Efficient hydrogen evolution reaction catalyzed by molybdenum carbide and molybdenum nitride nanocatalysts synthesized via the urea glass route

Abstract

Molybdenum carbide and molybdenum nitride nanoparticles have been developed for catalyzing the hydrogen evolution reaction (HER). These nanocatalysts were synthesized by the ‘urea glass’ route. By simply changing the molar ratio of the urea/metal precursor, α-Mo₂C and γ-Mo₂N nanoparticles, both of which are crystalline, phase pure and monodisperse in size, were obtained. Hydrogen evolution was performed in both 1 M KOH and 0.5 M H₂SO₄ electrolytes, and characterized by linear sweep voltammetry and electrochemical impedance spectroscopy. The as-synthesized Mo₂C showed excellent HER performance especially in KOH. At a catalyst loading of 102 μg cm⁻², a low overpotential of 176 mV was needed to produce 10 mA cm⁻² of H₂. Its measured currents and turnover frequencies for hydrogen evolution at different overpotentials compare favorably against many other recently reported non-precious metal HER catalysts. Online gas chromatography demonstrated that the current efficiency for H₂ production is ∼100%. Both Mo₂C and Mo₂N showed negligible overpotential losses after acceleration degradation tests in acid and alkali. This is noteworthy since very few catalysts are active in these two extreme pHs. An attractive aspect of the α-Mo₂C and γ-Mo₂N nanoparticles for electrochemical hydrogen evolution is that they are simple and well-characterized in chemical and physical composition. The excellent catalytic activity of the α-Mo₂C catalysts can be attributed to their small particle size, which will facilitate a rapid electron transfer for the hydrogen evolution reaction. Our study has placed the as-synthesized α-Mo₂C nanoparticles as highly promising alternatives to platinum in the alkaline water electrolyzer.