Establishing * CO/ * H Intermediate Descriptor for Selective CO 2 ‐to‐Ethanol Electroreduction Under Limited CO 2 Supply

ABSTRACT Electrochemical CO 2 reduction (CO 2 RR) to ethanol offers a sustainable route for carbon utilization and energy storage, yet achieving high ethanol selectivity under limited CO 2 availability remains a significant challenge. The central issue is the lack of quantitative understanding of how the interfacial * CO and * H intermediates jointly govern product selectivity. Here, we establish an interfacial coverage regulation strategy by embedding Cu nanoparticles into an imine‐functionalized covalent organic framework (Cu/Im‐COF). The imine‐rich interface enriches CO 2 adsorption while modulating proton transfer, enabling a balanced surface coverage of * CO and * H, identified as the key selectivity descriptor ( θ *CO/*H ) for ethanol formation. Operando Raman spectroscopy, density functional theory calculations, confirms that an optimized θ *CO/*H stabilizes the * HCCHOH intermediate and lowers its formation barrier. Quantitative integration of Raman features allows direct determination of θ *CO/*H , revealing a volcano‐type dependence of ethanol selectivity on this descriptor. A maximum ethanol Faradaic efficiency of 56% is achieved at log( θ *CO/*H ) ≈ 3.0, a current density of 700 mA cm − 2 , and a 15% CO 2 feed. This work establishes θ *CO/*H as a quantitative bridge between intermediate coverage and reaction kinetics, providing a mechanistic framework for rational design of CO 2 RR catalysts with high ethanol selectivity under CO 2 ‐constrained conditions.