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 CO 2 reduction reaction (CO 2 RR). Nonetheless, the inconsistent average Cu valence states are reported to significantly govern the product profile of CO 2 RR, 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 CO 2 RR 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 CO 2 RR. 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 Cu 2 O@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.