Temperature-induced relaxation for determining oxygen transport kinetics of nonstoichiometric oxides: fundamentals and experiments

Abstract

Oxygen transport properties are critical for mixed ionic–electronic conductors, which are applied in high-temperature devices such as solid oxide fuel cells and oxygen separation membranes. These properties are widely determined via chemical relaxation induced by changes in oxygen partial pressure through a gas switch. However, the abrupt atmospheric change leads to unstable boundary conditions and thus unreliable transport kinetics. To address this problem, we report the primary efforts to develop temperature induced relaxation (TIR), where the relaxation is induced by changing the temperature rather than pressure. The fundamentals of TIR are carefully discussed, showing that chemical relaxation can be induced at a relatively high rate. The TIR technique is experimentally demonstrated with a custom-made system using La_0.6Sr_0.4Co_0.2Fe_0.8O_3−δ as a model material. The results show that the relaxation process occurs at constant oxygen partial pressure and temperature, the relaxation curves match well with theoretical predictions, and the oxygen reduction reaction kinetics including the chemical surface exchange coefficient and chemical bulk diffusion coefficient can be determined by decreasing the temperature. Furthermore, the coefficients at different temperatures and oxygen partial pressures are determined and compared with those obtained with well-known electrical conductivity relaxation, revealing that TIR is a promising technique for determining the oxygen transport kinetics of nonstoichiometric oxides.