In Situ Investigations of the Chromium-Induced Degradation of the Oxygen Surface Exchange Kinetics of IT-SOFC Cathode Materials La_0.6Sr_0.4CoO_3−δand La_0.58Sr_0.4Co_0.2Fe_0.8O_3−δ

The long-term stability of the oxygen exchange properties of the IT-SOFC cathode materials La 0.6 Sr 0.4 CoO 3−δ and La 0.58 Sr 0.4 Co 0.2 Fe 0.8 O 3−δ vs. chromium poisoning is investigated at 600°C by in-situ measurements of the chemical surface exchange coefficient of oxygen k chem using the dc-conductivity relaxation method. The as-prepared samples show high values of k chem , 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 k chem 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 H 2 O(g). It can be concluded that La 0.6 Sr 0.4 CoO 3−δ (and to a lesser extent La 0.58 Sr 0.4 Co 0.2 Fe 0.8 O 3−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)O 3−δ -based cathodes has to be expected in humid atmospheres.