Surface exchange and bulk diffusion properties of Ba 0.5Sr 0.5Co 0.8Fe 0.2O 3− δ mixed conductor

Abstract

Ba 0.5Sr 0.5Co 0.8Fe 0.2O 3− δ (BSCF) is a mixed conducting oxide that shows high oxygen permeability to perform as a ceramic membrane and high electrochemical activity for oxygen reduction to perform as a cathode of solid oxide fuel cells. Both performances are closely related to the bulk and surface properties of the BSCF oxide. In this study, the chemical bulk diffusion coefficient ( D chem) and chemical surface exchange coefficient ( k chem) of BSCF at various temperatures and oxygen partial pressures are determined by an electrical conductivity relaxation (ECR) method. Both D chem and k chem are found to be dependent on p O 2 with positive effect. Ea of D chem and k chem are respectively 111 ± 5 and 110 ± 6 kJ mol −1 between 600 and 800 °C. Oxygen-ion diffusion and tracer diffusion coefficients are estimated from D chem and compared with the literature results. Ionic conductivities are further derived according to the Nernst–Einstein relation. The poisoning effect of CO 2 on the performances of BSCF is further investigated by the ECR method in combination with oxygen temperature-programmed desorption technique. The presence of CO 2 causes a substantial decrease in k chem, however, the surface kinetics can be recovered by performing re-calcination in an oxidative atmosphere at 900 °C, agreeing well with literature reports.