Cu^δ+ sites on the surface of oxide-derived copper (OD-Cu) are of vital importance in electrochemical CO₂ reduction reaction (CO₂RR). However, the underlying reason for the dynamically existing Cu^δ+ species, although thermodynamically unstable under reductive CO₂RR conditions, remains uncovered. Here, by using electron paramagnetic resonance, we identify the highly oxidative hydroxyl radicals (OH^•) formed at room temperature in HCO₃^- solutions. In combination with in situ Raman spectroscopy, secondary ion mass spectrometry, and isotope-labelling, we demonstrate a dynamic reduction/reoxidation behavior at the surface of OD-Cu and reveal that the fast oxygen exchange between HCO₃^- and H₂O provides oxygen sources for the formation of OH^• radicals. In addition, their continuous generations can cause spontaneous oxidation of Cu electrodes and produce surface CuO_x species. Significantly, this work suggests that there is a “seesaw-effect” between the cathodic reduction and the OH^•-induced reoxidation, determining the chemical state and content of Cu^δ+ species in CO₂RR. This insight is supposed to thrust an understanding of the crucial role of electrolytes in CO₂RR.

氧化物衍生铜 (OD-Cu) 表面上的Cu ^{δ+位点在电化学 CO} ₂还原反应 (CO ₂ RR)中至关重要。_{然而，尽管在还原性 CO 2} RR 条件下热力学不稳定，但动态存在的 Cu ^δ+物种的根本原因仍未被发现。在这里，通过使用电子顺磁共振，我们鉴定了在室温下在 HCO ₃ ^-中形成的高氧化性羟基自由基 (OH ^{• )}解决方案。结合原位拉曼光谱、二次离子质谱和同位素标记，我们展示了 OD-Cu 表面的动态还原/再氧化行为，并揭示了 HCO ₃ ^-和 H ₂ O 之间的快速氧交换提供了氧气OH ^•自由基的形成来源。此外，它们的连续生成会引起Cu电极的自发氧化并产生表面CuO _x物质。值得注意的是，这项工作表明在阴极还原和 OH ^•诱导的再氧化之间存在“跷跷板效应”，决定了CO _{2中 Cu} ^δ+物质的化学状态和含量RR。这一见解应该有助于理解电解质在 CO ₂ RR 中的关键作用。