Electrodeposition of MoS_x Hydrogen Evolution Catalysts from Sulfur-Rich Precursors

Amorphous molybdenum sulfides (a-MoS_x) are known to be active electrocatalysts for the hydrogen evolution reaction (HER), but the role stoichiometry of the sulfur atoms plays in the HER activity remains unclear. In this work, we deposited thin films of a-MoS_x from two thiomolybdate deposition baths with different sulfur ratios (MoS₄^2- and Mo₂S₁₂^2-) and showed that the sulfur stoichiometry, as determined by X-ray photoelectron spectroscopy, is controlled by the precursor of choice and the electrochemical method used to deposit the thin films. Using the Mo₂S₁₂^2- precursor allows access to a MoS₆ thin film, with a higher S/Mo ratio compared with that of any previously reported electrodeposited films. We also examined the effect of electrochemistry on the resulting S/Mo ratio in the as-prepared a-MoS_x thin films. Samples with S/Mo ratios ranging from 2 to 6 were electrodeposited on glassy carbon (GC) substrates by using anodic, cathodic, or cyclic voltammetry deposition. The a-MoS_x thin films deposited on GC substrates were tested as HER catalysts in acidic electrolytes. The overpotentials needed to drive current densities of 10 mA/cm² ranged from 160 mV for MoS₆ samples to 216 mV for MoS₂ samples, signifying the important role sulfur content plays in HER activity of the prepared films. Furthermore, we characterized the deactivation of the a-MoS_x films and found that the sulfur content is gradually depleted over time, leading to a slow deactivation of the a-MoS_x thin-film catalysts. We showed a facile procedure that affords a-MoS_x films with high sulfur content by using S-rich precursors and highlighted the role of sulfur in the prepared films for HER.