The thermochemical properties and long-term behavior of mixed electron-ion conducting materials for oxygen separation were explored under oxyfuel conditions in this thesis. Perovskite-type oxides, known for high oxygen fluxes, face sustainability challenges in the intermediate temperature range of 500-800 °C due to thermodynamic decomposition, which poses significant obstacles for coal-fired power plant applications. To address compatibility issues, a low expansion coefficient for membrane materials is recommended. Perovskite-structured BaxSr1-xCoyFe1-yO3-d (BSCF) materials were synthesized through solid-state reaction, and the sintering behavior of BSCF powders was analyzed before producing gastight membranes. Key membrane properties, including melting temperature, oxygen nonstoichiometry, and thermal expansion, were measured in synthetic air. The relationship between thermochemical properties, doping compositions, and temperature effects was investigated. Additionally, the degradation of BSCF membranes during long-term operation was examined under oxyfuel conditions. Oxygen permeation measurements revealed a slow exponential decay of flux at 800 °C, while higher temperatures stabilized oxygen permeability. The deterioration was linked to phase decomposition from cubic to hexagonal polymorph. Interestingly, an increase in oxygen flux was observed in the Ba0.4Sr0.6Co0.2Fe0.8O3-d membrane at 850 °C, despite ongoing kinetic deco
