Atomistic modeling of electrified interfaces remains a major issue for detailed insights in electrocatalysis, corrosion, electrodeposition, batteries, and related devices such as pseudocapacitors. In these domains, the use of grand-canonical density functional theory (GC-DFT) in combination with implicit solvation models has become popular. GC-DFT can be conveniently applied not only to metallic surfaces but also to semiconducting oxides and sulfides and is, furthermore, sufficiently robust to achieve a consistent description of reaction pathways. However, the accuracy of implicit solvation models for solvation effects at interfaces is in general unknown. One promising way to overcome the limitations of implicit solvents is going toward hybrid quantum mechanical (QM)/molecular mechanics (MM) models. For capturing the electrochemical potential dependence, the key quantity is the capacitance, i.e., the relation between the surface charge and the electrochemical potential. In order to retrieve the electrochemical potential from a QM/MM hybrid scheme, an electrostatic embedding is required. Furthermore, the charge of the surface and of the solvent regions has to be strictly opposite in order to consistently simulate charge-neutral unit cells in MM and in QM. To achieve such a QM/MM scheme, we present the implementation of electrostatic embedding in the VASP code. This scheme is broadly applicable to any neutral or charged solid/liquid interface. Here, we demonstrate its use in the context of GC-DFT for the hydrogen evolution reaction (HER) over a noble-metal-free electrocatalyst, MoS2. We investigate the effect of electrostatic embedding compared to the implicit solvent model for three contrasting active sites on MoS2: (i) the sulfur vacancy defect, which is rather apolar; (ii) a Mo antisite defect, where the active site is a surface bound highly polar OH group; and (iii) a reconstructed edge site, which is generally believed to be responsible for most of the catalytic activity. According to our results, the electrostatic embedding leads to almost indistinguishable results compared to the implicit solvent for the apolar system but has a significant effect on polar sites. This demonstrates the reliability of the hybrid QM/MM, electrostatically embedded solvation model for electrified interfaces.
Atomistic modelling of electrified interfaces remains a major issue for detailed insights in electrocatalysis, corrosion, electrodeposition, batteries and related devices such as pseudocapacitors. In these domains, the use of grand-canonical density functional theory (GC-DFT) in combination with implicit solvation models has become popular. GC-DFT can be conveniently applied not only to metallic surfaces, but also to semi-conducting oxides and sulfides and is, furthermore, sufficiently robust to achieve a consistent description of reaction pathways. However, the accuracy of implicit solvation models for solvation effects at interfaces is in general unknown. One promising way to overcome the limitations of implicit solvents is going towards hybrid quantum mechanical (QM)/molecular mechanics (MM) models. For capturing the electrochemical potential dependence, the key quantity is the capacitance, i.e., the relation between the surface charge and the electrochemical potential. In order to retrieve the electrochemical potential from a QM/MM hybrid scheme, an electrostatic embedding is required. Furthermore, the charge of the surface and of the solvent regions have to be strictly opposite in order to consistently simulate charge-neutral unit cells in MM and in QM. To achieve such a QM/MM scheme, we present the implementation of electrostatic embedding in the popular VASP code. This scheme is broadly applicable to any neutral or charged solid/liquid interface. Here, we demonstrate its use in the context of GC-DFT for the hydrogen evolution reaction (HER) over a noble-metal-free electrocatalyst, MoS2. We investigate the effect of electrostatic embedding compared to the implicit solvent model for three contrasting active sites on MoS2: (i) the sulfur vacancy defect which is rather apolar. (ii) an Mo anti-site defect, where the active site is a surface bound highly polar OH group and (iii) a reconstructed edge site which is generally believed to be responsible for most of the catalytic activity. Our results demonstrate that electrostatic embedding leads to almost indistinguishable results compared to the implicit solvent for apolar systems, but has a significant effect on polar sites. This demonstrates the reliability of the hybrid QM/MM, electrostatically embedded solvation model for electrified interfaces.
In this work, we present a detailed mechanistic study of HER at the sulfur vacancy Vs. We evaluate the Volmer, Tafel, and Heyrovsky transition states for the different possible reaction steps, considering the activation energy as a function of electrochemical potential. The results show that the Volmer and Heyrovsky steps depend on the electrochemical value and the activation energies decrease for more negative potential values, while this is not the case for the Tafel step, where the activation energy is essentially constant. From the activation energy values at -0.2 V, it can be concluded that to release H2 at Vs, we follow two Volmer steps and then a Heyrovsky step, since they have the lowest activation energies compared to the others. Heyrovsky is the rate-determining step. In addition, we investigate for the first time the effect of the support on the conductivity of MoS2 and the HER activity of sulfur vacancies. Our results show that copper, gold and graphite supports have no effect on the barrier energies of all steps of the HER mechanism.
Molybdenum disulfide (MoS2) is considered one of the most likely materials that could be turned into low-cost hydrogen evolution reaction (HER) catalysts to replace noble metals in acidic medium. However, several challenges prevent MoS2 from being truly applicable, including limited number of active sites (typically only the edges are active) and poor conductivity. In this work, we perform an extensive density func- tional theory (DFT) screening of substitutional doping as a possibility to activate the otherwise inert basal surface. We assess 17 Earth abundant elements for molybdenum doping and 5 elements (N, O, P, Se and Te) for sulfur substitution. Systematically de- termining the preference of the metallic dopants to be located on the edges rather than in the basal plane, we reveal that most dopants are much more likely to be incorpo- rated at the edges, suggesting that advanced synthesis methods are required to obtain basal-plane doped catalysts. The latter may, however, feature many more active sites per MoS2 formula unit, motivating our study on the properties of such substitutionally doped surfaces. For the first time for such a screening study, we explore not only the adsorption of H, but also of OH and H2O to explore the solvent effect since the reac- tion takes place in an aqueous medium. Two additional phenomena that could hinder the hydrogen production at these sites are investigated, namely H2S release and the (local) segregation/dispersion tendency of the dopants in the basal surface. Moreover, to assess the electrocatalytic activity, we take the electrochemical potential explicitly into account via grand canonical DFT in combinations. Compared with pristine MoS2 nanosheets, our results show that most doping elements significantly enhanced the elec- trocatalytic activity. Considering all assessed factors, we identify the most promising systems: Dimers of Ti, Zr and Hf and the substitution of S by P are predicted to lead to stable active sites on the basal plane with overpotentials of about 0.2 V.
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