CO2 electroreduction reaction (CO2RR) to chemicals and fuels is of both fundamental and practical significance since it would lead to a more efficient storage of renewable energy while closing the carbon cycle. Here we report enhanced activity and selectivity for CO2RR to multicarbon hydrocarbons and alcohols (~69 % Faradaic efficiency and −45.5 mA cm−2 partial current density for C2+ at −1.0 V vs RHE) over O2-plasma-activated Cu catalysts via electrolyte design. Increasing the size of the alkali metal cations in the electrolyte, in combination with the presence of subsurface oxygen species which favor their adsorption, significantly improved C-C coupling on CuOx electrodes. The co-existence of Cs+ and I− induced drastic restructuring of the CuOx surface, the formation of shaped particles containing stable CuI species, and a more favorable stabilization of the reaction intermediates and concomitant high C2+ selectivity. This work combining both experiment and density functional theory, provides insights into the active sites and reaction mechanism of oxide-derived Cu catalysts for CO2RR.