Re-evaluation of Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3-δ perovskite as oxygen semi-permeable membrane

Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3-δ (BSCF), a mixed oxygen ionic and electronic conducting ceramic derived from SrCo_0.8Fe_0.2O_3-δ (SCF), was re-evaluated for its possible application as an oxygen semi-permeable membrane and membrane reactor for partial oxidation of light hydrocarbons. The partial substitution of Sr²⁺ with Ba²⁺ in SCF led to an increase in the phase stability of the cubic perovskite structure. Both BSCF and SCF were found to have high oxygen nonstoichiometry. However, BSCF possessed a higher oxygen nonstoichiometry than SCF at room temperature, but a similar value at high temperatures. This resulted in the lower chemical expansion for BSCF than SCF. The doping of Ba²⁺ in SCF led to the decrease of the electronic conductivity, but an increase in the oxygen ionic conductivity. The oxygen permeation study of the BSCF membrane demonstrated that the permeation rate was mainly rate-determined by the slow surface exchange kinetics at the oxygen lean side (or reaction side) membrane surface. The high ionic conductivity and the slow surface exchange kinetics resulted in the surface oxygen partial pressure at the reaction side membrane surface to be much higher than the surrounded atmosphere, therefore protecting the BSCF membrane from reduction.