Active Site Evolution of the TM-N₄-C Single-Atom Catalyst during the Oxygen Evolution Reaction

It has been well documented that single-atom catalysts (SACs) with transition-metal atoms embedded in a N-doped carbon matrix (TM-N₄-C) exhibit great potential for the oxygen evolution reaction (OER). However, the nature of their active centers remains a long-standing conundrum due to the complex reaction under working conditions. Herein, the three most experimentally documented TM centers (i.e., Fe, Co, and Ni) were selected as paradigms to investigate their active site evolution by first-principles calculations. Thermodynamic and kinetic simulations collectively reveal that the components of active sites depend strongly on the location of metal d-band orbital, which decreases in the order of Fe, Co, and Ni. Specifically, the Fe center is covered by an *OH or *O, while Co is steadily covered by *OH. As for Ni-N₄-C, Ni is axially coordinated by an *OH and its adjacent C is covered by an *O, together forming a combined active site for the OER. Furthermore, these active site configurations remain stable under applied electrode potentials. This work provides a thorough exploration of the OER pathway and deep insight into the activity center under working conditions.