Revisiting the Active Sites at the MoS2/H2O Interface via Grand-Canonical DFT: The Role of Water Dissociation

Uncovering the Interfacial Strengthening Mechanisms of α-Mg/Mg2Sn/β-Li Interfaces Using First-Principle Calculations and HAADF-STEM

Previously, we reported our new invention of an ultralight (ρ = 1.61 g/cm3) and super high modulus (E = 64.5 GPa) Mg-Li-Al-Zn-Mn-Gd-Y-Sn (LAZWMVT) alloy. Surprisingly, the minor additions of Sn contribute to significant strength and stiffness increases. In this study, we found that Mg2Sn was not only the simple precipitate but also acted as the glue to bind the α-Mg/β-Li interface in a rather complicated way. To explore its mechanism, we have performed first-principle calculations and HAADF-STEM experiments on the interfacial structures. It was found that the interfacial structural models of α-Mg/β-Li, α-Mg/Mg2Sn, and β-Li/Mg2Sn composite interfaces prefer to form α-Mg/Mg2Sn/β-Li ternary composite structures due to the stable formation enthalpy (ΔH: -1.95 eV/atom). Meanwhile, the interface cleavage energy and critical cleavage stress show that Mg2Sn contribute to the interfacial bond strength better than the β-Li/α-Mg phase bond strength (σb(β-Li/Mg2Sn): 0.82 GPa > σb(α-Mg/Mg2Sn): 0.78 GPa > σb(β-Li/α-Mg): 0.62 GPa). Based on the interfacial electronic structure analysis, α-Mg/Mg2Sn and β-Li/Mg2Sn were found to have a denser charge distribution and larger charge transfer at the interface, forming stronger chemical bonds. Additionally, according to the crystal orbital Hamiltonian population analysis, the bonding strength of the Mg-Sn atom pair was 2.61 eV, which was higher than the Mg-Li bond strength (0.39 eV). The effect of the Mg2Sn phase on the stability and interfacial bonding strength of the alloying system was dominated by the formation of stronger and more stable Mg-Sn metal covalent bonds, which mainly originated from the contribution of the Mg 3p-Sn 5p orbital bonding states.

Next-Generation Green Hydrogen: Progress and Perspective from Electricity, Catalyst to Electrolyte in Electrocatalytic Water Splitting

Green hydrogen from electrolysis of water has attracted widespread attention as a renewable power source. Among several hydrogen production methods, it has become the most promising technology. However, there is no large-scale renewable hydrogen production system currently that can compete with conventional fossil fuel hydrogen production. Renewable energy electrocatalytic water splitting is an ideal production technology with environmental cleanliness protection and good hydrogen purity, which meet the requirements of future development. This review summarizes and introduces the current status of hydrogen production by water splitting from three aspects: electricity, catalyst and electrolyte. In particular, the present situation and the latest progress of the key sources of power, catalytic materials and electrolyzers for electrocatalytic water splitting are introduced. Finally, the problems of hydrogen generation from electrolytic water splitting and directions of next-generation green hydrogen in the future are discussed and outlooked. It is expected that this review will have an important impact on the field of hydrogen production from water.

Sulfur-deficient edges as active sites for hydrogen evolution on MoS2

A grand-canonical approach is employed to calculate the voltage-dependent activation energy and estimate the kinetics of the hydrogen evolution reaction (HER) on intrinsic sites of MoS2, including edges of varying S-coverage as well as S-vacancies on the basal plane. Certain edge configurations are found to be vastly more active than others, namely S-deficient edges on the Mo-termination where, in the fully S-depleted case, HER can proceed with activation energy below 0.5 eV at an electrode potential of 0 V vs. SHE. There is a clear distinction between the performance of Mo-rich and S-rich adsorption sites, as HER at the latter sites is characterized by large (generally above 1.5 eV) Heyrovsky and Tafel energy barriers despite near-thermoneutral hydrogen adsorption energy. Thus, exposing Mo-atoms on the edges to which hydrogen can directly bind is crucial for efficient hydrogen evolution. While S-vacancies on the basal plane do expose Mo-rich sites, the energy barriers are still significant due to high coordination of the Mo atoms. Kinetic modelling based on the voltage-dependent reaction energetics gives a theoretical overpotential of 0.25 V and 1.09 V for the Mo-edge with no S atoms and the weakly sulfur-deficient (2% S-vacancies) basal plane, respectively, with Volmer-Heyrovsky being the dominant pathway. These values coincide well with reported experimentally measured values of the overpotential for the edges and basal plane. For the partly Mo-exposed edges, the calculated overpotential is 0.6-0.7 V while edges with only S-sites give overpotential exceeding that of the basal plane. These results show that the overpotential systematically decreases with increased sulfur-deficiency and reduced Mo-coordination. The fundamental difference between Mo- and S-rich sites suggests that catalyst design of transition metal dichalcogenides should be focused on facilitating and modifying the metal sites, rather than activating the chalcogen sites.

Substantial impact of surface charges on electrochemical reaction kinetics on S vacancies of MoS2 using grand-canonical iteration method

The surface charges of catalysts have intricate influences on the thermodynamics and kinetics of electrochemical reactions. Herein, we develop a grand-canonical iteration method based on density functional theory calculations to explore the effect of surface charges on reaction kinetics beyond the traditional Butler-Volmer picture. Using the hydrogen evolution reaction on S vacancies of MoS2 as an example, we show how to track the change of surface charge in a reaction and to analyze its influence on the kinetics. Protons adsorb on S vacancies in a tough and charge-insensitive water splitting manner, which explains the observed large Tafel slope. Grand-canonical calculations report an unanticipated surface charge-induced change of the desorption pathway from the Heyrovsky route to a Volmer-Tafel route. During an electrochemical reaction, a net electron inflow into the catalyst may bring two effects, i.e., stabilization of the canonical energy and destabilization of the charge-dependent grand-canonical part. On the contrary, a net outflow of electrons from the catalyst can reverse the two effects. This surface charge effect has substantial impacts on the overpotential and the Tafel slope. We suggest that the surface charge effect is universal for all electrochemical reactions and significant for those involving interfacial proton transfers.

Electrocatalytic study of the hydrogen evolution reaction on MoS2/BP and MoSSe/BP in acidic media

Molecular hydrogen (H2) production by the electrochemical hydrogen evolution reaction (HER) is being actively explored for non-precious metal-based electrocatalysts that are earth-abundant and low cost like MoS2. Although it is acid-stable, its applicability is limited by catalytically inactive basal planes, poor electrical transport and inefficient charge transfer at the interface. Therefore, the present work examines its bilayer van der Waals heterostructure (vdW HTS). The second constituent monolayer boron phosphide (BP) is advantageous as an electrode material owing to its chemical stability in both oxygen and water environments. Here, we have performed first-principles based calculations under the framework of density functional theory (DFT) for the HER in an electrochemical double layer model with the BP monolayer, MoS2/BP and MoSSe/BP vdW HTSs. The climbing image nudged elastic band method (CI-NEB) has been employed to determine the minimum energy pathways for Tafel and Heyrovsky reactions. The calculations reveal that the Tafel reaction shows no reaction barrier. Thereafter, for the Heyrovsky reaction, we obtained a low reaction barrier in the vdW HTSs as compared to that in the BP monolayer. Subsequently, we have observed no significant difference in the reaction profile of MoS2/BP and MoSSe/BP vdW HTSs in the case of 2 × 2 supercell configuration. However, in the case of 3 × 3 and 4 × 4 configurations, MoSSe/BP shows a feasible Heyrovsky reaction with no reaction barrier. The coverages with 1/4H+ concentration (conc.) deduced high coverage with low conc. and low coverage with high conc. to be apt for the HER via the Heyrovsky reaction path. Finally, on observing the activation barrier of the Heyrovsky pathway along with that of second H adsorption at the surface, the Heyrovsky path is expected to be favoured.

An Electrostatically Embedded QM/MM Scheme for Electrified Interfaces

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.

pH- and Facet-Dependent Surface Chemistry of TiO2 in Aqueous Environment from First Principles

TiO₂ is a relevant catalytic material, and its chemistry in aqueous environment is a challenging aspect to address. Also, the morphology of TiO₂ particles at the nanoscale is often complex, spanning from faceted to spherical. In this work, we study the pH- and facet-dependent surface chemistry of TiO₂/water interfaces by performing ab initio molecular dynamics simulations with the grand canonical formulation of species in solution. We first determined the acid-base equilibrium constants at the interface, which allows us to estimate the pH at the point of zero charge, an important experimental observable. Then, based on simulated equilibrium constants, we predict the amount of H⁺, OH^-, and adsorbed H₂O species present on the surfaces as a function of the pH, a relevant aspect for water splitting semi-reactions. We approximated the complex morphology of TiO₂ particles by considering the rutile (110) and (011), and anatase (101), (001), and (100) surfaces.

Ion Solvation Free Energy Calculation Based on Ab Initio Molecular Dynamics Using a Hybrid Solvent Model

Free energy calculation of small molecules or ion species in aqueous solvent is one of the most important problems in electrochemistry study. Although there are many previous approaches to calculate such free energies, they are based on ab initio methods and suffer from various limitations and approximations. In the current work, we developed a hybrid approach based on ab initio molecular dynamics (AIMD) simulations to calculate the ion solvation energy. In this approach, a small water cluster surrounding the central ion is used, and implicit solvent model is applied outside the water cluster. A dynamic potential well is used during AIMD to keep the water cluster together. Quasi-harmonic approximation is used to calculate the entropy contribution, while the total energy average is used to calculate the enthalpy term. The obtained solvation voltages of the bulk metal agree with experiments within 0.3 eV, and the simulation results for the solvation energies of gaseous ions are close to the experimental observations. Besides the free energies, radial pair distribution functions and coordination numbers of hydrated cations are also obtained. The remaining challenges of this method are also discussed.

Nitric oxide electrochemical reduction reaction on transition metal-doped MoSi2N4 monolayers

Nitric oxide electrochemical reduction (NOER) reactions are usually catalyzed by noble metals. However, the commercial applications are limited by the low atomic utilization and high price, which prompt researchers to turn their attentions to single-atom catalysts (SACs). Recently, a novel two-dimensional semiconducting material MoSi₂N₄ (MSN) has been synthesized and is suitable for the substrate of SACs due to its high stability, carrier mobility and mechanical strength. Herein, we employed first principles calculations to investigate the catalytic properties of transition metal doped MoSi₂N₄ monolayers (labelled as TM-MSN, where TM is a transition metal atom from 3d to 5d except Y, Tc, Cd, La-Lu and Hg) in NO reduction. The calculated results demonstrate that the introduction of Zr, Pd, Pt, Mn, Au, or Mo atoms can greatly improve the catalytic NOER performance of a pristine MSN monolayer. Zr-MSN and Pt-MSN monolayers at low coverage exhibit the most superior catalytic activity and selectivity for NH₃ production with a limiting potential of 0 and -0.10 V. This work may help guide the application of MSN monolayer in the area of energy conversion.

Perspectives on the Competition between the Electrochemical Water and N2 Oxidation on a TiO2(110) Electrode

The electrochemical nitrogen oxidation reaction (NOR) has recently drawn attention due to promising experimental and theoretical results. It provides an alternative, environmentally friendly route to directly synthesize nitrate from N₂(g). There is to date a limited number of investigations focused on the electrochemical NOR. Herein, we present a detailed computational study on the kinetics of both the NOR and the competing oxygen evolution reaction (OER) on the TiO₂(110) electrode under ambient conditions. The use of grand canonical density functional theory in combination with the linearized Poisson-Boltzmann equation allows a continuous tuning of the explicitly applied electrical potential. We find that the OER may either promote or suppress the NOR on TiO₂(110) depending on reaction conditions. The detailed atomistic insights provided on the mechanisms of these competing processes make possible further developments toward a direct electrochemical NOR process.

Computational screening of transition-metal doped boron nanotubes as efficient electrocatalysts for water splitting

The search for efficient and low-cost electrocatalysts for the oxygen evolution reaction (OER) and the hydrogen evolution reaction (HER) is of utmost importance for the production of hydrogen and oxygen via water splitting. In this work, the catalytic performance of the OER and HER on transition metal doped boron nanotubes (BNTs) was investigated using density functional theory. It was found that the doped transition metal atoms determine the catalytic activity of the BNTs. Rhodium-doped BNTs exhibited excellent OER activity, while cobalt-doped BNTs displayed great catalytic activity toward the HER. Volcano relationships were found between the catalytic activity and the adsorption strength of reaction intermediates. Rhodium- and cobalt-doped BNTs exhibited great OER and HER catalytic activity due to the favorable adsorption strength of reaction intermediates. This work sheds light on the design of novel electrocatalysts for water splitting and provides helpful guidelines for the future development of advanced electrocatalysts.

Rhodium-doped BNTs demonstrated excellent OER activity, while cobalt-doped BNTs exhibited the best catalytic activity toward the HER among 12 different transition metal-doped BNTs.

Ab initiomodelling of interfacial electrochemical properties: beyond implicit solvation limitations

First-principles calculations are an important tool to investigate the complex processes occurring at solid/liquid interfaces which are at the heart of modern technologies. Currently, capturing the whole electrochemical environment at an interface, including the applied potential and solvation, still remains challenging as it necessitates to couple different approaches whose interactions are not fully understood. In this work, a grand canonical density functional theory approach is coupled with solvation models to investigate the electrochemical interfaces under applied potential. We show that a parametrized polarizable continuum model (PCM) which represent solvation in a mean field approach by a continuous polarizable media, possesses catastrophic limitations for the modelling of ionic and charged interfaces. We reveal the origin of PCM instabilities under chemical or electrochemical strong oxidation to be the consequence of a phase transition in the surface Li electronic structure. Thus, PCM undergoes an unphysical response to this phase transition by penetrating within the atomic radius of surface Li atoms. To recover a physical response, an explicit first solvation shell has to be included in addition to the PCM in order to properly describe the electrochemistry of the interface. The Fukui functions show that the first solvation shell becomes involved in the redox process as solvent electron doublet is transferred to the acidic Li⁺. If another explicit solvent layer is added, the interface electrochemical properties become independent of the PCM parameters: in particular capacitance can then be computed from a parameter-free electrochemical approach. This is an important conclusion as the experimental electrochemical capacitance are not easily found and thus the parametrization of the PCM for electrochemical interface can be difficult. This approach can easily be applied to investigate electrochemical properties at the atomic scale and generalized to any electrochemical device for which interfaces play a crucial role.

Theoretical Study of an almost Barrier-Free Water Dissociation on a Platinum (111) Surface Alloyed with Ruthenium and Molybdenum

A theoretical study based on density functional theory for H₂O dissociation on the metal surface of Pt(111) alloyed simultaneously with Ru and Mo was performed. The determination of the minimum energy path using the climbing image nudged elastic band (CI-NEB) method shows that the dissociation reaction of H₂O with this catalyst requires almost no energy cost. This dissociation reaction is not only kinetically favored but also almost thermodynamically neutral and somewhat exothermic. The electronic structure analysis showed that much more charge was released in Mo and was used to bind the adsorbed hydroxyl (OH_ad). Further analyses of the density of states (DOS) showed that the large number of orbitals that overlap when OH binds to Mo are responsible for the stabilization of the OH-surface bond. The stability of the OH_ad fragment on the surface is believed to be a descriptor for the dissociation of H₂O with an almost spontaneous process.