Oxygen stoichiometry of A-site-deficient perovskites for solid oxide cells

Abstract

The surface chemistry and thus in turn the oxygen exchange kinetics of perovskite-type electrode materials for solid oxid fuel cells (SOFCs) and solid oxid electrolysis cells (SOECs) is strongly affected by segregation phenomena. Typically, B-site cations segregate in form of catalytically active metal nanoparticles in reducing conditions, while A-site cations tend to segregate in oxidizing conditions and lead to performance loss. One possible way to enhance the desired segregation of B-site cations while simultaneously decreasing the undesired segregation of A-site cations is to employ A-site deficient materials. For a detailed understanding of the underlying mechanisms, precise knowledge of the oxygen vacancy concentration is of importance. In this study, A-site deficient La0.6Sr0.4FeO3-δ (LSF) and SrTi0.6Fe0.4O3-δ (STF) serve as model materials.We present a new miniature coulometric titration cell that allows for precise measurements of changes in the oxygen non-stoichiometry of LSF and STF powder samples over a wide pO2 range. Additionally, we applied the same principle to electrochemical cells with thin film STF electrodes, on which irreversible processes could be determined more precisely. The presented methods not only allow for the quantification of changes in oxygen stoichiometry, but also for the quantification of easily reducible secondary phases that either come from synthesis, or form in-situ during electrochemical reduction. Moreover, we can show how the bulk oxygen vacancy concentration is linked to transition metal oxidation states on the surface that were determined by in-situ X-ray photoelectron spectroscopy (XPS) measurements on electrochemically polarized model cells.