LAUSR

Carbon-based electrocatalysts with single-metal-site hold great potential for the mechanism exploration via mimicking molecular catalysts, due to their distinct catalytic sites. In addition to metal atoms, the neighboring non-metal heteroatoms such as N, S, and O atoms, which are widely detected in carbon-based single-atom catalysts, may also contribute to enhancing the electrochemical activity of single-metal center. In this work, the boosting effect of O-doping toward electrochemical oxygen reduction reaction (ORR) was evaluated by both experimental study and DFT calculations. O-doped carbon supported single-Fe-site catalysts possessing deep mesopores and desirable hydrophilic surface were achieved by confined carbonization in inert or reductive atmosphere (SAFe-NDC and SAFe-NDC-H). As compared to the-state-of-the-art Pt/C, these catalysts showed superior catalytic activity toward ORR in terms of half-wave potential, Tafel slope as well as long-term stability. Particularly, SAFe-NDC-H outperformed the SAFe-NDC counterpart. Considering these two catalysts possess comparable porous structure, surface properties, and local electronic structure of single-Fe-site, the dopant non-metal O atoms, specifically, carbonyl group (C=O), are revealed to affect the ORR activity of single-Fe-site exclusively. The introduced C=O facilitates the formation of *OOH as well as the reduction of *OH, thereby reducing the catalysts' overpotential.