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.

结合了均相和非均相催化剂优点的单原子催化剂（SAC）引起了人们的极大兴趣。SAC的合理设计和合成需要对不同pH溶液中的实际活性位点有清晰的了解。在这里，通过识别H的不同物理化学性质⁺和OH ^-中的氧还原反应（ORR），我们确定两个关键结构，FEN ₄和石墨型N-CH（g型N-CH）分别是在酸性和碱性溶液中对ORR的催化活性位。通过精心设计包含两个活性位点的碳负载的单个铁原子催化剂（Fe-SAC），可以通过实验证实这一见解，这些活性位点可协同工作。双活性位Fe-SAC显示增强的ORR活性，良好的长期稳定性和甲醇耐受性，以及预期的pH通用性。通过查明和澄清SAC的活性位点，这项工作可能会激发化学家为ORR设计和合成具有成本效益的SAC。