Highly Active Fe Sites in Ultrathin Pyrrhotite Fe₇S₈ Nanosheets Realizing Efficient Electrocatalytic Oxygen Evolution

Identification of active sites in an electrocatalyst is essential for understanding of the mechanism of electrocatalytic water splitting. To be one of the most active oxygen evolution reaction catalysts in alkaline media, Ni-Fe based compounds have attracted tremendous attention, while the role of Ni and Fe sites played has still come under debate. Herein, by taking the pyrrhotite Fe₇S₈ nanosheets with mixed-valence states and metallic conductivity for examples, we illustrate that Fe could be a highly active site for electrocatalytic oxygen evolution. It is shown that the delocalized electrons in the ultrathin Fe₇S₈ nanosheets could facilitate electron transfer processes of the system, where d orbitals of Fe^II and Fe^III would be overlapped with each other during the catalytic reactions, rendering the ultrathin Fe₇S₈ nanosheets to be the most efficient Fe-based electrocatalyst for water oxidation. As expected, the ultrathin Fe₇S₈ nanosheets exhibit promising electrocatalytic oxygen evolution activities, with a low overpotential of 0.27 V and a large current density of 300 mA cm^-2 at 0.5 V. This work provides solid evidence that Fe could be an efficient active site for electrocatalytic water splitting.