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

Abstract

International audienceLow temperature oxygen ionic conductors are key materials for the development of the next generation solid oxide fuel cells. In this regard, SrMO2.5 (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 SrFeO2.5 and SrCoO2.5 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 SrCoO2.5+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 SrFeO2.5+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 SrCoO2.5, whereas for SrFeO2.5, 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 SrCoO2.5 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

Anisotropy in the thermal hysteresis of resistivity and charge density wave nature of single crystal SrFeO3-δ: X-ray absorption and photoemission studies

“…The insets present corresponding EXAFS k 2 χ data. The Fe K -edge EXAFS spectra include main FT features ( A , B and C ) that correspond to the NN bond lengths of Fe-O, Fe-Sr and Fe-Fe, respectively, in the SrFeO 2.81 crystal 3, 39, 40 . To obtain detailed information about the temperature dependence of the local structure around the Fe atoms, Fig.…”

Synchrotron Radiation and Chemistry: Studies of Materials for Renewable Energy Sources

“…Oxides based on the brownmillerite structure, which is related to perovskite, have been proposed as cathodes. The oxygen conduction mechanism in the Sr 2 Fe 2 O 5 brownmillerite was investigated by resolving the local structure of Fe in situ during the electrochemical insertion of oxygen [68]. Other prospected candidates for SOFC cathodes also include Ruddlesden-Popper phases like La 2 NiO 4+x .…”

Hard X‐ray Nanotomography of Catalytic Solids at Work

“…The experimental Fe K-edge spectrum of the fresh sample was fitted to contributions of Fe 2 O 3 , Fe 2 TiO 5 , and Fe 3 O 4 reference spectra (see Figure S2 in the Supporting Information) at every 30 30 nm 2 pixel, according to the method described by Meirer et al [10] The Fe 2 TiO 5 , pseudo-brookite, is formed by reaction of Fe 2 O 3 and TiO 2 as a result of the high calcination temperature of the preparation procedure of the catalyst. [20][21][22] In this way, we identified by PCA three major iron components; that is, Fe 2 O 3 , Fe 2 TiO 5 , and Fe 3 O 4 . Additionally, we have used principal component analysis (PCA) to identify spectral features, originating from different chemical species, to establish the number of species required to achieve an excellent fit quality of the spectra.…”

“…Additionally, we have used principal component analysis (PCA) to identify spectral features, originating from different chemical species, to establish the number of species required to achieve an excellent fit quality of the spectra. [20][21][22] In this way, we identified by PCA three major iron components; that is, Fe 2 O 3 , Fe 2 TiO 5 , and Fe 3 O 4 . The XANES spectrum obtained for the bulk particle (Figure 2 a) in its fresh state indicates that the iron phase is mainly present in the trivalent oxidized phase (Fe 2 O 3 and Fe 2 TiO 5 ), showing the absorption peak at 7123 eV and the pre-edge feature at 7114 eV originating from the 1s-to-3d transition.…”