Enhanced reversible hydrogen storage performance of light metal-decorated boron-doped siligene: A DFT study

The use of nanomaterials for hydrogen storage could play a very important role in the large-scale utilization of hydrogen as an energy source. However, nowadays several potential hydrogen storage nanomaterials do not have a large gravimetric density and stability at room temperature. In this work, we have investigated the hydrogen storage performances of Na-, K- and Ca-decorated B-doped siligene monolayer (BSiGeML) using density functional theory calculations. The results show that boron doping improves the interaction between the metal adatom and the siligene monolayer (SiGeML). The K- and Ca-decorated BSiGeMLs can bind up to seven H₂ molecules per metal adatom, whereas Na-decorated BSiGeML only adsorb four H₂ molecules per adsorption site. The effect of temperature and pressure on the hydrogen storage capacity of BSiGeMLs was also evaluated. At room temperature, all the H₂ molecules adsorbed on Na-, and Ca-decorated BSiGeML are stable at mild pressure. The metal decoration of both sides of BSiGeML may lead to hydrogen gravimetric densities exceeding the target of 5.5 wt% proposed by DOE for the year 2025. K- and Ca-decorated BSiGeML could be efficient hydrogen molecular storage media compared to undoped SiGeML and other 2D pristine materials.     

The reversible hydrogen storage abilities of metal Na (Li,K, Ca,Mg, Sc,Ti, Y) decorated all-boron cage B28

The density functional theory is used to study the hydrogen storage abilities of alkali metal Li (Na, K), alkaline-earth metal Mg (Ca), and transition metal Ti (Ti, Sc, Y) decorated B₂₈, which is the possible smallest all-boron cage and contains one hexagonal hole and two octagonal holes. The most stable structure of B₂₈ explored by the calypso search is as same as that explored by Zhao et al. [Nanoscale 7(2015)15086]. It is calculated that the hollow sites outside of the cavities should be the most stable for all metals except for Ti. The average adsorption energy of H₂ molecules (E_ad) adsorbed by each Na (Ca, K, Mg, Sc, Y and Li) atom outside of the B₂₈ cage are in the range from 0.2 to 0.6 eV, which is suitable for hydrogen storage under near-ambient conditions. However, the largest hydrogen gravimetric density (HGD) for the B₂₈Sc₃-12H₂ structure is smaller than the target of 5.5 wt% by the year 2017 specified by the US Department of Energy (DOE). Therefore, the metal Ti (Sc) decorated all-boron cage B₂₈ should not be good candidates for hydrogen storage. The calculated desorption temperature and the molecular dynamic simulation indicate that the B₂₈M₃-nH₂ (M = Na, Li, Ca, K, Mg, Y) structures are easy to desorb the H₂ molecules at the room temperature (T = 300 k). Furthermore, the B₂₈ cages bridged by the sp²-terminated B₅ chain can hold Na (Li, Ca, K, Mg, Y) atoms to capture hydrogen molecules with moderate E_ad and HGD. These findings suggest a new route to design hydrogen storage materials under the near-ambient conditions.     

Density functional theory study on hydrogen storage capacity of metal-embedded penta-octa-graphene

H₂ storage capabilities of penta-octa-graphene (POG) adorned by lightweight alkali metals (Li, Na, K), alkali earth metals (Be, Mg, Ca) and transition metals (Sc, Ti, V, Cr, Mn) are studied by density functional theory. Metals considered, with the exception of Be and Mg, can be stably adsorbed to POG, effectively avoiding metal clustering. The average H₂ adsorption energies are calculated in a range from 0.14 to 0.95 eV for Li (Na, K, Ca, Sc, Ti, V, Cr, Mn) decorated POG. Because the H₂ adsorption energies for reversible physical adsorption lie in the range of 0.15–0.60 eV and the desorption temperatures fall in the range of 233–333 K under the delivery pressure, 4Li@POG and 2Ti@POG are found to be the most suitable for H₂ storage at ambient temperature. By polarization and hybridization mechanisms, up to 3 and 5 hydrogen molecules are stably adsorbed around each Li and Ti, respectively. The H₂ gravimetric densities can reach up to 9.9 wt% and 6.5 wt% for Li and Ti decorated POG, respectively. Our findings suggest that, with metal decoration, such a novel two-dimensional carbon-based structure could be a promising medium for H₂ storage.     

Alkali and transition metal atom-functionalized germanene for hydrogen storage: A DFT investigation

In this work, we have performed density functional theory-based calculations to study the adsorption of H₂ molecules on germanene decorated with alkali atoms (AM) and transition metal atoms (TM). The cohesive energy indicates that interaction between AM (TM) atoms and germanene is strong. The values of the adsorption energies of H₂ molecules on the AM or TM atoms are in the range physisorption. The K-decorated germanene has the largest storage capacity, being able to bind up to six H₂ molecules, whereas the Au and Na atoms adsorbed five and four H₂ molecules, respectively. Li and Ag atoms can bind a maximum of three H₂ molecules, while Cu-decorated germanene only adsorbed one H₂ molecule. Formation energies show that all the studied cases of H₂ molecules adsorbed on AM and TM atom-decorated germanene are energetically favorable. These results indicate that decorated germanene can serve as a hydrogen storage system.     

Metal adatoms-decorated silicene as hydrogen storage media

Hydrogen storage properties of 10 different adatom decorated silicene are carried out using density functional theory calculations with long-range van der Waals dispersion correction. It is found that the binding energy between metal adatoms and the silicene is greater than the cohesive energy of bulk metal so that clustering of adatom will not occur once it is bonded with silicene. The adsorption of H₂ on Li, Na, K, Mg, Ca, and Au decorated silicene is a weak physisorption. Differently, a weaker chemisorption is responsible for the adsorption of H₂ on Be, Sc, Ti, and V decorated silicene. In particular, silicene with Na, K, Mg, and Ca decorating on both sides leads to 7.31–9.40 wt% hydrogen storage capacity with desirable adsorption energy, indicating that the metal-decorated silicene can serve as a high capacity hydrogen storage medium.     

Tin carbide monolayers decorated with alkali metal atoms for hydrogen storage

In this work, a density-functional study of hydrogen storage in tin carbide monolayers (2DSnC) decorated with alkali metals atoms (AM) such as Li, Na, and K, is reported. The most stable adsorption site for these alkali metal atoms on the 2DSnC is above a tin atom. The results indicate that the alkali metal atoms are chemisorbed on the 2DSnC and that electronic charge is transferred from the decorating atom to the 2DSnC. In all the studied cases, the hydrogen molecules are physisorbed on the AM-2DSnC (AM = Li, Na, and K) complexes and then these systems could be used for hydrogen storage. In particular, it is found that the K-2DSnC monolayer has the highest hydrogen-storage capacity, where a single potassium atom can adsorb up to 6 hydrogen molecules, followed by Na-2DSnC with 5 hydrogen molecules and Li-2DSnC with 3 hydrogen molecules. Finally, it can be estimated that when the K, Na and Li adatom-coverings respectively attain 40%, 44% and 70%, the hydrogen-storage gravimetric capacities of AM-2DSnC could overcome the US-DOE recommended target of 5.5 wt% for onboard automotive systems.     

Hydrogen adsorption on Li metal in boron-substituted graphene: An ab initio approach

The characteristics of hydrogen adsorption on Li metal atoms dispersed on graphene with boron substitution is investigated including Li clustering, hydrogen bonding characteristics, and the open metal states of Li adatom using density functional theory calculations. It is found that Li atoms are well dispersed on boron-substituted graphene and can form the (2 × 2) pattern because clustering of Li atoms is hindered by the repulsive Coulomb interaction between Li atoms. One Li adatom dispersed on the double side of graphene can absorb up to 8 hydrogen molecules corresponding to a 13.2% hydrogen storage capacity. In addition, the adsorption behaviors of non-hydrogen atoms such as C and B are calculated to determine whether Li atoms can remain as the open metal state in boron-substituted graphene.     

Sc and Ti-functionalized 4-tert-butylcalix[4]arene as reversible hydrogen storage material

Using the idea of metal functionalized material for H₂ storage, 4-tert-butylcalix[4]arene (CA) functionalized with Sc and Ti atoms are explored. The first principles density functional theory (DFT) with M06 functional and 6-311G(d,p) basis set is used to explore the hydrogen storage properties of metal functionalized CA. Sc and Ti strongly binds with CA by Dewar coordination with high binding energy. It is found that maximum four hydrogen molecules are adsorbed on each metal site in Sc and Ti functionalized CA. Hydrogen molecules are adsorbed on metals by Kubas and Niu-Rao-Jena mechanism. In Sc functionalized CA system all 4 hydrogen molecules on each Sc bind in molecular fashion while on each Ti in Ti functionalized CA, the first hydrogen molecule binds in dissociative fashion and remaining three hydrogen molecules bind in a molecular form. The stability of Sc and Ti functionalized CA is studied by computing conceptual DFT parameters, which obeys maximum hardness and minimum electrophilicity principle. Hirshfeld charge analysis and electrostatic potential map explore the charge transfer mechanism during the hydrogen adsorption. Born-Oppenheimer molecular dynamics simulations are performed at temperature range 200–473 K to study the stability of the system and the reversibility of adsorbed hydrogen from the system. The calculated H wt% is found to be 10.3 and 10.1, respectively for Sc and Ti functionalized CA systems on complete H₂ saturation. This study explores that Sc and Ti functionalized CA systems are efficient reversible hydrogen storage material.     

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.     

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.     

Ab initio and periodic DFT investigation of hydrogen storage on light metal-decorated MOF-5

The effect of light metal (M = Li, Be, Mg, and Al) decoration on the stability of metal organic framework MOF-5 and its hydrogen adsorption is investigated by ab initio and periodic density functional theory (DFT) calculations by employing models of the form BDC:M₂:nH₂ and MOF-5:M₂:nH₂, where BDC stands for the benzenedicarboxylate organic linker and MOF-5 represents the primitive unit cell. The suitability of the periodic DFT method employing the GGA-PBE functional is tested against MP2/6-311 + G* and MP2/cc-pVTZ molecular calculations. A correlation between the charge transfer and interaction energies is revealed. The metal-MOF-5 interactions are analyzed using the frontier molecular orbital approach. Difference charge density plots show that H₂ molecules get polarized due to the charge generated on the metal atom adsorbed over the BDC linker, resulting in electrostatic guest-host interactions.Our solid state results show that amongst the four metal atoms, Mg and Be decoration does not stabilize the MOF-5 to any significant extent. Li and Al decoration strengthened the H₂-MOF-5 interactions relative to the pure MOF-5 exhibited by the enhanced binding energies. The hydrogen binding energies for the Li- and Al-decorated MOF-5 were found to be sensible for allowing reversible hydrogen storage at ambient temperatures. A high hydrogen uptake of 4.3 wt.% and 3.9 wt.% is also predicted for the Li- and Al-decorated MOF-5, respectively.     

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.     

Ab initio study of hydrogen storage on metal-decorated GeC monolayers

Bidimensional nanostructures have been proposed as hydrogen-storage systems owing to their large surface-to-volume ratios. Germanium carbide monolayers (GeC-MLs) can offer attractive opportunities for H₂ adsorption compared to graphene. However, this possibility has not been explored in detail. In this work, the adsorption of H₂ molecules on GeC-MLs decorated with alkali metal (AM) and alkaline earth metal (AEM) adatoms was investigated using the density functional theory. Results showed that the AM adatoms were chemisorbed on the GeC-ML, whereas AEM adatoms were physisorbed. The H₂ molecules presented negligible adsorption energies on the weakly adsorbed AEM adatoms. Conversely, the AM adatoms improved the H₂ adsorption, possibly due to a large charge transfer from the adatoms to the GeC-ML. The potassium-decorated GeC-ML exhibited the most optimal H₂ storage capacity, adsorbing up to six molecules and with a lower possibility of forming metal clusters than the other studied cases. These results may aid in the development of new efficient hydrogen-storage materials.     

Enhancement of hydrogen storage properties of metal-organic framework-5 by substitution (Zn,Cd and Mg) and decoration (Li,Be and Na)

Two strategies of decoration by three elements Z = Li, Be and Na in cyclic site, and substitution of Zn by Mg and Cd in unit cell were used in the framework of functional density theory to tune the hydrogen storage properties of metal-organic framework-5 (MOF-5) based on Zn whose decomposition temperature and initial gravimetric capacity are 300 K and 1.57 wt% respectively.Based on the adsorption of hydrogen molecules in the crystal surface at three different adsorption sites with three orientations of H₂, we show that our system may reach a maximum gravimetric storage capacity of 4.09 wt% for multiple hydrogen molecules. Moreover, the functionalization of Z combined to the substitution, shows an exceptional improvement of hydrogen storage properties. For example, Mg-MOF-5 decorated with Li₂ has a capacity up to 5.41 wt% and 513 K as desorption temperature.     

Hydrogen storage capacities of alkali and alkaline-earth metal atoms on SiC monolayer: A first-principles study

A detailed theoretical Density-Functional-Theory-based investigation of hydrogen adsorption on silicon carbide monolayers (SiC-ML) decorated with alkali and alkaline-earth metal atoms is presented. The results show that the favourable position for all adsorbed metal atoms is above a Si atom. These metal atoms are chemisorbed to the SiC-ML, except for Mg which is physisorbed. The adsorbed atoms act in turn as adsorption sites for H₂ molecules. The single-sided K-functionalized SiC-ML can store up to six H₂ molecules. For double-side K-decorated SiC-ML, up to ten H₂ molecules can be captured. In all cases, the H₂ molecules are physisorbed. This is beneficial because the breaking of chemical bonds, which otherwise would be needed to make use of the stored H₂, is energetically expensive. These results find decorated SiC-ML as a promising material for hydrogen storage systems.     

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.     

Hydrogen storage in lithium,sodium and magnesium-decorated on tetragonal silicon carbide

Motivated by the widespread application and fascinating properties of various silicon-carbon nanomaterials, we have extensively investigated the properties of tetragonal silicon carbides (t-SiC) monolayer as a novel 2D material for hydrogen storage.Using calculations of the density functional theory comprising van der Waals interactions of type vdW-DF2-C09_x, the structural stability, electronic properties and hydrogen molecules adsorption energies on the surface of pure t-SiC were investigated.The results show that adsorption energies of H₂ molecules in this system are stronger than that of graphene. We also found that the decoration with alkali (Li, Na) and alkaline-earth (Mg) metals atoms increases the stability of hydrogen compared to the pure system. The studied system decorated with 8 elements of Li/Na/Mg is able to adsorb up 24 molecules of hydrogen and reaches a gravimetric capacity of 6.50, 5.54 and 5.48 wt%, respectively. The desorption temperatures found are significantly high compared to other 2D systems.     

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.     

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.