Abstract
The long-term stability of the oxygen exchange properties of the IT-SOFC cathode materials La0.6Sr0.4CoO3−δ and La0.58Sr0.4Co0.2Fe0.8O3−δ vs. chromium poisoning is investigated at 600°C by in-situ measurements of the chemical surface exchange coefficient of oxygen kchem using the dc-conductivity relaxation method. The as-prepared samples show high values of kchem, which decrease moderately during the treatment in a dry atmosphere during 1000 h without and with the presence of a Cr-source. Humidification of the carrier gas results in a strong decrease in kchem of both materials. Post-test surface analysis of the degraded specimens by X-ray photoelectron spectroscopy identifies Cr- and Si-poisoning as well as Sr- and O-enrichment in depth ranges of about 100 nm as the origin of the degradation in a humid atmosphere. These results can be correlated with the strong enhancement of the gas phase transport of volatile Cr- and Si-species in the presence of H2O(g). It can be concluded that La0.6Sr0.4CoO3−δ (and to a lesser extent La0.58Sr0.4Co0.2Fe0.8O3−δ) cathodes retain a relatively high stability of the chemical surface exchange coefficients of oxygen at 600°C in dry atmospheres, even in the vicinity of Cr- and Si-sources. However, significant performance degradation of (La,Sr)(Co,Fe)O3−δ-based cathodes has to be expected in humid atmospheres.
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