Modulating water hydrogen bonding within a non-aqueous environment controls its reactivity in electrochemical transformations

Electrochemical carbon dioxide reduction (CO2R) can provide a sustainable route to produce fuels and chemicals; however, CO2R selectivity is frequently impaired by the competing hydrogen evolution reaction (HER), even for small concentrations of water. Here we tune water solvation and dynamics within a series of aprotic solvents featuring different functional groups and physicochemical properties to modulate HER activity and CO2R selectivity. We show that one can extend the HER onset potential by almost 1 V by confining water within a strong hydrogen bond network. We then achieve nearly 100% CO Faradaic efficiency at water concentrations as high as 3 M with a gold catalyst. Furthermore, under mildly acidic conditions, we sustain nearly 100% Faradaic efficiency towards CO with no carbonate losses over long-term electrolysis with an earth-abundant zinc catalyst. Our work provides insights to control water's reactivity and reveals descriptors to guide electrolyte design for important electrochemical transformations. Electroreduction of CO2 competes with the hydrogen evolution reaction; thus, controlling water's activity to exclusively act as a proton donor is a desirable yet challenging goal. Now the behaviour of water in aprotic solvents is shown to depend on the solvent's donor ability, which can modulate the hydrogen bond network and in turn promote the desired reactivity.