Long-term degradation of La0.6Sr0.4Co0.2Fe0.8O3-δ IT-SOFC cathodes due to silicon poisoning

The impact of silicon on the long-term stability of the intermediate temperature solid oxide fuel cell cathode material La0.6Sr0.4Co0.2Fe0.8O3-δ (LSCF) was investigated during 2400 h at 700 °C. The area-specific resistance (ASR) of symmetric cells of LSCF on Gd0.1Ce0.9O1.95-δ (GDC) was determined by electrochemical impedance spectroscopy (EIS) in ambient air without and with Si contamination of the electrodes. The ASR of the fresh LSCF electrode was 0.3 Ω cm2. During 1060 h without Si contamination, an increase in the cathode ASR by a factor of 4.5 was observed, which may be attributed to a decrease in the surface oxygen exchange coefficient of LSCF due to changes in the surface elemental composition. Subsequently, 10 nm thick silicon layers were sputtered onto the LSCF electrodes. This contamination resulted in a strong increase in the ASR by a factor of 5.9 during additional 1340 h at 700 °C. Post-test analyses by scanning electron microscopy (SEM) and transmission electron microscopy (TEM) revealed that the degradation of the cathode is caused by the phase decomposition of LSCF within an approximately 50 nm thick surface region. A continuous La-Sr-silicate layer is formed in addition to isolated (Co,Fe)xOy nanoparticles. In agreement with the changes in the shape of the impedance spectra, it is assumed that this impurity layer leads to limitations in the gas diffusion towards the electrode and/or oxygen adsorption on the surface of the electrode.