Mechanistic Understanding of the pH-Dependent Oxygen Reduction Reaction on the Fe-N-C Surface: Linking Surface Charge to Adsorbed Oxygen-Containing Species

The Fe-N-C catalyst, featuring a single-atom Fe-N₄ configuration, is regarded as one of the most promising catalytic materials for the oxygen reduction reaction (ORR). However, the significant activity difference under acidic and alkaline conditions of Fe-N-C remains a long-standing puzzle. In this work, using extensive ab initio molecular dynamics (AIMD) simulations, we revealed that pH conditions influence ORR activity by tuning the surface charge density of the Fe-N-C surface, rather than through the direct involvement of H₃O⁺ or OH^- ions. The acidic environment, combined with an elevated electrode potential, can result in a highly charged Fe-N-C surface. On this surface, the adsorbed *OH will spontaneously convert to *O and remain stable, accompanied by a change in the valence state of the Fe atom. This phenomenon makes the ORR step from *O to *OH the rate-determining step, thereby significantly reducing the corresponding ORR activity. Under fixed pH conditions and electrode potentials, the surface charge density of Fe-N-C can be tuned by changing the coordination environment of the Fe atom. Further calculations reveal that doping a Co₄ cluster near the Fe active center or creating an edge-type Fe-N-C structure can effectively reduce the local charge density around the Fe atom. This reduction hinders the transition of *OH to *O, thereby enhancing ORR activity at a high electrode potential in acidic environments. Our work revealed the underlying explanation of the pH-dependent ORR activity for the Fe-N-C catalyst and sheds light on the future design and synthesis of high-performance Fe-N-C catalysts.