O-Doping Boosts the Electrochemical Oxygen Reduction Activity of a Single Fe Site in Hydrophilic Carbon with Deep Mesopores

Carbon-based electrocatalysts with single metal sites hold great potential for mechanism exploration via mimicking molecular catalysts, due to their distinct catalytic sites. In addition to metal atoms, the neighboring nonmetal 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 centers. In this work, the boosting effect of O-doping toward the electrochemical oxygen reduction reaction (ORR) was evaluated by both experimental studies and DFT calculations. O-doped carbon-supported single-Fe-site catalysts possessing deep mesopores and desirable hydrophilic surface were achieved by confined carbonization in an 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 the ORR in terms of half-wave potential, Tafel slope, and long-term stability. In particular, SAFe-NDC-H outperformed its SAFe-NDC counterpart. Considering that these two catalysts possess a comparable porous structure, surface properties, and local electronic structure of a single Fe site, the dopant nonmetal O atoms, specifically, carbonyl group (C═O), are revealed to affect the ORR activity of the 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.