Cation Dehydration by Surface-Grafted Phenyl Groups for Enhanced C2+ Production in Cu-Catalyzed Electrochemical CO2 Reduction

The challenge to produce multicarbon (C₂₊) products in high current densities in the electrochemical reduction of carbon dioxide (CO₂RR) has motivated intense research. However, the ability of solvated cations to tune and activate water for C₂₊ production in the CO₂RR has been overlooked. In this study, we report the incorporation of a covalently grown layer of functionalized phenyl groups on the Cu surface that leads to a 7-fold increase in ethylene production (to -530 mA cm^-2) and a 6-fold increase in C₂₊ products (to -760 mA cm^-2). Our mechanistic study, notably by in situ infrared, isotope effect, and electrochemical impedance spectroscopy, reveals that the surface grafting provokes the reduction of the hydration shell around alkali cations at the electrode-electrolyte interface. This reduction weakens the hydrogen bond network and thus enhances water dissociation to provide a proton for the CO₂RR. Moreover, it increases cation density at the outer Helmholtz plane, resulting in a larger local electric field that further stabilizes intermediates en route to C₂₊ production. Our study brings to light the crucial role of cations in modulating the interfacial water structure, which has to be optimal for efficient C₂₊ production in CO₂RR.