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.

中文翻译：

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单原子催化剂在电催化中的动态行为：鉴定 Cu-N3 作为氧还原反应的活性位点

原子分散的 MNC（M 指过渡金属）材料是贵金属 Pt 的最有前途的催化剂替代品，用于氧的电化学还原（ORR），但 MNC 中真正的活性位点仍然难以捉摸。在这里，我们开发了一种两步法来制造具有均匀且明确定义的 Cu ²⁺ -N ₄结构的Cu-NC 单原子催化剂，与Pt/C 相比，该催化剂具有相当的活性和优异的耐久性。通过将操作X 射线吸收光谱与理论计算相结合，我们明确地确定了 Cu-N ₄到 Cu-N ₃以及进一步到 HO-Cu-N ₂的动态演变ORR工作条件，其中同时进行还原的Cu发生下²⁺成Cu ⁺，并通过所施加的电势来驱动。随着电位的降低，Cu ⁺ /Cu ²⁺比值的增加表明低配位的 Cu ⁺ -N ₃是真正的活性位点，这进一步得到 DFT 计算的支持，该计算表明在每个元素步骤中较低的自由能Cu ⁺ -N ₃上的ORR比Cu ²⁺ -N ₄上的ORR 高。这些发现提供了对 MNC 催化剂动态电化学的新理解，并可能指导更有效的低成本催化剂的设计。