Dynamic Proton Transfer Competition and pH-Dependent ORR Mechanism in γN-Modulated Fe-N-C Single-Atom Catalysts

The oxygen reduction reaction (ORR) at the cathode of alkaline fuel cells is governed by the dynamic coupling of catalyst active sites, interfacial solvation structures, and electrolyte ions, yet the atomic-level proton supply and transfer mechanism of Fe-N-C single-atom catalysts (SACs) in alkaline ORR remains elusive. In previous DFT studies, proton supply is assumed to solely rely on adsorbed water (*H₂O) dissociation, which suffers from high energy barriers and slow kinetics; free OH^- in alkaline electrolytes further competes with ORR intermediates for protons, hindering the reaction. Herein, combining DFT and ab initio molecular dynamics (AIMD) with an explicit water model, we study Fe-N₄ and γN-doped Fe-N₄-γN₂ SACs under neutral/alkaline conditions and propose a γN-mediated βC-modified proton supply mechanism. Two plausible proton transfer pathways are highly suggested: βC-adsorbed ^#OH may form hydrogen bonding with Fe-site *O to donate protons, or *O may gain protons via a water-centered hydrogen-bond-mediated proton relay. Furthermore, under alkaline conditions, free OH^- is observed to abstract protons from *OOH to regenerate *O₂, implying a potential retardation of the conventional 4e^- ORR pathway. Overall, this work indicates that pH and γN doping synergistically modulate ORR dynamics, providing a cautious yet insightful atomic-level mechanism for designing alkaline Fe-N-C SACs.