Chromium and silicon poisoning of La0.6Sr0.4CoO3-δ IT-SOFC cathodes at 800°C

The oxygen exchange kinetics of the intermediate temperature solid oxide fuel cell (IT-SOFC) cathode material La0.6Sr0.4CoO3-δ (LSC64) was measured in-situ for 3500 h by the dc-conductivity relaxation method (CR). The chemical surface exchange coefficient (kchem) and the chemical diffusion coefficient (Dchem) of oxygen were determined at 800°C in dry and humidified atmospheres in the absence and presence of Cr- and Si-sources. Post-test analyses of degraded samples were performed by scanning electron microscopy
(SEM) with energy and wavelength dispersive X-ray spectroscopy (SEM-EDXS/WDXS), X-ray photoelectron spectroscopy (XPS), and analytical scanning transmission electron microscopy (STEM) with EDXS and electron energy loss spectroscopy (EELS). In dry atmosphere (pO2=0.10 bar) at 800°C high values of kchem=1×10-3 cm s-1 and Dchem=2×10-5 cm2 s-1 were found. No degradation was observed during 1300 h without or with the presence of Cr- and Si-sources in the dry test gas. However, a significant
decrease in kchem and Dchem occurred when the atmosphere was humidified (pO2=0.10 bar; 30-60 % relative humidity). SEM analyses show that various crystallites are formed on the surface of the sample during the degradation. SEM-EDXS and -WDXS confirm the presence of significant amounts of Cr- and Si-impurities in the near-surface region. XPS elemental depth profiles give evidence of Sr- and Cr-enrichment and Co-depletion of the surface down to depths of approximately 900 nm. STEM shows that Cr- and Si-rich secondary phases are formed on the surface and at the grain boundaries in the nearsurface
region. Sr-chromate and La-silicate phases were identified in addition to Co-oxide by STEM-EDXS and –EELS cross-sectional analyses. It can be concluded that the decomposition of the oxygen exchange active LSC64 bulk material into inactive secondary phases results in the observed decrease of the oxygen exchange kinetics, and that gas phase humidity is a critical factor for the degradation.