Co and Fe co-doping influence on functional properties of SrTiO3 for use as oxygen transport membranes

Perovskite-structured powders of SrTi1-xCoxO3-δ (STC-x) with nominal stoichiometry of x = 0-0.75 as well as SrTi0.75Co0.25-yFeyO3-δ (STCF-y) where y = 0-0.25 were synthesized using the Pechini method. Thermal/chemical expansion behaviour, total electrical conductivities, and oxygen permeation rates were investigated. The substitution of Ti with Co leads to an increase in both electronic and ionic conductivities and, therefore, oxygen permeability. Thermal and chemical expansions also increase slightly. The optimum Co content was found to be 25-35% due to the trade-off between phase stability and permeability. The oxygen permeation rate of STC35 is comparable to that of state-of-the-art (La,Sr)(Co,Fe)O3-δ, whereas the expansion coefficients are lower. Co-doping in STCF-y did not produce any significant differences in oxygen permeability at both high temperature and sample thickness (1.0 mm), i.e. in a solid-state diffusion-limited regime. At lower temperatures (<800 °C), STC25 exhibits higher permeability than STF25 due to the higher catalytic activity of Co compared to Fe.