Effects of scandium doping concentration on the properties of strontium cobalt oxide membranes

Perovskite-type mixed conducting oxides of SrCo₁₋yScyO₃₋δ (y=0.02-0.7) were synthesized by a combined EDTA-citrate complexing method. Different scandium doping concentrations in SrCo₁₋yScyO₃₋δ have significant effects on the phase structure, electrical conductivity, surface properties and oxygen permeation behaviour of the resultant membranes. SrCoO₃₋δ without scandium incorporation displayed a 2H BaNiO₃-type structure with almost zero oxygen flux at high temperatures. Small amounts of Sc₂O₃ doping (y<0.1) efficiently stabilized the oxide in a cubic perovskite structure, leading to a sharp increase in the permeation flux, with a maximum value of 3.1ml.cm⁻².min⁻¹ achieved at 900ºC for SrCo₀․₉₅Sc₀․₀₅O₃₋δ and SrCo₀․₉Sc₀․₁O₃₋δ membranes. The permeation process could be rate-limited by either the oxygen bulk diffusion, oxygen surface exchange, or a combination of both depending on the scandium doping level. Further increases of the Sc³⁺ doping concentration (y>0.1) were found to lower the membrane oxygen fluxes, with y³≥0.4 doped SrCo₁₋yScyO₃₋δ ceramics no longer showing any oxygen permeation.