A Kinetic Study of Catalytic Activity Degradation of La0.6Sr0.4Co0.2Fe0.8O3-δ

The long term performance of La 0.6 Sr 0.4 Co 0.2 Fe 0.8 O 3-δ (LSCF) was evaluated using electrical conductivity relaxation (ECR). The degradation rate of the chemical exchange coefficient (kchem) was faster than that of the chemical diffusion coefficient (Dchem) when LSCF was aged at 800 oC, which indicates surface degradation during the test. No crystallographic phase changes were detected by X-ray diffraction (XRD). Scanning electron microscopy (SEM), however, showed a significant change in surface morphology. SEM energy dispersive spectroscopy (EDS) and Auger electron spectroscopy (AES) showed that the surface was separated into two regions: A Sr rich phase on the grain boundaries and a Sr deficient top surface layer surrounding it. Transition metals were deficient in both phases. It is proposed that Sr is initially enriched to the top surface and diffuses to the grain boundary by surface diffusion. The preferential growth of Sr rich particles might be related to the Cr poisoning from the thermocouple sheath. From defect chemistry it is shown that both Sr rich particles and Sr deficient surface can contribute to the degradation of the catalytic activity of the cathode material by the reducing the oxygen vacancy and transition metal concentration.