Sulfur promotes hydrogen evolution on molybdenum carbide catalysts

Abstract

We report the synthesis of sulfur-doped molybdenum carbide (Mo₂C) catalysts with high activity towards electrochemical hydrogen evolution and excellent stability under acidic conditions. We show that sulfur – a well-known catalyst poison – acts as a promoter by weakening the hydrogen binding energy of Mo₂C, which is generally considered to bind hydrogen too strongly. Physical and chemical characterizations of sulfur-doped Mo₂C catalysts suggest that sulfur is successfully doped into the Mo₂C crystal lattice. We found the optimal sulfur loading to be around 7 wt%, which required −92 mV overpotential for −10 mA cm⁻² with Tafel slope value of 51 mV dec⁻¹. In our analysis, the sulfur atoms in the lattice occupied the original hydrogen adsorption sites in the Mo₂C lattice. This weakens the hydrogen binding energy, to which we attribute the higher activity towards hydrogen evolution.