Activation of H2O tailored by interfacial electronic states at nanoscale interface for enhanced electrocatalytic hydrogen evolution

Despite the fundamental and practical significance of the hydrogen evolution reaction (HER), the reaction kinetics at the molecular level is not clear, particularly in basic media. Here, with ZIF-67 derived Co-based carbon frameworks (Co/NC) as a model catalyst, we systematically investigated the effects of different reaction parameters on the HER kinetics, and surprise found that HER activity was not directly dependent on the type of nitrogen in the carbon framework, but on the relative content of surface hydroxyl and water (OH-/H2O) adsorbed on Co active sites embedded in carbon frameworks. When the ratio of the OH-/H2O was close to 1:1, Co/NC nanocatalyst showed the best reaction performance under the condition of high-pH electrolytes, e.g., with an overpotential of only 232 mV at a current density of 10 mA cm-2 in 1 M KOH electrolyte. We unambiguously identified that, the structural water molecules (SWs) in the form of hydrous hydroxyl complex absorbed on metal centers {OH-∙H2O@M+} were catalytic active sites for enhanced HER, where M+ could be transition and/or alkaline metal cations. Different from the traditional hydrogen bonding of water, the hydroxyl (hydroxide) groups and water molecules in SWs were mainly transiently bonded together through the spatial interaction between the p orbitals of O atoms, showing characteristics of delocalized π bond with dynamic feature. These new formed surface bonds or transient states could be a new weak interaction, which can act as an alternative channel for concerted electron and proton transfer at electrode surface. The capturing of new surface states not only answers pH-, cation- and transition metal- dependent hydrogen evolution kinetics, but also provides completely new insights into the understanding of other electrocatalytic reduction involved by other small moleculesm, including CO2, CO and N2 etc.