High hydrogen storage ability of a decorated g-C3N4 monolayer decorated with both Mg and Li: A density functional theory (DFT) study

In this work, the hydrogen storage properties of a g-C₃N₄ monolayer decorated with both Mg and Li were thoroughly investigated by performing density functional theory (DFT) calculations. Along these lines, the projected densities of states (PDOS) and the Bader Charge analysis showed that both Mg and Li atoms can transfer their electronic charges to the g-C₃N₄ monolayer. Interestingly, the latter is transformed from a semiconductor material to a metallic conductor configuration, while a local electric field is formed around it. On top of that, the formed local electric field polarized hydrogen molecules and as a result, led to an enhanced hydrogen adsorption ability. Mg atoms have more outmost electrons, and more charges can be transferred to the monolayer, which leads to the creation of a stronger local electric field to adsorb an elevated number of hydrogen molecules than Li atoms. On the other hand, Li atoms are lighter, more active and easier to lose outmost electrons than Mg atoms. By considering these advantages, a g-C₃N₄ monolayer decorated with one unit of both Mg and Li was investigated, which has the ability to adsorb 10 hydrogen molecules, leading thus to a high hydrogen storage capacity of 10.01 wt %. Our work paves the way for the development of novel material configurations for improved hydrogen storage applications.     

Computational insights of alkali metal (Li / Na / K) atom decorated buckled bismuthene for hydrogen storage

The mechanism of hydrogen molecule adsorption on 2D buckled bismuthene (b-Bi) monolayer decorated with alkali metal atoms was studied using density functional theory based first principles calculations. The decorated atoms Li, Na and K exhibited distribution on surface of b-Bi monolayer with increasing binding energy of 2.6 eV, 2.9 eV and 3.6 eV respectively. The adsorption of H₂ molecule on the slabs appeared stable which was further improved upon inclusion of van der Waals interactions. The adsorption behaviour of H₂ molecules on the decorated slabs is physisorption whereas the slabs were able to bind up to five H₂ molecules. The average adsorption energy per H₂ molecules are in range of 0.1–0.2 eV which is good for practical applications. The molecular dynamics simulation also confirmed the thermodynamic stabilities of five H₂ molecules adsorbed on the decorated slabs. The storage capacity values are found 2.24 wt %, 2.1 wt %, and 2 wt %, for respective cases of Li, Na and K atoms decorated b-Bi. The analysis of the adsorbed cases pointed to electrostatic interaction of Li and H₂ molecule. The adsorption energies, binding energies, charge analysis, structural stability, density of states, and hydrogen adsorption percentage specifies that the decorated b-Bi may serve as an efficient hydrogen storage material and could be an effective medium to interact with hydrogen molecules at room temperature.     

Computational Evaluation of Li-doped g-C2N Monolayer as Advanced Hydrogen Storage Media

The emerging 2D g-C₂N obtained increasingly more popularity in functional materials design, and its natural porosity can easily accommodate metal atoms, making itself more suitable for energy gases storage. In this study, we employed DFT computational studies to systematically solve the electronic structure of Li-doped g-C₂N monolayer, and evaluate its performance in hydrogen storage. In our calculations, we found that each pore of g-C₂N can adsorb at most three Li atoms that bind with pyridinic N atoms. We also noticed that considerable amount of charges were transferred from the adsorbed Li to the pristine materials, potentially enhancing its overall conductivity. The change of electronic structure also leads to its improved performance in H₂ adsorption, due to the fact that the electrostatic interactions between the adsorbed H₂ and Li can be largely enhanced. The optimised configurations of the Li-doped g-C₂N with multiple adsorbed H₂ molecules were presented, and the fundamental mechanisms of adsorption were also investigated in details. The highest storage capacity of hydrogen by Li-doped g-C₂N can reach to 7.8 wt%, much higher than the target value of 5.5 wt %, defined by the U.S department of energy (DOE). Moreover, except Li, we also found that the nitrogen atoms or the N-C bonds can also serve as active adsorption sites. The computational explorations conducted in this study actually indicates a promising prospect of alkali metals decorated 2D materials in the area of hydrogen storage; and we believe the performance of these kinds of novel materials can be further enhanced via more decent modifications.     

Hydrogen adsorption and storage behaviors of Li-decorated PdS2 monolayer: An extended tight-binding study based on DFT

The hydrogen adsorption and storage of the lithium-decorated PdS₂ monolayer at nano-size has been investigated by using extended tight-binding (GFN1-xTB) based on density functional theory (DFT). The calculation results demonstrate that the average adsorption energies of 1–5H₂ change in 0.47–0.20 eV/H₂ range which decreases with increasing of adsorbed hydrogen molecule number. The gravimetric density for hydrogen storage adsorption with 12Li atom and 60H₂ molecules of Li-decorated PdS₂ monolayer is about 6.98 wt% considered as possible application in hydrogen storage. The examination of the hydrogen store mechanism between the monolayer and hydrogen molecules is presented by polarization between Pd and H₂, which can be effect on the adsorption behavior.     

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.     

Potassium decorated γ-graphyne as hydrogen storage medium: Structural and electronic properties

Different sites for K adsorption in γ-graphyne were investigated using density functional theory (DFT) calculations and optical and structural properties of the structures were examined. For the most stable structures, we put one H₂ molecule in different directions on the various sites to evaluate the hydrogen adsorption capability of them. Then, one to nine H₂ molecules in sequence were added to the best structure. Results show that clustering of the K atoms is hindered on the graphyne surface and the most desirable adsorption site for K atom is the hollow site of 12-membered ring with adsorption energy of 5.86 eV. Also, this site is the best site for H₂ adsorption onto K-decorated graphyne with E_das of −0.212 eV. Adding of number of H₂ molecule on this site shows that K atom can bind nine H₂ molecules at one side of the graphyne with the average adsorption energy of 0.204 eV/H₂. Therefore, for one side ca. 8.95 wt % and for both sides of the graphyne with a K atom in each side ca. 13.95 wt % of the hydrogen storage capacity can be achieved. This study shows that K-decorated graphyne can be a promising candidate for the hydrogen storage applications.     

First-principles study of superior hydrogen storage performance of Li-decorated Be2N6 monolayer

The potential application of pristine Be₂N₆ monolayer and Li-decorated Be₂N₆ monolayer for hydrogen storage is researched by using periodic DFT calculations. Based on the obtained results, the Be₂N₆ monolayer gets adsorb up to seven H₂ molecules with an average binding energy of 0.099 eV/H₂ which is close to the threshold energy of 0.1 eV required for practical applications. Decoration of the Be₂N₆ monolayer with lithium atom significantly improves the hydrogen storage ability of the desired monolayer compared to that of the pristine Be₂N₆ monolayer. This can be attributed to the polarization of H₂ molecules induced by the charge transfer from Li atoms to the Be₂N₆ monolayer. Decoration of Be₂N₆ monolayer with two lithium atoms gives a promising medium that can hold up to eight H₂ molecules with average adsorption energy of 0.198 eV/H₂ and hydrogen uptake capacities of 12.12 wt%. The obtained hydrogen uptake capacity of 2Li/Be₂N₆ monolayer is much higher than the target set by the U.S. Department of Energy (5.5 wt% by 2020). Based on the van't Hoff equation, it is inferred that hydrogen desorption can occur at T_D = 254 K for 2Li/Be₂N₆ (8H₂) system which is close to ambient conditions. This is a remarkable result indicating important practical applications of 2Li/Be₂N₆ medium for hydrogen storage purposes.     

A density functional theory and grand canonical Monte Carlo simulations study the hydrogen storage on the Li-decorated net-τ

To find ideal hydrogen storage media, hydrogen storage performance of Li decorated net-τ has been investigated by first-principles calculations. Maximum 6 Li atoms are adsorbed on net-τ, with the average binding energy of 2.15 eV for per Li atom. Based on 6Li-decorated net-τ, up to twenty H₂ molecules are adsorbed, with a high H₂ storage capacity of 12.52 wt% and an appropriate adsorption energy of 0.21 eV/H₂. Finally, H₂ uptake performance is measured by GCMC simulations. Our results suggest that Li-decorated net-τ may be a promising hydrogen storage medium under realistic conditions.     

Computational evaluation of Mg-decorated g-CN as clean energy gas storage media

Ab initio studies were conducted to evaluate the performance of hydrogen storage by Mg-decorated graphite carbon nitride (g-CN, heptazine structure). In our calculations, we found that each unit of this material can accommodate one Mg atom. Partial charges from Mg were transferred to the pristine material, making itself more electropositive. This is favorable for hydrogen storage, as the adsorbed H₂ molecules can be easily polarized, and the electrostatic interactions can be enhanced. The configurations of the Mg-decorated g-CN with multiple adsorbed H₂ molecules were presented in this study, and the related adsorption mechanisms were also discussed in details. Each unit can adsorb at most 7 H₂ molecules with adsorption energies ranging from −0.276 eV to −0.130 eV. In addition, besides Mg, we also noticed that the nitrogen atoms also perform well in hydrogen adsorption. For this novel material, its highest capacity of hydrogen storage can reach to 7.8 wt%, highly surpassing the target value of 5.5 wt% set by the U.S. department of energy (DOE)[1]. The computational results provided in this study indicates a promising prospect for alkali metal functionalized 2D materials in energy storage; and through decent explorations, the performance of this class of materials can be largely improved.     

A novel lithium decorated N-doped 4,6,8-biphenylene for reversible hydrogen storage: Insights from density functional theory

Two-dimensional (2D) carbon-based (C-based) and carbon-nitrogen (C–N) materials have great potential in the energy harvest and storage fields. We investigate a novel carbon biphenylene (C468) consisting of four-, six- and eight-membered rings of sp² carbon atoms (Fan et al., Science, 372:852-6 (2021)) for hydrogen storage. Using first-principles based Density functional theory calculations, we study the geometrical and electronic properties of C468 and N-doped C468. Lithium (Li) atoms were symmetrically adsorbed on both sides of the substrate, and their adsorption positions were determined. The maximum gravimetric density of hydrogen (H₂) adsorbed symmetrically on both sides of Li atom was studied within the scope of physical adsorption process (−0.2 eV/H₂ ∼ −0.6 eV/H₂). Li-decorated C468 can adsorb 8 upper hydrogen molecules and 8 lower hydrogen molecules, and Li-decorated N-doped C468 can adsorb 9 upper hydrogen molecules and 9 lower hydrogen molecules. The gravimetric densities of Li-decorated C468 and Li-decorated N-doped C468 can reach 9.581 wt% and 10.588 wt%, respectively. Our findings suggest significant insights for using Li-decorated C468 and Li-decorated N-doped C468 as hydrogen storage candidates and effectively expand the application scope of C-based materials and C–N materials.     

Hydrogen adsorption around lithium atoms anchored on graphene vacancies

Density functional theory and molecular dynamics were used to study the interaction of a lithium atom with a vacancy inside a graphene layer. It was found that the lithium atom is adsorbed on this vacancy, with a binding energy much larger than the lithium cohesive energy. Then, the adsorption of hydrogen molecules around lithium atoms was studied. We found that at 300 K and atmospheric pressure, this system could store up to 6.2 wt.% hydrogen, with average adsorption energy of 0.19 eV per molecule. Thus, this material satisfies the gravimetric capacity requirements for technological applications. A complete desorption of hydrogen occurs at 750 K. However, a multilayer of this system would be required for practical reasons. Under atmospheric pressure and at 300 K, we found that a system made of multiple layers of this material is stable. The storage capacity remained at 6.2 wt.%, but all adsorbed molecules were dissociated. The average adsorption energy becomes 0.875 eV/H.     

Hydrogen storage in scandium decorated triazine based g-C3N4: Insights from DFT simulations

Hydrogen is being considered a ‘fuel of the future,’ a viable alternative to fossil fuels in fuel cell vehicles. Using Density Functional Theory simulations, reversible, onboard hydrogen storage in Sc-decorated triazine-based graphitic carbon nitride (g-C₃N₄) has been explored. Sc atom binds strongly on the g-C₃N₄ structure with a binding energy of ?7.13 eV. Each Sc atom can reversibly bind 7 molecules of hydrogen, giving a net gravimetric storage capacity of 8.55 wt%, an average binding energy of ?0.394 eV per H₂, and a corresponding desorption temperature of 458.28 K, fulfilling the criteria prescribed by the US Department of Energy. The issue of transition metal clustering has been investigated by computing the diffusion energy barrier (2.79 eV), which may be large enough to hinder the clustering tendencies. The structural integrity of Sc-g-C₃N₄ has been verified through ab-initio Molecular Dynamics simulations. The interaction mechanism of Sc over g-C₃N₄ and H₂ over Sc-g-C₃N₄ has been explored using density of states and charge transfer analysis. A flow of charge from valence 3d orbitals of Sc towards vacant orbitals of g-C₃N₄ during the binding of Sc over g-C₃N₄ is observed. The binding of H₂ on Sc-g-C₃N₄ may be via Kubas type of interactions which is stronger than physisorption due to net charge gain by H 1s orbital from Sc 3d orbital. Our systematic investigations indicate that Sc-decorated g-C₃N₄ may be a high-performance material for reversible hydrogen storage applications.     

Yttrium-decorated C48B12 as hydrogen storage media: A DFT study

We report a density functional theory calculation dedicated to analyze the behavior of hydrogen adsorption on Yttrium-decorated C₄₈B₁₂. Electron deficient C₄₈B₁₂ is found to promote charge transfer between Y atom and substrate leading to an enhanced local electric field which can significantly improve the hydrogen adsorption. The analysis shows that Y atoms can be individually adsorbed on the pentagonal sites without clustering of the metal atoms, and each Y atom can bind up to six H₂. molecules with an average binding energy of −0.46 eV/H₂, which is suitable for ambient condition hydrogen storage. The Y atoms are found to trap H₂ molecules through well-known “Kubas-type” interaction. Our simulations not only clarify the mechanism of the reaction among C₄₈. B₁₂, Y atoms and H₂ molecules, but also predict a promising candidate for hydrogen storage application with high gravimetric density (7.51%).     

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.     

Stabilized Li-decoration and enhanced hydrogen storage on reduced graphene oxides

Hydrogen storage properties of Li-decorated graphene oxides containing epoxy and hydroxyl groups are studied by using density functional theory. The Li atoms form Li₄O/Li₃OH clusters and are anchored strongly on the graphene surface with binding energies of −3.20 and −2.84 eV. The clusters transfer electrons to the graphene substrate, and the Li atoms exist as Li⁺ cations with strong adsorption ability for H₂ molecules. Each Li atom can adsorb at least 2H₂ molecules with adsorption energies greater than −0.20 eV/H₂. The hydrogen storage properties of Li-decorated graphene at different oxidation degrees are studied. The computations show that the adsorption energy of H₂ is −0.22 eV/H₂ and the hydrogen storage capacity is 6.04 wt% at the oxidation ratio O/C = 1/16. When the O/C ratio is 1:8, the storage capacity reaches 10.26 wt% and the adsorption energy is −0.15 eV/H₂. These results suggest that reversible hydrogen storage with high recycling capacities at ambient temperature can be realized through light-metal decoration on reduced graphene oxides.     

Computational evaluation of superalkali-decorated graphene nanoribbon as advanced hydrogen storage materials

In this study, we proposed that homo superalkali NM₄ clusters with high tetrahedral geometry, can be applied to develop high-performance hydrogen storage materials. Moreover, their special bonding structures and chemical stability make them ideal units for decoration of different kinds of pristine monolayers. We made a trial to decorate the NLi₄ clusters onto the 1D graphene nanoribbon, and employed density functional theory (DFT) computational studies to solve its electronic structure, and further evaluate its applicability in hydrogen storage. We found that the electronic charges on Li atoms were successfully transferred to the pristine monolayer, thus a partial electronic field around each Li atom was formed. This subsequently leads to the polarization of the adsorbed hydrogen molecules, and further enhances the electrostatic interactions between the Li atoms and hydrogen. Each NLi₄ cluster can adsorb at most 16 hydrogen molecules. For this novel material, its total capacity of hydrogen storage can reach to 11.2 wt %, surpassing the target value of 5.5 wt %, set by the U.S department of energy (DOE) [1], making itself an ideal unit for advanced energy materials design.     

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.     

An investigation of Li-decorated N-doped penta-graphene for hydrogen storage

By making use of first principles calculations, lithium-decorated (Li-decorated) and nitrogen-doped (N-doped) penta-graphene (PG) was investigated as a potential material for hydrogen storage. The geometric and electronic structures of two types of N-doped PG were studied, and the band gaps were 1.86 eV and 2.06 eV, respectively, depending on the positions of the substitution. The probable adsorption sites for Li atoms on topside and downside were calculated. Hydrogen molecules were added one by one to Li-decorated N-doped PG to research the maximum hydrogen gravimetric density. It is found that up to 5 hydrogen molecules on topside and 8 hydrogen molecules on downside can be adsorbed around a Li atom, and the average adsorption energies are in the range of physical adsorption processes (0.1–0.4 eV). The gravimetric densities can reach 7.88 wt% for N-doped PG with Li decoration. Our results suggest that Li-decorated N-doped PG is a significantly promising material for hydrogen storage.     

Ultrahigh reversible hydrogen storage in K and Ca decorated 4-6-8 biphenylene sheet

By applying density functional theory (DFT) and ab-initio molecular dynamics (AIMD) simulations, we predict the ultrahigh hydrogen storage capacity of K and Ca decorated single-layer biphenylene sheet (BPS). We have kept various alkali and alkali-earth metals, including Na, Be, Mg, K, Ca, at different sites of BPS and found that K and Ca atoms prefer to bind individually on the BPS instead of forming clusters. It was found that 2?2?1 supercell of biphenylene sheet can adsorb eight K, or eight Ca atoms, and each K or Ca atom can adsorb 5H_2, leading to 11.90% or 11.63% of hydrogen uptake, respectively, which is significantly higher than the DOE-US demands of 6.5%. The average adsorption energy of H₂ for K and Ca decorated BPS is ?0.24 eV and ?0.33 eV, respectively, in the suitable range for reversible H₂ storage. Hydrogen molecules get polarized in the vicinity of ionized metal atoms hence get attached to the metal atoms through electrostatic and van der Waals interactions. We have estimated the desorption temperatures of H₂ and found that the adsorbed H₂ can be utilized for reversible use. We have found that a sufficient energy barrier of 2.52 eV exists for the movement of Ca atoms, calculated using the climbing-image nudged elastic band (CI-NEB) method. This energy barrier can prevent the clustering issue of Ca atoms. The solidity of K and Ca decorated BPS structures were investigated using AIMD simulations.     

Ruthenium decorated single walled carbon nanotube for molecular hydrogen storage: A first-principle study

Molecular hydrogen storage on Ruthenium (Ru) decorated single-walled carbon nanotube (SWCNT) has been studied by using spin-polarized density functional theory (DFT). When a Ru atom is adsorbed on SWCNT, the Bader analysis reveals that Ru transfers a charge of 0.44e⁻ to SWCNT. Accordingly, Ru acts as adsorption center for H₂ molecules; thus, it can hold up to four H₂ molecules with an adsorption energy (E_ads) of −0.93 eV/H₂. A uniform addition of Ru atoms on SWCNT shows that this nanomaterial can adsorb up to five Ru without clustering. Each Ru atom of 5Ru-decorated SWCNT system can bind up to four H₂ molecules involving an E_ads of −0.83 eV/H₂. After H₂ molecules adsorption, Ru atoms shifted from a near hollow site to a bridge site. Moreover, Ru-decorated systems reduce their magnetic moment when the number of H₂ molecules increase from 2 μ_B to 0 μ_B.