Catalytic Activities of Sulfur Atoms in Amorphous Molybdenum Sulfide for the Electrochemical Hydrogen Evolution Reaction

Abstract

The catalytic activities of sulfur sites in amorphous MoS x for the electrochemical hydrogen evolution reaction (HER) was investigated in aqueous 0.5 M H 2 SO 4 electrolyte. Using X-ray photoelectron spectroscopy and linear sweep voltammetry, we found the turnover frequency for H 2 production to increase linearly with the percentage of S atoms with higher electron binding energies. These S atoms could be apical S 2– and/or bridging S 2 2– . To distinguish the catalytic performances of these two types of atoms, we turn to quantum chemical simulations using density functional theory. The apical S 2– atoms were found to adsorb H weakly with a Gibbs free energy for atomic H adsorption (Δ G H ) in excess of +1 eV, and were thus ruled out as reaction sites for HER. In situ Raman spectroscopy of the model [Mo 3 S 13 ] 2– cluster further demonstrate the higher catalytic reactivity of the bridging S 2 2– over terminal S 2 2– (which have lower electron binding energy) for proton reduction.