Effect of A-site lanthanum doping on the CO2 tolerance of SrCo0.8Fe0.2O3a-I' oxygen-transporting membranes

The SrCo0.8Fe0.2O3aI' (SCF) perovskite was systematically doped with increasing lanthanum content up to 60 wt% on the A-site to investigate the effect on CO2 tolerance. Different powders were prepared by a sol-gel method and the materials were characterized by in-situ X-ray diffraction (XRD) and long-term oxygen permeation measurements in CO2-containing atmospheres. The microstructure was investigated using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). All powders exhibit cubic perovskite structure except the rhombohedral La0.6Sr0.4Co0.8Fe0.2O3aI' (LSCF 6482), which however, shows a phase transition into cubic perovskite structure at higher temperature. By doping 20 wt% lanthanum, the tolerance against CO2 is considerably increased and doping with 60 wt% lanthanum resulted in a stable oxygen permeation performance in CO2 atmosphere for at least 200 h. Oxygen permeation experiments in an air/helium gradient showed that with increasing lanthanum content the oxygen permeation flux decreases. Microstructure analysis of the membranes after CO2 operation showed that the carbonate preferentially forms a dense layer at the carbon dioxide exposed sweep side of the membranes.