Oxygen exchange kinetics of La0.6Sr0.4CoO3-δ affected by changes of the surface composition due to chromium and silicon poisoning

The long-term stability of the mixed-conducting perovskite oxide La 0.6 Sr 0.4 CoO 3-δ (LSC) was investigated for 3400 h at 800 °C. The oxygen exchange kinetics of LSC was studied in-situ in dry and humidified atmospheres in the absence as well as the presence of Cr and Si sources. The chemical surface exchange coefficient (k chem ) and the chemical diffusion coefficient (D chem ) of oxygen were measured by the dc-conductivity relaxation method. Degraded samples were analyzed by scanning electron microscopy (SEM) with energy and wavelength dispersive X-ray spectroscopy (EDXS/WDXS), X-ray photoelectron spectroscopy (XPS), and analytical scanning transmission electron microscopy (STEM). In dry atmosphere with 10% O 2 high values of k chem = 1 × 10 − 3 cm s − 1 and D chem = 2 × 10 − 5 cm 2 s − 1 were found. A stable performance was observed during 1300 h without or with the presence of Cr- and Si-impurities. However, a significant decrease in k chem and D chem occurred when the atmosphere was humidified (30–60% relative humidity). XPS depth profiles showed Sr-enrichment and Co-depletion of the surface already during 1300 h in dry atmosphere. After the treatment in humidified atmospheres for additional 2100 h significant amounts of Cr and Si contaminations were found. SEM and STEM showed crystallites of SrCrO 4 and La-silicate on the surface. SrCrO 4 and Co 3 O 4 were also found at the grain boundaries in the near-surface region. It can be concluded that the observed decrease in the oxygen exchange kinetics is closely related to significant changes of the surface composition as a result of Cr and Si poisoning which leads to the decomposition of the oxygen exchange-active LSC phase into inactive secondary phases.