Relation between NiMo(O) Phase Structures and Hydrogen Evolution Activities of Water Electrolysis

Abstract: NiMo(O) catalysts show extremely low overpotential at high current density for the electrocatalytic hydrogen evolution reaction (HER). However, the real reason for the remarkable electrocatalytic activity is unclear. A new perspective for revealing the relation between the phase structures of the electrocatalysts and their electrocatalytic HER performance provides a deep insight into the nature of the HER. Herein, the dehydration and oxygenation of as-synthesized nickel molybdate hydrate (NiMoO4·nH2O) are discussed and confirmed to be critical for evolving the catalytic phase structures in the subsequent reduction treatment. The typical phase evolution processes of the electrocatalysts were investigated using thermogravimetric (TG) analysis and H2 temperature-programmed reduction (H2-TPR). The crystalline phases were identified through X-ray diffraction (XRD), Raman spectroscopy, and high-resolution transmission electron microscopy (HRTEM) analyses. The phases of the electrocatalysts during the electrochemical tests were confirmed by in situ electrochemical XRD characterization. Three typical crystalline phases, MoNi4, β-NiMoO4, and α-NiMoO4, corresponding to significantly different HER activities, were proposed. The β-NiMoO4 dominant electrocatalyst (NiMoO4-400air-H2)exhibited the worst performance for alkaline water reduction, and an improvement was observed for the α-NiMoO4 electrocatalyst (NiMoO4-500air-H2). The NiMoO4-300air-H2 electrode derived from NiMoO4·(n−x)H2O exhibited the most active phase (MoNi4) and the best electrocatalytic HER performance. Moreover, the intrinsic electrocatalytic HER performance obtained from the electrochemical active surface area (ECSA) normalized activities exhibits the same tendency as the geometrically normalized ones. Varied adsorption capacities of the H2O, OH, and H intermediate species for water reduction on these typical phases are assumed to be responsible for the significantly different HER performance of the NiMoO4-(T)air-H2 electrodes through density functional theory analysis. Poor adsorption of H, OH radicals, and H2O on β-NiMoO4 impedes the water dissociation process, which may be the reason that it exhibits the worst electrocatalytic hydrogen evolution activity. Optimized adsorption abilities of H, OH, and H2O on α-NiMoO4 benefit the water reduction kinetics, leading to an enhanced electrocatalytic HER performance. MoNi4 forms the strongest interactions with H2O, H, and OH species, contributing to the best electrocatalytic hydrogen evolution activity. Further analysis of the energy barrier of the water-splitting reaction shows that these three crystalline phases exhibit different water dissociation ability, which is attributed to their varied adsorption capacities of the intermediate species for water reduction. Among them, MoNi4 and β-NiMoO4exhibit the lowest and highest water dissociation barriers, respectively, in line with their electrocatalytic hydrogen evolution activities. The phase-dependent HER activity identified in this work can provide guidelines for rationally designing and adjusting the structures of active NiMo(O) electrocatalysts.