Investigation of a Ba0.5Sr0.5Co0.8Fe0.2O3-delta based cathode SOFC II. The effect of CO2 on the chemical stability

The effect of carbon dioxide on the chemical stability of a Ba0.5Sr0.5Co0.8Fe0.2O3-delta cathode in the real reaction environment at 450 deg C was investigated by X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX), temperature programmed desorption (TPD), X-ray diffraction (XRD) and electrochemical impedance spectra (EIS) techniques. It was found that the presence even of very small quantities of CO2 seriously deteriorates the fuel cell performance at 450 deg C. XPS, TPD and XRD results strongly evidenced the formation of carbonates involving strontium and possibly barium after the BSCF cathode was operated in 1% CO2/O2 gas mixture at 450 deg C for 24h. SEM-EDX analysis of the BSCF cathode surface, after treatment in CO2/O2 environment at 450 deg C, showed small particles on the surface probably associated with a carbonate phase and a segregated phase of the perovskite. The corresponding EDX spectra confirmed the presence of a carbonate layer and also revealed the surface enrichment of strontium and barium elements. EIS results indicated that both ohmic and polarization resistances increased gradually with the introduction of carbon dioxide in the oxidant stream, which could be interpreted by the decreased oxygen reduction kinetics and the formation of carbonate insulating layer.