Establishing ^*CO/^*H Intermediate Descriptor for Selective CO₂-to-Ethanol Electroreduction Under Limited CO₂ Supply

Electrochemical CO₂ reduction (CO₂RR) to ethanol offers a sustainable route for carbon utilization and energy storage, yet achieving high ethanol selectivity under limited CO₂ 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 CO₂ 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% CO₂ 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 CO₂RR catalysts with high ethanol selectivity under CO₂-constrained conditions.