Remarkable dependence of electrochemical performance of SrCo0.8Fe0.2O3-δ on A-site nonstoichiometry

SrCo0.8Fe0.2O3-δ is a controversial material whether it is used as an oxygen permeable membrane or as a cathode of solid oxide fuel cells. In this paper, carefully synthesized powders of perovskite-type SrxCo0.8Fe0.2O3-δ (x = 0.80–1.20) oxides are utilized to investigate the effect of A-site nonstoichiometry on their electrochemical performance. The electrical conductivity, sintering property and stability in ambient air of SrxCo0.8Fe0.2O3-δ are critically dependent on the A-site nonstoichiometry. Sr1.00Co0.8Fe0.2O3-δ has a single-phase cubic perovskite structure, but a cobalt-iron oxide impurity appears in A-site cation deficient samples and Sr3(Co, Fe)2O7-δ appears when there is an A-site cation excess. It was found that the presence of the cobalt-iron oxide improves the electrochemical performance. However, Sr3(Co, Fe)2O7-δ has a significant negative influence on the electrochemical activity for intermediate-temperature solid oxide fuel cells (IT-SOFCs). The peak power densities with a single-layer Sr1.00Co0.8Fe0.2O3-δ cathode are 275, 475, 749 and 962 mW cm−2 at 550, 600, 650 and 700 °C, respectively, values which are slightly lower than those for Sr0.95Co0.8Fe0.2O3-δ (e.g. 1025 mW cm−2 at 700 °C) but much higher than those for Sr1.05Co0.8Fe0.2O3-δ (e.g. only 371 mW cm−2 at 700 °C). This remarkable dependence of electrochemical performance of the SrxCo0.8Fe0.2O3-δ cathode on the A-site nonstoichiometry reveals that lower values of electrochemical activity reported in the literature may be induced by an A-site cation excess. Therefore, to obtain a high performance of SrxCo0.8Fe0.2O3-δ cathode for IT-SOFCs, an A-site cation excess must be avoided.