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Dynamic Behavior of Single-Atom Catalysts in Electrocatalysis: Identification of Cu-N3 as an Active Site for the Oxygen Reduction Reaction
Journal of the American Chemical Society  (IF15.419),  Pub Date : 2021-08-31, DOI: 10.1021/jacs.1c03788
Ji Yang, Wengang Liu, Mingquan Xu, Xiaoyan Liu, Haifeng Qi, Leilei Zhang, Xiaofeng Yang, Shanshan Niu, Dan Zhou, Yuefeng Liu, Yang Su, Jian-Feng Li, Zhong-Qun Tian, Wu Zhou, Aiqin Wang, Tao Zhang

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.