Impact of humid atmospheres on oxygen exchange properties, surface-near elemental composition, and surface morphology of La0.6Sr0.4CoO3-δ

In-situ investigations of the long-term degradation of the oxygen exchange kinetics of the promising SOFC cathode material La0.6Sr0.4CoO3 - delta are performed at 600 degrees C in dry and in humid O-2-Ar atmospheres during subsequent time intervals of 1000 h. Extensive pre- and post-test analyses using X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM) are performed in order to assess the changes in the relevant surface-near zones with respect to elemental composition and morphology. The decrease of the chemical surface exchange coefficient of oxygen k(chem) during the exposure of the sample to a dry atmosphere at 600 degrees C for 1000 h is correlated to the formation of a 2 nm thick region at the surface, which is enriched with Sr and O, as well as to modifications of the surface morphology. The underlying mechanism cannot be unambiguously explained from this work. A possible interpretation would be the decomposition of La0.6Sr0.4CoO3 - delta at the surface into a Sr-enriched perovskite La(1 - x)S(r)xCoO(3 - delta) (0.4< x <1) and La2O3. During the treatment in humid atmospheres at 600 degrees C, a significant decrease in the kinetic parameters for oxygen exchange occurs. This is ascribed to the presence of a humid atmosphere together with a Si-source. Poisoning of the surface with silicon and a strong enrichment with Sr and Oat depths of 10-90 nm, depending on the exposure time are evident in the XPS elementary depth profiles. It is assumed that the reaction of Si with Sr is a strong driving force for the accumulation of Sr at the surface. When, instead of distilled water, a saturated aqueous NaCI solution is used for the humidification of the carrier gas no significant increase in the degradation rate is observed. The degradation is also accompanied by a re-structuring of the surface morphology. An increase in surface roughness by about one order of magnitude due to the progressive growth of crystallites up to 200 nm in diameter is detected during the course of 5000 h. The targeted technical application of La0.6 Sr0.4CoO3 - delta in IT-SOFCs operating at temperatures of around 600 degrees C involves ambient (humid) air as an oxidant and the presence of Si-sources (glass seals, thermal insulation materials). Under these circumstances, the mechanisms shown in this study can induce a significant decrease of the activity for oxygen exchange over time. The use of pre-dried air is suggested in order to increase the life-time of the SOFC cathode, since a significant transport of Si via the gas phase occurs almost exclusively in the presence of H2O(g). However, a time-dependent decrease of the performance of these cathodes can occur during several 1000 h periods of operation, even under dry conditions. (C) 2011 Elsevier B.V. All rights reserved.