An Electrostatically Embedded QM/MM Scheme for Electrified Interfaces

Abstract

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.

Electrochemical Potential-Dependent Stability and Activity of MoS3 during the Hydrogen Evolution Reaction

“…However, their optimal working regimes depend on the active sites involved. Note that modeling the solvent effects more explicitly (e.g., via QM/MM or DFT-based molecular dynamics , ) would provide an even more comprehensive understanding, especially if coupled with the computationally expensive determination of transition states. ,− When hydrogen release involves the SH site on the Mo 3 S 8 cluster, MoS 3 is more promising. By contrast, MoS 2 is more promising when water is involved, and hydrogen release occurs via OH sites located on the Mo-edge.…”

2023 Roadmap on molecular modelling of electrochemical energy materials

“…Then, DFT calculations are employed to extract the physicochemical properties of c-MOF and the corresponding reduction potentials of the representative species in the EDL. In addition to the previously postulated models, an alternative approach involving the amalgamation of quantum mechanical/molecular mechanical (QM/MM) principles can be employed to achieve commensurate functionality [98,99]. This model elucidates the interface by DFT, while solvent effects are captured through molecular mechanical frameworks [98,99].…”

“…In addition to the previously postulated models, an alternative approach involving the amalgamation of quantum mechanical/molecular mechanical (QM/MM) principles can be employed to achieve commensurate functionality [98,99]. This model elucidates the interface by DFT, while solvent effects are captured through molecular mechanical frameworks [98,99]. Currently, research on MOFs via QM/MM models predominantly fixates upon the intrinsic properties of MOF materials, such as reaction mechanisms [100,101].…”

Incorporating Electrolyte Correlation Effects into Variational Models of Electrochemical Interfaces

“…As a way out of this dilemma, the majority of methods utilize some kind of hybridization scheme that describes electrode and electrolyte regions at distinct levels of theory . While electronic effects in the metal must be treated quantum mechanically at the level of DFT, treatments of electrolyte effects vary from force field-based molecular dynamics methods (MD) − to approaches based on continuous density distributions for solvent and ions. ,− …”