Authors: Daniel Friebel, Mary W. Louie, Michal Bajdich, Kai E. Sanwald, Yun Cai, Anna M. Wise, Mu-Jeng Chen, Dimosthenis Sokaras, Tsu-Chien Weng, Roberto Alonso-Mori, Ryan C. Davis, John R. Bargar, Jens K. Nørskov, Anders Nilsson, Alexis T. Bell Year of publication: 2015 Journal: Journal of the American Chemical Society DOI: 10.1021/ja511559d 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.