Redox Behavior of Solid Solutions in the SrFe_1-xCu_xO_3-δ System for Application in Thermochemical Oxygen Storage and Air Separation

Perovskite oxides with temperature and oxygen partial pressure dependent non-stoichiometry δ, such as SrFeO_3-δ or its Cu-doped variants, can be applied as redox materials for two-step thermochemical processes, i. e. to reversibly store oxygen and thereby thermal energy, or separate air using concentrated solar power. We studied the redox state of Cu in SrFe_1-xCu_xO_3-δ samples using in-situ X-ray photoelectron spectroscopy (XPS) and X-ray absorption (XAS) measurements in oxygen atmospheres using synchrotron radiation, and characterized these materials through thermogravimetric analysis. By this means, we show how spectroscopic and thermogravimetric data are correlated, suggesting that Cu and Fe are reduced simultaneously for x=0.05, whereas the reduction of samples with x=0.15 is mainly driven by a change in the Fe oxidation state. Furthermore, we studied the re-oxidation kinetics of reduced SrFe_1-xCu_xO_3-δ, revealing very high reaction speeds with t_1/2=13 min at 150 °C for SrFeO_3-δ. Our results indicate that SrFe_1-xCu_xO_3-δ solid solutions can be applied for oxygen storage and air separation with high capacity at relatively low temperatures, which allows an efficient thermochemical process.