Bipolar membranes reveal surface-hydroxyl-structure-dependent water dissociation mechanism.

The high energy penalty for driving fast water dissociation hinders the implementation of emerging catalysis and energy conversion technologies with high sustainability, including electrolysis and fuel cells. This challenge stems in part from a limited understanding of how active-site structures influence water dissociation pathways. Here, we show that the structure of surface hydroxyls governs water dissociation activity in bipolar membranes. Using tin oxide quantum wires as model catalysts, we find that hydroxyl groups associated with bridging oxygen vacancies promote proton transfer and accelerate the alkaline water dissociation pathway at the membrane interface. By increasing the abundance of these hydroxyl structures through controlled synthesis, the bipolar membrane achieves a transmembrane voltage of 0.88 ± 0.02 V at 1 A cm-2, equivalent to an overpotential of approximately 60 mV. When incorporated into a pure-water electrolyzer, the resulting membrane enables a full-cell voltage of 1.98 ± 0.02 V and stable operation for more than 550 hours at 1 A cm-2. These findings provide guidance for the design of catalysts for efficient bipolar membrane devices.