Mechanistic study on nickel-molybdenum based electrocatalysts for the hydrogen evolution reaction

Abstract

A series of graphene nanosheet (GNS) supported nickel-molybdenum nanomaterials with controllable phases and compositions were synthesized for the hydrogen evolution reaction (HER) in both acidic and alkaline solutions. Experimental results indicate that the Ni-Mo2C/GNS catalysts show superior activity in 0.5 M H2SO4 owing to the presence of abundant heterogeneous interfaces. Notably, the 1Ni-0.5Mo2C/GNS possesses an apparent current density of 10 mA cm−2 at an overpotential of 49.6 mV and a turnover frequency (TOF) value of 0.86 s−1 at an overpotential of 200 mV. Such remarkable activity renders the 1Ni-0.5Mo2C/GNS to be one of the best non-precious HER electrocatalysts in acidic electrolytes reported in the literature. In contrast, the Ni4Mo/GNS catalyst exhibits better activity in 1 M KOH solution due to the formation of Ni4Mo alloy. It yields a current density of 10 mA cm−2 at an overpotential of 69 mV and TOF value of 0.11 s−1 at an overpotential of 100 mV. Density functional theory calculations indicate that the superior activity of the Ni-Mo2C/GNS in acidic solutions is related to the appropriate hydrogen adsorption free energy, which is highly beneficial for the Tafel step. However, the leaching of nickel from the catalysts was noted in 0.5 M H2SO4, thus leading to unsatisfactory durability. The better activity of the Ni4Mo/GNS in alkaline solutions was attributed to the low energy barrier of water activation on Ni4Mo alloy surface, which facilitates the dissociation of water molecules. The electrocatalysts showed enhanced durability in alkaline electrolytes. These findings from this work are valuable for the further exploration of advanced Ni-Mo based HER electrocatalysts.