Ca-containing CO2-tolerant perovskite materials for oxygen separation

Perovskites (La1−xCax)FeO3−δ and (La1−xCax)(Co0.8Fe0.2)O3−δ with varying La and Ca contents (x = 0.4–0.6) were designed by sol–gel route as model membrane materials to be an alternative to Ba- and Sr-based systems for operation in the presence of CO2. It was found that only the first members of the systems with x = 0.4 consisted of almost pure perovskite phases. The materials containing more Ca (x = 0.5–0.6) exhibited a considerable amount of bi-phase material, such as brownmillerite and/or spinel, after calcination at 1223 K. The orthorhombicly distorted (La0.6Ca0.4)FeO3−δ and rhombohedrally distorted (La0.6Ca0.4)(Co0.8Fe0.2)O3−δ perovskites showed relatively high oxygen permeation fluxes at 1223 K of 0.26 cm3 min−1 cm−2 and 0.43 cm3 min−1 cm−2, respectively. The oxygen-ionic conductivity of the materials was improved by about 50% via an asymmetric configuration using a porous support and an approximately 10-μm thick dense layer with the same chemical composition. In situ XRD in an atmosphere containing 50 vol.% CO2 and long-term oxygen permeation experiments using pure CO2 as the sweep gas revealed a high tolerance of Ca-based materials toward CO2. Thus, we suggest that Ca-containing perovskite can be considered promising membrane materials if operation in the presence of CO2 is required.