Perovskite Oxides for the Cathode in Solid Oxide Fuel Cells

The perovskite type oxides, Lnl-xAxM03-δ (Ln = lanthanides, A=Sr, Ca, M=Mn, Co), were studied as cathode materials in solid oxide fuel cells (SOFC) from a view point of compatibility with the solid electrolyte of yttria stabilized zirconia (8 mol% Y203 doped zirconia, 8YSZ). Generally, the pyrochlore, La2Zr2O7, can form at the boundary between the perovskite and 8YSZ. In the case of Lnl-xSrxMnO3-δ (Ln=La, Pr, Nd, Sm, and Gd), the formation of the pyrochlore, Ln2Zr2O7, was suppressed for the perovskites having smaller lanthanoids than La, especially for the Prl-xSrxMnO3-δ and Ndl-xSrxMnO3-δ systems. The Lnl-xAxCoO3-δ systems with samaller lanthanoid ions were also effective in suppressing the reaction with 8YSZ. However, the Lnl-xAxCoO3-δ systems, which are promising cathode materials for a low operating temperature SOFC, have larger thermal expansion rates than 8YSZ. The formation of a solid solution with Mn in the B-site of the perovskite such as Gdl-xAxMn1-y COyO3-δ (A = Sr and Ca) brought reasonable thermal expansion rates, compatible with 8YSZ and high oxygen reduction catalytic activity.