The contribution of water molecules to the hydrogen evolution reaction

Traditionally, water molecules act as solvents in most chemical reactions, whereas they act as solvents and reactants in the alkaline electrolyte for the hydrogen evolution reaction (HER). It is well known that there is a current plateau in the linear potential—current dependence for HER in neutral or near-neutral electrolytes, showing that the HER is governed by the mass transport of reactive hydronium species at a given overpotential. The sharp rise in the current signal after the plateau at a slightly higher overpotential indicates that HER is supported by a new reactant, namely the water molecules rather than the limited hydronium species. Herein, in combination with our own research experience in water electrolysis, we review the relevant literature in these years about the HER activity descriptor and mainly focus on the contribution of water molecules to the HER, including their dissociation, configuration, and composition in regulating the pH-dependent HER. Finally, we try to provide new insights into understanding the mechanism of the HER in terms of interfacial water enrichment, orientation, and configuration with the electric field strength of electrode/electrolyte interface and electrode compositions.