Elucidating Factors Governing the Discrepancy of Oxygen Vacancy Concentration Between La_0.6Sr_0.4Co_1−yFe_yO_3−δ (y = 0, 0.2, 0.4, 0.6, 0.8, 1) Dense Film and Bulk Electrodes

Dense film electrodes are used to understand the reaction mechanisms of solid oxide cells. However, the oxygen vacancy concentration, δ , in films differ from that in bulk materials, which can mislead the understanding of the reaction mechanism. Therefore, a comprehensive study was conducted to elucidate the differences in δ for the air electrode, La 0.6 Sr 0.4 Co 1− y Fe y O 3− δ (LSCF, y = 0, 0.2, 0.4, 0.6, 0.8, and 1), between film and bulk electrodes. Two key findings emerged: (1) For Co-rich compositions ( y ≤ 0.5), significant differences in δ were observed between films and bulk electrodes, while (2) for Fe-rich compositions ( y ≥ 0.6), there was almost no difference in δ . Plausible reasons for these discrepancies were investigated: (1) lattice stress in the film, (2) compositional variation within the film electrode, and (3) changes in the electronic conduction. High-temperature X-ray diffraction revealed no significant differences in lattice constant between films and bulk electrodes, and X-ray photoelectron spectroscopy and X-ray absorption spectroscopy indicated no changes in electronic structure. However, scanning transmission electron microscopy/energy dispersive X-ray spectroscopy revealed the presence of Co 3 O 4 formation within the film, suggesting local compositional variations. Therefore, the results indicate that local compositional differences are the most likely factor affecting δ in LSCF films.