Chemical Heterogeneities on La[subscript 0.6]Sr[subscript 0.4]CoO[subscript 3−δ] Thin Films--Correlations to Cathode Surface Activity and Stability

Abstract

La[subscript 0.6]Sr[subscript 0.4]CoO[subscript 3−δ](LSC) thin film cathodes synthesized by pulsed laser deposition at 450°C (LSC_450°C) and 650°C (LSC_650°C) exhibit different electrochemical performance. The origin of the differences in the oxygen reduction activity and stability of these cathodes is investigated on the basis of their surface chemistry and their surface atomic and electronic structures. Angle resolved X-ray photoelectron spectroscopy and nanoprobe Auger electron spectroscopy are used to identify the surface cation content, chemical bonding environment, and the spatial heterogeneities with nanoscale resolution. The higher electrochemical activity of LSC_450°C is attributed to the more stoichiometric cation content on the surface and the more uniform lateral and depth distribution of constituent cations. The poorly crystalline atomic structure of the LSC_450°C was found to prohibit the extensive segregation and phase separation on the surface because of the more favorable elastic and electrostatic interactions of Sr in the bulk. Upon annealing in air at 600 °C, the surface of the LSC_650°C undergoes a structural change from a Sr-rich LSC state to a SrO/Sr(OH)[subscript 2]-rich phase-separated state. The partial blockage of the surface with the heterogeneously distributed SrO/Sr(OH)[subscript 2]-rich phases, the decrease in oxygen vacancy content, and the deterioration of the electron transfer properties as evidenced from the Co oxidation state near the surface are found responsible for the severe electrochemical deactivation of the LSC_650°C. These results are important for advancing our ability to tailor the electrochemical performance of solid oxide fuel cell cathodes by understanding the relation of surface chemistry and structure to the oxygen reduction activity and stability, and the dependence of cation segregation on its driving forces including material microstructure.