Surface-Related Effects Limiting the Performance of Solid Oxide Fuel Cell Cathodes

The oxygen exchange kinetics of the solid oxide fuel cell (SOFC) cathode materials La0.6Sr0.4CoO3-δ (LSC) and La0.58Sr0.4Co0.2Fe0.8O3-δ (LSCF) is measured by in-situ conductivity relaxation experiments at 600 °C. The impact of silicon and chromium on the chemical surface exchange coefficient kchem and the chemical diffusion coefficient of oxygen Dchem is assessed in dry and humid atmospheres in the 1,000 h time scale. X-ray photoelectron spectroscopy (XPS) shows that the degradation goes along with changes of the chemical composition of the surface. Poisoning with Cr and/or Si, and Sr-enrichment occurs in 10–100 nm thick layers, depending on the pre-treatment. Scanning electron microscopy (SEM) reveals changes in the surface topography of the degraded specimen. Based on experimental evidence as well as on thermodynamic data, it can be concluded that the surface poisoning is triggered by H2O(g). Both Si and Cr form volatile gas phase species under humid conditions, which react with LSC or LSCF. Even though the degradation is confined to a thin surface layer, a strong decrease in the oxygen exchange activity of the cathode materials occurs. Since neither Si nor Cr are transported to a significant extent via a dry, oxidising gas phase at 600– 700 °C, the use of dried air at the cathode side would be an important factor to improve the durability and reliability of SOFCs.