Oxygen permeation properties of Co-free perovskite-type oxide membranes based on BaFe _1-yZr _yO _3-δ

Partially Zr-substituted BaFe _1-yZr _yO _3-δ membranes were developed as a Co-free oxygen permeable membrane. In order to stabilize the cubic perovskite structure, Fe sites in BaFeO _3-δ were partially substituted with Zr ⁴⁺. In the substitution range of y=0.01-0.1, the cubic perovskite structure was stabilized even at room temperature. Among the membranes prepared, a BaFe _0.975Zr _0.025O3 _-δ material (y=0.025) showed the highest oxygen permeation flux of 1.30 cm3 (standard temperature pressure) min-1 cm-2 at 930°C under an air/He gradient. The oxygen permeation flux was higher than that of partially Ce-substituted BaFe _1-yCe _yO _3-δ membranes reported previously. From the results obtained by chemical and scanning electron microscope analyses, it appears that the oxygen permeability for BaFe _1-yZr _yO _3-δ membranes was well correlated with the amount of oxygen defects in the lattice as well as the grain size. In addition, the oxygen permeation flux of the BaFe _0.975Zr _0.025O3 _-δ membrane was significantly increased after decreasing the thickness of the membrane from 2.0 to 0.4 mm. For thin membranes (0.4-1.0 mm), the thickness dependence of the oxygen permeability deviated from the Wagner equation, suggesting that the oxygen permeation of BaFe _0.975Zr _0.025O3 _-δ is controlled by not only bulk diffusion of oxide ions but also their surface reactions.