Time resolved in situ XAFS study of the electrochemical oxygen intercalation in SrFeO_2.5 brownmillerite structure: Comparison with the homologous SrCoO_2.5 system

Low temperature oxygen ionic conductors are key materials for the development of the next generation solid oxide fuel cells. In this regard, SrMO_2.5 (M = Fe,Co) systems with a Brownmillerite-type structure are able to reversibly intercalate oxygen in an electrochemical reaction at room temperature to reach SrMO₃ stoichiometry. To understand and characterize this behavior, at the atomic level, in situ X-ray absorption spectroscopy experiments during the electrochemical oxidation reaction were performed for both SrFeO_2.5 and SrCoO_2.5 compounds at the Fe and Co K-edge, respectively. The comparative analysis of the two experiments allowed us to emphasize the similarities and differences observed during electrochemical oxidation of the two parent compounds. The data were analyzed both in XANES and EXAFS regions to extract both electronic configuration and local order information. To extract as much information as possible from collected data, the standard linear combination of spectra was complemented by the principal component analysis advanced method, which allowed us to clarify some aspects of the reaction process that were otherwise hidden. As for SrCoO_2.5+x ordered intermediates (x = 0.25, 0.375) that could have been identified by neutron diffraction and XAFS experiments [J. Am. Chem. Soc. 2006, 128, 13161], no ordered intermediates of the homologous SrFeO _2.5+x different from an oxygen deficient perovskite phase have been reported to occur during the oxygen intercalation reaction, except on a very local level. However, a detailed fit of the EXAFS signals for starting and final phases showed that the final fully oxidized compound (with stoichiometry SrCoO₃) has been obtained for SrCoO_2.5, whereas for SrFeO_2.5, the reaction ended before the expected charge transfer. We interpret the formation of SrFeO₃ to be accompanied by the parasitic formation of an unknown phase, containing Fe(III). Furthermore we were able to highlight that, at a local level, the most probable space group for SrCoO _2.5 at ambient temperature is Imma, as the average local environments extracted from Pnma and I2mb models were not able to reproduce the experimental EXAFS spectrum.