Identifying the Active Sites of a Single Atom Catalyst with pH-Universal Oxygen Reduction Reaction Activity

Single-atom catalysts (SACs), combining the merits of homogeneous and heterogeneous catalysts, have attracted significant interest. Rational design and synthesis of SACs requires a clear understanding of the real active sites in different pH solutions. Here, by recognizing the distinct physicochemical properties of H+ and OH− in the oxygen reduction reaction (ORR), we identify two key structures, FeN4 and graphitic-type N-CH (g-type N-CH), to be the catalytic active sites toward the ORR in acidic and alkaline solutions, respectively. This insight is confirmed experimentally by the careful design of a carbon-supported single Fe-atom catalyst (Fe-SAC) containing both of the active sites, which are found to work in synergy. The bi-active site Fe-SAC shows enhanced ORR activity, good long-term stability, and methanol tolerance, as well as the anticipated pH universality. By pinpointing and clarifying the active sites of SACs, this work may inspire chemists to design and synthesize cost-efficient SACs for the ORR.