NBSIR 77-1270

Materials For Fuel Cells

L. H. Bennett, M. I. Cohen, A. L. Dragoo, A. D. Franklin, A. J. McAlister and K. F. Young

Institute for Materials Research

National Bureau of Standards

Washington, D.C. 20234

Annual Report - January 1976 to December 1976

Issued May 1 977

Prepared for Division of Conservation Research and Technology

U.S. Energy Research and Development Administration

Washington, D.C. 20545

NBSIR 77-1270

MATERIALS FOR FUEL CELLS

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

A. J. McAlister and K. F. Young

Institute for Materials Research

National Bureau of Standards

Washington, D C. 20234

Annual Report - January 1976 to December 1976

Issued May 1 977

Prepared for Division of Conservation Research and Technology

U.S. Energy Research and Development Administration

Washington, D.C. 20545

U.S. DEPARTMENT OF COMMERCE, Juanita M. Kreps, Secretary

Dr. Sidney Harman, Under Secretary

Jordan J. Baruch. Assistant Secretary for Science and Technology

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Acting Director
.

.
1. Introduction

The National Bureau of Standards has undertaken a program of

research on materials for fuel cells. This program includes studies of

electrocatalysis in liquid-electrolyte cells and of the solid electrolyte in high-temperature cells. 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.

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

The Materials for Fuel Cells Program at NBS consists of three major

elements, which taken together reinforce each other in techniques and

concepts; these elements embrace:

i. electrocatalysis, especially oxygen reduction, on

non-metallic surfaces;

ii. electrocatalysis, especially hydrogen oxidation, on

non-noble metals and alloys; and

iii. degradation mechanisms involving solid oxygen-transporting

electrolytes. Items ii and iii received ERDA funding during

the past year and are the subject of this report.

1
2. Subtask Reports

2.1 Non-Noble-Metal Electrocatalysts

2.1.1 Electrocatalysis on Non-Noble Metals and Alloys

Introduction

i A limiting factor in full scale implementation of the low temperature acid fuel cell for electric power production is the scarcity and

expense of Pt, the current material of choice for fuel and oxygen

electrocatalysts. In the search for adequate non-precious substitutes

for Pt, the "refractory hard metals" - alloys of transition metals with

carbon, boron, nitrogen, etc. - offer a promising area of investigation,

for they are transition-metal-like: they have d-band vacancies, have

refractory properties, and display considerable chemical stability. A

number of these materials have in fact been found to be active for the

oxidation of hydrogen in acid electrolyte. A survey is presented in

Table 1.

Table 1 lists the corrosion potential, where available, in volts

relative to the standard hydrogen potential, with the compound. Most of

these studies have been carried out in h^SO^. Activity in H^PO^ has

been observed for WC and (MQq qWq

2 )C> the latter for the first time in

our laboratory, in the course of the present work. A number of these

compounds have no practical use in fuel cells, either because of low

corrosion potentials, or because their activity is very low. The latter

are indicated in Table 1 by the notation l.a.

Our overall task under this contract is the search for non-precious

substitutes for Pt as fuel el ectrocatalysts , and proceeds along three

general lines: 1) preparation and characterization of materials; 2)

screening of these materials for stability in hot phosphoric acid; and

2
Table 1. Materials for which H

2

oxidation activity has been observed

in acid electrolyte.

IV V VI VII VIII

3d MnPg(0.02) (1 .a.

)

FeP(0.08)

FeP

2 (0.30)

FePS

CoP(0.05)

CoP

3 (0.25)

CoPS(0.30)

CoAsS

CoAs

2 (0.20)

CoAs

3 (0.25)

Ni

P

2 (0. 30) ( 1 .a.

)

4d

Mo 0.8

W0.2C(0 ' 6)

MoS

2 (l.a.)

5d

WC (0.6)

WS

2 (l.a.)

Numbers in parenthesis are oxidation potential in volts relative to the

standard hydrogen potential. Low activity is devoted by (l.a.). Materials

are classified according to column (Roman numerials) and row in the Periodic

Table of the host transition metal.

Sources: the present work, in particular MOg gWg

2 C;

H. Bohm and F. A. Pohl , Wiss. Ber. AEG-Telefunken, 41_, 46 (1968); K. Mund,

G. Richter, and F. v. Sturm, Coll. Czech. Chem. Com., 439 (1971 ); K. Mund,

G. Richter, R. Schulte, and F. v. Sturm, Ber. der Bunsen-Gesell , 77_, 839 (1973);

G. Luft, K. Murid, G. Richter, R. Schulte, and F. v. Sturm, Siemen Forsch. u. Entwickl. Ber., _3, 177 (1974).

3
3) examination of acid-stable materials for electrocatalytic activity

through potentiodynamic and steady-state current-voltage measurements.

A summary of our work in these areas for the past year follows.

Acid Stability Measurements

To date, 55 alloys have been tested for stability in hot (82 °C),

concentrated (85%) phosphoric acid. These materials are listed in Table

2. Of the 55, 18, denoted by blocks in the table, have shown stability,

in that no gas was evolved, and no change in color of solution or alloy

occurred. Several other materials (not noted in the table) reacted only

slowly, and they will be investigated electrochemical ly, since they may

have small but useful electrical ranges of stability.

Voltammetric Studies

Thus far, the following materials have been examined by cyclic

voltammetry, in 1M H^PO^ (22 °C): l^C, W^, WC, Nk^C, (MOq qWq MoB,

TaN, TaB, TaC, Pt, W, Mo, and two stainless steels, PH 15-7 Mo and PH

17-7. Hydrogen oxidation was observed only on Pt, WC, and (MQq qWq

2 K.

The activity of WC was found to be a function of preparation method;

results for three WC samples are given in Table 3. The compound

(MOq qWq ) C , which has the WC structure, was shown, for the first time,

in the course of this work, to catalyze the oxidation of hydrogen in

acid electrolyte. Its performance is compared with that of our most

active WC sample in Figure 1. Both samples were porous, flooded electrodes consisting of powdered material (-325 mesh) bonded by dilute

addition of Teflon. At the level of activity of these carbides, these

electrodes should operate near unit effectiveness factor [1]. The

4
Table 2. Stability of some metals, alloys and "refractory hard metals

in hot phosphoric acid.

5
Table 3. Dependence of electrocatalytic activity of WC on the method

of carbide preparation.

H

2 oxidation current per unit

SAMPLE NO. FORM PREPARATION CAPACITANCE AT +0 .3 VOLTS (RHE)

1 foil W + CH.

4

2.1 X io

-3

A/F

2 powder W + C 3.0 X 10” 3 A/F

3 powder wo

3 + C 17.1 X io

-3

A/F

Preparation and Support:

1) Sheet, formed by exposing W sheet, 13ym thick, to a 5 % CH^-95%

mixture for 4 hours at 1500 °C. Supported on epoxy.

2) Commercially obtained powder, formed by direct reaction of W with

carbon. Supported on Ta sheet.

3) Powder, formed by heating WO3 and carbon powder for 2 hours at

1800 °C in H

2

. Supported on Ta sheet, and on Au with Teflon bonding.

Current is normalized to capacitance, evaluated from the sweep rate and

current at +0.3 volts with respect to a relative hydrogen electrode (RHE).

6
Figure 1. Cyclic voltammetric curves (RHE) for (Mo

Q g

W

Q 2

)C and WC: H

2

oxidation in 1 molar (M) phosphoric acid (22 °C)

7
current scales in Fig. 1 are identical but the current amplitudes do not

directly reflect relative activity. The (MOq qWq

2

)C electrode was

smaller in gross surface area, and thinner. On this basis, we estimate

(MOq gW

Q 2 ^ to have a maximum specific activity from 1.5 to 3 times

greater than that of the WC sample, perhaps more, since we were forced

to clean Fe grinding contaminant from the Mo carbide, a process which

favors loss of fines from the powder. Several materials displayed

potentially useful stability in acid, though no catalytic activity.

Voltammetric data for three materials - Ta, TaN, and TaB - are shown in

Fig. 2. The two stainless steels, PH 15-7 Mo and PH 17-7, showed wide

ranges of stability, from -.4 to +3.2 volts (relative to a hydrogen

electrode, RHE) but displayed noticeable capacitance in this range.

2.1.2 Microprocessor Controlled Electrochemical Measurements

Introduction

The proper design of an automated measurement system for

laboratory use depends strongly on the parameters under investigation in

« the course of the controlled experiment. Decisions as to instrument

capabilities, however, must allow flexibility or the end result might be

a single purpose device whose usefulness is zero after completion of its

initial objective. With this in mind, we examined the experimental

technique used in electrochemical investigations and decided on what we

feel is a satisfactory approach to automatic control and data taking.

It is useful at this point to review the technique normally used so

that basic criteria governing the design of the automated system will be

evi need

8
Figure 2. Cyclic vol tammetric curves ( RHE ) for Ta, TaN and TaB showing

absence of catalytic activity: 1 M phosphoric acid (22 °C) with

bubbled nitrogen.

9
The potentiostat is the basic instrument used for this class of

experiments. It consists (Fig. 3) of an error amplifier which measures

the difference between an input potential and the potential drop between

a reference electrode and the working (sample) electrode; this potential

drop is a function of the current through the cell, i.e., from the

counter electrode to the working electrode. The difference (amplified)

is fed to a power amplifier which alters the cell current until the

difference (error) is zero. The input potential to the potentiostat may

be dc or ac voltages. The ac wave shape may be whatever the operator

desires with the frequency range (typically dc-10 Hz) governed only by

the characteristic impedances of the overall system [2].

The most common experimental arrangement consists of a motorized

potentiometer used to generate an input potential (triangular waveshape), which is fed to the potentiostat. With the addition of an X-Y

recorder and the cell, the equipment is complete. Going beyond this

experimental minimum, we can find varying arrangements of oscillators,

stepping-motor-driven voltage sources, etc., used to generate the input

potential. These allow the use of dc, linear sweeps (ramps), triangular

sweeps, stepped sweeps, sine wave input, square wave input, and pulse

inputs. Compensation for changes in potentials due to the IR drop

across the cell's internal impedance is sometimes added. We may also

find varying arrangements for output recording: two-pen recorders,

logarithmic recorders, digital voltameters, etc [2]. With all this in

mind, system design can proceed.

Basic Design Criteria

Experimentally, the system must be able to generate dc and ac

potentials of variable wave shape. It must be capable of both analog

10
POWER

COUNTER

ELECTRODE

REFERENCE

ELECTRODE

Figure 3. Simplified block diagram of a potentiostat . R

$

is the effective

series impedance produced by the counter electrode and the cell

electrolyte R

c is the effective impedance (due to the flow of

current through the cell) between the reference and working electrodes,

which develops the measured potential.
and digital outputs of data. Control of the format of both input and

output must be simple and fast. Once started, the apparatus must be

able to operate unattended and must be able to terminate the experiment

according to criteria established by the operator. It must be capable

of communication with a computer for data analysis or else be capable of

performing the analysis itself.

All of these functions can be performed by a system consisting of

appropriate digital-to-analog and analog-to-digi tal converters combined

with a digital computer. Although use of a central computer with a

communication line to the experiment was considered, we decided to use

a dedicated processor as a large computer would be under-utilized in the

control functions of our system. Similar considerations of utilization

also led to elimination of standard minicomputers.

The choice thus narrows to the usage of the desk calculator systems

or of the new microprocessors. Cost considerations mandated the microprocessor approach which in retrospect appears to be a wise one. Since

these devices are relatively new, a survey of their characteristics is

desirable for understanding their usage in this system.

The Microprocessor

The microprocessor [3] is a single-chip large-scale integrated

circuit (IC) which contains all the basic functions of a computer's

central processing unit (Fig. 4). The differences between the two

result mainly from in a difference in word size, with the microprocessor

having a word of 16 bits or less. The smaller word size means a smaller

instruction repertoire and smaller directly addressed memory capability.

The most commonly available units at reasonable cost use an 8-bit word.

12
ADDRESS

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r* co < ®

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o' oo < »- £ O < « N CM < N

CO CO <"^ 

Ifl If) CM

CO

O

CO

Q

co

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«- CM

CO

Q

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O'

CM

Q

00 CO

CM

Q

P* CO N a

CO Cm CM O

(/> D

CD<H<G

co r*co o ID CM

M

a IO r^to^

CO CO CO

’r cm ®; -e- * * o O O o

-I -I O O

c/1 o ui a: 3 t- O z

uj o

OC o Is“ H

“i co

GCI

ICO UJ CO IUJ — K I Q > <5> UJ

CC OC O§

UJ5<> i o

S-u

COu

Q.

>o

03Co

CON1 •i

c

COCD

S-o

cOc

s_a»

<u -CoE

cO

r- s_

Ol oo

r* CO

mz

UJZ

•1

“O Z LL n U u o

(j «•

«l o > > CO

aj

s_3

OO sna loaiNOO

processor

IC. H

indicates

the

uppermost 8

bits

ofa

16-bit

register.

L

is

the

lower 8

bits.
These 8-bit processors fall roughly into 2 classes. One class has

both control and arithmetic functions of a relatively high order (e.g.,

6800, 8080, Z-80, etc., processors), while the other class has restricted

arithmetic functions and is better suited for control purposes only (F8, SC/MP, etc.). Of the first class, the 6800 type is more readily

utilized where input-output functions are of prime importance. As

outlined under design criteria, there is some need for arithmetic

capability. Thus, the 6800 type of microprocessor was selected for our

appl ication.

All of these units utilize what is known as bus-oriented architecture. That is, all signals of a common type are run via a common set

of transmission lines to all devices requiring them. Each device has

decoders so that it responds only to those signals meant for it. As

indicated in Fig. 4 there are, in general, 3 buses: the address bus

(unidirectional), the control bus (unidirectional) and the data bus

(bidirectional )

When assembled with appropriate other devices: memory IC's,

peripherals, logic IC's, clock, etc. (Fig. 5), a microcomputer results

1 fi [4]. With the 6800 this arrangement has a capability of 2 memory

locations and a minimum instruction time of 2 ys. Data are received

from peripherals by a memory read operation, and data are sent to peripherals by a memory write operation. Communication with a large

computer or a very remote peripheral is via the modem.

Microcomputer Configuration

As noted above, we are using a 6800 microprocessor , with its

associated support circuits (clock, gating circuits, etc.), as the basic

element of our control system [5]. Along with this we have approximately

14
6800 MICROCOMPUTER FAMILY

BLOCK DIAGRAM

CONTROL ADDRESS DATA

BUS BUS BUS

Figure 5. Block diagram of the typical organization of a microcomputer.

15
4,000 bytes (1 byte = one 8-bit word) of ROM (read only memory) storage.

We also have about 12,000 bytes of RAM (random-access memory = read and

write memory) storage. An interface is provided which allows communication with either the operator or an external computer. We use a teletype

at present. Interface circuitry is also provided for use with a magnetic

type cassette recorder.

Two other interfaces are provided. One communicates with a software

programmable timer, while the other communicates with the conversion (or

control) printed circuit board. The overall configuration is shown in

Fig. 6. Each of the elements of the system will be discussed in detail

in subsequent sections.

Main Printed Circuit Board

The central processing unit (CPU) and the input/output interfaces

are contained on one (15 x 25 cm) board. The CPU consists of: the 6800

processor, a 1-MHz 2-phase clock, 1000 bytes of ROM, 640 bytes of RAM,

and signal gating circuitry [6]. The three buses (data, address and

control) are brought out through the 86-pin interconnection plug. There

are three interfaces built on the board. One enables communication with

the terminal at either 10 or 30 characters per second; communication

with a central computer is, at present, through this terminal. The

other two are reserved for future additions. One will handle a modem

for direct communication with the central computer, while the other will

provide high speed parallel data output [6].

An auxiliary (10 x 15 cm) board contains the power-on-reset circuit [6]

and the cassette tape recorder interface. This board will be removed in

the near future when a terminal with built-in cassette recorders arrives.

An overall view of the input/output arrangement is shown in Fig. 7.

16
Figure 6. Black diagram of overall configuration of the microprocessor

controlled electrochemical system.

17
I=!nterface circuitry

Figure 7. Input/output configuration of the system.

18
Memory Boards

There are three (15 x 25 cm) printed circuit boards, each containing

4,000 bytes of RAM. They are operable by simply inserting them into the

86-pin interconnection frame containing the buses, power, etc. At

present, one is used for program storage which is loaded via the input/output

arrangement. The others are used for data storage during the experiment.

A fourth memory board is designed and under construction. It will

contain up to 8,000 bytes of reprogrammable ROM (at present we have 3000

bytes). There also will be an on-board programmable circuit so that a new

program may be inserted when desired. Completion of this board will

prevent loss of program on power failure (or turn-off) but will allow

us the flexibility to change our operating system to accommodate needs.

Conversion (Control) Board

This (15 x 25 cm) board (Fig. 8) contains a programmable interval

timer and interface, 2 digital-to-analog converters and interfaces, an

analog-to-digi tal converter with its interface and a multiplexing

circuit [7].

The interval timer [6] has a timing range of 1 ys to 42.6 min.

The CPU sets the desired interval between points and starts the timer.

When the period has elapsed, the timer signals the CPU which then reads

the data via the analog-to-digital converter and checks to see if another

point is needed. If so, the CPU stores, then outputs the data, resets

the timer and repeats the process.

The two digital-to-analog converters (DAC) are identical 10-bit

units. They have an output range of -5 to +5 volts and have a conversion

time of about 1 ys. However, the CPU time to output a point is such

19
Figure 8. Configuration of the data conversion portion of the system.

Direction of data flow is shown by the arrows. Alternative methods

are indicated by dashed lines.

20
that the maximum frequency of a full scale triangular wave out of the

converter is 25 Hz. To allow for this, a multiplexer has been added,

and when desired the digi tal -to-analog output DAC #1 may be fed to a

voltage controlled oscillator (max. freq. 3 MHz). Digital -to-analog

converter #2 is used when a plot of log of the current vs. input potential

is desired.

Control of the output wave shape is achieved by suitable programming.

The most common output is in the form of a stepped triangular scan with

the residence time at each point preselected by the operator. This

enables determination of Tafel slopes by utilizing long step-delay

periods and allows dynamic scans by using short periods. Other wave

shapes can be obtained by entering the appropriate program.

The analog-to-digital converter (ADC) has an input range of -5 to

+5 volts and a conversion frequency of approximately 40 kHz. It is a tracking type of converter [8], which means it is on all the time and rapidly

tracks the input, except when halted by the computer to output data.

The input voltage may be either direct or through a voltage divider,

which effectively doubles the input range but reduces the resolution.

Operation of the System

Operation of the system will be described by referring to Fig. 9,

which is a typical printout of a run, and to Fig. 8.

At initiation of the program the CPU prints out an identifier

(DATA-TAKING-RAMP) and then requests an operator input (DELAY RATE).

Upon receiving this, the CPU sets the basic timing frequency of the

interval timer. The CPU then requests the number of basic counts (DELAY

COUNT) and sets the interval timer's counter. It then requests the

number of times the operator wishes to go up and down (SWEEPS), the step

21
*G

DATA- TAKING RAMP

DELAY RATE

>0

DELAY COUNT

>30

# SWEEPS

> 1 STEP MVLT

>100

HI/LOW MVLTS

>1000

>-310

10 SEC PEP. COUNT

30 COUNTS

MATERIAL:

ELECTROLYTE:

REFERENCE:

2ND AND THIRD

COLUMNS:

IS 1000 - 7 0 - 70

900 - 70 - 70

800 - 90 - 90

700 - 50 - 50

600 - 90 - 90

500 - 90 - 90

40 0 - 1 1 0 - 1 1 0 300 - 90 - 90

200 - 70 - 70

100 - 70 - 70

- 1 10 - 1 1 0 100 - 70 - 70

« 200 - 90 - 90

300 - 1650 - 1650

IS 300 - 1 330 - 1 330

200 - 1 30 - 130

100 - 1 1 0 - 1 10

- 90 - 90

100 - 90 - 90

200 - 90 - 90

300 - 70 - 70

400 - 70 - 70

500 - 70 - 70

600 - 90 - 90

700 - 70 - 70

800 - 30 - 30

900 - 50 - 50

000 - 90 - 90

Figure 9. Sample print-out from a measurement using

controlled electrochemical system.

M IN/SCANNED POINT

PLATINUM

1 M PHOSPHORIC ACID

CALOMEL

5000 MV = IMA

the microprocessor

22
size and the limits (HI/LOW). After receiving this information, the CPU

loads the parameters in the program and then signals that it is waiting

for a start (!). When ready the operator types an S and the experiment

begins

.

The CPU loads the upper limit value into DAC #1, outputs the analog

voltage to the input of the potentiostat and starts the timer. When the

interval is over, the timer signals the CPU which reads the ADC, checks

the point against the lower limit, substracts the step size from the

upper limit value and then outputs the next point. If the interval is

long enough (>1.5 seconds), the CPU outputs the data. If too short, the

CPU stores the data for subsequent output. The data format used is:

the first column is the potential input to potentiostat, the second

column is the output current of the potentiostat, and the third column

is either the log of the current, or when that is not required, a repeat

of column 2. When the lower limit is reached, the CPU asks for another

command to sweep up.

In addition to the printout, the X-Y recorder normally reads the

outputs of the potentiostat (voltage and current). If desired, these

may be digitally plotted by the CPU, allowing plots of the log of the

current vs. input potential.

Future Plans (Hardware)

As indicated earlier, provision has been made for addition of a

modem and high speed parallel output. These are to be implemented very

soon. Also, a programmable gain amplifier to increase the resolution of

the ADC is being designed. It would be preferable to increase the

resolution of the ADC and DAC's by purchase of higher resolution devices

23
(12 bit or greater), and if funds allow, this is the route we will take.

Initial design of interfaces for other types of measurements (mainly ac)

is underway. Memory enlargement is also desirable, both in form of more

RAM and by the addition of mass storage such as floppy-disc memory. As

mentioned earlier, the PROM board must be finished, and if possible, the

additional PROM's to fill it to its capabilities must be purchased. The

addition of a good digital plotter is also suggested.

Present plans also indicate that (with proper shielding) it would

be possible to mount the entire existing unit within the potentiostat

housing, and this may well be implemented.

Future Plans (Software)

Of first priority is the completion of the necessary programs for

successful operation of the existing system [9]. The addition of an

assembler and a higher-level programming language, such as BASIC or

FORTRAN, either resident in the system or time shared on a large computer is needed. Although machine-language programming is more economical

in terms of memory space needed, it is more wasteful by far of the

programmer's time.

In view of the need for operation of the system by more than one

individual; a larger body of non-volatile (PROM) programs in the unit is

desirable. Such a mode of operation requires that these resident these

are accessible by simple commands and are forgiving of improperly

operated switches, etc.

Conclusion

A system has been built that, for a relatively low cost, allows

electrochemical experiments to be performed under computer control. A

limited amount of data analysis can be performed in situ , and the results

24
can be conmunicated to a central facility for complete analysis. The

system provides for keyboard operational control and outputs both digital

data and analog plots.

2.2 High Temperature Solid Electrolytes

Introduction

Oxide solid solutions having a defective fluorite structure and

based on Zr0

2

, Ce02 and Th0

2

, doped with CaO, Y

2

0

3 or the tri valent 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 dopant may be added depending

upon the oxide system.

Zr0

2

-based solid solutions have been used as electrolytes for hightemperature electromotive force measurements since the mid-1930's [10,11],

and Th0

2

-based electrolytes, since 1957 [12]. Zr0

2

-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 [13,14] on Ce0

2

:Ln

2

0

3

, where Ln represents

a variety of lanthanide ion or Y 3+

, have revealed oxygen-ion conductivities

essentially equivalent to that of Zr0

2

:Ca0 but at temperatures about 200 °C

lower. Thus, fuel cells using a Ce0

2

~based electrolyte might operate at

about 800 °C as compared to one with a Zr0

2

~based electrolyte which

would require 1000 °C. However, Ce0

2 ~based electrolytes begin to

show appreciable electronic conduction at higher oxygen partial pressures

than do the Zr0

2 -electrolytes ; for example, one-half of the electrical

conductivity is electronic in (Ce0

2

) 0 95 ^2 ^ 3)0 05

at P0

= atm

25
(%10~^ Pa) for t = 800 °C, whereas for ZK^CaO, Pq = 2.5xl0~ 3 ^ atm

2.5xl0'

36

Pa) for t = 1000 °C [15].

At the high oxygen vacancy concentrations present in these electrolytes, the oxygen vacancies are probably nearly all associated with the

dopant cations [16]. 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 vacancies no longer exist. In actual consequence, 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 ZrC^-

based solid solutions [17-24]. Coherent structures in CeO^-based solid

solutions have not been identified.

Carter and Roth [21] found that the increase in resistance of

calcia stabilized zirconia (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

26
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

Ca ions in CSZ [25], 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

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

perhaps 1000 times larger at 1800 °C than 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 dopant ions along grain boundaries 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 study of high-temperature solid electrolytes was initiated

this year to investigate the possible aging of CeC^-based electrolyte

materials. Experiments were designed to study the aging of these materials

both by ordering of the vacancies and by migration of the dopant cations.

The principal research tool to be employed in the study will be the

measurement of ac impedance, which yields values for the bulk and grain

boundary conductances. Specimens will be annealed for a variety of

27
times and conditions. These measurements will be combined with structural studies and measurements of other physical parameters.

Design of the Experiments

With the help of Dr. John Mandel , experiments were designed to

examine the rate of aging of solid electrolyte materials for various

conditions of composition, temperature, oxygen partial pressure and

current density. For each condition at least one replicate experiment

will be performed to establish the uncontrolled variation in the experiments. With the exception of oxygen partial pressure, the range of the

sampling for each condition will include the probable condition under

which a ceramic electrolyte of such material would be used. The conditions to be sampled are summarized in Tables 4 and 5. The materials

selected for study are hot-pressed Ce0

2

:Y

2 03

and CeQ

2

:Gd

2

0

3

solid

solutions, with the Y

2 0

2

and Gd

2

0

3

in the range of 5 to 18 m/o.

Table 4 presents the conditions selected for the ordering experiment. In this experiment specimens will be extracted after the prescribed

annealing times, electrodes then will be applied to the specimens and ac

impedance measurements will be made. If ordering, for example, the

formation of coherent structures, occurs as a result of the annealing,

the bulk resistance is expected to increase; minor changes in the geometric capacitance may also occur. The ordering experiment will be

performed under zero current conditions.

Table 5 presents the conditions for the migration experiment. In

this experiment a DC electric field will be imposed on one set of specimens

so that a constant current through the specimens will be maintained; a

second set of specimens will be maintained under a field-free (zero

28
Table 4. Proposed ordering experiment.

Specimens: CeC^ + 10, 20 or 30 atom % YO-j ^ or GdO-j

40 specimens of each composition

-4

Oxygen partial pressure: ^10 atm

Temperature: 500, 700, 900, 1100°C

Sampling times: 1, 2, 4, 8, 16, and 32 weeks

Procedure: Two specimens of each composition removed from

each furnace at the end of each annealing period;

for electrical measurements; specimens not returned

to furnaces after electrical measurements are made.

Expected results: If ordering of the oxygen vacancies occurs,

a decrease in the ionic conductivity with

increased ordering is expected. Specimens

with significant changes in electrical

properties will be examined by other techniques.

29
Table 5. Proposed migration experiment.

Specimens: Ce0

2 + 10 or 20 atom % YO-j 5 or GdO-j

6 specimens of each composition

Oxygen partial pressure: ^0.2 atm

Temperature: Max. temperature allowed by stability of Pt

electrodes, perhaps %900°C.

2 2

Current density: ^1 A/cm and 0 A/cm (blank).

Procedure: Periodic electrical measurements will be made on the specimens without removing the specimens

from the furnace.

Expected results: Migration of the dopant cation (Y or Gd)

as a small secondary ionic current is

expected to deplete the dopant cations at

the cathode and to enhance the concentration at the anode, thus changing the

electrical characteristics of the specimen.

30
current) condition. The resistance of both sets of specimens will be

measured during the anneal without removal of the specimens from the

furnace.

The experiments as designed impose a number of requirements.

First, a large number of specimens is required: between 50 and 60 at

each of the compositions, including "blanks" which will not be annealed.

Second, annealing of 30 specimens at a single temperature in the ordering

experiment places two stringent constraints on the furnace: a large

isothermal zone, and the ability to insert and remove specimens with

little perturbation of the specimens in the furnace. Two transparent

furnaces were obtained for the ordering anneals at 500 and 700 °C. Two

furnaces were built for the anneals at 900 and 1100 °C; these are described in the next section.

To minimize complications in the measured ac impedance due to grain

boundaries and pores in the specimens, and to avoid changes in the

electrical characteristics of specimens due to further densification

during the anneals, hot-pressing was selected as the technique for

preparing specimens of nearly full density. Sintered specimens also

have been prepared to establish the hot-pressing conditions and to

obtain some preliminary samples for ac-impedance measurements. Details

are given in the section on electrolyte materials.

High Temperature Facility

A laboratory was remodeled to provide the high temperature facility

for the ordering and the migration experiments. To allow working space

and ease of access to the furnaces, a central island was installed which

31
consists of modular, counter-height relay racks to hold the controllers

and power supplies for the furnaces. Connection of the controllers and

power supplies is now in progress; the countertop has yet to be delivered

and installed. The five furnaces required for the experiment will be

emplaced on the island when it is completed.

Transparent furnaces were selected for the low- temperature ordering

anneals because these furnaces have a very flat temperature profile

through the hot-zone and because they permit observation of the specimens.

These furnaces can be operated at temperatures up to 1000 °C.

For the high-temperature ordering anneals conventional resistance

furnaces with three independent heat zones were constructed. A schematic

of these furnaces is shown in Fig. 10. The main heater and the two end

heaters are independently regulated. These furnaces can be operated up

to 1200 °C. For the migration experiment a Pt-Rh wound multiple-zone

tube furnace is available.

The system for the distribution of power to a three-zone furnace

was suggested by Mr. Harry Parker; a schematic of the system is shown in

Fig. 11. The output of a thermocouple in the main-heater zone is fed to

a controller which regulates the total power to the furnace. The output

of the power supply regulated by the controller energizes the primary of

a transformer. Power for the main heater is obtained from the full

secondary of the transformer; power for the two end heaters is taken

from the two halves of the secondary via variable autotransformers.

Electrolyte Materials

a. Preparation and Characterization

Initially specimens were prepared by mechanically mixing the oxide

powders; forming the powder into disks or billets; isostatical ly pressing

32
END

HEATER

MAIN

HEATER

END

HEATER

o

£ S

CD m

LU Z

o

O

<

§3

<

33

Figure

10.

Three-zone

resistance

furnace.
FURNACE

QC

C£ UJ cc

UJ I— UJ

S— < H— < UJ <

UJ UJ

T*a z< o az

UJ UJ

34

Figure

11.

Power

distribution

system

for

the

three-zone

resistance

furnace.
to achieve a high, uniform green density; and sintering or hot-pressing.

Mechanical mixing of the oxide powders was selected on the basis that it

would minimize the introduction of impurities during the preparation of

the specimens.

Densities and estimated porosities of sintered and hot-pressed

specimens are given in Table 6 along with composition, calcination

conditions for the powder, and sintering or hot-pressing conditions.

Two densities are given: the bulk density for which the relevant volume

includes both the open and closed pores, and the "intrinsic" density for

which the volume includes only the closed pores. The bulk density is

obtained from the mass of the specimen and the geometric volume, if the

specimen has a regular shape, or from a buoyancy measurement in mercury.

The intrinsic density is obtained from a buoyancy measurement using a

fluorocarbon liquid which penetrates the open pores of the specimen.

The maximum bulk density obtained by sintering the mechanically mixed

oxide powders was 91% of theoretical; and by hot-pressing, 93 to 94% of

theoretical

.

Calcination of the mixed powders to homogenize them prior to

sintering reduced the sinterabi 1 i ty of the powder.

Densities of about 93% of theoretical have been reported as routine

for sintered CeO^Y^ specimens [26]. Also, an X-ray diffraction study

of the hot-pressed material showed the presence of the Ce0

2 and the Y 2

0

3

phases, and not solely the solid-solution phase. Consequently, it appeared

that a chemical preparation of the mixed oxide powder was required to

achieve a higher density and to obtain a homogeneous solid solution.

35
Table

6.

Density

and

porosity

of

sintered

and

hot-pressed

specimens.

>-> -o +-> • <D I—

CO CO

O o o >

r— S_ O O cfS CL

r^. 00 • • r—

00 LO

CM CO CO

lt> r-~. CO LO

LD

>> CO

£C I

CL> to

O LO LO CO C\J CL O > r— • 1— . . . O i- CO CO CO CO o

CL

«=J" ro CO CO o o

LD o>-o

<o

O ro

•<- >> • CO +-> Sc: -i- o

•i— to a)

t- c -c

+-> a> -t-> c -o

coco co

jo

u

(XI ,

CO CM CO +1 +i 00 . • r— C\J CO «3" cn CO cn 03 oco

to >> • +-> sj*: •r- O CO i

co CL) +1 o C JC CM

CO CD -*-> r-'~ 00 a

CO

+1 COco

JO o

CO CO .— cn LO • • cti r^. oo cej co

o> 03

s_ CD o C CO CO JC Jf

•rC S_ 1— «vl- LD CM iLD CM r— CM *cf CM «3" LD •O" CD

CD woe r—~ i

r— i— r— i

CO to c CO -i— CL

•r” O) +-> s s- S- -1- Q) CL-O 00

J-> 1 C CD o o o o o o o o o o o o OO o o O CM

C +-> o O o o o o o o o o o o o o LD CM CD CM ID

•r— o o LD "Lj- LD LO CO LO LD CO LO LD LD LD LD *3" LD CO oo JC 1— 1

1— r— r— r— i— ii— i

i— i

r— r— i r— i

C CO CO o c t- CD 03 • 1- o

4-> -r- JZ •—

fO +J C •!—

•>- -a u c C_) o O

O r- O o o O o (O t_) LD ID CD r i

r—

CM

aj aj

03

cz c: o o c c O o o o o CO CM t

r—

r—

aO LO

co >- E O \ O

Cl cm 4-> E O ro o a> CD CO oCOo oo03 oCOor-v o o o o CM CM 1— CO O O o o

CO CO 03 oo03

<T3 JO o

36

Theoretical

densities

(g/cm 3

):

10

a/o

Y0

1#5 ,

6.98;

20

a/o

Y015,

6.74;

Based

on

the

theoretical

density

of

the

solid

solution,

see

footnote

a.

Based

on

the

theoretical

density

of

the

fully

dense

mixture,

6.92

g/cm 3
Micrographic examination. Fig. 12 and 13, also showed a two-phase

material. Coarse grains (gray in Fig. 12) are dispersed in a white,

fine-grain matrix. The black areas are pullouts from the polishing and

pores. In Fig. 13, etching of the specimen in a dilute mixture of

nitric acid and hydrofluoric acid reveals the fine-grain structure of

the matrix and the coarse grains of the second phase. The matrix is

presumably ceria, and the coarse grains, yttria. A scanning electron

microscopy (SEM) was used to examine the pullouts and pores in one

specimen. Fig. 14 is a typical SEM micrograph of one of the pullouts on

the surface of a hot-pressed specimen. Pores are evident in the bottoms

of the pullouts.

Before embarking on a, perhaps lengthy, chemical preparation of the

CeC^^Og powder, chemical analyses of the possible starting materials

was obtained as work on Ce 02 :Ta 20 5 at NBS had found sodium as a major

contaminant in the CeC^ used in that work. The mass fractions of

impurities in some mixed powders, in a cerous nitrate and a cerous

carbonate are given in Table 7.

The first chemical technique tried in this work used 6N sulfuric

acid containing some hydrogen peroxide to effect the dissolution of the

CeC^. This technique required several subsequent dissolutions in nitric

acid, followed each time by precipitation with ammonium hydroxide, to

remove the sulfate. In addition, the initial dissolution of the CeC^ by

the sulfuric acid was very slow. More recently, the use of concentrated

nitric acid containing a few drops of hydrofluoric acid was suggested to

us as a solvent for CeC^ [27]; this reagent has been found to work

successfully for our CeC^ powders. To reduce the contamination in the

37
Figure 12. Optical micrograph (320X) of hot-pressed Ce0 2 :Y 2 0 3

. Oxide

powders were mechanically mixed. A duplex structure of

fine, "white" grains and coarse, "gray" grains is present;

black regions are pullouts produced by polishing and pores.

Figure 13. Optical micrograph (800X) of hot-pressed Ce0 2 :Y 2 0 3 ; oxide

powders were mechanically mixed prior to hot-pressing. The

specimen was etched in dilute nitric acid/hydrofluoric acid

to reveal the grain boundaries.
Table

7.

Impurities

in

CeC^^O^,

Ce2(N0

3)3

and

Ce2(C0

3)3

powders.

•r-l LO

uo I v

LO LO V VLO LO V V

an r-. u_| CM

co cn

«3-

i

co CM

'3

-

LO CO

LO O

r- oco

cn

51

CO LO

• • • • • co CO <3- CO o

*3- LO LO

• • • • o cr> C\J CM CO

r— p— CM 1CO

lo cn

LO

lo r>. O co

cnCD O r-* CO LO E CO •3" CM CO r— o o o o (T\

PL • • • • •

rt3 o o o o LO

essed in

a> >, l. >> s- cn -a f— ai -o r— CL c

•r— r— -a cd oo r— 1 *r- X (O 2 S- CD « +j T3 S- o o O ra -O o o c

•J— CL a.v- •r- JC t- +-> c a; x c s- sa) cn ro x> s- o ra co cn o

f— cn -c CD CL -c i_ e

a. • o X u ai >, E cn cu •r- >,cn 0IT3O fl

(T3 cn e E i— cn E 2 c

CO i r- • 1 O T3 -r1 00 i co cn i a ai e fO ra co s_ m (j OO no J D cn cn o a> -o O -r- O T3 O 1—

CM-O CD CNI E E rsl CD i— 03 on cn >- 2 X >- a) o >- X 1— O CD

• • o •<- •• JC i- •• -r- O C <_> <M Q. E N U 4- N E4- IO ^—«* >*—

O o O CM CM CD ai ai 0) a> O C_) o o o

-a

a>

s-«3CL

OJ

s-CL

>>

ItJ u

s-0)o

c_>

<c

39
chemical preparation, the glass labware has been replaced by polyethylene

items. By dissolving the high-purity oxides in nitric acid, coprecipitating

from the mixed solutions and avoiding sources of sodium impurity, homogeneous mixed oxide powders with purities comparable to that of the

starting material are to be expected.

b. Annealing Experiment

Three of the hot-pressed specimens were annealed at 1400, 1500 and

1550 °C, respectively. The total annealing times were 100 hours at

1400, 48 hours at 1500 and 120 hours at 1550 °C. The two-phase appearance

was not distinguishable after 48 hours at 1400 °C; however, this may

have been due in part to the roughening of the surface which occurred

during the anneal. After 100 hours at 1400 °C, grains or crystallites

resembling clusters of grapes had begun to develop. These grape-like

clusters were even more evident after annealing at 1500 °C and at 1550 °C,

see Fig. 15. The size of the clusters and their number increased with

annealing temperature and time. These clusters may be the solid-solution

phase. The three specimens have been repolished and will be examined by

x-ray diffraction and by optical and electron microscopy.

Impedance Measurements

Our impedance measuring techniques have been improved markedly in

three important aspects. We now have a furnace for use in making

electrical measurements that has extended high temperature capability

and improved gaseous environmental control. Our network analyzer system

has extended the frequency range of our measurements and improved our

resolution. We have developed sputtered Pt electrodes that exhibit high

temperature stability.

40
Figure 14. SEM micrograph (5600X) of pullouts and pores in hot-pressed

Ce0 2:

Y

2 0 3

.

Figure 15. Optical micrograph (800X) of hot-pressed Ce0 2 :Y 2 0 3 after an anneal of 120 hr. at 1550 °C in air. The specimen originally

had a duplex structure of ceria and yttria grains. The

specimen shows growth of new grains (grape-like clusters).
We modified an existing furnace so that electrical impedance

measurements could be made with our network analyzer. This furnace was

then operated satisfactorily at temperatures up to 800 °C, and it also

has the potential of attaining somewhat higher temperatures. The gaseous

environment of the specimen chamber of this furnace can be controlled

either in a uniform way or by applying a differential partial pressure

of 0^ across the specimen.

Using the network analyzer system provides an extended frequency

range as compared to the time honored technique of using an ac-impedance

bridge. The network analyzer sacrifices some resolution in the mid

range of frequencies (1 kHz to 20 kHz), however, the extension of the

low frequency limit to 10 Hz and the high frequency limit to 13 MHz

makes it far more useful in sorting out multiple relaxation processes

and other related phenomena. We presently operate the network analyzer

system in a semiautomatic mode and data collection is about 5 to 10

times faster than using an ac bridge. Our system also offers the

potential of even more rapid automatic operation with the addition of a

programmable calculator and suitable interfacing equipment.

When a network analyzer is operated in an insertion loss mode, as

we use it, then its operation bears some resemblance to an ac-bridge

measurement technique. Fig. 16 shows schematically the circuit we use

to measure an unknown complex impedance Z*. It was drawn in such a way

as to show its resemblance to an ac bridge. The most fundamental

difference is that a bridge is adjusted to a null output for each desired

frequency, and a network analyzer just compares V^* with Vg* and gives

the result directly. From the circuit shown in Fig. 16 Z* can be

written as:

42
Z* = 100

F * _A__

E * l B

-1

( 1 )

where E^* and Eg* are complex electromotive forces (voltages). Since

the voltages vary sinusoidally then they can be expressed as:

E * = E e^1l

A

b

A

e

f * = f

p-i ) > l B

r'B e

where u = frequency, t = time and <\> = phase shift between E^* and Eg*.

( 2 )

The unknown Z* then becomes

Z* = 100

"E n

!a J<t> _

E

. B 1 (3)

Figure 16. Schematic diagram of the measuring circuit for a network analyzer.

43
Second, the surface of the specimens was much too rough and(or) porous

for the film to remain continuous. As our preparation of specimens

improved and we achieved higher density, then the ground and polished surfaces

were capable of being electroded with sputtered Pt that would withstand

temperatures of at least 800 °C in

Impedance Data Analysis

We developed computer programs for data analysis using a Hewlett

Packard 9830A progammable desktop calculator with a plotter attachment.

For each measurement frequency the network analyzer displays the voltage

ratio E./E d (in dB) and the phase shift <t> (in degrees) on a pair of

A D L.E.D. readouts. A set of frequencies is then scanned and the data

pairs are recorded in a logbook for later manual key board entry on the

HP 9830A. When entered, the data is formatted, and then it is stored on

magnetic tape cassettes. When the data are recalled for analysis, the

impedance, Z*, is calculated from eq. (3). The admittance, Y*, is then

easily calculated as the reciprocal of the impedance, Z* - 1/Y*.

Once these fundamental properties are obtained, then various

graphical displays can be easily generated on the plotter peripheral of

the HP 9830A . The complex impedance, Z*, or admittance Y* can be plotted

with either or both variables raised to any arbitrary power. Either the

real or imaginary parts of these complex quantities may be plotted

versus frequency or log (frequency) and again these may be raised to any

arbitrary power. This is most useful when attempting to sort out

diffusional properties from relaxational ones. Diffusion-controlled

electrical processes involve time raised to the 1/2 power, whereas.

44
single relaxation phenomena involve integer powers. When there are

phenomena with distributions of relaxation times then the appropriate

powers of dependence lie somewhere in between 1/2 and 1.

We use a linear least-square fitting procedure with 2 parameters in

several ways. If the imaginary part of the impedance, Y*, is plotted

against frequncy, w, then at high frequency a linear dependence would

indicate a capacitive effect is dominant. Fitting this linear portion

would give the value of the capacitance as the slope because the admittance, Y*, for a pure capacitor is equal to wC. If the complex impedance

plot, Y*, has a linear region with a slope of unity, then it can be

easily found by this procedure. This could indicate the presence of a

diffusional impedance mechanism and can easily be verified by separately

plotting the real and imaginary parts versus to 2

. A linear dependence

here would confirm this, and allow an estimate of the diffusional impedance constant, o, by fitting the slope of the linear portion.

In representing relaxational processes the model equivalent circuits

are composed of resistors and capacitors. For the case of a single

relaxation mechanism there is just one capacitor and one resistor, and

for the case of a distribution of relaxation processes there is some

combination of these circuit elements. The complex impedance of a

single resistor and capacitor connected in parallel gives rise to a

circular arc that passes through the origin with its center lying on the

real axis. Multiple relaxation processes add up in such a way that

either the center then falls below the real axis, or if the time constants

of the processes are sufficiently different then the arcs become a

series of "bumps" connected together. A circular-arc least-squares

fitting procedure was developed in order to extract the parameters for

45
an equivalent circuit. A single well-defined arc can be characterized

directly when it appears as the most prominent feature in the data.

However, in certain instances the data can be "corrected" by subtracting

frequency dependent capacitance effects as shown below in the next

section.

Impedance Data Interpretation

Impedance measurements as a function of frequency and temperature,

and in the presence of atmosphere composed of tank ^

or 02 , have been

made on several sintered specimens of Y-doped Ce02» These specimens are

somewhat porous, and the electrodes are somewhat irregular in outline.

No attempt has yet been made to reduce the data to intrinsic material

properties

.

Figure 17 shows typical data taken on the same specimen at several

temperatures in the same atmosphere. Here the complex impedance is

plotted in the complex plane. Physical and chemical processes normally

become faster as the temperature rises, while the frequencies available

for measurement, and therefore the region of the time scale sampled,

remain fixed. The result is that the "window" available to us to view

the relative time scale moves toward the slower, lower-frequency processes

as the temperature rises. This effect is clearly observable in Fig. 17,

where the data appear to move toward the low-frequency (right-hand) side of

the impedance diagram.

Within the portion of the time scale so far explored using the

frequencies (10 Hz to 13 MHz) and temperatures (22 °C to 600 °C) available,

the impedance plots exhibit two major arcs. At high frequencies, and

46
200 300 400 500 600

Z OHMS

Figure 17. Impedance plotted in the complex plane for a sintered specimen

(bulk density 85% of theoretical) of Ce

Q gY

Q

^

g

at three

temperatures shown.

47
low temperatures , there is a distorted arc independent of whether the

atmosphere is 0^ or Ng* and therefore probably due to conduction processes

in the bulk of the specimen, not involving electrode reactions. The

distorted shape of this arc suggests that it is the result of the

superposition of several processes, arising perhaps from the combined

effects of the geometric capacitance, internal inhomogeneities such as

grain boundaries, and the conductance of regions of "good" material

between the inhomogeneities [28]. At low frequencies, and high temperatures, there is an additional arc, depressed below the real axis, details

of which appear to depend upon the nature of the atmosphere. This arc

probably arises from electrode effects.

It appears at this stage possible to represent these data by the

equivalent circuit shown in Fig. 18. The part left of the dotted line

arises from the bulk processes while that to the right represents electrode effects.

Figure 18. Equivalent circuit to represent impedance of polycrystall ine

Ce

x

V l-x°2-x/2

specimens.

48
In the bulk part of the equivalent circuit as we interpret it at

the moment, C' represents the geometric capacitance, R' the resistance

of the "good" material between homogeneities, and Z*

b

the impedance due

to the inhomogeneities. The variable nature of these inhomogeneities has

been accounted for by writing Z*

b as resulting from a Cole-Cole [29]

distribution of relaxation times;

7* = aZ ( 4

)

^ 1 + ( jojx

)

n

,

where AZ is the total resistance contributed by the inhomogeneities at

zero frequency, a> is the circular frequency, t is the most probable

relaxation time, n is a parameter expressing the width of the distribution and j is equal to>/l. When n = 1, the distribution is a delta

function. For n<l the distribution broadens as n decreases.

A hand-waving rationale for treating the combined impedances of the

inhomogeneities and the "good" material between in this fashion can be

developed as follows. The bulk of the specimen can be thought of as a

bundle of roughly identical chains, in parallel. Each chain consists of

a string of inhomogeneities separated by "good" material. The inhomogeneities are thin, and must each be represented by a capacitor shunting a

resistance, while the sections of "good" material are relatively long

and contribute only resistances proportional to the length of the section.

The corresponding capacitance has in fact been included in the geometric

capacitance. When the impedance of this chain is calculated, the "good"

material resistances combine in simple series fashion to give R ' , while

49
the impedances of the inhomogeneities combine in the series calculation

1 as

Z (5)

where i is an index running over all inhomogeneities, Z(t) is the

impedance in this sequence corresponding to the range in relaxation

times from t to x+dx, and g(x) is the distribution function for x.

With g(x) given by the Cole-Cole distribution function, eq (5) reduces

to eq (4)

.

Figure 19 shows an example of how the equivalent circuit fits the

experimental data. The parameters of the equivalent circuit for this

case are:

They were obtained as follows. First, the very high frequency data were

converted to admittances, Y*, and the imaginary part Y" plotted against

w, the circular frequency. The slope of the resulting line (Fig. 20)

gives C' directly. The admittance values at all frequencies were then

reduced by the contribution from this estimate of C', and inverted to

yield the impedance of the R'+Z*^ branch of the equivalent circuit.

When plotted in the complex plane, this yielded a depressed arc that

could be fitted by the least-squares techniques described above to a

Cole-Cole equation. The results of this fit give the parameters of

Z*^. The high-frequency (left-hand) intercept of this arc on the real

axis gives R'. With these parameters the overall impedance, shown as

plus signs on Fig. 19, for the whole circuit was then calculated.

R' = 134 kft AZ = 658 kft n = 0.566

C = 6.454 pF t = 8.87 ys

50
i <\J o

Sl/\IHO 9 0l ‘ „Z

51

The fit appears to be quite good, and in particular offers the hope

of estimating the true conductance of the "good" material, needed to

monitor and assess the results of the degradation studies described

above. The fitting procedure described is clumsy and, because of slow

convergency, may not yield good estimates of the parameters in every

case. For this reason, future work will include the development of a

steepest-descents computer program to allow fitting of the whole equivalent circuit to all of the data simultaneously.

These techniques should be useful beyond the simple estimation of

R ' , which in turn will yield the conductance of the "good" material

between inhomogeneities. There is also, in Z*

b

, information about the

inhomogeneities themselves, if the model proves to be sound. The inhomogeneities may be grain boundaries, regions of a second phase, pores, and

perhaps even extended defects or stacking faults. The technique could

thus be used to guide efforts to develop optimum microstructures, or to

monitor changes in microstructure during aging. Experiments are planned

to try in the immediate future to prove the validity of the model.

At high temperatures and low frequencies an additional arc appears.

At 600 °C in 0^, as shown in Fig. 21, the complex impedance at low

frequencies is well fitted by a circular arc with center depressed below

the real axis. The angle between the real axis and the line joining the

left intercept of the arc on the real axis and the center of the arc is

44°. Fig. 22 shows that for these same data a very similar arc is

exhibited by the complex admittance. If we identify the left intercept

of this arc, in the impedance plot as R., the right intercept in the

admittance plot as R

t

, the right intercept in the impedance plot as

R^+R^, and the left intercept in the admittance plot as (R^R^)"^, then

53
54

Figure

21.

Complex

impedance

of

sintered

Ceno Yn

?0-,q

at

600
CMCM

CM

CM

<7> CO

lOIO

CD >

CD

lO

CM Ooo

55

Figure

22.

Complex

admittance

of

sintered

CeggY0g

at

600
the fits shown in Figs. 21 and 22 result in the following estimates

these parameters:

Impedance, Admittance,

R

t

,fi 486 485

R

t

+Rd’“ 609 610

These results suggest several alternate explanations. In the first

the part of the equivalent circuit in Fig. 16 to the right of the

dashed line is represented by Warburg impedance, representing a diffusional process, shunted by a resistance, R

rf

, representing the electrode

process involving oxidation of 0 ions and reduction of C^, and exchange with the atmosphere. The diffusional process might be the storage

of 0^ as solute in the Pt electrodes. At these low frequencies and high

temperatures the bulk processes have been reduced to simple conduction,

represented by the resistance R

t

in series with the electrodes.

The Warburg impedance is given by

Z* - od-jjuf15

where a is the intensity factor of the impedance. Then the impedance of

the entire equivalent circuit as described is

Z* = Rt + /

R

d \

\l+(ju)T )'5 /

which is the equation for an arc depressed below the real axis such that

the left (hign-frequency) intercept is R

t

, the right (low-frequency)

intercept is ( R

^ +R

ci)»

and the angle between the real axis and the line

joining the left intercept to the center of the arc is n/4.

(6

(7)

S

i

56
The admittance of this whole equivalent circuit is the inverse of

Z* given by eq (7)

:

Y* = (Z*)"

1 = R^

1 AY

l + ( jo)T ' ) h

9 ( 8 )

where

AY = R1 - (R^)-1 (9)

This is the equation for an arc with left intercept given by

(Rt+Rd

)~\ right intercept by R^""* , and the angle between the real axis

and the line joining the right intercept to the center of the arc given

by n/4. These descriptions for the behavior of eqs (8) and (9) appear

to match very closely the descriptions of the data given above. Furthermore,

the ratio of t'/t drawn from the fit of the equations to the experimental

o data is 0.71, where the ratio [R

t

/(R

t +R

d

)] is 0.64, satisfactory agree-

)' ment as called for by eq (9).

Essentially the same set of equations (7 and 8) can be arrived at

in a somewhat more general format by replacing the Warburg impedance by

an element consisting of a set of parallel admittances, each of which

If consists of a series resistor and capacitor, and which are distributed

according to a Cole-Cole distribution. The admittance for this element

is

Y* = y - Y

°°

» 0°)

“ H(ju>T) n

where Y^ is the conductance of the element at infinite frequency. The

57
impedance corresponding to eq (10) is

Z* = Y'

1

1 l+[T-nCos(2|)]u

' n -j[T'nSin(!!|)Vn

( (

which for n = h reduces to a Warburg impedance in series with a resistance

given by Y . This formulation could account for departures of the

exponent from hThis discussion has presented a model that can account for the data

observed so far. One of the immediate tasks will be to devise and carry

out experiments to test and refine this model, and to relate its parameters

to properties of the materials measurable in other ways.

58
References

1. P. Stonehart and P. N. Ross, Electrochim. Acta, 21_, 441 (1976).

2. Potentiostat and its Applications, J. A. Von Fraunhofer and

C. H. Banks, Butterworth, (London, 1972).

3. Electronics, 49, No. 8, (1976); Special Issue on Microprocessors.

4. An Introduction to Microcomputers, A. Osborne and Associates,

Inc., Berkeley, CA (1975).

5. M6800 Microcomputer System Design Data, Motorola, Inc. (1976).

6. M6800 Microprocessor Applications Manual, Motorola, Inc. (1975).

7. Analog-Digital Conversion Handbook, edited by D. H. Sheingold,

Analog Devices, Inc., Norwood, MA. (1972).

8. A Low Cost, High Performance Tracking A/D Converter, Application

Note AN-6, Precision Monolithics, Inc., Santa Clara, CA.

9. Software Design for Microprocessors , J. G. Webster and

W. D. Simpson, Texas Instrument Learning Center, Dallas, TX (1976).

10. W. Schottky, Wiss. Veroff. Siemenswerke, H, H2, 1 (1935).

11. E. Baur and H. Preis, Z. Elektrochem. , 43, 727 (1937).

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

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

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

123 , 415 (1976).

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

114, 752 (1967).
16 . 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)

.

17. J. Lef£vre, Ann. Chim., 8, 135 (1963).

18. R. Collongues, F. Queyroux, M. Perez y Jorba and J. C. Gilles, Bull.

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

19. M. R. Thornber, D. J. M. Bevan and J. Graham, Acta Cryst., B24 ,

1183 (1968).

20. J. G. Allpress and H. J. Russell, J. Solid State Chem., 1_5, 68 (1975).

21. R. E. Carter and W. L. Roth, "Conductivity and Structure in

Cal cia-stabi 1 ized Zirconia," in Electromotive Force Measurements in

High-Temperature Systems , ed. by C. B. Alcock, Elsevier (New York, 1968).

22. D. Michel, Mater. Res. Bull., 8, 943 (1973).

23. D. Michel, Rev. Int. Hautes Tempar. Refract., 9, 225 (1972).

24. B. Hudson and P. T. Moseley, J. Solid State Chem., ]9^, 383 (1976).

25. W. H. Rhodes and R. E. Carter, J. Am. Ceram. Soc., 49, 244 (1966).

26. A. S. Nowick, Columbia University, private communication.

27. J. Griffith, Columbia University, private communication.

28. J. E. Bauerle, J. Phys. Chem. Solids, 30, 2657 (1969).

29. K. S. Cole and R. H. Cole, J. Chem. Phys., 9, 341 (1941).
U.S. DEPT. OF COMM. 1. PUBI.K ATION OR REPORT NO. 2. Gov't Accession 3. Recipient's Accession No. BIBLIOGRAPHIC DATA No. SHEET NBSIR 77-1270

!l. TITLE AND SUBTITLE 5. Publication Date

Materials for Fuel Cells

6. Performing Organization Code

author(S) L. H. Bennett, M. I. Cohen, A. L. Dragoo,

A. D. Franklin. A. J. McAlister and IC .F^ Young

8 . Performing Organ. Report No. 3130456

>. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

10. Project/Task/Work Unit No.

11. Contract/Grant No.

2. Sponsoring Organization Name and Complete Address (Street , City, State, ZIP)

Division of Conservation Research and Technology

U.S. Energy Research and Development Administration

Washington, D. C. 20545

13. Type of Report & Period

Covered

Annual Report

14. Sponsoring Agency Code

5. SUPPLEMENTARY NOTES

6. ABSTRACT (A 200-word or less factual summary of most significant information. If document includes a significant

bibliography or literature survey, mention it here.)

This report describes the first year's progress of the NBS program on Materials for

: uel Cells.

Transition metal carbides, borides and nitrides ("refractory hard metals') were eximined with respect to stability and to catalytic oxidation of as non-precious subititutes for Pt as fuel electrocatalysts. Hydrogen oxidation was observed only on Pt,

/C and (MOq gW

Q

The activity of WC was found to depend upon the preparative method,

'he compound°(M6Q gW

Q 2 )C, which has the WC structure, was estimated to have a specific

ictivity from 1 .5‘io u3^times greater than that of the WC samples.

A versatile automated system for electrochemical analysis was designed and built

itilizing the precision of digital control and readout techniques to perform essentially

malog measurements. The automated system uses a microprocessor as the key element in

:he central processing unit. The various components and operation of the automated

;ystem are described.

Studies of CeOptY^Og ceramic electrolytes were designed to measure the influence of

innealing and prolonged current passage on electrical properties of these materials,

fork on a high temperature facility for these experiments and preparation of CeO

? :Y ? CL

:eramic specimens is described. d

AC impedance was measured on several CeC^YpOg sintered specimens as a function of

"requency using a network analyzer. The features of this instrument and the theory of

;he measurements are presenleTL

7. KEY WORDS (six to twelve entries; alphabetical order; capitalize only the first letter of the first key word unless a proper name; separated by semicolons)

\C impedance; automated electrochemical analysis; ceramic el ectrolytes ; ceria-yttria

:ycl ic voltommetry; electrocatalysis; equivalent circuit; fuel cells; microprocessor;

letwork analyzer; phosphoric —acid electrolyte; refractory hard metals.

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