Transport and electrocatalytic properties of La_0.3Sr_0.7Co_0.8Ga_0.2O_3−δ membranes

Abstract

Incorporation of gallium into the perovskite lattice of La_0.3Sr_0.7CoO_3−δ leads to increasing unit cell volume and to decreasing thermal expansion, total conductivity and oxygen permeability. At 973–1223 K, the oxygen permeation fluxes through La_0.3Sr_0.7Co_0.8Ga_0.2O_3−δ ceramics with 96.5% density are determined by the bulk ionic conduction and surface exchange rates. The total conductivity of La_0.3Sr_0.7Co_0.8Ga_0.2O_3−δ, predominantly p-type electronic, exhibits an apparent pseudometallic behavior due to oxygen losses on heating, whereas the p(O₂) dependencies of the conductivity and Seebeck coefficient suggest a small-polaron mechanism of hole transport. The average thermal expansion coefficients in air are 15.9×10⁻⁶ K⁻¹ at 360–710 K and 27.9×10⁻⁶ K⁻¹ at 710–1030 K. On decreasing oxygen pressure down to 4–30 Pa at 973–1223 K, perovskite-type La_0.3Sr_0.7Co_0.8Ga_0.2O_3−δ transforms into a brownmillerite-like modification, whose electrical properties are essentially p(O₂) independent. Further reduction results in the decomposition of the brownmillerite into a multiphase oxide mixture at p(O₂)=8×10⁻¹⁰–3×10⁻⁴ Pa, and then in the segregation of metallic cobalt. Due to surface-limited oxygen transport, La_0.3Sr_0.7Co_0.8Ga_0.2O_3−δ membranes are, however, kinetically stable under an air/CH₄ gradient up to 1223 K. The conversion of dry methane in model membrane reactors increases with oxygen permeation flux and temperature, but yields high CO₂ concentrations (>90%), indicating a dominant role of complete CH₄ oxidation on the membrane surface.