Among novel technologies under development as cost-effective alternatives to conventional oxygen production methods, mixed ionic–electronic conducting ceramic membranes offer great promise. These oxygen-transport membranes selectively separate oxygen from an air supply, or other source, at elevated temperature (700-1000°C) under an oxygen chemical potential gradient. Besides direct use in schemes for oxygen production, the membranes have attracted much interest for the conversion of natural gas into syngas (CO + H2).
Membranes fabricated from oxygen-deficient perovskites La1-xSrxCo1-yFeyO3-d can be considered as prototypes. Very high oxygen transport rates have been achieved, which relates to the high concentrations of mobile oxygen vacancies in the perovskite oxides at elevated temperatures. The presence of multivalent cations ensures a high, often predominating, electronic conductivity. Crucial to the performance as oxygen transport membrane is that the materials are capable of rapid oxygen exchange and diffusion of oxygen, and maintain structural and chemical integrity under the conditions of application.
SrCo0.8Fe0.2O3-d (SCF) is reported to provide one of the largest membrane oxygen fluxes in the series La1-xSrxCo1-yFeyO3-d. However, undesirable ordering of SCF into a brownmillerite type structure, Sr2Co1.6Fe0.4O5, has been reported to occur below 1073 K at a pO2 lower than 0.1 atm. This ordered state reduces the oxygen flux, whereas the associated lattice expansion leads to large mechanical stresses across the membranes. Literature reports have demonstrated an increase in oxygen flux and an apparent increase in the stability of the cubic perovskite phase upon 50% substitution of Ba for Sr, Ba05Sr0.5Co0.8Fe0.2O3-d (BSCF).
The present paper discusses the oxygen transport properties, oxygen stoichiometry, and phase stability of SCF and BSCF, and related compositions. Particular attention is drawn to the influence of CO2 adsorption on the kinetics of surface oxygen exchange, chemical expansion and the phenomenon of kinetic demixing. Prospects are discussed to use these materials as oxygen transport membranes.