AlllOl

AlllOS D3ti7SS

NBSIR 80-1991

Materials for Fuel Cells

't4

L. H. Bennett, C. K. Chiang, M. I. Cohen, A. L Dragoo,

A. D. Franklin, A. J. McAlister

National Measurement Laboratory

National Bureau of Standards

U.S. Department of Commerce

Washington, D.C. 20234

Annual Report—January 1978 to December 1978

Issued March 1980

Prepared for Division of Fossil Fuel Utilization

Department of Energy

"UC Washington, D.C. 20545

100

,U56

80-1991

1980

NBSIR 80-1991

W I I

Natfonsf Bureau cf Standards

Liffrfiry, >^-0; 5HP 2 9 1980

O-cr. ^

Cluoo

MATERIALS FOR FUEL CELLS . U6(^

lb-iqq(

io

L. H. Bennett, C. K. Chiang, M. I. Cohen, A. L. Dragoo,

A. D. Franklin, A. J. McAlister

National Measurement Laboratory

National Bureau of Standards

U.S. Department of Commerce

Washington, D.C. 20234

Annual Report—January 1978 to December 1978

Issued March 1 980

Prepared for Division of Fossil Fuel Utilization

Department of Energy

Washington, D.C. 20545

U.S. DEPARTMENT OF COMMERCE, Philip M. Klutznick, Secretary

Luther H. Hodges, Jr., Deputy Secretary

Jordan J. Baruch, Assistant Secretary for Productivity, Technology, and Innovation

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director
D ';?]!

oeer e s S'

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t>Wi j4h| ' » ' ,,, '-L'M ''';fc,

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Tables

Page

1. Electrochemical Parameters for several Mo, W C Electrodes. ... 9 1-x X

2 . H^-oxidation Rate Constants and Apparent Activation Energies

for Mo^ in 85% H^PO^ 11

0.7 0.3 3 4

3. H^-oxidation Rate Constants and Apparent Activation Energies

for WC in 85% H^PO, 12

3 4

4. Summary of Best Densities of Y^O^-doped CeO^ Ceramics 21

5. Activation Energies at Low Temperature for Types M and C Specimens

of Y^O^-doped CeO^ 36

6. Summary of Floppy Disc Unit Commands 55

7. Comparison of Successive Approximation and Tracking Type ADC. . . 68

Table of Contents

Page

Abstract 1

1 . Introduction 3

2. Subtask Reports 4

2.1. Electrocatalysis in Phosphoric Aid 4

Introduction 4

2.1.1 Materials Survey 4

2.1.2 Electrocatalytic Studies of

Mo. W C Alloys 5 1-x X

2.1.3 Refractory Hard Metals as Pt

Catalyst Supports 12

2.2 High Temperature Solid Electrolytes 14

Introduction 14

2.2.1 Electrolyte Materials 17

Preparation and Characterization of High

Density Ceria-Yttria 17

Development of Analytical Methods 22

Acknowledgements 24

2.2.2 Impedance Measurements 26

Introduction 26

Correlation of Preparation and Properties ... 28

Temperature Dependence of Resistances 33

Slow Transients 37
Page

2.3 Automated Electrochemical Measurement System 53

Introduction 53

2.3.1 Mass Storage 53

2.3.2 High Speed 12 bit Data Acquisition

System 60

2.3.3 Software 75

References 77
Figures

Page

1. Voltainmetric sweeps of a porous electrode of WP in and Hsaturated IM H_PO. at 22 °C, showing evidence of H

3 4 ^ oxidation activity. Sweep rate is 10.4 mV/sec. ..... 6

2. Hydrogen oxidation current vs. voltage for a thin porous

electrode of Mo W C. The first straight line segment

corresponds to a region of pure activation control; the

second to mixed activation and diffusion control. The

round off at higher voltage arises, not from diffusion '

limitations, but from a decrease in activity associated

with chemical change of the catalyst surface. • • 8

3. Voltammetric sweeps of a porous electrode of Mo W C

\j • i U • .3 in a saturated IM H^PO^ at 22 °C, at various sweep

rates as labelled. The moving peaks approach the

theoretical minimum separation of 60 mV corresponding

to unit charge exchange as the sweep rate is lowered.

Equilibrium potential for the surface reaction is

0. 32 ± 0.03 (RHE) 10

4. Preparation Scheme: Carbonate Process 18

5. Fracture surface of hot-pressed ceria-yttria ceramic 19

6. Ceri\am-yttria oxalate obtained by coprecipitation

from a homogeneous solution. 23

7. Cerium-yttrium oxide obtained by calcining the

oxalate to 650 °C 25

I
Page

8. Equivalent circuit for Y^O^-doped CeO^ Ceramic

specimens with Ft Electrodes 27

9. Complex pleme impedance plot for Type M Y^O^-doped

Ce^O^ Ceramic at 281 **C, in air. Frequency remge

50 Hz to 13 MHz 30

10. Complex plane impedemce plot for Type C Y^O^-doped

^®2°3 Frequency rauige

50 Hz to 13 MHz 31

11. Circles and solid lines (illustrative only): Ratio

of bulk to inhomogeneity resistances for Y^O^-doped

CeO^ cereunics at 'V'250 ®C vs. porosity. Open circles

3+ 3+ are for 20 cation % Y , rest for 10 cation % Y .

Squares and dotted line: Inverse ratio (Inhomogeneity

to bulk resistemce) for Type M specimens , line is least

squares fit to data 32

12. Arrhenius plots for bulk (R^) and inhomogeneity

resistances for Type M Ce Y 0, __ ceramic 34 0*9 0*JL X« 13. Arrhenius plots for bulk (R^) and inhomogeneity

resistemces for Type C Ce Y 0 _ ceramic 35

14. Schematic of 4-probe sample 38

15. Temperature dependence of 4-probe dc conductivity

of Type C Ce. ^Y^ ,0, ceramic in air. 39

0.9 0.1 1.95

16. Schematic of current and transient voltage pulses

at the potential probes in Type C Ce^ ^Y^

^0^^ cereunic. ... 40

17.

Initial portion of transient voltage pulse at potential

probes for Type C Ce Y 0 ceramic in air at 887 ®C.

Current 'V'lO yA. 42
Page

18. Transient voltage pulse at potential probes for Type C

Ce Y O ceramic in air at 887 °C. Current '\^10yA 43

0*3 0*1 X* 95

19. Dependence of saturation overvoltage at potential probes

on amplitude I of the current pulse for Type C Ce Y O

U*9 U*X X* 70

ceramic at 888 °C in air 44

20. Dependence of overvoltage at potential probes on square root

of time for Type C Ce„ ^Y^ ,0- ceramic at 888 °C with

0.9 0.1 1. 9d

log Po^ = -3.01. Current -lOyA 45

21. Transient voltage pulse on active electrodes, placed at

ends of specimen, for Type C Ce Y 0 ceramic 0*9 0«X X«95

at 888 °C in air. Current -10 yA 47

22. Transient voltage pulse on potential probes used as

active electrodes for Type C Ce Y 0 _ ceramic at 0*9 0*X X* 95

887 °C in air. Current ==10yA 48

23. Dependence of saturation overvoltage at potential probes

on oxygen activity for Type C Ce. _Y^ ,0, ceramic at

U.9 Q.X X. 95

888 °C. Current =^10yA 49

1/2 24. Dependence of the slope of the t

' plot on oxygen

activity for Type C Ce Y 0. ceramic at 888 °C. D«9 0«X X* 95

Current -lOyA 50

25. Arrhenius plot for delay time for Type^

C Ce„ ^Y^ ,0,

0. 90. 11. 95

ceramic in air. Current =10yA 51

26. a) Block diagram of PIA based interface for Floppy disc 57

b) Block diagram of Tracking Type ADC 57

27 . Modified clock circuit utilizing Integrated Crystal

controlled clock 58
Page

28. Schematic diagram of complete Floppy Disc Interface 59

29. Digital to Analog Converter Block Diagram 63

30. a) Block diagram of Successive Approximation ADC 67

b) Block diagram of Tracking Type ADC 67

31. Schematic diagram of clock, timer and address decode

section of converter board 70

32. DAC control section schematic 71

33. ADC control section schematic 72

34. DAC converters 73

35. ADC converter #1 schematic. Converter # 2 is identical

with the necessary connections to the control section

indicated by the number 2 in parenthesis 74
ABSTRACT

Transition metal-metalloid compounds involving V, Mo, Ta, and Nb

with P, Si and N were examined as potential electrocatalyts in hot

concentrated phosphoric acid. MoP, WP, MoSi

2

, MO

2

N/M0N, and W

2

N/V/N were

found to be stable in the Absence of applied potentials. The W

2N/VIN

was non-conducting. MoSi

2 and MO2

N/M0N vzere both inactive as catalysts

and unstable under cathodic potentials. MoP and WP corroded at about

0.2 volts RHE. Only WP showed even mild activity as a hydrogen oxidation

catalyst.

The electrochemical properties of Mo^W^ electrodes were studied

in some detail, and found to be independent of x for values of x near 0.7.

The electrochemical behavior is very similar to that of WC, with about the

same activity and the same indifference to CO poisoning.

Experiments are outlined for studying a possible "spillover" effect

from Pt to Mo^ W C and transition metal borides as supports, and for

using a Raman spectroscopic technique for characterizing the electrochemical interface of transition metal-metalloid compound electrodes.

The preparation and characterization of high-density (99.6 to 99.8%

of theoretical) Y202~doped Ce02 ceramics is described. The material

exhibits very sharp, single-phase, x-ray diffraction peaks, indicating

good compositional homogeneity. The influence of various preparation

methods on the. sintering characteristics of powders of these materials

is described. Chemical analytical techniques are given.

The frequency dependence of the total impedance of discs of Y-doped CeO

ceramic was used to separate the intrinsic impedance (internal to the

grains) from that arising from inhomogeneities in the specimen, presumably

grain boundaries.
2

The electrical properties of the inhomogeneities were very different for

materials prepared from powders produced by mixing the oxides from those

found when the powders were prepared by coprecipitation. A higher

resistance and a much higher capacitance were found in the latter case.

The temperature dependence of the inhomogeneity resistance in the

YzOi-doped CeO^ is somewhat different from that of the intrinsic resistance,

and since these resistances are in series, the larger resistance dominates

in a dc measurement. At low temperatures it is the Inhomogenelty resistance that controls the dc data, but at higher temperatures the intrinsic

resistance becomes the more important. Hence, an Arrhenius plot of the dc

conductivity exhibits a break, as often seen in the literature on these

materials. Only the portion at temperatures above the break, on this view,

represents the true properties of the substance Itself.

In the course of 4-probe dc conductivity measurements on some ^26^-

doped Ce02 materials a slow voltage transient was observed under constantcurrent conditions. This transient appears to involve a solid state

diffusion process.

Modifications to our automated electrochemical measurement system

are described that allow 1) . an Increase of the mass data storage

available to the system, so that the results of a number of repetitive

runs can be stored and later examined and compared; 2). an Increase in

the maximum sensitivity when data are being acquired at high speed, as

in pulse mode operation; and 3). the use of a complete driver system in

the software to coordinate all its aspects, and also the use of Fast

Fourier Transform techniques during pulse mode operations. These

modifications are described in detail.
3

1 . Introduction

The National Bureau of Standards has undertaken a program of

research on materials for fuel cells auid batteries. This program

includes studies of electrocatalysis in liquid-electrolyte cells and of

solid electrolytes, in high-temperature cells and in batteries. The

overall objectives of the program are:

1. to provide data allowing selection of optimum materials

for critical aspects of fuel cell operations;

2. to improve the measurement capability and standards

for assessment of performance (e.g., efficiency of electrode

processes and lifetime of components) of materials in fuel

cells and batteries.

This report will describe the results for the second year of the program.

The Materials for Fuel Cells Program at NBS during this year

consisted of three major elements, which taken together reinforce each

other in techniques and concepts; these elements embrace:

i. electrocatalysis, especially hydrogen oxidation, on non-noble

metals and alloys;

ii. degradation mechanisms involving solid oxygen-transporting

electrolytes; and

iii. Development of instruments.
4

2 . Svibtask Reports

2.1 Electrocatalysis in Phosphoric Acid

Introduction

In most fuel cell applications a major contribution to the overvoltage, and therefore to a reduction in useful voltage and to efficiency,

comes from electrode polarization. For use in hot no satisfactory

substitutes for Pt have been found, but Pt is not completely satisfactory.

In catalyst loadings sufficiently large to ensure adequate efficiency

euid lifetime it medces a significemt contribution to the cost. At the

anode its sensitivity to CO poisoning forces the use of higher temperatures

and catalytic shift conversion in the fuel processing train. At the

cathode a mixed potential situation seems to exist and the open-circuit

voltage lies below what should be possible. Thus a continued study of

electrocatalysis and electrocatalysts in H^PO^ is worthwhile. In our

program we have elected to study refractory metal-metalloid compounds

(e.g. WC) as potential anode catalysts, amd also in the role of Pt

catalyst supports, capitalizing on a possible synergistic effect.

2.1.1 Materials Survey

In 1978, a number of tremsition metal-metalloid compounds were

examined for stability in hot concentrated (82 °C, .85 by weight)

H^PO^, and materials of appau:ent stability examined for oxidation

activity in IMH^PO^ at 23 °C. Monophosphides of W, Mo, and Nb were

prepaured in powdered form by the method of Ripley [1]^. Mixtures of the

metal chlorides and calcium phosphide were heated at 1200 °C under Argon

for two hours, then leached in dilute HCl to remove debris. Crystal

structures were verified by X-ray diffraction. MoP has the crystal

structure of WC, a superior acid-stad}le non-noble oxidation catalyst.

• ^Figures in brackets Indicate the literature references given at the

end of this report.
5

WP and NbP have structures closely related to that of WC, differing in

the mode of stacking of a basic trigonal cell. The disilicides of W,

Mo, Ta, and NB, and the mixed nitrides M02N/M0N and W

2

N/WN were obtained

commercially in powder form. Of these materials, MoP, WP, MoSi

2

, and

both nitrides appeared stable in hot concentrated H^PO^. Porous electrodes

of these materials were prepared in the manner described in the next

section and examined by cyclic voltammetry for catalytic activity and

voltage stability in H^PO^ at room temperature. MoSi

2 and MO2

N/M0N were

inactive and unstable. The W nitride proved non-conducting. Both MoP

and WP reacted at 0.2V vs RHE; W showed a shift in the current in the

presence of H

2

that might indicate mild H

2 oxidation activity, as

illustrated in Fig. 1.

2.1.2 Electrocatalytic Studies of Mo^ W C Alloys

2

Our previous method of preparing porous electrodes on tefloncoated Au discs has been altered, owing to occasional contamination of

the electrodes by catalytically active precious metal contaminants in

the Au supports [2]. In our current method, weighed amounts of the’

sample powder are wetted with a known amount of diluted Teflon- 30. This

mixture is either prepared directly upon, or an aqueous slurry of the

mixture dripped upon, a teflon layer supported on Ta sheet, then

air dried. After sandwiching between A1 foils, the electrodes are hot

pressed at 3,000 pounds, and 310 ®C for several minutes. The A1 is then

dissolved in 20% NaOH and the electrode thoroughly rinsed in distilled

H

2

O and air dried. The electrode and its teflon support are then lifted

— __

^ Certain commercial equipment, instruments, or materials are identified in

this report in order to specify adequately the experimental procedure. In no case does such identification imply recommendation or endorsement by the

National Bureau of Standards, nor does it imply that the material or equipment identified is necessarily the best available for the- purpose.
6

I (N

a:

-oc

fO

I <N 2 D

c •X. Oi0 • s o

0) 01

0-1 Cj CO 0 c \

Q) >

(L T3 E

-o •rO 0 > "O’ u o • 4-1 o

u CP 0 c

rH H CO 0 3: •r-l 0

0i cu 3 ”(0 4J 0 (0 - 0 U

a, 0 a

CJ m rg CJ CN s

0-1 WJ 0 4J

10 10 • Cl

0) O 4J

GJ a •H s

CO I •H

4J u 2 U

10

+j T3 c

o Oj 0

£ 4-J •H £ IB 4J

IB 113 U 3 n 4-J •H 0 IB X > i/f

01DU

•rJ Cm

!
7

from the Ta sheet. Under test, the bare electrode surface is exposed to

gas-saturated electrolyte, with electrical contact made at the edges

with a foil of Ta, which we have shown [3] to be passivated in the

voltage range of interest here.

Four different production lots of Moq and one of MOq gW^

(kindly supplied by the manufactuer, Wa Chang-Teledyne Corp.) were

examined for H

2 oxidation activity. As received, the refractory metal

powders are contaminated to varying extents by Fe picked up in the

finishing process, and by varying amounts of N1 and/or Co, used as

catalysts in the forming reaction. These latter contaminants are

insoluble in the compound lattice, and tend to wet the surface of the

sample particles. Cleanup was accomplished by successive leachlngs in

hot and washings in distilled H

2

O, until no staining of the acid

occurs. Porous electrodes were fabricated in the manner described

above, with minimal teflon bonding, total teflon content being ^^. 4 % by weight.

Current voltage characteristic curves of these Mo W C electrodes

X J. X

were obtained in N»- and H„-saturated IM H„PO, at 22 °C. As all electrodes

2 2 3 4

behaved similarly, only a sample curve for H

2 oxidation current vs.

voltage for x = 0.7 is shown in Fig. 2. No difference in performance

was observed for different values of x. Electrode performance is consistent

with that expected for porous electrodes of catalysts obeying ButlerVolmer kinetics [4] up to about 0.3 V(RHE). The roll off above 0.3 V

is not associated with diffusion-limited maximum current densities

(judging from the far higher currents produced on Pt sheet under similar

experimental conditions). Rather, it appears to be due to a drop in

activity associated with reversible change in the chemical state of the

catalyst surface at this potential. Linear voltammetric scans of an x =
8

Volts (RHE)

Figure 2, Hydrogen oxidation current vs. voltage for a thin porous

electrode of Mo^ . The first straight line segment

corresponds to a region of pure activation control; the

second to mixed activation and diffusion control. The

round off at higher voltage arises, not from diffusion

limitations, but from a decrease in activity associated

with chemical change of the catalyst surface.
9

0.7 sample at various sweep rates, shown in Fig. 3, lend support to this

view. At lower sweep rates, the oxidation- reduction peak separation

approaches Nicholson's limit [5] of 60 mV for unit charge exchange.

Exchange current densities and Tafel slopes can be extracted from steady

state current voltage characteristics, and are tabulated for

samples at room temperature in IM in Table 1.

Table 1

Electrochemical parameters for several Mo, W C electrodes,

1-X X

obtained

in IM H^PO, at

3 4

22 °C.

b b' I o A

Sample (volts) (volts) (Amperes) / 2. (cm ) (Amp/cm^)

%.7"o.3=-3 .19 .43 2.3 X lO"^ 23 1.0 X lO"^

“°0.7”o.3''-=

.19 .30 0.85 X lo"^ 11 .8 X 10~^

%.7"0.3'^-® .18 . 36 3.6 X 10“^ 36 1.0 X lo”^

!

Mean of above .19 . 36 0.9 X lo"^

19 .40 1.8 X 10

-6

18 1.0 X 10

-7

Symbols;

b = Tafel slope

b' = slope of Tafel plot in region of mixed

activation and diffusion control

= Exchange current

A = Active area, inferred from manufacturer's

stated mean particle size.
SWEEP

RATE

(mV/sec)

10

lN3HdnO
11

Further valuable information can be obtained from current-voltage

characteristics obtained at various temperatures. One of the x = 0.7

electrodes, and an electrode of WC (prepared by reacting WO^ with

lampblack at 1200 °C for two hours in flowing H^) were studied in 85%

H^PO^ at 50, 70, 90, and 115 °C. Rate parameters and apparent activation

energies extracted from these measurements are tabulated for Mo^

in Table 2 and for WC in Table 3. In addition, measurements were performed

on the X = 0.7 sample with a fuel feed containing 3% CO at the same four

I temperatures. The CO had no detectable effect on the oxidation current.

(Pt sheet, however, was completely poisoned under these conditions.)

Table 2

H^-oxidation rate constants and apparent acitvation energies for

“°0.7"0.3= V°4-

°c 10^/T

(k"^)

b

(volts)

b'

(volts)

50 3.096 .245 .462

70 2.907 .282 .633

89 2.766 .257 .490

115 2.577 . 340 .645

io\ b'/b Io

i

Op

1 (Amp) (A/cm )

0.768 1.886 .16xl0"^ 0.4xl0“^

0.820 2.245 .71xl0”^ 2.0xl0”^

0.716 1.892 l.lOxlo"^ 2.8xl0”^

0.876 1.898 3.10xl0"^ 8.6xl0~^

mean: mean:

0.795 1.980

3 8.3x10 cal/mol. from current at 0.2 volts (RHE)

E = 3 a 10.5x10 cal/mol. from Io
12

Table 3

H

2 “Oxidation rate constants and apparent activation energies for

for WC in 85% H„PO,

3 4

“c lO^/T

(k‘^)

b

(volts) ^

b'

(volts)

^

3

10 b

T

b'/b Io

(A)

-5

i° 2

(A/cm )

.V ^-7

50 3.096 .201 .321 0.622 1.597 0.09x10 0.3x10

70 2.907 .302 .579 0.878 1.917 0.44x10“^ 1.7x10"^

89 2.766 .244 .521 0.675 2.135 0.47x10“^ 1.8x10“^

115 2.577 .353 .794 0.910 2.249 1.70x10"^ 6.6x10"^

mean : mean

:

0.771 1.974

3

6.8x10 cal/mol from current at 0.2 volts (RHE)

E « a

3

10.7x10 cal/mol from Io 2.1.3 Refractory Hard Metals as Pt Catalyst Supports

The catalytic activity of Pt has been found to be promoted on

certain supports (on perovskites for CO oxidation [6] and on sodium

tungsten bronze for 02reduction [7]). It is of considerable interest

and some practical significance to inquire whether any benefits might

accrue from supporting Pt on Mo^ w C alloys. It is known that the

Group VIII metals Co and Ni wet WC, though they are Insoluble in it. If

Pt behaves similarly, it may disperse stably, on an atomic scale, over

the WC surface, perhaps forming a well defined Pt-WC surface alloy. In

addition to such geometric promotion, there is the possibility that,

since WC is insensitive to CO poisoning, spillover of adsorbed CO onto

the WC support may occur, even if the supported Pt is clustered, with

subsequent improvement of the catalytic efficiency of Pt in the presence

of CO. Several methods can be employed in the fabrication of Mo W C
13

s'^Pported Pt catalysts:

a) , precipitation and reduction of a Pt salt on the compounds;

b) . alloying of Pt with the base metal (s) before carburization

(%!% of Pt is soluble in W at 200 °C, e.g.);

c) . doping of an oxide precursor before caburization;

d) . electrochemical deposition of Pt upon Mo^_^W^C.

Apparatus is being completed to carry out these preparative methods.

We have observed [3] that several metallic transition metal borides

remain passivated at remarkably high potentials at room temperature. It

is of considerable interest to investigate the stability of these materials

as Pt supports at high potential and temperature in the presence of in

concentrated H^PO^. Some contribution to the problem of carbon support

corrosion on the air side of phosphoric acid cells may be made in this way.

The preparative techniques outlined above should prove useful here.

We are testing the Raman technique as a method for characterizing

the electrochemical interface of transition metal-metalloid compound

electrodes. We have carried out a preliminary study of the Raman spectrum

of WC in air, and observed the presence of small amounts of W in the +6

oxidation state. Further encouragement comes from the work of other

investigators who have observed Raman spectra in air of related compounds

(TiN, Tic, ZrN, NbC, NbN, and V^Si [8]) via defect-induced scattering of

radiation whose wavelength falls in a region of low reflectivity, hence

of higher than average sample penetration. This strongly suggests that

surface alloying on WC should be observable via impurity vibrational

bands. We are preparing a half cell suitable for these Raman investigations.
14

2.2 High Temperature Solid Electrolytes

Introduction

Oxide solid solutions having a defective fluorite structure and

based on ZrO^, ^^^2' with CaO, ^^0^ or the trivalent

rare earth oxides, have high oxygen ion mobilities relative to the pure

constituent oxides. The high oxygen ion mobilities make these oxide solid

solutions potentially useful solid electrolytes for high temperature fuel cells.

The high oxygen ion mobility results from the large concentration of oxygen

vacancies introduced into the oxygen sublattice of the host oxide, about 5

to 15 mole % of dopauit may be added depending upon the oxide system.

ZrO^-based solid solutions have been used as electrolytes for hightemperature electromotive force measurements since the mid-1930's [9,10],

and Th02“based electrolytes, since 1957 [11]. Zr02”based electrolytes

have been studied for fuel cell applications principally at Westinghouse

and G. E. in the United States, and elsewhere in France, Germany and the

Soviet Union. Recent results [12, 13] on Ce02:Ln20^, where Ln represents

3+

a variety of lantheuiide ion or Y , have revealed oxygen-ion conductivities

essentially equivalent to that of Zr02 :Ca0 but at temperatures about 200

®C lower. Thus, fuel cells using a Ce02~based electrolyte might operate at

about 800 °C as compared to 1000 °C for Zr02“based electrolytes. However,

Ce-based electrolytes begin to show appreciable electronic conduction at

higher oxygen partial pressures than do the Zr02~electrolytes; for example,

one-half of the electrical conductivity is electronic in (Ce0-)_

at P = 10 atm ('^lO Pa) for T = 800 °C, whereas for ZrO ;CaO,

P - 2.5xl0~^^ atm (2.5xlo”^^ Pa) for T 1000 ®C [14].

2 At the high oxygen vacancy concentrations present in these electrolytes,

the oxygen vacancies are probably nearly all associated with the dopant

cations [15] . If these electrolytes undergo prolonged anneals at moderate
temperatures (<1000 °C) , the oxygen vacancies, and perhaps to a lesser

extent, the dopant cations, may order into a structure coherent with the

fluorite structure of the host oxide. Formally, in terms of a coherent

structure, the ionic conductivity of the electrolyte may decrease (resistance

may increase) because the vacancies, relative to the fluorite structure,

must then move cooperatively.

The existence of coherent structures is well documented for ZrO^-

based solid solutions [16-23] . Coherent structures in CeO^-based solid

solutions have not been identified.

Carter and Roth [20] found that the increase in resistance of

calcia stabilized (CSZ) when annealed at <1000 ®C was associated with

the formation of coherent structures, observed as superstructure in

neutron and x-ray diffraction. This "aging" of CSZ could be reversed

("deaged") by annealing at 1400 °C. In a high temperature fuel cell

aging of the electrolyte would reduce the efficiency of the cell.

A second aging process which may alter the long-term performance of

a high temperature fuel cell electrolyte is the redistribution of the

dopant cations along the grain boundaries. Relative to the lattice, the

dopant cations appear as negative charges. Thus, they can be expected

to migrate under the influence of an applied field. Although the

mobility of these cations within the interior of the grain is negligibly

small for practical purposes, their mobility in the grain boundaries can

be significant. For example, from the tracer diffusion measurements for

2+ 2+

Ca ions in CSZ [24]

,

the time required for Ca ions to cross a 20 ym

thick electrolyte film with a potential difference of 0.8 V is on the

order of hundreds of years. On the other hand, the same tracer diffusion
16

study showed that migration of Ca ions in the grain boundaries is

perhaps 1000 times larger at 1800 °C them the bulk diffusion rate, a

ratio that would be expected to be even larger at lower temperatures.

Hence, migration of the dopant ions along grain boundary paths over

significant distances during times of the order of a year or less can be

expected in all of the fluorite- structure oxides of current interest as

solid electrolytes.

The migration of dopemt ions along grain bovmdarles can be expected

to result in at least three deleterious effects: (1) a build-up of a

resistive layer at either electrolyte boundary, (2) a build-up of a

space charge at the electrodes resulting effectively in an overvoltage,

and (3) a possible interference with the electrode reaction due to a

change in electrolyte composition at the electrolyte-electrode boundaries.

The NBS program of studies on high-temperature electrolytes currently

contains two elements: a study of degradation mechanisms in CeO^-based

oxygen-ion electrolytes, and the development of the impedance measurement

as a method of analyzing fuel cell performance with the particular aim of

developing a lifetime-prediction test.
1 7 2.2.1 ExecLroiyte Materials

Preparation and Characterization of High-Density Ceria-Yttria

Using the carbonate process described in the previous report (NBSIR-78-1A72)

about 880 g of ceria-yttria powder was prepared by coprecipitation of the carbonates from homogeneous solution; the carbonate process is illustrated

schematically in figure A. The oxide powder which was obtained by calcining the

carbonate to 600 °C had the nominal composition Ce^

]^q^1 95* 8 of this

powder was formed into a billet; the billet was isostatically pressed at 69 MPa

(10,000 psi) and then was hot-pressed at 1350 °C and 28 MPa (AOOO psl) for 3/A

hour. The resulting ceramic was beige color, had a glassy luster and

fractured conchoidally. The bulk density was 99.6 to 99.8 percent of

theoretical (densities will be expressed as a percentage of theoretical, the

lattice density calculated from the x-ray lattice parameters and the mass of

the contents of the unit cell for the appropriate composition) . The billet

has cracked into several pieces during the hot-pressing, apparently as a result of excessive shrinkage and adherence of part of the billet to the walls

of the die.

Figure 5 shows a SEM micrograph of an etched fracture surface of

the hot-pressed material. The grains are equiaxed; only a small amount

of porosity is evident; and the fracture has occurred along the grain

boundaries.

Specimens were cut from the billet and were polished, cleaned,

etched and examined by optical microscopy and x-ray diffraction. Thick

specimens were observed to be translucent to the beam of the microscope;

thin sections, about 50 ym thick, were transparent. This optical quality

is consistent with the nearly theoretical density of the specimens. The

x-ray diffraction patterns showed very sharp fluorite structure peaks,

tbe K -K doublet was clearly resolved for 29>50°, indicating good

“l “2

homogeneity. Estimates of the lattice parameter suggested that the

yttrium content was less than 10 atom-percent of the cation content.
PREPARATION SCHEME: CARBONATE PROCESS

Figure 4. Preparation Scheme: Carbonate Process.
19

Figure 5. Fracture Surface of hot-pressed

ceria-yttria ceramic.

Ulr!
20

The shrinkage during hot-pressing may be affected (1) by the

reactivity of the powder, which may be adjusted by chemical preparation

and by calcining the powder prior to hot-pressing; (2) by the extent to

which the powder is compacted in forming the green body; and (3) by the

temperature and pressure during the hot-pressing stage. To study the

shrinkage, or densif ication, a sintering study v;as carried out. Powders

were prepared by the carbonate process, by a tartaric acid-ethylene

glycol (TA-EG) resin process and by a citric acid (CA) resin process.

Powder samples from the carbonate process were calcined at 600, 700,

800, 900 and 1000 °C for about six hours; powder samples from the other

two processes were calcined at 800 “C for about six hours. Isostatic

pressing to 69 MPa (10,000 psi) was used to compact further some of the

specimens. Specimens were randomly selected for the sintering anneals.

Anneals to 3 were carried out at about 1510 ®C for 24 hours. During

the fourth anneal the furnace burned out at about 1500 “C. Anneals //5

and 6 were carried out in a second furnace at about 1550 “C for 24

hours. Bulk densities were measured by an Archimedes technique with

mercury as the buoyant medium; each sample was measured three times to

obtain an estimate of the measurement error.

Relative treatment effects with associated 80 percent confidence

intervals were calculated from the data.

For the chemical processing effects, the TA-EG resin process did

not differ significantly from the carbonate process; whereas, the CA

resin process yielded a powder slightly less sinterable than the powder

from the carbonate process, resulting in a loss of density amounting to

4 ± 2 percent of theoretical.
21

The effects due to calcination temperature were compared to the

treatment effect at 600 °C. Calcining at 700 °C resulted in a loss of

density in the final ceramic of about 12 ± 8 percent of theoretical,

whereas, calcining at 800 °C may have improved the powder slightly, the

increase in density being 4.4 ± 4.2 percent of theoretical. Calcining

at 900 and 1000 °C did not significantly alter the densif ication as

compared to the 600 °C treatment.

Isostatic compaction of the green bodies improved the final relative

densities by 15 ± 5 percent of theoretical. Green densities before

isostatic pressing were 40 to 50 percent of theoretical; after pressing

they were 60 to 70 percent of theoretical.

Final densities were significantly higher, 13 ± 5 percent of theoretical,

for specimens sintered at 1550 °C in the second furnace as compared to those

sintered at about 1510 °C in the original furnace. As expected, the densities

of specimens from the aborted anneal, #4, were the lowest, the relative

treatment effect being -14 ± 9 percent of theoretical.

The best densities are summarized in table 4.

Table 4

Summary of Best Densities of Y202~doped Ce02 Ceramics

Powder Preparation Sintering Process

Density, % of

Theoretical

Carbonate Process Thermal 95.4 ± 3.4*

Carbonate Process Hot-Press 99.5

TA-EG Resin Process Thermal 95.2 ± 2.8

CA Resin Process Thermal 89.0 ± 2.6

k

Standard deviation.
'’2

Development of Analytical Methods

The possible loss of some yttria in the preparation of the oxide

powder by the carbonate process Indicated the need for routine analytical

procedures to verify the compositions of solutions and powders.

To analyze solutions for Ce and Y, an analytical procedure and

standard solutions were developed for atomic emission spectroscopy.

Cerium concentrations can be measured in the range of 100 to 500 yg/ml;

yttrium, In the range of 1 to 25 yg/ml.

For the analysis of the oxide powders work was begun on the development

of a procedure and of standards for x-ray fluorescence analysis. Lytle

and Heady [25] described an x-ray fluorescence analysis of hlgh-purlty

rare earth oxides. They prepared their standards by copreclpltatlon of

mixtures of rare earths as oxalates and then converting the oxalates to

oxides. In preparing the oxalates we have chosen to use a method

developed by Carron, Skinner and Steven [26] who homogeneously coprecipitated

rare earths from solution using the hydrolysis of dimethyl oxalate to

generate the oxalate Ion. They prepared the dimethyl oxalate reagent by

refluxing dehydrated oxalic acid In methanol.

This copreclpltatlon technique yielded a precipitate that settled

rapidly. When the precipitate was dried, a fine white powder was obtained.

Figure 6 Is a micrograph of a particle of the oxalate powder. X-ray

diffraction analysis of this powder showed many low angle peaks, some of

which could be assigned to the monocllnlc structure of Ce

2 (C20^)^* 10H20 but

with small changes of the lattice parameters, suggesting some incorporation

of yttrium into the structure. However, a second phase was found to be

present as indicated by the strongest diffraction peaks which could not be

indexed with the monocllnlc pattern.
23

Figure 6. Cerium-yttria oxalate obtained by coprecipitation from

a homogeneous solution.
24

Upon calcining to 650 “C a fine yellow powder was obtained. Figure

7 shows a micrograph of this powder. The powder retained the shape and

twinning features of the oxalate precursor. However, x-ray diffraction

analysis clearly showed that the powder had been tremsformed to the

fluorite structure.

Acknowledgements

We wished to thank Ms. Bethanne Warrack for her assistemce with

this work and, in particular, for her development of emalytical procedures

! and standard solutions for atomic emission analysis of solutions.
25

Figure 7. Cerium-yttrium oxide obtained

by calcining the oxalate to 650 °C.

uiri 0

3
26

2.2.2. Impedance Measurements

Introduction

1 We are studying the use of impedance measurements, using a network

analyzer operating in the frequency range from 10 Hz to 13 MHz, as a

method of characterizing solid electrolyte cells. To date, we have

concentrated on the bulk properties of the electrolyte, using sputtered

Pt electrodes and working in an atmosphere containing either air or

oxygen-argon mixtures ranging in oxygen partial pressure from 1 to about

-4

10 atm, at temperatures from about 200 ®C to about 800 °C. These

measurements appear to be interpretable in terms of a simple equivalent

circuit, shown in Fig. 8. In this circuit, Cg is the geometric capacitance of the specimen, the resistance arising from the intrinsic

* transport process of the crystalline grains, and the impedance

arising from Inhomogeneitles such as grain boundaries. We find the

latter can be represented using a Cole-Cole [27] equation:

l+aur)'"“

( 1)

where is the contribution of the inhomogeneities to the dc resistance,

T the most probable time constant, and a is a measure of the width of

the distribution of time constants. The circular frequency is w. The

electrode impedance is indicated by Little work has been done on

Z*^ during the past year and it will not be discussed further here. We

expect a post-doctoral fellow (James Bethin, from the University of

Illinois) to join the group early next year to begin studies of electrode

effects.
27

U

0.1 c.

I/!

ICo

CJr j CJ

Oj

C > c

-C.ro OC\J >-

S^. cU

£-

•f“ OT U GJ

--}O

s- a» +J f— u

rO OJ > (

•r- CD

r3cr 4-> LU c_

c.';

CL III
28

In principle. In Ce.0^ heavily doped with Y20^ only ionic conduction

is expected within the range of precision of these measurements. Tuller

and Nowick [12] show negligible deviation from unity of the ionic transport

4+ 3+ number for 10% of the Ce ions replaced by Y for temperatures up to

1000 "C and atmospheres containing more than 10 ^ atm oxygen. Thus, we

expect that measured values for both Rg and should be independent of

P_ , the oxygen partial pressure. This was found to be true for several

sets of measurements at 250 *C for two specimens (5 sets for one, 7 for the

other) as the oxygen partial pressure P_ was cycled between 'v-io and

^2

-3 7 ^

'^lO * atm. (Quarterly Progress Report for 1 April 1978 to 30 June 1978)

.

Correlation of Preparation and Properties

^2^3”^°^®^ Ce02 ceramic specimens have been prepared under a variety

of conditions, and we find that they fall clearly into two distinct

types

1. Specimens prepared by mixing the oxides Ce02 and '^ 2^2

in powdered form, ball milling an alcohol slurry of

the mixed oxides, calcining, and either Isostatlcally

pressing specimens at room temperature and sintering

at temperatures up to 1550 ®C for about 16 hours, or hot

pressing at temperatures near 1350 for 3/4 hpur. (Type M)

2. Specimens prepared by coprecipitating the combined oxides

from solution, calcining, and either sintering or

hot pressing as above. (Type C)

The impedance behavior for these two types, as revealed by plots of

the impedance 7. in the complex plane, are very different (Details are given

in the Quarterly Progress Report for 1 July, 1978 to 30 September, 1978 where

what is here labelled type M was called Type B. M and C stand for mixed

and coprecipitated). In brief, and in terms of the parameters of eqn. 1,
3/12/73

B-7

2SI

LPD2-.75

H5

31

M

5WHD / I iZ

1

/

DHM5
30

r*

CD

LD• I rvj a

CL.

CD

rvj m

bD

cn

rrvi m

->

w? rn CM /

DHM5
3/I2/7H

B-7

251

LPD2-.75

H5

31

f* +

9 5 h C £ I 5NHD ' i ^Z

I

/

aHMS
32

_ in

CM

in

N. (O in ^ CO CM • d d d • • o o d o

““zv/^u

u>8o

Q.

(C 4J

(T3 P T3 •H 0

0; <p 0 c P

(U OJ 0

O'

p •H P 0 OJ P • H e to P c in P £ <v tr, c o o

•H c w

p

p

p 3 0 •H 0 5

4J 0 > O' c X

AS c M

0)p 3 a (/; J3 c • • 3

OJG

M-l c «

> 0 • W 0 p» •rH •H •iH •o

•p in 0;

G

nj 0 P C

oe; P P •H 0 0 rH a V • t • •v X

in c c

rH > 05

G c B 0 U (fl •r^ 0 05 0

(Up

G > O to a

'H iTi 3 X

+J CN O’ m cn s pp P a; m <0 • Cl, 3

rH in ‘ +

nn rH o >< •H •H

p “-r e cK’ 0

IQ

p

(/) P c

(U 0) 0 , c O •H

G

•H P c

iH CN 05 c O O 3

T3 0) P

'H

u

O

X

rH •H 0 T3 X

in 01

p

G

a, c P

T3 0 p C '0

(0 1 p P

ro 0) 3

UJ

O X,

<u CN

p

rH >• c o p p P •t •H 0 f'l > u <4H p

•H

in o\<^

G • 0) C 1—

u c G I—

c G 0- m •H C

(V 4J P E p in 05 0 3 C JZ o> in c

•iH (U c •r^ IP p CN
33

AZgj^/Rg = -.36 + 31.3 P, ( 2 )

where P is the volume fraction of pores. While the form of this equation

is very similar to that e:^ected for non-interacting spherical pores in

an otherwise homogeneous conducting medium# the coefficient of P is very

different. Juretschke ejt al. 128) have given for the effective

conductivity of this model

o = 2(1-P) a /(2 P) (3)

m o

where is the conductivity of the medium. This gives for

the total low frequency resistance

Go

%

^ “

o ^2 (1-P)

o _ G(2-t-P)

(4)

m

where Ga is the geometric factor. At high frequencies no internal

o fields due to the pores occur and

1-P (5)

From eqns 4 and 5 we find

Whereas this model %«ould give a slope for the line in fig. 11 of 1/2#

the experimental value is 31.3. Since pores cire inclusions of the

highest possible resistivity, this result suggests that no model based

on a homogeneous conducting medium containing non-interacting inclusions

of different conductivity can account for these data. Rather, the

higher-resistivity material must consist of at least strongly interacting

regions, even perhaps continuously connected.

Temperature Dependence of Resistances

We have examined in somewhat greater detail the values for R^ and

AZgj^ for the two most porous specimens of each type. Figures 12 and 13

exhibit the temperature dependence of each quantity for the type M

specimen (specimen B-3) and the type C specimen (B-7) , respectively, in
34

1.3 1.4
Resistance/T,

ohm/K

35

Figure 13. Arrhenius plots for bulk (R ) and inhomogeneity (Az ) resistances for

y D

Type C

1°1 95 ceramic

.
36

the form of plots of J,n(Rg/T) and Hn(AZgb/T) against T where T is the

temperature in Kelvin units. Both quantities appear to obey the Arrhenius

equation, with greater uncertainty in the Type M specimens because of the

overlap of the arcs in the plot of impedance in the complex plane (cf Fig. 9)

.

The activation energies drawn from least- squa:res fits to the data

in Figs. 12 and 13 are given in Table 5. " ’ '

Table 5

Activation Energies at Low

Temperature for Types M and C Specimens of Y^O^-Doped CeO^

Specimen

B-3 B-7

AE

B

Az

gb

Type M C ’

AE„ 69.0±4.9 72.711.5 kJ/mole

10813.6 98.011.9 kJ/mole

Cross-over Temperature 380 °C 790 °C

For Rg the activation energies are close, and the Student's t-value is

2.15 with 14 degrees of freedom, implying that they may not be different

at the 95% confidence level. The values of the activation energy for

AZ^j^ produce a t-value of 6.54, and they clearly are different.

For each specimen the total dc resistance in a 4-probe experiment

would be controlled at low temperatures by and at high temperatures

by R . The cross-over point for each specimen is given in the last row

B I of Table 5. These data, and the existence of the cross-over between

bulk and inhomogeneity values are very similar to the findings of Kudo

and Obayashi [13] , whose dc conductivity measurements show bends in the

plots of Una vs T with straight-line portions above and below the

bend. The activation energies in Table 5 are very similar to their high
37

and lov7 temperature values. According to the interpretation offered

here, then, only the high temperature data, above the bend, represents

the true intrinsic properties of the material. The low temperature data

reflect the inhomogeneities, presumably grain-boundaries. Similar

conclusions were reached by Chu and Seitz [29] for Ca-doped ZrO^.

Slow Transients

We have begun examining 4-probe resistivity measurements in these

materials. Figure 14 is a schematic diagram of the type of sample used.

The dc electrical conductivity was measured from 180 °C to 1200 °C. The

sample dimensions were 1.7mm?x 1.7mm x 13mm. Electrical^ contacts were

made by using Engelhard #6082 platinum paste and platinum wires. A high

ac current (>100yA) was applied through the contacts in air at high

temperatures to form porous contacts. A dc current of less than lOyA

was used over the whole temperature range. All the contacts were ohmic.

The temperature was measured by a pair of Pt-Pt 10% Rh thermocouples

placed next to the sample.

As described below, the IR drop in the specimen is given by the

Instantaneous voltage rise between the potential probes when the current

step is applied. Such data for a Type C specimen (CHP-2) are shown in

fig. 15. The bend at 930 °C is quite clear, and the upper and lower

activation energies are similar to those in Table 5 for R^ and

Type C material. There appears to be reasonable consistency between the

ac and dc measurements at this stage.

When we started the small constant dc current through the sample.

a transient effect was observed in addition to regular IR voltage drop.

Typical behavior is shown in figs. 16-18. Figure 16 gives a schematic

representation of the current and voltage pulses observed at the potential
112 3

Figure 14. Schematic of 4-probe sample.
(OT)

(KIQ-cm)

T(C)

0.5 1.0 1.5 2.0

103x1/7 (K-l)

Figure 15. Temperature dependence of '^••;^robe dc cone of Tyoe C Ceg. aVg/|0-| g^ cera.i'iic i n air.

temperatures / and' +, failing ten.i.irral uro

I
Voltage

Current

40

Time

Time

Figure 16. Schematic of current and transient voltage pulses

at the potential probes in Type C Ce. _Y O ceramic.

u.y c)»j i.yb
41

probes. Four identical electrodes were used as described previously. A

constant dc current at the order of lOyA was applied through contacts 1

and 4 (fig. 14) , and the voltage response was measured at electrodes 2

and 3. A high- impedance electrometer was used to measure the voltage,

so that there was no current drawn at electrodes 2 and 3. Figure 16

shows a typical test cycle of applied current as function of time. The

corresponding measured voltage is shown also. Immediately after the

current is turned on, there appears the IR voltage, AA' . The transient

voltage follows the IR voltage after a short delay time. Figure 17

shows the recorder trace of such a treinsient, with its delay, on an

expanded time scale. The transient voltage increases with time in an Sshaped curve, and finally saturates. Figure 18 shows the recorder trace

of one of these transition voltage pulses at longer times. If one

reverses the applied current, the IR voltage, BB' (fig. 16) equals twice

AA' as expected. The transient voltage also reversed. The effect is

symmetrical with respect to the zero current axis. At time C, if one

turns off the applied current, the transient voltage relaxes slowly

after the IR voltage drop and the delay time as shown in Figure 16.

We shall define the saturation overvoltage as the steady state

excess voltage observed in the experiment, excluding the initial IR

voltage. Figure 19 shows the relation between and the applied

current. The saturation overvoltage is proportional to applied current

at low currents, but the curve bends toward lower overvoltages as the

current increases. The initial increase of overvoltage with time

appears to follow the square root of time. A large linear range can be

1/2 found in the t plot, as shown in fig. 20. The intercept of the linear

line on the time axis indicates the delay time involved in the process.
42

LO

lO

m

(0

N- CD in ^

CO

Aui ‘a6e)|OA

00 CM

Time,
43

tn

CTi

f—

I

+J O

ftj I—

I

0) o

in >H

(—1 CT'

P4 O

<U

(U U

tTi fC U

-P

rH (U o a^

+j

C, M

QJ O

•H 4-4 inC m

m (U M X!

tH O

u

ft

CO rH

i—

I

•H c

0) -p u p B

p (U to cn -p p

•H o OJ ft 0-1 o

in

air

at

887

®C.

Current

'^>10

yA.
I

ir> lO

yyuj ‘aBeiiOAjeAQ uoiiBjnies

I,mA
45

0 2 4 6 8 10 12 14 16

(s)
At long times the rate of increase in the overvoltage is reduced and

finally the steady value is reached. This overvoltage behavior is

different from that observed at a current-carrying (active) electrode.

A typical rise in overvoltage observed when the active electrodes are at

the ends of the specimen (positions 1 and 4, Fig. 16) is shown in fig.

21, and when the active electrodes are at positions 2 and 3 the rise is

as shown in Fig. 22. In both cases there is no delay, so that the time

dependence curve is smooth as shown. Note, however, that the rise is

slower when there is greater separation between the active electrodes.

The overvoltage was also studied as a function of the oxygen

partial pressure. Figure 23 shows the oxygen partial pressure dependence

of the saturation overvoltage. The ambient oxygen partial pressure has

a large effect on V^. The IR voltages are also shown in the graph

for comparison. As shown in fig. 23 at high oxygen partial pressures

and are comparable. However, the overvoltage became dominant at

low oxygen partial pressures.

1/2 The initial rise of the overvoltage in the t plot also depends

on the partial pressure of oxygen. The slopes obtained from plots such

as fig. 20 at different partial pressures of oxygen are plotted in loglog form in fig. 24. A straight line of slope between 1/2 and 1/3

appears to describe the data at least down to -log 2.

The temperature dependence of the delay time was also studied,

defined as the time when the recorded voltage at the potential probes

starts to deviate from the constant IR voltage. The result is shown in

fig. 25. A straight line of activation energy 80.1 kJ/mol describes the

data quite well.
Time, s

Figure 21. Transient voltage pulse on active electrodes, placed at ends of specimen, for Type C Ce^ ceramic

at 888 ®C in air. Current =^10 yA.
Figure 22. Transient voltage pulse on potential probes used as active electrodes for Type C CeQ ^Yq ^0^ gg

ceramic

at 887 °C in air. Current -10 yA.
49

AUl ‘96e)|OAJdAo uo^ejn^es

I

j

0

c. CO o X

rC OT 0

!>, -I-) r1-^ 'J

(rs H • H E

-1-1 Cl O u

J c

Cm LTl

(Ti

-1- • rH 3

D rH

CT' (Nl fO o

o

>4

cr o. >

f»4 o

U O U

o> u

o r- 0) .J 6

rH a

>1

1

+1 Fh

1

rO

!h S40

-U 4-1 fO

U1 • -P sC

4-4 •H P

0 >

-< O

Cj

r;

4-1 (—

O >1

(TJ 4-1 'V r: c

0 0)

0) TI P

r )j >1 P

X P n 0 a

j C

5h

':3
-Log

Slope

‘30

0.5 1.0 1.5 2.0 2.5 3.0 3.5

-Log Po

2

(atm-)

1/2 • • Figure 24. Dependence of the slope of the t plot on oxygen activity

for Type C Ce Y 0 ceramic at 888 ‘’C. Current -10 yA.

^

.1

j

1

I

,1
51

0.6 0.8 1.0 1.2 1.4 1.6x10-3

T'l (K)

Figure 25. Arrhenius plot for delay time for Type C Ce Y O

in air. Current ~]0 pA. 0.9 1.0 1.95
52

This transient effect appears to be governed by a diffusion process

1/2 involving diffusion in the solid state. This fits the initial t

behavior seen in fig. 20 and the Arrhenius behavior of ’the delay time in

fig. 25, for which the activation energy is very like tliose of thfe

oxygen transport processes observed in the bulk through and

It further agrees with the fact, shown by a comparison of figs. 21 and

22, that the rise time at active electrodes depends upon the dist|ance

between them. We are currently examining a model suggested by Fouletier

et al. [30] to account for oxygen semi-permeability in stabilized; zirconia

and recently invoked by Wang and Nowick [31] in somewhat modified^* form

to account for similar effects in doped CeO^. In this model, son^ of

the oxygen flux represented by the steady current is supplied by coupled

diffusion of electrons and oxygen vacancies. We believe this may be

taking place primarily in the grain boundaries, and plan to study, the

7;; effect intensively in the near future.

9

\
53

i

I r 2 . 3 Automated Electrochemical Measurement System

Introduction

^ Experiences with operation of our system [32] and proposals for use

; of the system in pulse measurement have resulted in several modifications

of the original instrument. A need for additional programming has also

' become evident, over and above that necessary for operating the modifications.

, The bulk of this section of the report is concerned with discussion of

' these facts and the resulting designs. For convenience, this discussion

I

li will be separated into three parts. This procedure will be followed

t since each part can stand alone even though it has been derived from

consideration of relationships with the others. These interconnections

I will be indicated where necessary.

2.3.1 Mass Storage

Very early in the operational history of the instrument, it became

obvious that mass storage of some kind was a needed adjunct. As an

example of the need, consider the situation where we are repetitively

performing an electrodynamic scan of electrode characteristics. We

i

!

j would like to be able to examine the data from each scan as well as the

I average of all scans. Depending upon scan speed, we can either print

I each run or store each run. If the scan speed is too fast, we can only

i

: store each run. Because of memory capacity limitations, in the original

I system we siammed each run with the previous ones. This allowed us to

look only at the average of all scans unless we could slow the scan rate

enough to allow printout of each scan.

I

I

Several approaches can be utilized to resolve this problem. All

1 are some kind of enlargement of the original memory. First we could

just enlarge the memory capacity of the instrument. This is relatively

I
54

expensive and subject to loss of data on a power- failure. Second, we

could store each scan on a magnetic tape cassette. We are limited here

by the speed of data transfer to tape [in the present arrangement a

maximum of 120 characters (bytes) per second], and by the fact that

cassette storage is sequential and therefore data retrieval is slow. A

third approach is to use "Floppy Disc" storage. As in the case of

cassette storage, data is sequentially-stored. However, we can access a

particular block of data directly without going through an operation

that passes all preceding data through the system as in a cassette

4

system. Data transfer is rapid going at speeds up to about 3 x 10

characters per second. This last approach was the one selected.

There are a large number of Floppy Disc units available, and

therefore, we must establish specifications for selection. The criteria

we established were:

(a) In order to minimize software overhead and interfacing, the

r unit selected must have a controller built in.

(b) For compatibility with other existing NBS equipment, the disc

should be single-sided, 8", single density and the recording

format should be IBM-3740 compatible [33].

(c) There should be a minimum of two drives, all operated by the

same controller.

(d) Data transfer between the disc unit and the instrument should

have the option of either direct transfer at maximum speed or

on a single byte basis under command of the Instrument CPU.

A Floppy disc unit meeting these specifications has been purchased. It

has resident microprocessor control and includes a 1024 character buffer

memory. Two sets of commands are operable (Table 6) . One involves
55

direct transfer of data between disc and CPU while the other operates

through the buffer on a character by character basis.

Table 6

S\ammary of Floppy Disc Unit Commands

Command Code Data Flow Data Transfer

per characters Control

Bootstrap 00 Disc to Host (CPU) DMAC

Read Status 01 Disc to Host CPU

Read Auxiliary Status 02 Disc to Host CPU

Load Sector Count 03 Host to Disc CPU

Read Last Access 04 Disc to Host CPU

Read Data 05 Disc to Host DMAC

Write Data 06 Host to Disc DMAC

Read Buffered Data 07 Disc to Host CPU

Write Buffered Data 08 Host to Disc CPU

Write Deleted Data 09 Host to Disc DMAC

Write Deleted Data Buffered OA Host to Disc CPU

Copy Date OB Disc to Disc —

** Initialize Indexed OC Host to Disc CPU

* * Initialize OD Host to Disc CPU

DMAC (Direct Memory Access Controller)

.

* * Initialize differs from Initialize Indexed in that Initialize formats all

tracks the same whereas Initialize Indexed formats track 00 in 26 sector

IBM format and the rest of the tracks in the format specified with the

command

.
56

Interfacing of the disc unit to the potentiostat control requires

(as a minimum) only a single parallel peripheral Interface adaptor

(PIA) , and some bus transceivers (fig. 26a). At a clock rate of 1 MHz,

the PIA programming to write (or read) one byte of data to (or from) the

disc unit takes an absolute minimum of 32 ys, a time which corresponds

to the maximiam data transfer rate of the disc unit (31,500 characters

per second) . There is no margin for error. However, Direct Memory

Access (DMA) was already being considered for a high speed data

acquisition board, and an interface for the disc unit using this method

is also relatively simple (fig. 26b) . The only problem here is that the

instriament clock requires modification. This difficulty is easily

surmounted by the use of a commercial clock module [34] which is already

configured for this use. Figure 27 shows the resulting modification.

The interface that we have designed (fig. 28) is capable of operation

in either of the above modes. [Table 6 includes a column which indicates

the planned mode for each disc command]. This system (interface, power

supplies, disc unit) has been assembled. Some of the needed programming

has been done, and the combination is presently being debugged. When

fully programmed and checked out, this interface will allow us to select

(in software) either of the two modes of operation. It should be noted

here that the DMA controller chip (6844) allows three modes of DMA. One

of these modes stretches the clock period and does not halt the CPU.

The other two modes both halt the processor, one for the time necessary

to transfer an entire data block and the other for the time needed for

one character. Transfer speed can be as fast as the basic CPU clock

(921.6 KHz in this case). It should also be noted that the new clock

circuit replaces the Band Rate Generator clock as well, and provides for

the use of Dynamic memory circuits if desired.
Fiaure 26a. Block diagram of PIA based interface for Floppy disc.

Figure 26b. Block diagram of Tracking Type ADC.
58

Figure 27. Modified clock circuit utilizing

Integrated Crystal Controlled clock.
59

Figure

28.

Schematic

diagram

of

complete

Floppy

Disc

Interface
60

2.3.2 High Speed 12-Bit Data Acquisition System

Experimental usage of the Microprocessor- controlled Electrochemical

Measurement System has exposed some drawbacks in the design of the data

acquisition board. These restrictions appear only when maximum sensitivity

at high speed is desired. However, for pulse mode operations, these

limitations are severe difficulties. In the subsequent discussion,

these restrictions will be examined and their solutions delineated in

terms of a general overview of digital production and measurement of

analog quantities. Prior to this, however, some general concerns of

Input-Output (I/O) to the data system must be expressed.

Experience has demonstrated that several aspects of I/O connection

between the system and the experiment need revision. First, the addition

of overvoltage (or overcurrent) protection circuitry for all inputs and

outputs is required. Although there have been no difficulties due to

inadequate protection, the possibility does exist. We must also provide

some fixed level input-output lines (logic lines) to be used for control

functions such as relay operation, limit switch detection or interrupt

lines. All such lines should be maskable by the CPU so that their

functions can be disabled without interfering with system operation.

Access to the data system should be via memory-management techniques.

By this is meant that the Data Acquisition System is to be treated as a

set of memory locations, and all control instructions, data input, and

data output are performed as normal memory accesses from the microprocessor

supervised by the usual progreun instructions. This technique simplifies

progrcunming and allows for high speed operation both by normal procedures

and by DMA. This implies, of course, that the data system must be, to a
6 ]

certain extent, self controlled. That is, it must be capable of operating

alone upon receipt of the appropriate instructions and parameters from

the microprocessor (yP) . Also, it must generate status signals indicating

what state it is in, when queried by the CPU.

The basic parameters governing the performance of a Digital to

Analog Converter (DAC) are resolution (number of bits) and settling time

[35,36]. This applies to either the straightforward use of the device

to generate an analog voltage or to its use as a component part of an

Analog to Digital Converter (ADC) . The resolution is the smallest

analog quantity that can be generated while the settling time is the

shortest interval allowed between full scale changes. If we wish to

generate a triangular wave of 30 KHz frequency at a resolution of 12

bits with full scale magnitude we find that a settling time of 4 ns is

12 12 required [33 ms divided by 8192 (2 points up and 2 points down)].

Granting that the settling time for a one bit change will be less than

for a full scale change, it is still unrealistic to expect to obtain

that performance when we note that available 12 bit DAC's have settling

times in the range of 100 ns to 100 ys. However, a square wave of the

same parameters with a rise time of 1 ys or less could easily be generated

by a large number of available DAC’s. This square wave processed by a

precision op- amp integrator [37] would yield a triangular wave of the

desired parameters. Clearly, we must be prepared to accept variations

in signal processing techniques if we wish to accommodate the limitations

imposed by the basic parameters of the DAC. In any case, we must

optimize these parameters in light of the desired experimental method.
62

All DAC's exhibit an undesired phenomenon known as a "glitch" [38],

It is an extraneous output pulse generated when the digital input changes

state and it is primarily observed when a number of bits change state at

the same time (e.g., 01111111 goes to 10000000). The energy contained

in this pulse is a function of the number of bits changing state and is

proportional to the width of the clock pulse. Thus to minimize the

glitch we minimize the clock pulse width. However, the glitch is always

there ! In low speed applications its influence is small and it can be

ignored. In high speed applications, on the other hand, its effects can

be serious and we must therefore deal with it. The best technique is to

utilize a "sample and hold" [39] device on the DAC output, which delays

a change in output until after the effects of the glitch have dissipated.

In ADC applications, the deglitching circuitry can be incorporated in

the comparator stage design.

The output magnitude of the DAC signal will depend upon the application,

and should therefore be controllable by the yP. This may be achieved by

the use of a programmable amplifier (or attenuator) on the output. Such

a device may be included in the deglitching circuit or may be separate.

Irrespective of the need for signal level control, this is a logical

location in the design for output protection circuitry, the need for

which has previously been discussed. Figure 29 summarizes the DAC design.

High speed ACS's present further problems, which must be resolved,

as well as the ones already discussed. These will be considered sequentially

from the analog input to the digital output.

The input stage, in addition to the input protection already discussed,

must contain filtering. This filter should be a lowpass type and its

cutoff frequency should be no greater than that determined by the Nyquist

condition [40]

,

in order to reduce "aliasing" [41] problems. This
Figure 29. Digital to Analog Converter

Block Diagram
64

frequency will of course be one of the original design parameters and as

such enters into the choice of basic DAC characteristics (settling time)

.

Ideally, the filter resonant frequency should be softwave programmable,

but manual changes are acceptable. The frequency rolloff of the filter

beyond cutoff should be as steep as possible (> 12 dB per octave) . This

leads to greater complexity of filter design and more susceptibility to

error than with simpler filters. Very steep rolloffs are probably best

attained with active filters. This implies the possiblity of including

signal amplification in the filter section.

The choice of measuring technique governs the next stage configuration.

This stage might contain circuits such as absolute value amplifiers [42];

pealc detectors, sample and hold circuits, programmable amplifiers [43];

polarity detectors or multiplexers. These functions do not need to be

described in detail since their names adequately describe their operation.

The discussion of measuring techniques will indicate which of these are

needed for each particular method.

The factors that are in control of the type of Analog to Digital

converter are primarily conversion speed, precision and flexibility. Of

these, conversion time is the most important. Depending upon the regime

under investigation, the optimal choice will be either the Successive

Approximation [44] or Traclcing type* [45] . In either case, consideration

must be given to both unipolar and bipolar operation. Determination of

which mode is preferable depends upon the desired resolution.

* Other conversion techniques will not be considered since they are slower than these two.
65

At this point, a short discussion of the principle of operation of

SA and Tracking type ADC's is worthwhile. Successive Approximation

converters operate by a bit by bit comparison of the output of a DAC to

the analog input. In other words, the operation consists of a comparison

of the input to the output of the DAC with the Most Significant Bit

(MSB) set. If the DAC output is lower, the next MSB is set and the

comparison repeated. If the DAC is higher, the MSB is cleared and the

comparison with the next MSB is performed. This process continues until

all bits down to the LSB have been compared. The process takes n + 1

clock cyles where n is the number of bits of resolution that the DAC

has. To a first approximation, the time to acquisition is t < (n + l)t

A “ S where t is the settling time for full scale input [A4].

s On the other hand, the Tracking type ADC uses an up /down counter

for the source of the digital input to the DAC. The count direction is

set by the output of a comparator whose inputs are the DAC output and

the analog voltage being measured. The counter always operates so that

at match the DAC output "dithers" ± 1 LSB. The time for acquisition is

thus given by t^ < ntg where n in this case is the number of counts

needed to match the input voltage. Since we are concerned here with

changes on the scale of one LSB at a time, the actual tg to use is

considerably less than that tg required for a full scale change [45].

Comparison of the two types shows that there are basically 2 distinct

aspects to consider. First, as regards precision, the Tracking type ADC

always "dithers" ± 1 LSB. The Successive Approximation ADC gives a

fixed answer for each conversion. The precision of this value however

will also vary ±1/2 LSB. Another way of stating this comparison is

that the Tracking type makes a conversion per clock pulse with a precision

of ± 1 LSB while the SA makes a conversion every n + 1 clock pulses with

a precision of ± 1/2 LSB.
66

The second aspect to consider is speed. The conversion time for an

SA-ADC is the same regardless of the magnitude of the analog input

change. Thus for input chcuiges greater them>(n + 1) LSB's the Tracking

type is faster. Thus the choice of type of ADC depends on the magnitude

of the "instantaneous" chauige in emalog input.

Table 7 emd fig. 30 are compsurisons of these two types indicating

the accessory devices needed, as well. This comparison shows that

consideration of some sort of hybrid of the two would be worthwhile.

Careful examination shows that the difference, in hardware, between the

two resides in the fact that the Tracking type uses an up/down counter

in place of the Successive Approximation Register. Since many presettable

up/down counters are availcible, it appears possible to use the SAR

output to load the counter and then switch to the Tracking configuration.

This is the approach we are using. Our design allows this switchover to

occur either automatically at the first End of Conversion signal from

the Successive Approximation Register, or by programmed input from the CPU.
G7

CMR - COMPARATOR Figure 30a. Block diagram of Successive

Approximation ADC

Figure 30b. Block diagram of Tracking Type

ADC.

69

As discussed earlier, accesses to all converters are via registers

that look like normal memory locations. However, the CPU actually loads

data into the conversion board via a set of registers that are not

I accessible to a CPU read operation. Similarly, the yP cannot write

into those registers that transfer data to the system bus. This procedure

is followed in order to reduce the complexity of the required logic

circuits. It also requires dedication of one DMA channel to data transfers

to the converter board and another channel to transfers from the board.

Figures 31 through 35 are schematics of the resulting circuits

after consideration of the above. A brief description of this board is

given to highlight its features.

The DAC section contains 3 converters. DAC # 1 is a high speed

(settling time less than 200 nsec.) 12 bit unit. Choice of either

unipolar or bipolar operation is programmable, as is the gain of the

output stage. Data can be input to the DAC by normal memory access or

by DMA. The output is overvoltage protected and provision is made for

low pass filtering if desired. The Sample and Hold is used for deglitching

the output and has an acquisition time of less than 1 ys. Overall

settling time for the entire system should be less than 4 ys, allowing

operation (for output square waves) at frequencies up to 250 kHz.

DAC //2 and DAC #3 are slow speed 12 bit units. Operational control

is similar to DAC #1 except that there is no DMA provided for. These

convertjers operate at system settling times of about 12 ys. Thus,

the maximum frequency out should be about 80 kHz (for square wave output)

.

Provision is made for simultaneous or independent load and turn on of

these devices. DAC //2 and DAC #3 are primarily intended for output to a

plotter or to an external device (e.g. voltage controlled oscillator).
220 220

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devices. Each can be configured under program control as either a

Successive Approximation or Tracking Type. They can be set to automatically

switch from Successive Approximation to Tracking after initial signal

acquisition, or to be switched at any desired time, if at all. Amplification

of the overvoltage protected input signal is programmable as is choice

of unipolar or bipolar operation. The input comes thru a low pass

filter which attenuates signals of greater than 20 kHz by more than 80

dB. DMA operations or normal memory access is provided for. However,

under DMA control both ADC’s are read. There is no provision for a DMA

read of only one converter (since all 4 DMA channels are being utilized)

.

This board provides for 2 logic level output lines and 2 selectable

lines, the direction of which is under program control. Also provided

are 3 maskable interrupt lines. Two of these have their transition

directions controlled (as a pair) independently of the third.

2.3.3 Software

As indicated in section 2.3.2 some programming for the mass storage

(Floppy disc) unit is being debugged. However, this is only the very

minimum needed to drive the system for test purposes. (This will be

adequate to read and write files to the disc but will not support all

functions). A complete driver system is being written, 'and investigations

into the possibility of obtaining a commercial Operating system are

underway. (An Operating System [OS] is a program that coordinates all

aspects of the software. It includes utility routines, file formating,

etc. The peripheral routines - floppy disc drivers, converter board

drivers, etc. - are assigned locations in the OS program, but must be

written by the user) . The ultimate driver program depends to some

degree on the format of the OS but this is not limiting.
76

Drivers for the converter board have not been written since there

is still some doubt as to the exact data form required (due to timing

considerations which must be experimentally checked) . Upon completion

of the board* they will be written. This program should not be difficult

due to the memory-mapping arrangement.

The primary programming effort has been devoted to Fast Fourier

Transform (FFT) [45] programming. Speed considerations mandate that the

program ultimately be written in assembly Icuiguage. However, we at

present have only an inadequate BASIC language program which will

manage a 128 point FFT. It is based on a program from the literature

[47] modified to suit our present purposes. An assembly language program

for a 256 point (8 bit data) FFT has also been found in the literature

[48]

. This program requires extensive modification emd revision before

it can be used. Major modification to the addressing scheme used is

needed before data of more than 8 bits com be used. Changes will also

be needed to handle more than 256 points. However, it represents a

useful starting point.

FFT programs for the present system are not expected to give us

"real-time" outputs since at best they will tcdce times measured in

seconds. In order to obtain speeds unnoticeable to the operator, major

hardware modifications wo\ild be net'dtul. Such cliani|« s .nc n»>i ,ntl

.1 1 this I i iiv'

.

*Completion is delayed due to delays in delivery of some parts.
77

References

1. R. L. Ripley, J. Less-Common Metals 496 (1962).

2. L. H. Bennett, M. I. Cohen, A. L. Dragoo, A. D. Franklin, A. J.

McAlister, and K. F. Young, Materials for Fuel Cells, NBSIR 78-1472,'

National Technical Information Service, Springfield, VA 22151.

3. L. H. Bennett, M. I. Cohen, A. L. Dragoo, A. D. Franklin, A. J.

McAlister, and K. F. Young, Materials for Fuel Cells, NBSIR 77-1270,

National Technical Information Service, Springfield, VA 22151.

4. W. Vogel, J. Lundquist, and A. Bradford, Electrochimica Acta 1^, 1735,

(1972) ; L. G. Austin and H. Lerner, Electrochimica Acta, 9, 1469 (1964)

.

5. R. S. Nicholson, Analytical Chem. 37 , 1351 (1965).

6. D. W. Johnson, P. K. Gallagher, G. K. Wertheim, and E. M. Vogel,

J. Catal. 48, 97 (1977).

7. J. O. Bockris and J. McHardy, J. Electrochem. Soc. 120 , 61 (1973).

8. See S. Schicktanz, R. Kaiser, and W. Spengler, Solid State Com.

28 , 935 (1978) and references therein.

9. W. Schottky, Wiss. Veroff. Siemenswerke, 14 , H2, 1 (1935)

.

10. E. Baur and H. Preis, Z. Elektrochem. , 43^/ 727 (1937).

11. K. Kiukkola and C. Wagner, J. Electrochem. Soc., 104 , 379 (1957).

12. H. L. Tuller and A. S. Nowick, J. Electrochem. Soc., 122 , 255 (1975).

13. T. Kudo and H. Obayashi, J. Electrochem. Soc., 122 , 142 (1975); 123 ,

415 (1976).

14. J. W. Patterson, E. C. Bogren and R. A. Rapp, J. Electrochem. Soc.,

114 , 752 (1967).

15. A. S. Nowick and D. S. Park, "Fluorite-Type Oxygen Conductors," in

Superionic Conductors , ed. by G. D. Mahan and W. L. Roth, Plenum Press

(New York, 1976)

.

16. J. Lefevre, Ann. Chim. , 8, 135 (1963)

.
78

17. R. Collongues, F. Quelyroux, M. Perez y Jorba and J. C. Gilles, Bull.

Soc. Chim. Fr., 1141 (1965).

18. M. R. Thornber, D. J. M. Bevan and J. Graham, Acta Cryst., B24 , 1183 (1968).

19. J. G. Allpress and H. J. Russell, J. Solid State Chem. , 3^, 68 (1975)

.

20. R. E. Carter and W. L. Roth, "Conductivity and Structure in Calciastabilized Zirconia," in Electromotive Force Measurements in HighTemperature Systems , ed. by C. B. Alcock, Elsevier (New York, 1968)

.

21. D. Michel, Mater. Res. Bull., 943 (1973).

22. D. Michel, Rev. Int. Hautes Temp. Refract., 225 (1972).

23. B. Hudson and P. T. Moseley, J. Solid State Chem., 19 , 383 (1976).

24. W. H. Rhodes and R. E. Carter, J. Am. Chem. soc., 49 , 244 (1966).

25. F. w. Lytle and H. H. Heady, Anal. Chem., 31 , 809 (1959).

26. M. K. Carron, D. L. Skinner and R. E. Steven, Anal. Chem., 27 , 1058 (1955).

27. K. S. Cole and R. H. Cole, J. Chem. Phys., 9, 341 (1941).

28. H. Juretschke, R. Landauer, and J. A. Swanson, Appl. Phys. 27 ,

838 (1956).

29. S. H. Chu and M. A. Seitz, J. Sol. State Chem. £3, 297 (1978)

30. J. Fouletier, P. Fabry, and M. Kleitz, J. Electrochem. Soc. 123 ,

204 (1976).

31. D. Y. Wang and A. S. Nowick, sxibmitted to J. Electrochem. Soc.

We thank Prof. Nowick for having allowed us to see this paper

before publication.

32. M. I. Cohen and P. A. Heimann, NBS J. Research 83 , 429 (1978).

33. IBM Diskette OEM Information GA 21-9190-1.

34. Motorola Corp. MC 6871A Microprocessor clock application Note

R29-3-10. Revised (L-30-76)

.
79

35. W. C. Jung, IC Converter Cookbook, Howard W. Sams & Co., Inc.

Indianapolis, Ind. p. 40, p. 49 (1978)

.

36. D. H. Sheingold, Ed. Analog-Digital Conversion Note Analog Devices,

Inc., Norwood, Mass. p. 218 (1977).

37. E. R. Hnatek, Applications of Linear Integrated Circuits, John Wiley

& Sons, New York, p. 59 (1975) .

38. Ref. [35] p. 47 and Ref. [36] p. 224.

39. Ref. [35] p. 210.

40. L. R. Rabiner and R. W. Schafer, Digital Processing of Speech Signals,

Prentice-Hall, Inc. Englewood Cliffs, NJ, p. 25 (1978)

.

41. Ibid. p. 26.

42. J. G. Graeme, Designing with Operational Amplifiers-Application

Alternatives, McGraw-Hill Book Co. , NY, p. 126 (1977)

.

43. J. Maxwell, Electronics, 50 , No. 4, 99 (1977)

.

44. Ref. [35] , p. 37.

45. Ibid p. 35.

46. E. 0. Brigham, The Fast Fourier Transform, Prentice-Hall Inc.,

Englewood Cliffs, NY, p. 148 (1974)

.

47. W. Waggener, EDN, 2^, No. 18, 80 (1978).

48. R. H. Lord, BYTE, 4, No. 2, 108 (1979).
aNBS>I14A (REV. s-781

U.S. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

SHEET

1. PUBLICATION OR REPORT NO.

NBSIR 80-1991

'-H"' [ \' ' •%? 'i

4. TITLE AND SUBTITLE

MATERIALS FOR FUEL CELLS

5 . Publication Date MARCH 1980

7. AUTHOR(S)

L. H. Bennett, C. K. Chiang, M. I. Cohen, A. L. Dragoo,

A. D. Franklin. A. J. McAlister

8. Performing Organ. Report No.

9. PERFORMING ORGANIZATION NAME AND ADDRESS "

'i ^

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF ^COMMERCE

WASHINGTON, DC 20234

11. Contract/Grant No.

12. SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS fSftreet, city, state, ZIP) 13. Type of Report & Period Covered

15. SUPPLEMENTARY NOTES

I I Document describes a computer program; SF-185, FIPS Software ^romary, is attached.

15. ABSTRACT (A 200-word or leaa factual aummary of moat migiiificant ir\formatlon. It documant inctudea a algnlflcant bibliography or

literatura aurvey, mention it here.) Transition metal-metalloid cpmpotinds involving V, Jto, Ta, and Nb with P, Si and N were examined as potential electrocatalyts in hot concentrated phosphoric acid» MoP, WP,

MoSi2 / M02N/M0N, and W2N/WN were found to be stable in the absence of cathodic potentials. The W2N/WN was non-conducting. M0SI2 and M02N/M0N were both inactive as catalysts and unstable under cathodic potentials. MoP and WP' corroded at about 0.2 volts

RHE. Only WP showed even mild activity as a hydrogen oxidation catalyst.

The electrochemical prof>erties of electrodes were studied in some detail,

and foxind to be independent Qf x near 0 . 7 , The electrocl^^ical behavior is very similar

to that of WC, with about the same activity and the seme indifference to CO poisoning.

Experiments are outlined for studying a possible "spillover" effect from Pt to

Mo2^_j^W^C and transition metal borides as supports, and for using a Raman spectroscopic

technique for characterizing the electrochemical interface of transition metal-metalloid

compound electrodes.

The preparation and characterization of high-densityi ( 99.6 to 99 . 8 % of theoretical)

Y203-doped CeOz ceramics is described. The material exhibits very sharp, single-phase,

x-ray diffraction peaks, indicating good compositional homogeneity. The influence of

various preparation methods on the sintering characteristics of powders of these materials is described. Chemical analytical techniques are given.

(Continued on attached sheet.)

17. KEY WORDS faix to twelre entriea; alphabetical order; capitalize only the Ural letter of the firat key word unleaa a proper name;

aeparaled by aemicolona)

Catalyst; cerium dioxide; electrocatalysis; fuel cells; impedance; solid electrolyte

tungston ceirbide.

18. AVAILABILITY (^Unlimited 19. SECURITY CLASS

(THIS REPORT)

21. NO. OF

PRINTED PAGES

1 1 For Official Distribution. Do Hot Release to NTIS UNCLASSIFIED 89

1 1 Order From Sup. of Doc., U.S. Government Printing Office, Washington, DC

20402, SD Stock No. SN003-003-

20. SECURITY CLASS

(THIS PAGE)

22. Price

[3 Order From National Technical Information Service (NTIS), Springfield, VA. 22161 UNCLASSIFIED

$8.00

USCOMM-DC
ABSTRACT (continued)

The electrical properties of inhomogeneities, presumably grainboundaries, in the Y2O3 -doped CeOz ceramics were very different for

materials prepared from powders produced by mixing the oxides from

those found when the powders were prepared by coprecipitation. A

higher resistance and a much higher capacitance were found in the

latter case. The temperature dependence of the inhomogeneity contribution to

the overall resistance in the Y2O3 -doped Ce02 is somewhat different

from that of the internal grain or crystal contribution, and since

they are in series, the larger resistance dominates. At low temperatures it is the crystal contribution. Hence an Arrhenius plot of the

dc conductivity exhibits a break, as often seen in the literature on

these materials. Only the portion at temperatures above the break,

on this view, represents the true properties of the substance itself.

In the course of 4 -probe dc conductivity measurements on some

Y2O3 -doped Ce02 materials a slow voltage transient was observed under

constant-current conditions. This transient appears to involve a solid state diffusion process.

Modifications to our automated electrochemical measurement system

are described that allow 1). an increase of the mass data storage

available to the system, so that the results of a number of repetitive

runs can be stored and later examined and compared; 2). an increase

in the maxim\om sensitivity when data are being acquired at high speed,

as in pulse mode operation; and 3 ) . the use of a complete driver

system in the software to coordinate all its aspects, and also the

use of Fast Fourier Transform techniques during pulse mode operations.

These modifications are described in detail.
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