Sulfur-Rich MoS₆ as an Electrocatalyst for the Hydrogen Evolution Reaction

In this work, we studied a sulfur-rich molybdenum sulfide (MoS6) hydrogen evolution reaction (HER) catalyst prepared by chemical oxidation of thiodimolybdate (Mo2S122–). Using X-ray photoelectron spectroscopy, X-ray diffraction, and other techniques, we found that the material had an overall S:Mo ratio of 6:1 and is likely an amorphous coordination polymer of molybdenum–sulfur (Mo3-like) clusters with the additional sulfur existing as bridging or terminal disulfide moieties. Used as a HER catalyst, MoS6 has an HER overpotential of 130 mV at 10 mA/cm2, compared to 178 mV for MoS4 samples made from tetrathiomolybdate (MoS42–). Electrochemical surface area measurements show that the improved performance of MoS6 over MoS4 was not due to the sulfur-rich material having a higher surface area. Instead, we ascribe the performance of the sulfur-rich MoS6 catalyst to the presence of additional disulfide active sites compared to other amorphous MoSx catalysts. This study highlights the possibility of using different thiomolybdate precursors to synthesize novel MoSx HER catalysts.