Hydrogen adsorption on pristine and platinum decorated graphene quantum dot: A first principle study

In the ever growing demand of future energy resources, hydrogen production reaction has attracted much attention among the scientific community. In this work, we have investigated the hydrogen evolution reaction (HER) activity on an open-shell polyaromatic hydrocarbon (PAH), graphene quantum dot “triangulene” using first principles based density functional theory (DFT) by means of adsorption mechanism and electronic density of states calculations. The free energy calculated from the adsorption energy for graphene quantum dot (GQD) later guides us to foresee the best suitable catalyst among quantum dots. Triangulene provides better HER with hydrogen placed at top site with the adsorption energy as −0.264 eV. Further, we have studied platinum decorated triangulene for hydrogen storage. Three different sites on triangulene were considered for platinum atom adsorption namely top site of carbon (C) atom, hollow site of the hexagon carbon ring near triangulene's unpaired electron and bridge site over C–C bond. It is found that the platinum atom is more stable on the hollow site than top and bridge site. We have calculated the density of states (DOS), highest occupied molecular orbitals (HOMO), lowest unoccupied molecular orbitals (LUMO) and HOMO-LUMO gap of hydrogen molecule adsorbed platinum decorated triangulene. Our results show that the hydrogen molecule (H₂) dissociates instinctively on all three considered sites of platinum decorated triangulene resulting in D-mode. The fundamental understanding of adsorption mechanism along with analyses of electronic properties will be important for further spillover mechanism and synthesis of high-performance GQD for H₂ storage applications.     

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, B₄N monolayer as efficient HER electrocatalysts based on first-principles computations. When the material is metal-free, the calculated Gibbs free energy (ΔG_H1) 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 B₄N monolayer is sensitive to the strains-driven. The single metal atom supported on B₄N can still make the value of ΔG_H1 close to 0 eV for Cr/B₄N and V/B₄N. These results reveal that the B₄N monolayer is a promising candidate for HER applications.     

The influence of pre-adsorbed Pt on hydrogen adsorption on B2 FeTi(111)

The hydrogen adsorption properties on a Pt covered Fe-terminated B2-FeTi (111) surface are studied using the Density Functional Theory (DFT). The calculations are employed to trace relevant orbital interactions and to discuss the geometric and electronic consequences of incorporating one Pt atom or a Pt monolayer on top of the FeTi surface. The most stable adsorption site is a distorted FCC hollow for one Pt atom and from this location we build the Pt monolayer (ML). The H-adsorption energy is very close among BRIDGE, HCP and FCC hollow sites (∼−0.45 eV) being lower for the TOP site (−0.34 eV) in the case of a Pt(111) fcc surface. In the case of a Pt ML/FeTi, the H more stabilized on a BRIDGE site (∼−1.13 eV) interacting with both a Pt and Fe atom. We also computed the density of states (DOS) and the overlap population density of states (OPDOS) in order to study the evolution of the chemical bonding after adsorption.     

The strain and transition metal doping effects on monolayer Cr2O3 for hydrogen evolution reaction: The first principle calculations

Chromic oxide (Cr₂O₃) monolayer is a promising alternative hydrogen evolution reaction (HER) catalyst compared with expensive platinum (Pt) due to its advantages such as low cost, large specific surface area, high reserves, and designability. In this study, the two practical strategies, strain engineering and transition metal (TM) doping (Mn, Fe, Zn, etc.), are proposed to activate the catalytic sites of Cr₂O₃ monolayer for the HER. The density functional theory (DFT) calculations demonstrate that the strained Cr₂O₃ monolayer can stimulate the HER activity with the Gibbs free energy of hydrogen adsorption (ΔG_H1) close to 0.09eV, which can be considered as a performable strategy to tune the HER catalytic behavior of Cr₂O₃ monolayer. For the TM doping, it also plays a role in the performance adjustment. These results provide a guideline to optimize the HER performance of Cr₂O₃ monolayer.     

Hydrogen evolution reaction and electronic structure calculation of two dimensional bismuth and its alloys

We present a systematic ab initio study of atomic hydrogen and oxygen adsorption on bismuthene monolayer and its alloys with arsenic and antimony through electronic structure calculations based on density functional theory within generalized gradient approximation. We systematically investigated the preferable adsorption site for hydrogen and oxygen atom on 2D Bi, BiAs and BiSb. It was found that the hydrogen atom prefers top site of bismuth atom and oxygen atom prefers to reside in the hexagonal ring of these 2D bismuth alloys. The free energy calculated from the individual adsorption energy for each monolayer subsequently guides us to predict the best suitable catalyst among the considered 2D monolayers. The 2D BiSb serves better for hydrogen evolution reaction (HER) with hydrogen adsorption energy as ?1.384 eV while 2D BiAs is suitable for oxygen evolution reaction (OER) with oxygen adsorption energy as ?1.092 eV. We further investigated the effects of the adsorbate atom on the electronic properties of 2D Bi, BiAs and BiSb. The adsorption of oxygen on 2D BiAs and BiSb was shown to reduce the bulk band gap by 40.56 and 67.79% respectively which will be advantageous for the observation of Quantum Spin Hall effect at ambient conditions.     

Catalytic activity and underlying atomic rearrangement in monolayer CoOOH towards HER and OER

For efficient hydrogen and oxygen production, design and synthesis of cost-effective, stable and active materials are inevitable. In this work, the catalytic activity of 2D CoOOH towards hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) has been investigated using first principles calculations based on density functional theory. The adatom induced structural rearrangement have been investigated from structural parameters as well as charge redistribution in 2D CoOOH. The preferred site for hydrogen and oxygen adsorption were found to be the top site of oxygen atom of 2D CoOOH. The catalytic activity of HER and OER towards 2D CoOOH was studied by calculating the Gibbs free energy. Our study revealed that the 2D CoOOH serve better as a catalyst for HER than OER with adsorption energy of −0.45 and −3.68 eV respectively suggesting its efficient use for hydrogen production. We further investigated the changes in electronic properties of 2D CoOOH on adsorption of hydrogen and oxygen 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 (ΔG_H1) is 0.06 eV, which is comparable or even better than that of Pt catalyst (ΔG_H1 = ?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 ΔG_H1. 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 (E_ave). 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.     

Platinum doped iron carbide for the hydrogen evolution reaction: The effects of charge transfer and magnetic moment by first-principles approach

In this work, the catalytic activity towards hydrogen evolution reaction (HER) was studied for hydrogen adsorption on Pt doped Fe₂C (001) surface configuration (Pt/Fe₂C) and compared with pure Pt (001). The adsorption of H on the pristine Fe₂C, Pt doped Fe₂C, and pure Pt in (001) slab was computed. The best and promising HER activity (ΔG_H1 = −0.02 eV) is obtained at the hollow site adsorption of Pt/Fe₂C (Fe₁₃Pt₃C₈) compared to the experimental value of pure Pt (ΔG_H1 = −0.09 eV) suggesting the possibility of the H₂ formation on the surface of Fe₁₃Pt₃C₈. The structural stabilities of Fe₂C and Pt/Fe₂C were investigated by the formation energy analysis. Also, it is observed that to enhance the HER mechanism, the modification of the d-electron structure of Pt atoms is essential which can be achieved by the increased Pt doping. The Bader charge analysis demonstrated the charge transfer between the substrate and the adsorbed H atoms. The density of states (DOS) of pure Fe₂C and optimal Pt/Fe₂C were calculated which revealed the magnetic and metallic nature of these materials. In addition, the adsorption and resulted activation of H₂ were facilitated by the elongation of H–H bond length in Fe₁₃Pt₃C_8. This work supports the HER over single atom catalysts (SACs) with lower Pt loading but with high catalytic activity and the maximum atom utilization of SACs.     

Reaction coordinate mapping of hydrogen evolution mechanism on Mg3N2 monolayer

In this work, we have envisaged the hydrogen evolution reaction (HER) mechanism on Mg₃N₂ monolayer based on electronic structure calculations within the framework of density functional theory (DFT) formalism. The semiconducting nature of Mg₃N₂ monolayer motivates us to investigate the HER mechanism on this sheet. We have constructed the reaction coordinate associated with HER mechanism after determining the hydrogen adsorption energy on Mg₃N₂ monolayer, while investigating all possible adsorption sites. After obtaining the adsorption energy, we subsequently obtain the adsorption free energy while adding zero point energy difference (ΔZPE) and entropic contribution (TΔS). We have not only confined our investigations to a single hydrogen, but have thoroughly observed the adsorption phenomena for increasing number of hydrogen atoms on the surface. We have determined the projected density of states (DOS) in order to find the elemental contribution in the valence band and conduction band regime for all the considered cases. We have also compared the work function value among all the cases, which quantifies the amount of energy required for taking an electron out of the surface. The charge transfer mechanism is also being investigated in order to correlate with the HER mechanism with amount of charge transfer. This is the first attempt on this material to the best of our knowledge, where theoretical investigation has been done to mapping the reaction coordinate of HER mechanism with the associated charge transfer process and the work function values, not only for single hydrogen adsorption, but also for increasing number of adsorbed hydrogen.     

New insights into the anti-disproportionation mechanism of ZrCo alloying with Ti,Hf, Sc,Cu, and Fe elements

First-principles calculations were performed to investigate the effects of alloying substitutions (i.e. Ti, Hf, Sc, Cu and Fe) on the anti-disproportionation ability of ZrCo alloy. For the first time, we revealed the disproportionation mechanism from the energy point of view and provided a new theoretical method to predict whether the substitution element has the ability to enhance the anti-disproportionation performance of ZrCo alloy. Based on the hydrogen atom occupancy behavior in ZrCoH₃ and the results of our calculation of binding energy, the hydrogen atom migration model during hydrogenation and theoretical computational model were established. Through structural optimization, a series of stable 2 × 1 × 2 ZrCoH₃ supercells were obtained, which contain four hydrogen atoms occupying 8e site and various substitution atoms with different amounts except for pure system. The binding energy of hydrogen atom in the 8e site and activation energy of diffusing from 8e site to 4c₂ site of these ZrCoH₃ supercells were calculated. The results showed that the substitution of Ti and Hf increased the binding energy of hydrogen atom in the 8e site, while the substitution of Fe, Cu and Sc decreased the binding energy of hydrogen atom in the 8e site. Meanwhile, both of Ti and Hf substitution reduced the activation energy of diffusing from 8e site to 4c₂ site, while all of Fe, Cu and Sc substitution increased the activation energy of diffusing from 8e site to 4c₂ site. These results indicated that hydrogen-induced disproportionation was the inherent property of ZrCo alloy, and element substitution can restrain or accelerate disproportionation by affecting both the binding energy and activation energy. With simultaneous consideration of the binding energy and activation energy, the effect of these alloying substitutions on the anti-disproportionation ability could be ascertained and the results were in good agreement with the previous theoretical and experimental results.     

First principles of Si-doped BC2N single layer for hydrogen evolution reaction (HER)

Understanding the Hydrogen Evolution Reaction (HER) process is fundamental to use hydrogen as a sustainable (clean and renewable) energy source. Using first-principles calculations, we study the HER process when Si-doped a h-BC₂N single layer. The pristine BC₂N presents semiconducting properties with a band gap of 1.6–2.0 eV, being appropriate as a catalytic in the water splitting process. When Si is incorporated into the BC₂N monolayer, we obtain that the most stable site (lower formation energy) occurs when the Si atom replaces a C atom (Si_C). The Si atom moves out of the plane forming a buckling structure and the semiconducting properties are maintaining without spin effects. However, Si_B and Si_N give rise to two unpaired spin electronic levels inside the band gap and a magnetic moment of 1 μ_B. The adsorption energies of an H₂ molecule on the top of the Si atom are in the range of 50–100 meV, which are greater than the calculated ones for H₂ adsorbed on graphene and h-BN nanosystems but still low to be considered as an optimized medium for hydrogen storage. In addition, we observe that dispersive forces (van der Waals interactions) are responsible for half part of the adsorption energies. Strain due to the difference between the atomic radius of Si and C as well as the less stability of the Si–H bonds compared to the C–H ones leads to the Gibbs free energy (ΔG1) for hydrogen adsorbed on Si_C near zero, showing that Si-doped h-BC₂N is a potential system for HER.     

Density functional theory study of reversible hydrogen storage in monolayer beryllium hydride by decoration with boron and lithium

The hydrogen storage capacity of M-decorated (M = Li and B) 2D beryllium hydride is investigated using first-principles calculations based on density functional theory. The Li and B atoms were calculated to be successfully and chemically decorated on the Surface of the α-BeH₂ monolayer with a large binding energy of 2.41 and 4.45eV/atom. The absolute value was higher than the cohesive energy of Li and B bulk (1.68, 5.81eV/atom). Hence, the Li and B atoms are strongly bound on the beryllium hydride monolayer without clustering. Our findings show that the hydrogen molecule interacted weakly with B/α-BeH₂(B-decorated beryllium hydride monolayer) with a low adsorption energy of only 0.0226 eV/H₂ but was strongly adsorbed on the introduced active site of the Li atom in the decorated BeH₂ with an improved adsorption energy of 0.472 eV/H₂. Based on density functional theory, the gravimetric density of 28H₂/8li/α-BeH₂) could reach 14.5 wt.% higher than DOE's target of 6.5 wt. % (the criteria of the United States Department of Energy). Therefore, our research indicates that the Li-decorated beryllium hydride monolayer could be a candidate for further investigation as an alternative material for hydrogen storage.     

High-throughput screening of transition metal single-atom catalyst anchored on Janus MoSSe basal plane for hydrogen evolution reaction

Considerable efforts have been made to enhance the hydrogen evolution reaction (HER) catalytic performance of Janus MoSSe monolayer, which have been considered to be a promising candidate due to the unique asymmetry structure. However, the activation effect remains non-optimal for the inert Janus MoSSe basal plane at present. Herein, a train of transition metal (TM) atoms were anchored on the S-/Se-/Mo-defective MoSSe basal plane to screen effective TM single-atom catalysts for HER through density functional theory (DFT) computations. Interestingly, the single Co atom anchored on Mo-defective MoSSe and the single Zn or Cd atom anchored on S-defective MoSSe were judged to possess excellent HER performance yielding a near-zero ΔG_H (ΔG_H = ?0.050, ?0.095, ?0.098 eV, respectively), which is comparable to the optimized Pt-SACs. The enhanced HER activity is attributed to the doping of TM atoms (Co, Zn and Cd) which improves the conductivity of the original MoSSe and offers unoccupied states near the Fermi level decreasing the energy barrier of electrons transfer between H and TMs@MoSSe surface. In addition, the change of unoccupied antibonding states of active atoms leads to appropriate interaction between the active sites and H. The hybridization between H-s orbital and the TMs@MoSSe systems around the Fermi level also suggests the formation of stable bonding-antibonding hydrogen adsorption states. This work reveals an effective way of activating MoSSe basal plane for HER.     

Defects and doping engineered two-dimensional o-B2N2 for hydrogen evolution reaction catalyst: Insights from DFT simulation

In this work, a detailed investigation of the structural and electronic properties and hydrogen evolution reaction (HER) activity of the pristine, vacancy and carbon (C) doped o-B₂N₂ monolayer is carried out using first-principles based density functional theory. The creation of vacancy and C doping modulates structural and electronic properties of the monolayers and enhances the HER activity of o-B₂N₂. The BN vacancy defect, C doping at B and N sites in the monolayer enhances the magnitude of HER activity by 77.34%, 86.71% and 83.59% as compared to pristine monolayer. The modulation in the HER activity of the o-B₂N₂ is due to the redistribution of charge after induction of vacancy and dopant. Our results suggest that the C doping makes o-B₂N₂ metallic which can be utilized as an “electrocatalyst” whereas BN vacancy defected o-B₂N₂ monolayer is semiconducting with a band gap of ～1 eV and can be used as “photocatalyst” for HER activity.     

2D transitional-metal nickel compounds monolayer: Highly efficient multifunctional electrocatalysts for the HER,OER and ORR

The atomically dispersed Ni metal on two-dimensional (2D) monolayer species have exhibited impressive catalytic properties towards oxygen evolution and oxygen reduction reactions (OER/ORR). In this work, nickel boride and nickel carbide monolayers, which is inspired by the more different active sites of monolayer surface, such as Ni atom, that efficiently catalyze the reduction of OOH to O₂ to some extent, while B/C atom can work in the reduction of H1 to H₂, are theoretically reported. Remarkably, the density functional theory (DFT) calculations showed that the random combination of Ni atom to two free-metal atoms such as B and C to form catalytically active double sites lead to a remarkable reduction of the first or last hydrogenation free energy barrier step. The resulting Ni₂B₅ and NiC₃ exhibited ultra-low Gibbs free energy (ΔG_H1) of only 0.096 V and 0.018 V for HER and lower onset potentials of only 0.39 V (0.62 V) and 0.60 V (0.31 V) for OER (ORR), respectively, while the NiB₆ exhibited appropriate OER and ORR electrocatalytic activity. The superior bifunctional even multifunctional catalytic performance in the overall water splitting is mainly attributed to both the electron donation from the Ni metal atoms to the key intermediates, which significantly polarizes and weakens the O–H bond, and to the synergistic effect of the B/C atoms that moderates the binding strength of terminal top of H atoms. This work constitutes the DFT study of the water electroreduction processes on diverse nickel compounds monolayer catalytic sites and, consequently, paves the way towards the rational design of highly efficient bifunctional even multifunctional electrocatalysts.     

Enhanced hydrogen evolution reaction performance of MoS2 by dual metal atoms doping

Molybdenum disulfide (MoS₂) has been considered a promising high-efficiency, low-cost hydrogen evolution reaction (HER) catalyst in acidic and alkaline media. However, the lack of active sites in the basal plane become the most significant obstacle hindering the widespread application of MoS₂. Here, we systematically studied the HER performance of MoS₂ plane or edge by co-doping Co atom and other 3d transition metals (TM = Ti–Fe, Ni) by density functional theory calculation methods. Interestingly, the dual atoms doping in both the basal plane and edges of MoS₂ is a feasible fabrication with small or negative formation energies. Compared with the pristine MoS₂ electrocatalyst, the HER performance in these doped systems is largely enhanced in both basal plane and edges due to the effective charge regulation on the S site by dual atom doping. Remarkably, close to zero H adsorption free energy (ΔG_H = ?0.161–0.119 eV) is identified for the TM-Co co-doped MoS₂ basal, indicating that they are potential alternate HER electrocatalysts of Pt. Our study provides a new strategy to design highly efficient non-noble metal electrocatalysts accessibility for energy-related applications.     

Monolayer palladium supported on molybdenum and tungsten carbide substrates as low-cost hydrogen evolution reaction (HER) electrocatalysts

Active and low-cost hydrogen evolution reaction (HER) electrocatalysts are needed to minimize capital costs associated with large-scale hydrogen production from water electrolysis. Catalysts based on monolayer (ML) amounts of precious metals supported on carbides are a promising concept for this purpose. In the current study Pd supported on tungsten carbide (WC) and molybdenum carbide (Mo₂C) were evaluated for HER activity. Carbide foils were synthesized using temperature programmed reaction of W or Mo in a CH₄/H₂ atmosphere. Physical vapor deposition was used to deposit Pd on WC or Mo₂C while X-ray Photoelectron Spectroscopy (XPS) was used to determine the Pd surface coverage. Linear sweep voltammetry and chronopotentiometry were used to evaluate the HER activity and electrochemical stability of the catalysts, demonstrating the possibility of using ML Pd on either WC or Mo₂C as active, stable and lower-cost HER catalysts.     

Copper induced phosphide for enhanced electrochemical hydrogen evolution reaction

Research on highly efficient catalysts for electrochemical hydrogen evolution reaction (HER) remains a challenge. In this work, we successfully wrap copper (Cu) inside of copper phosphide (Cu₃P) nanoparticle to form a copper/copper phosphide (Cu/Cu₃P) core/shell structure attached on carbon nanotubes (CNTs) for enhanced HER activity in acid. The average size of the core/shell particles is around 25 nm, with about 5 nm of Cu₃P as the outer layer. The catalytic activity of the core/shell structure is significantly promoted compared to the metallic Cu and Cu₃P pure phases nanoparticles on CNTs, requiring overpotentials of 84 and 161 mV to achieve 10 and 100 mA cm⁻² of current density, respectively. The core/shell structure also presents high HER durability and stability, with the polarization curve overlapped after 5000 cycles of CVs and steady current density at 25 mA cm⁻² for as long as 10 h. To account for the promoted HER performance, the Cu/Cu₃P structure is fully investigated by physical and electrochemical characterizations and density functional theory (DFT) calculations. The DFT results depict that the neutralized the adsorption Gibbs free energy of hydrogen atoms (ΔG_H1) is induced by the electronic interactions between metallic Cu and phosphide phase.     

Li-decorated B2O as potential candidates for hydrogen storage: A DFT simulations study

Two-dimensional (2D) B₂O monolayer is considered as a potential hydrogen storage material owing to its lower mass density and high surface-to-volume ratio. The binding between H₂ molecules and B₂O monolayer proceeds through physisorption and the interaction is very weak, it is important to improve it through appropriate materials design. In this work, based on density functional theory (DFT) calculations, we have investigated the hydrogen storage properties of Lithium (Li) functionalized B₂O monolayer. The B₂O monolayer decorated by Li atoms can effectively improve the hydrogen storage capacity. It is found that each Li atom on B₂O monolayer can adsorb up to four H₂ molecules with a desirable average adsorption energy (E_ave) of 0.18 eV/H₂. In the case of fully loaded, forming B₃₂O₁₆Li₉H₇₂ compound, the hydrogen storage density is up to 9.8 wt%. Additionally, ab initio molecular dynamics (AIMD) calculations results show that Li-decorated B₂O monolayer has good reversible adsorption performance for H₂ molecules. Furthermore, the Bader charge and density of states (DOS) analysis demonstrate H₂ molecules are physically absorbed on the Li atoms via the electrostatic interactions. This study suggests that Li-decorated B₂O monolayer can be a promising hydrogen storage material.     

Transition metal atoms anchored 2D holey graphyne for hydrogen evolution reaction: Acumen from DFT simulation

Here, we report a theoretical design of transition metals (TMs) anchored two-dimensional (2D) holey graphyne (HGY) based catalyst for the hydrogen evolution reaction (HER) through state-of-art density functional theory (DFT) simulation. The studied TMs (Co, Fe, Cr) are bonded strongly on HGY surface due to charge transfer from d orbital of metal to C 2p orbital of HGY. The HGY+TMs systems are stable at room temperature as evident from ab-initio molecular dynamics (AIMD) simulation. We predicted that the Co, Fe and Cr anchored HGY are highly active for HER activity with Gibbs free energy (ΔG) value as low as −0.21, −0.14, and −0.05 eV respectively and which are close to the best-known HER catalyst (Pt metal). The enhanced HER performance is attributed to the increased conductivity as well as redistribution of electrons. As pristine HGY is experimentally synthesized, HGY+TMs (Co, Fe, Cr) systems can be as an efficient catalyst for H₂ production.