Light metal functionalized two-dimensional siligene for high capacity hydrogen storage: DFT study

In this work, the hydrogen storage capacities of two-dimensional siligene (2D-SiGe) functionalized with alkali metal (AM) and alkali-earth metal (AEM) atoms were studied using density functional theory calculations. One AM (Li, Na, K) or AEM (Be, Mg, Ca) atom was placed on the 2D-SiGe surface, and several H₂ molecules were placed in the vicinity of the adatom. The results demonstrate that the most favorable siligene site for the adsorption of Li, Na, K and Be atoms is the hollow site, while for the Mg and Ca atoms is the down site. The AM atoms are the only ones with considerable binding energies on the SiGe nanosheets. Pristine 2D-SiGe slightly adsorbs one H₂ molecule per hollow site and, therefore, it is not suitable for hydrogen storage. In some of the AM- and AEM-decorated 2D-SiGe, several hydrogen molecules can be physisorbed. In particular, the Na-, K- and Ca-functionalized 2D-SiGe can adsorb six hydrogen molecules, whereas Li and Mg atoms adsorbed three hydrogen molecules, and the Be adatom only adsorbed one hydrogen molecule. The complexes formed by hydrogen molecules adsorbed on the analyzed metal decorated 2D-SiGe are energetically stable, indicating that functionalized 2D-SiGe could be an efficient molecular hydrogen storage media.     

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.     

Ca and K decorated germanene as hydrogen storage: An ab initio study

The hydrogen storage capacity and performance of Ca and K decorated germanene were studied using density functional theory calculation. The Ca and K adatoms were found to be sufficiently bonded to the germanene without clustering at the hollow site. Further investigation has shown an ionic bonding is apparent based on the charge density difference and Bader charge analysis. Upon adsorption of H₂ on the decorated germanene, it was found that the Ca and K decorated systems could adsorb 8 and 9 H₂ molecules, respectively. The adsorption energies of H₂ molecules were within the Van der Waals energy (400–435 meV), suggesting weak physisorption. The charge density profile revealed that the electron of H₂ moved toward the adatom decoration without leaving the local region of H₂. This suggests that a dipole-dipole interaction was apparent and consistent with the energy range found. Finally, the gravimetric density obtained from the adsorption of H₂ on the decorated germanene shows that this material is a potential for H₂ storage media.     

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.     

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.     

A comparative study of the reversible hydrogen storage behavior in several metal decorated graphyne

In virtue of the first-principle calculations, the hydrogen storage behavior in several metal decorated graphyne was investigated. It is found that the hydrogen storage capacity can be as large as 18.6, 10.5, 9.9 and 9.5 wt% with average adsorption energy of about −0.27, −0.36, −0.76 and −0.70 eV/H₂ for Li, Ca, Sc, Ti decorated graphyne, respectively. The results suggest potential candidates for hydrogen storage at ambient condition. The adsorption mechanism for H₂ on metal coated graphyne was mainly attributed to the polarization induced by electrostatic field of metal atoms on graphyne and the hybridization between the metal atoms and hydrogen molecules. Furthermore, the formation of super-molecules of hydrogen can enhance the adsorption energy.     

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.     

Calcium-decorated graphdiyne as a high hydrogen storage medium: Evaluation of the structural and electronic properties

Graphdiyne (GDY) is a new member of carbon allotropes consisting of sp and sp² hybridized carbon atoms. In this work, the hydrogen adsorption on Calcium (Ca) decorated GDY and the influence of adatom on structural properties of GDY are investigated, using first principles plane wave calculations with Van der Waals corrections. The results show that similar to graphyne (GY) and unlike carbon nanotube (CNT), fullerene and graphene, clustering of Ca on GDY hinders due to the higher binding energy of the adatom to the carbon frame than the Ca cohesive energy. It can be seen that the Ca-decoration promotes hydrogen storage capacity of GDY, extremely (E_ads = ?0.266 and ?0.066 eV for Ca-decorated and pristine GDY, respectively). It is concluded that, the best site for the Ca trapping is 18-membered ring in which, Ca lies in-plane of GDY (E_ads = ?3.171 eV). Fourteen H₂ molecules (with the average adsorption energy of ～0.2 eV/H₂) can be adsorbed on the Ca atom from one side. The hydrogen storage capacity is estimated to be as high as 17.95 wt% for the both sides of GDY. So, the Ca-decorated GDY is offered as a promising candidate for hydrogen storage applications.     

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.     

Tunable H2 binding on alkaline and alkaline earth metals decorated graphene substrates from first-principles calculations

Based on first-principles calculations, the H₂ adsorptions onto six types of modified graphene substrates decorated with light metals (Li, Na, K, Be, Mg, Ca) are investigated to shed light on the factors affecting the H₂ binding energies. It is demonstrated that the introduction of defects and dopants into graphene substrates is essential to prevent the metal clustering and achieve dispersed metal atoms desirable for H₂ adsorption. The interaction between H₂ and alkali/alkali-earth metal decorated graphene systems is attributed to the electrostatic effect induced by polarized dipole–-dipole interaction. Via introducing defects and hetero-atoms to modify the electronegativity of the local structure, the H₂ adsorption energy can be tuned by choosing the combination of suitable metals and substrates. The calculated H₂ binding strength is positively correlated to the charge transfer from the metal to the substrates and the dipole momentum of metal decorated substrates. Compared the cases with different metals decoration, Mg and Ca are expected to the most promising candidates for multiple H₂ adsorptions.     

Density functional theory study on hydrogen storage capacity of yttrium decorated graphyne nanotube

The hydrogen storage capacity of yttrium decorated graphyne nanotubes is calculated using spin polarized DFT method. The stabilities, electronic properties and the structures of Y attachment on graphyne tube are investigated. It is revealed that Y can be separately adsorbed on graphyne tube with the binding energy of 6.76 eV and the clustering of metal atoms is hindered. The geometry optimization shows that Y atoms decorated graphyne tube can capture 42H₂ molecules through Dewar-Kubas like interaction and the polarization under the electrostatic potential formed by Y and graphyne tubes. The weight percent capacity is 5.71 wt%, with an average hydrogen adsorption energy of −0.153 eV per H₂, indicating its potential application on hydrogen storage candidates.     

Li decorated penta-silicene as a high capacity hydrogen storage material: A density functional theory study

The H₂ adsorption characteristics of Li decorated single-sided and double-sided penta-silicene are predicted via density functional theory (DFT). The orbital hybridization results in Li atom strongly bind onto the surface of the penta-silicene with a large binding energy and it keeps the decorated Li atoms from aggregation. Moreover, Li decorated double-sided penta-silicene can store up to 12H₂ molecules with the average hydrogen adsorption energy of ?0.220 eV/H₂ and hydrogen uptake capacity of 6.42 wt%, respectively. The ab initio molecular dynamics (AIMD) simulations demonstrate the H₂ molecules are released gradually from the substrate material with the increasing simulation time and the calculated desorption temperature T_D is 281 K in the suitable operating temperature range. Our explorations confirm that Li decorated penta-silicene can be regarded as a promising hydrogen storage candidate for hydrogen storage applications.     

High-capacity reversible hydrogen storage properties of metal-decorated nitrogenated holey graphenes

Motivated by the need for an effective way of storing hydrogen (H₂), a promising energy carrier, we have performed density functional theory (DFT) calculations with different van der Waals corrections coupled with the statistical thermodynamic analysis and ab initio molecular dynamics (AIMD) on the light-metal decorated nitrogenated holey graphene (C₂N) monolayers. We have found that the decoration by selected light metals (Na, Mg, Ca) improves the H₂ adsorption on the C₂N to the desired levels (>150 meV/H₂). Moreover, the metal dopants strongly bonded with C₂N even at higher doping concentrations, which invalidates the metal clusters formation. Among considered metals, Na and Mg resulted in H₂ storage capacities of 5.5 and 6.9 wt%, respectively, which exceed the target set by the U.S. Department of Energy's for 2025. Thermodynamic analysis and the AIMD simulations were employed to investigate the H₂ sorption at varied conditions of temperature and pressure for practical applications.     

Li-decorated double vacancy graphene for hydrogen storage application: A first principles study

Lithium decoration is an effective strategy for improving the hydrogen adsorption binding energy and the storage capacity in carbon nanostructures. Here, it is shown that Li-decorated double carbon vacancy graphene (DVG) can be used as an efficient hydrogen storage medium by means of Density Functional Theory (DFT) based calculations. The Li binding energy in DVG is 4.04 eV, which is much higher than that of pristine graphene. A maximum of four hydrogen molecules adsorb on Li decorated on one side of DVG and this leads to a gravimetric storage capacity of 3.89 wt% with an average adsorption binding energy of 0.23 eV/H₂. When Li is decorated on both sides of DVG, the gravimetric storage capacity reaches 7.26 wt% with a binding energy of 0.26 eV/H₂ which shows that desorption would take place at ambient conditions.     

Superalkali NLi4 decorated graphene: A promising hydrogen storage material with high reversible capacity at ambient temperature

This paper investigates the decoration of superalkali NLi₄ on graphene and the hydrogen storage properties by using first principles calculations. The results show that the NLi₄ units can be stably anchored on graphene while the Li atoms are strongly bound together in the superalkali clusters. Decoration using the superalkali clusters not only solve the aggregation of metal atoms, it also provide more adsorption sites for hydrogen. Each NLi₄ unit can adsorb up to 10 H₂ molecules, and the NLi₄ decorated graphene can reach a hydrogen storage capacity 10.75 wt% with an average adsorption energy ?0.21 eV/H₂. We also compute the zero-point energies and the entropy change upon adsorption based on the harmonic frequencies. After considering the entropy effect, the adsorption strengths fall in the ideal window for reversible hydrogen storage at ambient temperatures. So NLi₄ decorated graphene can be promising hydrogen storage material with high reversible storage capacities.     

The effect of Na addition on the first hydrogen absorption kinetics of cast hypoeutectic Mg–La alloys

With superior properties of Mg such as high hydrogen storage capacity (7.6 wt% H/MgH₂), low price, and low density, Mg has been widely studied as a promising candidate for solid-state hydrogen storage systems. However, a harsh activation procedure, slow hydrogenation/dehydrogenation process, and a high temperature for dehydrogenation prevent the use of Mg-based metal hydrides for practical applications. For these reasons, Mg-based alloys for hydrogen storage systems are generally alloyed with other elements to improve hydrogen sorption properties. In this article, we have added Na to cast Mg–La alloys and achieved a significant improvement in hydrogen absorption kinetics during the first activation cycle. The role of Na in Mg–La has been discussed based on the findings from microstructural observations, crystallography, and first principles calculations based on density functional theory. From our results in this study, we have found that the Na doped surface of Mg–La alloy systems have a lower adsorption energy for H₂ compared to Na-free surfaces which facilitates adsorption and dissociation of hydrogen molecules leading to improvement of absorption kinetic. The effect of Na on the microstructure of these alloys, such as eutectic refinement and a density of twins is not highly correlated with absorption kinetics.     

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.     

Electronic properties and hydrogen storage application of designed porous nanotubes from a polyphenylene network

Based on a polyphenylene network, a series of porous graphene nanotubes (PGNTs) are created and optimised via density functional theory calculations. The calculated band dispersion of the two-dimensional porous graphene can be tuned by rolling it into nanotube form. To explore the energy application of PGNTs, we studied H₂ adsorptions on metal (Li, Ca, and Na) decorated structures of PGNTs as well as B-substituted PGNTs. The results indicate that both the curvature effect and B substitution can strengthen the metal binding and prevent the metal atoms from clustering. Particularly for H₂ adsorption, modification of the electronic property by the curvature effect is beneficial to provide more accessible space, leading to much higher adsorption energies of H₂ on PGNTs than that on planar porous graphene, which is promising for the practical application of hydrogen storage.     

Computational evaluation of Ca-decorated nanoporous CN monolayers as high capacity and reversible hydrogen storage media

Developing novel materials with high-capacity and reversible properties for storing hydrogen (H₂) is crucial for energy treatments. We here investigated comprehensively the H₂ storage performance of the Ca-decorated g-CN (Ca@CN) monolayers using first-principles calculations. The Ca atoms can be uniformly decorated into the center of the pores of g-CN monolayers without aggregation. The Ca@CN monolayer has an average H₂ adsorption energy of around 0.163–0.228 eV as well as high H₂ storage capacity of 10.1 wt%. The stabilities of the H₂ adsorption systems are confirmed by high hardness and low electrophilicity. The temperature of desorption is anticipated to be near the room temperature and ideal for fuel cell devices. The thermodynamic analysis along with desorption temperature reveal that the Ca@CN monolayer has promising potentials as reversible and high capacity hydrogen storage materials (HSM), which will motivate experimental efforts to synthesize the high-efficient HSM.