investigation of strontium-doped la(cr, mn)o 3 for solid oxide fuel cells

8/16/2019 Investigation of strontium-doped La(Cr, Mn)O 3 for solid oxide fuel cells

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J O U R N A L O F M AT E R I A L S S C I E N C E 2 7 ( 19 92 ) 5 8 3 7 - 5 8 4 3

In v e s t ig a t io n o f s t r o n t iu m - d o p e d L a (C r, M n ) O3fo r so l id ox ide fue l ce l l s

R A S I T K O C * , H . U . A N D E R S O NUniversity of Missouri-Rolla, Ceramic Engineering Department, Roll& M O 65401, U SA

The (La , Sr) (Cr, M n)O 3 sys tem w as inves t iga ted in an effor t to develop an in tercon nec t andcathode materials for sol id oxide fuel cel ls . Sintering studies were done in air at temperaturesbe low 1500 ~ S ign i f i can t improvemen t s i n dens i f ica t ion were obse rved wi th subs t i t u t ion o f50 mol% Mn fo r ch romium and a dens i ty o f 95% theo re ti ca l was ach ieved wi th t hesub s t i tu t ion o f 70 mo l% Mn for chromium in the La(Cr, M n) O 3 sys tem. Elec tr ica l con du ct iv i ty(d .c .) measurements were made as a fun ct ion of tempera tu re and oxyg en ac t iv i ty. At 10 00 ~and 1 a tm oxyg en, the e lec t rica l cond uct iv i ty ranged f rom 2 .2 -2 0 S cm - t for LaCr ¦and Lao.gSro.1Cro .2Mno.803, respect ive ly. Al l of the composi t ions showed s imi lar dependence

of e lec t r ica l conduct iv i ty on the oxygen ac t iv i ty. Dependence was smal l a t h igh oxygenact ivi t ies ; as the o xygen ac t iv i ty decreased , a break in e lec t rica l con du ct iv i ty a t 10 -12 a tm and100 0 ~ was observed, and then the e lec t r ica l cond uct iv i ty decreased asP¦ Sinter ing andelec t rica l co nd uc t iv i ty s tudies indica te tha t Lao.gSro .1Cro.3Mno.703 appears to be a candidatefor sol id oxide fuel cel l applicat ions.

1 . I n t r o d u c t i o nH i g h - t e m p e r a t u r e s o l i d o x i d e f u e l c e l l s ( S O F C ) h a v eb e e n e x t e n s i v e l y i n v e s t i g a te d f o r a n u m b e r o f y e a rs ,b e c a u s e t h e y h a v e t h e p o t e n t i a l o f c o n v e r t i n g c h e m i c a lene rg y i n to e l ec t r i c a l ene rgy. I n t he US , t he y a r e be ingu s e d t o i m p r o v e t h e e f f ic i e n cy o f p o w e r g e n e r a t i o n f o rs y s te m s u s i n g c o a l o r c o a l - d e r i v e d f u e ls [ 1 - 6 ] . A t t h ep re sen t t ime , t he mos t f r eque n t ly u sed sys t em usesy t t r i u m - s t a b i l iz e d Z r O 2 a s a n e l e c t ro l y t e , a m e t a l o rc e r m e t a s a n a n o d e , a n d e o n d u c t i n g o x i d e s as c a t h o -d e s a n d i n t e rc o n n e c t s. T h e f e a t u r e s t h a t n e e d i m p r o v e -men t i n so l i d ox ide fue l c e l l s i nc lude :

1 . cond uc t iv i t y o f t he e l ec t ro ly t e ;2 . c o n d u c t i v i t y o f t h e c a t h o d e a n d i n t e r c o n n e c t ;

3 . chem ica l and s t ruc tu ra l s t ab i l i t y o f t he i n t e r-c o n n e c t ;

4 . ga s t i gh tnes s o f t he i n t e r conne c t s ;5 . t h e r m a l e x p a n s i o n m a t c h o f c e ll c o m p o n e n t s ;6 . f a b r i c a t i o n c o m p a t i b i l i t y o f t h e c o m p o n e n t s o f

the cell.T h e r e q u i r e m e n t s f o r a f u e l c el l e l e c t r o l y te a r e v e r y

s t r i n g e n t . T h e t h e r m a l e x p a n s i o n m u s t m a t c h t h eo t h e r c e l l c o m p o n e n t s . I t m u s t b e c h e m i c a l l y s t a b l ew i t h r e s p e c t t o t h e o t h e r c e ll c o m p o n e n t s . I t m u s t b ec a p a b l e o f b e i n g f a b r i c a t e d to a n o n p o r o u s d e n s i t y.The i ons i t pa s se s mus t be oxygen , and t he e l ec t r i c a l

c o n d u c t i v i t y m u s t r e m a i n > 9 0 % i o n i c a t a l e ve l o f a tl e a st 0 .1 S c m - 1 o v e r t h e o x y g e n a c t i v it y r a n g e o f1 0 - 1 8 1 a t m [ 3 ] . A l t h o u g h t h e e l ec t r ic a l c o n d u c t i v i t y

o f t he m os t succes sfu l e l ec t ro ly t e (y t t r i um-s t ab i l i z edZrO2) cou ld be i nc rea sed , i t p r e sen t ly r ep re sen t s l e s stha n 10% o f t he ove ra l l c e ll r e s i s tance ; so a t t hem o m e n t i t d o e s n o t a p p e a r t o l i m i t t h e u t i l i z a t i o n o fh i g h - t e m p e r a t u r e f u e l c e l l s , b u t f o r i m p r o v e d p e r -fo rmance , t he e l ec t r i c a l conduc t iv i t y needs t o be i n -creased .

C u r r e n t l y, n i c k e l - b a s e d a l l o y s a r e b e i n g u s e d f o ra n o d e s ; h o w e v e r , b e c a u s e o f t h e o x i d a t i o n r e s i st a n c er e q u i r e m e n t s , o x i d e s a r e b e i n g c o n s i d e r e d . A n a n o d erep re sen t s abo u t 25% o f t he r e s i s t ance t o s se s o f a ce ll ,c o n s e q u e n t l y i m p r o v e m e n t s i n i t s el e c tr i c al c o n d u c t i v -i t y w o u l d b e h e lp f u l, b u t t h e 6 5 % c o n t r i b u t i o n t or e s i st a n c e b y t h e c a t h o d e , r e p r e s e n t s t h e p r i m a r y r e s-

i s t ance p rob l em in t he sys t em.A c a t h o d e h a s t o a c t n o t o n l y a s a c a ta l y s t fo r t h e

d i s s o c i a t io n o f o x y g e n b u t a s a n e l e c t r o d e a s w e ll .A p p l i c a t i o n s a r e b e i n g f o u n d f o r p e r o v s k i t e - t y p e o x i -des a s ca th ode s because o f t he i r h igh e l ec t r i c a l con -duc t iv i t y and ca t a !y t i c ac t i v i t y. To be u sed a s a ca th -o d e , a m a t e r i a l m u s t m e e t a n u m b e r o f r e q u i r e m e n t s : itm u s t ( 1) b e s t a b l e t o w a r d s o x i d a t i o n a n d r e d u c t i o n , ( 2)b e c h e m i c a l l y c o m p a t i b l e w i t h o t h e r c e l l c o m p o n e n t s ,( 3) b e a b l e t o m a t c h t h e t h e r m a l e x p a n s i o n o f o t h e rce l l componen t s , and (4 ) have e l ec t r i c a l conduc t iv i t y> 1 0 0 S c m - 1 .

A t p r e s en t , L a M n O 3 su b s t it u t ed w i t h 1 0 - 2 0 m o l %S r w i t h a c o n d u c t i v i t y o f a b o u t 11 0 S c m - 1 a t 1 0 00 ~a p p e a r s t o m a t c h t h e s e r e q u i r e m e n t s t h e b e s t [ 7 ] .

* Present address: N a t i o n a l R e n e w a b l e e n e rg y L a b o r a t o r y, 1 6 1 7 C o l e B l v d ., G o l d e n , C o 8 0 4 0 1 , U S A .

0 0 2 2 - 2 4 6 1 9 1992 Chapman & Hall 5837


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Thus , i t i s be ing u sed a s a ca thode . Th i s ma te r i a l ,howeve r, con t r i bu t e s t o h igh i n t e rna l r e s i s t ance l o s se sin the ce l l s (6 '5%). A low er in g o f these losses w ou ldg r e a t l y e n h a n c e t h e e f f e c ti v e n es s o f h i g h - t e m p e r a t u r efue l c e ll s. Such l o s ses cou ld be subs t an t i a l l y l ow ered : i f .t he e l ec t r i c a l r e s i s t ance o f t he ca th ode s cou ld be r e -d u c e d b y a f a c t o r o f 2 5 . To a c h i e v e a r e d u c t i o n o f t h ism a g n i t u d e r e q u i r e s t h e a l te r a t i o n o f t h e c o n d u c t i o np r o p e r t i e s o f t h e c a t h o d e s o r c h a n g e s i n t h e m a t e r i a l.

T h e s t a b il i ty o f t h e c a t h o d e a n d i n t e r c o n n e c t t ov a r i a t i o n s i n t h e o x y g e n a c t i v i t y a n d t e m p e r a t u r e a r eo f m a j o r c o n c e r n . L a M n O 3 - b a s e d c a t h o d e s a n dL a C r O 3 - b a s e d i n t e r c o n n e c t s a r e v e r y s e ns i ti v e t o r e -d u c t i o n . T h e o x y g e n l os s r e d u c e s th e c h a rg e c a r r i e rc o n c e n t r a t i o n a n d t h e r e b y i n c r ea s e s t h e e l e ct r ic a l re s -i s t an c e b y a b o u t a n o r d e r o f m a g n i t u d e .

T h e c a t h o d e , s t r o n t i u m - d o p e d L a M n O 3 , i s l e s ss t a b le t h a n L a C r O 3 a n d b e g i n s t o l o se o x y g e n a n d

e x p e r i e n c e s u b s t a n t i a l d e c r e a s e in c o n d u c t i v i t y atPo2 < 10 - s~ a tm . In fue l ga s a t 1000 ~ s t ron t ium -d o p e d L a M n O 3 d i ss o c ia t es i n t o L a 2 0 3 , S r M n O 3 ,

a n d M n O a n d b e c o m e s s t ru c t u r al l y u n s o u n d ( se eTa b l e I) [ 7 ] . I m p r o v e d r e s i s t a n c e o f t h e c a t h o d e t o -ward s r ed uc t io n was a l so a goa l o f t h i s s t udy.

Th e l o s s o f oxygen o n t he fue l s ide o f t he i n t e r-c o n n e c t m a y i n f l u e n c e b o t h t h e o x y g e n p e r m e a b i l i t yand mec han ica l s t ab i l i t y o f r e l a t i ve ly t h in (1 mm th i ck )s t r u c tu r e s . U n d e r r e d u c i n g c o n d i t i o n s o f 1 0 . 2 o a t m ,a s m u c h a s 5 m 0 1 % o x y g e n m a y b e l os t f r o m1 0 m o l % M g - d o p e d L a C r O 3. T h is c a us es a b o u t0 . 1 % e x p a n s i o n a t P o2 < 1 0 - 1 6 a t m a n d 1 0 0 0~Th i s r a i s e s a ques t i on a bo u t t h e s t ab i l i t y o f t he s t ruc -t u r e t o w a r d s c r a c k i n g a s a r e s u l t o f t h e o c c u r r e n c e o fpo t e n t i a l l y h igh s t r e s se s caused by t he l o s s o f oxyg en .T h e p r o b l e m i s e v e n m o r e p r o f o u n d f o r s t r o n t i u m -d o p e d L a C r O 3 b e c a u se a s m u c h a s 0 .4 % e x p a n s i o no c c u r s w i t h r e d u c t i o n [ 8 ] .

T h e r m a l e x p a n s i o n m i s m a t c h b e t w e e n t h e L a C r O 3a n d Z r O 2 a p p e a r t o b e t h e m o s t s e v e re p r o b l e m .Ta b l e I I c o m p a r e s t h e t h e r m a l e x p a n s i o n c o e f f i c i e n t( T E C ) o f y t t r i u m - s t a b i l iz e d Z r O 2 ( Y - P S Z ) a n d v a r i -o u s L a C r O 3 c o m p o s i t i o n s [ 8 ] . A s c a n b e s e e n( Ta b le I l) , t h e T E C o f m a g n e s i u m - d o p e d L a C r O 3c a n n o t b e m a d e t o m a t ch Y - P S Z f o r a n y m a g n e si u mcon te n t ; how eve r, a s l i t t l e a s 2 mo l % S r w i l l y i e ld aT E C m a t c h . T h i s s u g g e s t s t h a t s t r o n t i u m m i g h t b e ab e t t e r d o p a n t f o r L a C r O 3 t h a n t h e m a g n e s i u m t h a t i sc u r r e n t l y b e i n g u s e d [ 8 ] . T h e t h e r m a l e x p a n s i o n o f t h ec a t h o d e , L a M n O 3 , i s h i g h e r t h a n t h a t o f Y - P S Z ( se eTa b l e I I I) . T h i s m i s m a t c h i s a p o t e n t i a l p r o b l e m , b u t

TA B L E I I T h e r m a l e x P a n s i o n c o e f f i c ie n t s o f i n t e r c o n n e c t s (f r o m[ 8 ] )

L a ~ _ x S r x C r O 3 ( 35 0 1 0 0 0~ C ) ;~ L a C r l _ r M g r O 3 ( 3 5 0 - 1 0 0 0 ~

9 ,CQm position (x) TE C ~" TE C(i0 8oC - 1) Compo sition y) (10~ 8 C- 1)

0.02 10.24 0.00 9.480.05 10.89 0.02 9.460.10 10.74 0.05 9.57

0.15 10.84 0.10 9.480.20 11.10 0.15 9.55Y-PSZ 10 .30

TA B L E I I I T h e r m a l e x p a n s i o n c o e ff ic i en t s o f c a t h o d e ( f ro m[8])

La 1 _~ ,Sr, M nO 3 (2 5-1000 ~

C o m p o s i t i o n ( x ) T E C ( 1 0 - s ~ 1 )

0.00 11.2

0.05 11.70.10 12.00.20 12.40.30 12.8

t o da t e t he re i s no ev iden ce t o sup por t t h i s con j ec tu re .P e r h a p s t h e f a c t t h a t t h e c a t h o d e i s p o r o u s a l l o w ss t ress re l ie f a t the in ter fa ce .

A n o t h e r p r o b l e m i n h i g h - t e m p e r a t u r e f u e l c e l l s i st h e i n h e r e n t p r o c e s s i n g i n c o m p a t i b i l i t y o f t h e v a r i o u sc e ll c o m p o n e n t s . T h i s p r o b l e m e x i st s f o r b o t h t h ep l a n a r a n d t u b u l a r c e ll g e o m e t r i e s. T h e We s t i n g h o u s ec o m p a n y h a s t r i e d t o a v o i d t h i s b y u s i n g c h e m i c a lv a p o u r d e p o s i t i o n t e c h n i q u e s t o m a k e t h e ce lls. H o w -eve r, d i f f icu l t ie s occ u r w i th t he de pos i t i o n o f t he i n t e r-c o n n e c t . E v i d e n t l y, p o r o s i t y i s a p r o b l e m . T h e c o - f i r e dp l a n a r o r t u b u l a r g e o m e t r i e s p r e s e n t s o m e r e a l c h a l -l en g e s, b e c a u s e t h es e s t r u c t u r e s a r e m a d e i n a m o n o -l i t h i c manne r, e . g . f o r t he Argonne ce l l , a l l f ou r ce l lc o m p o n e n t s m u s t b e s i n t e r e d s i m u l t a n e o u s l y a n d f o rt h e We s t i n g h o u s e t u b u l a r c e l l , t h e c a t h o d e i n t e r -c o n n e c t a n d s u p p o r t t u b e m u s t b e s i n t e r ed s i m u l t a n -e o u s ly. T h i s m e a n s f o r t h e p l a n a r c e ll th a t t h e d e n s ep o r t i o n s , e l e c tr o l y t e a n d i n t e r c o n n e c L m u s t s h o w t h es a m e s i n t e r i n g s h r i n k a g e a s t h e p o r o u s a n o d e a n dc a t h o d e . F o r t h e t u b u l a r c el l, t h e i n t e r c o n n e c t m u s t b ed e n s e, c r a c k f r ee a n d a d h e r e t o t h e p o r o u S s u b s t r a t e

T A B L E I E l e c t r ic a l c o n d u c t i v i t y a n d o x y g e n l o s s o f c a t h o d e a n d i n t e r c o n n e c t a t 1 00 0 ~ a s a f u n c t i o n o f o x y ge n a c t i v it y ( f ro m [ 7 ] )

Po2 (a tm) Lao .8Sro .2M nO 3 LaCr0 .95M g0.o50 3

cr (S cm - 1) Ox ygen los t ( r (S cm - 1) Oxy gen los t(mol ) (mol )

10 ~ 150 0 3 .2 0

1 0 - s 150 0 3 .2 010-1 0 125 0.02 3.2 010-1 2 65 0.04 3.2 010 - 24 22 (dissoc iated) 0.15 2.5 0.00410-1 6 10 (dissoc iated) 0.20 0.32 0.017

5 8 3 8


3/7


4/7

Figure 2 Scann ing electron microg raphs of the polished and thermally etched surfaces of La0.gSr0.~Cra _=Mn=O3 sintered at 1475 ~ for 48 h:(a) x = 0.5, (b) x = 0.7.

s u r f a c e s o f s p e c i m e n s s i n t e r e d a t 1 4 7 5 ~ f o r 4 8 h a r eg i v e n in F i g . 2 . M i c r o s t r u c t u r e d e v e l o p m e n t w i t h

m a n g a n e s e s u b s t i t u ti o n i n L a C r O 3 c a n b e o b s e r v e df r o m t h e s e p h o t o m i c r o g r a p h s :

3.2. Electrical conductivity and Seebeckcoefficients

3.2. 1. Tem perature dep ende nceC o n d u c t i o n i n L a C r O 3 a n d L a M n O 3 is t h o u g h t t oo c c u r b y t h e d i f fu s io n o f p - t y p e s m a l l p o l a r o n s a m o n gc h r o m i u m o r m a n g a n e s e i o n s. A p o l a r o n is a s s o c i a te dw i t h a p a r t i c u l a r c h r o m i u m o r m a n g a n e s e s i te i f t h ei o n i s i n th e + 4 v a l e n c e s ta t e , r a t h e r t h a n t h e s t o i -c h i o m e t r i c + 3 v a l e n c e s t a te . T h e p o l a r o n h o p p i n g isc h a r a c t e r iz e d b y a t h e r m a l l y a c t i v a t e d m o b i l i ty. W h e nt h e c o n c e n t r a t i o n o f s m a l l p o l a r o n s is in d e p e n d e n t o ft e m p e r a t u r e c o n d u c t i v i t y e x p e c t e d t o t a k e t h e f o r m

= A / T e x p ( - - E ~ / k T ) (1)

w h e r e A i s b o t h a c h a r g e c a r r ie r c o n c e n t r a t i o n a n dm a t e r i a l c o n s t a n t , Ti s t h e a b s o l u t e t e m p e r a t u r e , s - - 1i n t h e a d i a b a t i c l i m i t a n d s = 3 / 2 i n t h e n o n - a d i a b a t i cr e g i m e , E ~ i s t h e a c t i v a t i o n e n e rg y, a n d k i sB o l t z m a n n ' s c o n s t a n t . T h e r e f o r e , fo r m a t e r ia l s w h i c ho b e y t h e s m a l l p o l a r o n m e c h a n i s m , A r r h e n i u s p l o t sa r e e x p e c t e d t o b e li n e a r , w i t h a s l o p e p r o p o r t i o n a l t ot h e a c t i v a t i o n e n e rg y a s s o c i a t e d w i t h s m a l l p o l a r o nh o p s . F o r t h i s s t u d y, t h e a d i a b a t i c f o r m w a s u s e d t oa n a l y s e t h e c o n d u c t i v i t y d a t a .

T h e e l e c t r i c a l c o n d u c t i v i t y d a t a f o rL a C r ~ _ ~ M n ~ O 3 a n d L 8 a r es h o w n i n F i g s 3 a n d 4 , r e s p e c t iv e l y, a s l o g o v e r s u sr e c i p r o c a l t e m p e r a t u r e . T h e d a t a w e r e s im i l a r i n b o t hs y s t e m s w i t h a n o r d e r o f m a g n i t u d e d r o p i n th ec o n d u c t i v i t y w h e n a s m a l l a m o u n t o f m a n g a n e s e w a ss u b s t i t u t e d f o r c h r o m i u m . F i g s 5 a n d 6 a re t y p i c a lA r r h e n i u s p l o t s o f l o g y v e r s u s r e c i p ro c a l t e m p e r -a t u r e . T h e a c t i v a t i o n e n e rg i e s f o r m o t i o n o f c h a rg e

c a r r i e r s d e t e r m i n e d f r o m F i g s 5 a n d 6 a r e p l o t t e d i nF i g . 7 as a f u n c t i o f f ' o f m a n g a n e s e c o n t e n t . T h e e n dm e m b e r s o f t h e s e il es , L a C r O 3 a n d L ” 3 , e x h i b its m a l l p o l a r o n b e h a v i o u r o v e r a w i d e t e m p e r a t u r ef a n g e a n d h a v e v e r y s i m i la r a c t i v a t io n e n e rg i es . T h e

5 8 4 0

1

IEo

0

"~ -1

¦g"9 - 3

-4 m

- - . . . . .

x = l . 0

+ ~-~ . [ ]

~- x=OA + ................

A + [ ] . . . . . . - - - - - .+&

+z~

+&

5 10 15 2 0 ;~5Reciprocal tempery (104 K 1)

Figure 3 Electrical conductivity of L aCr~ _~Mn~O3 as a function oftemperature. (- - ) End members, x: (98 0.2, ( + ) 0.3, (~ ) 0.4, (*)0.6.

A 1

I Eu

0

-3

x = l . 0

: . . . . ......... .................. = o+ + + 9 . . . . . . . . . . . . . 9

c] D[]

D

o

5 10 15 20 25ReciprocQI tempero ture (104 K 1)

Figure 4 Electrical condu ctivity of Lao.gSro.~Cr~_~MnxO as afunction of temperature. ( - - ) End members,x: ([]) 0.2 ,(*) 0.4, ( + )0.6, (B) 0.8.

c o n d u c t i v i ty o f u n d o p e d L a M n O 3 i s a b o u t 4 0 0 t im e sg r e a t e r t h a n t h a t o f L a C r O 3 . A l e a s t s q u a r e s f i t t o t h ed a t a g i ve s a n a c t i v a t i o n e n e r g y o f 0 .1 8 e V f o r L a C r O 3a n d 0 .1 9 e V f o r L a M n O 3 . T h e l i n e a r i t y o f t h e d a t a f o r


5/7

x = l . 0

~ 3 - ~ ~ + + . ~ ............... ~ .

~ ~ + ~ --- E Z ~

7 z x ~ + ~ ~'~'~~Ez ...

~. ~ ~ ~ \ ~ x=O

I i ~ ~ AS 10 15 20 25

Reoiproeal temperoture( 1 0 4 K - l )

Figure 5 Log c rT versus rec iproca lL a C r l _ ~ M n , O 3 . ( - - - ) E n d m e m b e r s, (the data. x: (A) 0.2, ( + ) 0.3, (D) 0.4, (*) 0.6.

t e m p e r a t u r e f o r) theor et ic” f i ts to

5 rT- . . . . . . . .] . . . . . . . . . . . . . . . . . . . . . . .

...................... . . . . . . X : t o

%*~ - ~ + =~ 2 2 ....... .. /- - 6 ' ~ ~ ~ i i < - J

v

o ~\

C7,

5 10 15 2'0 25R e r t e m p e r o t u r e 1 0 4 I - 1 )

Figure 6 Log orT versus rec iproca l t emp era ture fo rLao .gSro . lCq _xM n~O 3 . ( - - - ) End mem bers , ( ) theore t ica lfits to the data. x: (~) 0.2, (*) 0.4, ( + ) 0.6, (Ig) 0.8.

0 . 6

0.5

0.4g,,

0 . 39

0.2

yOA

0 0.2 0.4 0.6 0.8 1.0Mangonese r ( mol )

Figure 7 A c t i v a t i o n e n e rg y f o r c o n d u c t i o n v e r s u s m a n g a n e s e c o n -ten t in (La , Sr )C q _xMnxO3. (A ) 10% Sr, (O) 0 % Sr.

the end members over such an extended temperaturerange is consistent with the earlier identification of theconductivity in these materials as being due to atemperature-independent concentration of p-type

small polarons. In spite of the larger intrinsic elec-tronic cond uctivity of LaMn O 3, a small substituti onof manganese for chromium in LaCrO3 results in asharp drop in low-temperature conductivity from thatexhibited by either end member. This minimum, whichat room temperature is more than four orders ofmagnitude below that of LaCrO3, occurs at a man-ganese concentration of 20 mol %. In addition, thesesmall substitutions of manganese result in a nonlin ear

temperature dependence with increased slope. Themaximum slope observed occurs in the 20 mol % Mncomposition and corresponds to an activation energyof 0.48 eV. As can be seen from Fig. 7, the activationenergy increases with manganese content t o max imumat 20 mol % and then linearly decreases as the man-ganese content increases. The substitution of smallamount s of manganese for chromium have been foundto cause the room-temperature electrical conductivityto dr op order s of magnitude below that of either endmember and increase in activation energy. This beha-viour is attribute d to a l ower small polar on site energyat the manganese sites as compa red t o chr omium sites.At low manganese concentrations, the lower energymanganese sites act as traps for carriers diffusingamong chromium sites. Therefore, the activation en-ergy for conducti on is increased from that of LaCrO3.At higher manganese concentrations, direct transportamong manganese sites becomes possible and theconductivity increased and activation energy de-creased.

Thermopower measurements were made to deter-mine the type and concentration of charge carriers.Figs 8 and 9 show plots of the Seebeck coetticientsversus tempera ture for LaCr l_xMnxO3 andLao.9Sro.lCr 1 xMnxO3, respectively. The Seebeckmeasurements exhibit similar behaviour with respectto manganese substitution for chromium in both theundoped and strontium-doped series. There is anappropriate shift in the magnitude of the Seebeckcoefficient with stront ium addition in LaCrO3; how-ever, very little occurs in the manganese substitutedcompositions. The Seebeck coefficients show essen-tially no change with the t 0 mol % Sr substitution forthe compositions containing more than 30 mol % Mn.

1000

8 0 0

~ ~ ~

o o t

~-8 7 1 7 60 /4 0 0

. . k

x=¦ . . . . . . . . . . . . . . . . . . . .

I

~, 9 O O O O O i

O O O ~ iO O O O O A A A A A

A A A A A A A & A A f3 Q [] E3

9~- - - -- -~ - -- - - ~ s - - - -~ - - - - -~ - - -~ - - - - - -~ - - ,~ ~~I ;0 c~ , . . . .

60 0 800 1000 1200 1400Temperoture(K)

Figure 8 Seebeck coe t t i c ien t versus t empera ture fo rLaC r 1 _xMnxO 3 . ( - ) End memb ers , x : (O) 0 .2 , (A) 0 .4 , (E l ) 0 .6 .

5841


6/7

3 0 0

~..-- 2 50v.

~.2oo

2_~1 5 0

o

o 100

D=50

x=O

o o [ ] D D o o[]

D

D

0

O

AA

x = l . 0

4 0 0 6 0 0 8 0 0 1 0 0 0 1 2 0 0 1 4 0 0Temperature (K)

Figure 9 Seebe ck co eff ic ien t v er su s tempera ture forLao.gSro.lCr1_xMnxO3 (- - -) End mem bers, x: ([]) 0.2, (A) 0.4, (*)0.6.

0 . 5

9 . o.4o

~,0.3

~. 0.2o¦¦u

0 []

0 0 .2 0 .4 0 .6 0 .8 1 .0

Hanr r (mol)

Figure 10 Calculated fraction of occupied hopp ing sites as a func-tion of manganese content at 1000 ~ for LaCr 1_xMn~O87 9 0%Sr, (D) 10% Sr.

T h i s i n d i c a t e s t h a t s t r o n t i u m s u b s t i t u t i o n t o c o m p o s i -t io n s c o n t a i n in g m o r e t h a n 3 0 m o l % M n p r o b a b l yd i d n o t r e s u l t in t h e f o r m a t i o n o f c o n d u c t i n g s m a l lp o l a r o n s a t th e c h r o m i u m o r m a n g a n e s e s it es in a c -c o r d a n c e w i t h Ve r w a ye t a u s p r i n c i p l e [ 1 3 ] . T h e r e -l o r e , t h e a b s e n c e o f s ig n i f i c a n t i n c r e a s e i n t h e e l e c t r i c a lc o n d u c t i v i t y a n d d e c r e a s e i n t h e h i g h - t e m p e r a t u r eS e e b e c k c o e f f i c i e n t s w e r e o b s e r v e d f o r c o m p o s i t i o n sc o n t a i n i n g m o r e t h a n 3 0 m o l % M n . S e e b e c k c o e ff ic -i e n t s h a v e a n o n - l i n e a r t e m p e r a t u r e d e p e n d e n c e f o rt h e m i x e d c o m p o s i t i o n s . T h e S e e b e c k c o e f f i c i e n t s o fe n d m e m b e r s a r e l i n e a r w i t h r e s p e c t t o t e m p e r a t u r e .A c c o r d i n g t o H e i k e s ' s f o r m u l a t h i s ty p e o f b e h a v i o u ri n d i c a t e s a s m a l l p o l a r o n c o n d u c t i o n m e c h a n i s m .( H e i k e s 's f o r m u l a i s a t e m p e r a t u r e - i n d e p e n d e n t e x -p r e s s i o n . ) U s i n g t h e a s s u m p t i o n t h a t o n l y o n e e l e c t r o ni s a l l o w e d o n a g i v e n s it e a n d b o t h s p i n a n d o r b i t a ld e g e n e r a c y a r e n e g l ig i b le , y i e l ds a n e x p r e s s i o n f o r t h e

S e e b e c k c o e f l i c i e n t [ 1 4 ]

Q = ( k / e ) { l n [(1 - x ) / x ] + S * / k } (2)

w h e r e k is B o l t z m a n n ' s c o n s t a n t , e is th e u n i t c h a rg e , xi s t h e f r a c t io n o f h o p p i n g s i te s w h i c h a r e o c c u p i e d a n d

5 8 4 2

S * i s t h e v i b r a t i o n a l e n t r o p y a s s o c i a t e d w i t h t h e i o n ss u r r o u n d i n g a p o l a r o n o n a g i v e n s i t e . T h i s e q u a t i o nl e a d s to a t e m p e r a t u r e - i n d e p e n d e n t S e e b e c k c o ef fi -c i e n t . U s u a l l y, t h e e n t r o p y,S * , i s s m a l l e n o u g h t o b en e g l i g i b l e ; t h e r e f o r e , t h e S e e b e c k c o e f f i c i e n t d e p e n d so n l y o n th e c o n c e n t r a t i o n t e r m . U s i n g t h e S e e b e c kd a t a a n d H e i k e s ' s f o r m u l a , t h e f ra c t io n o f h o p p i n gs it e s a t 1 0 0 0 ~ w a s c a l c u l a t e d a s a fu n c t i o n o f m a n -g a n e s e s u b s t i t u t i o n ( F i g . 1 0 ) . A s c a n b e s e e n , t h e f r a c -

t i o n o f o c c u p i e d h o p p i n g s it e s i n c r e a s e d a s a f u n c t i o no f m a n g a n e s e s u b s ti t u ti o n . T h i s b e h a v i o u r a g r e e dw i t h t h e e l e c t r i c a l c o n d u c t i v i t y.

3 .2 . O x y g e n a c t iv i ty d e p e n d e n c eD . c . e l e c t r i c a l c o n d u c t i v i t y m e a s u r e m e n t s f o rL a C r o . 4 M n o . 6 0 8 7 L a o . g S r o .l C r o .4 M n o . 6 0 3 , a n dL a o . g S r o . l C r o . 6 M n o . 4 0 8 7 w e r e m a d e a s a f u n c t i o n o fo x y g e n a c t i v i ty a t 1 0 0 0 ~ T h e r es u l ts a r e s h o w ni n F i g . 11 i n w h i c h L a o . g S r o . l M n O 3 a n dL a C r o . 9 8 7 3 d a t a a r e i n c lu d e d f o r c o m p a r i s o n .

T h e e l e c t r i c a l c o n d u c t i v i t y d a t a s h o w e d s i m i l a ro x i d a t i o n - r e d u c t i o n b e h a v i o u r . In th e h ig h o x y g e na c t i v i ty r e g i o n , w i t h i n e x p e r i m e n t a l e r r o r , t h e e l e ct r i-c a l c o n d u c t i v i t y w a s n e a r l y c o n s t a n t . T h e c o n d u c t i v -i t y d e c r e a s e d a s a f u n c t i o n o f o x y g e n a c t i v i t y t o t h eo n e - q u a r t e r p o w e r a s r e d u c t i o n p r o g r e s s e d . T h e c o n -s t a n t e l e c t r i c a l c o n d u c t i v i t y w h i c h e x i s t s i n t h e h i g ho x y g e n a c t i v i t y r e g i o n m a y b e e a s i ly u n d e r s t o o d i f i t i sa s s u m e d t h a t t h e c a r r i e r c o n c e n t r a t i o n a n d e l e c t r o n i cc o m p e n s a t i o n p r e d o m i n a t e s . I n t h e l o w o x y g e n a c ti v -i t y r e g i o n , o x y g e n v a c a n c i e s a r e f o r m e d a n d t h e e l e c -t r i c a l c o n d u c t i v i t y b e g i n s t o d e c r e a s e a s a r e s u l t o f

i o n ic c o m p e n s a t i o n .S e e b e c k m e a s u r e m e n t s w e r e a l s o m a d e a s a f u n c -t i o n o f o x y g e n a c t iv i t y a t 1 0 0 0 ~ f o r s o m e c o m p o s i -t i o n s . I t w a s f o u n d t h a t t h e S e e b e c k c o e f f i c i e n t s w e r ea l w a y s p o s i ti v e ev e n a f t er th e c o m p o u n d s d e c o m -p o s e d . A t 1 0 0 0 ~ w i t h d e c r e a s i n g o x y g e n a c t iv i t y, t h eS e e b e c k c o e f fi c ie n t s i n c r e a s e d t o a m a x i m u m t h e ns t a r t e d d e c r e a s i n g a s a r e s u lt o f d e c o m p o s i t i o n . T h ec r i t i c a l o x y g e n a c t i v i t y f o r c o m p o s i t i o n s w e r e o b -s e rv e d t o b e c l o s e t o t h o s e f o r L a M n O 3 (a b o u t

?.S

- - 2 . 0"7

Sm l . 5

.w

> 1.0u.j

coo 0.5

0

0

9

-0.5

- 2 0

+ +

+

+

+

o o o

o D ~ ~ A + A ` A

A

+

o o o oA A A 'b

0 Q Q

i

-15 -10 -5 0Lo9 PO2 lotto)

Figure 11 Log conductivity versus log Po2 for compositions at1000 ~ ( + ) 10% Sr-LaM nO 87*) 5% Mg -LaCrO 879 10% Sr,60% Mn, (A ) 10% Sr, 40% Mn, (122) 0% Sr, 60% M n.


7/7

100

I

9 80

::L

609

4 08

~ z ~0

21

O o

B ~ q o

A

A

18 15 12 9 6 3Lo9 P02 lo t to)

0 0

B 11

1.5

1.2 'Ev

u ~

0.9

o,6

0 . 3

00

Figure 12 ( 9 Seebeck coefficient and (A) log conduc tivity as afuncti on of oxy gen activity a t 1000 ~C for Lao.gSro.lCro.6Mno.40 3.

10-15 a tm a t 1000 ~ An in it i a l d rop in t he e l ec tr i ca lc o n d u c t i v i t y o f c o m p o s i t i o n s w a s o b s e r v e d a s o x y g e nac t iv i ty was dec reaseck Th i s t aus t be du e to t he r educ -

t i o n i n t h e n u m b e r o f c h a rg e c a r r ie r s a s o x y g e n v a c a n -c i e s were i n t roduced . Th i s was con f i rmed by the oxy-gen a c t iv i ty depen denc e o f t he Seebeck coe ff i c i en t a t1000 ~ F ig . 12 shows the expe r ime n ta l ev idence o fs u c h a r e l a t i o n s h i p b e t w e e n c o n d u c t i v i ty a n d S e e b e c kcoe ff ic i en t fo r Lao .9Sro . lCro .6M no .403 . A s can be seenin F ig . 12 , t he Seebeck coe ff i c i en t i nc reased and thee l ec t r i ca l conduc t iv i ty dec reased wi th dec reas ingoxygen ac t iv i ty. Th i s was t rue un t i l decompos i t i onoccur red .

4 . C o n c l u s i o nT h e L a C r O 8 7 1 8 7 s y s te m f o rm s a s o li d s o lu t io n .M a n g a n e s e s u b s t it u ti o n f o r c h r o m i u m i n L a C r O 3s ign i f i can t ly imp rove d the s in t e r ab i l i ty i n a i r. Dens i t -i e s above 95 % theo re t i ca l were ach ieved a t 1475 ~ fo rc o m p o s i t i o n L a o . 9 S r o . l C r o . 3 M n o . v O 3 . H o w e v e r, t h es u b s t i t u t io n o f s u c h q u a n t i t ie s o f m a n g a n e s e o n t h ech romium s i t e has shown two nega t ive e ff ec t s :

1 . e l ec t ri ca l cond uc t iv i ty dec reased ;

2 . s t ab i l i t y aga ins t r educ t ion was ob ta ined on ly a tve ry h igh c h rom ium con cen t r a t ions (Lao .9Sro. lCro . s -M n o . 2 0 3 ) .

R e f e r e n c e s1. T. TAKHASHI, in "Physics of Electrolytes", edited by J.

Haadik (Academic Press, New York, 1972) p. 989.2. A.O. ISENBERG, Solid Stare Ionics 3/4 (1981) 432.3. I. J. ROHR, in "High Temperature Fuel Cells", edited by P.

Hagenmuller and W. Van Gool (Academic Press, New York,1987) p. 431.

4. W. FEDUS KA and A. O. ISENBER G, J. Power Sources 10(1983) 89.

5. M. WARSHAY, "Discussion of Potential of High Temper-ature Solid Oxide Fuel Cell Power Plan Systems", in theWorkshop on High Temperature Sotid Oxide Fuel Cells,Brookhaven National Laboratory, Upton, New York, 5-6May 1977.

6. D.C. FEE,"S olid Oxide Fuel Performance", in Proceedings ofthe Conference on High Temperature Solid Oxide Electro-lytes, Brookhaven National Laboratory, Upton, New York,16-18 August 1983, edited by D. Fee, J. Fillo and H. Isaacs,p. 4.

7. L. KUO, PhD thesis, Univer sity of Misso uri-R olla (1987).8. S. SRILOMS AK, D. P. SCHILLIN G, H. U. ANDERSON,in "Proceedings of the 1st International Conference on SolidOxide Fuel Cells", Hollywood, Florida, O ctober 1989, editedby S. C. Singhal (The Electrochemical Society, NJ, 1989)p. 129.

9. L. GRO UPP and H. U. ANDERSON, J. Amer. Ceram. Soc.59 (1976) 449.

10. M. PECHI NI, US Par. 3 330697 (1967).11. G. CARINI , MS thesis, Univers ity of Misso uri-R olla (1988).12. R. KOC, H. U. ANDE RSO N, S. A. HOW 93 and D. M.

SPARLIN, in "Proceedings of the 1st International Confer-ence on Solid Oxide Fuel Cells", Hollywood, Florida, Octobe r1989, edited by S. C. Singhal (Etectroch emical Society, NJ,1989) p. 220.

13. E. J. VERWAY, P. W. HAAI JAM, F. C. ROM EIJ H andG. W. VAN OOSTERHO UT, Philips Res. Report 5 (1950) 173.

14. R.R . HEIKES , in "Thermoelectricity: Science and Engineer-ing" edited by R. R. Heikes and R. W. Ure, Jr (Wiley Inter-science, New York, 1961) p. 37.

Received 14 Augustand accepted 28 No vember 1991

5843

investigation of strontium-doped la(cr, mn)o 3 for solid oxide fuel cells

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