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Abstract

Detailed knowledge about the relationship between the electronic structure and the catalytic properties of a material is a fundamental brick to rationally design better oxygen evolution reaction (OER) catalysts. Here, resonant photoelectron spectroscopy (RPES) is used to elucidate the electronic structure and active state of the most broadly employed OER catalyst, Ni-Fe (oxy)hydroxides. We implemented a graphene-capped catalyst-coated ionomer membrane approach. Starting from a well-characterized iron nickel oxide precursor, the changes in the electronic structure of oxygen and nickel species with different applied electrode potentials were studied. In particular, RPES measurements helped to distinguish between formal Ni³⁺ and Ni⁴⁺ species emerging upon catalyst oxidation, which indicate charge accumulation in adjacent hole states. Based on RPES, the core-level binding energy and partial electron yield absorption spectroscopy (PEY-XAS), we identify the contributions to the oxygen partial density of states (p-DOS) that are crucial for OER catalysis. Our results reveal the occurrence of metal-oxygen hybridized holes, which we can correlate to the Fermi-level under oxidizing potentials. These states potentially promote the active sites as indirect electron acceptors. The nature of this metal-oxygen charge redistribution influencing catalytically active surface-structure motifs is crucial for the formation of OER intermediates. Our findings allow to revisit the role of metal and oxygen species in the OER mechanism from the viewpoint of electronic structure.