Study of a solid oxide fuel cell with a ceria-based solid electrolyte

Solid State lonics 35 (1989) 285-291 North-Holland~ Amsterdam

STUDY OF A S O L I D O X I D E FUEL CELL WITH A C E R I A - B A S E D S O L I D E L E C T R O L Y T E T. I N O U E , T. S E T O G U C H I , K. E G U C H I and H. A R A I Department of Material Science and Technology, Graduate School of Engineering Sciences, l(vushu University, Fukuoka 816, Japan Received 3 October 1988

The ionic conductivity of the ceria-samaria system is higher than that o f y n r i a stabilized zirconia and other ceria-based oxides. The current density and power density of oxygen-hydrogen fuel cell with the ceria-samaria system was found to be high at 600800 :C, because of its low internal cell resistance. However, the open circuit voltage of the cell for ceria-samaria system was lower than the theoretical value, because of reduction with H2 and resulting decrease in ionic transference number. To suppress the reduction, a thin film of yttria-stabilized zirconia was coated on the fuel side of ceria-samarium oxide disk by the ion plating method. The fuel cell with zirconia-coated ceria-samaria exhibited high durability of the cell output and stable open circuit voltage.

1 Introduction

Solid electrolytes with a high oxygen ionic conductivity have been suggested for use in high temperature fuel cell. The solid oxide fuel cell has the advantage over the other types o f fuel cells because o f its low electrode polarization due to high working temperature, high conversion efficiency and absence o f liquid components. Stabilized zirconia has been most c o m m o n l y e m p l o y e d as a solid electrolyte o f the fuel cell [ 1 - 4 ] . Since the electrolyte layers are deposited on a porous electrode surface, their thickness should be more than 10 p m to attain sufficient mechanical strength. This type o f fuel cell must be operated at 1000 ° C or more even with relatively thin electrolyte layers ( ~ 100 ~ m ) , because o f its low conductivity (9 x'. 10 2 S c m - ~ at 1000 ° C). A solid electrolyte, with higher ionic conductivity than stabilized zirconia, would enable the fuel cell to be operated at lower t e m p e r a t u r e s for the same thickness o f electrolyte [5]. We have reported that the ionic conductivity o f the c e r i a - s a m a r i a system is the highest among the ceria-based oxides [6]. The low internal cell resistance of CeO2-Sm203 leads to a high current density as c o m p a r e d with zirconia based electrolytes [ 7,8 ]. However, the open circuit voltage o f c e r i a - b a s e d oxides fuel cell is smaller than the the-

oretical values, because o f partial reduction o f the oxide with the fuel [9]. In this paper, a thin film of stabilized zirconia was coated on the fuel side o f a CeO2-Sm203 disk, to suppress the reduction with the fuel. The preparation m e t h o d o f a stabilized zirconia thin film was investigated using an R F - i o n plating method.

2. Experimental

2. 1. Preparation of electrolyte Oxide specimens were p r e p a r e d by calcination of mixtures o f the c o m p o n e n t oxides by the previously described method. The oxide mixtures o f calculated a m o u n t s o f CEO2(99.9%) and Sm203(99.9%) or ZRO2(99.9%), were ball-milled for 24 h and then calcined at 1300°C for 10 h. The powders thus obtained were pulverized and subsequently pressed into a disk (20 m m in d i a m e t e r and 1.8 m m thick) at 2.7 t o n / c m 2 in vacuo. The pressed disk was sintered at 1650°C for 15 h. The crystal structure was determ i n e d by X-ray diffraction using CuKc~ radiation. The density o f sintered samples was estimated by a water p y c n o m e t r i c method.

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286

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2.2. I='/eetrica/ measuremen!

orescent X-ray analysis. Microstructures at the film surface were observed by' SEM (JOEL, J S M - 5 0 A ) .

The ionic transference n u m b e r o f oxygen, l,, is est i m a t e d from E M F of oxygen concentration cell by the previously described m e t h o d [6]. The electrical conductivity o f s i n t e r e d samples was measured in air by the de-4 probe m e t h o d as a function o f temperature. l - l" characteristics of an oxygen-hydrogen fuel cell were measured with the following cell structure, O , ( 1 a r m ) , Pt/(CeO~)<),~(SmO~ 5)o2/ /Pt, H:(1 atm) .

( 1)

Fig. 1 shows the schematic view of the cell. P l a t i n u m paste was applied to form the a n o d e and cathode. The pellet with the electrodes was fired at 800: C for one hour. Electrical contact between electrodes and Pt leads was m a d e using p l a t i n u m gauze. The disk was attached to a mullite tube by glass rings to ensure gas tightness. C a t h o d i c and anodic polarization curves were measured by the current interruption method.

2.3. PreparaHon of thin films Thin films of yttria-stabilized zirconia were prepared using an ion-plating a p p a r a t u s e q u i p p e d with an EB gun and an R F coil. The film thickness was d e t e r m i n e d by an interference microscope. The composition of ZrO2-Y20~ was estimated from flu-

3. Results and discussion

3. I. Electrical eonductiriO' of CeO,-based oxides The density of the sintered pellet, which was det e r m i n e d by a water pycnometric method, was more than 95% o f the theoretical density for every sample used in this study. The crystal structure of pure CeO2 and rare earth oxide- or alkaline earth o x i d e - d o p e d ceria is the cubic fluorite type. The diffraction patterns of s a m a r i a - d o p e d ceria was attributed to cubic fluorite type, but the slight shifts in diffraction angles due to dissolution of d o p a n t in ceria were observed. The ionic transference number of ceria-based oxides and yttria-stabilized zirconia was almost unity at 6 0 0 - 8 0 0 : C , whereas that of pure ceria was only, about 0.4. F o r c a t i o n - d o p e d ceria and stabilized zirconia, the Arrhenius plots of the ionic conductivities were quite linear at 5 0 0 - 1 0 0 0 ° C , as shown in fig. 2. The electrical conductivities of ceria-based oxide were about two orders of magnitude higher than those of stabilized zirconia. ( C e O e ) o . ~ ( S m O i . 5 ) o . 2 has the high-

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Fig. 2. Arrhenius plots for the electrical conducfivities of ( C e O e L . ( M ( ) , ) . 2 , YSZ and CeO,: M = ( C ) ) S m , ( A ) Y. (@)Ca, (&)Sr, ( ~ ) (ZrO:L s,(YO~ ,)~, is, ([])(e(-)>

72 Inoue et al. /Solid oxide fuell cell est ionic conductivity a m o n g zirconia- and ceriabased oxide systems. The apparent activation energy for ( C e O 3 ) o s ( S m O , 5)o.3 was 0.78 eV.

40 AA~2 AAO

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3i 2. Solid oxide fixel cell with CeO,-SmeO.~ electrolyte In a solid oxide fuell cell, one of the most important problems is how to lower the i n n e r cell resistance. A low electrolyte resistance leads to a high current density or to the ability to operate the cell at low temperature. The high ionic conductivity of (CeO_,)o8 (SmO~ s)o2 was expected to contribute to the reduction of the inner cell resistance. Fig. 3 shows the relation between voltage and current density for the oxygen-hydrogen fuel cell with (CeO2)o.s(SmOl.5)o.2 and (ZrO2)o.ss(YO,.5)o.15. The slope of the plot corresponds to the internal cell resistance. The cell resistance of (CeO2)o.8(SmO~.5)02 was lower than that of (ZrOa)o.85(YO[.5)o. i5i The power density is also plotted as a function of current density in fig. 4. The power density of (CeOa)o.8(SmOLs)0.2 was higher than that of (ZrOa)o.85(YO~.5)o.~5. The open circuit voltage for (CeO2)o 8(SmO~ s)o 2 was 0.89 V at 8 0 0 ° C which was smaller than that for (ZrOa)o.85(YOi.5)o.15. Low open circuit voltage seems to result from the deviation of oxygen transference n u m b e r from unity due to the reduction of the electrolyte with H2. The e q u i l i b r i u m oxygen partial pressure of the mixture of H2 and H 2 0 u n d e r the

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Fig. 4. I-P characteristics of the fuel cell at 800 : C: (C))He, Pt/ (CeOa)os(SmOt~)o.e/Pt, Oa: (A)Ha, Pt/(ZrOz)ll~s(YOi 5)0 i~ coated (CeO2)odSmO~5)o3/Pt, O3; (O)H> PI/ (ZrO2)0.s)(YOt ~)(, ~,/Pt, O2 (electrolyte thickness 1.8 mm ).

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Fig. 5. Relation of composition between deposit and evaporation source.

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standard condition is about 5 X 10 23 aim at 700°C. Our previous study indicated that (CeOa)oa (SmO~ 5)o.2 is a mixed ionic and n-type electronic conductor below Po_, = 10-15 atm. In the operating condition, the electronic conduction is not negligible at the surface which is exposed to H>

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Fig. 3.1-V characteristics of the fuel cell at 800°C: (O)H> Pt/ (CeO2)o s(SmO, 5)o.2/Pt, O2; ( A ) H > Pt/(ZrOa)o ss(YOi 5)o 15

coated (CeO,)o ~(SmO, s)o ,/Pt, O2; (O)Ha,Pt/ (ZrO,)o ss (YO,.5)t~,5/Pt. O2 (electrolyte thickness 1.8 mm ).

3.3. Coating o f stabilized zirconia on the ceriabased solid electrolyte Yttria-stabilized zirconia is less reducible than ceria-based oxides [ 10 ]. To suppress reduction near the hydrogen electrode, a thin film of the stabilized

7: lnoue et al. /Solid oxide l~tel cell

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Fig. 6. X-ray diffraction patterns of (CeO3)o s(SmO~ 5){~2 coated with (ZrO2)o.ss(YO, 5)o t5 by RF-ion plating (a) before and (b) after annealing al 800 °C for 10 min: thickness: (CeO3)o,(SmO~ 5)~2; 1.825 mm, (ZrO2)oss(YO~ 5)o,s: 1.86 lam: ( Q ) Sm-doped ceria phase, ( O ) Y-stabilized zirconia phase.

substrate

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: 200~C

substrate

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: 400 ° C

Fig. 7. SEM images of (ZrO3)ous(YOi 5)o ,5 thin film coated by RF-ion plating (a) before and (b) after annealing at 800 :'C for 1 h. Pressure, 2 × 10 4 T o r t (Ar:O2 = 1: 1 ) ; bias voltage, 300 V: RF-power, 400 W; EB-gun current, ~ 500 mA.

T. lnoue et al. / Solid o.ride fuell cell

zirconia was coated onto the surface of the ceria-samaria disk by ion plating as has been suggested previously. The stabilized-zirconia film should be thin enough not to increase the overall resistance and should be pin-hole free to avoid a contact of ceria with H2. In the present study, the thickness of stabilized zirconia film was a few micrometers. The chemical composition of the deposited stabilized zirconia film was estimated from fluorescent X-ray analysis and is plotted as a function of the composition of the evaporation source (fig. 5). The content of Y in the deposit tended to be lower than the source material. Hence, the source composition was con-

Film

trolled to obtain the thin film with the composition of (ZrO2)o.s5 (YO,.5) o., 5, at which the ionic conductivity is the highest among the yttria-stabilized zirconia. Fig. 6 shows the X-ray diffraction pattern of (ZrO2)o.ss(YOt.5)o.15 film deposited on the (CeO2)o.s(SmOLs)o.2 substrate. The diffraction lines from the film before annealing were obviously broader than those of substrate due to the poor crystallinity (fig. 6a). Annealing at 800°C promoted crystallization of zirconia in the film, as shown in fig. 6b. In the ion plating method, the quality of a thin film was influenced by (i) substrate temperature, (ii)

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Fig. 8. SEM images of (ZrOz)oss(YO,.5)o.,5 thin film c o a t e d by RF-ion plating at RF-power of (a) 100 W, ( b ) 200 W, (c) 300 W. Pressure, 2 × 10 4 T,orr (Ar:O2 = 1: 1 ) ; bias voltage, 300 V: EB-gun current, ~ 500 mA; substrate t e m p e r a t u r e , 400 ° C.

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7~ lnoue el al. /Solid oxtde./i4elcell

R F power, (iii) atmosphere, etc. In particular, (i) and (ii) were i m p o r t a n t factors for preparing a pinhole free thin film in the present case. As shown in fig. 7a. when a ( C e O 2 ) o s ( S m O , 5),,2 substrate was heated at 200°C during ion plating the thin film contained m a n y cracks alter annealing at 800: C. As the substrate t e m p e r a t u r e was raised to 400~C, cracks could be eliminated from the thin film. The cracks in the thin film result from the difference in thermal expansion between (ZrO_,)<,<,(YO~ 5), ,> and (CeO~)~.s(SmO,~)o2. The thermal expansion o f those materials was measured to be 0.97 × 10 ~ deg ' for ( Z r O e ) o s ~ ( Y O ~ 5 ) o j ~ a n d 1 . 2 2 × 1 0 - ~ d e g ~ for (CeO~),,~ ( S m O , , ) , , :. As the R F power was increased, the thin film tended to have a smooth surface, as shown in fig. 8. This suggests that a large p l a s m a concentration is necessary for d e p o s i t i o n o f dense film. An R F power of 300 W was sufficient to obtain a crack-free film. The I - V curve for the fuel cell with zirconia-coated ( C e O , ) ~ , s ( S m O ~ s ) o 2 disk is plotted in fig. 3. The open circuit voltage was enhanced by coating o f the stabilized zirconia and became similar in level to that of the ( Z r O ~ ) o s ~ ( Y O . 5 ) . ~ pellet. The thin film o f ( Z r O e ) o s s ( Y O , ~ ) ~ , , prevents direct contact between ceria and gaseous H, and protects ( C e O e ) o s ( S m O , ~ ) o 2 from the reduction. Theretore, the open circuit voltage a p p r o a c h e d the theoretical E M F after coating. The relation between power density and current density is shown in fig. 4. Although the cell resistance slightly increased by the zirconia coating, the output power of the fuel cell with the zirconia-coated ( C e O : ) o s ( SmOg5 )~2 electrolyte was larger than that for the (ZrOe)~ ~_s(YO, s)o ,5 electrolyte.

3.4. Cathodic and anodic overpotential qfi the fuel cell Fig. 9a and 9b show the cathodic and anodic overpotential and I R - d r o p o f the solid electrolyte(iR,.), which were measured by the current interruption method. The fuel cell with c e r i a - s a m a r i a electrolyte exhibited cathodic but not anodic overpotential, (fig. 9a). On the other hand, the fuel cell with the zirconia-coated sample had a large anodic overpotential, suggesting that the anodic reaction at ( Z r O ~ ) o ~ ( Y O , 5 ) o , 5 / P t interface has a larger o r -

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Fig. 9. Performance of the fuel cell at 800 : C: He, P t / e l e c t r o l y t e / Pt, Oe (O) 1-[" characteristics of the cell (a)

(CeO:)~/.s(SmO, 5)o> (b) (ZrO:)o ss(YO,.,)~ ,s coated (CeO)o s(SmO, , )o 2- The solid lines indicate contributing of iR drops (JR), cathodic overpotential q,. and anodic one tl:,, which are measured by the current interruption method. erpotential than that at (CeO2)o ~(SmO, s)oe/Pt interface. The voltage drop of the solid electrolyte, iR,., was larger for (ZrO2)..s>(YO~ s)o,5 than for zirconia-coated/(CeO2)os(SmO],)o2. Because the zirconia film was very thin, the electrolyte resistance did not increase significantly.

4. Conclusion Ionic conductivity o f s a m a r i a - d o p e d ceria, (CeO_,)o s(SmO] 5)o.> is higher than that o f yttriastabilized zirconia. Although the high conductivity leads to higher power densities for fuel cells with (CeO2)o.s(SmOl 5)o.2 electrolyte, the open circuit voltage was low because o f reduction with hydrogen during operation. The present study has shown that a yttria-stabilized zirconia thin film coated on the fuel side o f the solid electrolyte is quite effective in suppressing the reduction o f c e r i a - s a m a r i a . The thin film can be prepared by controlling substrate temperature and R F power during ion plating. The resulting electrolyte exhibited not only a high and stable open circuit voltage, but also higher power density than that of the stabilized zirconia system. The elec-

T. lnoue et al. / Solid oxide fuell cell

t r o l y t e l a y e r o f c e r i a - b a s e d o x i d e , w h i c h is c o a t e d w i t h sufficiently thin yttria stabilized zirconia, gave rise t o a cell w i t h l o w i n t e r n a l r e s i s t a n c e a n d h i g h p o w e r density.

References [ 1 ] T.H. Etsell and S.N. Flengas, Chem. Rev. 70 (1970) 339. [2]R.M. Dell and A. Hooper, Solid electrolyte, eds. P. Hagenmuller and W.V. Gool (Academic Press, New York, 1978) p. 291. [ 3 ] E.C. Subbarao and H.S. Maiti, Solid State lonics 11 ( 1984 ) 317.

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[4] J. Weissbert and R. Ruka, J. Electrochem. Soc. 109 (1962) 723. [5] T. Takahashi, H. lwahara and Y. Nagai, J. Appl. Electrochem. 2 (1972) 97. [6] H. Yahiro, Y. Eguchi, K. Eguchi and H. Arai, J. Appl. Electrochem. 18 (1988) 527. [ 7 ] H. Yahiro, Y. Baba, K. Eguchi and H. Arai, J. Electrochem. Soc. 135 (1988) 2077. [ 8 ] T. Kudo and H. Obayashi, J. Electrochem. Soc. 123 ( 1976 ) 415. [9] T. TakahashL I. lIo and H. Iwarahara, Denki Kagaku 34 ( 1966 ) 205. [10] Y. Saito, Surface Sci. 21 (1983) 635.