Effect of Ionic Conductivity of Zirconia Electrolytes on the Polarization Behavior of Various Cathodes in Solid Fuel Cells

Abstract

The polarization behaviors of porous platinum and La(Sr)MnO 3 (LSM) cathodes coupled with zirconia electrolytes with various ionic conductivities ( ion ) were investigated. The exchange current density, j 0 , on Pt cathode was not influenced by the ion at 900 and 1000ЊC, whereas j 0 increased proportionally to ion at a lower temperature of 800ЊC. However, the j 0 on LSM cathodes increaesd in proportion to the ion in the temperature region between 800 and 1000ЊC. The dispersion of nanometer-sized Pt catalysts on LSM particles greatly enhanced the performance, the magnitude of which depended on the temperature, the ion , and the microstructure of LSM. The observations are well explained kinetically, i.e., the cathode performance is controlled by the transport rate of O 2Ϫ at the interface when the surface reaction rate is sufficiently high. Consequently, the use of high-performance electrodes in combination with the solid electrolyte having high ion is very important for achieving the high performance of solid oxide fuel cells. © 1999 The Electrochemical Society. S0013-4651(98)03-090-0. All rights reserved.Manuscript submitted March 18, 1998; revised manuscript received September 1, 1998.Solid oxide fuel cells (SOFCs) have been intensively investigated since they are expected to yield fairly high energy conversion efficiencies. At present, the operating temperature of SOFC is restricted to temperatures of about 1000ЊC because of insufficient performance of the state-of-the-art electrolyte and electrodes at low temperatures. Lowering the operating temperature to around 800ЊC is desirable to overcome a limited choice of the component materials resistant to mechanical degradation due to thermal heat shock or to physical and chemical degradation due to oxidation/reduction of materials or to solid-phase reaction at an interface between different materials. However, two major obstacles must be solved to operate SOFCs at medium temperatures. The first is to reduce ohmic losses in solid electrolytes. Progress in this direction has been achieved by the development of a very thin film zirconia electrolyte 1-7 or novel solid electrolytes with higher ionic conductivity. [8][9][10][11] Besides the reduction of ohmic losses, it is very important to develop high performance electrodes since the electrode reaction rates at both anode and cathode are affected detrimentally at medium temperature range.We have been conducting a series of studies with the objective of developing high performance SOFC electrodes operating at relatively lower temperatures than those of the present level. Thus, it becomes essential to clarify the factors that control the polarization properties of these electrodes. It was shown earlier that the polarization loss at the electrodes can be reduced by using mixed-conducting ceria electrolytes, 12 or by introducing the mixed-conducting (reduced zirconia or ceria) layer on the conventional zirconia electrolyte surface. 4,5,[13][14][15] There is, however, no report available exclusively on the effect of ionic ...

Recent Achievements of NEDO Durability Project with an Emphasis on Correlation Between Cathode Overpotential and Ohmic Loss

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Functional Nanostructure Engineering of SOFC Cathode by Solution Infiltration

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Effect of Sr-Doped LaCoO3 and LaZrO3 Infiltration on the Performance of SDC-LSCF Cathode

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