of the electrolyte over 

significant distances: 

• In the Siemens Westinghouse technology, this current is conducted tangentially around the 

tube. Toto, in Japan, follows an almost identical approach. Each tube contains one cell. 

Tubes are connected either in series or in parallel. In a refinement on this approach to shorten 

the current path and increase volumetric power density, the tube can be flattened and ribs 

added 

• In micro-tubular SOFC technology (e.g. Acumentrics), current is conducted axially along the 

tube. Interconnections are made at the end of the tube using various proprietary 

interconnection systems that connect cells within the stack. To minimize the in-plane 

resistance on the cathode side, a metallic current collector (typically silver) is applied. 

Acumentrics has shown the technology to be capable of repeated thermal cycling. Typical 

tube dimensions and performance are shown in Figure 7-10. The cells have been integrated 

into 2 kW stacks. 
7-15 

Figure 7-10 Cell Performance and Dimensions of Accumentrics Technology (32). 

• In segmented-in-series tubular SOFC technology, the tube’s active cell area is segmented and 

connected in series. As a consequence, the length over which in-plane conduction occurs can 

be controlled by the cell segmentation pattern. Another consequence of segmentation in 

series is that the voltage per tube is higher, and hence the total current lower, requiring less 

heavy-duty interconnections between tubes. Mitsubishi Heavy Industries has developed this 

approach with cylindrical tubes and constructed both atmospheric and pressurized 10 kW 

stacks, achieving power densities of around 140 mW/cm2

 (35, 36). Rolls Royce is developing 

a version with flattened tubes (34). 

The remainder of this section on tubular SOFC focuses on the cell design furthest advanced in its 

development: the Siemens Westinghouse tubular SOFC technology. 

Tubular SOFC Cell Manufacturing Method 

A schematic cross-section of the Siemens Westinghouse cell is shown in Figure 7-11. Air is 

fed through an alumina feed tube, while fuel is supplied externally. The cell length has been 

gradually increased from 30 cm to about 150 cm. The cell has a diameter of 1.27 cm. 

Figure 7-12 shows a bundle of eighteen cells that features 3 cells in series with 6 cells in parallel. 

To ensure good contact between tubes, nickel felt is used. Because the current flows tangentially 

to the electrodes, a relatively large ohmic loss exists, especially in the cathode, which places an 

upper limit on the tube diameter. 
7-16 

Figure 7-11 Schematic cross-section of cylindrical Siemens Westinghouse SOFC Tube. 

To make a tubular SOFC, the cathode tube is fabricated first by extrusion and sintering. As 

shown in Table 7-1, it has a porosity of 30 to 40 percent to permit rapid transport of reactant and 

product gases to the cathode/electrolyte interface where the electrochemical reactions occur. The 

electrolyte is applied to the cathode tubes by electrochemical vapor deposition (EVD), which for 

many years has been the heart of Siemens Westinghouse technology (37). In this technique, 

metal chloride vapor is introduced on one side of the tube surface, and O2/H2O is introduced on 

the other side. The gas environments on both sides of the tube act to form two galvanic couples, 

as described in Equations 7-1, 7-2, and 7-3. 

 MeCly + 1⁄2yO= Æ MeOy/2 + 1⁄2yCl2 + ye-

 (7-1) 

 1⁄2O2 + 2e- Æ O=

 (7-2) 

 H2O + 2e- Æ H2 + O= (7-3) 

The net result is the formation of a dense, uniform metal oxide layer in which the deposition rate 

is controlled by the diffusion rate of ionic species and the concentration of electronic charge 

carriers. This procedure is used to fabricate the solid YSZ electrolyte. 

The anode consists of metallic Ni and YSZ. The latter inhibits sintering of the metal particles, 

with thermal expansion comparable to the other cell materials. The anode structure is fabricated 

with a porosity of 20-40 percent to facilitate mass transport of reactant and product gases. 
7-17 

Table 7-1 Evolution of Cell Component Technology for Tubular Solid Oxide Fuel Cells 

Component Ca. 1965 Ca. 1975 At Present a

Anode • Porous Pt • Ni/ZrO2 cermeta

• Ni/ZrO2 cermetb

• Deposit slurry, EVD fixedc

• 12.5 X 10-6 cm/cm °C CTE 

• ~150 μm thickness 

• 20 to 40 percent porosity 

Cathode • Porous Pt • Stabilized ZrO2

impregnated with 

praseodymium oxide 

and covered with SnO 

doped In2O3

• Doped lanthanum manganite 

• Extrusion, sintering 

• ~2 mm thickness 

• 11 X 10-6 cm/cm °C CTE from room 

temperature to 1000 °C 

• 30 to 40 percent porosity 

Electrolyte • Yttria stabilized ZrO2

• 0.5-mm thickness 

• Yttria stabilized ZrO2 • Yttria stabilized ZrO2 (8 mol percent 

Y2O3) 

• EVDd

• 10.5 X 10-6 cm/cm °C CTE from 

room temperature to 1000 °C 

• 30 to 40 μm thickness 

Cell 

Interconnect 

• Pt • Mn doped cobalt 

chromite 

• Doped lanthanum chromite 

• Plasma spray 

• 10 X 10-6 cm/cm °C CTE 

• ~100 μm thickness 

a - Specification for Siemens Westinghouse SOFC 

b - Y2O3 stabilized ZrO2

c - “Fixed EVD” means additional ZrO2 is grown by EVD to fix (attach) the nickel anode to the 

electrolyte. This process is expected to be replaced. 

d - EVD = electrochemical vapor deposition 

The cell interconnect (doped lanthanum chromite) must be impervious to fuel and oxidant gases, 

and must possess good electronic conductivity. The interconnect is exposed to both the cathode 

and anode environments. Thus, it must be chemically stable under O2 partial pressures of about 

1 to 10-18 atmospheres at 1,000 °C. The interconnect material is applied to the cathode tube as a 

narrow strip (see Figure 7-9, Figure 7-11) prior to depositing the electrolyte by masking the rest 

of the tube. Similarly, the interconnect strip is masked when the electrolyte is applied. 

The other cell components should permit only electronic conduction, and interdiffusion of ionic 

species in these components at 1,000 °C should not affect their electronic conductivity. Other 

restrictions on the cell components are that they must be stable in the gaseous environments in 

the cell and they must be capable of withstanding thermal cycling. The materials listed in Table 

7-1 appear to meet these requirements. 
7-18 

The resistivities of typical cell components at 1,000 °C under fuel cell gaseous 

environments (38) are 10 ohm-cm (ionic) for the electrolyte (8-10 mol percent Y2O3 doped 

ZrO2), 1 ohm-cm (electronic) for the cell interconnect (doped LaCrO3), 0.01 ohm-cm (electronic) 

for the cathode (doped LaMnO3), and 3 x 10-6 ohm-cm (electronic) for the anode (Ni/ZrO2

cermet). It is apparent that the solid oxide electrolyte is the least conductive of the cell 

components, followed by the cell interconnect. Furthermore, an operating temperature of about 

1,000 °C is necessary if the ionic conductivity of the solid electrolyte (i.e., 0.02/ohm-cm at 800 

°C and 0.1/ohm-cm at 1,000 °C) is to be within an order of magnitude of that of aqueous 

electrolytes. The solid electrolyte in SOFCs must be only about 25 to 50 μm thick if its ohmic 

loss at 1,000 °C is to be comparable to the electrolyte in PAFCs (39). Fortunately, thin 

electrolyte structures of about 40 μm thickness can be fabricated by EVD, as well as by tape 

casting and other ceramic processing techniques. 

Operation of SOFCs requires individual cell components that are thermally compatible so that 

stable interfaces are established at 1,000 °C, i.e., CTEs for cell components must be closely 

matched to reduce thermal stress arising from differential expansion between components. 

Fortunately, the electrolyte, interconnect, and cathode listed in Table 7-1 have reasonably close 

CTEs (i.e., ~10-5 cm/cm °C from room temperature to 1,000 °C). An anode made of 100 percent 

nickel would have excellent electrical conductivity. However, the CTE of 100 percent nickel 

would be 50 percent greater than the ceramic electrolyte and the cathode tube, which causes a 

thermal mismatch. This thermal mismatch has been resolved by mixing ceramic powders with 

Ni or NiO. The trade-off in the amounts of Ni (to achieve high conductivity) and ceramic (to 

better match the CTE) is approximately 30/70 Ni/YSZ by volume (40). 

Schematic representations of the gas manifold design and cross section of a typical tube 

bundle (41) are presented in Figure 7-12. In this design, the tubular cathode is formed by 

extrusion. The electrolyte and cell interconnect are deposited by electrochemical vapor 

deposition (EVD) and plasma spraying, respectively, on the cathode. The anode is subsequently 

formed on the electrolyte by slurry deposition. A major advantage of this design is that 

relatively large single tubular cells can be constructed in which the successive active layers can 

be deposited without chemical or material interference with previously-deposited layers. The 

support tube is closed at one end, which eliminates gas seals between cells. 
7-19 

Figure 7-12 Gas Manifold Design for a Tubular SOFC and Cell-to-Cell Connections 

in a Tubular SOFC (41) 

The oxidant is introduced via a central A12O3 injector tube and fuel gas is supplied to the 

exterior of the closed-end cathode tube. In this arrangement, the A12O3 tube extends to the 

closed end of the tube, and the oxidant flows back past the cathode surface to the open end. The 

fuel flows past the anode on the exterior of the cell and in a parallel direction (co-flow) to the 

oxidant gas. The spent gases are exhausted into a common plenum, where any remaining fuel 

reacts. The heat generated preheats the incoming oxidant stream and drives an expander. One 
7-20 

attractive feature of this arrangement is that it eliminates the need for leak-free gas manifolding 

of the fuel and oxidant streams. However, the seal-less tubular design results in a relatively long 

current path around the circumference of the cell. 

For the current YSZ electrolyte to provide sufficient oxygen conductivity, it must be heated to a 

high temperature (900 to 1,000 °C). This means that expensive, high temperature alloys must be 

used to house the fuel cell, increasing its cost substantially. These costs could be reduced if the 

operating temperature was lowered to between 600 to 800 °C, allowing the use of less expensive 

structural materials such as stainless steel. A lower operating temperature would also ensure a 

greater overall system efficiency and a reduction in the thermal stress in the ceramic structure, 

leading to a longer service life for the fuel cell. 

To lower the operating temperature, either the conductivity of the YSZ must be improved by 

thinner electrolytes, or alternative electrolytic materials must be developed that can replace YSZ. 

A concerted effort is being made by researchers around the world to find a better solution. 

7.2.1.1 Performance 

This section provides empirical information that can be used to estimate the performance of 

SOFCs based on various operating parameters. The SOFCs being developed, particularly the 

planar types, have unique designs, are constructed of various materials, and are fabricated by 

different techniques. This development process will result in further evolution of the performance trends summarized here. The electrochemical reactions associated with hydrogen fuel are 

expressed in equations (7-4) to (7-6): 

H2 + O= → H2O + 2e- (7-4)

at the anode, and 

1⁄2O2 + 2e- → O= (7-5)

at the cathode. The overall cell reaction is 

H2 + 1⁄2O2 → H2O (7-6)

The corresponding Nernst equation for the reaction in equation 7-6 is 

2 2

2

1/2

H O

H O

RT P P + ln 

2 P

Ε = Ε° F

(7-7)

In addition to hydrogen, carbon monoxide (CO) and other hydrocarbons such as methane (CH4) 

can be used as fuels. It is feasible that the water gas shift reaction involving CO (CO + H2O →

H2 + CO2) and the steam reforming of CH4 (CH4 + H2O → 3H2 + CO) in the high temperature 

environment of SOFCs produce H2 that is easily oxidized at the anode. The direct oxidation of 

CO in fuel cells is also well established. Because of the increased number of chemical species 

and competing reactions, however, derivation of cell performance as a function of temperature, 
7-21 

pressure, and composition effects is not straightforward. Data by Crucian, et al. (42), presents 

results for the direct oxidation of hydrocarbons on copper/ceria. 

The thermodynamic efficiency of SOFCs operating on H2 and O2 at open circuit voltage is lower 

than that of MCFCs and PAFCs because of the lower free energy at higher temperatures. On the 

other hand, the higher operating temperature of SOFCs is beneficial in reducing polarization 

resistance. 

The voltage losses in SOFCs are governed by ohmic losses in the cell components. The 

contribution to ohmic polarization (iR) in a tubular cell (assuming uniform current distribution in 

the electrolyte) is 45 percent from the cathode, 18 percent from the anode, 12 percent from the 

electrolyte, and 25 percent from the interconnect when these components have thicknesses of 

2.2, 0.1, 0.04 and 0.085 mm, respectively, and specific resistivities (ohm-cm) at 1,000 °C of 

0.013, 3 x 10-6, 10, and 1, respectively. The cathode iR dominates the total ohmic loss despite 

the higher specific resistivities of the electrolyte and cell interconnection because of the short 

conduction path through these components and the long current path in the plane of the cathode. 

In an effort to further improve performance, power density, and cost, Siemens Westinghouse 

initiated the development of a variant on its technology with a flattened tube (also schematically 

shown in Figure 7-9a). By shortening the current path the power density, on an active area basis, 

is substantially increased. In addition, the volumetric power density is increased (Figure 7-13), 

(42). 

Figure 7-13 Performance Advantage of Sealless Planar (HPD5) over Conventional 

Siemens Westinghouse Technology (42) 
7-22 

Effect of Pressure 

SOFCs, like PAFCs and MCFCs, show enhanced performance by increasing cell pressure. The 

following equation approximates the effect of pressure on cell performance at 1,000 °C: 

∆ p

2

1

V (mV) = 59 log

P

P

(7-8)

where P1 and P2 are different cell pressures. The above correlation was based on the assumption 

that overpotentials are predominately affected by gas pressures and that these overpotentials 

decrease with increased pressure. 

Siemens Westinghouse, in conjunction with Ontario Hydro Technologies, tested air electrode 

supported (AES) cells at pressures up to 15 atmospheres on both hydrogen and natural gas (42). 

Figure 7-14 illustrates the performance at various pressures: 

Figure 7-14 Effect of Pressure on AES Cell Performance at 1,000 °C (2.2 cm diameter, 

150 cm active length) 

Effect of Temperature 

The dependence of SOFC performance on temperature is illustrated in Figure 7-15 for a two-cell 

stack using air (low utilization) and a fuel of 67 percent H2/22 percent CO/11 percent H2O (low 

utilization). The sharp decrease in cell voltage as a function of current density at 800 °C is a 

manifestation of the high ohmic polarization (i.e., low ionic conductivity) of the solid electrolyte 

at this temperature. The ohmic polarization decreases as the operating temperature increases to 
7-23 

1,050 °C, and correspondingly, the current density at a given cell voltage increases. The data in 

Figure 7-15 show a larger decrease in cell voltage with decreasing temperature between 800 to 

900 °C than that between 900 to 1,000 °C at constant current density. This and other data 

suggest that the voltage gain with respect to temperature is a strong function of temperature and 

current density. One reference (43) postulates the voltage gain as: 

∆ VT 21 (mV) = 1.3(T - T )( C) ° (7-9)

for a cell operating at 1,000 °C, 160 mA/cm2

, and a fuel composition of 67 percent H2/22 

percent CO/11 percent H2O. In light of the strong functionality with respect to current density, it 

might be more appropriate to describe the voltage gain with the following relationship: 

∆ VT 21 (mV) = K(T - T )( C) * J ° (7-10)

where J is the current density in mA/cm2. 

Figure 7-15 Two-Cell Stack Performance with 67 percent H2 + 22 percent CO + 11 

percent H2O/Air 

The following values of K have been deduced from several references using a fuel composition 

of 67 percent H2/22 percent CO/11 percent H2O, and an air oxidant. 
7-24 

Table 7-2. K Values for ∆VT 

K Temperature (°C) Ref.

0.008 ~1000 43 

0.006 1000 - 1050 44 

0.014 900 - 1000 

0.068 800 - 900 

0.003 900 - 1000 45 

0.009 800 - 900 

By inspection, there is a reasonably large range in the value of K between these references. As 

the SOFC technology matures, these differences may reconcile to a more cohesive set of values. 

In the interim, the following expressions may help the reader if no other information is available: 

V (mV) = 0.008(T - T )( C) * J(mA/ cm )

2 ∆ T 2 1 ° 900 °C < T < 1050 °C (7-11)

V (mV) = 0.040(T - T )( C) * J(mA/ cm )

2 ∆ T 2 1 ° 800 °C < T < 900 °C (7-12)

V (mV) =1.300(T - T )( C) * J(mA/ cm )

2 ∆ T 2 1 ° 650 °C < T < 800 °C (7-13)

Equations (7-11) and (7-12) are for a fuel composed of 67 percent H2/22 percent CO/11 percent 

H2O. Experiments using different fuel combinations, such as 80 percent H2/20 percent CO2 (45) 

and 97 percent H2/3 percent H2O (49, 51), suggest that these correlations may not be valid for 

other fuels. Equation (7-13) is based on the average value of the data shown in Figure 7-13, i.e., 

an anode-supported PSOFC with a thin electrolyte using hydrogen as a fuel and air as an oxidant. 

This approach is indicative of the current development path being pursued in SOFC technology: 

planar, electrode-supported cells featuring thin (<10 μm) electrolytes of YSZ. It has been noted 

that new electrode and electrolyte materials are also under development. 

Figure 7-16 presents a set of performance curves for a fuel of 97 percent H2/3 percent H2O at 

various temperatures (43). Voltage actually increases with decreasing temperature for current 

densities below approximately 65 mA/cm2

. Other data (46) show that this inverse relationship 

can extend to current densities as high as 200 mA/cm2

. 
7-25 

Figure 7-16 Two Cell Stack Performance with 97% H2 and 3% H2O/Air (43) 

Effect of Reactant Gas Composition and Utilization 

Because SOFCs operate at high temperature, they are capable of internally reforming fuel gases 

(i.e., CH4 and other light hydrocarbons) without the use of a reforming catalyst (i.e., anode itself 

is sufficient), and this attractive feature of high temperature operation has been experimentally 

verified. Another important aspect is that recycle of CO2 from the spent fuel stream to the inlet 

oxidant is not necessary because SOFCs utilize only O2 at the cathode. 

Oxidant: The performance of SOFCs, like that of other fuel cells, improves with pure O2 rather 

than air as the oxidant. With a fuel of 67 percent H2/22 percent CO/11 percent H2O at 85 percent 

utilization, the cell voltage at 1,000 °C shows an improvement with pure O2 over that obtained 

with air (see Figure 7-19). In the figure, the experimental data are extrapolated by a dashed line 

to the theoretical Nernst potential for the inlet gas compositions (45). At a target current density 

of 160 mA/cm2

 for the tubular SOFC operating on the above-mentioned fuel gas, a difference in 

cell voltage of about 55 mV is obtained. The difference in cell voltage with pure O2 and air 

increases as the current density increases, which suggests that concentration polarization plays a 

role during O2 reduction in air. More recent data for planar cells at 800 o

C are presented in 

Figure 7-17. These data suggest that concentration polarization at open circuit conditions is not a 

significant factor with the new generation of cells. However, as expected, the differences in 

voltage between air and oxygen increases with increasing current density. 
7-26 

Figure 7-17 Cell Performance at 1,000 °C with Pure Oxygen (o) and Air (∆) Both at 25 

percent Utilization (Fuel (67 percent H2/22 percent CO/11 percent H2O) Utilization is 85 

percent) 

Based on the Nernst equation, the theoretical voltage gain due to a change in oxidant utilization 

at T = 1,000 °C is 

∆ Cathode

O 2

O 1

V = log (P )

(P )

2

2

63

(7-14)

where PO2 is the average partial pressure of O2 in the system. Data (43) suggest that a more 

accurate correlation of voltage gain is described by 

∆ Cathode

O 2

O 1

V = 92 log (P )

(P )

2

2

(7-15)

Fuel: The influence of fuel gas composition on the theoretical open circuit potential of SOFCs 

is illustrated in Figure 7-18, following the discussion by Sverdrup, et al. (39). The 

oxygen/carbon (O/C) atom ratio and hydrogen/carbon (H/C) atom ratio, which define the fuel 

composition, are plotted as a function of the theoretical open circuit potential at 1,000 °C. If 

hydrogen is absent from the fuel gas, H/C = 0. For pure CO, O/C = 1; for pure CO2, O/C = 2. 

The data in the figure show that the theoretical potential decreases from about 1 V to about 0.6 V 

as the amount of O2 increases and the fuel gas composition changes from CO to CO2. The 

presence of hydrogen in the fuel produces two results: (1) the potential is higher, and (2) the 

O/C ratio corresponding to complete oxidation extends to higher values. These effects occur 
7-27 

because the equilibrium composition obtained by the water gas shift reaction in gases containing 

hydrogen (H2O) and carbon (CO) produces H2, but this reaction is not favored at higher 

temperatures. In addition, the theoretical potential for the H2/O2 reaction exceeds that for the 

CO/O2 reaction at temperatures of about 800 °C. Consequently, the addition of hydrogen to the 

fuel gas will yield a higher open circuit potential in SOFCs. Based on the Nernst equation, the 

theoretical voltage gain due to a change in fuel utilization at T = 1,000 °C is 

∆ Anode

H H O 2

H H O 1

V = 126 log (P / P )

(P / P )

2 2

2 2

(7-16)

where PH2 and PH2 O are the average partial pressures of H2 and H2O in the fuel gas. 

Figure 7-18 Influence of Gas Composition of the Theoretical Open-Circuit Potential of 

SOFC at 1,000 °C 

The fuel gas composition has a major effect on the cell voltage of SOFCs. The performance data 

(47) obtained from a 15-cell stack (1.7 cm2

 active electrode area per cell) of the tubular 

configuration at 1,000 °C illustrates the effect of fuel gas composition. With air as the oxidant 

and fuels of composition 97 percent H2/3 percent H2O, 97 percent CO/3 percent H2O, and 1.5 

percent H2/3 percent CO/75.5 percent CO2/20 percent H2O, the current densities achieved at 80 

percent voltage efficiency were ~220, ~170, and ~100 mA/cm2

, respectively. The reasonably 

close agreement in the current densities obtained with fuels of composition 97% H2/3% H2O and 

97 percent CO/3 percent H2O indicates that CO is a useful fuel for SOFCs. However, with fuel 

gases that have only a low concentration of H2 and CO (i.e., 1.5 percent H2/3 percent CO/75.5 

percent CO2/20 percent H2O), concentration polarization becomes significant and the 

performance is lower. 

A reference fuel gas used in experimental SOFCs had a composition of 67 percent H2/22 percent 

CO/11 percent H2O. With this fuel (85 percent utilization) and air as the oxidant (25 percent 

utilization), individual cells (~1.5 cm diameter, 30 cm length and ~110 cm2

 active surface area) 

delivered a peak power of 22 W (48). Figure 7-19 (45) shows the change in cell voltage with 
7-28 

fuel utilization for a SOFC that operates on this reference fuel and pure O2 or air as oxidant (25 

percent utilization). The cell voltage decreases with an increase in the fuel utilization at constant 

current density. Insufficient data are available in the figure to determine whether temperature 

has a significant effect on the change in cell voltage with utilization. However, the data do 

suggest that a larger voltage decrease occurs at 1,000 °C than at 800 or 900 °C. Based on this 

and other data (48, 49), the voltage gain at T = 1,000 °C and with air is defined by 

Equation (7-17): 

∆ Anode

H H O 2

H H O 1

V = 172 log (P / P )

(P / P )

2 2

2 2

(7-17)

Figure 7-19 Variation in Cell Voltage as a Function of Fuel Utilization and Temperature 

(Oxidant (o - Pure O2; ∆ - Air) Utilization is 25 percent . Current Density is 160 mA/cm2 at 

800, 900 and 1,000 °C and 79 mA/cm2 at 700 °C) 

Effect of Impurities 

Hydrogen sulfide (H2S), hydrogen chloride (HCl) and ammonia (NH3) are impurities typically 

found in coal gas. Some of these substances may harm the performance of SOFCs. Early 

experiments (57) used a simulated oxygen-blown coal gas containing 37.2 percent CO/34.1 

percent H2/0.3 percent CH4 /14.4 percent CO2/13.2 percent H2O/0.8 percent N2. These 

experiments showed no degradation in the presence of 5,000 ppm NH3. An impurity level of 

1 ppm HCl also showed no detectable degradation. H2S levels of 1 ppm resulted in an 

immediate performance drop, but this loss soon stabilized into a normal linear degradation. 

Figure 7-2020 shows the performance of the experimental cell over time (50). Additional 

experiments showed that removing H2S from the fuel stream returned the cell to nearly its 

original level. It was also found that maintaining an impurity level of 5,000 ppm NH3 and 

1 ppm HCl, but decreasing the H2S level to 0.1 ppm eliminated any detrimental effect due to the 

presence of sulfur, even though, as mentioned above, 1 ppm H2S caused virtually no degradation. 
7-29 

Figure 7-20 SOFC Performance at 1,000 °C and 350 mA/cm2, 85 percent Fuel Utilization 

and 25 percent Air Utilization (Fuel = Simulated Air-Blown Coal Gas Containing 

5,000 ppm NH3, 1 ppm HCl and 1 ppm H2S) 

Silicon (Si), which also can be found in coal gas, has been studied (50) as a contaminant. It is 

believed to accumulate on the fuel electrode in the form of silica (SiO2). The deposition of Si 

throughout the cell has been found to be enhanced by high (~50%) H2O content in the fuel. Si is 

transported by the following reaction: 

SiO2 (s) + 2H2O (g) → Si(OH)4 (g) (7-18)

As CH4 reforms to CO and H2, H2O is consumed. This favors the reversal of Equation (7-18), 

which allows SiO2 to be deposited downstream, possibly on exposed nickel surfaces. 

Oxygen-blown coal gas, however, has a H2O content of only ~13 percent, and this is not 

expected to allow for significant Si transport. 

Effect of Current Density 

The voltage level of a SOFC is reduced by ohmic, activation, and concentration losses, which 

increase with increasing current density. The magnitude of this loss is described by the 

following equation that was developed from information in the literature (44, 51, 52 ,53, 54, 55): 
7-30 

∆Vj(mV) = -0.73∆J (T = 1000 °C) (7-19)

where J is the current density (mA/cm2

) at which the cell is operating. Air electrode-supported 

(AES) cells by Siemens Westinghouse exhibit the performance depicted in Figure 7-. 

Figure 7-21 Voltage-Current Characteristics of an AES Cell (1.56 cm Diameter, 50 cm 

Active Length) 

Effect of Cell Life 

The endurance of the cell stack is of primary concern for SOFCs. As SOFC technology has 

continued to approach commercialization, research in this area has increased and improvements 

made. The Siemens Westinghouse state-of-the-art tubular design has been validated by 

continuous electrical testing of over 69,000 hours with less than 0.5 percent voltage degradation 

per 1,000 hours of operation. This tubular design is based on the early calcia-stabilized zirconia 

porous support tube (PST). In the current technology, the PST has been eliminated and replaced 

by a doped lanthanum manganite air electrode tube. These air electrode-supported (AES) cells 

have shown a power density increase of approximately 33 percent over the previous design. 

Siemens Westinghouse AES cells have shown less than 0.2 % voltage degradation per 1,000 

hours in a 25 kW stack operated for over 44,000 hours (23,56), and negligible degradation in the 

100 kW stack operated in the Netherlands and Germany (>16,000 hours). 

Summary of Equations for Tubular SOFC 

The preceding discussion provided parametric performance based on various referenced data at 

different operating conditions. It is suggested that the following set of equations could be used 

for performance adjustments unless the reader prefers other data or correlations. 
7-31 

Parameter Equation Comments

Pressure ∆Vp(mV) = 59 log 2

1

P

P

1 atm < P < 10 atm (7-8) 

Temperature ∆VT(mV) = 0.008(T2 - T1)( °C) * J1

∆VT(mV) = 0.04(T2 – T1)( °C) * J1

∆VT(mV) = 1.3(T2 - T1)( °C) * J 

900 °C < T < 1050 °C (7-11) 

800 °C < T < 900 °C (7-12) 

650 °C < T < 800 °C (7-13) 

Oxidant ∆VCathode(mV) = 92 log (P )

(P )

2

2

O 2

O 1

0.16 P

P

 0.20 O2

Total

≤ ≤ (7-15) 

Fuel ∆VAnode = 172 log (P / P )

(P / P )

2 2

2 2

H H O 2

H H O 1

0.9 < PH HO 2 2 / P < 6.9 T = 1000 °C, 

with air (7-17) 

Current 

Density 

∆VJ(mV) = - 0.73∆J 50 < J < 400 mA/cm2

 (7-19)

P = 1 atm., T = 1000 °C 

7.2.2 Planar SOFC 

A variety of planar SOFC sub-types are distinguished according to construction: 

Structural support for membrane/electrolyte assembly: 

• Electrolyte-supported. Early planar cells were mostly electrolyte-supported. This requires a 

relatively thick electrolyte (>100 but typically around 200 μm, with both electrodes at about 

50 μm) which leads to high resistance, requiring high-temperature operation. Sulzer Hexis 

and Mitsubishi Heavy Industries (MHI) are actively pursuing this technology and have 

scaled-up the technology into 1 and 15 kW systems, respectively. Power density at 0.7 V is 

reported to be about 140 mW/cm2

 for the Sulzer stacks (57, 58, 59, 60, 61) and about 190 to 

220 mW/cm2

 for the MHI stacks (62, 63, 64, 65), both under commercially-relevant 

operating conditions. 

• Cathode-supported. This allows for a thinner electrolyte than electrolyte-supported cells, but 

mass transport limitations (high concentration polarization) and manufacturing challenges (it 

is difficult to achieve full density in a YSZ electrolyte without oversintering an LSM 

cathode) make this approach inferior to anode-supported thin-electrolyte cells. 

• Anode-Supported. Advances in manufacturing techniques have allowed the production of 

anode-supported cells (supporting anode of 0.5 to 1 mm thick) with thin electrolytes. 

Electrolyte thicknesses for such cells typically range from around 3 to 15 μm 

(thermomechanically, the limit in thickness is about 20 to 30 μm (the cathode remains around 

50 μm thick), given the difference in thermal expansion between the anode and the 

electrolyte). Such cells provide potential for very high power densities (up to 1.8 W/cm2

under laboratory conditions, and about 600 to 800 mW/cm2

 under commercially-relevant 

conditions). 

 

1

 Where J = mA/cm2, for fuel composition of 67% H2/22% CO/11% H2O 
7-32 

• Metal interconnect-supported. Lawrence Berkeley National Laboratory (66), Argonne 

National Laboratory, and Ceres (67) have pioneered metal-supported cells to minimize mass 

transfer resistance and the use of (expensive) ceramic materials. In such cells, the electrodes 

are typically 50 μm thick and the electrolyte around 5 to15 μm. While the benefits are 

obvious, the challenges are to find a materials combination and manufacturing process that 

avoids corrosion and deformation of the metal and interfacial reactions during manufacturing 

as well as operation. 

Interconnect material: 

• Ceramic (lanthanum or yttrium chromite) suitable for high-temperature operation (900 to 

1000 °C). These materials, while chemically stable and compatible with the MEA from a 

chemical and thermal expansion perspective, are mechanically weak and costly. 

• Cr-based or Ni-based superalloy for intermediate-high temperature operation (800 to 900 

°C). These materials are chemically stable at 900 °C, but they require additional coatings to 

prevent Cr-poisoning of the electrodes. In addition, they are expensive and difficult to form. 

• Ferritic steel (coated or uncoated) for intermediate temperature operation (650 to 800 °C). 

While uncoated steels are chemically unstable, especially during thermal cycling, coated 

steels provide corrosion resistance as well as acceptable conductivity when new. However, 

thermal cycling performance still requires improvement. 

Shape of the cell. 

• Rectangular, with gases flowing in co-flow, counter-flow, or cross-flow. 

• Circular, typically with gases flowing out from the center in co-flow, and mixing and burning 

at the edge of the cells. Spiral flow arrangements and counter-flow arrangements have also 

been proposed. 

Method for creating flow-channels: 

• Flat ceramic cell with channels in interconnect or flow-plate. 

• Corrugated ceramic with flat interconnects. 

Manifolding arrangement: 

• External manifolding. 

• Internal manifolding, through the electrolyte. 

• Internal manifolding through the interconnect, but not through the electrolyte. 

Figure 7-22 shows a sample of recently-pursued planar SOFC approaches. The anode-supported 

technology with metal interconnects will be described in some detail below. Mitsubishi tested a 

15 kW system with its all-ceramic MOLB design for almost 10,000 hours with degradation rates 

below 0.5 percent per 1,000 hrs, but without thermal cycles, and with power densities ranging 

from 190 to 220 mW/cm2

 (under practical operating conditions). Because the interconnect is flat 

and relatively thin (the flow-passage is embedded in the MEA), less of the expensive LaCrO3 is 

required than if the flow-passages were in the interconnect. Nevertheless, cost reduction is still 

one of the main priorities for this stack technology. Thermal cycling is also thought to be a 

challenge with the system, which is targeted to small-scale distributed stationary power 

generation applications. 
7-33 

Figure 7-22 Overview of Types of Planar SOFC: (a) Planar Anode-Supported SOFC with 

Metal Interconnects(68); (b) Electrolyte-Supported Planar SOFC Technology with Metal 

Interconnect (57,58,69); (c) Electrolyte-Supported Design with “egg-crate” electrolyte 

shape and ceramic interconnect (62,63,64,65). 

Sulzer Hexis built 110 1 kW demonstration units based on its electrolyte-supported technology 

with superalloy interconnects. The latest version of the units, integrated into a hot water/heating 

appliance, has shown a degradation rate of around 1 to 2 percent per 1000 hrs in continuous 

operation, and about 2x higher with thermal cycling (69). 

The planar anode-supported SOFC with metal interconnects has benefited from support for 

fundamental science and stack development under DOE’s SECA Program. The SECA Program 

is focused on developing technology required for competitive SOFC stack technologies that can 

be mass-customized for a wide range of applications, including stationary power generation, 

mobile power generation, military power applications, and transportation applications such as 

auxiliary power units (APUs). By commercializing SOFC stacks for a number of applications 

simultaneously, stack production could be increased more rapidly and, consequently, 

manufacturing cost reduced more quickly. The SECA Program has two interrelated components: 

(1) the core program in which universities, national laboratories, and private industry develop 

fundamental component and materials technologies for SOFC stacks that can be licensed with 

stack developers, and (2) a vertical program with teams of private stack developers with other 

parties to develop and demonstrate stacks that meet the SECA goals (70). Particularly useful, and 

broadly shared amongst the international SOFC development community, are the stack 

performance goals developed by SECA. 

(a) Anode-Supported Rectangular

(c) Electrolyte-Supported Circuclar (c) Electrolyte-Supported MOLB
7-34 

Table 7-3 SECA Program Goals for SOFC Stacks (70) 

Over the past ten years, this technology has developed from a scientific concept to cell 

technologies that can achieve 1.8 W/cm2

 under idealized laboratory conditions, and stacks that 

can achieve initial power densities of 300 to 500 mW/cm2

. The power density of this technology 

has allowed the engineering of integrated systems for small-scale stationary power and APU 

applications, making the hypothesis that these stack technologies can be customized for a wide 

range of high-volume applications. 
7-35 

7.2.2.1 Single Cell Performance 

A significant advance in the development of intermediate temperature PSOFCs has been the use 

of metallic “bipolar” interconnects in conjunction with thin electrolytes. Although originally 

conceptualized in the early 1990s, development of the anode-supported planar SOFC with 

metallic interconnects was significantly accelerated by the US DOE’s SECA Program. The 

benefits of the anode-supported approach with metallic interconnects were readily recognized 

(see summary in Table 7-4): 

• Sintering and Creep – Milder temperatures result in less sintering and creep of the stack 

materials. This helps maintain geometric stability and high surface area for reaction. 

• Thermally Activated Processes – Thermally activated processes such as chromium 

vaporization, elemental inter-diffusion and migration, metallic corrosion, and ceramic aging 

become problematic at higher temperatures. The lower the operating temperature is 

maintained, the less damage these processes will cause to the fuel cell. 

• Thermal Stress – Reduced width of the operating temperature band reduces thermal 

expansion and contraction stresses during cycling, thus maintaining geometric stability. 

• Increase in Nernst potential. 

• Heat Loss – Reduced heat loss from the more compact stack at lower operating temperature. 

• Material Flexibility – The range of potential construction materials is somewhat greater at 

lower temperatures. In particular, certain metals can be incorporated in SOFC stack designs. 

• Balance of Plant – The BOP costs may be less if lower cost materials can be used in the 

recuperators. In addition, the stack temperatures will be closer to typical reformer and sulfur 

removal reactor operating temperatures; this further reduces the load on the thermal 

management system. However, it must be remembered that the main factor driving the heat 

duty of the thermal management system is the amount of cooling air required for stable stack 

operation, which in turn depends on the internal reforming capability of the stack and on the 

acceptable temperature rise across the stack. 

• Start-up time may be reduced. Lighter weight and high thermal conductivity of the metal 

interconnects may allow more rapid heat-up to operating temperature. 

Some negative effects also result from reducing the operating temperature of the SOFC: 

• A proven interconnect material for operating in the intermediate temperature range (650 to 

800 o

C) does not yet exist. 

• Sulfur resistance decreases with temperature. However, recent work has shown that addition 

of certain materials provides adequate sulfur tolerance at lower temperatures. 

• Lower temperatures generally require a planar configuration to minimize resistance losses. 

This is accomplished using ultra-thin electrode and electrolyte membranes. In turn, effective 

seals for the planar configuration are needed.
7-36 

Table 7-4 Recent Technology Advances on Planar Cells and Potential Benefits 

Technology Advance Potential Benefit 

Design Electrode supported thin 

electrolyte unit cells – e.g., 

anode 

• Lower resistance of 

 electrolyte 

• Increased power density 

System Lower temperature of 

operation 

• Use of metallic 

 Interconnects and 

 manifolding possible 

Materials Metallic interconnect 

plates 

• Lower cost 

• Lower resistance 

 interconnect 

• Mechanical solution to 

 thermal expansion of 

 stack 

Materials More conductive 

electrolyte materials: 

 Sc – Zr Oxides 

 Ce – Gd Oxides 

• Reduced voltage drop 

 across electrolyte 

An example of a stack geometry is shown in Figure7-22a (68). The cassette-type repeat unit 

with a plain rectangular ceramic cell, a metal picture frame with cavities for manifolding, and a 

matching separator plate is not uncommon among developers of planar anode-supported SOFC 

with metal interconnects. Units such as the one shown typically result in a pitch of 5 to 10 unit 

cells per inch. The bipolar plate has several functions, including providing a gas barrier between 

the anode and cathode, providing a series electrical connector between the anode and cathode, 

and flow field distribution. 

Individual cell assemblies, each including an anode, electrolyte, and cathode are stacked with 

metal interconnecting plates between them. The metal plates are shaped to permit the flow of 

fuel and air to the membranes. The electrolyte and interconnect layers are made by tape casting. 

The electrodes are applied by the slurry method, by screen-printing, by plasma spraying, or by 

tape-casting/tape calendaring. Fuel cell stacks are formed by layers of unit cells, much like other 

fuel cell technologies. Tests of single cells and two-cell stacks of SOFCs with a planar 

configuration (5cm diameter) have demonstrated power densities up to 1.8 W/cm2

 (Figure 7-23) 

under ideal conditions. 
7-37 

Figure 7-23 Representative State-of-the-Art Button Cell Performance 

of Anode-Supported SOFC (1) 

To reduce resistivity of the electrolyte, development has focused on reducing its thickness from 

150 μm to about 10 μm. Wang, et al. (71), at the University of Pennsylvania, fabricated thinfilm YSZ electrolytes between 3 and 10 μm. Wang reported significant improvement in cell 

performance with mixed conducting-doped YSZ electrodes; Tb- and Ti-doped YSZs increased 

power densities between 15 to 20 percent. Other examples of this approach are also available in 

the literature (72, 73,74 ,75 ,76). 

Ball and Stevens (74) report that gadolinium-doped ceria is a good candidate for use as an 

alternative electrolyte when compared to zirconia, due to its higher conductivity at lower 

temperatures. However, doped ceria has a number of disadvantages, such as electronic 

conductivity and reduced strength. Results indicate that an increase in strength can be produced 

in the ceria by addition of zirconia particles that is dependent on the particle size and heat 

treatment. 

Research at the University of Texas at Austin (72) sought to develop electrolytes that have 

higher conductivity than YSZ. Goodenough and Huang (77) identified a system of LaSrGaMgO 

(LSGM) as a superior oxide-ion electrolyte with performance at 800 °C comparable to YSZ at 

1,000 °C. LSGM lacks the toughness of YSZ, which makes it more difficult to fabricate as an 

ultra-thin film, but its superior ionic conductivity allows thicker films to be used. Figure 7-24 

illustrates the performance of a single cell based on LSGM electrolyte. 
7-38 

Figure 7-24 Single Cell Performance of LSGM Electrolyte (50 μm thick) 

Barnett, Perry, and Kaufmann (75) found that fuel cells using 8 μm thick yttria-stabilized 

zirconia (YSZ) electrolytes provide low ohmic loss. Furthermore, adding thin porous yttriadoped ceria (YDC) layers on either side of the YSZ yielded much-reduced interfacial resistance 

at both LSM cathodes and Ni-YSZ anodes. The cells provided higher power densities than 

previously reported below 700 °C, e.g., 300 and 480 mW/cm2

 at 600 and 650 °C, respectively 

(measured in 97 percent H2 and 3 percent H2O and air), and also provided high power densities 

at higher temperatures, e.g., 760 mW/cm2

 at 750 °C. Other data (Figure 7-25) from the 

University of Utah (73) show power densities of 1.75 W/cm2

 with H2/air and 2.9 W/cm2 with 

H2/O2 at 800 °C for an anode-supported cell. However, no data is presented with regard to 

electrodes or electrolyte thickness or composition. 
7-39 

Figure 7-25 Effect of Oxidant Composition on a High Performance Anode-Supported Cell 

7.2.2.2 Stack Performance 

A number of planar cell stack designs have been developed based on planar anode-supported 

SOFC with metal interconnects. Typically, cells for full-scale stacks are about 10 to 20 cm 

mostly square or rectangular (though some are round). Stacks with between 30 and 80 cells are 

the state-of-the-art. Figure 7-26 shows examples of state-of-the art planar anode-supported 

SOFC stacks and selected performance data (68,78, 79). The stacks shown are the result of three 

to seven generations of full-scale stack designs by each of the developers. The capacities of these 

stacks (2 to 12 kW operated on reformate and at 0.7 V cell voltage) is sufficient for certain 

small-scale stationary and mobile (APU) applications. 

It is still difficult to compare performance figures for the stacks, as performance is reported for 

different (often vaguely described) operating conditions. However, it has been estimated that if 

the data were corrected for fuel composition and fuel utilization, the power density on a per unit 

area basis for these stacks is around 300 to 400 mW/cm2

. The differences in performance are 

modest compared with the differences in performance between this generation and previous 

generations of stacks. 

These three stack technologies can be considered to be among the most advanced of the planar 

anode-supported SOFC stacks. Interestingly, their stack architectures are rather similar: 

• All are rectangular cells, with a cassette-type multi-component repeat unit design 

• All use integrated manifolds that do not pierce the ceramics 

• All use some form of stack compression, although presumably the Jülich stack requires this 

for contact, not sealing (a glass seal is used) 
7-40 

(a) Delphi

• 30 cells x 106 cm2

• 3.5 liter, 13 kg

(b) Fuel Cell Energy

• 4 x 20 cells x 121 cm2

(c) Forschungs

Zentrum Jülich

• 60 x cells 361 cm2

On humidified Hydrogen

• 13.3 kWel

• 0.74 mA/cm2 at 0.83 V 

(0.6 W/cm2)

• 700 °C

On simulated reformate

• 11.9 kWel

• 0.74 mA/cm2 at 0.74 V 

(0.55 W/cm2)

• 720 °C

Figure 7-26 Examples of State-of-the-Art Planar Anode-Supported SOFC Stacks and 

Their Performance Characteristics (68,79,78) 

The stack performance lags behind the impressive performance demonstrated at the cell level. 

Results reported by Delphi are typical (Figure 7-27). Of course, the very high numbers for single 

cells (1.8 W/cm2

) were obtained with pure reactants, humidified hydrogen as a fuel, and with 

very low utilization. But still, if the performance in single cells were corrected for the operating 

conditions prevalent in a full cell at about 80 percent utilization with real reformate (data in 

Figure 7-27 represent a more modest level of utilization and idealized fuels), a power density of 

between 600 and 800 mW/cm2

 may be expected. However, measured power densities in multicell stacks (note Figure 7-27 shows only single-cell stacks) for such conditions range from 300 to 

400 mW/cm2

. Most of this discrepancy stems from high contact resistance caused by 

deterioration of the electrodes and the electrical interface with the interconnects. 
7-41 

Figure 7-27 Trend in Cell and Single-Cell-Stack Performance in Planar SOFC (68) 

Degradation rates observed by various groups for this type of stack range from about 0.8 to about 

3 percent per 1,000 hours, though experiments with coated ferritic steel interconnects reportedly 

achieve still lower degradation rates. The longest operating experience is currently around 6,000 

to 7,000 hours per stack. The effect of thermal cycling varies strongly from system to system, but 

it appears that about 5 to 10 thermal cycles are achievable. The effects of more thermal cycles 

combined with long-term operation are not well-characterized in the public literature. With 

respect to degradation rate, both chromia poisoning and interfacial resistances are issues that 

require further improvement. Because of the thin metal foils used in some of the designs, the 

effect of changes in chromium content of the bulk foil metal over long periods of time must be 

taken into account, and could influence corrosion behavior in a non-linear fashion (80). 

Although these are significant problems, they have been well-characterized. Structured publicprivate R&D programs are now under way in the U.S., Europe, and Japan to overcome these 

hurdles in the coming years. 

7.2.3 Stack Scale-Up 

Although some SOFC applications require systems no larger than the 2 to 10 kW to which many 

tubular and planar SOFC have been scaled-up, most stationary applications, especially those with 

the greatest potential impact on global energy use, will require systems ranging from about 200 

kW for medium-scale distributed generation to several hundred MW for utility-scale power 

stations. Table 7-5 lists the major SOFC system manufacturers worldwide; this list does not 

include research institutes, universities, and manufacturers of solely ceramic components (1). 

Tubular SOFC systems have been scaled-up and integrated into systems with capacities up to 

250 kW (Figure 7-28). This is accomplished by combining individual tubes into 3x8 tube 

modules with capacities of around 2 kW. These modules, in turn, are combined to form the 

stack. Mitsubishi Heavy Industries scaled-up its tubular segmented-in-series system to 10 kW 

(pressurized and atmospheric) and its all-ceramic planar design up to 15 kW. The planar design 

follows a scale-up approach that involves small ~2 kW units which are combined into larger 

stack units. Planar anode-supported stacks with metallic interconnects have been scaled-up to 

about 12 kW in a single stack. 
7-42 

Figure 7-28 Siemens Westinghouse 250 kW Tubular SOFC Installation (31) 

The question then arises how these stack technologies could be used to create systems with 

capacities ranging from 200 kW to at least 20 MW. One approach would be to simply combine 

~5 kW stacks in a modular fashion into a larger system. However, as recent studies have implied, 

this would lead to rather complex manifolding arrangements of very large numbers of cells (a 1 

MW system would require at least 200 5 kW stacks). Although feasible, the complexity, cost, 

and pressure loss associated with such massive modularization are not trivial. 

Scaling up cells and individual single stacks may have limits based on fundamental 

considerations: 

• The larger the cells, the more severe the effects of CTE mismatches. 

• As cells are scaled up, pressure drop will increase unless flow channels are made higher. 

Higher flowchannels will increase the cell resistance and, in most designs, increase the 

material intensity of the stack. 

• Scaling up the cells for certain applications makes it more difficult to mass-customize stack 

technology for a broad range of applications with different capacity requirements. 

• Increasing the number of cells has its limits because of mechanical stability concerns. 

• As the number of cells increases, minor imperfections in cell geometry (e.g. flatness) will 

lead to maldistributions of the contact or sealing pressure inside the stacks. 

• Manifolding the gas flow evenly to all cell levels will become difficult. 
7-43 

An alternative approach would be to build integrated stack units out of planar cells, for example 

using a windowpane design (Figure 7-29). Earlier in the development of planar SOFC, when 

developers of electrolyte-supported planar SOFC were focused on large-capacity applications, 

several players suggested this approach. It appears likely that cost, simplicity, and reliability 

advantages will ultimately drive developers of larger-scale systems. 

Figure 7-29 Example of Window-Pane-Style Stack Scale-Up of Planar 

Anode-Supported SOFC to 250 kW 

Window-Pane Layer

25 Cells

2.2 kW

Window-Pane Layer

25 Cells

2.2 kW

Cell

289 cm2

86 W

Cell

289 cm2

86 W

Stack

115 Layers

249 kW

Stack

115 Layers

249 kW

Stack

115 Layers

249 kW

Stack

115 Layers

249 kW

Stack

115 Cells

249 kW

Stack

115 Cells

249 kW

Air feed

Air 

Exhaust

Fuel Feed Fuel Exhaust
7-44 

Table 7-5 SOFC Manufacturers and Status of Their Technology 

Manufacturer Country Achieved Year Attributes and status Acumentrics Corp. USA 2 kW 2002 Microtubular SOFCs, 2kW for uninterruptible power Adelan UK 200 W 1997 Microtubular, rapid start-up and cyclable Ceramic Fuel Cells Ltd Australia 5 kW 

25 kW 

1998 2000 

Planar SOFC, laboratory stack testing, 600 operating hour40 kW fuel cell system 

Delphi/Battelle USA 5 kW 2001 Developing 5 kW units based on planar cells Fuel Cell Technologies (with Siemens Westinghouse Power Corporation) 

Canada 5 kW 

2 kW 

2002 2002 

5 kW prototype SOFC under test, 40 percent electrical eff

planned in Sweden, USA, Japan, etc. 

General Electric Power Systems (formerly Honeywell and Allied Signal) 

USA 0.7 kW 

1 kW 

1999 2001 

Planar SOFCs, atmospheric and hybrid systems 

Global Thermoelectric Canada 1 kW 2000 Planar SOFCs, 5000 hours fuel cell test MHI/Chubu Electric Japan 4 kW 

15 kW 

1997 2001 

Planar SOFC, laboratory stack testing, 7500 operating hou

MHI/Electric Power Development Co. Japan 10 kW 2001 Tubular SOFC, pressurized operation, 10 kW laboratory tRolls-Royce UK 1 kW 2000 Planar SOFC, laboratory testing, developing 20 kW stack Siemens Westinghouse Power Corporation 

USA 25 kW 

110 kW 220 kW 

1995 1998 2000 

Tubular SOFC, several units demonstrated on customer sistack operating hours, first hybrid SOFC demonstration 

SOFCo (McDermott Technologies and Cummins Power Generation 

USA 0.7 kW 2000 Planar SOFC, laboratory testing, 1000 operating hours, de

SOFC unit 

Sulzer Hexis Switzerland 1 kW 1998- 

2002 

Planar SOFC, field trails of many testing 

Tokyo Gas Japan 1.7 kW 1998 Planar design, laboratory testing TOTO/Kyushu Electric Power/Nippon Steel 

Japan 2.5 kW 2000 Tubular SOFC, laboratory testing, developing 10 kW syst
7-45 

7.3 System Considerations 

System design depends strongly on fuel type, application, and required capacity, but the stack 

has several important impacts on the system design and configuration: 

• The stack operating temperature range, degree of internal reforming, operating voltage, and 

fuel utilization determine the air cooling flow required, as well as level of recuperation 

required. This determines specifications for the blower or compressors and the thermal 

management system. 

• The stack geometry and sealing arrangement typically determine stack pressure drop and 

maximum operating pressure, which can influence the system design especially in hybrid 

systems. 

• The stack’s sulfur tolerance determines the specifications of the desulfurization system. 

• The degree of internal reforming that the stack can accept influences the choice and design of 

the reformer. 

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7-47 

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Electrochemical Society, Inc., Pennington, NJ, 1997. 

44. Ide, H., et al., “Natural Gas Reformed Fuel Cell Power Generation Systems – A Comparison 

of Three System Efficiencies,” Proceedings of the 24th Intersociety Energy Conversion 

Engineering Conference, The Institute of Electrical and Electronic Engineers, Washington, 

D.C., 1989. 

45. Data from Allied – Signal Aerospace Company, 1992. 

46. Zeh, C., private communication, 2nd Edition of Handbook, April, 1987. 

47. Khandkar, A., S. Elangovan, “Planar SOFC Development Status,” Proceedings of the Second 

Annual Fuel Cells Contractors Review Meeting, U.S. DOE/METC, May, 1990. 

48. Warde, C.J., A.O. Isenberg, and J.T. Brown, “High Temperature Solid Electrolyte Fuel Cells 

Status and Programs at Westinghouse,” in Program and Abstracts, ERDA/EPRI Fuel Cell 

Seminar, Palo Alto, CA, June, 1976. 

49. Dollard, W.J., and J.T. Brown, “Overview of the Westinghouse Solid Oxide Fuel Cell 

Program,” Fuel Cell Abstracts, 1986 Fuel Cell Seminar, Tucson, AZ, October, 1986. 

50. Uchuyama, F., et al., “ETL Multi-Layer Spray Coating for SOFC Component,” The 

International Fuel Cell Conference Proceedings, NEDO/MITI, Tokyo, Japan, 1992. 

51. Maskalick, N., “Contaminant Effects in Solid Oxide Fuel Cell,” Proceedings of the Fourth 

Annual Fuel Cells Contractors Review Meeting, U.S. DOE/METC, July, 1992. 

52. Minh, N., et al, “Monolithic Solid Oxide Fuel Cell Development: Recent Technical 

Progress,” Fuel Cell Seminar Program and Abstracts, Fuel Cell Seminar, Tuscon, AZ, 

November, 1992. 

53. Yashida, Y., et al, “Development of Solid Oxide Fuel Cell,” paper provided by Mitsubishi 

Heavy Industries, Ltd. 

54. Khandkar, A., et al., “Planar SOFC Technology Status and Overview,” Ceramatec, Inc., Fuel 

Cell Seminar Program and Abstracts, 1992 Fuel Cell Seminar, Tuscon, AZ, November, 1992. 

55. “Research and Development on Fuel Cell Power Generation Technology,” FY 1990 Annual 

Report, NEDO, April, 1991. 

56. Nakanishi, T., “Substrate Type, Planar Solid Oxide Fuel Cell,” Fuji Electric, Fuel Cell 

Seminar Program and Abstracts, 1992 Fuel Cell Seminar, Tucson, AZ, November, 1992. 

57. Forbes, C., Private Communication, (2002). 

58. R. Diethelm, Th. Gamper, M. Keller, R. Kruschwitz, D. Lenel, in Electrochemical Society, 

Vol. 97-18, 1997, p. 79. 

59. R. Diethelm, in Electrochemical Society, Vol. 99-19, 1999, p. 60. 

60. K. Honegger, M. Keller, and G.M. Christie, in Electrochemical Society, Vol. 99-19, 1999, p. 

1019. 

61. E. Batawi, A. Schuler, M. Keller, and C. Voisard, in Electrochemical Society, Vol. 2001-16, 

2001, p. 140. 
7-48 

62. E. Batawi, W. Kraussler, M. Janousek, B. Boggwiler and R. Diethelm, in Electrochemical 

Society, Vol. 99-19, 1999, p. 731. 

63. Y. Sakaki, M. Hattori, H. Miyamoto, H. Aiki, K. Takenobou, in Solid Oxide Fuel Cells VII 

(SOFC - VII) (S. C. S. H. Yokokawa, ed.), The Electrochemical Society, Tsukuba, Japan, 

2001, p. 72 . 

64. T. Satake, K. Watanabe, F. Nanjo, K. Takenobu, in USPTO, Mitsubishi Jukogyo Kabushiki 

Kaisha, United States, 1996. 

65. M. Hattori, M. Iio, Y. Sakaki, in Seventh International Symposium on Solid Oxide Fuel Cells 

(SOFC - VII) (S. C. S. H. Yokokawa, ed.), The Electrochemical Society, Tsukuba, Japan, 

2001, p. 1061 . 

66. Nakanishi, A., et al. Development of MOLB Type SOFC. in Eighth International Symposium 

on Solid Oxide Fuel Cells (SOFC VIII). 2003. Paris, France: The Electrochemical Society, 

Inc. 

67. Visco, S.J., et al. Developmet of Low-Cost Alloy Supported SOFCs. in Eighth International 

Symposium on Solid Oxide Fuel Cells (SOFC VIII). 2003. Paris, France: The 

Electrochemical Society, Inc. 

68. Brandon, N., et al. Metal Supported IT-SOFCs for Operation at 500-600C. in 2003 Fuel Cell 

Seminar. 2003. Miami Beach, FL, USA: Department of Energy. 

69. Schuler, A. An Intermediate Report on the Way to a Near-Series Sulzer Hexis Fuel Cell 

System. in Sixth European Solid Oxide Fuel Cell Forum. 2004. Luzern, der Schweiz: 

European Fuel Cell Forum. 

70. Shaffer, S. Development Update on Delphi's Solid Oxide Fuel Cell System. in SECA 2004 

Annual Meeting and Core Program Review. 2004. Boston: US DOE NETL. 

71. Surdoval, W., S.C. Singhal, and G.L. McVay. The Solid State Energy Conversion Alliance 

(SECA) - A U.S. Department of Energy Initiative to Promote the Development of Mass 

Customized Solid Oxide Fuel Cells for Low-Cost Power. in SOFC VII. 2000. Tsukuba, 

Japan: The Electrochemical Society Proceedings. 

72. Wang, C., W.L. Worrell, S. Park, J.M. Vohs, and R.J. Gorte, “Fabrication and Performance 

of Thin Film YSZ SOFCs Between 600 and 800°C,” paper presented at the 

JointFuelCellTechnologyReviewConference,1999 

http://www.netl.doe.gov/publications/proceedings/99/99fuelcell/fc7-1.pdf 

73. Goodenough, J.B. and K. Huang, “Lanthanum Gallate as a New SOFC Electrolyte”, 

Proceedings of the Fuel Cells ’97 Review Meeting. 

74. Surdoval, W.A., “SECA Core Technology Program,” presented at Solid State Energy 

Conversion Alliance 3rd Annual Workshop Proceedings, March 2002. 

http://www.netl.doe.gov/publications/proceedings/02/SECA/SECA3Surdoval.pdf 

75. Ball, R.J. and R. Stevens, “Novel Composite Electrolytes for Solid Oxide Fuel Cell 

Applications”, paper presented at the 26th Annual International Conference on Advanced 

Ceramics and Composites, Cocoa Beach, FL, January 13 – 18, 2002. 

76. Barnett, S., E. Perry, and D. Kaufmann, “Application of Ceria Layers to Increase Low 

Temperature SOFC Power Density”, Proceedings of the Fuel Cells ’97 Review Meeting. 

http://www.netl.doe.gov/publications/proceedings/97/97fc/FC6-6.pdf 

77. Bhide, S.V., W. Meng, and A.V. Virkar, “Stability of Mixed Perovskite Proton Conductors,” 

paper presented at Joint Fuel Cell Technology Review Conference, 1999. 

78. Goodenough, J., “Solid Oxide Fuel Cells with Gallate Electrolytes,” summary data on 

research supported by EPRI, 1998. 
7-49 

79. Steinberger-Wilckens, R., et al. Progress in SOFC Stack Development at Forschungszentrum 

Jülich. in Sixth European Solid Oxide Fuel Cell Forum. 2004. Luzern, der Schweiz: 

European Fuel Cell Forum. 

80. Patel. Thermally Integrated High Power Density SOFC Generator. in SECA 2004 Annual 

Meeting and Core Program Review. 2004. Boston: US DOE NETL. 

81. Huczkowski, P., et al. Growth Rate and Electrical Conductivity of Oxide Scales on Ferritic 

Steels Proposed as Interconnect Materials for SOFC. in Sixth European Solid Oxide Fuel 

Cell Forum. 2004. Luzern, der Schweiz: European Fuel Cell Forum. 
8-1 

8. FUEL CELL SYSTEMS

Although a fuel cell produces electricity, a fuel cell power system requires the integration of many 

components beyond the fuel cell stack itself, for the fuel cell will produce only dc power and 

utilize only certain processed fuel. Various system components are incorporated into a power 

system to allow operation with conventional fuels, to tie into the ac power grid, and often, to utilize 

rejected heat to achieve high efficiency. In a rudimentary form, fuel cell power systems consist of 

a fuel processor, fuel cell power section, power conditioner, and potentially a cogeneration or 

bottoming cycle to utilize the rejected heat. A simple schematic of these basic systems and their 

interconnections is presented in Figure 8-1. 

Figure 8-1 A Rudimentary Fuel Cell Power System Schematic 

The cell and stacks that compose the power section have been discussed extensively in the 

previous sections of this handbook. Section 8.1 addresses system processes such as fuel 

processors, rejected heat utilization, the power conditioner, and equipment performance guidelines. 

System optimization issues are addressed in Section 8.2. System design examples for present day 

and future applications are presented in Sections 8.3 and 8.4, respectively. Section 8.5 discusses 

research and development areas that are required for future systems. Section 8.5 presents some 

advanced fuel cell network designs, and Section 8.6 introduces hybrid systems that integrate fuel 

cells with other generating technologies. 
8-2 

8.1 System Processes 

The design of a fuel cell system involves more than the optimizing of the fuel cell section with 

respect to efficiency or economics. It involves minimizing the cost of electricity (or heat and 

electric products as in a cogeneration system) within the constraints of the desired application. For 

most applications, this requires that the fundamental processes be integrated into an efficient plant 

with low capital cost. Often these objectives are conflicting, so compromises, or design decisions, 

must be made. In addition, project-specific objectives, such as desired fuel, emission levels, 

potential uses of rejected heat (electricity, steam, or heat), desired output levels, volume or weight 

criteria (volume/kW or weight/kW), and tolerance for risk all influence the design of the fuel cell 

power system. 

8.1.1 Fuel Processing 

Fuel processing is defined in this Handbook as the conversion of a commercially available gas, 

liquid, or solid fuel to a fuel gas reformate suitable for the fuel cell anode reaction. Fuel 

processing encompasses the cleaning and removal of harmful species in the fuel, the conversion 

of the fuel to the fuel gas reformate, and downstream processing to alter the fuel gas reformate 

according to specific fuel cell requirements. Examples of these processes are: 

• Fuel Cleaning – Removal of sulfur, halides, and ammonia to prevent fuel processor and fuel 

cell catalyst degradation. 

• Fuel Conversion – Converting a fuel (primarily hydrocarbons) to a hydrogen-rich gas 

reformate. 

• Reformate Gas Alteration – Converting carbon monoxide (CO) and water (H2O) in the fuel 

gas reformate to hydrogen (H2) and carbon dioxide (CO2) via the water-gas shift reaction; 

selective oxidation to reduce CO to a few ppm, or removal of water by condensing to 

increase the H2 concentration. 

A fuel processor is an integrated unit consisting of one or more of the above processes, as needed 

for the fuel cell requirements37 and the fuel, that function together to be cost effective for the 

application. Design considerations may include high thermal efficiency, high hydrogen yield 

(for some fuel cells hydrogen plus carbon monoxide yield), multi-cycling, compactness, low 

weight, and quick starting capability, depending on the application. 

Figure 8-2 depicts the Processing steps needed for a low temperature cell.38 Most fuel processors 

make use of the chemical and heat energy left in the fuel cell effluent to provide heat for fuel 

processing thus enhancing system efficiency. 

 

37. Primarily determined by the cell’s operating temperature. 

38. Requires relatively complex fuel processing. 
8-3 

a) - For MCFC & SOFC, no high temperature shift, low temperature shift, or CO removal required. 

- For PAFC and circulating AFC, no CO removal required after low temperature shift. 

- For PEFC, all components required except that for high temperature CO removal eliminated or reduced in 

complexity. 

b) Possible to use residual air, water, and heat of fuel effluent from fuel cell and other downstream components. 

c) Vaporizer required for liquid fuels. 

d) Non-catalytic POX fuel processor does not require water. 

e) Temperature dependent on fuel, sulfur content of fuel, and type of reactor. 

f) Can be located prior to, within, or after the reactor; liquid desulfurizer located prior to the vaporizer. 

Figure 8-2 Representative Fuel Processing Steps & Temperatures 

Fuel conversion and alteration catalysts are normally susceptible to deactivation by impurities,39

thus the fuel cleaning process takes place upstream or within the fuel conversion process. The 

fuel conversion and reformate gas alteration processes can take place either external to the fuel 

cell or within the fuel cell anode compartment. The former is referred to as an external 

reforming fuel cell and the latter is referred to as an internal reforming fuel cell. Cells are being 

developed to directly react commercially available gas and liquid fuels, but the chemically 

preferred reaction of present fuel cells is via hydrogen-rich gas. This discussion will address 

external reforming fuel processors only. Descriptions of internal reforming are contained within 

the specific fuel cell sections. The system calculation section provides examples of heat and 

material balances for both externally and internally reforming fuel cells. 

Fuel processors are being developed to allow a wide range of commercial fuels suitable for 

stationary, vehicle, and military applications. Technology from large chemical installations has 

been successfully transferred to small, compact fuel cells to convert pipeline natural gas, the fuel 

of choice for small stationary power generators. Several hundred multi-kWe commercial fuel 

cell units are operating that contain fuel processors (see Section 1.6). Cost is an issue, as it is 

with the entire fuel cell unit, for widespread commercial application. Scaling of existing fuel 

processing technology to larger fuel cell power plants will reduce the specific cost of the fuel 

processor. 

Natural gas fuel reforming for fuel cells is essentially mature. Recent fuel processor research 

and development has focused on fuels for transportation and military applications. 

 

39. Referred to as poisoning in catalysis literature. Ni-based fuel processing catalysts are poisoned by 

“physiadsorbtion” of S onto the Ni surface, thus reducing performance. Pt catalysts are less susceptible to S 

poisoning because S does not physiadsorb as strongly as it does on Ni; thus affecting performance less. 

Airb

Reformate to

Fuel Cell

Gaseous 

Fuelc

Reactord

High °Ce

Sulfur 

Removalf

350 °C 

High Temp

Shift

260-370 °C 

CO 

Removal 

150-200°C 

Low Temp 

Shift 

200-260 °C

Waterb
8-4 

The issue with transportation is how to match a plausible commercial fuel infrastructure with the 

requirements of the fuel cell unit to be competitive. Economics drive the fuel of choice toward 

existing infrastructure, such as gasoline. Fuel cell requirements drive the fuel toward methanol 

or a “fuel cell friendly” gasoline. Environmental concerns drive the fuel of choice toward pure 

hydrogen40. Gasoline is a complex fuel, requiring high conversion temperature, and it has high 

levels of impurities that affect catalytic activity (see Appendix A). Methanol fuel processors 

(regarded by some as a necessary step towards an eventual liquid transportation fuel) are easier 

to develop than processors capable of converting gasoline. However, use of methanol or 

hydrogen would require major changes to the fuel supply infrastructure. Processors for both 

methanol and gasoline have been tested up to the 50 kWe level for vehicle application. What 

fuel to use onboard the vehicle is open to question at this time, but recent research in the fuel cell 

community points toward a modified gasoline tailored for fuel cell use that could be supplied 

through the existing fuel infrastructure (1). 

The U.S. military has a substantial fuel supply infrastructure in place. The two predominant fuel 

types in this infrastructure are diesel and jet fuel, a kerosene. It is highly improbable that the 

U.S. military would change these fuels to accommodate fuel cells. Use of a fuel more suitable to 

the fuel cell would limit the technology’s military use (there is R&D activity for fuel cell power 

packs to provide man-portable soldier power using hydrogen cartridges, or other hydrogencontaining forms, as well as methanol). Diesel and jet fuel are two of the most difficult 

conventional fuels to convert to a hydrogen-rich gas. They contain large amounts of sulfur that 

deactivate catalysts and require high conversion temperature. Fuel processors that convert diesel 

and jet fuel to a hydrogen-rich gas are in the early stages of development. The technology has 

been demonstrated at a 500 W size; 50 kWe units are being developed. Argonne National 

Laboratory (ANL) has operated a 3 kWe autothermal reformer with direct injection of diesel-like 

hydrocarbons – hexadecane and dodecane. Experiments with real diesel are anticipated 

shortly (2). 

Fuel Processing Issues 

Major issues that influence the development of a fuel processor are 1) choice of commercially 

available fuels suitable for specific applications; 2) fuel flexibility; 3) catalyst tolerance; 4) fuel 

cell size, and 5) vaporization of heavy hydrocarbons. Heavy hydrocarbons, such as diesel, 

require vaporization temperatures much in excess of 350 to 400 °C, at which temperature some 

of the heavier fuels pyrolyze. 

Fuel Choice and Flexibility: The fuel cell is a power generation technology that is in the early 

stages of commercial use. As a result, it is paramount to target applications that have the 

potential for widespread use (to attract adequate financial investment) with the simplest 

technology development (to minimize development cost). There is a strong relation between 

viable applications and the infrastructure of available fuels. 

 

40. The US FreedomCAR program is focused primarily towards hydrogen and secondarily towards “gasoline” as the 

onboard fuel. 
8-5 

High-value niche markets drove early fuel cell technology development. These included the use 

of fuel cells for on-board electric power in space vehicles, and to demonstrate that fuel cells are 

an efficient, environmentally-friendly technology for stationary on-site commercial power. 

The technology of choice for on-board electric power on mid-length space vehicle missions 

(several days to a year), including the important man-moon mission, was the fuel cell. This was 

because the use of batteries for more than a couple of days proved too heavy, combustion 

engines and gas turbines required too heavy a fuel supply, and the use of a nuclear reactor was 

only suitable for missions of a year or more. There was a simple choice of fuel for space fuel 

cells: it was hydrogen because it doesn’t require a fuel processor other than storage and 

pressurization, it is relatively lightweight when stored under pressure, and it was the best fuel for 

the early-developed alkaline fuel cell. Fuel flexibility was not an issue. 

It was logical to exploit fuel cell space development for terrestrial use. The initial terrestrial 

application was to increase power generation efficiency (in reaction to the oil crisis of the early 

1970s) and to improve the environment by lowering fossil-fueled power generation exhaust 

emission. Although coal-derived gas was recognized as a viable fuel, early fuel cell 

development was based on conveniently accessible pipeline gas prior to turing attention to coalderived gas. One of the major fuel cell sponsors at the time was the natural gas industry. 

Pipeline gas consists primarily of methane that is relatively easy to purify. The technology to 

convert methane to a H2-rich gas existed for large chemical plants. Developers had only to adapt 

existing technology to small fuel cell units, not easy due to several magnitudes of scale-down. 

Owners of stationary power plants usually desire fuel flexibility. Fortunately, the fuel processor 

on these early plants could convert a light distillate, such as naphtha, with minor changes (e.g., 

add a vaporizer, change-out the fuel nozzles). 

Once the niche markets were exploited to start fuel cells on their development path, it became 

necessary to target widespread potential applications while keeping technology development as 

simple as possible. General application areas of present interest to the fuel cell community are 

multi-kWe residential, commercial, and light industrial stationary power, transportation prime 

and auxiliary power, and military uses. 

In summary, these are the applications and coupled fuel choices of interest to fuel cell 

technology to date: 

• H2 is preferable for a closed environment such as space vehicle application. There are 

sources of H2-rich gases, such as an off-gas at a chemical plant, that require only fuel 

cleaning. Fuel flexibility is not applicable in either case. 

• The fuel choice for small, stationary power plants is pipeline gas due to its availability for 

multiple commercial, light-industrial, and residential applications. Some users request that 

the fuel processor convert at least one additional fuel, i.e., a light distillate. 

• Light vehicles are a key commercial target due to the large number of potential units; the fuel 

choice is open to question. Some proponents support the use of on-board hydrogen. There is 

a strong argument for liquid fuels due to on-board volume restrictions and existing fuel 

supply infrastructure. Candidate liquid fuels for light vehicles could be available gasoline or 
8-6 

a new gasoline, if driven by the infrastructure. Methanol may have an edge if it proves too 

difficult to process gasoline, provided the use of methanol compares favorably on a cost and 

environmental basis with present internal combustion engine (ICE) gasoline. Fuel flexibility 

in processors should be considered because of the indecision on fuel type and because the 

public is accustomed to a selection of different octane liquid fuels and diesel. 

• The present infrastructure fuel for heavy vehicles is high sulfur diesel (now ~500 ppm sulfur 

by weight) but this may change to a nearly sulfur-free diesel as proposed by the EPA. 

Beginning June 1, 2006, refiners must produce a diesel containing a maximum of 15 ppm 

sulfur (3). The fuel for this sector could also be a gasoline if such a fuel cell system could 

compete. 

• On-board vehicle auxiliary power is increasing dramatically to satisfy consumer convenience 

demands. Fuel selection for these applications parallel light and heavy vehicle fuels. 

• The military will continue with its fuel infrastructure of high sulfur diesel (up to 1,000 ppm 

sulfur by weight) and jet fuel (JP-8, up to 300 ppm by weight). Sulfur specification will 

remain high because the military has to consider worldwide fuel sources. High sulfur diesel 

and JP-8 are close in characteristics, so no fuel flexibility is required. However, there is a 

possibility that some parts of the military or the Coast Guard (a military service within the 

DOT) could use fuels more compatible to the fuel cell in limited applications. 

• As environmental regulation becomes more stringent for megawatt-size power stations and 

fuel cells are scaled larger in size, there is the possibility to use the U.S.’s most plentiful, 

indigenous fuel, coal. The term, coal, covers a broad spectrum of solid fuels that complicate 

fuel processing, particularly cleanup. 

• There is the possibility of using other available fuels such as light distillates, ethanol, 

anaerobic digester gas, biomass, and refuse-derived fuel. 

The market that has the greatest impact on fuel processor development at this time is in the light 

vehicle application sector, due to the potential large number of units. Some fuel processor 

developers are focusing on the development of methanol fuel processing either as the fuel of 

choice or as a development step toward processing gasoline. Others consider that it is best to 

develop a vehicle that uses the most environmentally attractive fuel, hydrogen. There are 

numerous opinions reagarding fuel and infrastructure best-suited for the light vehicle 

transportation market.

Methanol is unquestionably the easiest of the potential liquid fuels to convert to hydrogen for 

vehicle use. Methanol disassociates to carbon monoxide and hydrogen at temperatures below 

400 °C and can be catalytically steam reformed at 250 °C or less. This provides a quick start 

advantage. Methanol can be converted to hydrogen with efficiencies of >90 percent. But 

methanol is produced primarily from natural gas, requiring energy, and it is less attractive than 

gasoline on a well-to-wheels efficiency basis (5, 6). 

Gasoline has many advantages over methanol, but conversion to H2 requires temperatures in 

excess of 650 °C and produces greater amounts of CO, methane (CH4), and possibly coke. 

Without catalyst, the conversion temperature is 1,000 °C or higher. High temperatures require 

special materials of construction and significant preheating. Petroleum-derived fuels contain 

more sulfur and trace amounts of metal that could be harmful to the fuel cell. Natural gas is not 
8-7 

good for transportation because of its low relative energy density and 700 °C or higher 

processing temperature (7). 

ExxonMobil has presented a position paper (8) for liquid fuels that addresses the pros and cons 

of methanol versus gasoline. Paraphrased excerpts from this are: 

• Fuels that are most directly suited to the fuel cell are the most difficult and costly to produce 

and distribute. Gasoline and methanol are the leading candidates to power fuel cell engines. 

Both the gasoline and methanol fuel cell vehicles should be more fully developed prior to 

making a commercial decision on fuel choice. 

• Due to methanol's corrosivity and its affinity for water, it cannot be readily distributed in 

today's fuel infrastructure. Methanol burns with a nearly invisible flame. Available 

luminosity additives won’t reform in the low-temperature methanol steam reformers. 

Methanol is more acutely toxic than gasoline. Additives that are likely to be needed for 

safety and health reasons will impact the fuel processor’s performance and cost. 

• Gasoline fuel processing has the ability to utilize the existing infrastructure, a major 

advantage. It is inherently more flexible than the low temperature methanol processor, 

allowing multiple fuel use in the same system. The gasoline processor is also more tolerant 

of contaminants or additives contained in the fuel. Due to the higher energy density of 

gasoline, the gasoline system offers the potential for up to twice the vehicle range of the 

methanol system. Today’s mid-sized passenger cars are about 15 to 18 percent "well-towheels" energy efficient as indicated in Figure 8-3.41 Despite the increased vehicle 

efficiency of a methanol fuel-based system, the resultant "well-to-wheel" efficiency would be 

only 20 to 28 percent, lower than either gasoline hybrids or gasoline fuel cell vehicles. 

• A customized gasoline for fuel cells could offer better performance and be produced at lower 

cost because many of conventional gasoline’s more expensive ingredients would not be 

required. Naphtha is a common refinery stream that is an inexpensive alternative to 

conventional gasoline. Although its octane is too low for today’s ICE, naphtha is ideal for 

fuel cells and could be supplied to retail stations within the existing gasoline infrastructure. 

Fuel Cell and Fuel Processor Catalyst Tolerance: There are major fuel requirements for the 

gas reformates that must be addressed. These requirements result from the effects of sulfur, 

carbon monoxide, and carbon deposition on the fuel cell catalyst. The activity of catalysts for 

steam reforming and autothermal reforming can be affected by sulfur poisoning and coke 

formation; this commonly occurs with most fuels used in fuel cells of present interest. Other fuel 

constituents can also prove detrimental to various fuel cells. Examples of these are halides, 

hydrogen chloride, and ammonia. 

 

41. Editor’s note - The gasoline-fueled ICE well-to-wheel efficiency values apply to today’s technology and are 

averaged over the entire driving cycle. Advanced IC engine/vehicles are more efficient over the entire 

operating cycle than 18% (up to 20 some odd %). This implies that future IC engine/vehicle efficiency for light 

vehicles can be in excess of the 15 to 18% quoted in the ExxonMobil paper. Vehicle miles per gallon increase 

when the ICE is combined with a battery in developmental vehicles with very low drag coefficients. For 

example, the 60+ mpg for the Honda Insight, 40 to 50+ mpg for the Toyota Prius, 70+ mpg for the Ford 

Prodigy, and ~80 mpg for the GM Precept. The overall well-to-wheel efficiency over a standard city/highway 

driving cycle for a four passenger, production hybrid vehicle has been estimated to be about 25-30%, close to a 

fuel cell vehicle. The fuel cell engines for lightweight vehicles are likely to be hybrids, and therefore the 

projected efficiencies must be carefully considered. 
8-8 

There are discrepancies in the tolerance for harmful species specified by fuel cell developers, 

even for similar type fuel cells. These discrepancies are probably due to electrode design, 

microstructure differences, or in the way developers establish tolerance. In some cases, the 

presence of certain harmful species causes immediate performance deterioration. More often, 

the degradation occurs over a long period of time, depending on the developer’s permissible 

exposure to the specific harmful species. Here, the developer establishes an estimated cell life 

based on economics. The permissible amount of the harmful constituent is then determined 

based on economic return vs. fuel cell life expectancy. 
8-9 

Energy

Resource Recovery Processing Reformer Energy Conversion Power Management 

Natural

Gas

Extraction/

Cleaning/

Delivery

88%

Methanol

Conversion/

Storage

76-81%

Hydrogen

Conversion/

Storage

68-75%

Methanol

Steam

Reformer

80-85%

Fuel Cell

50-60%

Fuel Cell

50-55%

Motor/

Controller/

Accessories

75-84%

Crude

Oil

Extraction

96%

Gasoline

Processing/

Delivery

88%

Autothermal

Reformer

75-80%

Fuel Cell

50-55%

Motor/

Controller/

Accessories

75-84%

Gasoline IC Vehicle (current)

18%

Figure 8-3 “Well-To-Wheel” Efficiency for Various Vehicle Scenarios (9)
8-10 

Sulfur Effects 

Present gasolines contain approximately 300 ppm by weight of sulfur. New government 

standards will reduce the sulfur concentration to an average of 30 ppm and a maximum of 80 

ppm by 2006; however fuel gas produced from these gasolines may contain as high as 3-8 ppm 

of H2S. No. 2 fuel oil contains 2,200 to 2,600 ppm of sulfur by weight. Even pipeline gas 

contains sulfur-containing odorants (mercaptans, disulfides, or commercial odorants) for leak 

detection. Metal catalysts in the fuel reformer can be susceptible to sulfur poisoning, requiring 

that the sulfur in the fuel reformate be removed. Some researchers have advised limiting the 

sulfur content of the fuel from a steam reformer to less than 0.1 ppm, but note that the limit may 

be higher in an autothermal reformer (10). 

Sulfur poisons catalytic sites in the fuel cell also. The effect is aggravated when there are nickel 

or iron-containing components, including catalysts that are sensitive to sulfur and noble metal 

catalysts such as found in low temperature cell electrodes. Sulfur tolerances are described in the 

specific fuel cell sections of this handbook.42 In summary, the sulfur tolerances of the cells of 

interest, by percent volume in the cleaned and altered fuel reformate gas to the fuel cells from 

published data, are: 

• PEFC - <50 ppm sulfur as H2S (11), poisoning is cumulative and not reversible. 

• PAFC - <50 ppm sulfur as H2S + COS or <20 ppm sulfur as H2S at the anode. Poisoned 

anodes can be re-activated by polarization at high potentials. 

• MCFC - <0.5 ppm sulfur as H2S (at the cathode) equates to <10 ppm at the anode because of 

fuel exhaust being sent to the cathode in an MCFC (same amount of sulfur, more gas at the 

cathode), poisoning is reversible. 

• SOFC - <1 ppm sulfur as H2S, poisoning is reversible for the tubular SOFC. H2S levels of 1 

ppm result in an immediate performance drop, but this loss soon stabilizes into a normal 

linear degradation. Tests show that high temperature planar SOFCs with all-ceramic 

components can tolerate up to 3,000 ppm of sulfur. Sulfur, in H2S form, has been used as a 

fuel for an external reforming, all-ceramic SOFC operating at 1,000 °C (12). However, 

developers want to reduce the cell temperature to allow less expensive metal components, 

primarily interconnects, and improve cycle efficiency. There is a requirement to lower sulfur 

significantly if metal parts are used in an SOFC. For planar SOFCs, claims for sulfur 

tolerance vary among the developers. The range of sulfur has been published as 10 to 35 

ppm. Planar SOFC sulfur tolerance probably will be secondary to the fuel processor catalyst 

that, as mentioned, may be as low as 0.1 ppm. 

 

42. There is ambiguity in the way sulfur is reported in fuel cell literature that has caused confusion in the amount 

that can be tolerated. Reports often fail to distinguish whether the sulfur is measured by weight, as it would be 

before vaporization of a liquid fuel, or by volume, as it would be in a gas fuel or fuel gas reformate. An 

approximate rule of thumb is that the amount (by volume) of sulfur in a vaporized fuel is one-tenth the amount 

of sulfur measured by weight in the liquid fuel. 300 ppm sulfur (by weight) in the liquid fuel equates to 30 ppm 

sulfur (by volume) when the fuel is converted to a gaseous reformate. 
8-11 

Carbon Monoxide Effects 

Carbon monoxide, a fuel in high temperature cells (MCFC and SOFC), is preferentially absorbed 

on noble metal catalysts that are used in low temperature cells (PAFC and PEFC) in proportion 

to the H2:CO partial pressure ratio. A particular level of carbon monoxide yields a stable 

performance loss. The coverage percentage is a function of temperature, and that is the sole 

difference between PEFC and PAFC (13). Cell limits are: 

• PEFC – Consensus tolerance is <50 ppm into the anode. 

• PAFC – Major US manufacturer set tolerance limit as <1.0 percent into the anode. 

• MCFC – CO and H2O shift to H2 and CO2 in the cell as the H2 is consumed by the cell 

reaction due to a favorable temperature and catalyst. 

• SOFC – CO can be a fuel. However, if the fuel gas contains H2O, the shift reaction (CO + 

H2O → H2 + CO2) is chemically favored. 

Carbon Deposition Effects 

The processing of hydrocarbons always has the potential to form coke (soot). If the fuel 

processor is not properly designed or operated, coking is likely to occur (7). Carbon deposition 

not only represents a loss of carbon for the reaction, but more importantly results in deactivation 

of catalysts in the processor and the fuel cell due to deposition at the active sites. Thermodynamic equilibrium provides a first approximation of the potential for coke formation. The 

governing equations are: 

 C + CO2 ↔ 2CO (Boudouard) (8-1) 

 C + 2H2 ↔ CH4 (carbon-hydrogen) (8-2) 

 C + H2O ↔ CO + H2 (carbon-steam or gasification) (8-3) 

The possible formation of carbon using a particular fuel can be determined by the simultaneous 

solution of the above equations using their equilibrium coefficients.43 No solid graphitic carbon 

exists at low temperatures (~600 °C) in binary mixtures containing at least 2 atoms of oxygen or 

4 atoms of hydrogen per atom of carbon (14). 

Fuel Cell Unit Size: The size of the fuel cell is a characteristic that impacts fuel processor 

selection. There is a lower level of power output at which it is no longer advantageous to 

incorporate a fuel processor. The decision is also application-specific. It is likely that releasing 

H2 by chemical reaction from a solid compound when mixed with water is economical for small 

portable units (below 100 W). An H2 storage cartridge can be replaced in seconds (15). 

Actually the power level at which the tradeoff is likely to occur changes as processing and 

storage technology advances. One fuel processor developer has produced a 100 W partial 

oxidation (POX) methane reactor the size of a coffee can. The unit includes a reforming zone, 

shift reactors, and all heat exchangers. H2 is 36 percent (assume dry) and the CO level can be 

reduced to 1 percent. The unit runs on methane, propane, and ethanol (16). Another research 

project is investigating methanol reformers for sub-watt fuel cell power sources for the Army. 

 

43. Carbon is slightly less likely to be deposited than equilibrium coefficient calculations indicate, due to kinetics. 
8-12 

Fuel Processing Techniques 

The generic term most often applied to the process of converting liquid or gaseous light 

hydrocarbon fuels to hydrogen and carbon monoxide is “reforming”. There are a number of 

methods to reform fuel. The three most commercially developed and popular methods are 

1) steam reforming, 2) partial-oxidation reforming, and, 3) autothermal reforming. 

Steam reforming (SR) provides the highest concentration of hydrogen and can obtain a conversion efficiency. Partial oxidation (POX) is a fast process, good for starting, fast response, and a 

small reactor size. Non-catalytic POX operates at temperatures of appro ximately 1,400 °C, but 

adding a catalyst (catalytic POX or CPOX) can reduce this temperature to as low as 870 °C. 

Combining steam reforming closely with CPOX is termed autothermal reforming (ATR). 

Steam Reforming: Historically, steam reforming has been the most popular method of 

converting light hydrocarbons to hydrogen. The fuel is heated and vaporized, then injected with 

superheated steam into the reaction vessel. The steam-to-carbon molar ratio is usually in the 

neighborhood of 2.5:1 but developers strive for lower ratios to improve cycle efficiency. Excess 

steam is used to force the reaction to completion as well as to inhibit soot formation. Like most 

light hydrocarbons, heavier fuels can be reformed through high temperature reaction with steam. 

Steam reforming is usually carried out using nickel-based catalysts. Cobalt and noble metals are 

also active, but more expensive. The catalytic activity depends on metal surface area. For 

nickel, the crystals sinter quickly above the so-called Tamman temperature (590 °C), 

approaching a maximum size related to the pore diameter of the support. The crystal growth 

results in loss of surface area and activity (17). The steam reformer can operate with or without 

a catalyst. Most commercial applications of steam reforming use a catalyst to enhance reaction 

rates at decreased temperatures. Lower temperatures favor high CO and hydrogen concentration. 

The reforming catalyst also promotes the water-gas shift reaction. Steam reforming is 

endothermic, thus favored by high temperatures. But it is a slow reaction and requires a large 

reactor (4). As a result, rapid start and transients cannot be achieved by steam reforming due to 

its inherently slower indirect heating (18). Steam reforming suits pipeline gas and light distillate 

stationary fuel cell power generation well. 

The exothermic water-gas shift reaction occurs in the steam reformer reactor. The combined 

reaction, steam reforming and water gas shift, is endothermic. As such, an indirect high 

temperature heat source is needed to operate the reactor. This heat source usually takes the form 

of an adjacent, high-temperature furnace that combusts a small portion of the fuel or the fuel 

effluent from the fuel cell. Efficiency improves by using rejected heat from other parts of the 

system. Note that the intrinsic water-gas shift in the reactor may not lower the CO content to the 

fuel cell requirement, and additional shifting will be needed for lower temperature fuel cells. 

Steam reforming of higher hydrocarbons can be used to produce methane suitable for use in 

high temperature internal reforming fuel cells. Steam pre-reforming of hydrocarbons, as a 

process step in the manufacture of hydrogen, ammonia, methanol, carbon monoxide, and syngas, 

is an established technology. All higher hydrocarbons are converted over a nickel-based catalyst 

into a gas mixture containing hydrogen, methane, and carbon oxides. Establishment of 

methanation and shift reaction equilibria at the process conditions determines the composition of 
8-13 

the pre-reformed gas. By proper design of fuel processing systems, a wide variety of fuels may 

be converted to a suitable reformate. This reformate can then be used to promote internal 

reforming for high temperature fuel cell systems. For each type of fuel, optimum operating 

parameters such as temperature, steam/carbon ratio, and catalyst must be established (19). 

Partial Oxidation: A substoichiometric amount of air or oxygen is used to partially combust the 

fuel. Partial oxidation is highly exothermic, and raises the reactants to a high temperature. The 

resulting reaction products, still in a reduced state, are then quenched through the introduction of 

superheated steam. The addition of steam promotes the combined water-gas shift and steam 

reforming reactions, which further cools the gas. In most cases, and with sufficient pre-heating 

of the reactants, the overall reaction is exothermic and self-sustaining. For some applications 

however, particularly small-scale configurations, a catalyst can be used to increase reaction rates 

at lower reaction temperatures. As with steam reforming, additional, water-gas shift may be 

necessary to satisfy the fuel cell requirements. 

POX reactor temperatures vary widely. Noncatalytic processes for gasoline reforming require 

temperatures in excess of 1,000 °C. These temperatures require the use of special materials and 

significant preheating and integration of process streams. The use of a catalyst can substantially 

reduce the operating temperature, allowing the use of more common construction materials such 

as steel. Lower temperature conversion leads to less carbon monoxide (an important consideration for low temperature fuel cells), so that the shift reactor can be smaller. Lower temperature 

conversion will also increase system efficiency. 

For some heavy hydrocarbon fuels, typical values range from as low as 870 °C for catalytic POX 

upwards to 1,400 °C for non-catalytic POX. For sulfur-bearing diesel fuel, a catalytic POX 

reactor will usually operate at approximately 925 °C. This relatively elevated temperature is 

needed to overcome catalyst degradation due to the presence of sulfur. Non-catalytic POX 

reactors operate at around 1,175 °C on diesel fuel. 

Advantages of POX that make this type of fuel conversion suitable for transportation power are: 

• POX does not need indirect heat transfer (across a wall), so the processor is more compact 

and lightweight (7).

• Contrary to widely-held opinion, POX and ATR are capable of higher reforming efficiencies 

than are steam reformers (20). 

Partial oxidation should be reacted so that the overall reaction is exothermic, but at a low 

oxygen-to-fuel ratio to favor higher hydrogen yields. 
8-14 

It is a widely-held opinion that POX leads to lower efficiency than steam reforming due to the 

POX reaction being exothermic. However, a thorough examination of the thermodynamics 

shows that POX and ATR have higher reforming efficiencies than steam reformers. This raises 

the question why there is a need to use steam reforming or an ATR if the POX's efficiency is 

higher. The minimum allowable oxygen to carbon (O/C) ratio is 1 for the POX process. This 

generates high heat that leads to undesirable high temperatures (low H2, CO2 selectivity, 

materials of construction constraints, etc.). The steam reformer and ATR allow lower O/C ratios, 

keep the temperature down, and result in higher CO2 and H2 selectivity (more H2 yield per mole 

of fuel). 

Autothermal Reforming: The coupling of SR with POX is termed autothermal reforming 

(ATR). Some define ATR as a SR reaction and a POX reaction that take place over microscopic 

distances at the same catalytic site, thus avoiding complex heat exchange (21). Others have the 

less restrictive definition that ATR occurs when there is no wall between a combined SR reaction 

and catalytic POX reaction. ATR is carried out in the presence of a catalyst that controls the 

reaction pathways and thereby determines the relative extents of the POX and SR reactions. The 

SR reaction absorbs part of the heat generated by the POX reaction, limiting the maximum 

temperature in the reactor. The net result can be a slightly exothermic process. 

Autothermal reforming provides a fuel processor compromise that operates at a lower O/C and 

lower temperature than the POX; is smaller, quicker starting, and quicker responding than the 

SR, and results in high H2 concentration. A catalytic POX reaction must be used to reduce the 

temperature to a value compatible with the SR temperature. 

Other Reforming Combinations: There have been fuel processor configurations where a noncatalytic POX is placed in series with a steam reformer. Without catalyst, the POX reaction must 

be at a higher temperature than the steam reformer reaction. These reactions must take place in 

separate compartments with heat exchange and a wall between them (18). This configuration is 

not considered within the definition of autothermal reforming. 

State-of-the-Art Components 

Developers have brought fuel processing technology to the point where conversion of all fuels of 

interest to fuel cells have been demonstrated to a degree. Natural gas steam reforming is used in 

commercial fuel cell units. There has been equal success with steam reforming light distillates, 

although these fuels are not commonly used. Tests have been performed on reactors and 

complete small fuel processors using methanol, gasoline, and diesel, all suitable for vehicle use. 

These tests have not advanced to operation over prolonged periods. However, there have been 

tests that indicate these fuels can be processed in POX and ATR reactors with high levels of 

sulfur. Water-gas shift and methods to lower CO even to a few ppm have been developed, but 

the final CO cleanup processes are in an early stage of development. All fuel processors need 

additional engineering development to reduce volume, weight, and cost to allow widespread fuel 

cell power unit use. The state-of-the-art information below is based primarily on U.S. or closelyrelated fuel cell programs. 
8-15 

State-of-the-Art Components - Conversion of Fuels 

Generic Fuel Conversion: Considering the spectrum of fuel conversion from steam reforming 

to partial oxidation should convey a basic understanding of the reforming processes. An elegant, 

general equation published by the ANL describes fuel conversion throughout the spectrum. 

Autothermal reforming falls within this spectrum so that the equation encompasses processes of 

interest to fuel cells. The equation does not apply to complete combustion, but that conversion 

process is not relevant to fuel cells (20, 22, 23). The general, idealized equation is: 

 CnHmOp + x(O2 + 3.76N2) + (2n – 2x – p)H2O = nCO2 + (2n – 2x – p +m/2)H2 + 3.76xN2 (8-4) 

where x is the molar ratio of oxygen-to-fuel. This ratio is very important because it determines: 

• The minimum amount of water that is required to completely convert the carbon in the fuel to 

carbon dioxide (2n – 2x – p). Excess water is used in practice to ensure the conversion, 

resulting in water in the reformate (right side of the equation). Typically, one or two moles 

of water for every mole of oxygen are used. 

• The maximum hydrogen yield (2n – 2x – p +m/2) 

• The maximum concentration (percentage) of hydrogen in the reformate {[2n – 2x – p 

+m/2]/[n + (2n – 2x – p +m/2) + 3.76x] all times 100}

• The heat of reaction {∆Hr = n(∆Hf,CO2 )– (2n – 2x – p)∆Hf,H2O - ∆Hf,fuel}. 

Decreasing the oxygen-to-fuel ratio, x, results in increasing demand for water (water-to-fuel 

ratio), with commensurate increases in the yield and concentration of hydrogen in the reformate 

gas. When x = 0, the equation reduces to the strongly endothermic steam reforming reaction. 

The reaction becomes less endothermic with increasing oxygen. It becomes thermoneutral44 at 

x = x0 (0.44 for methane). Above this point, the reaction becomes increasingly exothermic. At x 

= 1 with methane, the pure POX reaction, the feed contains sufficient oxygen to convert all of 

the carbon in the fuel to CO2. No water needs to be added. The equation is a mix of the steam 

reforming reaction and the POX reaction at values of x between 0 and n. 

Beyond x = [n – (p/2)] = n (when p = 0), where water is a product, the heat of reaction is 

determined by the phase of the product water. At still higher values, the excess oxygen oxidizes 

the hydrogen to produce water. Finally, at stoichiometric combustion, all carbon and hydrogen 

are converted to carbon dioxide and water. Here, x = Xc = [n – (p/2) + (m/4)]. The value of x 

reduces to 2 with CH4 as the fuel. 

Equation 8-4 depicts a total reaction where the fuel input is converted to carbon dioxide. 

Actually, the initial reforming step is carried out at elevated temperatures, where a mixture of 

carbon monoxide and carbon dioxide is formed. In the subsequent reformate conversion step, the 

carbon monoxide is converted via the water-gas shift to carbon dioxide: 

 CO + H2O ↔ H2 + CO2 (8-5) 

 

44. The thermoneutral point (of oxygen-to-carbon ratio) is where the enthalpy of the reaction is zero, (∆Hf,298 = 0). 
8-16 

There may be additional, downstream inputs of water/steam and oxygen/air for water-gas shift 

and selective oxidation to further reduce CO, if needed. 

When the function of a fuel processor is to convert a fuel to hydrogen, the fuel conversion 

efficiency is 

Lower Heating Value of Fuel Used

Lower Heating Value of Anode Fuel(s) Produced Efficiency = (8-6) 

The fuel conversion efficiency for methane conversion to hydrogen is 93.9 percent at the 

thermoneutral point, x = 0.44 (an ATR reaction) and 91.7 percent at x = 0 (the SR reaction). The 

difference between the two efficiency values is exactly equivalent to the loss represented by the 

latent heat of vaporization of the H2O that escapes with the combustions products in the SR 

burner exhaust. The concentration of hydrogen is 53.9 percent at x = 0.44 (ATR) and 80 percent 

at x = 0 (SR). 

Equation 8-4 and related heats of reaction can be manipulated to show that the maximum 

efficiency is a state point function, regardless of path (steam reforming, partial oxidation, or 

autothermal reforming), and is achieved at the thermoneutral point. In practice, x is set slightly 

higher than the thermoneutral point so that additional heat is generated to offset heat losses from 

the reformer. Table 8-1 presents efficiencies at the thermoneutral point for various hydrocarbon 

fuels. 

Table 8-1 Calculated Thermoneutral Oxygen-to-Fuel Molar Ratios (xo) and 

Maximum Theoretical Efficiencies (at xo) for Common Fuels (23) 

CnHmOp n m p 

∆Hf,fuel

(kcal/gmol) m/2n 

Xo, 

∆Hr = 0 

Efficiency 

(percent) 

Methanol 

CH3OH(l) 

1 4 1 -57.1 2 0.230 96.3 

Methane 

CH4

1 4 0 -17.9 2 0.443 93.9 

Iso-Octane 

C8H18(l) 

8 18 0 -62.0 1.125 2.947 91.2 

Gasoline 

C7.3H14.8 O0.1(l) 

7.3 14.8 0.1 -53.0 1.014 2.613 90.8 

Because the components and design of a fuel processor depend on the fuel type, the following 

discussion is organized by the fuel being processed. 

Hydrogen Processing: When hydrogen is supplied directly to the fuel cell, the fuel processing 

section is no more than a storage and delivery system. However, in general applications, 

hydrogen must be generated from other fuels and processed to meet the system requirements. 

Natural Gas Processing: The major constituents of pipeline gas are methane, ethane, propane, 

CO2, and, in some cases, N2. Sulfur-containing odorants (mercaptans, disulfides, or commercial 

odorants) are added for leak detection. Because neither fuel cells nor commercial reformer 
8-17 

catalysts are sulfur tolerant, the sulfur must be removed. This is usually accomplished with a 

zinc oxide sulfur polisher and the possible use of a hydrodesulfurizer, if required. The zinc oxide 

polisher is able to remove the mercaptans and disulfides. However, some commercial odorants, 

such as Pennwalt's Pennodorant 1013 or 1063, contain THT (tetrahydrothiophene), more 

commonly known as thiophane, and require the addition of a hydrodesulfurizer before the zinc 

oxide sorbant bed. The hydrodesulfurizer will, in the presence of hydrogen, convert the 

thiophane into H2S that is easily removed by the zinc oxide polisher. The required hydrogen is 

supplied by recycling a small amount of the natural gas reformed product. Although a zinc oxide 

reactor can operate over a wide range of temperatures, a minimum bed volume is achieved at 

temperatures of 350 to 400 °C (660 to 750 °F). 

The CH4 in the natural gas is usually converted to H2 and CO in a SR reactor. Steam reforming 

reactors yield the highest percentage of hydrogen of any reformer type. The basic SR reactions 

for methane and a generic hydrocarbon are: 

 CH4 + H2O ↔ CO + 3H2 (8-7) 

 CnHm + nH2O ↔ nCO + (m/2 + n) H2 (8-8) 

 CO + H2O ↔ CO2 + H2 (8-9) 

In addition to natural gas, steam reformers can be used on light hydrocarbons such as butane and 

propane, and on naphtha with a special catalyst. Steam reforming reactions are highly 

endothermic and need a significant heat source. Often the residual fuel exiting the fuel cell is 

burned to supply this requirement. Fuels are typically reformed at temperatures of 760 to 980 °C 

(1,400 to 1,800 °F). 

A typical steam reformed natural gas reformate is presented in Table 8-2. 

Table 8-2 Typical Steam Reformed Natural Gas Reformate 

Mole 

Percent 

Reformer 

Effluent 

Shifted 

Reformate 

H2 46.3 52.9

CO 7.1 0.5

CO2 6.4 13.1

CH4 2.4 2.4

N2 0.8 0.8

H2O 37.0 30.4

Total 100.0 100.0

A POX reformer also can be used to convert gaseous fuels, but does not produce as much 

hydrogen as the steam reformers. For example, a methane-fed POX reformer would produce 

only about 75 percent of the hydrogen (after shifting) that was produced by an SR. Therefore, 

partial oxidation reformers are typically used only on liquid fuels that are not well suited for 

steam reformers. Partial oxidation reformers rank second after steam reformers with respect to 

their hydrogen yield. For illustration, the overall POX reaction (exothermic) for methane is 
8-18 

 CH4 + 1⁄2O2 → CO + 2H2 (8-10) 

When natural gas fuels are used in a PAFC or a PEFC, the reformate must be water-gas shifted 

because of the high CO levels in the reformate gas. A PAFC stack can tolerate about 1 percent 

CO in the cell before having an adverse effect on cell performance due to catalyst poisoning. 

The allowable CO level in the fuel gas for a PEFC is considerably lower. The shift conversion is 

often performed in two or more stages when CO levels are high. A first high-temperature stage 

allows high reaction rates, while a low-temperature converter allows for a higher conversion. 

Excess steam is used to enhance the CO conversion. A single-stage shift reactor is capable of 

converting 80 to 95 percent of the CO (24). The water gas shift reaction is mildly exothermic, so 

multiple stage systems must have interstage heat exchangers. Feed temperatures of high- and 

low-temperature shift converters range from approximately 260 to 370 °C (500 to 700 °F) and 

200 to 260 °C (400 to 500 °F), respectively. Hydrogen formation is enhanced by low 

temperature, but is unaffected by pressure. 

When used in a PEFC, the reformate must pass through a preferential CO catalytic oxidizer, even 

after being shifted in a shift reactor. Typically, the PEFC can tolerate a CO level of only 

50 ppm. Work is being performed to increase the CO tolerance level in PEFC. 

At least two competing reactions can occur in the preferential catalytic oxidizer: 

 CO + 1⁄2O2 → CO2 (8-11) 

 H2 + 1⁄2O2 → H2O (8-12) 

The selectivity of these competing reactions depends upon the catalyst and determines the 

quantity of required oxygen (25). 

Liquid Fuel Processing: Liquid fuels such as distillate, naphtha, diesel oils, and heavy fuel oil 

can be reformed in partial oxidation reformers. All commercial POX reactors employ 

noncatalytic POX of the feed stream by oxygen in the presence of steam with reaction 

temperatures of approximately 1,300 to 1,500 °C (2,370 to 2,730 °F) (24). For illustration, the 

overall POX reaction for pentane is 

 C5H12 + 5/2O2 → 5CO + 6H2 (8-13) 

The overall reaction is exothermic, and largely independent of pressure. The process is usually 

performed at 20 to 40 atmospheres to yield smaller equipment (24). A typical fuel composition 

for a fuel oil fed POX reformer is presented in Table 8-3. The CO contained in this reformate 

may need to be converted with a shift converter or selective catalytic converter, depending upon 

the specific fuel cell being fed. 
8-19 

Table 8-3 Typical Partial Oxidation Reformed Fuel Oil Reformate (24) 

Mole Percent 

(dry, basis) 

Reformer 

Effluent 

H2 48.0

CO 46.1

CO2 4.3

CH4 0.4

N2 0.3

H2S 0.9

Total 100.0

Alcohols are steam-reformed at lower temperatures (<600 °C) while alkanes45 and unsaturated 

hydrocarbons require slightly higher temperatures. Cyclic hydrocarbons and aromatics have also 

been reformed at relatively low temperatures, however a different mechanism appears to be 

responsible for their reforming. Blended fuels like gasoline and diesel, that are mixtures of a 

broad range of hydrocarbons, require temperatures of >700 °C maximum hydrogen production. 

Methanol, one of the fuels being considered for transportation applications, can be converted into 

hydrogen by steam reforming: 

 CH3OH = CO + 2H2 (8-14) 

 CO + H2O = CO2 + H2 (8-15)

The equivalent overall result of these two specific reactions is: 

 CH3OH + H2O = CO2 + 3H2 (8-16) 

The optimum choice of operating conditions is close to a steam to methanol ratio of 1.5 and a 

temperature range of 250 °C to 399 °C. Pressure does not influence the reaction rate, but very 

high pressures limit the equilibrium conversion, which otherwise is better than 99 percent at the 

preferred range of 5 to 15 bars. The Cu/Zn/Al and Cu/Zn/Cr based catalysts have been used in 

industrial units for many years (17). 

Coal Processing: The numerous coal gasification systems available today can be reasonably 

classified as one of three basic types: 1) moving-bed, 2) fluidized-bed, and 3) entrained-bed. All 

three of these types use steam and either air or oxygen to partially oxidize coal into a gas 

product. The moving-bed gasifiers produce a low temperature (425 to 650 °C; 800 to 1,200 °F) 

gas containing devolatilization products such as methane and ethane, and hydrocarbons including 

naphtha, tars, oils, and phenol. Entrained-bed gasifiers produce a gas product at high temperature (>1,260 °C; >2,300 °F composed almost entirely of hydrogen, carbon monoxide, and carbon 

dioxide. The fluidized-bed gasifier product gas falls between these two other reactor types in 

composition and temperature (925 to 1,040 °C; 1,700 to 1,900 °F). 

 

45. Alkanes are saturated hydrocarbons, i.e., no double carbon bonds. Examples are CH4, C2H6, C3H8, and 

C(n)H(2n+2). Alkenes have carbon-carbon double bonds such as ethene C2H4 and C(n)H(2n). 
8-20 

The heat required for gasification is supplied by the partial oxidation of coal. Overall, the 

gasification reactions are exothermic, so waste heat boilers often are used at the gasifier effluent. 

The temperature, and therefore composition, of the product gas depends upon the amount of 

oxidant and steam, as well as the design of the reactor. 

Gasifiers typically produce contaminants that must be removed before entering the fuel cell 

anode. These contaminants include H2S, COS, NH3, HCN, particulates, tars, oils, and phenols. 

The contaminant levels depend on both the fuel composition and the gasifier employed. There 

are two families of cleanup that remove the sulfur impurities: hot and cold gas cleanup systems. 

Cold gas cleanup technology is commercial, has been proven over many years, and provides the 

system designer with several choices. Hot gas cleanup technology is still developmental and 

would likely need to be joined with low temperature cleanup systems to remove the non-sulfur 

impurities in a fuel cell system. For example, tars, oils, phenols, and ammonia could all be 

removed in a low temperature water quench followed by gas reheat. 

A typical cold gas cleanup process following an entrained gasifier would include the following 

subprocesses: heat exchange (steam generation and regenerative heat exchange), particulate 

removal (cyclones and particulate scrubbers), COS hydrolysis reactor, ammonia scrubber, acid 

gas (H2S) scrubbers (Sulfinol, SELEXOL), sulfur recovery (Claus and SCOT processes), and 

sulfur polishers (zinc oxide beds). All of these cleanup systems increase process complexity and 

cost, while decreasing efficiency and reliability. In addition, many of these systems have 

specific temperature requirements that necessitate the addition of heat exchangers or direct 

contact coolers. 

For example, a COS hydrolysis reactor operates at about 180 °C (350 °F), the ammonia and acid 

scrubbers operate in the vicinity of 40 °C (100 °F), while the zinc oxide polisher operates at 

about 370 °C (700 °F). Thus, gasification systems with cold gas cleanup often become a maze of 

heat exchange and cleanup systems. 

Typical compositions for several oxygen-blown coal gasification products are shown in 

Table 8-4. 
8-21 

Table 8-4 Typical Coal Gas Compositions for Selected Oxygen-Blown Gasifiers 

Gasifier Type Moving-Bed Fluidized-Bed Entrained-Bed 

Manufacturer Lurgi (20) Winkler Destec KoppersTotzekTexaco Shell

Coal Illinois No. 6 Texas Lignite Appalachian 

Bit. 

Illinois No. 6 Illinois No. 6 Illinois No. 6 

Mole Percent 

Ar trace 0.7 0.8 0.9 0.9 1.1 

CH4 3.3 4.6 0.6 - 0.1 - 

C2H4 0.1 - - - - - 

C2H6 0.2 - - - - - 

CO 5.8 33.1 45.2 43.8 39.6 63.1 

CO2 11.8 15.5 8.0 4.6 10.8 1.5 

COS trace - - 0.1 - 0.1 

H2 16.1 28.3 33.9 21.1 30.3 26.7 

H2O 61.8 16.8 9.8 27.5 16.5 2.0 

H2S 0.5 0.2 0.9 1.1 1.0 1.3 

N2 0.1 0.6 0.6 0.9 0.7 4.1 

NH3+ HCN 0.3 0.1 0.2 - - -

Total 100.0 100.0 100.0 100.0 100.0 100.0 

Reference Sources: (26, 27) 

Note: All gasifier effluents are based on Illinois No. 6, except the Winkler, which is based on a Texas Lignite, and 

the Destec, which is based on an Appalachian Bituminous. 

Other Solid Fuel Processing: Solid fuels other than coal can be utilized in fuel cell systems. 

For example, biomass and RDF (refuse-derived-fuels) can be integrated into a fuel cell system as 

long as the gas product is processed to meet the requirements of the fuel cell. The resulting 

systems would be very similar to the coal gas system with appropriate gasifying and cleanup 

systems. However, because biomass gas products can be very low in sulfur, the acid cleanup 

systems may simply consist of large sulfur polishers. 

State-of-the-Art Components - Cleaning and Reformate Gas Alteration (Removal Of 

Contaminants): Besides their basic fuel reforming function, fuel processors require the removal 

of impurities that degrade the fuel processor or fuel cell performance. Sulfur is the major 

contaminant encountered. Carbon monoxide reduction for low temperature fuel cells and 

avoidance of carbon deposition are also addressed. A typical processing chain for a low 

temperature fuel cell will have a hydrodesulfurizer, a halogen guard, a zinc oxide sulfur 

absorber, a catalytic reformer, a high temperature shift converter, a second halogen guard, and 

low temperature shift converter. Figure 8-2 provides insight into how these may be arranged. 

The function of all these components, except the reformer, is to remove impurities. For the 

PEFC, an additional device is necessary to remove essentially all CO, such as a preferential 

oxidizer (28). 

Sulfur Reduction: There are high temperature and low temperature methods to remove sulfur 

from a fuel reformate. Low temperature cleanup, such as hydrodesulfurizing (limited to fuels 
8-22 

with boiling end points below 205 °C), is less difficult and lower in cost so should be used where 

possible, certainly with low temperature fuel cells. Sulfur species in the fuel are converted to 

H2S, if necessary, then the H2S is trapped on zinc oxide. A minimum bed volume of the zinc 

oxide reactor is achieved at temperatures of 350 to 400 °C. Thermodynamic and economic 

analyses show that it is appropriate to use high temperature cleanup with high temperature fuel 

cells. 

There is a vast difference between removing sulfur from a gaseous fuel and a liquid fuel. The 

sulfur in a liquid fuel is usually removed after it is converted to a gas. This by removing the 

sulfur in the reforming reactor at high temperature, or by incorporating sulfur resistant catalysts. 

Sulfur resistant catalysts are being developed, but none are mature enough for present use. ANL 

is developing catalysts to reform gasoline, and have demonstrated that their catalyst can tolerate 

sulfur. The ANL catalyst has been shown to tolerate (100s of hours) sulfur present in natural gas 

in an engineering scale reformer. 

At least one developer has a liquid-phase fuel desulfurizer cartridge that will be used to remove 

sulfur prior to fuel vaporization. Other developers remove the sulfur immediately after 

vaporization and prior to reforming. Hydrogen must be recirculated to the removal device to 

convert the sulfur species to H2S so that it can be entrapped on zinc oxide. Zinc oxide beds are 

limited to operation at temperatures below 430 °C to minimize thermal cracking of hydrocarbons 

that can lead to coke formation. Thermodynamics also favor lower temperatures. At higher 

temperatures, the H2S cannot be reduced to levels low enough for shift catalyst or to reach fuel 

cell limits. For sulfur removal in the reformer, the presence of significant concentrations of 

steam in the fuel gas has a negative impact on the reaction equilibrium, leading to a higher 

concentration of H2S than could be achieved with a dry fuel gas. 

Carbon Monoxide Reduction: The use of CO as a fuel in high temperature cells and water-gas 

shift reactions to lower carbon monoxide to conditions suitable for a PAFC or a PEFC have been 

previously described. Fuel gas reformate contains 0.5 to 1 percent by volume of CO even after 

the shift reactions. Present PEFCs operate below 100 °C. At these temperatures, even small 

amounts of CO are preferentially adsorbed on the anode platinum (Pt) catalysts. This blocks 

access of H2 to the surface of the catalyst, degrading cell performance (29). Reformate for PEFC 

stacks must contain very low (<50 ppm) CO to minimize Pt absorption to a reasonable value to 

maintain sufficient active sites for the oxidation of H2. This can be achieved in two ways, by air 

injection into the anode at up to about 4 percent of the reformate feed rate or by reducing CO 

concentration prior to the cell: even at 50 ppm, catalyst poisoning by CO must be mitigated by 

the injection of some air at the anode. For the latter approach, a preferential oxidizer (PROX) is 

used to reduce CO concentration prior to the cell. It has highly dispersed supported Pt or Pt-Ru 

(ruthenium) catalyst. Such catalysts act on the principle of selective adsorption of CO onto the 

active Pt or Pt-Ru (relative to H2), leading to CO being selectively oxidized by stoichiometric 

amounts of air co-fed to the catalyst bed. As the CO is oxidized, the gas temperature rises, 

which decreases the selectivity of CO adsorption on the catalyst and also increases the kinetics of 

the reverse water-gas shift reaction. In practice, the PROX process is carried out in stages to 

permit cooling between stages. The PROX is a relatively large unit that operates at 100 to 180 

°C (22). Preferential gas cleanup by selective oxidation results in 0.1 to 2 percent H2 lost (30). 
8-23 

Carbon Deposition Avoidance: The processing of hydrocarbons always has the potential to 

form coke. Coke formation is influenced by the composition of the fuel, the catalyst, and the 

process conditions (e.g., partial pressure of steam). Coke causes the greatest problems in gas 

flow paths and on catalyst. Carbon deposition not only represents a loss of carbon for the 

reaction, but more importantly also results in deactivation of the catalyst due to deposition at the 

active sites. Thermal cracking46 in over-heated preheaters and manifolds can easily form carbon. 

If the fuel conversion reactor is not properly designed or operated, coking is likely to occur. 

Thermo dynamic equilibrium provides a first approximation of the potential for coke formation. 

Free carbon in hydrocarbon fuels forms according to the three equations, (8-1), (8-2), and (8-3). 

Figures 8-4 and 8-5 show the effect of increasing steam on carbon deposition for methane and 

octane, respectively. Increasing steam, hydrogen, and carbon dioxide concentrations alleviates 

carbon deposition. Low contents of aromatics and alkenes help to maintain the activity of the 

catalyst (10). No carbon deposits at low temperatures (~600 °C) in mixtures containing at least 

two atoms of oxygen and four atoms of hydrogen per atom of carbon. At these conditions, all 

carbon is present as CO2 or CH4 (7).

Higher hydrocarbon fuels show a greater tendency for carbon formation than does methane. One 

method to alleviate carbon deposition problems in the fuel processor is to use special catalysts 

either containing alkali or based on an active magnesia support. With a highly active catalyst, 

the limit permitted on the final boiling point of the hydrocarbon feedstock is related mainly to the 

possibility of desulfurizing the feed to below 0.1 ppm, rather than to the reactivity of the 

hydrocarbons. With proper desulfurization, it has been possible to convert light oil into syngas 

with no trace of higher hydrocarbons in the reformate gas (17).

Coke formation resulting from higher hydrocarbon fuels can also be eliminated with an adiabatic 

pre-reformer. The adiabatic reformer is a simple fixed bed reactor. By adiabatic pre-reforming, 

all higher hydrocarbons are converted at low temperature (below ~500 °C) with steam into 

methane, hydrogen, and carbon oxides at conditions where carbon formation does not occur. It 

is possible to use a high pre-heating temperature (650 °C or above) for internal reforming in 

MCFC and SOFC without the risk of carbon formation. For natural gas containing only minor 

amounts of higher hydrocarbons, adiabatic pre-reforming at a steam to carbon ratio as low as 

0.25 mole/atom has been demonstrated. For heavier feedstocks such as naphtha, operation at a 

steam to carbon ratio of 1.5 has been proven in industry. Pilot tests have been carried out at a 

steam to carbon ratio of 1.0 with reformate recycle. 

 

46. Thermal cracking is the breaking of a hydrocarbon carbon-carbon bond through the free-radical mechanism. 

Cracking may result in the formation of lower chained hydrocarbons, the original "cracked" hydrocarbon, or 

further cracking of the hydrocarbon to soot. 
8-24 

 Steam Reforming Autothermal Reforming 

Steam-to-Carbon Mole Ratio

0.0 0.5 1.0 1.5 2.0 2.5

T (oC)

0

200

400

600

800

1000

Steam-to-Carbon Mole Ratio

0.0 0.5 1.0 1.5 2.0 2.5

T (oC)

0

200

400

600

800

1000

Figure 8-4 Carbon Deposition Mapping of Methane (CH4) 

(Carbon-Free Region to the Right of Curve)

 Steam Reforming 

Steam to carbon ratio

0123456

T carbon formation (oC)

0

200

400

600

800

1000

1200

Figure 8-5 Carbon Deposition Mapping of Octane (C8H18) 

 (Carbon-Free Region to the Right and Above the Curve)

Coking can be also be avoided by operating at high temperatures and at high oxygen-to-carbon 

ratios, where the ratio is based on the total atoms of oxygen contained in the steam and air feeds. 

For a given O/C ratio in the feed, it is preferable that the oxygen comes from water. Thus, for a 

given O/C, SR is preferred over ATR, which is preferred over POX; “preferred” meaning that 

coke formation can be avoided while still operating at a lower temperature (20, 23, 31).

Other Impurities Reduction: Halides in fuels such as naphtha have deleterious effects on steam 

reforming and low temperature shift, thus halogen guards must be included in fuel processing. 

There are many types of coal with different compositions, including harmful species. One 

common constituent, HCl, will cause formation of stable chlorides and corrosion in a MCFC. 

There has not been much work in SOFC yet on this topic. It is doubtful whether low temperature 

cells will be fueled by coal. 
8-25 

Research & Development Components 

There are two major areas where fuel processor developers are focusing their research and 

development efforts, catalyst development and process/engineering development. A smaller, 

long term effort on novel processing schemes is in the early stages of investigation. 

Catalyst Development. Performance targets for the fuel processor for transportation fuel cell 

systems will require that the reforming catalysts used in these processors exhibit a higher activity 

and better thermal and mechanical stability than reforming catalysts currently used in the 

production of H2 for large-scale manufacturing processes. To meet these targets, reforming 

catalysts will have to process the feed at a space velocity of 200,000/hr (based on the volumetric 

flow of the feed in the gaseous state at 25 °C and 1 atm) with a fuel conversion of >99 percent 

and a H2 selectivity of >80 percent (moles of H2 in product/moles of H2 “extractable” from the 

feed), and have a lifetime of 5,000 hr. Given the potential market for transportation applications, 

many of the major catalyst producers, such as Johnson-Matthey, Engelhard Corporation, and 

dmc2

 division of OM Group, Inc., have begun to develop new reforming catalysts (32). An ANL 

program is focused on improving long-term stability (minimize deactivation), an important, 

immediate goal, reducing coke formation for higher hydrocarbons, and improving catalyst sulfur 

tolerance while addressing cost issues. A major issue is to demonstrate that the catalyst can 

operate for 40,000+ hours in stationary applications and 4,000+ hours in transportation 

applications. It is believed that no one has successfully demonstrated these targets. Another 

issue is that coke formation will be problematic with higher hydrocarbons, especially diesel. 

Most industrial reforming catalysts are operated steam-rich to minimize coke formation. 

However, this increases the size of the reformer as well as the energy needed to vaporize the 

water. This option may not be viable for reformers used with fuel cells. Finally, <20 ppb of S is 

the target for use with nickel steam reforming catalyst. Most fuels being considered contain 

either sulfur at the ppm level, such as gasoline, or added as an odorant for safety reasons, such as 

to natural gas. The ability of the catalyst to process fuels containing ppm levels of sulfur would 

be beneficial. The ANL catalysts are based on solid oxide fuel cell technology, where a 

transition metal is supported on an oxide- ion-conducting substrate, such as ceria, zirconia, or 

lanthanum gallate, that has been doped with a small amount of a non-reducible element, such as 

gadolinium, samarium, or zirconium. Platinum was the transition metal used in the first 

generation of the ANL catalyst. Because of concerns over the cost associated with using a 

precious metal-based catalyst, work has begun on reducing the cost of the catalyst either by 

replacing Pt with a less expensive non-noble metal or by using a combination of a noble metal, at 

a considerably lower metal loading, and with a base metal without sacrificing performance. 

Work is proceeding on catalysts based on Ni, Rh, and combinations of Ni and Rh. Süd-Chemie, 

Inc. currently produces reforming catalysts based on this technology under a licensing agreement 

with Argonne (32).

There is also a need to develop better water gas shift catalysts (7, 33, 34), especially catalysts 

that operate at temperatures ranging from 200 to 300 o

C. Commercial shift catalysts based on 

FeCr and CuZn oxides are available, but are not designed for the rapid startups and frequent 

exposure to oxidizing conditions that will be experienced during normal operation of fuel 

processors developed for transportation applications. These commercial catalysts have fixed 

size, high density, and are susceptible to contaminant poisoning by ingredients found in 
8-26 

infrastructure fuels. Of primary concern is the need to reduce these catalysts in a well-controlled 

manner that minimizes temperature rise in order to achieve maximum catalyst activity and to 

prevent the exposure of the catalyst in the reduced state to oxidizing conditions. For example, 

the CuZn catalysts will sinter if exposed to >270 o

C and are pyrophoric when exposed to air in 

the reduced state. Present commercial catalysts are developed for process plant service where 

transient conditions are not a concern. There is a need for highly active catalysts that can be 

supported on a low density monolith that do not require reduction in order to be active and are 

stable when exposed to oxidizing conditions. ANL is developing a more robust shift catalyst that 

will work better under transient operating conditions than present catalysts developed for process 

plant service. The advantage of this catalyst over standard catalysts is that it is air stable, which 

is needed for many start-up and shutdown cycles. There is a trade-off of a moderate reduction in 

activity (35). 

There is also a need to demonstrate that the low-temperature, PROX catalysts have high 

selectivity toward CO and long term stability. 

Process/Engineering Development Numerous engineering and process issues are being 

addressed by fuel processor developers (20, 31, 36). Several major issues are: 

• As the size of the catalyst bed increases, the segregation within an ATR reactor bed toward 

over-oxidation and catalyst overheating in the front of the bed, and air starvation and carbon 

formation in the back end of the bed are important to consider. Maintaining a good 

temperature distribution in the bed, especially with a large reactor, is identified as one of the 

challenges facing this approach. 

• Fuel processor tests have been on the order of 40 hours, although the fuel processors have 

been tested for 1,000 hours on natural gas. There is a need is to show similar results at 

realistic operating conditions and further engineering development to enhance the catalyst 

activity and make the fuel processor lighter and smaller. 

• There is a need to investigate improved and simplified fuel processor designs. Examples are 

combining the reformer and the desulfurizer in a single stage to reduce weight and volume, 

producing an integrated vaporizer design, and designing for a wide variation of fuel 

vaporization temperatures to allow fuel flexible operation. 

• Transient issues are important in transport applications and should be addressed early by 

testing. The challenge is to demonstrate the operation at high sulfur content over the full 

operating envelope of the vehicle – start-up, transients, shutdown, sulfur spikes in the fuel, 

etc. using the same processor. 

Novel Processing Schemes: Various schemes have been proposed to separate the hydrogen-rich 

fuel in the reformate for cell use or to remove harmful species. At present, the separators are 

expensive, brittle, require large pressure differential, and are attacked by some hydrocarbons. 

There is a need to develop thinner, lower pressure drop, low cost membranes that can withstand 

separation from their support structure under changing thermal loads. Plasma reactors offer 

independence of reaction chemistry and optimum operating conditions that can be maintained 

over a wide range of feed rates and H2 composition. These processors have no catalyst and are 

compact. However, results are preliminary and have only been tested at a laboratory scale.
8-27 

Other: Although not R&D, it should prove beneficial for fuel cell developers to provide fuel 

tolerance specifications to fuel processor developers. Tolerances should be established by 

standard definition, determination methods, and measurement procedures. This would aid the 

fuel processor developer to deliver products compatible with various fuel cell units. Of 

particular importance are sulfur and CO limits. 

8.2 Power Conditioning 

Power conditioning is an enabling technology that is necessary to convert DC electrical power 

generated by a fuel cell into usable AC power for stationary loads, automotive applications, and 

interfaces with electric utilities. The purpose of this section is to explore power conditioning 

approaches for the following applications: 

 Fuel cell power conversion to supply a dedicated load 

 Fuel cell power conversion to supply backup power (UPS) to a load connected to a local 

utility 

 Fuel cell power conversion to supply a load operating in parallel with the local utility (utility 

interactive) 

 Fuel cell power conversion to connect directly to the local utility 

 Power conversion for automotive fuel cell applications 

 Power conversion architectures for a fuel cell turbine hybrid interfaced to local utility 

Figure 8-6 shows a block diagram of a representative fuel cell power plant. Natural gas flows to 

a fuel processor, where the methane is reformed to hydrogen-rich gas. The hydrogen gas reacts 

in the power producing section, which consists of a fuel cell. The DC power generated by the 

fuel cell must be converted to AC power; one of the power conditioning approaches identified 

above would be selected, based on the specific application. 

Figure 8-6 Block diagram of a fuel cell power system 
8-28 

8.2.1 Introduction to Fuel Cell Power Conditioning Systems 

Various power conversion “building” blocks, such as DC-DC converters and DC-AC inverters, 

are employed in fuel cell power conditioning systems. Figure 8-7 shows a typical variation of the 

output voltage of a fuel cell stack in response to changes in load current. Since the DC voltage 

generated by a fuel cell stack varies widely and is low in magnitude (<50V for a 5 to 10kW 

system, <350V for a 300kW system), a step up DC-DC converter is essential to generate a 

regulated higher voltage DC (400V typical for 120/240V AC output). The DC-DC converter is 

responsible for drawing power from the fuel cell, and therefore should be designed to match fuel 

cell ripple current specifications. Further, the DC-DC converter should not introduce any 

negative current into the fuel cell. A DC-AC inverter is essential to provide the DC to useful AC 

power at 60Hz or 50Hz frequency. An output filter connected to the inverter filters the switching 

frequency harmonics and generates a high quality sinusoidal AC waveform suitable for the load. 

Figure 8-7a Typical fuel cell voltage / current characteristics [1] 

Figure 8-7b Fuel cell power vs. current curve [1] 

Fuel Cell V-I Curve 

20

25

30

35

40

45

0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36

Current (A)

Fuel cell voltage (V)

Power vs. Current

0

100

200

300

400

500

600

700

800

900

1 3 5 7 9 11 13 15 17 19 21 23 25 27 29 31 33 35 37

Current (A)

Power (W)
8-29 

8.2.2 Fuel Cell Power Conversion for Supplying a Dedicated Load [2,3,4] 

Fuel cell power conversion for a 10kW stand-alone load is a representative example for 

distributed generation. Figure 8-7 shows the variation of fuel cell output voltage vs. load; fuel 

cell output DC voltage exhibits nearly 2:1 voltage range (Figure 8-7a). The power conversion 

unit must be capable of operating in this range and, in particular, be able to deliver rated power 

while regulating output voltage. Output from the power conversion unit is expected to be high 

quality power with less than 5 percent total harmonic distortion (THD). For domestic loads, a 5:1 

or better peak to average power capability for tripping breakers and starting motors is desired. 

This puts an additional constraint on the design of the power conditioning unit for stand-alone 

loads. Table 8-5 shows a typical specification for a stand-alone fuel cell power conditioning unit. 

.

Table 8-5: Specifications of a typical fuel cell power conditioning unit for stand-alone 

domestic (U.S.) loads 

Continuous output power: 10 kW continuous 

Output Phase (s): Split single-phase, each output rated for 0 to 5,000 VA, not to 

exceed 10,000 VA total 

Output voltage: 120V, 240V Sinusoidal AC. Output voltage tolerance no wider 

than ±6 percent over the full allowed line voltage and 

temperature ranges, from no-load to full-load. Frequency 60±0.1 

Hz. 

Output frequency: 60Hz (U.S.) or 50Hz (Europe) with enough precision to run AC 

clock accuracy 

Fuel Cell Current Ripple: 

(Fuel cell dependent) 

 120 Hz ripple: < 15 percent from 10 to 100 percent load 

 60 Hz ripple: < 10 percent from 10 to 100 percent load 

 10 kHz and above: < 60 percent from 10 to 100 percent load 

Output THD: < 5 percent 

Protection: Over current, over voltage, short circuit, over temperature, and 

under voltage. No damage caused by output short circuit. The 

inverter must shut down if the input voltage dips below the 

minimum input of 42 V. Inverter should not self-reset after a 

load-side fault. IEEE Standard 929 is a useful reference. 

Acoustic Noise: No louder than conventional domestic refrigerator. Less than 50 

dBA sound level measured 1.5 m from the unit. 

Environment: Suitable for indoor installation in domestic applications, 10 °C to 

40 °C possible ambient range. 

Electromagnetic 

Interference 

Per FCC 18 Class A -- industrial 

Efficiency: Greater than 90 percent for 5kW resistive load 

Safety: The system is intended for safe, routine use in a home or small 

business by non-technical customers. 

Life: The system should function for at least ten years with routine 

maintenance when subjected to normal use in a 20 °C to 30 °C 

ambient environment. 
8-30 

Currently, fuel cells supply only average power from the fuel cell. Thus, peak power must be 

supplied from some other energy source such as a battery or supercapacitor [5,6]. The power 

conditioning unit must therefore provide means for interfacing a battery and also ensure its 

charge maintenance. Figure 8-8 shows a block diagram of a typical fuel cell powered unit for 

supplying a load along with a battery interface. Figures 8-9 through 8-11 show three possible 

block diagrams and circuit topologies of power conversion units for this application. 

FUEL

SUPPLY

FUEL

PROCESSOR

(GASOLINE

OR METHANE)

FUEL

CELL

STACK

DC/DC

CONVERTER

DC/AC

INVERTER

THERMAL

MANAGEMENT

WASTEHEAT

Mgmt.

AIR Mgmt.

SENSORS

FUEL Mgmt.

AND

ELECTRONIC

CONTROLS

CONTROL ELECTRONICS

FOR DC/DC CONVERTER,

INVERTER

CENTRAL POWER CONTROL UNIT

BATTERY

H2 120V / 240V

60 HZ LOAD

Figure 8-8 Block diagram of a typical fuel cell powered unit for supplying a load 

(120V/240V) 

Power conditioning unit with line frequency transformer: Figure 8-9a shows the block 

diagram and Figure 8-9b shows the circuit topology of the power conditioning unit. In Figure 8-

9b, the fuel cell output DC (say 29V to 39V) is converted to a regulated DC output (say 50V) by 

means of a simple DC-DC boost converter. The output of the DC-DC converter is processed via 

a pulse width modulation (PWM) DC-AC inverter to generate a low voltage sinusoidal AC of ±

35 V AC (rms), a line frequency isolation transformer with a turns ratio of 1:3.5 is then 

employed to generate 120V/240V AC output as shown. A 42 to 48V battery is connected to the 

output terminals of the DC-DC converter to provide additional power at the output terminals for 

motor startups, etc. During steady state, the DC-DC converter regulates its output to 50V and the 

battery operates in a float mode. The fuel cell and the DC-DC converter are rated for steady state 

power (say 10kW), while the DC-AC inverter section is rated to supply the motor-starting VA. 

Assuming a motor-starting current of 3 to 5 times the rated value, the DC-AC inverter rating will 

be in the 15kVA to 25kVA range. The DC-DC boost converter is operated in current mode 

control. During a motor startup operation, the current mode control goes into saturation and 

limits the maximum current supplied from the cell. During this time, the additional energy from 

the battery is utilized. During steady state operation, the fuel cell energy is used to charge the 

battery when the output load is low. 
8-31 

a

b

n

AC Output

Fuelcell

&

Reformer

DC/DC

Converter

Battery

DC/AC

Converter Transformer

Figure 8-9a Block diagram of the power conditioning unit with line frequency transformer 

Fuel Cell

&

Reformer

DC/DC Boost

Converter

PWM DC/AC

Inverter

DC Link (50V)

& Battery

C

S1

L1

S2

S3

S4

S5

Fuelcell Output

(25-39V)

a

b

n

L-C Filter

AC Output

(35V)

Lf

Lf

Cf

Line Frequency

Transformer

120/240V

60Hz output

+

Vout

-

1:3.5

+

50V

-

Figure 8-9b Circuit topology of the power conditioning unit with line frequency 

transformer 

Efficiency calculation (approximate): Referring to Figure 8-9a, assume: 

DC-DC converter efficiency = η1

DC-AC inverter efficiency = η2

Line frequency isolation transformer efficiency = η3

The overall conversion efficiency of the power conditioning unit η = 1 2 3 η * η * η

Assuming η1 = 0.95 (95 percent) ; η2 = 0.95 (95 percent); η3 = 0.98 (98 percent), then η = 0.88 

(88 percent). 

The main limitation of this system is the low voltage of the entire power conditioning unit, which 

results in higher current and lower overall efficiency. Another disadvantage is the presence of a 

line frequency isolation transformer, which is large in size and weight (10kg/kw). Figure 8-9b 

shows the circuit topology of the power conditioning unit. 

Power conditioning unit with high frequency isolation transformer: Figure 8-10a shows a 

similar power conditioning block diagram. In this design, the low frequency isolation transformer has been eliminated by employing an additional DC-DC conversion stage. The 50V to 

400V DC-DC conversion stage includes a high frequency isolation transformer. The fuel cell and 

the first DC-DC converter are rated for steady state conditions. The second DC-DC converter, 

along with the DC-AC inverter, is rated for steady state and transient conditions. Figure 8-10b 

shows the circuit topology of the power conditioning unit. This approach suffers from three 

power conversion stages in the power flow path, which contributes to reduced efficiency. 
8-32 

a

b

n

AC Output

Fuelcell

&

Reformer

DC/DC

Converter

Battery

DC/AC

Converter

with Isolation

DC/AC

Inverter

Figure 8-10a Block diagram of the power conditioning unit with high frequency isolation 

transformer within the DC-DC converter stage 

Fuel Cell

&

Reformer

DC/DC Boost

Converter PWM DC/AC Inverter DC Link (50V)

& Battery

C

S1

L1

S4

S5

S6

S7

Fuelcell Output

(25-39V) L-C Filter 120/240V

60Hz output

+

Vout

-

L

L

S2

S3

DC/DC Converter

with Isolation

+

-

+

- C

C 200 V

200 V

a

CF CF

LF

LF

b

n

1:K

Figure 8-10b Circuit topology of the power conditioning unit with high frequency isolation 

transformer within the DC-DC converter stage (3,4) 

Figure 8-11a shows a power conditioning unit with fewer power conversion stages in series. In 

this approach, a push-pull type boost converter with a 1:10 gain employing a high-frequency 

isolation transformer is used. The output of the push-pull DC-DC converter is set to 400V with ±

200V. The DC-DC converter output is connected to two half bridge dual voltage DC-AC 

inverters to obtain 120/240V AC output. It is economical to install a ± 200V battery at the output 

of the DC-DC converter stage and regulate the push-pull DC-DC converter output to ±200V. The 

fuel cell and the push-pull DC-DC converter are rated to supply steady state load (10kW), while 

the DC-AC inverter stage is rated to supply the steady state as well as transient load demands 

such as motor starting, etc. Figure 8-11b illustrates the possible circuit topology for this 

approach. Assuming conversion efficiency of the DC-DC and DC-AC stages to be 96 percent 

each, an overall efficiency of 92 percent can be realized with this approach. Figure 8-12 shows 

the fuel cell power conditioner control system block diagram for powering a load. The DC-DC 

and DC-AC control blocks are controlled separately. The power consumed by the load is first 

computed by the reference signal generator block, and suitable reference signals for the DC-AC 

inverter and the fuel cell controller are generated. The DC-DC converter controller regulates the 

dc-link and draws power from the fuel cell. This block has the appropriate protection circuitry 

and limits to protect both the fuel cell and the dc-link against over-current and over-voltage, 

respectively. Fuel cell accessory loads are supplied via the dc-link. The battery also provides 

start-up power for this unit. 
8-33 

a

b

n

AC Output

Fuelcell

&

Reformer

DC/DC

Converter

with 1:10 Gain &

High Frequency

Transformer

Battery

DC/AC Inverter

Figure 8-11a Block diagram of the power conditioning unit with fewer power conversion 

stages in series path of the power flow 

 

Fuel Cell

&

Reformer

120V/240V, 20kHz

PWM DC/AC Inverter

Fuelcell Output

(42V to 72V) L-C Filter

120/240V

60Hz output

DC/DC Converter

with 1:10 Gain &

High Frequency Transformer

C

Lb

Lb

S3

S4

S5

S6

LF

CF

LF

CF

a

b

n

Battery

Backup

Vbatt

Vbatt

L

L

S1

S2

C

C

1:K

D1 D3

D2

D4

A

B

o

Figure 8-11b Circuit topology of the power conditioning unit with fewer power conversion 

stages in series path of the power flow 

Vdc

Output Power

Calculator &

Ref. Generator

FUEL

CELL

STACK

ACCESSORY

LOADS_

Gate Drive

DC/DC Control

Fuel Cell

Controller

Hydrogen

Input

PWM

I

a Ib Va Vb

I

dc

DC-DC DC-AC

120V/240V

60Hz

Load

a

n

b

Output

Filter LC

DC-AC Battery

DC-AC

Control Block

For Voltage & Current

Figure 8-12 Fuel cell power conditioner control system for powering dedicated loads 
8-34 

8.2.3 Fuel Cell Power Conversion for Supplying Backup Power to a Load 

Connected to a Local Utility 

Conventional uninterruptible power supply (UPS) systems employ engine generators and/or 

batteries as their main sources to provide electric power for critical functions or loads when the 

normal supply, i.e. utility power, is not available [7]. A typical UPS system consists of 

rechargeable batteries such as sealed lead-acid or nickel cadmium (Ni-Cd). However, these 

batteries contain toxic heavy metals such as cadmium, mercury, and lead that may cause serious 

environmental problems if they are discarded without special care [7]. Further, unlike batteries, 

fuel cells provide continuous power for as long as reactants are supplied. This feature is 

especially useful whenever the duration of the power outage is uncertain. 

Among the various kinds of fuel cells, proton exchange membrane fuel cells (PEFC) are compact 

and lightweight. They provide a high output power density at room temperature, and are easy to 

start-up and shut down in system operation [1,8,9]. 

In this section, design considerations for a 1-kW fuel cell powered line-interactive UPS system 

with one hour of backup power employing modular (fuel cell and power converter) blocks is 

discussed. Figure 8-13 shows two commercially available PEFC fuel cells (25 to 39V, 500W) [1] 

along with suitable DC-DC and DC-AC power electronic converter modules. Commercially 

available supercapacitors [5] supply transient power at the output terminals. Two possible 

power-conditioning architectures are investigated. 

SS1

Utility

Input

Fuel Cells

DC/DC

Boost

Converter

DC/AC

Inverter

BIdirectional

Converter

Hydrogen

Cylinder

Critical

Load

Fuel Cells

DC/DC

Boost

Converter

Super

capacitor

SS2

Non-Critical

Load

Figure 8-13 Diagram of a modular fuel cell power conversion unit for supplying backup 

power to a load connected to a local utility [10,11] 

Figure 8-13 shows a diagram of a line interactive UPS whose main function is to provide backup 

power to a load connected to a local utility. The load is connected via two static transfer switches 
8-35 

SS1 and SS2 either to the utility or to the fuel cell source. Normally the fuel cell inverter side 

static switch SS2 is rated for inverter output. However, the utility side SS1 is rated for 1500 

percent for approximately 15 milliseconds to provide current to open the down stream or load 

side circuit breaker in case of a short circuit [7]. Two commercially available 500W fuel cell 

stacks are considered in this example design. The use of a supercapacitor is explored to supply 

inrush current to the load (2kW for 20 seconds). A static bypass switch is configured to 

disconnect the utility in case of its failure or out of specified voltage range for the load. In steady 

state, the fuel cell power source is assumed to be in hot standby mode, i.e. supply 10 percent of 

its capacity; this allows the fuel cell subsystem to be ready to supply the load during a utility 

shutdown. It is assumed that pure hydrogen is available to supply the rated output power (1kW) 

for a 1hr utility outage. Figure 8-14 and Figure 8-15 show two possible circuit topologies for this 

application. The approach in Figure 8-15 is more suitable, as it employs modular DC-DC 

converters to interface each fuel cell output to a 400V dc-link. A 42V supercapacitor module 

[5,6] is connected to the dc-link via a bi-directional DC-DC converter. The bi-directional DC-DC 

converter module allows quick discharge and charging of the supercapacitor modules to supply 

inrush current demanded by the load. 

Table 8-6 Example specifications for the 1kW fuel cell powered backup 

power (UPS) unit [10,11] 

Continuous output power: 1 kW continuous for 1hr 

Peak power: 2kW for 20 seconds 

Output Phase (s): Single-phase, not to exceed 1,000 VA total 

Output voltage: 120V, Sinusoidal AC. Output voltage tolerance no wider than ±6 percent over 

the full allowed line voltage and temperature ranges, from no-load to full-load. 

Frequency 60±0.1 Hz. 

Output frequency: 60Hz (U.S.) or 50Hz (Europe) with enough precision to run AC clock accuracy 

Transfer time (typical/max): 4/6 milliseconds 

Load power factor (PF) range 

& crest factor (CF): 

PF: 0.6 to 1; CF: 3 

Fuel Cell Current Ripple: 

(Fuel cell dependent) 

 120 Hz ripple: < 15 percent from 10 to 100 percent load 

 60 Hz ripple: < 10 percent from 10 to 100 percent load 

Output THD: < 5 percent 

Protection: Over current, over voltage, short circuit, over temperature, and under voltage. 

No damage caused by output short circuit. The inverter must shut down if the 

input voltage dips below the minimum specified fuel cell voltage (29V). 

Inverter should not self-reset after a load-side fault. IEEE Standard 929 is a 

useful reference. 

Acoustic Noise: No louder than conventional domestic refrigerator. Less than 50 dBA sound 

level measured 1.5 m from the unit. 

Environment: Suitable for indoor installation in domestic applications, 10 °C to 40 °C 

possible ambient range. 

Electromagnetic Interference Per FCC 18 Class A -- industrial 

Efficiency: Greater than 90 percent 

Safety: The system is intended for safe, routine use in a home or small business by 

non-technical customers. 

Life: The system should function for at least ten years with routine maintenance 

when subjected to normal use in a 20 °C to 30 °C ambient environment. 
8-36 

Table 8-7 Specifications of 500W PEFC fuel cell stack (available from Avista Labs [1]) 

Power Output (Continuous) 500 W 

Output Voltage 25 to 39 DC 

Fuel Source Hydrogen 

Fuel Consumption 7.0L/min @500W(<1.0L/min @ no load) 

System Start Time 7 minutes @room temperature 

Turndown Ratio 500W to no load, infinity 

Operating Temperature Range 41 °F to 95 °F (5 °C to 35 °C) 

Dimension (W x D x H) 22.3” x 24.2” x 13.6” (0.056m x 0.0615m x0.0345m) 

Weight 97 lbs w/cartridges (44kg w/cartridge) 

Fuel

Cell 1

Fuel

Cell 2

Super

Capacitor

Fuelcell 1

Boost

Converter

Super

capacitor

Converter DC/AC Inverter Low Voltage

DC Link

+

50V

-

S1 S2

S3

S4

L1 L2 L3

S5

S6

S7

S8

+

-

Fuelcell 2

Boost

Converter

1 or 3

Isolation

Transformer

φ φ 1 120/240V

or

3 output

φ

φ

a

b

n

1:3.5

Figure 8-14 Modular power conditioning circuit topology employing two fuel cells to supply 

a load via a line frequency isolation transformer [10,11] 

Fuel Cells Input Filter DC/DC Converter

& Isolation DC/AC Inverter

Super

capacitor

High Voltage

DC LInk

Fuel Cell 1

+

-

Fuel Cell 2

400V

Lf 1

Lf 2

Cf 1

Cf 2

S3

S4

S5

S6

S7

S8

+

-

S11

S12

Bidirectional

Converter

+

-

+

-

1:K

1:K

L

L

L

L

S1

S2

S9

S10

a

n(c)

CF CF

φ

φ

1 120/240V

or

3 output LC Filter

LF

LF

b

Figure 8-15 Modular power conditioning circuit topology employing two fuel cells using a 

higher voltage (400V) dc-link [10,11] 
8-37 

8.2.4 Fuel Cell Power Conversion for Supplying a Load Operating in Parallel 

With the Local Utility (Utility Interactive) 

Figure 8-16 shows the block diagram of a fuel cell power conversion scheme supplying a load in 

parallel with the utility. In this configuration, the peak power (as well as the inrush current) 

demanded by the load is provided by the utility. By paralleling the fuel cell power output to the 

utility, the following advantages can be realized: 

• Power conversion rating is same as the fuel cell 

• Peak power (as well as the inrush current) demanded by the load is provided by the utility 

• A constant fuel cell power level can be set. 

While in the utility connect mode, the voltage is set by the utility and current control is used to 

manage the power flow from the fuel cell. 

In the event of a utility failure, the fuel cell system does not have the ability to supply inrush 

current to loads such as for motor starting etc. Special circuit breakers to isolate loads up to the 

capacity of the fuel cell are necessary. Furthermore, the utility interconnection must conform to 

IEEE Standard P1547 to prevent energizing a section of the utility by the fuel cell source under 

utility fault conditions (islanding). 

Electric

Utility

Fuel cell

&

Reformer

Load

Power

Conditioning

Unit

CB

CB

1:K

Figure 8-16 Fuel cell supplying a load in parallel with the utility

8.2.5 Fuel Cell Power Conversion for Connecting Directly to the Local Utility 

Fuel cell power systems can be configured with the purpose of connecting directly to the utility 

to supply power as shown in Figure 8-17. IEEE Standard P1547 [12] outlines the criteria and 

requirements for interconnecting distributed resources such as a fuel cell power plant with the 

electric power system. It provides requirements relevant to the performance, operation, testing, 

safety considerations, and maintenance of the interconnection. Islanding should be avoided at 

any cost: that is a condition in which a fuel cell power plant energizes a portion of the electric 

power system when the utility power is disconnected. In other words, the fuel cell power 

connected to the utility must be disconnected immediately in case of utility failure. In general, a 

fuel cell power converter that has to be interfaced with the utility should meet the following 

requirements: 

 When a distributed resource such as a fuel cell is synchronized with the utility, it should not 

cause the area electric power system voltage to fluctuate more than ±5 percent 
8-38 

C

Electric Utility

Ls

Ls

Ls Vdc

Reference Signal

Generator

P*ref

FUEL

CELL

STACK

ACCESSORY

LOADS_

Gate Drive

DC/DC Control

Fuel Cell

Control

Hydrogen

Input

abc

dq

I

d

I

q*

I

d*

PI

Vq

wLPI s

Vd

V*d

V*q I

q

wLs

abc

dq

SVPWM

PI

I

a Ib

A

Va Vb

I

dc

B

DC-DC DC-AC

Start-up Pow er

Controller

Transformer

Figure 8-17 Fuel cell power conditioner control system for supplying power to the utility 

(utility interface) 

 The paralleling device (static switch / mechanical circuit breaker) should be capable of 

withstanding twice the nominal peak utility voltage 

 Unintentional islanding: the fuel cell power conditioner must detect islanding and cease to 

energize the area electric power system within 2 seconds of the formation of an island. 

Figure 8-17 shows the fuel cell power conditioner control system for supplying power to the 

utility. The DC-DC converter and the DC-AC inverter are controlled separately. The required 

electric power to be injected into the utility grid is set by P*

ref signal, the signal generator block 

then generates appropriate reference signals for the DC-AC inverter and the fuel cell controller to 

generate more power. The DC-AC inverter control block shows a “d-q” control with space 

vector PWM. A line frequency isolation transformer is shown to match the output AC voltage of 

the fuel cell unit with that of the utility. Utility power can be utilized initially to perform the fuel 

cell startup operation. Upon satisfactory startup, current control of the fuel cell power 

conditioning unit can be utilized to set the power level to be supplied to the utility. Higher power 

fuel cell systems in the 50kW to 500kW range [13,14,15] are characteristic of commercial 

installations such as industrial facilities, hospitals, hotels, fast food outlets, etc. At these power 

levels, 480V three phase AC output is preferred (in the U.S.). It should be noted that a minimum 

dc-link voltage of 784V DC is essential to generate 480V AC output from a three phase power 

electronic inverter. The fuel cell voltage must be sufficiently high, or a suitable DC-DC 

converter can be employed to increase the dc-link voltage. If the fuel cell stack voltage can be 

boosted to 800V DC, then a transformer-less system can be envisioned. It should be emphasized 

that using an isolation transformer in higher voltage/higher power fuel cell systems 

interconnected to a utility offers an effective means of meeting the requirements of domestic and 
8-39 

international safety standards for electronic equipment. In the United States, for example, such 

standards are set by the Occupational Safety and Health Administration (OSHA), with product 

testing performed according to appointed laboratories, such as Underwriters Laboratories (UL). 

Throughout Europe, safety standards are established by the International Electro-technical 

Commission (IEC). 

8.2.6 Power Conditioners for Automotive Fuel Cells 

An announcement by the Secretary of Energy stated that $1.5 billion U.S. government subsidies 

would be re-allocated to develop fuel cell technologies for automotive applications. For the 

automotive fuel cell market to directly impact the stationary fuel cell market, fuel cell vehicles 

must achieve commercial success. A number of requirements are necessary to effectively 

commercialize fuel cell vehicles [16,17]. Most important are the need to further develop 

hydrogen-reforming technologies and the availability of low cost, reliable power conditioning 

systems. In view of this, it is motivating to explore common power conditioning systems that 

have dual use - both for stationary and automotive fuel cell applications. Development of 

common standards will be beneficial for the overall fuel cell market. 

Figure 8-18 shows a typical fuel cell vehicle system block diagram [16]. A fuel cell vehicle 

system consists of three main components: (a) fuel processor; (b) fuel cell stack, and (c) power 

conditioning unit (DC-DC or DC-AC) to power a traction motor (AC or DC). A fuel cell system 

designed for vehicular propulsion must have weight, volume, power density, start-up, and 

transient response similar to the present internal combustion engine-based vehicles. Proton 

exchange membrane (PEFC) fuel cells are gaining importance as the fuel cell for vehicular 

applications [16,17] because of their low operating temperature, relatively high power density, 

specific power longevity, efficiency, and relatively high durability. One problem in PEFC-based 

technology is that the carbon (CO) concentration in fuel should be reduced to less than 10 parts 

per million (PPM); higher CO content in hydrogen contributes to deterioration of the cell 

performance. 

Figure 8-18 A typical fuel cell vehicle system [16] 
8-40 

Fuel cell vehicle configurations: Fuel cell vehicles can be classified as fuel cell electric and fuel 

cell hybrid vehicles [16,17]. A fuel cell electric vehicle uses a fuel cell system as the power 

source without the use of the battery. A fuel cell hybrid vehicle consists of a battery or a 

supercapacitor [16] in addition to the fuel cell system. This configuration enables most efficient 

use of both the fuel cell and battery for vehicle propulsion. The battery (or supercapacitor) 

provides power during start-up and acceleration, and the fuel cell supplies the steady state load. 

A range extender-type fuel cell vehicle can also be designed with a low power fuel cell whose 

only function is to charge the batteries. In this case, the battery is designed to provide full power 

and the fuel cell is employed as an on-board battery charger. 

Power conditioning system for the fuel cell hybrid vehicle: Figure 8-19 shows a typical 

topology of a power conditioning unit for a fuel cell hybrid vehicle powering a three phase 

variable speed AC traction motor load. The bi-directional converter S2 and S3 can be eliminated 

if a 42V battery is employed. Figure 8-20 shows the fuel cell converter control system block 

diagram. The reference signal from the vehicle power controller is used to generate the current 

reference (Iref) as shown. The current reference Iref is used to control the hydrogen input to the 

fuel cell stack. Also, Iref regulates the current output of the DC-DC boost converter, which 

powers the DC-AC inverter stage. The power required to power fuel cell accessory loads are 

powered from the battery. The battery power is also used for system start-up to bring the fuel cell 

stack voltage to a nominal value. 

Automotive

Fuel Cell

&

Reformer

DC/DC Boost

Converter 3-Phase PWM Inverter

Regulated

42V DC Link

& Battery (12V)

C

S1

L1

S2

S3

S4

S5

3-Phase Traction Motor

(Induction / BLDC)

0 - 500Hz

+

42V

-

S6

S7

M

S8

S9

L2

Figure 8-19 Power conditioning unit for fuel cell hybrid vehicle 

Figure 8-20 Fuel cell power conditioner control system [16] 
8-41 

It is important to note that the vehicle power train for the fuel cell powered system is similar to 

that of battery powered vehicles. The power conditioning system requirements for vehicles 

include low EMI, high efficiency, low cost, and suitable for intermittent use over a 10 to 15 year 

lifetime. 

8.2.7 Power Conversion Architecture for a Fuel Cell Turbine Hybrid Interfaced 

With a Local Utility 

Systems studies to date indicate that fuel cell/turbine hybrids could realize a 25 percent increase 

in efficiency and 25 percent reduction in cost for a comparably sized fuel cell [14,15]. The 

synergy realized by fuel cell/turbine hybrids derives primarily from using the rejected thermal 

energy and combustion of residual fuel from a fuel cell to drive the gas turbine. This leveraging 

of the thermal energy makes the high-temperature molten carbonate (MCFC) and solid oxide 

fuel cells (SOFC) ideal candidates for hybrid systems. Use of a recuperator contributes to 

thermal efficiency by transferring heat from the gas turbine exhaust to the fuel and air. 

Integrating a pressurized SOFC with a micro-turbine generator to yield a system with a nominal 

capacity of 250kW has shown a power generating efficiency approaching 60 percent [14,15]. 

During normal steady state operation, the SOFC will produce about 80 percent of the electrical 

power and the micro-turbine will produce the remaining 20 percent. Effectively, for this design 

the fuel cell supplants the combustor of the micro-turbine, providing the added benefit of 

eliminating combustion and combustion by-product formation. 

Figure 8-21 shows the block diagram for the power conditioning unit of the example 250kW fuel 

cell turbine hybrid system. Three 100kW SOFC stacks, each generating 200V are connected in 

series to generate 600V DC. A DC-DC boost converter is employed to convert the 600V DC 

generated by SOFC to 750V dc-link. The 750V dc-link is then connected to the 360kVA threephase PWM inverter to generate 480V, 60Hz output. The micro-turbine, on the other hand, 

typically operates at high speed (96,000 rpm rated speed) and is connected to a 3-phase brushless permanent magnet generator [18]. The local utility supplies the necessary startup power for 

both the SOFC and the micro-turbine. 

DC Boost

Chopper 600V

DC

+ 360KVA

3-Phase

Inverter

90KVA

Microturbine

Static Isolator

3-Phase

Rectifier

3-Phase

Inverter

Transformer

Transformer

+

-

+

-

+

-

3 x 100 Kw

200V SOFC

cells in series

480V, 60Hz, 3φ

-

750V

DC

+

-

Figure 8-21 Power conditioning unit for the 250kW fuel cell turbine hybrid system 
8-42 

The electric power generated by the micro-turbine generator is at higher frequency (1.6kHz at 

rated speed) and is converted to DC via a 3-phase PWM rectifier. The DC power is then 

converted to AC by means of a 3-phase PWM inverter unit. The AC electrical outputs of both the 

SOFC and the micro-turbine are combined, as shown, and connected to the utility. A line 

frequency isolation transformer is employed at the output terminals of both the SOFC and the 

micro-turbine power conditioning units. Each transformer provides isolation as well as voltage 

matching with the utility. The hybrid power generating system is connected to the utility via a 

static isolator to facilitate rapid disconnection in the event of a fault. 

Shared dc-link power conditioning unit [19]: Figure 8-22 shows an alternative power 

conditioning unit for the fuel cell turbine hybrid system. In this approach a common dc-link 

system is envisioned. The SOFC DC-DC converter and the micro-turbine PWM rectifier stages 

output are paralled together to form a common dc-link. The main advantage of this approach is 

that only one common DC-AC power conditioning unit is necessary. This architecture has the 

potential to reduce the cost of the power conditioning unit of a hybrid system. 

Vdc Cd

U V W

Microturbine

Generator

(MTG)

SOFC

Fuel Cell

DC/DC

Converter

Heat

Figure 8-22 Alternative power conditioning unit for the fuel cell turbine hybrid 

system with shared dc-link [19] 

Power conditioning units for MW-range fuel cell turbine hybrid systems [19]: FuelCell 

Energy Inc. (FCE) is developing an ultra-high efficiency fuel cell/turbine hybrid power plant 

[20]. The power system is based on an innovative cycle utilizing an indirectly heated gas turbine 

to supplement fuel cell generated power. System development is being conducted under the 

Department of Energy through the Office of Fossil Energy, and managed by the National Energy 

Technology Laboratory (NETL). The project objectives include the design of a 40 MW system 

using an internally reformed fuel cell being commercialized by FCE [20]. 

The power conditioning unit for multi-megawatt fuel cell hybrid systems operates at medium 

voltage levels (2300V, 4160V, 6600V or 11000V in the U.S.). Medium voltage power 

conditioner units employing integrated gate-commutated thyristor (IGCT) type devices and/or 
8-43 

modular systems employing high voltage insulated gate bipolar transistors (IGBTs) are possible 

candidates. 

Figure 8-23 shows a possible power conditioning topology employing high voltage IGCT 

devices. It is envisioned that two fuel cell turbine hybrid systems with common dc-link are 

connected in series via their respective DC-DC converter stages to form a high voltage dc-link 

(6,000V). A neutral point clamped (NPC) type PWM inverter with 12 IGCT devices is employed 

to generate 4,160V three-phase AC voltage that is suitable for utility interface. Commercial 

IGCT based NPC inverters are widely available for powering variable speed medium voltage 

induction motors [21,22]. The stated technology is, therefore, viable. 

DC/DC

Vo2

Vo1

+

+

-

-

Neutral Point Clamped Inverter with IGCT Devices

Line

Frequency

Transformer

+

-

+

-

Fuel Cell

Turbine

Hybrid 1

Fuel Cell

Turbine

Hybrid 2

DC/DC

Transformer

Medium

Voltage Utility

1:K

DC/DC

Converter

Fuel Cell

Turbine

Hybrid

Figure 8-23 Possible medium voltage power conditioning topology for megawatt range 

hybrid fuel cell systems [19] 

8.2.8 Fuel Cell Ripple Current

An important variable in the design of the power conditioner for a fuel cell is the amount of 

ripple current the fuel cell can withstand. Since reactant utilization is known to impact the 

mechanical nature of a fuel cell, it is suggested [23] that the varying reactant conditions 

surrounding the cell (due to ripple current) govern, at least in part, the lifetime of the cells. Both 

the magnitude and frequency of the ripple current is important. For fuel cells powering single 

phase loads (60Hz), the ripple current of concern is twice the output frequency, i.e. 120Hz. A 

limit of 0.15 per unit (i.e. 15 percent of its rated current) from 10 to 100 percent load is specified 

[2]. In case of single phase inverters with dual output voltage (120V/240V), there is a possibility 

of 60Hz ripple current in the fuel cell under unbalanced loading conditions (i.e. one output phase 

loaded and the other unloaded). A limit of 0.1 per unit is specified for 60Hz ripple current from 

10 to 100 percent load [2]. Further, the magnitude of the low frequency ripple current drawn 

from the fuel cell by the DC-DC converter is largely dependent on the voltage loop response 

characteristics. Also the dc-link capacitor size determines the 120Hz voltage ripple on the dclink, which in turn has an impact on the input current drawn from the fuel cell. It should be noted 

that switching frequency components in the DC-DC converter can be easily filtered via a small 

high frequency capacitive filter. For balanced three phase AC loads at the inverter output, the 

possibility of low frequency components in the fuel cell input current is low. 
8-44 

Corrective measures for limiting fuel cell ripple current: The following corrective measures 

are suggested for limiting the fuel cell ripple current (especially for power conditioners with 

single phase AC output): 

 Install an input filter, as in Figure 8-15, to filter the 120Hz component of the ripple current to 

0.15 per unit: this approach contributes to additional size, weight, and cost of the unit. 

 Increase the size of the dc-link capacitor in the DC-AC inverter: size, weight, and cost are of 

concern. 

 Reduce the response time of the voltage loop of the DC-DC converter: this will affect the 

regulation of the dc-link and impact the quality of inverter AC output, and possibly increase 

the size of the output AC filter. 

8.2.9 System Issues: Power Conversion Cost and Size 

400V DC is required to produce 120V/240V AC. If a fuel cell can produce 400V DC, then only 

an inverter stage is required, resulting in lowest cost for power conditioning. Present day 

commercially available fuel cells produce low voltage (12V to 100V). Therefore, either a line 

frequency transformer to increase the AC voltage or a DC-DC converter to boost the DC voltage 

is required, adding to cost, weight, and volume. Figure 8-24 shows a representative cost per kW 

of the power conditioning unit, as the voltage and current values are varied for a certain power 

level. It is clear from this figure that extremes of voltage at low power and high current at high 

power levels does not result in an optimum design. In general, higher voltage levels are required 

at higher power outputs to minimize the cost of power conditioning hardware. The other issue is 

power density and size of power conditioning unit. Using higher switching frequency for power 

conversion should result in smaller size. However, the switching losses are higher and a design 

compromise becomes necessary. Employing power semiconductor devices with lower losses 

combined with active cooling methods should yield an optimum size. Power integrated circuits 

can also be considered for further size reduction and become viable, if the fuel cell systems are 

produced in high volume. 

Figure 8-24 Representative cost of power conditioning as a function of 

power and dc-link voltage 
8-45 

8.2.10 REFERENCES 

1. Avistalabs, “SR-12 Modular PEM Generator Operator’s Manual”, 2000. 

2. DOE Future Energy Challenge 2001, Low Cost Fuel Cell Inverter Design competition. 

3. “Analysis and Design of a Low Cost Fuel Cell Inverter for Fuel Cell Systems,” Texas A&M 

University, Report to DOE, August 2001. 

4. R. Gopinath, S.S. Kim, P. Enjeti, et al., “Development of a Low Cost Fuel Cell Inverter With 

DSP Control”, IEEE PESC Conference Record, June 2002. 

5. Montena, “http://www.montena.com/”, PC2500 Ultracapacitor Datasheet. 

6. J. L. Durán-Gómez, P. Enjeti, and A. von Jouanne, “An Approach to Achieve Ride-Through 

of an Adjustable Speed Drive with Flyback Converter Modules Powered by Super 

Capacitors”, IEEE Transactions on Industry Applications, Vol. 38, No. 2, March/April 2002, 

pp. 514-522. 

7. A. Kusko, “Emergency/Standby Power Systems”, McGraw-Hill Book Company, 1989, 

ISBN: 0-07-035688-0. 

8. E.Santi, et al., “A Fuel Cell Based Domestic Uninterruptible Power Supply”, IEEE Applied 

Power Electronics Conference and Exposition, APEC ’02 Proceedings, pp. 605-613, 2002. 

9. Sukumara, G.V., Parthasarathy, A., and Shankar, V.R, “Fuel Cell Based Uninterrupted 

Power Sources”, Power Electronics and Drive Systems, Proceedings Volume: 2, pp. 728 –

733, 1997. 

10. W. Choi, P. Enjeti, “Design of a Modular Fuel Cell Based UPS System”, paper submitted to 

IEEE APEC’03. 

11. P. Enjeti and J. Howze, “A 2kW Fuel Cell Based UPS System”, Texas State Energy 

Conservation Office and DOE sponsored project, Texas A&M University, 2002. 

12. IEEE Standard P1547 Document: http://www.ieee.org 

13. “220kW Solid Oxide Fuel Cell / Microturbine Generator Hybrid Proof of Concept 

Demonstration Report”, March 2001, California Energy Commission. 

14. McDermott Technology, Inc. and Northern Research and Engineering Corporation , “Fuel 

Cell/Micro-Turbine Combined Cycle”, Final Report for U.S. Department of Energy, 

December 1999. 

15. Siemens Westinghouse Power Corporation, “A High Efficiency PSOFC/ATS-Gas Turbine 

Power System”, Final Report for U.S. Department of Energy, February 2001. 

16. K. Rajashekara, “Propulsion System Strategies for Fuel Cell Vehicles”, Fuel Cell Power for 

Transportation 2000 Conference, SAE 2000 World Congress, March 2000, Ref: 2000-01-

0369. 

17. T. Matsumoto, et al., “Development of Fuel Cell Hybrid Vehicle”, Fuel Cell Power for 

Transportation 2002 Conference, SAE 2002 World Congress, March 2000, Ref: 2002-01-

0096. 

18. 30kW Capstone Microturbine Generator, http://www.capstone.com 

19. P. Enjeti, P. Krein, J. Lai, “Power Conditioning Approaches to Fuel Cells and Gas Turbine 

Hybrids”, NTU short course (Sponsored by DOE), March 2002. 

20. H.G. Ayagh, J.M. Daly, et al. “Critical Components for Direct Fuel Cell/Turbine UltraEfficiency system”, Presented at Turbine Power Systems Conference and Conditioning 

Monitoring Workshop, Galveston, TX, Feb 2002. Authors affiliation: Fuel Cell Energy Inc. 

Danbury, CT. 
8-46 

21. P. Lataire, “White Paper on the New ABB Medium Voltage Drive System, Using IGCT 

Power Semiconductors and Direct Torque Control”, EPE journal, Vol. 7, No. 3⁄4, December 

1998, pp. 40-45. 

22. J.P. Lyons. V. Vlatkovic, P.M. Espelage, F.H. Boettner, E. Larsen, (GE), “Innovation IGCT 

Main Drives”, IEEE IAS, Conf. Rec. 1999. 

23. Randall S. Gemmen, “Analysis for the Effect of Inverter Ripple Current on Fuel Cell 

Operating Condition”, ASME 2001 International Mechanical Engineering Congress and 

Exposition, November 11, 2001, New York. 

8.3 System Optimization 

The design and optimization of a fuel cell power system is very complex because of the number of 

required systems, components, and functions. Many possible design options and trade-offs affect 

unit capital cost, operating cost, efficiency, parasitic power consumption, complexity, reliability, 

availability, fuel cell life, and operational flexibility. Although a detailed discussion of fuel cell 

optimization and integration is not within the scope of this section, a few of the most common 

system optimization areas are examined. 

From Figure 8-25, it can be seen that the fuel cell itself has many trade-off options. A fundamental 

trade-off is determining where along the current density voltage curve the cell should operate. As 

the operating point moves up in voltage by moving (left) to a lower current density, the system 

becomes more efficient but requires a greater fuel cell area to produce the same amount of power. 

That is, by moving up the voltage current density line, the system will experience lower operating 

costs at the expense of higher capital costs. Many other parameters can be varied simultaneously 

to achieve the desired operating point. Some of the significant fuel cell parameters that can be 

varied are pressure, temperature, fuel composition and utilization, and oxidant composition and 

utilization. The system design team has a fair amount of freedom to manipulate design parameters 

until the best combination of variables is found. 

8.3.1 Pressure 

Fuel cell pressurization is typical of many optimization issues, in that there are many interrelated 

factors that can complicate the question of whether to pressurize the fuel cell. Pressurization 

improves process performance at the cost of providing the pressurization. Fundamentally, the 

question of pressurization is a trade-off between the improved performance (and/or reduced cell 

area) and the reduced piping volume, insulation, and heat loss compared to the increased parasitic 

load and capital cost of the compressor and pressure-rated equipment. However, other factors can 

further complicate the issue. To address this issue in more detail, pressurization for an MCFC 

system will be examined. 
8-47 

Figure 8-25 Optimization Flexibility in a Fuel Cell Power System 

In an MCFC power system, increased pressure can result in increased cathode corrosion. Cathode 

corrosion is related to the acidity of the cell, which increases with the partial pressure of CO2, and 

therefore with the cell pressure. Such corrosion is typified by cathode dissolution and nickel 

precipitation, which can ultimately result in a shorted cell, causing cell failure (1). Thus, the 

chosen pressure of the MCFC has a direct link to the cell life, economics, and commercial 

viability. 

Increasing the pressure in a MCFC system can also increase the likelihood of soot formation and 

decrease the extent of methane reforming. Both are undesirable. Furthermore, the effect of 

contaminants on the cell and their removal from a pressurized MCFC system have not been 

quantified. The increased pressure also will challenge the fuel cell seals (1). 

The selection of a specific fuel cell pressure will affect numerous design parameters and 

considerations such as the current collector width, gas flow pattern, pressure vessel size, pipe and 

insulation size, blower size and design, compressor auxiliary load, and the selection of a bottoming 

cycle and its operating conditions. 

These issues do not eliminate the possibility of a pressurized MCFC system, but they do favor the 

selection of more moderate pressures. For external reforming systems sized near 1 MW, the 

current practice is a pressurization of 3 atmospheres. 

The performance of an internal reforming MCFC also would benefit from pressurization, but 

unfortunately, the increase is accompanied by other problems. One problem that would need to be 

overcome is the increased potential for poisoning the internal reforming catalyst resulting from the 
8-48 

increase in sulfur partial pressure. The current practice for internal reforming systems up to 3 MW 

is atmospheric operation. 

Pressurization of an SOFC yields a smaller gain in fuel cell performance than either the MCFC or 

PAFC. For example, based on the pressure relationships presented earlier, changing the pressure 

from one to ten atmospheres would change the cell voltage by ~150, ~80, and ~60 mV for the 

PAFC, MCFC, and SOFC, respectively. In addition to the cell performance improvement, 

pressurization of SOFC systems allows the thermal energy leaving the SOFC to be recovered in a 

gas turbine, or gas turbine combined cycle, instead of just a steam bottoming cycle. Siemens 

Westinghouse is investigating the possibilities associated with pressurizing the SOFC for cycles as 

small as 1 to 5 MW. 

Large plants benefit the most from pressurization, because of the economy of scale on equipment 

such as compressors, turbines, and pressure vessels. Pressurizing small systems is not practical, as 

the cost of the associated equipment outweighs the performance gains. 

Pressurization in operating PAFC systems demonstrates the economy of scale at work. The 

IFC 200 kWe and the Fuji Electric 500 kWe PAFC offerings have been designed for atmospheric 

operation, while larger units operate at pressure. The 11 MWe plant at the Goi Thermal Power 

Station operated at a pressure of 8.2 atmospheres (2), while a 5 MWe PAFC unit (NEDO/ 

PAFCTRA) operates at slightly less than 6 atmospheres (3). NEDO has three 1 MWe plants, two 

of which are pressurized while one is atmospheric (3). 

Although it is impossible to generalize at what size a plant would benefit by pressurization, when 

plants increase in size to approximately 1 MW and larger, the question of pressurization should be 

evaluated. 

8.3.2 Temperature 

Although the open circuit voltage decreases with increasing temperature, the performance at 

operating current densities increases with increasing temperature due to reduced mass transfer 

polarizations and ohmic losses. The increased temperature also yields higher quality rejected heat. 

An additional benefit to an increased temperature in the PAFC is an increased tolerance to CO 

levels, a catalyst poison. The temperatures at which the various fuel cells can operate are, 

however, limited by material constraints. The PAFC and MCFC are both limited by life shortening 

corrosion at higher temperatures. The SOFC has material property limitations. Again, the fuel cell 

and system designers should evaluate what compromise will work best to meet their particular 

requirements. 

The PAFC is limited to temperatures in the neighborhood of 200 oC (390 oF) before corrosion and 

lifetime loss become significant. The MCFC is limited to a cell average temperature of 

approximately 650 oC (1200 oF) for similar reasons. Corrosion becomes significant in an MCFC 

when local temperatures exceed 700 oC (1290 oF). With a cell temperature rise on the order of 100 

oC (210 oF), an average MCFC temperature of 650 oC (1200 oF) will provide the longest life, 

highest performance compromise. In fact, one reference (4) cites "the future target of the operating 

temperature must be 650 °C +30 °C (1290 °F +55 °F)." 
8-49 

The high operating temperature of the SOFC puts numerous requirements (phase and conductivity 

stability, chemical compatibility, and thermal expansion) on material selection and 

development (5). Many of these problems could be alleviated with lower operating temperatures. 

However, a high temperature of approximately 1000 °C (1830 oF), i.e., the present operating 

temperature, is required in order to have sufficiently high ionic conductivities with the existing 

materials and configurations (5). 

8.3.3 Utilization 

Both fuel and oxidant utilizations47 involve trade-offs with respect to the optimum utilization for a 

given system. High utilizations are considered to be desirable (particularly in smaller systems) 

because they minimize the required fuel and oxidant flow, for a minimum fuel cost and 

compressor/blower load and size. However, utilizations that are pushed too high result in 

significant voltage drops. One study (6) cites that low utilizations can be advantageous in large 

fuel cell power cycles with efficient bottoming cycles because the low utilization improves the 

performance of the fuel cell and makes more heat available to the bottoming cycle. Like almost all 

design parameters, the selection of optimum utilization requires an engineering trade-off that 

considers the specifics of each case. 

Fuel Utilization: High fuel utilization is desirable in small power systems, because in such 

systems the fuel cell is usually the sole power source. However, because the complete utilization 

of the fuel is not practical, except for pure H2 fuel, and other requirements for fuel exist, the 

selection of utilization represents a balance between other fuel/heat requirements and the impact of 

utilization on overall performance. 

Natural gas systems with endothermic steam reformers often make use of the residual fuel from the 

anode in a reformer burner. Alternatively, the residual fuel could be combusted prior to a gas 

expander to boost performance. In an MCFC system, the residual fuel often is combusted to 

maximize the supply of CO2 to the cathode while at the same time providing air preheating. In an 

SOFC system, the residual fuel often is combusted to provide high-temperature air preheating. 

The designer has the ability to increase the overall utilization of fuel (or the oxidant) by recycling a 

portion of the spent stream back to the inlet. This increases the overall utilization while 

maintaining a lower per pass utilization of reactants within the fuel cell to ensure good cell 

performance. The disadvantage of recycling is the increased auxiliary power and capital cost of 

the high temperature recycle fan or blower. 

One study by Minkov, et al. (6) suggests that low fuel and oxidant utilizations yield the lowest 

COE in large fuel cell power systems. By varying the fuel cell utilization, the electric power 

generation split between the fuel cell, steam turbine, and gas turbine are changed. The low fuel 

utilization decreases the percentage of power from the fuel cell while increasing the fuel cell 

performance. The increased power output from the gas turbine and steam turbine also results in 

their improved performance and economy of scale. The specific analysis results depend upon the 

assumed stack costs. The optimal power production split between the fuel cell and the gas and 

steam turbines is approximately 35 percent, 47 percent, and 17 percent for a 575 MW MCFC 

 

47. Utilization - the amount of gases that are reacted within the fuel cell compared to that supplied. 
8-50 

power plant. The associated fuel utilization is a relatively low 55 percent. It remains to be seen 

whether this trend will continue to hold for the improved cells that have been developed since this 

1988 report was issued. 

Oxidant Utilization: In addition to the obvious trade-off between cell performance and 

compressor or blower auxiliary power, oxidant flow and utilization in the cell often are determined 

by other design objectives. For example, in the MCFC and SOFC cells, the oxidant flow is 

determined by the required cooling. This tends to yield oxidant utilizations that are fairly low (~25 

percent). In a water-cooled PAFC, the oxidant utilization based on cell performance and a 

minimized auxiliary load and capital cost is in the range of 50 to 70 percent. 

8.3.4 Heat Recovery 

Although fuel cells are not heat engines, heat is still produced and must be removed. Depending 

upon the size of the system, the temperature of the available heat, and the requirements of the 

particular site, this thermal energy can be either rejected, used to produce steam or hot water, or 

converted to electricity via a gas turbine or steam bottoming cycle or some combination thereof. 

Cogeneration: When small quantities of heat and/or low temperatures typify the waste heat, the 

heat is either rejected or used to produce hot water or low-pressure steam. For example, in a PAFC 

where the fuel cell operates at approximately 205 °C (400 °F), the highest pressure steam that 

could be produced would be something less than 14 atmospheres (205 psia). This is obviously not 

practical for a steam turbine bottoming cycle, regardless of the quantity of heat available. At the 

other end of the spectrum is the TSOFC, which operates at ~1000 °C (~1800 °F) and often has a 

cell exhaust temperature of approximately 815 °C (1500 °F) after air preheating. Gas temperatures 

of this level are capable of producing steam temperatures in excess of 540 °C (1000 °F), which 

makes it more than suitable for a steam bottoming cycle. However, even in an SOFC power 

system, if the quantity of waste heat is relatively small, the most that would be done with the heat 

would be to make steam or hot water. In a study performed by Siemens Westinghouse of 50 to 

2000 kW TSOFC systems, the waste heat was simply used to generate 8 atmospheres (100 psig) 

steam (7). 

Bottoming Cycle Options: Whenever significant quantities of high-temperature rejected heat are 

available, a bottoming cycle can add significantly to the overall electric generation efficiency. 

Should the heat be contained within a high-pressure gas stream, then a gas turbine potentially 

followed by a heat recovery steam generator and steam turbine should be considered. If the hot gas 

stream is at low pressure, then a steam bottoming cycle is logical. 

If a steam bottoming cycle is appropriate, many design decisions need to be made, including the 

selection of the turbine cycle (reheat or non-reheat) and the operating conditions. Usually, steam 

turbines below 100 MW are non-reheat, while turbines above 150 MW are reheat turbines. This 

generalization is subject to a few exceptions. In fact, a small (83 MW) modern reheat steam 

turbine went into operation (June 1990) as a part of a gas turbine combined cycle repowering 

project (8). 
8-51 

8.3.5 Miscellaneous 

Compressor Intercooling: Whether a compressor should be intercooled or not depends on the 

trade-off between the increased efficiency of the intercooled compressor and its increased capital 

cost. In general, intercooling is required for large compressors with pressure ratios that exceed 

approximately 5:1 (9). The designer also should consider whether the heat is advantageous to the 

process. For example, when near the 5:1 pressure ratio, it may not be appropriate to intercool if the 

compressed stream will subsequently require preheating as it would with the process air stream of 

an MCFC or SOFC system. 

Humidification/Dehumidification: Water often is added or removed in fuel cell systems to 

promote or prevent certain chemical reactions. For some reactions, excess water can help to drive 

the reaction, while too much requires larger equipment and can even reduce the yield of a reaction 

or decrease the performance of a fuel cell. Excess water often is utilized to increase the yield of 

reforming reactions and the water gas shift. 

In a natural gas fueled PAFC, water is condensed out of the fuel stream going to the fuel cell to 

increase the partial pressure of hydrogen. In coal gasification MCFC, water often is added to the 

fuel stream prior to the fuel cell to prevent soot formation. The addition of excess steam not only 

prevents soot formation, but also causes a voltage drop of approximately 2 mV per each percentage 

point increase in steam content (10). The use of zinc ferrite hot gas cleanup can aggravate the soot 

formation problem because of the catalytic effect of the sorbent on carbon formation, and requires 

even higher moisture levels (11). 

Maintaining the proper quantity of water within a PEFC is very important for proper operation. 

Too much, and the cell will flood; too little, and the cell membrane will dehydrate. Either will 

severely degrade cell performance. The proper balance is achieved only by considering water 

production, evaporation, and humidification levels of the reactant gases. Achieving the proper 

level of humidification is also important. With too much humidification, the reactant gases will be 

diluted, with a corresponding drop in performance. The required humidification level is a complex 

function of the cell temperature, pressure, reactant feed rates, and current density. Optimum PEFC 

performance is achieved with a fully saturated, yet unflooded membrane (12). 

8.3.6 Concluding Remarks on System Optimization 

System design and optimization encompass many questions, issues, and trade-offs. In the process 

of optimizing a power plant design, the engineer will address the selection of fundamental 

processes, component arrangements, operating conditions, fuel cell and bottoming cycle 

technologies and associated power production split, system integration, and capital and life cycle 

costs. The design will be governed by criteria such as output, weight, fuel basis, emissions, and 

cost objectives. Site and application specific criteria and conditions may strongly influence the 

cycle design criteria and resulting design. 

The objective of this system optimization discussion was not to present a detailed review of the 

subject of optimization, but simply to present select issues of system optimization as they apply to 

fuel cell power systems. 
8-52 

8.4 Fuel Cell System Designs 

The following five cycles are examples of current fuel cell offerings that reflect manufacturers' 

anticipated commercialization plans. These cycles are based on information available in relevant 

literature and may differ from the ultimate size of the commercial offering. 

8.4.1 Natural Gas Fueled PEFC System 

A natural gas PEFC power plant configuration is shown in Figure 8-26 and is a slight 

simplification of a cycle published in 1997 by a Ballard Researcher (13). In light of the PEFC 

sensitivity to CO, CO2 and methane, the fuel processing represents a significant portion of the 

cycle. Natural gas fuel enters a fuel compressor and a fuel cleanup device. (The reference 

document does not describe the cleanup device, but it is assumed to be a sulfur polisher to 

prevent poisoning of the fuel cell catalyst.) The cleaned gas is mixed with water in a vaporizer, 

which evaporates the liquid water into water vapor with waste heat from the reformer. This 

humidified fuel is reformed in the steam reformer. Because natural gas reformate is high in CO, 

the reformate is sent to a shift converter and selective oxidizer to reduce the CO to 10 to 50 ppm. 

This hydrogen rich/carbon monoxide lean fuel is fed to the PEFC stack where it reacts 

electrochemically with compressed air. 

C T

C

Vaporizer

Fuel

Gas

Water

Water

Tank

Fuel Gas

Cleanup

Intercooler

Air

Exhaust

C T

R

e

f

m

o

r

e

r Shift

Convertor

Selective

Oxidizer

A C

Spent

Fuel

Water

Separator

C

o

o

e

o

l

r

Fuel Gas

Air

Figure 8-26 Natural Gas Fueled PEFC Power Plant 

Ambient air is compressed in a turbocharger, powered by the expansion of the hot pressurized 

exhaust gases. Following this first compression stage, the air is intercooled by a fin fan air 

cooler and fed into a second turbocharger. The high-pressure air is fed directly to the PEFC 
8-53 

stack. The fuel cell water product is liberated to the oxidant gas stream. The spent oxidant 

stream exits the fuel cell where a water separator removes much of this water, which is 

subsequently used to humidify the fuel gas prior to the entering the reformer. The spent oxidant 

and fuel streams are combusted in the reformer burner to provide heat for the endothermic 

reforming reactions. The reformer exhaust also provides heat to the vaporizer. Finally, the 

residual heat and pressure of this exhaust stream are used in the turbochargers to drive the air 

compressor. 

The fuel cell itself liberates heat that can be utilized for space heating or hot water. The 

reference article did not list any operating conditions of the fuel cell or of the cycle. The PEFC 

is assumed to operate at roughly 80 oC. Another recent article (14) published by Ballard shows 

numerous test results that were performed at 3 to 4 atmospheres where fuel utilizations of 75 to 

85 percent have been achieved. Performance levels for an air fed PEFC are now in the range of 

180 to 250 mW/cm2

. Ballard Power Systems has performed field trials of 250 kW systems with 

select utility partners. Commercial production of stationary power systems is anticipated for the 

year 2002. Similarly sized transportation cycles also are anticipated for commercial production 

in the same year. 

8.4.2 Natural Gas Fueled PAFC System 

IFC has been marketing the PC25, a 200 kW atmospheric PAFC unit, since 1992. Details of this 

commercial cycle are proprietary and not available for publication. In order to discuss an 

example PAFC cycle, a pressurized (8 atm) 12 MW system will be presented (15). This cycle is 

very similar to the 11 MW IFC PAFC cycle that went into operation in 1991 in the Tokyo 

Electric Power Company system at the Goi Thermal Station, except that two performance 

enhancements have been incorporated. Limited data are available regarding the Goi power plant. 

However, it is understood that the average cell voltage is 750 mV and the fuel utilization is 80 

percent (16). The enhanced 12 MW cycle presented here utilizes values of 760 mV and 86 

percent. This enhanced cycle (Figure 8-27) is discussed below with selected gas compositions 

presented in Table 8-8. 

Natural gas (stream 100) is supplied at pressure and contains sulfur odorants for leak detection. 

A small hydrogen-rich recycle stream (stream 117) is mixed with the natural gas to hydrolyze the 

sulfur compounds to facilitate sulfur removal. The fuel stream (stream 102) is heated to 299 oC 

(570 oF) before entering the sulfur removal device. Superheated steam (stream 1) is mixed with 

the heated fuel to provide the required moisture for the reforming and the water gas shift 

reactions. The humidified stream (stream 105) is heated to approximately (705 oC) 1300 oF 

before entering the reformer. The effluent fuel stream (stream 107) leaves the reformer at 

approximately 760 oC (1400 oF) and is cooled in the heat exchanger used to preheat the 

humidified natural gas stream. This stream (stream 108) enters the high temperature shift 

converter (HTSC) at approximately 360 oC (680 oF), while leaving (stream 109) at about 415 oC 

(780 oF). The HTSC effluent is cooled in two heat exchangers before proceeding to the low 

temperature shift converter. A two-stage approach is utilized, allowing the HTSC to proceed at a 

faster rate, while the LTSC yields higher hydrogen concentrations. 
8-54 

Figure 8-27 Natural Gas fueled PAFC Power System 

Table 8-8 Stream Properties for the Natural Gas Fueled Pressurized PAFC 

Strm Description Temp. Press. Mole Flow Mass Flow Ar CH4 C2H6 CO CO2 H2 H2O N2 O2 Total

No. C atm Kgmol/hr kg/hr MW % % % % % % % % % %

1 Reformer Steam 243.3 10.00 418.8 7,545 18.02 100.0 100.0

100 NG Feed 15.6 13.61 115.1 1,997 17.34 90.0 5.0 5.0 100.0

106 Reformer Feed 712.8 9.93 562.6 9,846 17.50 18.3 1.0 trace 1.0 4.0 74.5 1.1 100.0

107 Reformer Effluent 768.3 9.59 755.9 9,846 13.03 2.4 trace 7.1 6.5 46.3 37.0 0.8 100.0

112 LTSC Effluent 260.0 8.72 755.9 9,846 13.03 2.4 0.5 13.1 52.9 30.4 0.8 100.0

114 Anode Feed 60.6 8.55 506.6 5,557 10.97 3.3 0.7 18.3 74.5 2.0 1.1 100.0

115 Anode Exhaust 207.2 7.95 181.4 4,901 27.02 9.3 1.9 51.2 28.8 5.7 3.1 100.0

118 NG to Aux Burner 15.6 13.61 1.59 27.5 17.34 90.0 5.0 5.0 100.0

200 Air Feed 15.6 1.00 1,156.5 33,362 28.85 0.9 trace 1.1 77.2 20.7 100.0

204 Cathode Feed 192.8 8.27 1,120.8 32,332 28.85 0.9 trace 1.1 77.2 20.7 100.0

205 Cathode Exhaust 207.2 8.09 1,283.4 32,987 25.70 0.8 trace 26.3 67.5 5.4 100.0

208 Cath. Gas to Heat Exch. 151.7 7.85 1,045.3 28,697 27.45 1.0 trace 9.5 82.8 6.7 100.0

209 Cath. Gas to Ref. Burner 243.9 7.81 1,045.3 28,697 27.45 1.0 trace 9.5 82.8 6.7 100.0

211 Cath. Gas to Heat Exch. 242.2 7.81 1,081.0 29,727 27.50 1.0 trace 9.2 82.6 7.1 100.0

301 Reformer Exhaust 380.6 7.71 1,234.6 34,629 28.05 0.9 9.2 15.9 72.8 1.2 100.0

302 Aux. Burner Exhaust 410.6 7.68 1,236.2 34,656 28.03 0.9 9.3 16.1 72.7 1.0 100.0

304 Exhaust 180.0 1.03 1,236.2 34,656 28.03 0.9 9.3 16.1 72.7 1.0 100.0

The LTSC effluent (stream 112) is utilized to superheat the steam required for the reformer and 

water gas shift reactions. The saturated steam sent to the superheater is supplied by the fuel cell 

water cooling circuit. The cooled stream (stream 113) is further cooled in a fuel gas contact 
8-55 

cooler (FGCC) to remove the excess moisture levels. This raises the partial pressure of hydrogen 

in the fuel before entering the fuel cell. Some of the hydrogen-rich fuel is recycled back, as 

mentioned previously, to the incoming natural gas, while the majority of the fuel (stream 114) 

proceeds to the fuel cell anode. Approximately 86 percent of the hydrogen in the fuel stream 

reacts in the fuel cell, where the hydrogen donates an electron and the resulting proton migrates 

to the cathode, where it reacts with oxygen in the air to form water. Key cell operating 

parameters are summarized in Table 8-9. The overall performance is summarized in Table 8-10. 

The spent fuel is combusted in the reformer burner and supplies heat for the endothermic 

reforming reactions. 

Table 8-9 Operating/Design Parameters for the NG fueled PAFC 

Operating Parameters Value 

Volts per Cell (V) 0.76 

Current Density (mA/cm2) 320 

No of stacks 12 

Cell Operating Temp. (oC) 207 

Cell Outlet Pressure (atm) 8.0 

Overall Fuel Utilization 

(percent) 

86.2 

Overall Oxidant Utilization 

(percent) 

70.0 

DC to AC Inverter efficiency 97.0 percent 

Auxiliary Load 4.2 percent 

Table 8-10 Performance Summary for the NG fueled PAFC 

Performance Parameters Value 

LHV Thermal Input (MW) 25.42 

Gross Fuel Cell Power (MW) 

 Fuel Cell DC Power 

 Inverter Loss

Fuel Cell AC Power 

13.25 

(0.40)

12.85 

Auxiliary Power 0.54 

Net Power 12.31 

Electrical Efficiency (percent 

LHV) 

48.4 

Electrical Efficiency (percent 

HHV) 

43.7 

Heat Rate (Btu/kWh, LHV) 7,050 

Note: The net HHV efficiency for the Goi Thermal Power 

Station is 41.8 percent (HHV) (1). 
8-56 

Ambient air (stream 200) is compressed in a two-stage compressor with intercooling to 

conditions of approximately 193 oC (380 oF) and 8.33 atmospheres (122.4 psia). The majority of 

the compressed air (stream 203) is utilized in the fuel cell cathode; however, a small amount of 

air is split off (stream 210) for use in the reformer burner. The spent oxidant (stream 205) enters 

a recuperative heat exchange before entering a cathode exhaust contact cooler, which removes 

moisture to be reused in the cycle. The dehumidified stream (stream 207) is again heated, mixed 

with the small reformer air stream, and sent to the reformer burner (stream 211). The reformer 

burner exhaust (stream 300) preheats the incoming oxidant and is sent to the auxiliary burner, 

where a small amount of natural gas (stream 118) is introduced. The amount of natural gas 

required in the auxiliary burner is set so the turbine shaft work balances the work required at the 

compressor shaft. The cycle exhaust (stream 304) is at approximately 177 oC (350 oF). 

Some of the saturated steam generated by the fuel cell cooling water is utilized to meet the 

reformer water requirements. Approximately 3,800 kg/hr (8,400 lb/hr) of 12.2 atmospheres 

(180 psi) saturated steam is available for other uses. 

Cycle performance is summarized in Table 8-10. The overall net electric conversion efficiency 

is 43.7 percent based on HHV input, or 48.4 percent on LHV. 

8.4.3 Natural Gas Fueled Internally Reformed MCFC System 

Fuel Cell Energy is developing initial market entry MCFC power systems, with mature megawatt 

class units projected to be available in 2004. These units will be produced in various sizes. 

Preliminary cycle information was received from FCE for a nominal 3 MW power plant. This 

cycle is presented in Figure 8-28 and is described below. 

Air

A C

Natural Gas

Steam

Anode

Exhaust

Converter

Fuel

Cleanup

Steam

Generator

59oF

47 lbmol/hr

Water

59oF

74 lbmol/hr

Exhaust or 

Waste Heat Boiler

700oF

831 lbmol/hr

Cleaned

Fuel NG/Steam

Spent

Fuel

CO2, H2O, H2 CO2, Air

Cathode

Feed

59oF

708 lbmol/hr C

Exhaust Gases

Figure 8-28 Natural Gas Fueled MCFC Power System 
8-57 

Natural gas is cleaned of its sulfur contaminants in a fuel cleanup device. Steam is added to the 

fuel stream prior to being fed to the internally reforming fuel cell. The fuel reacts 

electrochemically with the oxidant within the fuel cell to produce 3 MW of dc power. 

The spent fuel is completely combusted in the anode exhaust converter. This flue gas mixture is 

fed directly to the fuel cell cathode. The cathode exhaust has significant usable heat, which is 

utilized in the fuel cleanup and in steam generation. The residual heat can be utilized to heat air, 

water, or steam for cogeneration applications. Design parameters for the IR-MCFC are 

presented in Table 8-11. Overall performance values are presented in Table 8-12. 

Table 8-11 Operating/Design Parameters for the NG Fueled IR-MCFC 

Operating Parameters Value 

Volts per Cell (V) unknown 

Current Density (mA/cm2) unknown 

Operating Temperature (oC) unknown 

Cell Outlet Pressure (atm) 1.0 

Fuel Utilization (percent) 78.percent 

Oxidant Utilization (percent) 75.percent 

Inverter Efficiency 95.percent 

Table 8-12 Overall Performance Summary for the NG Fueled IR-MCFC 

Performance Parameters Value 

LHV Thermal Input (MW) 4.8 

Gross Fuel Cell Power (MW) 

 Fuel Cell DC Power 

 Inverter Loss

Fuel Cell AC Power 

3.0 

(0.15)

2.85 

Auxiliary Power (MW) 0.05 

Net Power (MW) 2.80 

Electrical Efficiency (percent 

LHV) 

58 percent 

Heat Rate (Btu/kWh, LHV) 5,900 
8-58 

8.4.4 Natural Gas Fueled Pressurized SOFC System 

This natural gas fuel cell power system is based on a pressurized TSOFC combined with a 

combustion turbine developed by Siemens Westinghouse48 (17). Most TSOFC power plant 

concepts developed to date have been based on atmospheric operation. However, as shown in 

Section 7, the cell voltage increases with cell pressure. Thus, operating with an elevated pressure 

will yield increased power and efficiency for a given cycle. In addition, the use of a pressurized 

SOFC will also allow integration with a combustion turbine. The combustion turbine selected 

for integration by Siemens Westinghouse is the unique 1.4 MW Heron reheat combustion 

turbine, a proposed product of Heron (18). 

A flow diagram for the natural gas fueled 4.5 MW class cascaded49 TSOFC power cycle is 

presented in Figure 8-29. A brief process description is given below, followed by a performance 

summary. Selected state point values are presented in Table 8-13. 

Filter

Compressor Compressor Turbine

Precooler

SOFC

System

SOFC

System

Turbine

Power

Turbine

Generator

Exhaust Recuperator / Fuel Heater

Fuel

Desulfurizers

Exhaust

Fuel

Air

Air

Compressor / Turbine Exhaust

Fuel

Fuel

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

Figure 8-29 Schematic for a 4.5 MW Pressurized SOFC 

 

48. The referenced Siemens Westinghouse publication presented the cycle concept and overall performance values. 

Neither specific stream information nor assumptions were presented. The stream data and assumptions presented 

here were developed by Parsons. The stream data were developed using an ASPEN simulation which yielded 

performance numbers in general agreement with the publication. 

49. The term "cascaded" fuel cells is used here to describe a fuel cell system where the exhaust of a high-pressure

 fuel cell is utilized as an oxidant feed stream in a low-pressure fuel cell after passing through an expander. 
8-59 

Table 8-13 Stream Properties for the Natural Gas Fueled Pressurized SOFC 

Strm Description Temp Press. Mass Flow Mole Flow Ar CH4 CO2 H20 N2 O2 Total

No. C atm kg/hr kgmol/hr MW % % % % % % % 

1 Fuel feed 15 8.85 508 30.9 16.44 97.4 0.4 0.9 100.0 

2 Pressurized Fuel 21 9.53 508 30.9 16.44 97.4 0.4 0.9 100.0 

3 Heated HP Fuel 399 9.42 508 30.9 16.44 97.4 0.4 0.9 100.0 

4 Cleaned HP Fuel 399 9.32 281 17.1 16.44 97.4 0.4 0.9 100.0 

5 Heated LP Fuel 399 9.42 227 13.8 16.44 97.4 0.4 0.9 100.0 

6 Cleaned LP Fuel 399 3.13 227 13.8 16.44 97.4 0.4 0.9 100.0 

7 Air Feed 15 0.99 18,536 642.3 28.86 0.9 trace 1.0 77.2 20.8 100.0 

8 Compressed Air 135 2.97 18,536 642.3 28.86 0.9 trace 1.0 77.2 20.8 100.0 

9 Intercooled Air 27 2.69 18,351 635.9 28.86 0.9 trace 1.0 77.2 20.8 100.0 

10 HP Air 160 8.80 18,351 635.9 28.86 0.9 trace 1.0 77.2 20.8 100.0 

11 Heated Air 555 8.66 18,167 629.5 28.86 0.9 trace 1.0 77.2 20.8 100.0 

12 HP FC Exhaust 860 8.39 18,448 646.5 28.53 0.9 2.7 6.2 75.2 15.0 100.0 

13 HPT Exhaust 642 3.11 18,631 653.1 28.53 0.9 2.7 6.2 75.2 15.0 100.0 

14 LP FC Exhaust 874 2.83 18,859 667.0 28.28 0.9 4.7 10.2 73.7 10.6 100.0 

15 LPT Exhaust 649 1.01 18,859 667.0 28.28 0.9 4.7 10.2 73.7 10.6 100.0 

16 Cycle Exhaust 258 1.00 19,044 673.4 28.28 0.9 4.6 10.1 73.7 10.7 100.0 

Reference Source: (30). 

The natural gas feed to the cycle (stream 1) is assumed to consist of 95 percent CH4, 2.5 percent 

C2H6, 1 percent CO2, and 1.5 percent N2 by volume along with trace levels of sulfur odorants. 

The odorants must be reduced to 1 ppmv before entrance into the fuel cell to prevent 

performance and cell life deterioration. Because the desulfurization requires elevated 

temperatures, the fuel (streams 3 and 5) is fed through a heat exchanger that recovers heat from 

the fuel cell exhaust stream (stream 15). The hot desulfurized fuel stream (stream 4) enters the 

anodes of the high-pressure fuel cell at approximately 399 oC (750 oF) and 9.3 atmospheres. The 

fuel entering the low-pressure fuel cell (stream 6) is approximately 399 oC (750 oF) and 3.1 

atmospheres. Ambient air (stream 7) is compressed to 3.0 atmospheres and 135oC (275 oF) 

(stream 8), subsequently intercooled to 27 oC (81 oF) (stream 9), compressed again to 8.8 

atmospheres and 160 oC (320 oF) (stream 10), and heated to 555 oC (1031 oF) prior to entering 

the high-pressure fuel cell cathode (stream 11). 

The hot desulfurized fuel and the compressed ambient air are electrochemically combined within 

the high-pressure fuel cell module with fuel and oxidant utilizations of 78 percent and 20.3 

percent, respectively. The SOFC high-pressure module was assumed to operate at 0.63 volts per 

cell. The spent fuel and air effluents of the Siemens Westinghouse tubular geometry SOFC are 

combusted within the module to supply heat required for the endothermic reforming reaction 

within the pre-reformer. The majority of the reforming takes place within the tubular fuel cell 

itself. The heat for internal reforming is supplied by the exothermic fuel cell reaction. A gas 

recirculation loop provides water for the internal reforming and to prevent soot formation. 

The combusted air and fuel stream (stream 12) from the high-pressure fuel cell are expanded 

(stream 13) in a turbine expander. The work of this turbine is used to drive the low- and 

high-pressure air compressors. The reduced pressure exhaust stream (stream 13) is utilized as 

the low-pressure fuel cell oxidant stream. Although vitiated, it still has 15 percent oxygen. The 

low-pressure TSOFC operates at 0.62 volts per cell, and fuel and air utilizations of 78 and 21.9 

percent, respectively. The spent air and fuel effluents are combusted and sent (stream 14) to the 

low-pressure power turbine. The turbine generator produces approximately 1.4 MW AC. The 

low-pressure exhaust (stream 15) still has a temperature of 649 oC (1200 oF) and is utilized to 
8-60 

preheat the fuel and oxidant streams. The resulting cycle exhaust stream (stream 16) exits the 

plant stack at approximately 258 oC (496 oF). 

Operating parameters are summarized in Table 8-14. Cycle performance is summarized in 

Table 8-15. The overall net electric LHV efficiency is 67 percent. 

The high efficiency of this TSOFC/Heron combined cycle is a result of synergism that exists 

between the SOFC and the Heron turbine. The TSOFC is able to fully replace the gas turbine 

combustor. That is, the waste heat of the SOFC exhaust is able to completely eliminate the need 

for the gas turbine combustor at the design point. As seen in Table 8-16, the Heron combustor 

design temperature of roughly 860 oC (1580 oF) is well within the TSOFC operating temperature 

range. Conversely, the Heron cycle is able to act as an efficient bottoming cycle without 

requiring a waste heat boiler or steam turbine. In simple cycle mode, the Heron cycle has a 

respectable LHV net electric efficiency of 42.9 percent. Together, the TSOFC/Heron cycle 

operates at an efficient 67 percent. Another advantage of this cycle is the low NOX emissions, 

because only the spent fuel is fired at the design point. The majority of the fuel reacts within the 

fuel cell. Overall NOX levels of less than 4 ppmv are expected. 

Table 8-14 Operating/Design Parameters for the NG Fueled Pressurized SOFC 

Operating Parameters HP FC LP FC 

Volts per Cell (V) 0.63* 0.62* 

Current Density (mA/cm2) NA NA 

Cell Operating Temp. (oC) 1000* 1000* 

Cell Outlet Pressure (atm) 8.4* 2.9* 

FC Fuel Utilization (percent) 78.0* 78.0* 

FC Oxidant Utilization 

(percent) 

20.3* 21.9* 

DC to AC Inverter Effic. 

(percent) 

96.0 

Generator Efficiency (percent) 96.0* 

Auxiliary Load (percent of 

gross) 

 1.0* 

Note: * assumed by Parsons to reasonably match the reference paper. 
8-61 

Table 8-15 Overall Performance Summary for the NG Fueled Pressurized SOFC 

Performance Parameters Value 

LHV Thermal Input (MW) 6.68 

Gross Fuel Cell Power (MW) 

 Fuel Cell DC Power 

 Inverter Loss

Fuel Cell AC Power 

3.22 

(0.13)

3.09 

Gross AC Power (MW) 

 Fuel Cell AC Power 

 Turbine Expander

Gross AC Power 

3.09 

1.40

4.49 

Auxiliary Power 0.04 

Net Power 4.45 

Electrical Efficiency (percent 

LHV) 

66.6 

Electrical Efficiency (percent 

HHV) 

60.1 

Heat Rate (Btu/kWh, LHV) 5,120 

Table 8-16 Heron Gas Turbine Parameters 

Performance Parameters Value 

Compressor Air Flow (kg/h) 18,540 

HP Combustor Temperature 

(oC) 

LP Combustor Temperature 

(oC) 

861 

863 

Compressor Pressure Ratio 8.8:1 

Power Turbine Exhaust 

Temp. (oC) 

620 

The cycle discussed here is based on a Siemens Westinghouse publication for a 4.5 MWe plant. 

Recent information from Siemens Westinghouse, plans for commercialization of a scaled down 

1 MWe version of this dual pressure TSOFC/Heron cycle. A 1 MW cycle was not available in 

the literature. 
8-62 

8.4.5 Natural Gas Fueled Multi-Stage Solid State Power Plant System 

The fuel cell system presented below is based on an innovative solid state fuel cell system 

developed by U.S.DOE (19). Conventional fuel cell networks, in order to effectively use the 

supplied fuel, often employ fuel cell modules operating in series to achieve high fuel utilization50

or combust the remaining fuel for possible thermal integration such as cogeneration steam or a 

steam bottoming cycle. Both of these conventional approaches utilize fuel cell modules at a 

single state-of-the-art operating temperature. In conventional fuel cell networks, heat exchangers 

are utilized between the fuel cell modules to remove heat so the subsequent fuel cell can operate 

at the desired temperature. 

In the multi-stage fuel cell, the individual stages are designed to operate at different temperatures, so that heat exchangers are not required to cool the effluent gases between stages. Each 

stage is designed to accommodate the next higher temperature regime. In addition, the multistage fuel cell concept does not attempt to maximize the fuel utilization in each stage, but allows 

lower utilizations in comparison to the state-of-the-art design. The number of stages and the fuel 

utilization per stage in the multi-stage concept is a matter of design choice and optimization. An 

example of the fuel utilization for a five stage concept is presented in Table 8-17. 

Table 8-17 Example Fuel Utilization in a Multi-Stage Fuel Cell Module 

 Fuel Balance for 100 Units of Fuel Fuel Utilization 

Stage Fuel Feed Fuel Out Fuel Used per Stage Cumulative 

1 100.0 81.0 19.0 19.0 % 19.0 % 

2 81.0 62.0 19.0 23.5 % 38.0 % 

3 62.0 43.0 19.0 30.6 % 57.0 % 

4 43.0 24.0 19.0 44.2 % 76.0 % 

5 24.0 6.0 18.0 75.0 % 94.0 % 

Overall 100.0 6.0 94.0 94.0 % 

A flow diagram for a natural gas fueled, 4 MW class, and solid state fuel cell power cycle is 

presented in Figure 8-30. A brief process description is given below, followed by a performance 

summary. Selected state point values are presented in Table 8-18. 

 

50. Current state-of-the-art SOFCs have fuel utilizations of 75 to 85%. By utilizing a second fuel cell in series,

 the total utilization could be theoretically increased to 93 to 98%. Note: Two cascaded fuel cells operating

 with a fuel utilization of 85% will have an overall utilization of 98%. 1-(0.15)2

 = 1-0.02 = 0.98 or 98%. 
8-63 

Fuel

Fuel

Fuel Processor

Pre-heated Air

Multi-staged

Fuel Cells

Combustor

Stage

Compressor Gas

Turbine Electric

Generator

Air

1

2

Water

3

4

6 5

7 8

9

10

11

12

13

14

15

16 17

18

19

20

Figure 8-30 Schematic for a 4 MW Solid State Fuel Cell System 

Table 8-18 Stream Properties for the Natural Gas Fueled Solid State Fuel Cell 

Power Plant System 

Strm Description Temp. Press. Mass Flow Mole Flow CH4 C2H6 C3H8+ CO CO2 H2 H20 N2 O2 Total

No. C atm kg/hr kgmol/hr MW % % % % % % % % % %

1 Fuel feed 25 3.74 373 21.64 17.23 93.9 3.2 1.1 1.0 0.8 100

2 Heated fuel 84 3.67 373 21.64 17.23 93.9 3.2 1.1 1.0 0.8 100

3 Humidification water 275 3.93 614 34.09 18.02 100.0 100

4 Humidified fuel 192 3.67 987 55.73 17.71 36.5 1.3 0.4 0.4 61.2 0.3 100

5 Heated fuel 725 3.60 987 55.73 17.71 36.5 1.3 0.4 0.4 61.2 0.3 100

6 Heated fuel 725 3.60 987 55.73 17.71 36.5 1.3 0.4 0.4 61.2 0.3 100

7 Processed fuel 494 3.53 987 63.70 15.50 29.1 0.0 0.6 6.0 ## 41.6 0.3 100

8 Spent Fuel 999 3.46 2,319 98.40 23.57 1.1 0.3 21.7 0.6 76.1 0.2 100

9 Air feed 25 1.00 7,484 259.42 28.85 79.0 21.0 100

10 Compressed air 175 3.47 7,484 259.42 28.85 79.0 21.0 100

11 Heated air 725 3.40 7,484 259.42 28.85 79.0 21.0 100

12 Spent air 999 3.33 6,149 217.69 28.25 94.1 5.9 100

13 FC exhaust 1119 3.33 8,471 315.78 26.83 7.2 24.7 65.0 3.2 100

14 Cooled exhaust 1119 3.33 8,471 315.78 26.83 7.2 24.7 65.0 3.2 100

15 Expanded exhaust 856 1.04 8,471 315.78 26.83 7.2 24.7 65.0 3.2 100

16 Cooled exhaust 328 1.02 6,438 239.99 26.83 7.2 24.7 65.0 3.2 100

17 Cooled exhaust 333 1.02 2,033 75.79 26.83 7.2 24.7 65.0 3.2 100

18 Combined exhaust 329 1.02 8,471 315.78 26.83 7.2 24.7 65.0 3.2 100

19 Cooled exhaust 152 1.01 8,471 315.78 26.83 7.2 24.7 65.0 3.2 100

20 Cycle exhaust 147 1.00 8,471 315.78 26.83 7.2 24.7 65.0 3.2 100

Reference Source: (20). 

The natural gas feed to the cycle (stream 1) is typical of pipeline quality natural gas within the 

U.S. containing both sulfur odorants and higher hydrocarbons (C2H6, C3H8, etc.). The odorants 

must be removed before entrance into the fuel cell to prevent performance and cell life 
8-64 

deterioration. Higher hydrocarbons are assumed to be pre-reformed to hydrogen and carbon 

monoxide in a mild reformer51 to avoid "sooting" or carbon deposition within the fuel cell. 

Because both the desulfurization and reforming require elevated temperatures, the fuel is fed 

through a series of heat exchangers that recover heat from the fuel cell exhaust stream 

(streams 13 to 20). Humidification steam (stream 3) is added to the fuel to provide the required 

moisture for the reforming and water-gas shift reactions. The heated and humidified fuel is 

desulfurized in a sorbent bed and partially reformed in a mild reformer catalyst bed. The balance 

of the reforming will occur between the stages of the multi-stage fuel cell module. The hot 

desulfurized and partially reformed fuel stream (stream 7) enters the fuel cell anode at 

approximately 500 oC (930 oF). 

Ambient air (stream 9) is compressed to 3.5 atmospheres and 175 oC (347 oF) (stream 10), and 

subsequently heated to 500 oC (932 oF) prior to entering the fuel cell cathode (stream 11). 

The hot processed fuel and the compressed ambient air are electrochemically combined within 

the fuel cell module. The fuel hydrocarbons still remaining after the mild reformer are reformed 

within the fuel cell. The heat required for the endothermic steam reforming reactions is supplied 

by the exothermic fuel cell reactions. The overall reactions are exothermic, and the fuel and 

oxidant temperatures rise to 999 oC (1830 oF) (streams 8 and 12). The fuel cell is capable of 

utilizing both H2 and CO as fuel and has an overall fuel utilization of 94 percent. 

The spent fuel (stream 8) and oxidant (stream 12) are combusted upon exiting the multi-stage 

fuel cell module. The resulting exhaust stream (stream 13) has a temperature of 1119 oC (2046 

oF) before being cooled in a fuel heater and expanded to 1.04 atmospheres and 856 oC (1573 oF) 

(stream 15). This nearly atmospheric exhaust stream passes through several additional heat 

exchangers before leaving the plant stack at 147 oC (300 oF). 

Operating parameters are summarized in Table 8-19. Cycle performance is summarized in 

Table 8-20. The overall net electric LHV efficiency is 80.1 percent. 

One advantage of this concept is the elimination of heat exchangers between fuel cell modules. 

This will minimize the cycle complexity, cost, and losses. Another advantage of the concept is 

the minimization of unreacted fuel leaving the fuel cell. By having discrete fuel cell stages, each 

operating with its own voltage and current density, fuel utilization can be pushed to very high 

levels without hurting the performance of the entire module. The voltage and performance 

degradation resulting from the low fuel concentrations (high utilization) is isolated to the latter 

fuel cell stage(s) whereas a single fuel cell module, the entire fuel cell performance is degraded. 

Experiencing a reduced voltage, power, and efficiency level in the latter stages of a multi-stage 

module is acceptable because it minimizes the heat released in the combustion stage, which is 

largely passed to the bottoming cycle, which typically has an efficiency of roughly 40 percent. 

That is, 60 percent of the heat liberated to the bottoming cycle is wasted. Thus, the minimization 

 

51. A "mild reformer" is assumedto eliminate of the higher hydrocarbons prior to entering the fuel cell to prevent 

sooting. This reformer is called a "mild reformer" to indicate that the reforming reactions are not pushed to 

completion, for it is desired that the methane be reformed in the fuel cell for better temperature management. 

Some of the methane, however, will be reformed with the higher hydrocarbons in the mild reformer. 
8-65 

of heat passed to the bottom cycle is desirable, even at the "cost" of reduced efficiency in a 

fraction of the fuel cell module. 

One obstacle for this concept is the uncertainty of fuel cell performance in a high utilization 

multi-stage concept. No testing has been performed to date utilizing a fuel cell in this manner. 

The exact loss of performance in the latter stages is not known. The reference document (21) for 

this multi-stage fuel cell concept did not attempt to specify the number of stages nor the fuel cell 

performance within each stage. Instead, an average fuel cell performance was assumed. This 

assumption may or may not represent of how a multi-stage fuel cell will perform. Additional 

development work of this novel and efficient concept is required. 

Table 8-19 Operating/Design Parameters for the NG fueled Multi-Stage Fuel Cell System 

Operating Parameters Value 

Volts per Cell (V) 0.800 

Current Density (mA/cm2) unspecified 

Number of Stages to be determined 

Cell Operating Temperature (oC) multiple temps 

(~650 to 850 oC) 

Cell Outlet Pressure (atm) 3.3 

Overall Fuel Utilization (percent) 94.0 percent 

Overall Oxidant Utilization (percent) 81.5 percent 

Steam to Carbon Ratio 1.5:1 

DC to AC Inverter efficiency 97.0 percent 

Generator efficiency 98.0 percent 

Fuel Cell Heat Loss (percent of MWdc) 1.7 percent 

Auxiliary Load 1.0 percent 

Table 8-20 Overall Performance Summary for the NG fueled Multi-StageFuel Cell System 

Performance Parameters Value 

LHV Thermal Input (MW) 4.950 

Gross Fuel Cell Power (MW) 

 Fuel Cell DC Power 

 Inverter Loss

Fuel Cell AC Power 

3.579 

(0.108)

3.471 

Gross AC Power (MW) 

 Fuel Cell AC Power 

 Net Compressor/Expander

Gross AC Power 

3.471 

0.534

4.005 

Auxiliary Power 0.040 

Net Power 3.965 

Electrical Efficiency (percent LHV) 80.10 percent 

Electrical Efficiency (percent HHV) 72.29 percent 

Heat Rate (Btu/kWh, LHV) 4,260 
8-66 

8.4.6 Coal Fueled SOFC System 

The coal fueled solid oxide fuel cell power system presented here is based on work performed 

for the Department of Energy’s program (22) to develop high efficiency, low emission, fuel 

flexible (including coal) processes. This cycle is a coal-fueled version of the Siemens 

Westinghouse TSOFC cycle presented in Section 8.4.4 consists of a Destec gasifier, cascaded 

SOFCs at two pressure levels, an integrated reheat gas turbine, and a reheat steam turbine 

bottoming cycle. The high-pressure portion of the cycle is designed to operate at 15 atmospheres 

to capitalize on a reasonable gas turbine expansion ratio and an advanced, but not unrealistic, 

fuel cell pressure. An operating pressure of 30 atmospheres would yield better fuel cell and gas 

turbine performance, but has been conservatively limited to 15 atmospheres; this is lower than 

the typical Destec design pressure. Higher pressure operation is feasible and would have better 

performance. The coal analysis is presented in Table 8-22. 

A flow diagram for the coal fueled 500 MW class cascaded TSOFC power cycle is presented in 

Figure 8-31. A brief process description is given below, followed by a performance summary. 

Selected state point values are presented in Table 8-23. 

Water

DESTEC

Gasifier

ASU

Fuel-Gas

Cooler

Transport-Bed

Desulfurization

Raw Fuel Gas

Air

Compressor

Turbine

SOFC

SOFC

Turbine

Power

Turbine

Generator

Recuperator

HRSG

Expander

Zinc Oxide 

Polisher

Reheat Steam

Turbine Bottoming

Cycle

Anode

Cathode

Anode

Cathode

Coal/Water

Slurry

Slag

To Asu

To Gasifier

Steam

IP Clean Fuel Gas 1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

17

18

19

20

21

22 23

24

25

26

Exhaust

Figure 8-31 Schematic for a 500 MW Class Coal Fueled Pressurized SOFC 
8-67 

Table 8-21 Stream Properties for the 500 MW Class Coal Gas Fueled Cascaded SOFC

Strm Description Temp Press Mass Flow Mole Flow CH4 CO CO2 H2 H20 H2S N2+Ar NH3 O2 Total

No. C atm t/h kgmol/hr MW % % % % % % % % % %

1 Coal Slurry Feed 18 23.8 151.2 - NA

2 ASU Oxygen 179 23.8 83.3 2,583 32.23 5.0 95.0 100.0

3 Slag Waste 93 19.1 11.6 - NA

4 Gasifier Effluent 1043 18.6 237.6 12,280 19.35 0.3 42.3 9.5 35.8 9.6 0.7 1.5 0.2 100.0

5 Raw Fuel Gas 593 17.6 237.6 12,280 19.35 0.3 42.3 9.5 35.8 9.6 0.7 1.5 0.2 100.0

6 Desulfurized Gas 593 16.6 236.2 12,280 19.23 0.3 42.3 9.6 35.8 10.3 trace 1.5 0.2 100.0

Recycle to Gasifier 399 15.0 9.4 491 19.23 0.3 42.3 9.6 35.8 10.3 trace 1.5 0.2 100.0

7 Polished Gas 399 15.0 226.7 11,789 19.23 0.3 42.3 9.6 35.8 10.3 trace 1.5 0.2 100.0

8 HP Fuel Gas 399 15.0 108.8 5,659 19.23 0.3 42.3 9.6 35.8 10.3 trace 1.5 0.2 100.0

9 IP Fuel Gas 221 3.7 117.9 6,130 19.23 0.3 42.3 9.6 35.8 10.3 trace 1.5 0.2 100.0

10 Ambient Air 17 0.98 1,270.1 44,024 28.85 trace 1.1 78.1 20.8 100.0

11 Compressed Air 409 15.1 1,146.2 39,732 28.85 trace 1.1 78.1 20.8 100.0

12 Heated Air 579 15.0 1,146.2 39,732 28.85 trace 1.1 78.1 20.8 100.0

13 HP SOFC Exhaust 979 14.7 1,255.1 43,181 29.07 6.9 7.1 trace 72.1 trace 13.9 100.0

14 HPT Exhaust 645 3.6 1,296.3 44,609 29.06 6.6 6.9 trace 72.3 trace 14.1 100.0

15 IP SOFC Exhaust 982 3.3 1,414.2 48,346 29.25 12.7 12.3 trace 66.9 0.1 8.0 100.0

16 IPT Exhaust 691 1.01 1,477.7 50,547 29.23 12.2 11.8 trace 67.4 0.1 8.6 100.0

17 Cooled Exhaust 573 0.99 1,477.7 50,547 29.23 12.2 11.8 trace 67.4 0.1 8.6 100.0

18 Cycle Exhaust 126 0.98 1,477.7 50,540 29.24 12.2 11.8 67.5 8.6 100.0

19 Gas Cooler Water 306 107.4 244.6 13,580 18.02 100.0 100.0

20 Gas Cooler Steam 317 107.4 244.6 13,580 18.02 100.0 100.0

21 HP Steam 538 99.6 301.4 16,730 18.02 100.0 100.0

22 Cold Reheat 359 29.3 298.4 16,563 18.02 100.0 100.0

23 Hot Reheat 538 26.4 298.4 16,563 18.02 100.0 100.0

24 ASU Steam 538 26.4 3.9 218 18.02 100.0 100.0

25 LP Steam 310 6.1 15.6 865 18.02 100.0 100.0

26 Gasifier Steam 307 5.4 32.0 1,774 18.02 100.0 100.0

Reference Source: (30) 

The Destec entrained bed gasifier is fed both coal water slurry (stream 1) and a 95 percent pure 

oxygen stream (stream 2) and operates with a cold gas conversion efficiency52 of 84 percent. 

The gasifier fuel gas product (stream 4) is cooled in a radiant heater, which supplies heat to the 

bottoming cycle. The cooled fuel gas is cleaned (stream 6) in a hot gas desulfurizer at 593 oC 

(1100 oF) and a polisher (stream 7) at 399 oC (750 oF) to less than 1 ppmv of sulfur prior to 

entering the high-pressure fuel cell (stream 8). Part of the polished fuel is expanded to 3.7 

atmospheres and 220 oC (429 oF) before being sent to the low-pressure fuel cell (stream 9). 

Ambient air (stream 10) is compressed to 15.1 atmospheres and 409 oC (275 oF) (stream 11), and 

subsequently heated to 579 oC (1075 oF) prior to entering the high-pressure fuel cell cathode 

(stream 12). 

The hot clean fuel gas and the compressed ambient air are electrochemically combined within 

the high-pressure fuel cell with fuel and oxidant utilizations of 90 percent and 24.5 percent, 

respectively. The SOFC module is set (sized) to operate at 0.69 volts per cell.53 The spent fuel 

and air effluents of the SOFC are combusted within the module to supply heat for oxidant 

preheating. Unlike the natural gas case, the fuel does not require a pre-reformer with only 0.3 

percent methane along with 36 percent hydrogen and 43 percent carbon monoxide. The carbon 

monoxide will be either water gas shifted to hydrogen or utilized directly within the fuel cell. A 

 

52. Cold gas conversion efficiency is the ratio of the gasifier fuel gas total heating value [i.e., (heating value)(mass

 flow)] to that of the coal feed, [(heating value)(mass flow)]. 

53. Siemens Westinghouse provided TSOFC performance values for the HP and LP conditions, which Parsons

 incorporated into the systems analysis. 
8-68 

gas recirculation loop for the fuel cell has not been assumed, for water is not required for prereforming nor internal reforming. 

The combusted air and fuel stream (stream 13) from the high-pressure fuel cell is expanded 

(stream 14) in a turbine expander. The work of this turbine is used to drive the low- and highpressure air compressors. The reduced pressure exhaust stream (stream 14) is utilized as the 

low-pressure fuel cell oxidant stream. Although vitiated, it still has 14 percent oxygen. The 

low-pressure SOFC operates at 0.69 volts per cell and fuel and air utilizations of 90 and 34.7 

percent, respectively (23). The spent air and fuel effluents are combusted and sent (stream 15) to 

the low-pressure power turbine. The turbine generator produces approximately 134 MWe. The 

low-pressure exhaust (stream 16) has a temperature of 691 oC (1276 oF) and is utilized to preheat 

the high-pressure oxidant. The resulting cooled exhaust stream (stream 17) still has a 

temperature of 573 oC (1063 oF) and is utilized to supply heat to a steam bottoming cycle. 

Steam generated in the bottoming cycle is utilized in a reheat turbine to produce 118 MWe, as 

well as to supply the steam required by the air separation unit (ASU) and the gasifier coal slurry 

heater. The cycle exhaust exits the heat recovery steam generator at 126 oC (259 oF) and 0.98 

atmospheres. 

Operating parameters are summarized in Table 8-23. Cycle performance is summarized in 

Table 8-24. The overall cycle net HHV efficiency is 59 percent. 

Table 8-22 Coal Analysis 

Coal Parameters Value 

Source Illinois No. 6 

Ultimate Analysis, (wt percent, 

a.r.) 

 Moisture 

 Carbon 

 Hydrogen 

 Nitrogen 

 Chlorine 

 Sulfur 

 Ash 

 Oxygen (by difference)

Total 

11.12 

63.75 

4.50 

1.25 

0.29 

2.51 

9.70 

6.88

100.00 

HHV (Btu/lb) 

LHV (Btu/lb) 

11,666 

11,129 
8-69 

Table 8-23 Operating/Design Parameters for the Coal Fueled Pressurized SOFC 

Operating Parameters HP FC LP FC 

Volts per Cell (V) 0.69 0.69 

Current Density (mA/cm2) 312 200 

Cell Operating Temp. (oF) 1794 1800 

Cell Outlet Pressure (atm) 14.7 3.3 

Overall Fuel Utilization 

(percent) 

90 90 

Overall Oxidant Utilization 

(percent) 

18.7 20.4 

DC to AC Inverter Efficiency 97.0 percent 

Generator Effic. - ST, GT 98.5 percent 

Generator Effic. - Expander 98.0 percent 

Auxiliary Load 7.2 percent 

Table 8-24 Overall Performance Summary for the Coal Fueled Pressurized SOFC 

Performance Parameters Value 

LHV Thermal Input (MW) 875.8 

Gross Fuel Cell Power (MW) 

 Fuel Cell DC Power 

 Inverter Loss

Fuel Cell AC Power 

310.9 

 (9.3)

301.6 

Gross AC Power (MW) 

 Fuel Cell AC Power 

 Combustion Turbine 

 Steam Turbine 

 Fuel Expander

Gross AC Power 

301.6 

133.7 

118.1 

 9.6

562.9 

Auxiliary Power 40.3 

Net Power 522.6 

Electrical Efficiency ( percent 

HHV) 

59.7 

Electrical Efficiency (percent 

LHV) 

62.6 

Heat Rate (Btu/kWh, HHV) 5,720 

This configuration has the potential to yield a very competitive cost of electricity. For example, 

for a fuel cell stack cost of $300 to $400/kW, it is estimated that the COE would range from 3.5 

to 3.9 cents/kWh (Assuming 20 percent equity at 16.5 percent, 80 percent debt at 6.3 percent, 

and a levelized carrying charge of 0.12.) 
8-70 

8.4.7 Power Generation by Combined Fuel Cell and Gas Turbine System 

In general, the oxidation of H2, CO, CH4, and higher hydrocarbons in fuel cells to produce power 

also produces reject heat. This heat arises from two sources: 

• the entropy decrease, ∆S, resulting from the overall oxidation reaction -- accompanying the 

usual decrease in the number of mols of gas, from reactants to products; and 

• the loss in work, or a conversion of "reversible" work from the oxidation process to heat, due 

to irreversible processes occurring in the operation of the cell. 

Heat from these two sources must be rejected from the fuel cell in order to maintain its 

temperature at a desired level. The heat can be removed and recovered by transferring it across a 

bounding surface to a heat transfer fluid, but care must be taken to maintain the cell at its desired 

temperature in this and adjacent regions. Alternatively, heat can be removed in one of the 

reactant streams passing through the cell -- most practically the air, oxidant stream. 

Also in the operation of a practical fuel cell, some unburned fuel must remain in the combustion 

products leaving the cell in order to maintain a significant generated voltage throughout the cell. 

In order to obtain the highest possible efficiency in electrical generation, both the thermal energy 

in the heat and the unburned fuel rejected from the cell must be recovered and converted into 

additional electrical energy. This can be accomplished by means of a heat engine cycle making 

use of a gas turbine operating in a regenerative Brayton or combined Brayton-Rankine cycle or a 

steam turbine operating in a Rankine cycle. The relative merits of these three heat engine cycles 

depend on their overall efficiencies and on the practical aspects of integration, operation, and 

cost of the power generation plant as a whole. 

8.4.8 Heat and Fuel Recovery Cycles 

Simple representations of three fuel cell based heat and fuel recovery cycles are shown in 

Figures 8-32, 8-33, and 8-36. 

Regenerative Brayton Cycle: The regenerative Brayton cycle, Figure 8-32, shows a gas turbine 

compressor for the air flow to the cell. The flow then passes through a countercurrent, 

recuperative heat exchanger to recover heat from the combustion product gases leaving the gas 

turbine. The air and the fuel streams then pass into the cathode and anode compartments of the 

fuel cell(s). The air and fuel streams leaving the cell(s) enter the combustor where they mix and 

the residual fuel burns. The combustion products enter the turbine, expand, and generate 

additional power. The turbine exhaust gases pass through the recuperative exchanger to the 

stack. 

The most significant variables characterizing the cycle are the fuel cell operating temperature 

range and the temperature and pressure at the gas turbine expander inlet. These variables are 

directly related to certain operating variables: the air/fuel ratio entering the fuel cell, the fraction 

of the fuel leaving the cell unburned, and the temperature difference between the combustion 

products and air at the high temperature end of the recuperative heat exchanger. The operating 

variables must be selected and controlled to allow effective operation of the fuel cell, combustor, 

and gas turbine. There may well be an optimal quantity of unburned fuel leaving the fuel cell, 

depending on the acceptable fuel cell operating temperature range and turbine inlet temperature. 
8-71 

Further insight can be gained from the idealized T - S diagram for the cycle, Figure 8-32. The 

compression of the air and fuel streams is represented here as a single adiabatic reversible 

(constant S) process in which the temperature of the gases rises above ambient. The heating of 

 Figure 8-32 Regenerative Brayton Cycle Fuel Cell Power System 

the air and also the fuel streams first in the recuperative exchanger, then in the fuel cell and 

finally in the combustor is assumed to occur along a single line of constant pressure. The 

subsequent expansion of the combustion gases in the turbine is also represented as an adiabatic 

reversible (constant S) process in which the temperature of the gases drops to a value close to 

that of the gases entering the fuel cell. The pressure ratio (PR) of the turbine (and of the 

compressor) is therefore established by the turbine nozzle inlet temperature (NIT) and the fuel 

cell operating temperature. In general, the pressure ratio of a regenerative Brayton cycle is low 

compared with that of a combined Brayton-Rankine cycle. A low pressure ratio allows a low 

outlet temperature of the exhaust gases from the recuperative exchanger as heat is transferred to 

the air leaving the compressor (and possibly also the fuel) and consequently results in low heat 

rejection and a high cycle efficiency. 

The practical aspects of the cycle involve the efficiencies of the gas compressors, the turbine 

expander, and the fuel cell; the pressure losses as the gases flow through the system; and the 

temperature differences and the difference in heat capacities of the streams flowing through the 

recuperative heat exchanger. Other aspects of the fuel cell operation must be considered in 

greater detail for the design and evaluation of the power system. These include the possible need 

for fuel reforming external to the cell and the recycle of combustion product streams to provide 

the steam required to carry out the reforming process, to avoid carbon deposition, and to provide 

H2 for effective cell operation. 
8-72 

Table 8-25 Performance Calculations for a Pressurized, High Temperature Fuel Cell 

(SOFC) with a Regenerative Brayton Bottoming Cycle; Approach Delta T=30 o

F 

The performance of a solid electrolyte fuel cell (SOFC) system (Hirschenhofer et al., 1994) 

operating with a regenerative Brayton bottoming cycle for heat and fuel recovery has been 

calculated. Table 8-25 illustrates the results. The work from the fuel cell burning CH4 is 

assumed to be 60 percent the theoretical maximum; the corresponding fuel cell voltage is 0.63 

volts. The efficiencies of the fuel and air compressors are 83 percent; and the expander of the 

turbine, 89 percent. It is assumed that the cell makes direct use of CH4 fuel, or that oxidation and 

reforming are coincident; operation of the cell thus provides both the heat and the H2O required 

for CH4 reforming. Pressure losses in the fuel cell, combustor, recuperative exchanger, and the 

ducts of the system are ignored. 

The results of the performance calculations are summarized in Table 8-26. The efficiency of the 

overall power system, work output divided by the lower heating value (LHV) of the CH4 fuel, is 

increased from 57 percent for the fuel cell alone to 82 percent for the overall system with a 30 o

F 

difference in the recuperative exchanger and to 76 percent for an 80 o

F difference. This 

regenerative Brayton cycle heat rejection and heat-fuel recovery arrangement is perhaps the 

simplest approach to heat recovery. It makes minimal demands on fuel cell heat removal and gas 

turbine arrangements, has minimal number of system components, and makes the most of the 

inherent high efficiency of the fuel cell. 

COM PRESSOR EFF = 0.83 n = num ber of m oles

TU RB EX PAN DER EFF = 0.89 Cp = specific heat

FUEL CELL EFF= 56.9 Hf = heat of form ation at standard conditions

CY CLE EFF= 82.1 So = entropy at standard conditions

 STREAM # 1 2 3 4 5 6 7 Cycle

 p, PRESSURE, atm 1 1.48 1.48 1.48 1.48 1 1

 T, TEM PERATURE, K 298 337 1200 1311 1332 1216 352

 CH4, n 1 1 1 0.07 0 0 0

 CO , n

 H 2, n

 CO2, n 0 0 0 0.93 1 1 1

 H2O, n 0 0 0 1.86 2 2 2

 O 2, n 16.23 16.23 16.23 14.37 14.23 14.23 14.23

 N 2, n 64.92 64.92 64.92 64.92 64.92 64.92 64.92

 SUM (n) 82.15 82.15 82.15 82.15 82.15 82.15 82.15

 SU M (nCp) 629.72 629.72 629.72 628.97 628.92 628.92 628.92

 SU M (nH f) -17.9 -17.9 -17.9 -196.181 -209.6 -209.6 -209.6

 SU M (nSo) 3813.11 3813.11 3813.11 3811.99 3811.91 3811.91 3811.91

 GAM M A 1.350 1.351

 Q , H EAT, kcal/m olCH 4 0.0 543.5 0.0 -0.2 0.0 543.5 1086.8

 W , W ORK , kcal/m olCH 4 -24.4 0.0 109.1 0.0 72.7 0.0 157.4
8-73 

Table 8-26 Performance Computations for Various High Temperature Fuel Cell (SOFC) 

Heat Recovery Arrangements 

Combined Brayton-Rankine Cycle: The combined Brayton-Rankine cycle, Figure 8-33, again 

shows the gas turbine compressor for the air flow to the cell. This flow passes through a heat 

exchanger in direct contact with the cell; it removes the heat produced in cell operation and 

maintains cell operation at constant temperature. The air and fuel streams then pass into the 

cathode and anode compartments of the fuel cell. The separate streams leaving the cell enter the 

combustor and then the gas turbine. The turbine exhaust flows to the heat recovery steam 

generator and then to the stack. The steam produced drives the steam turbine. It is then 

condensed and pumped back to the steam generator. 

General Conditions Notes

SOFC, solid oxide fuel cell PR = pressure ratio of the gas turbine

Operating temperature, 1700-1900 F NIT = nozzle inlet temperature of the turbine expander

Fuel cell output: 60% of theoretical maximum from CH4 fuel

Gas turbine compressor, expander efficiences: 83, 89%

Steam turbine efficiency: 90%

Work Output, % Overall

Heat Recovery Fuel Gas Steam System

Arrangement Cell Turbine Turbine Eff., % Remarks

 Regenerative Brayton Cycle 69.3 30.7 n/a 82.1 30 F Approach in Recuperative Exchanger

Gas Turbine PR=1.48, NIT=1938 F

 Regenerative Brayton Cycle 74.5 25.5 76.3 80 F Approach in Recuperative Exchanger

Gas Turbine PR=1.35, NIT=1938 F

 Combined Brayton-Rankine Cycle 75.3 10.3 14.3 75.6 Gas Turbine PR=12, NIT=2300 F

Steam Turbine: 1600 psia, 1000 F, 1.5" Hg

 Rankine Cycle 79.1 20.9 72.4 Steam Turbine: 1600 psia, 1000 F, 1.5" Hg
8-74 

Figure 8-33 Combined Brayton-Rankine Cycle Fuel Cell Power Generation System 

The air/fuel ratio entering the fuel cell and the fraction of the CH4 fuel consumed in the cell are 

selected to achieve the desired fuel cell operating temperature range and gas turbine NIT and PR. 

These are selected to correspond with those of a conventional, large-scale, utility gas turbine. 

Further insight can be gained from an idealized T- S diagram for the cycle, Figure 8-34, in which 

both the Brayton and the Rankine cycles are illustrated. Both the pressure and the temperature 

increase during fuel and air compression in this combined cycle will be significantly greater than 

in the regenerative Brayton cycle described above. The heating of the air and fuel, the operation 

of the fuel cell, and the burning of the residual fuel are assumed to occur at constant pressure. 

The expansion of the combustion product gases in the gas turbine again is represented as an 

adiabatic, reversible (constant S) process. Next, heat is removed from these gases at nearly 

constant pressure in the heat recovery steam generator; and they pass out through the stack. 
8-75 

Figure 8-34 Combined Brayton-Rankine Cycle Thermodynamics 

The Rankine cycle diagram placed adjacent the Brayton cycle in Figure 8-34 is indicated as a 

simple steam cycle with superheat, but no reheat and no multi-pressure steam generation. The 

thermodynamic advantage of the Rankine bottoming cycle is the lowered temperature of heat 

rejection, in the steam condenser, from the overall combined cycles. 

The performance of a SOFC system with a Brayton-Rankine bottoming cycle for heat and fuel 

recovery has been calculated. Gas turbine compressor and expander efficiencies of 83 percent 

and 89 percent and a steam turbine efficiency of 90 percent have been assumed. 

The significant operating conditions of the gas and steam turbines and the results of the 

computations are summarized in Table 8-26. The principal result is that the efficiency of the 

overall system, work output divided by the CH4 LHV, is increased from 57 percent for the fuel 

cell alone to 75 percent for the overall system. This combined Brayton-Rankine cycle heat-fuel 

recovery arrangement is significantly more complex and less efficient than the simple 

regenerative Brayton cycle approach. It does, however, eliminate the requirement for a large, 

high temperature gas to gas heat exchanger. 

The key link between the Brayton and the Rankine cycles is the heat recovery steam generator 

whose operation is illustrated by the temperature-heat (T-Q) plot in Figure 8-35. The 

temperatures of the gases and of the water, T, are plotted as a function of the heat, Q, transferred 

from the combustion product gases to the water-steam between their entrance and any point in 

the steam generator. The area between the temperature curves for the two flowing streams is an 

indication of the irreversibility, or loss in available work, resulting from the transfer of heat over 

a finite temperature difference. Reducing this area, moving the gas and steam curves closer, 

requires increased heat transfer surface area in the steam generator. Steam reheat and multipressure level heat recovery boilers are frequently proposed to minimize the loss in available 

work. 
8-76 

Figure 8-35 T-Q Plot for Heat Recovery Steam Generator 

(Brayton-Rankine) 

Rankine Cycle: The fuel cell Rankine cycle arrangement in Figure 8-36 employs a heat 

recovery steam generator operating on the exhaust combustion product stream from the fuel cell 

and combustor at atmospheric pressure. This exhaust stream first provides the heat required to 

preheat and reform the CH4 fuel, providing CO and H2 at temperature to the fuel cell. Partially 

combusted fuel from the cell is recycled to provide the H2O required for reforming the fuel. 

Depleted air from the cell exhaust is recycled to the air feed stream to raise its temperature to the 

desired value at the cell inlet. The operating conditions and the T - S diagram for the Rankine 

cycle are identical to those illustrated for the combined Brayton-Rankine cycle in Figure 8-34 

and Table 8-26. 

The results of the performance calculations for the fuel cell, Rankine cycle heat recovery system, 

summarized in Table 8-26, indicate that the efficiency of the overall system is increased from 57 

percent for the fuel cell alone to 72 percent for the overall system. This Rankine cycle heat-fuel 

recovery arrangement is less complex but less efficient than the combined Brayton-Rankine 

cycle approach, and more complex and less efficient than the regenerative Brayton approach. It 

does, however, eliminate the requirement for a large, high temperature gas to gas heat exchanger. 

And in applications where cogeneration and the supply of heat are desired, it provides a source 

of steam. 

The T - Q plot for the heat transfer processes involved in this fuel cell Rankine cycle 

arrangement is shown in Figure 8-37. Because heat is removed from the exhaust gases to heat 

and reform the CH4 fuel feed, the temperature of the hot gas entering the heat recovery steam 

generator in this 

-100.0

100.0

300.0

500.0

700.0

900.0

1100.0

1300.0

0123456

Q, Heat Transferred to Steam from Hot Gas, kcal

T, Temperature of Streams, F

water/steam in HRSG
8-77 

Figure 8-36 Fuel Cell Rankine Cycle Arrangement 
8-78 

Figure 8-37 T-Q Plot of Heat Recovery from Hot Exhaust Gas 

particular Rankine cycle fuel cell arrangement is significantly lower than in the previous 

combined Brayton-Rankine cycle arrangement. Increased surface area is, therefore, required in 

the heat recovery steam generator for this fuel cell Rankine cycle arrangement. 

These three approaches to reject heat and exhaust fuel recovery with power generation apply 

primarily to the higher temperature, solid oxide (1800 o

F) and molten carbonate (1200 oF), fuel 

cell systems operating on CH4 fuel. The lower operating temperatures of the phosphoric acid 

(400 o

F) and polymer electrolyte (175 oF) fuel cells severely limit the effectiveness of thermal 

cycle based power generation as a practical means of heat recovery. 

All three of the heat recovery arrangements have calculated overall efficiencies greater that 70 

percent as indicated in Table 8-26. None have been optimized in any sense -- in terms of 

efficiency, capital and operating costs, maintainability or availability. Each of the arrangements 

has its advantages and disadvantages. It appears, however, that the regenerative Brayton cycle 

has the advantage of greatest simplicity and highest potential overall efficiency over the 

combined Brayton-Rankine and Rankine cycle approaches. 

The consideration of heat recovery and use in such fuel cell systems requires some consideration 

of heat generation and transfer within the cells of the system. Direct oxidation of CH4 at the 

anode of the cell, if possible, would implement the overall process: 

CH4 + 2O2 = CO2 + 2H2O (v) 

This reaction, having equal number of mols of gas reactants and products, has a negligible 

change in entropy and thus a negligible heat effect if carried out reversibly at constant 

temperature. The maximum work available from a fuel cell under these circumstances would 

then be approximately the enthalpy change of the reaction, i.e., the heat of combustion of the 

0

200

400

600

800

1000

1200

1400

1600

1800

T, Temperature of Streams, K

gas leaving combustor

CH4 fuel gas feed

boiler water-steam
8-79 

CH4; the efficiency of the fuel cell power generation process could, therefore, approach 100 

percent. However, work is lost and a corresponding quantity of heat is produced by 

irreversibilities both in fuel cell operation -- 

• the electrical resistance of the electrolyte to ion flow and of the electrodes, current collectors, 

and leads to electron flow; 

• the kinetics of the processes involving reactants, ions, and electrons at the anode and cathode 

of the cell; 

• the transport, or diffusion, of reactants within the anode and cathode chambers to the 

electrode; 

• and also in overall system operation – 

• the preheating of the air and fuel streams; 

• the pretreating, or reforming, of the CH4 fuel to provide more reactive H2 and to prevent the 

deposition of carbon (C). 

The heat resulting from these irreversibilities must then be removed in order to maintain the fuel 

cells at a desired operating temperature. Irreversibilities and the resulting quantity of heat 

produced can be reduced, in general, by increasing the active area of the fuel cells, heat 

exchangers, and fuel reformer; but increased equipment costs result. 

In general, reforming of the CH4 fuel with excess H2O outside the cell has been practiced both in 

molten carbonate and solid oxide fuel cell systems in order to produce H2, more reactive on a 

fuel cell anode, and to avoid the possible deposition of C. This reforming reaction 

CH4 + H2O = CO + 3H2

is associated with an increase in entropy and absorbs heat. Excess H2O produces additional H2

and reduces the CO content of the reformed gases, which may adversely affect anode reactions, 

by the shift reaction 

H2O + CO = H2 + CO2. 

This reaction is thermally neutral. The heat absorbed in the CH4 reforming reaction is released 

by the subsequent reaction of the H2 product at the anode of the fuel cell. If, therefore, the 

reforming process can be carried out in close proximity to and in thermal contact with the anode 

process, the thermal neutrality of the overall CH4 oxidation process can be approximated. And 

the heat removal and recovery process for the fuel cell system can deal merely with the heat 

produced by its operational irreversibilities. 

Heat removal from fuel cells, and cell batteries, can be accomplished: 

• directly through the flow of reactants to and products from them. 

• indirectly through heat transfer surfaces in contact with the cell or included within a battery. 

A specific fuel cell system is viewed here as having a fixed range of operating temperature 

between a maximum and minimum; heat must therefore be removed in such a manner to 

maintain the temperature within these limiting values. If heat is removed directly by reactant 

flows, then the quantity of flow must be adjusted so that inlet and outlet temperatures (as well as 
8-80 

the intermediate temperatures) of the cell and of the flow streams are within the permissible 

range. Practically, the air stream is adjusted to achieve this result, since the purpose of the fuel 

cell is to consume the fuel in the production of electrical energy. Increasing the fuel flow to 

remove heat from the cell increases the quantity of unburned fuel in the exhaust from the cell. If 

heat is removed from the fuel cell indirectly through adjacent or embedded surface, then the flow 

and temperature of the coolant stream can be selected somewhat independent of the cell 

operating temperature. But the distribution of heat transfer surface in the cell (or battery) and the 

rate of heat transfer across that surface must be carefully adjusted and controlled to maintain the 

temperature throughout the cell (or battery) within the prescribed temperature range. 

The regenerative Brayton cycle, as presented, depends primarily on its fuel cell component for 

conversion of the fuel and thus for its overall efficiency. The gas turbine merely provides the 

means for recovery of the waste heat and residual fuel in the combustion product stream. The 

gas turbine operates, therefore, at a temperature only slightly elevated above that of the cell by 

the combustion of the residual fuel. The pressure ratio selected for the turbine in this 

regenerative cycle is determined by the ratio of the temperature of the gases leaving the auxiliary 

combustor to the temperature of the reactant gases entering the fuel cell. In general, for either 

molten carbonate or solid oxide cells, this selected pressure ratio will be less than two. The 

proposed method of cell cooling is air flow, which will be increased significantly, by a factor of 

4-8 above that required for oxidation of the fuel. The feasibility of this cycle will depend on the 

availability of air compressor and turbine expander units with: 

• the pressure ratio and temperature capability compatible with the fuel cell operation. 

• a capacity appropriate to market applications. 

The effectiveness of the regenerative Brayton cycle performance will depend on the efficiency of 

the fuel cell, compressor, and turbine units; the pressure loss of gases flowing through the 

system; the approach temperatures reached in the recuperative exchanger; and, most importantly, 

the cost of the overall system. 

The combined Brayton-Rankine cycle depends on both the fuel cell and the gas turbine 

components for conversion of the fuel and thus for its overall efficiency. The extent of 

conversion of the fuel occurring in the fuel cell increases as the cell operating temperature and 

the range of coolant temperature rise increase. For this reason, the cycle as presented is based on 

indirect heat removal from the cell, heating the air stream temperature from the compressor 

outlet to the cell operating temperature. This provision maximizes the cell contribution to the 

energy output of the combined cycle. The PR and NIT of the turbine are those selected to match 

those of the current utility scale equipment -- a PR of 12 and an NIT of 2300 o

F -- resulting in a 

combined cycle efficiency of perhaps 45 to 50 percent, not considering the electrical energy 

output of and the fuel input to the fuel cell. The fuel combustion occurring in the combustor and 

overall air/fuel ratio is then determined by the combination of the cell and the turbine inlet 

temperatures. 

The fuel cell Rankine cycle arrangement has been selected so that all fuel preheating and 

reforming are carried out external to the cell and air preheating is accomplished by mixing with 

recycled depleted air. The air feed flow is adjusted so that no heat transfer is required in the cell 
8-81 

or from the recycled air. Consequently, the internal fuel cell structure is greatly simplified, and 

the requirement for a heat exchanger in the recycle air stream is eliminated. 

Summary 

Advantages, Disadvantages of Various Fuel Cell, Power Cycles 

Regenerative Brayton 

 Advantages: 

• simple cycle arrangement, minimum number of components. 

• relatively low compressor and turbine pressure ratio, simple machines. 

• relatively low fuel cell operating pressure, avoiding the problems caused by anode/cathode 

pressure differential and high pressure housing and piping. 

• relatively low turbine inlet temperatures, perhaps 1950 oF for solid oxide and 1450 oF for 

molten carbonate fuel cell systems. Turbine rotor blade cooling may not be required. 

• relatively simple heat removal arrangements in fuel cells, accomplished by excess air flow. 

No internal heat transfer surface required for heat removal. 

• fuel conversion in cells maximized, taking full advantage of fuel cell efficiency. 

• adaptability to small scale power generation systems. 

 Disadvantages: 

• tailoring of compressor and turbine equipment to fuel cell temperature and cycle operating 

pressure required. (It is not clear to what extent available engine supercharging and 

industrial compressor and turbine equipment can be adapted to this application.) 

• large gas to gas heat exchanger for high temperature heat recuperation required. 

• efficiency and work output of the cycle sensitive to cell, compressor, and turbine 

efficiencies; pressure losses; and temperature differentials. 

Combined Brayton-Rankine 

 Advantages: 

• integrated plant and equipment available for adaptation to fuel cell heat recovery. 

• high efficiency system for heat recovery. 

 Disadvantages: 

• complex, multi component, large scale system for heat recovery. 

• adaptation of existing gas turbine required to provide for air take off and return of hot 

depleted air and partially burned fuel. 

• high pressure operation of the bulky fuel cell system required. 

• precise balancing of anode and cathode pressures required to prevent rupture of fuel cell 

electrolyte. 

• indirect heat removal required from fuel cells with compressed air, initially at low 

temperature, to enable significant conversion of the fuel flow in the cells. 

Rankine 

 Advantages: 

• ambient pressure operation within the fuel cell. 

• heat recovery in a boiler, avoiding the high temperature gas to gas exchanger of a 

regenerative Brayton cycle. 
8-82 

• no gas turbine required, only fans for air and exhaust product gas flow. 

• steam available for cogeneration applications requiring heat. 

 Disadvantages: 

• inherently lower efficiency than regenerative Brayton and combined Brayton-Rankine 

cycles. 

• requirement for cooling and feed water. 

• greater complexity than regenerative Brayton cycle arrangement. 

8.5 Fuel Cell Networks 

8.5.1 Molten Carbonate Fuel Cell Networks: Principles, Analysis and 

Performance 

The U.S. Department of Energy's National Energy Technology Laboratory (NETL) sponsors the 

research and development of engineered systems which utilize domestic fuel supplies while 

achieving high efficiency, economy and environmental performance. One of the most promising 

electric power generation systems currently being sponsored by NETL is the molten carbonate 

fuel cell (MCFC). 

NETL looked at improving upon conventional MCFC system designs, in which multiple stacks 

are typically arranged in parallel with regard to the flow of reactant streams. As illustrated in 

Figure 8-38a, the initial oxidant and fuel feeds are divided into equal streams which flow in 

parallel through the fuel cell stacks. 

In an improved design, called an MCFC network, reactant streams are ducted such that they are 

fed and recycled among multiple MCFC stacks in series. Figure 8-38b illustrates how the 

reactant streams in a fuel cell network flow in series from stack to stack. By networking fuel cell 

stacks, increased efficiency, improved thermal balance, and higher total reactant utilizations can 

be achieved. Networking also allows reactant streams to be conditioned at different stages of 

utilization. Between stacks, heat can be removed, streams can be mixed, and additional streams 

can be injected. 

Stacks in series approach reversibility. MCFC stack networks produce more power than 

conventional configurations because they more closely approximate a reversible process. To 

illustrate this fact, consider Figure 8-39, which compares the maximum power that could be 

generated by three different MCFC systems having identical feed stream compositions1

. 
8-83 

 

Figure 8-38 MCFC System Designs 
8-84 

Figure 8-39 Stacks in Series Approach Reversibility 

A graph of Nernst potential versus fuel utilization for the given feed stream compositions (60) 

was duplicated three times in Figure 8-39. The Nernst potential is the voltage which drives 

reversible electrode reactions. This reversible voltage, generated by the overall cell reaction, is a 

function of the local temperature, pressure, and reactant concentrations. As reactants are 

utilized, their concentrations change. Since Nernst potential is dependent upon the 

concentrations of reactants, it varies with the degree of utilization. 

Fuel utilization is directly proportional to the charge transferred across the electrolyte. 

Therefore, the shaded areas of the graphs represent power -- the product of voltage and current. 

If reversibility is assumed at the outlet of each stack, no voltage losses are deducted from the 

Nernst potential. Therefore, each shaded area represents the maximum power, which each cell 

could generate. 

System A in Figure 8-39 is composed of a single stack. Three stacks are arranged in series in 

system B. System C features many, or "n," stacks configured in series. In all three systems, the 

voltage of each stack corresponds to reactant concentrations at its outlet. 

For comparison, each system is assumed to have the same total stack membrane area. That is, 

the area of each stack in system B is one third the area of the stack in system A. Similarly, the 
8-85 

area of each stack in system C is one "nth" the area of the single stack in system A. For 

simplicity, each stack is considered to contain only one cell. 

Since each system achieves the same total fuel utilization (90 percent) across the same total area, 

each stack has the same average current density. Irreversible voltage loss is mainly a function of 

current density and stack temperature. Since these parameters are equivalent in each stack, it is 

assumed that the Nernst potential of each stack would be reduced by the same amount. 

In system A, 90 percent of the fuel is utilized in a single stack, and all the current is generated at 

a single voltage. The power that this system can achieve is represented by the graph's shaded 

region. 

In system B, three stacks in series each utilize 30 percent of the fuel. The current generated by 

each stack in system B is one third of the current generated in system A. Each stack in system B 

produces a different voltage. At the exit of the first stack, a high Nernst potential is generated 

because 70 percent of the fuel is still unburned. Likewise, at the exit of the second stack, 40 

percent of the fuel remains unburned, generating another improved Nernst potential. Only ten 

percent of the fuel remains at the exit of the third stack, yielding the same Nernst potential that 

the single stack in system A produced. The three stack network can produce more power 

because two-thirds of the total charge is transferred at increased voltages. Comparing the shaded 

areas of the graphs illustrates the additional power that can be produced by arranging stacks in 

series. 

In system C, many stacks are connected in series. Very small currents are generated at still 

higher voltages. As the number of stacks in series is increased, the maximum achievable power 

quickly approaches the power which a reversible system would generate, i.e. complete 

conversion of the available free energy. (A reversible system is reversible at every point in each 

stack, not just at the stack outlets.) The shaded area in the graph nearly fills the entire area under 

the curve - the reversible power. 

Each system in Figure 8-38 converts an equivalent amount of free energy (90 percent fuel 

utilization) into heat and electrical work. The key difference, however, is that the systems with 

MCFC stacks networked in series transfer charge at higher voltages, thus converting more of the 

free energy directly into electrical work, and less into heat. As the number of stacks in series is 

increased, a reversible process is approached which would convert all the free energy into work 

and none into heat. Although heat that is produced from free energy can be reconverted into 

electrical work (e.g. via a steam turbine), an MCFC stack's direct conversion of free energy is 

intrinsically more efficient. Therefore, networking MCFC stacks in series results in more 

efficient power production even when waste heat is recovered. 

Although each stack added to a series network would improve the system's efficiency, the 

incremental benefit obtained with each additional stack diminishes. A finite number of stacks 

could adequately, but not exactly, approach a reversible process. In a practical network, the 

number of stacks would be limited by economic, space, and design constraints. 
8-86 

In a similar study, Liebhafsky and Cairns (26) compared two arrangements of tubular, calciastabilized solid oxide fuel cells. In one arrangement, hydrogen and air were supplied to a single, 

30-cm cell. In the other arrangement, the same cell was segmented into three, 10-cm cells which 

were ducted such that the same reactant streams flowed through them in series. Each 

arrangement had a total fuel utilization of 90 percent and each cell had the same average current 

density. Each cell in the series arrangement accomplished one-third of the total fuel utilization. 

Calculations showed that the series arrangement produced 5 percent more power than the single 

cell, and that further sectioning would produce greater improvements. It was concluded that the 

increase in irreversibility associated with changes in gas composition has nothing to do with 

electrode kinetics, but is rooted in the Nernst equation. 

8.5.2 MCFC Network 

When designing an MCFC power system, several requirements must be met. An MCFC system 

must properly condition both the fuel and oxidant gas streams. Methane must be reformed into 

the more reactive hydrogen and carbon monoxide. Carbon deposition, which can plug gas 

passages in the anode gas chamber, must be prevented. To supply the flow of carbonate ions, the 

air oxidant must be enriched with carbon dioxide. Both oxidant and fuel feed streams must be 

heated to their proper inlet temperatures. Each MCFC stack must be operated within an 

acceptable temperature range. Excess heat generated by the MCFC stacks must be recovered 

and efficiently utilized. 

Figure 8-40 shows an MCFC network. The arrangement of stacks in series, as well as a unique 

recycle scheme, allows an MCFC network to meet all the requirements of an MCFC power 

system, while achieving high efficiency. 

8.5.3 Recycle Scheme 

In the network's recycle scheme, a portion of the spent fuel (Stream 5) and oxidant (Stream 4) is 

mixed and burned. The products of combustion (Stream 3) are then recycled through the cathode 

in order to provide the necessary carbon dioxide to the stacks. This eliminates the need for an 

external source of pure carbon dioxide. The cathode-cathode recycle (Stream 4) is large enough 

to cool the stacks, transferring excess energy to the heat recovery boilers. During the transfer of 

heat, enough energy is left in the oxidant recycle to heat the fresh air feed to the designated 

cathode inlet temperature. A second portion of the spent fuel (Stream 1) is recycled through the 

anode to provide enough steam to prevent carbon deposition and internally reform methane. 

This eliminates the need for steam to be supplied from another source. The anode-anode recycle 

also heats the fresh fuel feed to the designated anode inlet temperature. 

8.5.4 Reactant Conditioning Between Stacks in Series 

When MCFC stacks are networked in series, reactant streams can be conditioned between the 

stacks -- at different stages of utilization. The composition of reactant streams can be optimized 

between stacks by injecting a reactant stream (see Figure 8-40) or by mixing the existing reactant 

streams. 
8-87 

Figure 8-40 MCFC Network 

Between stacks networked in series, heat can be removed from the reactant streams to assist in 

controlling stack temperatures. The heat in a network reactant stream can be transferred to a 

cooler process stream in a heat exchanger or hot and cold reactant streams can be mixed directly. 

The recovered heat may be utilized in a combined cycle or for cogeneration. 

Methane can be injected into fuel streams between stacks networked in series. Since the 

reforming of methane into hydrogen is endothermic, its careful distribution among stacks in 

series is expected to improve the thermal balance of the system by allowing waste heat to be 

more evenly consumed throughout the total utilization of reactants. Improved thermal balance 

should allow stacks to be operated nearer their maximum temperature, reducing ohmic voltage 

losses. However, injecting portions of the fuel feed between stacks in series decreases the Nernst 

potential of every stack except the last one, since less fuel passes through each stack. (The 

amount of fuel which passes through the last stack does not change.) Optimizing the system 

requires an evaluation of the point at which the benefits of improved thermal balance outweigh 

the reduction in Nernst potential associated with such fuel redistribution. 

8.5.5 Higher Total Reactant Utilization 

The optimum total reactant utilization of stacks networked in series is higher than that of 

conventional, parallel stacks. Conventional designs avoid high utilization, because that would 

result in low voltages. In conventional configurations, the total utilization of reactants is 

accomplished in one stack. Therefore, when high utilizations are attempted, the low voltage 

which is generated adversely affects the total power production. In networks, however, the 
8-88 

utilization of reactants is accomplished incrementally, and the low voltage associated with high 

utilization is restricted to stacks which produce only a portion of the total power. 

Manifolding problems can further limit the practical reactant utilization of conventional MCFC 

systems. Ideally, fuel and oxidant streams are distributed equally among individual cells in a 

stack. Today's manifolds, however, have not been able to achieve this, and cells are typically 

supplied with unequal reactant flows. This causes the composition of outlet reactant streams to 

be variable among the cells. At high utilizations, this variability leads to a significant reduction 

in stack voltage. Therefore, conventional systems avoid such high utilizations. However, when 

stacks are networked in series, reactant streams can be thoroughly mixed between cells. This 

reduces the variability in reactant composition and helps to minimize the stack voltage loss. 

Another study (7) maximized the efficiency of conventional and series-connected fuel cell 

systems by optimizing cell voltage and current density. The study found that the optimum fuel 

utilization in the series-connected system was higher than that in the conventional system. Most 

importantly, the higher fuel utilization and lower current density of the series-connected system 

combined to give more efficient performance than the conventional system. 

8.5.6 Disadvantages of MCFC Networks 

For recycling to improve the performance of an MCFC network, it must provide benefits that 

outweigh its inherent disadvantages. If carbon dioxide is not separated from the anode-anode 

recycle, the concentration of carbon dioxide in the anode is increased. This reduces the Nernst 

potential. The Nernst potential is similarly reduced by the anode-cathode recycle if steam is not 

condensed out, since recycled steam dilutes reactant concentrations in the oxidant. In addition, 

part of the power generated by the network is consumed by the equipment necessary to circulate 

the recycle streams. Such circulation equipment, along with the additional ducting required by 

recycling, also increases the capital cost of the MCFC network. 

Given the same initial feed streams, the flowrate of reactants through stacks networked in series 

is much larger than the flowrate of reactants through stacks in a conventional system. 

Conventional fuel cell systems divide the initial feed streams among many stacks arranged in 

parallel. However, the initial feed streams in an MCFC network are not divided, but fed directly 

into the first of a series of many stacks. Perhaps the greatest disadvantage of MCFC networks is 

that this increased flowrate creates larger pressure drops. 

Another potential disadvantage of an MCFC network is the interdependence of the stacks in 

series. A problem with one stack could alter the performance of succeeding stacks. 

Furthermore, bypassing or isolating a problematic stack in a network could be a difficult control 

process. In the conventional parallel configuration, stack performance is not so interrelated. 

8.5.7 Comparison of Performance 

Two ASPEN (Advanced System for Process Engineering, public version) simulations compare 

the performance of conventional and networked fuel cell systems having identical recycle 

schemes and steam bottoming cycles. Each simulated system was composed of three MCFC 

stacks operating at the same temperature and pressure. The Nernst potential of each MCFC in 

both systems was reduced by 0.3 volts due to activation, concentration and ohmic voltage 
8-89 

polarizations. (This is a conservative estimate, representing a much higher outlet voltage 

polarization than would be expected.) Simple, single-pressure steam cycles produce secondary 

power. 

When the total fuel utilization of each system was optimized for maximum efficiency, the 

efficiency of the fuel cell stacks networked in series was nearly 10 percent greater than that of 

the stacks arranged in parallel (44.9 percent vs. 35.4 percent, LHV). When the power generated 

by each system's steam bottoming cycle was considered in addition to its fuel cell power, the gap 

in efficiency narrowed to 5.5 percent. The efficiency of the total networked system is 56.8 

percent, while that of the total conventional system was 51.3 percent. 

The fuel cell network which was simulated was not fully optimized. Optimization of flow 

geometry, operating pressure, stack fuel utilization and current, reactant conditioning, and other 

parameters would be expected to yield further significant increases in total system efficiency. 

8.5.8 Conclusions 

Key to the concept of networking is the arrangement of multiple fuel cell stacks relative to the 

flow of reactant streams. Conventional fuel cells systems have been designed such that reactant 

streams flow in parallel through fuel cell stacks. In a fuel cell network, however, reactant 

streams are ducted such that they are fed and recycled through stacks in series. 

Arranging fuel cell stacks in series offers several advantages over conventional fuel cell systems. 

Stacks networked in series more closely approach a reversible process, which increases the 

system efficiency. Higher total reactant utilizations can be achieved by stacks networked in 

series. Placing stacks in series also allows reactant streams to be conditioned at different stages 

of utilization. Between stacks, heat can be consumed or removed, (methane injection, heat 

exchange) which improves the thermal balance of the system. The composition of streams can 

be adjusted between stacks by mixing exhaust streams or by injecting reactant streams. 

Computer simulations have demonstrated that a combined cycle system with MCFC stacks 

networked in series is significantly more efficient than an identical system with MCFC stacks 

configured in parallel. 

8.6 Hybrids 

This section presents hybrids for generating electricity or for providing power in automotive 

vehicles. Hybrid systems that incorporate gas turbines build upon the outstanding performance of 

the fuel cell by utilizing the exhausted fuel cell heat. Hybrid electric vehicles utilize fuel cells to 

provide electric power to augment or replace exiting power sources. These systems are highly 

efficient and deliver superior environmental performance. Presented below is a general 

discussion of hybrid technology as well as specific initiatives in the gas turbine/fuel cell and 

electric power hybrid vehicle areas. 

8.6.1 Technology 

Advanced power generation cycles that combine high-temperature fuel cells and gas turbines, 

reciprocating engines, or another fuel cell are the hybrid power plants of the future. These 

conceptual systems have the potential to achieve efficiencies greater than 70 percent and be 
8-90 

commercially ready by the year 2010 or sooner. The hybrid fuel cell/turbine (FC/T) power plant 

will combine a high-temperature, conventional molten carbonate fuel cell (MCFC) or a solid 

oxide fuel cell (SOFC) with a low-pressure-ratio gas turbine, air compressor, combustor, and in 

some cases, a metallic heat exchanger (27). The synergistic effects of the hybrid fuel cell/turbine 

technology will also provide the benefits of reduced greenhouse gas emissions. Nitrous (NOX) 

emissions will be an order of magnitude below those of non-fuel cell power plants and carbon 

monoxide emissions will be less than 2 parts per million (ppm) (28). There will also be a 

substantial reduction in the amount of carbon dioxide produced compared to conventional power 

plants. 

The hybrid system is key to the Department of Energy’s program of achieving efficiencies 

greater than 75 percent (LHV) for natural gas. The higher efficiencies play a key role in 

reducing emissions. As a comparison, conventional coal-burning power plants are typically 35 

percent efficient and natural gas fired plants are now 40 to 50 percent efficient. Figure 8-41 

shows the estimated efficiency ranges of current and future power generation systems. 

The combination of the fuel cell and turbine operates by using the rejected thermal energy and 

residual fuel from a fuel cell to drive the gas turbine. The fuel cell exhaust gases are mixed and 

burned, raising the turbine inlet temperature while replacing the conventional combustor of the 

gas turbine. Use of a recuperator, a metallic gas-to-gas heat exchanger, transfers heat from the 

gas turbine exhaust to the fuel and air used in the fuel cell. Figure 8-42 illustrates an example of 

a proposed fuel cell/turbine system. 

There can be many different cycle configurations for the hybrid fuel cell/turbine plant. In the 

topping mode described above, the fuel cell serves as the combustor for the gas turbine while the 

gas turbine is the balance-of-plant for the fuel cell, with some generation. In the bottoming 

mode, the fuel cell uses the gas turbine exhaust as air supply while the gas turbine is the balance 

of plant. In indirect systems, high temperature heat exchangers are used (29). 

The hybrid plants are projected to cost 25 percent below comparably sized fuel cells, (30) and be 

capable of producing electricity at costs of 10 to 20 percent below today’s conventional plants 

(27). Operation of the plant is almost totally automatic. Therefore, it can be monitored and 

managed remotely with the possibility of controlling hundreds of the power plants from a single 

location (28).

Initial systems will be less than 20 MW, with typical system sizes of 1 to 10 MW. Future 

systems, in the megawatt class size, will boost efficiency even further by combining two solid 

oxide fuel cell modules with more advanced gas turbines and introducing sophisticated cooling 

and heating procedures. Another possibility of a hybrid power plant is to combine a solid oxide 

fuel cell with a polymer electrolyte (PEFC) fuel cell. The SOFC would produce both electric 

power and hydrogen. This hydrogen would then be utilized by the PEFC to generate more 

electric power (28). 
8-91 

Figure 8-41 Estimated performance of Power Generation Systems 

Figure 8-42 Diagram of a Proposed Siemens-Westinghouse Hybrid System 

(Taken from DOE Project Fact Sheet – Fuel Cell/ ATS Hybrid Systems) 

0

10

20

30

40

50

60

70

80

90

100

0.1 1 10 100 1000

Power Output, MW

Efficiency (LHV), %

Fuel Cells Gas Turbine Combined Cycle

SOFC/ Gas Turbine Hybrid System

Gas Turbine Simple Cycle

Microturbines

Gas Turbine w/ Cycle Improvements

Advanced Turbine System

Internal Combustion Engine
8-92 

8.6.2 Projects 

In 1997, a Program Research and Development Announcement (PRDA) was issued by the 

Department of Energy for conceptual feasibility studies of high-efficiency fossil power plants 

(HEFPPs). The terms of the conceptual power plant must be less than 20 MW in size, operate on 

natural gas and contain a high-temperature fuel cell. By late 1998, DOE awarded contracts to 

determine the feasibility of the highly efficient hybrid power plants. 

FCE, of Danbury, CT, teamed with Allison Engine Company to evaluate a carbonate fuel cell 

combined with a gas turbine and a steam turbine generator. The system was operated at ambient 

pressure. The net power of the hybrid system was 20.6 MW and the NOX levels were less than 

1 ppm. The process showed a 65 percent efficiency with off-the-shelf turbomachinery and 72 

percent efficiency with cycle specific machinery. The COE is predicted to be comparable to 

present day alternatives. 

Siemens-Westinghouse Power Corporation, of Pittsburgh, PA, with a subcontract to Allison 

Engine Company, evaluated a pressurized solid oxide fuel cell coupled with conventional gas 

turbine technology without a steam plant. The system was operated at a pressure of 7 atm. The 

fuel cell generated 16 MW of power and the gas turbine generated 4 MW of power. The process 

showed 67 percent efficiency as developed. An efficiency of 70 percent is deemed achievable 

with improvement in component design. The COE is predicted to be comparable to present day 

alternatives. NOX levels were less than 1 ppm. 

McDermott Technology, Inc., of Alliance, OH, developed a conceptual design of a high 

efficiency power plant system that joins planar solid oxide fuel cell technology with microturbine technology in a combined cycle. The system was operated at atmospheric conditions. 

The power plant had a combined cycle output of 700 kW with the turbine supplying 70 kW. The 

results indicate 70 percent efficiency is possible and the COE is comparable to present day 

alternatives. 

Siemens-Westinghouse Power Corporation, Pittsburgh, PA, and Solar Turbines developed a 

conceptual design of an economically and technically feasible 20-MW, 70- percent efficient 

natural gas-fueled power system that employs solid oxide fuel cells operating at elevated 

pressure in conjunction with an Advanced Turbine System gas turbine. The fuel cell, operated at 

9 atm pressure, generated 11 MW of power. Two Solar Mercury 50 gas turbines were used to 

generate 9 MW of power. The results of the study indicated system efficiency near 60 percent. 

A low COE relative to conventional power generation is predicted. 

In March of 1999, FCE, of Danbury, CT, with Allison Engine Company, Indianapolis, IN, and 

Capstone Turbine Corp., Woodland Hills, CA. was awarded a project to create a fuel cell/turbine 

system that meet or exceed DOE’s efficiencies and emissions goals. The 3-year program will 

include four steps: 

• Development of a high-utilization fuel cell, 

• Development of key system components, 

• Tests of the fuel cell/hybrid system to assess integration and system operation of an existing 

250-kilowatt fuel cell stack with a commercially available micro-turbine, and 

• Preparation of a conceptual design of a 40 MW ultra-high efficiency power plant. 
8-93 

A unique feature of the proposed system will allow the fuel cell and turbine modules to operate 

at independent pressures. The fuel cell will be operated at ambient pressure. This can increase 

the fuel cell stack life and save on piping and vessel costs. The turbine can then operate at its 

optimum pressure ratio. 

Countries around the world are developing interest in the high-efficiency hybrid cycles. A 320 

kW hybrid (SOFC and gas turbine) plant will enter service in Germany, operated by a 

consortium under the leadership of RWE Energie AG. This will be followed by the first 1 MW 

plant, which will be operated by Energie Baden-Wurttemberg AG (EnBW), Electricite de France 

(EDF), Gaz de France, and Austria’s TIWAG (29). 

Another project under development at the NETL is an advanced power plant system that 

combines a multistaged fuel cell with an extremely efficient turbine. Preliminary estimates show 

efficiencies greater than 80 percent (LHV). Studies showed that natural gas to electricity LHV 

efficiencies could break through an 80 percent barrier, while remaining cost competitive for a 4-

MW solid oxide plant (tubular or planar). The Advanced Fuel Cell concept directly coincides 

with the long-term goals of the Fuel Cell Program. These include system costs of $400/kW and 

efficiencies of 70 to 80 percent or more (LHV to AC electricity), with fuel flexibility and a stacklife of 40,000 hours. They are intended for commercial application in 2015, maintaining ultralow emissions. 

8.6.3 World’s First Hybrid Project 

Siemens-Westinghouse Power Corporation of Pittsburgh, PA developed and fabricated the first 

advanced power plant to combine a solid oxide fuel cell and a gas turbine. The microturbine 

generator was manufactured by Northern Research and Engineering Corporation of Woburn, 

Mass. The factory acceptance test was completed in April 2000. Southern California Edison is 

operating the new hybrid plant at The National Fuel Cell Research Center at the University of 

California-Irvine. A year of testing in a commercial setting will be performed at this site. The 

system cycle is expected to generate electric power at 55 percent efficiency. 

The pressurized system will generate 220 kilowatts of power and be operated at 3 atm of 

pressure. The fuel cell is made up of 1152 individual tubular ceramic cells and generates about 

200 kilowatts of electricity. The microturbine generator will produce an additional 20 kilowatts 

of electricity at full power. No sulfur dioxide pollutants will be released into the air. Nitrogen 

oxide emissions are likely to be less than 1 ppm. 

A 320-kilowatt hybrid system is also in the planning stages. An initial commercial offering of a 

one MW fuel cell-microturbine power plant in late 2002 will be the end results of this 

Department of Energy/Siemens Westinghouse partnership program (31). 

8.6.4 Hybrid Electric Vehicles (HEV) 

Hybrid Electric Vehicles (HEVs) typically combine the conventional internal combustion engine 

of the automobile with an energy storage device, such as a battery. However, there are many 

different arrangements for the HEV. The key components to an HEV are the energy storage 

system (batteries, ultracapacitors, and flywheels), the power unit (spark ignition engines, 
8-94 

compression ignition direct injection engines, gas turbines and fuel cells) and the vehicle 

propulsion system (electric motor). The benefits of HEVs, much like the hybrid power plants, 

are increased efficiency and lower emissions. 

As of July 2004, DOE has completed 1 million miles of hybrid electric vehicle testing. The 

number of each type of hybrid electric vehicle tested, the total miles accumulated, and average 

fuel economy to date include: 

• 4 Honda Civics, 284,000 miles and 38.0 mpg 

• 6 Honda Insights, 347,000 miles and 46.0 mpg 

• 6 Toyota Prius (model years 2002 and 2003) 380,000 miles and 41.1 mpg 

• 2 Toyota Prius (model years 2004) 16,000 miles and 44.6 mpg 

Fuel cell hybrid cars are not a new concept. In the early 1970s, K. Kordesch modified a 1961 

Austin A-40 two-door, four-passenger sedan to an air-hydrogen fuel cell/battery hybrid car (32). 

This vehicle used a 6-kW alkaline fuel cell in conjunction with lead acid batteries, and operated 

on hydrogen carried in compressed gas cylinders mounted on the roof. The car operated on 

public roads for three years and about 21,000 km. 

In 1994 and 1995, H-Power (Belleville, New Jersey) headed a team that built three PAFC/battery 

hybrid transit buses (33, 34). These 9 meter (30 foot), 25 seat (with space for two wheel chairs) 

buses used a 50 kW fuel cell and a 100 kW, 180 amp-hour nickel cadmium battery. 

The major activity in transportation fuel cell development has focused on the PEFC. In 1993, 

Ballard Power Systems (Burnaby, British Columbia, Canada) demonstrated a 10 m (32 foot) 

light-duty transit bus with a 120 kW fuel cell system, followed by a 200 kW, 12 meter (40 foot) 

heavy-duty transit bus in 1995 (35). These buses use no traction batteries. They operate on 

compressed hydrogen as the on-board fuel. In 1997, Ballard provided 205 kW (275 HP) PEFC 

units for a small fleet of hydrogen-fueled, full-size transit buses for demonstrations in Chicago, 

Illinois, and Vancouver, British Columbia. Working in collaboration with Ballard, DaimlerBenz built a series of PEFC-powered vehicles, ranging from passenger cars to buses (36). The 

first such vehicles were hydrogen-fueled. A methanol-fueled PEFC A-class car unveiled by 

Daimler-Benz in 1997 has a 640 km (400 mile) range. A hydrogen-fueled (metal hydride for 

hydrogen storage), fuel cell/battery hybrid passenger car was built by Toyota in 1996, followed 

in 1997 by a methanol-fueled car built on the same RAV4 platform (37). 

Ballard brought fuel cell technology into the public awareness a decade ago. Today, there are 

50 companies in North America, Europe, and Japan developing fuel cells and related systems 

and components for cars, buses, and specialty vehicles, like golf carts and fork lifts. The 

Breakthrough Technologies Institute, Inc. entered a cooperative agreement with the U.S. 

Department of Energy to survey fuel cell vehicle developers, selected energy and component 

suppliers, and interested government agencies. This survey identified nearly 20 companies 

developing light-duty fuel cell vehicles and components. The survey also identified at least 12 

companies or partnerships developing or demonstrating fuel cell buses. The results of the survey 

were published in February 2004 and the report can be viewed at 

http://www.fuelcells.org/info/charts/vehiclestudy.pdf. 
8-95 

Other major automobile manufacturers, including General Motors, Volkswagen, Volvo, Honda, 

DaimlerChrysler, Nissan, and Ford, also have announced plans to build prototype polymer 

electrolyte fuel cell vehicles operating on hydrogen, methanol, or gasoline (38). Honda’s FCX, a 

fuel cell prototype sedan, includes both hydrogen- and methanol-based systems. The GM 

Precept will use a hydrogen hydride storage system to help it to attain a 108 miles per gallon 

gasoline equivalent (39). A list of auto manufactures with information on their prototype can be 

found at http://www.fuelcells.org/info/charts/carchart.pdf. 

The Department of Energy’s Transportation Fuel Cell program is a collaboration between 

government and industry that supports the Partnership for a New Generation of Vehicles. 

Domestic automakers, fuel cell developers, national labs, universities, component suppliers and 

the fuel industry have created a Fuel Cell Alliance. This alliance helps in collaborating 

government sponsored research and development within the auto industry. Some of the goals of 

the program include developing fuel cell stack systems that are greater than 57 percent efficient 

at 25 percent peak power, more than 100 times cleaner than EPA Tier 2 emissions, and capable 

of operating on hydrogen or hydrogen-rich fuel from gasoline, methanol, ethanol and natural gas. 

By 2004, the program hopes to have fuel cell power systems that are reliable, safe and cost 

competitive with internal combustion engines (40). 

California has started a Fuel Cell Partnership with oil companies, automakers and fuel cell 

companies. They hope to have 50 fuel cell vehicles, both passenger cars and transit buses, on the 

road by 2003. The goals of the program include demonstrating vehicle performance, identifying 

fuel infrastructure issues and addressing commercialization challenges (41). 

DOD is interested in new or novel advanced power and propulsion systems that will reduce fuel 

consumption, improve performance, extend vehicle range, reduce emissions, and reduce support 

costs. The Navy and Army are considering hybrids for ships, land vehicles, helicopters, and 

battlefield power requirements. 

In 1997, the Office of Naval Research (ONR) initiated an advanced development program to 

demonstrate a ship service fuel cell (SSFC) power generation module. During Phase 1, 

competitive conceptual designs of 2.5 MW SSFC were prepared, along with critical component 

demonstrations. Phase 2 of the development program, scheduled for completion in 2002, will 

result in a nominal 500 kW fuel cell ship service generator demonstration module to be 

constructed and tested in a laboratory setting. The baseline concept is fueled by logistic fuel 

which is reformed in an adiabatic reformer designed and built by International Fuel Cells. 

Downstream of the reformer is a series of components that remove CO and H2S before the gas is 

sent to the fuel cell. The spent fuel and air are mixed and burned to drive a turbocompressor and 

recover compression work.54 

The Army has two programs that are looking at hybrids using fuel cells. In 1999, the Land 

Warrior Operational Combat System was approved. The goal is to develop a portable hybrid 

fuel cell system that weighs less than one kilogram and meets the power demand of the Land 

Warrior Power requirements. The second program is the Future Combat System. This program 

 

54 R.M. Privette, et al., “2.5 MW PEFC System for Navy Ship Service Power,” paper presented at the 1999 Review 

Conference on Fuel Cell Technology, Chicago, Illinois, August 3-5, 1999. 
8-96 

plans to develop technologies and systems for a lightweight, overwhelming lethal, strategically 

deployable, self-sustaining combat systems.55

8.7 Fuel Cell Auxiliary Power Systems 

In addition to high-profile fuel cell applications such as automotive propulsion and distributed 

power generation, the use of fuel cells as auxiliary power units (APUs) for vehicles has received 

considerable attention (see Figure 8-43). APU applications may be an attractive market because 

it offers a true mass-market opportunity that does not require the challenging performance and 

low cost required for propulsion systems for vehicles. In this section, a discussion of the 

technical performance requirements for such fuel cell APUs, as well as the current status of the 

technology and the implications for fuel cell system configuration and cost is given. 

Participants Application Size range Fuel /Fuel 

Cell type 

Nature of 

Activity 

BMW, International 

Fuel Cells1 

passenger car, BMW 

7-series 5kW net 

Hydrogen, 

Atmospheric 

PEM 

Demonstration 

Ballard, DaimlerChrysler2 

Class 8 Freightliner 

heavy-duty Century 

Class S/T truck cab 

1.4 kW net for 

8000 BTU/h A/C 

unit 

Hydrogen, 

PEM Demonstration 

BMW, Delphi, 

Global 

Thermoelectric3 

passenger car 1 to 5kW net Gasoline, 

SOFC 

Technology 

development 

program 

Figure 8-43 Overview of Fuel Cell Activities Aimed at APU Applications 

Auxiliary power units are devices that can provide all or part of the non-propulsion power for 

vehicles. Such units are already in widespread use in a range of vehicle types and for a variety of 

applications, in which they provide a number of potential benefits (see Figure 8-44). Although 

each of these applications could provide attractive future markets for fuel cells, this section will 

focus on application to on-road vehicles (specifically trucks). 

Vehicles Types Loads Serviced Potential Benefits 

• Heavy-duty & utility trucks 

• Airplanes 

• Trains 

• Yachts & Ships 

• Recreational vehicles 

• Automobiles & light trucks 

(not commercial yet) 

• Space conditioning 

• Refrigeration 

• Lighting and other cabin 

amenities 

• Communication and 

information equipment 

• Entertainment (TV, radio) 

• Can operate when main 

engine unavailable 

• Reduce emissions and noise 

while parked 

• Extend life of main engine 

• Improve power generation 

efficiency when parked 

Figure 8-44 Overview of APU Applications

 

55 J.C. Stephens, K. Gardner, and R. Jacobs, “US Army CECOM Fuel Cell Program,” presented at the World 

Association for Case Method Research and Application, Lucerne, Switzerland, January 5-7, 2000. 
8-97 

In 1997, the Office of Naval Research initiated an advanced development program to 

demonstrate a ship service fuel cell power generation module. The ship service generator 

supplies the electrical power requirements of the ship. This program will provide the basis for a 

new fuel cell-based design that will be an attractive option for the future Navy surface ships. 

This program will provide the Navy with a ship service that is more efficient and incorporates a 

distributive power system that will remain operating even if the engine is destroyed.

Fuel cells can serve as a generator, battery charger, battery replacements and heat supply. They 

can adapt to most environments, even locations in Arctic and Antarctic regions. One effort, 

being run in collaboration with the Army Research Office, has demonstrated a prototype fuel cell 

designed to replace in many applications a popular military standard battery. The target 

application is the Army's BA-5590 primary (i.e., use-once-and-dispose) lithium battery. The 

Army purchases approximately 350,000 of these batteries every year at a cost of approximately 

$100 per battery, including almost $30 per battery for disposal. Fuel cells, on the other hand, are 

not thrown away after each use but can be reused hundreds of times. Mission weight savings of 

factors of 10 or more are projected. The prototype fuel cell, which has the same size and delivers 

the same power as a battery, has been tested in all orientations and under simulated adverse 

weather conditions, and was enthusiastically received by Army senior management. 

8.7.1 System Performance Requirements 

A key reason for interest in fuel cell APU applications is that there may be a good fit between 

APU requirements and fuel cell system characteristics. Fuel cells could be efficient and quiet, 

and APUs do have the load following requirements and physical size and weight constraints 

associated with propulsion applications. However, in order to understand the system 

requirements for fuel cell APUs, it is critical to understand the required functionality (refer to 

Figure 8-44) as well as competing technologies. To provide the functionality of interest, and to 

be competitive with internal combustion engine (ICE) driven APUs, fuel cell APUs must meet 

various requirements; an overview is provided in Figure 8-45. 

Key Parameter Typical Requirements Expected fuel cell performance 

Power output 12 to 42 V DC is acceptable for 

most applications, 110 / 220 V 

AC may be desirable for 

powering power tools etc. 

DC power output simplifies the power 

conditioning and control for fuel cells 

System Capacity 1 to 5 kW for light duty vehicles 

and truck cabins 

up to 15 kW for truck refrigeration 

Fits expected range for PEFCs and 

probably also advanced SOFCs 

System Efficiency More than 15 to 25 percent 

based on LHV 

Efficiency target should be achievable, 

even in smallest capacity range 

Operating life and reliability Greater than about 5,000 hours 

stack life, with regular service 

intervals less than once every 

1,000 hours 

Insufficient data available to assess 

whether this is a challenge or not 

Figure 8-45 Overview of typical system requirements 
8-98 

Fuel cell APUs will likely have to operate on gasoline, and for trucks preferably on diesel fuel, in 

order to match the infrastructure available, and preferably to be able to share on-board storage 

tanks with the main engine. The small amount of fuel involved in fueling APUs would likely not 

justify the establishment of a specialized infrastructure (e.g. a hydrogen infrastructure) for APUs 

alone. Similarly, fuel cell APUs should be water self-sufficient, as the need to carry water for 

the APU would be a major inconvenience to the operator, and would require additional space and 

associated equipment. 

In addition to the requirement for stationary operation mentioned in Figure 8-45, fuel cell APUs 

must be able to provide power rapidly after start-up, and must be able to follow loads. While the 

use of batteries to accomplish this is almost a given, a system start-up time of about ten minutes 

or less will likely be required to arrive at a reasonable overall package. 

Finally, fuel cell APUs are clean. These attributes may well be the key competitive advantage 

that fuel cell APUs have over conventional APUs, and hence their performance must more than 

match that of internal combustion engines APUs. 

8.7.2 Technology Status 

Active technology development efforts in both PEFC and planar SOFC technology, driven 

primarily by the interest in distributed generation and automotive propulsion markets, have 

achieved significant progress in the development of these technologies. For distributed power 

applications refined and even early commercial prototypes are being constructed. However, in 

the case of planar SOFC a distinction must be made between different types of SOFC 

technologies. Neither the tubular nor the electrolyte supported SOFC technology is suitable for 

APU applications due to their very high operating temperature, large size and weight. Only the 

electrode supported planar SOFC technology may be applicable to APU applications. Since it 

has only been developed over the past nine years, as opposed to several decades for PEFC and 

other SOFC technologies, it is not developed as far, although it appears to be catching up quickly 

(See Figure 8-46). 

Figure 8-46 Stage of development for fuel cells for APU applications 

Research &

Development Production MarketEntry

Demonstration

Initial System

Prototypes

Refined 

Prototypes

Commercial 

Prototypes

Planar SOFC

(Residential)

PEM

(Residential)

Planar SOFC

(APU)

PEM

(APU)
8-99 

Fuel cell APU applications could benefit significantly from the development of distributed 

generation systems, especially from residential scale systems, because of the similarity in scale 

and duty cycle. However, distributed generation systems are designed mostly for operation on 

natural gas, and do not face as stringent weight and volume requirements as APU applications. 

As a result, fuel cell APUs are in the early initial system prototype stage. 

Several developers, including Nuvera, Honeywell, and Plug Power are active in the development 

for residential PEFC power systems. Most of the PEFC system technology can be adapted for 

APU application, except that a fuel processor capable of handling transportation fuels is 

required. However, most of the players in the residential PEFC field are also engaged in the 

development of PEFC systems for automotive propulsion applications, which are targeting the 

ability to utilize transportation fuels for PEFC systems. 

Relatively few developers of SOFC technology have paid attention to non-stationary markets. 

All are focused on small to medium sized distributed generation and on-site generation markets. 

Only Global Thermoelectric (Calgary, Canada) has been active in the application of its 

technology to APUs. A recently conducted a detailed conceptual design and cost estimate of a 5-

kWnet SOFC-based truck APU conclude that, provided continued improvement in several 

technology areas, planar SOFCs could ultimately become a realistic option for this mass-market 

application. 

8.7.3 System Configuration and Technology Issues 

Based on the system requirements discussed above, fuel cell APUs will consist of a fuel 

processor, a stack system and the balance of plant. Figure 8-47 lists the components required in 

SOFC and PEFC based systems. The components needed in a PEFC system for APU 

applications are similar to that needed in residential power. The main issue for components for 

PEM-based systems is the minimization or elimination of the use of external supplied water. For 

both PEFC and SOFC systems, start-up batteries (either existing or dedicated units) will be 

needed since external electric power is not available. 

Detailed cost and design studies for both PEFC and SOFC systems at sizes ranging from 5kW to 

1 MW were made that point to the fundamental differences between PEFC and SOFC 

technology that impact the system design and by implication the cost structure. These 

differences will be discussed in the following paragraphs. 

The main components in a SOFC APU are the fuel cell stack, the fuel processor, and the thermal 

management system. In addition there are several balance of plant components, which are listed 

in Figure 8-47. The relatively simple reformer design is possible because the SOFC stack 

operates at high temperatures (around 800 °C) and is capable of utilizing both carbon monoxide 

and certain hydrocarbons as fuel. Since both the anode and cathode exhaust at temperatures of 

600 to 850 °C, high temperature recuperators are required to maintain system efficiency. These 

recuperators are of expensive materials (high temperature reducing and oxidizing atmosphere), 

making it an expensive component in the system. However, if hydrocarbons are converted inside 

the stack, this leads to a less exothermic overall reaction so that the stack cooling requirements 

are reduced. 
8-100 

Further system simplification would occur if a sulfur-free fuel was used or if the fuel cell were 

sulfur tolerant, in that case, the fuel can be provided directly from the reformer to the fuel cell. 

In order to minimize system volume, (and minimize the associated system weight and start-up 

time) integration of the system components is a key design issue. By recycling the entire anode 

tailgas to provide steam, a water management system can be avoided, though a hot gas 

recirculation system is required. 

Figure 8-47 Overview of subsystems and components for SOFC and PEFC systems 

Figure 8-48 shows a simplified layout for an SOFC-based APU. The air for reformer operation 

and cathode requirements is compressed in a single compressor and then split between the unit 

operations. The external water supply shown in Figure 8-48 will most likely not be needed; the 

anode recycle stream provides water. Unreacted anode tail gas is recuperated in a tail gas burner. 

Additional energy is available in a SOFC system from enthalpy recovery from tail gas effluent 

streams that are typically 400 to 600 °C. Current thinking is that reformers for transportation 

fuel based SOFC APUs will be of the exothermic type (i.e. partial oxidation or autothermal 

reforming), as no viable steam reformers are available for such fuels. 

SOFC-Based System

Fuel

Preparation

Fuel preprocessing

Reformate

Conditioning

Sulfur

removal

Fuel Cell

Stack

Air

Preparation

Balance of Plant:

• Compressor/Expander

• Pumps

• Controls

• Insulation & Packaging

• Safety system

• Start-up battery

PEM-Based System

Fuel

Preparation

Sulfur

Removal

Water-Gas

Shift CO clean-up Fuel CellStack

Steam

Generation

Air

Preparation

Balance of Plant:

• Compressor/Expander

• Pumps

• Controls

• Water management

• Packaging

• Safety system

• Start-up battery

Reforming
8-101 

Figure 8-48 Simplified System process flow diagram of pre-reformer/SOFC system 

Due to the operating requirements of PEFC stack technology, shift reactors and a carbon 

monoxide removal step are required to produce reformate of sufficient quality. Similarly, the 

stack operating temperature and its humidity requirements require a water management system 

as well as radiators for heat rejection. Some developers are developing pressurized systems to 

the benefit from higher reactant partial pressures on both anode and cathode. Fuel processing for 

PEFC APU systems is identical to that needed in residential power or propulsion applications. 

The additional issue for PEFC is the minimization of steam needed for the fuel processor system. 

Since an APU is a mobile and/or remote unit, the need for external sources of water should be 

minimized. The reformate stream is further diluted by additional steam, if that water is not 

removed prior to the fuel cell stack. 

Another design integration issue in PEFC systems is water management for hydrating the 

electrolyte and providing the necessary steam for reforming and water-gas shift operations. 

Additional steam may be required for the CO clean-up device. Some reformate-based PEFC 

systems are run under pressure to increase the partial pressure of reactants for the PEFC anode 

and cathode, increasing efficiency. Pressure operation also aids in heat integration for the 

internal generation of steam at pressures greater than atmospheric (i.e. steam generated at 

temperatures greater than 100 °C). PEFC system integration involves the integration of a 

reformer (either exothermic or endothermic overall, ~850 to 1000 °C), shift reactors (exothermic, 

150-500 °C), CO-cleanup (primarily exothermic, 50 to 200 °C), and the fuel cell stack 

(exothermic, 80 °C). Each reaction zone operates at a significantly different temperature thus 

providing a challenge for system integration and heat rejection. To alleviate some of these 

drawbacks, and further reduce the cost of the PEFC systems, developers are now investigating 

the possibility of using higher temperature membranes (e.g. operating slightly above 100 °C). 

Fuel Cell

Reformer

Mixer

Gasoline or Diesel

Preheat

Air

water

Anode

Cathode

Burner

Anode

Recycle

Sulfur

removal

Q

Flow 

Splitter
8-102 

This would increase the carbon monoxide tolerance, potentially simplifying the fuel processor 

design, and simplify the heat rejection. 

The power requirements for auxiliary power applications require smaller fuel cell stack duties. 

The heat losses for a SOFC stack operating at a smaller power duty are a larger proportion of the 

gross rating that in a stationary power application. Insulation required for specified system skin 

temperatures requirements could conceivably result in large proportion of the total system 

volume. Integration of the high temperature components is important in order to reduce the 

system volume and insulation requirements. SOFC APU systems will require inexpensive high 

performance insulation materials to decrease both system volume and cost. 

8.7.4 System Cost Considerations 

As for any new class of product, total cost of ownership and operation of fuel cells will be a 

critical factor in their commercialization, along with the offered functionality and performance. 

This total cost of ownership typically has several components for power systems such as fuel 

cells. These components include fuel cost, other operating costs such as maintenance cost, and 

the first cost of the equipment. This first cost has a significant impact on fuel cells’ 

competitiveness. 

The main component of a fuel cell’s first cost is the manufacturing cost, which is strongly related 

to the physical configuration and embodiment of the system, as well as to the manufacturing 

methods used. System configuration and design in turn are directly related to the desired system 

functionality and performance, while the manufacturing methods used are strongly linked to the 

anticipated production volume. 

Arthur D. Little has carried out cost structure studies for a variety of fuel cell technologies for a 

wide range of applications, including SOFC tubular, planar and PEFC technologies. Because 

phenomena at many levels of abstraction have a significant impact on performance and cost, they 

have developed a multi-level system performance and cost modeling approach (see Figure 8-49). 

At the most elementary level, it includes fundamental chemical reaction/reactor models for the 

fuel processor and fuel cell as one-dimensional systems. 

Reformer model

M

O

M

O

C3H7 H

C3H8

Manufacturing

Cost Model

Fuel Cell Model

0.4 

0.5 

0.6 

0.7 

0.8 

0.9 

1 

cell potential (V)

0 0.2 0.4 0.6 0.8 1 1.2 1.4 

current density (A/cm2)

NG2000 H2 NG3000 H2 NG2000 ref NG3000 ref

Thermodynamic

 System Model

Conceptual

Design and

Configuration

46" 60"

53"

1 2

4

5

6 7

8

9

10

11

12

13

15

14

Illustrative

Tape Cast

Anode

Powder Prep

Vacuum

Plasma

Spray

Electrolyte

Small Powder

Prep

Screen

Print

Cathode

Small Powder

Prep

Sinter in Air

1400C Sinter in Air

Forming

of

Interconnect

Shear

Interconnect

Vacuum

Plasma

Spray

Slurry

Spray

Screen

Print

Slurry Spray

Slip Cast

Finish Edges

Note: Alternative production processes appear in gray to the

bottom of actual production processes assumed

Braze

Paint Braze

onto

Interconnect

Blanking /

Slicing

QC Leak

Check

Interconnect

Fabrication

Anode Electrolyte Cathode

Stack Assembly

$/kW

Figure 8-49 Multilevel system modeling approach. 
8-103 

Each of the detailed sub-models feed into the thermodynamic system model, and provides sizing 

information directly to the conceptual design and configuration. The thermodynamic system 

model provides a technical hub for the multi-level approach. It provides inputs on the required 

flow rates and heat duties in the system. Sizing information, together with information from the 

thermodynamic model then flows to the conceptual design. 

8.7.5 SOFC System Cost Structure 

The main difference in SOFC stack cost structure as compared to PEFC cost relates to the 

simpler system configuration of the SOFC-based system. This is mainly due to the fact that 

SOFC stacks do not contain the type of high-cost precious metals that PEFCs contain. This is 

off-set in part by the relatively complex manufacturing process required for the manufacture of 

the SOFC electrode electrolyte plates and by the somewhat lower power density in SOFC 

systems. Low temperature operation (enabled with electrode supported planar configuration) 

enables the use of low cost metallic interconnects which can be manufactured with conventional 

metal forming operations. 

The balance of plant contains all the direct stack support systems, reformer, compressors, pumps, 

and the recuperating heat exchangers. Its cost is low by comparison to the PEFC because of the 

simplicity of the reformer. However, the cost of the recuperating heat exchangers partially 

offsets that. 

To provide some perspective on the viability of SOFCs in APU applications from a cost 

perspective, NETL sponsored an estimate of the cost structure of small-scale (5 kW), simplecycle SOFC anode-supported system, operated on gasoline. The estimated manufacturing cost 

for such systems (see Figure 8-50) could well be close to that estimated for comparable PEFC 

systems, while providing somewhat higher system efficiency. 

While the stack, insulation and stack balance in this simple-cycle system is a key component; the 

balance of plant is also an important factor. The stack cost again mainly depends on the 

achievable power density. Small systems like these will likely not be operated under high 

pressure. While this simplifies the design and reduces cost for compressors and expanders 

(which are not readily available at low cost for this size range in any case) it might also 

negatively affect the power density achievable. 

One of the key challenges with small-scale SOFC systems is to overcome heat losses. The 

higher the heat losses are, the more recuperation is required to maintain the fuel cell within an 

acceptable operating temperature range and hence to ensure good performance. 

The large fraction of cost related to balance of plant issues is mainly due to the very small scale 

of this system, which results in a significant reverse economy of scale. While design work is still 

ongoing, it is anticipated that the cost structure of this system will change rapidly to reduce the 

cost of balance of plant further, and further improve the competitiveness of these systems. 
8-104 

Controls/ 

Piping/ 

Batte ry

Indirect, 

Labor, 

Depr.

Balance of 

Plant

Stack, 

insulation 

and stack 

balance

SOFC System 

Cost Structure:

Manufacturing Costs:

 $350-550/kW

Figure 8-50 Projected cost structure of a 5kWnet APU SOFC system. Gasoline fueled 

POX reformer, Fuel cell operating at 300mW/cm2, 0.7 V, 90 percent fuel utilization, 

500,000 units per year production volume. 

8.7.6 Outlook and Conclusions 

In conclusion, both PEFC and SOFC have the potential to meet the allowable cost targets, 

provided successful demonstrations prove the technology. It is critical however, that for the 

current technologies to be commercially successful, especially in small-capacity markets, high 

production volumes will have to be reached. APU applications might provide such markets. It is 

similarly critical that the technologies be demonstrated to perform and achieve the projected 

performance targets, and demonstrate long life. These are the challenges ahead for the fuel cell 

industry in the APU market segment. 

8.8 References 

1. James M. Douglas, Conceptual Design of Chemical Processes, McGraw-Hill, Inc., New 

York, NY, 1988. 

2. Max S. Peters, and Klaus D. Timmerhaus, Plant Design and Economics for Chemical 

Engineers, 3rd Edition, McGraw-Hill, Inc., New York, NY, 1980. 

3. Warren L. McCabe, Julian C. Smith, Peter Harriot, Unit Operations of Chemical 

Engineering, 4th Edition, 1985. 

4. M.C. Williams, and T.J. George, "Research Issues in Molten Carbonate Fuel Cells: 

Pressurization," presented at the 1992 IECEC, Vol. 3, pp. 263-267. 

5. "Overview of 11 MW Fuel Cell Power Plant," non-published information from Tokyo Electric 

Power Company, September 1989. 

6. T. Koshimizu, et al., "Development of 5000 kW and 1000 kW PAFC Plants," presented at the 

JASME-ASME Joint Conference (ICOPE-93), Tokyo, September 1993. 
8-105 

7. R. Shreve and J. Brink, Chemical Process Industries, fourth edition, McGraw-Hill, 1977. 

8. T. Okado, et al., "Study of Temperature Control in Indirect Internal Reforming MCFC Stack," 

presented at 25th IECEC, pp. 207-212, 1990. 

9. N.Q. Minh, "High-Temperature Fuel Cells, Part 2: The Solid Oxide Cell," Chemtech, Vol. 21, 

February 1991. 

10. V. Minkov, et al., "Topping Cycle Fuel Cells Effective Combined with Turbines," Power 

Engineering, July 1988, pp. 35-39. "Design and Economics of Large Fuel Cell Power Plants," 

presented at 1986 Fuel Cell Seminar, Tucson, AZ, p 255. 

11. F.G. Baily, "Steam Turbines for Advanced Combined Cycles," presented at the 35th GE 

Turbine State-of-the-Art Technology Seminar, 1991. 

12. M.S. Peters, and K.D. Timmerhaus, Plant Design and Economics for Chemical Engineers, 

Third Edition, McGraw-Hill, 1980. 

13. M. Farooque, "Development of Internal Reforming Carbonate Fuel Cell Stack Technology," 

Performed under Contract No. DE-AC21-87MC23274, DOE/MC/23374-2941, October 1990. 

14. M. Farooque, et al., "Comparative Assessment of Coal-Fueled Carbonate Fuel Cell and 

Competing Technologies," presented at the 25th IECEC, Vol. 3, pp.193-200, 1990. 

15. Dawn M. Bernardi, "Water-Balance Calculations for Solid-Polymer-Electrolyte Fuel Cells," 

Journal of Electrochemical Society, Vol. 137, No. 11, November 1990. 

16. David P. Wilkinson, (Ballard Power Systems) and David Thompsett (Johnson Matthey 

Technology Centre), "Materials and Approaches for CO and CO2 Tolerance for Polymer 

Electrolyte Membrane Fuel Cells," presented at the 1997 Proceedings of the Second 

International Symposium on New Materials for Fuel Cells and Modern Battery Systems, 

Montreal, Quebec, Canada, July 6-10, 1997. 

17. David P. Wilkinson, and Alfred E. Steck, "General Progress in the Research of Solic 

Polymer Fuel Cell Technology at Ballard," presented at the 1997 Proceedings of the Second 

International Symposium on New Materials for Fuel Cells and Modern Battery Systems, 

Montreal, Quebec Canada, July 6-10, 1997. 

18. Thomas L. Buchanan, John H. Hirschenhofer, David B. Stauffer, and Jay S. White, "Carbon 

Dioxide Capture in Fuel Cell Power Systems," September 1994, G/C Report 2981. 

19. "Overview of 11 MW Fuel Cell Power Plant," Non-published information from Tokyo 

Electric Power Company, September 1989. 

20. F. P. Bevc, W. L. Lundberg and D. M. Bachovchin, "Solid Oxide Fuel Cell Combined 

Cycles," ASME Paper 96-GT-447, presented at International Gas Turbine and Aeroengine 

Congress & Exhibition, Birmingham, UK, June 1996. 

21. R. Hendricks, "Heron Turbine Prototype Test Results," 20th International Congress on 

Combustion Engines, International Council on Combustion Engines (CIMAC), London, 

1993. 

22. T.J. George, K.D Lyons, and R. James III, "Multistaged Oxide Fuel Cell Power Plant 

Concept," May 1998. 

23. Fax Transmittal from J. Hirschenhofer of Parsons Energy & Chemicals Group to Tom 

George of USDOE/FETC, dated August 26, 1998, Re: Aspen Analysis Results of the 

Multistaged SOFC Concept. 

24. Parsons Energy & Chemical, work for the U.S. DOE, Spring 1998. 

25. Appleby, A.J.; Foulkes F.R. Fuel Cell Handbook; Van Nostrand Reinhold: New York, 1989. 
8-106 

26. " Fuel-Flexible Fuel Processor," J. Cuzens, J. Mauzey, and R. Woods, Hydrogen Burner 

Technology, Pg. 234 Fuel Cell Seminar Abstracts, Courtesy Associates, Inc., November 

1998. 

27. Theory and Application; Paper presented at the 1992 AIChE Annual Meeting: Miami Beach, 

FL, 1-6 November, 1992. 

28. “Developing Power Systems for the 21st Century – Fuel Cell/ATS Hybrid Systems,” U.S. 

Dept. of Energy, National Energy Technology Center & Office of Industrial Technologies, 

Project facts for Advanced Clean/ Efficient Power Systems, PS031.1099. 

29. U. Eberl, “Fuel Cells and Gas Turbines: A Marriage of Efficiency,” Research and 

Innovation, January 2000. 

30. R. Gemmen, et al, “Technical Development Issues and Dynamic Modeling of Gas Turbine 

and Fuel Cell Hybrid Systems,” Proceedings of the 1999 Review Conference on Fuel Cell 

Technology, August 1999. 

31. “Fuel Cells- Opening New Frontiers in Power Generation,” U.S. Department of Energy, 

Office of Fossil Energy, National Energy Technology Center, November 1999. 

32. DOE Techline Press Release, “Department of Energy Announces World’s First “Hybrid” 

Fuel Cell-Turbine,” April 2000. 

33. “Auto Show Attendees get a Glimpse of Future Concept Cars,” Alternative Fuel News, Vol.4 

No.1, US Department of Energy, Office of Energy Efficiency and Renewable Energy. 

34. K.V. Kordesh, “City Car with H2-Air Fuel Cell and Lead Battery,” 6th Intersociety Energy 

Conversion Engineering Conference, SAE Paper No. 719015, 1971. 

35. A. Kaufman, “Phosphoric Acid Fuel Cell Bus Development,” Proceedings of the Annual 

Automotive Technology Development Contractors’ Coordination Meeting, Dearborn, MI, 

October 1994, SAE Proceedings Volume P-289, pp. 289-293, 1995. 

36. R.R. Wimmer, “Fuel Cell Transit Bus Testing & Development at Georgetown University,” 

Proceedings of the Thirty Second Intersociety Energy Conversion Engineering Conference, 

July 1997, Honolulu, HI, pp. 825-830. 

37. N.C. Otto, P.F. Howard, “Transportation Engine Commercialization at Ballard Power 

Systems,” Program and Abstracts 1996 Fuel Cell Seminar, November 1996, Orlando, FL, 

pp.559-562. 

38. F. Panik, “Fuel Cells for Vehicle Applications in Cars – Bringing the Future Closer,” J. 

Power Sources, 71, pp.36-38, 1998. 

39. S. Kawatsu, “Advanced PEFC Development for Fuel Cell Powered Vehicles,” J. Power 

Sources, 71, pp. 150-155, 1998. 

40. Fuel Cell Technology: Powering the Future, Electric Line, November/December 1996. 

41. J. Milliken, “The DOE Transportation Fuel Cell Program: Recent Accomplishments and 

Future Plans,” U.S. Dept. of Energy, Office of Transportation Technologies. “Questions and 

Answers – California Fuel Cell Partnership,” www.drivingthefuture.org. 
9-1 

9. SAMPLE CALCULATIONS

This section presents sample problems to aid the reader in understanding the calculations behind a 

fuel cell power system. The sample calculations are arranged topically with unit operations in 

Section 9.1, system issues in Section 9.2, supporting calculations in Section 9.3, and cost 

calculations in Section 9.4. A list of conversion factors common to fuel cell systems analysis is 

presented in Section 9.5, and a sample automotive design calculation is presented in Section 9.6. 

9.1 Unit Operations 

The following examples are presented for individual unit operations found within a fuel cell 

system. Unit operations are the individual building blocks within a complex chemical process. By 

analyzing example problems for each unit operation, one can learn about the underlying scientific 

principles and engineering calculation methods that are applied to various processes. This 

approach will provide the reader with a better understanding of fuel cell power system building 

blocks as well as the interactions between unit operations. For example, the desired power output 

from the fuel cell unit will determine the fuel flow requirement from the fuel processor. This 

section starts by examining the fuel cell unit operation, and continues on to the fuel processor and 

power conditioner. 

9.1.1 Fuel Cell Calculations 

Example 9-1 Fuel Flow Rate for 1 Ampere of Current (Conversion Factor Derivation) 

What hydrogen flow rate is required to generate 1.0 ampere of current in a fuel cell? (This 

exercise will generate a very useful conversion factor for subsequent calculations.) 

Solution:

For every molecule of hydrogen (H2) that reacts within a fuel cell, two electrons are liberated at 

the fuel cell anode. This is most easily seen in the PAFC and PEFC because of the simplicity of 

the anode (fuel) reaction, although the rule of two electrons per diatomic hydrogen molecule (H2) 

holds true for all fuel cell types. The solution requires knowledge of the definition of an ampere 

(A) and an equivalence of electrons.56

H2 → 2H+

 + 2eThe moles of hydrogen liberated to generate one amp can be calculated directly: 

 

56 One equivalence of electrons is 1 g mol of electrons or 6.022 x1023 electrons (Avagadro’s number). This 

quantity of electrons has the charge of 96,487 coulombs (C) (Faraday’s constant). Thus, the charge of a single 

electron is 1.602 x10-19 C. One (1) ampere of current is defined as 1 C/sec. 
9-2 

( )

hr - kA

2 kg H

or 0.037605

hr - A

2 kg H -6 37.605x10

1000 g

1kg

2 1g mol H

2.0158 g

2

H

hr - A

g mol

0.018655

2 H m

2 H

hr - A

g mol

0.018655

1hr

3600 sec

- 2 equiv.of e

2

1g mol H

96,487 coulombs

- 1equivalence of e

1A

1coulomb/sec

1.0 A

2 Hn

= =

= =

⎟

⎠

⎞ ⎜

⎝

⎛

⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛

⎟

⎠

⎞ ⎜

⎝

⎛

⎟

⎠

⎞ ⎜

⎝

⎛

⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛

⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛

⎟

⎠

⎞ ⎜

⎝

⎛

The result of this calculation, 0.037605 kg H2 per hour per kA (0.08291 lb H2 per hour per kA), is 

a convenient factor that is often used to determine how much fuel must be provided to supply a 

desired fuel cell power output, as illustrated in the next example. 

Example 9-2 Required Fuel Flow Rate for 1 MW Fuel Cell 

A 1.0 MWDC fuel cell stack is operated with a cell voltage of 700 mV on pure hydrogen with a 

fuel utilization, Uf of 80 percent. (a) How much hydrogen will be consumed in lb/hr? (b) What 

is the required fuel flow rate? (c) What is the required air flow rate for a 25 percent oxidant 

utilization, Uox? 

Solution: 

(a) The solution of this problem will be simplified by assuming that the individual fuel cells are 

arranged in parallel. That is, the fuel cell stack voltage is the same as each individual cell 

voltage, and the fuel cell stack current is equal to the current of an individual cell times the 

number of cells. 

Recalling that power (P) is the product of voltage (V) and current (I), 

P = I x V 

Therefore, the current through the fuel cell stack can be calculated as 

I= P

V = 1.0 MW0.7 V

10 W

1 MW

1 VA

1 W

1 kA

1000 A

 1429 kA

6 ⎛

⎝

⎜ ⎞

⎠

⎟

⎛

⎝

⎜ ⎞

⎠

⎟

⎛

⎝

⎜ ⎞

⎠

⎟

⎛

⎝

⎜ ⎞

⎠

⎟ =

The quantity of hydrogen consumed within the fuel cell stack is 

( ) hr

lb H =118.4

hr - kA

0.08291lb H

m = 1429 kA 2 2

H ,consumed 2 ⎟

⎠

⎞ ⎜

⎝

⎛

Note that without the simplifying assumption that the fuel cells were arranged in parallel, the 

same hydrogen mass flow could have been calculated with a few extra steps. For example, if the 

fuel cell stack was composed of 500 cells in series, then the stack voltage would have been 350 

volts [(500 cells)(0.7 V/cell)], and the stack current would have been 2.858 kA/cell [1429 kA / 

500 cells]. Because this stack current passes through 500 cells arranged in series, the hydrogen 

consumption is calculated as 
9-3 

( ) hr

lb H 500 cells =118.4

hr - kA

0.08291lb H

cell

kA

m = 2.858 2 2

H ,consumed 2 ⎟

⎠

⎞ ⎜

⎝

⎛

⎟

⎠

⎞ ⎜

⎝

⎛

Thus, the reader may find it more expedient and less error prone to envision parallel arrangement 

when calculating the mass flow requirement of hydrogen. 

(b) The utilization of fuel in a fuel cell is defined as 

U = 

H

H f

2, consumed

2,in

Therefore the fuel flow rate required to generate 1 MWDC can be calculated as 

hr

2 lb H

148.0

80 %

118.4 = U

H

H = hr

lb H

f

2, consumed

2, in

2

=

(c) To determine the air requirement, first observe that the stoichiometric57 ratio of hydrogen to 

oxygen is 2 to 1 for H2O. Thus, the moles of oxygen required for the fuel cell reaction are 

determined by 

hr

lb mol O 29.38

2 lb mol H

1lb mol O

2.0158 lb H

1lb mol H

hr

lb H

n 118.4 2

2

2

2

2 2

O2 , consumed =⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛

⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛

⎟

⎠

⎞ ⎜

⎝

⎛ =

If 25 percent utilization is required, then the air feed must contain four times the oxygen that is 

consumed 

hr

lb mol O 117.5

0.25 lb mol O consumed

1lb mol O supplied

hr

lb mol O consumed

n 29.38 2

2

2 2

O2 ,supplied =⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛

⎟

⎠

⎞ ⎜

⎝

⎛ =

Because dry air contains 21 percent O2 by volume, or by mole percent, the required mass flow 

rate of dry air is 

hr

lb dry air 16,142

1lb mol of air

28.85 lb dry air

0.21lb mol O

1lb mol air

hr

lb mol O supplied m 117.5

2

2

air,supplied ⎟ =

⎠

⎞ ⎜

⎝

⎛

⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛

⎟

⎠

⎞ ⎜

⎝

⎛ =

Example 9-3 PAFC Effluent Composition 

A PAFC, operating on reformed natural gas (900 lb/hr) and air, has a fuel and oxidant utilization 

of 86 percent and 70 percent, respectively. With the fuel and oxidant composition and molecular 

weights listed below, how much hydrogen will be consumed in lb mol/hr? (b) How much 

oxygen is consumed in lb mol/hr? (c) What is the required air flow rate in lb mol/hr and lb/hr? 

 

57 The stoichiometric ratio is the ratio of atoms in a given molecule. 
9-4 

(d) How much water is generated? (e) What is the composition of the effluent (spent) fuel and 

air streams in mol percent? 

Fuel Data mol percent Air Data mol percent, 

dry 

mol percent, 

wet 

CH4 4.0 

CO 0.4 H2O 0.00 1.00 

CO2 17.6 N2 79.00 78.21 

H2 75.0 O2 21.00 20.79 

H2O 3.0 Total 100.00 100.00 

Total 100.0 

MW 10.55 MW 28.85 28.74 

Solution: 

(a) To determine the lb mol/hr of hydrogen, first determine the molar fuel flow 

hr

lb molfuel 85.29

10.55 lb fuel

1lb molfuel

hr

lb fuel

nfuel, supplied 900 ⎟ =

⎠

⎞ ⎜

⎝

⎛

⎟

⎠

⎞ ⎜

⎝

⎛ =

Thus, 

hr

lb mol H 55.01

100 lb mol H supplied

86 lb mol H consumed

100 lb molfuel

75 lb mol H

hr

lb molfuel

n 85.29 2

2

2 2

H2 consumed =⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛

⎟

⎠

⎞ ⎜

⎝

⎛

⎟

⎠

⎞ ⎜

⎝

⎛ =

(b) To determine how much oxygen is consumed, it is useful to note the overall fuel cell reaction 

H2 (g) + 1⁄2 O2 (g) → H2O (g)

Therefore, 

hr

lb mol O 27.51

1lb mol H

1⁄2 lb mol O

hr

lb mol H

n 55.01 2

2

2 2

O2 , consumed =⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛

⎟

⎠

⎞ ⎜

⎝

⎛ =

(c) The required air flow will be determined on a wet air basis, thus 

hr

lb mol wet air 189.01

20.79 lb mol O

100 lb mol wet air

70 lb mol O consumed

100 lb mol O supplied

hr

lb mol O

n 27.51

2 2

2 2

air, required =⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛

⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛

⎟

⎠

⎞ ⎜

⎝

⎛ =

hr

lb wet air 5,433

1lb mol wet air

28.74 lb wet air

hr

lb mol wet air

mair, required 189.01 ⎟ =

⎠

⎞ ⎜

⎝

⎛

⎟

⎠

⎞ ⎜

⎝

⎛ =
9-5 

(d) Per the overall fuel cell reaction above, the water generated is equal to the moles of hydrogen 

consumed 

hr

lb mol H

n n 55.01 2

H O generated H consumed 2 2 = =

(e) The composition of the effluent is developed in the table below, by working from the left to 

right. The composition is determined by converting the composition to moles, accounting for 

the fuel cell reaction, and converting back to the desired units, mol percent. (Note: 

mol percent is essentially equivalent to volume percent for low pressure gases.) 

Spent Fuel Effluent Calculation 

 mol percent lb mol/hr mol percent 

Gas FC inlet FC inlet FC reaction FC outlet FC outlet 

CH4 4.0 3.41 3.41 11.27 

CO 0.4 0.34 0.34 1.13 

CO2 17.6 15.01 15.01 49.58 

H2 75.0 63.97 -55.01 8.96 29.58 

H2O 3.0 2.56 2.56 8.45 

Total 100.0 85.29 -55.01 30.28 100.00 

In the PAFC, only the moles of hydrogen change on the anode (fuel) side of the fuel cell. The 

other fuel gas constituents simply pass through to the anode exit. These inert gases act to dilute 

the hydrogen, and as such will lower the cell voltage. Thus, it is always desirable to minimize 

these diluents as much as possible. For example, to reform natural gas, significant quantities of 

steam are typically added to maximize the reforming reactions. The wet reformer effluent would 

commonly have a water composition of 30 to 50 percent. The reformate gas utilized in this 

example has been “dried” to only 3 percent moisture via condensation in a contact cooler. 

The spent oxidant composition is calculated in a similar manner. Note that in both the PAFC and 

PEFC, the water is generated on the cathode (air) side. This can be seen from the cathode 

reaction listed below and the following table listing the fuel cell reaction quantities. 

1⁄2O2 + 2H+

 + 2e- → H2O 

 Spent Air Effluent Calculation 

 mol percent lb mol/hr mol percent 

Gas FC inlet FC inlet FC reaction FC outlet FC outlet 

H2O 1.00 1.89 55.01 56.90 26.28 

N2 78.21 147.82 147.82 68.27 

O2 20.79 39.30 -27.51 11.79 5.44 

Total 100.00 189.01 27.51 216.51 100.00 
9-6 

Example 9-4 MCFC Effluent Composition - Ignoring the Water Gas Shift Reaction 

An MCFC operating on 1,000 lb/hr of fuel gas and a 70 percent air/30 percent CO2 oxidant has a 

fuel and oxidant utilization of 75 percent and 50 percent respectively. With the fuel and oxidant 

composition and molecular weights listed below, (a) How much hydrogen will be consumed in 

lb mol/hr? (b) How much oxygen is consumed in lb mol/hr? (c) What are the required air and 

oxidant flow rates in lb mol/hr? (d) How much CO2 is transferred from the cathode to the 

anode? (e) What is the composition of the effluent (spent) fuel and oxidant streams in mol 

percent (ignoring the water gas shift reaction)? 

Fuel Data Mol percent Air Air + CO2

CH4 0.0 Oxidant Data mol percent, 

wet 

Mol percent, 

wet 

CO 0.0 CO2 0.00 30.00 

CO2 20.0 H2O 1.00 0.70 

H2 80.0 N2 78.21 54.75 

H2O 0.0 O2 20.79 14.55 

Total 100.0 Total 100.00 100.00 

MW 10.42 MW 28.74 33.32 

Solution:

(a) To determine the lb mol/hr of hydrogen, first determine the molar fuel flow 

hr

lb molfuel 96.02

10.42 lb fuel

1lb molfuel

hr

lb fuel

nfuel, supplied 1000 ⎟ =

⎠

⎞ ⎜

⎝

⎛

⎟

⎠

⎞ ⎜

⎝

⎛ =

Thus, 

hr

lb mol H 57.61

100 lb mol H supplied

75 lb mol H consumed

100 lb molfuel

80 lb mol H

hr

lb molfuel

n 96.02 2

2

2 2

H2 consumed =⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛

⎟

⎠

⎞ ⎜

⎝

⎛

⎟

⎠

⎞ ⎜

⎝

⎛ =

(b) To determine how much oxygen is consumed, it is useful to note the overall fuel cell reaction 

H2 (g) + 1⁄2 O2 (g) → H2O (g) 

Therefore, 

hr

lb mol O 28.81

1lb mol H

1⁄2 lb mol O

hr

lb mol H

n 57.61 2

2

2 2

O2 , consumed =⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛

⎟

⎠

⎞ ⎜

⎝

⎛ =

(c) The required air flow will be determined on a wet air basis: 
9-7 

hr

lb mol wet air 277.11

20.79 lb mol O

100 lb mol wet air

50 lb mol O consumed

100 lb mol O supplied

hr

lb mol O

n 28.81

2 2

2 2

air, required =⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛

⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛

⎟

⎠

⎞ ⎜

⎝

⎛ =

The oxidant flow rate will be calculated knowing that air is 70 percent of the total oxidant flow: 

hr

lb mol oxidant 395.86

70 lb mol wet air

100 lb mol oxidant

hr

lb mol wet air

noxidant, required 277.11 ⎟ =

⎠

⎞ ⎜

⎝

⎛

⎟

⎠

⎞ ⎜

⎝

⎛ =

(d) Per the overall fuel cell reaction presented below, the quantity of CO2 transferred from the 

cathode to the anode side of the fuel cell equals the moles of hydrogen consumed: 

H O CO H O CO 2, anode 2, cathode 2, cathode 2 , anode 2, anode + + →+ 1

2

Therefore, 

hr

lb mol

n n 57.61 CO transferred H consumed 2 2 = =

(e) The composition of the fuel effluent is developed in the table below, by working from left to 

right. The composition is determined by converting the composition to moles, accounting for 

the fuel cell reaction, and converting back to the desired units, mol percent. 

Spent Fuel Effluent Calculation 

 mol percent lb mol/hr Mol percent 

Gas FC inlet FC inlet FC reaction FC outlet FC outlet 

CH4 0.0 0.00 0.00 0.00 

CO 0.0 0.00 0.00 0.00 

CO2 20.0 19.20 57.61 76.82 50.00 

H2 80.0 76.82 -57.61 19.20 12.50 

H2O 0.0 0.00 57.61 57.61 37.50 

Total 100.0 96.02 -57.61 153.63 100.00 

The oxidant effluent composition is calculated in a similar manner. Note that in the MCFC, both 

oxygen and carbon dioxide are consumed on the cathode (air) side. This can be seen from the 

cathode reaction listed below and the following table listing the fuel cell reaction quantities. 

1⁄2O2 + CO2 + 2e- → CO3

= (MCFC cathode reaction)
9-8 

Spent Oxidant Effluent Calculation 

 mol percent lb mol/hr Mol percent 

Gas FC inlet FC inlet FC reaction FC outlet FC outlet 

CO2 30.00 83.13 -57.61 25.52 13.38 

H2O 0.70 1.94 1.94 1.02 

N2 54.70 151.71 151.71 79.56 

O2 14.6 40.33 -28.81 11.52 6.04 

Total 100.00 277.11 -86.42 190.69 100.00 

Example 9-5 MCFC Effluent Composition - Accounting for the Water Gas Shift Reaction 

For the above example, determine the composition of the effluent (spent) fuel stream in mol 

percent including the effect of the water gas shift reaction. Assume an effluent temperature of 

1200 oF and that the water gas shift reaction proceeds to equilibrium. 

Solution:

For convenience, the water gas shift reaction is presented below: 

CO + H2O ⇔ CO2 + H2

The double headed arrow is used to indicate that the reaction is in equilibrium. That is, the 

reaction does not proceed completely to the left or to the right. Instead, the reaction proceeds to 

an equilibrium point, where both “products” and “reactants” remain. The equilibrium 

composition depends on the initial composition and final temperature and pressure. Fortunately, 

the equilibrium concentrations can be determined by a temperature dependent equilibrium 

constant, K, and the following equation: 

[ ][ ]

[ ][ ] K= 

CO H

CO H O

2 2

2

The quantities in brackets represent the thermodynamic activities of the reacting species. 

Because the reaction is equimolar, the quantities in brackets are also equal to the mole fractions 

of the respective components. At 1200 oF, the equilibrium constant is 1.96758. A check of the 

compositions from the preceding example shows that those concentration levels are not in 

equilibrium. 

[ ][ ]

[ ][ ]

[ ][ ]

[ ][ ]

CO H

CO H O

0.50 0.125

0.0 0.375

 1.967 2 2

2

= =∞ ≠

 

58 Equilibrium constants can be calculated from fundamental chemical data such as Gibbs free energy, or can be 

determined from temperature dependent tables or charts for common reactions. One such table has been 

published by Girdler Catalysts (1). The following algorithm fits this temperature dependent data to within 5% 

for 800 to 1800 oF, or within 1% for 1000 to 1450 oF: Kp= e(4,276/T -3.961). Kp(1200 oF or 922K) equals 1.967. 
9-9 

Because the numerator contains the products of the reaction and the denominator contains the 

reactants, it is clear that the reaction must proceed more towards the reactants. By introducing a 

variable, x, to represent the extent of the reaction to proceed to the right and rewriting the 

equilibrium equation as: 

[ ][ ]

[ ][ ]

[ ][ ]

[ ][ ] K= 

CO H

CO H O

0.50 + x 0.125 + x

0.0 - x 0.375 - x

2 2

2

= = 1967 .

This can be solved algebraically as follows: 

[ ][ ]

[ ][ ] K= 

CO x H x

CO - x H O - x

2 2

2

+ +

can be written as 

K CO - x H O - x = CO x H x [ ][ ] [ ][ ] 2 22 + +

which can be expanded as 

K x - CO H O CO H O = x CO H CO H { } ( ) [ ][ ] [ ][ ] ( ) [ ][ ] [ ][ ] 2

2 2

2 + + +++ 2 2 22 x x

which can be combined to 

{ } [ ][ ] [ ][ ] ( ) { } [ ][ ] [ ][ ] (1- K)

a

x + CO H K CO H O

b

x + CO H CO H O K

c

= 0 2

123 2 2 2 22 2 1 2 444444 3 444444 1 2 44444 3 44444 ++ + −

This is in the standard quadratic form of 

ax2

 + bx + c= 0 

which can be solved by the quadratic formula 

x

b b ac

a = −± − 2 4

2

Substituting the appropriate values for K and the concentrations yields two roots of -0.0445 and 

1.454. The larger root is physically impossible; it “wants to” react more CO and H2O than are 

initially present. The remaining root of -0.0445 is used to compute the equilibrium gas 

composition, which is shown in the following table. 
9-10 

Spent Fuel Effluent Calculation 

 mol percent Lb mol/hr, assuming 100 lb mol/hr basis Mol percent 

Gas 

FC outlet 

w/o shift. 

FC outlet

w/o shift

effect of 

shift rxn

FC outlet in 

shift equil.

FC outlet in 

shift equil. 

CO 0.00 0.00 4.45 4.45 4.45 

CO2 50.00 50.00 -4.45 45.55 45.55 

H2 12.50 12.50 -4.45 8.05 8.05 

H2O 37.50 37.50 4.45 41.95 41.95 

Total 100.0 100.00 0.00 100.00 100.00 

Example 9-6 SOFC Effluent Composition - Accounting for Shift and Reforming Reactions 

An SOFC operates at 1800 °F on 100 percent methane (CH4) and a fuel utilization of 85 percent. 

What is the composition of the effluent (spent) fuel in mol percent? Assume that the methane is 

completely reformed within the fuel cell, and the moisture required for reforming is supplied by 

internal recirculation. 

Solution:

There are many different ways to approach this problem, some of which may seem rather 

complex because of the simultaneous reactions (fuel cell, reforming, and water gas shift 

reactions) and the recycle stream supplying moisture required for the reforming reaction. The 

solution to this problem can be simplified by focusing on the fuel cell exit condition. 

First, write the relevant reactions: 

SOFC

Recycle

Fuel Feed Point of Interest

CH4 + 2H2O → 4H2 + CO2 (Steam Reforming Reaction) 

H2, anode + 1⁄2O2, cathode → H2O, anode (Fuel Cell Reaction) 

CO + H2O ⇔ CO2 + H2 (Water Gas Shift Reaction) 

Next combine the reforming reaction and the fuel cell reaction into an overall reaction for that 

portion of the fuel that is consumed (i.e., 85 percent). The combined reaction is developed by 

adding the steam reforming reaction to 4 times the fuel cell reaction. The factor of four allows 

the hydrogen molecules to drop out of the resulting equation because it is fully utilized. 
9-11 

CH4, anode + 2H2O, anode → 4H2, anode + CO2, anode (Steam Reforming Reaction) 

4H2, anode + 2O2, cathode → 4H2O, anode (Fuel Cell Reaction) 

CH4, anode + 2O2, cathode → 2H2O, anode + CO2, anode (Combined Reforming and FC 

Reactions) 

For ease of calculation, assume a 100 lb/hr basis for the methane. 

hr

lb molCH 6.23

16.043 lb CH

1lb molCH

hr

lb CH

n 100 4

4

4 4

fuel, supplied =⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛

⎟

⎠

⎞ ⎜

⎝

⎛ =

Thus, 85 percent, or 5.30 lb mol CH4 /hr, will be reformed and consumed by the fuel cell. The 

remainder will be reformed but not consumed by the fuel cell reaction. These changes are 

summarized in the following table:

Spent Fuel Effluent Calculation 

 mol percent lb mol/hr mol percent

Gas FC inlet FC inlet Ref / FC rxn Reforming FC outlet FC outlet

CH4 100.0 6.23 -5.30 -0.93 0.00 0.00

CO 0.0 0.00 0.00 0.00 0.00 0.00

CO2 0.0 0.00 5.30 0.93 6.23 33.33

H2 0.0 0.00 0.00 3.74 3.74 20.00

H2O 0.0 0.00 10.60 -1.87 8.73 46.67

Total 100.0 6.23 10.60 1.87 18.70 100.00

This intermediate solution reflects only two out of three reactions. Now apply the water gas shift 

reaction to determine the true exit composition. Use the quadratic equation listed in Example 9-5 

to determine how far the reaction will proceed, where x is the extent of the reaction in the 

forward direction as written: 

CO + H O CO + H 2 22

x

← →⎯

x

b b ac

a = −± − 2 4

2

The equilibrium constant, K, at 1800 °F (1255 °K) is 

K = e (4276/1255-3.961) = 0.574 

a = (1- K) = (1- 0.574) = 0.426

b = { } [ ][ ] [ ][ ] CO2 + H2 + K( ) CO + H2O = 0.3333 + 0.2000 + 0.574 *(0.00 + 0.4667) = 0.8012

c = − { } [ ][ ] CO H CO H O K = (0.3333)(0.20) - (0.00)(0.4667)(0.574) = 0.0666 22 2 [ ] [ ]
9-12 

x

b b ac

a = −± − −± − 2 2 4

2

08012 08012 4 0 426 0 0666

2 0 426

 = = - 0.0873 and -1.794 . ( . ) ( . )( . )

(. )

The only root that is physically possible is x = -0.0873. The following table summarizes the 

effect of accounting for the water gas shift equilibrium: 

Spent Fuel Effluent Calculation 

 mol percent Lb mol/hr, assuming 100 lb mol/hr basis Mol percent 

Gas 

FC outlet 

w/o shift. 

FC outlet

w/o shift

Effect of 

shift rxn

FC outlet in 

shift equil.

FC outlet in 

shift equil. 

CO 0.00 0.00 -(-8.73) 8.73 8.73 

CO2 33.33 33.33 -8.73 24.61 24.61 

H2 20.00 20.00 -8.73 11.27 11.27 

H2O 46.67 46.67 -(-8.73) 55.39 55.39 

Total 100.00 100.00 0.00 100.00 100.00 

Example 9-7 Generic Fuel Cell - Determine the Required Cell Area and Number of Stacks 

Given a desired output of 2.0 MWDC and the desired operating point of 600 mV and 

400 mA/cm2

, (a) How much fuel cell area is needed? (b) Assuming a cell area of 1.00 m2 per 

cell and 280 cells per stack, how many stacks are needed for this 2.0 MW unit? 

Solution:

(a) Recalling that power is the product of voltage and current, first determine the total current 

for the fuel cell as 

I= 

P

V

 = 

2.0 MW

0.600 V

10 W

1 MW

1 VA

1 W

1 kA

1000 A 3,333 kA 

6 ⎛

⎝

⎜ ⎞

⎠

⎟

⎛

⎝

⎜ ⎞

⎠

⎟

⎛

⎝

⎜ ⎞

⎠

⎟

⎛

⎝

⎜ ⎞

⎠

⎟ =

Because each individual cell will operate at 400 mA/cm2

, determine the total area as 

Area = I

Current Density = 3,333 kA

400 mA / cm

1000 mA

1 A

1000 A

1 kA 8,333,333 cm 2

⎛ 2

⎝

⎜ ⎞

⎠

⎟

⎛

⎝

⎜ ⎞

⎠

⎟

⎛

⎝

⎜ ⎞

⎠

⎟ =

b) The number of required cells and stacks are calculated simply as 

( )

( ) No. of Cells = 

8,333,333 cm

1 m per cell

1 m

10,000 cm

 = 833 cells

2

2

2

2

⎛

⎝

⎜ ⎞

⎠

⎟

( )

( ) No. of Stacks = 

833 cells

280 cells per stack

 = 2.98 stacks 3 stacks ≅
9-13 

9.1.2 Fuel Processing Calculations 

Example 9-8 Methane Reforming - Determine the Reformate Composition 

Given a steam reformer operating at 1400 oF, 3 atmospheres, pure methane feed stock, and a 

steam to carbon ratio of 2 (2 lb mol H2O to 1 lb mol CH4), (a) List the relevant reactions; (b) 

Determine the concentration assuming the effluent exits the reactor in equilibrium at 1400 oF; 

(c) Determine the heats of reaction for the reformer's reactions; (d) Determine the reformer's heat 

requirement assuming the feed stocks are preheated to 1400 oF; (e) Considering LeChâtelier's 

Principle, indicate whether the reforming reaction will be enhanced or hindered by an elevated 

operating temperature; (f) Considering LeChâtelier's Principle, indicate whether increased 

pressure will tend to promote or prevent the reforming reaction. 

Solution:

(a) The relevant reactions for the steam reformer are presented below: 

CH4 + H2O ⇔ 3H2 + CO (Steam Reforming Reaction) 

CO + H2O ⇔ CO2 + H2 (Water Gas Shift Reaction) 

A third reaction is presented below; this reaction is simply a combination of the other two. Of 

the three reactions, any two can be used as an independent set of reactions for analysis, and can 

be chosen for the user's convenience. 

CH4 + 2H2O ⇔ 4H2 + CO2 (Composite Steam Reforming Reaction) 

(b) The determination of the equilibrium concentrations is a rather involved problem, requiring 

significant background in chemical thermodynamics, and will not be solved here. One 

aspect that makes this problem more difficult than Example 9-6, which accounted for the 

steam reforming reaction within the fuel cell, is that the reforming reaction cannot be 

assumed to proceed to completion as in the former example. In Example 9-6, hydrogen was 

consumed within the fuel cell, thus driving the reforming reaction to completion. Without 

being able to assume the reforming reaction goes to completion, two independent 

equilibrium reactions must be solved simultaneously. The solution to this problem is most 

easily accomplished with chemical process simulation programs using a technique known as 

the minimization of Gibbs free energy. To solve this problem by hand is an arduous, timeconsuming task. 

The ASPENTM computer solution to this problem is provided below: 

 Inlet Composition 

(lb mols/hr) 

Effluent Composition

(lb mols/hr) 

Effluent Composition 

(mol fraction) 

CH4 100 11.7441 2.47 

CO 0 64.7756 13.59 

CO2 0 23.4801 4.93 

H2 0 288.2478 60.49 

H2O 200 88.2639 18.52 

Total 300 476.5115 100.00 
9-14 

(c) This problem is rather time-consuming to solve without a computer program, and will 

therefore be left to the ambitious reader to solve59 from thermodynamic fundamentals. As an 

alternative, the reader may have access to tables that list heat of reaction information for 

important reactions. The following temperature-dependent heats of reaction were found for 

the water gas shift and reforming reactions in the Girdler tables (1). 

 CH4 + H2O ⇔ 3H2 + CO ∆Hr(1800 oF)= 97,741 Btu/lb mol 

 

CO + H2O ⇔ CO2 + H2 ∆Hr(1800 oF)= -13,892 Btu/lb mol 

Note: a positive heat of reaction is endothermic (heat must be added to maintain a constant 

temperature), while a negative heat of reaction is exothermic (heat is given off). 

(d) With knowledge of the equilibrium concentration and the heats of reaction, the heat 

requirement for the reformer can be approximated. Knowing that for each lb mol of CH4

feed, 88.3 percent [(100-11.7)/100= 88.3 percent] of the CH4 was reformed, and 26.6 percent 

[23.5/88.3= 26.6 percent] of the formed carbon monoxide shifts to carbon dioxide, then the 

overall heat generation for each lb mol of methane feed can be developed from 

( ) 1 88 3%

100%

 lbmol CH 97 741

 CH reacted

 CH feed

Btu

lbmol reformed CH = 86,300 Btulbmol CH feed 4

4

4 44

. , ⎛

⎝

⎜ ⎞

⎠

⎟

⎛

⎝

⎜ ⎞

⎠

⎟

( ) feed 4 lbmolCH

Btu = - 3,300

lbmolCO rxn

-13,982 Btu

lbmolCO feed

26.6% CO shifts

rxtd 4 lbmolCH

1lbmolCO

feed 4 100% CH

rxtd. 4 88.3% CH

4 1lbmolCH ⎟

⎠

⎞ ⎜

⎝

⎛

⎟

⎠

⎞ ⎜

⎝

⎛

⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛

⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛

Summing these results, the heat requirement for the reformer is about 83,000 Btu/lb mol of CH4

fed to the reformer. Because this value is positive, the overall reaction is endothermic and heat 

must be supplied. This approximate value neglects the change in sensible heat in taking the 

reactants from 1400 °F to the reference temperature of 1800 °F, and then the products from the 

reference temperature (1800 °F) back to 1400 °F. 

(e) LeChâtelier's Principle simply states that "if a stress is applied to a system at equilibrium, 

then the system readjusts, if possible, to reduce the stress". In this reforming example, 

LeChâtelier's Principle dictates whether higher or lower temperatures will promote the 

reforming reaction just by knowing that the reaction is endothermic. To facilitate the 

application of the principle, write the endothermic reforming reaction (which is the dominant 

heat of reaction) with a heat term on the left side of the equation. 

CH4 + H2O + Heat ⇔ 3H2 + CO 

 

59 The reader can refer to Reference 2, Example 4-8 for the solution of a related problem. 
9-15 

Consider that raising the temperature of the system is the applied stress; the stress will be 

relieved when the reaction proceeds forward. Therefore, the reforming reaction is 

thermodynamically favored by high temperatures. 

(f) To solve this application of LeChâtelier's Principle, write the reforming reaction in terms of 

the number of gaseous molecules on the left and right sides. 

CH4(g) + H2O(g) ⇔ 3H2(g) + CO(g)

2Molecules(g) ⇔ 4Molecules(g)

Now imagine the reformer at equilibrium, and increase the pressure (the applied stress), then the 

reaction will try to proceed in a direction that will reduce the pressure (stress). Because a 

reduction in the number of molecules will reduce the stress, elevated pressure will tend to inhibit 

the reforming reaction. (Note: reformers often operate at moderate pressures, for operation at 

pressure will reduce the equipment size and cost. To compensate for this elevated pressure, the 

designer may be required to raise the temperature.) 

Example 9-9 Methane Reforming - Carbon Deposition 

Given the problem above, (a) List three potential coking (carbon deposition, or sooting) 

reactions, and (b) Considering LeChâtelier's Principle, indicate whether excess steam will tend to 

promote or inhibit the coking reactions. 

Solution:

(a) Three of the most common/important carbon deposition equations are presented below. 

CH4 ⇔ C + 2H2 (Methane Coking) 

2CO ⇔ C + CO2 (Boudouard Coking) 

CO + H2 ⇔ C + H2O (CO Reduction) 

(b) Considering LeChâtelier's Principle, the addition of steam will clearly inhibit the formation 

of soot from the CO Reduction reaction. The introduction of excess steam will encourage 

the reaction to proceed towards the reactants, i.e., away from the products, of which water is 

one. Since water does not participate in the other two reactions, excess steam does not have 

a direct effect on either the Methane coking or the Boudouard coking reactions except that 

the presence of steam will dilute the reactant and product concentrations. Because neither 

reaction is equimolar with respect to gaseous species, the effect will be ambivalent; the 

Methane coking reaction will be driven forward while the Boudouard coking reaction will 

reverse. In addition, the reverse reaction of CO-reduction stimulated by excess steam will 

increase the presence of CO, driving the Boudouard coking reaction forward. Overall, the 

addition of steam is useful at preventing soot from ruining the expensive catalysts used in 

reformers and fuel cell systems. Too much steam, however, simply adds an unnecessary 

operating cost. 
9-16 

Determination of the minimum steam to carbon ratio that will inhibit carbon deposition is of 

interest to the fuel cell system designer. The interested reader is referred to references (4), (5), 

and (6). 

The quantity of steam that would preclude the formation of soot based upon thermodynamic 

equilibrium could be calculated based on minimization of Gibbs free energy. However, it may 

not be necessary to add as much steam as is implied by this method. Although soot formation 

may be thermodynamically favored under certain conditions, the kinetics of the reaction can be 

so slow that sooting would not be a problem. Thus, the determination of sooting on a kinetic 

basis rather than equilibrium basis is of significant interest. The interested reader is referred to 

reference (6). When temperature drops to about 750 oC, kinetic limitations preclude sooting (7). 

However, above this point, the composition and temperature together determine whether sooting 

is kinetically precluded. Typically, steam reformers have operated with steam to carbon ratios of 

2 to 3, depending on the operating conditions in order to provide an adequate safety margin. An 

example calculation presented in reference (6) reveals that conditions requiring a steam to carbon 

ratio of 1.6 on a thermodynamic basis can actually support a steam to carbon ratio of 1.2 on a 

kinetic basis. 

9.1.3 Power Conditioners 

Example 9-10 Conversion between DC and AC Power 

Given a desired output of 1.0 MWAC, and an inverter efficiency of 96.5 percent, what DC output 

level is required from the fuel cell stack? 

Solution:

(a) The required DC power output level is found simply as the quotient of AC power and the 

inverter efficiency as demonstrated below. 

MW = 1.0 MW ( ) 1 MW

96.5% MW DC AC

DC 1.036 MW

AC

DC

⎛

⎝

⎜ ⎞

⎠

⎟ =

9.1.4 Others 

Numerous other unit operations and subsystems can be found in fuel cell processes. These 

operations and subsystems are well documented in many references (2,8,9,10). For convenience, 

the unit operations that are commonly found within fuel cell power system are listed below: 

• heat exchangers • intercoolers 

• pumps • direct contact coolers 

• compressors • gasification 

• expanders • gas clean up 

9.2 System Issues

This section covers performance issues such as higher heating value (HHV), lower heating value 

(LHV), cogeneration efficiency, heat rate, and cogeneration steam duty calculations. 
9-17 

9.2.1 Efficiency Calculations 

Example 9-11 LHV, HHV Efficiency and Heat Rate Calculations 

Given a 2.0 MWAC fuel cell operating on 700 lb/hr of methane, what is (a) the HHV60 thermal 

input of the methane gas, (b) the LHV thermal input, (c) the HHV electric efficiency, (d) the 

LHV electric efficiency, and (e) the HHV heat rate? Assume the higher and lower heating value 

of methane as 23,881 and 21,526 Btu/lb respectively. 

Solution:

(a) The HHV thermal input of the methane gas is 

( ) 16.716 MMBtu/hr

10 Btu 

1MMBtu

1lb CH

23,881Btu, HHV HHV Thermal Input = 700 lb/hr CH 6

4

4 ⎟ =

⎠

⎞ ⎜

⎝

⎛

⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛ 

or 

( ) 4.899 MWt 3.412 MMBtu 

1MW HHV Thermal Input = 16.716 MMBtu/hr ⎟ =

⎠

⎞ ⎜

⎝

⎛

(b) The LHV thermal input of the methane gas is 

( ) 15.068MMBtu/hr

Btu 6 10

1MMBtu

4 1lb CH

21,526 Btu, LHV

4 LHV ThermalInput = 700 lb/hr CH ⎟ =

⎠

⎞ ⎜

⎝

⎛

⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛ 

or 

( ) 4.416 MWt 3.412 MMBtu 

1MW LHV Thermal Input = 15.068 MMBtu/hr ⎟ =

⎠

⎞ ⎜

⎝

⎛

(c) The HHV electrical efficiency is 

= 40.8% HHV

4.899 MWt, HHV

2.0 MW = Input, HHV

Output Electrical Efficiency (HHV) = AC ⎟

⎠

⎞ ⎜

⎝

⎛ ⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛

(d) The LHV electrical efficiency is 

= 45.3% LHV

4.416 MWt, LHV

2.0 MW = Input, LHV

Output Electrical Efficiency (LHV) = AC ⎟

⎠

⎞ ⎜

⎝

⎛ ⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛

Note: Because a fuel's LHV is less than or equal to its HHV value, the LHV efficiency will 

always be greater than or equal to the HHV efficiency. 

(e) Heat rate is the amount of heat (Btu/hr) required to produce a kW of electricity. 

Alternatively it can be thought of as an inverse efficiency. Because 1 kW is equivalent to 

3,412 Btu/hr, a heat rate of 3,412 Btu/kWh represents an efficiency of 100 percent. Note 

 

60 Heating values are expressed as higher or lower heating values (HHV or LHV). Both higher and lower heating 

values represent the amount of heat released during combustion. The difference between the HHV and LHV is 

simply whether the product water is in the liquid phase (HHV), or the gaseous phase (LHV). 
9-18 

that as the efficiency goes up, the heat rate goes down. The HHV heat rate for this example 

can be calculated easily from either the HHV efficiency or the thermal input. Both methods 

are demonstrated below: 

(HHV) kWh

Btu = 8,360 40.8%

3412 Btu/kWh = Efficiency, HHV

3412 Btu/kWh Heat Rate (HHV) = ⎟

⎠

⎞ ⎜

⎝

⎛ ⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛

or 

(HHV) kWh

Btu = 8,360 2,000 kW

16,716,000 Btu/hr = Output

Input, HHV Heat Rate (HHV) = ⎟

⎠

⎞ ⎜

⎝

⎛ ⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛

Note: The LHV to HHV ratio of 90 percent for methane (21,526/23,881 = 90 percent) is typical 

for natural gas, while this ratio is roughly 94 percent for fuel oils. Common coals typically have 

a LHV to HHV ratio of 92 to 96 percent depending upon the hydrogen and moisture content61. 

Typically, gas turbine based cycles are presented on an LHV basis. Conventional power plants, 

such as coal-, oil-, and gas-fired steam generator/steam turbine cycles are presented on an HHV 

basis within the U.S. and on an LHV basis throughout the rest of the world. 

Example 9-12 Efficiency of a Cogeneration Fuel Cell System 

Given the system described in Example 9-11, what is the combined heat and power efficiency 

assuming that cycle produces 2 tons/hr of 150 psia/400 oF steam? Assume a feedwater 

temperature of 60 oF. 

Solution:

Before calculating the cogeneration efficiency, first determine the heat duty associated with 

steam production. This requires knowledge of the steam and feed water enthalpies, which can be 

found in the ASME Steam Tables (11) as indicated below: 

Temperature (oF) Pressure (psia) Enthalpy (Btu/lb) 

Steam 400 150 1219.1 

Feedwater 60 180 28.6 

The steam heat duty is calculated as 

( )( ) ( )( ) 4.762 MMBtu/hr

Btu 6 10

1MMBtu

Heat Duty = mass flow Change in enthalpy = 4000 lb/hr 1219.1 − 28.6 Btu/lb ⎟ =

⎠

⎞ ⎜

⎝

⎛

Alternatively, this heat duty can be expressed as 1.396 MWt, [4.762 / 3.412 = 1.396 MW]. Thus, 

the combined heat and power efficiency is calculated as 

 

61 The difference between the LHV and HHV heating values can be estimated by (1055 Btu/lb)*w, where w is the 

lbs moisture after combustion per lb of fuel. Thus, w can be determined from the fuel's hydrogen and moisture 

content by w= moisture + 18/2 * hydrogen. [e.g., for a fuel with 10% moisture and 4% hydrogen, the LHV to 

HHV difference is 485 Btu/lb, [i.e., 1055*(0.10 + 0.04*9)=485.] 
9-19 

= 69.3% HHV

4.899 MWt, HHV

+1.396 MWt AC 2.00 MW = Input, HHV

Output Combined Heat & ElectricalEfficiency (HHV) = ⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛ ⎟

⎠

⎞ ⎜

⎝

⎛

9.2.2 Thermodynamic Considerations 

Example 9-13 Production of Cogeneration Steam in a Heat Recovery Boiler (HRB) 

Given 10,000 lb/hr of 700 oF cycle exhaust gas passing through a heat recovery boiler (HRB) (a) 

How much 150 psia, 400 oF steam can be produced? (b) How much heat is transferred from the 

gas in the HRB? (c) What is the exhaust temperature of the gas leaving the HRB? and (d) Sketch 

the T-Q (temperature-heat) diagram for the HRB. Assume a gas side mean heat capacity of 

0.25 Btu/lb-oF, an evaporator pinch temperature of 30 oF, a feedwater temperature of 60 oF, and 

an evaporator drum pressure of 180 psia to allow for pressure losses. 

Solution:

(a) Develop a solution strategy by examining a typical HRB T-Q diagram presented below. 

From this diagram, observe that the pinch point (the minimum temperature differential 

between the gas and saturated steam) limits the steam production. To produce more steam, 

the lower steam line would be stretched to the right until it "bumped" into the hot gas line. 

At the point of contact, both the hot gas and saturated steam would be at the same 

temperature. This is thermodynamically impossible, because heat will only "flow" from a 

higher temperature to a lower one. In practice, the temperature approach at the pinch point is 

kept large enough (15 to 40 oF) to prevent an unusually large and expensive evaporator. 

Because the pinch limits the steam production, the sensible heat available in the exhaust gas 

from 700 °F to the pinch point will determine how much steam can be produced. 

0

100

200

300

400

500

600

700

800

0 20 40 60 80 100 120 Q

Temperature

Exhaust Gas

Saturated steam

Superheated 

 steam

Pinch

Tsat

TF

Tg,1

Feedwater

Tg,

Tg,

QSH QEvap QEcon

Tg,

TS

Q0 Q1 Q2 Q3

The governing equations for the heat available in the gas down to the pinch point (Tg,0 to Tg,2), 

and the corresponding heat absorbed by the superheated and saturated steam are presented 

below. 

Q (m )(C )(T - T ) SH + Evap

gas = gas p g,0 g,2

Q (m )(h - h ) SH + Evap

steam = steam superheated f
9-20 

Q Q SH + Evap

gas 

SH + Evap

steam =

Calculate QSH + Evap

gas based on the steam saturation temperature from the steam tables. By using 

the ASME steam tables (11), determine the saturation temperature and enthalpies of interest: 

hsuperheated (150 psia, 400 oF) = 1219.1 Btu/lb 

hf (180 psia, saturated water) = 346.2 Btu/lb 

Tsat (180 psia, saturated steam/water) = 373.1 oF 

Tg,2 = Tsat + 30 = 403.1 °F 

Now solve for QSH + Evap

gas

( ) hr

Btu 700 - 403.1 F 742,000 lb F

Btu 0.25

hr

lb Q 10,000 o

o

gas

SH + Evap ⎟ = ⎠

⎞ ⎜

⎝

⎛

⎟

⎠

⎞ ⎜

⎝

⎛ =

Substitute this heat value into the steam side equation to solve directly for the steam mass flow 

rate: 

( ) m =

 Q

(h - h ) = 742,000 

1219.1 - 346.2 

 = 850 lb

hr steam

SH + Evap

steam 

superheated f

Btu

hr

Btu

lb

(b) Knowing the water/steam mass flow rate, the HRB heat duty can be calculated using the 

following equations: 

hfeedwater (60 oF) = 28.6 Btu/lb 

Q (m )(h - h ) = 850 - 28.6 = 1,012,000 ( )( ) Btu

hr

Total

steam steam superheated feedwater

lb

hr

Btu

lb = 12191. 

(c) The gas temperature leaving the HRB (Tg,3) is now easily calculated, because the total heat 

transferred to the steam is equivalent to that lost by the gas stream: 

Q (m )(C )(T - T ) gas p g,0 g,3

gas

Total = 

Thus, 

( ) 700 F- T

lb F

Btu 0.25

hr

lb gas 10,000 hr

Btu 1,012,000 g,3

o

o ⎟

⎠

⎞ ⎜

⎝

⎛

⎟

⎠

⎞ ⎜

⎝

⎛ =
9-21 

Solving, Tg,3 = 295 oF. 

(d) Because a constant mean Cp was assumed for the exhaust gas over the temperature range of 

interest, simply draw a straight line from 700 oF to 295 oF, with the 295 oF corresponding to 

a transferred quantity of heat of 1.01 MMBtu/hr. On the steam side, separately determine 

the heat absorbed by the superheater, the evaporator, and the economizer. These heats are 

determined by the following equations: 

Q (m )(h - h ) SH

steam = steam superheated g

Q (m )(h - h ) Evap

steam = steam g f

Q (m )(h - h ) Econ

water = water f feedwater

Substitute the known flow and enthalpy data and solve for these three quantities: 

hg (180 psia, saturated steam) = 1196.9 Btu/lb 

hr

Btu Q (850 )(1219.1-1196.9 ) = (850 )(22.2 ) 18,900 lb

Btu

hr

lb

lb

Btu

hr

steam lb

SH = =

hr

Btu Q (850 )(1196.9 - 346.2 ) = (850 )(850.7 ) = 723,100 lb

Btu

hr

lb

lb

Btu

hr

steam lb

Evap =

hr

Btu Q (850 )(346.2 - 28.6 ) = (850 )(317.6 ) = 270,000 lb

Btu

hr

lb

lb

Btu

hr

water lb

Econ =

Use these values to calculate cumulative heat duties: 

 at 373.1 F

hr

MMBtu = 0.019

hr

Btu Q = Q 18,900 steam o

1 SH =

at 373.1 F

hr

MMBtu = 0.742

hr

Btu = 742,000 hrBtu Q = Q + Q 18,900 + 723,100 steam o

2 1 Evap =

at 60 F

hr

MMBtu =1.012

hr

Btu =1,012,000 hrBtu Q = Q + Q 742,000 + 270,000 water o

3 2 Econ =

Plotting these points on the chart below yields the following T-Q diagram. 
9-22 

0

100

200

300

400

500

600

700

0 0.2 0.4 0.6 0.8 1

Q, MMBtu/hr

Temperature, F

403.1F

373.1F

30F Pinch

295F

60F

9.3 Supporting Calculations 

Example 9-14 Molecular Weight, Density and Heating Value Calculations 

Given the fuel gas composition presented below, what is (a) the molecular weight, (b) the higher 

heating value in Btu/ft3

? (c) the density of the gas in lb/ft3 at 1 atm and 60 oF? (d) the higher 

heating value in Btu/lb, and (e) the lower heating value in Btu/ft3

? 

Fuel 

Constituent 

mol percent 

CH4 4.0 

CO 0.4 

CO2 17.6 

H2 75.0 

H2O 3.0 

Total 100.0 

Solution:

(a) Before determining the molecular weight of the fuel gas mixture, develop the molecular 

weights of each of the gas constituents in the following table: 

Fuel 

Constituent 

MW Derivation MW 

CH4 (12.01) + 4*(1.008) = 16.04 16.04 

CO (12.01) + 1*(16.00) = 28.01 28.01 

CO2 (12.01) + 2*(16.00) = 44.01 44.01 

H2 2*(1.008) = 2.016 2.016 

H2O 2*(1.008) +1*(16.00) = 18.02 18.02 
9-23 

The molecular weight for the gas mixture is calculated below for a 100 lb mol basis: 

 100 lb mol basis 1 lb mol 

Fuel 

Constituent mol 

percent 

lb mols 

MW 

(lb/lb mol) 

Weight 

(lb) 

MW 

(lb/lb mol) 

CH4 4.0 4.0 16.04 64.16 

CO 0.4 0.4 28.01 11.20 

CO2 17.6 17.6 44.01 774.58 

H2 75.0 75.0 2.016 151.20 

H2O 3.0 3.0 18.02 55.06 

Total 100.0 100.0 1056.2 10.56 

b) The higher heating value of the fuel gas can be reasonably predicted from the composition. 

The following table presents the higher heating value for common fuel gas constituents: 

Table 9-1 HHV Contribution of Common Gas Constituents 

 Higher Heating Value 

Gas Btu/lb Btu/ft3

H2 60,991 325 

CO 4,323 321 

CH4 23,896 1014 

C2H6 22,282 1789 

C3H8 22,282 2573 

C4H10 21,441 3392 

H2O, CO2, N2, O2 0 0 

Reference (12) 

HHV (Btu/ft3

) at 1 atm and 60 oF. 

Using these HHV contributions, the gas composition, and the ideal gas law, calculate the overall 

HHV on a basis of 100 ft3

 in the following table: 

 100 ft3 Basis 1 ft3 Basis 

Fuel 

Constituents 

mol 

percent 

Volume 

(ft3

) 

HHV 

(Btu/ft3) 

Heat Input 

(Btu) 

HHV 

(Btu/ft3) 

CH4 4.0 4.0 1014 4056 

CO 0.4 0.4 321 128 

CO2 17.6 17.6 0 0 

H2 75.0 75.0 325 24,375 

H2O 3.0 3.0 0 0 

Total 100.0 100.0 28,559 285.6 
9-24 

Thus, the higher heating value for the specified fuel gas is 285.6 Btu/ft3

. 

(c) The density of any ideal gas can be calculated by modifying the ideal gas law, presented 

below: 

PV nRT =

Because density is simply the mass of a substance divided by its volume, multiply both sides of 

the ideal gas equation by the molecular weight, MW, of the gas mixture. Recall that the moles of 

a substance, n, times its molecular weight equals its mass. 

PV(MW) n(MW)RT =

PV(MW) mass)RT = (

Rearrange this equation to derive an ideal gas law equation that will calculate the density of any 

ideal gas given the temperature, pressure and MW: 

density = mass 

volume = P(MW)

RT

The selection of the ideal gas constant, R, in convenient units such as (atm-ft3

)/(lb mol-R) will 

simplify the density calculation in units of lbs per ft3

(at 1atm, 60 F) ft

lb = 0.02781

(0.7302 )(60 + 460 R)

(1atm)(10.56 ) = RT

P(MW) density = o

3

lbmol-R

atm-ft

lbmol

lb

3

(d) The HHV in Btu/lb can be calculated from the HHV in Btu/ft3

 and the density: 

lb

Btu 10,270 0.02781lb

1ft

ft

Btu HHV 285.6

3

3 =⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛

⎟

⎠

⎞ ⎜

⎝

⎛ =

(e) The LHV can be calculated by recalling that the fundamental difference between HHV and 

LHV is the state of the product water. That is, HHV is based on a liquid water product, 

while LHV is based on a gaseous water product. Because energy is consumed to evaporate 

liquid water into gaseous water, LHV values are always less than or equal to HHV values. 

To convert liquid water to water vapor at 1 atm and 60 oF requires approximately 

1050 Btu/lb, or 50 Btu/ft3 of water vapor. For a given gas mixture, the quantitative 

difference between the HHV and LHV is, obviously, a function of how much water is 

produced by the given fuel. So the first step in converting HHV to LHV is the 

determination of the amount of water produced by the fuel. This is done in the table below. 

The LHV to HHV adjustment is calculated by multiplying the water volume times the 

change in enthalpy going from liquid to vapor (50 Btu/ft3

): 
9-25 

 Basis: 1.0 ft3 of Fuel Gas 

Fuel 

Constituent 

mol 

percent 

Fuel Gas 

Volume 

(ft3

) 

Stoichiometric 

Factor62 for 

Gas to H2O 

Water 

Volume 

(ft3

) 

LHV to HHV 

Adjustment 

(Btu/ft3

) 

CH4 4.0 0.04 2.0 0.08 4.0 

CO 0.4 0.004 0.0 0.00 0.0 

CO2 17.6 0.176 0.0 0.00 0.0 

H2 75.0 0.75 1.0 0.75 37.5 

H2O 3.0 0.03 0.0 0.00 0.0 

Total 100.0 1.00 0.83 41.5. 

Thus, the LHV can be estimated from the HHV of 285.6 Btu/ft3

 as 246.1 Btu/ft3

(285.6 - 41.5= 244.1 Btu/ft3

). 

9.4 Cost Calculations 

This section presents information on developing the Cost of Electricity (COE), as well as 

information for the development of capital costs. 

9.4.1 Cost of Electricity 

Three major components are considered in the computation of the COE for a fuel cell power 

plant: 1) capital cost, 2) fuel cost and 3) operation and maintenance costs. The cost of electricity 

($/MWh) can be calculated using these parameters as follows: 

COE = 0.125CC

H

 + 

3.412 FC

 + 

O& M

ε s H

where 0.125 is a typical capital recovery rate (excluding taxes and insurance), CC is the capital 

cost ($/kW), FC is the fuel cost ($/106

Btu), 3.412 is the theoretical heat rate for 100 percent 

efficiency (3412 Btu/kWh) divided by 1000 for units consistency, εs is the fractional efficiency, 

H is the annual operating hours divided by 1000, and O&M is the operating and maintenance 

cost ($/kW-yr total, including fixed and variable costs). 

Example 9-15 Cost of Electricity 

Given a capital cost of $1000/kW, a fuel cost of $2 per MMBtu, a net plant efficiency of 40 

percent (LHV), 6000 operating hours, and a total O&M cost of $20/kW-yr, what is the estimated 

cost of electricity? 

Solution: 

COE = (0.125)(1000)

6

 + (3.412) (2) + (20)

0 40 . 6

 

62 The stoichiometric factor is the number of water molecules produced per fuel molecule in complete combustion. 

For example, for CH4, which combusts to 2 H2O, the stoichiometric factor is two. 
9-26 

COE = 20.8 + 17.1 + 3.3 = $41.2 / MWh, or 4.1 cents / kWh

9.4.2 Capital Cost Development 

There is a need for an easily understood, flexible, and reasonably accurate methodology for 

rapidly estimating the cost of conceptual fuel cell power plants. 

One method proposed for estimating the cost of fuel cell power plants is to calculate distributive 

(bulk) costs as a function of the equipment cost using established factors based on conventional 

generating technologies. When applied to compensate for the differences associated with a fuel 

cell plant, this approach can yield reasonable results. Based on the international prominence of 

the Association for the Advancement of Cost Engineering (AACE), this approach is useful for 

conceptualizing the costs for fuel cell/turbine power plant systems. 

Typical factors in common use are listed in Table 9-2. These factors apply to processes 

operating at temperatures in excess of 400 o

F at pressures of under 150 psig, and are taken from 

the AACE Recommended Practice No. 16R-90, Conducting Technical and Economic 

Evaluations in the Process and Utility Industries. 

Table 9-2 Distributive Estimating Factors 

Area Material Labor 

Foundations 0.06 1.33 

Structural Steel 0.05 0.50 

Buildings 0.03 1.00 

Insulation 0.02 1.50 

Instruments 0.07 0.75 

Electrical 0.06 0.40 

Piping 0.40 0.50 

Painting 0.005 3.00 

Misc. 0.04 0.80 

The suggested material factors are applied to direct equipment costs, whereas the labor factors 

apply to the corresponding material item. Because the distributive factors are based on large 

scale field-built plants, an alternative factory fabrication adjustment can be made to reflect a 

modular construction approach requiring less field fabrication, as would likely be the case with 

smaller plant configurations. This approach is illustrated in reference (16). 

The approach discussed above does not preclude the use of alternate methodologies. One such 

alternate methodology, currently in the early stages of development, is based on the premise that 

fuel cell plant costs could be more accurately estimated using factors developed specifically for 

fuel cell applications, rather than factors based on conventional generating technologies. An 

overview of this approach along with a “first cut” at developing new fuel cell specific factors is 

presented in reference (18). Fuel cell-specific factors developed to date are based on limited data 

and should be considered highly preliminary. Continued refinement will be required as 

additional fuel cell plant costing information becomes available. 
9-27 

9.5 Common Conversion Factors 

To Convert 

From 

To Multiply by To Convert 

From 

To Multiply by 

A (amperes) Faradays/sec 1.0363E-05 Joule (J) V-coulomb 1 

A/ft2 mA/cm2 1.0764 

atm kg/cm2 1.0332 

atm lb/in2 14.696 

atm bar 1.01325 kg lb 2.2046 

atm Pa 101,325 kg/cm2 lb/in2 14.223 

Avagadro's 

number 

particles/g mol 6.022E+23 

bar atm 0.98692 Kcal Btu 3.9686 

bar lb/in2 14.504 kPa lb/in2 0.14504 

bar kg/cm2 1.0197 kW Btu/hr 3412.1 

bar Nm2 100,000 kW kcal/sec 0.23885 

bar Pa 100,000 kW hp 1.3410 

Btu cal 251.98 lb grams 453.59 

Btu ft-lb 778.17 lb kg 0.45359 

Btu J (Joules) 1055.1 

Btu kWh 2.9307E-04 

Btu/hr W 0.29307 lb/in2 kg/cm2 0.070307 

Btu/lb-°F cal/g-°C 1.0000 lb/in2 Pa 6894.7 

°C °F °C*(9/5)+32 l (liter) m3 1.0000E-03 

°C °K °C+273.16 m (meter) ft 3.2808 

cal J 4.1868 m (meter) in 39.370 

cm ft 0.032808 m2 ft2 10.764 

cm in 0.39370 m3 ft3 35.315 

°F °C (°F-32)*(5/9) m3 gal 264.17 

Faradays C (coulombs) 96,487 mA/cm2 A/ft2 0.92903 

Faradays/sec A 96,487 MMBtu/hr MW 0.29307 

ft m 0.30480 MW MMBtu/h 3.4121 

ft cm 30.480 Pa lb/in2 1.4504E-04 

ft2 cm2 929.03 R (gas constant) atm-ft3/lbmol-R 0.73024 

ft2 m2 0.092903 R (gas constant) Btu/lb mol-R 1.9859 

ft3 liters 28.317 R (gas constant) cal/g mol-K 1.9857 

ft3 m3 0.028317 R (gas constant) ft-lbf/lb mol-R 1545.3 

ft3 gal 7.4805 R (gas constant) J/g mol-K 8.3144126 

gal liters 3.7854 R (gas constant) l-atm/g mol-K 0.082057 

grams (g) lb 2.2046E-03 tonne kg 1000.0 

hp ft-lb/sec 550.00 tonne lb 2204.6 

horsepower (hp) kW 0.74570 Watts Btu/hr 3.4121 

hp W 745.70 Watts hp 1.3410E-03 
9-28 

9.6 Automotive Design Calculations 

The total power, P, needed from a vehicle’s power system must be sufficient for vehicle 

acceleration, aerodynamic drag losses, rolling resistance, changes in elevation, and auxiliary 

power for vehicle accessories (19, 20). These power terms are, respectively: 

P = (mav + 0.5kCDAFv

3

 + mgCRv + mgv . sin(α)) / ε + Paux

Where P = total power (W) 

 m = vehicle mass (kg) 

 a = vehicle acceleration (m/sec2

) 

 v = vehicle velocity (m/sec) 

k = air density (kg/m3) 

 CD = aerodynamic drag coefficient 

 AF = vehicle area normal to direction of travel (m2

) 

 g = gravitation constant (9.8 m/sec2

) 

 CR = coefficient of rolling resistance 

α = inclined angle of road (radians) 

ε= efficiency of motor, controller, and gearing 

 Paux = auxiliary power for lights, radio, wipers, air conditioner, cigarette lighter, etc. (W) 

The power system may consist of the fuel cell plus peak power storage device(s). Criteria 

established by the Partnership for a New Generation of Vehicles (PNGV) specify that: 

• The fuel cell system (without peak power device) must provide enough power to sustain a 

speed of 55 mph (24.58 m/sec) on a 6.5 percent grade, and 

• The output of the fuel cell system plus peak power device must allow acceleration for high 

speed passing of 3 mph/sec (1.34 m/sec2

) on a level road at 65 mph (29.05 m/sec) 

These values are computed for a conventional mid-size passenger vehicle using the following 

assumptions: 

 m = 1360 kg (vehicle weight) + 272 kg (weight of passengers plus cargo) 

k = 1.29 kg/m3 (at standard temperature and pressure) 

 CD = 0.3 

 AF = 2.0 m2

 g = 9.8 m/sec2

 CR = 0.0085 

ε = 0.77 

 Paux = 400 W (= 400 kg-m2

/sec3) 

Substituting these values into the equation above, the minimum power needed by the fuel cell 

alone to sustain 24.58 m/sec on a 6.5 percent grade (0.0649 radians) is 

PS = ((0.5)(1.29)(0.3)(2.0)(24.58)3

 + (1632)(9.8)(0.0085)(24.58) + 

(1632)(9.8)(24.58)sin(0.0649))/0.77 + 400 
9-29 

PS = 45,339 kg-m2

/sec3 = 4.53 kW 

The minimum power needed by the power system to accelerate on a level road at 1.34 m/sec2

 at 

29.05 m/sec is 

PA = ((1632)(1.34)(29.05) + (0.5)(1.29)(0.3)(2.0)(29.05)3

 + (1632)(9.8)(0.0085)(29.05))/0.77 + 

400 

PA = 100,355 kg-m2

/sec3 = 10.03 kW 

9.7 References

1. "Physical and Thermodynamic Properties of Elements and Compounds," Girdler Catalysts, 

Chemetron Corporation, Catalysts Division. 

2. J. M. Smith, H. C. Van Ness, Introduction to Chemical Engineering Thermodynamics, Third 

Edition, McGraw-Hill, 1975. 

3. Chemistry: Principles and Applications, M. J. Sienko, R. A. Plane, McGraw-Hill, New 

York, NY, 1979. 

4. D. B. Stauffer, J. S. White, J. H. Hirschenhofer, "An ASPEN/SP MCFC Performance User 

Block," DOE Contract DE-AC21-89-MC25177, Task 7, July 1991. 

5. D. B. Stauffer, R. R. Engleman Jr., J. S. White, J. H. Hirschenhofer, "An ASPEN/SP SOFC 

Performance User Block," DOE Contract DE-AC21-88-FE-61684, Task 14, September 

1993. 

6. E. S. Wagner, G. F. Froment, "Steam Reforming Analyzed," Hydrocarbon Processing, July 

1992, pp. 69 -77. 

7. Fuel Cell Systems, Edited by L. J. M. Blomen, M. N. Mugerwa, Plenum Press, New York, 

NY, 1993. 

8. W. L. McCabe, J. C. Smith, P. Harriot, Unit Operations of Chemical Engineering, 4th

Edition, 1985. 

9. Chemical Engineers' Handbook, Edited by R. H. Perry, D. Green, 6th Edition, McGraw-Hill, 

1984. 

10. M. S. Peters, K. D. Timmerhaus, Plant Design and Economics for Chemical Engineers, 3rd

Edition, McGraw-Hill, Inc., New York, NY, 1980. 

11. C. A. Meyers, R. B. McClintok, G. J. Silvestri, R. C. Spencer, Jr., 1967 ASME Steam 

Tables, New York, 1967. 

12. Combustion, Fossil Power: A Reference Book on Fuel Burning and Steam Generation, 4th

Edition, edited by J. G. Singer, P.E., Combustion Engineering, 1991. 

13. B. J. McBride, "Coefficients for Calculating Thermodynamic and Transport Properties of 

Individual Species," NASA Technical Memorandum 4513, October 1993. 

14. B. J. McBride, "Thermodynamic Data for Fifty Reference Elements," NASA Technical 

Paper 3287, January 1993. 

15. H. M. Spencer, Ind. Eng. Chem., 40:2152 (1948), as presented in Introduction to Chemical 

Engineering Thermodynamics, Third Edition, J. M. Smith and H. C. Van Ness, McGrawHill, 1975. 
9-30 

16. T. J. George, R. James III, K. D. Lyons, "Multi-Staged Fuel Cell Power Plant (Targeting 

80% Lower Heating Value Efficiency)," Power Generation International 1998 Conference, 

December 9-11, 1998, Orange County Convention Center, Orlando, Florida. 

17. Recommended Practice No. 16R-90, Conducting Technical and Economic Evaluations in 

the Process and Utility Industries. 

18. L. L. Pinkerton, "Express Method for Estimating the Cost of Fuel Cell Plants," 1998 Fuel 

Cell Seminar, November 16-19, 1998, Palm Springs Convention Center, Palm Springs, 

California. 

19. J. M. Ogden, M. M. Steinbugler, and T. G. Kreutz. 1999. “A Comparison of Hydrogen, 

Methanol, and Gasoline as Fuels for Fuel Cell Vehicles: Implications for Vehicle Design 

and Infrastructure Development.” Journal of Power Sources, 79 (1999) 143-168. 

20. K. H. Hauer, D. J. Friedmann, R. M. Moore, S. Ramaswamy, A. Eggert, and P. 

Badranarayanan, March 6-9, 2000. “Dynamic Response of an Indirect-Methanol Fuel Cell 

Vehicle.” Fuel Cell Power for Transportation 2000. Society of Automotive Engineers 

World Congress, Detroit, Michigan. 
10-1 

10. APPENDIX

10.1 Equilibrium Constants 

Figure 10-1 presents the temperature dependence of the equilibrium constants for the water gas 

shift reaction, 

CO2 + H2 = CO + H2O (10-1)

the carbon deposition (Boudouard reaction) reaction, 

2CO → C + CO2 (10-2)

the methane decomposition reaction, 

CH4 → C + 2H2 (10-3)

and the methane formation reaction, 

CO + 3H2 → CH4 + H2O (10-4)
10-2 

Figure 10-1 Equilibrium Constants (Partial Pressures in MPa) for (a) Water Gas Shift, 

(b) Methane Formation, (c) Carbon Deposition (Boudouard Reaction), and (d) Methane 

Decomposition (J.R. Rostrup-Nielsen, in Catalysis Science and Technology, Edited by 

J.R. Anderson and M. Boudart, Springer-Verlag, Berlin GDR, p.1, 1984.) 

10.2 Contaminants from Coal Gasification 

A list of contaminant levels that result from various coal gasification processes is presented in 

Table 10-1. The contaminant levels obtained after a first stage of hot gas cleanup with zinc ferrite 

also are listed. 
10-3 

Table10-1 Typical Contaminant Levels Obtained from Selected 

Coal Gasification Processes 

Parameters Coal Gasification Process 

LURGI 

Fixed Bed 

METC (raw gas) 

Fixed Bed 

Cleaned 

Gas 

Max. Product 

Temp. (EC) 

750 1300 <800 

Gasification O2 blown Air blown Regenerative 

Pressure (psi) 435 220 150 

Product Gas (EC) 600 650 <700 

Methane (vol percent) 11 3.5 3.5 

Coal type Sub-bitum. 

Navajo 

Sub-bitum. 

New Mexico 

(Humidified 

Output) 

Particulates (g/l) 0.016 0.058 0.01 est. 

Sulfur (ppm) 

 (Total H2S, COS, 

 CS2, mercaptans) 

2,000 5,300 <10 

NH3 (vol percent) 0.4 0.44 0.25 

Trace metals (ppm) 

As 2 NSa NS 

Pb 0.8 2 1.7 

Hg 0.4 NS NS 

Zn 0.4 NS 140 

Halogens (ppm) 200 700 500 

Hydrocarbons (vol percent) 

C2H6 1 NS NS 

C2H4 1 0.3 NS 

C2H2 1 NS NS 

Oil tar 0.09 NS NS 

a - Not specified 

Source: A. Pigeaud, Progress Report prepared by Energy Research Corporation for U.S. 

Department of Energy, Morgantown, WV, Contract No. DC-AC21-84MC21154, 

June 1987. 
10-4 

10.3 Selected Major Fuel Cell References, 1993 to Present 

Books on Fuel Cells: 

1. A.J. Appleby, F.R. Foulkes, Fuel Cell Handbook, Van Norstand Reinhold, New York, N.Y., 

1989. Republished by Krieger Publishing Company, Melborne, FL, 1993. 

2. L.J. Blomen, M.N. Mugerwa, editors, Fuel Cell Systems, ISBN 0-306-44158-6, Kluwer 

Academic Publishers, 1994. 

3. M. Corbett, Opportunities in Advanced Fuel Cell Technologies – Volume One – Stationary 

Power Generation 1998-2008, Kline & Company, Inc., Fairfeild, NJ, 1998.

4. EscoVale Consultancy Services, Fuel Cells: The Source Book. 

5. S. Gottesfeld, T.A. Zawodzinski, "Polymer Electrolyte Fuel Cells," Advances in 

Electrochemical Science and Engineering, Volume 5, edited by R.C. Alkire, et al., Wiley-VCH, 

1998.

6. G. Hoogers, Fuel Cell Technology Handbook, CRC Press, ISBN: 0849308771 August, 2002.

7. T. Koppel, Powering the Future: The Ballard Fuel Cell and the Race to Change the World, 

John Wiley & Sons, ISBN: 047-1646296, 2001.

8. K. Kordesch, G. Simander, Fuel Cells and Their Applications, VCH Publishers, New York, 

N.Y., ISBN: 3-527-28579-2, 1996. 

9. J. Larminie, A. Dicks, Fuel Cell Systems Explained, John Wiley and Sons, ISBN: 0-471-

49026-1, 2000. 

CD’s on Fuel Cells: 

1. Fuel Cell Handbook, 6th Edition - November 2002. The latest technical specifications and 

description of fuel cell types. Prepared by EG&G Technical Services and Science 

Applications International Corporation for the National Energy Technology Laboratory. 

2. Distributed Generation Primer – May 2002. This CD provides the background for 

decision makers to evaluate the options, market conditions, drivers and issues related to 

successful use of distribution generation. Prepared by Science Applications International 

Corporation and EG&G Technical Services for the National Energy Technology 

Laboratory. 

3. Hybrid Fuel Cell Technology Overview - May 2001. Prepared by Energy and 

Environmental Solutions for the National Energy Technology Laboratory. 
10-5 

Periodicals and Newsletters: 

1. Advanced Fuel Cell Technology – monthly. Published by Seven Mountains Scientific, Inc. 

Boalsburg, PA 16827. Online subscription available at http://www.7ms.com/fct/index.html

2. Alternative Fuel News – quarterly. Published by U.S. Department of Energy’s Alternative 

Fuel Data Center and the Clean Cities Program. Available online at 

http://www.eere.energy.gov/cleancities/ccn/archive/archive.html#afn

3. Clean Fuels and Electric Vehicles Report – Published 4 times per year. Published by Energy 

Futures, Inc. Boulder, CO 80306. Online subscription available at www.energy-futures.com/

4. Electrifying Times – 3 issues per year. Published by Bruce Meland, Bend, Oregon 97701. 

Table of contents and past issue archives available online at www.electrifyingtimes.com/

5. Fuel Cell Catalyst – quarterly. Published by U.S. Fuel Cell Council and National Fuel Cell 

Research Center and sponsored by National Energy Technology Laboratory. To subscribe 

http://lb.bcentral.com/ex/manage/subscriberprefs.aspx?customerid=9927 

6. Fuel Cell Connection – monthly e-mail. Published by U.S. Fuel Cell Council and National 

Fuel Cell Research Center and sponsored by National Energy Technology Laboratory. To 

subscribe http://lb.bcentral.com/ex/manage/subscriberprefs.aspx?customerid=9927 

7. Fuel Cell Industry Report – monthly. Published by Scientific American Newsletter, New 

York, NY 10003. To subscribe http://www.sanewsletters.com/fcir/FCIRinfo.asp

8. Fuel Cell Magazine – a supplement in March, June, September 2002 issues of Battery Power 

Products and Technology. Will be standalone bimonthly magazine in October 2002. To 

subscribe http://www.fuelcell-magazine.com

9. Fuel Cell Quarterly – quarterly. Published by Fuel Cells 2000, Washington DC 20006. To 

subscribe: http://www.fuelcells.org/info/pub_fcq.html 

10. Fuel Cell Technology News – monthly. Published by Business Communications Company, 

Inc. Norwalk, CT 06855. 

11. Fuel Cell World – quarterly (in German). Published by World Fuel Cell Council, Frankfurt am 

Main, Germany. 

12. Fuel Cells Bulletin – monthly. Published by Elsevier Advanced Technology, Kidlington, 

Oxford OX5 1AS, United Kingdom. For ordering information: 

http://www.sciencedirect.com/science?_ob=JournalURL&_cdi=6202&_auth=y&_acct=C0000

58399&_version=1&_urlVersion=0&_userid=2638189&md5=cf981e59da40bc59c7a73e84e6

72c33e
10-6 

13. Fuel Cells: From Fundamentals to Systems – Journal. Published by Wiley – VCH. Online 

ordering information: http://www.interscience.wiley.com/jpages/1615-6846/

14. Hybrid Vehicles – bimonthly. Published by Energy Futures, Inc. Boulder, CO 80306. Online 

subscription available at www.energy-futures.com/

15. Hydrogen and Fuel Cell Letter – monthly. Published by Peter Hoffman, Rhinecliff, NY 

12574. Headlines and ordering information available online at www.hfcletter.com/

16. The Hydrogen – Gazette – Newsletter published by HyWeb and the German Hydrogen 

Association. Available online at www.hydrogen.org

17. Hydrogen Mirror / Wasserstoff-Spiegel – bimonthly. Published by Deutscher-WasserstoffVerband (German Hydrogen Association). To subscribe http://www.dwv-info.de/e/

18. International Journal of Hydrogen Energy – monthly. Published by Elsevier Advanced 

Technology, Kidlington, Oxford OX5 1AS, United Kingdom. For ordering information: 

http://www.sciencedirect.com/science?_ob=JournalURL&_cdi=5729&_auth=y&_acct=C0000

58399&_version=1&_urlVersion=0&_userid=2638189&md5=d022b0d976d72b556eb2f49a97

c97601

19. Journal of Power Sources - monthly. Published by Elsevier Advanced Technology, Kidlington, 

Oxford OX5 1AS, United Kingdom. For ordering information: 

http://www.sciencedirect.com/science?_ob=JournalURL&_cdi=5269&_auth=y&_acct=C0000

58399&_version=1&_urlVersion=0&_userid=2638189&md5=32db478e73fce31e72368c9928

505ff4

20. Platinum Metals Review – quarterly. Published by Johnson Matthey PLC, London, United 

Kingdom. Available online at http://www.platinum.matthey.com/publications/pmr.php

21. SCNG News – monthly. Published by the U.S. Department of Energy’s Strategic Center for 

Natural Gas. Available online at http://www.netl.doe.gov/scng/news/news_toc.html

Proceedings and Abstracts from Major U.S. Fuel Cell Conferences: 

1. Fuel Cell Seminar, Programs and Abstracts, Fuel Cell Seminars, sponsored by Fuel Cell 

Seminar Organizing Committee. For information visit web site at: 

http://www.fuelcellseminar.com/index.asp

 November /December 1994 – San Diego, California. 

 November 1996 – Orlando, Florida. 

 November 1998 – Palm Springs, California. 

 November 2000 – Portland, Oregon. 

 November 2002 – Palm Springs, California. 

 November 2003 – Miami, Florida. 

 November 2004 – San Antonio, Texas 
10-7 

2. Proceedings of the Annual Fuel Cells Review Meeting. Meetings held annually at the U.S. 

DOE Morgantown Energy Technology Center (now the National Energy Technology 

Laboratory), Morgantown, WV, until 1998, then at U.S. locations: 

DOE/METC-94/1010, August 1994 

DOE/METC-95/1020, August 1995 

DOE/METC CD-ROM, August 1996 

DOE/FETC–98/1054 CD-ROM, August 1997 

 Joint DOE/EPRI/GRI Workshop on Fuel Cell Technology, May 1998, San Francisco, CA 

 Joint DOE/EPRI/GRI Workshop on Fuel Cell Technology, August 1999, Chicago, IL 

3. EPRI/GRI Fuel Cell Workshop on Technology Research and Proceedings, Cosponsored by 

EPRI and GRI, Proceedings by EPRI, Palo Alto, CA, March 1994. 

 March 1994, Atlanta, Georgia 

 April 1995, Irvine, California. 

 April 1996, Temple, Arizona 

 In 1997, the EPRI/GRI Workshop joined with the DOE Annual Fuel Cells Contractors 

Meeting. See Item 2 for information in 1997 and beyond. 

4. J.R. Selman, et al., ed. Carbonate Fuel Cell Technology IV, Proceedings Vol. 97-4, Montreal, 

Canada, The Electrochemical Society, Inc., Pennington, NJ, 1997. 

5. K. Hemmes, et al., Proceedings of the Fifth International Symposium on Molten Carbonate 

Fuel Cell Technology, Honolulu, Hawaii, The Electrochemical Society, Inc., Pennington, NJ, 

October 1997.

6. S.C. Singhal, et al., Proceedings at the Fourth International Symposium on Solid Oxide Fuel 

Cells, Proceedings Vol. 95-1, Yokohama, Japan, The Electrochemical Society, Inc., 

Pennington, NJ, 1995. 

7. S.C. Singhal, et al., Proceedings of the Fifth International Symposium on Solid Oxide Fuel 

Cells, Proceedings Vol. 97-40, Aachen, Germany, The Electrochemical Society, Inc., 

Pennington, NJ, 1997. 

8. S.C. Singhal, et al., Proceedings of the Sixth International Symposium on Solid Oxide Fuel 

Cells, Proceedings Vol. 99-19, Honolulu, Hawaii, The Electrochemical Society, Inc., 

Pennington, NJ, 1999. 

9. A.R. Landgrebe, S. Gottesfeld, First International Symposium on Proton Conducting 

Membrane Fuel Cells, Chicago, IL, Proceedings Vol. 95-23, The Electrochemical Society, 

Inc., Pennington, NJ, 1995. 

10. S. Gotts, et al., Second International Symposium on Proton Conducting Membrane Fuel Cells, 

Boston, MA, The Electrochemical Society, Inc., Pennington, NJ, 1998. 
10-8 

11. Proceedings of the Workshop on Very High Efficiency Fuel Cell/Gas Turbine Power Cycles, 

edited by M.C. Williams, C.M. Zeh, U.S. DOE Federal Energy Technology Center, 

Morgantown, WV, October 1995. 

12. Proceedings of the National Hydrogen Association Meetings, National Hydrogen Association, 

usually in Alexandria, VA, annually in spring. 

13. Proceedings of the Intersociety Energy Conversion Engineering Conference. Sponsorship of 

meeting rotates among six technical societies. Meetings are held annually (usually in August) 

in different cities of the United States: 

 29th - Part 2, Sponsor - American Institute of Aeronautics and Astronautics, Monterey, CA, 

August 1994. 

 30th - Volume 3, Sponsor - American Society of Mechanical Engineers, Orlando, Fl, 

August 1995. 

 31st - Volume 2, Sponsor - Institute of Electrical and Electronics Engineers, Washington, 

D.C., August 1996. 

32nd - Sponsor - American Institute of Chemical Engineers, Honolulu, Hawaii, July/August 

1997. 

 33rd - CD-ROM, Sponsor - American Nuclear Society, Colorado Springs, Colo., August 

1998. 

 34th - CD-ROM, Sponsor – Society of Automotive Engineers, Vancouver, BC, August 

1999. 

35th – Sponsor - American Institute of Aeronautics and Astronautics, Las Vegas, NV, July 

2000. 

 36th – Sponser - American Society of Mechanical Engineers, Savannah, Georgia, July 

2001. 

14. Proceedings of the 58th American Power Conference, Volume 58-1, Sponsored by Illinois 

Institute of Technology, Chicago, IL, 1996. 

15. Proceedings of U.S. Russian Workshop on Fuel Cell Technologies, Sandia National 

Laboratories, Albuquerque, N.M., September 1995. 

16. Lake Tahoe Fuel Cell Conference Proceedings, Desert Research Institute, Energy & 

Environmental Engineering Center, P.O. Box 60220, Reno, NV 69506-0220, July 1998. 

17. Next Generation Fuel Cells Workshop: Workshop Proceedings – National Energy Technology 

Laboratory (formerly Federal Energy Technology Center), Morgantown, WV, December 1998. 

18. Proceedings of the NETL Workshop on Fuel Cell Modeling, National Energy Technology 

Center, Morgantown, WV, April 2000. 

19. Proceedings of the Second Annual Small Fuel Cells & Batteries Conference, New Orleans, 

LA, The Knowledge Foundation, Brookline, MA, April 2000. 
10-9 

20. Proceedings of the U.S. DOE Natural Gas/Renewable Energy Hybrids Workshops. 

Morgantown, WV. Proceedings can be found online at 

http://www.netl.doe.gov/publications/proceedings/01/hybrids/hybrid01.html

21. Proceedings of the Second DOE/UN International Conference and Workshop on Hybrid 

Power Systems. April 16-17, 2002. Proceedings can be found online at 

http://www.netl.doe.gov/publications/proceedings/02/Hybrid/hybrid02.html

22. Proceedings of the Solid State Energy Conversion Alliance Workshops, SECA is coordinated 

by the National Energy Technology Center and Pacific Northwest National Laboratory. 

Proceedings can be found online at: http://www.netl.doe.gov/scng/seca/past-events.html

First Workshop Baltimore, Maryland June 2000 

Second Workshop Arlington, Virginia March 2001 

Third Workshop Washington, DC March 2002 

Fourth Workshop Seattle, Washington April 2003 

23. Proceedings of the Fuel Cell Summits on Codes and Standards, Originated in the Department 

of Energy’s Office of Building Equipment, State and Community Programs, but has now 

transferred to the Departments Office of Power Technologies. Results of the Summits can be 

found online at: http://www.pnl.gov/fuelcells/summits/

Summit I April 1997 

Summit II May 1998 

Summit III April 1999 

Summit IV May 2000 

Summit V May 2001 

 Summit VI May 2002 

 Summit VII May 2003 

 Summit VIII June 2004 

Other Important Information on Fuel Cells: 

1. U.S. DOE, Fuel Cell Program Plans, published each Fiscal Year by U.S. Department of 

Energy, Assistant Secretary of Fossil Energy: 

 1994 - DOE/FE-0311P 

 1995 - DOE/FE-0335 

 1996 - DOE/FE-0350 

2. NEDO, Research and Development on Fuel Cell Power Generation Technology, published 

yearly by the New Energy and Industrial Technology Development Organization, Tokyo, 

Japan. 
10-10 

3. Fuel Cell RD&D in Japan, Published annually by the Fuel Cell Development Information 

Center c/o The Institute of Applied Energy, Tokyo, Japan, usually in August. 

4. Proceedings of the Grove Anniversary Fuel Cell Symposium, London, UK, September 1995, 

Journal of Power Sources, Elsevier Sequoia Science, The Netherlands, January 1995. 

5. Proceedings of the Grove Anniversary Fuel Cell Symposium, London, UK, September 1997, 

Journal of Power Sources, Elsevier Sequoia Science, The Netherlands, March 1998. 

6. Proceedings of the 6th Grove Anniversary Fuel Cell Symposium, London, UK, September 

1999, Journal of Power Sources, Elsevier Sequoia Science, The Netherlands, March 2000. 

7. Proceedings of the 7th Grove Anniversary Fuel Cell Symposium, London, UK, September 

2001, Journal of Power Sources, Elsevier Sequoia Science, The Netherlands. For information 

see http://www.grovefuelcell.com/

8. U. Bossel, editor, Proceedings of the European Solid Oxide Fuel Cell Forums, European Fuel 

Cell Group and IEA Advanced Fuel Cell Programme, 1994, 1996, 1998. 

9. Various Technical Reports Posted on the Strategic Center for Natural Gas Fuel Cell Reference 

Shelf. Available online at http://www.netl.doe.gov/scng/enduse/fc_refshlf.html

10.4 List of Symbols 

Abbreviations: 

® registered 

AACE Association for the Advancement of Cost Engineering 

A.R. as received 

ABS acrylonytril-butadiene-styrene 

AES air electrode supported 

AFC alkaline fuel cell 

AFV alternative fuel vehicle 

ANL Argonne National Laboratory 

APU auxiliary power unit 

ASF amps/ft2

ASME American Society of Mechanical Engineers 

ASR area specific resistance 

ASU air separation unit 

CC capital cost 

CEC California Energy Commission 

CFD computational fluid dynamics 

CHP combined heat and power 

COE cost of electricity 

CVD chemical vapor deposition 

DIR direct internal reforming 
10-11 

DOE Department of Energy 

EMF electromotive force 

EV electric vehicle 

EVD electrochemical vapor deposition 

FC fuel cost 

FCE Fuel Cell Energy 

FEP fluoro-ethylene-propylene 

FETC Federal Energy Technology Center 

GDL gas diffusion layer 

H annual plant operating hours 

HEV hybrid electric vehicle 

HHV higher heating value 

HR heat rate 

HRB heat recovery boiler 

IIR indirect internal reforming 

IEC International Electrotechnical Commission 

IEEE Institute of Electrical and Electronics Engineers 

INEEL Idaho National Engineering and Environmental Laboratory 

iR ohmic loss 

ITSOFC intermediate temperature solid oxide fuel cell 

J-M Johnson Mathey Technology Center 

LHV lower heating value 

MCFC molten carbonate fuel cell 

MEA membrane/electrode assembly 

NETL National Energy Technology Laboratory 

NFPA National Fire Protection Association 

NREL National Renewable Energy Laboratory 

O&M operating and maintenance costs 

ODS oxide dispersion strengthened anode 

OS/IES on-site/integrated energy systems 

PAFC phosphoric acid fuel cell 

PC phthalocyanines 

PEFC polymer electrolyte fuel cell 

PEM proton exchange membrane 

PMSS pyrolysis of metallic soap slurry 

PNNL Pacific Northwest National Laboratory 

PR pressure ratio 

Pt platinum 

PTFE polytetrafluoroethylene 

RDF refuse derived fuel 

SAE Society of Automotive Engineers 

SCF standard cubic feet 

SMR steam methane reforming 

SOFC solid oxide fuel cell 

TAA tetraazaannulenes 

TBA tetrabutyl ammonium 
10-12 

tcf trillion cubic feet 

TFMSA trifluoromethane sulfonic acid 

THT tetrahydrothiophene (thiophane) 

TMPP tetramethoxyphenylporphyrins 

TPB triple phase boundary 

TPP tetraphenylporphyrins 

TSOFC tubular solid oxide fuel cell 

TZP tetragonal phase 

TM trade mark 

UL Underwriters’ Laboratories, Inc. 

U.S. United States of America 

WSF watts/ft2

YSZ yttria stabilized zirconia 

ZEV zero-emission vehicle 

Letter Symbols:

∆E potential difference 

∆G Gibbs free energy 

∆Hc heat available from combustion of fuel gas 

∆Hr enthalpy of reaction 

∆Sr entropy of reaction 

∆V voltage difference 

<D> equilibrium pore size 

a (-2.3RT/αnF) log io

a acceleration 

A area 

a coefficient 

AC alternating current 

b 2.3RT/αnF

b coefficient 

b Tafel slope 

Btu British Thermal Unit 

c coefficient 

CB bulk concentration 

CD drag coefficient 

Cp heat capacity 

CR coefficient of rolling resistance 

CS surface concentration 

D diffusion coefficient 

D pore diameter 

dBA average decibles 

DC direct current 

e

-

 electron 

E equilibrium (reversible) potential 

E° standard potential 
10-13 

Ea activation energy 

F Faraday's constant 

f fugacity 

f gas flow rate 

g gravitational constant 

G Gribbs free energy 

H enthalpy 

hrs hours 

I current 

i current density 

iL limiting current density 

io exchange current density 

J current density 

K equilibrium constant 

k(T) constant, function of temperature 

kW kilowatt 

lb pound 

m mass 

m mass flow rate 

MM million 

mol mole 

MW molecular weight 

MW megawatt (1000 kW) 

MWhr megawatt-hour 

n mole flow rate 

n number of electrons participating in a reaction 

nmax maximum stoichiometric value 

p power 

P pressure 

Pi partial pressure 

ppm parts per million 

PT total pressure 

Q heat duty 

R cell resistance 

R universal gas constant 

S entropy 

t electrolyte thickness 

T temperature 

U utilization 

V cell voltage 

v rate at which reactant species are consumed 

v velocity 

V volume 

Vc voltage of single cell 

vol volume 

Wel maximum electrical work 
10-14 

wt weight 

X mole fraction 

yr year 

Greek Letter Symbols: 

α road angle of incline 

α electron transfer coefficient 

β hydrogen utilization 

ε motor efficiency 

γ interfacial surface tension 

γ oxidant utilization 

δ diffusion layer thickness 

η thermal efficiency 

ηact activation polarization 

ηconc concentration polarization 

ηohm ohmic polarization 

θ electrolyte contact angle 

θCO CO coverage 

k air density 

Subscripts: 

a anode 

c cathode 

e electrolyte 

f fuel 

i species 

in cell inlet 

out cell outlet 

ox oxygen or oxidant 

p pressure 

t temperature 

10.5 Fuel Cell Related Codes and Standards

10.5.1Introduction 

The rapid development and application of fuel cells throughout the world has created the need 

for fuel cell technology related codes and standards. Several organizations and committees are 

currently working on the development of codes and standards related to fuel cells. 

According to the National Fire Protection Agency (NFPA) Regulations Governing Committee 

Projects, codes and standards are defined as follows: 

Code: A standard that is an extensive compilation of provisions covering broad subject matter 

or that is suitable for adoption into law independently of other codes and standards. 
10-15 

Standard: A document, the main text of which contains only mandatory provisions using the 

word "shall" to indicate requirements and which is in a form generally suitable for mandatory 

reference by another standard or code or for adoption into law. Non-mandatory provisions shall 

be located in an appendix, footnote, or fine-printnote and are not to be considered a part of the 

requirements of a standard. 

This section provides a brief overview of the existing and developing codes and standards related 

to fuel cell technologies. The discussion focuses on participating organizations, specific codes 

and standards, and more generally applied codes and standards (e.g., the Uniform Building 

Code) that apply to system installation. 

10.5.2 Organizations 

Below is a listing and brief description of organizations involved in the development of codes 

and standards pertaining to fuel cell technology. 

American National Standards Institute (ANSI): ANSI has served in its capacity as 

administrator and coordinator of the United States private sector voluntary standardization 

system for 80 years. The Institute is a private, non-profit membership organization supported by 

a diverse constituency of private and public sector organizations. ANSI Z21.83 has been 

published and provides a means of testing and certifying the safety of stationary fuel cell power 

plants having a capacity of less than 1 MW. 

American Society of Mechanical Engineers (ASME): ASME is an international engineering 

society that conducts one of the world's largest technical publishing operations. ASME 

International is a non-profit educational and technical organization serving a worldwide 

membership. Its mission is to promote and enhance the technical competency and professional 

well-being of engineers through programs and activities in mechanical engineering. To this end, 

ASME has developed the Boiler and Pressure Vessel Code, which is referenced as part of the 

AGA certification. Additionally, ASME issued a fuel cell standard, ASME PTC 50, 

Performance Test Code on Fuel Cell Power Systems Performance. 

Institute of Electrical and Electronics Engineers (IEEE): The mission of IEEE is to advance 

global prosperity by promoting the engineering process of creating, developing, integrating, 

sharing, and applying knowledge about electrical and information technologies. IEEE Standards 

Coordinating Committee 21 (SCC21) oversees the development of standards in the area of fuel 

cells, photovoltaics, distributed generation, and energy storage. SCC21 coordinates efforts in 

these fields among the various IEEE societies and other appropriate organizations to ensure that 

all standards are consistent and properly reflect the views of all applicable disciplines. Working 

Group 1547 - Standard for Distributed Resources Interconnected with Electric Power Systems - 

establishes criteria and requirements for interconnection by distributed resources with electric 

power systems. The purpose is to provide a uniform standard for interconnection of distributed 

resources with electric power systems and requirements relevant to the performance, operation, 

testing, safety considerations, and maintenance of the interconnection. 
10-16 

International Code Council (ICC): The International Code Council was established in 1994 as a 

non-profit organization dedicated to developing a single set of comprehensive and coordinated 

national model construction codes without regional limitations. 

International Electrotechnical Commission (IEC): The IEC is the world organization that 

prepares and publishes international standards for all electrical, electronic, and related 

technologies. The membership consists of more than 50 participating countries, including all of 

the world's major trading nations and a growing number of industrialized countries. The IEC’s 

mission is to promote, through its members, international cooperation on all questions of 

electrotechnical standardization and related matters, such as the assessment of conformity to 

standards, in the fields of electricity, electronics, and related technologies. The IEC charter 

embraces all electrotechnologies including electronics, magnetics and electromagnetics, 

electroacoustics, telecommunication, and energy production and distribution, as well as 

associated general disciplines such as terminology and symbols, measurement and performance, 

dependability, design and development, safety, and the environment. 

The National Fire Protection Association (NFPA): NFPA is non-profit organization that 

publishes the National Electrical Code®, the Life Safety Code®, the Fire Prevention CodeTM, the 

National Fuel Gas Code®, and the National Fire Alarm Code®. The mission of NFPA is to 

reduce the worldwide burden of fire and other hazards on the quality of life by providing and 

advocating scientifically based consensus codes and standards, research, training, and education. 

NFPA 853, “Standard for the Installation of Stationary Fuel Cell Power Plants” covers the 

design, construction, and installation of stationary fuel cells of at least 50 kW output. 

Society of Automotive Engineers (SAE): SAE is a resource for technical information and 

expertise used in designing, building, maintaining, and operating self-propelled vehicles for use 

on land, sea, air, or in space. Composed of nearly 80,000 engineers, business executives, 

educators, and students from more than 97 countries, the network of members share information 

and exchange ideas for advancing the engineering of mobility systems. Technical committees 

write more new aerospace and automotive engineering standards than any other standardswriting organization in the world. In late 1999, a Fuel Cell Standards Forum was created to 

establish standards and test procedures for fuel cell powered vehicles. It will address the safety, 

performance, reliability, and recyclability of fuel cell systems in vehicles with an emphasis on 

efficiency and environmental impact. 

Underwriters Laboratories Inc. (UL): UL is an independent, not-for-profit product safety testing 

and certification organization. UL has tested products for public safety for more than a century 

with more than 14 billion UL Marks applied to products worldwide. UL has developed a 

standard for inverters that can be applied to fuel cells. 

10.5.3 Codes & Standards 

A summary of existing and pending fuel cell related codes and standards is presented in 

Table 10-2. More detailed descriptions are provided subsequently based on their specific area of 

application. 
10-17 

Table 10-2 Summary of Related Codes and Standards 

CODE/STANDARD ORGANIZATION SUMMARY 

1. PTC 50 ASME Performance Test Code - Provides test 

procedures, methods, and definitions for 

the performance characterization of fuel 

cell power systems. 

2. IEEE SCC 21 IEEE Standards coordinating committee - Fuel 

cells, photovoltaics, dispersed generation 

and energy storage 

3. IEEE P1547 

 

IEEE DG Interconnection Standard - 

Establishes criteria and requirements for 

interconnecting distributed resources with 

electric power systems 

4. ANSI Z21.83-1998 

 

ANSI Product Standard - Provides detailed test 

and examination criteria for fuel cell power 

plants that use natural and liquefied 

petroleum gases. 

5. NFPA 853 NFPA Installation Standard - Applies to 

installation of stationary fuel cell power 

plants. 

6. NEC/NFPA 70 

 Article 690,691 & 

 705 

NFPA 690 – Solar Photovoltaic Systems 

691 – Fuel Cells 

705 – Interconnected Power Production 

Sources 

7. IEEE SCC 36 IEEE Standards Coordinating Committee - 

Pertains to utility communications 

8. UL 1741 UL Electric Inverters - Standard for testing, 

listing and safety certification for inverters 

9. SAE Standards 

 Forum 

SAE Vehicle Standards - In the early stages of 

developing standards for safety, 

performance, reliability, and recyclability. 

Also establish testing procedures. 

10. IEC TC 105 IEC Technical Committee 105 – Seeking to 

expand the scope of ANSI Z21.83 for 

international basis and additional fuel cell 

technologies. 

11. IMC 2000/ 924 ICC Installation Standard – Must be in 

compliance with ANSI Z21.83. 

10.5.4 Codes and Standards for Fuel Cell Manufacturers 

ANSI Z21.83: American National Standard - Fuel Cell Power Plants provides a means of testing 

and certifying the safety of stationary fuel cell power plants with a nominal electric capacity not 

exceeding 1.0 MW. This standard is intended for applications other than residential when 

installed outdoors and operated on a gaseous hydrocarbon as the reactant. The current version of 
10-18 

the standard is based on two specific fuel cell technologies and is being revised to take into 

consideration the characteristics of additional fuel cell power plant technologies. Many state and 

local regulatory authorities have adopted this standard. 

ASME PTC 50: ASME Performance Test Code 50 - Fuel Cell Power Systems provides test 

procedures, methods, and definitions for the performance characterization of fuel cell power 

systems. The code specifies the methods and procedures for conducting and reporting fuel cell 

system ratings. Specific methods of testing, instrumentation, techniques, calculations and 

reporting are presented. 

IEC TC 105: The International Electrotechnical Committee has established a Technical 

Committee charged with the preparation of an international standards regarding fuel cell 

technologies for all fuel cell applications including stationary power plants, transportation 

propulsion systems, transportation auxiliary power units, and portable power generation systems. 

The standards will have four parts: Terminology and Definitions, Stationary Fuel Cell Systems, 

Fuel Cell Systems in Transportation, and Portable Fuel Cell Systems. The committee was 

established in 2000 and plans to have the standards approved and published in 2004. 

IEEE SCC21/P1547: The Institute of Electrical and Electronic Engineers has established a 

Standards Coordinating Committee (SCC 21) chartered with the development of a standard for 

the interconnection of distributed resources. This standard focuses on electrical interface 

standards for the application of distributed generation technologies described as fuel cells, 

photovoltaics, dispersed generation, and energy storage. The resulting standard will be IEEE 

P1547, which will establish criteria and requirements for the interconnection of distributed 

resources with electric power systems. 

IEEE SCC 36: This committee reviews, recommends, and solicits the development of standards 

relevant to the gas, water, and electric utility industries on a worldwide basis with respect to 

utility communication architecture. This SCC coordinates standards-development activities with 

other relevant IEEE groups and sponsors standards-development activities that are appropriate to 

the needs of the utility industry. 

IMC 2000/ 924.1: The International Code Council develops the International Mechanical Code. 

Section 924.1 of the IMC 2000 requires stationary fuel cell power plants not exceeding 1,000 

kW to be tested and listed to ANSI Standard Z21.83. 

ISO TC 197: The International Organization for Standardization has developed a committee to 

develop international safety standards for the production, storage, transport, measurement, and 

use of hydrogen. 

UL 1741: Underwriters Laboratory 1741 is a standard for the testing, listing, and safety 

certification for electric inverters. This standard is for static inverters and charge controllers for 

use in photovoltaic power systems, but may be used for fuel cells. 
10-19 

10.5.5 Codes and Standards for the Installation of Fuel Cells 

NFPA 853: National Fire Protection Association 853 - Standard for Fuel Cell Power Plants 

provides a standard for the design, construction, and installation of stationary packaged, self 

contained, and field constructed fuel cell power plants with a capacity greater than 50 kW. 

NFPA 70: National Fire Protection Association 70 is also known as the National Electric Code 

(NEC). Revisions and addenda to the code have been developed that specifically address fuel 

cells. Article 690 - Solar Photovoltaic Systems has been targeted for revision to include fuel 

cells and alternate energy sources systems. This proposal is not expected to be approved since 

the technological and operational differences between fuel cells and photovoltaic systems are 

considerable. A new article, Article 692 deals with rules covering fuel cell systems for buildings 

or residential dwellings. This standard addresses the electrical interface between the fuel cell 

system and a building’s electrical distribution panel. NFPA Article 705 - Interconnected 

Electrical Power Production Sources has also been revised to address fuel cell power sources. 

10.5.6 Codes and Standards for Fuel Cell Vehicles 

SAE established a Fuel Cells Standard Forum that is chartered with the establishment of 

standards and test procedures for fuel cell powered vehicles. The committee was established in 

1999. The standards will cover the safety, performance, reliability, and recyclability of fuel cell 

systems in vehicles with emphasis on efficiency and environmental impact. The standards will 

also establish test procedures for uniformity in test results for the vehicle/systems/components 

performance, and define interface requirements of the systems to the vehicle. Working Groups 

have been formed in the areas of safety, performance, emissions, recyclability, interface, and 

terminology. The working groups have created the following documents: 

J2572 – Draft - Recommended Practice for Measuring the Exhaust Emissions, Energy 

Consumption and Range of Fuel Cell Powered Electric Vehicles Using Compressed Gaseous 

Hydrogen. 

J2574 – Published March 2002 - Fuel Cell Electric Vehicle Terminology. 

J2578 – Published December 2002 – Recommended Practices for General Fuel Cell Vehicle 

Safety. 

J2579 – Draft – Recommended Practices for Hazardous Fluid Systems in Fuel Cell Vehicles. 

J2594 – Published September 2003 – Recommended Practice to Design for Recycling Proton 

Exchange Membrane (PEM) Fuel Cell Systems. 

J2600 – Published October 2002 – Compressed Hydrogen Surface Vehicle Refueling 

Connection Devices. 

J2601 – Draft – Compressed Hydrogen Vehicle Fueling Communication Devices. 

J2615 – Draft – Performance Test Procedure of Fuel Cell Systems for Automotive Applications. 

J2616 – Draft – Performance Test Procedure for the Fuel Processor Subsystem of Automotive 

Fuel Cell System. 

J2617 – Draft – Performance Test Procedure of PEFC Stack Subsystem for Automotive 

Applications. 

10.5.7 Application Permits 

The installation of stationary fuel cells requires adherence to a variety of building codes. In 

April 2001, The National Evaluation Service published a “Protocol for Evaluation of Stationary 
10-20 

Fuel Cell Power Plants.” This is used by NES to facilitate the process of evaluating stationary 

fuel cell power plant technology for compliance to all codes. A few of the major codes are 

summarized below. 

International Mechanical Code 2000: Published by the International Code Council. At the 

present time, it is the only code to provide specific guidance on stationary fuel cell power plants. 

Uniform Building Code: The Uniform Building Code (UBC) is the most widely-adopted model 

building code in the world, and is a proven document meeting the needs of government units 

charged with enforcement of building regulation. Published triennially, the UBC provides 

complete regulations covering all major aspects of building design and construction relating to 

fire and life safety and structural safety. The requirements reflect the latest technological 

advances in the building and fire and life-safety industry. 

Uniform Mechanical Code: Provides a complete set of requirements for the design, 

construction, installation, and maintenance of heating, ventilating, cooling and refrigeration 

systems, incinerators, and other heat-producing appliances. 

Uniform Plumbing Code: Published by the International Association of Plumbing and 

Mechanical Officials (IAPMO), the Uniform Plumbing Code covers all aspects of plumbing, 

including requirements for plumbing materials and IAPMO installation standards. 

 

National Electric Code: The National Electrical Code (NFPA 70) provides "practical 

safeguarding of persons and property from hazards arising from the use of electricity." More 

specifically, the National Electric Code covers the installation of electric conductors and 

equipment in public and private buildings or other structures (including mobile homes, 

recreational vehicles, and floating buildings), industrial substations, and other premises (such as 

yards, carnivals, and parking lots). The National Electric Code also covers installations of 

optical fiber cable. Wiring, general electrical equipment, the use of electricity in specific 

occupancies (from aircraft hangars to health care facilities), and equipment (ranging from 

elevators to hot tubs) are covered, as well as special conditions (emergency and stand-by power, 

or conditions requiring more than 600 volts, for example) and communication systems. 

National Fire Code: The National Fire Code consists of approximately 300 codes and standards 

as published by the National Fire Protection Association (NFPA). These codes address the 

practices to reduce the burden of fire on the quality of life by advocating scientifically based 

consensus codes and standards, research and education for fire and related safety issues. The 

most widely applied codes are: 

 (1.) NFPA 70 – National Electric Code 

 (2.) NFPA 101 – Life Safety Code 

 (3.) NFPA 30 – Flammable and Combustible Liquids Code 

 (4.) NFPA 13 – Standard for the Installation and Maintenance of Automatic Fire Sprinkler 

Systems 
10-21 

10.5.8 References 

1. M. Glass, Fuel Cell Codes and Standards Summit III Summary, April 5-7, 1999, Pacific 

Northwest National Laboratory. 

2. ASME, Object & Scope for the Proposed Code on Fuel Cell Power Systems, August 2000, 

http://www.asme.org. 

3. IEEE, Distributed Resources and Electrical Power Systems Interconnection Working 

Group - Meeting Minutes, June 7-8, 2000, http://grouper.ieee.org. 

4. SAE, SAE Initiates Activities in Area of Fuel Cells, August 1999, http://www.sae.org. 

5. UL, Standard for Safety for Static Inverters and Charge Controllers for Use in 

Photovoltaic Power Systems, 2000, http//:ulstandardsinfonet.ul.com. 

6. National Evaluation Service, National Evaluation Protocol for Stationary Fuel Cell Power 

Plant, June 5, 2000, http://www.nateval.org. 

7. International Electrotechnical Commission, IEC TC 105 Strategic Policy Statement, March 

2000, CA/1719A/R, http://www.iec.ch. 

8. R. Bielen, Telephone Contact, August 9, 2000, NFPA. 

9. S. Kazubski, Telephone Contact, August 14, 2000, CSA International. 

10. D. Conover, Telephone Contact, August 9, 2000, National Evaluation Service. 

10.6 Fuel Cell Field Site Data 

This section of the handbook contains field site information. Most of the worldwide summaries 

were extracted from an IEA paper63 and updated with information taken from “Fuel Cell 

Technology News”64. Information on the U.S. Department of Defense (DoD) Fuel Cell 

Demonstration was taken from the following web site: www.dodfuelcell.com. Finally, updating 

the information for Fuel Cell Energy, IFC, and Siemens Westinghouse was taken from “Fuel 

Cell Technology News”. The IFC PAFC summary includes a number of projects reported by 

DoD. In the DoD demonstration program, a total of 30 PAFC units were installed at DoD sites 

across the United States. These were model B and C PC-25 units. 

 

10.6.1 Worldwide Sites 

Worldwide information reported in this handbook is for stationary application of fuel cells in 

different countries. Data on PEFC, PAFC, AFC, MCFC, and SOFC has been collected. The 

main worldwide projects are summarized below: 

PEFC 

Canada: Canada has focused primarily on PEFC research and development over the last decade. 

To commercialize its PEFC technology, Ballard Power Systems has developed a major 

international network of strategic partners, including DaimlerChrysler, Ford Motor Company, 

GPU international (US), Alstom SA (a UK company based in France), and Ebara Corporation 

(Japan). Ballard 250 kWe stationary prototypes are developed by Ballard Generation Systems. 

The first prototype operating is in Vancouver, Canada. Ballard delivered a second 250-kilowatt 

PEFC power system to Cinergy Technology. This is the first field trial unit built by Ballard. The 

 

63 K. Kono, “Implementing Agreement “Advanced Fuel Cells,” Annex IX Fuel Cells for Stationary Applications, 

Subtask 2,” draft IEA paper, April 1999. 

64 Fuel Cell Technology News, January 2002, published by Business Communications Company, Inc. 
10-22 

unit runs on natural gas, and was commissioned in 1999 at the Naval Surface Warfare Center in 

Crane, Indiana. A third unit in Berlin, Germany at Bewag Treptow Heating Plant started 

operating the second half of 1999. In 2001, Ballard completed 10-kW and 60-kW engineering 

prototype stationary fuel cell generators. 

Japan: In 2002, Japan nearly doubled its fuel cell R&D budget to $220 million from $119 

million. It created a new large-scale Polymer Electrolyte Fuel Cell Development Program to 

develop PEFC fuel cells for both transport and stationary applications, leaving molten carbonate 

fuel cell and solid oxide fuel cell R&D under the New Sunshine Program. 

Ministry of International Trade and Industry (MITI, now METI) has been sponsoring five 

companies to develop PEFC technologies since 1992. Sanyo, Toshiba, and Mitsubishi Electric 

have been developing 2-kW, 10-kW, and 30-kW stacks, respectively. Asahi Chemical Industry 

and Asahi Glass are developing fuel cell components, including polymer membranes and 

separators, according to METI documents. In 2002, Japan nearly quadrupled its budget for 

PEFCs to $156 million and created a 10-year Polymer Electrolyte Fuel Cell Development 

program, which aims to develop PEFCs for both stationary and transport applications. Japan will 

equally focus on both stationary and transportation applications. 

United States: Plug Power has installed over 300 residential systems for unattended operation. 

Plug Power delivered more than 106 5-kW grid-parallel systems through October 31, 2001, 

against its milestone of 125 to 150 units for the year. Deliveries included 44 units to the New 

York State Research & Development Authority and 57 systems to the Long Island Power 

Authority. 

PAFC 

Europe: The Energetic Utility of Milan, the National Agency for Energy, New Technology and 

Environment (ENEA), and Ansaido Ricerche designed, built, and tested a 1.3 MWe PAFC 

system in Milan. The powerplant had an actual capacity of 930 kW and an energy efficiency of 

38 percent (LHV). It has operated for over 5,000 hours. 

Japan: Fuji Electric developed a 100 kWe on-site system. To date, they have tested a 50 kW 

power plant using an innovative cell design that improves electrolyte management. They tested 

this stack (154 cells) for about 2,000 hours. They have tested 65, 50 kWe units for a total 

cumulative operating tome of over 1 million hours. They have tested 3, 500 kWe units for a total 

of 43,437 hours. Their latest design, FP100E, has been shown to have a net AC efficiency of 

40.2 percent (LHV). 

Mitsubishi Electric developed a 200 kWe class on-site powerplant. As of 2002, 11 units were 

operated in the field with applications ranging from an electric utility to a brewery factory. Four 

of the units operated for more than 2,000 hours. 

AFC 

United Kingdom: ZeTek Power, an UK based company with plants in the US and Europe, is 

developing alkaline fuel cells. They are putting AFCs in fleet vehicles and boats in Europe. 
10-23 

AFCs are getting greater than 50 percent efficiency over most of the power curve (5 to 95 

percent). Capital cost for the AFC stack is $300/kWe, and approximately $700 for the system. 

MCFC 

Japan: Japan has been providing support to three companies to develop MCFC technology. 

Ishikawajima-Harima Heavy Industries and Hitachi developed 250-kW stacks and built a 1-MW 

MCFC pilot plant with an external reformer at Kawagoe, Mie Prefecture, consisting of four 250-

kW stacks. The test operation started in July 1999 and ended in January 2000 after 5,000 hours 

of test operations. Since 2000, Japan has focused on commercialization and is supporting 

development of a pressurized 300-kW MCFC cogeneration system, followed by a 750-kW 

system to be completed in March 2004. Japan hopes to see commercialization start immediately 

thereafter, according to METI officials. Japan’s budget was $17 million on MCFC R&D in 2002. 

Europe: Italy and Spain have been working on research and development of MCFC systems as a 

collaborative project called MOLCARE program. The project has a budget of 10 billion pesetas 

(35 percent by Spain and 65 percent by Italy). They have partnered with industry to develop and 

conduct a 1,000-hour test on a 100 kWe unit. 

The European Direct Fuel Cell Consortium carries out the largest European program for the 

commercialization of MCFC. They are developing an innovative direct fuel cell process which 

is internally reformed and operates on humidified hydrocarbon fuels. They have successfully 

tested a 292 cell, 155 kW stack (60 percent of maximum power). 

United States: FuelCell Energy is developing an externally manifolded internally reformed 

MCFC. FuelCell Energy has reached the 50 MW manufacturing capacity and plans to have 400 

MW capacity by 2004. They have also constructed a 400 kWe test facility. They have 

successfully completed the manufacture and test of 16 stack (4 modules), 2 MWe test in Santa 

Clara, California, for 4,000 hours. Details on Fuel Cell Energy field site are found in Table 10-3. 

SOFC 

Japan: The Kansai Electric Company has tested a four-cell stack and accumulated 10,529 hours 

of operation at high current densities and completed 101 thermal cycles. Tokyo Gas started 

research and development of a planar SOFC in 1993. They conducted a 1.7 kW module test with 

stable performance. 

Australia: Ceramic Fuel Cells Limited demonstrated a 5 kWe laboratory prototype fuel cell in 

1997. Their system has thin sheet steel components as interconnects in a planar fuel cell design. 

They are scaling up to a 25 kWe pre-commercial stack module. 

Canada: Ontario Hydro has tested a single Siemens-Westinghouse cell for 1725 hours. Over 

1425 of the hours were at elevated pressure of 5 atm. 

Europe: The ELSAM/EDB project for a 100 kWe Siemens-Westinghouse SOFC field unit has 

operated from January 1998. The unit was to operate until January 2000 with a total of 17,500 

test hours according to the plan. 
10-24 

Spain: A consortium called SEGE is developing an intermediate temperature planar fuel cell. 

United States: Siemens Westinghouse projects on SOFC include a 250 kWe tubular prototype at 

the Irvine University campus (California), that is operated by Southern California Edison 

Company. It is pressurized to 3.5 bar and gives 200 kWe; a coupled microturbine gives an 

additional 50 kWe. The have operated a tubular SOFC at pressures up to 15 atm. Worldwide 

fuel cell installations and general information as of 2004 for installations are reported by Fuel 

Cells 2000 at http://www.fuelcells.org/ info/charts/FCInstallationChart.pdf. 

10.6.2 DoD Field Sites 

DoD’s Climate Change Fuel Cell program included purchasing and installing 30 ONSI PC25 

200 kWe PAFC at DoD installations in addition to providing rebates of $1,000/kW (up to 1/3 of 

the installed cost). Many factors determine the availability and efficiency of individual units; 

maintenance programs and application are two factors. The summary table, Table 10-3, provides 

information on operating hours, efficiency, and availability. The following website provides 

additional information on DoD field sites: www.dodfuelcell.com. 

10.6.3 IFC Field Units 

IFC provided DOE with information on their 59 fuel cell unit operating in North America. This 

information is provided in Table 10-4. Several of these units operate on DoD field sites and are 

reported in Table 10-3. 

10.6.4 FuelCell Energy 

FuelCell Energy provided DOE with information on their fuel cell field units. This information 

is provided in Table 10-5. 

10.6.5 Siemens Westinghouse 

Siemens Westinghouse provided DOE with information on their fuel cell field units. This 

information is provided in Table 10-6. 
10-25 

Table 10-3 DoD Field Site 

Through January 31, 2002 

SITE NAME SERVICE START 

DATE

OPER. 

HOURS

MWHRS 

OUTPUT

AVG 

kWe

ELEC. 

EFF.

AVAIL.

MODEL B UNITS 

Naval Station Newport Navy 1/23/95 42,375 6,387.537 150.7 30.2% 76.1% 

U.S. Army Soldier Systems 

Center

Army 1/27/95 38,608 6,379.235 165.2 31.2% 61.2% 

US Military Academy Army 11/17/95 28,393 4,872.371 171.6 31.5% 63.0% 

934th Airlift Wing Air Force 2/1/95 26,777 4,653.232 173.8 29.7% 48.2% 

Picatinny Arsenal Army 10/11/95 32,053 5,316.291 165.9 30.9% 62.4% 

Naval Hospital 

MCB Camp Pendleton

Marines 10/6/95 26,859 4,507.218 167.8 33.9% 55.1% 

Naval Hospital 

MCAGCC Twentynine Palms

Marines 6/20/95 21,652 3,522.419 162.7 32.3% 44.6% 

Nellis AFB Air Force 9/23/95 19,996 3,383.481 169.2 32.5% 38.7% 

Watervliet Arsenal Army 10/29/97 28,875 4,117.735 142.6 31.4% 77.3% 

Fort Eustis Army 9/12/95 27,705 4,256.532 157.2 31.9% 50.7% 

Kirtland AFB Air Force 7/20/95 16,713 2,502.970 149.8 31.0% 32.5% 

Naval Oceanographic Office Navy 10/7/97 19,641 3,574.854 182.0 34.6% 51.4% 

Pine Bluff Arsenal Army 10/21/97 9,343 1,747.040 187.0 34.9% 39.6% 

CBC Port Hueneme Navy 9/18/97 18,001 3,332.808 185.1 34.3% 46.3% 

B's TOTAL/AVG: 356,991 58,553.723 164.3 32.2% 53.4% 

MODEL C UNITS*

911th Airlift Wing Air Force 12/18/96 35,234 6,037.038 171.3 31.6% 79.8% 

Naval Hospital 

NAS Jacksonville

Navy 3/18/97 33,284 6,193.403 186.1 31.6% 78.4% 

NAS Fallon Navy 3/30/97 31,054 4,880.720 157.2 30.8% 80.1% 

Subase New London Navy 9/30/97 32,848 5,884.840 179.2 31.8% 84.8% 

Fort Richardson Army 12/17/96 30,593 5,617.251 183.6 31.5% 68.1% 

Little Rock AFB Air Force 8/17/97 23,104 4,336.428 187.7 31.7% 68.2% 

Westover AFB Air Force 9/19/97 32,844 6,316.483 192.3 30.7% 86.3% 

Barksdale AFB Air Force 7/24/97 28,554 5,289.629 185.2 31.1% 72.0% 

Fort Huachuca Army 7/28/97 31,776 5,744.980 180.8 32.4% 80.3% 

Laughlin AFB Air Force 9/16/97 29,558 5,584.936 188.9 32.4% 77.0% 

US Naval Academy Navy 9/22/97 37,928 4,736.374 124.9 27.9% 80.9% 

Edwards AFB Air Force 7/5/97 23,866 4,603.664 192.9 32.6% 59.9% 

Fort Bliss Army 10/10/97 23,973 3,936.077 164.2 32.0% 63.6% 

Davis-Monthan AFB Air Force 10/14/97 26,462 4,513.985 170.6 32.1% 71.5% 

NDCEE Other 8/14/97 17,167 2,083.041 121.3 30.5% 58.1% 

C's TOTAL/AVG: 438,245 75,758.849 172.9 31.4% 73.9% 

B+C TOTAL/AVG: 795,236 134,312.570 169.0 31.8% 64.0% 
10-26 

Disclaimer

Electrical efficiency calculations include fuel cell idle time (such as when the fuel cell is 

awaiting the return to operation of the utility grid, etc.). If values were adjusted for idle time, 

fuel cell electrical efficiencies would be higher. ONSI fuel cells passed DoD Fuel Cell Program 

electrical efficiency criteria during unit acceptance tests (range = 33.5 to 37.2 percent, Higher 

Heating Value). 

Availability values are not adjusted for times when the fuel cell was down for extended periods 

unrelated to typical fuel cell operation (delays in maintenance personnel response, site operating 

conditions, etc.). Adjusting for these times would result in higher availability values. 
10-27 

Table 10-4 IFC Field Units 

 PC25 C Fuel 

Cell Power Plant (Run hours, etc. as of 8/4/00) 

North America 

 Status Country/State Site Start Date Load hrs MW

1 Active SOUTH WINDSOR, CT PROTOTYPE FOR R&D N/A N/A 2 Active DEL RIO, TX HOSPITAL 9/6/97 20,143 3 Active LITTLE ROCK, AR HOSPITAL 10/6/97 21,408 4 Active SHREVEPORT, LA HOSPITAL 7/18/97 19,577 5 Active GROTON, CT CENTRAL BOILER 

PLANT 

9/27/97 23,175 

6 Active ANNAPOLIS, MD DORMITORY 9/20/97 20,274 7 Active STATEN ISLAND, NY CHEMICAL PLANT 8/22/96 27,412 8 Active ANCHORAGE, AK YMCA 11/18/96 21,589 9 Active JACKSONVILLE, FL HOSPITAL 3/17/97 24,396 10 Active EL PASO, TX LAUNDRY 10/7/97 16,775 11 Active STATEN ISLAND, NY CHEMICAL PLANT 8/27/96 29,333 12 Active PITTSBURGH, PA CENTRAL BOILER 

PLANT 

12/16/96 28,105 

13 Active SYRACUSE, NY SCHOOL 1/22/97 27,222 14 Active CAPE COD, MA COLLEGE 3/31/99 10,995 15 Active OMAHA, NE BANK 3/25/99 11,084 16 Active YONKERS, NY ANAEROBIC DIGESTER 

GAS 

4/8/97 18,321 

17 Active OMAHA, NE BANK 3/24/99 11,030 18 Active ANCHORAGE, AK ARMORY BUILDING 12/24/96 9,046 19 Active ANCHORAGE, AK ARMORY BUILDING 12/11/96 22,321 20 Active DEER ISLAND, MA ANAEROBIC DIGESTER 

GAS 

9/4/97 2,760 

21 Being Installed ANN ARBOR, MI RESEARCH LAB 0 22 Active FALLON, NV GALLEY BUILDING 2/28/97 25,781 23 Active OMAHA, NE BANK 3/24/99 10,648 24 Active SPOKANE, WA HOTEL 6/11/97 22,680 25 Active CHICOPEE, MA CENTRAL BOILER 9/15/97 22,230 
10-28 

PLANT 

26 Active TUCSON, AZ CENTRAL BOILER 

PLANT 

10/18/97 20,577 

27 Active ROSAMOND, CA CENTRAL BOILER 

PLANT 

6/19/97 19,325 

28 Active SIERRA VISTA, AZ BARRACKS 7/28/97 20,812 29 Will be restarted 

Fall ‘00 

JOHNSTOWN, PA OFFICE/RESEARCH LAB 7/28/97 9,637 

30 Active HARTFORD, CT OFFICE BUILDING 6/18/97 26,023 31 Active WINDSOR LOCKS, CT DATA CENTER 12/19/97 19,634 32 Active MERIDAN, CT OFFICE BUILDING 9/21/97 20,987 33 Being Installed ALCORN STATE, MS UNIVERSITY 0 34 Active BRAINTREE, MA LANDFILL 9/10/99 5,211 35 Being Installed BRONX, NY HOSPITAL 0 36 Active SOUTH WINDSOR, CT INDUSTRIAL SPACE 

HEATING 

3/9/98 19,689 

37 Active PORTLAND, OR WASTE WATER 

TREATMENT PLANT 

5/21/99 7,259 

38 Active OMAHA, NE BANK 3/25/99 11,068 39 Owner sold 

property; being relocated 

HARVEY, LA COMMERCIAL FACILITY 3/13/99 6,823 

40 Active HOUSTON, TX MANUFACTURING 5/12/98 17,871 41 Not Yet Installed NY, USA TBD 0 42 Active GULFPORT, MI DINING FACILITY 5/13/99 7,775 43 Not Yet Installed NJ, USA COLLEGE 0 44 Active NEW YORK, NY SKYSCRAPER OFFICE 

BUILDING 

12/15/99 5,220 

45 Active NEW YORK, NY SKYSCRAPER OFFICE 

BUILDING 

12/16/99 5,553 

46 Active RAMAPO, NJ COLLEGE 3/29/00 2,448 47 Active NEW YORK, NY POLICE STATION 4/17/99 11,108 48 Active MESA, AZ MUNICIPAL BUILDING 4/29/00 2,192 49 Active ANCHORAGE, AK POST OFFICE 

DISTRIBUTION CENTER 

6/28/00 3,396 
10-29 

50 Active ANCHORAGE, AK POST OFFICE 

DISTRIBUTION CENTER 

6/28/00 3,329 

51 Active ANCHORAGE, AK POST OFFICE 

DISTRIBUTION CENTER 

6/28/00 3,939 

52 Active ANCHORAGE, AK POST OFFICE 

DISTRIBUTION CENTER 

6/28/00 4,123 

53 Active ANCHORAGE, AK POST OFFICE 

DISTRIBUTION CENTER 

6/28/00 3,563 

54 Active CALABASAS, CA ANAEROBIC DIGESTER 

GAS 

12/15/99 6,613 

55 Active CALABASAS, CA ANAEROBIC DIGESTER 

GAS 

12/15/99 8,322 

56 Active JOHNSTOWN, PA RESEARCH LAB 1/6/00 3,655 57 Active SOUTH KINGSTOWN, 

RI 

HOSPITAL 10/18/99 6,532 

58 Active SYRACUSE, NY HIGH SCHOOL 2/4/00 4,427 59 Active BELLAIR, TX INDUSTRIAL BUILDING 5/24/00 1,867 
10-30 

Table 10-5 FuelCell Energy Field Sites (mid-year 2000) 

Fuel Cell Type Location Status 

Start 

Date 

Operating 

Hours 

MWhrs 

Output 

Size, kw 

Design Actual* 

Eff. 

Percent 

Avail. 

Percent

Direct Fuel Cell Santa Clara, CA Completed 3/1996 5,800 2,570 1,800 1,930 44 99** 

Direct Fuel Cell Danbury, CT Completed 2/1999 11,800 1,906 250 263 45 93 

Direct Fuel Cell Bielefeld, Germany Continuing 11/1999 4,300+ 500+ 250 225 45 90 

* Maximum attained 

** BOP availability 

Table 10-6 Siemens Westinghouse SOFC Field Units (mid-year 2002) 

Year Customer 

Size, 

kWe Fuel 

Cell 

Type 

Cell 

Length 

(cm) 

Operating 

Hours 

Cell 

Number

MWH

(DC) 

1986 TVA 0.4 H2+CO TK-PST 30.0 1,760 24 0.5 

1987 Osaka Gas 3.0 H2+CO TK-PST 36.0 3,012 144 6 

1987 Osaka Gas 3.0 H2+CO TK-PST 36.0 3,683 144 7 

1987 Tokyo Gas 3.0 H2+CO TK-PST 36.0 4,882 144 10 

1992 JGU-1 20.0 PNG TK-PST 50.0 817 576 11 

1992 Utilities–A 20.0 PNG TK-PST 50.0 2,601 576 36 

1992 Utilities-B1 20.0 PNG TK-PST 50.0 1,579 576 26 

1993 Utilities-B2 20.0 PNG TK-PST 50.0 7,064 576 108 

1994 SCE-1 20.0 PNG TK-PST 50.0 6,015 576 99 

1995 SCE-2 27.0 PNG/DFJP-8 AES 50.0 5,582 576 118 

1995 JGU-2 25.0 PNG AES 50.0 13,194 576 282 

1998 SCE2/NFCRC 27.0 PNG AES 50.0 3,394+ 576 73+ 

1997 EDB/ELSA

M-1 

125.0 PNG AES 150.0 4,035 1152 471 

1999 EDB/ELSA

M-2 

125.0 PNG AES 150.0 12,577 1152 1,153+ 

2000 SCE 180.0 PNG AES 150.0 770+ 1152 25+ 

2001 RWE 125.0 PNG 150.0 3,700+ 1152 

PNG = Pipeline Natural Gas 

TK-PST = Thick Wall Porous Support Tube 

TN-PST = Thin Wall Porous Support Tube 

AES = Air Electrode Support 

+ = Testing Continues 
10-31 

10.7 Hydrogen 

10.7.1 Introduction 

The use of hydrogen in the United States energy infrastructure has been considered for decades. 

For economic reasons, the hydrogen economy has not developed; for environmental reasons, the 

potential of hydrogen becoming a major commodity exists. In 1990, the United States Congress 

passed the Matsunaga Hydrogen Research and Development Act. The Act required the 

Department of Energy to develop critical hardware for hydrogen technology. The Act also 

established the Hydrogen Technical Advisory Panel, which is composed of experts from industry 

and academia, to advise the Secretary of Energy on the status and recommended direction of 

hydrogen energy development. In 1996, Congress passed the Hydrogen Future Act; the Act 

authorized the spending of $164.5 million between 1996 and 2001 on the research, development, 

and demonstration of hydrogen production, storage, transport, and use. In February 2003, 

President Bush announced the FutureGen initiative to build the world's first integrated 

sequestration and hydrogen production research power plant with zero-emissions. When 

operational, the prototype will be the cleanest fossil fuel-fired power plant in the world. 

The demand for hydrogen grew 23 percent per year between 1994 and 1999 and is projected to 

continue to grow by 14 percent per year through 2004. Oil refining accounts for 67 percent of 

the hydrogen usage in the United States. The manufacture of petrochemicals accounts for 26 

percent, and the final 7 percent is used in the reduction of metals, electronics, glass, rocket fuel, 

food processing, laboratory use, and power generation. Many believe that the demand for 

hydrogen will continue to grow for the following reasons: 

• As domestic reserves of oil decline and heavier imported crude oil is refined, increased 

amounts of hydrogen will be required 

• As higher sulfur crude oils are refined, additional hydrogen for desulfurization to meet 

existing and more stringent future regulations will be required 

• The number of hydrogen-powered vehicles will increase 

• Electricity produced by hydrogen-fueled technology will increase 

• The increased use of hydrogen will reduce the dependency on imported oil 

The interest in hydrogen as pollution-free energy has sparked legislation. The following is some 

of the Federal and state legislation: 

• The "Hydrogen Future Act of 1996" focuses Federal hydrogen research on the basic 

scientific fundamentals needed "to provide the foundation for private sector investment and 

development of new and better energy sources.” 

• California’s “zero-emission” standard for passenger cars requires that 2 percent of new cars 

in the state be non-polluting. 

• As part of California’s Clean Transportation Fuels Initiative, the California Energy 

Commission (CEC) will assist in establishing publicly accessible clean-fuel refueling 

facilities to serve clean-fuel fleets and vehicles in California. Eligible projects include all 

non-petroleum fuels such as natural gas, alcohol, and hydrogen (for fuel cell applications). 

• In April 2000, the Arizona Legislature passed SB 1504 - an important piece of legislation for 

the alternative fuels movement and most particularly the hydrogen program. The Hydrogen 

Grant Program allows up to $500,000 for hydrogen programs that benefit the public.
10-32 

• The State of Georgia offers an income tax credit of $5,000 for the purchase or lease of a zero 

emission vehicle (ZEV). ZEVs include battery-only electric vehicles (EVs) and hydrogen 

fuel cells. 

• New York's Alternative-Fuel (Clean-Fuel) Vehicle Tax Incentive Program offers tax credits 

and a tax exemption for people who purchase alternative fuel vehicles (AFVs). Purchasers of 

compressed natural gas, liquefied petroleum gas, methanol, ethanol, and hydrogen-powered 

vehicles, as well as hybrid electric vehicles (HEVs), are eligible for a tax credit worth 60 

percent of the incremental cost. 

• In 1992, the state of Pennsylvania established a program to reduce Pennsylvania's 

dependence on imported oil and improve air quality through the use of alternative fuels. 

Eligible alternative motor fuels and fuel systems are compressed natural gas, liquefied 

natural gas, liquid propane gas, ethanol, methanol, hydrogen, hythane, electricity, coalderived liquid fuels, fuels derived from biological materials, and fuels determined by the 

Secretary of the U.S. Department of Energy as meeting the requirements of Section 301 of 

the Energy Policy Act of 1992. After July 1, 2001, qualified projects will receive funding for 

20 percent of eligible project costs. 

• Effective January 1, 1996, Virginia’s sales and use taxes were reduced by 1.5 percent for any 

motor vehicle that has been manufactured, converted, or retrofitted to operate on compressed 

natural gas, liquefied natural gas, liquefied petroleum gas, hydrogen, or electricity. 

• The University of Wisconsin-Milwaukee Center for Alternative Fuels offers a Congestion 

Mitigation Air Quality Alternative Fuels Grant Program for the incremental cost of 

purchasing AFVs. Wisconsin municipalities, in an 11 county area (including Milwaukee, 

Waukesha, Racine, Kenosha, Walworth, Washington, Ozaukee, Sheboygan, Manitowoc, 

Kewaunee, and Door counties), are eligible to participate in the grant program. Eligible 

vehicles include dedicated, bi-fuel, or flexible fuel vehicles. Eligible fuels include ethanol, 

methanol, hydrogen, compressed natural gas, liquefied natural gas, propane, biodiesel, and 

electricity. Grant awards are allocated through a competitive grant application process. The 

maximum grant award per passenger vehicle is $6,500 and $12,000 per truck, van, or bus 

with a total of $50,000 per municipality. 

The opportunities for R&D to advance hydrogen production, utilization, and storage hold great 

potential. “Much of the recent ferment over hydrogen and fuel cells has taken place in the auto 

industry. DaimlerChrysler has committed $1 billion over 10 years to fuel cell development, and 

is working with Ford and Ballard Power Systems to put transit fuel cell buses on the road in 

Europe. General Motors aims to be the first car company to sell one million fuel cell vehicles, 

beginning mass production in 2010, and has announced major investments in two companies 

specializing in hydrogen storage and delivery. Toyota sent shock waves through the industry by 

announcing it would start selling its fuel cell car in Japan. The energy industry is also getting 

serious about hydrogen. Both Shell and BP have established core hydrogen divisions within 

their companies. ExxonMobil is teaming up with GM and Toyota to develop fuel cells. Texaco 

has become a major investor in hydrogen storage technology (4).” For additional information on 

industry announcements, see The Hydrogen and Fuel Cell Letter. 

10.7.2 Hydrogen Production 

A number of hydrogen manufacturing plants are sited (see Table 10-7) across the United States. 

Any carbonaceous material can be used to make hydrogen from steam reforming, but they are 
10-33 

more likely to contain contaminants than natural gas, and would require cleanup before use. The 

main reason natural gas is used is that the supply of natural gas is abundant and the price 

continues to remain low. If the prices or availability of natural gas becomes prohibitive, water is 

another abundant source of hydrogen. Several forms of energy can be used to make hydrogen: 

• Thermal: Thermal decomposition of water into hydrogen and oxygen occurs at temperatures 

around 2,500 o

C. The process isn’t attractive because few materials can withstand that 

temperature. In the plasma arc process, water is heated to 5,000 o

C by an electric field 

resulting in the cracking products H, H2, O, O2, OH, HO2, and H2O. A fraction of 50 percent 

by volume of H and H2 is possible. The plasma gases are quenched with a cryogenic liquid 

to prevent the gases from recombining. This process consumes a lot of energy and is very 

expensive to operate. 

• Thermochemical: Today, hydrogen is produced mainly from natural gas by steam methane 

reforming. Steam methane reforming (SMR) is not only the most common, but is also the 

least expensive method of producing hydrogen; almost 48 percent of the world’s hydrogen is 

produced from SMR (1). Refineries produced and used 2,500 billion scf in 1998. 

• Electrochemical: Water electrolysis passes a direct current between two electrodes in water. 

The water is made more conductive by adding an electrolyte such as potassium hydroxide. 

Hydrogen gathers around the negative electrode (cathode) and oxygen gathers around the 

positive electrode (anode). The gases are collected separately. 

• Photoelectrochemical: Sunlight (photons) provides the source of energy for this process. 

Photons interact with dissolved chemicals to produce activated species, which in turn 

deactivate by releasing hydrogen from water. This is solar-powered electrolysis. 

• Photobiologial: Sunlight provides the source of energy for this process. Living organisms, 

such as green algae, make enzymes. The pigment of algae absorbs solar energy, and the 

enzyme in the cell acts as a catalyst to split the water molecules. 

Table 10-7 Hydrogen Producers3 

Producer Capacity* 

Merchant Cryogenic Liquid 

Air Products and Chemicals, New Orleans, LA 26,800 

Air Products and Chemicals, Pace, FL 11,500 

Air Products and Chemicals, Sacramento, CA 2,300 

Air Products and Chemicals, Sarnia, Ont. 11,500 

BOC, Magog, Quebec 5,900 

HydrogenAL, Becancour, Quebec 4,600 

Praxair, East Chicago, IN 11,500 

Praxair, McIntosh, AL 11,500 

Praxair, Niagara Falls, NY 15,000 

Praxair, Ontario, CA 8,500 

Total Merchant Cryogenic Liquid 109,100 

Merchant Compressed Gas 

Air Liquide (11 locations) 67,960 

Air Products and Chemicals (20 locations) 740,440 

BOC (6 locations) 14,650 

Brown Industries (3 locations) 460 

General Hydrogen, Natrium, WV 200 
10-34 

Producer Capacity* 

Holox, Augusta, GA 400 

Industrial Gas Products, Sauget, IL 1,500 

Javelina, Corpus Christi, TX 35,000 

Jupiter Chemicals, Westlake, LA 35,000 

Lagus, Decatur, AL 9,000 

Equistar, Channelview, TX 80,000 

MG Industries (3 locations) 1,300 

Praxair (22 locations) 425,960 

Prime Gas, Delaware City, DE 200 

Rohm and Haas, Deer Park, TX n.a. 

T&P Syngas Supply, Texas City, TX 32,400 

Total Merchant Compressed Gas 1,444,470 

Total Merchant Product 1,553,570 

* Thousands standard cubic feet (SCF) per day merchant hydrogen from steam reforming of light 

hydrocarbons or recovered as by-product from chloralkali plants or chemical synthesis 

operations. 

Hydrogen Utilization 

Hydrogen can be used to power vehicles, run turbines or fuel cells to produce electricity, and 

generate heat and electricity for buildings. Hydrogen is used as a chemical in the petrochemical, 

electronics, and food industries. The zero-emission potential of using hydrogen as a fuel has 

sparked interest in the utility and transportation sectors. 

10.7.3 DOE’s Hydrogen Research 

Concerns about air pollution, global warming, and long-term fuel availability have focused 

international attention on the development of alternative fuels. Hydrogen will be an important 

part of future energy systems addressing these concerns. Whether processed in a fuel cell or 

burned in a combustion process, hydrogen represents an exceptionally clean energy source. 

Development is underway on processes that economically produce hydrogen from methane, coal, 

water, and other abundant sources. 

DOE’s hydrogen research draws upon core competence in several engineering and technology 

areas, including systems engineering, safety and risk assessment, chemical and mechanical 

engineering, manufacturing and materials, sensors and controls, plasma processing, fuel cell 

technology, biotechnology engineering, and alternative fuel vehicle fueling infrastructure 

development. Hydrogen programs are managed at the National Energy Technology Laboratory, 

the Idaho National Engineering and Environmental Laboratory (INEEL), and the National 

Renewable Energy Laboratory (NREL). Promising technologies related to production, 

infrastructure, and utilization of hydrogen are: 

• Production of hydrogen from coal (NETL) 

• Thermal-plasma/quench process for converting methane to hydrogen, with solid carbon 

produced as a by-product (INEEL) 

• Biotechnology processes for production of hydrogen from carbon-containing waste and 

renewable resources (INEEL) 
10-35 

• Photoconversion production uses either biological organisms (bacteria or algae) or 

semiconductors to absorb sunlight, split water, and produce hydrogen (NREL) 

• Thermochemical production uses heat to produce hydrogen from biomass and solid waste 

(NREL) 

• Low-pressure storage of hydrogen in the use of metal ion intercalated graphite fibers as a 

medium (INEEL) 

• Fleet and fueling systems engineering analysis of hydrogen-powered buses and supporting 

fueling stations (INEEL) 

• Safety and risk assessment of hydrogen as transportation fuel (INEEL) 

• Demonstration of hydrogen-powered vehicles and related transportation system 

infrastructure, including hydrogen production, storage, and fueling 

• Demonstration of hydrogen-fueled, small-scale power generation for local (distributed) 

electricity production 

• Since hydrogen can neither be seen nor smelled, as an added safety precaution for hydrogenfueled vehicles, hydrogen sensors are being developed. To detect hydrogen, a very thin 

sensor that reacts to hydrogen by changing colors is applied to the end of a fiber optic cable. 

The sensors can be placed throughout the vehicle to relay information on leak detection to a 

central control panel (NREL) 

As research and development progresses, collaboration with private sector partners to conduct 

demonstration testing of hydrogen-fueled vehicles, and demonstration testing of prototype 

hydrogen-fueled distributed electric power stations will be done. A list of worldwide hydrogen 

fuel stations can be viewed at http://www.fuelcells.org/info/charts/h2fuelingstations.pdf. 

10.7.4 Hydrogen Storage 

The four most common methods for storing hydrogen are: 

• Compressed gas in pressure vessels: New materials have allowed pressure vessels and 

storage tanks to be constructed that can store hydrogen at extremely high pressures. 

• Hydrogen absorbing materials: 

1. Metals (pure and alloyed) can combine with hydrogen to make a metal hydride. The 

hydride releases hydrogen when heated. Hydrogen stored in hydrides under pressure has 

a very high energy density. 

2. Hydrogen molecules that have been absorbed on charcoal can approach the storage 

density of liquid hydrogen. 

3. Small glass spheres (microspheres), carbon nanotubes, and fullerenes can hold hydrogen 

if it is induced at high pressure and temperature. The hydrogen is held captive in the 

solid matrix when the temperature lowers. Hydrogen can be released by heating the 

solid. 

• Liquid storage: Hydrogen can be converted into a liquid by reducing the temperature to –

253 o

C. This can save cost in transportation, but requires additional energy and cost to keep 

the hydrogen at the low temperature. Refrigerating hydrogen in liquid form uses the 

equivalent of 25 to 30 percent of its energy content. A concern of storing liquid hydrogen is 

minimizing loss of liquid hydrogen by evaporation. 

• Underground storage in depleted oil and natural gas reservoirs, aquifers, and salt 

cavities: For underground storage of hydrogen, a large cavern of porous rock with an 
10-36 

impermeable caprock above it would be needed to contain the gas. As much as 50 percent of 

the hydrogen pumped into the formation would remain in the formation. 

10.7.5 Barriers 

A number of key barriers must be addressed by federal, state, and local governments along with 

industry and academia (2). These barriers are listed below: 

• The primary constraint on remote fuel cells generating electricity from hydrogen is 

economical. Power is inexpensive in the United States. For a fuel cell to compete with other 

generation sources, its price must be reduced dramatically. Remote power applications offer 

the best opportunities for fuel cells to compete economically. Generally speaking, the cost of 

hydrogen should be under $10/MMBtu to be competitive with other energy sources. Fuel 

cells at customer sites with a use for the waste heat must be acquired and installed at a cost 

under $2,000/kW. 

• Research and development is required to improve the performance and reduce the cost of 

renewables, storage, and fuel cell technologies. Technologies are needed that can produce 

hydrogen for the same price as gasoline. Storage technologies must be developed to allow 

cheap, safe hydrogen storage. Finally, fuel cell technology must advance to improve 

efficiency. 

• Safety is a prime consideration for stationary fuel cells. As fuel cells come closer to the 

customer, codes must be written and building inspectors educated to allow the introduction of 

fuel cell power systems. Standards are being developed for on-board hydrogen, but these 

efforts must be expanded to include standards in building codes and for on-site hydrogen 

production, storage, and use at industrial sites. Codes and standards activities along these 

lines are underway. 

• Difficulty in obtaining insurance is of prime concern for siting hydrogen projects. Efforts 

must be undertaken for the government to provide a layer of insurance coverage. In addition, 

insurance companies must be educated as to the proper handling of hydrogen and the 

associated risks. This would allow for property, liability, and efficacy insurance to be 

offered at reasonable rates. 

• Public outreach is necessary for the development of hydrogen technologies. The public 

perception is that hydrogen is dangerous. EPA lists hydrogen as a hazardous chemical. The 

public requires positive experiences in using hydrogen at work or in transportation to 

overcome negative perceptions. Children can be educated at school with a curriculum that 

includes studying hydrogen as a renewable, non-polluting energy source. 

10.8 The Office of Energy Efficiency and Renewable Energy Work in Fuel 

Cells 

The Office of Energy Efficiency and Renewable Energy (EERE), whose mission is to develop 

and deploy efficient and clean energy technologies, is part of the United States Department of 

Energy. EERE partners with the private sector, state and local governments, DOE national 

laboratories, and universities to conduct its program activities. To help accomplish its mission, 

EERE is aided by the Golden Field Office and six regional offices, each of which serves a 

specific geographic region of the United States and its territories. 
10-37 

In early 2002, the Office of Energy Efficiency and Renewable Energy organized into eleven 

Program Offices. These offices include: 

1. Solar 

2. Wind and Hydropower 

3. Geothermal 

4. Distributed Energy, Electricity, Infrastructure, and Reliability 

5. Biomass 

6. Industrial Technologies 

7. FreedomCAR & Vehicle Technologies 

8. Hydrogen, Fuel Cells and Infrastructure Technologies 

9. Building Technologies 

10. Weatherization and Intergovernmental Grants 

11. Federal Energy Management Programs 

EERE’s fuel cell research is focused on low temperature fuel cells for transportation applications 

and distributed energy systems. A short summary of the activities conducted in the focus areas 

follows: 

• Transportation Systems: Conduct R&D and analysis activities that address key barriers, 

including cost and reliability, to fuel cell systems for transportation applications. Activities 

support the development of individual component technology critical to systems integration, 

as well as systems-level modeling activities that guide R&D activities, benchmark systems 

progress, and explore alternate systems configurations on a cost-effective basis. 

• Distributed Energy Systems: Develop high-efficiency polymer electrolyte fuel cell power 

systems as an alternative power source to grid-based electricity for buildings and other 

stationary applications. Activities focus on overcoming the barriers to stationary fuel cell 

systems, including cost, durability, heat utilization, startup time, and managing power 

transients and load-following requirements. 

• Fuel Processing: Develop onboard fuel processors as an alternative to the direct hydrogen 

approach for transportation and stationary applications. 

• Stack Components: In collaboration with partners, research and develop technologies to 

overcome the most critical technical hurdles for polymer electrolyte fuel cell stack 

components for both stationary and transportation applications. Critical technical hurdles 

include cost, durability, efficiency, and overall performance of components such as the 

proton exchange membranes, oxygen reduction electrodes, advanced catalysts, bipolar plates, 

etc. 

• Technology Validation: Validate component R&D in a systems-context under real-world 

operating conditions to quantify the performance and reliability, document any problem 

areas, and provide valuable information to researchers to help refine and direct future R&D 

activities related to fuel cell vehicles. 

DOE’s Office of Energy Efficiency and Renewable Energy (EERE) develops fuel cell 

technologies with an emphasis on the polymer electrolyte fuel cell for both stationary and 

transportation applications. In general, PEFC technology, a low temperature fuel cell system, has 

attractive performance characteristics for smaller-scale systems, while the high temperature 

systems developed under the Fossil Energy program are most attractive in larger sized systems. 

The FreedomCAR partnership between DOE and USCAR (a pre-competitive research 
10-38 

organization consisting of General Motors, Ford, and DaimlerChrysler) is the vehicle through 

which PEFC fuel cells are being developed for use in automotive applications. EERE also has 

the responsibility for developing PEFC fuel cells for portable and distributed generation 

applications as well as the technologies required for the hydrogen energy infrastructure that is 

important in the long-term for large-scale use of PEFC fuel cells. 

EERE addresses a recommendation in the National Energy Policy to integrate hydrogen and fuel 

cell activities by creating the Hydrogen, Fuel Cells & Infrastructure Technologies Program. The 

program recognizes the direct linkage between a robust cost-effective hydrogen infrastructure 

and the effective utilization of fuel cell technologies. This program consists of three teams: 

Hydrogen Production, Hydrogen Storage, and Fuel Cells. Similarly, the FreedomCAR 

Partnership now includes a Hydrogen Storage and Refueling Interface Technical Team, a Fuel 

Cell Technical Team, and a new team formed to address hydrogen production and infrastructure 

issues. The teams consist of automotive and energy industry professionals along with DOE 

personnel to ensure adequate industry input in the planning and evaluation of program activities. 

10.9 Rare Earth Minerals 

10.9.1 Introduction 

In an effort to reduce fuel cell manufacturing cost, low-priced rare earth minerals are being 

considered. Rare earth minerals such as lanthanum are used in making cathodes for the solid 

oxide fuel cell. Lower purity minerals, such as lanthanide manganite, are being tested to 

determine whether these materials will perform without serious degradation of fuel cell 

performance. 

The rare earth minerals are composed of scandium, yttrium, and the lanthanides. The 

lanthanides comprise a group of 15 elements that include: lanthanum, cerium, praseodymium, 

neodymium, promethium, samarium, europium, gadolinium, terbium, dysprosium, holmium, 

erbium, thulium, ytterbium, and lutetium. Cerium is the most abundant element in the rare earth 

group at 60 ppm, followed by yttrium at 33 ppm, lanthanum at 30 ppm, and neodymium at 28 

ppm. Thulium and lutetium are the least abundant at 0.5 ppm. 

Molycorp, a wholly owned subsidiary of Unocal Corp., was the only company to mine rare earth 

minerals in the United States in 2002. The rare-earth separation plant operations stopped in 

2003. Molycorp mined bastnasite, a rare earth fluorocarbonate mineral, as a primary product at 

Mountain Pass, California. The value of domestic ore production was estimated at $31 million 

in 2002; the estimated value of refined rare earth minerals was more than $1 billion. The end 

uses for rare earth products in 2000 were as follows: automotive catalytic, 22 percent; glass 

polishing and ceramics, 39 percent; permanent magnets, 16 percent; petroleum refining catalysts, 

12 percent; metallurgical additives and alloys, 9 percent; rare earth phosphors for lighting, 

televisions, computer monitors, radar, and x-ray intensifying film, 1 percent, and miscellaneous, 

1 percent. 

Rare earth minerals are relatively abundant in the Earth’s crust, but discovered minable 

concentrations are less common than for most other ores. U.S. and world resources are 

contained primarily in bastnasite and monazite. Bastnasite deposits in China and the United 
10-39 

States constitute the largest percentage of the world’s rare earth economic reserves, while 

monazite deposits in Australia, Brazil, China, India, Malaysia, South Africa, Sri Lanka, 

Thailand, and the United States constitute the second largest segment. Xenotime, rare earth 

bearing clays, loparite, phosphorites, apatite, eudialyte, secondary monazite, cheralite, and spent 

uranium solutions make up most of the remaining resources. Undiscovered resources are 

thought to be very large relative to expected demand. Table 10-8 provides world mine 

production and reserves.65

Table 10-8 World Mine Production and Reserves 

Country Mine Production, 2003 Reserves 

United States -- 13,000,000 

Australia -- 5,200,000 

Brazil -- 110,000 

Canada -- 940,000 

China 90,000 27,000,000 

Commonwealth of 

Independent States 

2,000 19,000,000 

India 2,700 1,100,000 

Malaysia 450 30,000 

South Africa -- 390,000 

Sri Lanka 120 12,000 

Other Countries -- 21,000,000 

World Total (rounded) 95,000 88,000,000 

Rare earth prices are quite competitive, causing product prices to be quoted on a daily basis. 

Table 10-9 shows Rhodia, Inc. quoted prices66 during 2002. 

Table 10-9 Rhodia Rare Earth Oxide Prices in 2002 

Product (oxide) Percentage purity 

Standard package 

quantity (kilograms) 

Price (dollars per 

kilogram) 

Cerium 96.00 25 19.20 

Cerium 99.50 900 31.50 

Dysprosium 99.00 3 120.00 

Erbium 96.00 2 155.00 

Europium 99.99 1 990.001 

Gadolinium 99.99 3 130.00 

Holmium 99.90 10 440.002 

Lanthanum 99.99 25 23.00 

Lutetium 99.99 2 3,500.00 

Neodymium 95.00 20 28.50 

 

65 U.S. Geological Survey, Mineral Commodity Summaries, January 2004. 

66 U.S. Geological Survey, Mineral Yearbook, 2002 edition. 
10-40 

Praseodymium 96.00 20 36.80 

Samarium 99.90 25 360.00 

Samarium 99.99 25 435.00 

Scandium 99.99 1 6,000.00 

Terbium 99.99 5 535.00 

Thulium 99.90 5 2,300.00 

Ytterbium 99.00 10 340.00 

Yttrium 99.99 50 88.00 

1 Price for quantity greater than 40 kilograms is $900.00 per kilogram 

2 Price for quantity less than 10 kilograms is $485.00 per kilogram 

10.9.2 Outlook 

The demand for rare earth minerals is expected to increase as demand for products such as 

automobiles, computers, electronics, and portable equipment grow. Rare-earth markets are 

expected to require greater amounts of higher purity mixed and separated products to meet the 

demand. Growth in autocatalysts has been strong in response to legislation on lower emission 

levels, and between 1997 and 2000 the demand for rare earth magnets grew at 21 percent per 

year in spite of the uncertainties created by the financial crisis in Asia. Over the past 5 to 6 years 

China has increased its dominance of the world market, supplying an estimated 95 percent of 

world demand in 2003. 

World reserves are believed to be sufficient to meet forecast world demand well into the 21st

century. Several world class rare earth deposits in Australia and China have yet to be developed 

because world demand is currently met by existing production. The long-term outlook is for an 

increasing competitive and diverse group of rare earth suppliers. As research and technology 

continue to advance the knowledge of rare earth minerals and their interactions with other 

elements, the economic base of the rare earth industry is expected to continue to grow. New 

applications are expected to be discovered and developed. 
10-41 

10.10 References 

1. K. Kono, “Implementing Agreement ‘IEA Advanced Fuel Cells,’ Annual Report 2001.” 

February 2002. 

2. Fuel Cell Technology News, January 2002, published by Business Communications 

Company, Inc. 

3. “Hydrogen Rising in Energy Policy Debate: Global race for “tomorrow’s petroleum” heats 

up,” Worldwatch News Release, August 2, 2001. 

4. C Padro and V. Putsche, “Survey of the Economics of Hydrogen Technologies,” 

NREL/TP-570-27079, September 1999. 

5. National Renewable Energy Laboratory web site: http://www.nrel.gov/hydrogen 

6. Idaho National Engineering and Environmental Laboratory web site: 

http://www.inel.gov/energy/fossil/hydrogen 

7. National Hydrogen Association Near-term Hydrogen Implementation Plan 1999-2005; 

http://www.hydrogenus.com/implementationplan.asp 

8. U.S. Geological Survey, Mineral Commodity Summaries, January 2002. 

9. U.S. Geological Survey, Mineral Yearbook, 1999 edition. 

10. Source: www.roskill.co.uk/rey.html
11-1 

11. INDEX 

acid, xviii, 1-9, 1-10, 1-22, 3-2, 3-4, 5-2, 

5-3, 5-5, 5-6, 5-11, 5-12, 6-7, 8-94, 10-12 

activation losses, 2-18 

alkali, 1-11, 6-5, 6-6, 6-7, 6-10, 6-28, 6-29 

alkaline, 1-7, 1-10, 1-22, 8-94, 10-10 

anode, 1-2, 1-9, 1-10, 1-11, 1-12, 2-4, 2-5, 

2-6, 2-7, 2-16, 2-17, 2-18, 2-21, 2-22, 3-6, 

3-7, 3-8, 3-9, 3-10, 3-12, 3-13, 3-16, 3-19, 

3-20, 5-1, 5-2, 5-9, 5-10, 5-11, 5-13, 5-15, 

5-16, 5-17, 5-20, 6-1, 6-2, 6-3, 6-4, 6-5, 

6-7, 6-8, 6-9, 6-10, 6-15, 6-16, 6-18, 6-19, 

6-20, 6-21, 6-23, 6-24, 6-26, 6-28, 6-29, 

6-30, 6-31, 6-32, 6-37, 7-20, 7-25, 8-49, 

8-55, 8-57, 8-64, 9-1, 9-5, 9-6, 9-7, 9-10, 

9-11, 10-14 

anodic, 2-6, 2-21, 5-16, 5-19, 6-28 

Ansaldo, 1-16, 6-1 

applications, 1-13, 1-15, 1-16, 1-17, 1-23, 

2-19, 3-3, 3-8, 3-11, 3-15, 5-7, 8-1, 8-2, 

8-57, 9-26 

availability, 1-14, 1-17, 8-46 

balance, 1-3, 1-9, 2-7, 3-9, 6-3, 8-49, 8-51, 

8-64 

Ballard Power Systems, 1-18, 1-22, 1-23, 

1-34, 8-53, 8-94, 8-105 

bipolar, 3-13, 5-4, 6-8, 6-9 

bottoming cycle, 8-1, 8-47, 8-48, 8-49, 8-50, 

8-51, 8-60, 8-62, 8-64, 8-66, 8-67, 8-68 

Cairns, 2-23, 2-34, 5-24, 6-41 

carbon, 2-4, 2-21, 3-10, 3-12, 5-1, 5-2, 5-3, 

5-9, 5-10, 5-11, 5-12, 6-16, 6-17, 6-19, 

6-33, 7-26, 8-51, 8-52, 8-64, 8-67, 9-7, 

9-13, 9-14, 9-15, 9-16, 10-1 

carbon black, 5-1, 5-2, 5-3, 5-9, 5-10, 5-11 

carbon composite, 3-10 

carbon monoxide, 2-4, 3-10, 8-52, 8-64, 

8-67, 9-14 

catalyst, 2-6, 2-7, 3-10, 3-11, 3-12, 3-13, 

3-19, 5-1, 5-8, 5-9, 5-10, 5-11, 5-13, 6-28, 

6-31, 6-32, 7-25, 8-47, 8-48, 8-64 

catalysts loading, 1-9 

cathode, 1-2, 1-9, 1-10, 1-11, 1-12, 2-4, 2-5, 

2-6, 2-16, 2-17, 2-18, 2-21, 2-22, 2-23, 

2-24, 3-7, 3-9, 3-11, 3-12, 3-15, 5-1, 5-2, 

5-6, 5-9, 5-11, 5-12, 5-14, 5-17, 5-20, 6-1, 

6-2, 6-3, 6-4, 6-5, 6-7, 6-9, 6-10, 6-11, 

6-12, 6-13, 6-15, 6-18, 6-20, 6-21, 6-22, 

6-23, 6-24, 6-28, 6-29, 6-37, 7-20, 7-25, 

8-47, 8-49, 8-55, 8-56, 8-57, 8-59, 8-64, 

8-67, 9-5, 9-6, 9-7, 9-10, 9-11, 10-14 

cathode dissolution, 6-10, 8-47 

cation, 3-2 

ceramic, 1-11, 1-12, 6-6, 6-10, 7-1 

cermet, 1-12 

characteristics, 1-14, 1-15, 1-18, 3-9, 6-37 

chemisorption, 3-8, 3-14 

cleanup, 1-19, 5-10, 6-10, 6-13, 6-26, 6-27, 

8-51, 8-52, 8-57, 10-2 

coal gasification, 6-25, 8-51, 10-2 

coflow, 2-22 

cogeneration, 1-9, 1-13, 1-15, 1-17, 5-1, 8-1, 

8-2, 8-57, 8-62, 9-16, 9-18 

coking, 9-15 

commercialization, 1-18, 3-20, 6-30, 7-30, 

8-52, 8-61 

concentration losses, 2-18, 5-19, 6-30, 7-29 

contaminants, 1-19, 3-10, 5-10, 5-13, 6-13, 

6-25, 6-26, 6-30, 8-47, 8-57 

converter, 5-10, 8-18, 8-21 

cooling, 1-17, 3-10, 3-11, 5-5, 8-50, 8-54, 

8-56 

corrosion, 1-9, 1-12, 2-19, 5-3, 5-4, 5-9, 

5-11, 5-12, 5-13, 6-3, 6-7, 6-9, 6-21, 6-29, 

7-1, 8-47, 8-48 
11-2 

cost of electricity, 8-2, 8-69, 9-25, 10-10 

counterflow, 2-22 

creepage, 6-3 

crossflow, 2-22 

crossover, 6-12 

current density, 2-12, 2-18, 2-19, 3-11, 5-7, 

5-12, 5-14, 5-19, 6-7, 6-14, 6-18, 6-20, 

6-21, 6-28, 6-30, 6-36, 7-22, 7-23, 7-25, 

7-28, 7-29, 7-30, 8-46, 8-51, 8-64, 10-13 

Daimler-Benz, 1-22, 8-94 

degradation, 1-7, 3-15, 5-4, 5-8, 5-9, 5-10, 

5-13, 5-20, 6-25, 6-30, 7-28, 7-30, 8-64 

demonstration, 5-2 

desulfurization, 6-27, 8-59, 8-64 

dielectric, 5-5 

diluent, 5-19 

direct internal reforming, 6-31, 10-10 

Dow Chemical, 3-4, 3-11 

drag, 3-9 

DuPont, 3-3, 3-11 

efficiency, xvii, 1-12, 1-13, 1-15, 1-16, 1-17, 

1-18, 1-19, 2-7, 2-8, 2-9, 2-19, 2-21, 3-9, 

3-15, 6-10, 6-12, 6-16, 6-17, 6-26, 7-21, 

8-1, 8-2, 8-46, 8-50, 8-51, 8-55, 8-56, 

8-58, 8-60, 8-64, 8-65, 8-66, 8-67, 8-68, 

9-16, 9-17, 9-18, 9-25 

electrocatalyst, 1-10, 3-10, 3-16, 5-1, 5-2, 

5-3, 5-6 

electrochemical performance, 1-3 

electrodes, 1-2, 1-3, 1-7, 1-12, 2-13, 2-22, 

3-1, 3-6, 3-9, 3-10, 3-12, 3-13, 3-14, 3-20, 

5-1, 5-3, 5-10, 5-11, 5-16, 5-18, 5-19, 6-3, 

6-4, 6-9, 6-10 

electrolyte management, 6-3 

emissions, 1-12, 1-19, 1-22, 8-51, 8-60 

endothermic, 3-8, 6-19, 6-31, 6-32, 8-49, 

8-53, 8-55, 8-59, 8-64, 9-14 

equilibria, 6-23 

equilibrium, 2-4, 2-5, 2-6, 2-10, 2-15, 2-21, 

2-23, 2-24, 6-3, 6-16, 6-19, 6-20, 6-22, 

6-23, 6-24, 6-28, 6-32, 7-27, 9-6, 9-8, 9-9, 

9-12, 9-13, 9-14, 9-15, 10-1, 10-12, 10-13 

Europe, 1-22, 6-1 

exchange current, 2-12, 2-16, 5-12, 10-13 

exothermic, 3-14, 6-19, 6-31, 6-32, 8-59, 

8-64, 9-14 

external, 2-7, 3-7, 3-9, 6-12, 6-31, 6-32, 

8-47 

Faraday, 2-1, 6-37, 9-1, 10-13 

flat plate, 1-4, 1-12, 5-7 

flooded, 6-3 

Foulkes, 1-2, 1-32, 2-33, 10-4 

fuel, xvii, xix, 1-2, 1-3, 1-7, 1-9, 1-10, 1-11, 

1-12, 1-13, 1-14, 1-15, 1-16, 1-17, 1-18, 

1-19, 1-22, 1-23, 1-32, 2-1, 2-3, 2-4, 2-6, 

2-7, 2-8, 2-9, 2-12, 2-13, 2-17, 2-18, 2-19, 

2-20, 2-21, 2-22, 2-23, 2-24, 3-2, 3-8, 3-9, 

3-10, 3-11, 3-13, 3-15, 3-18, 3-20, 5-1, 

5-3, 5-4, 5-9, 5-10, 5-11, 5-12, 5-15, 5-16, 

5-17, 5-19, 5-20, 6-6, 6-7, 6-9, 6-10, 6-13, 

6-16, 6-18, 6-19, 6-20, 6-23, 6-24, 6-25, 

6-26, 6-27, 6-28, 6-29, 6-30, 6-31, 6-32, 

6-33, 6-35, 7-1, 7-20, 7-22, 7-23, 7-24, 

7-25, 7-26, 7-27, 7-28, 7-29, 7-31, 8-1, 

8-2, 8-46, 8-47, 8-48, 8-49, 8-50, 8-51, 

8-52, 8-53, 8-54, 8-56, 8-57, 8-58, 8-59, 

8-60, 8-62, 8-63, 8-64, 8-65, 8-66, 8-67, 

8-68, 8-69, 8-94, 9-1, 9-2, 9-3, 9-4, 9-5, 

9-6, 9-7, 9-8, 9-10, 9-11, 9-12, 9-13, 9-15, 

9-16, 9-17, 9-18, 9-23, 9-24, 9-25, 9-26, 

10-10, 10-11, 10-12, 10-14 

fuel cell stacks, 1-17, 9-2 

fuel electrode, 7-29 

fuels, 1-2, 1-13, 1-22, 2-6, 3-10, 5-15, 6-3, 

6-17, 6-18, 6-29, 6-31, 7-24, 7-27, 8-1 

Fuji Electric Corporation, 5-1

gas turbine, 1-13, 1-18, 8-48, 8-49, 8-50, 

8-60, 8-66, 9-18 

gasification, 9-16 

gasified coal, 6-17 

gasifiers, 1-19, 5-17 

Germany, 1-33, 5-23, 10-7 

Girdler, 9-8, 9-14, 9-29 

graphite, 3-13, 5-3, 5-4 

Grove, 10-10 

Grubbs, 3-2 

Halides, 6-26, 6-29, 6-42 

heat exchanger, 8-53, 8-59, 8-62, 8-64, 9-16 

heat rate, 6-3, 9-16, 9-17, 9-25, 10-11 

heat removal, 5-5 

heat transfer, 3-10, 6-13, 9-20, 9-21 
11-3 

higher heating value, 2-9, 9-22, 9-23, 9-24, 

10-11 

Hitachi, 6-39 

hybrid, 1-18, 1-22, 1-23, 8-94 

hydrogen, 1-10, 1-17, 1-22, 1-23, 1-32, 2-4, 

2-5, 2-6, 2-8, 2-9, 2-17, 2-21, 3-7, 3-8, 

3-10, 3-13, 3-20, 5-10, 5-15, 6-31, 7-22, 

7-26, 7-28, 8-51, 8-52, 8-53, 8-55, 8-64, 

8-67, 8-94, 9-1, 9-2, 9-3, 9-4, 9-5, 9-6, 

9-7, 9-10, 9-13, 9-18, 10-14 

impurities, 2-18, 3-19, 5-11, 5-16, 5-17, 

7-28 

indirect internal reforming, 6-31, 10-11 

interconnect, 1-4, 1-7, 7-21 

interconnections, 8-1 

intercooled, 8-51, 8-52, 8-59 

internal, 1-14, 2-7, 2-18, 3-14, 6-12, 6-30, 

6-31, 6-32, 8-47, 8-59, 8-68, 9-10 

internal manifolding, 6-12 

internal reforming, 1-14, 2-7, 6-30, 6-31, 

6-32, 8-47, 8-59, 8-68 

International Fuel Cells Corporation (IFC), 

1-16 

inverter, 9-16 

ionomer, 3-12 

JANAF, 2-3, 2-7, 2-33 

Japan, 1-16, 1-17, 1-22, 5-1, 5-23, 5-24, 6-1, 

6-31, 6-39, 10-7, 10-9, 10-10 

Johnson Matthey, 3-20, 5-9, 8-105 

kinetics, 1-10, 1-12, 2-12, 2-13, 2-18, 5-13, 

6-10, 9-15, 9-16 

life, xviii, 1-7, 1-11, 1-17, 2-18, 5-2, 5-4, 

5-5, 5-7, 5-9, 5-10, 5-11, 5-20, 6-3, 6-12, 

6-21, 6-26, 8-46, 8-47, 8-48, 8-51, 8-59, 

8-63 

loss, 2-7, 2-9, 2-20, 2-21, 2-23, 3-13, 3-14, 

3-15, 5-10, 5-11, 5-14, 5-16, 5-17, 5-19, 

5-20, 6-9, 6-10, 6-12, 6-16, 6-21, 6-22, 

6-25, 6-29, 6-30, 7-21, 7-28, 7-29, 8-46, 

8-48, 8-65 

lower heating value, 1-13, 9-17, 9-22, 10-11 

management, 1-9, 1-10, 3-10, 3-12, 6-3, 

6-32, 7-1, 8-64 

manifold, 5-5, 6-12 

manufacturing, 1-16, 1-17, 3-6 

M-C Power, 6-40 

membrane, 1-7, 1-9, 3-1, 3-2, 3-3, 3-6, 3-9, 

3-11, 3-12, 3-14, 3-18, 8-51 

membranes, 3-4, 3-11, 3-12 

methanation, 3-18, 6-16, 6-19 

methane (CH4), 9-10 

methanol, 1-6, 1-22, 1-23, 3-13, 3-20 

migration, 5-11, 6-3 

Mitsubishi Electric Corporation, 5-1, 5-8 

molten carbonate, xviii, 1-7, 6-3, 6-4, 6-7, 

6-8, 6-9, 6-29, 6-30, 6-31, 10-11 

multi-stage, 8-62, 8-64, 8-65 

Nafion, 3-12, 3-14 

Nafion membranes, 3-14 

natural gas, 1-6, 1-15, 1-16, 1-17, 1-18, 

1-19, 1-22, 1-32, 3-13, 5-15, 6-13, 6-20, 

6-32, 7-22, 8-51, 8-52, 8-53, 8-55, 8-56, 

8-58, 8-59, 8-62, 8-63, 8-67, 9-3, 9-5, 

9-18, 9-22, 9-23, 9-24 

Nernst, 2-3, 2-4, 2-5, 2-15, 2-21, 2-22, 2-23, 

3-15, 5-14, 6-15, 6-20, 6-24, 7-20, 7-25, 

7-26, 7-27 

nitrogen compounds, 5-19 

odorants, 8-53, 8-59, 8-63 

ohmic, 2-13, 2-17, 2-18, 3-14, 3-15, 5-11, 

5-12, 5-13, 5-19, 6-7, 6-12, 6-21, 6-30, 

7-21, 7-22, 7-29, 8-48, 10-11, 10-14 

ohmic loss, 2-13, 2-18, 5-12, 5-13, 6-7, 

6-12, 7-21, 8-48, 10-11 

ohmic polarization, 2-17, 2-18, 6-7, 6-21, 

7-21, 7-22, 10-14 

ohmic resistance, 3-14, 6-7 

ONSI, 1-33 

overpotential, 5-14, 5-16, 6-10 

oxidant, 1-2, 1-3, 1-4, 2-21, 2-22, 2-23, 

2-24, 5-4, 5-11, 5-12, 5-14, 5-15, 5-16, 

5-19, 5-20, 6-13, 6-17, 6-19, 6-20, 6-21, 

6-22, 6-23, 6-24, 6-25, 6-26, 7-23, 7-25, 

7-26, 7-27, 8-46, 8-49, 8-50, 8-53, 8-56, 

8-57, 8-58, 8-59, 8-64, 8-67, 8-68, 9-2, 

9-3, 9-5, 9-6, 9-7, 10-14 

oxidation, 2-5, 2-6, 2-21, 3-13, 3-18, 3-20, 

5-13, 5-16, 5-18, 6-1, 6-5, 6-31, 6-32, 

7-20, 7-26 

oxygen, 1-2, 1-10, 1-12, 2-4, 2-5, 2-6, 2-8, 

2-9, 3-11, 3-13, 3-15, 3-18, 5-10, 5-11, 
11-4 

5-12, 5-13, 5-16, 7-26, 8-55, 8-59, 8-67, 

8-68, 9-3, 9-4, 9-6, 9-7, 10-14 

phosphoric acid, 1-7, 1-10, 5-1, 5-5, 5-9, 

5-10, 5-11, 6-3, 10-11 

planar, 1-12 

poison, 1-9, 1-10 

polarization, 2-12, 2-13, 2-16, 2-17, 2-18, 

3-15, 5-11, 5-12, 5-13, 5-14, 5-16, 5-17, 

6-20, 6-21, 6-22, 6-24, 7-21, 7-22, 7-25, 

7-27, 10-14 

polymer, xviii, 1-7, 1-9, 1-22, 1-23, 3-2, 3-4, 

3-8, 3-10, 3-11, 3-20, 8-94, 10-11 

porous electrodes, 1-3, 5-2, 5-3, 5-4, 6-3 

potential, 1-14, 1-19, 2-1, 2-3, 2-4, 2-5, 2-6, 

2-10, 2-16, 2-17, 2-18, 2-20, 2-22, 2-23, 

3-15, 5-3, 5-12, 5-13, 5-14, 5-19, 6-2, 6-3, 

6-9, 6-12, 6-15, 6-16, 6-17, 6-19, 6-20, 

6-23, 6-24, 6-28, 6-30, 7-25, 7-26, 8-2, 

8-47, 8-69, 9-15, 10-12 

power conditioning, 1-6 

pressure, 1-7, 1-10, 1-17, 1-32, 2-1, 2-2, 2-4, 

2-5, 2-9, 2-18, 2-20, 2-22, 2-23, 3-8, 3-9, 

3-11, 3-14, 3-15, 3-16, 3-18, 5-2, 5-8, 

5-11, 5-12, 5-14, 5-15, 5-16, 5-17, 5-21, 

6-8, 6-10, 6-13, 6-15, 6-16, 6-17, 6-18, 

6-20, 6-21, 6-26, 6-29, 6-31, 6-32, 6-37, 

7-22, 7-26, 8-46, 8-47, 8-48, 8-50, 8-51, 

8-53, 8-55, 8-58, 8-59, 8-61, 8-66, 8-68, 

9-5, 9-15, 9-19, 9-24, 10-13, 10-14 

pressurization, 2-20, 5-10, 5-12, 8-46, 8-47, 

8-48 

processing, 1-6, 1-19, 3-10, 6-6, 8-52 

production, 1-17, 3-9, 8-49, 8-51, 8-53, 

9-18, 9-19 

ramp, 1-17 

Rare Earth Minerals, 10-38 

reactants, 1-2, 1-3, 2-2, 2-3, 2-4, 2-18, 2-21, 

3-10, 6-14, 6-16, 8-49, 9-8, 9-9, 9-15 

reformate, 3-13, 3-15, 6-32, 8-52, 9-5 

reformer, 1-32, 3-20, 6-31, 8-49, 8-52, 8-53, 

8-54, 8-56, 8-64, 9-5, 9-13, 9-14 

reservoir, 5-7 

resistivity, 3-11, 6-12 

seals, 2-21, 5-7, 8-47 

separator plate, 1-4 

shift, 2-6, 2-16, 2-21, 2-22, 2-23, 2-24, 3-10, 

3-13, 5-10, 5-15, 6-1, 6-16, 6-19, 6-20, 

6-21, 6-23, 6-27, 6-28, 6-32, 7-20, 7-27, 

8-51, 8-52, 8-53, 8-54, 8-64, 9-6, 9-8, 

9-10, 9-11, 9-12, 9-14, 10-1 

Siemens Westinghouse, xix, 1-18, 7-22, 

7-30, 8-48, 8-50, 8-58, 8-59, 8-61, 8-66, 

8-67 

sintering, 5-13, 6-9 

siting, 1-16 

solid oxide, xviii, 1-7, 8-66, 10-11 

sorbent, 8-51, 8-64 

space, 1-10, 1-15, 1-17, 1-22, 1-23, 3-3, 

5-10, 8-53, 8-94 

stability, 1-10, 1-11, 2-18, 3-4, 5-2, 5-4, 5-9, 

6-3, 6-7, 8-49 

stack, 1-17, 1-19, 3-3, 3-10, 3-11, 3-14, 

3-19, 5-4, 5-5, 5-6, 5-7, 5-8, 5-9, 5-13, 

5-20, 6-4, 6-7, 6-9, 6-12, 6-13, 6-14, 6-18, 

6-21, 6-22, 6-25, 6-30, 6-37, 7-22, 7-27, 

7-30, 8-1, 8-49, 8-52, 8-53, 8-60, 8-64, 

8-69, 9-2, 9-12, 9-16 

stacking, 1-17 

stationary, 1-15, 1-18, 1-19, 5-1, 8-53 

steam reforming, 2-6, 5-15, 6-23, 6-31, 6-32, 

7-20, 8-64, 9-10, 9-13 

steam turbine, 8-49, 8-50, 8-60, 8-66, 9-18 

structure, 1-2, 1-3, 3-8, 3-10, 3-12, 5-3, 5-5, 

6-3, 6-4, 6-6, 6-7, 6-9, 6-10, 6-12, 6-29 

sulfonic, 1-9, 3-4, 10-12 

sulfur, 5-16, 5-17, 5-19, 6-26, 6-28, 6-29, 

6-30, 7-28, 8-48, 8-52, 8-53, 8-57, 8-59, 

8-63, 8-67 

system efficiency, 1-13, 2-19, 6-10 

Tafel, 2-12, 2-13, 10-12 

tape casting, 6-6, 6-7 

temperature, 1-9, 1-10, 1-12, 1-13, 1-14, 

1-15, 2-1, 2-3, 2-4, 2-5, 2-6, 2-7, 2-8, 

2-16, 2-18, 2-20, 2-23, 3-1, 3-7, 3-8, 3-9, 

3-10, 3-14, 3-15, 3-16, 5-2, 5-4, 5-10, 

5-11, 5-12, 5-13, 5-16, 5-19, 6-3, 6-8, 6-9, 

6-10, 6-12, 6-13, 6-18, 6-19, 6-20, 6-21, 

6-24, 6-26, 6-31, 6-32, 7-20, 7-21, 7-22, 

7-24, 7-25, 7-28, 8-46, 8-48, 8-49, 8-50, 

8-51, 8-53, 8-59, 8-60, 8-62, 8-64, 8-68, 
11-5 

9-8, 9-13, 9-14, 9-15, 9-16, 9-18, 9-19, 

9-20, 9-21, 9-24, 10-1, 10-13, 10-14 

thermodynamic, 2-1, 6-3, 7-21, 9-14, 9-15, 

9-16 

three phase interface, 1-3, 5-3 

Tokyo Electric Power, 5-7, 5-23, 8-53, 

8-104, 8-105 

Toshiba Corporation, 1-16, 5-1

UltraFuelCell, 8-62, 8-63, 8-65 

vehicle, 1-10, 1-22, 1-23, 8-94 

voltage, 2-6, 2-9, 2-17, 2-18, 2-19, 2-21, 

2-22, 2-23, 3-14, 3-15, 3-16, 5-2, 5-3, 5-4, 

5-9, 5-12, 5-13, 5-14, 5-15, 5-16, 5-19, 

6-4, 6-7, 6-12, 6-13, 6-14, 6-17, 6-18, 

6-19, 6-20, 6-21, 6-22, 6-25, 6-28, 6-29, 

6-30, 6-36, 6-37, 7-21, 7-22, 7-23, 7-25, 

7-26, 7-27, 7-29, 7-30, 8-46, 8-48, 8-49, 

8-51, 8-53, 8-58, 8-64, 9-2, 9-5, 9-12, 

10-12, 10-13 

voltage efficiency, 7-27 

Westinghouse, 1-33, 5-24, 7-30, 8-58, 8-61 

zirconia, 7-30, 10-12