Oxygen Nonstoichiometry and Defect Chemistry of Perovskite-Structured Ba_xSr_1–xTi_1–yFe_yO_3–y/2+δ Solid Solutions

The oxygen nonstoichiometry of mixed conducting perovskite-structured Ba x Sr 1– x Ti 1– y Fe y O 3– y /2+δ (BSTF) ( x = 0, 0.1, 0.5 and y = 0.05, 0.35) was measured by means of thermogravimetry as a function of oxygen partial pressure, p O 2, in a temperature range typical for solid oxide fuel cell (SOFC) cathode applications. With increasing Ba content, the nonstoichiometry curve was shifted to higher p O 2, indicating enhanced reducibility. Based on a defect chemical analysis of thermogravimetry derived nonstoichiometry data, the substitution of Ba for Sr on the A-site of STF was found to result in a decrease in both the reduction enthalpy and band gap energy, consistent with expectations that substitution of Sr by the larger Ba leads to a reduction in bond strength. A consequent increase in minority electron density and oxygen vacancy concentration are expected to result in enhancements in oxygen surface exchange kinetics and diffusivity and thereby cathode performance. The nonstoichiometry data obtained in this study also brought to light a previous underestimation of the minority electron density, by approximately a factor of 10 3, a key parameter believed to impact cathodic performance.