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

Abstract

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.