Influence of M cations on structural, thermal and electrical properties of new oxygen selective membranes based on SrCo_0.95M_0.05O_3-δ perovskite

In this work, various M cations like Bi⁵⁺, Zr⁴⁺, Ce⁴⁺, Sc³⁺, La³⁺, Y³⁺, Al³⁺ and Zn²⁺ were incorporated into SrCoO_3-δ (SC) lattices one by one via doping strategy to form a series of new SrCo_0.95M_0.05O_3-δ (SCM) mixed oxides for membrane separation. In general, the M cations have significant effects on the crystal structure, electrical conductivity, sintering behavior and the oxygen permeability of the SCM membranes even though only 5% of Co ions are exchanged. The phase structure of SCMs in the air atmosphere is seemingly determined by the electron configuration in the outer orbits of M cation. In the current cases, the doping of M cation with d¹⁰ configuration results in 2H-hexagonal structure while that with p⁶ gives rise to a cubic perovskite or brownmillerite. The formation of cubic perovskite induced by M cation increases the electrical conductivity and the oxygen permeability by 1-2 orders of magnitude due to the high density of oxygen vacancy. In particular, disk-shaped perovskite SrCo_0.95Sc_0.05O_3-δ membrane demonstrates the highest oxygen flux and theoretical analysis indicates that the oxygen permeation process is rate-determined by the surface reactions.