Influence of Ca Doping on the Performance of Ruddlesden-Popper Oxides Sr3.2-XCaxLn0.8Fe1.5Co1.5O10-δ as Cathode Materials for It-SOFCs

Solid oxide fuel cell (SOFC) is an attractive electrochemical device for energy conversion with high efficiency and fuel flexibility at high operating temperatures (~1000 °C). Current operating temperatures of SOFC technology limit its wide commercialization due to the selection of materials used and degradation of cell components. This challenge has generated enormous interest in SOFCs operating at intermediate temperatures (600 – 800 oC). However, the catalytic activity of the conventional La1-xSrxMnO3 perovskite cathode drops significantly at temperatures < 800 °C. Mixed ionic-electronic conducting perovskites are widely investigated as cathodes for SOFCs due to their high catalytic activity for oxygen reduction reaction (ORR) at intermediate operating temperatures. Cobalt-based perovskite oxides exhibit good mixed ionic-electronic conductivity and catalytic activity at 600 – 800 oC, but they suffer from a huge thermal expansion mismatch between the electrolyte and electrode. Another class of materials with mixed ionic and electronic conducting property and catalytic activity for ORR is the perovskite-related intergrowth oxides belong to the Ruddlesden-Popper (R-P) series. Here we present R-P oxides of the n = 3 series Sr3.2-xCaxLn0.8Fe1.5Co1.5O10-δ with x = 0 and 0.4 and Ln = La, Pr, and Nd as potential cathodes for intermediate temperature SOFCs. Specifically, the effects of Ca substitution for Sr on the crystal chemistry, oxygen content, thermal expansion coefficient (TEC), and electrocatalytic activity in SOFC are presented. The substitution of Ca for Sr in Sr3.2-xCaxLn0.8Fe1.5Co1.5O10-δ enhances the stability of phase at the operating temperatures and decreases the amount of oxygen loss on heating. Comparing different lanthanides with and without Ca in Sr3.2-xCaxLn0.8Fe1.5Co1.5O10-δ, the Ln = Nd samples exhibit superior cathode performance which is ascribed to the high concentration of oxygen vacancies. Among the different compositions studied, the composite cathode Sr2.8Ca0.4Nd0.8Fe1.5Co1.5O10-δ + GDC exhibits an enhanced performance with good phase stability due to high oxygen vacancy concentration and enhanced oxide-ion transport.