Highly active catalysts for the oxygen evolutionreaction (OER) are required for the development ofphotoelectrochemical devices that generate hydrogen effi-ciently from water using solar energy. Here, we identify theorigin of a 500-fold OER activity enhancement that can beachieved with mixed (Ni,Fe)oxyhydroxides (Ni1−xFexOOH)over their pure Ni and Fe parent compounds, resulting in oneof the most active currently known OER catalysts in alkalineelectrolyte. Operando X-ray absorption spectroscopy (XAS)using high energy resolution fluorescence detection (HERFD)reveals that Fe3+ in Ni1−xFexOOH occupies octahedral siteswith unusually short Fe−O bond distances, induced by edge-sharing with surrounding [NiO6] octahedra. Using computational methods, we establish that this structural motif results in near optimal adsorption energies of OER intermediates and low overpotentials at Fe sites. By contrast, Ni sites in Ni1−xFexOOH are not active sites for the oxidation of water.
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