Hydrogen Coverage Research Articles

Transition Metal Atoms Anchored on CuPS3 Monolayer for Enhancing Catalytic Performance of Hydrogen Evolution Reactions

The noble metal such as Pt has been used as the catalysts for hydrogen evolution reaction (HER), but with problems such as scarcity of resources and high cost. Anchoring transition metal atoms onto the catalysts is regarded as a potential approach to solve this problem and enhance the electrocatalytic performance of HER. For this purpose, two-dimensional materials, such as CuPS3 monolayer, are regarded as one of the most ideal carriers for adsorption of metal atoms. However, there is no previous study on this topic. In this paper, we systematically studied microstructures, electronic properties, and electrocatalytic performance of the CuPS3 monolayer anchored with transition metal atoms (e.g., Sc, Ti, V, Cr, Mn, Fe, Co, and Ni) using a density functional theory (DFT). Results showed that all the transition metal atoms are favorably adsorbed onto the CuPS3 monolayer with large binding energies at the top of the Cu atom. The pristine CuPS3 monolayer has a large catalytic inertia for hydrogen evolution reactions, whereas after anchored with transition metal atoms, their catalytic performances have been significantly improved. The Gibbs free energy (ΔGH) is 0.44 eV for the H atom absorbed onto the pristine CuPS3 monolayer, whereas the ΔGH values for the V, Fe, and Ni atoms anchored onto the CuPS3 monolayer are 0.02, 0.11, and 0.09 eV, respectively, which is close to the ΔGH of H atom adsorbed on Pt (e.g., −0.09 eV). At the same time, the influence of hydrogen coverage rate was calculated. The result shows that V adsorbed on CuPS3 monolayer is catalytic active for HER for a large range of hydrogen coverage. Our results demonstrate that anchoring of V atom onto the CuPS3 monolayer is a potentially superior method for making the catalyst for the HER.Graphical abstract

Bandgap engineering of ZnX (X = O, S, Se, Te) QDs/Graphene nanocomposites: Towards the designing of a highly efficient light-harvesting device

Increasing the power conversion efficiency (PCE) of photovoltaic devices (PVDs) is a topic of fundamental importance not only for academia but also for industry. In this study, we systematically investigated the electronic structure of ZnX (X = O, S, Se, and Te) quantum dots (QDs)/Graphene (G) nanohybrid systems by employing the density functional method. To understand the key issues, associated with photoinduced charge generation, generated charge separation, charge transfer process, recombination probability, and variations of PCE, we have considered different combinations of hybrid nanostructures composed with ZnX QD and G. The tuning of the electronic properties of hybrid systems has been done as a function of the size of ZnX QD's, chalcogenides, and hydrogen coverage percentage on G to design a highly efficient hybrid solar cell. We have also explored the photovoltaic efficiency of ZnX/ZnX’ (X/X' = O, S, Se, and Te) core/shell QDs/G to investigate the effect of core/shell QDs in composite nanomaterials. Bandgaps of ZnX QDs/G nanocomposites are very low (0.028eV–0.158 eV) which implies the probability of recombination of photo-induced separated charges at the heterojunction region is very high. After passivation with hydrogen atoms partially on the G, this unsavory circumstance is overcome. We have performed the bandgap and band alignment engineering of ZnX QDs/G hybrid nanostructures by varying the size of QD's and H-coverage percentage on G to form a type-II material which is very important to reduce the recombination rate and ultimately increase the PCE of the hybrid solar cells. Considering hydrogenated graphene (HG)-ZnX QDs nanocomposites we were achieving high PCE values (up to 30.25%), which implies our explored materials efficiency is exceptionally good and competitive with recently explored tandem/hybrid/latest perovskite solar cells.

Substrate effect on hydrogen evolution reaction in two-dimensional Mo2C monolayers

The physical and chemical properties of atomically thin two-dimensional (2D) materials can be modified by the substrates. In this study, the substrate effect on the electrocatalytic hydrogen evolution reaction (HER) in 2D Mo2C monolayers was investigated using first principles calculations. The isolated Mo2C monolayer shows large variation in HER activity depending on hydrogen coverage: it has relatively low activity at low hydrogen coverage but high activity at high hydrogen coverage. Among Ag, Au, Cu, and graphene substrates, the HER activity is improved on the Ag and Cu substrates especially at low hydrogen coverage, while the effects of the Au and graphene substrates on the HER activity are insignificant. The improvement is caused by the charge redistribution in the Mo2C layer on the substrate, and therefore the HER activity becomes high for any hydrogen coverage on the Ag and Cu substrates. Our results suggest that, in two-dimensional electrocatalysis, the substrate has a degree of freedom to tune the catalytic activity.

Synergistic Effects of V and Ni Catalysts on Hydrogen Sorption Kinetics of Mg-Based Hydrogen Storage Materials: A Computational Study

Adding transition metals (TMs) in Mg-based hydrogen storage materials has been proposed as a promising approach to improve their storage performance. It was experimentally shown that adding Ni and V catalysts in Mg dramatically decreased the formation enthalpies and activation energies of hydrogenation and dehydrogenation. Herein, we aim to unravel the roles of Ni and V catalysts in improving the hydrogen absorption process in Mg-based storage materials using first-principles methods. Mg2Ni and V clusters deposited on Mg2Ni structures were modeled, as evidenced by experimental observations. The results indicate that both V and Ni facilitate spontaneous H2 dissociation and stabilize hydrogen adsorption. Such strong interactions stem from the strong hybridization between the molecular orbital of adsorbed hydrogen and the Ni and V 3d states. The addition of the V cluster on the Mg2Ni surface also induces surface reconstruction, and consequently, more strong adsorption sites are available and the sites with connected Ni are formed, which could promote more facile diffusion paths of hydrogen spillover from the cluster to the surface and surface diffusion. Although hydrogen diffusion to a subsurface is the most kinetically limited step at low hydrogen contents, increasing hydrogen coverages reduces such barriers by a half. The high hydrogen coverage also drives surface, subsurface, and under-subsurface diffusion to be highly thermodynamically favorable. The computational results suggest that hydrogen absorption into the V/Mg2Ni material is kinetically and thermodynamically appreciable at operating conditions of high H2 pressure. The catalytic roles of Ni and V for the hydrogen absorption process also agree with the phenomenon seen in ab initio molecular dynamics simulations where the hydrogen absorption process occurs at a significantly faster rate on the Mg2Ni structure and even faster on the V/Mg2Ni structure compared to the pure Mg structure. Through systematic computational investigations, our findings provide in-depth theoretical insights and guidance on using a combination of TM catalysts to improve the performance of Mg-based hydrogen storage materials.

Hydrogen Spillover-Bridged Volmer/Tafel Processes Enabling Ampere-Level Current Density Alkaline Hydrogen Evolution Reaction under Low Overpotential.

Water-alkaline electrolysis holds a great promise for industry-scale hydrogen production but is hindered by the lack of enabling hydrogen evolution reaction electrocatalysts to operate at ampere-level current densities under low overpotentials. Here, we report the use of hydrogen spillover-bridged water dissociation/hydrogen formation processes occurring at the synergistically hybridized Ni3S2/Cr2S3 sites to incapacitate the inhibition effect of high-current-density-induced high hydrogen coverage at the water dissociation site and concurrently promote Volmer/Tafel processes. The mechanistic insights critically important to enable ampere-level current density operation are depicted from the experimental and theoretical studies. The Volmer process is drastically boosted by the strong H2O adsorption at Cr5c sites of Cr2S3, the efficient H2O* dissociation via a heterolytic cleavage process (Cr5c-H2O* + S3c(#) → Cr5c-OH* + S3c-H#) on the Cr5c/S3c sites in Cr2S3, and the rapid desorption of OH* from Cr5c sites of Cr2S3 via a new water-assisted desorption mechanism (Cr5c-OH* + H2O(aq) → Cr5c-H2O* + OH-(aq)), while the efficient Tafel process is achieved through hydrogen spillover to rapidly transfer H# from the synergistically located H-rich site (Cr2S3) to the H-deficient site (Ni3S2) with excellent hydrogen formation activity. As a result, the hybridized Ni3S2/Cr2S3 electrocatalyst can readily achieve a current density of 3.5 A cm-2 under an overpotential of 251 ± 3 mV in 1.0 M KOH electrolyte. The concept exemplified in this work provides a useful means to address the shortfalls of ampere-level current-density-tolerant Hydrogen evolution reaction (HER) electrocatalysts.

Modulation of the B4N monolayer as an efficient electrocatalyst for hydrogen evolution reaction

Exploring stable catalysts with an efficient hydrogen evolution reaction (HER) arises intense concerns due to its renewable and low-cost properties. In this work, we have systematically investigated a two-dimensional (2D) material, namely, B 4 N monolayer as efficient HER electrocatalysts based on first-principles computations. When the material is metal-free, the calculated Gibbs free energy (ΔG H∗ ) corresponding to hydrogen coverages of 2/4 reaches to 0.005 eV, which is better than that of the Pt catalyst. Moreover, we also find that the HER activity of the B 4 N monolayer is sensitive to the strains-driven. The single metal atom supported on B 4 N can still make the value of ΔG H∗ close to 0 eV for Cr/B 4 N and V/B 4 N. These results reveal that the B 4 N monolayer is a promising candidate for HER applications. • The B 4 N monolayer is a promising candidate for HER applications. • Hydrogen coverage and external strain engineering can improve HER activity. • The HER performance can be enhanced with doping of single transition metal (TM) atom.

Two-dimensional B7P2: Dual-purpose functional material for hydrogen evolution reaction/hydrogen storage

It is well known that the development of dual-purpose materials is more significant and valuable than single-use materials due to the diversity of their use purposes. Based on density functional theory (DFT), the hydrogen evolution/hydrogen storage characteristics of two-dimensional (2D) B7P2 monolayer are systematically studied in this paper, focusing on the key word of clean energy-“hydrogen”. The results show that the B7P2 monolayer can be used as a stable metal-free decorated catalyst for hydrogen evolution reaction (HER), which is renewable and environmentally friendly. The calculated Gibbs free energy (ΔGH∗) is 0.06 eV, which is comparable or even better than that of Pt catalyst (ΔGH∗ = −0.09 eV). In addition, we also found that the increase of hydrogen coverage and strain driving (−2%–2%) did not further enhance the HER activity of B7P2 monolayer, showing a poor ΔGH∗. In the aspect of hydrogen storage, we have investigated the hydrogen storage performances of alkali-metal (Li, Na and K) doped B7P2. It is found that in the fully loaded case, B7P2Li6 is a promising hydrogen storage material with a 7.5 wt% H2 content and 0.15 eV/H2 average hydrogen adsorption energy (Eave). Moreover, ab initio molecular dynamics (AIMD) calculations show that there is no dynamic barrier for H2 desorption of Li-decorated B7P2 monolayer. In conclusion, our results indicate that the B7P2 monolayer is not only an excellent catalyst for HER, but also a promising hydrogen storage medium.

Hydrogen Adsorption/Desorption Isotherms on Supported Platinum Nanoparticles Determined by in‐situ XAS and ΔXANES Analysis

AbstractHydrogen atoms on supported metal nanoparticles (NPs) play a significant role in heterogeneous catalysis. Despite the extensive previous research, however, the adsorption of hydrogen onto practical supported NP catalysts is still unclear especially in the atmospheric pressure conditions. In this study, adsorption/desorption isotherms of hydrogen on Pt NPs supported on alumina (Al2O3) were obtained in gas‐flow conditions using the analysis of the difference spectra of Pt LIII‐edge X‐ray absorption spectroscopy (XAS) at 303 K. From the analysis of extended X‐ray absorption fine structure (EXAFS), a positive linear relationship was observed between the hydrogen coverage and Pt−Pt bond distance. Additionally, the re‐adsorption experiment of hydrogen after the first adsorption/desorption cycle and the transient change of dispersive XAS spectra in the hydrogen adsorption suggests that the hydrogen adsorption induces the surface reconstruction of Pt NPs on Al2O3 at around room temperature and atmospheric pressure.

Borophane Polymorphs

Hydrogenated borophenes─borophanes─have recently been synthesized as a new platform for studying low-dimensional borides, but most of their lattice structures remain unknown. Here, we determine the structures of borophane polymorphs on Ag(111) by performing extensive structural search using the cluster expansion method augmented with first-principles calculations. Our results reveal rich borophane polymorphs whose stability depends on hydrogen pressure. At relatively low hydrogen pressures, borophane structures with rhombic patterns of two-center-two-electron B-H bonds are energetically preferred, in excellent agreement with two experimentally observed phases. In a wider range of hydrogen pressures, the structure with a combination of two-center-two-electron B-H and three-center-two-electron B-H-B bonds is a deep global minimum, rationalizing its experimental prevalence. For all these borophane polymorphs, their hydrogen "skin" raises the energy barriers for oxidation above 1.1 eV, while their work functions can be reduced by more than 0.5 eV through varying the hydrogen coverage.

Theoretical insight into the interaction between hydrogen and Hägg carbide (χ-Fe5C2) surfaces

In heterogeneous catalysis, the interaction between adsorbates and substrate surface plays an important role on surface structure evolution and even the reaction mechanism. Herein, unified DFT calculation and ab initio atomistic thermodynamics were adopted to explore the interaction between hydrogen and multiple Fe5C2 surfaces. The computation indicates that the dissociation of hydrogen molecule is both thermodynamically and kinetically favorable on Fe5C2 surfaces. The adsorption energy of hydrogen atoms is related to the summed H bond valance (SBVH), a higher SBVH corresponding to a more stable adsorption strength. Based on calculated surface free energy with different hydrogen coverage, the equilibrium crystalline morphology evolution of Fe5C2 under dynamic conditions was constructed with Wulff theorem. C-rich facets expose more area in Wulff construction with decreasing hydrogen chemical potential. The generation of carbon vacancy through reducing to methane has also been investigated, and the reduction of (111) and (11-1) facets is more facile than other facets. We anticipate this work could complement some knowledge to help understanding the surface structure and performance of iron carbide catalysts under experimental conditions.

Electrolyte pH-dependent hydrogen binding energies and coverages on platinum, iridium, rhodium, and ruthenium surfaces

The weakened Hatop binding strength and increased Hatop coverage are universal phenomena on Pt, Ir, Rh, and Ru surfaces from acidic to alkaline media, which are important factors in the pH-dependent hydrogen reaction kinetics.

On the thermodynamics of hydrogen adsorption over Pt(111) in 0.05M NaOH.

The reasons for the sluggish kinetics of the hydrogen adsorption reaction in alkaline media remain a question still to be solved. This information is important to achieve a complete understanding of the mechanistic details that could lead to the production of key catalytic materials necessary for the development of a future hydrogen economy. For a better understanding of this reaction, it is important to acquire information about the thermodynamic parameters characteristic of the different steps in the reaction. Among these, the hydrogen adsorption is a key step in the process of hydrogen evolution. Although some debate still remains about the difference between adsorbed hydrogen in the underpotential deposition (UPD) region and at the overpotential deposition region, there is no doubt that understanding the former can help in the understanding of the latter. Making use of charge density measurements, we report on this paper a thermodynamic study of the hydrogen UPD process on Pt(111) in 0.05M NaOH over the range of temperatures from 283 ≤ T/K ≤ 313. The coulometric features corresponding to HUPD allow for the calculation of the hydrogen coverage and a fit to a Generalized Frumkin isotherm. From these values, different thermodynamic functions for the UPD reaction have been calculated: ΔGads, ΔSads, ΔHads, and the Pt-H bond energy. From extrapolation, a value of ΔSads ◦=-7.5±4Jmol-1K-1 was found, which is very close to 0, much lower than previously reported measurements both in acid and in alkaline solutions. Such value has an effect on the enthalpy and bond energy calculations, the latter having a decreasing tendency with pH and coverage. This tendency is completely different from the acidic systems and implies that the change in the thermodynamic functions due to the formation of the double layer and the reorganization of interfacial water has a strong influence on the process in high pH solutions.

Structure-dependent adsorption and desorption of hydrogen on FCC and HCP cobalt surfaces

The interaction of hydrogen with cobalt surfaces is of fundamental interest for Fischer-Tropsch synthesis. In the present work, the adsorption and desorption of hydrogen was studied on various cobalt single crystal surfaces that together represent the surface structures exposed by FCC and HCP cobalt nanoparticles used in applied catalysis. Dissociative hydrogen adsorption is activated on flat Co(0001), especially for hydrogen coverages beyond 0.5 ML. A tungsten filament creates hydrogen atoms and hot hydrogen molecules that increase the dissociative sticking probability and make it possible to obtain hydrogen coverages above 0.5 ML. Hydrogen in excess of 0.5 ML binds more weakly and desorbs in a separate low temperature desorption peak, in line with theoretical predictions. A third desorption peak appears above 1 ML and is attributed to subsurface hydrogen, the formation of which is attributed to hydrogen atoms produced by the tungsten filament. Adsorbed hydrogen atoms form (islands) of an ordered (2 × 2)-2H honeycomb structure for coverages between 0.3 and 0.8 ML which points to a specific stability of this structure. Step and kink sites on vicinal close-packed surfaces provide a low energy path for both hydrogen adsorption and desorption which results in a much higher dissociative sticking probability and a lower desorption temperature. The hydrogen adsorption strength on various FCC and HCP cobalt surfaces varies between 30 and 45 kJ/mol Had and is strongest on threefold hollow sites on the close-packed terraces while it is significantly lower on fourfold hollow sites on FCC-(100) and on threefold hollow sites on various open HCP surfaces. Under reaction conditions, the structure-dependent adsorption energy translates to a two to three orders of magnitude variation of the equilibrium constant for hydrogen.

Mechanism of Selective Hydrogenation of 4-Nitrophenylacetylene Using Pt–Zn Intermetallic Nanoparticles: The Role of Hydrogen Coverage

Mechanism of Selective Hydrogenation of 4-Nitrophenylacetylene Using Pt–Zn Intermetallic Nanoparticles: The Role of Hydrogen Coverage

Computational examination of the kinetics of electrochemical nitrogen reduction and hydrogen evolution on a tungsten electrode

Elucidating the elementary kinetics of nitrogen reduction on transition metal surfaces can guide the search for materials capable of efficiently catalysing electrochemical ammonia synthesis at ambient conditions. Density functional theory calculations are used here to explore the elementary kinetics of nitrogen reduction and hydrogen evolution on W(110). All proton-electron transfer barriers are calculated at −0.7 V vs. SHE, where the cathodic potential is explicitly modelled using a solvation bilayer of water with hydronium ions. The protonation of adsorbed NH is determined to be rate-limiting for nitrogen reduction, with a barrier of 0.65 eV. The same reaction step is potential-limiting based on the thermochemistry of adsorbed intermediates only, showing that thermochemical calculations suffice to predict catalytic trends for nitrogen reduction. The largest barrier found for hydrogen evolution at high hydrogen coverage is 0.36 eV. Thus, hydrogen evolution is expected to dominate over nitrogen reduction on W(110) at negative potentials.

Probing active sites on MnPSe3 and FePSe3 tri-chalcogenides as a design strategy for better hydrogen evolution reaction catalysts

Electronic structure and catalytic activity in hydrogen evolution reactions (HER) of two ternary tri-chalcogenide nano-ribbons, MnPSe3 and FePSe3, have been investigated using first-principles electronic structure calculations. Specific edge sites have been identified as the catalytic centers in both these materials. HER catalytic activity has been predicted through determination of the hydrogen adsorption free energy following Nørskov's approach. This has been done both with and without considering effects of the aqueous solvent. Hydrogen coverage dependency of the catalytic activity have also been studied. Identification of the catalytically active edge sites on these materials bridges experimental observations and theoretical predictions on these materials.

Interfacial Proton Transfer for Hydrogen Evolution at the Sub-Nanometric Platinum/Electrolyte Interface.

Understanding the dynamic process of interfacial charge transfer prior to chemisorption is crucial to the development of electrocatalysis. Recently, interfacial water has been highlighted in transferring protons through the electrode/electrolyte interface; however, the identification of the related structural configurations and their influences on the catalytic mechanism is largely complicated by the amorphous and mutable structure of the electrical double layer (EDL). To this end, sub-nanometric Pt electrocatalysts, potentially offering intriguing activity and featuring fully exposed atoms, are studied to uncover the elusive electrode/electrolyte interface via operando X-ray absorption spectroscopy during the hydrogen evolution reaction (HER). Our results show that the metallic Pt clusters derived from the reduction of sub-nanometric Pt clusters (SNM-Pt) exhibit excellent HER activity, with an only 18 mV overpotential at 10 mA/cm2 and one-magnitude-higher mass activity than commercial Pt/C. More importantly, a unique Pt-interfacial water configuration with a Pt (from Pt clusters)-O (from water) radial distance of approximately 2.5 Å is experimentally identified as the structural foundation for the interfacial proton transfer. Toward high overpotentials, the interfacial water that structurally evolves from "O-close" to "O-far" accelerates the proton transfer and is responsible for the improved reaction rate by increasing the hydrogen coverage.

Effects of hydrogenation on the tensile and shear mechanical properties of defective penta-graphene

Penta-graphene (PG) is a new theoretical two-dimensional metastable carbon allotrope composed entirely of carbon pentagons. In this paper, molecular dynamics simulations are performed to investigate the effects of the hydrogenation on the tensile and shear mechanical properties, together with the failure mechanism of PG with vacancy defects. The results show that hydrogenation can effectively tune the mechanical properties and failure mechanism of PG with vacancy defects. The defective PG (DPG) with low hydrogenation coverages exhibits obvious plastic deformation features under tensile and shear loading, and pentagon-to-polygon structural transformation is observed, while complete hydrogenation can change the failure mechanism of DPG from plastic deformation to brittle fracture. Both the tensile and shear moduli and elastic limit of DPG first decrease dramatically and then increase slowly with the increase of hydrogenation coverage, while tensile and shear strain increases almost monotonically with rising hydrogenation coverage. Complete hydrogenation can result in large enhancement of tensile and shear elastic stress limit and strain. These results may provide an important guideline for effectively tuning the mechanical properties of PG and other two-dimensional nanomaterials.

A reactive molecular dynamics study of the hydrogenation of diamond surfaces

Hydrogenated diamond has been regarded as a promising material in electronic device applications, especially in field-effect transistors (FETs). However, the quality of diamond hydrogenation has not yet been established, nor has the specific orientation that would provide the optimum hydrogen coverage. In addition, most theoretical work in the literature use models with 100% hydrogenated diamond surfaces to study electronic properties, which could be unreachable experimentally. In this work, we have carried out a detailed study using fully atomistic reactive molecular dynamics (MD) simulations on low indices diamond surfaces i.e. (001), (013), (110), (113), and (111) to evaluate the quality and hydrogenation thresholds on different diamond surfaces and their possible effects on electronic properties. Our simulation results indicate that the 100% surface hydrogenation on these surfaces is hard to achieve because of the steric repulsion between the terminated hydrogen atoms. Among all the considered surfaces, the (001), (110), and (113) surfaces incorporate a larger number of hydrogen atoms and passivate the surface dangling bonds. Our results on hydrogen stability also suggest that these surfaces with optimum hydrogen coverage are robust under extreme conditions and could provide homogeneous p-type surface conductivity on the diamond surfaces, a key requirement for high-field, high-frequency device applications.

Hydrogen evolution reaction on transition metal nanoparticles from first-principles

An atomic-level understanding of hydrogen evolution reaction (HER) on transition metal nanoparticles is of great importance in exploring highly efficient and potential electro-catalysts for water splitting. We herein present a study of HER on some Ni- and Cu-group metal nanoparticles constructed from (100), (110) and (111) surfaces based on the principle of Wulff structure. The Gibbs free energies of hydrogen adsorption on three typical surfaces are calculated by using density functional theory (DFT) calculations. The coverage effect on the surface energy at different hydrogen coverage is considered due to the important influence of the hydrogen–hydrogen interaction. The Ni-group metal nanoparticles prefer the (111) surface exposed and the Cu-group metal nanoparticles prefer the (110) surface exposed. The Wulff structures of nanoparticles at different hydrogen coverage clearly show the different reaction rates. These theoretical results are helpful for understanding the HER mechanism on metal nanoparticles.