Oxygen separation membranes based on intergrowth structures

The perovskite-related intergrowth oxides Sr4(Fe,Co)6O13+δ and (La,Sr)n+1(Fe,Co)nO3n+1 (Ruddlesden–Popper (R–P) series) with n=1, 2, 3 have been investigated for use as oxygen separation membranes. The crystal chemistry, degree of oxygen nonstoichiometry δ, total conductivity, and oxygen permeability have been investigated to evaluate the feasibility of these compounds for oxygen permeation membranes. The oxygen permeation flux of the Sr4Fe6−yCoyO13+δ samples with y>1.8 is due to the presence of a secondary perovskite phase, which has higher oxygen flux than the intergrowth oxide Sr4Fe6−yCoyO13+δ with y≤1.5. While the n=2 and 3 members of the R–P series show oxygen flux values one order of magnitude lower than the perovskite phase SrCo0.8Fe0.2O3−δ, the n=1 member shows negligible oxygen permeability. The (La,Sr)3(Fe,Co)2O7−δ (n=2) phases show good structural stability at high temperature and in the pO2 range of the oxygen permeation measurements. The oxygen permeation flux is found to be bulk limited for La0.3Sr2.7Fe1.4Co0.6O7−δ and the ionic conductivity σi has been estimated by assuming that this observation is valid for the other n=2 compositions as well.