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.