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

Abstract 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.