Interfacial Hydrogen-Bond Networks at Bi-H2O Interfaces Govern Hydroxylamine Selectivity in Nitrate Electroreduction.

Metal-H2O interfaces strongly influence electrocatalytic selectivity. However, how interfacial H2O mediates coupled interactions among catalyst surfaces, electrolytes, and intermediates remains unresolved. Here, by combining theory and experiment on Bi-H2O interfaces, we show that pH-dependent reconstruction of the interfacial hydrogen-bond network governs hydroxylamine (NH2OH) selectivity during nitrate electroreduction. Under acidic conditions, a strengthened hydrogen-bond network induced by fully hydrogen-bonded H2O shortens proton-transfer distances and interfacial charge redistribution and promotes protonation of adsorbed NOx species while weakening *NH2OH binding, thereby favoring NH2OH formation and desorption. Under neutral conditions, the weaker hydrogen-bond network from partially hydrogen-bonded H2O suppresses *NO stabilization and protonation, enabling NO release, whereas electron transfer from interfacial H2O to Bi strengthens *NH2OH adsorption and drives its further reduction to NH3. These findings identify interfacial hydrogen-bond networks as key regulators of product selectivity at metal-H2O interfaces and provide a mechanistic basis for catalyst design for selective NH2OH electrosynthesis.