Understanding Why SrCo_0.9Ta_0.1O_3−δ is a Better Perovskite Oxygen Electrocatalyst than BaCo_0.9Ta_0.1O_3−δ

Oxygen-deficient perovskites are highly active catalysts for oxygen catalysis needed for high-performance solid oxide cells (fuel cells and electrolyzers), metal-air batteries, and air-oxygen separation membranes. In this study, we systematically investigated Ta-doped BaCoO₃ (BaCo_1-xTa_xO_3−δ) perovskites as a potential class of oxygen-active materials. Among the three Ta-doping levels (x = 0.1, 0.3, and 0.5), a primitive cubic structure is achieved, but the x = 0.1 (BCT10) sample exhibits the largest oxygen nonstoichiometry, highest electronic conductivity, and thus the lowest polarization resistance. Interestingly, unlike other cobaltite perovskites, all BCT samples show a lowthermal expansion coefficient close to the commonly used electrolytes (10-12 ppm/K). A further comparison with SrCo_0.9Ta_0.1O_3−δ (SCT10), an analogue to BCT10, reveals that SCT10 exhibits superiority over BCT10 in electrocatalysis-relevant properties such as oxygen nonstoichiometry, electronic conductivity, and electrode polarization resistance. Long-term stability testing further indicates that BCT10 is significantly less stable than SCT10 due to its thermal decomposition into the oxygen-ordered, less active phase BaCoO_2.26 and subsequent formation of BaCO₃. However, SCT10 does exhibit a much higher TEC than does BCT10. A close comparison of crystallography reveals that SCT10 has a larger Co-O octahedron than BCT10, even though the latter has a larger unit cell, implying that the oxygen catalytic activity is controlled by the size of the Co-O octahedron, not the size of the unit cell of the perovskite.