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.