Transition Metal Doped LaGaO 3 Perovskite Fast Oxide Ion Conductor and Intermediate Temperature Solid Oxide Fuel Cell
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electrolyte film without gas leakage and an excessively high operating temperature such as 1273 K, are essential for the high power density of SOFCs when YSZ is used as electrolyte. On the other hand, all advantages of SOFC such as a high efficiency and a variety of usable fuel can be obtained at decreased temperatures such as 1073 K. Furthermore, choice of the materials for cell stacking becomes wider; in particular, cheap refractory metals such as a stainless steel will be usable by decreasing the operating temperature down to 900 K. Consequently, decrease in operating temperature is of importance for the development of cheap but reliable cells (1). Decreasing operating temperature requires the active electrode, in particular, cathode catalyst, and the electrolyte with a low resistance. Ceria doped with Gd or Sm is generally considered for the electrolyte of SOFC operable in the decreased temperature range (1). However, ceria based oxide exhibits the ntype semiconduction in the reducing atmosphere (2). Consequently, n-type semiconduction drastically decreases the open circuit potential from the theoretical values (3) resulting in the decreased efficiency. It is also reported that reduction expansion forms a severe stress on the electrolyte which sometimes become higher than the intrinsic mechanical strength of CeO2 based oxide (4,5). On the other hand, preparing very thin YSZ film is also investigated for intermediate temperature SOFC (6), however, reliability becomes low when the thickness of electrolytes becomes extremely thin. It is, therefore, of great importance to develop new electrolyte materials which exhibit In our previous study, oxide high oxide ion conduction over a wide oxygen partial pressure. ion conductivity in oxide with perovskite structure was investigated and it was found that the LaGaO3-based perovskite type oxide exhibits high oxide ion conductivity (7,8), which is 283 Mat. Res. Soc. Symp. Proc. Vol. 575 c 2000 Materials Research Society
comparable with that of CeO2-based oxide. In particular, LaGaO 3 doped with Sr for La and Mg for Ga sites exhibits high oxide ion conductivity stably over a wide oxygen partial pressure range (9-12). The advantage of this oxide is that almost pure oxide ion conductivity is exhibited in both reducing and oxidizing atmosphere. It is generally believed that doping transition metal cations such as Co or Ni enhances electron or hole conduction. Therefore, doping these cations seems not to be desirable from the oxide ion conduction point of view. However, it is possible to get increased oxide ion conductivity without decreasing the transport number of oxide ion provided that the amount of dopants are small. EXPERIMENTAL All specimens used in this study were prepared by conventional solid state reaction techniques employing powders of La2O 3, Ga20 3, MgO, SrCO 3, and MeOx (Me=Fe, Co, Ni, Cu, Mn). Details of the preparation method can be found elsewhere (4). The sintering was achieved
at 1773 K for
6 hours in air. Platinum paste was applied on both faces of sint
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