Effect of calcination temperature on oxidation state of cobalt in calcium cobaltite and relevant performance as intermediate-temperature solid oxide fuel cell cathodes

(Graph Presented) Calcium cobaltite materials are synthesized by calcining the mixture of CaCO₃ and Co₃O₄ with the Ca: Co ratio of 3:4. The reactivity of CaCO₃ with Co₃O₄ is evaluated by thermogravimetric analysis (TGA), X-ray powder diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). Thermal expansion coefficient (TEC), electrical conductivity and electrochemical performance as intermediate-temperature solid oxide fuel cells (IT-SOFCs) cathode of as-prepared materials are characterized. The experiment results show that simultaneous decomposition of CaCO₃ with calcium cobaltite formation occurs at 650-900°C. The average valence for Co ions of calcium cobaltite increases with temperature in the range of 750-900°C, involved in the formation of the compounds Ca₃Co₄O₉ and Ca₉Co₁₂O₂₈ at 800 and 900°C, respectively. The performance of calcium cobaltite cathodes applied in IT-SOFCs is significantly effected by the oxidation state of cobalt ions. As a result, Ca₉Co₁₂O₂₈ cathode has a lower area specific resistance (e.g. 41.8% lower at 800°C) and higher peak power density (e.g. 45.0% higher at 800° C) than the cathode of Ca₃Co₄O₉.