Oxygen transport and stability of asymmetric SrFe(Al)O3−δ-SrAl2O4 composite membranes

In order to appraise a two-stage compaction procedure using pore-forming additives for the fabrication of asymmetric mixed-conducting membranes where the porous and dense layers are made of the same composition, the oxygen permeability of a series of (SrFeO3−δ)0.7(SrAl2O4)0.3 composite samples with varying architecture was studied at 1073–1223 K. The preparation route for the crack-free supported membranes included pressing of the starch-containing and pure dual-phase composite powders, sintering at 1623 K, and subsequent surface modification of the dense layers having the thickness of 0.12–0.15 mm. Analysis of the oxygen permeation fluxes show a significant limiting effect of oxygen diffusion through the support, where the porosity and average pore size are 20% and 2–4 μm, respectively. The overall level of oxygen transport, higher than that in the symmetric surface-activated membranes, was only achieved at 1173–1223 K for the porous layer thickness of 0.4 mm. Slow microstructural degradation due to the support sintering, evidenced by dilatometry, leads to a moderate decrease in the oxygen fluxes with time. At 1973 K, the corresponding changes were approximately 16% during 220 h. The results suggest that increased total porosity, preferential pore orientation perpendicular to the dense layer and incorporation of nano-sized catalyst particles into the pores are needed to increase the performance of asymmetric ferrite-based membranes.