Chain-Length-Dependent Amine Regulation of CuOx Interfaces for Electrochemical CO2-to-C2 Conversion

The electrochemical reduction of CO2 to multicarbon products using copper-based catalysts is severely hindered by the over-reduction of Cu+ species, which critically degrades C2 selectivity. Herein, we report a series of chain-length-varied amine-modified CuOx catalysts using ethylamine (EA), ethylenediamine (EDA), diethylenetriamine (DETA), triethylenetetramine (TETA), and tetraethylenepentamine (TEPA) as molecular modifiers. A distinct nonmonotonic chain-length-dependent regulation effect is revealed: shorter amines provide insufficient interfacial regulation, whereas excessively long amines lead to overly strong surface confinement and reduced catalytic accessibility. DETA, with an intermediate molecular length and nitrogen-donor density, uniquely achieves the optimal balance among Cu+/Cu0 interfacial stabilization, interfacial-water regulation, and active-site accessibility. As a result, DETA-CuOx delivers exceptional performance in an alkaline flow cell, achieving a C2 Faradaic efficiency of 63.5% at −0.8 V vs RHE with a partial current density of −126 mA cm–2 and stable operation over 65 h. Beyond simply stabilizing the Cu+/Cu0 interface, we reveal that the amine modifiers critically regulate the hydrogen-bonding network of interfacial water, thereby tuning local proton availability. Short-chain amines (EA, EDA) insufficiently disrupt hydrogen bonding, leading to excessive protons that favor H2 evolution. In contrast, long-chain amines (TETA, TEPA) may impose excessive interfacial confinement, affecting active-site accessibility, interfacial proton transfer, and intermediate transport, thereby limiting C2 formation. DETA achieves an optimal balance, sustaining moderate proton availability to preserve *CO coverage and promote *OCCOH intermediate formation. These results identify optimal intermediate-chain amine regulation rather than a simple chain-length enhancement, establishing molecular-chain engineering of the CuOx interface as an effective strategy for selective CO2 reduction to C2 products (CO2-to-C2) conversion.