In-Situ Generated High-Valent Iron Single-Atom Catalyst for Efficient Oxygen Evolution.

Oxygen evolution reaction (OER) plays an important role in renewable energy supplies as the anodic reaction for electrochemical transformation of various chemicals. Iron-based OER catalysts are potential candidates due to their abundance but suffer from poor activity. Here we demonstrate that a single-atom iron catalyst with in-situ generated Fe4+ centers is highly active toward OER. Only an overpotential of 320 mV was needed to reach 10 mA cm-2. The catalyst exhibited an ultrahigh turnover frequency of 0.62 s-1 at an overpotential of 0.35 V, which is comparable to currently reported transitional-metal based OER catalysts. Experimental and theoretical studies revealed that the valence state of the metal center transferred from Fe3+ to highly active Fe4+ prior to the OER process. This transformation was originated from the strong interaction between atomic Fe and carbon support via C-O-Fe bonding, leading to a lower energy barrier of the rate-limiting *OOH formation.