Behavior of Ba(Co, Fe, Nb)O<sub>3-δ</sub> Perovskite in CO<sub>2</sub>-Containing Atmospheres: Degradation Mechanism and Materials Design

This study investigates the degradation behavior and mechanism of perovskite-type BaCo<sub>0.4</sub>Fe<sub>0.4</sub>Nb<sub>0.2</sub>O<sub>3-δ</sub> membranes in CO<sub>2</sub>-containing atmospheres at 800−1000 °C and examines the influence of cation substitution on the CO<sub>2</sub> resistance. The oxygen permeation flux deteriorates rapidly upon switching the sweep gas from Ar to CO<sub>2</sub>. During exposure to CO<sub>2</sub>, the membrane material decomposes to form a compact BaCO<sub>3</sub> surface layer and a subjacent porous decomposed zone which consists of CoO and a Co-depleted Ba(Co, Fe, Nb)O<sub>3-δ</sub> perovskite phase. Within this zone, the composition of the perovskite product varies with depth, with more pronounced cobalt depletion found closer to the carbonate layer. The growth of the product layers is found to be diffusion-controlled and can be enhanced by the presence of oxygen. Outward diffusion of barium from the unreacted perovskite bulk appears to rate limit the growth. A drop of the barium chemical potential is concurrent with a larger degree of cobalt depletion in the Ba(Co, Fe, Nb)O<sub>3-δ</sub> phase. This suggests that Co substitution by Fe, or particularly by Nb, results in better CO<sub>2</sub> resistance. The effectiveness of the Fe/Nb substitution was experimentally proved and may be ascribed to increase in both the oxygen stoichiometry and acidity of the perovskite. A strategy for development of CO<sub>2</sub>-resistant materials is then proposed.