Molybdenum disulfide (MoS2) based catalysts for the electrochemical hydrogen evolution reaction (HER) have been widely studied as alternatives to platinum based catalysts due to the earth-abundance and great catalytic activity. Since the edge sites of 2H phase MoS2 were shown to be the active sites for HER, a great number of studies have focused on maximally exposing catalytically active edge-sites through diverse engineering process. Recently, we demonstrated that active sites could be created on the basal plane of 2H-MoS2 by generating sulfur (S)-vacancies. At the S-vacancy sites, the under coordinated Mo atoms introduce gap states that allow for favorable hydrogen binding, introducing the highest per site turnover frequency (TOF) reported for any MoS2-based catalyst for HER. However, the S-vacancies in the basal plane have so far only been generated using controlled argon (Ar) plasma exposure and H2 annealing. A more industrially viable alternative to the argon plasma desulfurization process is needed. To effectively utilize S-vacancies in MoS2 catalysts for industrial applications, a facile, general, and scalable route for generating S-vacancies in MoS2 of any morphology is needed.

Electrochemical desulfurization is one of the possible methods for generating S-vacancies by removing sulfur atoms from the basal plane of MoS2. This method removes the sulfur atoms in the basal plane of 2H-MoS2 to form hydrogen sulfide (H2S) gas through a desulfurizing activation cycle. In this work, we show an electrochemical (EC) desulfurization method for generating S-vacancies in monolayer as well as polycrystalline multilayer MoS2 supported on diverse electrodes. Density functional theory (DFT) calculations show that S-vacancies are expected to be thermodynamically favorable relative to the pristine basal plane at a sufficient negative potential. The concentration of S-vacancies can be controlled by changing the applied desulfurization voltage. These theoretical predictions are experimentally verified with the continuously grown MoS2 monolayers on gold (Au) showing that electrochemically generated S-vacancies are comparable to the recent work about Ar-plasma treated ones. In addition, we demonstrate the generality of the electrochemical desulfurization approach by generating S-vacancies on MoS2 multilayers supported on flat carbon rods and porous carbon foams leading to a significant HER activity enhancement. Finally, we experimentally show that the HER activity is stable under extended desulfurization durations as well as operating durations and that the concentration of S-vacancies and activity can be varied using the applied potential in polycrystalline multilayer MoS2 on carbon foam electrode.