Rare-earth materials for Solid Oxide Fuel Cells (SOFC)

In this paper, we have shown that rare earths are used in most of the candidate materials for SOFCs. However, it should also be noted that most of the SOFC materials are complex oxides of rare earths and other metals. The oxide ion conductivities of zirconia or ceria are controlled by rare earths substitutions, however, the electrical conductivity or catalytic activity are rather controlled by transition metals or alkaline earths. The most important problem of rare earths is their high costs. The reduction in the cost of lanthanum or samarium would lead to a reduction of the total fabrication cost of the SOFC cell stacks. The possibility of using rare earth mixtures as an alternative of pure lanthanum have been tried, however, it resulted in the degradation of the performance of the SOFC. Another issue for SOFC material research is to increase the available thermodynamic data for rare-earth containing complex oxides. Although there has been intensive researches on the binary oxides, there is a paucity of data for complex oxides such as the perovskites, and the fluorite oxides with some substitutions. Most of the SOFC materials are complex oxides with several metal components which have been reported in the past twenty years. However, it is a matter of regret that there are only a few reports concerning calorimetric studies of these materials. Fundamental research on these advanced materials is necessary especially from the viewpoint of evaluating chemical stability and durability of the SOFC components.