Oxygen exchange kinetics of La0.58Sr0.4Co0.2Fe0.8O3 at 600 °C in dry and humid atmospheres

The time-dependent degradation of the oxygen exchange kinetics of the solid oxide fuel cell cathode material La0.58Sr0.4Co0.2Fe0.8O3 − δ (LSCF) is investigated at 600 °C. Special emphasis is placed on systematic long-term dc-conductivity relaxation measurements (t > 1000 h) in dry as well as in humidified atmospheres in order to obtain representative trends for the application of LSCF in intermediate-temperature SOFCs. The determination of the chemical surface exchange coefficient kchem of oxygen is combined with investigations of the elemental surface compositions and depth profiles of fresh and degraded samples by X-ray photoelectron spectroscopy (XPS), providing further insight into the mechanisms of degradation. The slow decrease of kchem by a factor of 2 during exposure of the sample to a dry O2–Ar reference atmosphere for 1000 h at 600 °C can be ascribed to an enrichment of La and Sr in correlation with an elevated oxygen concentration within about 30–35 nm depth. The interpretation of the XPS core level spectra indicates the formation of SrO and La2O3 secondary phases in this zone. The subsequent treatment in a humidified atmosphere for 1000 h results in a pronounced initial decrease of kchem by an additional factor of 10, followed by a time dependent decay of about 15% kh− 1. A Sr-rich silicate layer of about 10 nm thickness is identified by XPS as the major cause of the degradation in humidified atmosphere. The evidence of Si-poisoning over the whole sample surface could also be confirmed by post-test SEM analysis. In addition, indications of a re-structuring of the sample surface during the degradation are shown. These results indicate, that with LSCF as a cathode in ambient (humid) air in SOFC stacks containing various Si-sources, such as glass or glass-ceramic seals, and thermal insulation materials a significant decrease of the surface oxygen exchange coefficient can occur, even at temperatures as low as 600 °C. In order to prevent a severe Si-induced degradation, dry air should be used as an oxidant. However, even in dry atmosphere a minor decrease of kchem can occur during long-term operation due to changes in the relative cation and oxygen content at the surface.

Research highlights
► Oxygen exchange of La0.58Sr0.4Co0.2Fe0.8O3 − δ at 600 °C on > 1000 h time-scale.
► Decrease of kchem within 1000 h due to changes of surface element composition.
► kchem strongly decreased in humidified atmosphere.
► XPS identified severe Si-poisoning within a 10 nm thick surface zone.
► High relevance for application of LSCF as SOFC cathode in ambient (wet) air.