Catalytic activity of MS₂ monolayer for electrochemical hydrogen evolution

Finding earth-abundant and inexpensive materials to catalyze the electrochemical hydrogen evolution reaction (HER) is critical for turning hydrogen into a competitive clean energy source. Breakthrough discoveries of high activity toward HER have been reported using two-dimensional transition metal dichalcogenides. Here, we performed a comprehensive investigation on the three most common MS₂ single-layer materials, with M being Mo, W, V, and with ab initio calculations to lay a theoretical framework on the dynamic stability and HER activity of the common polymorphs (1H, 1T, and ZT phases) of these MS₂ monolayers. First, our calculations show that the HER catalytic activity of these MS₂ monolayers generally reduces with the increase of hydrogen coverage, except that, although the HER performance of 1T-VS₂ is indeed best at low hydrogen coverage, the HER performance of 1H-VS₂ is best when the coverage is large. In addition to the effects of hydrogen coverage, we show that the HER activity of WS₂ can be improved more effectively by strain than the other two counterparts. Finally, but also most importantly, our calculations show a ranking of HER activity among these three types of MS₂ monolayers being VS₂ > MoS₂ > WS₂, at least for their basal planes under standard conditions. This theoretical analysis calls for additional experimental studies of HER activity in this subject despite the current belief of the superiority of ZT-WS₂. Turning our attention to the case of VS₂, we show that both the 1H and the 1T phases of VS₂ monolayers are dynamically stable and HER-active. Further, by taking spin polarization into consideration, we show that VS₂ monolayers, unlike their WS₂ and MoS₂ counterparts, possess spin-specific HER activity; as such, ignoring spin effects will not get an accurate picture of HER properties of VS₂ monolayers.