Abstract
An explicit potential model is applied to all thermodynamic states and kinetic barriers of the hydrogen evolution reaction (HER) on Au(111) and graphene supported molybdenum disulfide (MoS2). Due to computational expense, explicit potential models are often only applied to electrochemical barriers in computational studies on electrocatalysis. However, new methods implemented in the solvation extension to the Vienna ab initio simultaion package (VASPsol) offer a computationally tractable way of applying an explicit potential. Here, the VASPsol software is employed to calculate the reaction energies and activation barriers of the HER on supported MoS2 in the presence of an explicit potential.
The optimal thermodynamic hydrogen adsorption sites were first calculated. The results showed that the energy required to adsorb a hydrogen atom calculated with an explicit potential model differs significantly when compared to literature values calculated using an implicit potential model. This was true for most adsorption sites and across both supported MoS2 systems. These results indicate that it may be important to include an explicit potential model when studying the thermodynamics of electrocatalytic reactions computationally. However, further work extending this thermodynamic study to other types of catalysts both within and outside of the dichalcogenide family of MoS2 is needed to confirm that this result is more general.
Thereafter, a number of protocols taking advantage of transition state theory methods were developed for finding and refining transition states in VASPsol. These protocols were then applied to the Volmer-Heyrovský and Volmer-Tafel mechanisms of the HER on supported MoS2. Quantitatively, the calculated rates for these mechanisms were several orders of magnitude smaller than experiment, suggesting that, under experimental conditions, other processes are likely to contribute to the rate of reaction beyond the lowest E pathway calculated in this work. In qualitative agreement with experiment it was found that the HER occurs most readily on Au(111) supported MoS2 via the Volmer-Heyrovský mechanism.
As with the thermodynamics, the explicitly applied potential had a significant effect on the reaction kinetics. Interestingly, these kinetic results did not agree with a similar study by Ruffman et al., where all barriers bar the electrochemical Heyrovský desorption barrier were modelled with an implicit potential. This was an interesting finding as even those barriers that were not electrochemical in nature were affected by the explicit potential model.
These results indicate that an explicitly applied potential has a significant effect on the reaction energies and activation barriers of the electrocatalytic HER. This may have wider implications for computational studies on electrocatalytic processes, such as N2 and CO2 reduction, where an implicit potential is often applied. However, further work is required to ascertain whether these results arise due to the explicitly applied potential model or perhaps a property of the particular catalyst, MoS2.