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

Abstract

The long-term stability of the mixed-conducting perovskite oxide La0.6Sr0.4CoO3-δ (LSC) was investigated for 3400h 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 (kchem) and the chemical diffusion coefficient (Dchem) 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% O2 high values of kchem=1×10−3cms−1 and Dchem=2×10−5cm2s−1 were found. A stable performance was observed during 1300h without or with the presence of Cr- and Si-impurities. However, a significant decrease in kchem and Dchem occurred when the atmosphere was humidified (30–60% relative humidity). XPS depth profiles showed Sr-enrichment and Co-depletion of the surface already during 1300h in dry atmosphere. After the treatment in humidified atmospheres for additional 2100h significant amounts of Cr and Si contaminations were found. SEM and STEM showed crystallites of SrCrO4 and La-silicate on the surface. SrCrO4 and Co3O4 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.