Introduction

Solid oxide fuel cells (SOFCs) have been intensively investigated because, in principle, their energy conversion efficiency is fairly high. Lowering the operating temperature of SOFCs from 1000°C to around 800°C is desirable for reducing serious problems such as physical and chemical degradation of the constructing materials. It is very important to develop high performance elcctrodcs because the electrode reaction rates decrease extremely in such temperature regions.

The object of a series of the studies is to find a clue for achieving higher electrode performances at a low operating temperature than those of the present level. Although the polarization loss at elcctrodcs can be reduced by using mixed-conducting ceria electrolytes (I), or introducing the mixcd-conducting (reduced zirconia or ceria) layer on the conventional zirconia electrolyte surface (2-4), no reports are available on the effect of such an ionic conductivity of electrolytes on electrode polarizations. High ionic conductivity of the electrolyte, of course, reduces the ohmic loss. However, we have found that the IR-free polarization of a platinum anode attached to zirconia electrolytes is greatly influenced by the ionic conductivity, cr,„„, of the electrolytes used (5). The higher the a,on. the higher the exchange current density, j0, for the Pt anode in H2 at 800 ~ 1000°C. It was indicated that the H2 oxidation reaction rate was controlled by the supply rate of oxide ions through the Pt/zirconia interface, which is proportional to the cw Recently, we have proposed a new concept of the catalyzed-reaction layers which realizes both high-performances of anodes (6,7) and cathodes (6,8) for medium-temperature operating SOFCs. From the viewpoints of both fundamental electrochemistry and practical application, it is very interesting to clarify such an effect of aw„ on the polarization of various electrodes. In this paper, we present the interesting dependence of the polarization properties of various electrodes (the SDC anodes with and without Ru microcatalysts, Pt cathode, La(Sr)Mn03 cathodes with and without Pt microcatalysts) on the a,on of various zirconia electrolytes at 800 ~ 1000°C.

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