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.