The role of iron in the electronic configuration of mixed nickel iron oxides for the oxygen evolution reaction

Nickel-based oxides are among the best performing catalysts for the alkaline O2 evolution reaction (OER). It has long been recognized that iron enhances the catalytic activity of nickel-based catalysts, though only recently intensive research has been done on the interplay between the two transition metals, leading to the excellent performance, surpassing that of either pure metal. It is still not clear how the electronic configuration in mixed metal compounds changes to enhance their catalytic activity for the OER. We carried out a systematic study of the electronic configuration of thin film mixed metal oxides Ni(1-x)FexOyHz with varying contents x of iron. In this investigation we employed X-ray absorption and resonant valence photoelectron spectroscopy (XAS and resPES) to gain knowledge on the changes induced in the electronic structure by introduction of iron, both before and after electrochemical activation. Based on density functional calculations we found iron species to induce a highly oxidizing environment that facilitates generation of oxo species on iron and neighboring nickel sites. The reduced electron density around Ni-O bonds creates in-gap states near the Fermi level. The magnitude of these in-gap states scales linearly with the OER performance and thus can be used as an activity descriptor. Contrary to literature we see the in-gap states even before electrochemical activation and conclude that they are a consequence of Ni-O-Fe motifs present already before anodization. Beyond 50% metal content the number of Ni-O-Fe motifs is decreasing again, resulting in an interval of 10-30% iron metal content to be optimal for the OER.