Sulfur promotes hydrogen evolution on molybdenum carbide catalysts

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.