Crystal Structure and Composition-Dependent Electrocatalytic Activity of Ni–Mo Nanoalloys for Water Splitting To Produce Hydrogen

Electrocatalytic water splitting presents an exciting opportunity to produce environmentally benign fuel to power human activities and reduce reliance on fossil fuels. Transition metal nanoparticles (NPs) and their alloys are emerging as promising candidates to replace expensive platinum group metal (PGM) catalysts. Herein, we report the synthesis of distinct crystal phases and compositions of Ni1–x Mo x alloy NPs as low-cost, earth-abundant electrocatalysts for the hydrogen evolution reaction (HER) in alkaline medium. Phase-pure cubic and hexagonal Ni and Ni1–x Mo x alloy NPs, with sizes ranging from 18 to 43 nm and varying Mo composition (∼0–11.4%), were produced by a low-temperature colloidal chemistry method. As-synthesized NPs show spherical to polygonal morphologies and a systematic shifting of Bragg reflections to lower 2θ angles with increasing Mo, suggesting the growth of homogeneous alloys. XPS analysis indicates the dominance of metallic Ni(0) and Mo(0) species in the core of the alloy NPs as well as the presence of higher valent Ni n+ and Mo n+ surface species, stabilized by surfactant ligands. The cubic alloys exhibit significantly higher HER activity in comparison to the hexagonal alloys. For a current density of −10 mA/cm2, the cubic alloys demonstrate overpotentials of −62 to −177 mV compared to −162 to −242 mV for the hexagonal alloys. The overpotentials of cubic alloys are comparable to the commercial Pt-based electrocatalysts for which the overpotentials range from −68 to −129 mV at −10 mA/cm2. In general, a decrease in overpotential and an increase in HER activity were observed with increasing concentration of Mo (up to 6.6%) for the cubic alloys. The cubic Ni0.934Mo0.066 alloy NPs exhibit the highest activity as alkaline HER electrocatalysts.