Determination of Factors Governing Surface Composition and Degradation of La0.6Sr0.4Co0.2Fe0.8O3-δ Electrode under Sulfur-Contained Air.

Factors governing the change of the surface composition and the degradation of transport properties of La_0.6Sr_0.4Co_0.2Fe_0.8O_3-d (LSCF) under low concentration sulfur-contained air (0.01 ppm) were examined as functions of water vapor pressure (p(H₂O)) and oxygen partial pressure (p(O₂)). Bulk LSCF samples were heat-treated at 973 K in dry or wet air containing SO₂ and more enhanced SrSO₄ formation/growth was observed under wet compared to dry air. When the p(O₂) was changed from 0.21 to 0.01 bar, the SrSO4 precipitates became smaller in size and higher in number, indicating that the p(H₂O) and the p(O₂) have different effects on secondary phase formation. Detailed analysis of the surface composition indicates the presence of CoFe2O4 in the vicinity of the SrSO₄ particle, suggesting the Sr and Co depletion in the subsurface region. Based on the analysis, the water vapor affects the secondary phase growth due to the possibility of additional reaction pathways that may enhance the growth rate. This behavior is correlated with the logarithmic surface oxygen exchange coefficient, log k*, that was observed to decrease monotonically within 20 h of annealing time but its degradation was found to be faster under wet air, in accordance with Sr and Co depletion rate. [ABSTRACT FROM AUTHOR]