NBSIR 76-1091

Materials for Fuel Cells

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

Institute for Materials Research

National Bureau of Standards

Washington, D. C. 20234

Quarterly Report - February to April 1976

May 1976

Issued August 1 976

Prepared for Division of Conservation Research and Technology

U.S. Energy Research and Development Administration

Washington, D. C. 20545

NBSIR 76-1091

MATERIALS FOR FUEL CELLS

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

Institute for Materials Research

National Bureau of Standards

Washington, D. C. 20234

Quarterly Report - February to April 1976

May 1976

Issued August 1 976

Prepared for Division of Conservation Research and Technology

U.S. Energy Research and Development Administration

Washington, D. C. 20545

V N

^fAU 0*

U.S. DEPARTMENT OF COMMERCE, Elliot L. Richardson, Secretary

Edward O. Vetter, Under Secretary

Dr. Betsy Ancker-Johnson, Assistant Secretary for Science and Technology

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Acting Director

Table of Contents

Abstract

y 1. Introduction

2. Technological Motivation

3. Program and Progress

3.1 Non-Noble-Metal Electrocatalysts

3.1.1 Electrocatalysis on Non-Noble Metals and Alloys

Objectives

Scientific Motivation

Research Program

Progress to Date

3.1.2 Electrocatalysis on Non-Metal lie Surfaces

Objectives

Scientific Motivation

Research Program

Progress to Date

3.2 High Temperature Solid Electrolytes

Objectives

Scientific Motivation

Research Program

Progress to Date

4 . References

1

ABSTRACT

This report describes the research plans and initial progress on the

NBS program of research on Materials for Fuel Cells.

Transition metal carbides, borides, and nitrides ("refractory hard

metals", or RHM) are being examined as potential hydrogen oxidation electrocatalysts for use in acid fuel cells. Preliminary screening of 19 carbides

and borides has revealed apparent stability in hot (up to 160°C) phosphoric

acid for TaB, MoB, NbC, TaC, Mo^C, and WC. More sophisticated tests for

acid stability for these materials are under way. Equipment for examination

of the electrochemical behavior of these materials as hydrogen oxidation

electrocatalysts in phosphoric acid electrolytes at temperatures up to

150°C is being completed, and tests will be started on these materials

shortly. Additional materials in the general RHM class, including a number

of nitrides, have been ordered and will be screened for acid stability.

More extensive exploration will be made of the properties of TaC and WC

as a function of stoichiometry and presence of other metals and of oxygen.

Plans exist for a parallel study of oxides as oxygen reduction electrocatalysts for acid fuel cells, supported by NBS. The current effort

is devoted to improving and expanding the facility for electrochemical

measurements.

The long-term stability of CeO^-based solid electrolytes, which are

promising new candidate materials for the high-temperature fuel cells, is

under study. Materials for fabricating specimens of CeO^ doped with Gd^O^

or Y

2 O

2 have been obtained or ordered, and arrangements made to hot press

them. Experiments have been designed to examine degradation of the electrical

properties at temperatures from 500°C to 1100°C, with and without a constant
2

current throughout the degradation anneals. Compositions will be chosen around

those giving a maximum conductivity, and the oxygen partial pressure will be

fixed at a convenient value. AC admittance measurements will be used to monitor

the electrical properties. Furnaces and ancillary equipment are being assembled

or constructed.
3

1. Introduction

The National Bureau of Standards is undertaking a program of research

on materials for fuel cells, with an especial, long-range, emphasis on problems

relating to lifetime and the assessment or testing of expected life of

particularly the electrically active components of fuel cells. Such components

include, for instance, the electrocatalysts in liquid-electrolyte cells, and

the solid electrolyte in high-temperature cells. This report will describe

the entire program in order to provide the context for the subject project,

but will report detailed progress on the latter only.

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

elements, which taken together reinforce each other in techniques and concepts,

but each of which may also stand alone. These embrace:

i. Electrocatalysis, especially oxygen reduction, on

non-metal lie surfaces;

ii. Electrocatalysis, especially hydrogen oxidation, on

non-noble metals and alloys; and

iii. Degradation mechanisms involving solid oxygen-transporting

electrolytes

The subject project contains work on items ii and iii.

2. Technological Motivation

Fuel cells offer considerable potential benefit in terms of conservation

of fuel and minimum environmental impact for the generation of electricity

from fossil fuels.

The major potential immediate uses for fuel cells include the delivery

of peak power in central station operation, the distributed generation of

electricity on the neighborhood or local level [1], and replacement of
4

small generators for rural and small municipal power plants and in urban

locations [2]. These potentialities draw upon the considerably higher

efficiency of even present fuel cells systems, as compared to conventional

generators, when operating under partial load, and their low environmental

impact. In addition, as liquid primary fuels become scarce, fuel cells,

like batteries, may have potential for vehicular use.

The major present thrust in the electric utility field [3,4] is the

hot (~ 1 50- 1 80°C ) concentrated phosphoric acid fuel cell. This cell uses

hydrogen and air, and with a reformer can burn hydrocarbon gases. It has

the great advantage of lack of sensitivity to CO^ in the fuel or the air.

Alkaline fuel cells have also been highly developed, but, because of their

sensitivity to C0 9 , only for special applications such as the Apollo program.

The major limiting problems [5] in these cells are concerned in the first

instance with cost, and therefore also lifetime, and secondly, with efficiency.

In the acid cell the only electrocatalyst so far capable of achieving the

needed performance is platinum. With Pt electrocatalysts of particle size

large enough to be stable over the desired lifetime (5 years, or 4 x 10^

hours) the loading is so high that the cost is prohibitive. If the particle

size is made sufficiently small so that the cost is reduced enough to make

first-generation (1975-80) cells commercially attractive, the lifetime cannot

be achieved because of sintering of the Pt particles. Ultimately, also, the

Pt will probably have to be replaced altogether to meet the cost requirements

of the more general use contemplated beyond 1980. Other metals or alloys,

and metal oxides or other compounds are possibilities.

Furthermore, at the air electrode in both acid and alkaline media,

present catalysts (including Pt) reduce the oxygen to peroxide rather than

to water. The working voltage for the peroxide reaction is lower than that
5 possible with the water reaction, and therefore the achievable efficiency of

conversion of chemical to electrical energy is also lowered. The development

of catalysts for oxygen reduction, particularly in the acid cell, that avoid

the peroxide reaction or minimize it could result in significant increases

in efficiency that may be necessary in the second-generation cells and beyond.

Besides the low-temperature acid cell, another fuel cell with rather

large potential for fuel saving is the high-temperature solid-electrolyte cell,

directly coupled into hot product gases of a coal combustor. This combination

on paper should produce efficiencies better than 60% in the conversion of fuel

chemical energy to electrical energy.

The materials problems at the high operating temperatures (~ 1000°C)

are severe, however. In particular, the electrical connection between cells

must be a good electronic conductor at this temperature over a range of

equivalent oxygen pressure from that of air to that of the fuel. At the

same time it and all other components must also, in the interest of economic

operation, be not too costly and be stable over the design lifetime. These

problems have proved so severe that serious development in this country

ceased for a period and only recently has been renewed.

Reduction in operating temperature would help considerably in meeting

these problems. In the past, this temperature was dictated by the temperature

dependence of the conductivity of solid electrolyte (stabilized ZrO^) and

the practical lower limit to the thickness of films formed under production

conditions. Recent work [6,7] on CeC^-based materials has demonstrated the

possibility of oxygen-conducting solid electrolytes capable of operating at

significant lower temperatures. However, these indications are quite

preliminary and little is known of the stability of such materials under
6

current-carrying conditions. Furthermore, although electrode polarization

is not a serious problem at the higher temperatures, some is evident at both

electrodes. It must be expected to become more serious as the temperature

is reduced, with a resultant loss in efficiency.

From the above discussion, it is evident that some of the most important

problems standing in the way of realizing practical fuel cells involve the

electrodes and the catalytic and other processes that occur there. On the

other hand, successful development of fuel cells for power generation could

provide real savings in the use of fuel.

3. Program and Progress

3.1 Non-Noble-Metal Electrocatalysts

3.1.1 Electrocatalysis on Non-Noble Metals and Alloys

Objectives

To use the results of combined studies of electrochemistry and solid

state properties (particularly the relationship between the electronic

properties such as the densi ty-of-states in the d-bands and electrocatalytic

activity) of the refractory hard metals to identify materials with promise for

efficient electrocatalysts. Emphasis is on hydrogen or fuel oxidation and

the replacement of Pt for this purpose in acid fuel cells.

Scientific Motivation

Correlations have been noted between the electrocatalytic efficiencies

of Group VIII metals and rough features of the electronic density of states.

For example, the 0^ reduction rate in orthophosphoric acid of a number of pure

precious metals and their alloys among themselves has been shown [8] to be

maximal at a d-band vacancy level of ~ 0.6. Correlations of this sort are

of limited use, in that the parameters are neither well defined nor sufficient

to describe the complex electronic structure of the materials studied. Other
7

features may be important to electrocatalytic activity, such as the magnitude

of the density of states at the Fermi level N(Ep), or the occurrence of

exchange enhancement. In fact, the importance of N(Ep) has been stressed

by Levich [9].

Refractory hard metals -- compounds of transition metals with the

metalloids B, C, and N and hereafter referred to as RHM -- have transitionmetal-like physical properties. They are electrical conductors, often with

higher conductivity than the transition metal hosts; and generally have

higher melting points and greater chemical stability than the host transition

metal. While little is yet known of the detailed electronic structure of

these materials (partly due to difficulties in preparing pure, single phase

samples), the available data do indicate that in these compounds, transition

metal site d-densities of states resemble those of pure metals of higher

group number. For example, we have observed such d-band behavior in studies

of the state densities of first transition series metal diborides [10] and

of the carbides WC and M02 C [11].

Thus, the RHM have electronic structures similar to that of Pt in the

essential feature of incomplete d-bands, and may well be catalytically active

for hydrogen oxidation (at least two are know to be: WC [12], N

i ^ B [13]) and

may well be acid resistant as well (at least one is known to be: WC [12]).

Available data on the hydrogen oxidation activity and acid resistance of the

RHM are far from exhaustive, and in some instances questionable, largely from

the standpoint of sample quality. To date, only WC of the RHM is in active

use, [14], and even here it is not yet clear what the exact composition of

the active form is; there is some evidence to suggest that pure WC is

catalytically inactive [15], and that the active species (activity being a

very sensitive function of preparation method [1 6] ) i s perhaps tungsten rich
8

[15] or has at least a partial substitution of oxygen at carbon sites

[17].

Research Program

The work will consist of a direct evaluation, using wel 1 -characterized

specimens of RHM which, by virtue of their electronic structures, promise to

be catalytically active. A parallel effort is planned, currently under NBS

sponsorship, to continue to study oxides as oxygen reduction catalysts and

to initiate studies of the activity of WC, alloyed at both metal and metalloid

sites, with a view to determining the precise nature of the active species

and the effect of alloying, and to conduct studies of the acid resistance

and activity of the alloy system TaC , 0.8 < x < 1.0 (which displays [18] a A wide variation in conductivity and other physical properties with x) to

determine the effects of stoichiometry.

Our immediate aim is the study of the compounds Mo^O and Cr^ for

which we expect catalytic activity and anticipate good acid stability. We

shall conduct a screening program for acid stability of various, well

characterized RHM as they become available to us. Presently, these include

M02 C and Cr^C^ as mentioned, as well as the di borides of Sc, Ti, V, Cr, Zr,

Hb, Hf, and Ta; the monoborides of V, Cr, Ta, Nb, and Mo; and the monocarbides of V, Nb, and Ta. Other materials, including nitrides, phosphides,

and silicides, will be screened as acquired. Validity of literature data

will be tested, particularly where doubt of a sample quality exists.

Alloyed WC, particularly with oxygen substituted at the metalloid site,

and various TaC samples will be prepared at NBS, characterized, and tested

as available.

In background studies, not part of the subject project but to be carried

on as part of the larger program, it is proposed to focus attention on rather
9

complete investigations of the transition metal diborides, and to initiate

studies of the cubic monocarbides. Theoretical interpretation by band

theoretical techniques will stress systematic variation in d-band densities

through these isostructural sequences- of compounds; similarities between their

electronic structures and those of the Group VIII metals; and correlations of

their densities of states and electrode performance as measured by relatively

standard electrochemical test methods. The results of electrochemical testing

will be used as feedback for suggesting which electrode properties and

structures should be looked for.

Sample preparation and characterization must be emphasized in such an

investigation. Our own experience indicates that preparation of pure,

single phase materials is difficult, and leads us to question much of the

available data on the physical and chemical properties of these materials.

As a specific example, consider high temperature oxidation data on transition

metal diborides. All samples of these materials that we were able to obtain

for our studies contained significant amounts of BN and BC; it is therefore

questionable as to whether high temperature oxidation data on these materials

reflect properties of the diborides or of the occluded contaminants. Similar

uncertainties must be avoided in the present investigation.

Experimental tools for study of the electronic density of states include

x-ray photoemission, which probes the local transition metal d-state density,

both bulk and surface via variation of x-ray incidence and electron takeoff

angles; soft x-ray emission which probes the local p-like state density at

metalloid sites and yields information on the d- 1 ike structure through

metalloid s- and metal d-hybridization; and nuclear magnetic resonance.

Mossbauer effect and perturbed angular correlation studies will be used where

appropriate; the latter, owing to its great sensitivity to local variations
10

in electric field gradient is an appropriate tool for study of sintering of

electrode particles. Magnetic susceptibility, x-ray diffraction, and

metallography will also be employed.

Progress to Date

Our work in this area involves 1) preparation and characterization of

materials, 2) screening of these materials for stability in hot phosphoric

acid and 2) examination of stable materials for electrocatalytic activity

through potentiodynamic current-voltage measurements. The first two of these

efforts are well underway, and we shall describe our results to date. For the

second phase, equipment is very nearly complete, as described elsewhere in

this report, and work will begin very soon on candidate materials (5 to date)

suggested by the screening program. Some problems and progress on sample

and electrode fabrication will be discussed here as well.

Our screening procedure for acid stability consists of a) visual

inspection for signs of instability (bubbling, change of color of solution,

or changes in color or state of candidate material) in 85% H^PO^ at room

or elevated temperature, and b) comparison of initial sample mass with

insoluble residual mass after long exposure to 85% phosphoric acid at 80°C.

To date, some 19 materials have been examined, with five showing

promising stability. All are listed in Table 1, the promising materials

being starred. Long term weight loss studies are now being conducted on

the latter.
11

Table 1

Stability of Refractory Hard Metals in H^PO^

Cr

3

C

7 fc CrB fh

Cr

7

C 3

fh VB fc

NbB fc

FeB fc TaB (good) P*

Fe

2 B fh TaB (bad) fh

MoB P*

Ni

3

p fh

Ni

3 MgB

2 fc TiC IP

zrC IP

Ti B

2 fh, Iw VC lp

ZrB

2

1 w NbC P*

HfB

2

1 w TaC P*

vb

2

1 w Mo

2 C P,1P*

nb

2

1 w WC P,1P*

TaB

2 P» IP w

2 c fc

CrB

2 fc , 1 w

Key:

Present work: in 85% H

3

P0

Z

1 fc failed, visible reaction in cold (22°C) H

3

P0

4

fh failed, visible reaction in hot (80 or 160°C) H

3

P0

P no visible reaction, to be given long term weight

test in 80°C H^PO^

* long term wt. loss tests underway.

Literature

Iw excessive weight loss reported in literature, drop.

Ip small weight loss reported in literature, consider further.
12

Other materials for study are now being obtained or fabricated. In

Table 2 are listed materials ordered for screening tests. (Many of these

purchased materials will be of less than ideal quality, but should be

adequate for the screening tests).

Table 2

List of Ordered Materials

Cr

23

C

6

Cr

2 N WB

HfC HfN w

2 b

NbC NbN w

2 b

Nb

2 C TaN

TaC NiN

Ta^C ZrN

TiC

Work is underway to produce samples of known surface area by gaseous

carburization, bonding, or nitriding of metal sheet. High quality sheet

samples of WC and TaC have been produced in this way. We are attempting

now to vary stoichiometry in TaC so produced, principally by control of gas

pressure. Scheduled for early trial are attempts at alloying WC with other

metals at W sites and with oxygen at carbon sites. We suspect that the latter

effect is achieved by pretreatments and preparation methods currently used to pro

duce active WC for fuel cell electrodes. Oxygen substitution for carbon is our

interpretation of the recent experiments of Ross and Stonehart, who found

that of a variety of WC samples obtained from different sources, only that

specially prepared by AG-Telefunken was active in hydrogen adsorption.
13

The problem of preparing test electrodes of known geometrical area is being

pursued in two ways. One, the preparation of sheet samples has already been

mentioned. We have tried a variety of binding materials for use with powder

samples, with the aim of producing a conducting, non-porous electrode whose

active geometric area can be estimated by metallographic techniques. The most

stable binder in hot phosphoric acid, teflon, generally yields porous electrodes.

Electrochemical area determinations will also be carried out where possible.

3.1.2 Electrocatalysis on Won-Metallic Surfaces

This part of the overall program is not under the subject contract, but

will be discussed here to present as complete a picture as possible of the

entire program.

Objectives

1. To elucidate the mechanisms of oxygen reduction on non-metal lie

electrocatalytic surfaces.

2. To find efficient non-metallic electrocatalysts as replacements

for Pt, preferably capable of electroreducing oxygen directly to hydroxyl

rather than through peroxide, and of functioning in hot concentrated acid

electrolytes.

Scientific Motivation

Semiconductors have long been used as heterogeneous catalysts, especially

for oxidation and reduction reactions. Early materials used as electrocatalysts

[19] included the oxides of Cu and Ga. Silver has found extensive use at the

oxygen electrode in alkaline fuel cells [20], and may well operate as an oxide.

Lithiated NiO at temperatures above about 100°C has also operated successfully

as an oxygen electrode in alkaline media [21]. More recently, work has been

done using spinels [21] such as CoAl^O^ and perovskites such as Sr-doped

LaCoO^ [22]. The latter use is obviously related to the use of the
14

perovskites ABO^, where A represents rare earth, Pb, or alkaline earth ions,

and B is Co or Mn, for oxidation of CO and reduction of No in automotive

X exhaust catalysts [23]. Some of the early, exciting non-metallic

electrocatalysts, for both hydrogen oxidation and oxygen reduction, were

the tungsten bronzes and related compounds [24, 25, 26], However, more

recent work has shown that the observed high catalytic activity depends

upon the presence of Pt [27, 28], which in some way activates the oxide in

a synergistic fashion.

A fairly detailed body of theory has been worked out for electrochemical processes on semiconductors and insulators [29], but most of the

application has been to very well -characterized semiconductors such as Ge,

Si, GaAs, ZnO, and CdS. Recently Tench and Yeager [30] have applied these

ideas to a study of lithiated NiO. The present proposal is to carry this

sort of work forward to the study of materials more closely related to those

that may play a role in acid electrolytes as oxygen catalysts.

Previous work in these laboratories [31] has uncovered a number of

candidate materials exhibiting stability in concentrated phosphoric acid

at 100°C. These include oxides of Ta and Nb with various transition metals,

and also BaRuO^. The latter also showed some promise as a catalyst for

oxygen reduction, although under strong oxidizing conditions in the hot acid

a deterioration of the electrochemical activity occurred. A new class of

compounds based on CeTaO^ also appears interesting in that considerable

conductivity can be built in by adjusting the oxygen content and also

presumably by doping, and yet acid stability is to be expected. The materials

to be studied will be chosen from among such materials, with perhaps also

IrO,) or RuO^, which although not acid stable do give some evidence of

catalyzing oxygen reduction without going through peroxide [32].
15

Research Program

Because the mechanisms of reduction reactions on non-metallic electrode

materials are not well enough understood to serve as much of a guide in the

selection of electrocatalytic materials, it is proposed to combine a study

of electroreduction mechanisms on non-metallic electrodes with a continued

search for promising non-metallic electrocatalysts, especially for oxygen

reduction in acid media.

1. Mechanism of electroreduction

Oxygen reduction is a complex process that may involve a number of

simultaneous reactions [33]. As a probe for at least the initial phases of

a study of electrocatalytic mechanisms, oxygen reduction may present too

difficult a problem in interpretation. For this reason it is proposed to use

also one or more simpler redox reactions, of which a number are available.

3+ 2+ 4+ 3+

For instance, the Fe /Fe and Ce /Ce couples exhibit simple mechanisms

from the viewpoint of reaction kinetics [34]. On the other hand, the

Fe(CN)

6 ^"/Fe(CN)g

4 " couple, although not as well studied, is relatively

stable over a wide pH range and would allow examination of the influence

of hydrogen ion concentration on the electrode kinetics [35].

The initial experiments, conducted in supporting electrolyte only,

without the redox system present, will be designed to elucidate the potential

distribution within the combined electrode-electrolyte system (space charge,

double layer, etc.) and the changes taking place in the electrode surface

(adsorbed species, valence changes, etc.) as a function of potential [29].

Conventional voltammetry will be combined with ac impedance measurements.
16

el 1 ipsometry , and perhaps electroreflectance to yield this information [30].

Electrolytes of various pH's will be used, and the influence of temperature

and adsorbable ions studied. Changes in the chemical composition of the

electrolyte will be monitored.

The electrochemical measurements in the presence of the redox

couple will be performed with a rotating electrode to allow study of the

mass transport aspects of the faradaic currents. The study will involve

the dependence of the current on potential, rotation, pH, and other

compositional variables to determine the mechanisms of reduction and the

exchange current (reaction rate) as a function of pH and other measures of

the electrolyte composition. The impedance measurements in the initial

experiments should allow correction to the applied potential across the

cell for that portion across the electrode itself and therefore not effective

in the electrical reactions.

To interpret the electrochemical phenomena we will need to know

more about the electronic properties of the electrode material [36]. If

possible, single crystals will be grown for this part of the study.

Measurements as a function of temperature, atmosphere, and doping will

be made of such electronic quantities as conductivity. Hall effect, and

thermopower. For the most part the materials of interest appear to

contain transition metal ions and magnetic susceptibility and perhaps EPR

may be very valuable in sorting out the nature of the important electronic

states. These data will be used to explore the electronic structure

(including the band or localized nature of the d-levels, the density of

states, etc.), to identify the charge carriers and conduction mechanisms,

to determine the energy levels of donor or acceptor states and of the Fermi

energy, and to obtain data on the numbers and mobilities of charge carriers.
17

Finally, because the electrocatalytic reactions of interest take

place on the interface between the electrode and the electrolyte, details

of the chemistry and structure of that interface will also be studied.

The macroscopic topographic features of the initial surfaces can be

explored by a variety of available microscopic techniques, and the

initial chemistry by x-ray emission analysis, etc. Something of the

nature of the adsorption chemistry can be learned by studies of the

relatively simple species adsorbed from the gas phase, using optical

spectroscopy together with measurements of the adsorption isotherms.

With the expected transition metal ions present, there may be a real

opportunity to learn of the electronic interactions of adsorbate groups

with the surface using sensitive magnetic susceptibility measurements

or EPR. Also needed will be in-situ studies of the behavior of the

surface exposed to the electrolyte and under varying potential. Optical

methods (e. g., electroreflectance, el 1 ipsometry, reflectivity) should

be particularly suited to exploring the surface electronic states of the

electrode material, the development of surface layers, and the nature of

adsorbed species.

The results of these efforts should be a picture of the electronic

structure of the electrode material and its behavior under electrochemical conditions. The way this structure and behavior influences

the kinetics of the redox reactions studied should then be better

understood [29], and clues should be obtained as to how to modify the material

to optimize its performance as a redox catalyst. At this point in the

program it should then be profitable to turn to a systematic examination

of oxygen reduction as the electrochemical process.
18

2. Search for acid-resistant potential oxygen-reduction electrocatalysts.

Research will be directed toward the preparation and characterization

of new, acid stable, oxide materials which also are reasonable electronic

conductors. Previous work [31] has demonstrated that mixed oxides containing

Ta^Og and/or Nb^O^ exhibit remarkable stability in concentrated phosphoric

acid at 100°C. For this reason, research efforts will continue to focus

on oxide systems containing these components. Furthermore, systems which

afford reasonable potential for development of conducting materials will be

selected for study. Electronic conductivity in crystalline oxides often

results from gross nonstoichiometry compensated by cations which exist in

multiple oxidation states. Candidate systems, therefore, also will include

oxide components such as those of the transition metals and some of the

rare earths. Additionally, attempts will be made to enhance the conductivity

of new oxide materials by doping with suitable components.

Although the primary objective of these studies will be to develop

new oxide materials for electrocatalysis , it is anticipated that important

correlations will be made among phase nonstoichiometry, electrical and

magnetic properties, and catalytic activity. These correlations will

serve as input to determine the direction of continued exploratory work.

Experiments will be done, using such techniques as x-ray diffraction,

thermogravimetry , DTA, etc., to outline the phase relations of the candidate

systems, including oxygen partial pressure as a variable. Promising phases

will then be screened for acid stability by following the loss in weight of

pressed specimens rotated in concentrated phosphoric acid at 100° to 150°C.

Changes in near-surface chemistry as a result of this treatment will be

examined using x-ray diffraction. The influence of compositional changes

within promising phases on the electronic properties will be followed using

measurements of conductivity .magnetic susceptibility, etc.
19

Progress To Date

Previous work on this part of the program is reported [31] under

Contract EPA-IAG-D4-0528. Since then the work has been solely on upgrading the facility for electrochemical measurements on electrocatalytic

materials.

The central measurement facility is a potentiostatically - driven cell

capable of operating both as in the potentiodynamic scan mode, primarily with

linear sweep but also with a variety of waveforms, and also in a galvanostatic mode for steady-state voltammetric measurements. Studies us i rig cyclic

voltammetry, steady-state current-voltage measurements, and adsorption

at the electrodes will be possible.

This apparatus has been modified to allow operation at temperatures up

to 150°C while controlling the nature and flow of gas (N^, 0^, H^) to the

cell. An optical window allows illumination of the experimental electrode.

The output is recorded either with an x-y recorder, or for very rapid scans

with a storage oscilloscope.

It is planned in the future to add rotating disk and ring electrodes to

study formation of intermediates during the electrode reactions. Such

information will be of greater value for the study of materials as

electrocatalysts for oxygen reduction than for hydrogen oxidation. It

can also be very useful for studies of the chemical stability of the

electrode itself.

Finally, it is planned to add modern equipment for determining the

complex impedance of the electrodes over a wide range of frequencies,

temperatures, bias potentials, and electrolyte compositions. In order to

pursue these and other measurements most efficiently, an automated system

based on microprocessor control has been designed and is being constructed.
20

3.2 High Temperature Solid Electrolytes

Objectives

1. To determine the long-term stability of oxygen-transporting solid

electrolyte-electrode systems at high temperatures and to elucidate the

mechanism producing whatever degradation occurs in bulk transport properties

and in electrode-electrolyte interfaces.

2. To develop reliable lifetime tests for these systems as needed.

Scientific Motivation

Developmental work on high temperature fuel cells using solid electrolytes

has been concentrated almost wholly on the use of ZrO^ stabilized with CaO

(about 15 mole %) or (about 10 mole %). Work has been done [37]

principally at Westinghouse and G. E. in the United States, and elsewhere in

France, Germany, and Russia. In the United States, only Westinqhouse has

carried on development work in the recent past.

The Westinghouse cell [38] consists of a thin membrane of stabilized

Zr0

2 electrolyte, which conducts electricity almost completely by oxygen

ion transport, with an air electrode of a metal oxide (e. g., In^O^ doped

with Sn, Sb, or Te) and a Ni fuel electrode. At the temperature of operation,

1000°C, both H

2 and CO are oxidized. Attainable voltage efficiencies are

estimated as better than 70 percent. The limiting electrochemical factors

in attaining the design characteristics appear to be the electrolyte

resistivity, the interconnection resistance, and the fuel electrode

polarization voltage drop. Lifetime appears to be the major unattained

design requirement, and, of course, is closely tied to cost and economic

feasibility. The Westinghouse Final Report [38] quotes a target lifetime

in service of five years. Life tests were described in which cells operated

for 50 days or more at temperature with no apparent deterioration in

performance. However, the interconnection between cells was not stable.
21

A reduction in operating temperature would undoubtedly bring great

improvement in lifetime of materials and components. The ZrO^ electrolyte

requires an operating temperature of 1000°C to attain the necessary

conductivity, about 10 ft

""

1 m"\ Recent results [6,7] on CeC^Ln^,

where Ln represents a variety of lanthanide ions, have revealed that

essentially equivalent oxygen conduction occurs at substantially lower

temperatures. The door being open to lower-temperature operation,

there is hope that some of the stability problems can be overcome.

However, the long-term stability of the candidate electrolyte materials

and of the electrolyte-electrode interfaces should be investigated.

Indeed, solid electrolytes of this kind, containing deliberatelyadded impurity ions, must be expected under continual exposure to the

high temperatures (700° - 1000°C) and constant electrical potential

differences (about 0.8 volts across approximately 20 microns, or fields

of the order of 400 volts/cm) to undergo several slow changes that

should lead to degradation of desirable fuel -cell characteristics and

substantial loss of efficiency. These include:

2+ 3+ 3 +

1. Ordering of Dopant Ions (Ca , Y , Ln , etc.)

This effect is known to occur in most (but not all) stabilized

zirconia specimens with compositions near that exhibiting maximum conductivity r 39 ,40] . For instance, for annealing times of several thousand

hours at 1000°C (the proposed Westinghouse Fuel Cell operating temperature),

polycrystall ine samples of calcia-stabilized zirconia develop x-ray and

neutron diffraction patterns corresponding to the ordered arrangement of

2 + ... Ca ions on the cation sites. At the same time the resistivity increases

strongly. The effect occurs much more rapidly in single crystals than in
22

polycrystal 1 ine specimens. In the latter, the rate varies from specimen to

specimen, and in some, was not observed after three months annealing. Similar

effects probably occur in the ceria-based materials as well [6]. It seems

likely that judiciously chosen impurities can probably inhibit the effect.

For attainment of a consistent five-year life, however, further examination

of the effect to insure that lifetime seems required. While similar studies

of CeC^-based electrolytes don't seem to have been made as yet, it seems

likely that ordering of impurity cations can also occur in that system.

2. Migration of Dopant and Impurity Ions

The desirable high-conductivity cubic phase of ZrO^ occurs only

with at least about 7-10 mole percent of added oxides such as CaO or

YO-j g (1/2 Y^O^), and the conductivity exhibits a fairly sharp maximum as

the composition changes across the cubic phase. Similar maxima occur with

doped CeOg [41] compositions. Under the electric fields that will be imposed

on the fuel cells in continuous operation, these added cations can be expected

to migrate slowly toward the cathode (air electrode). The tracer diffusion

coefficient of Ca in cal cia-stabil ized zirconia has been measured [42]

at temperatures well above the 1000°C operating temperature of the

Westinghouse fuel cell. Extrapolation down to 1000°C and use of the

Nernst-Einstein relation to convert diffusivity to conductivity suggests

that the time it would take Ca ions to cross the 20y electrolyte film

under the expected 0.8 volts potential difference might be very long, of

the order of hundreds of years. On the other hand, Rhodes and Carter

2 + found that the rate of migration of Ca ions in grain boundaries is

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

ratio that would be expected to be even larger at lower temperatures. Hence,
23

migration of impurity or stabilizing ions along grain boundary paths over

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

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

solid el ectrolytes

If such a migration away from the anode proceeded to the point

where the content of impurity or stabilizing ions changes appreciably,

resistive layers would appear at the electrodes or grain boundaries. In

the extreme case in ZrC^ the composition limit at the cubic phase boundaries

could be violated and a destabilized layer formed at the anode and a

zirconate layer at the cathode. Since oxygen migration is much slower in

the non-cubic than in the cubic phases of ZrOg, and in the zirconates, the

result would be the deposition of essentially non-conducting layers under

the electrodes and a deterioration of the cell efficiency.

Even in solid electrolytes other than ZrO^, the impurity ions

necessary to produce high ionic conductivity must be expected to migrate

at least to some extent in the solid electrolyte under the influence of

the steady applied field. A number of consequences can result. In

particular, the accumulation of such impurities near the electrodes can

lead to the development of resistive layers or to interference with the

processes whereby electrons are interchanged with oxygen ions by the

electrodes. The result would be either an increase in the electrolyte

resistivity or in the electrode polarization.

It must be emphasized that these processes are known to occur in

stabilized zirconia and are quite likely in other similar oxides. It seems

very probable they would result in deterioration. What cannot be predicted

on the basis of present knowledge is the rate at which they will occur in

the particular solid electrode formulations to be used in fuel cells, and
24

the extent of property degradation they will produce. However, prudent

management would seem to require an effort to explore probable degradation

effects as the fuel cell development proceeds, to provide evaluation techniques,

and if possible to understand the processes as a basis for developing counter

measures

Research Program

The research program is based upon study of the longevity of fuel cell

components for use at high temperature with low BTU gases of the kinds produced

by coal gasification processes. In particular, work will be concentrated upon

ceria solid electrolytes and processes occurring within the electrolyte and

leading to degradation of the electrolyte resistivity, the electrodeelectrolyte contact resistance, or the electrode polarization.

The major thrusts of the research will be:

1. Evaluation of real electrolyte-electrode materials with

respect to longevity;

2. Development of an understanding of the degradation processes

that occur;

3. Development of test methods for the evaluation of candidate

fuel cell materials and structures with respect to these

degradation processes, with particular emphasis on shorttime tests capable of predicting longevity.

In greater detail, the work will be:

1. Evaluation with respect to longevity.

The initial work on the project will consist of a series of

experiments aimed at detecting, in as short a time as possible, the

operation of degradation processes in real electrode materials.

a. Annealing Procedures
25

In these experiments, solid electrolyte specimens, both fully

electroded and in contact with the appropriate fuel (H^ plus CO) and air

atmospheres, and also provided with sputtered Pt electrodes and in contact

with an 0

£ atmosphere, will be annealed at a variety of temperatures around

the working temperature. For instance. Carter and Roth [ 3.9] have found that

an ordering reaction in stabilized zirconia occurs, but only below about

1100°C, and the rate depends on temperature. Furthermore, more complex

annealing schedules, consisting of nucleation anneals at lower temperatures

followed by growth anneals at higher temperatures, may be needed to produce

most rapidly the ordering that might otherwise occur slowly. These anneals

will be designed to explore how the ordering proceeds, if at all, in real

solid electrolyte specimens.

Some of the anneals will be performed with continuous dc

electric fields applied across the specimens, in order to examine the

potential migration of stabilizing cations and impurity ions. This can

be most conveniently done with specimens with symmetrical electrodes

(probably sputtered Pt) in a simple air or oxygen atmosphere, with the

dc field impressed by an outside source of potential. Increase in

temperature and field strength can be used to hasten and therefore render

rapidly detectable otherwise extremely slow processes.

Cerium dioxide (CeO^) doped with trivalent cations [6,7] has

been selected as the material for this study. The work will be carried out

on hot-pressed specimens of CeC^ doped with 10 to 30 mole percent yttrium

oxide (Y^) or gadolinium oxide (Gd^O^). These specimens will be hotpressed outside our laboratory to our specifications. In addition, hotpressed and sintered specimens of materials being used in high temperature

fuel cell development projects will be obtained, where possible, from the
26

appropriate industrial laboratories carrying out the development work. The

frequency-dependent conductivities of these specimens will be examined first.

They will be characterized further with respect to composition, dopant

distribution, microstructure, and mechanical strength.

b. Measurements

Greatest reliance will be placed upon the use of ac complex

admittance measurements, during the anneals, to monitor changes taking place

in the specimens. The complex admittance can be used, as shown by Bauerle [43],

to obtain information about the intrinsic conductivity of the electrolyte,

the influence of impurities on the grain boundaries and the rate of electron

ion transfer at the electrodes. As Raleigh [44] pointed out, these measurements

can be made in the presence of a dc bias and used to explore the limitations

of reactant transport to and from the electrodes. We plan to make these

measurements during the anneals to monitor the electrochemical properties and

to signal changes that arise from degradation processes. These ac admittance

measurements will be supplemented by measurements of oxygen transport through

the oxide. Electrode polarization will be further studied with cyclic

voltammetry. Measurements of electrical properties will be correlated

with chemical analyses, SEM studies of microstructure and dopant distribution,

and other measurements which may elucidate the defect structure of the

electrolytes.

c. Degradation Experiments

Degradation of the ceramic electrolytes will be studied as

a function of dopant concentration, oxygen activity, temperature, current

density, and time. Two types of studies will be conducted. In the first type

of experiment, the current-independent decrease in electrical quality

(decrease in ionic conductivity) of the electrolyte will be studied as a

function of concentration, temperature, and time. The initial experiments
27

will be carried out at an oxygen pressure of about 10"^ atm. Subsequent

experiments may be carried out at both lower and higher oxygen pressures and

/ at values of concentration and temperature as indicated by the previous

experiments. For these experiments, specimens will be removed from the

annealing furnace at selected times over the course of a 32-week period for

the electrical measurements. The second type of experiment will be carried

out on specimens in situ to observe the possible migration of dopant cations

when the electrolyte is subjected to high current densities.

This study will attempt to show whether degradation of doped

ceria electrolytes can be observed within a period of 32 weeks and, if

degradation is observed, to describe the degradation and to determine the

conditions which influence it.

2. Studies of Degradation Mechanisms

If long-term degradation of the important fuel cell properties is

found to occur, the source of the changes will be sought in the mechanisms

discussed above. In particular, the development of an ordered structure

can in principle be detected by x-ray, electron or neutron diffraction

experiments. Special preparation would be required to remove electrodes and

prepare the very thin (20 micron) electrolyte layers, and the measurement

would only be made when the probable existence of an ordering reaction

had been well established by the electrical monitoring.

Impurity and dopant ion migration can be detected and studied by

a number of techniques. For the dopant cations, in CeO^, the concentration

changes should be large enough to be detectable by use of the electron

microprobe. In addition, the occurrence of second phases might be detectable

by x-ray diffraction and optical and/or electron microscopy. Attempts will
28

be made to use reversible Pt/0

2 electrodes to probe the dc electrical

conductance across the solid electrolyte film to correlate the conductance

with observed compositional changes. In particular, transmission electron

microscopy should be very valuable in helping to understand the arrangement

of grains and ordered domains and their boundaries in the electrolyte. Early

detection of changes in this microstructure, including the development and

distribution of second phases, will be attempted to correlate with the

chemical and electrical measurements.

Where concentrations are small, as in the case of impurity ions,

direct detection of migration may not be possible, and resort may be had to

indirect experiments on radioactive tracer migration of probable impurities,

to correlate with changes observed in the electrical properties.

3. Development of Life Tests

Out of the work proposed above should come an understanding of the

basic long-term processes occurring and their impact on the desirable properties.

This information will be used to guide an effort to find means to accelerate

and characterize these processes, or to find other indicators of their

eventual occurrence. The aim is to provide tests that can be run over short

times but that may be used reliably to predict the behavior of the electrolyteelectrode systems, and thus to serve quality control and screening functions.

Progress to Date

During this past quarter the initial experiments have been designed,

the starting materials have been obtained or ordered, and necessary equipment

assembled or ordered.

Ceria (CeC^) doped with yttria (Y

2 03

) or gadolinia ( Gd^O^ ) will be the

material studied. Oxide powders of 99.9 and 99.99% purity have been obtained,

except for the yttria, for which 99.9% powder has been ordered but not yet
29

delivered. Arrangements have been made with a near-by commercial laboratory

for hot-pressing of specimens from these powders, to achieve high density with

uniform grain size.

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

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. In addition, we hope that the range is broad

enough so that some specimens will degrade within the period of a few weeks.

Since it is necessary to keep the number of experiments and specimens

manageable, and since the materials appear to be oxygen-ion conductors

over wide ranges of oxygen partial pressure, a single oxygen partial

-A

pressure, about 10 ' atm, will be used for all experiments.

In selecting the experimental conditions, two modes of degradation

have been considered. The first is a decrease in oxygen-ion conductivity

due to the ordering of the oxygen vacancies or of the dopant cations.

"Ordering" experiments will be carried out to measure the change in ionic

conductivity with time for various zero-current conditions. Temperature

and composition are expected to be the principal conditions which will

influence the rate and the extent of ordering of oxygen vacancies or of

dopant cations. The second mode of degradation is the development of

regions of reduced ionic conductivity due to the build-up or the depletion

of dopant cation concentration as a result of the migration of these

cations as small secondary currents. "Migration" experiments will be

performed to examine the effect of large current densities over long

periods on the ionic conductivity of doped ceria.
30

The two types of degradation experiments can be summarized as follows:

1. Ordering experiments

Specimens: CeO^ + 10, 20 or 30 mol. % YQ-j 5 or GdO-j ^ 40 specimens of each composition

-4

Oxygen partial pressure: ~ 10 atm

Temperature: 500, 700, 900, 1 1 00°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

propers will be examined by other techniques.

2. Migration experiments

Specimens: Ce0

2 + 10 or 20 mol. % YO^ or GdO^; 6 specimens

of each composition

Oxygen partial pressure: dry air

Temperature: Maximum 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 anode and to enhance the concentration at the

cathode, thus changing the electrical characteristics of the

snpcimen
31

Two furnaces for the 900° and 1 1 00°C anneals of the ordering experiments

are under construction. Two transparent furnaces have been ordered for the

500° and 700° anneals of the ordering experiments. A Pt-wound tube furnace

is being rebuilt for the migration experiments. The necessary control

equipment, etc., is being assembled.
32

4.

References

1. Research News* Science 178 , 1273 (1972).

2. W. J. Lueckel and P. J. Faris, presented at the IEEE Power Engineering

Society, July 15, 1974.

3. Chemical and Engineering Mews, January 7, (1974), p. 31; Wall Street

Journal, Thursday, December 20 (1973).

4. W. J. Leuckel , L. G. Eklund, and S. H. Law, IEEE Transactions on Power,

Apparatus, and Systems, Vol . PAS-92, 230 (1973).

5. Discussion at the Catalyst Workshop, Electric Power Research Institute,

Jan. 15-17 (1975).

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

7. T. Kudo and H. Obayashi, J. Electrochem. Soc. 122 , 142 (1975); 123 , 415 (1976).

8. A. J. Appleby, 7th Biennial Fuel Cell Symposium, 1969.

9. V. G. Levich, Adv. Electrochem. and Electrochem. Eng. 4_, 249 (1966).

10. A. J. McAlister, J. R. Cuthill, M. L. Williams, and R. C. Dobbyn, Proc.

Int'l. Symposium X-Ray Spectra and Electronic Structure of Matter, A. Faessler

and G. Wiech, eds. (Munich, 1973) Vol . II, p. 426.

11. L. H. Bennett, J. R. Cuthill, A. J. McAlister, N. E. Erickson and

R. E. Watson, Science 184 , 563 (1974). WC; Mo,; C, unpublished data of same authors,

12. H. Bobm and F. A. Pohl , Wiss. Ber. AEG-Tel efunken, 41_, 46 (1968).

13. R. J. Jasinski, in Fuel Cell Systems , G. J. Young and H. R. Linden,

eds. (Am. Chem. Soc., Washington, D. C., 1965), p. 95.

14. L. Baudendistel , H. Bohrn, J. Heffler, G. Louis, and F. A. Pohl,

Proc. 7th Intersociety Energy Conversion Engineering Conf. (American Chemical

Society, Washington, D. C., 1972).

15. P. N. Ross, Jr., and P. Stonehart, J. Catal . , 39, 298 (1975).

16. H. Bohm and F. A. Pohl, Lecture 3, Journees int. d' Etude des Piles

a Combustible Brussels, June 1969.
17. L. H. Bennett, A. J. McAlister, J. R. Cuthill, N. E. Erickson, and

R. E. Watson, to be published.

18. L. Ramqvist, Jernkont, Ann. 1 52 , 465 (1968).

19. G. J. Young and R. B. Rozelle, Ind. Eng. Chem. 52_, 298 (1960).

20. W. Vielstich, Fuel Cells (Wiley, N. Y. 1970), p. 129.

21. F. T. Bacon and T. M. Fry, Proc. Roy. Soc. A334 , 427 (1973).

22. D. B. Meadowcroft, Nature 226 , 847 (1970).

23. R. J. H. Voorhoeve, J. P. Remeika, P. E. Freeland, and B. T. Matthias

Science, 177, 353 (1972).

24. D. B. Sepa, A. Damjanovic, and O'M. Bockris, Electrochim. Acta,

12_, 746 (1967).

25. A. Damjanovic, D. Sepa, and J, O'M. Bockris, J. Res. Inst. Catalysis

Hokkaido Univ. 16_, 1 (1968).

26. B. Broyde, J. Catalysis 1_0, 13 (1968).

27. R. D. Armstrong, A. F. Douglas, and D. E. Williams, Energy Conversion

11, 7 (1971).

28. J. O'M. Bockris and J. McHardy, J. Electrochem. Soc. 120 , 61 (1973).

29. H. Gerischer, in Physical Chemistry , Vol . IXA, H. Eyring, ed.

(Academic Press, N. Y. 1970) p. 463.

30. D. M. Tench and E. Yeager, J. Electrochem. Soc. 1 20 , 164 (1973);

E. Yeager and J. Kuta, in Physical Chemistry , Vol. IXA, H. Eyring, ed.

(Academic Press, N. Y. 1970) p. 346.

31. U. Bertocci, M. I. Cohen, W. S. Horton, T. Negas, and A. R. Siedle,

Mixed Oxides for Fuel Cell Electrodes, NBSIR 75-742, Final Report on

Contract EPA-IAG-D4_0528, January 1976.

32. Discussions at The Catalyst Workshop, Electric Power Research

Institute, January 1975.
34

33. A. Damjanovic, in Modern Aspects of Electrochemistry , No. 5, J. O'M.

Bockris and B. E. Conway, ed. (Plenum, N. Y., 1969) p. 369.

34. K. J. Vetter, Electrochemical Kinetics (Academic Press, N. Y., 1967)

pp. 455-8.

35. D. M. Tench and E. Yeager, J. Electrochem. Soc. 1 21 , 318 (1974).

36. P. J. Boddy, in Progress in Solid State Chemistry , Vol . 4, p. 81

(1967).

37. M. Vielstich, Fuel Cells (Wiley, London, 1970), p. 246 ff.

38. Final Report, Project Fuel Cell, OCR Contract 14-01-0001-303,

Westinghouse Electric Co. (1970)

39. R. E. Carter and W. L. Roth in Electromotive Force Measurements

in High Temperature Systems , C. B. Alcock, ed. (Institution of Mining

and Metallurgy, London, 1968), p. 125.

40. R. C. Garvie in Refractory Materials, Vol. 5-11, A. M. Alper ed.,

(Academic Press, N. Y. 1970), p. 117.

41. B. C. H. Steele, in Solid State Chemistry , L. E. J. Roberts, ed.

(Buttersworth, London, 1972), p. 117.

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

43. J. E. Bauerle, J. Phys. Chem. Solids, 30_> 2657 (1969).

44. D. 0. Raleigh in El ectroanal.ytical Chemistry , A. J. Bard, ed. Vol. 6

(Dekker, 1973), p. 87.
S-1 14A (REV. 7-73)

J.s. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

SHEET

1. PUBLICATION OR REPORT NO.

NBSIR 76-1091

2. Gov’t Accession

No.

3. Recipient’s Accession No.

TITLE AND SUBTITLE

JERIALS FOR FUEL CELLS

5. Publication Date August 1976

6. Performing Organization Code

AUTHOR(S)

E. Bennett , M. I. Cohen, A. L. Dr aeoo. A. D. Franklin. and A . J .McAlister

8. Performing Organ. Report No.

PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

10. Project/Task/Work Unit No. 3130456

11. Contract/Grant No.

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

[vision of Conservation Research and Technology

. S. Energy Research and Development Administration

ashington, D. C. 20545

13. Type of Report & Period Covered

Quarterly Feb/Arp 1976

14. Sponsoring Agency Code

. SUPPLEMENTARY NOTES

i. 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 research plans and initial progress on the NBS program of

isearch on Materials for Fuel Cells. Transition metal carbides, borides, and nitrides

'refractory hard metals", or RHM) are being examined as potential hydrogen oxidation

.ectrocatalysts for use in acid fuel cells. Preliminary screening of 19 carbides and

)rides has revealed apparent stability in hot (up to 160 C) phosphoric acid for TaB,

)B, NbC ,TaC ,Mo_C ,and WC . More sophisticated tests for acid stability for these materials

:e under way. Equipment for examination of the electrochemical behavior of these materia It

3 hydrogen oxidation electrocatalysts in phosphoric acid electrolytes at temperatures upj

3 150 C is being completed , and tests will be started on these materials shortly. Addi- ! Lonal materials in the general RHM class , including a number of nitrides , have been orderi and will be screened for acid stability. More extensive exploraton will be made of the

roperties of TaC and WC as a function of stoichiometry and presence of other metals and

E oxygen. Plans exist for a parallel study of oxides as oxygen reduction electroatalysts for acid fuel cells not under support of this project. The current effort is

evoted to improving and expanding the facility for electrochemical measurements. The

Dng-term stability of CeO^-based solid electrolytes, which are promising new candidate

aterials for the high-temperature fuel cells, is under study. Materials for fabricating

pecimens of Ce0„ doped with Gd^O^ or Y^O^ have been obtained or ordered, and arrangeents made to hot press them. Experiments have been designed to examine degradation of the

CONTINUED

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 er ^um dioxide; degradation; electrocatalyeis ; fuel cells;

efractorv hard metal; solid electrolytes

8. AVAILABILITY jX

~ Unlimited

£2 For Official Distribution. Do Not Release to NTIS

1~ Order From Sup. of Doc., U.S. Government Printing Office Washington, D.C. 20402, SD Cat. No. Cl 3

IX Order From National Technical Information Service (NTIS)

Springfield, Virginia 22151

19. SECURITY CLASS 21. NO. OF PAGES

(THIS REPORT)

37

UNCL ASSIFIED

20. SECURITY CLASS 22. Price

(THIS PAGE)

$4.00

UNCLASSIFIED

USCOMM-DC 29042- P 74
CONTINUED

electrical properties at temperatures from 500°C to 110Q°C,with and without a constant

current throughout the degradation anneals. Compositions will be chosen around those

giving a maximum conductivity, and the oxygen partial pressure will be fixed at a convenient value. AC admittance measurements will be used to monitor the electrical

properties. Furnaces and ancillary equipment are being assembled or constructed.

\
'