Solid State Chemistry of Energy Conversion and Storage - American

18 High-Temperature Electrolysis/Fuel Cells:

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Materials Problems H. OBAYASHI and T. KUDO Central Research Laboratory, Hitachi, Ltd., Higashi-Koigakubo, Kokubunji, Tokyo, 185, Japan The motivation for electrolysis/fuel cell technology is sum marized, and problems regarding the development of high -temperature electrolysis cells and medium-temperature fuel cells are discussed: (a) a suitable solid electrolyte, (b) a cathode for operating in a highly oxidizing atmosphere mechanically compatible with the electrolyte, (c) a cell design that minimizes material and heat transfer problems, and (d) an electronic conductor stable in a wide range of oxygen partial pressures (1-10 atm PO2) for series connec tion. Materials research addressed to solving these problems is reviewed. -20

T 7 u e l cells convert

chemical energy

d i s s i p a t i o n of heat;

electrolysis

e n e r g y i n t o c h e m i c a l energy.

i n t o e l e c t r i c a l energy

cells c o n v e r t

w i t h the

heat a n d e l e c t r i c a l

I f t h e e n t h a l p y of t h e c h e m i c a l r e a c t i o n is

AH = AG + Τ AS, the electrical energy derived f r o m a fuel c e l l ( o r consumed i n a n electrol ysis c e l l ) is AG a n d t h e heat d i s s i p a t e d ( o r c o n s u m e d ) is Τ AS.

I n the

absence of overvoltages a n d other c i r c u i t losses a n e n g i n e c o n s i s t i n g of a h i g h - t e m p e r a t u r e electrolysis c e l l a n d a l o w - t e m p e r a t u r e f u e l c e l l w o u l d c o n v e r t heat to e l e c t r i c i t y a n d / o r f u e l w i t h a n efficiency a p p r o a c h i n g t h e C a r n o t factor.

I n p r a c t i c e , losses at t h e electrodes are c o m p a r a b l e

with

t h e n e t e n e r g y d e r i v e d f r o m s u c h a c y c l e , so s u c h h e a t engines h a v e n o t been developed.

H o w e v e r , as t h e s u p p l y of c h e a p f o s s i l fuels b e c o m e s

e x h a u s t e d , t h e r e is r e n e w e d

i n c e n t i v e to d e v e l o p

cells w i t h

reduced

e l e c t r o d e loss, f o r t h e v e r s a t i l i t y o f a n e n g i n e that c a n c o n v e r t h e a t t o e i t h e r e l e c t r i c i t y o r f u e l w o u l d h a v e t r e m e n d o u s advantages i n a n u c l e a r 316

In Solid State Chemistry of Energy Conversion and Storage; Goodenough, J., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1977.

18.

OBAYASHi

energy

A N D

High-Temperature

K U D O

economy.

Two

approaches

to

Electrolysis/Fuel this

problem

d e v e l o p m e n t of b e t t e r c a t a l y t i c electrodes a n d ( b )

Cells

are

(a)

317 the

o p e r a t i o n at h i g h e r

temperatures.

B e c a u s e l i q u i d electrolytes b e c o m e too corrosive at h i g h

temperatures,

s o l i d electrolytes

offer

great

advantages

for

use

with

l i q u i d a n d gaseous fuels. T w o factors h a v e i n h i b i t e d the use of s o l i d e l e c t r o l y t e s : ( a )

adequate

c e l l d e s i g n a n d ( b ) a d e q u a t e i o n i c m o b i l i t y . B e c a u s e ions m o v e t h r o u g h


solids v i a a d i f f u s i o n process, the i o n i c m o b i l i t y contains a n e x p o n e n t i a l f a c t o r e x p ( — EJkT), Unless E

&

where E

&

is r e f e r r e d to as t h e a c t i v a t i o n e n e r g y .

is u n u s u a l l y s m a l l ( 0 . 1 - 0 . 3 e V ) , the i o n i c m o b i l i t y gives a j o u l e

loss i n t h e e l e c t r o l y t e that is t o l e r a b l e o n l y at h i g h t e m p e r a t u r e s .

This

s i t u a t i o n is a c c e p t a b l e for a h i g h - t e m p e r a t u r e electrolysis c e l l , a n d t h e reasons s u c h cells h a v e not b e e n d e v e l o p e d are, essentially, ( a )

inade

q u a t e c e l l d e s i g n a n d ( b ) insufficient i n c e n t i v e g i v e n a n a v a i l a b l e s u p p l y of c h e a p fossil fuels. W e s h o u l d a n t i c i p a t e b e t t e r designs of h i g h - t e m p e r a t u r e electrolysis cells as t h e i n c e n t i v e for d e v e l o p i n g s y n t h e t i c fuels b e c o m e s m o r e intense. T o date, e n g i n e e r i n g d e s i g n has c o n c e n t r a t e d o n f u e l cells, since t h e y offer the p o s s i b i l i t y of c o n v e r t i n g the c h e m i c a l e n e r g y of a n a v a i l a b l e f u e l into electrical a n d m e c h a n i c a l energy w i t h o u t the customary restriction of the C a r n o t efficiency factor. M a n y types h a v e b e e n p r o p o s e d ( I , 2, 3, 4, 5 ) , a n d e a c h has advantages f o r a p a r t i c u l a r a p p l i c a t i o n .

However,

f u e l cells f o r extensive t e r r e s t r i a l a p p l i c a t i o n a r e o n l y n o w

becoming

commercially available from a single supplier ( 6 ) .

T h e y use a l i q u i d

electrolyte a n d c a n a c c e p t a f a i r l y w i d e r a n g e of fuels. T h e i r efficiency is h m i t e d b y t h e t e m p e r a t u r e of o p e r a t i o n , w h i c h is r e s t r i c t e d b e c a u s e of t h e corrosive c h a r a c t e r of the electrolyte.

C e l l s that e m p l o y e i t h e r s o l i d

or m o l t e n - s a l t electrolytes f o r o p e r a t i o n a b o v e 3 0 0 ° C are c a l l e d h i g h t e m p e r a t u r e ( or m e d i u m - t e m p e r a t u r e ) f u e l cells ( 7, 8, 9 ). S i n c e m o l t e n salt electrolytes are corrosive, a t t e n t i o n is g i v e n i n this a r t i c l e to cells u t i l i z i n g s o l i d electrolytes. T h e h i g h - t e m p e r a t u r e o p e r a t i o n m a d e p o s s i b l e b y s o l i d electrolytes has several advantages, acceptably

(a)

I m p r o v e d k i n e t i c s at the electrodes a l l o w

h i g h c u r r e n t densities w i t h m e t h a n e ,

propane,

and

other

h y d r o c a r b o n fuels that d o n o t react s m o o t h l y at a m b i e n t t e m p e r a t u r e s , (b)

I n the absence of a l i q u i d phase i n t h e system, m a i n t a i n i n g the

" t e r n a r y phase b o u n d a r y r e g i o n " to a v o i d w e t t i n g of t h e e l e c t r o d e n e e d not be considered, system (d)

is m a d e

(c)

F a b r i c a t i o n , o p e r a t i o n , a n d m a i n t e n a n c e of t h e

easier b y

reduced

corrosion

and

sealing

problems,

C o n t r o l of t h e w a t e r / f u e l b a l a n c e is easier b e c a u s e t h e c o m p o s i t i o n

of the e l e c t r o l y t e is i n v a r i a n t a n d i n d e p e n d e n t of the c o m p o s i t i o n of t h e f u e l gas.

(e)

A r e d u c e d p o l a r i z a t i o n of the system p e r m i t s o p e r a t i o n at

h i g h e r c u r r e n t densities, a n d h e n c e h i g h e r p o w e r densities.


318

SOLID

STATE

CHEMISTRY

O n t h e other h a n d , w o r k i n g at e l e v a t e d temperatures has d i s a d v a n tages: a l o w e r o u t p u t v o l t a g e a n d p r o b l e m s w i t h m i s m a t c h of the t h e r m a l e x p a n s i o n of e l e c t r o d e a n d e l e c t r o l y t e m a t e r i a l s . A l t h o u g h the l a t t e r problem can be engineered around b y appropriate cell design,

both

p r o b l e m s m a k e i t d e s i r a b l e to operate a f u e l c e l l at as l o w a t e m p e r a t u r e as possible, c o m p a t i b l e w i t h the p o w e r r e q u i r e m e n t s . A m e d i u m - t e m p e r a t u r e r a n g e ( 2 0 0 ° - 4 0 0 ° C ) appears m o s t d e s i r a b l e w e r e i t p o s s i b l e to Downloaded by UNIV OF CALIFORNIA SAN DIEGO on August 26, 2015 | http://pubs.acs.org Publication Date: June 1, 1977 | doi: 10.1021/ba-1977-0163.ch018

find a s o l i d e l e c t r o l y t e h a v i n g a c c e p t a b l e i o n i c m o b i l i t i e s i n this t e m p e r a t u r e r a n g e . T h e m o b i l e i o n i c species of t h e s o l i d e l e c t r o l y t e w o u l d b e either O ' or K 2

+

ions ( m o s t fuels offering H

+

as t h e m o b i l e c o m p o n e n t ) .

Because no k n o w n proton conductors w i t h acceptable H - i o n mobilities +

are stable a b o v e 2 0 0 ° C , t h e e v a l u a t i o n of s o l i d electrolytes i n f u e l cells has b e e n c o n f i n e d to 0 " - i o n c o n d u c t o r s . 2

Unfortunately, acceptable O ' 2

ion mobilities occur only above 800°C, a temperature considerably above the o p t i m u m f o r f u e l - c e l l o p e r a t i o n .

Nevertheless, the materials con

siderations f o r a h i g h - t e m p e r a t u r e f u e l c e l l are s i m i l a r to those f o r a h i g h - t e m p e r a t u r e electrolysis c e l l , a n d the 0 " - i o n c o n d u c t o r s 2

h e r e are excellent c a n d i d a t e s for this latter a p p l i c a t i o n .

considered Clearly

the

d i s c o v e r y of a s o l i d e l e c t r o l y t e c a p a b l e o f t r a n s p o r t i n g O ' or H ions w i t h 2

+

a c c e p t a b l e m o b i l i t i e s i n the t e m p e r a t u r e r a n g e 2 0 0 ° - 4 0 0 ° C w o u l d r e v o l u t i o n i z e the d e s i g n a n d use of f u e l cells. Development

of a c o m m e r c i a l e l e c t r o l y s i s / f u e l c e l l u s i n g a s o l i d

e l e c t r o l y t e r e q u i r e s the s o l u t i o n of s e v e r a l p r o b l e m s : ( a ) s u i t a b l e m a t e r i a l f o r the s o l i d e l e c t r o l y t e ; ( b )

s e l e c t i o n of a

i d e n t i f i c a t i o n of a c h e m

i c a l l y a n d m e c h a n i c a l l y c o m p a t i b l e c a t h o d e that r e m a i n s a g o o d elec t r o n i c c o n d u c t o r i n a h i g h l y o x i d i z i n g atmosphere, is t h e r m o d y n a m i c a l l y stable, a n d contains a b u n d a n t , i n e x p e n s i v e elements; d e s i g n t h a t a l l o w s series

connection

of

(c)

simple cell

i n d i v i d u a l units a n d

makes

a c c e p t a b l e a n y m i s m a t c h i n t h e r m a l e x p a n s i o n of the electrolyte a n d the electrodes; a n d ( d ) i d e n t i f i c a t i o n of a n i n e x p e n s i v e e l e c t r o n i c c o n d u c t o r stable i n a w i d e r a n g e of o x y g e n p a r t i a l pressures ( 1 - 1 0 " series c o n n e c t i o n .

20

atm p

)

0 2

f

o r

O f these, t h e most i m p o r t a n t i m m e d i a t e p r o b l e m is

t h e s o l i d electrolyte. A t a n o p e r a t i n g t e m p e r a t u r e n e a r 3 5 0 ° C the s o l i d 0 " - i o n electrolyte s h o u l d h a v e a h i g h 0 " - i o n m o b i l i t y a n d a h i g h 0 " - i o n 2

2

2

transference n u m b e r ( r a t i o of 0 " - i o n c o m p o n e n t to t o t a l e l e c t r i c a l c o n 2

d u c t i v i t y ) o v e r a w i d e r a n g e of o x y g e n p a r t i a l pressures.

It must be

c h e m i c a l l y stable o v e r a w i d e t e m p e r a t u r e range, m a d e of i n e x p e n s i v e m a t e r i a l s , a n d easily f a b r i c a t e d i n t o dense c e r a m i c m e m b r a n e s of a r b i t r a r y shape. T h i s p a p e r r e v i e w s the present status of m a t e r i a l s r e s e a r c h i n t o 0 " - i o n s o l i d electrolytes, electrodes, a n d c e l l interconnectors. 2


18.

OBAYASHi

A N D

High-Temperature

K U D O

Electrolysis/Fuel

319

Cells

&~-Ion Solid Electrolytes General Considerations. The d i s c o v e r y a n d u t i l i z a t i o n of o x y g e n - i o n c o n d u c t i o n i n a c e r a m i c o x i d e dates b a c k to b e f o r e 1900, w h e n N e r n s t used yttria-stabilized zirconia, χ Υ 0 · ( 1 — 2 x ) Z r 0 2

fluorite

3

2

crystallizing i n the

structure, as a g l o w e r element, k n o w n as t h e N e r n s t mass

M a n y w o r k e r s h a v e c o n t r i b u t e d to this field since t h e n (11).

(10).

Although

N e r n s t ' s g l o w e r element was n o t a c o m m e r c i a l success, oxides c o n d u c t i n g O " ions h a v e b e e n i n v e s t i g a t e d for a v a r i e t y of other a p p l i c a t i o n s , i n c l u d Downloaded by UNIV OF CALIFORNIA SAN DIEGO on August 26, 2015 | http://pubs.acs.org Publication Date: June 1, 1977 | doi: 10.1021/ba-1977-0163.ch018

2

i n g t h e n o w c o m m e r c i a l l y a v a i l a b l e o x y g e n sensors u s e d f o r the s t u d y of the t h e r m o d y n a m i c p r o p e r t i e s of oxides at e l e v a t e d t e m p e r a t u r e s

(8,12).

T h e use of c e r a m i c 0 " - i o n c o n d u c t o r s as s o l i d electrolytes i n f u e l cells 2

has b e e n s t u d i e d i n t e n s i v e l y at several institutes (13, 14, 15, 16), c i a l l y d u r i n g the late 1960s.

A l t h o u g h the c e l l designs w e r e

espe

inadequate

a n d the h i g h - t e m p e r a t u r e a p p l i c a t i o n is b e t t e r s u i t e d to electrolysis, these studies h a v e p r o v i d e d i m p o r t a n t i n f o r m a t i o n a b o u t the p e r f o r m a n c e

of

0 " - i o n c o n d u c t o r s u n d e r o p e r a t i n g c o n d i t i o n s a n d s o m e of the p r o b l e m s 2

to b e e n c o u n t e r e d because of m a t e r i a l s m i s m a t c h b e t w e e n electrodes a n d electrolyte.

T h e 0 " - i o n conductors

extensively s t u d i e d are

the

O t h e r systems w i t h the fluorite s t r u c t u r e , f o r

2

e x a m p l e those b a s e d o n T h 0 The

most

2

Z r 0 - b a s e d systems (13).

a n d C e 0 , h a v e also b e e n i n v e s t i g a t e d .

2

2

0 ~ - i o n conductors

that have been

2

s t u d i e d most i n t e n s i v e l y

g e n e r a l l y e x h i b i t a n e g l i g i b l e c a t i o n i c c o n d u c t i v i t y (17, e r a l m o d e l for s u c h a n 0 " - i o n 2

conductor

consists of

18, 19). a

A gen

fixed

cation

s u b a r r a y w i t h i n w h i c h the anions m o v e b y j u m p i n g f r o m one e n e r g e t i c a l l y e q u i v a l e n t site to another. The mobility μ cient D

0

of t h e i o n i c m o t i o n is r e l a t e d to t h e d i f f u s i o n coeffi

t h r o u g h the N e r n s t - E i n s t e i n r e l a t i o n ,

0

μ

ο

=

2eD /kT

(1)

0

w h e r e —2e is the charge o n the m o b i l e O " i o n a n d the s i g n of μο is 2

chosen f o r c o n v e n i e n c e . D where n

D

0

=

F r o m r a n d o m - w a l k theory,

0

=

(2n

D O

(2)

)- [Vo' ]ô vo 1

,

2

1, 2, or 3 is the d i m e n s i o n a l i t y of the i o n i c m o t i o n ,

[V "] 0

is the c o n c e n t r a t i o n of d o u b l y i o n i z e d o x y g e n vacancies ( e a c h of w h i c h is c a p a b l e of t r a p p i n g t w o e l e c t r o n s ) , z

0

nearest-neighbor

is t h e n u m b e r of

equivalent

sites a j u m p distance flo a w a y , a n d vo is t h e

jump-

a t t e m p t f r e q u e n c y of the O " ions. T h e e x t r i n s i c 0 " - i o n c o n d u c t i v i t y is 2

a o

( e x t ) — Nod

2

-

[V "])e/io 0


(3)

320

SOLID

STATE

CHEMISTRY

w h e r e JV is t h e d e n s i t y of e n e r g e t i c a l l y e q u i v a l e n t o x y g e n sites o n w h i c h 0

t h e ions m o v e . K i n g e r y et a l . ( 2 0 ) h a v e d e m o n s t r a t e d (see below) t h a t 0 " - i o n c o n d u c t i o n i n Zro.85Cao.15O1.s5 takes p l a c e v i a t h e d o u b l y i o n i z e d 2

o x y g e n v a c a n c i e s V " , a n d w e assume this to b e t r u e for a l l 0 ~ - i o n 2

0

c o n d u c t o r s c o n s i d e r e d here. F i n a l l y , since a free e n e r g y A G = 0

TaS

0

vo = Downloaded by UNIV OF CALIFORNIA SAN DIEGO on August 26, 2015 | http://pubs.acs.org Publication Date: June 1, 1977 | doi: 10.1021/ba-1977-0163.ch018

Δί/

0

—

is r e q u i r e d to m a k e a j u m p , t h e j u m p f r e q u e n c y is

where E = a

v e x p ( — AGo/kT)

— v o o e x p ( A S / f c ) e x p ( — EJkT)

o 0

ΔΗ

0

0

(4)

is r e f e r r e d to as the a c t i v a t i o n e n e r g y f o r i o n i c m o b i l i t y .

F r o m the f a c t o r ( 1 — [ V " ] ) [ V " ] a p p e a r i n g i n a o ( e x t ) , i t is c l e a r 0

0

t h a t t h e i o n i c c o n d u c t i v i t y vanishes i f the N o e q u i v a l e n t sites are e i t h e r c o m p l e t e l y o c c u p i e d or c o m p l e t e l y e m p t y . S u c h a s i t u a t i o n is analogous to a s t o i c h i o m e t r i c s e m i c o n d u c t o r or i n s u l a t o r i n w h i c h i n t r i n s i c elec t r o n i c c o n d u c t i v i t y r e q u i r e s t h e c r e a t i o n of m o b i l e electrons a n d holes b y t h e t h e r m a l e x c i t a t i o n of electrons f r o m t h e v a l e n c e b a n d to the c o n d u c t i o n b a n d . I n t h e e l e c t r o n i c a n a l o g y , t h e p r o d u c t of the concentrations of i n t r i n s i c electrons a n d holes is [e]i„ [h] t

where K

l n t

-

tf exp[-

E /kT] g

0

is a n e q u i l i b r i u m constant a n d E

0

(5)

is the e n e r g y g a p b e t w e e n

g

t h e t o p of t h e v a l e n c e b a n d a n d the b o t t o m of t h e c o n d u c t i o n b a n d . S i m i l a r l y , i n t r i n s i c i o n i c c o n d u c t i v i t y r e q u i r e s t h e r m a l e x c i t a t i o n of O ' 2

ions f r o m n o r m a l positions to i n t e r s t i t i a l positions across a n e n t h a l p y barrier Δ ί ^ :

O " ^± V o " +

Oi "

2

(6)

2

w h e r e O i " is a n i n t e r s t i t i a l i o n a n d t h e e q u i h b r i u m constant is 2

KdT)

-

[Vc/'HOi -] 2

X

i 0

e x p ( - ΔΗι/kT)

(7)

A s i g n i f i c a n t c o n c e n t r a t i o n of m o b i l e e l e c t r o n i c c h a r g e carriers at n o r m a l temperatures can be i n t r o d u c e d into a semiconductor b y a suitable c h e m i c a l s u b s t i t u t i o n . S i m i l a r l y , i t is c u s t o m a r y i n i o n i c c o n d u c t o r s to i n t r o duce m o b i l e ionic charge carriers b y appropriate c h e m i c a l substitutions. I n t h e s t a b i l i z e d z i r c o n i a s a l r e a d y m e n t i o n e d , the f o l l o w i n g s u b s t i t u t i o n s were made: Zr 2Zr

4 +

4 +

+

O " by C a

+

O " by 2 Y

2

2

2 +

3 +

+ +

V "

(8a)

Vo'

(8b)

0


18.

OBAYASHi

A N D

High-Temperature

K U D O

Electrolysis/Fuel

321

Cells

A n o x y g e n p o s i t i o n adjacent to a s u b s t i t u t i o n a l c a t i o n is n o t c r y s t a l l o g r a p h i c a l l y , a n d h e n c e e n e r g e t i c a l l y , e q u i v a l e n t to

one

t h a t is

not.

H o w e v e r , w i t h sufficient c a t i o n s u b s t i t u t i o n V o " is a b l e to m i g r a t e to a p o s i t i o n n e i g h b o r i n g another s u b s t i t u t i o n a l c a t i o n v j a a n e a r e s t - n e i g h b o r j u m p , a n d the b i n d i n g energy b e t w e e n d o p a n t a n d V " m a y b e n e g l e c t e d 0

at o p e r a t i n g temperatures. T h e s i t u a t i o n b e c o m e s analogous to e x t r i n s i c e l e c t r o n i c or h o l e c o n d u c t i o n i n a h i g h l y d o p e d s e m i c o n d u c t o r ,

except


that the c h a r g e - c a r r i e r m o b i l i t y μο is a c t i v a t e d ( E q u a t i o n s 1-4)

as i n

t h e case of a s m a l l - p o l a r o n c o n d u c t o r . T h e p o p u l a t i o n s of o x y g e n v a c a n c i e s a n d m o b i l e electrons a n d holes i n s u c h a c a t i o n - s u b s t i t u t e d 0 ' - i o n c o n d u c t o r are also i n f l u e n c e d b y t h e 2

c h e m i c a l e q u i l i b r i u m of the s a m p l e w i t h gaseous o x y g e n 0 ( g ) 2

*0 (g) +

V

2

0

" i

h

0

- +

2

(21):

2h

(9a)

0 - S i 0 ( g ) + V " + 2e 2

2

(9b)

0

F r o m the l a w of mass a c t i o n the e q u i l i b r i u m constants a r e

=

K (T) h

K (T) e

where p

02

[h] =

2

• [Vo"]"

[e]

2

• Po -

1

2

· [Vo"] • p

m

0 2

1 / 2

-

#

-

KêxpiAHJkT)

h o

e x p ( - AHJkT)

(10b)

is the p a r t i a l pressure of o x y g e n a n d A / / , AH

are t h e e n t h a l

e

h

(10a)

pies f o r o x i d a t i o n a n d r e d u c t i o n , r e s p e c t i v e l y . T h e c o r r e s p o n d i n g

elec

t r o n i c c h a r g e - c a r r i e r concentrations are

[h] = X [e] -

h

1 / 2

Xe

[V "]

1 / 2

0

1 / 2

[V "]0

p

1 / 2

0 2

Po 2

e

(Hb)

1 / 4

I f the v a l e n c e b a n d of the o x i d e is a n 0 " : 2 p b a n d , A f / 2

(Ha)

1 / 4

h

is l a r g e a n d

[h]

is n e g l i g i b l e . H o w e v e r , i f the v a l e n c e b a n d is a n a r r o w d or f b a n d , as m a y o c c u r i n t r a n s i t i o n - m e t a l a n d r a r e - e a r t h oxides, A H

h

m a y be small

e n o u g h to g i v e a significant c o n c e n t r a t i o n of m o b i l e holes [ h ] at h i g h e r o x y g e n p a r t i a l pressures.

S i m i l a r l y , i f t h e c o n d u c t i o n b a n d associated

w i t h t h e l o w e s t e m p t y orbitals o n the c a t i o n a r r a y is r e l a t i v e l y stable, Aff

e

m a y b e s m a l l e n o u g h to g i v e a significant c o n c e n t r a t i o n of m o b i l e

electrons [e] at l o w e r o x y g e n p a r t i a l pressures.


322

SOLID STATE

CHEMISTRY

T h e total electrical conductivity is the s u m of the partial

conduc

t i v i t i e s c a u s e d b y holes, electrons, a n d O " i o n s : 2

«n, = σ


σ where N

tf [h]

e/x

h

= i V [ e ] e/x

β

e

(12a)

h

(12b)

e

— ΛΓ (1 — [ V ' ' ] )2e/*o + ^ [ ( V ^ e / V

0

0

(12c)

0

i s the d e n s i t y o f c r y s t a l l o g r a p h i c sites a v a i l a b l e t o c h a r g e -

m

c a r r i e r species m , e is t h e m a g n i t u d e o f the e l e c t r o n i c c h a r g e , a n d t h e signs of the m o b i l i t i e s /A are so d e f i n e d (see E q u a t i o n 1) t h a t the p r o d u c t m

of t h e m o b i l i t y a n d t h e c h a r g e c a r r i e d is a l w a y s p o s i t i v e . T h e m o b i l i t y /Ao refers t o t h e i n t e r s t i t i a l O i ' ions, a n d N is t h e d e n s i t y o f i n t e r s t i t i a l 1

2

{

sites. T h e transference n u m b e r for 0 " - i o n c o n d u c t i o n is d e f i n e d as 2

to = σ / ( a 0

h

+ σ + σ ) β

(13)

0

F o r a s o l i d e l e c t r o l y t e i t is d e s i r a b l e t o h a v e t =

1. T h i s i d e a l m a y b e

0

a p p r o a c h e d i n oxides w i t h l a r g e b a n d gaps (E

> 100&Γ), p r o v i d e d A H

G

and Δ Η

β

h

are also l a r g e .

F o r a g i v e n AH a n d Δ Η , the transference n u m b e r o f a n e x t r i n s i c h

β

0 " - i o n c o n d u c t o r is 2

to =

(«Po

1 / 4 2

[V '']-

1 / 2

0

+

/W

/

4

[V

0

"]-

3

/

2

1

+

D"

(14)

where a = K^W^HO

a n d β = Κ"*μ*/2μο

(15)

F i g u r e 1 shows s c h e m a t i c a l l y the d e p e n d e n c e o f ίο o n the o x y g e n p a r t i a l pressure p

0 2

(22).

S i g n i f i c a n t l y , the r a n g e o f o x y g e n pressures at w h i c h — —

• u n d o p e d oxide = doped -

ff

NX

y /

6" ion

\--/

1.0

n i / / 1

.20.5 Figure 1. Schematic show ing the effect of doping on conductivity and ionic trans ference number

\

1

I'

*

/ '

»

//

\

P

0

y

\

-

\ \

\

2


18.

OBAYASHi

High-Temperature

A N D K U D O

Electrolysis/Fuel

c o n d u c t i o n is p r e d o m i n a n t l y i o n i c c a n , f o r a g i v e n A i /

h

323

Cells and Δίί , β

be

widened b y increasing [Vo"] w i t h chemical doping. Schmalzried (23)

has d e f i n e d p * σ

h

to =

a n d p * as t h e o x y g e n

h

pressures at w h i c h a =

0

{1 +

and σ =

σ

β

(Po /Ph*) 2

e

(i.e. t

0

0

1 / 4

+

=

0.5).

WPe*)"

1 7 4

a n d i t is sufficient to k n o w p * a n d p * to c a l c u l a t e t


h

}-

(16)

1

at a n y g i v e n o x y g e n

0

e

partial

Then

p a r t i a l pressure. C o n v e r s e l y , p * a n d p ° c a n b e o b t a i n e d f r o m c o n d u c h

e

tivity measurements that give t

0

pressures (24, 2 5 ) .

u n d e r a f e w different o x y g e n

S c h m a l z r i e d ' s (23, 26)

partial

o r i g i n a l e q u a t i o n t o estimate

these p a r a m e t e r s is b a s e d o n t h e e m f of a n o x y g e n c o n c e n t r a t i o n c e l l :

(17)

w h e r e F is F a r a d a y ' s constant a n d ρ

θ 2

' , P o " are, r e s p e c t i v e l y , t h e o x y g e n 2

p a r t i a l pressures at t h e a n o d e a n d c a t h o d e

(po " 2

>

Ρο ')· 2

If the ionic

defects are f u l l y i o n i z e d , as is g e n e r a l l y t h e case at e l e v a t e d t e m p e r a t u r e s (20), η

y .

—

4

T h e " e l e c t r o l y t i c d o m a i n " is d e f i n e d as the d o m a i n of o x y g e n p a r t i a l pressures i n w h i c h to is greater t h a n some s p e c i f i e d v a l u e s u c h as 0.99 (27, 28).

M o s t 0 " - i o n conductors have a n 0 " : 2 p 2

h e n c e a large &H . h

2

e

valence b a n d a n d

I n these cases i t is p r a c t i c a l l y m o r e i m p o r t a n t to

d e t e r m i n e the v a l u e of p * i n o r d e r to c h e c k t h e s u i t a b i l i t y of a m a t e r i a l e

for a f u e l / e l e c t r o l y s i s c e l l or a n o x y g e n sensor (29, 30,

31).

I n a d d i t i o n t o c h e m i c a l s t a b i l i t y against o x i d i z i n g a n d r e d u c i n g atmopsheres ( l a r g e AH

a n d Δ Η ) a n d a large intrinsic electronic energy

h

β

g a p E g , a l l to m i n i m i z e a n y e l e c t r o n i c c o n t r i b u t i o n to t h e c o n d u c t i v i t y , a good 0 " - i o n electrolyte must have a h i g h μ , a n d hence a l o w activation 2

0

energy E

a

for i o n i c m o b i l i t y . E x t e n s i v e studies o n 0 " - i o n electrolytes 2

h a v e s h o w n t h a t the 0 ' - i o n m o b i l i t i e s m a y v a r y g r e a t l y w i t h c r y s t a l 2

s t r u c t u r e . A l t h o u g h l i t t l e a t t e n t i o n has b e e n g i v e n to l e a r n i n g w h y c e r t a i n c r y s t a l structures are m o r e f a v o r a b l e t h a n others f o r h i g h i o n i c m o b i l i t y , i t has b e e n f o u n d e m p i r i c a l l y t h a t m o s t of t h e g o o d 0 " - i o n 2

k n o w n crystallize i n the The B i 0 2

3

fluorite,

conductors

p e r o v s k i t e , o r p y r o c h l o r e structures.

a n d C - t y p e l a n t h a n i d e structures, w h i c h are r e l a t e d to

fluorite

w i t h V " i n a n o r d e r e d a r r a y , are also g o o d host structures. I n w h a t 0

f o l l o w s , r e p r e s e n t a t i v e s o l i d electrolytes f r o m e a c h s t r u c t u r a l g r o u p are d i s c u s s e d i n some d e t a i l . Fluorites.

S T R U C T U R E .

The cubic

fluorite

s t r u c t u r e corresponds

c h e m i c a l f o r m u l a M X . I t consists of a f a c e - c e n t e r e d - c u b i c 2

to

a r r a y of

cations M w i t h anions X o c c u p y i n g a l l of t h e t e t r a h e d r a l interstices. T h e


324

S O L I D

S T A T E

C H E M I S T R Y

o c t a h e d r a l sites are e m p t y ; t h e y represent i n t e r s t i t i a l positions f o r

the

anions. I n t r o d u c t i o n of v a c a n c i e s i n t o the t e t r a h e d r a l sites p e r m i t s a n i o n m o b i l i t y , a n d the most p r o b a b l e j u m p p a t h is v i a a n o c t a h e d r a l site t h a t shares c o m m o n faces w i t h t h e t w o n e i g h b o r i n g t e t r a h e d r a l sites. activation energy E

a

The

increases as t h e e n e r g y r e q u i r e d t o p l a c e a n a n i o n

i n t o a n adjacent i n t e r s t i t i a l site increases. T h e C - t y p e l a n t h a n i d e s t r u c t u r e ( M X ) contains 2 5 % a n i o n v a c a n 2

3


cies t h a t are o r d e r e d a m o n g the t e t r a h e d r a l sites.

I n such an

ordered

s t r u c t u r e the tetrahedral-site vacancies are not c r y s t a l l o g r a p h i c a l l y e q u i v a l e n t a n d m u s t b e c o n s i d e r e d i n t e r s t i t i a l positions. D O P E D

ZR0 . 2

Pure Z r 0

has t w o c r y s t a l l o g r a p h i c m o d i f i c a t i o n s :

2

it

is m o n o c l i n i c at l o w t e m p e r a t u r e s a n d t e t r a g o n a l at h i g h temperatures (32, 3 3 , 3 4 ) .

I n t r i n s i c e l e c t r o n i c c o n d u c t i v i t y competes w i t h i n t r i n s i c

i o n i c c o n d u c t i v i t y , a n d the e l e c t r o n i c m o b i l i t i e s are m u c h greater t h a n t h e i o n i c m o b i l i t i e s . E l e c t r o n i c c o n d u c t i v i t y d o m i n a t e s the phase

monoclinic

(35, 36, 3 7 ) ; the t e t r a g o n a l m o d i f i c a t i o n s h o w s a m i x e d i o n i c /

e l e c t r o n i c c o n d u c t i o n of c o m p a r a b l e m a g n i t u d e

(38).

t h e i o n i c a n d e l e c t r o n i c c o n d u c t i v i t i e s of p u r e Z r 0 p a r t i a l pressure of o x y g e n p o

2

2

F i g u r e 2 shows

as a f u n c t i o n of the

for t w o temperatures, 6 0 0 ° C a n d 9 0 0 ° C .

A t h i g h e r p , i n t e r s t i t i a l o x y g e n ions O i " are i n t r o d u c e d ; these are the 2

0 2

m o b i l e i o n i c species. A t l o w e r p o , o x y g e n vacancies V o " are i n t r o d u c e d , 2

a n d t h e n o r m a l - s i t e O " ions are m o b i l e .

A t intermediate po

2

2

b o t h the

i o n i c a n d e l e c t r o n i c c o n d u c t i v i t i e s are i n t r i n s i c . O n the other h a n d , i o n i c c o n d u c t i v i t y dominates i n c a t i o n - s u b s t i t u t e d Z r 0 , e v e n i n the m o n o c l i n i c phase. 2

a transference n u m b e r t

0

>

F o r example, 0 . 0 1 Y O 0 . 9 9 Z r O 2

three orders of m a g n i t u d e l a r g e r t h a n that of p u r e Z r 0

0

-4

3

2

has

0.9 at 6 0 0 ° C a n d a t o t a l c o n d u c t i v i t y n e a r l y

-8

-12

-16

-20

-24

2

(37).

-28

Figure 2. Electronic and ionic partial conductivity isotherms for pure Zr0 . Adapted from Ref. 37. 2


18.

OBAYASHi

A N D

High-Temperature

K U D O

Τ

ίο-·-

1

Electrolysis/Fuel

1

1

325

Cells

Γ

Calculated from electrical conductivity


10 - 7

10-·

10-·

io-

-L

1 0

0.7

0.6

0.5

10

0.9

0.8

U

îoocvr Figure 3. Comparison of directly measured diffusion constants with a line calculated from electrical conductivity and the Nernst-Einstein relation. Adapted from Ref. 20.

K i n g e r y et a l . ( 2 0 ) ductivity i n doubly mobility

h a v e d e m o n s t r a t e d t h a t the h i g h 0 ~ - i o n 2

Zro.s5Cao.15O1.85

ionized oxygen

vacancies

at 7 0 0 ° - 1 1 0 0 ° C

employing

con

is c a u s e d b y n o r m a l - s i t e Ό " ions h o p p i n g to 2

by

mass-spectrometer

[Vo"].

They

introducing O analysis.

They

1 8

measured

the

oxygen

v i a i o n exchange also m e a s u r e d

and

the total

e l e c t r i c a l c o n d u c t i v i t y a n d c o m p a r e d i t w i t h the i o n i c c o n d u c t i v i t y c a l c u l a t e d f r o m the m e a s u r e d i o n i c m o b i l i t y a n d E q u a t i o n 3. T h e c o m p a r i s o n is s h o w n i n F i g u r e 3.

W i t h i n e x p e r i m e n t a l error, t h e entire e l e c t r i c a l

c o n d u c t i v i t y c a n b e a t t r i b u t e d to a n e x t r i n s i c i o n i c c o n d u c t i v i t y i n w h i c h t h e i o n i c m o b i l i t y is p r o p o r t i o n a l to the c o n c e n t r a t i o n of d o u b l y i o n i z e d vacancies, [ V " ] . 0

S i m p s o n a n d C a r t e r ( 1 8 ) o b t a i n e d s i m i l a r results w i t h

s i n g l e - c r y s t a l a n d p o l y c r y s t a l l i n e Zr .858Ca .i42Oi.858. 0

0


326

SOLED

STATE

CHEMISTRY

W i t h l a r g e r c a t i o n s u b s t i t u t i o n s a c u b i c fluorite s t r u c t u r e is s t a b i l i z e d r e l a t i v e to t h e m o n o c l i n i c a n d t e t r a g o n a l m o d i f i c a t i o n s of p u r e Z r 0 . T h e 2

a n i o n - d e f i c i e n t fluorite s t r u c t u r e has a s i g n i f i c a n t l y l a r g e r O ^ - i o n c o n d u c t i v i t y , a n d i t is this phase that is of interest f o r s o l i d electrolytes.

I n the

s y s t e m C a . Z r i . 0 . . , t h e c u b i c phase is s t a b i l i z e d a b o v e 1 7 0 0 ° C f o r χ > a?

a

2

a

0.12, a b o v e 1 3 0 0 ° C f o r χ >

0.16 ( 3 9 ) .

M a x i m u m c o n d u c t i v i t i e s are

o b s e r v e d i n t h e r a n g e 0.13 > χ > 0.15 (40).

T h e systems M


w h e r e M is a t r i v a l e n t c a t i o n of c o m p a r a b l e size ( M = G d . . .) h a v e also b e e n e x t e n s i v e l y i n v e s t i g a t e d (41).

2 a ;

Zri. .0 . , 2 a

2

a r

Se, Y , N d , S m , F i g u r e 4 shows

c o n d u c t i v i t y v s . d o p a n t c o n c e n t r a t i o n at 8 0 0 ° C f o r several of these sys tems

I n most

(42).

cases t h e c o n d u c t i v i t y m a x i m u m o c c u r s at t h e

m i n i m u m d o p a n t l e v e l n e e d e d to s t a b i l i z e t h e c u b i c phase. T h i s o b s e r v a t i o n seems to i n d i c a t e that t h e m a g n i t u d e of t h e m o b i l i t y is n o t o b t a i n e d b y o p t i m i z i n g t h e p r o d u c t [ V o " ] ( 1— [ V " ] ) w i t h t h e s i m p l i f i e d a s s u m p 0

t i o n of r a n d o m o x y g e n v a c a n c i e s i n t h e fluorite s t r u c t u r e (43, 44). =

0.15/2 =

In

f o r e x a m p l e , t h e o x y g e n - v a c a n c y c o n c e n t r a t i o n is [ V o " ]

Cao.15Zro.85O1.g5,

0.075.

A t the large vacancy

concentrations

required for

s t a b i l i z a t i o n o f t h e c u b i c phase, t h e d i l u t e - s o l u t i o n a p p r o x i m a t i o n , w h i c h neglects i n t e r a c t i o n s b e t w e e n t h e vacancies, m a y n o t h o l d . I n d e e d i t has b e e n suggested t h a t a c u b i c phase w i t h

disordered

o x y g e n vacancies is n o t stable b e l o w a b o u t 1 0 0 0 ° C ( 4 5 , 46, 47, 48, 49). Baukal

f o u n d that a decrease w i t h t i m e i n t h e c o n d u c t i v i t y of

(50)

( Z r 0 ) 0 . 9 1 ( Y 0 ) 0 . 0 9 o n l o w e r i n g t h e t e m p e r a t u r e to 8 0 0 ° C c o u l d b e 2

2

3

described b y

first-order

kinetics a n d that the activation energy E

for

&

ionic conduction remained unchanged

d u r i n g t h e a g i n g process.

This

r e s u l t p r o v i d e s c l e a r e v i d e n c e f o r some k i n d of o r d e r i n g o f t h e o x y g e n vacancies,

a n d this i n f e r e n c e

observation specimens

from

has b e e n

partially confirmed

x-ray a n d neutron-diffraction

studies

by

for the

(40, 48, 4 9 ) . T h i s a g i n g effect, o b s e r v e d f o r m a n y

direct aged

systems,

a p p e a r s b e l o w a c r i t i c a l t e m p e r a t u r e b u t has a cutoff at l o w t e m p e r a t u r e s w h e r e t h e i o n i c m o b i l i t y b e c o m e s too l o w f o r a g i n g to t a k e p l a c e . I n t h e s y s t e m CSL ZT . 0 .X, X

1 x

2

χ (47, 48, 51, 52); occur for Τ >

o r d e r i n g occurs m o r e r e a d i l y at h i g h e r v a l u e s of i t has a m a x i m u m rate of 1 0 0 0 ° C ( 5 3 ) , i t does n o t

1 2 5 0 ° C , a n d i t is too s l o w to b e o b s e r v a b l e

for Τ

3

Pr

349

Cells

2.5) causes s p a l l i n g of t h e

cathode during cooling. S h i r a i et a l . (130)

investigated C e 0

C r , C o , a n d N i as w e l l as L a 0 2

wt % Zr0

2

3

2

doped with Y 0 , F e 0 , L a 0 , 2

3

2

3

2

doped with C r , Co, and N i . A

3

60-40

L a 0 - C o s a m p l e h a d the l o w e s t r e s i s t i v i t y , a n d adhesiveness to 2

was

3

much improved by

substituting Y

for

L a i n the

system

L a . . Y 0 - C o . T h e y c l a i m that a f u e l c e l l e m p l o y i n g this c a t h o d e c o u l d


2

a

; c

3

b e o p e r a t e d a b o v e 1000°C, b u t details w e r e n o t g i v e n . I t w o u l d a p p e a r t h a t t h e i r c o m p o s i t i o n contains L a C o 0 , i n w h i c h case r e a c t i v i t y w i t h 3

the solid w o u l d be a problem. S v e r d r u p et a l . ( 13, 128) %

f o u n d a r e s i s t i v i t y m i n i m u m n e a r 1-2 m o l

S n , S b , or T e as a d o p a n t i n l n 0 2

at 1000°C, a n d S n - d o p e d l n 0 2

d u c t o r . It has t h e C - t y p e L n 0 2

r e l a t e d to

fluorite.

3

In 0 2

3

is stable

s t r u c t u r e of F i g u r e 2 1 , w h i c h is closely

3

=

0

sions of the t w o c o m p o u n d s 2

Figure 25).

A t 25 ° C , its p s e u d o f l u o r i t e l a t t i c e p a r a m e t e r , a

5.06 A , m a t c h e s w e l l that of Z r 0 Moreover, l n 0

(see

3

is u s e d as a t r a n s p a r e n t e l e c t r o n i c c o n

3

2

(a

=

0

5.10 A ) , a n d t h e t h e r m a l e x p a n

are s i m i l a r (131),

as seen i n F i g u r e 26.

is essentially u n r e a c t i v e w i t h Z r 0

2

at 1000°C.

Polariza

t i o n losses at t h e e l e c t r o d e c a n b e s i g n i f i c a n t l y r e d u c e d b y a s i m p l e r e v e r s e - c u r r e n t t r e a t m e n t , as c a n b e seen i n the v o l t a g e - c u r r e n t curves of F i g u r e 27 for a film of l n 0 2

3

deposited on stabilized Z r 0

2

by chemical

v a p o r t r a n s p o r t . P o l a r i z a t i o n losses w e r e also r e d u c e d w i t h a c o m p o s i t e electrode

c o n s i s t i n g of a d o p e d - I n 0 2

1

1

1

3

current collector covered b y 1

1

1

a

Γ

Temperature, ° C

Figure 26. A comparison of the linear thermal expansion characteristics and stabilized zirconia. Adapted from Ref. 13.

of


In O t

s

350

SOLID

STATE

CHEMISTRY

'600

1 Ί Test In 10 Electrode weight: 21 mg/cm Operating ρ/δ -0.68Ω 1400H Τ • 1000°C

î

N


e

Q

- 200cc/min (26 C, latm)

Acuve electrode area.- 1.3 cm

1200

>

2

Total drop before treatmen

1000

1

2

Total drop following reverse current treatment

800

ιι a.

UJ

o 600

Resistive drop following reverse current treatment R -0.67Q

o 400

ÛJ

^ ^ ^ ^ Resistive drop prior to reverse current treatment

200

200

400 600 800 Electrode Current, ma

1000

1200

Figure 27. Voltage-current characteristic of In O air electrodes. "Re verse-current" treatment reduces the potential drop. Adapted from Ref. 13. s

p o r o u s l a y e r of Z r 0

2

i m p r e g n a t e d w i t h o n e of t h e " c u b i c " p e r o v s k i t e s

P r C o 0 , N d C o 0 , P r N i 0 , or N d N i 0 3

3

doped l n 0 2

3

s

3

3

(13).

F r o m these observations,

is a p r o m i s i n g a i r e l e c t r o d e f o r h i g h - t e m p e r a t u r e f u e l /

electrolysis cells o p e r a t i n g at a b o u t 1000°C. A n a l t e r n a t i v e to a p o r o u s e l e c t r o d e is a m i x e d e l e c t r o n i c / i o n i c conductor. A s illustrated i n F i g u r e 28 for a n anode ( a similar argument a p p l i e s to c a t h o d e s ) , T a k a h a s b i et a l . (124)

have p o i n t e d out that w i t h

s u c h a n o n p o r o u s e l e c t r o d e t h e e l e c t r o d e r e a c t i o n takes p l a c e o v e r t h e entire electrode surface; i t is n o t c o n f i n e d t o t h e p o r t i o n a d j a c e n t to t h e


18.

OBAYASHi

electrolyte.

High-Temperature

A N D K U D O

T h i s s i t u a t i o n has b e e n

c a t h o d e h a v i n g a d i f f u s i o n constant D 119).

I f the D

0 "-ion

0

Cells

351

demonstrated w i t h a molten A g 0

— 10~ — 10" c m / s e c 5

4

2

(117,118,

o f t h e e l e c t r o d e is c o m p a r a b l e w i t h t h a t o f t h e e l e c t r o l y t e ,

d i f f u s i o n t h r o u g h t h e electrode

2

Electrolysis/Fuel

i n c r e a s i n g t h e effective

electrode

is n o t r a t e - d e t e r m i n i n g , a n d

area reduces

t h e p o l a r i z a t i o n losses.

S i n c e d o p e d a n d n o n s t o i c h i o m e t r i c perovskites h a v e b e e n s h o w n t o h a v e h i g h 0 - i o n conductivities 2 _


cathode materials,

and P r C o 0

(101, 102, 103)

to b e good

3

i t is reasonable to suspect t h a t " P r C o 0 " is a m i x e d 3

electronic/ionic conductor. Kudo

e t a l . (132)

have

shown

that

electronically

conducting

Ndi^Sra-CoOa-e a n d s i m i l a r p e r o v s k i t e systems a r e g o o d c a t h o d e m a t e r i a l s

Electrolyte

Particle of the electronic conductor

/H 2

L-H20

Electrolyte

(b)

Particle of the mixed conductor

Figure 28. Models of the anodic reaction zone showing the superiority of a mixed conductor: (a) electronic conductor, (b) mixed conductor. (The hatched parts indicate the active reaction zone.) A simihr model is applicable to the cathodic reaction. Adapted from Ref. 124.


352

SOLID

STATE

CHEMISTRY

f o r r o o m - t e m p e r a t u r e , a l k a l i n e - s o l u t i o n a i r batteries; a n d O b a y a s h i et a l . (133)

h a v e m e a s u r e d 0 M o n diffusivities at 2 5 ° C i n t h e r a n g e

ÎO^-IO

2

- 1 1

c m / s e c , w h i c h are v e r y m u c h l a r g e r t h a n those of o r d i n a r y oxides. 2

S i m i l a r results h a v e b e e n o b t a i n e d f o r " L a C o 0 " (134). 3

I t is reasonable

to assume t h a t t h e 0 ~ - i o n c o n d u c t i o n is h i g h e n o u g h at e l e v a t e d t e m 2

peratures to c h a r a c t e r i z e these systems as m i x e d e l e c t r o n i c / i o n i c ductors. Schwarz a n d A n d e r s o n (105) have reported a D

=

0

5

c m / s e c i n r e d u c e d r u t i l e i n t h e i n t e r v a l 700° < Γ < 950 C (see Downloaded by UNIV OF CALIFORNIA SAN DIEGO on August 26, 2015 | http://pubs.acs.org Publication Date: June 1, 1977 | doi: 10.1021/ba-1977-0163.ch018

3

4

above).

2

O b a y a s h i a n d K u d o ( 1 3 5 ) h a v e s h o w n that L a N i 0

con

10" -10"

loses o x y g e n

s t e p w i s e o n r a i s i n g t h e t e m p e r a t u r e i n a r e v e r s i b l e , v e r y fast r e a c t i o n , s u g g e s t i n g that this m e t a l l i c c o m p o u n d is also a m i x e d c o n d u c t o r . has b e e n s u c c e s s f u l l y u s e d at 8 0 0 ° C as t h e c a t h o d e of a oxygen meter using stabilized Z r 0 compares

t h e a g i n g of L a N i 0

3

as t h e s o l i d electrolyte.

2

It

flow-through

a n d p o r o u s - P t electrodes

F i g u r e 29 The

(136).

d e v i a t i o n w i t h t i m e of t h e m e a s u r e d f r o m t h e t h e o r e t i c a l e m f at t h e p o r o u s - P t electrodes porous L a N i 0 Anodes.

3

is c a u s e d b y a g g r e g a t i o n of t h e p l a t i n u m . N o n -

cathodes c a n b e u s e d successfully f o r m o r e t h a n 1 0 h o u r s . 4

T h e c o n d i t i o n s u n d e r w h i c h a n o d e m a t e r i a l s operate are

m u c h less severe, a n d g o o d p e r f o r m a n c e has b e e n r e p o r t e d f o r v a r i o u s m e t a l s s u c h as T i , M n , F e , N i , C u , a n d P t (137)

as w e l l as s e v e r a l oxides

s u c h as N i O , C r 0 , C o O , F e 0 , r e d u c e d T i 0

(J38), V 0

3

U0

2

3

T a k a h a s h i et a l . (140)

(139).

reported

2

excellent

2

2

(138),

3

a n d T e d m o n et a l . (138)

depolarization i n the mixed electronic/ionic

and have con

d u c t o r s C e 0 - L a 0 a n d C e 0 - Y 0 , as s h o w n i n F i g u r e 30. 2

2

3

Interconnectors.

2

2

3

I n p r a c t i c e , i n d i v i d u a l e l e c t r o l y s i s / f u e l cells m u s t

b e c o n n e c t e d i n series to m a k e a c e l l stack (8, 13). o n e p o s s i b l e c o n f i g u r a t i o n (141).

F i g u r e 3 1 illustrates

T h e m a t e r i a l u s e d to i n t e r c o n n e c t t h e

0,

one y e a r ± 3 i

-201

102 time ( h )

10

10

3

10*

Figure 29. Comparison of a LaNiO and a porousplatinum cathode: deviation of observed emf s from the theoretical value of the cell p . = 10' atm, (Pt)/Zr Ca O /cathode, air. s

0

015

4

085

lm85


18.

OBAYASHi

A N D

High-Temperature

K U D O

Electrolysis/Fuel

353

Cells

06

S 0 4

g

02

Φ

ο


è

οι

I

10

100 (m A/cm )

Current density

2

Electrode material (I) ( C e 0 2 W Y 0 , . ) o 4

(2) ( C e 0 W

(3)Pt

(4)(Zr02)o«(YO,.5)o.,8

Figure

30. Anodic polarization trode materials at 1000°C.

a i r a n d f u e l electrodes

of

L

2

5

a

0

i 5>04

characteristics of various Adapted from Ref. 124.

adjacent

cells m u s t b e

elec

a good

electronic

c o n d u c t o r stable at o p e r a t i n g t e m p e r a t u r e s i n b o t h the r e d u c i n g atmos p h e r e of the a n o d e a n d the o x i d i z i n g a t m o s p h e r e of t h e c a t h o d e .

It

m u s t also b e r e a d i l y m a d e n o n - p o r o u s , since n e i t h e r f u e l n o r a i r s h o u l d penetrate through it. S v e r d r u p et a l . (13)

have

studied the normal spinel C o C r 0 , 2

w h i c h is stable at o x y g e n p a r t i a l pressures ρ

> 10"

θ2

it b e c o m e s a p - t y p e s e m i c o n d u c t o r ; at l o w p

15

atm. A t h i g h

4

p

02

i t is a n η-type s e m i c o n

02

d u c t o r . F i g u r e 32 shows r e s i s t i v i t y isotherms at 1000°C as a f u n c t i o n of Po

2

(141,

D o p i n g w i t h V or M n l o w e r s the r e s i s t i v i t y one to t w o

142).

orders of m a g n i t u d e , e s p e c i a l l y at l o w e r po .

A n i r r e v e r s i b l e loss of

2

occurs f o r ρ

θ2

>

1 2 3 4 5

10~

10

V

a t m v i a o x i d a t i o n a n d v a p o r i z a t i o n , w h i c h raises

interconnecter electrolyte cathode anode support

^ connection of a high-temperature cell. Adapted from f

F

i

g

m

e

3

1

F o

Re

f c f e

1

4

s e r i e s

1


354

SOLID

STATE

CHEMISTRY

1000°C

-

-

Undoped CoC r0 Cr/Co= 1 9; ?

4

1 1


10'

>pe

•

-

r

-

10'

10°

-8 log

P Q

-12 2

-16

(atm.)

Figure 32. Comparison of resistivities of two types of cobalt chromite. Adapted from Ref. 141. the resistivity. O n the other h a n d , M n - d o p e d C o C r 0 2

(2-4 mol %

4

Μη)

w a s stable f o r o v e r 26 days i n t h e r e d u c i n g f u e l a t m o s p h e r e , a n d its r e s i s t i v i t y w a s i n d e p e n d e n t of c y c l i n g to a n o x i d i z i n g a t m o s p h e r e F i g u r e 33)

(141).

I t appears t h a t M n - d o p e d C o C r 0 2

(see

has t h e c h e m i c a l

4

s t a b i l i t y a n d a n e a r l y t o l e r a b l e r e s i s t i v i t y at 1 0 0 0 ° C f o r u s e as a n i n t e r connector material i n a high-temperature electrolysis/fuel cell. K l e i n s c h m a g e r a n d R e i c h (143)

have investigated the electronically

c o n d u c t i n g perovskites d o p e d a n d / o r r e d u c e d L a N i 0 w e l l as t h e i n t e r l a y e r c o m p o u n d L a N i 0 . 2

4

3

and L a C r 0

as

3

A t 1 0 0 0 ° C the resistivities

of t h e n o m i n a l c o m p o s i t i o n s L a . 8 S r . C r o . 8 N i o . 0 a n d L a i ^ N i . 9 C o . i O . 6 0

0

2

2

3

0

0

c h a n g e d v e r y l i t t l e o v e r the r a n g e of o x y g e n p a r t i a l pressures 10~


3

17

ι


Ζ 100

Figure 35. Performance of a 100W solid-electrolyte fuel-cell battery operated at 1020°C. Adapted from

g

50 u.o

υ

13.

Ref.

IJU

CURRENT

(A)

systems h a v e b e e n e v a l u a t e d b y W e s t i n g h o u s e E l e c t r i c C o . o n a 1 0 0 - W scale ( 1 3 , 1 4 1 ) . T h e institutions where cell-performance

studies h a v e b e e n c a r r i e d

out i n c l u d e G e n e r a l E l e c t r i c C o . , W e s t i n g h o u s e

Electric Co., B r o w n -

B o v e r i C o . , B a t t e l l e L a b o r a t o r i e s at F r a n k f u r t a n d G e n e v a , a n d A . E . R . E . Harwell.

W e select t h e f u e l cells f a b r i c a t e d b y W e s t i n g h o u s e E l e c t r i c

C o . a n d B r o w n - B o v e r i C o . to i l l u s t r a t e c e l l - p e r f o r m a n c e d a t a . T h e W e s t i n g h o u s e u n i t consisted of t w o c e l l stacks c o n n e c t e d p a r a l l e l to s u p p l y p o w e r to a l o a d . connected

cells.

in

E a c h stack c o n t a i n e d 200 series-

F r o m the p e r f o r m a n c e

d a t a of F i g u r e 35, the

open-

c i r c u i t v o l t a g e at 1020°C is 190 V , a n d t h e m a x i m u m p o w e r is 110 W . T h e B r o w n - B o v e r i C o . has d e v e l o p e d b o t h p l a n a r a n d c y l i n d r i c a l f u e l cells, e s p e c i a l l y t h e latter. T h e cells use a s t a b i l i z e d z i r c o n i a electro lyte ( Z r 0 cathode.

2

— Y 0 2

3

F i g u r e 36

c o n n e c t e d cells (146,

— Yb 0 ) 2

w i t h a N i anode a n d a

3

complex-oxide

illustrates a c y l i n d r i c a l s t r u c t u r e of 147).

100

series-

A i r is i n t r o d u c e d to the i n n e r s i d e of the

e l e c t r o l y t e tubes a n d p u r g e d at the t o p of t h e c e l l stack. F u e l gas, w h i c h is o x i d i z e d n o n - e l e c t r o c h e m i c a l l y , is i n t r o d u c e d to t h e outer c o m p a r t m e n t . B o t h t h e i n l e t a n d t h e f u e l gas are p r e h e a t e d i n a c o u n t e r - f l o w

heat

e x c h a n g e r b y the exhaust. F i g u r e 37 gives the c e l l c h a r a c t e r i s t i c ; p o w e r densities of 105 m W / c m

2

decreased to 35 m W / c m

2

after 1 0

4

h o u r s of

operation. E l e c t r o l y s i s C e l l s . O p e r a t i o n of a h i g h - t e m p e r a t u r e electrolysis c e l l to o b t a i n h y d r o g e n f r o m w a t e r is essentially the same as that of a f u e l c e l l , b u t i n reverse. W a t e r v a p o r is i n t r o d u c e d i n t o t h e " f u e l " c o m p a r t m e n t , a n d a n e x t e r n a l v o l t a g e is a p p l i e d across the electrodes to the f o l l o w i n g reactions. cathode: H 0 + 2 e - » H 2

anode: 0 ~ - > \0 2

2

2

+

O " 2

+2e

S u c h cells h a v e b e e n d e m o n s t r a t e d b y m a n y w o r k e r s

(148,149,150).


give


18.

OBAYASHi

A N D

K U D O

High-Temperature

Electrolysis/Fuel

357

Cells

Figure 36. Structure of a stack containing 100 series-connected ceils developed by Brown-Boveri Co. (a) Mixing nozzle, (b) thermal isolation, (c) afterburning catalyst layer, (d) reforming catalyst layer, (e,h) current collector, (f) flue gas recycling, (g) fuel-cell module. B r o w a l l a n d H a n n e m a n (151)

have used a m i x e d e l e c t r o n / 0 " - i o n 2

c o n d u c t o r a n d a r e d u c i n g gas to p r o d u c e h y d r o g e n f r o m w a t e r w i t h o u t t h e n e e d f o r electrodes.

T h e y h a v e n a m e d t h e process G E Z R O .

The

f u n c t i o n of the r e d u c i n g gas is to l o w e r the t e m p e r a t u r e at w h i c h w a t e r dissociates. W i t h C O , for e x a m p l e , the reactions Inside: H 0 + 2e-> H 2

2

+

Outside: C O + Ο " - » C 0 2

2

0 " 2

+

2e

J 0 0

160 |AO

I

20

Figure 37. Performance of H + 3% HJD/air high-temperature fuel cell using a Zr0 -Y 0 Yb O solid electrolyte 0.8 mm thick operated at 830°C. Adapted from Ref. 147. 2

0

J 0

100

2

200

300

400

current density ( m Acm"2 )

2

s


2

5

358

SOLID


2

0-2+C0—C0J+ 2f

2

NET REACTION* HgO • CO—> H • C0 2

Figure 38. production

proceed

2

Schematic of self-dnven mode of hydrogen using an oxygen-ion/electron mixed con ductor. Adapted from Ref. 151.

at 1000°C w i t h o u t any a p p h e d voltage.

a " s e l f - d r i v e n " c e l l i s s h o w n i n F i g u r e 38. of 6 0 - 7 0 %

CHEMISTRY

"ANOOE" REACTION

"CATHOOE" REACTION H 0 +2Γ—HÔ'

STATE

A schematic of such

O v e r a l l t h e r m a l efficiencies

a r e a n t i c i p a t e d . T h e o p t i m u m transference n u m b e r s f o r t h e

m i x e d e l e c t r o n i c / 0 " - i o n c o n d u c t o r are t 2

0

= U = 0.5, b u t l i t t l e w o r k has

been done to obtain an o p t i m u m material. Summary R e a l i z a t i o n o f c o m m e r c i a l h i g h - t e m p e r a t u r e cells a w a i t s t h e s o l u t i o n of s e v e r a l m a t e r i a l s p r o b l e m s .

I d e a l l y , h i g h - t e m p e r a t u r e cells w o u l d b e

u s e d f o r e l e c t r o l y s i s , m e d i u m - t e m p e r a t u r e cells f o r p o w e r

generation.

Development o f a medium-temperature cell u t i l i z i n g a solid electrolyte p r o b a b l y awaits t h e discovery

and development

c o n d u c t o r , a p r o b l e m n o t d i s c u s s e d i n this p a p e r .

of a suitable proton However, with the

r i s i n g cost o f f o s s i l fuels, t h e m o t i v a t i o n f o r d e v e l o p i n g a h i g h - t e m p e r a t u r e electrolysis c e l l i s s t r o n g , a n d s u c h cells c a n p r o v i d e a t e s t - b e d f o r o p t i m i z i n g c e l l designs. T h e 0 " - i o n c o n d u c t o r s u s e d as s o l i d electrolytes are n o t satisfactory 2

at present. too

T h e most w i d e l y investigated, the stabilized zirconias, have

h i g h a r e s i s t i v i t y ( 1 0 - 2 0 Ω-cm

at 1000°C),

a n d efforts

to

find

a l t e r n a t e electrolytes o f l o w e r r e s i s t i v i t y h a v e b e e n o n l y p a r t i a l l y s u c cessful. T h e d o p e d cerias s u c h as C e i _ a , G d 2

2iP

0 _ , ( x = 0.115) h a v e b e t t e r 2

p

0 " - i o n c o n d u c t i o n b u t are n o t sufficiently r e s i s t i v e t o r e d u c i n g a t m o s 2

pheres. Gd

2 a ;

(a) 0 . 2

A satisfactory e l e c t r o l y t e m u s t h a v e t h e f o l l o w i n g p r o p e r t i e s : a n 0 " - i o n c o n d u c t i o n a t 7 0 0 ° C greater t h a n t h e best a n d w i t h a n o x y g e n transference n u m b e r to-*1.0; 2

i p


Cei. «2

18.

OBAYASHi

A N D

High-Temperature

K U D O

Electrolysis/Fuel

359

Cells

( b ) c h e m i c a l s t a b i l i t y over a w i d e r a n g e of o x y g e n p a r t i a l pressures, £ P02 * 1 a t m ; (c) mechanical stability, especially no crystallographic modification o v e r a w i d e r a n g e of t e m p e r a t u r e s ; ( d ) c h e m i c a l inertness to f u e l a n d electrodes as w e l l as a t h e r m a l e x p a n s i o n m a t c h i n g that of the electrodes; a n d ΙΟ"

30

(e)

i n v a r i a n c e d u r i n g c u r r e n t transfer.

T h e c h o i c e of electrode m a t e r i a l s is i n f l u e n c e d b y t h e

electrolyte


u s e d . I n a d d i t i o n to m a t c h e d t h e r m a l expansions the electrodes m u s t b e c h e m i c a l l y stable a n d c a t a l y t i c a l l y r e a c t i v e at the o p e r a t i n g t e m p e r a t u r e of t h e c e l l .

K n o w n electrode

temperatures

of

inadequate.

700°C,

materials are acceptable

for

operating

b u t at 1 0 0 0 ° C t h e i r c h e m i c a l stabilities are

T h e most p r o m i s i n g c a t h o d e

m a t e r i a l s are e l e c t r o n i c a l l y

c o n d u c t i n g oxides, a n d the m i x e d e l e c t r o n / 0 " - i o n c o n d u c t o r s give e x c e l 2

l e n t d e p o l a r i z a t i o n . A s e a r c h for m o r e s u i t a b l e m i x e d c o n d u c t o r s s h o u l d be encouraged. Interconnectors

m u s t b e c h e m i c a l l y stable o v e r a w i d e r a n g e

of

o x y g e n p a r t i a l pressures, a n d a s e a r c h for a c c e p t a b l e m a t e r i a l s has b a r e l y begun.

T h e c o n d u c t i v i t y of d o p e d C o C r 0 2

4

is m u c h too l o w . A l t h o u g h

several institutions have studied fuel-cell performance,

c e l l d e s i g n has

n o t b e e n o p t i m i z e d for heat transfer a n d m e c h a n i c a l s t r e n g t h . H i g h - t e m p e r a t u r e a n d m e d i u m - t e m p e r a t u r e cells u t i l i z i n g s o l i d elec trolytes offer great p o t e n t i a l for t h e e c o n o m i c m a n u f a c t u r e of s y n t h e t i c fuels

a n d for

more

efficient

power

generation

with

built-in

storage

c a p a c i t y for l o a d a v e r a g i n g . T h e r e q u i r e d m a t e r i a l s research s h o u l d b e supported. Acknowledgment H . O b a y a s h i thanks the P e t r o l e u m R e s e a r c h F u n d of t h e A m e r i c a n C h e m i c a l Society f o r

financial

assistance i n a t t e n d i n g the N e w

York

M e e t i n g of the Society.

Literature Cited 1. Liebhafsky, Η. Α., Cairns, E. J., "Fuel Cells and Fuel Batteries—A Guide to Their Research and Development," Wiley, New York, 1968. 2. Young, G. J., Linden, H. R., Eds., "Fuel Cell Systems," Advan. Chem. Ser. (1965) 47. 3. Young, G. J., Ed., "Fuel Cells," Vol. 1, Reinhold, New York, 1960. 4. Young, G. J., Ed., "Fuel Cells," Vol. 2, Reinhold, New York, 1963. 5. Baker, B. S., Εd., "Fuel Cell Systems—II," Advan. Chem. Ser. (1969) 90. 6. Rogers, L. J., "Status of the 4.8 Megawatt Demonstration Program," National Fuel Cell Seminar, Boston, June 21-23, 1977. 7. Markin, T. L., in "Power Sources 4, Research and Development in NonMechanical Electrical Power Sources," D. H . Collins, Ed., p. 583, Oriel Press, New Castle upon Tyne, 1973.


360

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☼

+



18. OBAYASHI AND KUDO High-Temper at ure Electrolysis/Fuel Cells 361

45. Michel, D., Jorba, M . P., Collonges, R., C. R. Acad. Sci. (1968) 266, 1602. 46. Rhodes, W. H., Carter, R. E., J. Am. Ceram. Soc. (1966) 49, 244. 47. Tien, T. Y., J. Am. Ceram. Soc. (1964) 47, 430. 48. Tien, T. Y., Subbarao, E. C., J. Chem. Phys. (1963) 39, 1041. 49. Subbarao, E. C., Sutter, P. H., J. Phys. Chem. Solids (1963) 24, 128. 50. Baukal, W., Electrochim. Acta (1969) 14, 1071. 51. Cocco, Α., Danelon, M., Ann. Chim. (Rome) (1965) 55, 1313. 52. Wachtman, J. B., Corwin, W. C., J. Res. Nat. Bur. Stand. (1965) 694, 457. 53. White, D. W., Rev. Energ. Primaire (1966) 2, 10. 54. Carter, R.E., Roth, W. L., in "Electromotive Force Measurements in High-Temperature Systems," C. B. Alcock, Ed., p. 125, Institution of Mining and Metallurgy, London, 1968. 54a.Dixon, J. M., LaGarange, L. D., Merten, U., Miller, C. F., Porter, J. T., J. Electrochem. Soc. (1963) 110, 276. 54b.Nuimin, A. D., Pal'guev, S. F., Tr. Inst. Elektrokhim., Akad. Nauk SSSR, Ural. Fil. (1964) 5, 154; Chem. Abstr. (1965) 62, 8472. 54c.Guillou, M., Rev. Gen. Elec. (1967) 76, 58. 54d.Stricker, D. W., Carlson, W. G., J. Am. Ceram. Soc. (1965) 48, 286. 54e.Tannenberger, H., Schachner, H., Kovacs, P., Rev. Energ. Primaire (1966) 2, 19. 54f.Möbius, H.-H., Proeve, G., Z. Chem. (1965) 5, 431. 55. Schmalzried, H., Z. Electrochem., Ber. Bunsenges. Phys. Chem. (1962) 66, 572. 56. Yanagida, H., reported in Yuan, D., Kröger, F. Α., J. Electrochem Soc. (1969) 116, 594. 57. Etsell, T. H., Flengas, S. N., J. Electrochem. Soc. (1972) 119, 1. 58. Komatsu, S., Yonehara, M., Kouzuka, Z., Bull. Metall. Soc. Jpn. (1972) 36, 674. 59. Patterson, J. W., Bogren, E. C., Rapp, R. Α., J. Electrochem. Soc. (1967) 114, 752. 60. Steele, B. C. H., Alcock, C. B., Trans. Metall. Soc. AIME (1965) 233, 1359. 61. Shores, D. Α., Rapp, R. Α., J. Electrochem. Soc. (1971) 118, 1107. 62. VanHandel, G. J., Blumenthal, R. N., J. Electrochem. Soc. (1974) 121, 1198. 63. Tuller, H. L., Nowick, A. S., J. Electrochem. Soc. (1975) 122, 836. 64. Panlener, R. J., Blumenthal, R. N., Garnier, J. E., J. Phys. Chem. Solids (1975) 36, 1213. 65. Kevane, C. J., Holvesson, E. L., Armstrong, J. B., Proc. Rare Earth Res. Conf., 5th Ames, Iowa, August, 1965, Book 4, 43. 66. Noddack, W., Walch, H., U. Phys. Chem. (1959) 211, 194. 67. Rudolph, J., Z. Naturforsch. (1959) 14A, 727. 68. Greener, E. H., Wimmer, J. M., Hirthe, W. M., in "Rare Earth Research II," K. S. Vorres, Ed., Gordon and Breach, New York, 1964. 69. Blumenthal, R. N., Pinz, Β. Α., J. Appl. Phys. (1967) 38, 2376. 70. Blumenthal, R. N., Lee, P. W., Panlener, R. J., J. Electrochem. Soc. (1971) 118, 123. 71. Kofstad, P., Hed, A. Z., J. Am. Ceram. Soc. (1967) 50, 681. 72. Blumenthal, R. N., Hofmaier, R. L., J. Electrochem. Soc. (1974) 121, 126. 73. Croatto, U., Mayer, Α., Gazz. Shim. Ital. (1943) 73, 199. 74. Takahashi, T., Ito, Κ., Iwahara, Η., Proc. J. Int. Etude Piles Combust. (1965) 1-III, 42. 75. Takahashi, T., Iwahara, H., Denki Kagaku (1966) 34, 254. 76. Singman, D., J. Electrochem. Soc. (1966) 113, 502.



362

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77. Kudo, T., Obayashi, H., J. Electrochem. Soc. (1975) 122, 142. 78. Blumenthal, R. N., Brugner, F. S., Garnier, J. E., J. Electrochem. Soc. (1973) 120, 2340. 79. Seitz, Μ. Α., Holliday, T. B., J. Electrochem. Soc. (1974) 121, 122. 80. Tuller, H. L., Nowick, A. S., J. Electrochem. Soc. (1975) 122, 255. 81. Pal'guev, S. F., Volchenkova, Z. S., Tr. Inst. Elektrokhim., Akad. Nauk SSSR, Ural. Fil. (1961) 2, 157; Chem. Abs. (1963) 59, 12267. 82. Kudo, T., Obayashi, H., to be published in J. Electrochem. Soc. (1976) 123. 83. Takahashi, T., Ito, K., Iwahara, H., Electrochim. Acta (1967) 12, 21. 84. Alcock, C. B., Steele, B. C. H., in "Science of Ceramics," Vol. II, G. H. Stewart, Ed., Academic, London, 1965; Trans. Br. Ceram. Soc. (1964) 397. 85. Laskar, M. F., Rapp, R. Α., Ζ. Phys. Chem. (1966) 49, 211. 86. Choudhury, N. S., Patterson, J. W., J. Am. Ceram. Soc. (1974) 57, 90. 87. Diness, A. M., Roy, R.,J.Mater. Sci. (1969) 4, 613. 88. Mehrotra, A. K., Maiti, H. S., Subbarao, E. C., Mater. Res. Bull. (1973) 8, 899. 89. Etsell, T. H., Z. Naturforsch. Teil A (1972) 27A, 1138. 90. Volchenkova, Z. S., Pal'guev, S. F., Electrochem. Molten Solid Electrolyte (1964) 2, 53. 91. Möbius, H. -H., Witzmann, H., Witte, W., Z. Chem. (1964) 4, 152. 92. Swinkels, D. A. J., J. Electrochem Soc. (1970) 117, 1267. 93. Hardaway, J. Β., III, Patterson, J. W., Wilder, D. R., Schieltz, J. D., J. Am. Ceram. Soc. (1971) 54, 94. 94. Tretyakov, Y. D., Muan, Α.,J.Am. Ceram. Soc. (1969) 116, 331. 95. Wells, A. F., "Structural Inorganic Chemistry," 3rd Ed., pp. 668-670, Clarendon, Oxford, 1962. 96. Takahashi, T., Iwahara, H., Nagai, Y., Proc. Annu. Meet. Electrochem. Soc. (Jpn.), 37th, Tokyo, May, 1970, Paper A209. 97. Takahashi, T., Iwahara, H., Arao, T., Proc. Annu. Meet. Electrochem. Soc. (Jpn.), 38th, Osaka, May, 1971, Paper D111. 98. Takahashi, T., Iwahara, H., Proc. Annu. Meet. Electrochem. Soc. (Jpn.) 39th, Tokyo, March, 1972, Paper C305. 99. Goodenough, J. B., Longo, J. M., Landolt Börnstein New Series III/4a, p. 126, Springer-Verlag, Berlin, 1970. 100. Galasso, F. S., "Structure, Properties and Preparation of Perovskite-Type Compounds," Int. Ser. of Monographs in Solid State Physics, 5, Pe amon, Oxford, 1969. 101. Takahashi, T., Iwahara, H., Denki Kagaku (1967) 35, 433. 102. Takahashi, T., Iwahara, H., Imaichi, T., Denki Kagaku (1969) 37, 857. 103. Takahashi, T., Iwahara, H., Imaichi, T., Aoyama, H., Annu. Rep. Asahi Glass Found. Contrib. Ind. Technol. (1969) 15, 237. 104. Matsuo, Y., Sasaki, H., Jpn. J. Appl. Phys. (1964) 3, 799. 105. Schwarz, D. B., Anderson, H. U., J. Electrochem. Soc. (1975) 12, 707. 106. Stephenson, C. V., Flanagan, C. E., J. Chem. Phys. (1961) 34, 2203. 107. Roth, R. S., J. Res. Nat. Bur. Stand. (1956) 56, 17. 108. Aleskin, E., Roy, R.,J.Am. Ceram. Soc. (1962) 45, 18. 109. Mazelsky, R., Kramer, W. E., J. Electrochem. Soc. (1964) 111, 528. 110. Wells, A. F., "Structural Inorganic Chemistry," 3rd Ed., p. 465, Clarendon, Oxford, 1962. 111. Etsell, T. H., Flengas, S. N., J. Electrochem. Soc. (1969) 116, 771. 112. Tedmon, C. S., Jr., Spacil, M. S., Mitoff, S. P., J. Electrochem. Soc. (1969) 116, 1170. 113. Yanagida, H., Brook, R. J., Kröger, F. Α., J. Electrochem. Soc. (1970) 117, 593. 114. Etsell, T. H., Flengas, S. N., J. Electrochem. Soc. (1961) 118, 1890.

g



18. OBAYASHI AND KUDO High-Temperature Electrolysis/Fuel Cells 363

115. Kröger, F. Α., J. Electrochem. Soc. (1973) 120, 75. 116. Yanagida, H., Brook, R. J., Kröger, F. Α., J. Electrochem. Soc. (1970) 117, 593. 117. Hirsch, Η. H., White, D. W., Ref. 4 in Tedmon, C. S., Jr., Spacil, H . S. Mitoff, S. P., J. Electrochem. Soc. (1969) 116, 1170. 118. Tannenberger, H., Siegert, H., "Abstracts of Papers," 154th National Meeting, ACS, Sept. 1967, FUEL 042. 119. Tannenberger, H., Siegert, H., in "Fuel Cell Systems—II," B. S. Baker, Ed., Advan. Chem. Ser. 90, 281. 120. Goodenough, J. B., "Metallic Oxides," in Prog. Solid State Chem. (1972) 5, 145. 121. Verwey, E. J. W., Haaijman, P. W., Romeijin, F. C., Van Oosterhout, G. W., Philips Res. Rep. (1950) 5, 173. 122. van Houten, S., J. Phys. Chem. Solids (1960) 17, 7. 123. Takahashi, T., Suzuki, Y., Ita, K., Hasegawa, H., Denki Kagaku (1967) 35, 201. 124. Takahashi, T., Iwahara, H., Suzuki, Y., Proc. J. Int. Etude Piles Combust. (1969) 3, 113. 125. Böhm, R., Kleinschmager, H., Z. Naturforsch. (1971) 26A, 780. 126. Heikes, R. R., Miller, R. C., Mazelsky, R., Physica (1964) 30, 1600. 127. Goodenough, J. B., Raccah, P. M., J. Appl. Phys. (1963) 36, 1031. 128. Sverdrup, E. F., Archer, D. H., Glasser, A. D., in "Fuel Cell Systems— II," B. S. Baker, Ed., Advan. Chem. Ser. (1969) 90, 301. 129. Bosman, A. J., Crevecoeur, C., Phys. Rev. (1966) 144, 763. 130. Shirai, K., Ihara, S., Sato, H., Bull. Electrotech. Lab., Tokyo (1974) 38, 378. 131. Weiher, R. L., Ley, R. P., J. Appl. Phys. (1963) 34, 1833. 132. Kudo, T., Obayashi, H., Gejo, T., J. Electrochem. Soc. (1975) 122, 159. 133. Obayashi, H., Kudo, T., Gejo, T., Jpn. J. Appl. Phys. (1974) 13, 1. 134. Similar results were obtained for LaCoO by G. H . J. Broers, et al., private communication. 135. Obayashi, H., Kudo, T., Jpn. J. Appl. Phys. (1975) 14, 330. 136. Obayashi, H., et al., unpublished data. 137. Eysel, H . H., "Extended Abstracts," No. 36, Electrochem. Soc. Meeting, Atlantic City, Oct., 1970. 138. Tedmon, C. S., Jr., Spacil, H . S., Mitoff, S. P., G. E. Report No. 69-C-056 (1969). 139. Rohland, B., Möbius, H . H., Naturwissenschaften (1968) 55, 227. 140. Takahashi, T., Suzuki, Y., Ito, K., Yamanaka, H., Denki Kagaku (1968) 36, 345. 141. Sun, C. C., Hawk, E. W., Sverdrup, E. F., J. Electrochem. Soc. (1972) 119, 1433. 142. Schmalzried, H., Ber. Bunsenges. Phys. Chem. (1963) 67, 93. 143. Kleinschmager, H., Reich, Α., Z. Naturforsch. (1972) 27A, 363. 144. Boll, R. H., Bhada, R. K., Energy Convers. (1968) 8, 3. 145. Kudo, T., Obayashi, H., Energy Convers. (1976) 15, 121. 146. Fischer, W., Kleinschmager, H., Rohr, F. J., Steiner, R., Eysel, H . H., Chem. Ing. Tech. (1972) 44, 726. 147. Eysel, H. H., Kleinschmager, H., BBC-Nachr. (1972) 54, 13. 148. Spacil, H. S., Tedmon,C.S., Jr., G. Ε. Report No. 69-C-176 (1969). 149. Chem. Eng. News (1968) 46, 48. 150. Allison, H. J., Hughes, W. L., Intersoc. Energy Convers. Eng. Conf., Rec., 10th, 1975 (1975) 104. 151. Browall, K. W., Hanneman, R. E., G. E. Report No. 75CRD012 (1975). 152. Takahashi, T., Iwahara, H., Energy Convers. (1971) 11, 105. 3

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July 26, 1976.


Solid State Chemistry of Energy Conversion and Storage - American

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