ИСТИНА

Transition metal oxides with mixed ionic and electronic conductivities (MIEC) are of particular interest due to their potential application in high-temperature electrochemical devices including solid oxide fuel cells (SOFCs). This is especially true for cathode material in intermediate temperature solid oxide fuel cell (IT-SOFC) operated at 550-750oC. Major requirements for cathode material in IT-SOFC are adequate electronic and oxide-ion conductivities, catalytic activity in the oxygen reduction reaction, thermal expansion coefficient (TEC) close to that of the electrolyte and chemical inertness toward the electrolyte [1]. In the present work influence of the crystal structure and chemical composition on the properties of perovskite-related cobalt and copper oxides important for their use as cathode materials for IT-SOFC is discussed. Perovskite-related cobaltates with Co3+ perfectly meet the basic requirements for cathode material for IT-SOFC listed above. However, they suffer from high TEC due to thermally activated transition between low (LS) and high-spin (HS) state of Co3+. Influence of this transition on high-temperature thermal expansion of perovskite-related oxides is discussed using the example of layered (Pr,Sr)2(Ni,Co)O4 oxides [2]. One way to reduce TEC of cobaltates is to stabilize HS Co3+ in the ground state. Such compounds can be found among cobaltates with the brownmillerite-type structure. We have discovered the presence of HS Co3+ in the ground state in CoO6 octahedra of Sr2Co1.2Ga0.8O5 oxide with the brownmillerite structure [3]. This cobaltate is found to possess as low TEC as 13.1 ppm K-1 in comparison with ~20 ppm K-1 for LaCoO3. Oxides R2CuO4, where R is rare-earth cation with layered crystal structures having perovskite slab alternating with rock-salt slab (R=La, so called Т-phase) or fluorite slab (R=Nd-Gd, so called T’-phase) are considered as prospective cathode materials for IT-SOFC due to their low TEC (~12 ppm K-1) and moderate high-temperature electrical conductivity (>100 S/cm for Pr2CuO4). Correlation between the crystal structure of layered cuprates and their high-temperature oxide-ion conductivity is discussed.