Probing metal disproportionation and oxygen coupling in Fe-based Ruddlesden-Popper-type perovskites for the oxygen evolution reaction

This investigation aims to probe metal charge disproportionation and oxygen-to-metal charge transfer in Fe-based Ruddlesden-Popper-type perovskites (Sr3FeMO7−δ, M = Fe, Co, Ni) and relate these properties to the catalytic activity of the materials in oxygen evolution reaction (OER). Neutron powder diffraction (NPD) measurements would allow to confirm the charge disproportionation of the metal sites and oxygen-to-metal negative charge transfer (i.e., lattice oxygen oxidation) through identification of non-equivalent Fe/Co/Ni sites and an accurate band valence sum (BVS) evaluation. Furthermore, NPD due to its high sensitivity to crystal symmetry and to oxygen content and position in a lattice, is proposed to be a highly suitable characterization to identify reduced O‒O distances (including the presence of O‒O peroxo-like dimers which are expected in the materials containing oxidized oxygen species). The knowledge to be obtained through our study would have profound fundamental importance as a rare direct structural evidence for the lattice oxygen oxidation induced by the metal charge disproportionation in the highly catalytically active oxides, and will be used to instruct the design of the compositions with improved oxygen evolution reaction activity.