Electrical conductivity and oxygen permeation properties of SrCoFeO_x membranes

The total conductivity and oxygen permeation properties of dense SrCoFeO_x membranes synthesized from the solid state method were studied in the temperature range of 700-900 °C. The SrCoFeO_x membranes consist of an intergrowth (Sr₄Fe_{6 - x}Co_xO_{13 ± δ}), perovskite (SrFe_{1 - x}Co_xO_{3 - δ}), and spinel (Co_{3 - x}Fe_xO₄) phase. SrCoFeO_x exhibits n-type and p-type conduction at low and high oxygen partial pressures, respectively, and has a total conductivity of 16.5 S/cm at 900 °C in air. The oxygen permeation fluxes for SrCoFeO_x and SrFeCo_0.5O_x membranes were measured with either an inert or carbon monoxide sweep gas. The oxygen permeation fluxes were higher through SrCoFeO_x membranes than SrFeCo_0.5O_x membranes and can be attributed to a difference in the amount and makeup of the perovskite phase present in each composition. The oxygen permeation fluxes with a carbon monoxide sweep gas were approximately two orders of magnitude larger than the fluxes measured with an inert sweep gas for both compositions. The large oxygen permeation fluxes observed with a carbon monoxide sweep are due to a higher driving force for oxygen transport and a reaction on the sweep side of the membrane that maintains a low oxygen partial pressure.