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-NiMoO₄ exhibit 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.

Key words: Electrocatalysis, Water splitting, Hydrogen, Nickel molybdate, Crystalline Phase