Ca substitution in Ca_1‐xSr_xTi_0.8Fe_0.2O_3‐δ perovskites for oxygen transport membranes: A promising strategy to improve CO₂ tolerance

Abstract

Oxygen transport membranes (OTMs) can be operated as efficient oxygen separators and oxygen distributors, but their diffusion is hampered by the lack of appropriate materials with sufficient chemical stability in operating conditions. Mixed ionic‐electronic conducting SrTiO₃‐based perovskite are a promising option due to their remarkable resistance in reducing environments, but they are prone to degradation in CO₂‐containing atmospheres owing to carbonate formation. In this study, we propose new Ca_1‐xSr_xTi_0.8Fe_0.2O_3‐δ perovskites where the Sr²⁺ cation is partially substituted by Ca²⁺ to improve the tolerance to CO₂. We show that, upon increasing Ca molar fraction, the phase stability in CO₂ is progressively improved thanks to the mitigation of carbonate growth. The manufacturing process of the membranes benefits from Ca introduction, which promotes powder densification during sintering. A membrane made of Ca_0.1Sr_0.9Ti_0.8Fe_0.2O_3‐δ exhibited a stable oxygen flux for 340 h at 900°C when swept with He and for 170 h at 900°C when swept with a gaseous stream containing 40% CO₂. Post‐test characterizations confirmed the retention of the crystal structure after the long‐term permeation tests. Ca‐substituted SrTi_1‐yFe_yO_3‐δ perovskites can be considered promising materials for OTMs working in harsh environments and in the presence of CO₂.