Oxygen-Bearing Defects in Graphene-Supported M–N–C Single-Atom Catalysts: A Hidden Determinant of Catalytic Performance

In the study of M–N–C single-atom catalysts (SACs), a series of unresolved issues point to the structural role of inevitable oxygen-bearing defects in the carbon lattice beyond the MN4 moieties. Through explaining experimental observations on the enhanced oxygen reduction reaction (ORR) activity of hydrogenated Fe–N–C SACs, we demonstrated the necessity of incorporating such oxygen-bearing defects in modeling frameworks. Furthermore, the sp3-hybridized carbon atoms near these defects readily adsorb hydrogen atoms and exhibit competitive ORR performance. Therefore, this discovery also offers an alternative strategy for increasing the density of active sites. Subsequently, we performed systematic calculations to revisit the mechanism of reduction reactions of small molecules (O2, CO2, and N2) over Fe–N–C SACs. The varying synergistic effects between different oxygen-bearing defect configurations and the FeN4 moiety lead to distinct magnetic moments of Fe centers, unraveling the origin of the previously observed discrepancies in ORR performances of such catalysts. Furthermore, we established a unified modeling strategy based on oxygen-bearing defects and extended it to elucidate the experimentally observed enhancement in the catalytic activity for F-doped NiN4 and Cl-doped ZnN4 SACs. Overall, our calculations address three long-standing challenges in the field of M–N–C SACs: (1) discrepancies in reported catalytic performances despite identical MN4 active sites, (2) low active site density, and (3) the absence of standardized modeling strategies for functionalized M–N–C SACs. This study provides a deep understanding of the structure–property relationship in graphene-supported SACs, emphasizing the crucial role of the inevitable oxygen-bearing defects in electrochemical reactions.