The enhancement of hydrogen storage capacity in Li,Na and Mg-decorated BC3 graphene by CLICH and RICH algorithms

New hydrogen adsorption states on Li, Na, and Mg-decorated graphene-type BC₃ sheet have been investigated by first-principles calculations. The structural, electronic and binding properties, metal binding and nH₂ (n = 1–10) adsorption states of these systems are studied in detail with the Mulliken analysis, charge density differences, and partial density of states. To enhance the number of the adsorbed H₂ molecules per metal atom, and also to generate the better initial coordinates for reducing the simulation time, we present two masthematical algorithms (CLICH and RICH). The tested results on BC₃ sheet and boron-doped graphene (C₃₀B₂) show that these algorithms can increase the number of adsorbed hydrogen molecules by minimizing the computational time. It is seen that nH₂ (n = 1–10) adsorbed Li,/Na and/Mg-decorated BC₃ single- and double-sided systems are industrial materials for hydrogen storage technology, their adsorption energies fall into the acceptable adsorption energy range (0.20–0.60 eV/H₂). It is concluded from the optimized geometries and charge density differences for the higher number of H₂ adsorbed systems that not only decorated metal atom but also the sheet plays an important role in hydrogen storage process, because the boron atoms in the sheet expand the induced electric field between the adatoms and BC₃ sheet. From Mulliken analysis, there is a charge transfer mechanism between H₂ molecules and metal atoms. Moreover, the resonant peaks for the sheet, metal atoms and H₂ molecules in partial density of states curves indicate the possible hybridizations. Additionally, these adsorption processes are supported by charge density difference plots.     

Activated HER performance of defected single layered TiO2 nanosheet via transition metal doping

Doping engineering has been considered as a viable strategy to obtain highly efficient photocatalysts for hydrogen evolution reaction (HER). Since transition metal doping usually introduces of oxygen vacancy defect in oxide compounds, the first-principles calculations were performed to investigate the HER activity of transition metal-doped lepidocrocite-type TiO₂ nanosheets with oxygen vacancies. Different hydrogen adsorption sites are taken into account here. When adsorbed hydrogen occupies the oxygen vacancy, an optimal HER activity, similar to ideal Pt metal, was obtained in Fe-doped nanosheets. The tunable HER performance was correlated with the d-band center level of dopants. On the other hand, O-p_z band center level is responsible for the hydrogen adsorption free energy in the case of hydrogen adsorbed with the surface oxygen atom. Unexpectedly, for Mn doping cases, the pre-adsorbed hydrogen could activate surface O atom with additional hydrogen adsorption free energy being close to zero. In addition, the strain engineering also could effectively adjust HER activity in defected nanosheets.     

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.     

Potential and support-dependent hydrogen evolution reaction activation energies on sulfur vacancies of MoS2 from GC-DFT

We present a detailed mechanistic study of HER at the sulfur vacancy V_S of 2H–MoS₂. We evaluate the Volmer, Tafel, and Heyrovsky transition states for the different possible reaction steps, determining the activation energy as a function of the electrochemical potential via grand-canonical density functional theory. The results show that the Volmer and Heyrovsky steps depend on the electrochemical potential and the activation energies decrease for more negative potentials, while this is not the case for the Tafel step, for which the activation energy is constant. From the activation energies at ?0.2 V vs SHE, it can be concluded that during HER on V_S a first hydrogen atom is adsorbed as a spectator via a Volmer step. Then, the catalytic cycle consists of a Volmer and a Heyrovsky step, with the latter being rate determining. In addition, we investigate for the first time the effect of a conductive support on the HER activity of these sulfur vacancies. Our results show that copper, gold and graphite supports have little effects on the activation energies of all steps. Hence, we conclude that cheap, acid-stable, high-surface area carbon supports are well suited for MoS₂-based HER catalysts.     

Unusual adsorption behavior of hydrogen molecules on Zr–doped perfect and oxygen–vacancy defective rutile TiO2(110) surfaces: Periodic DFT study

Adsorptions of Zr atom onto the perfect rutile TiO₂(110) and the oxygen vacancy rutile TiO₂ (110) ([TiO₂+V_o]) to form Zr–TiO₂ and Zr‒[TiO₂+V_o] were studied using periodic density functional theory (DFT) method. Three configurations of both Zr–TiO₂ and Zr‒[TiO₂+V_o] surfaces were found and binding energies of Zr atom of the most stable Configurations of Zr–TiO₂ and Zr‒[TiO₂+V_o] surfaces are respectively −3.36 and −3.26 eV. The most stable Configurations of the Zr–TiO₂ and Zr‒[TiO₂+V_o] surfaces were selected in hydrogen adsorption study. Adsorption energies of single H₂ molecule on the most stable Zr–TiO₂ and Zr‒[TiO₂+V_o] are −1.43 and −1.45 eV, respectively. Based on the second H₂ molecular adsorption on the hydrogen pre‒adsorbed Zr–TiO₂ and Zr‒[TiO₂+V_o] surfaces, adsorption energies of −1.90 and −2.55 eV were found, respectively. The second H₂ molecule adsorption was found to be much stronger than the first H₂ molecule adsorbed onto the Zr–TiO₂ and Zr‒[TiO₂+V_o] surfaces by 32.9% and 75.9%, respectively. Either the Zr–TiO₂ or Zr‒[TiO₂+V_o] surface is suggested as hydrogen–storage material and the Zr–TiO₂ can be proposed as an electrical resistance‒based hydrogen sensor.     

First-principles study of hydrogen storage on Si atoms decorated C60

Hydrogen storage capacity of Si_nC₆₀ is studied via first-principles theory based on DFT and Canonical Monte Carlo Simulation (CMCS). It is shown that Si atoms strongly prefer D-site rather than other sites and in these structures maximum number of hydrogen molecule onto any Si atom is one. Each Si atom adsorbs one hydrogen molecule in molecular form and with proper binding energies when Si atom is placed in any D-site of C₆₀. Si atoms enhance remarkably hydrogen storage capability in fullerene.     

Perception of Mg adsorption on the BC2N nanotube as a anode for rechargeable Mg ion batteries

Owing to their cost-effectiveness and the natural abundance of magnesium, magnesium-ion batteries (MIBs) were introduced as encouraging alternatives to Lithium-ion batteries. Following the successful synthesis of carbon nano-tube, its B and N doped derivatives which were doped with B and N enjoyed the attention of researchers as novel anode materials (AM) for MIBs. Here, we investigated a BC₂N nano-tube (BC₂NNT) as an encouraging AM for MIBs. To have a deeper understanding of the electrochemical properties, cycling stability, specific capacity (SC) and the adsorption behavior of this nano-tube, first-principles density functional theory computations were performed. By performing NMR calculations, we identified two types of non-aromatic hexagonal rings, namely B₂C₂N₂ (I) and BC₄N (II). Magnesium was adsorbed onto I with the adsorption energy of ?40.38 kcal/mol and on II with the adsorption energy ?20.15 kcal/mol. The SCs were as high as 783 mAh/g. The predicted average open-circuit voltage for BC₂NNT was 1.94 V, which was greater than that of other 2D materials. The findings demonstrated the possibility of utilizing the BC₂NNT as an AM for MIBs. The results can provide useful insights into the design of boron-carbon-nitrogen-based AMs for MIBs.     

A first-principle study of calcium-decorated BC2N sheet doped by boron or carbon for high hydrogen storage

Hydrogen adsorption and storage on calcium-decorated BC₂N sheets doped by Boron or Carbon were investigated using the first-principles calculations. Unlike the weak bond between Ca atoms and pristine BC₂N, doping boron or carbon atoms on BC₂N sheet can significantly strengthen the Ca atoms on the BC₂N, especially for BC₂NB_C and BC₂NC_N. It is observed that Ca decorated BC₂NB_C and BC₂NC_N possess strong donation and back-donation of Ca with the sheets, which is responsible for enhanced binding energy to eliminate the clustering problem. Up to four hydrogen molecules can stably attach to a Ca atom with an average adsorption energy of ∼0.3 eV, which is in the range that permits hydrogen recycling at ambient conditions. The Ca decorated BC₂NB_C and BC₂NC_N complexes can work as high-capacity hydrogen storage materials with the practical usable capacities of 8.36 wt% and 8.38 wt%, respectively.     

Hydrogen storage in Ca-decorated carbyne C10-ring on either Dnh or D(n/2)h symmetry. DFT study

We computationally investigate the hydrogen storage properties of carbyne C₁₀-ring structure on either D_nh or D_(n/2)h symmetry decorated with calcium (Ca) atoms adsorbed on its outer surface. The calculations are carried out on DFT-GGA-PW91 and DFT-GGA-PBE levels of theory as implemented in Biovia Materials Studio modeling and simulation software. To account for van der Waals interactions we also carried out calculations using DFT-D method of Grimme. Dmol³ is used to calculate total energies, HOMO-LUMO electronic charge density, Mulliken population analysis, and electrostatic potential fitting charges (ESP). Based on these results: i) the average binding energy of Ca atom doping to C₁₀-ring is ~2.3 eV (PW91) and ~2.1 eV (PBE). ii) Up to seven H₂ molecules per Ca atom can be physically adsorbed with an average energy of ~0.2 eV per H₂ molecule. iii) This physisorption leads to 8.09 wt percentage (wt. %) for the gravimetric storage capacity. According to these results, calcium-decorated carbyne C₁₀-ring structure is excellent candidate for hydrogen storage at ambient conditions with application to fuel cells.     

Modulating oxygen electronic orbital occupancy of Cr-based MXenes via transition metal adsorbing for optimal HER activity

Modulating the surface electronic properties of the 2D MXenes is of significant importance to boost their hydrogen evolution reaction (HER) activity. Herein, a series of transition metal adatoms are employed to tune the surface electronic properties of Cr-based MXenes with oxygen function group for realizing impressive HER performance. The Results show that the charge of surface oxygen atoms, which is affected by both the host metal atoms and the adsorbed transition metal atoms, play critical role in the adsorption strength of hydrogen. The optimal performance is achieved by depositing Cr atom on the Cr₂TiC₂O₂ MXene, which results in the adsorption free energy of hydrogen very close to zero (0.03 eV). Systematic electronic structure analyses confirm that the charge transfer from the adsorbed transition metals to the neighboring surface oxygen atoms could tune the orbital occupancy of oxygen and their adsorption strength to hydrogen atom and therefore the HER activity. These findings and concepts may be useful for the design of advanced MXene-based HER catalysts.     

Complex interaction of hydrogen with the monolayer TiS2 decorated with Li and Li2O clusters: an ab initio random structure searching approach

We have applied ab initio random structure searching to study the structure, stability and hydrogen storage properties of monolayer TiS₂ coated with Li and small Li₂O clusters. For the low Li covered system we found a complex adsorption mechanism: some hydrogen molecules were adsorbed due to polarization with Li, others due to polarization with S near the surface of TiS₂. The peculiarities of the interaction of the H₂ molecules with each other and the preferred adsorption sites allowed us to formulate a series of recommendations that can be useful when selecting the material for the most effective support. Moreover, the findings also show that the storage capacity of this system can reach up to 9.63 wt%, presenting a good potential as hydrogen storage material. As for the Li₂O clusters supported on TiS₂, we found that the polarization of the Li–O bond increases upon the adsorption of the Li₂O nanocluster. Moreover, the polarized Li–S bonds appear in addition to the already existing Li–O bonds. All this is possible due to the extraction of 1.46 electrons from the S atom of the substrate by O atom of the cluster, and should contribute to an increase in both the adsorption energy and the maximum capacity. The adsorption energies of H₂ for the systems studied here are within 0.11–0.16 eV/H₂ which is a recommended range for reversible hydrogen physisorption under standard test conditions. This study may stimulate experimental efforts to check the claims of high-capacity, stable and reversible hydrogen adsorption reported here.     

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.     

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.     

Activating basal plane of Janus VSSe for efficient hydrogen evolution reaction by non-noble metal element doping: A first-principal study

Searching electrocatalysts with excellent hydrogen evolution reaction (HER) performance is very important for developing clean hydrogen energy. Two-dimensional (2D) materials have been widely studied as HER electrocatalysts, however, the basal planes of 2D materials, which dominate the surface area, are usually with poor activity. In this work, we theoretically studied the HER activity of Janus 2H–VSSe with or without non-noble metal element doping. Density functional theory (DFT) calculations suggest that doping As and Si atoms in the S or Se sites of VSSe and the C and Ge atoms in the Se site of VSSe greatly promote the HER performance of the basal plane of VSSe, resulting in hydrogen adsorption free energy close to zero (i.e. ?0.022, ?0.040, 0.066, 0.065, ?0.030, 0.058 eV, respectively), which are better than the Pt catalyst (?0.09 eV). The doped atoms strengthen the interaction between their pz-orbital and the hydrogen s-orbital, resulting in a lower bonding state in energy and higher bind strength for the hydrogen atom. This work opens up a new way to design highly efficient and low-cost catalysts for HER.     

Accelerating water dissociation kinetic in Co9S8 electrocatalyst by mn/N Co-doping toward efficient alkaline hydrogen evolution

Electrocatalytic hydrogen evolution under alkaline media holds great promising in hydrogen energy production. Transition-metal sulfides (TMSs) are attractive for electrocatalytic alkaline hydrogen evolution, yet their catalytic performance is unsatisfactory owing to the sluggish water dissociation kinetics. Herein, a Mn/N co-doping strategy is proposed to regulate the water dissociation kinetics of Co₉S₈ nanowires array grown on nickel foam thus improve the activity of hydrogen evolution reaction (HER). The optimal Mn/N co-doping Co₉S₈ (Mn–N–Co₉S₈) catalyst achieves low overpotentials of 102 and 238 mV at 10 and 100 mA cm^?2 in the 1 M KOH solution, respectively, remarkably higher than the single-doping Mn–Co₉S₈ and N–Co₉S₈ as well as superior to many reported Co₉S₈-based HER electrocatalysts. Density functional theory (DFT) calculation results confirm that the water dissociation barrier of the Mn–N–Co₉S₈ is reduced significantly owing to the synergistic co-doping of Mn and N, which accounts for the enhanced alkaline HER performance. This study offers an effective strategy to enhance the alkaline HER activity of TMSs by accelerating water dissociation kinetic via the cation and anion co-doping strategy.     

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.     

Computational exploration of magnesium-decorated carbon nitride (g-C3N4) monolayer as advanced energy storage materials

Density functional theory (DFT) computational studies were conducted to explore the hydrogen storage performance of a monolayer material that is built on the base of carbon nitride (g-C₃N₄, heptazine structure) with decoration by magnesium (Mg). We found that a 2 × 2 supercell can bind with four Mg atoms. The electronic charges of Mg atoms were transferred to the g-C₃N₄ monolayer, and thus a partial electropositivity on each adsorbed Mg atom was formed, indicating a potential improvement in conductivity. This subsequently causes the hydrogen molecules’ polarization, so that these hydrogen molecules can be efficiently adsorbed via both van der Waals and electrostatic interactions. To note, the configurations of the adsorbed hydrogen molecules were also elucidated, and we found that most adsorbed hydrogen molecules tend to be vertical to the sheet plane. Such a phenomenon is due to the electronic potential distribution. In average, each adsorbed Mg atom can adsorb 1–9 hydrogen molecules with adsorption energies that are ranged from ?0.25 eV to ?0.1 eV. Moreover, we realised that the nitrogen atom can also serve as an active site for hydrogen adsorption. The hydrogen storage capacity of this Mg-decorated g-C₃N₄ is close to 7.96 wt %, which is much higher than the target value of 5.5 wt % proposed by the U.S. department of energy (DOE) in 2020 [1]. The finding in this study indicates a promising carbon-based material for energy storage, and in the future, we hope to develop more advanced materials along this direction.     

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.     

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.     

Investigating hydrogen evolution reaction properties of a new honeycomb 2D AlC

The hydrogen due to its high mass energy density is a new renewable, economically viable and clean resource. The most eco-friendly and economical approaches for the generation of hydrogen through hydrogen evolution is electrochemical water splitting. The two-dimensional (2D) nanomaterials have been recently found as potential candidates as non-noble metal catalyst for hydrogen evolution. In this work, we have systematically studied the structural and electronic properties of the newly predicted hexagonal-aluminium carbide monolayer (h-AlC ML) under the framework of dispersion-corrected density functional theory (DFT) calculations. The calculated electronic total density of states (TDOS) of h-AlC ML predict its metallic nature in contrast to other polar honeycomb 2D materials which are either semiconducting or semimetallic. The metallic behavior of h-AlC monolayer which motivates us to investigate its HER activity results due to the presence of delocalized charge density near Fermi level. Thus, we have investigated the HER activity of h-AlC ML by calculating hydrogen (H) adsorption energy (ΔE_H) and Gibbs free energy (ΔG_H) at three different sites of the 3 × 3 and 4 × 4 supercells of h-AlC ML; top of carbon atom (E_H-C), top of aluminium atom (E_H-Al) and hollow site (E_H-Hollow). Our results show that the hollow site is most catalytically active site in both supercells of h-AlC ML. We believe that our results will inspire experimentalists to fabricate this new 2D material for achieving the desired range of HER activity.