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NASA Conference Publication 3125

Space

Electrochemical

Research and

Technology

Lewis Research C_,nler

Clcuela,d, Ohio

Procccding_ o1' a c()nrcrcncc hchl a!

NASA I_ewi_ Research ('cnlt.'r

April 9 10, 1991

N/LNA

National Aeronautics and

Space Administration

Office of Managemenl

Scientific and Technical

Information Program

1991
"l_atle name_ or matlufaclurers' iI_u]c_ are lised ill Ihig reDorl fo_ ideltlifiuaHon

onl','. ]hi _, ii_,age ctoe_ t_+i COll_tilule _lfl official cndorseme111, either expressed or

implied, b', Ihe Nat ional Acr(_llLlllliC_, _llld _,p;ICe ,,\dHliniq_ali_n.
TABLE OF CONTENTS

Page

Electrolytic Production of Oxygen from Lunar Resources

R. Keller, EMEC Consultants .....................................................

The Au Cathode in the System Li2C03-C02-C0 at 800 to 900 °C

N.H. Hagedorn, NASA Lewis Research Center .......................................

SPE Water Electrolyzers in Support of Mission from Planet Earth

J.F. McEIroy, United Technologies Corp. Hamilton

Standard Division ................................................... . ........ 23

Space Water Electrolysis: Space Station Through Advanced Missions

RJ. Davenport, FH. Schubert, and D.J. Grigger,

Life Systems, Inc .............................................................

37

The Effects of Platinum on Nickel Electrodes in the Nickel Hydrogen Cell

A. Zimmerman, The Aerospace Corp ...............................................

57

Physical and Chemical Analysis of a Ni/H 2 Cell

H. Vaidyanathan, M.W. Earl, and T.D. Kirkendall,

Comsat Laboratories .......................................................... 73

Raman Spectral Observation of a "New Phase" Observed in Nickel

Electrodes Cycled to Failure

P.L. Loyselle, X. Shan, and B.C. Cornilsen, Michigan Technological

University, and M.A. Reid, NASA Lewis Research Center ............................... 81

Impedances of Nickel Electrodes Cycled in Various KOH Concentrations

M.A. Reid and P.L Loyselle, NASA Lewis Research Center ............................. 89

High Energy Efficiency and High Power Density Proton Exchange Membrane

Fuel Cells - Electrode Kinetics and Mass Transport

S. Srinvasan, O.A. Velev, A. Parthasarathy, D.J. Manko, and

A.J. Appleby, Texas A & M University .............................................

101

I-BIEM Calculations of the Frequency Dispersion and AC Current Distribution

at Disk and Ring-Disk Electrodes

B.D.Cahan, Case Western Reserve University ....................................... 123

Regenerative Fuel Cell and Architectures for Lunar Surface Power

D.W. Harris, S.P. Gill, T.M. Nguyen, and JJ. Vrotyk, Rockwell

International ................................................................ 147

Oxygen Electrodes for Rechargeable Alkaline Fuel Cells - III

L. Swette, N. Kackley, and S.A. McCatty, Giner, Inc .................................... 155

Some Recent Studies w_th the Solid-lonomer Electrochemical Capacitor

S. Sarangapani, J. Forchione, A. Griffith, and A.B. LaConti, Giner, Inc.,

and R. Baldwin, NASA Lewis Research Center ...................................... t75

iii
Performance of a Dual Anode Nickel-Hydrogen Cell

R.F. Gahn, NASA Lewis Research Center ...........................................

Multiple Celt Common Pressure Vessel Nickel Hydrogen Battery

J.P. Zagrodnik and K.R. Jones, Johnson Controls

Battery Group, Inc ...........................................................

Small Capacity, Low Cost (Ni-H2) Design Concept for Commercial, Military

and Higher- Volume Aerospace Applications

J.R. Wheeler, W.D. Cook, and R. Smith, Eagle-Picher

Industries, Inc ................................................................

Sodium - Metal Chloride Battery Research at JPL

BV. Ratnakumar, A.I. Attia, and G. Haipert, Jet

Propulsion Laboratory .........................................................

Advances in Li-TiS 2 Cell Technology

S. Surampudi, D.H. Shen, C.-K. Huang, F. Deligiannis,

A. Attia, and G. Halpert, Jet Propulsion Laboratory ...................................

195

209

221

227

235

iv
N91-32550

ELECTROLYTIC PRODUCTION OF OXYGEN FROM LUNAR RESOURCES

Rudolf Keller

EMEC Consultants

Export, PA 15632

Abstract

Oxygen is a valuable commodity in space. Substantial savings have been projected for future

missions if oxygen could be produced on the moon from local resources.

Some of the most promising approaches to extract oxygen from lunar resources involve

electrochemical oxygen generation. In a straight-forward concept called "magma electrolysis",

suitable oxides (silicates) are molten at 1300-1500 °C and the melt electrolyzed. Residual melt can

be discarded after partial electrolysis. Alternatively, lunar soil may be dissolved in a molten salt and

electrolyzed. In this approach, temperatures are lower and melt conductances higher, but

electrolyte constituents need to be preserved. Both possibilities are being studied in the laboratory.

In a different approach, ilmenite is reduced by hydrogen and the resulting water electrolyzed

(Carbotek Process TM).

Although know-how from terrestrial technology can be applied, the process task is unconventional

and requires many innovative solutions. Time and effort needed for successful development should

not be underestimated.

Oxygen from Lunar Resources

Due to the effort necessary to transport it there from Earth, oxygen is a precious commodity in

space. Oxygen is needed for life support and, in larger amounts, also for propulsion. In situ

preparation of oxygen from local resources in space can reduce costs for future missions in

space. The lunar surface is an appropriate site to establish an oxygen manufacturing capability.

Molecular oxygen may be prepared from oxides by electrolysis. In the absence of volatile oxides,

solid oxides may be used. Such oxides are readily available on the lunar surface, mainly as silicates

of various compositions. Lunar soil may be regarded as an already adequately comminuted raw

material. Highland soils are anorthositic, feldspar-like, with compositions similar to the one given as

an example in Table 1. Soils of the mare region have a more basaltic character and are

represented in Table 1 by a composition indicated by Washington University authors [1] as representative.

In the electrolytic decomposition of the oxides, oxygen gas is produced at the positive electrode,

the anode. The reaction may be formulated as follows:

Me O - 2y e- ----> x Me 12y_×I* + y/2 02 .

x y

Metallic components,,meanwhile, are reduced at the cathode:
Table 1. Compositionsof LunarSoils

AnorthositicSoil [2] BasalticSoil [ 1]

SiO2 449 wt% 46.2 wt%

AI203 27.6 wt% 12.6wt%

FeO 503 wt% 17.4wt%

TiO2 0.55 wt%

MgO 535 wt%

CaO 158 wt%

2.8 wt%

10.4wt%

105 wt%

Na20 039 wt%

K 0 O, 10 wt%

2

Me O { 2y e- ----> x Me _ y 02.

x y

Various metals are reduced with different ease. Theoretical decomposition potentials for the

oxides can be calculated from thermodynamic data, but they are modified as the oxides combine to

mixed oxide compounds. For the major components of the oxides discussed, the following order

of decreasing (more negative) cathodic deposition potential appears to exist:

Fe > Ti > Si > AI > Mg > Na > Ca .

The cathodic products may be of no substantial interest and may even be discarded Silicon and

aluminum, on the other hand, may be the products of major interest Depending on the desirability

to produce and collect the metals, the process may be refined to various degree [3], as illustrated

by Figure 1

Figure 1. Refinement of Lunar Molten Salt Electrolysis Depending

on Metallic Products Desired
Two major approachesto the electrolysisof lunaroxides are being explored:(1) a molten silicate

electrolysis in which the oxides are melted and the melt electrolyzed, and (2) a fused salt

electrolysis in which the oxides are dissolvedin a molten salt (flux) and electrolyzed.

Electrolysis of Molten Silicates

Lunar raw material may be molten and electrolyzed at temperatures of 1400 - 1500 °C This can

be accomplished in a batch mode, whereby the electrolyte composition changes during the course

of electrolysis; at a certain point, the residual melt is discarded. Such an approach was discussed

in an earlier publication by Washington University authors [1].

Alternatively, the electrolysis may be conducted in a quasi-continuous mode. Fresh ore is added

to an electrolyte which represents partially electrolyzed raw material. While components that

reduce most easily are continuously electrolyzed, electrolyte is gradually removed from the system.

This approach is represented in Figure 2.

Oxygen

O

2

Feed: l

FeO

SiO_ Discarded oxides:

AI20j t Si02

TiOz -- Electrolysis _- AIzO_

MgO MgO

CaO CaO

1

Fe

Ti

Si

Figure 2. Continuous Electrolysis of Molten Oxides

A cell design for such an electrolysis, with a molten silicon-iron cathode, has been suggested by

McCullough and Mariz [4].

Electrolyte conductivities vary significantly with composition. Haskin et al [1] give the following

regression equation (for 1425 °C):

In X = 5.738 - 126[SIO 2] - 10.0[AIO1. _] - 3.7[TIO 2] + 1.89[FEO] + 0.07[MgO] - 1.25[CAO]

(), in ohm-lcml; symbols for oxides stand for mole fractions).

High iron oxide contents obviously lead to relatively high conductances. Excessive silicon contents

result in low conductivities In practice, one may expect to be operating with electrolytes of

specific conductivities of about 0.3 ohm-lcm -_.

So-called "magma electrolysis" experiments have been conducted by L. A. Haskin and his co-
workers at WashingtonUniversity. They gatheredessentialbasic dataon the electrolyte properties

and electrolyzed small molten charges suspendedin platinumwire loops. Recently,experiments

were conducted in small spinel crucibles. EMECConsultantsstarted investigationsusing alumina

crucibles,a platinumanodeand a graphitecathode. Cathodicdepositswill be examinedand several

anodecandidatesand other materialstested in future work.

Electrolysisin Molten Salt

Lunar soil may be dissolved in a suitable electrolyte of molten fluorides and electrolyzed at temperatures of about 1000 °C. It is important that all components added to the electrolyte are

removed again from the system. In the approach envisioned by EMEC Consultants, this is accomplished by complete electrolysis (separation of some components as undecomposed oxides

would also be possible). Electrolysis conditions are such that the component most difficult to

decompose reacts. As this component is calcium, a cathode potential, therefore, is maintained

which permits reduction of calcium; the other metals are co-reduced at the rate they reach the

cathode, i.e. at the mass transport limited rate. This approach has, thus, been called a "calcium-plus

electrolysis".

Calcium fluoride, with a very high theoretical decomposition voltage, would be a desirable

electrolyte, but its melting point is high. Also metal solubilities leading to current efficiency losses

would be excessive. A mixture of calcium fluoride and lithium fluoride is preferable as electrolyte,

as added lithium fluoride reduces operating temperatures. The presence of lithium fluoride,

however, leads to a cathodic co-deposition of lithium which has to be recovered in an auxiliary

metal separation step, presumably by vacuum distillation

Oxygen is evolved at the anode as a gas Some electrolyte components will evaporate into this

gas but may be retained by process feed through which the off-gases are bled, similar to the

recovery of fluorine values in the fume treatment of commercial aluminum production. Traces of

sulfur dioxide may remain with the oxygen and should not affect the performance of the oxygen

as propellant. It may be possible to obtain oxygen feasible for life support by simple reevaporation of liquefied oxygen.

The envisioned molten salt electrolysis process to produce oxygen is summarized in Figure 3

The development of this process actually started with work on a concept to produce silicon and

aluminum from lunar resources [5]. In a stepwise reduction process, silicon of the anorthite feed

is reduced chemically by aluminum metal. Aluminum oxide and calcium oxide are electrolyzed in a

second major process step. Auxiliary process steps involve the beneficiation of lunar soil by magnetic separation to yield a good-quality anorthite (CaAI2Si20 a) feed, the separation of silicon from a

hypereutectic Si-AI alloy, the purification of oxygen by exposing the cell gases to feed ore, and

the separation of lithium from the cathode metal.

Other Approaches

llmenite reduction with hydrogen is practiced in the Carbotek process [6]. This process includes

an electrochemical step, as the water produced by the reaction of hydrogen with ferrous oxide _s

electrolyzed. An electrolysis at high temperatures is proposed but has not been explored in

Carbotek's experimental work.
LI

HEAVY

METAL

METAL

SEPARATION

;I CALCIUM-PLUS

q ELECTROLYSIS

JLUNAR

I OXYGEN PURIFICATION LUNAR SOIL

(LU!AR) I

METALS 02

WASTE HEAT

ELECTRIC POWER

SOIL

Figure 3 'Calcium plus Electrolysis" to Produce Oxygen From Lunar Soil

Concluding Remarks

There does not appear to be a clearly favored process to produce lunar oxygen at this time All

approaches discussed above include unconventional process elements with various degree of uncertainty regarding their technical feasibility [7] Additional research is required Materials stability

problems need to be addressed. In particular, a suitable anode for the evolution of oxygen needs

to be identified Such work is in progress at EMEC Consultants

After investigating the chemistry of individual process steps, processes shall be demonstrated on

the bench scale It is estimated that 8 to 12 years of additional work w_ll be required before a

successful pilot demonstration may be undertaken [8] Time periods for the development of new

processes or process variants in terrestrial extractive metallurgy, which is directly related to the

discussed approaches, are considerable Time and effort necessary to develop a lunar oxygen extraction process should not be underestimated

Acknowledgements

The author gratefully acknowledges the contributions of his co-workers and assooates

D L. Anthony, C N Cochran, W C Cochran. W E. Haupin, and K T Larimer Thanks also to Prof

L A Haskin and Dr R. O. Colson of Washington University for fruitful discussions

EMEC appreciates the support of the NASA Johnson Space Center through the SBIR program, of

the University of Arizona's Space Engineering Research Center for Utilization of Local Planetary

Resources, and of the Space Studies Institute Particular thanks to Dr David S McKay for his in

terest and support
[I]

[2]

[3]

[4]

[5]

[6]

[7]

[8]

References

L. A, Haskin, R O, Colson, D J. Lindstrom, R. H. Lewis, and K W. Semkow .

Electrolytic Smelting of Lunar Rock for Oxygen, Iron, and Silicon.

Revised Manuscript , 1989 .

R. J. Williams and J. J. Jadwick, editors

Handbook of Lunar Materials .

NASA Reference Publication 1057 , 1980 .

D L. Anthony, C N Cochran, W C Cochran, W. E Haupin, R. Keller, and K. T Latimer .

Products from Lunar Anorthite .

Space Manufacturing 7; Proceedings of the Ninth Princeton/A/AA/SSI Conference :86-89

• 1989.

E McCullough and C Mariz .

Lunar Oxygen Production Via Magma Electrolysis .

In Proceed/ngs of Engineering, Construction and Operations in Space II, Space 90, Albuquerque NM . April , 1990

D. L Anthony, W. C. Cochran, W. E. Haupin, R. Keller, K. T. Latimer .

Dry Extraction of Silicon and Aluminum from Lunar Ores .

Lunar Bases & Space Activities in the 2lst Century Paper No. LBS-88-066 : 1988

M A Gibson and C. W. Knudsen .

Development of the Carbotek Process for Lunar Oxygen Production.

In Proceedings of Engineering, Construction and Operations in Space //, Space 90, Albuquerque NM April , 1990

R Keller

Lunar Production of Aluminum, Silicon and Oxygen

Metallurgical Processes for the Year 2000 and Beyond, edited by H. Y. Sohn and

E. S. Oeskin, published by The Minerals, Metals & Materials Society 551-562 .

1989

EMEC Consultants

Production of Oxygen by Electrolysis of Lunar Soil in Molten Salt

NASA Phase II Proposal 1990
N91-32551

THE Au CATHODE IN THE SYSTEM Li2003-002-00 AT 800 TO 900 "C

Norman H. Hagedorn

NASA Lewis Research Center

Cleveland, Ohio 44135

SUMMARY

The lithium-CO battery has recently been proposed as an advanced concept for

2

missions to Mars or Venus, planets with carbon dioxide atmospheres. This article

discusses the initial evaluation of gold as a catalyst for the cathodic reduction of

carbon dioxide in molten lithium carbonate. An attempt is then made to rationalize

the experimental results by proposing a phenomenological model for the sequence of

reactions at and around the electrode. Finally, inferences are made as to the

viability of gold as a cathode catalyst for the proposed battery.

INTRODUCTION

The possibility of missions such as a manned outpost on Mars, or a probe to tile

surface of Venus, has motivated researchers to consider developing electrochemical

devices which might utilize the carbon dioxide atmosphere common to both planets.'

One such device is an alkali metal - carbon dioxide battery in which the carbon

dioxide serves as the cathodic reactant. As presently envisioned, the device would

utilize lithium as the anodic reactant, and the cell reaction product would be

lithium carbonate, from which the lithium anode would be shielded by an as-yet

hypothetical lithium-ion-conducting solid. The battery could be treated as a primary

device, serving only until its supply of stored lithium was exhausted; as a secondary

device, recharged chemically by resupply with lithium transported from Earth (which

would, because of the low mass of lithium, be quite efficient); or, for the Mars

mission, as a secondary device recharged electrochemically from an available nuclear

central station power supply to regenerate the lithium. The last possibility is

quite unlikely, since it would require a very high potential, evolving not only

carbon dioxide at the anode, but oxygen as well. However, if energy were abundant

and a premium were placed on the availability of an oxygen-containing process stream,

perhaps an argument could be made for this option.

The following presents current results of experimental work to evaluate gold as

a possible catalyst for the cathodic reduction of carbon dioxide. An attempt is then

made to rationalize the experimental results by proposing a phenomenological model

for the sequence of reactions at and around the electrode. Finally, inferences are

made as to the viability of gold as a cathode catalyst for the proposed battery.

EXPERIMENTAL

Apparatus

The test cell used in this experiment is shown in figure I. The 5-cm-diameter

gold-plated nickel cup which served as the counter electrode was supported by a

nickel disk suspended by nickel tie rods from a water-cooled cell cover. The cup was

insulated electrically from the support disk by an intervening alumina disk. Alumina

baffles were spaced above the cup to minimize thermal gradients due to radiation and

convection. The water-cooled cell cover made possible the use of O-ring comp]_ession

fittings, enabling the vertical positioning of cell components such as the working
and reference electrodes, reactant gas feed tubes, and the cell thermocouple. The

cell was enclosed in an alumina sheath and suspended in an Inconel 600 container

which was, in turn, positioned vertically in a tubular furnace. The portion of the

Inconel container that extended above the furnace was also water-cooled, ensuring the

integrity of the rubber O-ring flange seal between the cell cover and the container,

as well as that of those in the fittings in the cell cover. The use of O-ring seals

made operation possible at pressures greater than one atmosphere.

The working electrode was 99.985 wt% gold foil (Johnson Matthey, Inc.), 1 cm by

1 cm by 0.5 mm. After several configurations were tried, the reference electrode was

modeled after Borucka (ref. i). When this electrode was lowered to the bottom of the

cell, it trapped a portion of the carbonate melt and isolated it from the reaction

products of the working and counter electrodes in the bulk melt. The reference

electrode consisted of an outer 6.35-mm-diameter tube and an inner 3.18-mm-diameter

tube, both of 99.8 wt% alumina (Coors); 0.813-mm-diameter, 99.9 wt% gold wire

(Johnson Matthey); and assorted plastic fittings for sealing the electrode and

connecting it to the gas supply system. The reference gas entered the electrode via

the annulus between the gold wire and the inner tube, and exited between the inner

and outer tubes. The outer tube, as described above, rested flush on the bottom of

the counter electrode cup, trapping melt yet providing a thin-film high-impedance

path for continuity between the trapped melt and the bulk melt of the cell. The

inner tube extended about halfway down from the cell cover, and the gold wire continued to about 1 mm from the bottom of the counter electrode cup. Gold wire leads

to the external circuit from the working and counter electrodes were enclosed in

3.18-mm-diameter alumina tubes and were externally sealed at the tube ends with epoxy

resin. Prior to cell assembly, all gold electrode surfaces were washed in acetone,

isopropyl alcohol, and deionized water.

The chromel-alumel (type K, ±6 °C accuracy) thermocouple was sheathed in a

3.18-mm-diameter Inconel 600 tube with the lower 7.5 cm gold-plated. During the

experiment, the thermocouple was positioned above the melt except when the melt

temperature was being measured. The reactant gas feed was a 3.18-mm-diameter, 99.8%

alumina tube that could be positioned to bubble the gas through the melt. The melt

depth was 2.5 cm, and the working electrode was vertically centered in the melt and

horizontally centered in the counter electrode cup.

Two reactant gases were used in the experiment: 99.99 mol% CO2, and an equimolar

mixture (±i percent mixing accuracy) of 99.99 mol% CO 2 and 99.3 mol% CO. In use, the

gases were dried in a molecular sieve column (type 4A, Union Carbide) and purified

over copper turnings in an in-line tube furnace. Flows were set by hand and monitored with rotameters (Cole-Parmer, ±2 percent accuracy). The Li2CO 3 electrolyte

(Fischer Certified ACS grade) was used as purchased.

Power to the cell furnace was regulated by a zero-firing solid-state relay

actuated by a digital, solid-state microprocessor (Therm-Pro, Inc.). Stability was

±2 °C. A model 273 potentiostat (EG&G Princeton Applied Research Corp.) was used for

the electrochemical experiments, and that company's Electrochemistry Demonstration

Program, Vol. I, in conjunction with an Apple IIe personal computer, was used to

control transient tests and for the associated acquisition and presentation of data.

Procedure. - The reference electrode was designed to permit the use of a

separate reference gas, isolated from the working electrode reactant. However, for

the experiments being reported, the composition of the reference gas was identical to

that of the reactant gas stream, and cell potentials are reported as polarizations
away from the rest potential. In practice, the reference electrode quickly became

blocked, probably because the carbonate melt crept up the annular flow passages to a

location cool enough for it to solidify. It thus became standard procedure to

occasionally raise the reference electrode above the melt so that the gas in the

outer tube of the electrode could become representative of the reactant stream, and

then to return it to its operating position. Stability of the reference under this

stagnant flow condition presented no difficulty during the short duration of the

linear potential sweeps used as the standard technique to monitor the working electrode. Such stability could not be assumed during the acquisition of steady-state

polarization data, when one data point could require half an hour or more. It was

fortuitous under these conditions that the counter electrode, having a much greater

active area than the working electrode, polarized only a few millivolts anodically

under load and quickly returned to its rest potential when curren£ flow ceased. In

effect, it became a "reference for the reference" and made it possible to sort out

the recorded data.

As mentioned above, linear sweep voltammetry was used to evaluate the performance of the working electrode in response to changes in pressure, temperature, and

reactant gas composition, and to the presence or absence of bubbling through the

melt. Generally, cathodic sweeps were made at 20, 40, and 80 mV/sec to a polarization of -i00 mV from the rest potential. The test parameters were total pressures

of 1.2x105 and 2.1xlO 5 N/m2; melt temperatures of 800, 850, and 900 °C; reactant

feeds of CO 2 and an equimolar CO2-CO mixture; and a bubble rate, when used, of about

30 ml/min. Because of the cell configuration and the high conductance of the molten

Li2CO3, about 5 (_-cm) -i at 850 °C (ref. 2), IR-corrections have not been applied to

the measured currents. Because of the nonuniform current distribution on a flag

working electrode surrounded by its counter electrode, currents are presented simply

in milliamperes and not as current densities.

When switching from one reactant gas to the other, a laboratory vacuum pump

(Leybold, Inc., Model DI.6B) was used to exhaust the existing gas from the cell,

and the cell was then repressurized with the replacement gas. In order to avoid

bulk dissociation of the carbonate melt according to

CO_ + CO 2 + O" (i)

it was necessary first to cool the melt below its solidification temperature of

723 °C. When switching from pure CO 2 to the mixture, it was sufficient to carry out

only one or two vacuum/repressurization cycles. However, going in the other direction, in order to completely remove the CO component from the cell, four of the

cycles were executed, theoretically reducing the gas phase CO concentration to the

order of i0 N/m 2.

To avoid melt dissociation according to equation (1) while at operating temperatures, it is necessary to maintain the partial pressure of CO 2 in the gas above the

melt at values greater than the equilibrium dissociation pressure at the existing

temperature. Some dissociation pressure data for Li2CO 3 are presented by Janz et al.

(ref. 2), but their results contain some uncertainties and extend only to about

850 °C. No data have been discovered relating the dissociation rate of the molten

carbonate to the deficit between the existing CO 2 partial pressure and the equilibrium dissociation pressure, but it has been reported that the corresponding recombination rate for the ternary eutectic carbonate melt is generally quite slow

(ref. 3). In the light of the uncertainities, it is possible that, when operating at

900 °C and 1.2xlO 5 N/m 2 with an equimolar mixture of CO and CO 2, melt dissociation
was occurring. However, there is no experimental evidence for this, and no straightforward way to estimate possible effects if melt dissociation were occurring.

Another chemical process that one must be cognizant of is the Boudouardreaction

for the partitioning of CO (ref. i):

2C0_C + CO2 (2)

The reaction is favored thermodynamically by high COpressure and low temperature. It has been pointed out (ref. i) that, regardless of the cover gas composition, an electrochemical reaction at the surface of an electrode, by consuming CO 2

and generating CO, could create local conditions for carbon deposition. To estimate

the "worst case" scenario for the present experiment, a calculation based on data

from the Joint Army-Navy-Air Force (JANAF) tables (ref. 4) for an equimolar mixture

of CO and CO 2 at 827 °C and 2.1x105 N/m 2 total pressure indicates that a cathodic

polarization of -150 mV from the rest potential could result in carbon deposition.

This assumes very fast kinetics and negligible resistive loss, so that total

polarization is Nernstian. Later discussion will argue that the reaction at the

electrode surface does not produce the situation proposed above.

The working electrode surface was examined before and after its 9-day exposure

to the carbonate melt at temperatures of 800 °C or greater. Scanning electron microscopy, with both secondary and backscattered electron imaging, along with energy dispersive x-ray spectroscopy, were used in the analyses.

RESULTS AND DISCUSSION

Experimental results

Figure 2 presents linear sweep voltammograms for the gold cathode in Li CO 3 at

800, 850, and 900 °C, under an equimolar atmosphere of CO and CO 2 at a total pressure

of about 1.2x105 N/m 2 (1.2 atm). These data represent a baseline for the effects of

other parameters, including aging at cell temperature. Some of these effects were

quite surprising, in light of the original assumption that the cathodic process would

reduce dissolved CO 2 diffusing from the melt. In the simplest case, the linear sweep

voltammograms for such a process would have shown a current peak as the diffusional

gradients developed, then a fall off to a steady limiting current until the potential

became adequate to promote some new reaction (ref. 5). However, the potential sweeps

in figure 2 indicate no approach to a limiting current. Rather, the inflection of

the curves toward the ends of the sweeps shows an increasing gradient of current with

applied polarization. At much greater cathodic polarizations, one can expect the

deposition of carbon, or even alkali metal, with very rapid reaction kinetics

(ref. 6). But, at the cathodic limit of -I00 mV adhered to for the cathodic sweeps

in this experiment, it is assumed that such depositions have not occurred, and that

the overall thermodynamic process can be represented as

2CO 2 + 2e-_ CO + CO_ (3)

However, the actual sequence of physical, chemical, and electrochemical steps

comprising this process becomes a matter of conjecture.

The most arresting result of this work was that electrode performance was very

poor in the presence of CO 2 alone, but quite good when the reactant gas was a mixture

I0
of CO 2 and CO. Figure 3 presents cathodic potential sweeps at -40 mV/sec before and

after changes in the reactant gas, and reveals several salient characteristics:

(i) A loss in performance (e.g., from -18 to -7.5 mA at a polarization of

-i00 mV) after switching from the CO2-CO mixture to CO 2 alone

(2) A further loss in performance, from -7.5 to -2 mA, during six days of

evaluation of the Au cathode in CO

2

(3) A recovery, from -2 to -4 mA, due to return to the CO-CO mixture

2

(4) A subsequent additional recovery to -12 mA following several hours of

quasi-steady-state operation under the CO-CO 2 mixture, during which steadystate polarization data were acquired

This last characteristic, an improvement engendered by extended periods under

10ad, was consistently observed when working with the CO-CO 2 gas mixture, as shown in

figure 4. The two curves at each temperature indicate performance, at a sweep rate

of -40 mV/sec, before (upper curve) and after (lower curve) the collection of steadystate polarization data. This improvement was not consistently in evidence when

operating with CO2, alone. Rather, in that case, the time spent under quasi-steady

load conditions resulted almost equally in small gains or losses in sweep performance

and seemed to be more affected by other aspects of electrode history, such as prior

time at the existing temperature, time at open circuit, or time raised from the melt.

The results of steady-state polarization measurements are shown in figure 5 for

the CO-CO 2 mixture. Figure 5 shows excellent steady-state performance of the gold

cathode when CO is present equally with the CO 2. In the same figure, single data

points at 800 and 900 °C indicate the effect when the reactant gas stops bubbling.

Also, the data for 900 °C include measurements at 2.1x105 N/m 2, and unexpectedly show

no pressure effect. Furthermore, it can be seen in figure 6 that increasing pressure

actually inhibited sweep voltammograms, whereas one would anticipate, on the basis of

equation (3), that increased total pressure would enhance performance.

Both systems were slow to equilibrate after load changes. With the CO-CO 2 gas

mixture, tens of minutes could be required for the rate of change of potential under

load to diminish to 1 mV/2 to 3 min, at which time data would be recorded; and up to

hr was required to return to a stable open circuit value. With CO 2 alone, attainable currents were at least an order of magnitude less than with the gas mixture, and

polarizations were greater. Response was so slow in both directions that a considerable degree of uncertainty exists for the recorded data. For this reason, figure 7

simply shows envelopes encompassing the data at a given temperature, without attempting to distinguish between the effects of total pressure or bubbling of the gas

through the melt.

A phenomenological model. - In order to rationalize the performance characteristics exhibited by the gold cathode in this experiment, it is hypothesized that the

primary source of CO 2 being reduced at the cathode surface is not diffusion from the

melt, but the dissociation reaction:

CO_ _CO 2 + O = (i)

At the electrode surface, reaction (I) would be promoted by the initial electrochemical consumption of any dissolved CO 2 at, or near, the electrode. (Note that

II
reaction (I) would be retarded by an increase in total pressure or in CO 2 partial

pressure, with the inhibiting effect noted in fig. 6.)

It is also hypothesized that the buildup of the oxide ion at the electrode surface can strongly impede cathodic performance. Reaction (i) is an example of a LuxFlood (refs. 7 and 8) acid-base system in which the carbonate ion (the oxide ion

donor) is the base, and carbon dioxide is its conjugate acid. Depending on the

operating conditions and the processes occurring locally, the melt can become

extremely basic near the electrode. This could significantly affect the overall

reaction path by influencing the variety of electrochemically active species that

could be present, including carboxy-ion intermediates such as CO + and CO2 (ref. 9).

The two systems (CO2/Au and CO2,CO/Au) in the molten ternary eutectic mixture of

alkali carbonates were evaluated by Borucka (ref. i0) with regard to their suitability as reference electrodes in molten carbonates. Micropolarization was measured

anodically and cathodically, with current densities of the order of 1 to i0 _A/cm 2.

Exchange currents were estimated, and possible electrode processes were suggested.

It was determined that the CO2/Au electrode was extremely irreversible, with an

overall process thermodynamically equivalent to a very dilute oxygen electrode,

1⁄2o 2 +c% ÷2e_CO_ (4)

governed by a very slow activation step possibly involving interaction between the

gold surface and oxygen, a 200 ppm impurity in the CO 2. The exchange current

for this electrode was shown to be about 1/30th that of CO2,CO/Au.

Although the stability of gold oxides at the temperatures and potentials under

consideration is difficult to defend thermodynamically, the observed behavior of CO 2

on the gold cathode supports the hypothesis of a process that is at least generically

similar to that described. Therefore, for the sake of the present argument, it is

simply assumed that oxide ions are preferentially adsorbed on the gold surface, presenting a physical impediment for access of CO_ to the electrode and for the charge

transfer process.

The preference for this explanation is partly based on the extreme slowness of

the return to open circuit potential, especially in the presence of CO 2 alone, after

sustaining a quasi-steady-state cathodic load. The timeframe appears more typical of

relaxations associated with chemisorption bonds than of the diffusional dispersal of

concentrated species in solution, especially when the melt is agitated because the

reactant gas is bubbling.

The remainder of the proposed reaction model draws heavily on the work of

Borucka (ref. 3) and Borucka and Appleby (ref. 9), in which the C02,CO/Au/CO _ system

was examined in relation to the oxidation of CO, as in a molten carbonate fuel cell

anode. In particular, the CO_ ion, proposed in reference 3 and strongly supported

by experiment and thermodynamic and kinetic arguments (ref. 9), is invoked here.

12
The CO_ ion was shown to be formed by the rapid reaction of CO and the O" ion:

CO + O" _ CO_ (fast)

(5)

The CO_ was further shown to be quite soluble in the molten carbonate, with

solubility comparable to that of CO 2, about 3 times greater than that of CO (ref. 6).

Thus, equation (5) presents the process by whi9h CO is able to depolarize the cathode

of O" ions, promoting the supply of CO 2 reactant via equation (i). CO_ is also produced in the cathodic charge transfer sequence, which can be indicated by the overall

process

CO 2 + 2e • CO_ (6)

the reverse of which represented the "second wave" in the oxidation of CO (ref. 3).

by

Finally, the reaction sequence is completed a short distance from the electrode

c<+co2•co÷c< (7)

producing the expected products of the overall reaction and regenerating the CO for a

continuation of the depolarization process at the cathode.

Obviously, equations (I), (5), and (7), when simply added, cancel each other.

However, they convey a picture of the CO molecule functioning over a short distance,

transporting O" ions from the cathode surface into the bulk melt via the CO_ ion, and

there being regenerated for return to the cathode. Thus, in spite of its relatively

low solubility in the melt, the flux of CO to the cathode can be sufficient to

depolarize the cathode because of the short distance the CO must diffuse from the

saturated bulk melt.

In summary, the proposed cathodic sequence for the C02,CO/Au/CO _ system, with

(s) denoting the cathode surface and (b) the bulk melt, is, near the electrode,

CO_(b) _CO2(s ) + O'(s)

(8)

CO(b) + O'(s) _ CO_(b)

(9)

Summing,

CO2(s ) ÷ 2e + CO_(b)

CO_(b) + CO(b) + 2e _ 2CO;(b)

(i0)

(ii)

13
In the melt,

and overall,

2CO_(b) + 2C02(b ) _ 2CO(b) + 2CO_(b) (12)

2CO2(b ) + 2e • CO(b) + CO_(b) (13)

In the absence of CO to effectively depolarize the cathode, the cathode must be

depolarized via the slow (refs. 3, 6, and 9) acid-base equilibrium (equation (I)) by

CO 2 diffusing from the melt. A possible sequence might be

CO_(b) _CO2(s ) + O=(s) (8)

CO2(b ) + O'(s) _CO_(b) (slow) (14)

CO2(s ) + 2e • CO (b) (slow, in the presence of excess O (s))

(15)

with equations (14) and/or (15) controlling.

Electrode morphology. - Figure 8 is a secondary electron image of the gold

cathode surface after about 240 hr at temperatures between 800 and 900 °C, during

most of which the electrode was immersed in the molten Li2CO 3. The large grains

visible in the micrograph were not present at the beginning of the test. Figure 9 is

an optical image of a cross section of a similar electrode that had not been at

temperature as long but had been used as both an anode and a cathode. The picture

reveals that the large gold grains can extend the full thickness of the 0.5-mm

electrode flag. (Borucka (ref. i0) notes that after 6000 hr at 800 °C in a ternary

alkali carbonate eutectic, a gold electrode remained "perfectly bright," developed

very large surface grains, and did not dissolve, according to a spectroscopic

analysis of the melt to a detection limit of 0.05 ppm.)

The Au flag working electrode had fused to it a 6-cm-long, 0.813-mm-diameter Au

wire at the other end of which, above the melt, was fused another Au flag for connection to the external circuit. The upper flag was never immersed and does not

visibly appear to have undergone grain development, as did the working electrode.

Thus, the grain development is not simply the result of exposure to high temperature,

but is due to some combination of temperature, exposure to the molten carbonate, and

electrochemical usage.

Another indication of the reordering of the gold electrode surface during testing is seen in figure i0, a scanning electron microscope image which shows a region

with a large population of small crystals in the process of being "swallowed up," as

indicated by the contour lines encompassing each crystal. X-ray analysis of the

crystals reveals aluminum and oxygen. Based on the bipyramid structure, it is

assumed that the crystals are 7-LiAIO 2, since lithium is not detectable by the

analysis and the 7-form is stable above 750 °C (ref. 6). This material would be

formed by surface disintegration of the alumina tubes of the reference electrode and

the reactant gas bubbler, followed by the reaction

AI203 + Li2CO 3 _ 2LiAIO 2 + CO 2 (16)

14
CONCLUSIONS

In regards to the suitability of gold as a catalyst for the CO 2 cathode of a

lithium-CO 2 battery, the results of the experiment reported here are generally

encouraging. The most significant result is the evidence that CO must be present in

the gaseous reactant feed stream of CO 2 to the battery cathode. In a functioning

system this does not represent a problem, since CO is a product of the cathodic

reaction and would be continuously rejected into the reactant gas stream. It would

be a simple matter to recycle a portion of the effluent stream to build up the

desired CO content. This is analogous to the recycling done in hydrogen-oxygen fuel

cells to humidify a reactant inlet stream. The relationship between the amount of CO

present and the activity of the gold electrode has not been established.

Of equal importance is the fact that, given the presence of CO in the reactant

feed, gold allows the reduction of CO 2 to proceed at a high rate with moderate

polarization. There was some loss of electrode activity during the 9-day experiment,

but it is not clear whether this was due to an intrinsic characteristic of the electrode or was an artifact of the experiment. It was shown that the gold undergoes

considerable morphological change at the test conditions and in a molten alkali

carbonate environment. How this would manifest itself in a practical electrode

design is open to conjecture. One can envision a finely divided gold catalyst dispersed on an appropriate current collector in such a way that the effect of the

morphological changes would not be detrimental.

The conclusion from the experiment, that the primary cathodic reactant is, in a

mechanistic sense, the carbonate melt, itself, has significance with regard to the

design of a practical electrode. The electrode would not be subject to the traditional fuel cell electrode requirement of the "three-phase interface," with its

vulnerability to flooding and drying, for example, thus giving the electrode designer

considerably more flexibility in approach. Nonetheless, access of the CO 2 and CO to

and from the near-electrode region must be quite unrestricted. Considerable conceptual, experimental, and modeling efforts will be required for development of a

practical electrode.

The observed sluggishness in reaching steady-state operation after a change in

load also requires mentioning. It may be that this is merely an artifact of the test

cell geometry. If so, it must be resolved in the design of a practical electrode.

If the problem is not so tractable, perhaps it can be resolved in a complete system

by adequate power conditioning. This points out the need for some degree of system

analysis early in the development of a defined mission.

ACKNOWLEDGMENTS

The author would like to thank Russel Gemeiner for his assistance and valuable

design suggestions which made it possible to reposition cell components while at

operating temperature, and Patricia Book for the microscopic and spectrographic

analyses of the test electrode.

i. Borucka, A.:

Carbonates.

REFERENCES

Electrochemical Behaviour of CO/CO 2 Gas Electrode in Molten Alkali

Electrochimica Acta, vol. 13, no. 3, Mar. 1968, pp. 295-307.

15
2. Janz, G.J., et al.: Physical Properties Data Compilations Relevant to Energy

Storage-If, Molten Salts: Data on Single and Multi-Component Salt Systems.

NSRDS-NBS-61-PT-2, U.S. Government Printing Office, Washington, 1979,

pp. 102-112.

3. Borucka, A.: Evidence for the Existence of Stable CO_ Ion and Response Time of

Gas Electrodes in Molten Alkali Carbonates. J. Electrochem. Soc., vol. 124,

no. 7, July 1977, pp. 972-976.

4. JANAF Thermochemical Tables. 3rd ed., American Chemical Society, 1985.

5. Bard, A.J.; and Faulkner, L.R.: Electrochemical Methods, Wiley, 1980,

pp. 213-248.

6. Selman, J.R.; and Maru, H.C.: Physical Chemistry and Electrochemistry of Alkali

Carbonate Melts with Special Reference to the Molten-Carbonate Fuel Cell.

Advances in Molten Salt Chemistry, Vol. 4, G. Mamantou and J. Braunstein, eds.,

Plenum, 1981, pp. 159-390.

7. Lux, H.: "Acids" and "Bases" in a Fused Salt Bath: The Determination of OxygenIon Concentration. Z. Elektrochem., vol. 45, 1939, pp. 303-309.

8. Flood, H.; and Forland, T.: The Acidic and Basic Properties of Oxides. Acta

Chem. Scand., vol. i, 1947, pp. 592-604.

9. Borucka, A.; and Appleby, A.J.: Kinetics and Mechanism of Electrochemical

Oxidation of Carbon Monoxide in Molten Carbonates. Confirmation of the

Existence of the Carbon Dioxide Dianign. J. Chem. Soc. Faraday Trans. i,

73, vol. no. 9, 1977, pp. 1420-1435.

i0. Borucka, A.: Single Gas Electrodes in Molten Carbonates. Fuel Cell Systems II

(Advances in Chemistry, Vol. 90), American Chemical Society, 1969, pp. 242-268.

16
ORIGINAL PAGE

BLACK AND WHITE PHOTOGRAPi-I

¢

:!i__ _i

C-90-05657

Figure 1.--Experimental cell showing water-cooled cover, tie rod suspension,

thermal baffles, thermocouple, reactant feed, and electrodes.

17
-10 I

-20' _ Rate of change __._,..._

I r - of potential, _ _..,_'___'_" '-3o - ,,,v/s_ _

..-4n 0 -8o LJT 8 -- a -40 'r"

-5O-- [] -20 L.

-6o (a) I I I I I 1(b) I I I I I

-100 .-80 --60 --40 -20 0 -100 -80 -60 -40 -20 0

Polarization, mV Polarization, mV

-10

_-_

_ tf J(o) t I I I

-100 -80 --60 -40 -20 0

Polarization, mV

(a)800°C.

(b) 8SO °C.

(c) 900 °C.

Figure 2._Baseline linear sweep voltammograms. Gold electrode with li2CO 3

melt; gas feed, CO/CO2; pressure, 1.2 arm; with bubbling.

<c

E

E

8

o-- J

-10

-15

polarization measurements

Closed symbols denote CO 2

-20 I 1 I O_nsymb_,sdenote_ui,_o,arC_/CO._• I 1 I

-100 -8O -60 -40 -20 0

Polarization, mV

Figure 3._Effect of reactant composition on performance, Gold electrode with

Li2CO 3 melt; potential sweep rate, -40 mV/sec; temperature, 800 °C; pressure,

1.2 arm.

18
o

-10

-20 r_.---'cJ_ _ _--- temperature,

T _.i_/- _ 9o0

I ._ _A_f_b_S denote performance before

-40 J-----,'J"_,,"_ steady-state measurements.

}y,,'_/,/_ Closed symbols denote performance after

steady-state measurements,

_50 ! I_ I I I I I I I 1

-100 -80 -60 --40 -20 0

Polarization, mV

Figure 4.--Performance before (upper curve) and after (lower curve) acquisition

of steady-state data at 800, 850, and 900 °C. Gold electrode with Li2CO- melt;

potential sweep rate, -40 mV/sec; gas feed, CO/CO2; pressure, 1.2 atrn. _

0 -- _-No n_

C_ ! bubbti

-'° - ',® -_ _ _ O_,a_ng

I / / temperature,

/ / oc

,k - #A o 80o

/ [] 850

_ atrn.

Closed symbols denote 2.1 atm.

-40 , . I I I I I I I l

-200 -160 -120 -80 -40 0

Polarization, mV

Figure 5.--Steady-state polarization data for equimolar CO/CO2 mixture. Gold

electrode with Li2CO 3 melt.

-20

lg
0 [-- Pressure,

-10 F • 2.1

_ --30

-40

_5o I l I t I

-100 -80 -60 -40 -20 0

Polarization, mV

Figure 6.---Effect of total pressure on electrode performance. Gok:l elect) ode

with Li2CO 3 melt; potential sweep rate, -40 mV/sec; temperature, 900 °C; gas

feed, CO/CO 2.

0 m []

-.5

_: -1.0 E

t_

-1.5

-2.0

/ _+ "*'*(_ j/

/ t

._-_" _ _-

/ I / /

////I /t/ I

-- / i I I ii

i / ii/j I III

- []

Operating

temperature,

°C

O 80O

[] 850

A g00

-2.5.,, I 1 I I I ! 1 I 1 1 I

-275 -250 -225 -200 -175 -150 -125 -100 -75 -50 -25 0

Polarization, mV

Figure 7.---Steady-state polarization data for 002 alone. Gold electrode with

Li2CO 3 malt; pressure, 1 to 2.1 arm; with and without bubbing.

2O
ORIGINALPAGE

BLACK AND WHITE PHOTOGRAPH

Figure 8.--Scanning electron microscope imageof gold cathode surface after experiment

showing development of large grains.

_mm

Figure g._Optical microscope image of cross section of gold electrode (lighter central

segment) showing grain development to full depth.

21
ORIG!NALPAGE

BLACK AND WHITE PHOTOGRAPH

Figure 10.--Scanning electron microscope image of gold cathode surface after experiment

showing encapsulation of ceramic crystals.

22
SPEe WATER ELECTROLYZERS

IN SUPPORTOF

MISSION FROM PLANET EARTH

J.F.McEIroy

UnitedTechnologiesCorp.

HamiltonStandardDivision

WindsorLocks,Connecticut06096

Navies was incorporated

demonstrators for NASA's

Program. Amongthesewere:

ABSTRACT

During the1970's, the SPE water electrolyzer,

which uses ion exchange membranes as its sole

electrolyte, was developed for nuclear

submarine metabolic oxygen production.

These developments included SPE water

electrolyzer operation at up to 3,000 psia and at

current densities in excess of 1,000 amps per

square foot. The SPE water electrolyzer system

is now fully qualified for both the U.S. and UK.

Navies with tens of thousands of system hours

accumulated at sea.

During the 1980's, the basic SPE water

electrolyzer cell structure developed for the

into several

Space Station

• The SPE regenerative fuel cell for

electrical energy storage

• The SPEwater electrolyzer for metabolic

oxygen production

The high pressure. SPE water

electrolyzer for reboost propellant

production

In the 1990's, the emphasis will be the

development of SPE water electrolyzers for

Mission from Planet Earth. Currently defined

potential applications for the SPE water

electrolyzer include:

SPE water electrolyzers operating at

high pressure as part of a regenerative

fuel cell extraterrestrial surface energy

storage system

SPE _ is a Registered Trademark of Hamilton

Standard Division, United Technologies

Corporation

, 91-92552

SPE water electrolyzers for propellant

production from extraterrestrial

indigenous materials

SPE water electrolyzers for metabolic

oxygen and potable water production

from reclaimed water

SPE WATER ELECTROLYSIS TECHNOLOGY

OVERVIEW

The heart of the SPE water electrolyzer is the

electrolysis cell which consists of an ion

exchange membrane with teflon bonded, finely

divided metal electrodes. Figure 1 shows this

arrangement along with the water electrolysis

reactions. Since the fixed acid ion exchange

membrane has neither a traditional bubble point

nor free electrolyte, operating pressures and

hydrogen-oxygen differentials are limited only

by the surrounding structures. This affords a

significant safety factor in maintaining positive

separation of the hydrogen and oxygen

products.

The introduction of the perfluorocarbon cation

exchange membrane in the late 1960's enabled

the development of the SPE electrolyzer. In prior

years, water electrolyzers made with existing ion

exchange membranes had useful lifetimes of

only a few hundred hours. With the use of

perfluorocarbon ion exchange membranes, the

SPE water electrolyzer cell life has been

demonstrated to be in excess of 12 years and

projected to over 30 years depending on

operating conditions. Figure 2 displays the

longest lifetime SPE water electrolyzer cell at its

100,000 hour milestone in 1989. This cell and

two others have now accumulated in excess of

100,000 operational hours without disassembly

or modification. These three cells continue to

accumulate additional operational hours.

In most practical applications, a number of cells

are stacked in a filter press arrangement with as

many as 100 or more cells electrically connected

in series while the fluids are passed through the

cells in parallel. Figure 3 displays a pair of SPE

water electrolyzers, each with 81 cells, in a filter

press arrangement. Without any free

electrolyte, the parallel fluid flows can be

conducted without fear of shunt currents

inducing stray water electrolysis and its

potentially deleterious result of product gas

mixing. The purity of the product gases from

SPE water electrolyzers is typically greater than

99.99%.

23
NUCLEAR SUBMARINE OXYGEN

GENERATORS

Both the U.S. Navy and the U.K. Royal Navy

have sponsored the development of SPE water

electrolyzers for oxygen generation in nuclear

submarines. In the case of the U.K. Royal Navy,

the SPE water electrolyzer system is fully

qualified with more than 30 systems delivered to

date. The SPE water electrolyzer module

equipment is supplied by Hamilton Standard

and the supporting system equipment supplied

by CJB Developments of Portsmouth, England.

The modules previously shown in Figure 3 are

the type used in the U.K. Royal Navy system and

the overall oxygen generation system itself is

depicted in Figure 4. The operational

experience of the SPE water electrolyzer has

been exceptional with over 41,000 operational

system hours without a single malfunction. The

longest operational service for any single 150

psia SPE electrolyzer module is 8,539 hours as

of December 31, 1990.

The U.S. Navy SPE water electrolyzer system,

which operates at pressures up to 3000 psia, has

passed all qualification testing, including shock,

vibration and sea trials. The U.S. Navy SPE

water electrolyzer modules and overall oxygen

generation plant are shown in Figures 5 and 6

respectively.

During the course of developing the two Naval

oxygen generation systems and the subsequent

operation in the U.K. Royal Navy, over 8 million

cell hours have been accumulated on the basic

0.23 ft2 cell design. This high level of maturity is

further described in Table 1.

SPACE STATION DEMONSTRATORS

During the decade of the eighties, a series of

demonstrators were fashioned, delivered and

tested at NASA. Each of these demonstrators

made use of the identical 0.23 ft2 SPE water

electrolyzer design with its naval maturity.

SPE Regenerative Fuel Cell

The SPE regenerative fuel cell for electrical

energy storage was the first of the

demonstrators to be delivered to NASA in

support of Space Station Freedom. Figure 7

shows the three subsystems making up the

demonstrator as follows:

• SPE fuel cell subsystem

• SPE water electrolyzer subsystem

• Microprocessor controller

24

The SPE fuel cell module consists of eight cells,

each of an active area of 1.1 ft 2. The SPE water

electrolyzer module contains 22 cells, each of

the 0.23 ft2 design.

The SPE regenerative fuel cell demonstrator,

with its 1 to 2 kW rating, underwent parametric

testing at the factory prior to its delivery to

NASA/JSC. NASA/JSC conducted extensive

testing of the system accumulating 1,630

simulated low earth orbit charge/discharge

cycles [1]. Including the pre-delivery factory

cycles, over 2,000 cycles were accumulated on

the combined SPE water electrolyzer and SPE

fuel cell.

Other demonstrated features included:

• Closed system fluid cycle balance

° Direct solar array/electrolyzer

voltage/current control compatibility

(i.e., no power conditioning required)

• An electric energy storage efficiency of

48% recorded with the SPE water

electrolyzer at ambient temperature

At the successful conclusion of the SPE

regenerative fuel cell demonstration, the SPE

fuel cell was replaced with a Space Shuttle

alkaline development fuel cell subsystem. This

hybrid of alkaline fuel cell and acid SPE water

electrolyzer was operated by NASA through an

additional 100 low earth orbit charge/discharge

cycles [2]. Both subsystems displayed stable

performance throughout the 100 cycles and

proved the compatibility of the hybrid approach.

The most recent activity with the SPE water

electrolyzer subsystem was to retrofit two

random cells of the 22 cell SPE water

electrolyzer module with high performance cells

using a membrane manufactured by Dow

Chemical. Following the factory modifications,

the SPE water electrolyzer module with 20

standard 0.23 ft2 cells and the two high

performance 0.23 ft2 cells underwent parametric

testing at NASA/JSC [3]. The testing at various

temperatures and pressures showed a

significant performance improvement with the

Dow membrane cells, especially at the higher

current densities. The curves in Figure 8 are

typical of the improvement.

SPE Metabolic Oxygen Generator

Under contract to the Boeing Aerospace and

Electronics Company, an oxygen generator

assembly technology demonstrator was

constructed and is being evaluated. The heart

of the oxygen generator is a 12 cell SPE water

electrolyzer module of the identical 0.23 ft 2 SPE
cell configurationused on the Navyprograms.

Figure 9 displays the oxygen generator

assemblytechnologydemonstratorwithitsSPE

electrolyzermodule. The operatingpressure,

temperature and current density of the

technology demonstrator are well within the

technology maturity establishedby the Navy

experiences. Where this technology

demonstratordiffersfrom the Navydatabaseis

in the need to operate in a microgravity

environment and to use processed hygiene

waterasthe feedstock.

In the normal operation of the SPE water

electrolyzer,liquidwater is circulatedthrough

the oxygenanode. This loop requiresa phase

separatorasanoxygen/watermixisdischarged

from the module. Also, as hydrogen protons

pass through the cell membranes,water is

carried to the hydrogen cathode and thus a

phaseseparatorfor hydrogen/waterisrequired.

In the microgravitysituation, the functions of

gravity type pressurevessel phaseseparators

mustbeaccomplishedbyothermeans_norder

to makeuseofthe highperformanceSPEwater

electrolyzer.Priordesignshave performedthe

microgravityfunctionoftheonegravitypressure

vessel phase separatorwith a combinationof

bellows accumulators and motor driven

centrifugaldevices. Althoughthis arrangement

has been used successfully,the drawbacks

include lower reliability and higher power

consumption. The approach taken in the

BoeingTechnologyDemonstratorutilizestwo

membranestatic phase separatorsto replace

the pressure vessel phase separators. This

arrangementis displayedin Figure10.

Threebasictypesofmembranesareusedinthe

construction of the membrane phase

separators:

• Hydrophobic Membrane - This

membrane easily passes gas with a

smalldifferentialpressurebutblocksthe

passageof liquid waterup to thewater

_ntrusionpressure of the membrane.

The chosen material for the Boein_

TechnologyDemonstratorisGore-Tex_,

having a water intrusion pressure of

approximately75psi differential.

• Ion Exchange Membrane - Nation®

membranewith attachedelectrodesisa

very efficient hydrogen compressor.

With an appliedvoltageof between0.5

and 1.0 volt, hydrogen is rapidly

transferredthrough the membraneat a

rateproportionalto the electricalcurrent

draw.

TheTechnologyDemonstratorwasactivatedat

NASA/MSFC in November 1990 and, in

operating for 529 hours, exceeded the test

objectiveof 450 hours. The waterelectrolysis

was conductedat aneight-man rate,withboth

deionizedwater and shower waterprocessed

through an ultrafiltration/reverse osmosis

subsystem. Throughoutthe operation of the

Technology Demonstrator, the microgravity

phaseseparatorsworked in a very satisfactory

manner. The electrical performance of the

TechnologyDemonstratorisshownin Figure11.

Continuingtestsand evaluationsareinprocess

to improvetheSPEcellvoltageperformanceon

processedhygienewater.

SPEPropellant Generator

Under contract to NASA/JSC, a 3,000 psi

hydrogen-oxygen generator based on the

Naval 0.23 ftz SPE water electrolyzer cell

configurationwasdesignedanddelivered.The

purposewas to demonstratethe feasibilityof

producing3,000 psi hydrogenand oxygen on

orbitfor periodicrocketmotorfiring to maintain

SpaceStationFreedomorbitalaltitude.

Hydrophilic Membrane - This

membrane easily passes liquid water

with a small differentialpressure but

blocks the passage of gas up to the

bubble point of the membrane. The

chosen material for the Boeing

Technology Demonstrator is Supor_,

having a bubble pressure of

approximately25 psidifferential.

Supor ® is a registered trademark of Gelman

Sciences, Inc.

Gore-Tex ® is a registered trademark of W. L.

Gore & Associates, Inc.

Nation ® is a registered trademark of E. I. DuPont

DeNemours & Co.

25
In high pressureSPEelectrolyzers,a pressure

vesselisusedto enclosethemodule. Fillingthis

pressure vessel with high pressure nitrogen

precludesthe necessityof designingcell seals

to withstandthe high pressuredifferential.

Toreducethe massand decreasethevolume,

as comparedto the U.S.Navydesign,several

configuration changes were made to the

supportingpressurevesseland fluid manifold.

The resultant SPE propellant generator

demonstratoris significantlysmallerand lighter.

In the SPEpropellantgeneratordemonstrator,

the pressurevesselis two torisphericaldomes

opposed on eitherside of a centralfluid plate.

This configurationis shown in Figure 12. The

domeddesignallowsfor a wallthicknessof as

lowasonequarterof aninchwhenusingInconel

or otherhighstrengthmaterials.

The fluid plate manifold is pressurebalanced

betweenthetwo pneumaticdomes,eliminating

the needfor athick plateto resistthepneumatic

load,asis usedinthe U.S.Navyhardware.The

demonstratorfluid manifold is only one inch

thick. In the demonstrator,the cell stack is

locatedon one side of the fluid platewithinthe

volumeof one dome.

Compressionspringsarelocatedinthevolume

of the opposingdome. Volumeis availablefor

incorporation of gas/water phase separators

and/or other system ancillaries, allowing for

additionalsavingsinsystemweightandvolume.

In addition, the cell stack incorporatededge

electricalconnectionsto a low profile positive

terminalplateinsteadof usinga plateand post

assembly for additional mass and volume

savings.

The SPEpropellantgeneratordemonstratoris

showninFigures13and 14. Thedimensionsof

the demonstrator are 13 inches across the

domes and 13inchesin diameterat the dome

flanges. The total weight of the cell stack for

spacestationpropulsionis 193Ibsor20% ofthe

navalversion. Thevolumeisreduced70%from

the navalstack.

Thedemonstratorisdesignedto produce2pph

normalrate/4pphemergencyrateof propellant

(i.e., oxygenand hydrogen)gas at 3000 psia,

120°F at an efficiency of greater than 70%.

PerformanceisshowninFigure15forconditions

of 3100 psia, 120°E This performancewas

establishedat the factory prior to deliveryto

NASA/JSCin 1990.

This demonstrator has been set up and

operatedintermittentlyat NASA/JSCoverthe

last few months. NASA personnel have

expresseda highdegreeof satisfactionwiththe

current performance.

MISSION FROM PLANET EARTH

APPLICATIONS

The technology maturity gained from the 8

million cell hours of operation of the 0.23 ft2

hardware, combined with the experience

obtained from the Space Station Freedom

demonstrators has placed the SPE water

electrolyzer in position to support Mission from

Planet Earth. Three potential applications for the

SPE water electrolyzer are described in the

following sections:

Extraterrestrial Surface Energy Storage

Recent studies have shown that, short of nuclear

power, solar energy combined with an

oxygen-hydrogen regenerative fuel cell is a

mission enabling and preferred technology for

Lunar and Mars bases [4]. Figure 16 displays

the relative mass of three leading candidates for

electrical energy storage as presented by NASA

Lewis Research Center. The long occult

periods, 14 days and 12 hours, respectively,

make the separation of power and energy in the

oxygen-hydrogen regenerative fuel cell

decisive. In Figure 17, showing an overall power

plant schematic, energy storage mass is related

to the tankage and stored fluids whereas the

power rating mass is related to the modules and

thermal management. The electrolysis and fuel

cell modules can be of either the alkaline or acid

type; however, the acid SPE water electrolyzer,

of the Naval 0.23 ft 2 configuration in particular,

has demonstrated the life, stability, reliability and

high pressure capability required of an

extraterrestrial surface energy storage system.

The schematic for 3000 psi SPE water

electrolysis is much the same as the low

pressure schematic except that the electrolyzer

module is enclosed in the nitrogen filled

pressure vessel. Figure 18 displays the overall

SPE water electrolyzer efficiency at various

temperatures. The 1000 amps per square foot

(ASF) current density, which is below the Naval

design point of 1300 ASF, will provide an

efficiency in excess of 70% at the 20,000 hour

end-of-mission point.

A fuel cell operating at 70% overall fuel cell

efficiency will require approximately 21.6

pounds per hour of hydrogen-oxygen reactants

in a 1 to 8 weight ratio to produce 25 kW direct

current. If one assumes an equal charge

discharge time for either the lunar or Mars

application, the electrolyzer will have to convert

26
amaximumof21.6poundsperhourof water into

hydrogen and oxygen. The mass of the SPE

water _lectrolyzer subsystem with 138 cells

would be about 200 kg using the proven cell

structure with DuPont's Nation ® 120 ion

exchange membrane. Figure 19 shows that a

decreased mass can be obtained by the use of

higher performance membranes and/or

advanced cell structures. However, the low

mass is gained at the expense of design

maturity.

A single SPE water electrolyzer subsystem

would probably not be considered because of

reliability aspects. Over a five-year period, the

loss of a pump or gas regulator is predicted.

These difficulties can be overcome by

redundancy within the subsystem at a small

weight penalty. Reliability is further enhanced by

having multiple SPE water electrolyzer

subsystems. Preliminary estimates show that

three subsystems, each with selected

component redundancies, would be highly

reliable for a multiple year mission.

Propellant Production

In any round trip to Mars, the propellant for the

return trip is a most significant mass factor when

the propellant is brought from Earth. For

manned missions to Mars, multiple launches

with Earth orbital vehicle assembly will be

required in mission architectures which bring the

return propellant to Mars from Earth. By

contrast, the use of in-situ propellant production

can greatly reduce mission mass and, together

with a heavy lift launch vehicle, can eliminate the

need for on-orbit assembly.

A system which combines a SPE water

electrolyzer with Sabatier and carbon formation

reactors can produce methane and oxygen from

carbon dioxide in the Martian atmosphere and

hydrogen delivered from Earth [5]. This

arrangement can increase the Earth return

propulsive reactant mass leverage by up to 18

fold. Each of the required subsystems have

been demonstrated individually with high

confidence that they can be combined to

produce the desired results.

A top level schematic with subsystem reactions

is shown in Figure 20. In this mission

architecture, liquid hydrogen is transported from

the Earth directly to the Martian surface. This

hydrogen is reacted with atmospheric carbon

dioxide to produce methane and water in the

exothermic Sabatier reactor. The water is

delivered to the SPE water electrolyzer and the

methane is liquified and stored. Hydrogen and

oxygen are produced within the SPE water

electrolyzer with the oxygen being liquified and

stored and the hydrogen returned to the

Sabatier for the formation of more methane and

water. With these two subsystems alone, a 12 to

1 propellant mass leverage is obtained.

The combined Sabatier and SPE water

electrolyzer produce the oxygen and methane at

a2to 1 mass ratio. Burning this mixture provides

a specific impulse of about 340 S at a nozzle

expansion ratio of 100. However, an optimum

oxygen to methane combustion mixture ratio is

about 3.5 to 1. This mixture would provide a

specific impulse of 353 S. To obtain the 3.5 to 1

ratio, which increases the propellant to

hydrogen mass leveraging to 18 to 1, additional

oxygen must be obtained. The carbon

formation reactor provides this opportunity.

Referring back to Figure 20, a portion of the

methane produced in the Sabatier is borrowed

and delivered to the carbon formation reactor.

In the reactor, the methane is endothermically

converted to solid carbon and hydrogen. The

hydrogen is returned to the Sabatier/SPE water

electrolyzer combination to replace the

borrowed methane and produce the additional

oxygen. By continuing this process, any desired

level of excess oxygen can be produced.

However, the 36 extra tonnes of excess oxygen

needed for a manned mission would require a

huge carbon formation reactor to hold all the

solid carbon produced. This difficulty is

overcome by having two carbon formation

reactors which alternately cycle between carbon

formation and an endothermic regeneration

reaction with carbon dioxide.

Preliminary mass estimates for the required

subsystems have been made for a Mars Sample

Return Mission and a Mars Manned Mission.

Table 2 provides these estimates. Since the

subsystems, once on the Martian surface, will

have to produce propellants for up to one year,

a degree of redundancy is required. The mass

estimates assume two complete units each with

100% mission capacity for the system used in

the Mars Sample Return Mission and three units

each with 50% capacity for the system

employed on the manned Mars mission.

Metabolic Oxygen and Potable Water

Production

The production of metabolic oxygen for space

applications has been under investigation for a

number of years. Recently, water feedstock with

various organic contaminants have been tested

with the SPE water electrolyzer to assess the

impact on the voltage stability. In these tests, the

27
contaminated water feedstock has been

introduced intothe oxygenanode chamberin

the same fashion as the Naval SPE water

electrolyzers. As discussedearlier,continuing

testsand evaluationsareinprocessto improve

the SPE cell voltage performanceon water

feedstockwithorganic contaminants.

Testsconductedon the qualityof the waterat

various points in the system indicated that

organic specieswerebeingoxidizedwithinthe

oxygenchamberand that the proton pum.ped

water was free of any detectable organics.

Theseobservationshaveledto thespeculation

that the SPE water electrolyzer can be

configuredto producepotablewateraswell as

metabolic oxygen. Figure 21 displays the

metabolic oxygen generator schematic as

modifiedto showdeliveryof potablewater.The

rateofprotonicallypumpedwaterissuchthatup

to eight pounds of potable water can be

deliveredfor each poundof oxygenproduced.

For manned Lunar and Mars bases and the

LunarandMarsmannedtransportationvehicles

a combined oxygen generator and potable

water processor could have significantmass

advantages.

SUMMARY

Although the Mission From Planet Earth is still in

the early planning stage, several unique and

potentially enabling uses of the SPE water

electrolyzer have been identified. The maturity

of the SPE water electrolyzer cells gained from

the Naval applications should give mission

lanners the confidence to take advantage of the

veraging effects of the SPE cell technology.

Although the inherent capabilities of this

technology have been proven, significant

development effort remains to package these

cells for the Mission From Planet Earth

applications.

REFERENCES

[1]: NASA/JSC Internal Note

Document No. JSC-22044, dated 4/28/86

[2]: NASA/JSC Internal Note

Document No. JSC-22632, dated 9/30/87

[3]: NASA/JSC Internal Note

Document No. JSC-24411, dated 7/25/90

[4]: NASA Conference Publication 3016;

September 12-13, 1988, p. 206

[5]: Zubrin, R. et al: "Mars Direct: A Simple,

Robust and Cost Effective Exploration Initiative"

(AIAA 91-0326), 29th Aerospace Sciences

Meeting, January 7-10, 199t

28
TABLE 1

DEMONSTRATED MATURITY-NUCLEAR

AS OF DECEMBER 31, 1990

Type of Service/Product Operating

Description Pressure

Navy Electroly'zers PSI

U.S. Navy:.

Nuclear Submarine

O= Generation

3000 psi System

Qualification Unit

(OGP#2)

3000 psi System

Preprototype (OGP#I)

Development System

(Breadboard)

Development Stack

OGP Program

Development Stack

OGP Program

Development Stack

OGP Program

Development Stack

OGP Program

Development Stack

NSSC 4

Development Stack

NSSC 7

Development Stack

NSSC 9

United Kingdom Navy

Nuclear Submarine

Oz Generation

Development Stack

S/N#1

Development Stack

S/N#2

Production Stacks:

Various Production

Longest Life UK Stack

Electrolysis Cell

Rate Area

Ibe/hr HzO ft2

21 0.23

21 0.23

21 0.23

1 0.23

2 0.23

4 0.23

7.4 0.23

0.2 0.23

0.2 0.23

0.2 0.23

12.5 0.23

15 0.23

15 to 17 0.23

15 to 17 0.23

SUBMARINE SPE WATER ELECTROLYZERS

Number Number Total System Stack

of of Cells Number Hours Hours

Stacks Per Stack Of Cells

Cell

Hours

300 to 3000 2 100 200 5.000 5,000 500,000

300 to 3000 2 100/83 183 10,000 13.000

.300 to 3000 2 100 200 14,000 28,000

300 to 3000 1 5 5 1.900 1,900

300 to 30(X) 1 10 10 1,500 1,500

300 to 3(X)0 1 20 20 2,500 2.500

300 to 3000 1 35 35 3,900 3,900

ambient 1 1 1 107,100 107,100

ambient 1 1 1 103,200 103,200

ambient 1 1 1 108,500 108,500

150 1 58 58 3,OOO 3,080

150 1 70 70 6,000 6,000

150 35 70 or 81 2700 41,632 41,632

150 1 81 81 8,539 8.539

TOTAL 50 3474 408,239 425,239

With over _ cell hours of successful operation, the SPE 0.23 ft2 cell design is well established

TABLE 2

MASS ESTIMATES FOR CH./O2,, PLANT

REACTOR

Sabatier

Electrolysis

Carbon Reactor

TOTAL

MARS SAMPLE

RETURN

36 kg

90 kg

231 kg

3.6 kg/day

7.2 kg/day

M_tEDJ_U_

164 kg

477 kg

1,091 kg

360 kg/day

540 kg/day

Propellant

Generation

Requirement

Capability

1,181,000

2.800,000

9,500

15,000

50,000

136,500

107,100

103.200

108,500

174,000

420,000

3.122,400

691,659

8,727200

29
ION EXCHANGE

MEMBRANE

CATHODE

HYDROGEN y ANODE

I / OXYGEN

ELECTRODE _ _ELECTRODE

HYDC I OXYGI_., I If=,,EN

4H + +4e _ 2H2 I 4H + + 4e+ 0 2

4H +

 ROCE S

DC SOURCE

FIGURE 1. SPE WATER ELECTROLYZER

REACTIONS

ORIGINAL PAGE

BLACK AND WHITE PHOTOGRAPN

FIGURE 3. SPE WATER ELECTROLYZER

MODULES

TIE $

FIGURE 2. SPE ELECTROLYZER LIFE TEST

FIGURE 4. A DUPLEX LOW PRESSURE

ELECTROLYZER (LPS) OXYGEN

GENERATING SYSTEM FOR NAVAL

USE

3O
ORIGINAE PAGE • t

BLACK AND WHITE piioTOGRAPH

ORIGINAL PAGE

8LACK AND WHITE PHOTOGRAPh

SPE ELECTROLYZER

SPE FUEL CELL MICROPROCESSOR

CONTROLLER

FIGURE 5. 225 SCFH U.S. NAVY SPE OXYGEN

GENERATOR MODULES

FIGURE 7. SPE REGENERATIVE FUEL

CELL DEMONSTRATOR

1.9

1,8

u)

1.7

0

1.6

1.5

0

BEST

STANDARD

CELL --_

1 DOW

CELLS

10 20 30 40 50

AMPERES

FIGURE 8. COMPARISON OF CELLS AT

150 PSI AND 105°F

6O

FIGURE 6. HIGH PRESSURE OXYGEN

GENERATING PLANT (OGP)

FOR NAVAL USE

31
ORIGINALPAGE

_LACK AND WHITE PHOTOGRAPt,-I

OXYGEN GENERATOR ASSEMBLY SPE ELECTROLYZER MODULE

FIGURE 9. OXYGEN GENERATOR ASSEMBLY TECHNOLOGY DEMONSTRATOR

H_zo MEM_U_NE SEPARATOR

H_

r'<

I I

_o

FEEDWATER

ACCUMULATOR

ACCUMULATOR

PUMP

SPE

ELECTROLYSIS

MODULE

CATHODE AJNODE

(H2) (02)

]

HEAT

EXCHANGER

U

PUMP

mE

1"t20

O2/t.120 MEMBRANE SEPARATOR

ACrJJMULATOR

MRB186.003

O 2

FIGURE 10. SIMPLIFIED MICROGRAVlTY FLUID SCHEMATIC (SPE ELECTROLYZER SYSTEM)

32
25

u) 20

__._"1.5

uJ

c3

i 1.0

0.5--

D!

__ WATER --

FEEDSTOCK

0 1

_ I _ PROCESSED HYGIENE

-/- WATER FEEDSTOCK

_r

3

HOURS X 100

FEEDSTOCK

CURRENT 80 AMPS

(8 MAN RATE)

I

5

MFB186.004

FIGURE 11. SPE ELECTROLYZER TECHNOLOGY

DEMONSTRATOR PERFORMANCE

ORIGINAL P_,Q E'

BLACK AND WHITE PHOTOGRAPH

o,.,.os,.,_,,,_sso,,_ _ I - (_,,,os,_,_TE ___.. _: I _ssu,,

END PLATE

SPRING ASSEMBLIES

FOR LOADING

CENTRAL FLUID PLATEI (--) BUS

FIGURE 12. CROSS-SECTION, SPE

PROPELLANT GENERATOR DEMONSTRATOR

FIGURE 13. SPE PROPELLANT GENERATOR,

DOMES REMOVED

o>

FIGURE 14, ASSEMBLED SPE PROPELLANT

40

38  ̧

32

30

28

GENERATOR

J

J

3100 PSIA OXYI IEN

3000 PSIA HYDROGEN

120"F

J

J

50 100 150 20O 250 -300 350

STACK CURRENT(AMPS)

FIGURE 15, SPE PROPELLANT GENERATOR

PERFORMANCE (STACK VOLTS VS. AMPS)

TECHNOLOGYIMFACTS

1.2 25kW SURFACEPOWERSYSTEM

1,0

!

<_ 0.8

0,6

0.4

LUNAR MARS

0.2

0

' r---_]

Ni/H 2 N_S RFC Ni/H 2 Na/S RFC

SOA PV SOA PV SOA PV SOA PV SOA PV SOA PV

FIGURE 16. RELATIVE MASS OF ENERGY

STORAGE TECHNOLOGIES

33
H 2

ION EXCHANGE

MEMBRANE

J_

02

\ ,

,,,,,

H20

_ _1=1- +

9 E'EOTR,OBOS 91

I

VOL_GEI

REGULATOR I ! ,, \

_LECTRO'¥S,S _,OO_AGE

i

HEAT EXCHANGER

ITHEAT

REJECTION

Hz

MODULE{S) FUEL CELL _._

ION EXCHANGE

MEMBRANE

i

\

H_

FLUID LINE

-- _ -- ELECTRICAL LINE

FIGURE 17. POWER PLANT SCHEMATIC INCLUDING HYDROGEN/OXYGEN REGENERATIVE FUEL

CELL ENERGY STORAGE SYSTEM

9O

3000 PSl DESIGN

NAFION 1;t0 POINT

CURRENT

80 -

_ 150°F _

_ 7o

/

l//

0 200 400 600 800 1000 1200

ASF

FIGURE 18. OVERALL SPE WATER ELECTROLYZER EFFICIENCY AT 3000 PSI

34
ELECTROLYZER

SUBSYSTEM

DESCRIPTION

• STATE OF THE ART"

WITH STATIC

SEPARATORS

ADVANCED DESIGN

WITH STATIC

SEPARATORS

NAFION 120

MEMBRANE

LBSj

_ KG

439_

NAFION 125/117

MEMBRANE

144

ADVANCED

MEMBRANE

137

BASED ON:

• 21.6 HR IF WATER ELECTROLYZED

• 70% THERMAL EFFICIENCY FOR 20,000 HRS

• 3,000 PSIAGAS GENERATION PRESSURE

• THERMAL VACUUM COMPATIBLE

• EQUAL CHARGE/DISCHARGE TIMES

NOTES:

• ONLY NAFION 120 HAS THE PEDIGREE

• ADVANCED DESIGN NEEDS DEVELOPMENT VERIFICATION

FIGURE 19. SPE WATER ELECTROLYZER

SUBSYSTEM MASS SUMMARY

FOR 25 KW SYSTEM

INPUTS OUTPUTS

H2

CO_

CO_ .....

N2

r

SABATIER

(CO z + 4H z $

CH 4 + 2HzO)

H20

SPE

WATER

ELECTROLYZER

(2H20 0 2H2 + 02)

L_-H-2_ (CH,cARBON0 C + 2H2)

FORMATION

REACTOR

(CO2 + C $ 2CO)

L

r f -"-0 2

I

I

I

I

..... "-_CO

FIGURE 20. TOP LEVEL SCHEMATIC OF

MARTIAN FUEL AND OXYGEN

GENERATOR

CH 4

/H2/H20 MEMBRANE SEPARATOR

/-

PURE

WATER_

TAP

H20

REGULATOR

ACCUMULATOR

ACCUMULATOR

FEED'WATER

(ORGANICALLY CONTAMINATED)

SPE

ELECTROLYSIS

MODULE

O2/H20

CATHODE AN(DOE

(H2) (02 )

HEAT

EXCHANGER

PUMP

O2/H20 MEMBRANE SEPARATOR

:::::::::::::::::::::

ACCUMULATOR

o

PUMP

FIGURE 21. METABOLIC OXYGEN/POTABLE WATER GENERATOR

-_ 02 WITH

TRACE CO2

35

N91-82558

SPACE WATER ELECTROLYSIS:

SPACE STATION THROUGH ADVANCE MISSIONS

Ronald J. Davenport , Franz H. Schubert, and David J. Grigger

Life Systems, Inc.

Cleveland, Ohio 44122

Static Feed Electrolysis technology has long been recognized as being importal_t in

meeting the National Aeronautics and Space Administration's requirements for life

support within the Space Station Freedom Program and future, advanced missi,ms.

The Static Feed ElectroJysis technology makes it possible to very ell iciently

generate oxygen to sustain the crew, 0nd hydrogen for the efficient and economical

operation of other space systems.

More recently, additional applications for the Static Feed l'loctroly,';is technolc_l;y

have been evaluated and tested. ]his paper summarizo,; tlm result.'; of thos:e

studies. The applications addressed her,: are those involving energy stor;igc,

propulsion, extravehicu]ar activity and other, specialized _pplicatinns, ,_ttcl| ;],,;

providing support for experimentation :_nd manufacturing. 'lhe Environmental

Control and I,ife Support System application is included for comparison.

1NTROI)UC'I- ION

This paper summarizes how Static Feed Electrolyzer (,qFE) technology cm_ ",at i,qfy

the need for oxygen (0 2 ) and hydrogen (tt2) in the Space Station Freedom and

future, advanced missigns. These applications are identified in _ig. I. The

efficiency with which the SFE technology can be used t,_ generate O and H, is one

2 2

of its major advantages. In fact, the SFE is baselined for the O×ygen (;eneration

Assembly (OGA) within the Space Station Freedom's Environmental Control and life

Support System (ECI,SS).

Figure 2 shows the conventionaJ SFE process. An alkaline electr_lyte is contained

within the matrix and is sandwiched between two porc u,_ electrodes. The electrodes

and matrix make up a unitized ceil core. The eJectrolyte provides tLe necessaJ-y

path for the transport of water and ions b_tween the electrodes, and form_: a

barrier to the diffusion of 0 2 and H 2.

A hydrophobic, microporous membrane permits water vapor to difft:se from tll_, teed

water to the cell core. This membrane separate_ tt_e liquid feed water fr<,,, the

product H^, and, therefore, avoids direct contact of the electrodes by tho feed

water. T_is eliminates the possibility of catalyst poisunin;: 1;v cc,_t_min,_ts in

the feed water. The feed water is also circulated through an t:xter,al h_L,t

exchanger to control the temperature oi the col].

Electrical power, when applied to the electrodes, consumuF: watc'r witl_[l_ tl_e cell

core. This increases the difference between the water vap_w l, ressure-:_ !t, the

" Author to whom correspondence should be addres.';cd.

37
electrolyte in the cell core, and in the liquid in the feed compartment. As a

result, water vapor diffuses from the feed water compartment into the cell core.

An external water supply tank replenishes water consumed from tile feed compartment. The operation and performance of the SFE have been previously described in

greater detail [I-3].

In a typical SFE subsystem, the electrochemical cells are combined to form an

electrolysis module. The cells are arranged electrically in series, but the

fluids flow through the cells in parallel. In addition to the module, the other

mechanical components within the subsystem consist of the Thermal Control Assembly

(TCA), a Fluids Control Assembly (FCA) and a Pressure Control Assembly (PCA).

The TCA supplies a constant flow rate of feed water to the electrochemical module.

The temperature of this water is varied to maintain the module at the desired

temperature. Temperature control is achieved by proportioning the flow of the

feed water between a bypass and a heat exchanger.

The FCA controls and monitors the flow of feed water and purge gas into the SFE

subsystem. The PCA maintains the absolute and differential pressures in the

subsystem and controls pressurization of the system during startup and shutdown.

The TCA, FCA and PCA are mechanically integrated assemblies, made up of valves,

pumps, pressure regulators and sensors which monitor and control the subsystem.

These components were developed and refined by Life Systems as part of SFE subsystem development efforts performed during the last 20 years. The integrated

design of these components offers significant savings in volume and weight,

compared to the use of discrete components [4]. Equally important, the use of

integrated components enhances the maintainability of SFE subsystems [5].

The operation of the SFE subsystem is controlled and monitored by microprocessorbased instrumentation. The instrumentation performs process control, automatic

mode transition, fault diagnostics and data acquisition functions.

APPI_ICATIONS

This section discusses tile application of the SFE technology to the energy

storage, ECLSS, propulsion, extravehicular activity (EVA) and other, special

applications. Typical O^ and H 2 production and delivery requirements for some of

these applications are listed in Table i. This table shows the diversity of the

requirements that be accommodated by SFE technology.

This section also addresses future studies that are planned to demonstrate the

performance of SFE water electrolyzers under low-g conditions.

Energy Storage

The energy storage application requires the SFE to provide 02 and H 2 for use

_n a Regenerative Fuel Cell Subsystem (RFCS) [6]. Electrical power is converted

by the SFE into O^ and H 2, which the fuel cells use later to generate electrical

power when there _s a power demand.

38
A process schematic for the SFE in this application is shown in Fig. 3. The 02

and H 2 product gases are stored in tanks at 2,172 kPa (315 psia), ready to be

consumed by the fuel cell. The fuel cel] provides feed water to the SFE. An

advantage of using the SFE in this application is that II2 from the fuel eel1,

dissolved in the feed water, is not a problem.

To demonstrate the readiness of tile SFE techm_logy for thi._; npplication, endur;mce

tests of SFE single cells and modules were performed [7]. A total test time of

238,616 cell-hr were accumulated (Table 2). Part of this testing (160,710 cellhr) was performed in a continuous-operation mode, but an additional 77,906 cell-hr

were obtained under cyclic operation to simulate the operation ol tlw SFE in

orbit. The SFE hardware was operated 54 rain with current on, and 36 rain with

current off.

An integrated Breadboard RFCS (Fig. 4) was assembled, ,.qing a 2.12 kW .ql,uttl_

Orbiter fuel cell power plant (X708)[8]. The SFE contained a 6-cclI, l.(l ft

water electrolysis module, which is sized for oper;ltion at 3 kW. 'l'h{, ,';FE in this

subsystem was operated for 635 hr at I, ife '.;ystcms, including 602 hr t_[ cyclic

operation. A light cycle time (SFE current on) of 56 rain was u._sed, with a ct_l_

cycle time (SFE current off) of 36 rain. Figure 5 shows the cell w,ltatw pcrf_,rmance and the variation of O._-to-.';ystem and tl2-to-System pre._;sure d J[ferential,'_

during this cyclic operation.

The projected configuration of a SFE sub,qy,qtem, sized to pr_duce 3. t_ kb'

(8.5 lb) O2/hr is shown in Fig. 6. ]'lie characteristics _,! this ._;ub,,_yst,,m ar_

summari_ed in Table 3. Utilization of cells having an active area of 0.(.;9 m"

(1.0 ft ) achieves the high production rates required tor tills _lppl ice, titre.

ECLSS

Extensive development has been performed during the pa_t 20 year,', t_ u.qe St,'I(

technology to generate 02 for life support. The splice .';tatinn I_CI c° rcquire._; (_.)

for the crew, pressurization of the air lock ;_nd to repleni.c;h other uNIt-rl_,ql '

leakage, ttydrogen is required for the efficient _,peratton ol other sub,-y,,:t:.'ms.

A process schematic of the SFE is shown in Fig. 7. lx_w pr_,,_:sure water zip.d

electricity are used to produce the 02 and |!.. Feed water is circulatL.d t hr,,IIF.l_

an external heat exchanger in the TCA to control the t,_mpt.rature ,_f tl,e re(Mule.

During normal operation, coolant is used to remove wa.,;te l_eat i);od_,c_,d hy the

electrolysis reaction, and a small amount of nitr(_gen (N..) is u:,t.d d,ring st,_rt,: I,

to pressurize the water tank. The dew point of the prodtJct gab;e::, aftt,r ,._:I.;i_.';itm

to ambient pressure is within tile range of 278 to 289 K (40 t_, bO I:), s_ a drv,-r

or condenser/separator is unnecess;ary for remowll of moisture I r_if_ t]_t: i,r,,duct

gases.

Technology Demonstration hardware has been produced (Fig. 8) to s,_pp_rl Ila

development of the OGA for Space Station Freedom. The Tect,_¢_logy llem_,vc_,! __1 i_,

hardware is being tested at tile National Aeronautics _nd Space Admini:;tv_,t i_n

(NASA) George C. Marshall Space Flight Center (MSFC) and at 1. ile System_;. _a_,,.

unit is undergoing endurance testing at l.ife Systems, ;_r.d _,n O3/O4/01 it ::,,rp._:-:;cd

2,500 operating hours.

39 ORiGiNAL PAGE IS

OF POOR QUALITY
Life Systems also initiated other tests before the Technology Demonstrator

hardware was available for testing. For example, a module that contains cells

identical to those to be used in the OGA also has been under endurance test, and

more than 10,190 operating hours have been accumulated as of 03/04/91.

Based on this experience and other design data, the OGA is projected to have the

characteristics listed in Table 4. The baseline operating level for the OGA is

production of 02 at a four-person rate (plus 02 to make up for leakage and air

lock repressurization) of 4.12 kg/day (9.09 ibTday). By adjusting the current

density and operating temperature, the OGA can increase production by 73% to a

45-day emergency level of 7.14 kg/day (15.75 ib/day).

Propulsion

The SFE is currently being evaluated for use in the Space Station Freedom's

Propulsion System. In this application 02 and H 2 propellants must be generated

at a pressure of 20,680 kPa (3,000 psia).

A schematic of tlle SFE for propulsion is shown in Fig. 9. The module is contained

within a pressure containment vessel. Inert fluid within the pressure containment

vessel is pressurized with NA to the same pressure as the product gases, so the

module experiences a minimalZinternal-to-external pressure differential. A

pressure intensifier, external to the pressure containment vessel, pumps low

pressure feed water into the water tank to make up for water electrolyzed by the

module. The feed water in the pressure vessel is then pressurized by the inert

fluid. Because of tlle low temperatures to be experienced by the Propulsion

System, the product gases must be dried prior to storage.

The SFE_for propulsion is projected to have circular cells with an active area of

0.046 m z (0.5 ft-). The major objective of the current evaluation is to demonstrate the performance of a 15-celi SFE module having cells of this size

(Fig. ]0), and operating at 20,680 kPa (3,000 psia). The normal operating

pressure of previo, s SFE subsystems is 1,241 kPa (180 psia), although one

subsystem has operated for nearly 1,200 hr at 2,172 kPa (315 psia) in tests at

IAfe Systems and NASA MSFC. Tests of SFE single cells have been performed at

higher pressures. For example, Fig. II illustrates the voltage characteristics of

one of these cel]s, operating at ambient temperature and 20,857 kPa (3,025 psia).

Its performance has been projected to that which would be achieved at a normal

operating temperature of 355 K (180 F).

The chnr_cteristics projected for a prototype-level SFE for propulsion are listed

in Table 5.

EVA

In the EVA app]icstlon, production of Op at 41,369 kPa (6,000 psia) is necessary

to recharge the O,_ bottle in the Extravehicular Mobility Unit (EMU). To achieve

this, Life System_ has modified the original SFE concept to incorporate a solid

metal cathode (SMC) (Fig. 12). The SMC serves as a solid separator between the 02

and H_ compartments, and is capable of safely withstanding very large pressure

Z

differentials, with 41,369 kPa (6,000 paid) actually demonstrated.

40
The SMC is constructed of a proprietary alloy which only allows transmission of

H 2. As a result, the SMC electrolyzer has the unique ability to simultaneously

generate 02 at 41,369 kPa (6,000 psia) and generate ultra-high purity H 2 at low

pressure. These characteristics make the SMC SFE ideally suited for use in the

EVA application.

Feasibility tests were conducted in a high pressure electrolysis cell test vessel

shown in Fig. 13. These tests indicated that it is possible to simultaneously

generate O_ at 41,369 kPa (6,000 psia) and H? at ambient pressure with a current

efficiencyZat 93.7%. Life Systems used a prSprietary electrode catalyst, which

produces a nominal cell voltage of 1.865 V at 358 K (184 F) at a current density

of 47.8 mA/cm_ (44.4 ASF) (Fig. 14). Subsequent testing at NASA Lyndon B. Johnson

Space Center (JSC) has confirmed these results.

A schematic of the SFE for the EVA application is shown in Fig. 15. The schematic

is nearly identical to that for the propulsion application, with the exception

that the H^ gas dryer is not required because the HA product contains no water.

z

Projected characteristics of the SMC SFE for EVA are listed in Table 6.

The SMC SFE also might be the technology of tile future for the energy storage,

propulsion and life support applications. It is potentially highly reliable and

tolerant of hardware failures downstream of the SFE, since it can withstand the

loss pressure in either the 02 or H 2 streams without being damaged by the

resulting pressure differential, up to at least 41,369 kPa (6,000 psid). More

effort is required to assess the SMC SFE for these applications.

Special Applications

Future space habitats, such as the Lunar Base, w_ll have all the applications and

requirements discussed above. In addition, scientific and manufacturing requirements for O^ and HA that will arise aboard Space Station Freedom are likely to be

expanded an_ diversified within these habitats. The development of new emergency

systems (e.g., On for emergency atmosphere repressurization) will probably

introduce additi_na] requirements.

These requirements will doubtless necessitate both small-scale and large-scale

hardware. The SFE has already demonstrated its ability to efficiently accommodate

both needs. For example, evaluations of SFE technology for large-scale H 2 generation applications addressed the capability of the SFE to electrolyze impure water

without costly purification equipment and expendables. In those evaluations [9]

SFE modules electrolyzed Cleveland, Ohio tap water for 1,750 hr without any

pretreatment and no adverse effects. Simulated sea water was also electrolyzed

i,i00 hr, and real sea water was electrolyzed for 1,550 hr without adverse effects

on the module.

The above discussion has shown that the SFE can be used to generate 02 and H^z at a

variety of pressures and production rates. Therefore, nearly all experimentation

and manufacturing needs can be satisfied by the SFE technology. In addition, a

special capability exists within the SMC SFE for the generation of ultra-high

purity, dry Hp and 02 at very high pressures. Special needs can be satisfied with

this technology.

O (GINAL PAGE IS

Poor QUALJTy. 41
Electrolysis Performance Improvement Concepts Studies

An effort is being planned for the testing of three integrated SFE units aboard

the Shuttle Orbiter. The objectives of the Electrolysis Performance Improvement

Concepts Studies (EPICS) is to investigate the ways that a low-g enviro_unent may

improve SFE performance by increasing the uniformity of thermal gradients and mass

transport within the cells.

Each integrated electrolysis unit in the EPICS consists of an electrolysis cell, a

thermal control plate, and accumulators for H 2 and 02 (Fig. 16). Each unit will

contain electrodes with a different catalyst microstructure. By comparing the

performance of each unit, it is expected that knowledge will be gained on how to

optimize the electrodes for a low-g environment, to result in decreased cell

voltage and increased operational current densities (Fig. 17).

Figure 18 shows the EPICS packaging concept, consisting of the three integrated

electrolysis units contained in a pressure control enclosure with external

control/monitor instrumentation•

CONCLUDING REMARKS

Static Feed Water Electrolysis technology has been demonstrated to satisfy the

02 and H 2 generation requirements of energy storage, life support, propulsion, EVA

and other applications. Prototype and preprototype SFE subsystems have shown that

SFE technology efficiently satisfies both small- and large-scale requirements. In

addition to low power consumption, compact dimensions and low weight, these

subsystems have demonstrated the ability to operate using impure water sources.

Without the need for special water purification equipment, gas/liquid separators

and other components, the SFE subsystems have the benefits of simplicity and

reliability. Additional efficiency improvements are envisioned as a result of the

planned 1ow-g Electrolysis Performance Improvement Concepts Studies. Finally,

applications requiring ultra-high pressure On and/or ultra-high purity Hp at low

pressure, such as the extravehicular activit_ application, can be satisfied by the

Solid Metal Cathode Static Feed Electrolyzer. An evaluation of the Solid Metal

Cathode for energy storage, propulsion and life support applications is also

recommended to evaluate how its ability to withstand large O2-to-H 2 pressure

differentials could improve the reliability of those systems.

ACKNOWLEDGMENTS

The authors wish to acknowledge NASA Lewis Research Center, NASA MSFC, NASA JSC,

Boeing, the Department of Energy and Life Systems, Inc. in the development of SFE

subsystems. In addition, the authors wish to acknowledge the assistance of

Dr. B-j Chang and Andrew J. Kovach in the preparation of this paper.

REFERENCES

i • F.H. Schubert, R.A. Wynveen and F.C. Jensen, "Development of an Advanced

Static Feed Water Electrolysis Module," Paper No. 75-ENAs-30, Intersociety

Conference on Environmental Systems; San Francisco, CA; July 21-24, 1975.

42
•

o

o

,

o

•

So

J

F.H. Schubert, M.K. Lee, R.J. Davenport and P.D. Quattrone, "Water

Electrolysis Systems: H 2 and 02 Generation," Paper No. 78-ENAs-03,

Intersoclety Conference on Environmental Systems; San Diego, CA; July 10-13,

1978.

J.T. Larkins and A.J. Kovach, "Static Feed Water Electrolysis System for

Space Station O^Z and H2 Generation," Paper No. 851339, Intersociety

Conference on Environmental Systems; San Francisco, CA; July 16-17, 1985.

P.D. Quattrone, F.H. Schubert and D.B. Heppner, "Air Revitalization System

Integration," Paper No. 840959, Intersociety Conference on Environmental

Systems; San Diego, CA; July 16-19, 1984.

F.A. Fortunato, A.J. Kovach and L.E. Wolfe, "Static Feed Water Electrolysis

System for Space Station Oxygen and Hydrogen Generation," Paper No. 880994,

Intersoclety Conference on Environmental Systems; San Francisco, CA;

July 11-13, 1988.

B.J. Chang, F.II. Schubert, A.J. Kovach and R.A. Wynveen, "Engineering Model

System Study for a Regenerative Fuel Cell," Study Report, Contract

NAS 3-21287, NASA CR-174801, TR-376-30; Life Systems, Inc., Cleveland, OH;

September, 1984.

A.J. Kovach, F.H. Schubert and B.J. Chang, "Endurance Test and Evaluation of

Alkaline Water Electrolysis Cells," Final Report, Contract NAS3-21287,

TR-614-3; Life Systems, Inc., Cleveland, OH; April, 1987.

B.J. Chang, A.J. Kovach, F.H. Schubert and J.T. Larkins, "Water Electrolysis

Subsystem for a Regenerative Fuel Cell System Breadboard for Orbital Energy

Storage," Final Report, Contract NAS9-16659, TR-529-4; Life Systems, Inc.,

Cleveland, Oil; March, 1986.

Life Systems, Inc., "Development of Static Feed Water Electrolysis for Large

Scale Hydrogen Production," Final Report, Contract 522723-S, BNL 51578,

TR-470-4; Life Systems, Inc., Cleveland, OH; July, 1982.

43
TABLE l

Typical application requirements

Gas Delivery O^ Gas

Pressure, Production,

Application kPa (psia) kg/hr (Ib/hr)

ECLSS _a_ 138 (20) 0.17 (0.38)

Energy Storage (Regen. Fuel Cell) _c_ 2,172 (3t5) 18.64 (41.09)

Propulsion _) 20,685/3,000) 0.81 ('I.78)

EVA 02 Bottle Recharge {e) 41,370 (6,000) 0.11 (0.24)

H_ Gas

Proc_uction,

kg/hr (Ib/hr)

0.02 (0.04) (hI

2.35 (5.19)

0.101 (0.22)

(a)

(b)

(c)

(d)

(e)

Based on four-person crew: Metabolic O_ at 0.83 kg (1.84 Ib) O/person-day,

Air lock pressurization at 0.54 kg (1.20 lb) O/day,

Station leakage at 0.24 kg (0.52 Ib) O_/day,

O_ Delivery Pressure 138 kPa (20 psia),

H 2 Delivery Pressure 172 kPa (25 psia).

Required for CO Sreduction.

For 75 kW nominal bus power to user.

Nominal rate of 0.91 kg (2.0 Ib) waterJhr.

Nominal rate of 3.0 kg (6.5 Ib) water/day.

44
CD

(J

cD

o

u

CO _ _ 0 CO

QO (kl tad O_

C_ _@ CW _D

I_ _ . . _ _::D_0 _ . .

C

.m

C

0

ID

I::_0_0 0 0900 0 0

I_ v- O_ 0 tad

I

iii

_D _D

d

Z

0

0 0

0

llt

L) 0 00

u_ _ hL

0 0 OJ

L i I

QO

rCP"0

O0

I I

LO

,@

0

O_

CO

G_

(D

0

h_

h_

I I

(M 0

I_ _--

e_ c_

v- QO

"0

N

3 0 0

0

!

C_

w--

QO

CO

C_

CO

O_ CO

0 O_

c_ o

if)

45
TABLE 3

Projected SFE energy storage elect_olysls subsystem

characterlst_cs for a 91 kW RFCS TM

Number of Cells

Oxygen Generation, kg/day (Ib/day)

Current Density, mA/cm 2(ASF)

Cell Area, m2 (ft2)

Operati'ng Temperature, K (F)

Operating Pressure, kPa (psia)

Subsystem Weight, kg (Ib)

Subsystem Volume,/hi m3 (ft3)

Subsystem Power, t°_kW

107

112.1 (247)

157 (146)

0.09 (1.0)

339 (150)

2,172 (315)

247 (544)

0.3 (10.6)

22.7

(a) Four units are required, for 75 kW nominal bus power to user.

(b) Includes mechanical and electrical subassemblies (cables excluded).

(c) Nominal level for electrolysis only (excludes ancillary components).

TABLE 4

ECLSS subsystem characteristics

Number of Cells

Oxygen Generation, kg/day (lb/day)

Current Density, mA/cm 2(ASF)

Cell Area, m2 (ft2)

Operating Temperature, K (F)

Operating Pressure, kPa (psia)

Subsystem Weight, Ial kg (Ib)

Subsystem Volume, Ibl m3 (ft3)

Subsystem Power,lC_W

20

4.12 (9.08)

124(115)

0.023 (0.25)

339 (150)

1,241 (180)

56.8 (125.0)

0.094 (3.32)

876

(a) Design goal. Current weight is 86.6 kg (190.7 Ib).

(b) Includes mechanical subassembly (electrical subassembly and cables excluded).

(c) Nominal level for electrolysis only (excludes ancillary components).

46
TABLE 5

Projected propulsion electrolysis subsystem characteristics

Number of Cells

Oxygen Generation, kg/day (Ib/day)

Current Density, mA/cm 2 (ASF)

Cell Area, m 2 (ft2)

Operating Temperature, K (F)

Operating Pressure, kPa (psia)

Subsystem Weight, _a,bl kg (Ib)

Subsystem Volume, (a,b) m 3 (ft 3)

Subsystem Power, _c>kW

45

19.4 (42.7)

134 (124)

0.046 (0.5)

356 (180)

20,685 (3,000)

487 (1,072)

0.26(9.1)

4.28

(a) Design goal.

(b) Includes mechanical and electrical subassemblies (gas dryers and cables excluded).

(c) Nominal level for electrolysis only (excludes ancillary components).

TABLE 6

Projected SMC electrolysis subsystem characteristics

for the EVA application

Number of Cells

Oxygen Generation, kg/day (Ib/day)

Current Density, mA/cm 2 (ASF)

Cell Area (Cathode), m 2 (ft 2)

Operating Temperature, K (F)

Operating Pressure, kPa (psia)

Subsystem Weight, kg (Ib)

Subsystem Volume, m 3 (ft 3)

Subsystem Power, W

7

2.6 (5.8)

53.8 (50)

0.058 (0.63)

383 (230)

41,370 (6,000)

272 (600)

0.078(2.74)

537

47
Static Feed

Water Electrolysis

Technology

I I I

Environmental

Control

Life Support

System

Energy

Storage Propulsion

Extravehicular

Activity

Special

Applications

Fig. l. Major SFE technology development applications

I Supply Power

eH2 ._,____------_

Circulating

Feed Water As._-_i_ ''-'1.'

Coolant _i_ii ._

iiii

Electrolyte Iiii

Matrixes_ _ _'

._ H_-

Water Feed

Membrane_

H20-4,_-_. H20

_-_ _ 02

i

__[Anode ( + ):

Cathode (-):4H20 + 4e---,,-

Overall: 2H20

Reactions

4OH-.--._ 02 + 2H20 + 4e2H2 + 4OH2H2 + 02

Fig. 2. Conventional static feed electrolysis process

48
Water

From

Fuel Cell

Circulation

Pump

N 2

N,

Coolant

Heat

Exchanger

Coolant Out

-lectrolysis

Module

Fig. 3. SFE process schematic for the energy storage application

0 2 TO

Jel Cell

H2 To

Fuel Cell

ORIGINAL _ PAGE

BLACK AND WHITE PHOTOGRAPH

Fuel Cell

Water

Fuel Cell Electrolysis Mec

Me Assembly

Assembly

Water Electrolysis

_:: Module

System

Controller

iuii

Fig. 4.

3kW regenerative fuel cell system breadboard

ORIGINAL PA(_E

BLACK AND WHITE PHOTOGRAPH

49
E 40

"" 30

co-_ _.._ 1

E _ 3(

0")o 10

2.6

_> 1.8 O .

_1.6

_ 1.4

_: 1.2

I I I I I I I I I _

00 1 2 3 4 5 6 7 8 9 10

Operating Time, h

Fig. 5. WES cyclic performance versus operating time in RFCS normal mode

Electrolyzer

Module

Fluids Pressure

Controller -_

I

I

Fluids Control

Assembly -_ 4-

Water Stora(

Tank

/

/

'\

Thermal

Control Assembly

=

02 Supply, Ib/hr: 8.50 (at 300 ASF, 1.52V/Ceil, 180 F)

Weight, Ib : 296

Volume, ft 3 : 6.4

Dimensions, ft : 2.9 x 1.3 x 1.7 (L x W x H)

No. of Cells : 46

Fig. 6. Conceptual representation of a large production electrolysis subsystem

5O
Water

Circulation

Pump

Coolant In Coolant Out

Fig. 7. SFE process schematic for the ECLSS application

ORIGIN./_L P,-,_E

BLACK AND WHITE PHOTOGRAPH

....................... J17 _"

Fig. 8. SFE technology demonstration hardware

51 OR[GfNAL PAGE

BLACK AND WHITE PHO!OGRAPH
Hi0h ssure __ -.

i/_TTaantl-_r _ Fluid \

Inert /

I I

i-.. I

Intens_ier l H Circulation _: _////'/'./_/

i ___ )PuHr_ptE_eE I ll]ll I1£" - _ j

x\ q/---I_ L ltllIlIII _J/" I/electr°lysis Module //Pressure Containment

Coolant In _--_C;ool'-ant Out .J Vessel

Fig. 9. SFE process schematic for the

propulsion application

ORIGINAL ""

......................................... SLACK AND WHITE PHOTOGRAPH

Fig. i0. Preprototype SFE propulsion electrolysis module

52
2.6

>

03

,4-,

O

>

2.4

2.2

2.0

1.8

1.6

1.4

1.2

1.0

0.8

0.6

0.4

0.2

0

0

I

f

I

Active Area:

Pressu re:

Temperature:

19.4 cm 2 (3.0 in 2)

20,857 kPa (3,025 psia)

294 K (70 F)

Projected Performance at 355 K (180 F)

I

50

Fig.

I I i I I I

100 150 200 250 300 350

Current Density, ASF

tl. Performance of 3,000 psia circular SFE cell

4OO

Circulating O2"__ e

Feed Water ._- z ..... t

As Coolant _.:," IE Electrolyte & ..... i !

Retention _ -_._ Matrix --,

Water Feed _': ::

Membrane

I Supply Power 1

eI__ H2

'21JAnode (+):

Cathode (-):

i Step 1

Reactions

4OH "-* 02 + 2H20 + 4e

I Step 2 (4Hads) electrolyte side

4H20 + 4e- '-* (4Hads) electrolyte side+ 4OH-

"-* (4Ha_s) gasside

Step 3 (4Hads) gasside'_ 2H2

Overall: 2H20 "-* 2H 2 + 02

Fig. 12. Modified static feed electrolysis with a solid metal cathode

53
Electrical

Hydrogen Ports_ ................. i_ Feed-Through

-:-:-- Oxygen Port

Vessel

Fig. 13. High-pressure solid metal cathode electrolysis test vessel

0

100

90

_, 80

c

_. 70

U.I

c

_ 6O

o

5O

QO

Pressure Differential, KPa

10,000 20,000 30,000 40,000

t 1 I I

m_....O-.---- m

mn_ •

A

___,i__ Conditions

" Current Density, m_SF)

Electrolyte Concentration, wt% KOH

Temperature, K (F)

(a) Testing At NASA JSC

47.8 (44.4)

: 45

• 352(174)

• 356!a_(180)

• 358(184)

I i l I I l J

1000 2000 3000 4000 5000 6000

Pressure Differential, psid

Fig. 14. Current efficiency versus pressure differential

for a high-pressure SMC cell

54
High Pressure N 2

/ ,

_e_

\

Inert Fluid I

I

I

i 02

)q

-- H 2

Pressure 1 Circulation

Q_ Pump

/ Heat I I P_Unrem,-,nt I _ Containment

k E_er / t vessel

--I[-_L -//electr°lysis Module /

\\ _-_ ..... J /

Coolant In _ _ Coolant Out

Intensifier

Fig. 15. SFE process schematic for the EVA application

Electrolysis Cell f- Thermal Control Plate

L i / / I /

_/////////////./// ._/__

N _""-_ 02 Path

- I-

'/ \ _ / _-0 Accumulator

\ H2Accumulator_ / 2

L_ H2 Path v_ N2 Interface

Fig. t6. Cross-section of integrated electrolysis unit

55
(D

{33

,,l-w

0

>

(D

o

Added

Operational

Flexibility

I -_-t

I

I

Existing Current I I

Density Range I I

, =7

Voltage / i

Performance--. _._ I

i

i I

I t

f I

increased Current I I

Density Range ' t

I d

I I

i I

Current Density--_,-

Fig. 17o Projected SFE performance improvements in O-G environment

ORIGINAE pA13I_

BLACK AND WHITE PHOTOGRAPH

Three Integrated

Electrolysis

Pressure Control/--_

Safety Enclosure "_,_ -----

Characteristics

weight, Ib ! 37.6

Volume, ft 1.9

Power, W " 100

Fig. 18,

Standard Middeck

L Modular Locker

Modified Locker Door

EPICS experiment packaging concept

56

i
N91"32554

THE EFFECTS OF PLATINUM ON NICKEL ELECTRODES

IN THE NICKEL HYDROGEN CELL

Albert H. Zimmerman

The Aerospace Corp.

El Segundo, California 90245

_n_rod_cti0n

The performance of the nickel electrode used in nickel hydrogen cells has

been subject to considerable variation by the incorporation of various

additives, dopants, and other variations in the solid-state structure of the

active material in the electrodes. Examples include the use of cobalt

additives to the nickel hydroxide active material, the incorporation of

potassium or lithium (Ref. I) into the gamma-NiOOH charged material, and the

interaction of hydrogen gas itself with the nickel electrode (Refs. 2, 3).

Depending on conditions of cell potential and state of charge, numerous such

reactions are possible that can result in changes to the active material.

This study addresses the possible interactions of platinum and platinum

compounds with the nickel electrode that are possible in the nickel hydrogen

cell, where both the nickel electrode and a platinum catalyst hydrogen

electrode are in intimate contact with the alkaline electrolyte.

Normal operation of the nickel hydrogen cell maintains the nickel electrode

at highly oxidizing potentials, and the hydrogen electrode at quite reducing

potentials. Under these conditions, the only possible path for platinum to

interact with the nickel electrode is by physical transfer of platinum

catalyst particles from the hydrogen electrode, through the separator, to the

nickel electrode. Such transfer, which may occur by "popping" (Ref. 4), can

leave platinum catalyst particles in contact with the nickel electrode. Such

particles of platinum are then oxidized at the nickel electrode to platinum

oxide Pt02, by electrochemical reaction (Ref. 5). Platinum oxide however,

has some solubility in alkaline solutions (Ref. 6), resulting in the

Z

formation of the platinum complex ion Pt(OH) 6 While the platinum is

likely to eventually be re-plated back onto the platinum catalyst hydrogen

electrode by electrochemical reduction of the dissolved platinum species,

until the slow transfer of platinum back to the hydrogen electrode occurs,

both the PtO 2 and Pt(OH)6 = species can be present in the nickel

electrode for possible interactions with the active material.

Another condition that the nickel hydrogen cell may be in for considerable

periods of time is storage, typically discharged and either open circuit or

short circuited. With a hydrogen precharge in the cell, both the nickel and

hydrogen electrodes develop a quite reducing potential during storage,

conditions under which platinum metal is stable at both electrodes. No

reactions are likely in this situation for interaction of platinum with

active nickel electrode material.

With a nickel precharge in a stored nickel hydrogen cell, the hydrogen

electrode rises to the highly oxidizing potential of the nickel electrode as

the hydrogen gas becomes depleted in the cell. Since the partially charged

nickel electrode will maintain a slight oxygen pressure in the cell, the

57
platlnum catalyst electrode becomes an oxygen electrode at about the same

potential as the nickel electrode. At the oxidizing potential maintained by

the nickel electrode the platinum metal in the catalyst electrode is not

thermodynamically stable, and can undergo slow oxidation to PrO 2 .

Furthermore, in alkaline solutions the PrO 2 will not be effective for

passivating the platinum metal because the oxide is slightly soluble, again

giving the Pt(OH) 6 species. This ionic species can then diffuse through

the cell electrolyte, thus making itself available for interactions with the

active material in the nickel electrode.

Thus, there is certainly opportunity for both platinum oxides and soluble

platinum complex ions to interact with the nickel electrode under a variety

of conditions. The question remains, however, as to whether such interactions

are either energetically or klnetlcally feasible under any of these

conditions. Addressing this issue viii be the primary purpose of this study.

_l_tlnum Interactions with Nickel Electrodes -- Data from NiH2 Cells

Data have been obtained from a variety of nickel hydrogen cells supporting

the formation of a material not previously seen in nickel electrodes. This

compound can be identified in tests on electrodes removed from nickel

hydrogen cells by its unique electrochemical signature during the reduction

of residual capacity in the nickel electrode, as indicated in Fig. I. Since

the cells from which these electrodes were removed were totally discharged,

the existence of residual nickel electrode capacity indicates that this new

material (compound x) is only formed in nickel hydrogen cells having a nickel

precharge. The amount of compound x typically formed is in direct proportion

to the amount o£ nickel precharge in the cell, as determined by the amount of

residual charge in the nickel electrodes.

As indicated in Fig. I, compound x may be recognized by its characteristic

reduction at a potential of about 0.15 volts vs. Hg/HgO in 31Z KOH

electrolyte. If the electrode potential is increased immediately after

reduction, it is possible to apparently re-oxldlze at least some of the

compound x at a potential of about 0.30 volts vs. Hg/HgO as indicated in Fig.

2. However, it is clear that i, the reduced form compound x is not stable.

After 24 hr at 0.0 volts, no compound x remains that can be either oxidized

or reduced upon cycling the nickel electrode. This material has apparently

decomposed when left in the reduced state. In contrast, the stability of

compound x in the oxidized state appears to be quite high. Repetitive cycling

of the nickel electrode does not alter compound x, as long as the electrode

voltage is kept high enough so that compound x will not be reduced, i.e.

above about 0_.2 volts vs. Hg/HgO. Compound x seems to form in the nickel

hydrogen cell in material that is never discharged during cell operation.

Thus it is a more stable phase of active material that never undergoes

discharge, and the increase in the amount of compound x tends to consume

whatever nickel precharge that exists in the cell. It should be pointed out

that the presence of compound x in NiH 2 cells is not necessarily a problem,

in fact the ceils containing the largest quantities of compound x were some

of the best performing cells of a given design that have been made.

In nickel hydrogen cells containing approximately 20Z nickel precharge, about

I0_ of the total capacity of the nickel electrode (about 5 mAh/cm 2) was

found to be present in the oxidized form of compound x. Because this seemed

58
to be a quite large amount of modified active material, x-ray diffraction

measurements were done on isolated active material (Ref. 7) in an attempt to

identify the structure of the modified active material. The x-ray digfractlon

patterns, however, indicated no significant differences between active

material from new nickel electrodes and that from the nickel electrodes

containing compound x. Spectrographic chemical analysis of the bulk active

material indicated that the only unexpected contaminant present in

significant quantities was platinum, which comprised about 0.5Z of the total

active material weight.

A more precise chemical analysis of the modified active material was done by

leaching the normal active material from the electrodes containing compound

x, and analyzing the residue left behind. The normal active material was

dissolved in 10Z acetic acid at 70 deg C. The residue left after this

dissolution process was filtered, washed, and dried. Analysis of the residue

was done using EDAX, x-ray diffraction, and spectrographic analysis. The

x-ray diffraction results were of little use, since no well defined

diffraction peaks could be seen except those from small fragments of the

zlrcar separator used in the nickel hydrogen cell. This result indicated that

the compounds making up this residue were essentially amorphous rather than

crystalline. EDAX analysis of the residue revealed that its major

constituents were Ni, Co, Pt, and oxygen. The platinum was not present in the

metallic state as catalyst particles, but was uniformly dispersed throughout

the residue, most likely in the form of an oxide. Every part of the residue

analyzed by EDAX had essentially identical proportions of Ni, Co, Pt, and

oxygen.

The composition of the residue was quantified using semlquantitatlve

spectrographic analysis. The weight percentages of each element found in the

residue are indicated in Table 1. Consistent with the EDAX results, the

primary constituents were Ni, O, Pt, and oxygen, with smaller amounts of

zirconium oxide (from the cell separator), silicates, and iron.

Table I. Elemental Composition of Nickel Electrode Residue

__ Weight percent Atom. percent

Ni 25.2 19.1

Co 27.2 20.6

Pt 25.5 5.8

0 18.8 52.6

Zr 2.5 1.2

Si 0.3 0.5

Fe 0.5 0.6

It is assumed that this residue bears some relationship to the composition of

compound x, suggesting that this compound is some oxide of Ni, Co, or Pt.

Since the amount of charge associated with compound x (from electrochemical

measurements) was about 5Z of the total electrode capacity, the amount of Pt

found in the active material would have to undergo transfer of about 10

electrons per platinum atom if a platinum oxide were to be responsible for

compound x. Clearly this is impossible, indicating that if Pt is involved in

compound x at a11, it is as a binary or ternary oxide with cobalt or nlckel.

59
The redox potentlals observed for compound x provide a signature that is

characteristic of the specific compound undergoing redox reactions, and thus

can be used to help identify compound x, assuming it is not a heretofore

unknown material. Detailed review of redox potentials for oxides of Ni, Co,

and Ft, as well as binary or ternary combinations of these elements, gave

only one compound having redox potentials consistent with those observed for

compound x (Ref. 8). This compound has a nominal formulation of

NiCoO2(OH)2, based on the preparation of this compound in Ref. 8 by

co-precipltatlon and oxidation of mixed nlckel and cobalt hydroxides. This

material is also quite consistent with the residue analysis results of Table

I, where Ni and Co are present in about a I:I ratio. The platinum is likely

to be present as a hlghly dispersed oxide mixed with the NiCoO2(OH) 2.

While the identification of compound x as NiC002(OH) 2 appears to be fully

consistent with the available chemlcal and electrochemical data from modified

nickel electrodes, the detailed mechanism by which this compound forms and

the role, if any, of Pt in this mechanism is not clear. In addition, a

number of details related to the electrochemical performance of the modified

nickel electrodes are not fully consistent with this simple picture of an

isolated NiCoO2(OH) 2 phase somehow being separated from otherwise stable

active materlal. First of a11, if IOZ of the electrode capacity is present as

NICoO2(OH)2, as was observed in extreme cases for electrodes containing

5Z Co additive, then essentlally all the cobalt has separated from the normal

active materlal and has been incorporated into this modified phase. The loss

of all cobalt from the remaining 90Z of the active materlal should result in

both significant increases in redox potentials (Ref. 8) and significant

decreases in electrochemical utilization. Neither of these effects is seen.

As indicated in Fig. 3, very little shift in potentlal for the 90Z "normal"

active material is seen, and if anything the utilization of this material is

significantly increased relatlve to that in an unmodified electrode. The only

significant modified characteristlc for the normal charge/discharge processes

is that the modified active material seems to be much more readily charged

into the gamma-NiOOH phase, which discharges at potentials about 30 my lower

than the beta-NiOOH phase (see Fig. 3). The primary goal of the remainder of

this study is to resolve these apparent inconsistencies, and to understand

the mechanism and impact of NiCoO2(OH) 2 formation in nickel hydrogen

cells.

Mechanism of Nick¢_-Cobalt Oxybydrpx_¢ F@rmation in NiH_ Cells

Before detailed studies related to the mechanism by which nickel electrode

active material is modified to form NiCoO2(OH) 2 in nickel precharged

nickel hydrogen cells could begin, it was necessary to identify laboratory

conditions under which this compound could be produced from normal active

material. Because platinum oxides were always found in modified active

material, it was assumed that platinum species played some role in the

reactions that occurred.

Initial tests simply involved placing a nickel electrode in 38Z KOH with a N£

sheet counter electrode, and adding powdered platinum hydroxide or platinum

oxide to the electrolyte. T_e nickel electrode was then continuously charged

for 2 weeks at 0.2 ma/cm _. After the two weeks the nickel electrode was

removed from this cell, placed in a test cell with 31Z KOH and a reference

electrode, and reduced as in Figs. I and 2. Wlth either platinum hydroxide or

6O
platinum oxide in the electrolyte, no evidence for nickel electrode active

material modification was found. The only significant results were that after

2 weeks with these platinum compounds present in the electrolyte, the nickel

counter electrode was coated with a uniform layer of platinum metal. This

confirms that these platinum compounds dissolve slightly in KOH electrolyte,

and that the Pt(OH) 6 solution species is readily reduced to Pt metal by

electrochemical processes.

The nickel electrode environment during storage of nickel precharged nickel

hydrogen cells was then better simulated by placing a nickel electrode in a

partially charged state in contact with a zircar separator and a platlnum

black catalyst electrode. The atmosphere over this simple cell was set to

contain the ambient 0.2 atm of oxygen gas. All simulated cells were then

wetted with KOH electrolyte and sealed in plastic containers while the

reactions were allowed to occur. Initial simulations used nickel electrodes

charged to 30, 60, 80, and I00_ states of charge in 38Z KOH in these test

cells. After 30 days of stand time in a test cell, each nickel electrode was

reduced as in Figs. 1 and 2. No evidence for modification of the active

material was found for any of these nickel electrodes.

A truly accurate simulation of the storage condition of a nickel precharged

nickel hydrogen cell must simulate the solld-state phase structure of the

active material that is present in the oxidized and reduced materials that

exist when the cell goes into storage. The state of the precharged active

material consists of whatever active material remains undischarged followlng

numerous charge/dlscharge cycles. As such, this material is the most stable,

or lowest potential active material phase present in the nickel electrode. To

simulate this situation, a nickel electrode was cycled I00 times in _8Z KOH

electrolyte. Each cycle involved dis charge of 70_ of the 2_ mAh/cm rated

capacity at I0 ma/cm , followed by recharge at 2 ma/cm to a charge

return of 125Z of the capacity discharged. After I00 cycles the 70Z

discharged electrode was put into a sealed storage condition with a platinum

catalyst electrode wetted with 38Z KOH electrolyte. After 60 days of storage,

the nickel electrode was removed and reduced in 31Z KOH as in Figs. I and 2

to determine whether any modification had occurred to the active material.

The results are indicated in Fig. 4, and clearly show some modified active

material had formed, as recognized by the peak at about 0.15 volts during

electrode reduction.

A repeat of the above test involved cycling a nickel electrode in 31_ KOH

electrolyte I00 times, instead of in 38_ electrolyte. In this case after 4

weeks of storage with a Pt electrode, the somewhat different reduction

behavior indicated in Figure 4 was obtained. In this case a reduction peak is

seen at about 0.22 volts vs. Hg/HgO, suggesting that either the active

material structure that can undergo modification is quite sensitive to KOH

concentration, or that 4 weeks was insufficient time to fully modify the

active material.

Based on these exploratory studies a parametric matrix of 32 nickel

electrodes was prepared as above, by cycling i00 times in 38Z KOH, then being

placed in a sealed storage cell in contact with platinum black electrodes.

For control purposes, 2 electrodes were also stored without a platinum

electrode. The variables that were included in this parametric study

included: (1)storage time, 39 to 157 days; (2)storage temperature, 0, 20, and

40 deg C; (3)cobalt concentration in the active material, O, 5, and 10Z; and

61
(4)electrolyte concentration during storage, 31, 38, and 45Z KOH. After

various storage times, electrodes were removed from the stored cells and

reduced using the same method indicated in Figs. i and 2. Modified active

materlal was detected, when present, by the observation of a reduction peak

near 0.15 volts in 31Z KOH. The quantity of NiCoO2(OH) 2 that was present

in each electrode was evaluated from the number of coulombs of charge

required to fully reduce the NiCoO2(OH) 2 material. To evaluate the effect

of KOH concentration on the redox potential for NiCoO2(OH) 2 reduction,

some electrodes were reduced in test cells containing either 26 or 38Z KOH

electrolyte.

It was expected that there would be some variation in the amount of

NiCoO2(OH) 2 formed as a function of storage time. However, as indicated

in Fig. 5p there was no clear correlatlon with storage time from 39 to 157

days. The amount of this modified phase found in 5_ Co containing electrodes

varied from quite small amounts up to quite high levels, irrespective of the

storage time allowed. It appears that the amount of this material formed

depends on the amount of active material formed in a phase structure that can

be readily converted to NiCoO2(OH)2, an amount that seemed to vary quite

widely for different electrode samples.

Figs. 6 and 7 indicate the relationships between the amount of

NiCoO2(OH) 2 formed in nickel electrodes and the KOH concentration and

temperature during storage. Within the variability in the data, there is no

clear dependence on either of these two variables. Fig. 8 indicates the

dependence of NiCoO2(OH) 2 formation on the amount of cobalt additive in

the active material. As would be expected, this compound is not formed at all

when there is no cobalt present in the active material. The amount of

material that is formed increases sharply as the cobalt level in the active

material goes from 5Z to 10Z.

The dependence of the reduction potentlal of NiCoO2(OH) 2 on

concentration is indicated in Fig. 9, where at each concentration the average

reduction potential for several electrodes was used. The observed dependence

of reduction potentlal in Fig. 9 on KOH concentration clearly indicates that

hydroxide ions are involved in the rate limiting step of the reduction

process. These observations as well as the observation that no

NiCoO2(OH) 2 was formed in the control electrodes after up to 75 days of

storage, allow a hypothesis concerning the mechanism for the formation of

NiCoO2(OH) 2 in nickel electrodes to be developed.

P0_tulated Reaction Mechanism

The fact that NiCoO2(OH) 2 is not found to form in the nickel electrode

except when the electrode shares electrolytic contact with oxidized platinum

metal surfaces, indicates that platinum species are clearly involved in the

reaction mechanism. Because the reaction only requires contact with

electrolyte that is also in contact with platinum metal at oxidizing

potentlals, it is concluded that the Pt(OH)_ = ionic species catalyzes the

formation of NiCoO2(OH) 2 from the oxldized nickel and cobalt

oxyhydroxldes in the solid-state lattice. This solid-state catalytic process

results in the NICoO2(OH) 2 compound because this is a more energetically

stable structure. However, for this catalytic process to occur at a

significant rate, it is necessary that adjacent Ni and Co centers in the

62
oxidized lattice have the proper physical proximity. This is why it is key

that the proper phase structure be formed in the active material before

NiCoO2(OH) 2 can form. The proper phase structure is clearly related to

the gamma-NiOOH structure, although it is not clear that any gamma-NiOOH

structure will engage in this reaction.

A sequence of reactions may be postulated to give NiCoO2(OH) 2 in the

nickel electrode. First, platinum dissolution in an oxidizing environment

produces Pt(OH)6 =

Pt + 02 _ PtO 2 [I]

PtO 2 + 2OH" + 2H20 > Pt(OH) 6 [21

The ionic platinum species can then diffuse to the nickel electrode, where

they can react with adjacent Ni and Co centers in the solid-state lattice.

This reaction is facilitated by the adjacent Ni and Co centers having the

necessary geometry to form a complex with the octahedral platinum ions,

according to reaction [3].

NiOOH + CoOOH + Pt(OH)6 = Pt(OH)6(NiO)(CoO) + 2OH- [3]

This platinum complex is in equilibrium with the ionic platinum species in

solution. Therefore if the platinum is plated back onto the hydrogen

electrode when a nickel hydrogen cell is brought from storage back into

normal operation (or for these test cells, when the nickel electrode is put

into a platinum free solution), the platinum complex formed in reaction [3]

will fall apart, leaving the adjacent Ni and Co species associated in a new

and more stable compound, NiCoO2(OH)2.

Pt(OH)6(NiO)(CoO) + 2OH" _I NiCoO2(OH) 2 + Pt(OH)6 = [4]

These compounds are structurally illustrated in Fig. I0.

Electrochemical reduction of NiCoO2(OH) 2 must at least convert the nickel

center to a divalent oxidation state. It is not fully clear whether the

cobalt remains in a trivalent oxidation state. However, in any case it is

clear that the when these metal species are converted into a divalent

oxidation state, the hydroxide structure becomes the most stable and they

gradually revert back to a configuration much like their original structure

in the solid-state lattice. Thus there is no irreversible degradation in the

active material lattice as a result of these reactions, although the

resultant structure does appear to make the lattice more easily oxidized to a

gamma-phase active material. The improvement in electrode capacity associated

with these reactions is likely to arise from the increased formation of

gamma-phase charged material (which has a higher oxidation state) as well as

from the high electrical conductivity of the NICoO2(OH)2 that is

dispersed throughout the active material.

63
C0nclusion_

This study suggests that platinum complex ions that can form in nickel

precharged nickel hydrogen cells under some conditions, can migrate to the

nickel electrode, where they are capable of interacting with adjacent nickel

and cobalt lattice sites that have an appropriate structural configuration.

The appropriate lattice configuration appears to require that the active

material be in a gamma-phase structure, and may also require a sufficiently

large interlayer spacing for platinum ions to penetrate into the active

material structure. The appropriate lattlce_configuration may also involve

unique interlayer cobalt sites that have been suggested in recent EXAFS

studies (Ref. 9). The platinum complex ions appear to make it energetically

possible for adjacent nickel and cobalt centers in the lattice to

reconfigure, thus catalyzing the formation of the compound NiCoO2(OH)2

within the lattice. This compound has well defined redox voltage signatures

that allow it to be readily identified in nickel electrodes.

The effects of these reactions on nickel hydrogen cell performance appear to

be beneficial, based on all data presently available. Capacity is improved by

making the gamma-phase active material more easily formed during cell

recharge, and utilization is improved by the added conductivity imparted by

the electrochemically active and highly dispersed NiCoO2(OH) 2. While the

effects of these changes on ultimate cell cycle life are not yet fully

established, limited life testing of nickel precharged cells that have also

experienced some storage have generally given good performance. More

intensive life testing in the future will address this issue more fully, as

well as the issue of what state these unique compounds eventually end up in

late in cell life, when corrosion processes have destroyed the initial nickel

precharge in the cell.

References

I. R. Barnard, C. F. Randell, and F. L. Tye, J. Applied Electrochem. I__0_0

(1980) 109; J. Appl. Electrochem. i__!I(1981) 517.

2. A. H. Zimmerman,

Zimmerman, eds., The

1990, p. 311.

Nickel Hydroxide Electrodes, D.A. Corrigan and A.H.

Electrochemical Society, Inc., Proceedings Vol. 90-4,

3. A. H. Zimmerman and R. Seaver, J. Electrochem. Soc. 137 (1990) 2662.

4. "Popping" is explosive reaction between bubbles of oxygen gas and hydrogen

gas at the platinum catalyst surface of the hydrogen electrode, and can occur

when a nickel hydrogen cell is in overcharge if there is sufficient

electrolyte in the separator to enable oxygen to gather into bubbles.

5. M. S. Antelman, The Encyclopedia of Chemical Electrode Potentials, Plenum

Press, New York, 1982.

6. M. Pourbaix, Atlas of Electrochemical Equilibria in Aqueous Solutions,

National Assoc. Corr. Engineers, Houston, Texas, 1974, p. 379.

7. Active material was isolated from the sinter by grinding it into a fine

slurry in DI water, then magnetically separating the nickel metal particles

64
from the active material in water solution. The active material was then

filtered from the solution, dried, and used for further chemical analysis.

8. A. H. Zimmerman, Extended Abstracts of the Fall Meeting of the

Electrochemical Society, 142 1987) 206.

9. D. A. Corrigan, T. W. Capehart, K. I. Pandya, and R. W. Hoffman, The

Nickel Hy4roxide Electrode, D. A. Corrlgan and A. H. Zimmerman eds., The

Electrochemical Society, Inc., Proceedings Vol. 90-4, 1990, p. 97.

65
0.3-

o 0.25-

>

'o

"- 0.2-

0

"D

m

,- 0.15-

a

_ 0.1

tO

-I

-o 0.05

rr

-0.4 -0.3 -0.2 -(_.1 -(),0 011 012 0.3

Voltage vs. Hg/HgO

(--) From Ni precharged

cell

(---_ New Ni electrode

KOH = 31% by weight

Scan rate = 0.002 mV/sec

Figure i. Electrochemically reducible state density vs. potential for nickel

electrodes. Total charge in the peak near 0.15 volts is about 5 mAh/cm 2.

0.25

-0.05 0.10 0.' 5 0. 0 0. 5 0. 0 0.35

Voltage vs. HglHgO

(--) Initial reduction

and re-oxidation

(.... ) After full redox

cycle

KOH = 31% by weight

Scan rate = 0.002 mV/sec

Figure 2. Electrochemically active state density vs. potential for electrodes

from a Ni precharged NiH 2 cell, before and after reduction and re-oxidation

66
45

°'°.20 025 0. 0 0. ,0

Voltage vs. Hg/HgO

(_) FrON Ni precharged

cell

( .... ) New Ni electrode

KOH = 31% by weight

Scan rate = 0.002 mV/sec

Init. oxidation was to

0.Sv at 0.002 mV/sec

_45

Figure 3. Density of reducible states for normal discharge of the capacity

in e new nickel electrode and an electrode from a Ni precharged NiH 2 cell.

0.03-

0

O_ 0.02

>

"o

0

"o 0.02-

ID

Q

m 0.01.

O9

0

"_ 0.01

n0"00.10 0.12 0.14 0.16 0.18 0.20 0.22 0.24

Voltage vs. Hg/HgO

(--) After 100 cycles in

38% KOH

(.... ) After I00 cycles in

31% KOH (x0.1)

( ..... ) Electrode of Fig. I

after 2 yr storage

KOH = 31% by weight

Scan rate = 0.002 mV/sec

Figure 4. Density of reducible states for storage of nickel electrodes with

Pt electrodes after several different electrode preparation methods.

67
0.25

O,I

E 0.20-

0

n

E 0.15-

X

"O

1-

_ 0.10-

E

8

"6

.c 0.05

E2J

D

[]

[]

DKI []

O

C1

D

[]

C]

C]

[]

[]

[21 t7 C1 T [3 °'°°0 4'o 0'0 8'0 100 140 100

Days in Storage at 20 deg C

Temperature = 20 deg C

KOH = 38% by weight

during storage

Cobalt = 5%

Figure 5. Amount of modified active material formed in nickel electrodes from

simulated cell storage as a function of storage time.

0.16-

e4 0.14- 4c

4_

E

U

0.12-

0..

"lD

_. 0.10-

0

LL

x 0.08

"{3

C

o 0.06- 0..

E

0

o 0.04.

v,..

O

r

z_ 0.02-

E3

LJ

E3

EJ

13

I-]

El

0"000 32 3'4 3'6 3_8 4'0 4'2 4'4 46

Wt% KOH During Storage

Temperature = 20 deg C

Cobalt = 5%

Figure 6. Amount of modified active material formed in nickel electrodes from

simulated cell storage as a function of KOH concentration during storage.

68
0.30-

"k_' 0.25-

E

-o 0.20-

× 0.15-

r'-

:D

O

O.

E 0.10-

O

O

0.05-

LJ

L_

[11

[J

E]

I ]

I)

0.00 -5 6 _ 1'0 1_ 2'0 2'5 3'0 _5 4'0 45

Storage Temperature (deg C)

KOH = 38% by weight

during storage

Cobalt = 5%

Figure 7. Amount of modified active material formed in nickel electrodes from

simulated cell storage as a function of storage temperature.

3.0-

c4

2.5

C_

2.0

E

o

x 1.5-

C

O

OE 1.0-

O

o

"6

t'-m 0.5-

0.0 1

0 _ 4 _ 8 1_ 12

Percent of Cobalt Additive

Total electrode capacity

was about 30 mAh/cm 2

Figure 8. Average amount of modified active material formed in nickel

electrodes during cell storage as a function of amount of cobalt additive.

69
0.18

©

O_

"1-

03

3-

05

>

°_

¢-

O

13..

cO

o

-O

(D

rr"

0.17-

0.16-

0.15-

0.14-

0.13-

0.12-

0.11

0.16 I I I I I I I

24 26 28 30 32 34 36 38

Wt% of KOH for Reduction

4O

Figure 9. Reduction potential at 20 deg C for the modified active material

phase formed during cell storage as a function of KOH concentration. These

measurements were made in a cell flooded with KOH electrolyte, with the same

electrolyte concentration in the reference electrode compartment as in the

cell. The voltage was scanned at 0.002 mV/sec, which was slow enough that the

peak reduction potential was independent of scan rate.

7O
Pt (OH) 6 (COO) (NiO) Complex

F] Fl

[7 E] Fl [I

F1 Fl [] Fl

[] II 11

I i i i i

i r

NiCoO 2 (OH) 2 Compound ( I_----u )

• D H n)

0 [] []

n

[7 [] t)

i I i i i i

Figure I0. Illustrations of likely configurations for platinum-nickel-cobalt

oxyhydroxide and nickel-cobalt oxyhydroxide materials in relation to the

active •aterial lattice.

Ni sites = O

Co sites = g

71

N91"32555

PHYSICAL AND CHEMICAL ANALYSIS OF A Ni/H 2 CELL*

H. Vaidyanathan,** M.W. Earl, and T.D. Kirkendall

Comsat Laboratories

Clarksburg, Maryland 20871-9475

Abstract

A cycled aerospace nickel-hydrogen (Ni/H z) cell was subjected to destructive physical analysis to determine the reason

for a capacity loss after 5,967 cycles at 60-percent depth of discharge. The positive plates in the cell were analyzed in terms

of chemical composition, active material utilization, charge efficiency, and thickness increase. The microstructure of a cross

section of the positive plate was determined by backscattered electron image analysis. The results suggest that the capacity

loss in the cell is caused by low charge acceptance and low active material utilization at the positive plate. The oxidized nickel

species content of the positive plate increased due to corrosion of the nickel sintered skeleton. This appears to circumvent the

orderly reacdon of the active material. Microstructural analysis has indicated that a new phase of active material is formed

with cycling.

INTRODUCTION

Tear-down analysis of an aerospace rechargeable cell is normally carried out for purposes such as preflight evaluation

of design and manufacturing techniques, tracking the degradation of components in a life test experiment, and determining

the reason for failure. The major elements of such an analysis include physical measurements at the cell level, as well as

chemical, microstructural, and specialized examination of cell components. Some of the procedures used to analyze the cell

are an extension of those used for aerospace nickel-cadmium (Ni/Cd) cells and are documented in the literature [1],[2].

Since nickel-hydrogen 0NiAt2) cells last longer and cycle longer than any other battery, there is tremendous interest in

understanding their gradual degradation with cycling and their failure mechanisms. Premature failure of Ni/H 2 cells in a cycling

experiment has been reported, and in one instance the failure was attributed to the rapid reaction of oxygen with hydrogen

[3]. In the study described here, a flight-model Ni/H 2 cell which was cycled in a simulated low earth orbit (LEO) regime at

4-percent overcharge and 60-percent depth of discharge (DOD) for 5,967 cycles was analyzed to determine the reason for

capacity decline.

ELECTROCHEMICAL PERFORMANCE

The subject cell is a COMSATflNTELSAT design which has asbestos separators in a back-to-back stack configuration.

The cell was withdrawn from a LEO cycling test when it's end-of-discharge voltage decreased below 1.0 V. Details and

pertinent observations made during the LEO test are given in Ref. 4. As a first step, cell capacity was determined using a

charge rate of C/10 and a discharge rate of C/2 at 10°C. Fig. 1 compares the voltag_ profile of the cell at the beginning of

the cycling and after 5,967 cycles. As a result of cycling, the cell exhibited markedly lower discharge capacity, lower discharge

voltages, and higher charge voltages. It is apparent from the capacity decay curve presented in Fig. 2 that cell capacity declined

gradually after about 3,000 cycles. The cell was subjected to open-circuit stand in the fully charged condition at 10°C. More

than 79 percent of the cell capacity was retained after 72 hr of stand, which is comparable to the value exhibited at the

beginning of cycling. Thus, the capacity decay in the cell cannot be attributed to soft shorts.

CELL DISSECTION

The cell was dissected, and the stack was transferred to a Soxhlet extractor. After extraction of the electrolyte, the stack

was dried and disassembled. Examination of cell mechanical components such as the seals, end plates, center rod, inside can

wall, weld ring, and bus bars did not reveal any damage due to cycling. Evaluation of the anodes, cathodes, and separator

from the cell revealed the following:

*This paper is based on work performed at COMSAT Laboratories under the stxmsorship of the World Systems Division of the Communications

Satellite Corporation.

**Please address any correspondence to Dr. Vaidyanathan at COMSAT Laboratories, 22300 Comsat Drive, Clarksburg, MD 20871-9475.

73
• Sticking of the separator to the positive and negative plates; however, no fluidization of asbestos or diffusion of

asbestos particles through either electrode was observed.

• Melted areas comprising 30 percent of the polypropylene gas distribution screen.

• Bum holes in the negative plates, and several cracks and delamination in the Gore-Tex TM hacking.

• A high degree of extrusion of active material in the positive plates.

The sticking of the separator probably occurred as a result of compression in the stack. The burn holes and melted areas

indicate abnormal heat generation. Delamination of the Gore-Tex" probably occurred as a result of a rapid oxygen and

hydrogen reaction.

ANALYSIS OF POSITIVE PLATES

The positive plates were based on slurry plaque impregnated with Ni(OH) 2 and Co(OH) 2 using an aqueous electrochemical technique. They were analyzed to determine changes in their physical, chemical, and microstructural properties, as

described below.

Swelling

The thickness of the plates was measured and compared against values obtained for similar uncycled plates. Swelling

was calculated to be 10.5 percent. One important effect of swelling is compression in the stack, which leads to squeezing of

the separator, thereby reducing the amount of electrolyte at the interface.

Electrolyte Absorbency

The amount of 30-percent KOH absorbed by a plate was determined using an immersion technique. A value of

0.20 gm of KOH per gram of plate was obtained, which was 23-percent higher than the absorbency of an uncycled plate. It

was concluded that swelling of the positive plate leads to an increased number of voids in the positive plates, which capture

electrolyte from the separator. In other words, the electrolyte in the slack components is redistributed in the cell.

Chemical Composition and Active Material

The positive plate was quantitatively analyzed, first by extracting the active material with acetic acid, and later by

dissolving in HNO 3. The percentage of nickel and cobalt were then determined using atomic absorption spectroscopy. The

amount of oxidized nickel species such as active Ni(OH)2 was 22 percent higher in the cycled plate than in an uncycled

positive plate, while the metallic nickel content due to the plaque structure had decreased by approximately a stoichiometrically

equivalent amount. This was direct evidence of corrosion of the sintered plaque skeleton.

The porous sintered plaque void volume was calculated based on analytical data and thickness, yielding a value 25

percent higher than the void volume of the sintered plaque that was used prior to electrochemical impregnation. This value

is in agreement with the increased plate electrolyte absorption discussed earlier. The loading of the active material [Co(OH) 2

and Ni(OH) 2] was 1.57 gm/cm 3 of void--a value close to that for the uncycled plate. Therefore, the increase in void volume

due to corrosion and swelling was offset by the increased amount of oxidized nickel species resident in the pores.

ELECTROCIIEMICAL PROPERTIES

The capacity of the plate in 30-percent KOH was determined at 10°C to be 1.30 Ah, which is 15-percent less than the

minimum capacity obtained for plates of similar design. When this value is factored into Faraday's law, a value of 81.6 percent

is obtained for oxidized nickel species utilization, which is lower than the 115 percent obtained for an uncyclcd plate. The

reported utilization in excess of 100 percent is due to the difference between the assumed valency change of 1 and the actual

valency change, which is between 1 and 2. Even though the mass of the oxidized nickel species has increased by 22 percent,

it is clear that this material is not available electrochemically. On the contrary, it appears that the presence of this corrosion

product and the loss of conductive metallic nickel hinders the charge transfer reaction of the original active material. The

charge efficiency of the plate was determined by charging at C/10 to a 50-percent state of charge, and capacity was determined

by discharging at C/'2. The charge efficiency was 81 percent, as compared to 100 percent for an uncycled plate.

74
MICROSTRUCTURAL ANALYSIS

An image analysis technique based on computer analysis of digitized backscattered electron (BSE) images generated in

a scanning electron microscope was employed to study the cross section of the positive plate [5]. Samples were prepared for

analysis by encapsulating positive plate sections in epoxy and then cross-sectioning them using metallographic polishing

techniques. In the BSE micrograph of a positive plate cross section (Fig. 3), the gray level is directly related to the local

composition of the plate. White areas indicate nickel sinter, gray areas indicate active material, and black areas indicate voids

(which were filled with epoxy in the sample preparation step). The most remarkable features of this micrograph are the

segregation of active material near the surface, the presence of voids, an increase in thickness, and a blistered region close

to the surface.

The active material component can be separated from the sinter and voids and displayed separately by computer

selection of the gray level in the digitized BSE image. Fig. 4 is a histogram of the gray levels of the 65,536 pixels displayed

in the BSE image of Fig. 3. The three peaks correspond to voids, active material, and sinter particles. By dividing the broad

central peak obtained for the active material into separate bins of varying gray level, digitized images of the active material

distribution can be obtained. When this selectivity was applied, it was discovered that the active material comprises a range

of gray levels in the BSE signal.

Figs. 5 and 6 were reconstructed from the BSE image of Fig. 3, using the above selection technique. The distribution

of the lighter-appearing active material associated with bin 1 of Fig. 4 is'shown in Fig. 5, and the darker active material

corresponding to bin 2 is shown Fig. 6. The lighter active material is fairly evenly distributed across the plate, except for the

blistered region near the edge. This region contains primarily dark active material. Because the scale in the BSE image is a

nearly monotonic function of the average atomic number of the local material composition, this variation in the gray level of

the active material is interpreted to mean that the blistered region contains active material of a different structure. The

difference between this structure and the generally observed active material may bc due to the H20 and KOt{ content of Ni(OH) 2.

This chemical change and redistribution of active material has not been observed in uncycled or low-cycled plates.

CYCLIC VOLTAMETRY

The voltage profile of a miniature (1-cm 2) positive plate was obtained using cyclic voltametry. The voltage of the test

electrode was scanned from -0.5 to +0.7 V (vs a Hg/HgO reference electrode) at a rate of 0.1 mV/s in 30-percent KOH.

Fig. 7 shows two voltage profiles. Curve A corresponds to an uncycled positive plate, and curve B to a plate from the cycled

cell. The uncycled plates exhibited the following features: an anodic peak at +0.557 V corresponding to Ni(OH) 2 oxidation,

another anodic peak at +0.70 V corresponding to oxygen evolution, and a cathodic peak at +0.183 V corresponding to the

reduction of the oxidized active material. The voltage profile of the cycled plate also contains a second cathodic peak which

is very broad at -0.120 V. The peaks associated with active material oxidation and reduction are polarized by 19 and 36 mV

(respectively) for the cycled plate, compared to the uncycled plate. The second cathodic peak could be due to the reduction

of a new phase of charged active material which reacts at a much lower cathodic potential. This may be the phase of active

material identified in the BSE image analysis.

The results of the positive plate analysis are summarized in Table 1.

ANALYSIS OF ELECTROLYTE

The electrolyte obtained from the Soxhlet extraction was analyzed for KOH and carbonate. The alkali content (OH,

HCO3. and COl 2) was 41.31 percent, of which 3 percent was carbonate. The electrolyte content translates into 2.92 cm3/Ah.

There were no abnormalities in the carbonate content or in the normalized volume per ampere-hour of theoretical capacity.

DISCUSSION

The objective of this study was to explain the decline in capacity and average discharge voltage of a Ni/H 2 cell after

5,967 cycles. In addition to close examination of the electrode stack, a range of experiments were conducted to probe the

positive plate. The results show that the sintered plaque has corroded, producing additional oxidized nickel species which do

not contribute to usable capacity. Also, the plate has swelled, the active material is extruding, and electrolyte absorbency has

increased. Digitized BSE analysis was used to obtain chemical information, based on the fact that the yield in number of

electrons backscattered from a material increases with the average atomic number. BSE image analysis shows two types of

75
active material. This observation, in combination with the second cathodic peak obtained in the cyclic voltametry, suggests

that a new phase of active material is produced with cycling which contains more water, hydroxyl, or potassium. The lower

charge efficiency and lower active utilization can be attributed to a combination of factors, including the formation of a new

phase, as well as slightly lower electrolyte content in the separator as a result of squeezing. The lower charge efficiency has

resulted in the production of more oxygen, which is given off in streams and reacts with hydrogen, producing "pops"

(microexplosions) that damage the negative electrode backing layer.

CONCLUSIONS

A systematic and regular analysis of a Ni/H 2 cell which exhibited gradual degradation in performance for 5,967 cycles

has shown that the origin of the failure resides in the positive plates. The conspicuous signs of degradation, such as lower

charge efficiency and lower active material utilization of the positive plate, are explained by hypothesizing the formation of

a new structure which contains more hydroxyl and potassium. The oxidized nickel species have increased in the positive plate,

and appear not only to be less active electrochemically, but also to impede the activity of otherwise good Ni(OH) 2.

ACKNOWLEDGMENTS

The authors wish to thank the following colleagues for their contributions: B. Shaw for BSE analysis, K. Butch for

chemical analysis, and W. Nakhleh for cyclic voltametric experiments.

REFERENCES

I. J.D. Dunlop and M. Earl, "Evaluation of INTELSAT IV Ni/Cd Cells," 25th Power Sources Symposium, PSC Publications

Committee, Red Bank, New Jersey, 1972, Proc., pp. 40-42.

2. G. Halpert and V. Kunnigahalli, "Procedures for Analysis of Ni/Cd Cell Materials," NASA Publication X-711-74-279,

NASA-God "dard Space Flight Center, Greenbelt, Maryland, 1980.

3. K. H. Fuhr, "Failure Analysis of 3.5 inch, 50 Ah Nickel-Hydrogen Cells Undergoing Low Earth Orbit Testing," 22nd

Intersociety Energy Conversion Engineering Conference, Philadelphia, August 1987, Proc., pp. 889-892. Available from

AIAA, New York.

4. H. Vaidyanathan, "Effect of Design Variables on the Cycling Characteristics of Ni/H 2 Cells," 24th lntersociety Energy

Conversion Engineering Conference, Washington, D.C., August 1989, Proc., pp. 1405-1409. Available from IEEE, New

York.

5. T.P. Remmel and T. D. Kirkendall, "Quantitative Analysis of Positive Battery Plates by Means of BSE Image Analysis,"

Microbeam Analysis, A. D. Romig and J. I. Goldstein, eds., San Francisco Press, 1984, pp. 165-168.

Table 1. Changes in Positive Plate From the Cycled Cell

Item Uncycled Plate Cycled Plate

Swelling

Electrolyte Absorbency (gin KOH/gm)

Plaque Porosity (%)

Charge Efficiency (%)

Active Material utilization (%)

BSE Image Analysis

Cyclic Voltametry

Initial thickness (stable) 10.5% increase in thickness

0.162 0.20

78 80.7

100 81

115 81.6

Gray active material Gray and dark active material

No second cathodic peak Second cathodic peak at -0.120 V

76
1.6

1.4

1.2

_" 1.0

_ 0.8

_ 0.6

O.I

0.2

0.0 I ! 1 I I I I I I I._ I

0 8 16 24 32 40 48

CAPACITY (Ah)

i I I I I I I I i i

8 16 24 32 40

Fig. 1. Chargedischarge voltage profiles for the cell before

and after cycling

40

35

o 30

o

2O

0

I I n I , I I I n I I

1,000 2,000 3,000 4,000 5,000 6,000

ORDINAL NUMBER OF CYCLES

Fig. 2. Variation of capacity (1.0 V) with cycling

(discharge rate C/2)

77
E0=20k V

Fig. 3. Digitized BSE image of a cross-sectioned positive plate

9

LU

X

0..

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GRAY-SCALE

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LIGHT

Fig. 4. ttistogram of gray levels in the BSE image of the cross-sectioned positive plate

shown in Fig. 3

78
Fig. 5. Active material distribution from bin 1 of Fig. 4

Fig. 6. Lower atomic number active material from bin 2 of Fig. 4

79
0.4

A 0.1

I-'--

z

t_ 0

I-- • I ' I ' I ' I ' I

PLATE FROM

UNCYCLED //'/2_--/. jf_,

"_ il l l

J _

-t?.1-- _

-0,2

-0.3 f

-0.4

-0.5

....... 1 i I = 1 = I 1 I l

-0.4 -0.3 -0.2 -0.1 0 0,1 0,2 0.3 0.4 0.5

VOLTAGE (V) vs Hg/HgO

Fig. 7. Voltage profiles of cycled and uncycled positive plates

(generated using cyclic voltarnetry)

1

0.6 0.7

_=

80
N91-32556

RAMAN SPECTRAL OBSERVATION OF A "NEW PHASE" OBSERVED IN NICKEL

ELECTRODES CYCLED TO FAILURE

P.L. Loyselle,* X.Shan, and 8.C. Cornilsen**

Michigan Technological University

Houghton, Michigan 49931

and

M.A. Reid

NASA Lewis Research Center

Cleveland, Ohio 44135

A "new phase" is reported in nickel electrodes from Ni/H 2 boilerplate

cells which were cycled to failure in electrolyte of variable KOH

concentration (21-36%). Raman spectra clearly show the presence of

this phase, and these spectra have been used to estimate the amounts

present on these electrodes. Ten of 12 electrodes examined contain

this new phase. The cycle life at higher KOH concentrations (31 and

36%) was greatly reduced, and nickel electrodes from these cells

exhibited extensive amounts of this new phase. The presence of this

"new phase" correlates with cell failure defined by low end of

discharge voltages. It is proposed that the lowered capacity and

failure of these electrodes was caused by loss of active mass and

formation of a phase with reduced electrochemical activity. These

results indicate that formation of this new phase is accelerated at

higher KOH concentrations.

INTRODUCTION

Ni/H 2 boilerplate cells containing 26% KOH displayed the longest

cycle life in a recent study by Lim and Verzwyvelt [i]. Electrodes

were cycled to failure in electrolyte of variable KOH concentration

(21-36% KOH), at 80% depth of discharge in an accelerated cycle regime

(6.24 Ah nominal capacity). The cycle life of the 26% KOH cells was

nine times greater than the cycle life of the conventional 31% KOH

cells.

The Raman spectra of electrodes from these cells have been

studied to characterize the active mass structure, as well as any

other phases that might be present. Observation of a second phase is

important because such a phase can be detrimental, adversely

influencing cycle life and contributing to failure. Raman spectra

provide a signature which is characteristic of each solid phase (or

compound) present [2]. Furthermore, each spectrum carries structural

information about each phase.

* Current Address: NASA, Lewis Research Center, Cleveland.

** Author to whom correspondence should be addressed.

81
In previous work, it has been shown that Raman spectra can be

used to characterize the NiO2 layer stacking in nickel hydroxides and

oxyhydroxides as close packed (ABAB...) or non-close packed

(ABBCCA...) [2,3]. E-nickel hydroxides are close packed. Active

mass, charged or discharged, is non-close packed. X-ray powder

diffraction patterns and EXAFS spectra have been successfully

interpreted using a non-close packed layer structure, supporting this

structural conclusion [3,4]. Raman spectroscopy also allows

structural differentiation of precursor materials and "formed" active

mass [2]. The precursor structure with cobalt differs from the

precursor structure without cobalt.

EXPERIMENTAL

Raman spectra discussed herein are ex situ scans of electrodes

which were removed from the cells in the discharged state. Spectra

have been collected for both the front (non-screen side) and the back

(screen side) of sectors taken from these electrodes. Two electrodes

were studied from each cell (positions i, top, and 6, bottom of cell

stack). The electrode ID numbers, KOH concentrations, and cycle lives

are given in Table 1 [i]. The 26% KOH cell containing electrodes 21-

05 and 03-02 was removed from cycling before failure. A low end of

discharge voltage (EODV) accounted for failure of the 31 and 36% KOH

cells. The remaining cells (_26% KOH) failed by a soft short.

The Raman instrumentation and scan conditions have been

previously described [2]. The compositions (weight percent "new

phase") have been estimated from the relative peak intensities (I553

vs. I470 ratio), using a technique developed previously for

quantitative analysis of coal dust - diesel particulate mixtures [5].

Material at or near the surface (in the volume illuminated and sampled

by the laser beam) is measured. Material that is deeper within the

electrode is not sampled. The compositions for the volumes sampled

are believed to be representative, as an approximate measure of the

amount of new phase.

RESULTS AND DISCUSSION

A Raman spectrum was scanned of each face of the 12 electrodes.

Most spectra exhibit the expected doublet spectrum of discharged

active mass (Fig. la) [2]. A four-peak spectral signature, which has

not previously been documented, is observed in spectra scanned on 12

of 24 electrode faces (Table i). This spectrum is characteristic of

the presence of an additional phase, a phase not previously reported.

A spectrum of an electrode displaying a predominant amount of this new

phase is shown in Fig. lb. This "new phase" shall be referred to

hereafter as "phase-X."

Ten of the twelve electrodes studied contain phase-X (Table I).

Two of these show phase,X on both sides. In general, the front side

exhibited the larger amount of phase-X. The front side faces the KOH

and hydrogen electrode. The fact that the unused electrode (#08-09)

showed no phase-X suggests that phase-X is produGed during cycling in

KOH electrolyte.

82
To facilitate electrode comparison, average compositions are

given in Table i, averaging over eight sides (for four electrodes) and

over four sides (for two electrodes). The 31% and 36% KOH electrodes

contain extensive amounts of phase-X on the average (56% average, over

8 electrode sides), and on both the front and back sides. By

comparison, the 26% KOH electrodes display both a lower average (29%)

and lower absolute percentages of phase-X.

First, the 26% KOH samples will be compared with the higher

percent KOH samples (31% and 36% KOH). The high-KOH samples exhibited

lower cycle life and more phase-X. The failure mode for these 4 highKOH electrodes was a low end of discharge voltage (EODV) at low cycle

life (<4000 cycles using either the 0.9 or 0.5 V EODV criterion) [i].

The observation that these electrodes display the most phase-X

suggests that the phase-X may be instrumental in reducing cycle life,

perhaps by inducing the low EODV. Also, a KOH concentration

dependence is implied, with the 31% and 36% KOH concentrations

favoring phase-X formation.

Phase-X is displayed on only one side, the front, of the lower

KOH electrodes (_ 26% KOH). These electrodes all cycled to greater

than 28,000 cycles with the 0.5 EODV criterion [i]. Except for the

cell removed before failure, they all failed due to a soft short.

When phase-X is not formed on both sides of the electrode, it appears

to have a less detrimental effect on cycle life, and a low EODV is not

induced. This observation suggests phase-X is first formed on the

front of the electrode. If formation begins on the front side and

propagates through to the back side, observation on both sides

indicates that much more phase-X is present, throughout the electrode.

Therefore, observation of phase-X on both sides of the 31 and 36% KOH

electrodes is indicative of a much more extensive formation of phaseX. This is consistent with the greatly reduced cycle lives.

A KOH concentration dependence is implied, with higher KOH

concentrations favoring phase-X formation. The 26% KOH electrodes all

have a lower amount of phase-X than the 31% and 36% KOH electrodes.

The 21% KOH electrodes exhibit no phase-X on either side. The 23.5%

KOH electrodes appear to break this trend. The 23.5% KOH electrodes

display more phase-X on the front side than the 26% KOH electrodes.

However, the semi-quantitative nature of these values suggests these

numbers may not differ significantly. That is, the 46% phase-X value

(average of 4) for the 23.5% KOH is not that much greater than the 33%

phase-X value for the first 26% KOH cell (and is less than the 56%

phase-X value (average of 8) for the 31 and 36% KOH cells). This is

especially true in light of the fact that the phase-X is only observed

on one side for the 23.5 and 26% KOH electrodes, and the amounts on

the interior are not estimated. Therefore, the amount of phase-X

increase can be taken in the order:

21% < 23.5% = 26% < 31% = 36% KOH.

We, therefore, propose that the higher KOH concentrations favor and

accelerate phase-X formation.

83
Phase-X is not simply induced by extensive cycling, because it is

clear that the phase-X content does not correlate with _ cycle

life. 31% and 36% KOH, with the most phase-X, show the lowest cycle

life. The phase-X content is lower in the 26% KOH electrodes, which

display the longest cycle life. The 21% KOH electrodes were cycled

extensively, but display no phase-X.

Furthermore, since 21% and 23.5% KOH electrodes have drastically

different amounts of phase-X, it appears that phase-X does not account

for the secondary plateau formation reported below 26% KOH by Lim and

Verzwyvelt [i].

The fact that the front of the electrode (which is closer to the

counter electrode) has more phase-X than the back is consistent with

greater utilization of the active mass on the side facing the hydrogen

electrode and separator containing KOH. This front of the nickel

electrode is directly exposed to KOH. This exposure as well as

differences in current densities and voltage drops between the front

and the back side of an electrode may be factors influencing phase-X

formation.

CONCLUSION

The presence of phase-X is correlated with cell failure at high

KOH concentration (31-36%) due to a low end of discharge voltage.

That is, these electrodes display a reduced capacity at low cycle

life. It is proposed that this lowered capacity is caused by a phase

transformation from active mass to a phase which is less

electrochemically active, thereby reducing electrode utilization. It

is apparent that higher KOH concentrations favor formation of this new

phase.

These results suggest that avoidance of the soft short failure

mechanism, observed for <26% KOH electrodes, might allow even greater

electrode cycle life for the KOH concentrations below 26% KOH. It

becomes obvious that the apparent maximum performance at 26% KOH is

controlled by multiple failure modes, not only phase-X formation.

It has often been proposed that a "less active phase" may be

formed and may be detrimental to the electrochemical behavior of the

nickel electrode. This is, to our knowledge, the first time that this

phase has been directly observed in active mass. The structural

identification of this phase is in progress.

Support by NASA, Lewis Research Center under Grant No. NAG3-519 is

gratefully acknowledged.

84
REFERENCES

i. H. S. Lim and S. A. Verzwyvelt, J. Power Sources, 29 (1990) 503-

519.

2. B. C. Cornilsen, X. Shan, and P. L. Loyselle, J. Power Sources, 29

(1990) 453-466.

3. B. C. Cornilsen, P. J. Karjala, and P. L. Loyselle, J. Power

Sources, 22 (1988) 351-357.

4. B. C. Cornilsen, X. Shan, and P. L. Loyselle, in D. Corrigan and

A. Zimmerman (eds.), Nickel Hydroxide Electrodes, Vol. 90-4, The

Electrochemical Society, Pennington, NJ, 1990, pp. 82-96.

5. B. C. Cornilsen, J. H. Johnson, P. L. Loyselle, and D. H. Carlson,

in R. E. Glenn (ed.), Proceedings of the VIIth International

Pneumoconioses Conference, DHHS (NIOSH) Publication No. 90-108,

Part I, U.S. Dept. of Health and Human Services, NIOSH, Pittsburgh,

PA, 1990, pp. 656-662.

85
TABLE 1

Quantitative Determination of "New-Phase" Content (%X) for Hughes

Electrodes with Variable KOH Concentration, using Raman Spectral

Analysis

ID %KOH Position

Side Averaqe {%X)

No. of Front Back

Cycles[l]* (Non-screen) (Screen) of 4 of 8

,,.

27-03 36 1 1,845/ 1,268 49 63

35-09 36 6 62 0 44

56

25-09 31 1 3,275/ 2,979 71 0 68

27-07 31 6 i00 I00

20-01 26 1 >39,573/39,230 52 0

19-06 26 6 80 0 33

29

21-05 26 1 >30,549/>30,549 27 0 24

03-02 26 6 69 0

04-03 23.5 1 >28,495/ 4,803 i00 0

16-09 23.5 6 85 0 46

23

28-05 21 1 >38,191/ 5,047 0 0 0

2401 21 6 0 0

08-09 -- - 0 0 - -

* No. of cycles to 0.5 V EODV / No. of cycles to 0.9 V EODV.

86
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N91"32557

IMPEDANCES OF NICKEL ELECTRODES CYCLED IN VARIOUS KOH

CONCENTRATIONS

Margaret A. Reid and Patricia L. Loyselle

NASA Lewis Research Center

Cleveland, Ohio 44135

ABSTRACT

Recent tests at Hughes have shown that Ni/Hp cells cycled in 26% KOH have much

longer lives than those cycled in other conSentrations. As part of an ongoing

program to try to correlate the impedances of nickel electrodes with their life

and performance, impedances were measured of a number of electrodes from these

tests that had been cycled in concentrations from 219 to 369 KOH. These had

been cycled under double-time LEO conditions at 809 DOD to failure. Cell life

ranged from about 1000 to 40,000 cycles. After cycling ten times to reduce possible changes due to storage, impedances were measured at five voltages corresponding to low states of charge. The results were analyzed using a standard

circuit model including a Warburg impedance term. Lower kinetic resistances and

Warburg slopes were found for several electrodes which had been cycled in 269

KOH even though they had been cycled for a much longer time than the others.

Interpretation of the data is complicated by the fact that the cycle lives, storage times, and failure mechanisms varied. Several other circuit models have

also been examined, but the best correlations with life were found with parameters obtained from the simple model.

INTRODUCTION

The effects of electrolyte concentration on the life of Ni/H? cells have recently been studied at Hughes Aircraft Corp. [I,2]. Cycle lives of up to 40,000

cycles were achieved with 219 KOH. Since no impedances have ever been measured

on electrodes that had been cycled for such long times, these electrodes were

investigated to determine the relationship of impedance to KOH concentration and

cycle life. Impedances of nickel electrodes have been studied by many investigators [3-7]. It has been found that impedances increase by several orders of

magnitude as the electrode goes from the fully charged to the fully discharged

state. Results from different investigators are similar qualitatively but not

quantitatively. Our earlier studies have shown this is not due to lack of reproducibility of the impedance measurements but is due to differences in the

preparation and storage of the electrodes. Cells from a given lot from each

manufacturer have been shown to have impedances that are very similar [7] provided that they have been cycled and stored in the same way, but cells from different manufacturers [8] and electrodes that have been stored for relatively

long periods of time [3] have different impedance characteristics. Thus, impedance measurements provide a promising method for the study of nickel electrodes

from different sources and under different operating conditions, especially

since many other methods of study, i.e., X-rays, do not show clear distinctions

between electrodes. However, considerable care must be taken in _ electrodes and cells from different sources to insure that the cyclin_rage

conditions are the same. An additional advantage of impedance measurements is

that they are electrical in nature; hence, they show the promise of providing

information for constructing cell models to predict charge and discharge curves

and other electrical characteristics. These models would ideally incorporate

89

PRECEDING PAGE BLANK NOT FILMED
differences due to cell manufacturer, KOH concentration, cycle regime, and

numbers of cycles. Such models are not available as yet, but are needed by

system designers to predict the behavior of cells and electrodes under various

pulse and steady-state loads.

EXPERIMENTAL

The electrodes examined in this study were provided by Hughes Aircraft Corp.

from their study on the effects on life of various KOH concentrations [1,2j

The cells were boilerplate cells with six electrodes each and were cycled in a

double-time LEO regime at 804 DOD. All electrodes were from the same lot. Each

cell had six electrodes, of which the top and bottom electrodes were used in

this study. The results of the cycling experiments are summarized in Table I.

One uncycled electrode from the same lot was also studied.

Measurements were made using a Solartron 1250 Frequency Response Analyzer and a

Solartron 1286 Electrochemical Interface from 1000 Hz to 0.001Hz (six decades).

At high frequencies a i mV RMS signal was used in order to keep the current

within the limitations of the instruments. Some measurements at the lower frequencies where the impedances were much larger were made using a 5 mV signal to

improve the sensitivity of the measurements. Data were taken using ZPLOT

software (Scribner Associates, Charlottesville, VA).

A Hg/HgO reference electrode was used. For comparison with voltages in actual

cells, Table 2 lists the voltages used along with the same voltages with respect

to that of a H2electrode at the pressure that would be found in a sealed cell [3].

The electrodes studied here had been removed from the boilerplate cells after

failure, rinsed and dried, and stored until the complete set of cells had

failed. It is known that storage affects the impedance [3], but the magnitude

of the effect and the effects of dry vs wet storage are unknown. In order to

bring the electrodes as much as possible to a consistent state, they were cycled

ten times, starting with four cycles at a C/I0 charge rate for 18 hours and a

C/4 discharge rate to a cut-off voltage of -0.50 V. The next five cycles were

at a C/4 charge rate with I0_ overcharge and a C/2 discharge rate to 804 DOD.

They were then charged again at a C/4 rate with 104 overcharge before taking the

impedance measurements, starting with the highest voltages. Before measurements

were taken, the electrodes were equilibrated at the desired voltage, usually

until the DC current fell below 10 mA. The electrode that had not been used in

the cycling tests was conditioned in the same way.

The top and bottom electrodes from each cell were studied. In general the impedances from the top electrodes were somewhat higher. When the cells had been

dismantled, it was noted that the top electrodes were drier [I]. The lesser

amount of electrolyte during cycling is probably the cause of the higher impedance. The differences were generally much smaller than those between cells, so

the parameters from the two electrodes were averaged for subsequent analysis.

The choice of potentials for the cell measurements was made based on the results

of earlier measurements with electrodes from different manufacturers [8,9]. It

has been found that at high states of charge the impedances are low, and differences between manufacturers are also small. However at low states of charge

(in the voltage range from about 0.330 V to 0.170 V vs a Hg/HgO electrode or

from 1.275 to 1.115 V in a cell) they increase by several orders of magnitude,

90
and there are significant differences between manufacturers. At voltages below

0.170 V the impedances are still higher, but there are smaller differences between electrodes from different manufacturers. There is also much more scatter,

and a much longer time is required to reach equilibrium, so that the voltage

range from 0.330 to 0.170 seems to be the best range for comparison.

RESULTS AND DISCUSSION

Bode plots and complex plane plots for one of these electrodes are shown in

Figure I. Figure Ic shows the overall complex plane curves, while Figure Id

shows the expanded portions at the higher frequency range. These figures are

for an electrode cycled in 264 KOH for 30,549 cycles, at which point the cell

was removed for further analysis. The same trends were observed for all the

electrodes as the voltage was changed.

The simplest equivalent circuit that can be postulated for a battery electrode

is shown in Figure 2a where Zw represents a Warburg impedance due to slow diffusion processes. The complex plane plot for such a circuit is given in Figure

2b, where the ohmic resistance is the intercept of the curve with the x-axis,

and the kinetic resistance is the diameter of the semicircle. Other parameters

that can be obtained by further analysis,are the Warburg slope and the c_p_citance. For a planar electrode the Warburg slope is proportional to I/CD 11_ where

C is the concentration of diffusing species and D is the diffusion coefficient.

This model is satisfactory for simple reactions on planar electrodes but is not

adequate for more complicated reactions and for porous or rough electrodes.

Severa] models have been proposed for porous electrodes [10,11], and modeling

studies are being carried out for electrodes such as the nickel oxide electrode

where the metallic substrate is covered with relatively thick layers of oxides

of mixed valences and conductivities [5,12]. Until the models for these electrodes are developed further, we will use some simple equivalent circuits as

approximations, first looking at the circuit of Figure 2a.

The high frequency portion of the experimental curves in the complex plane

representation often cannot be fit very satisfactorily by a semicircle, especially at higher voltages, as can be seen in Figure id. In such cases we have

graphically fit the best semicircle to the initial part of the curve. The

Warburg slope is interpreted here as an empirical parameter related qualitatively rather than quantitatively to the diffusion resistance, where a higher

slope signifies a slower rate of diffusion and a low slope a more rapid rate of

diffusion. The capacitance values were also obtained from the original ZPLOT

figures by drawing the best semicircles. Plots of the kinetic resistances, Warburg slopes, and capacitances for the electrodes (given in Fig. 3) illustrate

that, with the exception of the uncycled electrode, the impedances at the lower

voltages are smallest for the electrodes cycled in 264, which lasted for 30,549

cycles. It is believed that the high values for the other electrodes cycled in

264 KOH are due to their poor condition and that they probably failed by soft

shorts. These figures also illustrate the large changes in impedance parameters

with voltage. With few exceptions the parameters vary regularly as the voltage

is changed.

All but three of the electrodes in this study had been cycled to failure (the

electrodes from the cell which had been cycled over 30,000 cycles in 264 KOH

before the ceil was removed and the uncycled electrode). The fact that the

gl
impedances are different indicates that the impedance parameters are probably

not an indication of failure, otherwise they would all be similar.

Intuitively, one would expect that impedances would increase as the number of

cycles increases and the cell approaches failure. However, this does not appear

to be the case here. The effects of the KOH concentration seem to be more important than the numbers of cycles. We have very little other information about

impedance as function of cycle history. Two Ni/Cd cells that have been cycled

for 6,500 cycles showed decreases in impedance with cycling up to 1,500 cycles

and then stayed fairly constant [13]. However, these cells had been stored for

about five years before the measurements were taken, so the effects of storage

cannot be separated from the effects of cycling. We have taken measurements on

about 18 flightweight Ni/H 2 cells being tested for NASA missions after acceptance and after about 500-BOO LEO cycles. It has not been possible to interrupt

the cycle test and take further measurements to any extent since then; however,

a few measurements indicate that the impedance is increasing somewhat after

about 5,000 cycles.

Although some of the cells failed by soft shorts and others by loss of capacity

(Table 1), this is not thought to be a major factor. The fact that a cell failed

by a soft short does not mean that each electrode in the cell failed _his

way, only that at least one electro e(]e-ofthe cell had a soft short. From observations of the physical condition of the electrodes that we tested, we believe

that only the ones that had been cycled for 39,000 cycles had soft shorts, those

indicated on Figure 3 as being in poor condition. We feel that if these electrodes and the uncycled electrode are excluded, the conclusion that lower impedance is related to longer life and to the KOH concentration is probably valid.

The capacitances obtained from this analysis are very large at the higher voltages and probably reflect not only the double layer capacitances but diffusion

factors and any adsorption capacitances. They are obtained from plots of Y'/w

vs Y"/w where Y' and Y" are the real and imaginary parts of the complex admittance, i.e., the inverse of the complex impedance. (In a circuit with only one

parallel RC circuit and no diffusion elements, the double layer capacitance is

obtained unambiguously from this procedure.) If other circuit elements are

present, one can consider the capacitance obtained by this method as an "effective capacitance" rather than a true double layer capacitance. As will be seen

below, the use of other more detailed circuit models gives similar values for

ohmic resistances and kinetic resistances, but the capacitances vary greatly

depending on the model.

It is obvious from examination of the curves that a more complicated circuit is

needed, particularly at higher voltages. Several circuits have been examined

using commercially available programs which use a nonlinear least squares procedure to fit the data (ZFIT-CNRS from Schlumberger Instruments and EQUIVALENT

CIRCUIT from Princeton Applied Research). Preliminary estimates of the parameters are required as inputs to these fitting programs, which were obtained in

part from the graphical analysis of the data. In an attempt to reduce the subjectivity of the initial analysis of the simple circuit of Figure 2a, the EQUIVALENT CIRCUIT program was used to find the parameters rather than obtaining

them from the graphs as before. The experimental and calculated curves are

shown in Figure 4 for three voltages for the uncycled electrode. The calculated

curves do not fit the experimental curves well, and the parameters derived from

92
the computer analysis do not show as consistent trends with voltage as did those

obtained graphically.

Two other equivalent circuits have been extensively studied [14], and several

others have been tried. The circuit in Fig. 5 incorporates two constant phase

elements (CPE). These have been suggested for modeling fractal electrodes and

for certain cases of diffusion, but the interpretation of this element is still

not clearly understood. The calculated curves agree better with the experimental curves as shown in Fig. 6. Many of the parameters, as with the former analysis, do not show a regular trend with voltage. However, the capacitance associated with the CPE element which is in series with the kinetic resistance does

increase regularly with voltage and may prove to be a suitable parameter for

comparison.

Another equivalent circuit is suggested by examination of the curves in the Bode

phase angle plot in Figure lb. The plateau in the data at 255 mV suggests that

a circuit with more than one RC circuit might be a better fit to the experimental curves. Figure 7 gives the circuit used. Again, we can fit the experimental curves more closely (Fig. 8), but the parameters still do not always vary

regularly with voltage. Table 3 gives the results of this analysis for the uncycled electrode. Similar results were obtained for the other electrodes. The

best parameters for comparison with cycle life so far seem to be the Warburg

slopes at the lower voltages from the analysis of the simple circuit, since

these can be determined accurately with a minimum of subjective judgement. The

capacitance associated with the CPE element of the second circuit may also prove

to be useful.

As mentioned above, the double layer capacitances obtained with these more detailed circuit models are much less than the "effective capacitances" obtained

with the simplistic model, often by several orders of magnitude, whereas the

kinetic resistances obtained by the different models do not vary by more than a

factor of two. The double layer capacitance values obtained with the more detailed model of Figure 7 are of the same order of magnitude as those calculated

using the BET surface area and are probably a good indication of the true double

layer capacitance, since the adsorption and diffusion terms have been separated

out. We feel that this latter model is a better physical approximation to the

electrode, but because it consists of more elements, a least-squares computer

fit requires extremely accurate data which are not always obtainable for these

electrodes. One must remember that none of these equivalent circuit models can

represent the true situation in the highly complex porous nickel oxide, but in

the absence of an adequate theoretical model, a simple model is needed to obtain

parameters for comparison of electrodes and cells.

The measurements here are intriguing, but obviously a much larger data base is

needed to determine the effects of KOH concentration and cycle life on the impedance. We are planning to carry out experiments with individual electrodes

to obtain a more complete set of impedance data as functions of manufacturer,

KOH concentration, cycle life, and storage conditions.

SUMMARY

Impedances were measured of electrodes from boiler-plate cells that had been

cycled in KOH concentrations from 214 to 364. These cells had been cycled under

double-time LEO conditions at 804 DOD to failure. Cell life ranged from about

93
1,000 to 40,000 cycles. Measurements were made at five voltages corresponding

to low states of charge. The results were analyzed using a simple circuit model

including a Warburg impedance term. With the exception of the uncycled electrode and the electrode that is believed to have failed by a soft short, the

kinetic resistances and Warburg slopes were greater for those electrodes that

had failed earlier after cycling in KOH concentrations other than 264 and were

smallest for the electrode that had been cycled for over 30,000 cycles in 264

KOH. The impedances are not an indication of failure since the electrodes had

different impedances although they had all been cycled to failure. The results

are not conclusive, since in addition to the varying numbers of cycles that the

electrodes had been subjected to, the cells had been stored after failure for

varying lengths of time, which is known to affect the impedance. In order to

minimize the effects of storage, the electrodes were cycled 10 times before the

impedance measurements were taken.

Several more detailed circuit models were also tried. The data can be fit more

satisfactorily with these, but the parameters obtained have greater scatter and

do not correlate as well with the cycle lives as those obtained from the simple

_circuit. To improve the data base we are initiating a comprehensive study of

the effects on the impedance of electrodes as functions of storage conditions,

cycle life, KOH concentration, and manufacturer.

REFERENCES

[i] H.S. Lim and S.Z. Verzwyfelt, J. Power Sources, 2_22(1988) 213-220.

[2] H.S. Lim and S.Z. Verzwyfelt, J. Power Sources, 2__99(1990) 503-519.

[3] M.A. Reid, Nickel Hydroxide Electrodes (D.A. Corrigan and A.N. Zimmerman,

eds.), Proceedings Vol. 90-4, The Electrochemical Society, Pennington, NJ,

1990, pp. 296-310, and Extended Abstracts of the 176th Electrochemical

Society Meeting, Hollywood, FI, 1990, pp. 117-118.

[4] A.H. Zimmerman, M.R. Martinelli, M.C. Janecki, and C.C. Badcock,

J. Electrochem. Soc., 12____99 (1982) 289-293.

[5] S.J.Lenhart, D.D.Macdonald, and B.G. Pound, J. Electrochem. Soc., 135

(1985) 1063-1071.

[6] R. Haak, C. Ogden, and D. Tench, J. Power Sources, I_22(1984) 289-303.

[7] M.A. Reid, Proceedings of the 25th Intersociety Energy Conversion

Engineering Conference, Reno, NV, Aug. 1990. Vol. 3, pp. 48-53.

[8] M.A. Reid, Impedances of Electrochemically Impregnated Nickel Electrodes

as Functions of Potential, KOH Concentration, and Impregnation Method,

NASA TM 103283 (1990).

[9] M.A. Reid, Extended Abstracts of the 174th Electrochemical Society

Meeting, Chicago, IL, Oct. 1988, pp. 122-123.

[10] R. de Levie, Electrochim. Acta, 10, 113 (1965).

94
[II] H. Keiser, K.D. Beccu and M.A. Gutjahr, Electrochim. Acta, 2__II(1976) 539-

543. _ '

[12] S. Bhakta, D.D. Macdonald, B. Pound, and M. Urquidi-Macdonald, Extended

Abstracts of the 178th Meeting of The Electrochemical Society, Seattle,

WA, October 1990, p. 114.

[13] M.A. Reid, Proceedings of the 2nd SERT Conference, NASA Lewis Research

Center, NASA CP-3056, pp. 197-209; and J. Power Sources, 2__99(1990) 467-

476.

[14] M.A. Reid and P.L. LoyseIle, Extended Abstracts of the 178th Meeting of

The Electrochemical Society, Seattle, WA, October ]9go, pp. ]17-]18.

95
CELL

NO.

BP1

BP2

BP3

BP4

BP5

BP6

BP7"

BP8

BP9

BPIO

TABLE 1. LIFE TEST RESULTS OF NI/H 2 CELLS AT 80%

DEPTH-OF-DISCHARGE

[KOH]

%

21

26

26

31

31

36

21

26

26

23.5

NO.CYCLE NO.CYCLE TOTAL FAILURE

TO 09 V TO 0.5 V NOCYCLES MODE

5,047

39,230

4,329

2,979

3,620

1.268

1,037

>30,549

23,598

4,803

>38,191

>39,573

9,241

3,275

4,230

1,845

6,508

>30,549

>24,594

28,495

38,191

39,573

9,241

3,286

4,230

1,845

9,402

30,549

24,594

28,495

SOFT SHORT

SOFT SHORT

LOW E ODV

LOW EODV

LOW EODV

LOW E ODV

LOW EODV

REMOVED

SOFT SHORT

SOFT SHORT

"CYCLED AT 70% DEPTH-OF-DISCHARGE (DOE)) FROM 1644 TO 4644

CYCLES AND AFTER 6508 CYCLES.

TABLE 2. VOLTAGES OF Ni/H2CELLSCORRESPONDING

TO VARIOUS HALF-CELL VOLTAGES.

Ni vs Hg/HgO Comparable Ni/H2

0.170

0,200

O255

0.290

0.330

1.115

1.145

1.200

1.235

1,275

TABLE 3. IMPEDANCE PARAMETERS AT SEVERAL VOLTAGES

OBTAINED BY ANALYSIS WITH CNLS AND ZFIT

V vs Hg/HgO

Rkin/.0.

Cdt/ F

RadsorpJ

Cadsorp./ F

Rdilf/,O.

T

Phi

0.330

i

O.OO52

1.19

0.0066

41

0.17

479

O.7O

0.290

0.00073

3.65

0.00087

75

0.15

436

0.67

0.255

i •

0.057

0.48

0.0078

123

0.56

420

0.65

O.200 0.170

0.048 0.118

0.43 0.5

0.011 0.0097

3.7 350

0.59 1.00

443 477

0.63 0.6t

96
16

14

12

1

_- 08

o

06

O4

02

0

-02

• J,30 mV

a)

-3 -I I

LOG FRI/QUENC'r', HZ

290 mV o :,'55 mV 8 200 mV * i70 mV

:t

::I

70 I

11 D i

- I

LOC FR[OUtI_K3Y. HZ

290 mV D 2bb _l_V • 200 mV

3

• 170 rnV

1.6  ̧

t.4

1.2

0.8

o

E 0.6"

0.4'

0.2

0 - i i n u ,

0 0.2 0.4 0.6 0.8 I 1.2 1.4 f.6

Z reol (ohm)

• 330 rnV _ 290 mY , 255 rnV , 200 rnV = t70 rnV

0,04

0.03

.__ 0,02

.__

N

I

0.01

0

d)

0.02 0.03 0,04 0.05 ' 0.06

Z reoi (ohms)

= 330 rnV * 290 rr, V , 255 rnV , 200 rnV I t70 mV

Fig. 1 Changes of Impedance with Voltage for Electrode Cycled 30,549 Cycles

in 26_ KOH. (a) Bode Magnitude Plot, Normalized Data. (b) Bode Theta

Plot. (c) Complex Plane Plot. (d) Expanded High Frequency Portion of

C°mplex2Plane7 _lot. (c) and (d) are Nonnormalized Data. Electrode Area : .9 cm=.

C d

.E

0

E_

N

I

Z

I w

Kinetic D_ffus_on

Control . Control

/ j.Vi

I J-__L-L kJ

Rohm Rohm + Rk_

Z reol

Fig. 2 (a) Simple Circuit Used for Initial Analysis of Impedance Data.

(b) Complex Plane Plot for Typical Circuit of Figure 2a.

97
$000"

2_00

2000

a)

%

\ Legend

• 21_ KOHr 5,047 cyclel _ --

\ o 2j_._Ko,4_a_.

\ 0 26% KOH, 30_.3_0_5454g c d*y._.y__

!,_° %• _ \ \ x _'" .3.6.z KgH,._,L.._ ,_: o..--T ..................

I000 "

\\

500 "<--.... _ _.. - _ _-.._,,

- --'_'_:--_="__ 0:3 o'_ - o_4

Potential (V vs Hg/HgO)

_E

\ 2

II 40-

",.., -.. _ 20

0 --_,*_'"_r--_'-_f_-_'- Y " "- _ '_ "% ' "_34 0

o _ om o.;e o2;e o.;e.4 o._s o._'" 0.3 o..,.2 .

Potonfiol(Vvs Hg/_gO)

,,+

C) :"

/

., ........

: //

.......... ::,,'---

_t:::':__..,.': ....

_-__-7 -.-..:-.-..'-_-/._I_ , ,

0|_ 0,111 O._ 0r22 0.24 0+21' 0,211 0.3 0,32

Po|onlio] (V vs Hg,/_'lgO)

Fig. 3 Impedance Parameters for Various Electrodes Obtained from Graphical

Analysis Using Circuit of Fig. 2. All Parameters are Normalized.

(a) Kinetic Resistances, (b) Warburg Slopes, (c) Capacitances.

98
_°I 330 mV vs Hg/HgO 7

20 30 4O

Zreal (mOhms)

_0

E

6

E

_' so

:f

255 mV vs Hg/HgO u

/

/

/ u

/

/ °

i 0

20 30 40 50

Zreol (mOhm 5)

200

E

0

0

170 mV vs Hg/HgO /

i 0

/

/

_-_'_._ ./_ °

/ "15 _/o o

1O0 2OO 3O0

Zreol (mOhm5)

/_R _E

kin

Fig. 5 (a) Circuit with Two Constant

Phase Elements Used for Further

Analysis of Data.

R_

Fig. 5 (b) Circuit Incorporating Adsorption Capacitance and Resistance for Further Analysis of

Data.

Fig. 4 Comparison of Some Experimental

and Simulated Complex Plane

Plots Using Best Parameters

Obtained Using EQUIVALENT CIRCUIT Program and Circuit of

Fig. 2(a). Uncycled Electrode.

o, Experimental; +, Simulated.

99
LmO D_

0

2O

m

Z"

i! '°

330 mV vs Hg/HgO /o

_ n_/°

30 ,lo 5o

Zreol (rnOhrns)

ii _°°

255 mVvs Hg/HgO o

/

/ II

_0 40 50

Zreol (mOhms)

200 ................................

170 mV vs Hg/HgO /

y'

u

/

/o

/o

100 200 300

Zreol (rnOhrns)

2O

10

Y

0

v

"E 10

0

330 mV vs Hg/HgO

20 30 4O 5O

Z,eot (rnO_ms)

w

E

t"

._ 100

.[ •

't

0

170 mVvs Hg/HgO

° °1

Zreol (rnOhrns)

Fig. 6 Same as Fig. 4 Using Circuit of

Fig. 5(a).

Fig. 7 Same as Fig. 4 but Using ZFITCNLS Program and Circuit of

Fig. 5(b).

100
N91-32558

HIGH ENERGY EFFICIENCY AND HIGH POWER DENSITY PROTON EXCHANGE

MEMBRANE FUEL CELLS -

ELECTRODE KINETICS AND MASS TRANSPORT

Supramaniam Srinivasan, Omourtag A. Velev, Arvind Parthasarthy,

David J. Manko, and A. John Appleby

Center for Electrochemical Systems and Hydrogen Research

Texas Engineering Experiment Station

Texas A & M University System

College Station, Texas 77843-3402

The development of proton exchange membrane (PEM) fuel cell power plants with high energy

efficiencies and high power densities is gaining momentum because of the vital need of such high levels of

performance for extraterrestrial (space, underwater) and terrestrial (power source for electric vehicles)

applications. Since 1987, considerable progress has been made in achieving energy efficiencies of about

60% at a current density of 200 mA/cm 2 and high power densities (> 1 W/cm 2) in PEM fuel cells with

high (4 mg/cm 2) or low (0.4 mg/cm 2) platinum loadings in electrodes. This article focuses on (1) methods

to obtain these high levels of performance with low Pt loading electrodes - by proton conductor

impregnation into electrodes, localization of Pt near front surface; (2) a novel microelectrode technique

which yields electrode kinetic parameters for oxygen reduction and mass transport parameters;

(3) demonstration of lack of water transport from anode to cathode; (4) modeling analysis of PEM fuel cell

for comparison with experimental results and predicting further improvements in performance; and

(5) recommendations of needed R&D for achieving the above goals.

1. HIGH ENERGY EFFICIENCY AND HIGH POWER DENSITY FUEL CELL SYSTEMS

1.1 ViIal Needs, Essential Criteria, and Candidate Fuel Cells

High energy efficiency and high power density fuel cells are vitally needed for extra-terrestrial

(space, underwater) and terrestrial (electric vehicles, stand-by or remote power, portable power source)

applications to minimize fuel consumption, weight, volume, and capital cost of power plants. The relative

importance of energy efficiency and of power density depends on the application. In some cases, as for

example, for a hydrogen energy storage system to be coupled with photovoltaic power plants for NASA's

long lunar and Mars missions (14 days of darkness, 14 days of light), the energy efficiency is more critical

than the power density because the weight of the energy storage sub-systems exceeds that of the fuel cell

power conversion system by more than a factor of five. For military applications, where pulsed power is

required, the power density is the over-riding factor. For the terrestrial transportation application, both the

energy efficiency and power density have to be as high as possible.

The essential criteria for the attainment of high energy efficiencies and high power densities in fuel

ceils are (i) low activation overpotentials which can be achieved by (a) using electrocatalysts with high

exchange current densities for the oxygen reduction (ORR) reaction (the exchange current density for the

hydrogen oxidation reaction is sufficiently high and thus activation overpotential losses dne to this

electrode reaction are minimal in all types of fuel cells); and (b) maximizing the electrochemically active

surface areas; (ii) minimal mass transport overpotential, for example, by optimization of electrode structure

to accelerate the transport of reactants to and products away from the active sites; and (iii) low ohmic

overpotentials, for example, by minimization of the thickness of electrolyte layer and use of membranes

with higher proton conductivities. All forms of overpotential can be reduced by operation at elevated

temperatures and pressures. The fundamental limitation in fuel cells operating at low to intermediate

temperatures (i.e., fuel cells with alkaline, phosphoric acid, proton exchange membrane electrolytes) is the

101
low exchangecurrentdensityfor theoxygenreductionreaction,while in thehighertemperaturesystems

(moltencarbonateandsolidoxide),masstransportandohmicoverpotentialsimpedetheattainmentof high

energyefficiencies and high power densities. In order to achievehigh power densities,it is vitally

importantto minimizemasstransportandohmicoverpotentials[1,2].

Thephosphoricacidfuel cell system,whichis themostadvancedfuel cell technologyandcloseto

commercializationfor terrestrialapplications,suffersfrom theproblemsof slowoxygenelectrodekinetics,

comparedto that in alkaline andsolid polymerelectrolytefuel cells. Thus, this systemdoesnot show

promisefor attainmentof high energyefficienciesandhigh powerdensities.Themoltencarbonateand

solidoxide fuel cells exhibit goodoxygenelectrodekineticsat thehighoperatingtemperaturesof 650"C

and 1000"C,respectively. However,the masstransportandohmic problemsarequite severein these

systems,thusmakingit difficult to attainbothhighenergyefficienciesandhighpowerdensities.Theonly

systemsin which high energy efficiencies and high power densities have been simultaneously

demonstratedarethealkalineandprotonexchangemembrane(PEM) fuelcells.

1.2 Demonstration of High Power Densities in Alkaline and .Proton Exchange Membrane Fuel Cells

Significant progress has been made in achieving high energy efficiencies and high power densities

in alkaline and proton exchange membrane fuel cells, particularly because (i) the ORR is considerably

faster in these electrolytes than in any other electrolytes; (ii) the structures of the porous gas diffusion

electrodes have been optimized to minimize mass transport overpotential; and (iii) very thin electrolyte

layers are used to reduce ohmic overpotentials to negligible values. International Fuel Cells (IFC) has

demonstrated that in advanced alkaline fuel cells the cell potential is as high as 1.0 V at 100 mA/cm 2 while

it is 0-5 V at 8 A/cm 2 [2]. The high performances of the PEM fuel cells at Ballard Power Systems (BPS)

may be represented by a cell potential of 0.9 V at 100 mA/cm 2 and 0.5 V at 6 A/cm 2 [3]. In the IFC and

BPS fuel cells, the noble metal loading in the electrodes is quite high (_> 4 mg/cm2). Relatively high

energy efficiencies and high power densities in PEM fuel cells with low platinum loading (= 0-4 mg/cm 2)

electrodes (necessary for transportation applications) were demonstrated first at Los Alamos National

Laboratory (LANL) and later at Texas A&M University (TAMU) by Srinivasan and coworkers [4,5]. The

best performance, reported by these workers to date, is 0.88 V at 100 mA/cm 2 and 0.62 V at 2 AJcm 2. In

all the above cases, the pristine gases H2 and 02 were used as reactants. When using H2]Air as reactants

in the fuel cells with low platinum loading electrodes, the cell potential at 100 mAJcm 2 was 40 mV (the

theoretical value) lower and at 2 A/cm 2 was 250 mV lower than when H2/O2 were used as reactants - the

deviation from the theoretically expected value of 40 mV starts to occur at about 1.5 A/cm 2.

1.3 Methods of Attainment of High Power Densities in PrQtgn Exchange Membrane Fuel Cells with

Low Platinum Loading Electrodes

1.3.1 Extension of Three-Dimensional Reaction Zon__y_I_m_pregnation of Proton Conductor into

Active Layer of Electrode and Hot-Pressing of Electrode to Proton Conduction Membrane,

An ingenious method, which was developed by Raistrick [6] to extend the three-dimensional

reaction zone, is the impregnation of a proton conductor (say, Nation®) into the active layer of the

electrode (say, Prototech electrode with a platinum loading of 0.4 mg/cm2). In this work, it was shown

that such an electrode, mechanically pressed to a Nation® membrane, exhibited a considerable

improvement in oxygen reduction kinetics, as compared to that at an unimpregnated electrode/Nation®

interface. However, the first demonstration of high power densities in fuel cells with such impregnated

electrodes was by Srinivasan, Ticianelli, Derouin, and Redondo [7], who found it necessary to hot-press

the electrodes onto the proton conducting membrane at a slightly higher temperature than the glasstransition temperature of the membrane and at a pressure of about 15 atm. Fig. l(a) and (b) illustrate the

significant improvement in cell performance and the increase in electrochemically active surface area

respectively by Nation® impregnation of the electrode. These figures also demonstrate the comparable

102
performances and electrochemically active surface areas of single cells with high platinum loading and

Nation® impregnated low platinum loading electrodes.

1.3.2 Localization of Platinum Near Front Surface to Minimize A_Iivntion, Mass-Transport, and

Ohmic Overpotentials

From the cyclic voltammetric results, shown in Fig. l(b), as well as from Rutherford Back

Scattering experiments on a cross-section of the electrodes, it was shown that the proton conductor

penetrates only about 10/am within the front surface. The active layer thickness in the state-of-the-art low

platinum loading electrodes (say, from Prototech or ETEK) is about 100/am. These electrodes contain

10% Pt on carbon supports. In order to minimize activation, mass transport and ohmic overpotentials,

which is vital for the attainment of high energy efficiencies and high power densities, Srinivasan and

coworkers utilized (i) custom-made fuel cell electrodes with 20% or 40% (instead of 10%) platinum on

carbon, while maintaining the same platinum loading (0.4 mg/cm2); and (ii) sputter-deposited a thin layer

of platinum (0.05 mg/cm 2) on the front surface of these electrodes 14]. The results of these studies, which

are presented in Fig. 2 and Table 1 signify that (i) the platinum utilization of the electrodes is improved;

and (ii) current densities of 1 A/cm 2 at a cell potential of 0.6 V can be achieved. It has recently been

shown [5] that electrodes with thin layers of platinum, deposited using chemical or electrochemical

techniques, yield performances close to those of the sputter-deposited electrodes (Fig. 3).

1.3.3 Use of Thinner and Alternate Mcmbran¢_

In most studies, which have been reported, DuPont's Nafion® 117 membranes (thickness of

175/am) have been used. To minimize ohmic and mass transport overpotentials in the membrane of a

PEM fuel cell, two approaches, which were taken at LANL and TAMU, are the use of (i) thinner Nation@

membranes; and (ii) Dow® membranes with lower equivalent weight. Typical results are presented in

Fig. 4. The cell with the 50/am thick Nation® membrane exhibits the highest cell potential at 2 A/cm 2.

However, the open-circuit potential in this cell was 50-100 mV lower than in the cells with the other

membranes because of cross-over of the hydrogen gas. The cross-over effect is also reflected by a lower

potential in the cell with the 50/am thick membrane than in the cell with a 175 or 100/am thick membrane,

at low current densities. The cell with the Dow® membrane, 125/am thick, showed the next best result.

Due to its lower equivalent weight than that of Nation@, it has a better conductivity and water retention

capability. It is also striking that the cells with the Nation® membrane (thicknesses 100 and 175/am)

encounter mass transport limitations at higher current densities but the cell with the Dow® membrane does

not.

2. OXYGEN ELECTRODE KINETICS AT PLATINUM/NAFION® INTERFACE

2.1 Taf¢l Parameters; on MicroeleOirode.. from Slow Potential Sweep and Electrochemical Impedance

Spectroscopic Experiment,s

The objectives of this work [8] were to detemfine the electrode kinetic parameters for the reduction

of oxygen at the Pt/Nafion® interface under totally solid state conditions (i.e., no contacting liquid

electrolyte phase). Thus, these investigations were carried out under the same conditions as in a practical

fuel cell. The concentration and diffusion coefficient of oxygen in Nation® were also determined. Earlier

studies of the platinum/solid polymer electrolyte interface using platinum gauze electrodes indicated that

kinetic information was masked by large contributions from ohmic and mass transport effects [9]. Such

problems were addressed in this investigation by using a novel solid state cell, incorporating a platinum

microelectrode (Fig. 5). Cyclic voltammetric and potentiostatic transient measurements were made at the

Pt/Nafion® interface. From the high scan rate cyclic voltammetric experiments, the purity of Nation® and

of the electrode was ascertained and the roughness factor of the electrode was calculated. The slow sweep

experiments yielded the Tafel parameters for oxygen reduction. From the two-section Tafcl plot (Fig. 6),

103
theexchangecurrentdensitieswerecalculatedandfound to behigherthanthoseobtainedat any other

Pt/acid interface. The electrode kinetic parameters obtained therefrom are presented in Table 2.

These results were also substantiated by electrochemical impedance spectroscopic (EIS)

investigations [10]. A typical impedance plot (Nyquist plot) at the open-circuit potential is shown in

Fig. 7. From the charge transfer resistance Rct at this potential, one can calculate the corrosion current

density according to the Eq.

RT

Rct - n icorr F (1)

The corrosion current density (icorr) corresponds to the current density for oxygen reduction, as well as

that for the platinum oxide formation, at the open-circuit potential. From EIS measurements over a range

of potentials and a plot of log Rct vs. current density, the Tafel slope which was obtained was found to

agree with the results of the pseudo-steady state method (Table 2). From the values of icorr and Tafel

slope, the exchange current density for oxygen reduction on the platinum microelectrode was calculated; its

value was consistent with that obtained by the pseudo-steady state method (Table 2).

2.2 Taf¢l Parameters on..Fuel Cell El¢qtrgdes from Galvanostatic Measurements

Electrode kinetic studies of oxygen reduction in PEM fuel cells (single cells) have been conducted

at LANL since 1986 and at TAMU since 1988. A systematic study of ORR in fuel cells, as a function of

temperature and pressure was recently carried out at TAMU. The fuel cell electrodes contained either high

(4-10 mg/cm 2) or low (= 0.4 mg/cm 2) platinum loadings. The plot of the potential of the oxygen

electrode, with respect to the reversible hydrogen electrode potential, as well as that of the single cell

versus the current density fits the Eq.

E = Eo - b log i - Ri (2)

where Eo = Er + b log io (3)

and Er is the reversible potential for the half or single cell; io and b are the exchange current density and

Tafel slope for the ORR and R represents the resistance (predominantly ohmic as well as the charge

transfer resistance for the hydrogen electrode) which causes a linear variation of the potential with current

density. The Tafel plots for ORR in single cells with high and low platinum loading electrode at 50°C are

presented in Fig. 8. From such plots, the exchange current densities (based on geometric areas of

electrodes) and Tafel slopes were determined. In these experiments, cyclic voltammograms were also

recorded 6n the electrodes; from the coulombic charge required for hydrogen adsorption or desorption, the

electrochemically active surface areas and hence of the roughness factors of the electrodes were calculated.

Using these roughness factors, the exchange current densities, based on the real surface area of the

electrode were obtained. Table 2 also illustrates a comparison of the electrode kinetic parameters for ORR

on fuel cell electrodes at 50°C and on the microelectrodes at 25°C. The fuel cell electrodes exhibit only the

low Tafel slope, 60 mV/decade, because even at the high current density of 2 A/cm 2 (based on geometric

areas of the electrode) the half cell potential is still in the platinum oxide covered region, where the low

Tafel slope is observed on smooth platinum microelectrode. The exchange current densities (based on the

real surface area of the platinum) are nearly the same on the fuel cell electrode and on the microelectrode.

More details of this work, including that of the effects of pressure and temperature will be published

shortly [11].

104
3. MASS TRANSPORT AND OHMIC PHENOMENA

3.1 Oxygen Solubility and Diffusion Coefficient in Proton Exchange Membrane from Microelectrode

The microelectrode technique is a unique method for determining the electrode kinetic parameters

(as described in Section 2.1) and mass transport parameters. It involves chronoamperometry - the

potential of the microelectrode is stepped from a value where no oxygen reduction occurs (open,circuit

potential) to that at which oxygen reduction occurs at the limiting current density. According to

microelectrode theory [12], the dependence of the current density (i) on time (t) for such an experiment

follows the Eq.

nFnI/2D 1/2C r2

+ nnFDCr (3)

i = tl/2

where n is the number of electrons transferred in the overall reaction (n = 4 for ORR); D and C are the

diffusion coefficient and solubility of oxygen in the PEM; and r is the radius of the microelectrode. Thus,

a plot of the i vs. t-1/2 is linear and from the values of the slope and of the intercept, the diffusion

coefficients and solubilities can be easily calculated. It must be noted that this is not the case when

transient methods (AC impedance, cyclic voltammetry) are used on macroelectrodes. In these cases, only

the product D1/2C is obtained and unless some other method can be used to find D or C independently, the

two values cannot be individually determined. A typical plot of i vs. t-1/2 for ORR on Pt in Nation® is

shown in Fig. 9 [8]. The values of the diffusion coefficient and solubility, calculated from this plot are

presented in Table 3. The difference between these values and those reported by other workers are

probably due to different experimental conditions - water content of membrane, measurements with other

acids in membrane, etc.

3.2 Dependence on Temperature and Pressure

Both the temperature and pressure of operation of the cell have a significant influence on its

performance. The effect of temperature on performance is illustrated in Fig. 10 for four temperatures from

50°C to 95°C at a pressure of 5 atm. The slope of the linear region of the cell potential vs. current density

plot decreases, which is indicative of a lowering of the internal resistance of the cell; this decrease is

predominantly due to the decrease ohmic resistance of the electrolyte. Further, the plots show that mass

transport losses are also significantly reduced at the higher temperature, as seen by the extension of the

linear region with increase of temperature. The mass transport limitations are caused by the slow diffusion

of the reactants through the PEM to the active platinum sites and the slow transport of protons from the

anode to the cathode. At a temperature of 95"C and pressure of 5 atm, it has recently been shown in our

laboratory that the linear region extends to over 3 A/cm 2, indicating that the mass transport limitations are

minimal up to this current density.

3.3 Dependence on Cathodic Reactant - Oxygen vs. Air

By necessity, the source of oxygen for terrestrial applications is air and it is the pure gas (cryogenic

or compressed gas) for extraterrestrial (space, underwater) applications. Experiments using both these

gases in single cells, operating in the temperature range of 25°C to 95°C and at pressures from 1 to 5 atm,

show the striking influences of mass transport phenomena in the cell and half cell potential vs. current

density plots (Fig. 11 and 12). Fig. 11 demonstrates the effect of pressure at 50°C, while Fig. 12

represents the effect of temperature, at a pressure of 5 atm on the cell potential vs. current density plot

using H2/O2 and H2/Air as reactants. Two effects are apparent on passing over from oxygen to air as the

cathodic reactant: (i) the slope of the linear region of the E-i plot is 50% higher, and (ii) deviations from

105
the linear region are visible at lower current densities. In the higher temperature fuel cells (phosphoric,

molten carbonate, solid oxide) the oxygen gain in potential is constant at all current densities in the linear

region and thus the slopes of the E-i plot are the same with H2/O2 and H2/Air as reactants. The

dependence of the slope of the E-i line on the oxygen partial pressure is indicative of mass transport

limitations, which also cause a linear variation of E with i at lower current densities. It is very probable

that there is a "nitrogen barrier layer effect" which causes this mass transport limitations. In a previous

study, it was also shown that the dependence of the cell potential (Ep) on pressure (P) at a constant current

density may be expressed by the Eq.:

Ep = E1 + b log P (4)

where E1 is the cell potential at 1 atm and b is the Tafel slope for the reaction. The derivation of this

equation is based on a reaction order of unity for the ORR. This equation is valid over the entire region of

P, investigated with oxygen but at pressures between 1 and 1.5 atm there is a more rapid increase of E

with P, when using air.

The second effect, i.e., deviations from the linear region of the E vs. i plot at lower current

densities with air than with oxygen is clearly due to the effect of partial pressure of oxygen. The "nitrogen

barrier layer effect" could still play a role. A challenge is to be capable of operating a cell at least at

2 A/cm 2 and a cell potential above 0.5 using steam-reformed hydrogen and air as reactants. Optimization

of the structure of the electrode to minimize mass transport limitations is essential.

3.4 Dependence on Thickness and Conductivity of Membrane

The spade work on proton exchange membrane fuel cells has been carded out using DuPont's

Nafion® 1100 as the electrolyte layer. Nafion® membranes, with different thicknesses (50, 100 and

175 t.tm) have been incorporated as the proton exchange membrane in single cells and their performances

determined (see Section 1.3.3). As one may expect, the thinner the membrane, the lower is the slope of

the linear region of the cell potential vs. current density plot (Fig. 4). However, another significant effect

is that mass transport limitations are g-ready reduced or even eliminated by using thinner membranes. The

mass transport effects are clearly due to phenomena within the membrane - water and/or proton transport.

The emerging proton exchange membrane for fuel cells is the one with a lower equivalent weight,

manufactured by Dow Chemical Company than that of Nafion® 1100. The chemical structures of the

DuPont and Dow membranes are illustrated in Fig. 13. The presence of more sulfonic acid groups per

CF2 in the Dow membrane than in Nation® enhances the electrolyte conductivity and water retention

capability. A well humidified membrane in PEM fuel cells is vital as otherwise (i.e., drying out) problems

of increase in electrolyte resistance and consequent loss in performance are encountered. Fig. 4 also

shows that the cell with the Dow membrane (thickness 125 _tm) shows a lower slope in the linear region

of the E-i plot than that in the cell with the Nation® membrane (thickness 100 I_m). Further, mass

transport limitations are visible at high current densities in the latter but not in the former cell. The higher

conductivity and the better mass transport (proton and water) capabilities of the Dow membranes are

reflected in Fig. 4.

3.5 Evidence for Lack of Net Wat¢r Tronsport from An0*J¢ to Cathode

It has been reported that in the PEM fuel cell, a proton crossing the membrane carries with it four

to six water molecules. In the present study, the amount of water transported across the membrane during

the cell operation was measured in order to evaluate more accurately the mass balance conditions in the cell

[13]. A PEM fuel cell with a 50 cm 2 electrode geometric area was used in the experiments with two

different types of proton conducting membranes - Nafion® from DuPont and Dow experimental type. The

electrodes used in this study were supplied from Prototech, Inc., and contained 20% Pt on carbon

(0.4 mg/cm 2) on a carbon cloth substrate. Details of the experimental set up are published elsewhere [5].

106
In theseexperiments,theoxygen gasenteringthe cell wasdry. The hydrogengaswas saturatedwith

water vapor. The gasexiting the cell from the cathodechamberwas passedthrough a condenser

consistingof acopperspiralimmersedin iced-water.Thewaterexiting thecondenserwascollectedand

itsratecalculated.Theresultsof experimentswith aDowexperimentaltypemembrane(125gm thick)are

presentedin Fig. 14. The solidline presentstherateof waterproductioncorrespondingto Faraday'slaw.

The points representthe experimentally measuredcollection rates. At low current densities, the

measurementsarenot very accuratebecauseof the long periodof time neededto collect measurable

amountsof water. As canbe seenfrom Fig. 14,the amountof waterproducedat atmosphericpressure

and50"Ccorrespondsto Faraday'slaw. Therearetwo possibleexplanationsfor this effect. Thefirst is

that there is no water transportacrossthe membraneand that the proton transportis by a hopping

mechanism(Grothusconduction). The secondoneis that due to the concentrationgradientof water,

whichwill besetupduringcurrentflow, theback-diffusionof watercompensatesfor thewatermolecules

draggedwith the protons. Thefirst explanationis themost likely one. Experimentsareunderwayto

determineif thereis watertransportacrossthemembraneat higheroperatingtemperaturesandpressures

(3-5atm),whicharenecessaryto attainthehighpowerdensitiesrequiredof fuel cells for transportation,

spaceanddefenseapplications.

4. PROGNOSISOFATTAINABLE ENERGYEFFICIENCYAND POWERDENSITIES

4.1 BestPerformances Obtained With High and Low Platinum Loading Electrodes

The cell potential vs. current density plot up to the end of its linear region fits equation (2). In

Table 4 are presented the valued of Eo, b and R, evaluated by nonlinear least squares fits to the

experimental points, for the cells exhibiting high levels of performance: cells with (i) high platinum

loading (10 mg/cm2), electrodes and 125 pm thick Dow membranes (Fig. 15); (ii) low platinum loading

electrodes and 125 gm thick Dow membranes (Fig. 4); and (iii) low platinum loading electrodes and

Nation® membranes with thicknesses of 100 or 175 l.tm (Fig. 4). Also in Table 4 are presented the

current densities at a cell potential of 0.9 V. All these cells were operated at 85"C and 5 atm pressure.

Table 4 illustrates that Eo is generally approximately equal to 1.0 V, b is between 0.05 and 0.06 V/decade

and R is between 0.1 and 0.2 ohm cm 2. The electrochemically active surface area is about 10 times higher

for the high platinum loading than for the low platinum loading electrode, even though the platinum

content in the former case is 20 times higher than in the latter. The effective utilization of the platinum is

about 5% for the cell with the high platinum loading electrodes and about 10% for the ones with low

platinum loading electrodes. It is also worthwhile noting that the current density in the predominantly

activation-controlled region, based on the electrochemically active surface area of the electrodes, is

approximately the same for the cells with the high and low platinum loading electrodes. In order to attain

high power densities, the value of R has to be as low as possible, as in the case 125 lam thick Dow

membrane.

4.2 Prediction Qf Performance Improvements - Modeling Analysis

Since Eq. 2 is quite representative of the cell potential vs. current density plot up to the end of the

linear region, a simplified modeling analysis was carded out to determine the effects of the parameters Eo,

b and R. The results of this analysis are presented in Fig. 16(a) to 16(c). Figure 16(a) shows that

increase of the value of Eo (for constant b and R values) causes a parallel shift of the cell potential vs.

current density plot and thus for a 50 mV increase, the energy efficiency is increased by about 4%. This

figure also shows that the power density can be significantly improved by higher values of Eo. However,

the power density is also influenced by the value of R. For R = 0.1 ohm cm 2, which value was used for

the plots in Fig. 16(a), the current density at a cell potential of 0.8 V increases by a factor of only two for

Eo increasing from 1.0 V to 1.05, whereas it will be close to two orders of magnitude higher if R were

equal to zero (i.e., when fuel cell is only activation-controlled). It must be noted that Eo is expressed by

Eq. 2 and hence the basic parameter which determines Eo is the exchange current density for the ORR

(based on the geometric area of the electrode). Two possible approaches to increasing io are (i) finding

107
better electrocatalysts; and (ii) increasing the effective utilization of platinum. Eo can also be increased by

increasing value of Er the thermodynamic reversible potential. Operation at higher pressures is the only

way of obtaining a higher Er. The Nernst Eq. shows that for a ten fold increase of pressure, Er increases

for the hydrogen/oxygen fuel cell by 45 mV.

According to Eq. 2 if the current density is expressed in mA/cm 2, Eo is the value of E at 1 mA/cm 2

(geometric). At this current density the contribution of ohmic overpotential to the cell potential is

negligible. The current density, based on the electrochemically active surface area of the electrode is about

fifty times lower for the cell with the low platinum loading electrodes and 500 times lower for the one with

high platinum loading electrodes. At these low values of current densities, the value of Eo will be very

close to the open circuit potential. The open circuit potential is a mixed potential involving oxygen

reduction (and to a small extent platinum dissolution) as the cathodic reaction and the platinum oxide

formation as the anodic reaction. When using carbon in the active layer of the electrode, as with supported

electrocatalysts, the electrochemical oxidation of the carbon also influences the open circuit potential. This

is one of the main reasons for the Eo values being lower for electrodes with supported than with

unsupported electrocatalysts. A problem, which may thus arise by attaining higher Eo values, is that the

rates of the above mentioned anodic reactions will increase and cause degradation of structure of electrode

and hence of performance.

Fig. 16(b) shows the effect of variation of Tafel slope for the ORR on the cell potential vs. current

density plot. The values of the Tafel slope chosen are in the range of those experimentally observed. The

lower the Tafel slope, the better is the performance and it is more significant at higher current densities

because the slope of the cell potential vs. current density plot increases with increase of Tafel slope. The

Tafel slope of 60 mV/decade is representative of oxygen reduction on the oxide covered platinum in acid

medium [14], including the PEM [8]. Tafel slopes, lower than the assumed values, have rarely been

observed for oxygen reduction. One can expect lower Tafel slopes if an intermediate step, further down

the reaction sequence, say after 2 of the 4 electrons have been transferred to the oxygen molecule, is the

rate-determining step [15]. An alternative mechanism which could yield low Tafel slopes is via redox

catalysis which may be possible by use of alloys such as PtCr, PtCo, PtNi, etc., where the electrochemical

reaction is the reduction of the transition metal ion from a higher valence state to a lower one and oxygen

chemically oxidizes the lower valent metal ion to the higherone.

The most difficult challenge in achieving higher power densities is exemplified in Fig. 16(c), i.e.,

the marked effect of R, which represents the effective resistance causing a linear variation of overpotential

(predominantly ohmic) with current density. As stated in Section 3.4, the best performance has been

achieved in the single cell with the Dow membrane and hence in Fig. 16(c), the highest R value used is

that for the best performing cell (Fig. 4). The other extreme is for the case when R = 0, which is a

hypothetical case, where the overpotential loss is only due to activation control of the ORR. The

intermediate value chosen is R = 0.08 ohm cm 2. The corresponding values of the cell potential at a current

density of 2 A/cm 2 are 0.80, 0.60 and 0.64 V, respectively. The assumption that R = 0 is invalid because

R includes at least the contributions of the ohmic resistance of the electrolyte layer and of the charge

transfer resistance of the hydrogen oxidation reaction. Further, it will be extremely difficult to completely

eliminate resistances of contacts, fixtures, including bipolar plates in multi-cell stacks. Assuming that

(i) these resistances are negligible; (ii) the electrolyte resistance is 0.05 ohm cm 2 (using a specific

resistance of 5 ohm cm and thickness of the 100 i.tm for the electrolyte); and (iii) the charge transfer

resistance for hydrogen oxidation is 0.025 ohm cm 2 (which is the best value obtained in our laboratory,

the total value of R is 0.075 ohm cm 2 (i.e., very close to the assumed intermediate value of

0.08 ohm cm2). This brief analysis gives a realistic estimate of a cell potential of 0.64 V at a current

density of 2 A/cm 2, corresponding to an energy efficiency of 43% and a power density of 1.28 W/cm 2

(assuming Eo = 1.0 V and b = 0.055 V/decade). The maximum power density for this cell will be about

3 W/cm 2. If, however, Eo is 1.10 V instead of 1.0 V, the cell potential will increase to 0.74 V at 2 A/cm 2

and thus the energy efficiency and power density at this current density will be 50% and 1.48 W/cm 2.

108
Fig. 16(d)showsthevalidity of thepresenttreatmentfor thebestperformancesachievedin single

ceils with high andlow platinumloadingelectrodes.Thereis closeagreementbetweentheresultsof the

numericalsimulationandexperimentalresults.

4.3 Needed Research and Development to Attain Projected Performance Levels

4.3.1 Increase of Platinum Utilization and Use of Alloy Electrocatalysts

One of the main contributions to the loss in efficiency and power density in PEM fuel cells is the

activation overpotential at the cathode, which is highly visible from the cell and half cell performance in

Fig. 15 (single cell with electrodes having a Pt loading of 0.45 mg/cm2). This figure also shows that,

over the entire current density range, the potentials in a cell with the high platinum loading electrodes are

about 50 mV higher at all current densities than those in a cell with low platinum loading electrodes.

However, because of the vital importance to develop fuel cells with low platinum loading electrodes, a two

pronged attack should be followed: (i) increase the effective utilization of platinum, at least by a factor of 2

over the present value (= 10%); and (ii) use alloy electrocatalysts with higher exchange current densities.

Possible approaches for the former are use of (i) still thinner active layers with a higher percent platinum

on carbon (say, 40% Pt/C); and (ii) vacuum impregnation or incorporation of the proton conductor (dr, ring

fabrication) into the electrode to further extend the three-dimensional reaction zone. In the case of the

latter, alloys of platinum with Co, Cr, or Ni, which have been successfully used to enhance oxygen

reduction kinetics in phosphoric acid fuel cells, need be evaluated [16].

4.3.2 Optimization of Structure of Elet;trode and of Membrane and Electrode Assembly to

Minimiz_ Mas_ Tr_ans.por_ and Ohmic Overpotentials with Air as Reactanl

One of the challenges in PEM fuel cells is to minimize mass transport and ohmic overpotentials.

As seen from Fig. 11 and 12, when using hydrogen/air rather than hydrogen/oxygen as reactants, the

slopes of the linear region in the cell potential vs. current density plots are about 50% higher. Further,

these figures show that at higher current densities mass transport limitations are encountered with H2/air as

reactant, but not with H2/O2. To minimize these rate-controlling processes, optimizations of the structure

of the electrode and of the membrane and electrode assembly are essential. Possible solutions are

(i) altering the Teflon content of the electrode; (ii) use of electrodes with thinner diffusion and active layers;

and (iii) incorporation of a proton conductor during the fabrication of electrodes.

4.3.3 CO-Tolerant Electrocatalysts for Hydrogen Oxidation and Methanol Oxidation

Electrocatalysts

In the above sections, hardly any mention has been made about necessary improvements in the

hydrogen electrode, because most of the experimental results quoted are when pristine hydrogen is used as

the anodic reactant and oxygen or air is the cathodic reactant. Pure hydrogen and oxygen are, invariably,

the reactants for extraterrestrial applications. However, for transportation and other terrestrial applications

where PEM fuel cells will be used, methanol is presently the fuel of choice. Because of the low activity of

the best electrocatalysts for methanol oxidation, the indirect route (reformer - fuel cell) is most promising

for the near to intermediate term time-frame (say, up to the year 2010). The CO content of the reformed

fuel is 1%, which cannot be tolerated by the anode. Ballard Power Systems made excellent progress in

reducing this value to about 100 ppm by selective oxidation - passing the reformed gas with 1% to 2%

oxygen over a platinum/alumina catalyst at 160°C. Even 100 ppm of CO poisons the platinum anode, but

platinum/ruthenium alloy electrocatalysts are tolerant to this level of CO. It is essential that investigations

be made on alloy electrocatalysts and/or oxide supports, which enhance CO oxidation and reduce the

poisoning problem.

109
Finding CO tolerant electrocatalystsand/or oxide supports may also solve the "Fuel Cell

Researcher'sDream" - the direct oxidation of methanolin fuel cells. FTIR studiesreveal that the

intermediate,which is formedduringmethanoloxidationandcausesa poisoningproblem,appearsto be

the sameasthepoisoningspeciesadsorbedfrom carbonmonoxide[17]. Potentialelectrocatalystsfor

methanoloxidation,which areof theredoxtype(i.e.,electrochemicaloxidationof themetallicspeciestoa

highervalencestate,whichchemicallyoxidizesthemethanol)arePt-Ru,Pt-Sn,Pt-Moalloys.

5. CONCLUDINGREMARKS

Theconclusions,whichmaybedrawnfrom thisarticlemaybesummarizedasfollows:

(i) PEM andalkalinefuel cellsaretilestrongestcandidatesfor achievinghighenergyefficiencies

andhigh powerdensities,vitally neededfor extraterrestrialandterrestrialapplications.The

PEM fuel cell hasthe advantageoverthe alkalinefuel cell becauseof CO2toleranceand

longerlifetimes.

(ii) Significant progresshasbeenmadein the attainmentof high energyefficienciesandhigh

powerdensitiesin PEM fuel cells with high (4.0mg/cm2) andlow (0.4 mg/cm2)platinum

loadingelectrodes.

(iii)

(iv)

Novel approacheshavebeenusedin PEM fuel cells with low Pt loadingelectrodesto attain

the samelevels of performanceas thosewith high Pt loading - (a) proton conductor

impregnationintotheelectrode;(b) optimizedconditionsfor thehot-pressingof electrodesto

PEM'sandof pressureandtemperature;(c) localizationof platinumnearthe front surface;

and(d) useof thinmembraneswith betterconductivityandwaterretentioncapability.

Electrode kinetic and masstransportparametersfor ORR at a Pt/PEM interface were

determinedusing a Pt microelectrodeand pseudo-steadystate,transientand impedance

spectroscopictechniques.Valuesof theelectrodekinetic parametersarecomparableonthe

smoothmicroelectrodesandhighsurfaceareafuel cell electrodes.

(v) Theabsenceof netwatertransportfrom anodeto cathodewasdemonstrated.

(vi) A modelinganalysiswascarriedoutto demonstratethattheperformancelevelsobtainedin

PEM fuel cells areapproachingthoseprojected,which arebasedonrealisticvaluesfor the

electrodekineticandohmicparameters.

(vii) In order to achievethe projectedperformancelevels, it is necessaryto (a) improve the

effectiveutilization of platinumfrom 10%to about50%and/orusePt alloy electrocatalysts

for the ORR; (b) find membraneswith betterconductivity; and(c) discoverCO tolerant

electrocatalystsfor hydrogenoxidationandelectrocatalystswith ahigh activity for thedirect

oxidationof methanol- thelatteris the"FuelCell Researcher'sDream". Boththeseareasare

vitally importantforthetransportationapplication.

ACKNOWLEDGEMENTS

Theauthorswish to expresstheir sincereappreciationto Dr. EdsonTicianelli of theUniversityof

SaoPauloatSaoCarlos,Mr. CharlesR. Derouinof LosAlamosNationalLaboratoryandDr. Mohammad

A. Enayetullah of Johnson Matthey, Inc., who made significant contributions to the work carried out at

LANL and/or TAMU. The research work at LANL was sponsored by the U.S. Department of Energy and

at TAMU by the Defense Advanced Research Project Agency, Energy Research in Application Program -

Texas Higher Education Coordinating Board, Electrochem, Inc., Center for Space Power - TAMU and

ii0
NASA Johnson Space Center. The authors are also grateful to the Dow Chemical Company (Drs. Jeffrey

A. Gunsher, Glenn A. Eisman and Alan Cisar) and DuPont Chemical Company for providing the proton

exchange membranes for this work.

REFERENCES

1. A.J. Appleby and F.R. Foulkes, "Fuel Cell Handbook", Van Nostrand Reinhold, New York (1989).

2. S. Srinivasan, J. Electrochem. Soc., 136 (1989), 41C.

3. K. Prater, "Fuel Cells - Grove Anniversary Symposium: 1989", D.G. Lovering, Ed., Elsvier

Applied Science Division, London, New York (1990), 239.

4. E.A. Ticianelli, C.R. Derouin and S. Srinivasan, J. Electroanal. Chem., 251 (1988), 275.

5. S. Srinivasan, D.J. Manko, tt. Koch, M.A. Enayetullah and A.J. Appleby, J. Power Sources, 29

(1990), 367.

6. I.D. Raistrick, U.S.P. 4,876,115 (10/24/1989).

7. S. Srinivasan, E.A. Ticianelli, C.R. Derouin and A. Redondo, J. Power Sources, 22 (1988), 359.

8. A. Parthasarathy, C.R. Martin and S. Srinivasan, J. Electrochem. Soc., In press.

9. W. Paik, T.E. Springer and S. Srinivasan, J. Electrochem. Soc., 136 (1989), 644.

10. A. Parthasarathy, B. Dave, S. Srinivasan, A.J. Appleby and C.R. Martin, Paper to be submitted to

J. Electroanal. Chem.

11. A. Parthasarathy, S. Srinivasan, A.J. Appleby and C.R. Martin, Paper in course of preparation.

12. C.P. Winlove, K.H. Parker and R.K.C. Oxenham, J. Electroanal. Chem., 170 (1984), 293.

13. H. Koch, A. Nandi, N.K. Anand, O. Velev, D.H. Swan, S. Srinivasan and A.J. Appleby, Extd.

Abstracts of the 178th Meeting of the Electrochemical Society, Seattle, Washington, October 14-19,

1990, Abst. # 115.

14. A. Damjanovic and V. Brusic, Electrochim. Acta, 12 (1967), 615.

15. J.O'M. Bockris and S. Srinivasan, "Fuel Cells: Their Electrochemistry", McGraw Hill Publishing

Company, New York (1969).

16. D.A. Landsman and F.J. Luczak, U.S.P. 4,316,944 (2/23/1982); U.S.P. 4,373,014 (2/8/1983).

17. K. Kunimatsu, Ber Bunsenges Phys. Chem., 94 (1990), 1025.

111
TABLE 1

Effectsof LocalizationofPlatinumNearFrontSurfaceof Electrode

on HydrogenAdsorption Charge and Roughness Factor as Determined

by Cyclic Voltammetry and on Percentage of Platinum Utilization

Method of Pt Localization QH/mC cm "2 Roughness a Factor Utilization of ptb/%

10% Pt/C 4.4 20 4

10% Pt/C+50 nm Pt f'dm 9.6 44 9

20% Pt/C 11.4 53 11

20% Pt/C+50 nm Pt Film 15.3 70 14

40% Pt/C 17.0 77 15

40% Pt/C+50 nm Pt trim 19.2 87 17

Calculated assuming 220 gC/cm 2 for QH on smooth Pt.

Estimated assuming a 2.0 nm particle size for the Pt on C catalyst, as was shown by electron

microscopy of the materials. % of Pt utilization = (roughness factor/total Pt surface area x

100).

TABLE 2

Electrode Kinetic Parameters for Oxygen Reduction at the Pt/Nafion interface

using Fuel Cell Electrodes and Microelectrodes.

Electrode/Membrane Interface

@ 50°(2, 1 atm pressure

Unsupported Pt black (10mg/cm2on carbon cloth

(ElectroChem Inc.)/125-I.tm Dow membrane

Platinum supported on carbon (20%Pt/C),

Pt loading 0.4 mg/cm 2 ; Nation

impregnated-4 mg/cm 2 0Etek,Inc)/

I25-I.tm Dow membrane

100-[.tm Pt microelectrode/175-1am Nation

@ 25_C, 1 atm pressure.

Pseudo-steady state voltarnmetry

100-1am Pt microelectrode/175-l.tm Nation

@ 25 C, 1 atm pressure.

Electrochemical Impedance Spectroscopy

Tafel slope

b (mV/dec)

-58

-64

-63

-118

Exchange current

density, i o (A/cm 2 ) *

3.09 x 10 -7

1.09 x 10 "7

2.05 x 10 -9

7.8 x 10 "7

-65 4.09 x 10-8

-133 5.9 x 107

r.f. 773

r.f. 52

r.f. • roughness factor

• - Normalized for electrochemically active surface area

112
TABLE 3

Transport Parameters for Oxygen in Nation from

Measurements at Pt/Microelectrode Interface

Diffusion coefficient,D

(cm2/s)

Solubility, C(mM)

7.4 x 10

26

-7

TABLE 4

Electrode Kinetic Parameters for PEM H2/O2 Fuel Cells, As Evaluated by Non-Linear

Least Squares Fit of Experimental Results. Also Included are

Current Densities at Cell Potential of 0.9 V, Temperature 85"C, Pressure 5 atm.

Electrode, Platinum Loading,

Membrane

Electrochem, Inc., 10 mg/cm 2 Dow

Membrane, 125 I-tm

Prototech, 0.4 m_cm 2 Nation®,

175 gm

ibid, 100 gm

ibid, Dow Membrane 125 I.tm

F.o

V

1.065

1.007

1.008

1.002

b

V/decade

0.055

0.046

0.045

0.053

R

ohm cm 2

0.110

0.298

0.201

0.111

i0.9

A/cm 2

3O0

7O

61

62

113
1.0

0.9

0B

O.7

ruJ

I--

o

06

05

B _- 0.4

.J

.J

w 0.3

O

0.2

0.1

0

0 40 80 120 160 200

CURRENT DENSITY (mA/cm 2)

Fig.[a. Cell potential/current density plots for H2/O2 fuel cells at 50 °C and 1 atna pressure (electrodes 5 cna_-): A and C, Nafion-inapregnated and as-received Prototech electrodes (0 - 35 nag cna -z Pt) hot-pressed to Nafion membranes; B, GE/HS-UTC membrane

and electrode (4 nag cna -z Pt). Solid lines, conaputer generated plots; *, experimental

points.

>-

m

tair

cJ

I I l ! ] I I

I OXYGENELECTRODE/50"C

20maitre2 _ PEMD-120

-- - PEM# 3

..... PEPdw5

I

I

I

II

I1

Jl

......................... o;t; -:: ;-.J....,.

,o/

I 1 ! I I .1 i

0 0.2 0.4 0.6 0,1 1.0 1.2

HALFCELLPOTENTIAL(VIRHE)

Fig. Ib.Cyclic volt.amnaograms on oxygen electrodes in fuel cells (electrodes 5 cna2). PEM

D-120-cell with Nafion-impregnated Prototech electrodes (0.35 nag cm -2 PL)hot-pressed

to Dow membrane; PEM #3-cell with GE/HS-UTC membrane and electrode assembly

(4 nag cm -z Pt); and PEM #5-cell with as-received Prototech electrodes (0.35 nagcna -2 Pt)

hot-pressed to Nafion membrane.

114
1.2{ i

I 1

1.0

0.8

0 0.6

0

0.4

l-

(}.2 -

0 I I ]

0 200 400 600 800 1000

CURRENT DENSITY/mA.cm "2

Fig. 2. Effect of localization of Pt near front surface of Prototech electrodes on performance of

oxygen electrode in H2/O2 single ceil at 80"C and at 3/5 atm. 40 wt% Pt/C plus 50 nm

sputtered f'flm of Pt (o); 20 wt% Pt/C plus sputtered f'dm of Pt (_); 20 wt% Pt/C (A); and

10 wt% Pt/C (0). The H2 counter electrode in each case was the same as the working

electrodes (4).

1.0

_>

© 0.8

C

@

,oJ

o 0.6

(p

r_ 0.4

0.2

0

o I'L,PtCI, brushed

L * P{_in l_flion sol.

v Electroplated Pt "

i-_,_,_ v Sputtered PL

" _ _ o Control Prototech

''0

- _" _._.'_-.L...__ electrode

" "0" "0 '0_

-- "O- ,O..O "

"='O.

''_..

'O,

o.

, J , , J = L J | , I J J i I J , 1 . i J _ , , , , I I . , J , . . ]

500 1000 1500 2000

Currentdensity,mA/cm _

Fig. 3. Cell potential vs. current density plots for single cells with electrodes having localized Pt

layer deposited by different methods, all operating at 95"C and with H2/O2 at ,#/5 atm.

115
|.0

,.2

© 0.8

Ig

o 0.6

_J

0.4

0.2

0

. v I)_" 125-,'-

• He.flo,,50 =m

u_ a HLI'IonI00

::_ _ o II_io= 175-,_

500 tO00 [500 2000

Current density, mA/cm 2

Fig. 4. Cell potential vs. current density plots for single cells with different membrane materials,

operating at 95"C with H2/O2 at 4/5 atm. Pt loading on each electrode: 0.45 mg/cm 2 (5).

TO I:X)lt'¢_l_1;tJ !

Fig. 5: Schematic of the solid state cell incorlxn'ating a platinum microelectrode (8).

116
Fig. 6:

Fig. 7:

,;.:2

o

t.:.l

0.7

0._'

-63 •V/decade

' 1

-1.5

0.5 -2.5

.4.5 -3.5

log kft" i )J

Mass transfer corrected Tafel plot for oxygen reduction at the platinum

microelectrode/Nafion interface, at 25 "C (8).

lt_,?

•m

| t-_.7

!

L,_ -'" '_ - 77Cobn'a)

h

$._

o,,., k ._;._)

I

I

u_lKi

,mid•

| i

_1_5 '1_.3 3-,_+5 3/c+$

77(Ohms)

Nyquist plot obtained at open circuit at the platinum/Nation interface showing charge

transfer relaxation; inset shows high frequency membrane relaxation process (10)-

117
0.9

0.8'

0.7 i i ! n

0.0 0.5 I.O 1.5 2.0 22

logi (mA/c_2j • u_mppa-_ _

• ,_,,tZtxx',,d=,,'_"_

Fig.8: iR correctedTat'elplotsforoxygen reductionatmembrane electrodeassembliesusing

unsupportedand supportedpladnum gasdiffusionelectrodes(II).

<

..5

E

t=.

¢.,)

50O

00

300'

2OO

m

o....d.

°

°.o"

00 i t _ n

0.0 0.2 0.4 0.6 0.8 1.0

t-1/2 ($eC-I/2)

Fig. 9: Current versus t-1/2 plot at the platinum/Nation interface to determine mass transport

parameters of oxygen in Nation (8).

118
i.i

i.O

0.9

'-" 0.8 a_

,r.,I

0.7

0

0.6

0.5

0.4

I i''''i !

• 95°C

• 85°C

" _" _S"

" .,,o._: -'r:

-o, -v

"0, "l " .

-0 -v

"0 .

o 70°C

o

o 50C

. -:.. -:.

"0 - . "V

-e

"0 .

"0

"0

500 1000 1500 2000

Current Density, mA/cm 2

Fig. 10 Effect of temperature on the performance of a PEM II2/02 fuel

cell, Pt loading of electrodes 0.45mg/cm 2, Dow membrane, 5 arm.

1.1

1.0

0.9 _: "-.

> 0.8

© 0.7

0.6

o

o.5L

0.4

0.3

0.2

I I I''''I

• 5 atm 0 z • 5 arm Air

o I arm 0 z - 1 arm Air

- "v - "6.

" _ , "4D.

"V "

"I=. "v .

_0 , "V .

• "0.

-0.

"at

_V

0

"ID.

"e .

"_r. "e .

-o .

"" "o .

_v -o .

-e .

-o

"o

-o

v

.,¢ZI_,,I__JJA|JlJJ--L

500 1000 1500 2000

Current Density, mA/cm 2

Fig.11 Effect of pressure and reactant gas on the performance of a PEM

fuel cell, Pt loading of electrodes 0.45mg/cm 2, Dow membrane,

50oc.

119
1>

a_

O

D_

1.1

1.0

0.9

0.8

0.7

0.6

0.5

0.4

0.3

0.2

| = | = i

. 95°C O;z

"-o

' I ' '

- 95°C Air

,, 50°C /dr

-Q °

-0 .

-i

"0

• "w

0 500 1000 1500 2000

Current Density, mA/cm 2

Fig.12 Effect of temperature and reactant gas on the performance of a

PEM fuel cell, Pt loading of electrodes 0.45 mg/cm 2, Dow

membrane, 5 atm.

•--[(cF2CF2)dCF2_F)J.cOCF2?FCF3

OCF2CF2SO3H

/1'= 6.6

-[(CF2CF2L(CF2_F)JzOCF2CF2SO3H

n = 3.6--.10

DuPont's Nation® Dow Perfluorosulfonate Ionomers

Fig.13 The skeleton structure of DuPont's NAfion@ and Dow's

perfluorosulfonate membrane

120
8

6

O

0

2

0

o

o o y

• 0 0 0 0

'8 law

_ , , • , , , , • , , I J • I • i l • , • , , •

5 10 15 20 25 30

Current, A

Fig. 14 Water production rate in a 50 cm 2 area PEM H2/O 2 fuel cell

with Dow membrane, 50oc, l atm.

i.O

0.8

0.6 o E_

,....1

,.,.a

o 0.4

0.2

0.0

for 0.45 mg/cm s PL loaded

a go. elecLrodes.

• Ea,

P" ,r,, - --- , _ f _ , _ T "

0 500 1000 1500 2000

Current Density, mA/cm z

Fig.15 Half cell and single cell performance in PEM H2/O 2 fuel cell,

95°C, Dow membrane, 5 atm.

121
•_ ,_ c_ d ci o

A '[_11ualod

.... I .... I .... I .... I .... I "'"

-'>i>> I,'I/ " ,4 ._ "

i i t I I

i t • /

-_ '/// /

/,

/_//

i/'l/" F ,(,if/

.i 0 Ol _ l'- ID ll3

A '[u!lua10d

0

0

• o

o

0

o_0 o

° A

o U

_3

o

• '_',' b',,', '_ _w..'... ,_. •., ....

II _ i I I_ _ • J

-_ _.=_-_-." _ /

I,'._ fI__ d /

_. ,_ I_1.i', . /

T I "lj /

•4 .._ d o ci o o

A 'l_llUOlOd

.... I .... I .... I .... I .... I ....

N

° I

-'_ " C!

oS°!

l t • • /

'//

o

ci o d o o

o

c_

g

i'_

_'_ _1

0 0_"_

122
N91-32559

i •

i-BIEM CALCULATIONS OF THE FREQUENCY DISPERSION AND AC CURRENT

DISTRIBUTION AT DISK AND RING-DISK ELECTRODES

Boris D. Cahan

Case Center for Electrochemical Sciences and

The Department of Chemistry

Case Western Reserve University

Cleveland, Ohio 44106

ABSTRACT

The Iterative Boundary Integral Equation Method (I-BIEM) has been applied

to the problem of frequency dispersion at a disk electrode in a finite geometry. The

I-BIEM permits the direct evaluation of the AC potential (a complex variable) using

complex boundary conditions. The point spacing was made highly non-uniform, to

give extremely high resolution in those regions where the variables change most

rapidly, i.e. in the vicinity of the edge of the disk. Results are analyzed with respect

to "IR correction", equipotential surfaces and reference electrode placement. The

current distribution is also examined for a ring-disk configuration, with the ring and

the disk at the same AC potential. It is shown that the apparent impedance of the

disk is inductive at higher frequencies. The results are compared to analytic

calculations from the literature, and usually agree to better than 0.001%.

INTRODUCTION

One of the more useful electrode configurations for use in electrochemical

studies is the Rotating Disk Electrode (RDE) and the Rotating Ring-Disk Electrode

(RRDE). As such, they are often available in many laboratories and are used as

general purpose electrodes, even without rotation, and often for applications for which

the geometry is not well suited. Although a number of papers have appeared in the

literature which treat the frequency dispersion problem of such electrodes, most have

been analytic 1'2'3 solutions with restrictive boundary conditions, and/or restrictive

boundary geometries. The choice of geometry and boundary conditions used in finding

analytic solutions is often governed as much by the feasibility of formulating the

proper relationships so as to yield a solution as by their relevance to the problem at

hand. Real cells are usually of finite dimension, and rarely do electrochemical

interfaces have the linear responses invoked in heat transfer problems.

Some of the problems that still need attention are the questions of:

1. The local distribution of current, and the partition of current between ring and

disk,

2. The placement of the reference electrode, and the meaning of AC equipotentials,

3. The influence of the insulating gap,

4. The interaction of the ring and disk when they are driven by different potential

regimes,

5. The effect of real non-linear kinetics both in terms of overall reactions, and in

pseudo-capacitive substeps.

This paper will deal primarily with items 1 and 2. The last three will be the sul)j(_cl

of a later paper.

The I-BIEM4 has proven to be a powerful algorithm for solving problems of

this type with difficult boundary conditions and geometries. It has been applied here

to the analysis of the impedance of the RDE and the RRDE. A number of changes

have been made to the original algorithm which place the method on a more rigorous

footings and eliminate some of the initial approximations. These are discussed briefly

123
below. Details will be given in a separate publicationS.

MODIFICATIONS TO I-BIEM

We are seeking the solution to Laplace's equation for a domain fl with a

-boundary F

V2¢ = 0 (etlcl where ¢ is the dependent variable (e.g. potential, temperature, concentration

with general boundary conditions

where D is a differential operator, and x" the space variable (x,y,z). On a Diric et

e.g. conducting) boundary, f = f (¢,x) while on a Neuman (e.g. insulating) segment

= f (Dl¢l,x) at most. Choosing a function G to be a free space Green's function,

G(x,x'), where x and x' are coordinates at two points, such that _7_G(x,x ) =

6(x,x'), we can show from the divergence theorem that

_¢(x) = f,v2¢(x,)C(x,x,)_ + [¢(x,)C.(x,x,) - ¢.(x,)a(x,x')ldr (3)

where a is the Cauchy principal value of the integration of the Green's function

singularity. [For example, in two dimensions, e = 0 for points outside F, c_ = 2r

inside, and a = _r on a straight boundary.] Here ¢* and G* are derivative functions

of ¢ and G respectively. In cases where V2¢ = 0, the first integral in (3) vanishes

and

_¢(x) = [¢(x,)a (x,x,) - ¢ (x,)a(x,x,)]er (4)

For the function ¢ on F, we write

# # #

¢(x) = Y,Ni(x)¢i , ¢ (x)= EYi(x)¢i (5)

i i

where ¢i and ¢i are the values at discrete points on the boundary and Ni are shape

functions. If we integrate piecewise over F, using a finite number of points xj, eqn.

(5) becomes

nl m

_aijCj = _bij¢'j + ci i=l,2,...,m (6)

j:l i:1

where

o,J=J.r

rj

bij - Ni(x')C (x,x/)dr

J

Ci = _ F(xk)O(x,xk)

k

In a conventional BIEM, the boundary conditions are introduced into the

system of equations (6) and then assembled into the form

Ill

_dijuj = i= 1,2,..-,m (7) ei

j=l ¢* nodes. Equation (7) is then where the uj represent the unknown ¢ or at the m

124
inverted to obtain these unknown quantities. For problems with non-linear boundary

conditions equation (7) becomes a non-linear system of equations, and the assembling

process may even have to be repeated a number of times to use linear matrix

inversion techniques. In a system where the point spacing is highly non-uniform, the

inversion of the matrices can introduce near-singularities.

The approach of the I-BIEM is different since the assembly and inversion

symbolized by equation (7) is never performed. Rather, equation (6) is utilized

directly. To solve for the unknown ¢ and ¢* at the nodes, we start by guessing

values Ce and ¢* for these quantities. The actual values chosen are rather

e

unimportant. For Dirichlet (known Ce) or Neuman (known ¢_) boundaries the values

can be used once and for all and absorbed into the ci of eqn. (6). For non-linear

mixed boundary conditions, the iterative process described below readily accepts an)'

root finding process in the solution. When the guesses are used in the equation for

the ith node as origin of integration, the result is an error:

m ii1

= --E aiJCej- __EbiJCej 4- ci (8)

j= l j:l

Since the largest contribution to ei comes from the j = i terms, the j _ Z

terms of equation (8) can be used to get an expression for updating the unknowns at

the ith node. Thus:

_ '

Neuman Boundary ¢i : ¢ei Aaii

* * - A c (gb) Dirichlet Boundary ¢i -- eel

Mixed Linear Boundary aii (¢i- eel ) - bii (¢i - eel ) = -)_ci (9(')

Non--linear Boundary aii * Aei (9d) (¢i - eel ) - (B(¢i) - eel ) =

where the B(¢i) represent the functional relationship between the ¢ and ¢' from the

boundary conditions and A is an over-relaxation factor which can be shown 5 to be

z 0.6 - 0.75.

The updating process is continued for all points in sequential fashion for k

passes, until a suitable convergence criterion, ek = Ze_ < _ (j = 1 to N) is satisfied,

where _ is an arbitrarily small number, as desired. [_he smaller the value of _, the

more accurate the solution, but the longer the iteration time.]

For two dimensional problems the G(x,x') function is

a(x - x') = In r (10)

where r is the distance between x and x',

G * = _n (In r) = 7_-_ 1 Or = 71 (Vr.n) = -7 1 (11)

and equation (4) becomes (see equation (3) of ref [4])

fr Or_ *

= ¢ (x')ln fldr

Similarly, in three dimensions G(x - x') = 1/R and

125
IMPLEMENTATION OF THE ALGORITHM

In order to evaluate Cei (or ei) in eqn. (8) at a point x, we select a finite

number of points Pj (j = 1,2,3,..-N) on the boundary P and make initial guesses for

the ¢(x')j and ¢*(x')j. For simplicity, we choose a linear interpolation (or basis)

function for the variable, i.e. the values are chosen to vary linearly between x' at j

and j+l. (For non-linear boundary conditions this is not strictly true, but if the

points are chosen sufficiently close, this error is negligible.) Equations (12) and (13)

then become

_¢(x)= J:, j j¢(x,) _ _ dr - xJi¢'(x') In r dr

(14)

and

remembering that x1',, i - x]. Since we have used a linear basis function, the terms in

Cj and Cj can be taken out of the integral and the integrations, which are functions

only of the geometry and not of the ¢% are then simple linear combinations of

terms like equation (8),

-_e(x)j j:, (lSa)

f xi *'1 f'i., For 2-D Or

xl 7 _ dr = H2j - HIj and In r dr = Hgj - H3 i and

. X i '

fl i''° [_] f xi*'l -.sj for 3-D O-h dF = tt2j - Itlj and , _ dP= H4 i

• Xj

where the Hnj are constants which can be stored in a one-dimensional sequential

(and therefore dense) array. For the two dimensional case, the integrals can be

evaluated analyticallyL or solved by some other means of quadrature. We then use

the relations (9) to update the boundary values sequentially.

:¢

For an axi-symmetric geometry, the functions G and G (= OG/On) can be

rewritten7 in terms of the cylindrical coordinates (r,z) in the form

G(r,z) - 4 K(m) (16)

where a = _ + r 2 + z,] , b -- 2rri , zd = z- zi ,

4rri (I 7)

m = (r + ri) 2 + Z

126
OG OG OG

Also, _ = _-F nr + -0"-z- _

_ 2 nr [E(m)- U(m)]-4 n[_ r-ri)+ nzZ] E(m) (18)

(a - b)4a + b

where (n,_,n._.) are direction cosines, K(m) and E(m) are complete elliptic integrals of

the first and second kind. When these are substituted into eqn. (15), the expression

can no longer be integrated analytically, but can be approximated adequately using

e.g. Gaussian_ or adaptive quadrature. The elliptic integral can be approximated by

suitable polynomial expansions 8 or from Lamdens' transformation °. ]Note that the

constants for the expressions used for Gaussian quadrature and for the elliptic

integrals in ref. (7) contain several typographic errors, and should be obtained

directly from ref. (8) or some other suitable source.] Although these, integrals are

much more complicated than those for the two dimensional case, their values can

also be stored in a dense linear array, and evaluated once for the entire process.

In the direct BIEM there are several classes of singularities which must often

be treated carefully. [In a recent conference on Boundary Element Technology_0 seven

papers dealt with aspects of this problem.] For example:

1. Discontinuities or multiple valued points.

In the integration of each segment, Fi, there is, in effect, a term in Ax in tlw

denominator. If Ax -_ 0, The matrix becomes singular on inversion. If the

spacing is very small, the matrix becomes near-singular and imprecise' on

inversion.

2. Acute or re-entrant corners.

At any corner where the two adjacent sides are not orthogonai or straight, the

normal derivative has multiple (two or more, depending on the get,merry)

values, but only one independent equation.

3. Integrable singularities at intersections with different boundary conditions.

At an edge between two different boundaries, such as the one trealed in this

paper, the value of the current density can go t_ an integrable infinity. Such is

the case for primary current distributions. This is evidently not possible wilh

the finite number set of computer calculations. It also does not conform well t_)

a linear basis function.

In the I-BIEM, many of these simply do not occur, or can be safely ignored.

1. Multiply defined values.

Since the matrix (7) is never inverted, multiple values cause no problems. Each

element is simply integrated and summed. A multiply defined value is

integrated using the value appropriate to its element, and the "missing" element

of zero size is ignored.

2. Non--orthogonal intersections.

Consider any one corner where such a condition occurs. If the value at thai one

point were known, and all of the other values known, then from eqn. (14) a

value can be determined which uniquely satisfies the equation, such that all

sub-integrals containing that point are defined. This situalion actually occurs at

any curved boundary which is represented by a set of linear elements. In the

process of iteration, as long as e.g. the normals are interpreted consisler_lly and

the process is convergent, all values will be the "best fit" to the pr_bh.'nl ir_ lh¢ '

sense of a "least error".

3. Integrable singularities.

Similar reasoning can be used to show that the integral(s) over any _'lcm_'n_(s)

with the singular point as an end point must be ezactly equal t_, lh_, t_,

integral over the integrable singularity, even though the numerical value ,it th,ql

point is finite.

127
In all such cases, the element containing the "problem" may be made as small as

desired, without difficulty.

It may be noted here that the a's are sometimes difficult to eval,_lp, e.cpecinlly

when the boundaries are highly structured. However they may be evaluated simply

from the same equation (14 or 15) by the simple expedient of setting all of the 4>

terms to unity, and all of the _b• terms to zero.

NEAR-BOUNDARY ERRORS

One of the problems associated with solutions found with the BIEM (or BEM)

is that values computed for points just inside the boundary after solution suffer from

reduced accuracy. It is stated in ref. [7] pages 31-32, "...there exists a 'boundary

layer' where the numerical accuracy suffers...As a rule of thumb, a degeneration of

accuracy may occur at distances less than one element length from a boundary." This

problem does not appear to exist in 2-D problems (where the exact integral4 can be

used) with the I-BIEM, or at least is much less severe. For the axi-symmetric 3-D

case the problem does exist, although to a very small degree, and only on some

boundaries. We have not yet been able to establish a fundamental reason for this

discrepancy.

It has been found that precision of the math used is very significant. With

single precision, the results with the axi-symmetric case are much less accurate,

dropping to 0.1-1.0% agreement for some cases. We also use 16 point Gaussiarn

quadrature for the integration, to preserve precision as much as possible.

A related question deals with the value of _b on the boundary, but between

points. On a straight orthogonal segment, the problem is solved using linear

interpolation between the end-point values of each element. When such a value is

computed for _, it does not yield a value on the straight line. This is especially

noticeable on boundaries with a high curvature of the potential, or a low number of

points on a segment. Rather, the curve generated by computing values at closely

spaced intervals along the curve after convergence for a coarser grid is very smooth.

Some problems have been experienced on curved non-orthogonal boundaries.

This is clearly related to the problem of deciding whether a point is on, inside or

outside of a line with finite precision math. Even with a high precision, a point

which is mathematically on a line does not in general coincide ezactly_l with

computer floating point numbers.

APPLICATION TO TIlE RDE AND RRDE IMPEDANCE PROBLEM

1. Choice of Geometry

In order to verify the technique and at the same time to perform a useful

calculation, it was decided to make a comparison to an existing solution of the RDE

problem, constructing the geometry so as to approach that used for the analytic

solution of Newman 1. However, his solutions were generated with the counter

electrode "at infinity". With any numerical technique it is difficult to know where to

terminate the calculations with such a boundary. A real cell is never infinite,

however, and it is desirable to be able to incorporate "real" cell geometries into the

solution. The equipotential surfaces for primary current distribution (including the

"infinite frequency" behavior of an RDE) with the counter electrode at infinity, in

cylindrical coordinates

z = air/

r = a_(l+_ 2) (1-r/_) (21l)

where r is the distance from the axis of symmetry, z is the normal distance from the

disk, a is the radius of the disk, and _ and 77 are rotational elliptic coordinates, were

shown 12 to be

¢/¢0 = 1 - (2/_r) tan-l_ (21)

where _b is the potential at some 4, and ¢0 is the potential difference between

/

128
infinity (counter electrode) and the disk. Rearranging (21) for the conditions (19),

- tan [(r/2)(¢/¢ce)] (22)

where ¢_e = 1 - ¢/¢0. Many o_the calculations were done u,_ing l.he localion of

these equipotentials as the location of the counter electrode. For the primary current

case or for "infinite" frequencies the solutions are identical to Newman's results,

differing only by a scale factor. [It will be shown later that at finite frequencies, this

is no longer true. Indeed, the concept of an equipotential surface of the complex

variable _b has no meaning except in very special cases.] The scale factor actually

used in the calculations was determined by the desirability (for plotting purposes) of

maintaining the vertical dimension, z, as unity. Then, after the choice of ¢/¢ce and

the determination of _ from eqn. (22), a, the disk radius can be found from eqn.

(20), when r/=l (i.e. on the vertical axis) as a=l/_. The radius of the equipotential

at the plane of the disk is thus r = a4(1+_2 ) = _/_.

Figure la shows the usual geometry used. Other-geometries (Fig. lb) were also

used, because they were more amenable to the rectangular coordinates used for some

of the more difficult graphics routines. These represent finite cylindrical cells with the

counter electrode as a large disk parallel to the RDE, with a cylindrical electrode

coaxial with the RDE, or both.

In all calculations, the current in the outer portion of the disk was integrated

separately to model the RRDE used as a segmented electrode with both segments

driven at the same potential. In some calculations a small insulating segment was

interposed between the ring and the disk.

2. Choice of Boundary Conditions

The impedance of the RDE or RRDE is a problem of a complex variable

¢ = ¢_ + J¢l where CR (= _b) and ¢I (= 3¢) are two independent variables in the

domain f_. In this paper, only a purely capacitive boundary at the disk is treated.

The real and imaginary components are related only on the capacitive boundary,

F,:_I_,, by the relations

= 1 = 1

¢R w--'C_n I ¢, - w---C,]_-nll on rca,, (19a/

The potential inside the electrode is taken to be zero.

The counter electrode is taken to be a Dirichlet boundary. This is equivalent to

the statement that the counter electrode is perfectly reversible or unpolarized, or that

the counter electrode is at "infinity" and that this boundary, Fce is at the position

of an equipotential surface. Insulating boundaries Fins (and symmetry planes) are

taken to be Neuman boundaries. Thus:

{¢R = V, ¢, = 0} on roe; {¢_ = ¢; = 0} on Pins (19b)

3. Choice of point location and density.

The I-BIEM allows an economy of spacing not readily obtainable with other

techniques. A uniform point spacing in the r direction along the z = 0 surface is

undesirable, since the area of the annuli corresponding to this spacing increases

quadratically with the radius. A more desirable spacing is one where the Ar decreases

with the square of r. This is equivalent, for e.g. 64 points per segment to a linear

resolution of 4096 points at the disk near the edge. At the higher frequencies covered

in this work, the local structure of the potential near the edge (singular for primary

current distributions) varies so strongly that a finer resolution is required. In some, a

quartic spacing was used. This is equivalent to a resolution at the edge of better

than 1:10 -8 with 64 points. Such fine spacing in a conventional BIEM (or BEM)

could create severe stability problems as a result of the near-singularity induced by

the inversion of the matrix. Similar spacings were used for the calculation of the

potential at the insulator in the region of the edge.

129
Spacin.g and resolution at the counter electrode is not as critical, as the region

of interest _s primarily near or at the disk. Indeed, numerous sets of calculations

with various sets of spacings showed negligible effects on the overall solution. For

those calculations using; a primary current equipotential as the counter electrode,

points were chosen to have equal 77 values. For those cases with cylindrical counter

electrodes, equally spaced intervals were usually chosen.

In the axi-symmetric case, point spacing on the axis is irrelevant, since these

values are not used in the integration, and need be calculated only for display of the

values.

With quadratic spacing, "satisfactory" (see below) results can often be obtained

with surprising accuracy even with a limited number of points, especially at low

frequencies, where current distribution is more uniform. At the highest frequencies,

where the structure of the potential profile has significant detail within one point

spacing, the algorithm loses accuracy, as expected with low point density.

Specific values were used for the calculation, in contrast to the dimensionless

variables used in [11], but the results can be scaled as desired. The conductivity, _,

of the electrolyte was taken as unity, while the capacitance of the electrode was

taken as 100 #Fd/cm2 (actual area for 3-D calculations and capacitance/cm per

linear cm for 2-D). Frequencies solved ranged from three decades above the corner

frequency (where 0 z 45 °, and the resistive component is approximately equal to the

capacitive impedance) to three decades below. In most cases, this resulted in a

frequency range of 102 to 10s Hz, and up to 10t0 Hz for quartic spacing. It should

be remembered that all frequencies can be scaled to actual conditions. Thus, for

example, a decrease in conductivity of a factor of 100 lowers all frequencies by two

decades.

CRITERIA FOR ACCURACY AND VALIDITY OF SOLUTIONS

In the case of the solutions using primary current equipotentials, the analytic

solutions of Newmant were used to check the results where applicable. This was not

considered to be sufficient, since it is conceivable that overall numerical values could

be correct, yet errors could exist in the local values of the variables. Even in this

case, the choice of a finite counter electrode vs. one at infinity produces differences

at finite frequencies. Other criteria must therefore be utilized for all cases.

1. Continued minimization of residual errors.

The sum of the absolute values of the residual error _;q (j=I-_N) over the

contour F must continue to decrease without limit with increasing number of

iterations. Oscillatory convergence is allowed, but not a finite asymptotic value

(until the residuals are determined by the truncation error of the precision of

the real number set chosen). Thus the final solution will satisfy Laplace's

equation and the boundary conditions as closely as desired at all points.

2. Equality of anodic and cathodic currents.

A necessary consequence of the validity of the solution is that Gauss' law be

obeyed. That is, even though all points on the boundary are computed

independently, the integrals of both the real and imaginary currents into and

out of the domains must be separately equal. This _s a less stringent

requirement than the first, since local variations could integrate to the same

overall value. In practice, when the problem is set up correctly, these currents

agree within the specified convergence tolerance. [This is typically less than

0.01% and often better than 0.001%] In all cases where equality was not

observed, some error in bookkeeping was "discovered".

3. Other criteria.

a. Interior right-angled corners between conductors- In Fi_. lb, when the top

(BC) and right (CD) side segments are both equipotentials, the derivative in

the x and y directions must both be zero, and the computed current density in

130
the corner should be zero. In practice, remembering that the value at a point is

only valid in terms of the integral over the line segments containing it., 1he

typical value computed without any imposed constraints is always less than 1

part in 105 of the average current density on the remainder of the electrode.

b. It is often possible to derive other special and/or limiting relationships for

particular parts of the problem which can be tested separately. It will be shown

later that this can be done for the complex current density at the edge of the

disk.

RESULTS OF COMPUTATION AND DISCUSSION

Although the analytic solution given by Newman' could not be duplicated

completely because of the finitenumber limitations of a numerical solution, a number

of points of similarity could be replicated. As pointed out above, at "infinite"

frequency the choice of a counter electrode at the location of an equipotential surface

as determined from eqn.. (22) should have a series solution resistance equal to

R_,.¢,_e/¢0 (= i/4_.¢_.e/¢0). A series of calculationswere done at several resolutions

at values for ¢/¢0 of 0.5, 0.7, 0.9, 0.95 and 0.98 with resolutions of 8, 16, 32 and 64

quadratically spaced points on the disk. Frequencies from 103 to 108 Hz (with 4 to

16 points per decade) were used for most of the calculations,although some sets were

checked with frequencies as low as 10 and as high as I0'0 Hz [It should be. pointed

out that a ¢c_/¢0 value of 0.95 with 64 point resolution with quartic spacing ,s a

severe test of the robustness of the algorithm, since dimensional ratios of the order of

3x10 -D to I are involved.] Direct comparison can be made between the calculated

values at high frequency and R® from the analytic solution. The ratio of low

frequency to high frequency resistance can also be compared. Some results are

summarized in Table I.

It can be seen that at high frequencies the series Reff approaches very closely

to the theoretical value. As ¢ce/¢0 -' ! (and r-, 0), the corner frequency increases

and a frequency of 10s Hz is not sufficientto achieve "infinite frequency behavior"

for these large cell/electrode aspect ratios. On the other hand, the use of a small

aspect ratio causes significant departure from the anticipated behavior at low

frequencies.

Because the solution generates large amounts of data in the form of complex

variables, representation in a 2-D plot can be difficult. Referring to Fig. 1, the

rotational symmetry of the system means that all points can be represented in terms

of the highlighted area. Using this cross section as the X-Y plane in an orthographic

projection, the real and imaginary components can be plotted as vectors up and down

in the +Z and -Z direction, respectively. The left hand side is the axis of rotation.

After solution of the boundary values, the values of the variable internal to the

boundary can be calculated_using the same algorithm (14a) or (15a) and plotted as

two independent surfaces. [These surfaces appear similar to those produced by a

Finite Difference calculation, but are the exact solutions for the linear basis function

boundaries.] Any desired resolution can be obtained by the simple expedient of

choosing the points. At high resolution, the initial integration time can be 10 to 20

minutes on a VAX 2000 microcomputer, but the coefficients are saved in an array

for use at all frequencies. [Because of complexities of programming the plotting of the

surface near the curved contour of the counter electrode, these points in the

immediate vicinity of the counter electrode are omitted in the figure, I)ut are

calculated.]

Figure 2 shows the complex potential profiles for a disk electrode, and the

corresponding surfaces for a 20 x 20 array with an "equipotential" counter electrt)de

at ¢,:e./¢0 = 0.7 for a set of six frequencies (3x103, 104, 3x104, 105, 3x105, and 10 n)

selected to show the development of the complex potentials at the boundaries as the

131
frequency is decreased.These curves can be comparedto Fig. 3 of ref. 1 remembering

that the _? of that paper is in stretched coordinates. [_? = _?J-ff where 12 = xCro/a

and 7/ = 41-(r/a)_ ] Thus the apparent reverse curvature of Uor and Uoi as r-_0

there can be misleading and is not present in linear coordinates. From the boundary

conditions (19a) it can be seen that the real current densit_y is proportional to the

imaginar_¢ potential and vice versa. The maximum of the real current density is thus

not at the edge for any finite frequency! Indeed at all frequencies lower than 104 Hz

the highest real current is at the center of the diskY

Of even more interest is the observation that Oor is negative on the interior of

the disk at frequencies above 105 Hz. [This can be seen from Newman's figure on

close examination and on ours before reduction for publication, but is not

immediately evident. When we first obtained negative values in our calculations we

spent many fruitless hours looking for the "bug".] This means that .the imaginary

current on the inner part of the disk is negative, and that portion oi the electrode

appears inductive.

Once the values at a regular array inside the domain were determined, standard

routines (DEC LGP software) were used to generate equipotential plots of the real

and imaginary parts of the variable. In these routines, a uniform grid and linear

interpolation is used and thus the resultant curves have no higher resolution than the

chosen set of data points. They can be used only as a guide, and do not represent

the true contours close to the region of the edge. They are, however, useful in giving

an overall picture of the complex potential distribution, and can be generated at any

resolution desired (see below). Figures 3a-f show such contours for cases equivalent to

those of Fig. 2, but for frequencies of 104 to 108 Hz. The real contours are plotted

with light lines, and the imaginary contours are drawn heavier. The position of the

counter electrode and of the disk are marked with an even heavier line.

Figure 4 shows a surface calculated at 10 ,_ Hz for a much finer grid in the

vicinity of the edge. These were also calculated for a 20 x 20 grid, but only plotted

as a 10 x 10 grid, for clarity in publication after reduction. Figures 5a-f show

contour plots of these restricted areas at higher resolution.

Several important points can be observed from these contour plots.

1. It is only at the highest frequencies, that the real contours approach the

equipotentials (ellipsoids of revolution) predicted for primary distribution. At these

frequencies, the imaginary contours are more like tightly spaced and skewed hemitoroids centered over the edge.

2. At very low frequencies, the real and imaginary contours approach the same shape,

but are never identical, since at these frequencies the real current is highest at the

center and the imaginary current is highest at the edge. A significant fraction

intersect the surface of the disk, and the solution immediately outsiae the disk is no

longer an equipotential.

3. At all intermediate frequencies, the contours move and change shape from

frequency to frequency, and are no longer ellipsoids.

4. At frequencies above the corner, where the disk becomes "inductive", the

imaginary contours intersect the surface at an acute angle. Since the current

streamlines are normal to the contours, this means that a portion of the imaginary

part of the current actually flows out of the central part of the disk and inlo l he

edge region!

Even for the low frequency limit, there is no such thing as an equipotential

surface for this system, since any surface where the real part is constant will have

significantly different imaginary parts over this surface. Likewise, the concept of an

"IR" drop is meaningless. At intermediate frequencies, each annular segment has its

own resistive element which is different than all others, and is a function of the

132
frequency, and is different from the IF{ drop at infinite frequencies.

Without defined equipotential surfaces, the question of where to place the

reforence electrode takes on new meaning. In a system without frequency di._l)er,eion, a

reference electrode anywhere along an equipotential will give values equivalent to

those at any other point on the same surface. In this case, only the axially

symmetric lines (points in cross section) are degenerate. In general, no two

asymmetric points will yield the same complex potential values at all frequencies.

While these calculations have been done in the frequency domain, similar

considerations apply in the time domain, and will be discussed in a future

publication.

The impedance, Z (= Ell), or the admittance, Y (= 1/Z = 1lEt, of the entire

electrode or any portion of it can be determined by simple integration of the real

and imaginary components of the current over the desired fraction of the electrode,

using linear interpolation between points as needed. The admittance is proportional

to this value and the impedance is its complex reciprocal (since the applied voltage

is 1 + 03), and it can be expressed in terms of an effective series admittance, G,.fr

(= R)_), and capacitance, Cerf (= w_Y). Both Bode and complex plane admittance

plots were generated from the calculations. In most cases, a cubic spline interpolation

was used to smooth the data between points for presentation. This procedure was

checked by comparison with curves generated with more data points O_p to ID per

decade) and no difference was detected.

Fig 6a,b show a Bode plot of the impedance of the disk between l() 4 and 10_

Itz. The solid line is the calculated curve and the dotted line is the theoretical curve

for a series R (= R._) and C (= Co). It is clear that there are significant deviati()ns

from the "ideal", as expected.

Fig. 7 shows data similar to that of Fig 6 but over the range 10._ to lll_ tlz

and plotted in the complex admittance plane. These plots show:

A. The admittance of a simple RC network with the value of 1t = [(i,f and

C = Co.

B. The admittance of the entire disk and electrolyte.

Also shown are:

C. The admittance of the outer 50% of the area as a ring and its share of the

electrolyte resistance.

D. The corresponding admittance of the inner 5f1% as a disk.

The top dotted curve (at is the ideal semi-circle for the series R_Co, while the solid

line (b) is for the actual electrode. Curves (c) and (d) are the impedances of the

ring and disk _(outer and inner 50% of the area) as if they were measured

independently with two potentiostats driven at the same potential. The ring

impedance shows a significantly "flattened" semi-circle, while the disk shows a clear

pseudo-inductive loop at higher frequencies, as indicated above. At the low frequency

end of the curve, all four curves intercept the X-axis perpendicularly, while at high

frequencies, the ring and the whole disk intercept at a finite angle.

The effect of resolution (number of points per segment) can be seen in Fig. 8a

where C/C_.rf is plotted vs. log w for resolutions of 64, 32, 16 and 8 p()ints.

Comparing this curve to Fig. 4 of ref. 1, O(C/C_f_)/Ologw has the expe('ted limiting

value of 2.303/4 = 0.576 at high frequencies up to zl0_ tlz for 64 l)()inls, z2.1(): f,,r

32, z5xl0_ for 16, zl.5xl0_ for 8 points. Clearly, if the point spacing f()r lh(, p(,irlls

near the edge is not less than that required for D = 1, which is th(' al)l)r¢)xi))l:_l¢'

location of the maximum in 13"oi, a discrete model cannot adequately repres(,tll (h('

situation. [This is similar to the Nyquist criterion for harmonic content ()f digiliz('d

signals which states that the sampling rate must be at least twice the frequency ()f

133
the highest harmonic]. Using eqns. [9], [16], and [29] of ref. 1, we can obtain

ft - 2 _/2

= = = (23)

With the geometry used to obtain Fig. 8a, C = 81.56#Fd/cm2, r0 = 0.509525 cm,

and for the first point inside the disk is r63 = 0.50940. Thus the maximum frequency

for which satisfactory results can be expected (for [7 = 1) is ~4.9x10 r Hz. For 32

points ra_ = 0.50903, yielding 1.24x10 r Hz, for 16 points r15 = 0.50754 and OJmax z

3.ix10 _ Hz and for 8 points Wmax z 7.8x105 Hz , in excellent agreement with the

observed values. In Fig. 8a, vertical dotted lines are drawn at these frequencies,

showing that the computed curve deviates observably at about twice this number.

The algorithm gives accurate solutions up to frequencies where fine structure of the

actual boundary values is smaller than the resolution utilized. Doubling the resolution

near the edge increases the frequency range of validity by a factor of four. tit should

be noted that even above _max the solution still converges, i.e. is stable, but the

values are not correct.] A similar curve for a 64 point 2-D calculation with the same

geometry is also shown in Fig. 8a. The C/Ceff curve is also straight, but with an

obviously different,_ slope. This is in contradiction to the implication in ref. [1] that

the high frequency limit should be valid for any fiat electrode in a plane with an

edge. Fig. 8b shows the C/C,_ff curves for a 2-D geometry like that of fig. lb. In

this case, the segment AB is a plane of symmetry, and the counter electrodes BC

and CD are vertical and horizontal planes. Impedances are calculated per cm of

electrode length. The two curves represent the values when segments BC or CD were

used for the counter electrode. The third curve is the 2-D curve from fig. 8a

computed with the same shape "Equipotential" electrode as for the 3-D curves.

Clearly the proximity of the counter electrode to the edge changes this slope by more

than 55%. This illustrates the desirability of using a geometry as close as possible to

reality when modeling.

Table 2 shows the local current density at selected points from the edge to the

center for a _b,.,/q_0 ratio of 0.7 at three different resolutions at 10 e Hz. It can be

seen that agreement is very good where changes are slow, and where changes are

more rapid (such as at r = 0.5075) the low resolution value is higher to give an

overall integrated current value equivalent to that at higher resolution. Note the

cross-o_er from capacitive currents to inductive currents between 0.4916 and 0.4777,

marked with a "_" A "t" marks the maximum in the real current density.

That the I-BIEM converges to the best integral fit for straight line elements

can be seen from Table 3 which shows the agreement between several integrated

quantities for the same geometry at 10_ Hz as a function of resolution. It can be

seen that even with only 8 points on a segment, which has an O-'max below 10_, this

agreement is typically better than 0.5%.

In order to obtain a higher resolution near the edge for higher frequencies, a

quartic spacing was used. Table 4 shows the local values of the variables for

quadratic and quartic spacings for several frequencies, using 64 points per segment.

[Note that for quadratic spacing, 10 a Hz is at the limiting frequency, Wmax. For

quartic spacing, however, Wmax =10 t_ Hz, and the results should still be valid. Table 5

shows that this appears to be the case.

From Fig 3 of ref. 1 it can be seen that at the edge (_7 = 0), the values _f

trot and 13oi are equal in the high frequency limit. From the boundary conditi_gns

(19a), this also implies that _913or/0n and tglJoi/0n are equal and opposite in sign,

and their magnitudes should increase as _-_. Examination of the values of the local

current density at the edge from Tables 2 and 4 show that this is indeed the case.

The limiting current density at the edge approaches (1-3).oo, not infinity, with a

134
constant phaseangle of-45°!

It may be instructive to consider the effect of real dimensions on the current

density at the edge. A real electrode, polished with real abrasives on real pc,lishing

cloth is unlikely to have a mathematically sharp edge. Repeated metallographic

examination of well polished electrodes in this laboratory has shown that the edge

will round to a radius of the order of a micron or more. Even a disk made by e.g.

evaporation through a mask is likely to have a thickness, and therefore an edge size

of the order of 0.1#. This would put the system in the range of the first column of

Table 2, and the current density at the edge would likely be no more than 10-30

times that at the center at the highest frequencies attainable in the laboratory. There

is still a substantial problem with frequency dispersion, but the conceptual problem of

"infinite" current density disappears, as does the problem of the "singularity" there.

Once the value of R_¢ has been found, it is interesting to do an "IR correction"

of the data. If one had a "perfect" potentiostat, with Itl correction (not

compensation) capabilities, one could, in principle, subtract the "111 droll" an(l

examine the behavior of the residual impedance of the double layer, and the rest ()f

the included uncompensated solution resistance. Fig. 9 shows the Bode plot of lhe

results of such a calculation. At low frequencies, there is a small series resistance,

R®- R left uncorrected, tlowever, this is small compared to the expected

impedance of the capacitor (with 0 inlZI/O lnw z -1) below the "corner frequency"

at w z 104F -l0 ,_ Hz, where this residual resistance is beginning to drill) off, as shown in ig. 10. Instead, above this frequency, the residual impedance curve has a

smaller slope and the corresponding phase angle drops to a phase angle of z -77 ° t()

-85 ° . It can be shown simply that this behavior is compatible with the KramersKronig relations using the algorithm in the Appendix.

With four points per decade, and integrating from the values of the residual

impedance curve of Fig. 9, using values of-1 for the slopes below and above the

minimum and maximum calculated frequencies, it can be seen from Figure I I thai

excellent agreement is obtained up to about 107 ttz. The deviation above that

frequency is caused by the assumed slope of -1, and by small residual errors in the

infinite frequency subtraction.

DISCUSSION

The finiteness of the geometry of the rotating disk and its associated cell

configuration has been shown to have a significant effect on its high frequency

behavior. While the analytic solution of this problem is useful in determining trends

in behavior, the idealizations and approximations required for these solutions can lead

to conclusions that are not necessarily realistic for real systems. A real cell d()es not

have infinite extent, nor does it have ideally sharp corners or smooth surfaces, nor

are frequencies much above 1MHz readily produced in the typical electrochemical

potentiostatic setup.

With these restrictions in mind, the often referred to problem of the current at

the edge going to infinity is far from the real situation. This is not to imply that

the RDE is a good configuration for AC or for transient measurements, The current

distribution is distorted as a result of the general configuration and not just because

of the edge. The pseudo-inductive behavior of the central part of the (tisk will

complicate any attempt at analysis of non-steady state behavior.

ACKNOWLEDGMENTS

The author would like to acknowledge the support of NASA l,ewis lCeseaw(h

Center and the Ohio Aerospace Institute for part of this work.

135
APPENDIX

From Bode 13, Oc = _1 f_o 0ulnlZ[ In ctnh-[-_ du (24)

-OC

where u = ln(w/w,.). If o_nlZl/Ou is constant, it can be taken out of the integral.

Since the definite integral has the value _r2/2, eqn. (24) then becomes

f_ln ctnh-_ du- 1 O lzl 1 0 o lnlZl 0c- r v. 7r '2" = 0u (2S)

-OQ

For an ideal capacitor with a slope of-1, the phase angle is -90 ° , and for a slope

slightly less, the phase angle will decrease correspondingly. Similarly, for an ideal

resistor, the slope is zero and the phase angle is zero.

In this case, the expected phase angle at a frequency wc can be calculated from

eqn. (24) by splitting it into three parts. Thus

1 In ctnh du +

0,.=_ 0__

N-1 ' Un_ 1 00 ]

+_Atnu_-- n f In ctnhj-_j- du + Aln[/xu]Zl , f In ctnhJ_ du j (26)

n= l U n O N

where u,, = ln(w,,/w,,), w,, is a parameter of integration, and Aln[Z[/Au[,, is the

average slope over the nth interval. If, as in this problem, we have chosen equally

spaced frequencies on a logarithmic scale, the evaluation in this form is simple in a

computer. Remembering that the function F(u) = In ctnh([u[/2) is symmetric around

u,. = ln(w(./ax') = 0, and has a logarithmic singularity at that point, we can evaluate

the sub-integrals of the second (summation) term for each n from 1 to some

appropriate number, M, with adaptive (or other) quadrature, and stored in an array,

KK. [In practice, three to four decades is completely adequate, since the value of the

integral is less than 10 -9 beyond that point.] Choosing values for Aln[Zl/Aul0 and

&ln|Zl/_ulN as logical extensions of the low and high frequency limits or by

evaluation of some analytic extension, the following pseudo-algorithm can be used to

find the expected values of 0c, the Ith term.

(J = 0 to M + N)

=:, KK[J] (J = 0 to M z 30)

DZ[I] (I = 1 to N-l)

DZ[0]

DZ[N]

l.Zero INT[I] and Zero KK[J]

2. (Values of sub-integral)/Tr

3. AlnlZ /Au[,,

4. Aln Z /Au o

5. Aln Z /Au I_ :=*

6. m=I-M-1

6. For k = -M to 0 do

7a. if m < 1 then

7b. else if m >= N then

7c. else

7d.m=m+l

8. For k = 0 to M do

9. Return INT[I]/Tr

{left side of integral}

INT[I] := INT[I] + DZ[0]*KK[ABS(k)]

IN r[I] := INT[I] + DZ[N]*KK[ABS(k)]

INT[I] := INT[I] + DZ[m]*KK[ABS(k)]

steps 7. {right side}

136
Table 1. Effective resistanceas a function of domain size (quadratic spacing)

¢,_'_./¢0 Radius R,:(10_Hz) R®(anal.) Rc/R_¢ R(IOS)/R(IO 8)

0.5 1.00000 0.12500 0.125 1.00000 1.15776

0.7 0.509525 0.34349 0.343454 1.00011 1.11511

0.9 0.158384 1.42110 1.42060 1.00035 1.08970

0.95 0.078702 3.01975 3.01771 1.00068 1.08536

_R__R__ 1.08076

Table 2. Local current density along disk

Point radiusn i.(64 pt.) in(32 pt.)

0.5095 11.025-311.455 11.049-311.434

0.5094 11.219-311.131

0.5090 11.537-310.401 11.561-310.380

0.5084 11.818-39.414

0.5075 11.981-38.268 t12.009-38.245

0.5064 t11.986-37.045

0.5050 11.817-35.817 11.846-35.789

0.5034 11.483-34.645

0.5016 11.008-33.573 11.033-33-537

0.4916 8.388-30.695 8.391-30.652

0.4777 _6.011+10.226 _5.994+30.253

0.4598 4.521+30.297 4.504+30.304

0.4379 3.663+30.206 3.653+30.207

0.4120 3.127+30.135 3.122+30.135

at 106 Hz

i_,(16 pt.)

11.135-311.349

_12.107-38.150

11.123-33.398

8.399-30.489

_5.926+30.359

4.434+30.337

3.614+30.207

3.103+30.131

0.2229 2.002+)0.030 2.001+30.030 1.999+30.029

0.1783 1.912+30.025 1.912+30.024 1.909+30.024

0.1194 1.848+30.021 1.848+30.021 1.846+30.021

0.0617 1.809+30.019 1.809+30.019 1.807+30019

Center 1.795+30.018 1.796+30.018 1.793+30.018

Table 3. Calculated values for macroscopic variables at 106 Hz

(64 pt.) (32 pt.) (16 pt.) (8 pt.)

Integrated

Current

Me -2.86756+30.1595 -2.8682+30.1596 -2.8705+30.1599 -2.9264+30.1613

lw_ 2.86753-30.1595 2.8680-30.1596 2.8698-30.1600 2.9242-30.1615

l_ing 2.0057-30.1751 2.0063-30.1752 2-0089-30'1755 2"0182-301763

Idisk 0.8619+30.0156 0.8619+30.0156 0.8616+300155 0'8528+30015()

Reff (ohms) 0.34765 0.34758 0.34729 0.34616

Ceff (#Fd) 51.71674 51.71975 51.68149 51.59226

Phase shift 3.1835" 3.1839 ° 3.1889" 3.2048'

137
Spacing

(Hz)

Table 4. Local

Quadratic

10 8

values for different

f

10_

spacings and frequencies

Quartic

_N

10o 101o"

cm from

edge

0.0

6.0x10-8

9.5x10-7

4.8x10-_

1.5x10-5

3.7x10-_

7.7x10-5

1.4x104

2.4x10-4

3.9xi0-4

6.0x10-4

8.7x10-4

9.8x10-4

1.2x10-3

1.7xlOa

2.2x10-3

2.3x10-3

3.0xlO-,a

3.9x10-,_

Center

_118.48-3105.95

99.76-35.8.

_38.32+36.47

112.87-3112.49

112.95-3112.38

113.75-3111.06

115.95-j106.56

119.16-397.03

_121.63-381.36

120.39-358.55

112.56-336.95

97.65-315.84

78.66-32.22

I60.58+13.49

46.82+34.05

357.08-3352.41

358.83-3349.57

371.77-3319.89

t385.72-3232.86

352.53-3102.99

254.93-35.013

1155.84+315.96

i07.11+37.85

80.68+33.45

63.95+31.84

5i.70+ji.21

42.54+3O.74

1147.75-31104.8

1178.20-31046.8

t1197.26-3554.08

_615.64+3118.47

333.33+318.73

194.07+34.65

140.33+32.504

104.83+31.721

78.84+31.067

63.10+30.092

51.38+30.148

42.43+30.131

37.65+32.91 35.75+30.32 35.67+30.077

31.45+31.90 30.51+30.27 30.41+30.U37

26.47+30.96

26.87+31.28 26.32+30.19 26.25+30.1)20

23.28+30.905 22.92+J0.16 22.89+30.014

20.09+30.58 20.39+30.65 20.14+30.12 20.14+30.012

,.1 i,, .,. m,,

1.7844+30.00048 1.7845+30.00049 1.7845+33x10-4 1.7844+jlx10-_

Table 5. Calculated

Spacing Quadratic

(Hz) lO8

values for macroscopic variables at

Quartic

g , A

108 100

several frequencies

1010"

In tegrated

Current

lee -2.9114+30.0028

Iwe 2.9114-30.0028

Iring 2.0585-30.0032

ldisk 0.8529+30-0004

Reftr (ohms) 0,34348

C,4f (/zFd) 30.09175

Phase shift 0.0554"

-2.9113+30.0028

2.9113-30.0028

2.0583-30.0032

0.8530+30.0004

0.34349

30.24806

0.0551"

-2.9117+30.00034

2.9117-30.00034

2.0587-30.00039

0.8529+30.00005

0.34345

25,03443

0.0067 °

2.9117+24x10 -5

2.9117-34x10-5

2.0589-35x10-s

0.8528+31x10-5

0.34345

21.4904,1

0.00O8 °

138
REFERENCES

1 j. Newman, J. Electrochem. Soc., 117, 198 (1970).

2 Glarum, Calculation from variational calculus.

3 Other refs here.

4 B. D. Cahan, D. Scherson, and M. A. Reid, J. Electrochem. Soc., 135, 285 (1988)

s "On the Iterative Boundary Element Method", B. D. Cahan and O. Lafe, in

"Computational Engineering with Boundary Elements", Vol. 2, 367-376, A. tt-D.

Cheng, C. A. Brebbia, and S. Grilli, eds., Proceedings of the 5th International

Conference on Boundary Element Technology, July 1990.

B. D. Cahan, O. Lafe, in preparation.

T j. A. Liggett and P. L-F. Liu,"The Boundary Integral Equation Method for Porous

Media Flow", Chap. 6, Allen and Unwin, Boston (1983).

8 M. Abramowitz and I. A. Stegun, "Handbook of Mathematical Functions", New York,

Dover, 1974.

g R. Bullirsch,"Numerical Calculation of Elliptic Integrals and Elliptic Functions",

Handbook Series Special Functions, Numerische Mathematik Vol. 7, 1965, pp.78-90.

10 "Computational Engineering with Boundary Elements", A. H-D. Cheng, C.A.

Brebbia, and S. Grilli, eds., Proceedings of the 5th International Conference on

Boundary Element Technology, July 1990.

1_Scientific American...

1: j. Newman, J. Electrochem. Soc., 113, 501 (1966).

l._ H. W. Bode, Network Analysis and Feedback Amplifier Design, D. van Nostrand Co.

(1945)

139
140

•= E_"=I

,.. ,,, _"

. I:.I_ I:

,__,_

I_ r,.)_ "il
/ /

J

t_D

_LD

0

_LJ

[L.U

i ] • L

N

, L I L * l . J , i

J

141

o _ © "_. N. o

Q

0

N

0

[._.=

_R

_B

_N

I_ "_}

0 (P

• . II
, i

0.20

0.16

0.12

0.08

0.04

L_ 0,00

0.20

0.16

0.12

0.08

0.04

0.00

,; , ! _ J, , ' I : | 0.20 "' r ,

0.16

o.12

0.08 . -, ", . , ' "',,\

• _ • '_ 0.04 X

0.40 0.44 0.48 0.52 0.56 0.60 0.40 0.44 0,48 0.52 0.56 0.60

Fig. 5. Contour plots of surfaces similar to those of Fig. 4. Expanded in the region of

the edge from Fig. 3. Frequencies are 10t, 105, 3x105, 10_, 10 T and 10B Hz. Disk edge

is at 0509.

142
I

PSa_ A_k,-Frequ_y Plot

8000

7000

(tO.O0

50.00

40.00

3Q O0

20 O0

1004)

000

\

\

\

\

3.00 4 ()0 _ OU _,.UO

Lug. 4Frequ_-ut ._ )

Imp,'danee-Frequen(.v Plol

ft..,,

m

v

2OO

0 O0

\

\

\

-I O0

3.00 I rm 5 NO 6.0¢_

L(,I_ I Frpqupu¢)-)

Fig6. Bode plots of the cell impedance over the frequency range of IUs to I(1_ Itz

S(,lid lines - total cell impedance, dotted line - impedance ,Ji' a series equivalent

circuit uf R and Co. a. Top:- Phase angle vs. log _ b. Bottom:- log IZl vs log .,.

143
I I I I

\\ !"\

ji! i \

O _ll O a O

u'3/3

I I I I

It; \',\

_'_, , i

N

0

gl

e

0

Je

ft_

I

t0

.=__ -,,._

_. _

,,.,_, "_-,,

_ ,,,,'__ -_

'_ I 8._ I._!

o

_4

_4

0

f.-J

l II /

41 :-.,,/

4- t _ # "

• \ ,

!

(_= _),_,_ .h_

.,_

_ Jd ,,_

_,d

"=_1#

_S ._"

°_,"_

144
¢.a

tae

0.2..5

0.20 -

015 -

o 1o

0.05

R - Frequency Plot

[ I [ I

0.00 t _ I t

3.00 4.00 5.00 6.00 "7.00 8,00

Log (Frequency)

Fig.9. Residual resistance of fig. 4 (after subtraction of R®) vs. log w.

ResidualImpedance-Frequency PIol

0.00

-2 oo

\

Log Frequency (Hz)

ig. 10. Solid line:- Bode plot of residual impedance from Fi . 4 after subtraction of • Dotted line:- Theoretical impedance of ideal capacitor wiul value equal to Co.

®

145
$$

I

90 O0

85. O0

80.00

75.00

ResidualPhase - FrequencyPltbt

i I 1 J i

70.00 i i I i J

3.00 4.00 5.00 6.00 7.00 8.00 9.00

Log (Frequen('y)

Fi_.ll. Result of K-K integration of data from Fig. 10. Solid line:- calculated

values. Dotted line:- results of K-K integration.

146
N91"32560

REGENERATIVE FUEL CELL ARCHITECTURES

FOR LUNAR SURFACE POWER

D.W. Harris, S.P. Gill, T.M. Nguyen, and J.J. Vrolyk

Rocketdyne Division

Rockwell International

Canoga Park, California 91303

ABSTRACT

Power needs projected for the lunar and Mars exploration missions range from a few kilowatts for initial

manned outposts and rovers to hundreds of kilowatts for permanent bases and in-situ resource utilization.

The 354 hour lunar night presents a formidable challenge in energy storage systems for non-nuclear power

systems. The Photovoltaic/Regenerative Fuel Cell (PV/RFC) power system is currently being considered as

an option to meet the initial low power requirements.

For lunar applications, the RFC mass can be a critical driver in the overall PV/RFC system mass. For the

long duration nighttime operation missions, the hydrogen/oxygen (H2/O2) reactants and storage are the

dominant components from a mass standpoint. It is important, therefore, to examine options which may

reduce the reactant and storage mass and, thereby, result in mass savings for the overall PV/RFC system.

Various RFC configurations for the stationary lunar missions has been examined using Rocketdyne's RFC

computer model. For the stationary applications, a GaAs/Ge PV array with a 3000 psi gas storage proton

exchange membrane (PEM) RFC providing 25 kWe during the day and 12.5 kWe at night was designed.

PV/RFC systems utilizing super-critical H2/O2 storage and Cryogenic H2/O2 storage for the RFCs were

then compared with the baseline high pressure gas storage RFC system. Preliminary results indicate that for

long duration nighttime operation missions, the super-critical H2/O2 storage RFC systems offer over 20%

mass advantage over the high pressure gas storage while the mass savings for the cryogenic H2/O2 storage

RFC systems can be as high as 30%.

INTRODUCTION

To meet the challenge of president Bush's Space Exploration Initiative (SEI), NASA has conceptualized a

set of different options that will facilitate the human exploration of the moon and Mars. Power needs

projected under these options range from a few kilowatts for the initial manned outposts and mobile

systems to hundreds of kilowatts for permanent bases and in-situ resource development. Figure 1

illustrates the range of power needs identified by NASA for their options 1, 5, and 5A. As shown, the

maximum nighttime power requirements may be as high as 320 kWe. This presents a formidable challenge

in energy storage system technologies for non nuclear power systems.

The objective of this work was to compare the mass of several methods of storing the reactants in a

Regenerative Fuel Cell (RFC). The reactants were limited to a single set, namely water/hydrogen/oxygen.

The storage conditions were further restricted to storing the water as liquid and the hydrogen and oxygen to

one of the following three states: 1) as room temperature gas at high pressure (3000 psia and 60 F), 2) as

cryogenic liquids at low pressure (315 psia,-419F(O2),-292F(H2)), and 3) as super-critical gas at

cryogenic temperatures and at high pressure (-325F and 3000 psia).

147
Although initially, this study was for a Lunar mission environment, it was believed that the results would

be applicable for use in preliminary studies for a Mars mission, since the percentage differences between

storage methods should be comparable. In addition, this would reduce the Mars study cost since only

relatively simple modifications would be required to adapt existing Lunar study calculations to the Mars

conditions.

100o

800

.J

,tS,, 400

L

2OO

0

Range of Power Needs

(To Date)

Day

Night

MISSION OPTIONS

Figure 1: Lunar Surface Power Requirements

ANALYSIS

Modeling

Regenerative Fuel Cell energy storage has been examined extensively at Rocketdyne. Spreadsheet models

were developed to characterize the performance of an RFC system. The cell stack description for this study

was for PEM-type (acid) fuel and electrolyzer cells. However, the spreadsheet algorithms are intended to

deal equally with both acid and alkaline type cells. Major participants in the cell business are Hamilton

Standard, International Fuel Cells, and Life Systems. These organizations are our primary sources of

informatioa contained in this model. The fuel cell and electrolyzer cell models calculate the size of the cells

and stacks based on the user defined load. From this load the model calculates the number and size of the

cells stacks. From "standard" cell designs the mass of the cells and stack can be calculated. Added to this

mass is the mass of other parts of the stack that are necessary .to hold the units together (heads, tie rods,

springs). The electrolyzer mass also includes mass for a contamment vessel or dome. The size and mass

of the dome is based on the electrolyzer stack size and pressure.

Because the mass of the reactant tanks can be the single greatest mass for the RFC (e.g., -45%), the

reactant tank mass calculations are carried out in considerable detail. The tanks are modeled as metal-lined,

148
high-strength,fiber-woundvessels;theshapeis that of a cylinder with semi-elliptical heads, this shape

being superior in reduced mass-to-volume ratio as compared to spheres (this is characteristic of fiberwound structures). The approach taken for the tank model was to develop empirical equations which

would fit design parameters for a number of preliminary tank designs prepared by Structural Composites

Industries (SCI). The spreadsheet model for the tanks calculates fiber wrap mass is somewhat higher (-20-

40%) than those from SCI. Even so, the spreadsheet predicts some excellent figures of merit (PbV/W) for

the tanks. Because the spreadsheet model predicts heavier tanks masses, the model is considered to be

conservative.

General Assumptions

The following assumptions and ground rules were made for all systems:

1) Conditions are assumed suitable for operation on the Lunar surface somewhere near the equator. The

Lunar day is assumed to be 354.8 hrs long with the night 354.0 hrs long.

2) The nominal delivered electrical power to the user was assumed to .be 25 kW during the day and 12.5

kW during the night.

3) The PV array and the PMAD masses are assumed to have the same specific mass in lb/kW output, all

based on a value derived from Ref. 1. This was done to even out the relative influence of the PV and

PMAD masses while comparing the storage system masses of the three system considered here.

System Specific Assumptions

Tank Sub-System Mass

Warm Gas Storage Tanks

All the data for the warm-gas storage option was taken directly from the spread sheet model.

Super Critical Storage Tanks

The super-critical tank mass was calculated from the same spread sheet model for the warm-gas system,

except that a 4 in thick Multi-layer Insulation (MLI) layer was added to the hydrogen tank and a 3 in thick

MLI layer was added to the oxygen tank. MLI mass was calculated by using the tank surface area and MI,I

thickness to determine MLI volume, and then the mass factor of 3.5 lb/cu.ft was applied. This resulted in

an approximate 20 % increase in the tank mass. It should be noted that these tanks are "wrapped" tanks

and have never been flown when filled with fluids at cryogenic temperatures. This is an obvious new

technology area, although no problems are envisioned.

Cryogenic Storage Tanks

The Cryogenic System storage tank masses are based on linear scaling from the paper by L. Kohout.(Ref.

l)

Refrigeration Sub-System Mass

Warm Gas Refrigeration

Refrigeration is not required for the warm-gas storage sub-system.

Super-Critical Refrigeration

The amount of refrigeration needed to cool both the hydrogen and oxygen to their cold super-critical states

at the high exit pressure of the electrolyzer was calculated from thermodynamic property tables. The

149
coolant, or working fluid in the cooling loop is liquid nitrogen and cold nitrogen gas. Because the storage

temperatures are not extremely low, a nitrogen refrigeration system was selected. It is assumed that the

liquid nitrogen enters the coldest super critical storage tank at the bottom and is evaporated there and the

boiled off cold gas proceeds upward to cool the surrounding gas in the tank. It then exits the tank at the top

and enters the first storage tank, to cool its contents. The nitrogen gas exiting at the tank top, which will

have a temperature somewhat below the stored reactant gas which is entering the tank (at the bottom) from

the electrolyzer, is then returned to the nitrogen refrigeration system for re-liquefaction and re-use. Figure

2 shows this tank arrangement in the charge and discharge modes.

Closed Open

I

I

Coolant Input On

Charging

"41-1

Condensed

Water & Ice

0 J

Warm

Gas In

Tanks In Series

(Charging)

More Cool

Gas Out

Evaporating

Ice & Water

Warm Gas

In Both

w

Y

Closed

Tanks in Parallel

(Discharging)

Figure 2: Super-cfitcal Storage Tank Arrangement

150
The super-critical system refrigerator mass is based on combining a value of power required at the plant

input per unit of nitrogen re-liquified (Ref.2) the reported data. First the power input requirements were

calculated from the above data and the mass was then estimated from data in Ref.3, which contained the

value of mass per unit power input for a very similar oxygen liquefaction facility. This is justified by the

fact that the boiling points of oxygen and nitrogen are relatively close together and also because their

molecular weights are also close together (only a 14% difference in molecular weight) and the physical

properties of their gaseous and liquid states are also similar.

The power required to liquify one pound of nitrogen gas starting at room temperature was calculated from

an existing plant described in Ref. 2. To obtain the estimated mass (since this was not given in Ref. 2) of

such a plant, the mass per unit of input power was obtained for a similar plant designed for liquifying

oxygen, (Ref. 3). The information from the above two references was then combined to obtain the mass of

the refrigeration system. This mass was then doubled to allow for the mass of ancillary equipment. Even

then the resulting mass required of the subject RFC refrigeration system was only 200 lb.

Cryogenic Refrigeration

Data from a paper by L. Kohout, Ref. 1, was linearly scaled based on user power requirements. Only the

radiators associated with the reactant storage system are considered in the comparison.

PV Array and PMAD Sub-System Masses

The warm-gas system does not need any significant electrical power for its storage system. Therefore, no

mass was allotted for the PV-PMAD required to obtain this power.

The super-critical system PV-PMAD mass was determined by first determining the power required by the

storage system, and then a specific mass factor was utilized scaled from the values given in Ref. 1.

The mass of the PV-PMAD assignable to the cryogenic system was similarly treated based a scaling factor

derived from data in Ref. I.

Radiator Sub-System Mass

Warm Gas Radiator Mass

The warm-gas sub-system does not require a radiator for the storage system, at least any cooling

requirements are usually assigned to other sub-systems.

Super-Critical Radiator Mass

Waste heat loads were calculated and the mass was scaled linearly from data in Ref. 1.

Cryogenic Radiator Mass

Mass was determined by linear scaling from data in Ref. 1.

RESULTS

The results of the analysis are shown in table 1. The PV/RFC mass includes all of the system mass except

those pans related to H2/O2 storage. The super-critical storage shows a 16% savings in the over system

mass while the Cryogenic system shows a 22% savings. If just the storage systems are compared then the

savings increase to 40% and 55% for the super-critical and Cryogenic storage respectively.

151
Table 1. System Mass for Three Reactant Storage Options

Component

Mass, Ibs

PV/RFC System

(No Storage)

Storage

Tanks

Refrigerator

PV Array and PMAD

Radiator

Total

Total System Mass

Savings (compared to

warm gas system)

Total Mass

Storage Mass

Volumes, cu. ft

Reactant Tanks

Volume Savings

(compared to warm gas

system)

Warm Gas

System

9199.2

6112.3

6112.3

15311.5

Super Cdtical

S_'stem ,,

9199.2

1850.5

400.0

538.0

867.0

3655.5

12854.7

2456.8

16.0%

40.2%

223.8 69.1

154.7

69.1%

Cryogenic

System

9199.2

1214.6

295.6

2944.0

500.5

4954.7

11981.2

3330.3

21.8%

54.5%

49.5

174.3

77.9%

CONCLUSION

Storing fuel cell reactants at super-critical or cryogenic conditions can save system mass over a warm-gas

storage. The mass savings can be translated into life cycle cost savings in terms of launch cost and delivery

to the lunar surface. The advantages and disadvantages of each type of storage system are listed in table 2.

Even though super-critical and Cryogenic storage have the mass advantage, warm-gas storage has the big

advantage of system simplicity. The simpler the system the higher the reliability. If reliability becomes an

issue it may become necessary to add a backup refrigeration system. Adding a second refrigeration system

eliminates any mass advantage over the warm-gas system. For the cryogenic system an important

advantage is in the area of Lunar Base architecture. Current plans for the Lunar base include cryogenic

refrigeration systems for long term storage of Lunar Lander propellant. Cryogenic storage would allow for

a commonality of equipment for the Lunar Base which could alleviate the back-up system problem. If

water and it's elements (hydrogen and oxygen) become a "common" resource of Lunar and Mars surface

system, many surface system will contain technology common to the PV/RFC power system. One issue

not examined is technology readiness. Fuel cells and indirectly electrolyzer cells have space experience, but

cryogenic refrigeration systems have little or no space experience.

Cryogenic and super-critical storage show major mass savings advantages over warm-gas storage, but in

selecting the "best" system for Lunar/Mars missions, system reliability and the impact of the overall

mission architecture must be evaluated.

152
Table 2. System Advantages/Disadvantages

Warm Gas Storage

Super Critical Storage

Cryogenic Storage

Advantages

• Simple system

No refrigeration system

Higher reliability

• Smallest PV array

• Lighter mass compared to

Warm Gas Storage

Disadvantages

• High Pressure

Safety Issue

• High storage volume

Large storage tank mass

• Highest mass system

• More complex - requires

refrigerator which in turn:

• Smaller storage size

compared to Warm Gas

Storage

• Safer

Nitrogen working fluid

Requires electrical power -

increases PV size and

maSS

Requires cooling by

radiation - increases

radiator size and mass

provides shield between

oxygen and hydrogen

No Joule -Thompson type

valves having small

clearances (In oxygen and

hydrogen Cases) which

may plug up system.

Nitrogen refdgerator

contains hermetically

sealed dry Nitrogen

• No dnjers required

• Lightest mass system

• Smallest storage size

• Safer

Low system pressure

• Compatible with propellant

storage systems

• High Pressure

Safety Issue

• More complex - requires

refrigerator which in turn:

Requires electrical power -

Increases PV size and

mass

Requires cooling by

radiation - increases

radiator size and mass

1)

2)

REFERENCES

Kohout, Lisa L.,"Cryogenic Reactant Storage for Lunar Base Regenerative Fuel Cells", NASA

Technical Memorandum 101980 June,1989.

Mulholland, G.T., et al., "Design and Performance of a 90,000 Kilogram Per Day Nitrogen Reliquefier

for the Fermilab Energy Saver", Advances in Cryogenic Engineering, Vol. 33d, 1987, pp. 827-836.

153
3)

4)

Brock, E.H., and J.G. Fisher, "In-Space Propellant Processing Using Water Delivered as Shuttle

Contingency Payload", AIAA, 1978. (Authors were at General Dynamics when this was published.)

Robinson, J.W., et al., "Long Term Cryogenic Storage Study", report # AFRPL TR-83-082 prepared

by Martin Marietta, Final report, Sept. 1983, p.78. (AD-B089-763).

154
N91-32561

OXYGEN ELECTRODES FOR RECHARGEABLE

ALKALINE FUEL CELLS - III

*

L. Swette, N. Kackley, and S.A. McCatty

Giner, Inc.

Waltham, Massachusetts 02254-9147

ABSTRACT

The investigation and development of electrocatalysts and

supports for the positive electrode of moderate-temperature singleunit rechargeable alkaline fuel cells is described with focus on

chemical and electrochemical stability and 02 reductlon/evolution

activity.

INTRODUCTION

The work presented here represents an update of the continuing

investigation of candidate materials for moderate temperature single-unit

rechargeable alkaline fuel cells, some of which have been described

previously [1-3]. The focus is on new and previously studied materials which

have undergone testing to either qualify them for further investigation and

potential development or disqualify them for use in the rechargeable alkaline

fuel cell application.

Viable candidate materials must meet the following requirements: I) good

electrical conductivity (a more demanding requirement for supports than

electrocatalysts); 2) high resistance to chemical corrosion and electrochemical oxidation and/or reduction; 3) electrocatalysts, in addition, must exhibit

high bifunctional electrocatalytic activity (0 2 evolution and reduction).

Advanced development requires that the materials be prepared in high-surfacearea forms, and may also entail integration of various candidate materials,

e.g., one or two electrocatalysts unsupported or distributed on a stable

conductive support.

Candidate support materials have been drawn from transition metal

carbides, nitrides and oxides which have high electrical conductivity. Some

candidate support materials exhibit catalytic activity for 02 evolution and/or

reduction. These materials, generally noble metal-containing oxides, are

termed catalytic supports. Candidate catalyst materials have been selected

largely from metal oxides of the form ABO x (where A - Pb, Mn, Ti, Ni, Co, Zr,

Na, and B - Pt, Ir, Rh, Ru). Some of these have been investigated and/or

developed for one function only, either 02 reduction or 0 2 evolution. The

electrical conductivity requirement for catalysts may be lower, especially if

integrated with a higher conductivity support. For initial evaluation,

materials have been purchased when available; subsequently, In-house preparations have been attempted, to affect surface area and composition, when

necessary.

* Author to whom correspondence should be addressed

155
Candidate materials of acceptable conductivity are typically subjected to

corrosion testing in three steps. Preliminary corrosion testing consists of

exposure to 30_ KOH at 80°C under oxygen for about 5 days. Materials that

survive chemical testing are examined for electrochemical corrosion activity;

the material is held at 1.4 V versus reversible hydrogen electrode (RHE) in

30_ KOH at 80°C for 15 to 20 hours. An acceptable anodic current is on the

order of a few microamps/mg of material. For more stringent corrosion

testing, and for further evaluation of electrocatalysts (which generally show

significant 02 evolution at 1.4 V), samples are held at 1.6 V for about

I00 ho_rs and at 0.6 V for about 16 hours. The surviving materials are then

physically and chemically analyzed for signs of degradation (visual

examination, electron microscopy, X-ray diffraction).

To evaluate the bifunctional oxygen activity of candidate catalysts,

Teflon-bonded electrodes are fabricated and tested in a floating electrode

configuration [4]. Many of the experimental materials being studied have

required development of a customized electrode fabrication procedure. For

preliminary testing, catalysts of interest should show <500 mV polarization

(from 1.2 V) in either mode at 200 mA/cm 2. In advanced development, the goal

is to reduce the polarization to about 300-350 mV.

MATERIALS INVESTIOATED

Materials were originally assigned to the categories of support

electrocatalyst based on reported or anticipated performance. Some of

metal ratios shown are the targeted synthesis compositions only.

or

the

Candidate Supports:

LiNiO x TiN ZrC ZrN

Candidate Catalytic Supports:

LaNiO 3 PbPdO 2 ColrO 3 NilrO 3 Rh02 frO 2

Candidate Electrocatalysts:

NaxPt304 IrMnO x PtTil.50x RuTiOx

Pb2(Ir2.xPbx)O7_y _rNiO x PtTi20 x RuMn20 x

Rh black IrNi30 x PtZr2Ox RuMn3Ox

Pt black IrTil.25Ox (Rulr)Mn2Ox

Reference Materials:

02 reduction

i0% Pt/Au (Johnson-Matthey, II m2/g)

02 evolution

Pt black (Englehard, 25 m2/g)

Several candidate materials have been eliminated from further consideration for various reasons, as shown below. Some of the noble-metal/transitionmetal mixed oxide preparations resulted in two-phase materials consisting of a

noble metal or noble metal oxide (e.g., Pt, RuO 2 or IrO 2) and a transition

156
metal oxide (e.g., Ti02, ZrO2, Mn203). These materials may show adequate

conductivity and even some catalytic activity due to the noble metal or noble

metal oxide component, but they were not investigated further.

Inadequate Electrical Conductlvit i

IrNi30 x IrTil.250 x

Chemical Instability

RuTiO x (RuIr)Mn20 x

Two-Phase Mixed Oxide

PtTil.50 x PtTi20 x

RuMn30 x

PtZr20 x IrNiO x

SOURCES OF CANDIDATE MATERIALS

Candidate materials for both catalysts and supports were purc_ased, if

commercially available in powder form, as the most efficient approach for p_eliminary evaluation (electrical conductivity, chemical and electrochc_mical

stability). Such materials offer the advantage of an economical purch_ise of a

material of known purity in a quantity (5-25 g) sufficient for preliminary

qualification, and particularly for disqualification. The disadvantage is

that most of the commercially available materials have surface areas in tl_e

range from low (<i0 mZ/g) to very low (<i m2/g). Consequently, most

commercial materials that survive preliminary screening must be prepared in a

higher surface area form, in-house or by a custom fabricator, for effective

evaluation as potential catalysts and/or supports.

In many cases, especially for candidate catalysts, commercial materials

were not available. Preparation methods described in the literature, eitl_r

specific for the material or as a general model, were used when deemed

appropriate to the material requirements in terms of yielding an electrically

conductive, high-surface-area powder, frO 2 and RhO 2 were prepared by t|_e

Adams method [5]. NaxPt304 was prepared typically by firing Na2CO 3 and PrO 2

at >600°C [1-3,6,7]. Synthesis of nickel-iridium oxide was attempted by tI,e

Adams method of fusion of the metal chlorides in sodium nitrate [5], but tl_is

yielded a two-phase product. As an alternative, NilrO 3 and CoIrO 3 were

prepared following the method of Chamberland and Silverman [8]; this consis__s

of co-precipitation of the hydroxides and firing at 350°C. X-ray diffr,'_ct i,,n

analysis has not been carried out because prior work [8] has indicated tl,_,t,

although the materials have the elemental compositions of ilmeI_ites, _l_cy al-e

not crys_alline. Both materials had good surface area and cotlductivity, l_ut

the CoIrO 3 material showed higher values in both measurements. No chemic_l _r

electrochemical stability tests have been completed on these two materials to

date.

157
CHARACTERIZATION OF CANDIDATE SUPPORT MATERIALS

Candidate material preparations are typically analyzed by X-ray

diffraction (XRD) for chemical characterization. The objective of preparing

fine powder materials, however, is generally in conflict with obtaining sharp

XRD patterns (because of the line-broadenlng characteristic of hlgh-surfacearea powders); thus the quality of these results is sometimes compromised.

Firing materials for a longer time or at a higher temperature usually

increases the crystallinity and improves XRD results, but at the expense of

decreased surface area and some uncertainty about the composition of the

original higher surface area material. In some instances, materials of

interest have been re-analyzed by XRD and/or scanning electron microscopy

(SEM) after extended corrosion testing to check for changes in composition,

reaction products and changes in morphology. SEM has also been used

occasionally to observe the particle size range of powders.

A summary of the measured physical characteristics of candidate materials

is presented in Table i. The electrical conductivity of candidate materials

is estimated by compressing a small volume (e.g., 0.5-1 cm 3) of the powder at

about 12,000 psi between metal pistons within an insulating cylinder. The

resistance is determined by measuring the voltage drop across the powder under

the flow of sufficient current to generate easily measured current and voltage

signals. The more accurate 4-point method of measuring resistance has not

been used because of the larger sample volume requirement. Surface areas of

candidate materials are determined by the BET nitrogen adsorption method using

a Micromeritics Flowsorb II 2300 instrument.

STABILITY TESTING

A preliminary assessment of the chemical stabilit__ of the candidate

support materials is made by exposing the as-prepared powder to 30% KOH at

80°C under an oxygen atmosphere for about 5 days, as described in previous

publications [1-3]. This test is useful for eliminating the more unstable

candidate materials.

For an initial assessment of the electrochemical stability of candidate

support materials and catalysts, a wettable electrode composition is prepared

on gold mesh and the steady-state anodic current is measured in the range of

1.0 to 1.4 V versus RHE in 30% KOH at 80°C. If the anodic current observed,

after initiation of potentiostatic control at 1.4 V, drops to the microamp

range, the system is allowed to equilibrate overnight; the steady-state

anodic current is then recorded. In a second stage of testing, candidate

materials are subjected to higher potentials (1.6 V) representative of oxygen

evolution conditions for about I00 hours, and lower potentials (0.6 V)

representative of oxygen reduction conditions for about 16 hours. The latter

is intended to place electrochemical stress on the materials used in the oxide

form. The current values measured are combined with other observations such

as weight loss or gain, color changes, changes in conductivity, microscopic

examination (SEM, TEM) and analysis (EDAX, XRD, etc.) to assess the stability

of the material.

Observations on chemical stability are summarized in Table 2.

chemical stability measurements are recorded in Tables 3 and 4.

Electro158
LaNIO_3, LINiO x and grN are potentially promising support materials, as

described previously [1-3], but have not been investigated further.

The following observations were made on candidate materials tested:

TI___NN: In previous work, the coarse commercial powder (<40 microns) and

the low-surface-area Giner, Inc. preparation (i micron, 2.3 m2/g) appeared to

be quite stable at oxygen evolution potentials [2,3]. Post-test XRD analysis

indicated a strong TiN pattern (a - 4.240 A), no TiO2, and a few lines

possibly due to a silicate, phosphate or carbonate. The higher-surface-area

material, TiNx, (x = 0.72 to 0.86)* similarly appeared relatively stable in

the chemical test and anodically up to 1.4 V. However, dissolution occurred

when the electrode was held at 1.6 V. A gold-catalyzed TiN electrode was also

found to be very reactive. After electrochemical testing, the surface of tile

electrode had degraded to a refractory non-conductive material. Post-test XRD

analysis showed a strong second phase, possibly potassium titanate. Tracking

of electrical conductivity through electrode processing in air revealed that

the material increased in resistivity by about a factor of 1000 at 325°C,

indicating oxidation. Sintering under 5% }I2/N 2 preserved the conductivity,

but did not improve stability [2].

A stoichiometric form of TiN may prove to be suitable as a support for 02

reduction catalysts, but it seems unlikely that it will be a viable candidate

support for 02 evolution catalysts.

ZrC: As described previously, a commercially available, coarse

(<40 microns), low-surface-area powder with about 2% Hf impurity was

evaluated [2,3]. Gas generation was noted during the initial period of tile

chemical stability test, but this was attributed to dissolution of the 1{f

impurity (weight change measurements were not successful). Very low anodic

currents were measured at 1.4 V, although in testing at 1.6 V vs. R}IE from

50 to i00 hours, there was a visible loss of material, as well as a measurable

weight loss (e.g., -40%). This may have been due to physical shedding during

oxygen evolution, as there was no other evidence of reaction.

A "Hf-free" ZrC with similar conductivity, surface area and particle

size, obtained from another vendor, was also investigated [2]. In chemical

testing, no gassing or color changes were observed, but there was a weight

loss of about 16% and a small amount of white flocculent precipitate. The

electrical conductivity was higher after chemical stability testing (700 vs.

125 ohmcm'l). In electrochemical stability testing, the anodic currents

measured at 1.4 V were low, 0.5 to 1.0 microamp/mg. The electrode was tested

at 1.6 V vs. RHE for approximately 81 hours. There was a visible thinning of

the electrode and a weight loss of 36%, similar to that seen in tests of Illcontaining material. X-ray analysis of the post-test electrode showed broad

lines for a ZrC phase, plus a strong secondary phase which could not be

positively identified, but may be a Teflon or carbonaceous species.

* TiN x prepared by Dr. J. Michaels, Dept. of Chem. Eng.,

Univ. of Calif. Berkeley.

159
Preparation of high-surface-area ZrC by self-propagatlng high-temperature

synthesis (SPLITS) was investigated. High-surface-area carbon and zirconium

metal powder were combined and ignited with a resistance coll following the

method of Mullins and Riley [9]. The SPHTS was partially successful, but

X-ray diffraction analysis indicated that the product contained about 504

unreacted Zr. No further investigation of ZrC is planned.

PbPd02: A preparation of thls compound by the Adams method, reported

previously [2,3], had a very low surface area, but appeared to be quite stable

in electrochemical testing. Attempts to prepare a higher-surface-area

material have been unsuccessful to date.

NaxPt304: Some initial preparations of this compound, described in

previous publications [1-3], were determined by XRD to be single-phase

NaxPt304 with apparent x-values of 0.7-0.8. In a 17-day electrochemical

stability test, this material showed no evidence of degradation. In • more

demanding test, two NaxPt304 electrodes were cycled between oxygen reduction

and evolution for about 90 hours (45 one-hour cycles; see Fig. 2 and

description in discussion of oxygen performance). At the conclusion of this

test, both electrodes were subjected to XRD analysis (the final state of each

electrode, reduction or evolution, was not recorded). Both catalysts

exhibited the crystallographic pattern for NaxPt304 , but the value of x for

the catalyst from one electrode may have been reduced slightly. X-ray

diffraction peak broadening allows only very approximate determination of the

unit cell parameter needed to establish x-values (±0.2), thus we have not been

able to establish a correlation between this parameter and physical

characteristics and performance. For several more recent preparations, we

have performed elemental analysis by inductively coupled plasma spectroscopy

(ICP) to determine the sodium content. These measurements indicate a much

lower value for x (-0.5) than estimated from XRD data. A preliminary

conclusion is that the NaxPt304 compositions with higher x-values are more

conductive and may also be more catalytically active.

Pb2_2_xPbx)_Q7_y: In prior work [I], material prepared by a lowtemperature precipitation method, based on a procedure described by Horowitz,

et al. [i0], was found to be unstable in 304 KOH at 80°C and showed substantial physical changes at 1.6 V. Refiring at 500°C improved crystallinity

and decreased reactivity below 1.4 V. The test electrode survived 24 hours at

1.6 V, but delaminated from the current collector at 0.6 V [2,3]. Another

method to prepare Pb2(Ir 2 xPbx)O 7 , by fusion of the respective salts in - -y

sodium nitrate [5], was investigated. The product had good surface area

(95 m2/g) and conductivity (64 ohmcm'1). The as-prepared material could not

be analyzed due to lack of crystallinity. After heat treatment in oxygen at

750°C to increase particle size and crystallinity, X-ray diffraction analysis

showed a 50/50 mixture of frO 2 and the Pb-substituted pyrochlore. The effect

of the heat treatment on the original composition is unknown. The lattice

parameter of the pyrochlore was 10.316 _, which leads to an approximate

formula of Pb2(Irl.9Pb0.1)O7.y. In chemical stability tests, a 209 weight

loss was measured. This material showed some catalytic activity, as described

in the last section.

160
Rh02: This material was also prepared by fusion of a Rh-salt in sodium

nitrate [5]. The product showed excellent surface area (187 m2/g) and

conductivity (14 ohmcm'1). X-ray diffraction and thermogravimetric analyses

showed the material to be RhO2-(0.48)H20. In chemical stability tests, no

weight loss was seen. Electrochemical stability testing also showed no

significant weight loss, after 87 hours at 1.6 V and 18 hours at 0.6 V. The

current densities measured were relatively high, at 1.3 to 1.6 V, due to the

catalytic activity of the material. As described in the next section, RhO 2

shows potential as a catalyst and/or catalytic support material.

frO2: The sodium nitrate fusion method [5] was also used to prepare two

different batches of high-surface-area frO 2 (160 to 198 m2/g) with excellent

conductivity (33 to 70 ohmcm'1). X-ray analysis confirmed that the material

(Batch #2) was single-phase frO 2. Scanning electron microscopy showed a mix

of thin platelets of several microns in width, as well as agglomerations of

submicron-sized particles. In the chemical stability test, the Batch #i

material showed no visible change, but exhibited a 39% weight loss. This

result is not consistent with the electrochemical test and may be a result of

experimental error. In the electrochemical stability test, the IrO 2 electrode

showed i to 8 mlcroamps/mg at 1.0 to 1.3 V. There is significant oxygen

evolution activity above 1.3 V. After a total test time of 83 hours at 1.6 V

and 18 hours at 0.6 V in 30% KOH at 80°C, the electrode showed no significant

deterioration and a weight loss of only 5%. This material shows significant

promise, both as a catalyst and as a catalytic support and has been combined

with Pt to form a bifunctlonal electrode as described in the next sectioh.

IrMn2Qx: This material had higher conductivity (12 ohmcm -I) but slightly

lower surface area than RuMn20 x. In preliminary chemical stability tests, it

appeared more stable and did not cause a color change in the KOH solution.

However, a yellow coloration was noted during the electrochemical stress test,

along with a 17% weight loss from the test electrode. The IrMn20 x displayed

considerable catalytic activity, as described below, and may merit further

investigation. This material could not be analyzed in the as-prepared form

because of its high surface area (small particle size) and possible lack of

crystallinity. A heat-treated sample (900°C in 02) showed a broad-line rutile

phase in X-ray diffraction analysis. An approximate composition of

Irl_xMnx02, where x was 0.05 to 0.i0, was determined. However, the effect of

the heat treatment on the chemical composition is unclear.

RuMn2Ox: This material showed evidence of instability in chemical and

electrochemical stability tests (weight loss and color change of the KOH).

After leaching in KOH in order to remove any soluble component, the material

still showed a 13% weight loss on exposure to 80°C KOH and produced coloration

of the KOH solution. Another sample of the as-prepared RuMn20 x was heat

treated at 650°C in oxygen. This treatment caused a large loss in surface

area from 62 m2/g to 12 m2/g, and increased the conductivity slightly from

2.9 ohmcm "I to 5.6 ohmcm'_ As before, the chemical stability was not improved

(15% weight loss and coloration of the KOH solution were observed). The

leached and heat-treated samples were not subjected to electrochemical stress

testing.

161
OXYGEN ELECTRODE PERFORMANCE TESTING

Oxygen electrode performance testing was performed in a floating

electrode cell [4] in 30% KOH at 80°C using a I cm x I cm electrode sample, as

described in earlier publications [1-3]. The reference materials for 02

reduction and 02 evolution were 10% Pt/Au and Pt black, respectively. All

candidate catalysts were compared to these reference materials as well as to

each other.

The testing sequence is usually an oxygen reduction polarization test

followed by an oxygen evolution polarization test, applying small potential

steps sufficient to yield a few data points in each log-decade of current

density from i to I000 mA/cm 2, The potentials are controlled, and compensated

for iR loss, with a Princeton Applied Research model 173 potentiostat.

Collecting oxygen evolution data entails frequent interruptions to remove

trapped gas bubbles. After an oxygen evolution test, electrodes that have not

been optimized for blfunctional operation are sometimes too flooded to rerun

an oxygen reduction test. Development of an electrode structure adequate for

bifunctlonal operation with these candidate electrocatalysts is an independent

research task. Some electrodes have been prepared using processing methods

designed for optimization of bifunctlonal performance. Results on 02

reduction and 02 evolution tests on these electrodes are very promising, as

described at the end of this section.

Nilr03 add ColrO3: Both of these candidate support materials showed some

oxygen redox activity in preliminary testing, as shown in Fig. I. The ColrO 3

was poor in 02 reduction, but is promising for 02 evolution. After stability

testing, further electrode development will be pursued if warranted.

IrMn20 x displayed excellent 02 reduction and 02 evolution performance, as

shown in Fig. i, but this level of performance proved difficult to reproduce

on subsequent tests. As discussed above, this material shows signs of

instability in anodic corrosion testing and requires further development, but

it remains an interesting candidate for bifunctlonal oxygen catalysis.

IrO 2 was tested for comparison to the other iridium-based catalysts

prepared. It can be seen that the oxygen evolution performance of IrMn20 x is

similar to Ir02, but the oxygen reduction performance of IrMn20 x is better.

The latter effect may be due to electrode structural differences for the two

materials.

Pb2(Ir2_xP_bbx)OT_y: As shown in Fig. I, this Adams fusion preparation

(which turned out to contain two phases, Pb2(Irl_xPbx)O7_y and IrO 2) showed

better 02 reduction performance than frO 2 alone, but was not as good as the

IrMn20 x material. For 02 evolution, it was slightly poorer than IrO 2 alone or

IrMn20 x. At low current densities, this material showed performance similar

to single-phase Pb2(Ir I 33Pb0.67)O7.$ prepared previously by the Horowitz

method [I0]. At higher current densities, it showed much improved performance

(e.g., above about 50 mA/cm 2 in O2-evolution mode and above about 200 mA/cm 2

in O2-reduction mode). Some of these differences in performance may be due to

differences in the electrode structure.

162
NaxPt304: The NaxPt304 catalyst [1-3,6,7] was evaluated in an 02/02 cell

under cyclic 02 evolutlon/reductlon conditions. Two NaxPt304 electrodes were

separated by a matrix material soaked in 30% KOH. The full cell was operated

under a positive pressure oxygen atmosphere (50 psi) at 80°C. The cell

voltage was measured at current densities of 50, I00 and 200 mA/cm 2,

One electrode underwent 30 minutes on 02 reduction, 5 minutes on open

circuit, then 30 minutes on 02 evolution, while the other electrode was undergoing the opposite cycle. The measured voltage represents the difference

between the electrode potentials, which may be positive or negative, depending

on which electrode is at the 02 evolution potential. Figure 2 shows the cell

potential on progressive cycling at each current density. The cell polarization was 515 to 575 mV at 50 mA/cm2; 670 to 710 mV at i00 mA/cm2; and 900 to

995 mV at 200 mA/cm 2. The total cell potential increased slightly over time

at 50 and i00 mA/cm 2. The cell potential increased more rapidly over time at

200 mA/cm Z. The difference between the absolute values of cell voltage on

positive and negative portions of the cycle was near zero, indicating that the

two electrodes were performing nearly identically, as expected.

This material is one of the best candidate bifunctional oxygen electrode

catalysts developed to date in terms of both stability and oxygen reduction/

evolution performance. The emphasis in the current development effort is on

preparing larger quantities of single-phase material and correlating synthesis

conditions, crystalline structure, sodium deficiency and performance. A

number of our earlier attempts, for reasons we have not clearly established,

contained a second phase of free Pt, probably from partial decomposition of

the PtO 2 reactant. Generally this two-phase material exhibited lower

conductivity and surface area and poorer performance than single-phase

NaxPt304. The Pt phase can be partially dissolved out with hot aqua regia,

which improves conductivity, surface area and performance, but not

significantly. Several recent preparations have yielded slngle-phase,

conductive material, but the performance is lower than observed earlier. At

the present time, we are trying to determine Na/Pt ratios by elemental

analysis (ICP) for greater accuracy than that obtained from typical XRD backreflection data. Some oxygen electrode performance data for recent

preparations of NaxPt304, compared to data presented in earlier publications,

is shown in Fig. 3.

Rh-RhO 2. Pt-lrO 2 . Pt-RhO2: Three noble-metal/noble-metal-oxide electrodes were prepared; Rh-RhO2, Pt-lrO 2 and Pt-RhO 2. The fabrication methods

used to make these electrodes were chosen as our best estimate of a balanced

mix of hydrophobic/hydrophilic character. Oxygen reduction performance often

benefits from a more hydrophobic electrode character while a hydrophilic

electrode may be better for oxygen evolution. Giner, Inc. has developed a

unique proprietary process for controlling the hydrophilic/hydrophobic

properties at the particle level and applied this technology for achieving

superior bifunctional oxygen electrode performance.

The oxygen electrode performance of these three mixed-character

electrodes was tested in evolution and reduction modes, as discussed below.

In each case, earlier data on the performance of the individual component

materials (Pt, Rh, RhO 2 and Ir02) as oxygen electrodes is compared to the

performance of the mixed electrode structure. (The slngle-catalyst electrodes

163
were also single-character structures, which usually compromises the

performance in both modes.) In addition, the performance of the three mixedcharacter electrodes are compared directly.

In Fig. 4, the oxygen electrode performance of Rh black, RhO 2 and Rh/RhO 2

are compared versus the standard baseline materials: Pt for 02 evolution and

i0_ Pt/Au for 02 reduction. The Rh black, RhO 2 and Rh/RhO 2 electrodes all

showed very similar 02 evolution performance, achieving 200 mA/cm 2 at about

1.43 V vs. RHE. The Rh black and Rh/RhO 2 electrodes also showed very similar

02 reduction performance, 200 mA/cm 2 at about 0.8 V; the RhO 2 alone did not

perform as well. The level of improvement in 02 reduction performance of the

Rh/RhO 2 electrode over that of the RhO 2 electrode, was about 50 mV at

200 mA/cm 2. The mixed character of the Rh/RhO 2 electrode is believed to be

the reason for the performance improvement.

The first mixed-character Pt/RhO 2 electrode (4) showed little improvement

over the RhO 2 electrode, as shown in Fig. 5. The performance of the two

electrodes in both evolution and reduction modes is very similar, 200 mA/cm 2

at about 0.77 to 0.78 V in 02 reduction and at about 1.42 to 1.43 V in 02

evolution. Post-test examination of the electrode suggested that the

hydrophobic/hydrophilic character was not balanced in this trial (too

wettable). Therefore, a second Pt/RhO 2 electrode was fabricated using a

slightly different mixture ratio (B). As shown in Fig. 5, the resulting

improvement in 02 reduction performance was dramatic. The electrode was

almost as good as the baseline 109 Pt/Au material. The 02 evolution

performance was basically unchanged.

In Fig. 6, pure Pt black and pure frO 2 electrodes are compared to Pt/IrO 2

electrodes prepared by two different approaches. The catalyst for Electrode

was prepared in an Adams co-fuslon of Pt and Ir chlorides and was fabricated

as a single-character electrode. The mixed-character Pt/IrO 2 (_) electrode

showed excellent 02 reduction performance, greatly improved over that of pure

IrO 2 or the co-fuslon single-character Pt/IrO 2 electrode. This optimized

electrode sustained 200 mA/cm 2 at about 0.87 V. This level of performance is

close to that of the baseline catalyst, 109 Pt/Au, which held about 0.89 V at

200 mA/cm 2. In 02 evolution, the mixed-character Pt/IrO 2 electrode also

showed good performance, 200 mA/cm 2 at about 1.47 V. This level of

performance was slightly lower than that for frO 2, but was better than the cofusion single-character Pt/IrO 2 electrode.

The performance of the three mixed-character electrodes is shown in

Fig. 7. In 02 reduction mode, the Pt/RhO 2 and Pt/IrO 2 electrodes are very

similar in performance and very close to the i0_ Pt/Au baseline electrode. In

02 evolution mode, all three electrodes show similar superior performance; the

Pt/RhO 2 and Rh/RhO 2 electrodes showed slightly better performance than the

Pt/IrO 2 electrode.

CONCLUDING REMARKS

Two materials of lower catalytic activity that have been identified as

potential bifunctional alkaline oxygen electrode supports are LiNiO x and ZrN.

Two other materials, TiN and ZrC, do not appear to be sufficiently stable at

oxidation potentials, but ZrC may be suitable as a catalyst support for oxygen

reduction. Of the more catalytic candidate supports, LaNiO 3 and PbPdO 2 appear

164
to be stable but need surface area development; ColrO 3 and NilrO 3 have been

prepared in hlgh-surface-area forms and show some oxygen evolution activity

but need stability testing; RhO 2 and frO 2 have been prepared in very highsurface-area forms, they appear to be stable and show high activity for oxygen

evolution.

Recent work has focused on more complete development of the most promising

blfunctlonal oxygen electrode catalyst, NaxPt304, and the application of

wettabillty control at the catalyst/Teflon particle level to achieve superior

bifunctional electrode operation. The latter approach has been successfully

implemented with two catalysts integrated into "dual-character" electrodes,

e.g., Pt/IrO2, Pt/RhO 2 or Rh/RhO 2. The same approach can be applied to a

single bifunctional catalyst also, e.g., a "dual-character" NaxPt304

electrode. Such electrodes show highly efficient bifunctlonal oxygen

electrocatalysis.

ACKNOWLEDGMENTS

This work was supported by NASA Lewis Research Center under the direction

of Dr. William Fielder and Dr. Richard Baldwin, Contract No. NAS3-24635.

We would like to acknowledge the assistance of Professor B.L. Chamberland

of the University of Connecticut in the selection and identification of

numerous materials tested in this program.

REFERENCES

I. L. Swette and J. Giner, "Oxygen Electrodes for Rechargeable Alkaline Fuel

Cells," J. of Power Sources, 22 (1988) 399-408.

2. L. Swette and N. Kackley, "Rechargeable Alkaline Oxygen Electrode

Development," Proceedings of American Institute of Chemical Engineers

Annual Meeting, Nov. 5-10, 1989, San Francisco, CA.

3. L. Swette and N. Kackley, "Oxygen Electrodes for Rechargeable Alkaline

Fuel Cells II," J. of Power Sources, 29 (1990) 423-436.

4. J. Giner and S. Smith, "A Simple Method for Measuring the Polarization of

Hydrophobic Gas Diffusion Electrodes," Electrochem. Technol., 5 (1967)

59-61.

5. R. Adams and R.L. Shriner, "Platinum Oxide as a Catalyst in the Reduction

of Organic Compounds. IIl. Preparation and Properties of the Oxide of

Platinum Obtained by the Fusion of Chloroplatinic Acid with Sodium

Nitrate," J. Am. Chem. Soc., 45 (1923) 2171-2179.

6. K.B. Schwartz, C.T. Prewitt, R.D. Shannon, L.M. Corliss, J.M. Hastings

and B.L. Chamberland, "Neutron Powder Diffraction Study of Two Sodium

Oxides: Nal.OPt304 and Nao.73Pt304," Acta Cryst., 38 (1982) 363-368.

7. R.D. Shannon, T.E. Gier, P.F. Carcia, P.E. Bierstedt, R.B. Flippen and

A.J. Vega, "Synthesis and Properties of Platinum Metal Oxides of the Type

MxPt304," Inorg. Chem., 21 (1982) 3372-3381.

8. B.L. Chamberland and S. Silverman, J. Less Common Metals, 65 (1979)

41-48.

9. M.E. Mullins and E. Riley, "The Effect of Carbon Morphology on the

Combustion Synthesis of Titanium Carbide," J. Mater. Res., 4 (1989)

408-411.

i0. H.S. Horowitz, J.M. Longo and J.T. Lewandowski, "I.ead Ruthenium Oxide,

Pb2[Ru2_xPb_+]06.5 ,'' Inorganic Synthesis, Vol. 22, John-Wiley & Sons,

New York, 1983, pp. 69-72.

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173

N91"82562

SOME RECENT STUDIES WITH THE SOLID-IONOMER

ELECTROCHEMICAL CAPACITOR

S. Sarangapani*, J. Forchione, A. Griffith, and A.B. LaConti

Giner, Inc.

Waltham, Massachusetts 02254-9147

and

R. Baldwin

NASA Lewis Research Center

Cleveland, Ohio 44135

ABSTRACT

Giner, Inc. has developed a high-energy-density, all-solid-ionomer

electrochemical capacitor, completely free of liquid electrolyte. The novel

features of this device include I) a three-dimensional metal oxideparticulate-ionomer composite electrode structure, and 2) a ur_itized repeating

cell element. The composite electrode structures are bonded to opposite sides

of a thin sheet of a solid proton-conducting ionomer membrane and fo[-m an

integrally bonded membrane and electrode assembly (MEA). Individual MEAs can

be stacked in series as bipolar elements to form a multiple cell device. The

discharge characteristics and energy storage properties of these devices are

described. Typical capacitance measured for a unit cell is 1 F/cm 2. I.ife

testing of a multicell capacitor on an intermittent basis has shown, that over

a 10,000 hour period, the capacitance and resistance of the cell has remained

invariant. There has been no maintenance required on the device since it was

fabricated. Other multicell units of shorter life duration have e×hibited

similar reliable performance characteristics.

Recent work has focused on increasing the capacitance of the unitized

structure and improving the low-temperature characteristics. The approaches

and experimental results will be presented.

Some possible advanced NASA applications for these unique all-solidionomer devices will be discussed,

INTRODUCTION

Electrical pulse power generating systems include electrostatic (dielectric) and electrochemical capacitors. The former accumulate substaI_ti:_l

energy only by developing a very high voltage difference across the dielectric

gap. These very high potentials make them impractical for certain batt(_ry/

capacitor high-energy-density pulse power applications. Electrocl_emical

capacitors, on the other hand, can accumulate substantial charge (arid deliver

* Author to whom correspondence should be addressed

175
it within the required times) at much lower voltages, compatible with the

voltage of batteries. The classical "electrochemical supercapacitor" is a

symmetric device in which the electrolyte is placed between two identical

electrode systems (i.e., with the same electrode material under conditions

causing the same process in each electrode but "occurring in opposite

direction). Pragmatically, the term "electrochemical capacitors" can be

extended to asymmetric devices, i.e., devices with different electrode

reactions at each electrode, although these latter devices may be closer to an

ultra-hlgh-power battery.

Symmetric electrochemical capacitors show respectable capacities from the

double layer charging of their electrodes, which act as "perfectly polarizable

electrodes" (i.e., electrodes, which can operate in an extended potential

range without a faradaic reaction). Considerably higher capacities are

obtained, however, with electrode systems having a substantial potential

region over which a faradaic reaction takes place at a potential which depends

more or less linearly, and reversibly (independent of the value of the current

and without substantial hysteresis) on the charge that is transferred between

the phases. The charge vs. potential slope represents the "pseudo capacity"

of the system, which can be considerably higher than the capacity of the

double layer (ile., of a purely capacitative electrode interface). Since an

electrochemical capacitor cell is made of two capacitors in series, each

representing one electrode-electrolyte interface, the overall capacity of the

cell is one-half the capacity of the individual electrode interface. But the

stored energy is augmented by the _ell voltage being twice that of the single

electrode interface.

Symmetric electrochemical capacitors have electrical behavior similar to

electrostatic capacitors. Of particular relevance to the proposed work is the

question of energy (E) delivered by the capacitor, which is related to the

integral capacity (C) and initial potential (Vi) and the final potential (Vf)

of the capacitor by the expression:

or

E - I/2C [V_ - V_] [i]

2

E - 112CV_ [1 -a ] [2]

where is the fraction of the initial voltage retained in the capacitor. If

the internal resistance of the capacitor (Ri) is not negligible when compared

with the external resistance (Re) , and both of these resistances are constant,

the useful energy (Eu) is given by:

2 Re

Eu -I12CV_ [i - a ] [3]

R + R.

e 1

For the purpose of the following comparison of the energy of symmetric and

asymmetric devices, the supercapacitor's energy can be expressed as a function

of the charge, Q, which can be obtained by the complete discharge of the

capacitor (Q = CV i). Accordingly-, Equation [2], for instance, can be

converted to:

E - I/2QV i [I - =2] [4]

Symmetric electrochemical capacitor have a number of characteristics not

exhibited by dielectric capacitors. As mentioned above, electrochemical

capacitance is orders-of-magnitude higher than dielectric capacitance. On the

176
other hand, they have a relatively low limit to the voltage that they can

tolerate before undesired reactions (e.g., H20 electrolysis) occur.

Asymmetric devices have different reactions at each cell electrode. The

reactions can be a) both capacitative in nature, b) one capacitative and the

other battery-llke or, c) both battery-llke. The latter type represents an

ultra-hlgh power battery. The hlgh-power characteristic can be obtained by

the selection of materials with very reversible reactions (such as proton

insertion in NiO to produce NiOOH, or in Raney nickel, or in palladium to give

a hydride) and by the spreading the active materials over large electrode

areas so that ohmic resistances within the electrodes, and the mass diffusion

within the active material, are low.

The main advantage of the ultra-high-power battery is that it delivers

the energy at a substantially invariant potential. Its energy is related to

charge and voltage by:

E - QV i [5]

Comparison of Equations [4] and [5] shows that the energy obtained from the

ultra-high-power battery operating at a voltage, V, is 2/(1 -= 2) larger than

that of the ultra-capacitor with the same initial voltage, V.

In principle, the potential electrochemical limitations of the supercapacitor discussed above apply to the ultra-high-power battery. In practice

a specific system has to be selected with very high reversibility (e.g., a

system involving only simple proton insertion reactions), with very low

internal resistance, etc.

REVIEW OF PRIOR WORK

Carbon Capacitors

One of the first electrochemical capacitors developed had carbon paste

electrodes and liquid sulfuric acid electrolyte, together with a microporous

separator. Relatively thick (36 mil) electrodes were used to get 2 F/cm 2

capacitance from this system at low frequencies and/or low charge/discharge

rates [I]. Later developments in electrode technology for the acid electrolyte electrochemical capacitor included lead counter electrodes [2] and

polyoxometallate-deposited carbon paste electrodes, stabilized with an organic

amine [3]. These modifications add either additional weight or manufacturing

complexity to the capacitor system and resulted in poor stability during

cycling.

Several workers have attempted to improve the performance of carbon-based

electrochemical capacitors [4,5,6,7]. John Currie investigated several

carbons with different micropore volumes and compared the performance of H2SO 4

and K2SO 4 as electrolytes. An improvement in capacity with micropore surface

area was noticed and the self-dlscharge rate was significantly reduced with

K2SO 4 electrolyte. A drawback of the later approach was the considerable

increase in internal series resistance with the neutral electrolyte.

Researchers ,at Space Power Institute, Auburn University have been

conducting research to minimize series resistance in carbon-based capacitor

systems [5,6]. They point out that one of the main drawbacks of carbon black

177
as electrode material is its high resistivity arising out of poor particle-toparticle contact. The energy density (KJ/kg) is directly proportional to

accessible surface area of electrode materials, and carbon black is the only

known electrode material with surface areas in the range 1000-2000 m2/gm.

They show that the series resistance can be decreased considerably by making a

composite of nickel or stainless steel fiber (20 microns in diameter) and

carbon black/fibers. More than an order-of-magnltude improvement in capacitance for unit weight is realized by this approach.

Noble Metal Capacitors

Oxides of noble metals, mostly RuO 2, have been investigated extensively

as an electrode material. RuO 2 has a high capacity of about 150 microfarads/

real cm 2 [8]. This is substantial due to pseudocapaclty from surface

reaction, which can be written approximately as:

2RuO 2 + 2H + + 2e" --> Ru203 + H20 [61

This type of surface reaction, in combination with double-layer

processes, has been shown to be capable of sustaining high current densities

on the millisecond scale [9,10]. RuO 2 can be made into a high-surface-area

coating [ii] or as high-surface-area particulates [12]. Bonding of particulates to an ionomer membrane has been demonstrated for RuO 2 anodes for use in

chlor-alkali cells [13] where membrane and electrode assemblies with active

areas of up to 35 ft 2 have been prepared.

Craig [14] describes a supercapacitor based on ruthenium oxide and

mixtures of Ru and Ta oxides, and reported capacitances as high as 2.8 F/cm 2.

Sierra Alc_zar, et al. [15] have described an electrochemical capacitor

cell using high-surface-area RuO 2 having a surface roughness (real area/geometric area) of greater than I0,000; this device used a liquid sulfuric acid

electrolyte.

Other researchers have worked to increase the capacitance of electrode

structures. A summary of some of the most applicable work is as follows:

The work of Craig, Pinnacle Research, and Giner, Inc. with RuO2-based

systems are attempts to improve the power output of capacitors while improving

_he charge storage capacity by one or two orders of magnitude. McHardy, et

al. [16,17] at Hughes reported essentially on a similar system with IrO 2 as

the electrode material, except that they paired it with a Pt/H system. This

is fundamentally different from an electrochemical capacitor. The Pt-H

electrode uses the fast electrode kinetics of the hydrogen reaction and the

large pseudocapacitance associated with adsorbed hydrogen. The IrO 2 electrode

is essentially the same as RuO 2 except that it has higher charge density and

power capability, if properly prepared. McHardy, et al. report a typical

capacity of 20 mC/cm 2 for their Pt-H/IrO 2 system for 5 mS pulses.

Rauh, et al. [18] have investigated frO 2, and to a lesser degree RuO 2, as

part of a program to screen several materials for pulse power applications. A

sputtered IrO 2 film gave ii mF/cm 2 for 0.I micron thickness; a 5-micron film

of thermally prepared IrO 2 gave 35 mF/cm 2.

178
Non-Noble Metal Ox_d_ Capacitors

Rauh, et al. [19] investigated a nickel oxide film in combination with

LaNi 5 as the hydrogen electrode. Sputtered, electrochemical, and chemical

nickel oxides were examined -- only electrochemically formed oxide gave significant capacity. Several problems of scaling up these systems were noted

which included current distribution, reslstive/inductlve losses, heat rejection, side reactions and limited cycle llfe for pulse power applications.

A recent investigation of the nickel oxide electrode was by the CM

research labs [20]. A nlckel-zinc battery designed for electric vehicles was

tested for pulse power capabilities in this work. The power output was

limited by the nickel oxide electrode, and depended on the state of charge of

the battery. While slntered nickel oxide electrodes showed no degradation

after 300-500 deep discharge cycles, pocket-type nickel oxide electrodes

showed severe power capability degradation after only several cycles.

Rauh, et al. [18] also investigated WO 3. This and other hydrous oxide

films they claim, take advantage of both non-faradalc and faradaic processes.

For example, a one-micron thick WO 3 film should give 22 mF/cm 2 in non-faradalc

capacitance and 220 mF/cm 2 in faradalc capacitance. This translates into an

energy density of 7.8 Whr/Ib (258 kJ/l) and a power density of 434 kW/Ib

(1500 kW/l). Thicker films, unfortunately, do not appear to have linear

scaling of capacity.

Craig, in his patent application [14] has also claimed supercapacitance

behavior (in cyclic voltammetry studies) with metal oxides such as Mo, W, Co

and Ni.

Several other battery systems such as Li/metal sulfide, Li/S02, Pb/acid

and several conducting polymer based systems have been investigated as pulse

power sources. These suffer from either inadequate safety, poor cycle life,

or low power capabilities.

Non-Aqueous Capacitors

Non-aqueous solutions, such as propylene carbonate with a dissolved salt

have been used in capacitors. These electrolytes have been successfully used

in the low-rate Dynacap capacitor [7] to achieve single-cell voltage of up to

2.8 V with a specific capacitance of i00 F/g. They are limited for pulse

power applications because of their low conductivity. Also there is concern

about the long-term stability of these materials.

Capaci_prs Based on Conducting Polymers

Conducting polymers are a class of materials that have interesting

possibilities for capacitor applications. Polyaniline polymer films formed on

Pt and nickel foils have been used as supercapacitor devices. Stability

problems of these polymers appear to limit the cycle life at the present stage

of development. In addition, they typically exhibit a much higher resistance.

Polyaniline is an electronic conductor only when immersed in acid

electrolyte, between 0.2 and 0.8 V (vs. SCE) [21]. Cottesfeld, et al. [22]

reported a capacitance of 800 Coulombs/cm 3 in aqueous acidic electrolyte for a

179
10-60 nm polyanillne film, cycling between 0 and 0.55 V (vs. Ag wire). In

recent work, Bhakta, et al. [23] have described conductive polymers for use in

super capacitors based on aniline copolymers. Polymer films were formed on

Naflon coated platinum and nickel substrates using different aniline derivatives.

DEVICE CONCEPT AND DEVICE FABRICATION

The device concept and fabrication methods are fully explained in earlier

publications [24,25]. The capacitor includes I) a three-dimensional metal

oxide-partlculate-lonomer composite electrode structure, whereby the ionomer

coats the individual particles and provides a mechanism for continuous proton

transport throughout the composite structure and 2) a unitized repeating cell

element that includes this three-dlmensional composite electrode structure.

The composite electrode structures are bonded to opposite sides of a thin

sheet of a solid proton-conductlng ionomer membrane such as the Dow XUS

13204.10 and form an integrally bonded membrane and electrode assembly (MEA).

An exploded view of the cell components and assembly of a 35-celi stack are

shown in Fig. I. In this device, the membrane electrode assemblies (MEAs) are

stacked In a bipolar arrangement. An electrically conductive pressure pad is

used at one end to ensure isostatic conditions on all MEAs. The assembly is

held in place using titanium bolts and nuts, torqued to the desired force. A

photograph of this device is shown in Fig. 2.

EXPERIMENTAL/DISCUSSION

Capacitance test consisted of discharging the MEA across a 10-ohm load

and measuring the RC time constant the time to decay from 0.9 to 0.331V

(0.9/e). The MEA was charged to a potential of +I.000 V using a PAR 173

Potentiostat/Galvanostat in conjunction with a PAR 276 Interface. The cell

was then disconnected and allowed to discharge across a 10-ohm load while the

PAR 276 recorded the voltage decay curve. Internal resistance was determined

by applying a square wave current pulse of ±0.5 Amps, I00 Hz and measuring the

instantaneous voltage decay at the break points.

Performance and Life Test Results of Multlcell Stacks

Breadboard capacitor stacks containing 5, I0 and 35 cells were fabricated, as shown in Fig. i, using Naflon-coated RuO x particulate electrodes

bonded to Dow XUS 13204.10 membranes. Individual (unit) cell area was 25 cm z.

Typical capacitance per unit cell ranged from 0.7 to I F/cm 2. The results of

cycle testing (up to 1500 cycles) of the 5-cell unit on a 60-second charge/

discharge cycle are shown in Fig. 3 together with the energy efficiency. The

unit was charged at a constant current of 0.6 A to 5 V, then, after about an

8-second live hold, the unit was discharged into a 0.134 ohm load. This was

repeated at 60-second intervals until 1574 cycles were completed. The energy

efficiency for this very high load is -35%. While part of the efficiency loss

is due to iR dissipation, a significant portion of the energy loss can be

attributed to slow discharge processes, which may be the charge/dlscharge of

the inner core of the RuO x particles. The efficiency improves to 82% for a

lO-ohm load and about 89% for a 100-ohm load (Table I). Charging rate does

not seem to affect the energy efficiency appreciably at moderate rates

(Table 2). The self-dlscharge behavior (or leakage current) is illustrated in

180
Table 3. Comparison of these data with that presented in Table I shows that

approximately 15% of the energy is lost for a holding time of 5 minutes. The

effect of temperature on capacitor efficiency is shown in Table 4. Efficiency

decreases from approximately 66 to 61% as the temperature decreases from 46 to

10°C. At -4°C, the efficiency decreases dramatically to 29%. The approach to

decreasing this efficiency loss at low temperatures is discussed in a subsequent section.

Figure 4 shows the capacitance and resistance of the 5- and 35-celi units

as a function of time. Both the capacitance and resistance are steady over

the duration of the test which is over a year for the 5-cell unit.

Figure 5 shows the gravimetrlc and volumetric power densities of our

capacitors as a function of pulse width. For a 0.7 V cutoff, power densities

as high as 2 x 103 watts/kg and 4 x 104 watts/L can be obtained for 5-msec

pulses. This result can be improved further as the specific capacitance of

these devices are increased as discussed below.

Approaches to Increasing Energy Density

The maximum capacitance that has been realized currently with the RuOx*

I00 m2/g Nafion-coated structures [24,25] is in the range of 75-80 F/g.

Calculations based on 150 microfarads/cm 2 show that a RuO x particulate

material of i00 m2/g surface area should yield capacitances as high as

150 F/g. For a 20 mg/cm 2 RuO x electrode structure this should give a

calculated capacitance of 3 F/cm 2. The observed capacitance, however, is only

-I F/cm 2 . Through TEM studies in the above cited reference it has been

observed that for the RuO× particles the typical particle diameter is

approximately 50,000 _ with a porous substructure which is only partially

accessible to the solid ionomer. Thus there is a significant portion of the

I00 m2/g surface area that is not available for capacitive charging. If the

particle size of RuO x is reduced to 40 _ size, then assuming spherical

particles, and using the equation below,

6 p = 7 g/cc for RuO

Surface area = --, x [7]

p D D = 40 A (particle diameter)

a surface area of 214 m2/g is calculated. Of more importance is that

essentially all of the surface area of the 40 _ particle should be accessible

for coating with solid ionomer. For this surface area, capacitances of the

order of 320 F/g are obtainable. Assuming a 20 mg/cm 2 RuO x electrode

structure (40 _ particles coated with solid ionomer) the calculated capacitance is 6.4 F/cm 2.

Table 5 :_hows our initial results on our attempts to increase the

specific capacitance. Two approaches have been experimented in obtaining the

data presented in Table 5. The first two entries in this table use RuOx-lrO ×

* Technically, RuO x is a more correct expression of the chemical compound than

RuO 2 since the x-value can range from 1.9 to above 2.0. In add[tioll, the

charging/discharging cycle of the capacitor may cause variations in the

oxidation state of the compound.

181
alloy and fro x pure material to enhance capacltance. The addition of IrO x to

RuO x increases the surface area and, perhaps, results in increased specific

capacitance. The capacitance of a solld-ionomer cell fabricated with IrO x was

1.4 F/cm 2, leading to a specific capacitance of 45 F/g. The second approach

is to disperse the RuO x to 20-200 A range particle size. To do this, we need

to use a substrate on which the RuO x can be deposited. Activated carbon was

chosen as a support and RuO x deposited onto it using various methods. The

increase in specific capacitance is approximately one order of magnitude. One

disadvantage in this approach is the loss of volume energy density.

Approaches are being investigated to at least partially preserve the volume

energy density while decreasing the RuO x content.

Approaches to IncreaslnK Low-Temperature Performance

The influence of temperature on the performance of a PEM

ionomer capacitor is shown in Table 4. The capacitor contains only

water which is part of the ionomer structure. The performance

capacitors drops abruptly below the freezing point of water. Two

appear to contribute to the performance decrease: the conductivity

electrolyte and the rate of the proton transfer reaction

H +

RUOx(OH)y + + e - RuO x-1 (OH) y+1

We attempted to improve the performance of these devices by lowering

freezing point of water through the addition of certain solutes.

all-solid

hydrated

of these

factors

of the

[8j

the

Figure 6 shows the capacitance and ESR behavior as a function of

temperature for MEAs treated in several ways. The capacitance of the baseline

Nafion 117 MEA (contains only water) dropped drastically from 0.693 F/cm 2 at

room temperature (25°C) to 0.048 F/cm 2 at -14°C, while internal resistance

increased from 17 to 220 milliohms. The test results obtained after soaking

the MEAs in phosphoric acid, glycol and N-methylformamide solution are

summari'zed below.

DOW MEAs were tested after treating by soaking overnight in 43% H3PO 4

(50% by volume solution of Fisher 85% Phosphoric Acid), 40%, 50% and 70%

solutions of ethylene glycol (J.T. Baker #3-9300), and 50% N-methylformamide,

then tested at various temperatures down to approximately -40°C to obtain

performance curves. Treatments with all the solutions above were carried out

at room temperature. Some testing was also conducted by soaking the MEAs in

glycol solutions for 1 hour at 25°C. In one set of these experiments, a

treatment solution containing 5% Naflon in glycol/water was used.

Phosphoric Acid Treatment of MEAs: The DOW MEA treated in 43% H3PO 4 was

clearly superior at retaining capacitance and internal resistance characteristics. From 25°C temperature, down to -38°C, Capacitance dropped from

0.577 F/cm 2 to 0.543 F/cm 2 (94% retention) while internal resistance rose from

4.2 to 25 milliohms.

Ethylene Glycol Treatment of MEAs: The MEAs treated with 40% and 50%

ethylene glycol in water for one hour at 25°C appear to show good low-temperature performance as shown in Fig. 7. Also included in these tests was a MEA

treatment with a 5% Nafion ionomer in 50/50 glycol water. The 40% ethylene

182
glycol in water showedthe best overall capacitance from 25°C down to a

temperature of approximately -25°C. From 25°C, down to -20°C, capacitance

dropped from 0.701 to 0.532 F/cm 2 (76% retention). However, at a temperature

of -30°C, capacitance dropped to 0.164 F/cm 2. The effect of using I) 50%

ethylene glycol in water and 2) 5% Naflon ionomer in the 50/50 glycol/water is

also shown in Fig. 7. The capacitance data for the MEA soaked in 50% glycol

in water have been normalized for making a direct comparison of the data. It

appears some improvement in -30°C low-temperature performance for the glycol

systems is obtained when using 509 rather than 404 glycol in water. Addition

of dissolved Nafion ionomer (54) to the 504 glycol solution appears to improve

the MEA performance compared to the 504 glycol alone. Further work is

required to define the optimum water/glycol ratio and treatment conditions. A

potential advantage of the water/glycol system versus the H3PO 4 system for

improving low-temperature capacitor performance is that the glycol is not a

liquid electrolyte, thus minimizlng the possibility of shunt currents,

corrosion and leakage.

N-Methylformamide Treatment of MEAs: The performance of an MEA soaked in

a 509 N-methylformamide solution is shown in Fig. 6. Generally, the overall

performance of the cell as to capacitance and resistance was poor, compared to

the H3PO 4 and the 409, 509 glycol solutions.

A summary of the low-temperature results is shown in Table 6. In the

above experiments, we have used a new MEA for each low-temperature experiment

in order to avoid contamination from one experiment to another. However, the

capacitance of the several MEAs used in this test program varied somewhat.

The best low-temperature results were obtained for phosphoric acid; the

capacitance value at -30°C for the MEA soaked in phosphoric acid is approximately 954 of the 25°C value. Both 40 and 504 ethylene glycol treated samples

retain 60-804 at -20°C of the 25°C capacitance; however at -30°C, 504 glycol

shows better performance.

AEROSPACE APPLICATIONS

Although the projected energy densities for electrochemical capacitors

are about two orders of magnitude lower than that of batteries, the highpower-density characteristics of these devices renders them as pote_tially

viable candidates for meeting pulse or peak electrical power requirements for

some anticipated aerospace mission scenarios, especially those with discharge

times on the millisecond to second time scale. On a volumetric or gravimetric

basis, the advantages of utilizing electrochemical capacitors rather than

batteries for meeting the peak power demands associated with a specific

mission scenario will largely depend upon the total and pulse durations of the

power peaks.

Projected energy and power densities for electrochemical capacitors ai_d

batteries [25] can be used to make approximate volumetric comparisons between

these two power sources for peaking applications. For example, for a defined

peak power level and total duration, the total energy required for storage in

an electrochemical capacitor can be calculated. If one assumes that the total

charge will be stored in the cell structure of the capacitor, the[1 _he vole,he

of the capacitor which satisfies both its energy and power density characteristics can be estimated. A similar calculation can be made to estimate tJi_

volume of the reserve portion of a battery power source, i.e., the additional

183
size to meet the peaking demands which exceed the steady-state power needs.

Based on the data cited above, the estimated volumes of an electrochemical

capacitor and a battery for supplying the peak power needs would be equal for

a total peak power duration of 2.0 minutes. For a total duration of

30 seconds, the volume of a battery peaking power source would be four times

larger than that of an electrochemical capacitor. If a mission scenario is

such that there is sufficient time for recharging between transients, then the

required capacitor volume could be reduced even further.

Besides the potential for volume and/or mass reductions, other inherent

characteristics of electrochemical capacitors could be critical drivers for

selection of this technology for specific missions. High reliability and

safety have significant impact on technology selection, and solid-ionomer

electrochemical capacitors contain no toxic or corrosive electrolyte nor are

any appreciable reaction products formed. They have a very high cycle life,

and thus, can be utilized'as a rechargeable secondary power source, if this is

a feasible option for a mission scenario, Assessments of power source

technologies will also need to consider effects of the thermal environment

which will be encountered upon the power source, as well as any operational

constraints which may be imposed by incorporation of the power source in highspeed circuitry.

Specific applications for electrochemical capacitor technology may be

envisioned within the advanced launch systems (ALS) development and Orbiter

upgrade programs which are presently being pursued within NASA. With such

goals as increasing safety, reducing turnaround time, lowering recurring

costs, reducing vehicle weight, and increasing operational efficiency in mind,

a major objective of these programs is to replace existing auxiliary power

unit (APU)/hydraulic actuator systems with electromechanical actuator (EMA)

systems.

An advanced EMA system, which is depicted in Fig. 8, will require a

highly reliable power source to meet its unique electrical power requirements,

which include high peak power pulses for short durations (<i00 milliseconds).

The advanced EMA system, which would be composed of independent power channels

to meet failure-tolerant design requirements, would be developed to have highpower and high switching-speed capabilities. Multi-phase induction motors

would be controlled via a hlgh-frequency AC link using resonant power processing technology. For shuttle evolution programs, the proposed EMA system would

power and control all the flight control surfaces, landing gear, and the

Orbiter's thrust vector control (TVC) systems. These systems are powered just

before launch, during ascent, and throughout re-entry and roll-out.

As part of a Shuttle EMA system study performed for the Johnson Space

Center, several candidate electrical power system configurations were conceived to meet EMA power requirements and were compared to determine which

would be the most suitable option [26]. The electrical power requirements per

power channel which were used to synthesize the power systems included:

3.5 kW average power

5.5 kWh energy

120 kW peak power (400 ms total; 40-80 ms pulses)

270 VDC

184
The candidates for the conceptual study included several hlgh-powerdensity battery systems, modified Orbiter fuel cells in both dedicated and

integrated design approaches, and advanced (high power density) alkaline fuel

cells.

In the above study, the modified Orbiter fuel cell design approaches were

favored with respect to imposing the least impact upon the implementation

schedule and initial cost of implementation. Both of the conceptual design

approaches incorporated the solld-ionomer electrochemical capacitor for

providing the 120-kW peak power pulses specified for the ascent and descent

phases of EMA operation. The scaled-up 270-volt capacitor required per power

channel, which was sized based upon existing data, would contain 300 cells,

weigh approximately 60 pounds, and occupy a volume of approximately 0.5 ft 3.

A schematic of an integrated design approach is shown in Fig. 9.

As mentioned earlier, the replacement of hydraulic actuators with EMA

systems is also a critical element in NASA's ALS program, which addresses TVC

technology for solid rocket boosters, AtlasCentaur launch vehicles, etc.

Electrical power system trade studies have also been performed within this

program, and candidate power sources have included hlgh-rate primary and

secondary batteries, fuel cells, turboalternator systems, and flywheel energy

storage systems [27]. The recommended system for development based on these

studies, which did not include electrochemical capacitor technology, was the

bipolar lead-acid battery. The electrochemical capacitor could prove to be a

viable competitor for meeting peak power demands, and an approach incorporating electrochemical capacitor technology needs to be evaluated with respect

to satisfying overall mission requirements.

CONCLUDING REMARKS

Life testing of a multicell capacitor on an intermittent basis has shown,

that over a lO,000-hour period, the capacitance and resistance of the cell has

remained invarlant. There has been no maintenance required on the device

since it was fabricated. Other multicell units of shorter-life duration have

exhibited similar reliable performance characteristics.

Test results obtained with multlcell solid-ionomer electrochemical

capacitors indicate gravimetrlc and volumetKic power densities in the range of

2 x l0 S W/g and 4 x 104 W/L, respectively, can be obtained for 5-msec pulses.

Initial testing indicates that the specific capacitance of these devices can

be further increased by I) use of additives, or alloying of the RuO x particles

to result in a higher-surface-area particulate electrode structure and

2) dispersion of the RuO x to the 20 to 200 A range particle size by depositing

the RuO x on hlgh-surface-area carbon. This should result in further increase

in power and energy densities. Further work in this area is recommended.

Testing indicates there is loss in efficiency (joules out/joules in) when

discharging the capacitor devices across a low-resistance load. The efficiency decreases from 83% for a lO-ohm load to 35% for a O.134-ohm load. The

formation of smaller RuO x particles (20-200 A) and improved ionomer coating

should help access more of the RuO x surface structure and improve efficiency.

185
Some initial testing of the self-discharge characteristics of the solidionomer capacitor indicate that approximately 15% of the energy is lost for a

holding time of 5 minutes. This loss can likely be decreased by using

improved quality procedures when fabricating the individual cells to minimize

electrical shunt paths.

The performance of the solld-ionomer capacitor, containing only hydrated

water, drops abruptly below the freezing point of water. Two factors appear

to contribute to the performance decrease: the conductivity of the electrolyte and the proton transfer reaction. The performance of the capacitors was

improved by lowering the freezing point of water through the addition of

certain solutes. Addition of phosphoric acid to the MEAs extends the lowtemperature limit of our capacitors to -30°C, without any significant loss in

performance. Treatment of the MEAs with certain concentrations of glycol in

water also improves the low-temperature performance significantly compared to

the baseline MEA. The low-temperature performance of capacitors equilibrated

with glycol appears to depend on the quantity of glycol in the membrane and

treatment conditions. A range of 40 to 50% glycol in water and one-hour

treatment at 25°C gave the best results in this study. The addition of

dissolved Nafion ionomer (5%) to the 50% glycol solution appears to improve

MEA performance compared to 50% glycol alone. A potential advantage of the

water/glycol system versus the H3PO 4 system for improving low-temperature

capacitor performance, is that the glycol is not a liquid electrolyte, thus

minimizing the possibility of shunt currents, corrosion and leakage. A

systematic study of the proton mobility and ionic conductance at low temperatures in proton-exchange membranes equilibrated with different types and

quantities of non-electrolyte solutes may lead to optimum low-temperature

behavior of our capacitors and is recommended for future studies.

ACKNOWLEDGEMENTS

This work was supported in part by the following: DARPA/Office of Naval

Research Contract N00014-88-C-0391, Sandla National Laboratories, and National

Science Foundation Grant: ISI-9060142.

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N91-32563

PERFORMANCE OF A DUAL ANODE NICKEL-HYDROGEN CELL

Randall F. Gahn

NASA Lewis Research Center

Cleveland, Ohio 44135

An experimental study was conducted to characterize the voltage performance of a

nickel-hydrogen cell containing a hydrogen electrode on both sides of the nickel

electrode. The dual anode cell was compared with a conventional single anode

cell using the same nickel electrode. Higher discharge voltages and lower

charge voltages were obtained with the dual anode cell during constant current

discharges to IOC, pulse discharges to 8C, and polarization measurements at 504

state-of-charge.

INTRODUCTION

Nickel-hydrogen batteries are currently being used in numerous geosynchronous

orbit (GEO) applications and also on the Hubble Space Telescope in low-Earth

orbit (LEO) [i]. GEO applications require approximately 100 charge/discharge

cycles per year whereas LEO applications require about 6000 cycles per year.

Recent improvements in the cycle life indicate nickel-hydrogen batteries could

meet the five-year life requirements for a LEO satellite (e.g., Space Station

Freedom) and replace nickel-cadmium batteries for other LEO applications [2,3].

Reduction in battery weight to increase the specific energy is also being

addressed through the development of lighter weight nickel electrodes [4].

This investigation focused on the improvement in the voltage performance of the

nickel-hydrogen cell by reducing the polarization factors of the components.

Voltage losses in a cell are a function of the ohmic resistance of the components and the electrochemical polarization parameters of the electrodes [5].

The ohmic resistance fraction of the total polarization is determined by the

separator conductivity, the electrolyte concentration, the compression of the

components, and the resistivity of the electrode current collectors. The electrochemical polarization portion of the total polarization is attributed to the

kinetics of the reactions and mass transport of the ionic species. Reduction in

the effect of any of these variables will improve the voltage performance of the

cell.

In an attempt to reduce cell voltage losses, a second hydrogen electrode was

introduced into the cell on the other side of the nickel electrode. (A second

separator was also added between the second hydrogen electrode and the nickel

electrode.) The addition of the second anode reduces the current density on the

hydrogen electrode by one-half, and the additional separator provides

electrolyte on the "backside" of the nickel electrodes and in effect shortens

the electrolyte diffusion path into the nickel electrode by one-half. Thus,

these modifications should reduce the overall polarization losses by about onehalf.

The purpose of this investigation was to experimentally evaluate the voltage

characteristics of a nickel-hydrogen cell containing a dual anode. The performance was compared to that of a cell using a single anode and the same nickel

electrode. (The evaluation of the same nickel electrode eliminates any differences in nickel electrode characteristics.)

!

195
A second objective of this study was to evaluate the dual anode cell design

using thick nickel electrodes for potential pulse power applications. However,

only the analysis of a standard thickness (30 mils/O.76 mm)electrode has been

completed and is reported herein.

EXPERIMENTAL

Sketches of a conventional nickel-hydrogen cell and the dual anode cell used in

this experiment are shown in Fig. I. The dual anode sketch shows the additional

hydrogen electrode and separator placed on top of the nickel electrode. Cell

components were 3.5 in. (8.9 cm) diameter pineapple-slice design. The nickel

electrode was obtained from the Hughes Aircraft Co. The hydrogen electrodes

were obtained from Eagle-Picher Industries. The same nickel electrode was used

in both cells. A layered-type separator was used with each design and consisted

of one layer of Zircar cloth next to the hydrogen electrode and a layer of

radiation-grafted polyethylene film next to the nickel electrode. Voltage leads

were spot-welded to the base of each electrode. Following assembly the cells

were vacuum filled with 26% potassium hydroxide electrolyte and installed in a

boilerplate pressure vessel. The current leads and voltage sensing leads were

fed through a Conax fitting mounted in the lid. The vessel was evacuated and

then pressurized with 150 psi of hydrogen.

Performance of each cell design was determined with constant current discharges

from C/4 to IOC, constant current charges at C/2, pulse discharges at 2C, 4C and

8C and voltage-current relationships for 15-sec charges and discharges at 50%

depth of discharge.

RESULTS AND DISCUSSION

The ampere hour (Ahr) capacity of each cell design was determined by discharging

the cell at the C/4 rate to 0.1V. Capacities measured for the single anode

cell and the dual anode cell were 1.25 and 1.27 Ahr, respectively. For both

designs, however, performance of the cell was based on a capacity of 1.3 Ahr.

Charging for all cycles was done at the C/2 rate with a 5% to 10% overcharge.

Following all discharges the cell was returned to the same reference point by

discharging at the C/4 rate to 0.I V before starting the next cycle.

Discharge performance at constant current as a function of cell capacity was

measured at C/4, C/2, C, 2C, 4C, 6C, 8C and I0C for both cell designs. Voltage

profile for the single anode cell is shown in Fig. 2 and for the dual anode cell

in Fig. 3. As expected, the discharge amp-hour capacity decreased as the current increased, but both cell designs were similar over the entire range. At

the I0C rate the capacity to 0.1V was 1.09 Ahr for both cells. (Note that the

cell voltage was measured at the base of the electrodes and does not reflect

voltage losses associated with the current leads and connections when the measurement is taken outside the pressure vessel.)

Comparison of constant current discharges for the two designs at the C and 10C

rates are shown in Figs. 4 and 5. At both rates the voltage of the dual anode

cell is higher than the conventional single anode cell. The discharge capacities at the C rate were similar (about 1.2 Ahr) for both designs. The I0C

discharge capacities were also unaffected by the cell design. The voltage difference between the C rate discharge curves over the entire discharge is 10 to

196
20 mV, but at the 10C rate the voltage difference increased with the depth-ofdischarge. Figures 6 and 7 show the difference in the constant current discharge voltages between the two cells at the midpoint of the discharge and at

754 DOD. At 509 DOD the difference between the dual anode cell and single anode

cell was i0 mV at the C rate and 160 mV at the I0C rate. At 754 DOD the dual

anode cell C rate voltage was still about i0 mV higher but at IOC the difference between the two increased to 230 mV. These data indicate the dual anode

cell has a higher voltage throughout the discharge and has less polarization at

the deeper depths-of-discharge than does the single anode cell. The slopes of

the middischarge voltage curves were 12.9 mn for the dual anode and 26.7 mn for

the single anode cell. The slopes of the curves in Fig. 7 were 20.1 m_ for the

dual anode and 37.3 mn for the single anode cell. For both figures the dual

anode cell resistance is about one-half of the conventional cell resistance

which is what was anticipated. The change in slope of the single anode cell

resistance between 50_ DOD and 754 DOD was ]0.6 m_ (37.3 to 26.7), whereas the

dual anode cell only charged 7.2 mn (20.] to 12.9).

A comparison of the charge performance at the C/2 rate is shown in Fig. 8. Up

to approximately 75_ SOC the dual anode voltage was 15 to 20 mV lower than the

single anode cell. The rapid rise in the voltage is probably associated with

the production of oxygen which was consistent for both designs. The C/2 charge

was used as the standard charge rate for the evaluation of the discharge performance. No other charge rates have been investigated.

Another objective of this study was to evaluate the dual anode cell design for

pulse power applications. A comparison of the performance of both designs

during an 8C standard constant current discharge and an 8C pulse discharge is

shown in Fig. 9. During the pulse test the cell was alternately discharged at

8C (10.4 A) for 5 sec then switched to open-circuit for 10 sec until the cell

voltage reached 0.1V. The discharge voltage was taken at the end of the 5 sec

discharge period. The standard discharge was a continuous 10.4 A current unti]

the cell voltage reached 0.] V. The pulse discharge voltage for the dual anode

cell was only about 10 mV higher than the voltage obtained during the constant

current discharge. However, the pulse discharge voltage for the dual anode cell

was about ]20 mV higher than the pulse voltage for the single anode cell,

indicating a significant decrease in polarization effects with the dual anode

design. This figure also shows the pulse discharge voltage of the single anode

cell is about 30 mV higher than the constant current discharge voltage.

Comparison of the voltage difference between the standard and pulse discharges

for both designs demonstrates the significant reduction in polarization in the

cell when the second anode is used.

Both designs were also evaluated on pulse discharge at the 2C and 4C rates.

Voltage curves were in the same order as the 8C discharges but the differences

were less. A comparison of the pulse discharge voltage performance at 2C and 4C

for the dual anode cell are compared with the 8C performance in Fig. 10. The

improvement in performance of the dual anode cell over the conventional ceil is

shown in Fig. i]. Midpoint voltages for the pulse discharges are plotted

against the discharge rate. At 2C, 4C and 8C rates, the middischarge voltages

of the dual anode cell were about 30, 40, and 140 mY, respectively, higher than

the single anode cell.

The last test which was used to characterize the performance of the dual anode

design was to make short duration charges and discharges at increasing currents

while keeping the cell at approximately 50_ SOC. After charging to 50_ SOC the

197
cell was charged at the first current for 15 sec, switched to open-circuit for 3

min and 45 sec, and then discharged at the samecurrent for 15 sec followed by

another open circuit period. A comparison of the 15-sec charge and discharge

voltage performance is shown in Fig. 12. The charge characterization test was

terminated after the voltage reached ].6 V. In order to maintain the 509 SOC

condition for the higher current discharges, the ceil was charged at the C rate

for the time necessary for each succeeding discharge.

The single anode cell reached ].6 V on charge at 8 A whereas the dual anode cell

did not reach 1.6 V until the charging current was 15 A. During the discharge

test the single anode cell voltage dropped to ].0 V at 21 A, however, the dual

anode cell voltage did not reach 1.0 V until the current was about 33 A. This

test, although not tied to any particular application, demonstrates the improved

voltage performance on both charge and discharge for the dual anode cell design.

Summariesof the discharge power obtained from the two cell designs on the constant current and ]5-sec discharge tests are shownin Figs. ]3 and 14. Figure 13 comparesthe power from the cell at the midpoints of the constant current

discharges. The power outputs, plotted as a function of the discharge current,

of the two designs are about the samebelow the 2C discharge rate (2.6 A) but

gradually separate as the discharge current increases. At ]3.0 A (]OC) the

power of the dual anode cell is 184 (]5.0 Wversus 12.7 W) higher than the

single anode cell.

A comparison of the power output as a function of the current of the two cell

designs during the 15-sec discharge test is shownin Fig. ]4. The single anode

cell reached a maximumpower of 27 Wat about 37 A. The dual anode cell's

maximumpower was 37 Wat about 45 A which is a 374 higher power than the single

anode cell. (Since the voltage measurementson these single nickel-hydrogen

cells were taken at the base of the electrodes to eliminate voltage losses in

the current collectors, these power values should not be used to project performances of higher capacity cells containing multiple cells.)

The results reported in this study were obtained using the samenickel electrode. The experiments were repeated twice, and the results were in agreement.

Other types of nickel electrodes and large nickel-hydrogen cell stack sizes need

to be evaluated in order to assess the applicability of the dual anode concept.

CONCLUDINGREMARKS

The voltage performance of a single IPV nickel-hydrogen cell constructed with a

hydrogen electrode on both sides of the nickel electrode was determined and compared to a cell with the conventional design using a single hydrogen electrode.

The samenickel electrode was tested in both cell designs. Constant current

discharge voltages were comparedfrom C/4 to I0C and 5-sec pulse discharges were

comparedat 2C, 4C and 8C. Constant current charge performance was comparedat

the C/2 rate only. Fifteen sec charges and discharges at 504 SOCwere used to

characterize the cells over a large range of currents and to determine the maximumdischarge power of each cell.

In each test the dual anodecell voltage was higher on discharge and lower on

charge than the single anode cell. Voltage anomalies at low currents (C/4, C/2)

need to be clarified. Further studies with other nickel electrodes and multi198
cell stacks are warranted to determine if the reduction in cell polarizations

with the dual anode design justify the additional component weight.

ACKNOWLEDGEMENT

The author would like to acknowledge the assistance of Timothy Ryan and Michael

Degen with the experimental work and data reduction for this study.

I •

2,

3.

•

,

REFERENCES

L. Miller, Proceedings 23rd IECEC, Vol. 2, AMSE, New York, 1988, pp. 489-

492.

H. S. Lim and S. A. Verzwyvelt, J. Power Sources, 29 (1990) 503-519.

J. J. Smithrick and S. W. Hall, Proceedings 25th IECEC, Vol. 3, AIChE, New

York, 1990, pp. 16-21.

D. L. Britton, Proceedings 34th International Power Sources Symposium,

IEEE, N.J., 1990, pp. 235-238.

M. A. Reid, J. Power Sources, 29 (1990), pp. 467-476.

199
Single Anode Cell Dual Anode Cell

FIG. le

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SchematiCElectrode diagr_mSarea 50.0°fcm_.thsingle anode and dual anode cells.

200
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AMPERE HOURS

Constant current discharge voltage profile of the conventional

single anode nickel-hydrogen cell. Voltage measured at base of

electrodes. C = 1.3

FIG. 3.

1.5

1.4

1.2

1.1

1

0 0.2 0.4 0.5 0.8 1 1.2 1.4

AMPERE HOURS

Constant current discharge voltage profile of the dual anode

nickel-hydrogen cell. Voltage measured at base of electrodes.

C = 1.3

201
1.6

1.4

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FIG. 4. Comparison of the C rate discharge performance of the single anode

and dual anode cells. C = 1.3

FIG. 5.

1.4

1.3 --

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Comparison of the I0C discharge performance of the single anode

and dual anode cells. C - 1.3

202
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SINGLE ANODE CEIL'-""

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F]G. 6. Plot of mid-discharge cell voltages as a function of discharge

rate to compare polarizations of the dual anode and single anode

cell designs. C = 1.3

1.2 --

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(m .. 0.0373 ot_m=)

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FIG. 7. Plot of cell voltages at 75% DOD as a function of discharge rate

to compare polarizations of the dual anode and single anode cell

designs. C = 1.3

203
FIG. ,

1.45

SINGLE ANODE

•DUAL ANOOE CELL

1.35

O 0.2 0.4 0.6 0.8 1

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Charge performance comparison at the C/2 rate.

C discharge, C/4 drain. C = 1.3

1.2 1.4 1.6

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PULSE - 5 SECONDS DISCHARGE

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SHE - SINGLE HYDROGEN ELECTRODE CELL

DHE -, DUAL HYDROGEN ELECTRODE CELL

I I I I I I I

0.2 0.4 0.6 0.8

CAPACITY (Amp Hours)

FIG. g. Comparison of the 8C pulse discharge and 8C constant current

discharge voltages. Pulse - 5 seconds discharge, 10 seconds opencircuit. C - 1.3

204
0

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FIG. i0.

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CAPACITY (Amp Hours)

Pulsed-current discharge voltage profile of the dual anode nickelhydrogen cell. Voltage measured at base of electrodes. C = 1.3

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205
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Charge and discharge polarization measurements at 50% S0C.

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FIG. 13. Comparison of power outputs at 50% DOD during constant current

discharges. C - 1.3

206
L

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Comparison of power outputs during discharge polarization test

50% SOC. C - 1.3

at

207

N91-$2564

MULTIPLE CELL COMMON PRESSURE VESSEL NICKEL HYDROGEN BATFERY

Jeffrey P. Zagrodnik and Kenneth R. Jones

Johnson Controls Battery Group, Inc.

Nickel Hydrogen Battery Division

Milwaukee, Wisconsin

ABSTRACT

Johnson Controls Battery Group, Inc. has developed a multiple cell common pressure

vessel (CPV) Nickel Hydrogen battery that offers significant weight, volume, cost and interfacing

advantages over the conventional individual pressure vessel (IPV) Nickel Hydrogen configuration

that is currently used for aerospace applications. The baseline CPV design was successfully

demonstrated through the testing of a 26-cell prototype, which completed over 7,000 44% depthof-discharge LEO cycles at COMSAT Laboratories. Two-cell boilerplate batteries have now

exceeded 12,500 LEO cycles In ongoing laboratory tests. CPV batteries using both nominal 5"

and 10" diameter vessels are currently available. The flexibility of the design allows these

diameters to provide a broad capability for a variety of space applications.

KEY FEATURES OF THE JOHNSON CONTROLS CPV DESIGN

Nickel Hydrogen batteries are well established as an energy storage subsystem for

commercial communication satellites. The standard design has been the Individual Pressure

Vessel (IPV) which provides an Independent vessel for each cell of the battery. The comparative

advantages of a Common Pressure Vessel (CPV) design configuration, in which many series

connected cells are contained In a single vessel, are widely recognized. These Include higher

specific energy, higher system energy density, simplified interfacing, and reduced cost as

compared to the IPV (1). However, historical concerns related to electrolyte and thermal

management had previously prevented the introduction of a reliable CPV design.

Johnson Controls has successfully developed a patented (2) CPV battery design which

overcomes the histodcal concerns. A radial heat fin provides a pathway for heat transfer from the

center of each cell to the pressure vessel wall. Since the metal fins make direct contact with the

vessel, they actually provide an Improved thermal interface as compared to the IPV design where

heat must be transferred either through the polymeric wall wick or through a hydrogen gap In

order to pass from the cell to the vessel wall.

The heat transfer interface at the exterior of the vessel is simplified slnce only one vessel is

used for a multicell battery. Since IPV design specific energies are often reported without

including their thermal baseplate, there is an additional specific energy advantage for the CPV

which might not be readily apparent from a direct comparison of reported specific energies.

The CPV's vessel is electrically neutral so no insulating layer Is required on the outside of

the vessel. This allows a direct metal to metal contact between the outer vessel wall and the

mounting bracket, further enhancing heat transfer. In an IPV, the vessel wall is electrically live

and must be insulated from the mounting bracket. Prior to launch condensation can occur on the

outside of the IPV vessels creating a possible shorting path between the IPV cells.

209
Although thin-walled polymeric cell container called an electrolyte containment system

(ECS) addresses the Issue of electrolyte management in the CPV configuration. The thermally

sealed ECS isolates the electrolyte within each cell. An intercell connect between each of the

sedes connected cells employs a specially designed compression seal to prevent electrolyte

bddging between cells. Examination of the seals removed from the 26-cell Prototype #1 battery

after 7,400 cycles showed no evidence of any leakage.

A hydrogen vent sealed onto the ECS face of each cell allows hydrogen to pass into the

vessel plenum from the cells during charge and vice versa during discharge. The vent is designed

to prevent wetting with electrolyte and to ensure recombination within a given cell of any oxygen

that is generated within that cell during overcharge. This prevents the development of electrolyte

Imbalances between cells during extended cycling.

INITIAL PROTOTYPE DESIGN

As an initial demonstration of the capabilities of the CPV design, a 26-cell 22 amp-hour

prototype was fabricated in a joint effort with COMSAT Laboratories in 1988 (3). The CPV

prototype Is shown in relation to its IPV counterpart in Figure 1. This battery was composed of

two 13-cell half-stacks which are connected in sedes within the single common vessel to provide a

nominal 32 volts. The 10" diameter cells have a semicircular geometry and employ a double tab

design to enhance current distribution, as shown in Figure 2.

The half-stacks were inserted into two fixed heat fin cavities (Figure 3) which each contain

13 slots, one for each of the 26 cells. The cells are slipped into the fixed cavity prior to the

addition of electrolyte. Upon addition of electrolyte, the asbestos separators in the cells swell

providing the desired cell compression and an intimate thermal contact between the cell face and

the heat fin. Heat generated in the cell passes axially through the cell face to the heat fin, then

radially through the heat fin to the vessel wall. It is the direct thermal contact between the heat fin

and the vessel wall that provides a thermal advantage over the IPV design which provides no

direct radial pathway for heat conduction to the vessel. It is believed that this thermal advantage

will translate to extended life in LEO applications. In GEO applications, where long life is not

required, additional improvements in specific energy can be achieved due to the Improved thermal

pathway.

The two half-stacks for the prototype battery were inserted into a hydroformed Inconel 718

vessel cylinder/dome section. Springs pushed the half-stacks outward against the vessel wall,

maintaining the intimate contact between the heat fin cavity and the vessel wall. After insertion of

the half-stacks a second dome was welded in place using the same general weld ring design

approach that Is applied in the IPV vessels.

INITIAL PROTOTYPE TESTING

The Prototype battery was put on a real time LEO life test at COMSAT Laboratories. Over

7,000 44% depth-of-discharge (DOD) cycles were completed at 10°C. Voltage performance was

relatively stable over the first 6,400 cycles, prior to the rapid voltage degradation which ultimately

caused the battery to reach the 1.0 volt]cell battery failure criteda (Figure 4).

210
Subsequentdestructivephysicalanalyses(DPA)showedthat someof thecellECS'shad

beendamagedat thetime of insertionintothe fixed heat fin cavity, leading to electrolyte leakage

from the cells. The resulting drying out of the positives and separators is believed to have caused

the voltage decline and failure of the battery. All cell components, Including the negative

electrodes were in excellent physical condition. No pinholes or other signs of popping were

observed on the negative electrodes. No signs of blistering or other physical degradation were

observed on the positives. In retrospect, the ability to complete over 7,000 LEO cycles with a

battery which was leaking electrolyte from the onset of testing provides testimony to the resilience

of the CPV design.

Two 2-cell laboratory test battedes of the same baseline design (4) continue on a 44% DOD

LEO life test, one each at Johnson Controls and COMSAT. They have now exceeded 9,000 and

12,500 cycles, respectively with no significant performance degradation.

FEATURES OF IMPROVED LOOSE HEAT FIN CPV DESIGN

A new loose heat fin design was developed to overcome the problems encountered with

Insertion of the cells into the fixed heat fin cavity. The cell design was also modified by providing a

double ECS with a staggered vent pathway to further enhance the electrolyte management

reliability. These approaches were Introduced using a 5" diameter vessel, circular cell component

design. The circular cell with its loose heat fin is shown in Figure 5. The cells and heat fins are

assembled into a stack using a special alignment fixture. The ten cell stack, shown In Figure 6,

has a 9.6 Ah capacity, is 9.7" long and weighs 3 kg. A 22-celi version offers a 13.4 Ah capacity is

20.6" long, and weighs 7.9 kg. In general, the higher the capaclty and/or voltage, the better the

specific energy.

The 5" cell stack is also Inserted into a cylinder, but in this case two separate end domes

and weld rings are welded in place. This approach allows an unlimited vessel length for design

flexibility. Although the cell stack for the 22-cell battery Is only 9.7" long, the vessel length is 20.6"

to provide the required void volume to maintain a 700 psi maximum operating pressure.

Significant Improvements in energy density can be achieved by using a thicker, up to 0.060 inch,

Inconel 718 shell to allow higher operating pressures.

GENERAL DESIGN CHARACTERIZATION

A computer model is used to help optimize battery design parameters In the initial design

stages for a new battery. The model defines optimum design parameters Including vessel

diameter, positive electrode thickness, number of modules per cell, battery length and weight,

given Input on the desired battery voltage and capacity. Several design curves for common

aerospace design ranges have been developed using this model. Examples are provided in

Figures 7-9 for a 22-cell LEO battery In a 5" diameter vessel, a 22-cell LEO battery in a 10"

diameter vessel and a 26-cell GEO battery in a 10" diameter vessel. These curves can be used to

obtain an initial estimate of the available specific energy and length for a variety of designs. Such

estimates serve as a convenient starting point for more detailed design analyses.

211
OTHERAPPLICATIONS

Thecore CPV design concept has been expanded to provide a family of CPV batteries

(Figure 10) for a variety of uses Including aerospace, aircraft starting, and terrestdal applications.

The combined volume offered by this array of markets provides the potential to eliminate product

consistency problems related to intermittent production schedules, minimize the use of batch

processing techniques, and allow the introduction of statistical process control (SPC) into the

component fabdcation processes. These factors will result in a higher level of quality, improved

product consistency, and ultimately lower cost than is presently achieved in the IPV Nickel

Hydrogen battery industry.

The reduced cost provided by the CPM configuration coupled with other cost reducing

design refinements developed under contract to Sandia National Laboratories (5) have made the

system viable for a number of terrestrial applications. Among these is a 2 kWh battery designed

for photovoltaic applications. This battery employs a composite fiber-wound vessel design. Four

2-kWh prototypes are presently undergoing photovoltalc tests, two each at facilities In New Mexico

and Florida (4).

Aircraft starting battery designs also use the fiber-wound vessel approach, but employ a

carbon filament to enhance heat transfer and minimize weight. Initial tests suggest that the CPV

battery will provide performance equal to or better than Nickel Cadmium in this application. The

CPV battery will also provide a reliable measurement of state-of-charge and significantly reduced

maintenance. In addition to life-cycle cost savings, the limited maintenance requirements would

free aircraft designers to locate the battedes In a remote area since accessibility will no longer be

a primary concern. A 20-cell aircraft starting battery prototype Is presently being fabricated for test

by the U.S. Air Force.

CONCLUSION

In summary, a family of CPV battery designs have been developed for a wide variety of

applications. Aerospace designs are presently available in 5" and 10" diameter vessels. Although

the database is still limited, tests to date indicate that the significant advantages of the CPV

design can be realized in a reliable package.

Johnson Control's present aerospace CPV designs cover a wide range of voltages, 12 to

100 volts, and capacities, 10 to 125 ampere-hours. Vessel diameters have been limited to 5" and

10" diameter versions to date, although a 3.5" diameter design is being advanced to optimize the

configuration for battery capacities below 10 ampere-hours.

212
°

,

3.

,

.

REFERENCES

James Dunlop and Richard Beauchamp, "Making Space Nickel/Hydrogen Batteries

Lighter and Less Expensive," AIANDARPA Meeting on Lightweight Satellite Systems,

Monterey, CA, August 1987, NTIS No. N88-13530.

U.S. Patent No. 4,957,830.

Marry Earl, James Dunlop, Richard Beauchamp, Jack Sindorf and Ken Jones, "Design

and Development of an Aerospace Ni/H 2 Battery," 24th Intersociety Energy Conversion

Engineering Conference, August, 1989.

Jeffrey Zagrodnik and Kenneth Jones, " Development of Common Pressure Vessel

Nickel/Hydrogen Batteries," 25th Intersociety Energy Conversion Engineering

Conference, August, 1990.

Richard Beauchamp and Jack Sindorf, "Cost reductions in the Nickel/Hydrogen Battery,"

Journal of Power Sources, 1988, Issue 22.

213
0},, _:INALP_E

BLACKAND WHITE PrlOTOGRAPt-_

Figure i: 26-celi CPV Prototype Battery and IPV Counterpart

Figure 2: i0" Diameter Semicircular Cell

214
BL_,OK AND WHITE F'r'tGTO,..qRAPH

Figure 3: 13-Cell Half-Stack Fixed Heat Fin Cavity

CPVBFITTERY# I - CYCLINGTEST

B

R

T

T

E

R

Y

/

v

5O

45

40

3S

30

25

20

BRTTERY VOLTRGE vs NUMBER OF CYCLES

_'wT, ,. El. i.,,.,,:..i.,, ,., ,,. i.,,,.,,.,i,.. .... ,, i . , , , , ._'m. i , . , ,. . , , , i , . ,, ._-T_-w

-- (N0-OF'-CHRRG¢" VOLTRGr" I 40.S52 V

........ _.._ ............... .J,

-.L._..£ ., Al*

i

-- ENI]-OF'-I]ISCHRRC( VOLTRGIr s I_I.025 v

L

0 leee 2090 3000 4000 5000 6000 7000 eeeo

NUMBER OF CYCLES

Figure 4: CPV Prototype #i - Life Cycle Test

215
ORIGINALPAGE

8LACK AND WHITE PHOTOGRAPH

Figure 5: 5" Diameter Circular Cell and Loose Heat Fin

Figure 6: 5" Diameter 10-Cell Stack

216
L)

a.

(/)

NOMINAL AMP-HOUR CAPACITY

--e--- specific energy, 30 mtl vessel wall thickness

__A__ specific energy, 40 roll vessel wall thickness

specific energy, 50 roll vessel wall thickness

specific energy, 60 roll vessel wall thickness

- - e-- length, 30 roll vessel wall thickness

- - A- - length, 40 roll vessel wall thickness

- - e- - length, 50 mll vessel wail thickness

- - I_- - length, 60 mll vessel wall thickness

Figure 7: Design Curves for a 22-Celi LEO

CPV Nickel Hydrogen Battery in a 5" Diameter Vessel

217
U,I

z

U.I

4_

NOMINAL AMP-HOUI_ CAPACITY

-_e--- specific energy. 30 mll vessel wall thickness

--A-- specific energy, 40 mll vessel wall thickness

speclflc energy, 50 rollvesselwall thickness

-_ specificenergy,60 mllvesselwallthickness

--e-- length,30 rollvesselwallthickness

--A-- length,40 rollvesselwallthlckness

--e-- length,50 rollvesselwallthlckness

--II-- length,60 rollvesselwallthlckness

Figure 8: Design Curves for a 22-Celi LEO

CPV Nickel Hydrogen Battery in a I0" Diameter Vessel

218
$

NOMINAL AMP-HOUR CAPACITY

--e--- s.e., 30 mll vessel

--A-- s.e., 40 mll vessel

---e--- s.e., 50 mll vessel

$.e., 60 roll vessel

-- e-- length,30 mllvessel

-- A-- length,40 mllvessel

-- e-- length,50 mllvessel

--D- length,60 mllvessel

z

Figure 9: Design Curves for a 26-Celi GEO

CPV Nickel Hydrogen Battery in a i0" Diameter Vessel

219
ORIGINAL PAGE

BLACK AND WHITE PHOTOGRAPH

Figure i0: Family of CPV Batteries for Multiple Applications

Clockwise from Upper Left: 26-Celi, 22 Ah GEO Battery

20-Cell, 50 Ah Aircraft Starting Battery

10-Cell, 160 Ah Terrestrial Battery

22-Celi, 13 Ah LEO Battery

Conventional Lead Acid Starting Battery

220
N91-32565

SMALL CAPACITY, LOW COST (Ni-H2) DESIGN CONCEPT FOR COMMERCIAL,

MILITARY AND HIGHER-VOLUME AEROSPACE APPLICATIONS

James R. Wheeler, William D. Cook, and Ron Smith

Eagle-Picher Industries, Inc.

Joplin, Missouri 64801

N ickel-Hydrogen (Ni-H2) batteries have become the technology of choice for both commercial and defense-related satellites in geosynchronous orbits. Their use for low-earth- orbit

(LEO) applications is not as advanced, but seems just as

inevitable because of their inherent advantages over nickelcadnfium batteries. These include superior energy density,

longer cycle lile, and better tolerance to over-charge and

reversal. Ni-}i 2 cells have the added advantage in b()lh

construction and operation of not presenting the environmental possibility of cadmium pollution. Unfbrtunately, but

necessarily, the design of these cells has been driven to high

cost by the sophistication of the satellites and their uses.

Now, using most of the same concepts but less costly

materials and techniques, a low-cost, small cell design has

been developed. Combined with the concept of the common

pressure vessel, this new design promises to be ideal for the

"small-sat" and commercial markets which, increasingly,

are calling for large numbers of less-expensive satellites.

Introduction

The development of Ni-H 2 cells in capacities of 20 amperehours (AH) or less has been delayed because of the initial

demand for development of the technology for larger satellites. Now, however, much of the same technology can be

applied to smaller cells. Two cell design are manufactured at

Eagle-Picher which are tailored to the specific requirements

of the small satellite, not only in terms of power and volume,

but also in terms of economic feasibility. These designs are

based upon proven manufacturing processes and component

heritage, and therefore retain the reliability and performance

advantages of their predecessors. Details of these two

designs are described herein.

Backllround and Forecast

Nickel-Hydrogen batteries are well-established as the technology of choice for geocentric satellite applications, For

example, there are now approximately 30 operational satellites presently flying nickel-hydrogen batteries. EaglePicher has achieved more than 48 million fault-free cell hours

of operation for Ni-H 2 power in orbit. Mid- and Low-orbit

(LEO) applications f()r Ni-ll 2 batteries such ;its nnilitany

programs and the tlubble Space Telescope arc .just now

entering service. Both commercial and g,wernmcnt pr,)

grams have required extensive development et'ftu-ts I_ cnstuc

reliability and optinfization of weight and perf_rnit:mcc.

While these more expensive systems continue It) advauwc, it

is not too soon t. exph)it this tcclmoh)gy. In fact, tlw n,.'_.',.I

for smaller and cheaper satellites and battc, ies appc:tr>, It)I__.

acccler,fling with the burgeoning c()n)municali_ns husinc_.

Direct broadcast satellites, dedicated ph_ine service,; :in,.]

government concepts such as Brilliant Pcl,blcs and th illian[

Eyes, and many others demand just such dcvcl_q_nwnt.

Coupled with this demand are increasingly- practical methods of launch such ;its rail-guns and the rcccntly-succcs,,l'ul

Pegassus aircraft platlk)rm. These shcufld _o f_r to di,mni,,h

some of the constraints on launching lurge ntlnlhcrs ()t t,-',,,.,-

expensive satellites.

RNi!-5- I Design

There ;ire several areas ofconstruclit)n which clcV;dc the co,.i

of production:

Close-tolerance mechanical part fahricati(m

Electrodes of exacting speciticati_m

Pressure vessels with Mil-Spcc requirement,,

Labor-intensive construction

The new prot_type design has incorpor,tted impr()vcmczH,, in

these and other areas in order to reduce cost and czu,,¢ _1

assembly while maintaining the necessary i_cl f_)rnY, tnlcc alld

long life. This represents Phase I _dthc devchqmlcnt. Thc,,c

cells have not, however, bccn optimized Ik_r mass. That x_ ill

be pursued in the near future in a subscqtitent pha,,¢.

Calculations indicate that the energy cfliciency ',','ill bc S/UlWrior to present technology. Please see Figure I fi_r a lahlc _1

energy conlparisons between present systems ;lilt] :.ith,,l_-

thetical, optimized two-cell CPV unit. Please m)tc Ih_fl the

table does not reveal the principal advantayex _d, ('I'V

design, i.e., that when built into a battery there arc _mly hall

as many interccll COlln,...'ct(lrs t(I conlrihtfl,.: l_ Ixitlt,.'ry fill('-.'...

and that the f,_)tprint of the hattcry will occupy only II:tl f the

221 ORIGeNAL P&GE IS

OF POOR QUALITY
Cell Energy Weight

Efficiency Comparison

WH/LB

Equivalent Sealed Lead 9.25 10.14 10.50 10.74

EP Sealed Lead 9.10 9.45 9.87 11.41

EP NiCad 9.27 14.55 12.77 10.00

EP Nilt2* 14.00 16.67 19.74 22.52

* Rated @ I O°C, C/2 Discharge

FIGURE 1

EP 4/23/90

area of one made with IPV cells. The operational feasibility

of two-cell CPV modules has already been established (I).

For a cut-away view of the 5 AH CPV module, see Figure 2.

This unit contains two cells of equal size mounted in tandem

fashion. All of the cell internals are attached to a header, or

base plate, which constitutes the principal structural component. The header contains both terminals, the electrolyte

filling port, and the mechanical attachment stud for the cell

slack assembly. Therefore several machining operations are

consolidated in one piece. Cell activation is accomplished

with automatic equipment through a small hole in the header.

This will be subsequently sealed by insertion of a metal ball.

This sealant method was successfully developed and tested at

over 2,000 psi for a different type of pressurized cell. Those

cells are presently in production with well overa million cells

prodttced.

5AH CPV

FIGURE 2

Negative electrodes are constructed on less-expensive nickel

substrates and have a reduced quantity of platinum catalyst

which, tests show, does not degrade performance but which

substantially reduces cost. The positive electr(_es are designed to be impregnated on a new, automated impregnation

line which minimizes human handling. Thisnot only reduces

time, but handling damage as well.

Because all components are assembled on the header as a base

plate, construction is simplified. Further, the basic design is

rugged for improved resistance to launch-vibration damage

and can be made even more so by optional internal bracing.

This bolsters the inherent advantage of smaller mass which,

in itself, makes the unit more vibration resistant. This is an

obvious complement to recent innovations in less- expensive

launch methodologies. The basic design has the flexibility to

incorporate IPV (individual pressure vessel) or CPV arrangements and can vary in size from 5 to 20 AH's in capacity.

When stacking is complete, the pressure vessel is simply slid

over the cell stack assembly and mated to the header.

The pressure vessel is die-formed of stainless steel, it is

electron-beam welded to the header. Due to the thickness and

edge design of the header however, it can be easily arcwelded instead and that would reduce welding cost.

Prototype Operation

As Phase I, several dozen prototype modules were constructed to develop the smaller size and less-expensive materials. Five units of the most recent design iteration are

undergoing electrical testing at the time of this writing.

These units are designed to operate at a maximum pressure of

375 psig. Operation, as expected, conforms to the very good

performance previously experienced with 40 AH CPV nxv,lules

(1) Unlike those modules these do not have a Teflon inner

coating. However, the activation process was effective in

overcoming the effects of electrolyte bridging. If desired for

added assurance however, the Teflon coating could be easily

added. It is also notable that the capacity-retention characteristics of two-module CPV cells are no different from those of

IPV cells (I). Figure 3 is a photograph of the prototype

modules on test and of the preliminary Phase II design.

Electrical discharge characteristics at different current densities, and pressures are shown in Figure 4.

Phase II is the weight-optimization phase. Modules for this

are being built at the time of this writing (April '90) and the

results should be available in the future.

The prototype modules described herein offer a significant

improvement in ease of manufacture of small inexpensive

nickel-hydrogen cells or CPV modules. Significant cost

savings are realized from a combination of manufacturing

methods and design improvements. This concept has the

potential to reduce the cost of cells by a factor of 70% or

more. The attributes which make Ni-H 2 superior to Ni-Cd

222
FIGURE

and sealed lead acid batteries, i.e. higher energy density,

longer cycle life and better tolerance to overcharge and

reversal, will now also become cost effective for applications

such as small satellites and remote power systems which

require long life (2 to 20 years) and maintenance-free operation.

For terrestrial applications the small Ni-H 2 module is less

toxic in nature than Ni-Cd's, both in production and operalion. Naturally the cell operating pressure can be adjusted by

design as desired: higher for space use and lower for less

weight-sensitive terrestrial applications.

As for use in space, the fortunate availability of CPV design

at this time is especially beneficial because of the weight

savings of battery-packaging. Halving the number of intercell connectors, and the half-size footprint, both serve to

make this battery even more ideally-suited to smaller-diameter, smaller throw-weight launch vehicles.

RNH-12-1 D_iltn Concept

Another design concept which is suited for use in the small

satellite is Eagle-Picher's RNH-12-1, a 12 ampere-hour

common pressure vessel cell. This cell design has been

manufactured, tested, and delivered to lntraspacc ('orpc_ration (North Salt Lake City, Utah) for use as a power sc)ur_.,.

on a SPINSAT (Special Purpose Inexpensive Satellite) mission. The SALT spacecraft, designed to carry an .'tltinlctcr,

has a mission life specification of three years.

The RNH-12-1 is a standard 3.5 inch diameter colnuu_m

pressure vessel cell, with two 12 amp-hour stacks c_lTncctctl

in series, as depicted in the accompanying sketch, Figtwc 5.

The CPV design offers significant advantages for thi_ bat|cry

configuration which are of particular vahJe to tht: _lnal]

spacecraft. By eliminating half the number of units which

would be required to construct an equivalent power syMcM {_1

IPV (individual pressure vessel) units, the CPV tlc,_i_n

provides a 30% improvement in specific energy, a 39 f,_ improvement in energy density, and a 9.9 pound retltl_.lio_l izl

overall mass.

Figure ,5

ORIGINAL PAGE IS

OF POOR QUALITY

223
Vollage

32

5 AH CPV CELL

Discharge I.IA

Time (Min)

Voltage

3.33

3.0

2.67 ?

2.33

2.0

1,67

1.33

1.0

0.67

0.33

0

5 AH CPV CHARGE VOLTAGE/PRESSURE

C/10 CHARGE O°C

PSI

500

450

J t:

/ t" "_300

/

t j

J

/

//

J

//.'/

/, /.

.t.- S

I I 1 _1 1 I I I I 1 I l 1 I I 1 I !

0 2

250

2OO

150

IO(}

50

4 6 8 10 12 14 16 18 20

Chg. VDC Time (Hrs) Pressure

224
Using this concept, sizes up to 30 AH are obtainable. This

cell features performance-proven components which have

been used in the manufacture of over 14000 nickel-hydrogen

cells at Eagle-Picher. Several of the key components are

described herein.

A high mechanical-strength nickel slurry sinter positive

electrode is used. The plate is manufactured to a thickness of

.030 inches and loaded by an aqueous impregnation process.

The negative electrode used is the standard catalyzed nickel

substrate collector with a Tefloh film barrier backing. The

catalyst is a fuel cell grade platinum mixture.

The separator is 10 mil thick and die-punched in the same

profile as the electrodeswith provision for electrolyte redistribution.

An lnconel 718 pressure vessel is used, hydroformed in the

standard domed cylinder configuration with axial terminal

ports and a flame-sprayed ceramic wall wick (reservoir) on

the inner surface. The pressure vessel is hermetically sealed

at the girth using an electron beam weld and meets the

requirements of MIL-STD-1522A, having a burst test strength

in excess of 5 times the maximum expected operating

pressure of the unit at full state of charge.

The RNH-12-1 CPV is fitted with an aluminum thermal

sleeve/flange as shown in Figure 9 which provides a path for

heat dissipation and serves as an interface for installation into

the power system.

A select number of the cells were delivered with cell pressure

Characteristic

Cell Diameter

Overall Length

Cell Mass

=:

Maximum Operating

Pressure (MEOP)

RNH-5-1

(CPV)

2.51 in.

5.5 in.

1.1 lbs

350 psig

5:1

RNH-12-1

(CPV)

3.51 in.

9.0 in.

2.25 Ibs.

Safety Factor

Capacity

@ 10°C to 1.1V 6.3AH 15AH

Nominal Voltage

Specific Energy

2.5V

i4.3WH/lb.

480 psig

5:!

2.5V

16.6WH/Ib.

Energy Density 0.69WH/in 3 0.81WHlin 3

Max. Charge

Voltage 3.2V 3.2V

Small Satellite Cell Features

Figure 6

monitors attached to the surface of the vessel which provide

an electronic signal that indicates the state-of-charge for the

battery as shown in Figure 9. The unit consists of four selftemperature-compensated strain gages connected in a Wheatstone bridge configuration which is calibrated prior to cell

activation.

RNH-12-1 Testine

Twenty RNH-12-1 cells (Lot 1) were tested and found to be

compliant with all the requirements of the SALT detail

specification. Tests were designed which would evaluate the

cell's Coulombic efficiency, voltage characteristics, and

capacity, as well as tolerance of environmental conditions

(random vibration and thermal cycling tests).

The cell is proven capable of maintaining full state-of-charge

using a 1.05 charge/discharge ratio in a range between 5 and

80% depth of discharge. Reference Figures 7 and 8.

10°C Charge at 1.2 Amos

RNH-12-1 Cells

3.Z

rl.I

D.O

8, e.g

3

Z.8

2.7

2.B

_.5

0

glmrJm_m r IgBD

High Cell

. "-'" --o.

AI_ .... ! .... i Low Cell

L

4 8 IZ 10 30

Timm (Hourm]

Figure 7

o

3._

10°C Discharge at 6 Amps

RNH-12-1 Cells

225
OR_G_NAEPAGE

BLACK A,_D WHITE pHOTOGRAPP,

Figure 9

RNH-12-1

With Strain gage and Mounting Flange

Voltage characteristics for the CPV cell are easily predicted

by doubling the normal voltage values of an IPV cell at any

point during the electrical cycle. The CPV cell design has

shown through demonstration that plate-to-terminal current

paths are adequate to avoid IR drop which would affect cell

voltage.

Qualification units from Lot 1 of the RNH-12-1 cells were

subjected to random vibration levels up to 18.5 grms in each

of the three axes with no evidence of subsequent performance

degradation. Similarly, no difference in performance was

seen during or after a thermal exposure test, where the cells

were subjected to 8 temperature cycles between -15 ° C and

+ 35 ° C.

Post-Storaee Testine

At the time of delivery for RNH-12-! Cell Lot 1 (January

1990) the residual _lnits were placed in refrigerated storage

(ap.proximately 0° '(_) in the fully discharged open- circuit

condition. Following an approximate 5 months of such

226

storage a standard capacity check was performed (at I0 _' C)

and the results indicated no performance degradation secondary to storage. In fact, capacities for the two cells which

were tested were slightly improved from the pre-smrage

acceptance tests results.

Acceptance Post-Storage

Testing

S/N 10 13.5 AH 15.0 AH

S/N 22 13.7 AH 15.1 AH

(n)

References

James R. Wheeler, Burton M. Otzinger, "Common Pressure Nickel-Hydrogen Battery Development, Proceedings of the 24th lntersociety Energy

Conversion Engineering Conference (August 1989),

Vol. 3, pp. 1381-1386.

OR!G!N_.L PAGE iS

OF POOR QUALITY
N91-32566

SODIUM - METAL CHLORIDE BATI'ERY RESEARCH AT JPL

B.V. Ratnakumar, A.I. Attia, and G. Halpert

Jet Propulsion Laboratory

California Institute of Technology

Pasadena, California 91109

Sodium - metal chloride batteries have certain distinct

advantages over sodium - sulfur batteries such as

increased safety, inherent overcharge capability and

lower operating temperatures. Two systems, i.e.,

Na/FeCl 2 and Na/NiCl 2 were developed extensively

elsewhere and evaluated for various applications

including electric vehicles and space. Their performance

has been very encouraging and prompted a detailed

fundamental study on these cathodes here at the Jet

Propulsion Laboratory. A brief review of our studies on

these new cathode materials is presented here. The

initial efforts focussed on the methods of fabrication of

the electrodes and their electrochemical

characterization." Subsequent studies were aimed at

establishing the reaction mechanism, determining the

kinetics and identifying the rate-limiting processes in

the reduction of metal chloride cathodes. Nickel

chloride emerged from these studies as the most promising

candidate material and was taken up for further detailed

study on its passivation - a rate limiting process -

under different experimental conditions. Also, the

feasibility of using copper chloride, which is expected

to have higher energy density, has been assessed. Based

on the criteria established from the voltammetric

response of FeCl2, NiCl 2 and CuCI2, several other

transition metal chlorides were screened. Of these,

molybdenum and cobalt chlorides appear promising.

INTR0PUCTION

The disclosures by Coetzer et al (I02) on the use of transition metal

chlorides in chloroaluminate melts as positive electrodes in rechargeable

sodium batteries have generated a wide interest on the sodium-metal

chloride battery systems at various laboratories around the world (3-6).

These battery systems have attractive electrochemical characteristics

such as high voltages, high energy and power densities and a long cycle

life. Besides, some of the problems often encountered with the sulfur

cathode in Na/S batteries are noticeably absent in these systems, making

them viable alternatives to Na/S batteries in several applications.

The advantages stem mainly from the way these systems are configured: A

Na/MCI 2 cell consists of a solid (insoluble) transition metal chloride in

NaAICl 4 electrolyte separated from the anode (molten sodium) by a Na* ion

conducting solid electrolyte (beta" alumina ceramic). Some of these

..... 1227
advantages are : i) about i00 ° lower operating temperatures, 2) reduced

corrosion by the catholyte, 3) increased safety during internal shorts

or thermal excursions, 4) tolerance to overcharge and overdischarge and

5) ease of fabrication in the discharged state with little elemental Na.

Not surprisingly, sodium - metal chloride batteries, aided by the above

attractive features are being considered for various applications

including load leveling and electric vehicles. Both Na/FeCl 2 and

Na/NiCl 2 batteries have been developed in 40 - i00 Ah size for electric

vehicles application (7). Further, Na/NiCl 2 batteries are currently being

successfully evaluated for GEO and LEO space applications (8).

Considering the promise the Na-MCl 2 battery systems hold for space

applications, a program has been initiated at the Jet Propulsion

Laboratory to evaluate these systems for NASA's future missions. The

program has hi£herto focussed on gaining a fundamental understanding of

the electrochemistry of the metal chloride cathodes to augment the

developmental effort being pursued elsewhere in the U.S. and other

countries. In this paper, a brief review of such studies carried out on

different metal chlorides is presented.

EXPERIMENTAL

Various methods of fabricating the metal chlorides electrodes have been

examined. These methods include i) hot pressing powders of NaCl and a

transition metal with a Teflon binder (stable below 250°C) to obtain

electrodes in the discharged state; (2) impregnation of a metal chloride

in a sintered matrix of the same or nobler metal; and 3) cosintering of

pressed powders of NaCl and a transition metal at 800°C under argon

atmosphere for 6-8 hours to obtain the electrodes in the discharged

state. The impregnation method has the advantage of providing a suitable

morphology for extended cycle life, i.e., the active material is pocketed

in the pores of the plaque as in Ni-Cd batteries. In the cosintering

method, however, oxidation of the grid is likely as well thus reducing

its mechanical integrity during cycling. For the single electrode

studies aimed at obtaining fundamental understanding of the metal

chloride cathodes, the metal chloride is formed on the metal wire

electrode from an in-situ oxidation in the electrolyte. Other

experimental conditions are similar to those reported in our earlier

communications (914).

RESULTS AND DISCUSSION

Before embarking on the detailed fundamental studies, initial experiments

were focussed on the electrochemical characterization of the metal

chlorides, i.e., mainly on the discharge behavior, rechargeability, rate

- limiting processes etc. Three metal chlorides. FeClz, NiCl 2 and CuCl z

have been chosen for these studies. As mentioned above, FeCI 2 and NiCl 2

have been extensively developed elsewhere. Copper chloride, on the other

hand, hasn't been studied much and was included in this program with an

expectation that its voltage (as well as energy density) would be higher

than FeCl 2 and even NiCI 2.

228
CHARGE - DISCHARGE BEHAVIOR

The charge - discharge curves of FeCl 2 electrodes, formed by cosintering

of Fe and NaCl, and NiCl 2 and CuCl 2 electrodes formed by

impregnation(Figs. 2-5 in Ref.9) illustrate the high coulombic yields and

good reversibility of these cathodes. Both FeCI. and NiCl. have flat

charge - discharge curves in the range of current _ensities s_udied (1-8

mA/cm2). Copper chloride, on the other hand, exhibits a higher voltage

than FeCl 2 and NiCl 2 but sloping charge - discharge curves, suggesting the

possible formation of monovalent copper during reduction of CuCl.. CuCl 2

appeared to be chemically stable in NaAICI 4 electrolyte, which is an

essential requirement for a successful operation of the metal chloride

cathode in the above sodium batteries.

POLARIZATION STUDIES ON SINTERED ELECTRODES

DC linear polarization studies were performed on the sintered metal

chloride electrodes to determine the exchange current densities for their

reduction (Fig. 7 in ref. 9 and Fig. 1 in ref. I0). The exchange current

densities are in the range of 1-2 mA/cm 2 for NiCl 2and FeCI 2 and 0-i mA/cm 2

for CuCl.. The values are in good agreement with those derived from the

potentlodynamlc polarization data. The potentiodynamic polarization

curves also show mass transfer effects, evident from limiting currents,

in the reduction of metal chlorides. Furthermore, the Tafel plots of

NiCl 2 and CuCl 2 contain discontinuities during reduction, which, in the

case of CuCl2, may be due to the possible formation of monovalent copper.

In the case of NiCI2, these discontinuities may be attributed to a kinetic

effect (described later on as passivation) by the discharge products,

especially NaCI. Due to these kinetic effects, the NiCl 2 electrode

exhibits lower exchange currents at lower states of charge.

AC impedance studies on FeCl z and NiCl 2 reveal a single charge transfer

step with exchange current densities comparable to those derived from DC

polarization data. The complex impedance plots of CuCl 2, on the other

hand, contain a second relaxation loop especially at lower potentials

which may be related to the formation of monovalent copper during the

reduction of CuCl 2.

SINGLE - ELECTRODE STUDIES

Single electrode studies were performed on transition metal wire

electrodes in NaAICl 4 electrolyte with an aluminum reference electrode

(1.643 V vs. Na) and a nickel foil counter electrode in a pyrex cell.

These studies were aimed at verifying the reversibility, establishing the

reaction mechanisms, determining accurately the rate parameters and

identifying the rate limiting processes in the electrode reactions (11).

These studies broadly include cyclic voltammetry, dc polarization and ac

impedance.

The cyclic voltammograms of Ni (Fig. i) consist of sharp and symmetric

peaks both during oxidation and reduction, implying a high degree of

reversibility for this system. A single peak is observed during

oxidation as well as reduction with a peak splitting close to 48 mV

expected for a two-electron reaction, which is consistent with the phase

229
diagram of NiC_, Ni and NaCl. The peak current varies linearly with the

square root of scan rate implying mass transfer control over the

reaction. The above voltammetric behavior is explained in terms of

deposition of the oxidation product, by virtue of its insolubility in the

electrolyte, over the electrode manifesting itself in a mass - transfer

limited situation. From the slope of the peak current against (scan

rate) I/2, the diffusion coefficient for Ni +÷ ions across the layers of

NiCl 2 deposited was estimated as 4.5 x i0 "8 cm2/s. The cyclic

voltammograms of Fe (Fig. 2) have symmetric, but broad peaks splitting

into two peaks at slower scan rates. The peak splitting is 84 mV, which

is expected for two single electron transfer steps reported to be apart

by 12 mV in the reduction of FeCI_. The peak currents once again vary

linearly with the square root of t_e scan rate suggesting mass transfer

controlled kinetics. The diffusion coefficient for Fe ++ across FeCl 2

layers on the electrode was estimated from the slope of peak current

against (scan rate) I/2 to be 5.2 x 10 .8 cm2/s. The voltammograms of Cu show

expected higher voltages, but also poor reversibility evident from

smaller reduction peaks. Also, the voltammograms contain two peaks; the

products formed in the second step possibly monovalent copper, appear to

be soluble in the electrolyte. This is further confirmed by the presence

of dissolved copper in the electrolyte of a Na-CuCl 2 laboratory test cell,

especially when the charging is extended beyond (2.83 V vs Na) the first

step. The reversibility of CuCl z improves if the oxidation is limited to

the first step (12)

From the cyclic voltammetric behavior of FeCl 2 and NiCi2, certain criteria

have been established to predict the feasibility of using a metal

chlorlde as cathode material in sodium batteries _3). These criteria

include a low oxidation current subsequent to the peak, implying low

solubility for the chloride, and symmetry in shape and size between the

oxidation and reduction peaks even at low scan rates. Using these

criteria, several transition metal chlorides including vanadium, cobalt,

chromium, manganese, molybdenum and silver have been screened. Cobalt

(Fig. 3) and molybdenum (Fig. 4) appear to meet those requirements, with

the latter also showing promise as a stable current collector for FeCl 2

and NiCl 2 electrodes.

DC micropolarization studies carried out on NiCl 2 and FeCI 2 wire

electrodes gave more accurate exchange current densities of 0.73 and 0.43

mA/cm 2 respectively. Potentiodynamic polarization curves of NiCl 2 exhibit

'passivation' akin to reactive metal anodes, i.e., the current rises

sharply and falls down subsequently (Fig.5). This passivation behavior

is attributed to the deposition of NaCl formed during reduction on the

electrode surface causing significant ohmic polarization at such sites.

This is in fact responsible for the 'segmented' Tafel behavior observed

with sintered NiCl 2 electrodes. This aspect is addressed in more detail

later on. The current - potential curve of FeCl 2 shows reduced passivity

as compared to NiCl2, probably due to the intermediates formed during

reduction being less resistive than NaCl.

The exchange current density derived from the ac impedance data of FeCl 2

is 0.66 mA/cm 2, in agreement with the values obtained from dc data.

Furthermore, the diffusion coefficient calculated from the Randles'

impedance plot for Fe ÷+ ions across FeCl z is 4.4 x 10 .8 as compared to 5.2

230
x 10 .8 cm2/s derived from cyclic voltammetric data. The ac impedance

(Nyquist) plot of NiCl2is similar to that of a passive system, e.g., iron

in sulfuric acid, with the relaxation loop bending inward towards the

imaginary axis at low frequencies. This characteristic profile (Fig. 6)

of the Nyquist plot may serve as a qualitative index for the onset of

passivation of NiCI 2.

PASSIVATION OF NICI z

From the single electrode studies, NiCl 2 emerged as the candidate cathode

material for further study due to its near-ideal reversibility, high

exchange currents, simple reaction mechanism free of intermediates and

superior electrochemical characteristics. However, NiCl 2 shows a tendency

to passivate during reduction at high current densities, which warrants

a proper design of electrode geometry to achieve high power densities.

A detailed study on the passivation has been carried out under different

experimental conditions (_) .

DC micropolarization curves obtained at different states of charge show

increasing polarization resistance at low states of charge, implying

lower exchange currents. The open - circuit potential, on the other

hand, remains the same (Fig. 7). This decrease in the exchange current

is explained in terms of a progressive reduction in the active (nonpassive) electrochemical area due to deposition of the reduction product

NaCl on the electrode. Potentiodynamic polarization curves (Fig. 8) at

different states of charge also demonstrate the increasing passivation at

lower states of charge as is evident from a sharp decrease in the current

at overpotentials exceeding i00 mV.

AC impedance measurements carried ut at different states of charge also

gave lower exchange currents (Fig. 7) at lower states of charge, in

agreement with the results obtained from linear polarization. Also, at

lower states of charge, the Nyquist plots exhibit the typical profile

characteristic of a passive system, wherein the capacitive component

bends inward towards the imaginary axis at low frequencies. Ac impedance

plots obtained at different electrode potentials suggest that at lower

(reduction) potentials, i.e., _ 950 mV vs AI, the electrode tends to

passivate, as is evident from the characteristic shape of the Nyquist

plot. The exchange current appears to decrease with a decrease in

electrode potential until 950 mV vs A1 after which the electrode has a

high impedance due to passivation.

CONCLUSIONS

Transition metal chlorides have several attractive features and are

viable alternatives to sulfur as positive electrodes in sodium batteries.

Our initial studies on metal chloride cathodes were focussed on the

methods of fabrication and electrochemical characterization including

discharge behavior, rechargeability etc. Detailed fundamental studies

carried out in single-electrode configuration gave useful information on

the reaction mechanisms, rate parameters and rate-limiting processes.

NiCl 2 emerged from our studies as the candidate cathode material for

further study and development for NASA's requirements, whereas copper

chloride appears to be chemically unstable during extended charge. Among

231
various other metal chlorides screened, cobalt and molybdenum are likely

to be successful as cathodes with the latter also having a potential as

a stable current collector for NiCl 2 electrodes. The passivation of NiCl2,

a rate limiting phenomenon, increases at lower states of charge but can

be alleviated by a proper electrode design or, as our recent studies

show, by increasing the operating temperature to around 275°c. Higher

operating temperatures also improve the kinetics, the exchange current

increasing by 10% for each 10°C rise in temperature.

ACKNOWLEDGEMENTS

The work described here was carried out at the Jet Propulsion

Laboratory, California Institute of Technology, under contract with the

National Aeronautics and Space Administration. This program is sponsored

by the Office of Aeronautics and Space Technology, Code RP.

REFERENCES

I. J. Coetzer, J. Power Sources, i_88, 377 (1986).

2. J. Coetzer, R. J. Bones, R. C. Galloway, D. A. Teagle and P. T.

Moseley, U.S. Pat. 4,546,055 (1985).

3. R. J. Bones, J. Coetzer, R. C. Galloway and D. A. Teagle, J.

Electrochem. Soc., 13___44,2379 (1987).

4. R. M. Dell and R. J. Bones, Bull. Electrochem., 4, 319 (1988).

5. R. J. Bones, D. A. Teagle, S. D. Brooker and F. L. Cullen, J.

Electrochem. Soc., 13___66,1274 (1989).

6. P. A Nelson, J. Power Sources, 29, 565 (1990).

7. J. L. Sudworth and R. N. Bull, Pro. DOE/EPRI Beta Battery Workshop

VI__!, 53-1 , Toronto (1988); J. L. Sudworth, R. C. Galloway and R. N.

Bull, Proc. DOE/EPRI Beta Battery Workshop VII, 52-1, Toronto

(1988).

8. B. Hendel and G. Dudley, NASA Aerospace Battery Workshop,

Huntsville, AL., Dec. 4-6 (1990).

9. S. Di Stefano, B, V. Ratnakumar and C. P. Bankston, J. Power

Sources, 2__99,301 (1990).

i0. B. V. Ratnakumar, S. Di Stefano and C. P. Bankston, Proc. 24th

Intersoc. Energy Conversion and Enq. Conf. (IECEC), Vol. 6, p. 2921,

Washington, D.C., August 6-11, 1989.

ii. B. V. Ratnakumar, S. Di Stefano and G. Halpert, J. Electrochem.

Soc., 13___/7,2991 (1990).

12. B. V. Ratnakumar, A. I. Attia and G. Halpert, Abst. # 8,

177thElectrochem. Soc. Meetinq, Montreal, Canada, May 6-11 (1990).

13. B. V. Ratnakumar, A. I. Attia and G. Halpert, J. Electrochem. Soc.

138, 883 (1991).

14. B. V. Ratnakumar, A. I. Attia and G. Halpert, Proc. 34thTntl. Power

Sources Symp., pp 17, June 25-28 (1990).

232
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. 91-32567

ADVANCES IN Li-TIS 2 CELL TECHNOLOGY

S. Surampudi, D.H. Shen, C.-K. Huang, F. Deligiannis,

A. Attia, and G. Halpert

Jet Propulsion Laboratory

California Institute of Technology

Pasadena, California 91109

ABSTRACT

JPL is involved in a NASA sponsored program to develop ambient

temperature secondary cells for future space missions. After several

years of research on various cathode materials, Titanium Disulfide (TiSz)

was selected in view of its intrinsic reversibility and high faradaic

utilization. In the last two years, the efforts were focussed on

improving the cycle life of the system and developing 1 Ah cells.

Several approaches including the use of mixed solvent electrolytes, the

operation of cells at low temperature, and the cycling of cells under

different voltage limits, were initially examined to improve the cycle

life performance of the Li-TiS 2 system. Spiral wound 1 Ah cells

fabricated incorporating the improvements from the above studies have

delivered more than 600 cycles at 50% DOD. Work is in progress to

identify alternate anode materials that can improve the cycle life of

the cells to i000 cycles at 50% DOD. This paper summarizes the advances

made in the Li-TiS 2 technology at JPL since 1989.

INTRODUCTION

Ambient temperature secondary lithium batteries have a number of

intrinsic advantages such as higher energy density, longer active shelf

life, lower self discharge, etc. over the conventional Ni-Cd, Pb-Acid,

and Ag-Zn batteries. The pay-offs are 2-3 fold increase in energy

storage capability and a longer active shelf life of 5 to i0 times that

of Ni-Cd.

Under a NASA OAET sponsored program, Jet Propulsion Laboratory (JPL) is

developing ambient temperature secondary lithium cells for future space

applications. The main objective of the program is to develop ambient

temperature secondary lithium cells with greater than i00 Wh/kg specific

energy while delivering i000 cycles at moderate depths of discharge

(50%) by 1995. Some of the projected space applications of these

batteries are for planetary rover, planetary spacecraft/probes,

astronaut equipment, and Geo-Syn-Spacecraft.

After a detailed examination of various cathode materials, TiS 2 was

selected in view of its intrinsic reversibility and high faradaic

utilization (i). The state-of-art Li-TiS2 cells have a limited cycle

life performance (100-200 cycles at 100% DOD),and the cells were found

to lose capacity on cycling (2). The capacity loss is found to be

235
mainly due to the increased cell polarization with cycling. This

increased polarization is understood to be due to the electrolyte

degradation with cycling. Hence any approach, that can reduce the

electrolyte degradation and/or improve electrolyte stability towards

lithium will result in improved cycle life. A number of approaches such

as the use of mixed solvent electrolytes, the operation of cells at low

temperature, and the cycling of cells under different voltage limits,

were examined to improve the cycle life performance of the Li-TiS 2

system. Spiral wound 1 Ah cells were fabricated incorporating the

improvements from the above studies. The cells have delivered more than

600 cycles at 50% DOD. Other approaches are required to achieve the

goal of I000 cycles. Lithium alloys and intercalation compounds have

lower reactivity towards organic electrolyte than the elemental lithium

(3). The use of these materials as candidate anode materials can result

in improving the cycle life performance of the cells. A number of

lithium alloys and intercalation compounds are being evaluated as

candidate anode materials. This paper summarizes these advances made at

JPL since 1989 in Li-TiS 2 technology

MIXED SOLVENT ELECTROLYTES

The limited cycle life performance of the ambient temperature Li-TiS 2

cells is mainly due to the high reactivity of the organic electrolyte

towards lithium and cathode material(4). JPL used 1.5M LiAsF6/2-MeTHF

as bench mark electrolyte for secondary Li-TiS 2 cells. Use of additive

and co-solvents were investigated to improve the stability of THF, 2-

MeTHF, and dioxolane electrolytes. The additives and co-solvents

investigated were 2-methyl furan (2-MeF), ethylene carbonate (EC),

propylene carbonate (PC), and 3-methyl sulfolane (3-MeS). A number of

electrolytes with different combinations of additive and co-solvents

were evaluated for their stability towards lithium by open circuit

stand, AC impedance spectroscopy, and microcalorimetry methods (5).

Three electrolytes were selected for detailed assessment in view of

their improved stability and conductivity. The selected electrolytes

are 1.5 M LiAsF 6 EC/2-MeTHF, 1.5 M LiAsF6 THF/2-MeTHF/2-MeF, and 1.5 M

LiAsF 6 dioxolane/2-MeF. The cycle life performance of the experimental

cells(150mAh) activated with these electrolytes is given in Figure i.

The cells activated with 1.5 M LiAsF 6 EC/2-MeTHF and 1.5 M LiAsF 6 THF/2-

MeTHF/2-MeF electrolytes showed higher cathode utilization and minimum

capacity decline compared to the cells activated with 1.5 M LiAsF 6

dioxolane/2-MeF and 1.5 M LiAsF6 2-MeTHF bench mark electrolytes. The

rate capability, operating temperature range, and safety of the cells

are presently being evaluated. Final selection of the electrolyte will

be made based on the results of these studies.

LOW TEMPERATURE OPERATION

The reactivity of the organic electrolyte towards lithium can be reduced

by operating the cells at low temperature. It was felt that the reduced

reactivity of the electrolytes at low temperature can result in improved

cycle life performance (6). In the present investigation, we have

236
evaluated the influence of temperature on the stability and lithium

cycling efficiency of the electrolytes.

The electrolyte investigated are: THF, EC/THF, 2-MeTHF, EC/2-

MeTHF,THF/2-MeTHF, and EC/THF/2-MeTHF. All the electrolytes contained

1.5M LiAsF 6. The stability of the electrolytes at three different

temperatures (i0 °, 25° and 70°C) was evaluated by open circuit stand test

method. Results of these studies are summarized in Table I. Some of

the important observations are: i) the electrolyte showed higher

stability towards lithium at 10°C, 2) the reactivity of electrolyte

towards lithium increased with temperature, 3) 2-MeTHF electrolytes

exhibited higher stability towards lithium compared to THF electrolytes,

and 4) addition of EC improved the stability of THF, 2-MeTHF, and

THF/2-MeTHF electrolytes. From these results, it was expected that

cycling of the cells at low temperature may result in improved cycle

life performance.

Cycle life performance of the cells activated with these six

electrolytes was evaluated at i0 ° and 25°C. These two temperatures were

chosen to verify the hypothesis of benefits of low temperature cycling.

Sealed 150 mAh experimental Li-TiS2cells were cycled by constant current

method between 1.7V and 2.7V. The current densities were 2 and 1 mA/cm 2

for discharging and charging respectively. Cycling efficiency of

lithium in various electrolytes was determined from the cycle life data.

The lithium cycling efficiency/figure of merit (FOM) was calculated

using the relationship:

F.O.M.Li =

Total Accumulated Discharqe=CapaGity

Theoretical Li Capacity

Lithium cycling efficiency (FOM) in these electrolytes at I0 ° and 25°C

is given in Table II. From the results it can be observed that no

significant difference in FOM was observed by cycling cells at I0 ° and

25°C.

The lithium cycling efficiency results are not in agreement with the

prediction of the open circuit stability stand tests. This is probably

due to the fact that the open circuit stand test can only provide

qualitative information about the reactivity of the electrolytes towards

passivated lithium. During cycling, finely divided lithium is formed

and the reactivity of the electrolytes towards the freshly formed

lithium deposits is significantly higher.

OPERATING VOLTAGE LIMITS

It is known that at voltages lower than 1.5 V, the salt, LiAsF6,

undergoes reduction and at voltage higher than 3 V, some solvents

undergo oxidation. Thus, the degradation of the electrolyte can be

minimized by selecting optimum cycling limits (end of charge and

discharge voltages). To investigate this phenomenon, 1 Ah Li-TiS 2 cells

obtained from a commercial source were cycled under three different

voltage regimes (1.7-2.7V, 1.9-2.7 V, 1.9-2.5V). These cells were

237
activated with 1.5 M LiAsF 6 2-MeTHF/THF/2-MeF. Cycle life performance

of these cells under three different voltage regimes is given in Figure

2 (7). From the results, it can be observed that the cycle life

performance of the cells is significantly dependent on the cycling

voltage limits. The lower voltage limit seems to have higher influence

on the cycle life performance. Detailed cycling studies are needed for

each electrolyte to identity the optimum operating limits and the

degradation mechanisms involved.

1 Ah CELL DEVELOPMENT

Experimental cells (150 mAh) were used until 1989 for the performance

evaluation of electrolytes and cathode materials. Spiral wound 1 Ah

cells were developed in 1990 for design optimization studies and

performance evaluation of components. These cells had an electrode

capacity ratio of 6:1 (Li:TiS2) and were activated with 1.5 M LiAsF 6

EC/2-MeTHF electrolyte. This electrolyte was selected in view of its

higher stability and lithium cycling efficiency compared to the other

mixed electrolytes investigated at JPL. The cycle life performance of

the cells at 50% and 100% DOD was evaluated at C/10 charge and C/5

discharge. The cells had delivered more than 600 and 350 cycles at 50%

and 100% DOD, respectively (Figure 3&4). Even though the cycle life

performance of these cells is attractive for many commercial

applications, it is still inadequate for space applications. Alternate

anode materials are being examined to achieve the desired cycle life

performance goal.

ANODE MATERIALS

Lithium alloys and intercalation components have lower reactivity

towards organic electrolytes than the elemental lithium(3). The use of

these materials as candidate anode materials can result in improving the

cycle life performance of the cells. Further, the use of lithium alloy

anodes may also improve the safety of the cells.

A suitable lithium alloy anode material must have several desirable

properties such as high Li diffusivity, low equivalent weight (molecular

weight/no, of Li reversible), high reversibility, and low voltage (E ° vs.

Li). In addition, the material must have good thermal and chemical

stability in the cell environment. Detailed analysis of the variables

that control the specific energy of the Li alloy-TiS2 system resulted in

establishing selection criteria. The equivalent weight and voltage (vs.

Li) of the alloy must be less than 60 gm/eq, and 200 mY, respectively,

to achieve a practical specific energy of 70-80 Wh/Kg. The equivalent

weights of different alloys were calculated assuming that 80% of the

lithium in the alloy is available for reaction. This assumption was

made because of the lack of actual experimental data in the literature.

The properties of several lithium alloys and the corresponding

theoretical specific energy of the Li alloy-TiS 2 system are summarized

in Table III. Among the various alloy systems examined, the Li-Si and

Li-Cd alloys were only found to satisfy the above criteria. The Li-AI

238
alloy, with a voltage higher than 200 mV vs. Li, has a theoretical

specific energy of more than 300 Wh/Kg because of its lower equivalent

weight. In practical cells, Li-AI, Li-Si and Li-Cd alloys are projected

to provide greater than 75 Wh/Kg (1/4 of theoretical). Li-C, Li-Pb, and

Li-Sn appear to be the next best among the alloys considered.

Li-AI, Li-Cd, Li-Pb, Li-C, Li-Si, Li-Sn materials were evaluated for the

stability, and reversibility. The stability of the alloys was evaluated

by microcalorimetry and open circuit voltage measurement techniques.

Among the alloy materials examined Li-Si and Li-Cd were found to be

unstable towards 1.5M LiAsF6/EC+2-MeTHF electrolyte. The usable capacity

and reversibility of the remaining four alloy materials were evaluated

in experimental cells by galvonostatic cycling. The realizable specific

energy of the Li alloy-TiS 2 system was also calculated. The results of

these studies is summarized in Table IV. From the results it can be

observed that Li-AI and Li-C alloys are the only materials that can be

considered for further evaluation in view of their reversibility and

higher specific energy. Detailed assessment of these materials in

experimental Li-alloy/TiS 2 cells is in progress.

CONCLUSIONS

Several approaches were examined to improve the cycle life of ambient

temperature secondary Li-TiS 2 cells . Some of the important findings of

the study are : i) Li-TiS 2 cells activated with 1.5 M LiAsFdEC+2-MeTHF

electrolyte showed improved cycle life performance compared to the cells

activated with other mixed solvent electrolytes investigated 2) The

cycling of the cells at low temperature did not result in improving the

cycle life of the cells and 3) Charge and discharge voltage limits were

found to have significant influence on the cycle life of the cells.

Spiral wound 1 Ah cells activated with 1.5 M LiAsF6/Ec+2-MeTHF

electrolyte have delivered more than 600 cycles at 50% DOD. Work is

still in progress to improve the safety and cycle life performance of

the cells through the use of alternate anode materials.

ACKNOWLEDGEMENTS

The work described here was carried out at the Jet Propulsion

Laboratory, California Institute of Technology, through an agreement

with the National Aeronautics and Space Administration. The authors

would like to thank E. Davies and D. Perrone for their assistance in the

cell assembly and testing.

REFERENCES

(1) S. Subbarao, D. H. Shen, F. Deligiannis, C.-K. Huang and G.

Halpert, Jo Power Sources, 29 (1990) 579 - 587.

(2) S. Subbarao, D. H. Shen, C.-K. Huang, F. Deligiannis, G. Halpert

and E. Peled, Proceedings of the 34th International Power Sources

Symposium, 1990, p. 98.

239
(3)

(4)

(5)

(6)

(7)

Y. Geronov, P. Zlatilova, R. V. Moshtev, J. Power Sources, 12

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V. R. Koch, Advances in organic electrolytes for rechargeable

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D. H. Shen, S. Subbarao, F. Deligiannis, C.-K. Huang and G.

Halpert, Proceedings of the Symposium on Lithium Batteries, ECS

Fall Meeting 1990, Edited by S. Subbarao, V. R. Koch, B. B. Owens

and W. H. Smyrl, Proceedings Vol. 90-5, p. 114 (1990).

D. H. Shen, S. Subbarao, C.-K. Huang, F. Deligiannis and G.

Halpert, Proceedings of the 34th International Power Sources

Symposium, 1990, p. 90.

C.-K. Huang, S. F. Dawson, D. H. S_en, F. Deligiannis, S. Subbarao

and G. Halpert, Proceedings of the Symposium on Lithium Batteries,

ECS Fall Meeting 1990, Edited by S. Subbarao, V. R. Koch, B. B.

Owens and W. H. Smyrl, Proceedings Vol. 90-5, p. 42 (1990).

240
100 [

90"

oo

80

_! 70

60

50

40

U

" 30 <

rr 20

O

z 10

0

DIOXOLANE/2-MeF

! ' I I L I I

0 50 100 150 200 250

CYCLE NUMBER

3)O

FIGURE 1. Cycle Life Performance of Experimental Cells

Activated with Mixed Solvent Electrolytes

1.2

0.8

O

13..

O 0.6

0.4

0.2

0

FIGURE 2.

28 56 84 112 140

CYCLES

Cycle Life Performance of Li-TiS2 Cells Operated

under DifferentVoltage Limits.

241
3.00

2.75

2.50

co 2.25

I--

1.75

1.501

1.25

1.00

0

! !

100 200

I I I I

300 400 500 600

5_/oDODCYCLES

FIGURE 3. Cycle Life Characteristics of 1

Li-TiS2 Cell at 50% DOD

Ah

700

16t

1.4

1.2

1.0

0.8

0.6

0.4

0.2

0.0

0

I II ! | ! !

50 100 150 200 250 300

100% DOD CYCLES

FIGURE 4. Cycle Life Characteristics of 1 Ah

Li-TiS2 Cell at 100% DOD.

350

242
TABLE I. Effect of Tem0erature on the Stability of Electrolytes

Towards Lithium (O0en Circuit Stand Test Results)

Electrolyte I__QoC ;30oc

Li ELECt. Li ELEC.

20oc

L_j

THF BRN LT YEL I)RK BRN

EC+THF S* C BRN SPOTS C**

2-MeTHF S C(ppt) S C

EC+2-MeTHF S C S C

THF+2-MeTHF BRN SPOTS LTYEL DRK BRN YEL

EC+THF+2-MeTHF S C S C

BRN ppt DRK BRN

BRN SPOTS

BLK STRIPE

S

DRK BRN

BRN SPOTS

* S: shining

** C: clear

ELEC.

BRN ppt

BRN

LT YEI.

C

YEI+ ppt

I.T YILI.

TABLE II. Effect of Tem oerature on the Lithium Cycling Efficiency

ELECTROLYTE FIGURE OF .MERIT WOM)*

10o(2 _25oc

THF 3.3 3

EC+THF 4.6 4

2-MeTHF --- 29.5

EC+2-MeTI-IF 32 38.5

THF+2-MeTHF 5. l 4.5

EC+THF+2-MeTHF 7.4 7.6

* F.O.M.Li = (Total Accumulated Discharge Capacity) / (Theoretical Li Capacity)

243

7
TABLE III. Properties of Alternate Li Anode Material (Theoretical)

Material E vs. Li M.W. No. of Li Specific

(mV) (_mole) Reversible (Wh/Kg)

Li 0 6.941 1 472

]3-LiA1 385 35.32 1 312

Lil.]Zn 191 73 0.6 220

Li4.3Sn 411 148.5 2 243

Li4.5Pb 388 238.4 3.5 254

Li2.85Cd 140.6 132.2 2.6 322

Li0.] 5C 200 13.05 0.15 255

LiALMn 386 36 1 311

LiAICr 386 36 1 311

LiAICdPb 200 96.5 1 247

TABLE IV. Erooerties of Alternate Li Ano_le Material (ExDerimental)

Material Stability* Ave. E vs. Li No. of Li** Speci.fic Energy

(mV) Reversible (Wh/Kg)

Li 1.2AI good 380 0.9 300

Li2.85Cd poor 0 ......

Li3.0Pb good 449 < 1.5 167

Li0.13C good 200 < 0.08 186

Li4.4Si poor .........

Li2.5Sn good 520 < 1.0 171

Lio.5Zn good 256 < O. 12 62

* Microcalorimetric and OCV measurements.

** Galvanostatic cycling studies.

244


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Space Electrochemical Research and Technology

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Lewis Research Center

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This document contains the proceedings of NASA's third Space Electrochemical Research and Technology (SERT)

Conference, held at the NASA Lewis Research Center on April 9-10, 1991. The objective of the conference was to

assess the present status and general thrust of research and development in those areas of electrochemical technology

required to enable NASA missions into the next century. The conference provided a forum for the exchange of ideas and

opinions of those actively involved in the field, in order to define new opportunities for the application of electrochemical

processes in future NASA missions. Papers were presented in three technical areas: the electrochemical interface, the

next generation in aerospace batteries and fuel cells, and electrochemistry for nonenergy storage applications. This

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Electrochemistry; Batteries; Fuel cells

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