Oxygen Exchange Kinetics of (Bi,Sr)(Co,Fe)O3-δ Thin-Film Microelectrodes

Abstract

Mixed-conducting (Bi,Sr)(Co,Fe)O3-δ (BiSCF) perovskites are considered to be promising candidates for the application as cathode materials in solid oxide fuel cells (SOFCs) operating in the intermediate-temperature regime. In this study, the effective rate constants of surface oxygen exchange of both Co-free and Co-containing BiSCF perovskites were measured by electrochemical impedance spectroscopy (EIS) on geometrically well-defined thin-film microelectrodes on Y2O3-doped ZrO2 (YSZ) single crystal substrates. Oxygen exchange rate constants in the range of 3.0 × 10−7 cm s−1 to 2.4 × 10−6 cm s−1 at 750°C and an oxygen partial pressure p(O2) of 0.2 bar were obtained, which are higher in particular for Co-containing BiSCF than those of La-containing perovskites. The interrelation between the oxygen exchange rate constant and other material properties was investigated to derive information about the oxygen exchange mechanism. A correlation between the oxygen exchange rate constant and the ionic conductivity from thin-film electrochemical polarization experiments and measurements on bulk samples in the range of 1.4 × 10−2 S cm−1 to 2.6 × 10−1 S cm−1 at 650°C was observed that is similar to the correlation found for Ba-containing perovskites. This suggests oxygen vacancy diffusion to be involved in the rate-determining step (rds) of the oxygen exchange reaction.