Dynamic Behavior of Single-Atom Catalysts in Electrocatalysis: Identification of Cu-N₃ as an Active Site for the Oxygen Reduction Reaction

Atomically dispersed M-N-C (M refers to transition metals) materials represent the most promising catalyst alternatives to the precious metal Pt for the electrochemical reduction of oxygen (ORR), yet the genuine active sites in M-N-C remain elusive. Here, we develop a two-step approach to fabricate Cu-N-C single-atom catalysts with a uniform and well-defined Cu²⁺-N₄ structure that exhibits comparable activity and superior durability in comparison to Pt/C. By combining operando X-ray absorption spectroscopy with theoretical calculations, we unambiguously identify the dynamic evolution of Cu-N₄ to Cu-N₃ and further to HO-Cu-N₂ under ORR working conditions, which concurrently occurs with reduction of Cu²⁺ to Cu⁺ and is driven by the applied potential. The increase in the Cu⁺/Cu²⁺ ratio with the reduced potential indicates that the low-coordinated Cu⁺-N₃ is the real active site, which is further supported by DFT calculations showing the lower free energy in each elemental step of the ORR on Cu⁺-N₃ than on Cu²⁺-N₄. These findings provide a new understanding of the dynamic electrochemistry on M-N-C catalysts and may guide the design of more efficient low-cost catalysts.