Customizable Interfacial Solvation via Hydrogel Mediation for Enhanced C-C Coupling in CO₂ Electroreduction

Interfacial water structure critically influences CO₂ electroreduction pathway and product selectivity, yet molecular-level strategies to precisely regulate interfacial solvation and quantitatively correlate it with C-C coupling remain limited. Here, we report a hydrogel-mediated, customizable interfacial solvation strategy by integrating an ultrathin, ion-cross-linked, water-retentive hydrogel layer on Cu (Cu-IWH), formed via coordination between chitosan and divalent metal cations. The resulting charged and hydrated interface enables controlled modulation of the interfacial electrostatics, ion hydration, and local chemical environment, thereby steering CO₂ electroreduction toward C₂₊ products. In situ Raman spectroscopy reveals that solvated K⁺(H₂O)_n species interact with adsorbed hydroxyl groups (OH_ad), leading to a reorganization of the interfacial hydration structure and the formation of Cu·OH_ad·K⁺(H₂O)_n* interfacial complexes. By tuning the hydrogel cross-linking density, an optimal balance between OH_ad coverage and cation accessibility is achieved, maximizing the population of these complexes, which shows a quantitative correlation with enhanced C₂₊ selectivity. Density functional theory calculations further show that these complexes induce interfacial charge redistribution, stabilize *CO adsorption in configurations favorable for C-C coupling, and lower the kinetic barrier for *CO-*CO dimerization. Meanwhile, the hydrogel matrix enriches interfacial OH^-, creating a locally alkaline microenvironment that promotes *CO_L/*CO_B coadsorption and facilitates C-C coupling. As a result, the optimized Cu-IWH_M catalyst delivers a C₂₊ Faradaic efficiency of 87.5% at -1.2 A cm^-2 and a single-pass carbon efficiency of 94.3% at -1.0 A cm^-2, while sustaining stable operation for over 800 h at 800 mA in a 4 cm² MEA electrolyzer. When scaling to a 100 cm² MEA, the system achieves 81.5% C₂₊ selectivity with a C₂₊ energy efficiency of 34.0% at 40 A and operates stably for 82 h at 20 A. This work establishes hydrogel-mediated interfacial solvation engineering as a tunable approach for regulating interfacial environments and promoting C-C coupling in CO₂ electrocatalysis.