Effect of A-site lanthanum doping on the CO2 tolerance of SrCo0.8Fe0.2O3−-δ oxygen-transporting membranes

The SrCo0.8Fe0.2O3−δ (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 La0.6Sr0.4Co0.8Fe0.2O3−δ (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.