Dynamic (Sub)surface-Oxygen Enables Highly Efficient Carbonyl-Coupling for Electrochemical Carbon Dioxide Reduction

Nowadays, high-valent Cu species (i.e., Cu^δ+) are clarified to enhance multi-carbon production in electrochemical CO₂ reduction reaction (CO₂RR). Nonetheless, the inconsistent average Cu valence states are reported to significantly govern the product profile of CO₂RR, which may lead to misunderstanding of the enhanced mechanism for multi-carbon production and results in ambiguous roles of high-valent Cu species. Dynamic Cu^δ+ during CO₂RR leads to erratic valence states and challenges of high-valent species determination. Herein, an alternative descriptor of (sub)surface oxygen, the (sub)surface-oxygenated degree (κ), is proposed to quantify the active high-valent Cu species on the (sub)surface, which regulates the multi-carbon production of CO₂RR. The κ validates a strong correlation to the carbonyl (*CO) coupling efficiency and is the critical factor for the multi-carbon enhancement, in which an optimized Cu₂O@Pd_2.31 achieves the multi-carbon partial current density of ≈330 mA cm⁻² with a faradaic efficiency of 83.5%. This work shows a promising way to unveil the role of high-valent species and further achieve carbon neutralization.