NBSIR 75-742

Mixed Oxides for

Fuel Cell Electrodes

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

Institute for Materials Research

National Bureau of Standards

Washington, D. C. 20234

January 1976

Final Report

Prepared for Industrial Environmental Research Laboratory

Office of Research and Development

U.S. Environmental Protection Agency

Research Triangle Park, North Carolina 2771

1

NBSIR 75-742

MIXED OXIDES FOR

FUEL CELL ELECTRODES

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

Institute for Materials Research

National Bureau of Standards

Washington, D. C. 20234

January 1976

Final Report

Prepared for Industrial Environmental Research Laboratory

Office of Research and Development

U.S. Environmental Protection Agency

Research Triangle Park, North Carolina 2771

U.S. DEPARTMENT OF COMMERCE, Rogers C.B. Morton, Secretary

James A. Baker, III, Urtder Secretary

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

NATIONAL BUREAU OF STANDARDS. Ernest Ambler, Acting Director

ABSTRACT

This report presents studies of the preparation, chemical stability,

and electrochemical behavior of mixed oxides in order to determine if

these are potential candidates as oxygen-reducing electrocatalysts in an acid fuel cell. Materials studied included strontium and barium cobaltates

and manganates with and without added titanium; lanthanum titanates, with

and without calcium or strontium; calcium, strontium, and bari\am ruthenates; and mixed oxides of the systems Ti-Ta-0, V-Nb-0, Ce-Ta-0, Pr-Ta-0

,

Ce-Nb-0, and Ce-Pr-Ta-0. The choices were based upon producing variable

valence for a given transition metal and upon conferring stability at

elevated temperatures 150 °C) in phosphoric acid using oxides of known

stability. Barium ruthenate and the systems Ti-Ta-0, V-Nb-0, V-Ta-0,

Ce-Ta-0 were hot-acid stable. The high temperature reactions of CeTaOi^+jj with

0 $ X $ 0.5 were studied in air up to about 1960 °C.

Potentiodynamic and galvanostatic studies are reported for materials

from the Ti-Ta-0 system, Ti02 as grown, Ti02 reduced with hydrogen, Ti02

with 0.1% Nb , lanthaniam titanates with and without calcium or strontiiam,

a tungsten bronze, barium ruthenate, and strontium titanate with 0.03%

and with 0.15% Nb.

Because the reduction of oxygen is easier than that of nitrogen,

and because biochemical reduction of nitrogen takes place rather easily,

preparation of inorganic compounds with ternary metal-sulfur arrays

similar to the arrays in nitrogen reductase was attempted. These

compounds were also to be tested in a manner similar to the mixed oxides

The following were made: (Ph3P) 4CU2W2S5 , (PhsP) 5Ag5W2Sg02 , and

[ (Ph3P) 3Ag] 2W2SS , where PhaP refers to the triphenylphosphine moiety.

Also prepared were (Ph3PAu) 2W2S4 , (Ph3AsAu) 2WSi^ , (diphos) NiW02S2 , (diphos) 2Pd3W2S602 , and (Ph3P) 3PtW2S602 , where "diphos" refers to

1 , 2-bis (diphenylphosphino) ethane

iii

TABLE OF CONTENTS

Page

Abstract ii

List of Figures v Acknowledgements vi

I. CONCLUSIONS 1

II. KECOrMENDATIONS 2 III. INTRODUCTION 3 General 3 Objectives 4

Sections 4

IV. ACID STABILITY 6

Method of Test 6

Results 7 V. PHASE STUDIES AND MATERIALS PREPARATION 9

VI. ELECTROCHEMICAL MEASUREMENTS . . 16

Introduction 16

Experimental Arrangement 16

Description of Experimental Set-up 16

Description of Cells Used 16

Electrodes 18

Experimental Methods 23

Potentiodynamic Measurements 23

Titanates

Tungsten Bronze 26

Barium Ruthenate 26

Redox Measurements 33

Titanates

Tungsten Bronze -^4 Barium Ruthenate 35

Discussion 39

V
Page

VII. MATERIALS OTHER THAN OXIDES 42

Introduction 42

Results 42

Aqueous Systems 42

Nonaqueous Systems 43

VIII. REFERENCES 46

IX. LIST OF PUBLICATIONS 48

vi
LIST OF FIGURES

1. Block diagram of electrochemical instrumentation. Page 17.

2. Electrochemical cell used at room temperature with Type I electrode.

Page 19.

3. Type II electrode shown in front view and cross-section. Page 21.

4. Current-potential curve for 95Ti02 : 5Ta205 in 5 mol/1 HsPO^ at room

temperature. Page 24.

5 . Current-potential curves for the lanthanum titanates at room

temperature. Page 25.

6. Current-potential curves for a Type II BaRu03 electrode in concentrated phosphoric acid at 60 °C. Page. 27.

7. Effect of oxygen and the current-potential curve for a Type II

BaRu03 electrode. Page 29.

8. Current-potential curve for a Type II BaRu03 electrode in neutral

solution. Page 30.

9. Comparison of two BaRu03 electrodes (Type II) in 6 mol/1 NaOH at 60 °C.

Page 31.

10. The effect of 0.007M H2O2 on a Type II BaRu03 and on smooth Pt.

Page 32.

11. Steady-state performance of Type II BaRu03 electrode with

ferro-ferri cyanide reaction; semilog scale. Page 36.

12. Steady-state performance of Type II BaRu03 electrode with

ferro-ferricyanide reaction; linear scale. Page 37.

13. Steady-state performance of Type II BaRu03 electrode with

iodine-iodide reaction. Page 38.

vii
•ACKNOWLEDGEMENTS

The work reported herein was performed by Dr. U. Bertocci,

Mr. M. I. Cohen, Dr. T. Negas , and Dr. A. R. Siedle, with the assistance of Mr. N. K. Adams, Mr. J. L. Mullen, Mr. C. D. Olson, and

Mr. J. Broussalian.

Dr. A. D. Franklin provided overall scientific guidance. Dr.

H. P. R. Frederikse participated constructively in a number of the

discussions particularly those dealing with catalytic effects of a variety of perovskites . Dr. R. S. Roth provided valuable crystallographic data and advice on the cerium tantalates and the metal

thiotungstate derivatives. Mr. H. S. Parker gave valuable assistance

with preparation of single crystals. Dr. G. A. Candela performed and

interpreted magnetic measurements . All of the above collaborators

are at NBS.

Mr. N. L. Loeffler, University of Tennessee, engendered the

interest in and collaborated in the preparation of the lanthanum

titanates

viii
SECTION I CONCLUSIONS

On the basis of consideration of stoichiometry and the expected

effect on electron transport properties , the mixed oxides containing

rare earths (cesium and/or praeseodynium) and certain transition metals

(titanium, vanadium, niobiuir. , and tantalum.) may be quite interesting as candidates for electrocatalytic applications in hot-acid fuel cells.

It has been shown that somie are stable in hot, concentrated phosphoric

acid. The high temperature phase relations important to their preparation have been studied.

Barium ruthenate, although exhibiting stability in hot, phosphoric

acid, appeared to change chem.ically during the passage of electric

current. This material may not be useful as a catalyst for the oxygen

electrode of an acid fuel cell.

Alkaline earth titanates , manganates , and cobaltates are soluble

in hot, phosphoric acid. They are, therefore, unsuitable as catalysts

for fuel cells of the t^^pe considered here . Although some materials appear promising, the work reported here was

too preliminary to decide definitely that oxide materials will or will

not serve adequately as oxygen electrodes in a hot phosphoric acid

fuel cell.

1
SECTION II

RECOMMENDATIONS

There are several fruitful areas of research in which appropriate

work may well lead to a substitute for platinum and increased understanding of the electro-catalytic process involved. Of the materials

studied here, barium ruthenate appeared to have promise as a model

material, but much remains to be done to provide an understanding

of its chemistry as an electrode.

The phase relations for the mixed oxides containing rare earths

[e.g., cesium and/or praeseodymium) and certain transition metals

{e.g., titanium, vanadium, niobium, and/or tantalum) should be elucidated in order to prepare suitable materials for testing. Then the

acid stability tests should be made where still needed, and the

electrochemical studies performed.

Appropriate electrodes of triphenylphosphine copper thiotungstate should be prepared and studied electrochemically

2
SECTION III

INTRODUCTION

GENERAL

The fuel cell is related to the well-known "storage battery"

through the fact that both convert chemical energy into electrical

energy. The principal difference is pointed up by the words fuel and

storage. The storage battery, a regenerative device, stores electrical

energy by converting it to chemical energy for later release again as electricity. The fuel cell, on the other hand, can convert fuel into

electrical energy continuously.

In ordinary use, fuel is burned and the heat thereby generated

is used directly, or among other possibilities, it may be converted

to steam which is used to power an electric generator. Other means

to burn fuel and produce electric energy include magnetohydrodynamic generation. In a fuel cell, on the other hand, the chemical reaction between the fuel and oxidant produces , in the appropriate electrochemical device, electrical energy directly. For the

simplest devices hydrogen and oxygen serve directly as fuel and

oxidant. More complex systems are required in order to use fossil

fuels or derivatives therefrom. Air is a practical and economical

oxidant

The fuel cell offers advantages of high efficiency at partial

and at full load, silence, low-pollution, unattended operation, and

construction in modular units to allow for optimum distribution of

sites. There are, however, a number of areas where further

research and development are needed to make fuel cells economically

competitive so that they will make a significant impact on the

energy conversion technology of the United States. A large factor

in the economics of these devices is the cost of the catalyst used

on the electrodes. Currently, platinim is the most widely used catalyst

3
in the acid fuel cell , the leading candidate for widespread use in

electric power generation. For example, it is the catalyst for the

United Aircraft device for power generation"'' which has had the most

development, testing, and demonstration to date. The great cost of

platinum affects the fuel cell economics by leading to high initial

cost and to high replacement cost because platinum has a relatively

short life-time. Another factor is that its catalytic action is not

perfect and permits formation of intermediate peroxides cutting

efficiency by about 40% . For these reasons a fruitful program of

research would be to look for alternate catalysts which may not suffer

from some of the disadvantages of platinum. An important auxiliary

benefit could be to gain further understanding of the electrochemical

processes involved and of the nature of pertinent catalytic materials.

The work reported here was based upon the assumption that the

pertinent fuel cell for power generation would have hot phosphoric

acid as an electrolyte. Again, this is used in the cell which is

the forerunner"^. Furthermore, attention was focussed upon the

oxygen electrode, where the electrochemical efficiency loss appears

to be greatest. Consequently, two required properties are stability

in hot phosphoric acid and catalytic activity for the electroreduction

of oxygen . . : • ^ ,-f>Y; '

OBJECTIVES

The objectives of the project were to assess the potential of some selected transition metal oxides as electrocatalysts for fuel

cells and to develop an understanding of the important material

parameters for this application. It is a long-term goal, beyond

the term of the contract, to establish an interrelationship between

the structure, defect content, and concentration of dopants of

metal oxides and their behavior as electrodes in low temperature

fuel cells , with emphasis upon the cathode
ORGANIZATION OF THE REPORT

In the remainder of the report, separate sections will be

devoted to phase studies related to the discovery and preparation

of new nixed oxide materials . Brief studies of the acid stability

of some of these materials are reported. Electrochemical studies

of those materials which appeared at the time to be potential

catalysts for oxygen reduction at the cathode of a fuel cell are included.

Early in the course of the project the idea was conceived that

the molecular structure of nitrogen-reducing enzymes might offer a clue to oxygen reduction catalysts. Consequently, some effort was aimed at producing pertinent compounds . A nuriiber of such compounds

were made, and some of these are being subjected to tests involving

solid electrodes. The concept and the chemistry involved in the

preparation are sufficiently interesting to be reported here

5
SECTION IV

; ACID STABILITY

INTRODUCTION

A systematic program that integrated efforts in the acid

stability and materials preparation (see Section V) studies was

followed. Initially, acid stability testing was conducted using

available and newly-synthesized oxide materials which showed potential for suitable electronic properties. These materials primarily

were oxides containing first-row transition metals. This avenue of

approach proved unsuccessful and emphasis, therefore, was shifted

toward materials having potential for acid stability. Materials

in this group were oxides containing tantalum or niobium. These

oxides proved to be acid stable but are poor electronic conductors.

Research, therefore, was focussed on new Ta- and Nb-containing

oxides which could be modified chemically to enhance their electronic

properties .

METHOD OF TEST

Stability in 85% phosphoric acid at a minin\;im of 100 °C was

chosen as an initial screening test for candidate oxide materials

This test was conducted in two stages. A qualitative determination

first was made by simply immersing powdered oxides in a bath of hot

acid. Stability of a material was noted by observing apparent

degradational processes (i.e., dissolution/decomposition, solution

color changes) . Residues from these tests were examined by powder

x-ray diffraction to determine any changes in phase constitution

from the original , single phase , untreated material . A material

which did not show apparent degradation during this stage was fashioned into a rod- or disc-shaped body and sintered at an appropriate temperature . This body was placed in a Pt-wire basket

which then was immersed and rotated in a bath of hot acid. Possible

degradation of the material was monitored gravimetrically . In most

6
cases this test was performed first at 110 °C and then at 150 °C

for durations of up to one month.

RESULTS

The initial stage of this investigation concentrated on the

acid stability characteristics of oxides containing first-row

transition metals previously prepared in this laboratory. For this

stage, however, only those materials were chosen which, from stoichiometric

considerations, had at least the potential to be metallic or semiconducting.

Materials from the following systems were tested:

a. SrMn03_xr 0 ^ x $ 0.35 (see ref. 2 for complete details of

the system)

b. BaMn03-xf 0 $ x $ 0.28 (see ref. 3 for complete details)

c. Bai_ySryMn03_x/ (see ref, 4 for complete details)

d. BaCo03-x and BaNi03_x (see ref. 5 and 5 for complete details)

e. SrMn03_j^ - "manganese oxide" (see ref. 7 for complete details)

These systems feature materials having numerous crystal structures

related to and including the perovskite type. Moreover, the transition metal (Mn, Ni, or Co) can exist in multiple oxidation states,

a phenomenon (nonstoichiometry) recently shown to be important in some types of catalytic activity (i.e., CO oxidation and NOx

reduction) . None of the materials in these systems withstood attack

by hot phosphoric acid. It was noted, however, that the low temperature form of SrMn03_x (0 ^ x $ 0.10) , with a so-called four-layer

2 hexagonal structure was not attacked at room temperature . The

compound began to dissolve at 60 °C. It was thought that this

material might be rendered more acid stable by suitable partial

substitution (solid solution) of acid stable, oxide end-members

having similar, perovskite-related structures. BaTi03 and SrTi03 were chosen as candidate materials for this purpose in view of

their resistance to some acids. However, this effort was curtailed

when tests showed that even these materials undergo dissolution in

hot phosphoric acid.

7
Another series of nonstoichiornetric transition inetal oxide

materials of the perovskite type, La ^2/3) +x'^-'-^x'^-''- 3^-3x^3 ^ 1/3)

appeared promising. This series contains solid solutions with

properties ranging from metallic (LaTi^"''03) through semiconducting

to insulator (La2/3Ti'^'^03) . It had been suggested^ that these

materials were stable in cold HCl , HNO3 , perchloric acid and in

hot, dilute HNO3 , and in boiling perchloric acid. They were slowly attached by aqua regia. Phases in this series, as well as Gd and Sm analogues, were prepared either in this laboratory or obtained from an external source (Mr. Neil Loeffler, University

of Tennessee Space Institute). Similar materials,

Lao .7 5Ao.2 5TiSt7 5Ti^t2 503 ~ Sr or Ca) were also prepared in

cooperation with Mr. Loeffler. All materials were tested and

found to dissolve in hot phosphoric acid and 3N sulfuric acid.

In the second stage of this study, oxide materials containing

Ru, Ta, and Nb were investigated. BaRu03 , SrRuOs , and CaRuOs

9 10 characterized as having "good" conductivity ' and metallic

conductivity"^^ were tested. Only BaRu03 was found to be stable

(110 °C, 5 days) in phosphoric acid. All of the niobium- and

tantalum-containing phases that were prepared (see Section V)

showed remarkable resistance to attack by hot phosphoric acid.

Many of these phases, however, are poor conductors and it would

be necessary to enhance their electronic properties , perhaps , by

suitable doping.

8
SECTION V

PHASE STUDIES AND MATERIALS PREPARATION

Perovskite materials in the series La (2/3) +x'^i^x'^il -3x'-'3 (x $ 1/3) and in the series Lag ^ysAg ^25'^i^t75'^^^t2503

(A = Sr or Ca) were synthesized. The latter materials were chosen

hoping to enhance the acid stability of the first

series without a sacrifice in electronic conductivity. Materials

were prepared from La203 [from La (OH) 3], Ti02 , Ti metal, and

SrTi03 or CaTi03 starting materials, proportioned to yield the

desired stoichiometries . The starting materials were mixed then

melted and cooled in high purity Mo crucibles in an induction furnace

containing a "gettered" argon atmosphere. The resulting products,

examined by powder x-ray diffraction, were found to be single-phase

perovskites (non-cubic) . These materials were utilized subsequently

for electrochemical and acid stability experiments.

BaRu03, SrRuOs, and CaRu03 were prepared from the appropriate

alkaline-earth carbonate and RUO2 or Ru metal . The blended

starting materials were reacted in gold envelopes at 1000 °C in

air for one day. SrRuOs and CaRuOs have the perovskite structure

while BaRu03 has a so-called nine-layer rhombohedral -hexagonal

structure in which face-sharing of Ru-containing oxygen octahedra

9 occurs . As indicated in Section IV only BaRu03 remains stable in

phosphoric acid and was used in subsequent electrochemical

experiments. The acid stability of BaRu03 was somewhat unexpected

as the phase Bag .gSrg . iMn03 (< 1265 °C in air) has the same

4

crystal structure yet is attacked by phosphoric acid. Phase

transitions in or reduction of BaRuOs were not detected up to

1600 °C in air by differential thermal and thermogravimetric

analysis. Significant vaporization, presumably of Ru through

oxidation to gaseous RUO2 or RUO3, was observed above 1500 °C.

This vaporization precludes the preparation of dense sintered discs

at these elevated temperatures. For example, a pressed disc of

9
initially single phase BaRuOs was fired 22 hours at 1500 °C. The

product consisted of two phases, BaRu03 plus a ruthenium-containing

phase richer in barium. If dense discs are necessary for electrochemical testing, BaRu03 could be hot-pressed or isostatically

pressed at lower temperatures (< 1500 °C) to prevent loss of

compositional integrity.

Past experience in this laboratory has shown that oxide

materials containing Nb205 or Ta205 resist attack by many acids.

For this reason, several of these oxides were synthesized and

tested with phosphoric acid prior to initiating an investigation

12 of new materials. In the system Ta205-Ti02, Roth and Waring at

NBS showed that Ti02 (rutile) can accommodate as much as 10 mole

percent Ta205, depending on temperature, without gross structural

changes. A composition midway in this range, Ta205 •19Ti02 , was chosen and prepared using high purity Ti02 and Ta205 starting

materials . After a preliminary calcination at 1000 °C for

three days, this mixture was fired at 1600 °C for one day and

quenched in water. The final blue-black product consisted of

single-phase rutile solid solution as revealed by powder x-ray

diffraction analysis. This material was utilized for acid and

electrochemical testing. The compounds V205*9Nb205 and

V205*9Ta205 were also prepared according to methods in reference

13 and tested in hot phosphoric acid.

In view of the apparent resistance of Ta205- and Nb20 5 -containing

oxides to attack by hot H3PO1+ , an investigation of new materials based

on these oxides was initiated. Cerium oxide was chosen as an additional component because the cerium cation can exist in

crystalline phases in mixed 3+ , 4+ oxidation states . This could

enhance the probability for reasonable electronic conductivity of

potential electrode materials . Furthermore , cerium oxide was

14 snown to be a catalyst for related gas phase reactions . When Ce02

reacts with Ta205 (or Nb205) at elevated temperatures, compounds

are formed that contain the reduced oxidation state, Ce^"^, even

10
under oxidizing conditions as in air. In the Ta205-ceriura oxide

system, three compounds, CeTayOig, CeTasOg , and CeTaO^ are stable.

The crystallographic and magnetic properties of these , as well

as additional, unusual phases, were detailed fully in this

laboratory. The phases CeTasOg and CeTaOi^ resist attack by hot

H3P0i+, but, apparently, are not good conductors. Attempts,

therefore, were made to enhance the electrical conductivity by

altering the Ce^"*" content toward mixed Ce^'''/Ce^"'" variants for

each phase.

CeTasOg can be reformulated as Cei/3Ta03. Single crystal

and powder x-ray diffraction studies revealed that this material

has the perovskite structure which is distorted to orthorhombic

symmetry. The formulation, Cei/3Ta03, indicates that the A-sites

of this perovskite (ideally ABO3 , A/B = 1.0) are not fully

occupied, ^^g/^ip^ ~ 1/3. Attempts, therefore, were made to

establish Ce'^"'' cations at the A-sites by appropriate substitutions

of A^"*" cations. This would lead to mixed Ce^"'"/Ce^'^ oxidation

states and possibly to enhanced conductivity. The substitution

mechanism can be formulated as

[Ce^t/3)-xCe;^j2Al/2]Ta03, x < 1/3 (1)

The feasibility of this type of substitution was tested by

additions of Sr^"^ according to,

[Ce^;;;30Ce^;^20Sri/20]TaO2, x = 1/10 in (1) (2)

Single phase perovskite products could not be synthesized and this

effort was discontinued. It is noted, however, that Cei/3Ta03,

an acid stable material, might be used as a component to enhance

the acid stability of perovskite and perovskite-like phases of

the first-row transition metal oxides previously tested (see

Section IV)

Attempts to vary the Ce^''"/Ce^"'" content of CeTaOi+ met with

success . Phase equilibrium studies (reaction and

quenching of materials from elevated temperatures) , single crystal

and powder x-ray diffraction analysis, thermogravimetric analysis,

11
and magnetic susceptibility measurements were utilized to

completely define phase relations and oxidation-reduction

reactions in the system CeTaOi^ -oxygen (in air) . CeTaO^ is stable

in air from 1265 °C to a melting point near 1960 °C. At 1265 °C,

the material oxidizes (reversibly) according to,

1265 °C

3CeTaOi+ + 1/2 O2 »- CeTasOg + 2Ce02 (3)

Notice that the bulk composition of the decomposition products is

expressed by Ce3Ta30i3 or CeTa04^33. The reaction proceeds to

the right rapidly within the range 1265° -1100 °C but sluggishly

between 1100°-1000 °C. If CeTaOi+ is quenched to room temperature

and reheated below 1000 °C, or, if the material is rapidly cooled

from above 1265 °C to below 1000 °C, the oxidation-decomposition

reaction (3) is completely by-passed. Instead, CeTaOt+ absorbs

oxygen (oxidizes) according to,

CeTaOi+ + 1/2 x O2 CeTaOtt+x (4)

The X parameter in CeTaOt++x is variable but temperature dependent.

Four distinct reaction series v;ere defined for the system

CeTaOi+ -oxygen (CeTaOi++x) • A summary of the observed reactions and

reaction paths is given in the flow diagram provided. The

following symbols are used:

(-* ») , rapid, reversible reaction

^) ^ sluggish, time dependent, but reversible reaction

( ^) ^ sluggish, non-reversible reaction

^) ^ reversible reaction but rapid or sluggish in the

direction indicated.

It is emphasized that every phase shown in the flow diagram can be

prepared by the pertinent reaction path and can be quenched (and

retained) to room temperature. Within series 2, 3, and 4, x varies

continuously with temperature. For this reason, only typical x parameters in CeTaOt++jj are given for selected temperatures

.

Ce^'''/Ce^''" contents can be computed for given x parameters using

Cejl2x^®^x'^^°4+x' This formulation was confirmed by magnetic

12
CeTaOi.

The System CeTaOi^+x (0 $ x $ 0.5) in Air

1265'

CeTasOg + Ce02 (decomposition products) Series 1

350° CeTaOi4 (quenched from > 1265° to room temp.)

CeTaO^.5 0

500'

CeTaOt+.i+gi o

575°

CeTaO1+ .1+80

600'

CeTa04.i55

A

800'

/

950'

CeTa04.o58

950'

CeTa04.3t,3

800'

CeTa04.36 7

i

700°

CeTaOi, .38 3 A

700° to room temperature

CeTaOtt .i+02

Series 2 Series 3 Series 4

13
susceptibility measurements

Starting with preformed CeTaO^ (> 1265 °C, quenched to room

temperature) any reaction series can be initiated by heating the

materials within the pertinent temperature range indicated. Once

series 4 phases (not related crystallographically to CeTaOi+) are formed, however, series 3 and 2 materials subsequently will not

form. Series 4 materials will transform to series 1 phases at

elevated temperatures . Within each series , oxidation-reduction

reactions are almost instantaneous and reversible , with x being a function of temperature. Reactions between any two series are generally sluggish and not necessarily reversible. For example,

reactions in series 3 can be followed (thermogravimetrically)

continuously to at least 950 °C (x = 0.058) . Phases in this series

are crystallographically related to CeTaOi^ . If the sequence is

stopped within the 900-950 °C interval, and the series 3 material

is maintained at constant temperature , series 4 materials will

slowly develop over several hours. Thus the transition from

series 3 to series 4 materials, although indicated at 950 °C, can proceed sluggishly between at least 900-950 °C. Another example

is the oxidation reaction which results in the formation of com- pletely oxidized cerium phase CeTaOi+,50 (350 °C) from CaTaOt+ or from

any series 3 phase. This reaction proceeds over periods of at

least several days. Indeed, one sample of CeTaOi^ was maintained

at 350° for 61 days to insure formation of a completely oxidized

material which could be used for further experimentation. Phases

in series 2, 3, and 4 will transform to series 1 phases when reacted

above 1000 °C. Once CeTaOi^ is decomposed (series 1, < 1265 °C) , further reactions at lower temperatures, of course, are precluded.

The complex reactions observed for the CeTaOt++jj system provided

the impetus to investigate similar Ta- and Nb-containing materials.

The phases Pr^'''Ta04 and Ce^"*'NbOi+ were reported by Bodiot^^. PrTaOi^ was found to have an x-ray diffraction powder pattern (unindexed)

similar to that of CeTaO^ . CeNbO^ , however, has the m6noclinic

(M) -fergusonite structure (see, for example, ref. 16) at room

14
temperature. These materials were also prepared attempting to vary

their Ln^"*"/Ln^''' contents by appropriate annealing at low temperatures . PrTaOit is isostructural with CeTaOi4 , and the magnetic and

crystallographic properties of the material have been characterized.

In air, PrTaOi+ does not landergo the reaction series shown by CeTaOi^

This can be attributed to the difficulty of oxidizing Pr^"*" to Pr^"*". Indeed, Pr50ix (= PrOi^sss) the highest oxide that is stable in an air environment. PrTaOi^^^^ might be prepared at much higher oxygen

pressures. As PrTaO^ is isostructural with CeTaO^. and because it does

not oxidize, it is conceivable that this component can be utilized to

stabilize CeTaOm-x phases at specific x values and at lower temperatures.

For exair.ple, series 3 CeTaO^+x phases are stable only between 600 °C

to about 900 °C, but may be quenched to room temperature. If these

materials are subjected to high oxygen activities at lower temperatures, complete oxidation to CeTaOii.50 ultimately results. Suppose

PrTaOit is introduced in solid solution with CeTaOi^ , to give , for

example, the composition (Ce^'^sPr^'J's) TaOi+ . If it were possible

to completely oxidize this phase the maximum oxygen content would be

(Ce^t5P^3t5)Ta04.25/ ^ = 0.25, but not (CeJtsPr^ts) TaOi+ . 5 . The

stoichiometry , when x = 0.25, however, is never encompassed by the

X parameter variation within any given CeTaOi^+x series . If phases

with X > 0.25 cannot be obtained, then x parameters typical only

of series 3 phases remain possible. Single-phase (Ce^'^sPr^'^s) TaO^ was prepared and annealed at temperatures < 900 °C in air. Only

series 3 type phases were obtained.

The Ce'^'^/Ce^"*" content in CeNbO^ (not isostructural with

CeTaOi^) also can be varied according to preliminary data. Furthermore, x-ray diffraction data suggest that more than one reaction

series is operative. One series appears to be characterized by

oxidation together with exsolution of Ce02 from CeNbOt+ . Such a process would yield materials having variable Ce^''"/Ce'^''" contents

and vacancies on the Ce sublattice

15
SECTION VI

ELECTROCHEMICAL MEASUREMENTS

INTRODUCTION

Potential materials for the oxygen electrode in fuel cells

must be screened, under conditions similar to those encountered in

a real cell, for both stability and catalytic activity. Although,

ultimately, any electrode found must be tested by construction

of an actual cell, there are simpler methods of screening available that can be used to select the most likely candidates. Such

techniques were initiated during the course of this study.

Two basic approaches to the electrochemical characterization

of oxide electrodes were utilized. The primary method was to

subject the samples to a potentiodynamic scan ranging from the

hydrogen evolution potential to anodic potentials greater than the

expected potential for oxygen evolution. This was performed with

the sample in electrolytes similar to those used in real fuel cells.

If a cathodic current were to be observed in the potential range of

oxygen reduction, the possibility that the current was due to chemical

instability would have to be checked before it could be ruled out.

A secondary, galvanostatic , method was to observe the potential at

various fixed currents on both the anodic and cathodic side of

the equilibrium potential for several oxidation-reduction couples

dissolved in varying electrolytes

.

EXPERIMENTAL ARRANGEMENTS

Description of Experimental Set-up

As shown in Figure 1, the experimental arrangement is straightforward. Minor details such as a compensator for the recorder and

switching circuits for AC work have been omitted. A detailed

discussion of the cell and of the electrode format used will follow

later in this section.

The potentiostat is a commercial unit capable of providing

slightly more than 100 ma. Its control voltage is provided by the

output of the control voltage amplifier which is driven by the

16
Potentiostat

CE Ref.

Cell

Control Voltage

Amplifier

WE

Differential

Amplifier

Ramp

Generator

2- Pen

Recorder

X - Y

Recorder

C R 0

Switch positions

! Potentiostatic

2" Galvanostatic

Figure 1. Block diagram of electrochemical instrumentation.
ramp generator. The ramp generator consists of a motorizec muititurn potentiometer wired as a voltage divider. Pamp rates range

from 1.6 X 10"^ V/sec to 8 x 10"^ V/sec. Adjustment of the gain

of the control voltage amplifier gives the capability of rates

up to 1.6 V/sec.

Description of Cells Used

Depending on the choice of sample electrode, two types of

cells were used. The first, used with electrodes of Type I (described later) is shown in cross section in Figure 2 . This

cell consists of four interconnected compartments. The sample-tocounter electrode connection is through a fritted glass disc . The

two remaining compartments are for a Pt and a calomel reference

electrode, respectively. Provisions were made in the sample

container for stirring and for bubbling gasses over the sample

.

The sample is electrically connected to the system by a threaded

stainless steel rod which is protected from the electrolyte by

glass tiibing sealed to the sample by a Teflon gasket, in the manner

17

of Stern and Makrides

The second type of cell which was used, is intended for Type

II electrodes (described later). Essentially, it was a beaker

contained in a water bath. The entire assembly was enclosed in

an insulated box with feed-throughs for electrical contact and

gas input. The counter electrode was platinum black and of

Type II , and the reference electrode was either a smooth Pt strip

or a calomel electrode. For most of the experiments, the reference

was placed in a compartment connected to the electrolyte near

the sample by a capillary.

Electrodes

For convenience, in this report the two kinds of electrodes

used are labelled as Type I and Type II. Type I electrodes (shown

in cross section as part of Fig. 2) were made from poly- or mono18
Stainless Steel

Figure 2 . Electrochemical cell used at room temperature with Type I electrodes. This is a cross section of three of the four

compartments . Compartment 4 , which contains the Calomel

reference, is situated behind the compartment for the Pt

reference. A type I electrode is also shown in cross section. The heavy black line, connecting sample and Cu

block, denotes a Pt wire for electrical contact.

19
crystalline samples or from sintered powders . A Pt wire lead was attached to the rear surface of the sample by appropriate means

(indium soldering, peening, silver paste, etc.) , and the other

end of the lead was soldered to a Cu block about 1 cm. on each edge.

This assembly was placed in a mold made from Al foil and coldsetting epoxy mounting compound was poured in. The mold was then

placed in an evacuated chamber in order to remove air from any

pores and allow them to be filled by the epoxy. After a few

minutes , the mold was returned to atmosphere and allowed to set

for 24 hours. When set, the Al foil was stripped off and the

mounted sample was ground to a convenient shape. A hole was drilled

through one end into the Cu block, and this was then threaded to

receive the stainless steel rod previously mentioned. The sample

surface was then ground with 600 grit.

Type II electrodes (Fig. 3) were made from powders and are porous . These electrodes were manufactured by a modification of

18 the method of Neidrach and Alford . The substrates for Type II

electrodes were strips 1.5 cm by 15 cm of either electrodeposited

Ni or stainless steel screen, 0.017 cm thick with approximately

20% open area. This was coated with a 0.005 cm thick layer of

Teflon film except for two regions. At one end, about 5 cm were

left uncoated for purposes of electrical connection to the

measuring instriiments . Close to the other end, an area of about

0.5 cm by 1.5 cm was left clear on one side of the strip. A paste

composed of the sample powder mixed with #30 Teflon emulsion was placed on this area and covered with Al foil. The electrode was hot pressed at 320 °C under an applied load of 2 x 10^ kg for

2 minutes. When cool, the Al foil was removed by immersion in

6 mol/1 NaOH.

Table 1 shows the electrochemical systems that were examined.

20
Ni

Figure 3. Type II electrode shown in front view and cross section.
Table 1. ELECTROCHEMICAL EXPERIMENTS

Material Electrode Type Study Method

Ti02:5% Ta I potentiodynartiic

Ti02:5% Ta II potentiodynamic

Ti02:H2 reduced I potentiodynamic

galvanostatic

TiO2:0.1% Nb I potentiodynamic

galvanostatic

Lao,92Ti03 (nom.) I potentiodynamic

galvanostatic

Lag 75Ti03 (noiti.) ,•1'

,

potentiodynamic

galvanostatic

Lao.7 5Cao.2 5Ti03 I potentiodynamic

Lao.75Sro .25Ti03

Nag 7WO3

'l

I

potentiodynamic

potentiodynamic

galvanostatic

BaRu03 I potentiodynamic

BaRu03 II potentiodynamic

galvanostatic

SrTi03 :0.03% Nb I potentiodynamic

galvanostatic

SrTiO3:0.15% Nb I potentiodynamic

galvanostatic

Ti02 as grown I potentiodynamic

galvanostatic

Ni II potentiodynamic

Pt black 'II potentiodynamic

22
EXPERIMENTAL METHODS

Potentiodynamic Measurements

Titanates - Voltammetric scans of the titanates were done in both 5M and

concentrated phosphoric acid electrolytes with voltage scan range

of the order of 10 volts . Normal procedure was to sweep the potential

at a linear rate from values more negative than required for

hydrogen evolution to those more positive than needed for oxygen

eyolution. This was done at room temperature and at 60 °C nominal.

On the basis of behavior, the samples studied may be separated

into three groups. The first of these, (SrTi03 :0 .03% Nb and

SrTiO3:0.15% Nb) is soluble in phosphoric acid and will not be

discussed in this section. The second group (Ti02 as grown, Ti02

5% Ta, TiO2:0.1% Nb, Ti02:H2 reduced) gave I-V curves typified by

Figure 4 . Starting at a potential yielding good evolution of

H2 and scanning in an anodic direction, a peak due to oxidation

of H2 was first observed followed by a region of zero current. At

about 2.5 volts positive vs. S.C.E., a sizeable current began to flow.

On the reversed sweep direction there was no observable negative

current until the potential for H2 evolution was reached. Bubbling

of oxygen gas over the electrode yielded no detectable response

The third group of samples (LaQ.ysTiOs, Lag. 92^103,

Lag .ysSrg . 25*^^03 , and Lag . ysCag ,25'^i03 - all values nominal) gave

a family of I-V curves (Figure 5) that appear to depend on the

value of X in the generalized formula LaxTi03. (Addition of Sr ^

or Ca"*"^ yields an effective new x value.) Starting with Lao^75Ti03

fairly large anodic currents were drawn at potentials on the order

of +2 volts. As indicated in Fig. 5, however, these currents fell

off with continued cycling of the potentiodynamic sweep . As x was increased, there was a shift toward higher anodic potentials for

23
POTENTIAL (vs SCE)

VOLTS -

Figure 4. Current-potential curve for 95 TiO^ : 5 Ta^O^ in 5 mol/1 H^PO

at room temperature. Type I electrode configuration.

24
Figure 5. Current-potential curves for the Lanthanum Titanates at room temperature. Type I electrodes in 5 mol/1 H PO .
current flow, until x = 0.92 where the observed current approached

zero. There was no visible evolution of oxygen, and bubbling of

oxygen over the electrode gave no response.

Tungsten Bronze - A Nag 7WO3 single crystal was subjected to the same conditions

as above . Starting at the hydrogen evolution potential , a large

negative current was observed. As the scan went toward anodic

potentials the positive current peak due to oxidation of hydrogen

appeared (much larger than in the case of the titanates) . The

current then fell to zero and as the potential went positive, the

surface of the electrode changed color and after a short positive

transient the current returned to a low value and remained low.

There was no indication of response to oxygen.

Barium Ruthenate - Barium ruthenate, BaRu03 , was the subject for the major portion

of the experiments . Potentiodynamic scans were perfommed with

samples in a number of different electrolytes at both room temperature

and 60 °C. The electrolytes used were concentrated phosphoric acid,

6 mol/1 NaOH, 1:1 H3B03-Na2B407 , and 4 mol/1 NaOH with H2O2 additions.

Although there was a great deal of scatter in the magnitudes

of the currents observed at particular potentials , the overall shape

of the curve for each BaRu03 electrode was essentially the same. Figure 6 shows a typical result for concentrated phosphoric acid

at 60 °C. Visual observation of the electrode showed both

hydrogen and oxygen evolution. Oxygen evolution began at

potentials less anodic than the peak at + 0.9 V. In addition,

visual observation indicated dissolution of the BaRuOs as current

flowed. This is also evident in the I-V curves, since the observed

currents decrease linearly with number of scans (ranging from

hydrogen evolution to oxygen evolution) . The scatter in the

current for a fixed potential scan was about one order of magnitude

when all electrodes were compared.

26
1 1 1

1

- Pt Ref.

run 1 / /

-

run ^ / /

run 8 1 / /

/ A/:

—

/ ' t

^'

/ ^ /

/

BaRuOj

nom. area»5.l c

1

+ 40

+ 20

D

E

0

0)

3

o

-20

-2 0 +2

Potential (Volts)

Figure 6, Current-potential curves for a Type II BaRuO electrode in

concentrated phosphoric acid at 60 °C. The decrease in

current with tine is probably related to dissolution of

the sample.

27
Observed responses to oxygen bubbling vary with individual

electrodes. Some electrodes exhibit a change (Figure 7) while

others appear inert.

Voltammetry in 1:1 saturated H3BO3 :Na2B407 electrolyte at

room temperature differs from both acidic and alkaline (discussed

later) media. Figure 8 shows the results over a limited sweep

range. A wider potential sweep shows that the results of the

limited sweep illustrated are extended to greater potentials with

only small changes in shape. Interestingly, the sharp changes in

slope associated with hydrogen and/or oxygen evolution, are missing.

Also missing are the peaks observed in acid or alkaline media.

Figure 9 shows the results of voltammetry for two electrodes in

6 mol/1 NaOH at 60 °C. Note that the presence of O2 gives rise to

fairly significant changes. The observed shape of the curves is

different from that observed in concentrated phosphoric acid. Other

electrodes yielded similar responses although in one case the

reaction to oxygen addition was small. It should be noted that the

overall shape of these curves is similar to that observed for RUO2

. . 19 on Ti m 4 mol/1 NaOH, by 0' Grady, et al . Because of this similarity,

the effects of additions of 0.007 mol/1 H2O2 were studied for a 4 mol/1 NaOH

electrolyte on BaRuOs electrodes. Figure 10 shows the effect as compared to a smooth Pt electrode. Note that there is only a small

increase in cathodic current at potentials less than about -0.2 V

for BaRu03 in the presence of H2O2 , and no changes above that

potential . Compare with the large changes on Pt electrodes

.

Finally, it should be noted that a large hysteresis in current

was obsejTved with all media when the scan was halted. On stopping

the scan, the current rapidly fell toward zero. On restarting, the

current returned rapidly to its initial value

.

. - is-;.

28
Figure 7. Effect of oxygen and the current-potential curve for a Type II BaRuO electrode.
...

J 1

22'C Pt. Ref.

- -

— ^

Du r\ u

2

nom. area - 4.5 cm *"

i -1.60 0 + 1.85

Potential (Volts) X,

+ 50

0

-50

Figure 8. Current-potential curve for a Type II BaRuO^ electrode in

neutral solution. Scan range limited. Curve 1 is for an air saturated electrolyte. Curve 2 is for 0^ bubbling over

electrode surface

.

4M NaOH 22

Calomel Ref.

+ .59

Argon

-J-.007M H2O2

BaRuO

5.1 cm^

-1.52

1/ .

-+20

I -.20

/

/

/

-1.18

/ Pt (smooth)

+10

0

o

E

o

-10

-20

Potential (Volts)

Figure 10. The effect of 0.007 M H^O^ on a Type II BaRuO^ and on smooth Pt,

M = mol/1.
Redox Measurements

In general, measurements of the performance of electrode

materials with respect to single redox reactions were made in

neutral solutions of 0.1 mol/1 K3[Fe(a:)5] +0.1 mol/1 KL[Fe(OT)5],

0.1 mol/1 X3[Fe(CN)5] +0.02 mol/1 K^[Fe(C:0 5], and 0.1 mol/1 KI3 + 0.5 mol/1 KE

Most of the measurements were carried out by potentiodynamic

sweeping except in the cases of Type II BaRuO? electrodes, where,

because of their porous nature, long times at constant current or voltage were required to attain steady state conditions.

In K3 [Fe (CN) 5]/Ku [Fe (CN) g] solutions for all electrodes

tested, the open circuit potential corresponded to the equilibrium

potential for the redox couple, so that the potential difference

between the working electrode and the Pt reference electrode was

at most a few millivolts. In KI/KI3 solutions, however, the

SrTiOs and Ti02 electrodes tended to drift to -400 mV (vs . Pt)

indicating that the potential was not determined by the I~/I~

equilibriumv.

With the exception of tungsten bronze in KI3/KI, all materials

were stable in these redox solutions. Some electrodes, however,

showed increasing resistance with repeated anodic oxidation,

indicating that the properties of the surface were changing,

probably because of the formation of an insulating oxide layer.

Larger currents could at times be restored by cathodic polarization to sufficiently negative potentials so that hydrogen evolution

occurred. This behavior was seen with E2 reduced Ti02, 0.1% Nb

doped Ti02, Lao . 75Cao .25Ti03 , ^=^Z .7 5^^0 .IS'^^^S .

Titanates - For some of these materials a pronounced dissymmetry was observed, i.e., the anodic currents were much sm.aller than the

cathodic ones. Indeed, in some cases the rate of oxidation of

[Fe(CN)5]'^~ was negligible, as with La;^75TiC3 and LaQ^92Ti03.

33
Rutile (as grown) also exhibited similar dissymmetry, but the

anodic current was larger. In contrast, a fairly symmetric

current-potential curve was observed on Ta-doped Ti02 , as well as on two samples of SrTi03 with different amounts of Nb doping (0.03

and 0 .15%)

.

At equal polarization, current densities varied considerably

from material to material. To give an idea of their magnitude,

exchange current densities very roughly estimated from Ari/Ai

(where n is the over-voltage) are reported in Table 2. In some

cases, however, the dissymmetry is such that it is impossible to

derive reliable values for the exchange current density.

Table 2. EXCHANGE CURRENT DENSITIES

Electrode i (A/cm^) Behavior

Ti03 :5% Ta 10-7 . Symmetry

SrTiO3:0.03% Nb X 10"^ Symmetry

SrTiO3:0.15% Nb 2 X 10-^ Symmetry

Lao ,92^103 2 X 10-3

,

- Dissymmetry

Lao.75Ti03 7 X Dissymmetry

Lao.7 5Cao.2 5Ti03 . . 3 X 10-7 w;: Symmetry

None of the Ti02 electrodes tested in KI3/KI solution showed

any detectable current (< lyA) around the equilibrium potential,

at least, from -300 to +700 mV. Nb doped SrTi03 showed no anodic

current, and cathodic current only below -600 mV vs. Pt, and as already mentioned, anomalous behavior at open circuit.

Tungsten Bronze - Tungsten bronze. Nag 7WO3 showed peculiar behavior. Around the

equilibrixam potential currents were large, but on the anodic side a limiting current of 8 mA at an over-voltage (n) of about 300 mV was

reached. For polarizations higher than 0.6 V the current rapidly

decreased to 500 uA or less (for n of the order of 1.2V) and further

34
increases in potential had no effect. It is unlikely that this

behavior was due to chemical changes on the surface layers, but

could have been due to the formation of a depletion layer. The

exchange current density close to the equilibrium potential was fairly

large, and can be estimated at about 4 mA/cm^ . This corresponds to a rate constant of 4 x 10"^ cm/sec, in good agreement with the values

20

of Amjad and Fletcher in acidified solution (0.4 mol/1 H2SQ) . As

already mentioned, in KI3/KI, the tungsten bronze dissolved; (the

dissolution reaction seemed to occur in a regular fashion, leaving a flat and smooth surface) . Because of this , the results are difficult

to interpret, but they are consistent with the existence of a cathodic

limiting current due to charge-carrier depletion on the electrode

Barium Ruthenate - Barium Ruthenate - Steady-state data for a BaRu03-Type II electrode in two con- centrations, O.IM/O.IM and 0.1M/0.02M K3 [Fe (CN) g 1 /Kt^ [Fe (CN) 5 ] , are reported in Figures 11 and 12, where M = mol/1. It is evident

that a limiting current was reached on the cathodic side which was

not due to transport in solution; (as a further indication, stirring

had no effect on the limiting current) . The effect was probably due

to limited supply of charge-carriers in the electrode. The exchange

current for the redox reaction was of the order of 500 yA for the

first solution and 200 yA for the second. The current density

cannot be established because the surface area is not known.

The behavior of BaRuOs with respect to the [Fe(CN)g]^~/

[Fe(CN)5]^~ redox reaction is very interesting when compared with

the behavior in a O.lM KI3/O.5M KI solution. The results, for the

latter system, shown in Figure 13, indicate that on the cathodic

side the limiting current was of the order of 3 to 4 mA, a value

consistent with diffusion control. The current, therefore, was not

limited by charge carrier supply on the electrode side

35
Figure 11. Ferro-ferricyanide reaction. Steady-state performance of

Type II BaRuO^ electrode. Plot of overvoltage versus logarithm of current, m = mol/l.
+ 200

I

UJ?

o

>

UJ>

o

-200

BaRuOj Electrode

Reference

K,[Fe(CNL]/K^tFe(CNL ] 3 6 4 6

0.1 M/O.l M

• 0.1 M/0.02M

-I 0

CURRENT mA

Figure 12. Ferro-ferricyanide reaction. Steady-state performance of

Type II. BaRuO^ electrode. Plot of overvoltage versus current,

M = mol/1.

37
+

1

o

>

UJ

o

>

q:UJ>

o

-

1

-2.0

BaRuO^ Electrode

Pt Reference

0.1 M K 1^ / 0.5M K I

0.2 mA

0.1 0.2 0.5 1.0 2

CURRENT— mA

Figure 13. Iodine-iodide reaction. Steady-state performance of Type II

BaRuO^ electrode. Plot of overvoltage versus logarithm

of current. M = mol/1.
DISCUSSION

The electrochemical measurements carried out to screen the

materials for electrocatalytic properties have been perhaps more

extensive than anticipated at the beginning of the contract, since

less time than ejcpected was used in readying the necessary instrumentation. Nevertheless, for the purpose of both forming a systematic picture of electrochemical behavior as related to

chemical composition, and of investigating the reaction mechanisms

occurring on non-metallic electrodes, the research done so far

must be considered very preliminary, and few conclusions can be

drawn at this point.

The following discussion, therefore, will be aimed at outlining a few general conclusions that emerge from the results, but

even more at describing the research problems that have been

uncovered, so as to help in directing future efforts in the field.

A first conclusion that can be drawn concerning experimental

techniques is that electrodes produced from powders hot-pressed

with binders tend to be porous with considerable disadvantages for

the achievement of reliable data in a reasonable time: a large

amount of the current observed on these electrodes during

21 potentiodynamic scanning can be attributed to capacitative effects

Even more troxjblesome than their long relaxation times is the

difficulty of reproducible fabrication. For these reasons it is

suggested that great effort be made to produce massive specimens as electrodes , and alternatively — or better , concurrently — to find

methods to make electrodes from powders more reproducible and

suitable for laboratory research.

As far as stability to hot, concentrated H3PO1+ is concerned,

a number of materials have shown increasing electrical resistance,

that is, lower currents for the same scanning voltages particularly

at more positive potentials . This indicates slow deterioration

39
of the surface properties, probably with formation of insulating

oxides . These results point to the need to test for chemical

stability under realistic polarization conditions.

Some of the materials tested, such as the various La titanates

and BaRu03, exhibit metallic conductivity. However, in the electrochemical studies semiconducting properties have been observed.

This is particularly evident in solutions containing redox couples

,

where, in many cases, the current tended to flow dissymmetrically

in anodic and cathodic directions for reasons that appeared to be

due to saturation of the charge-carrier current within the electrode

.

Asymmetric behavior was also observed for some titanium oxide

electrodes in phosphoric acid: on rutile (as grown) as well as 0.1% Nb doped Ti02 no significant anodic current was detected.

Whether this occurs because of surface depletion of electrons

(these materials are n-semi conductors) or because of formation of

an insulating phase is not clear. The situation is further confused

since anodic current and oxygen evolution were obtained on 5% Ta

doped Ti02 and on H2 reduced Ti02 , which are supposed to possess

similar semiconducting characteristics.

As shown in Figure 5 , the anodic behavior of the lanthanum

titanates in H3P0^ seems to show an increase in anodic current with

decreasing concentration of lanthanum, which corresponds to an increase in the valency of Ti from 3 (for LaTi03) to 4 (for

Lao.5 7Ti03. This could be attributed to a more pronounced

n-semiconducting behavior for the more electron-rich titanate.

However, if the effect is related to the formation of a depletion

layer, the n-character appears to be a complicated function of

composition, since the nominal valency of Ti is the same in

Lao. 9 2^103 Lao .7 5Cao .2 5Ti03 , the anodic current is

larger on the latter. An alternative explanation might be based

on a greater ease in forming an insulating layer (possibly Ti02)

by anodic oxidation in the case of lower valency titanate.

40
Most of the materials under investigation have also been

tested as electrodes for two single reaox reactions K3[Fe(CN)g]/

Kt+[Fe(Cl0 6] and KI3/KI. The results obtained with BaRuOq and

sodiuia tungsten bronze have been described in more detail and are quite interesting. Saturation currents are of opposite sign

(cathodic for BaRuOs and anodic for ^<a;,75W03) . Since BaRu03

exhibits metallic conductivity, the semiconducting behavior might

be explained by assuming that BaRu03 is a p-metal , and cathodic

bias is sufficient to deplete holes at the surface. No saturation

currents could be detected for BaRu03 in KI3/KI. The reason might

be linked to the equilibrium- potential being more positive in

KI3/KI (+285 mV vs. SCE) compared with the f erri-ferrocyanide

couple (+285 mV for the O.lM/O.liM solution and +235 mV for the

0.LM/0.02M), but the differences in equilibrium, potential seem, rather small compared with the substantial (several tenths of a volt) polarizations applied, although it is not known how the

potential difference is partitioned inside and outside the electrode.

The W bronze, which should tend to exhibit n-semiconductor

properties, appeared to develop an exhaustion layer by anodic

polarization in f erri-ferrocyanide , so that the current dropped

by increasing bias . The other redox system used was not suitable

since the bronze underwent dissolution, probably caused by hole

injection on the part of the reducing iodine.

More redox systems have to be tested before a meaningful

correlation with the oxygen reaction can be attempted, but it

appears that m.aterials on which little if any current can be drawn

in H3PO1+ exhibit similar behavior in f erri-ferrocyanide . The

significance of the behavior in KI3/KI is much more confused,

since for many of the materials tested even the equilibriuri

potential is not determined by the equilibriuiri and no current is obtained.

41
SECTION VII

MATERIALS OTHER THAN OXIDES

INTRODUCTION

In agreement with molecular orbital theory, the reduction of

oxygen is easy and that of nitrogen difficult. Biochemical reduction of nitrogen, however, takes place under mild conditions, the

final product being ammonia. The ordering of molecular orbitals in

nitrogen and oxygen is very similar, leading to the expectation that

similarities between the reduction of oxygen and nitrogen may exist,

and that the nitrogen reductase enzyme (nitrogenase) might serve as

a useful model for compounds capable of catalyzing the reduction of

oxygen at electrode surfaces. The active site of nitrogenase

consists of a molybdenum-nonheme iron-sulfur array in which molybdenum is thought to be involved in the final electron transfer

steps. In order to pursue this analogy further, the synthesis

of ternary metal sulfide arrays , in which one of the metals is

molybdenum or tungsten, was undertaken with a view to providing an alternative approach to the central thrust of this program.

RESULTS

1. Aqueous Systems. Metathetical reactions in water, e.g.,

++ oFe + MoS^ *- FeMoSi+

failed and the products were ferrous sulfide and molybdenum disulfide. Analogous reactions were attempted with dithiotungstate (2-)

,

W02S^~, in the hope that the transition metal oxythiotungstates

would be more stable. However, unusual solvolysis reactions

supervened and sulfur-free products were isolated, e.g.,

Cr"*"^"*" + W02S^~ » Cr2 (WO4) 3 •3H2O

A publication describing these results is in press. In order to

avoid complications introduced by reactions with water, nonaqueous

syntheses were studied.

42
2. Nonaqueous Syterns . In order to solubilize transition

metal ions in organic solvents, it is usually necessary to employ

large, hydrophobic ligands. Triphenylphosphine was chosen for use

in initial experiments, primarily because of its ready availability.

Use of tertiary phosphines confers another potential advantage

for these ligands are often labile. Thus dissociation in solution

of the phosphine may uncover a coordination site on a metal atom

to produce a species, in equilibrium concentration, too reactive

to be isolated.

A simple metathetical reaction between (PhsP) 3CUCI and

(Ph3PCH3) 2WO2S2 in dichloromethane was attempted. The product,

(Ph3P) itCu2W2S5 , isolated by chromatography on silica gel, was oxygen-free. This suggested that triphenylphosphine could serve

as a reducing agent to remove oxygen and sulfur from W02S^~

to produce the desired ternary metal sulfides; the by-products are Ph3P0 and PhsPS.

This copper-tvmgsten-sulfur complex has been studied by a variety of physical techniques and some of its physical properties

are tabulated below:

yellow-orange crystalline solid, soluble in chlorinated

hydrocarbons

mp. 227-230° (dec.) (under vacuum)

{1h}^^P NMR: broad singlet at -8.1 ppm from H3PO1+

NMR: complex multiplet centered at -7.5 ppm

IR: 448 cm"^ (v ) ws

Raman: 467(s), 254(w)

Electronic spectriam: A at 440, 267 nm max

E (oxidation) - 1.14 V; E (reduction) - 0.94, - 1.5 V (vs.

P P

SCE, irreversible; by cyclic voltametry)

The general synthetic route (metal ion , triphenylphosphine

and W02S^~) has been extended to other coinage metals. The

Ag-W-S system has proved to be particularly instructive . The

43
initial product is a yellow solid whose elemental analysis

corresponds to (Ph3P) 6Ag5W2S602 . Attempts to further purify this

product led to its decomposition into a mixture of an orange and

a yellow material. The elemental analysis of the yellow material,

separated by fractional crystallization, corresponds to

[ (Ph3P) 3Ag] 2W2S6 . It is converted on boiling in dichloromethaneacetone to the orange material, whose analysis corresponds to

(Ph3P) 3Ag3WS3 . It is possible that the orange compound is not

a single phase and this question is being pursued. These observations suggest that the initial product in the Ag-Ph3P-W02S^~

system is a complex oxythiotungstate which undergoes further

reaction on the acidic sites on silica gel to form a W2S6 complex.

Using PhsPAuCl as the starting material, the only product

isolated was {Ph3PAu) 2W2Si+ . Like the related copper complex, this

material had a molecular weight half that of the expected value,

suggesting that the phosphine ligands are indeed labile. Only in

the case of gold was a new product formed when triphenylarsine was used as a reducing agent. It appears to be a simple tetrathiatungstate (2-) derivative , (PhsAsAu) 2WS1+

.

The physical techniques employed to date do not provide an adequate structural characterization of these ternary metal

sulfides, which are new and quite novel materials, and x-ray

diffraction studies are needed. The triphenylphosphine complexes

generally have low molar solubilities, making it difficult to

grow crystals and those that are obtained are usually thin flakes

.

Preparation of tri-p-tolylphosphine and diphenylmethylphosphine

analogues is currently underway. Good single crystals of the

diphenylmethylphosphine-gold-tungsten sulfide have been obtained.

o o They are monoclinic with a = 15.46 A, b = 13.51 A, and 3 = 96°.

An x-ray study of this material has just begun. Some single

crystals of orange (Ph3P) 3Ag3WS3 have also been grown and cell

parameters are now being measured. These more soluble derivatives

44
will be useful for studies of ^^P NMR spectra, since the triphenylphosphine complexes are so poorly soluble that exhorbitant amounts

of spectrometer time are required to obtain even marginal data.

Materials characterization has now proceeded sufficiently far

that applications in real systems can be considered. Arrangements

have been made for fabrication of composite electrodes containing

(PhsP) 4CU2W2S2 and graphite. These will be examined in a working

cell to test for catalytic activity in oxygen reduction.

A limited effort has been made to prepare ternary arrays

containing nickel, palladium, or platinum. Metathesis between

W02S^" and (diphos)NiCl2 is straightforward and (diphos) NiV702S2

is produced. This material appears to be a mixture of geometric

isomers. Similar reactions with (diphos) PdCl2 and cis-

(Ph3P)2PtCl2 led to (diphos) 2Pd3W2S602 and (Phs?) 3PtW2S502

These yellow microcrystalline materials are virtually insol\able

in nonreactive solvents and their characterization is extremely

difficult [diphos = 1 , 2-2?is (diphenylphosphino) ethane] .

45
SECTION VIII

• 1 REFERENCES

1. Gillis, A. E. Advanced Development. In: Seventh Status

Report on Fuel Cells, Huff, J. R. (ed.) . U.S. Army Mobility.

Equipment Research and Development Center, Fort Belvoir, Va.

Report 2039. NTIS No. AD-755 106. October 1972. 174 p.

2. Negas, T., and R. S. Roth. J. Solid State Chem. 1_:409-418,

1970.

3. Negas, T., and R. S. Roth. J. Solid State Chem. 3_: 323-339,

1971.

4. Negas, T. J. Solid State Chem. 6^:136-150, 1973.

5. Negas, T., and R. S. Roth. Proceedings of 5th Materials

Research Symposivim, National Bureau of Standards, Washington,

D.C. NBS Special Publication No. 364. July 1972. 31 p.

6. Ccindela, G. A., A. H. Kahn, and T. Negas. J. Solid State

Chem. 2*360-369,1973.

7. Negas, T. J. Solid State Chem. 7_: 85-88, 1973.

8. Kestigian, M., and R. Ward. J. Am. Chem. Soc. 77:6199-6200,

1955.

9. Donohue, P. C, L. Katz, and R. Ward. Inorg. Chem. 4_: 306-310,

1965.

10. Callaghan, A., C. W. Moeller, and R. Ward. Inorg. Chem.

5_: 1573-1576, 1966.

11. Bouchard, R. J., and J. L. Gillson. Mat. Res. Bull.

2:873-878, 1972.

12. Waring, J. L., and R. S. Roth. J. Res. NBS. 27A ;175-186,

• 1968.

13. Waring, J. L., and R. S. Roth. J. Res. NBS. 69A :119-129,

1965.

14. Nicks, L. J., and D. J. MacDonald. Report of Investigations

7841, U. S. Department of Interior, Bureau of Mines,

Washington, D.C. 1973. 9 p.
15. Bodiot, D. Revue de Chemie Minerale. 5_: 569-607, 1968.

16. Graham, J. Am. Mineral. 59^:1045-1046, 1974.

17. Stern, M., and A. C. Makrides. J. Electrochem. Soc.

107:782, 1960.

18. Neidrach, L. W. , and H. R. Alford. J. Electrochem. Soc.

112:117-124, 1965.

19. 0' Grady, W. , C. Iwakura, J. Huang, and E. Yenger. Ruthenium

Oxide Catalysts for the Oxygen Electrode. ONR Technical

Report 37, Case Western Reserve, Cleveland, Ohio. 1974. 22 p.

20. Amjad, M., and D. Fletcher. Electroanalytical Chem. and

Interfacial Electrochem. 59_:61-67, 1975.

21. Austin, L. G., and E. G. Gagnon. J. Electrochem. Soc.

120:251-254, 1973.

22. Gagnon, E. G. J. Electrochem. Soc. 120 :1052-1056, 1973;

Ibid. 121,512-515, 1974; Ibid., 122:521-525, 1975.

47
SECTION IX

LIST OF PUBLICATIONS

Siedle, A. R., T. Negas , and J. Broussalian. "Reaction

Transition Metal Ions with the Dithiotungstate (2-) Ion.'

J. Inorg. Nucl. Chem. In press.

48
NBS.114A (REV. 7-73)

U.S. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

SHEET

1. PUHLK ATION OR Rl-PORT NO.

NBSIR 75-742

2. Gov't Accession

No.

3. Recipient's Accession No.

4. TITLE AND SUBTITLE

MIXED OXIDES FOR FUEL CELL ELECTRODES

5. Publication Date January 1976

6. Performing Organization Code

7. AUTHOR(S) u. Bertocci, M. Cohen, W. S. Horton, T. Negas, and

A. R. Siedle

8. Performing Organ. Report No.

9. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

10. Project/Task/Work Unit No. 3130402

11. Contract/Grant No. EPA-IAG D4-0528

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

EPA, Office of Research and Development

Industrial Environmental Research Laboratory

Research Triangle Park, NC 27711

13. Type of Report & Period Covered

Final: 5/74-5/75

14. sponsoring Agency Code

15. SUPPLEMENTARY NOTES

EPA Report No: EPA-600-2-76-007

16. ABSTRACT (A 200-word or less factual summary of most si^ificant infbrmation. If document includes a significant

bibliography or literature survey, mention it here.)

Studies of mixed oxides were made in order to determine if such materials could act as oxygen-reducing electrocatalysts in an acid fuel cell. Included were strontium and

barium cobaltates and manganates with and without added titanium; lanthanum titanates,

with and without calcium or strontium; calcium, strontium, and barium ruthenates; and

mixed oxides of the systems Ti-Ta-0, V-Nb-0, Ce-Ta-0, Pr-Ta-O, Ce-Nb-0, and Ce-Pr-Ta-O,

Choices were based upon producing variable valence and upon conferrring stability at

elevated temperatures ($ 150 °C) in phosphoric acid. Barium ruthenate and the systems

Ti-Ta-0, V-Nb-0, V-Ta-0, Ce-Ta-0 were hot-acid stable. The thermal reactions of

CeTaO^+x with 0 $ x $ 0.5 were studied in air up to about 1960 °C.

Potentiodynamic and galvanostatic studies are reported on materials from the Ti-Ta-0

system, Ti02 as grown, Ti02 reduced with hydrogen, Ti02 with 0.1% Nb, lanthanum

titanates with and without calciura or strontiiam, a tungsten bronze, barium ruthenate,

and strontium titanate with .03% and with .15% Nb.

Preparation of inorganic compounds with ternary metal-sulfur arrays similar to the

arrays in nitrogen reductase was attempted. The following were made: (Phs?) 1+CU2W2S5 , (PhsP) 5Ag5W2S502 / and [ (Ph3P) 3Ag] 2W2S5 , where PhsP refers to the triphenylphosphine

moiety. Also prepared were (PhsPAu) 2W2S1+ , (PhsAsAu) 2WS4 , (diphos) NiW02S2 / (diphos) 2Pd3W2S602 , and (Ph3P) 3PtW2Sg02 , where "diphos" refers to 1 , 2-bis (diphenylphosphino) ethane

.

17. 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

)

Electrocatalysis ; fuel cells; mixed oxides, oxygen reduction; phosphoric acid; ternary

metal-sulfur arrays; transition elements; triphenylphosphine; tungsten bronze

18. AVAILABILITY [XX Unlimited

1 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. C13

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