Surface exchange and bulk diffusion properties of Ba_0.5Sr _0.5Co_0.8Fe_0.2O_3-δ mixed conductor

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 pO₁2 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 CO₂ 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.