Behavior of Ba(Co, Fe, Nb)O3-δ Perovskite in CO2-Containing Atmospheres: Degradation Mechanism and Materials Design

This study investigates the degradation behavior and mechanism of perovskite-type BaCo0.4Fe0.4Nb0.2O3-delta membranes in CO2-containing atmospheres at 800-1000 degrees C and examines the influence of cation substitution on the CO2 resistance. The oxygen permeation flux deteriorates rapidly upon switching the sweep gas from Ar to CO2. During exposure to CO2, the membrane material decomposes to form a compact BaCO3 surface layer and a subjacent porous decomposed zone which consists of CoO and a Co-depleted Ba(Co, Fe, Nb)O3-delta 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)O3-delta phase. This suggests that Co substitution by Fe, or particularly by Nb, results in better CO2 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 CO2-resistant materials is then proposed.