Ultra-high capacity hydrogen storage of B6Be2 and B8Be2 clusters

The B₆Be₂ and B₈Be₂ clusters, adopting fascinating inverse sandwich-like geometries, were recently predicted with quantum chemical calculations. Both systems exhibit the high stability and double aromaticity with 4σ/6π or 6σ/6π delocalized electrons. The hydrogen storage of two systems is studied in the present paper. Our calculations show that B₆Be₂ and B₈Be₂ clusters have the ultra-high capacity hydrogen storage, each Be site can bound up with seven H₂ molecules, corresponding to a gravimetric density of 25.3 wt percentage (wt%) for B₆Be₂ and 21.1 wt% for B₈Be₂, respectively, which far exceeds the target (5.5 wt%) proposed by the US department of energy (DOE) in 2017. The average absorption energies of 0.10–0.45 eV/H₂ for B₆Be₂ and 0.11–0.50 eV/H₂ for B₈Be₂ at the wB97XD level suggest that both systems are ideal for reversible hydrogen storage and release. The reversibility of H₂ molecules on B₆Be₂ and B₈Be₂ complexes are faithfully demonstrated with the Born-Oppenheimer molecular dynamics (BOMD) simulations. The Be-doped boron nanostructure is a promising candidate for ultra-high hydrogen storage 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.     

The high-capacity hydrogen storage of B6Ca2 and B8Ca2 inverse sandwiches

The B₆Ca₂ and B₈Ca₂ clusters adopt interesting inverse sandwich architectures, featuring a prolate B₆ (or perfect B₈) ring jammed with two capping Ca atoms. Both clusters show the high thermodynamic stability due to the double (σ and π) electronic delocalization. In present paper, we computationally studied the hydrogen storage of them. The results suggest that each Ca site in B₆Ca₂ and B₈Ca₂ clusters could store up six H₂, yielding a gravimetric density of 14.2 wt% for B₆Ca₂ and 12.6 wt% for B₈Ca₂. The average adsorption energy for H₂-adsorbed B₆Ca₂ and B₈Ca₂ complexes is within the scope of 0.12–0.15 eV per H₂ at wB97XD level, hinting that two clusters could reversibly store and release hydrogen, which is positively confirmed by the Born-Oppenheimer molecular dynamics simulations. Both B₆Ca₂ and B₈Ca₂ nanoclusters are feasible hydrogen storage media under the ambient condition.     

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.     

Structures and hydrogen storage properties of AeVH3 (Ae = Be,Mg, Ca,Sr) perovskite hydrides by DFT calculations

The capability of hydrogen to be an energy source has made the hydrogen storage as one of the most investigated research fields during the recent years, and novel perovskite materials have become the current focus for hydrogen storage applications. Here we study the AeVH₃ (Ae = Be, Mg, Ca, Sr) perovskite-type hydrides to explorer their potential for hydrogen storage applications using the density functional theory (DFT) implemented CASTEP code along with exchange correlation potential. The study examines the electronic structure, optical properties, elastic features and mechanical stability of the materials. The crystal structure of AeVH₃ compounds is found to be cubic with lattice constant as 3.66, 3.48, 3.76 and 3.83 for Ae = Be, Mg, Ca and Sr compounds, respectively. The calculated electronic structures of these compounds show ionic bonding and no energy bandgap. The mechanical characteristics of compounds are also investigated as to meet the Born stability criterion, these compounds should be mechanically stable. The Cauchy pressure and Pugh criteria revealed that these materials have a brittle character and rather hard. In low energy range, all optical properties are found to be suitable as needed for storing the hydrogen. Furthermore, the gravimetric ratios suggested that all the compounds are suitable for hydrogen storage as a fuel for a longer time and may provide remarkable contributions in diversity of power and transportation applications.     

Properties of BaYO3 perovskite and hydrogen storage properties of BaYO3Hx

In this study, Density Functional Theory (DFT) calculations have been performed for BaYO₃ perovskite with the generalized gradient approximation (GGA) as implemented in Vienna Ab-initio Simulation Package (VASP). The structural optimization of BaYO₃ perovskite have been studied for the five possible phases: cubic, tetragonal, hexagonal, orthorhombic and rhombohedral to determine the most stable phase of BaYO₃ perovskite. It has been found that the cubic phase is the most stable one and electronic and mechanical properties of this phase have been investigated. Moreover, the elastic anisotropy has been visualized in detail by plotting the directional dependence of compressibility, Poisson ratio, Young's and Shear moduli for cubic phase. Then, hydrogen bonding to BaYO₃ perovskite has been conducted and hydrogen storage properties of BaYO₃H_x (x = 3 and 9) such as: formation energy, cohesive energy and gravimetric hydrogen storage capacity have been analyzed. Having no study about BaYO₃ perovskite and hydrogen bonding in the literature makes this study the first considerations of BaYO₃ perovskite. Hence, this work could enlighten the possible future studies.     

Yttrium doped covalent triazine frameworks as promising reversible hydrogen storage material: DFT investigations

Through Density Functional Theory (DFT) simulations, we have explored the possibility of yttrium (Y) doped Triazine (Covalent Triazine Frameworks i.e., CTF-1) to be a promising material for reversible hydrogen storage. We have found that Y atom strongly bonded on Triazine surface can adsorb at the most 7H₂ molecules with an average binding energy of ?0.33 eV/H₂. This boosts the storage capacity of the system to 7.3 wt% which is well above the minimum requirement of 6.5 wt% for efficient storage of hydrogen as stipulated by the US Department of Energy (DoE). The structural integrity over and above the desorption temperature (420 K) has been entrenched through Molecular Dynamics simulations and the investigation of metal-metal clustering has been corroborated through diffusion energy barrier computation. The mechanism of interactions between Y and Triazine as well as between H₂ molecules and Y doped Triazine has been explored via analyses of the partial density of states, charge density, and Bader charge. It has been perceived that the interplay of H₂ molecules with Y on Triazine is Kubas-type of interaction. The above-mentioned analysis and outcomes make us highly optimistic that Y doped Triazine could be employed as reversible hydrogen storage material which can act as an environmentally friendly alternate fuel for transport applications.     

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.     

High capacity and reversible hydrogen storage on two dimensional C2N monolayer membrane

Searching advanced materials with high capacity and efficient reversibility for hydrogen storage is a key issue for the development of hydrogen as a clean energy. Here, we have explored the potential application of C₂N monolayer using as a promising material for hydrogen storage through a comprehensive density functional theory (DFT) investigation. Our calculational results indicate that hydrogen molecule can only form weak interaction on neutral C₂N monolayer with the adsorption energy of 0.06 eV. However, if extra charges (5 e^?) are introduced to the system, the adsorption energy of hydrogen molecule on C₂N will be dramatically enhanced to 0.27 eV. Moreover, once the extra charges are moved from the system, the adsorbed hydrogen molecule will be spontaneously released from C₂N monolayer without any barrier. Interestingly, the average adsorption energy for each of the 48 absorbed H₂ molecules is 0.28 eV with the charge injection (8 e^?). This adsorption energy meets the criterion of the Department of Energy (DOE) for hydrogen storage (0.2–0.6 eV). Moreover, C₂N has a high hydrogen storage capacity of 10.5 wt %. Overall, this investigation demonstrates that the new fabricated C₂N can be used as an efficient material for hydrogen storage with high capacity and reversibility by modifying the charges that it carried. The narrow band gap (1.70 eV) of C₂N also ensures the electrochemical methods can be easily realized in experiment.     

Lithium-functionalized boron phosphide nanotubes (BPNTs) as an efficient hydrogen storage carrier

In this work we have carried out extensive Density Functional Theory (DFT) and ab-initio Molecular Dynamics (AIMD) simulations to study the structural and electronic properties, thermal stability, and the adsorption/desorption processes of hydrogen H₂ molecules on Lithium (Li) functionalized one-dimensional boron phosphide nanotubes (BPNTs), for possible use as materials of H₂-storage media. Our results show that Li atoms can be adsorbed on the hollow sites of (7,7)-BPNT with binding energy ranging from 1.69 eV, for one Li, to 1.65 eV/Li for 14 Li atoms adsorbed on (7,7)-BPNT. These large energies of Li prevent the formation of clusters on the nanotube sidewall. The investigation of the electronic behavior showed that (7.7)-BPNT semiconductor turns metallic upon the Li-adsorption. Furthermore, the average binding energy of H₂-molecules adsorbed on nLi@BPNT(mH₂) systems (with n = 1, 2, 4, 6, 8, 14 and m = 1, 2, 3, 4, with m the number of H₂ for each Li) lies within a range of 0.13–0.20 eV/H₂ which is compatible to the required range for adsorption/desorption of H₂-molecules at room conditions. A H₂-storage gravimetric capacity up to 4.63% was found for 14Li@BPNT(4H₂) system. In addition, AIMD simulation strongly indicates that given adequate monitoring of the temperature, the charge/release process of H₂-molecules can be controlled. Our findings suggest that Li-functionalized (7,7) boron phosphide nanotubes can provide a valuable underlying material for H₂-storage technologies and therefore must certainly be the subject of further experimental exploration.     

Stability and hydrogen storage properties of Sc6O8 and Y6O8 cage-like complexes

To study the dihydrogen storage capacity of Sc₆O₈ and Y₆O₈ complexes, the stability and hydrogen adsorption behavior have been investigated by using density functional theoretical calculations. The lowest-lying isomers are cage-like complexes Sc₆O₈ 01 and Y₆O₈ 01, which are energetically much low-lying by at least 40.43 kcal/mol than the other isomers, respectively. Sc₆O₈ 01 can adsorb 26H₂ with gravimetric uptake capacity of 11.64 wt%. The average adsorption energy (ΔE_ave) is 0.12 eV/H₂, which is in the reversible adsorption range. The Y₆O₈ 01 seem have little ability to adsorb hydrogen molecules, because the ΔE_ave Y₆O₈ 01 (1H₂) is just only 0.065 eV. However, the binding capacity increases with the number of adsorbed H₂ increasing. Y₆O₈ 01 can adsorb 32H₂ with ΔE_ave of 0.11 eV/H₂, and the gravimetric uptake capacity is 8.89 wt%. Various characterization methods indicate that both transition metals and nonmetals in Sc₆O₈ 01 and Y₆O₈ 01 can effectively adsorb hydrogen molecules, and these two compounds can be regarded as candidate materials of dihydrogen adsorption under suitable condition.     

Computational insights into the energy storage of ultraporous MOFs NU-1501-M (M = Al or Fe): Protonization revealing and performance improving by decoration of superalkali clusters

In 2020, Chen et al. reported the synthesis of a series of promising metal–organic frameworks (MOFs) based on Al/Fe trinuclear clusters, known as NU-1501-M (M = Al or Fe). Both the gravimetric and volumetric Brunauer-Emmett-Teller (BET) areas of this novel structure are in an ideal range, making it highly promising for hydrogen storage. However, the physical chemistry of its adsorption processes has not been investigated. In this work, we applied grand canonical Monte Carlo (GCMC), density functional theory (DFT), and ab initio molecular dynamics (AIMD) to study their adsorption behaviours in details. These simulations suggest that the balance between the chemical porosity and the electronic structure is critical in determining the quality of the designed MOFs materials in deliverable energy storage. Moreover, theoretical predictions reveal the possible protonization of oxygen atoms from M trinuclear nodes by hydrogen molecules. To protect MOFs from being protonized, we proposed to employ NAl₃ clusters to decorate the MOFs. Simulations reveal that this novel strategy can not only stablize the oxygen atoms, but also significantly improve the hydrogen storage performance by almost one order of magnitude. Our work proposes an important and promising way to improve the energy storage performance of these MOFs.     

B12@Mg20Al12 core-shell molecule: A candidate for high-capacity hydrogen storage material

Based on the first-principles density functional theory, we proposed a stable icosahedral B₁₂-containing core-shell structure of B₁₂@Mg₂₀Al₁₂. The vibrational frequency analysis and the molecular dynamics (MD) simulations indicate the good stability of B₁₂@Mg₂₀Al₁₂ structure. Analysis of the chemical bonding characters shows that there are multi-center two-electron σ bonds formed by p electrons of B and Al atoms and s electrons of Mg atoms, which can form strong connections in the whole B₁₂@Mg₂₀Al₁₂. This can be the powerful evidence of the structural stability. The potential application of B₁₂@Mg₂₀Al₁₂ in hydrogen storage has also been investigated. Calculation results show that about 146 hydrogen molecules, which present a double-shell distribution, can be absorbed at most, corresponding to a high hydrogen capacity of 23.7 wt%, which means the B₁₂@Mg₂₀Al₁₂ can be a promising candidate of high-capacity hydrogen storage materials.     

First-principles studies concerning optimization of hydrogen storage in nanoporous reduced graphite oxide

By means of Density Functional Theory (DFT) calculations we investigated the optimal pore size for reduced graphite oxide (GOH) to favor hydrogen storage and to prevent oxygen interference. The interlayer distance of GOH is found to increase with oxygen content, given by the number of hydroxyl groups. Four types of GOHs were considered, with O/C ratio within a 0.09–0.38 range. In the case of the highest O/C ratio considered, 0.38, a spontaneous redox-reaction between hydroxyl groups delivering a water molecule and an epoxy group was found. Thus, GOHs with high O/C ratio are not recommended for hydrogen storage. In these materials the absorption energy values of hydrogen is in the range of −0.2 and −0.5 eV/molecule, that is within the values expected to allow an efficient storage. The best GOH for hydrogen storage was found to be that with a 0.09 O/C ratio since it has the largest void space and adequate absorption energy, −0.52 eV/molecule. On the other hand, oxygen absorption energy is lower in absolute value than that of hydrogen, which favors absorption of the latter, thus creating adequate conditions for its storage without oxygen interference.     

Stabilization of Ca7Ge-type magnesium compounds by alloying of non-metal elements: A new family material for reversible hydrogen storage applications

A new family material of Mg_nTMX₂ consisted by alloying non-metal element X into the Ca₇Ge-type Mg_nTM (TM = V, Ti, or Nb; X = C, N, O, F, P, S, or Cl; n = 6 for TM = Nb and V or 7 for TM = Ti) to keep its structural stability during the de/hydrogenations was proposed. Formation energy of Mg_nTMX₂ alloys was calculated using density functional theory to clarify the possibility of synthesizing these compounds. Calculations illustrated that compounds with X = F, O, and S satisfy both the thermal stability with negative formation energies and mechanical stability at zero pressure. The hydrogen absorption is proceed in a manner of stepwise, and finally all tetrahedron interstices were filled by 56 hydrogen atoms in the unit cell forming formula hydrides as Mg_nTMX₂H₁₄. It is obvious that the interstice composition plays an important role in hydrogen adsorption performance. The tetrahedral interstices composed by three Mg atoms and one transition metal own more strong ability to capture H atoms than those constituted by four Mg atoms in the initial hydrogenation process Results indicate that new family compound Mg₆VO₂ could be possibly ideal hydrogen storage materials in terms of dehydrogenation temperature (170–246 K) and hydrogen storage capacity (∼5.81 mass %).     

High performance material for hydrogen storage: Graphenelike Si2BN solid

Recently, two dimensional graphenelike i.e. Si₂BN solid monolayer have attracted much attention for the use of hydrogen developments. The work is based on first principles calculations using density functional theory with long range van der Waal (vdW) interactions. The optimized structure is energetically more stable due to high formation energy 45.39 eV with PBE and 50.82 eV with HSE06 functionals, respectively. Our ab-initio studies show that Pd (palladium) adatoms secured graphenelike Si₂BN solid via two types of interactions; physisorption and chemisorptions reactions, which engrossing up to 3H₂ molecules signifying gravimetric limits of ≈6.95–10.21 wt %. The absorption energies vary from ?0.31 eV to ?1.93 eV with Pd-adatom and without Pd-adatom respectively, and it varies up to ?1.24 eV. The work function of pure Si₂BN is 5.36 eV while metal-adatom on monolayer Si₂BN with (1 to 6)H₂ molecules is 3.53 eV–4.99 eV and reaches up to 5.85 eV. The theoretical study suggests that the functionalized graphenelike Si₂BN is efficient for hydrogen storage and propose a possible improvement for advantageous storage of hydrogen at ambient conditions.     

Hydrogen adsorption and dissociation on the TM-doped (TM=Ti,Nb) Mg55 nanoclusters: A DFT study

The slow hydrogenation kinetics and high reaction temperature of Mg primarily limit its application for mobile hydrogen storage. H₂ adsorption and dissociation on the pure and TM-doped (TM = Ti, Nb) Mg₅₅ nanoclusters are systematically studied by using density functional theory (DFT) calculations. It is found that the introduction of Ti and Nb atoms into Mg₅₅ nanocluster can greatly modify the electronic structure of Mg₅₅ nanocluster and enhance the stability of system. Through the analyses of results from the climbing image nudged elastic band (CI-NEB) and reaction rate constant, we also find that the energy barriers of H₂ dissociation on TM-doped Mg₅₅ nanoclusters can be significantly decreased due to the addition of Ti and Nb. Adding Ti and Nb atoms can dramatically improve the rate constant of H₂ dissociation, especially for H₂ dissociation on Mg₅₄TM2 (TM atom replacing the inner shell position), Mg₅₄TM3 (TM atom replacing the outermost vertex) and Mg₅₄TM4 (TM atom replacing the outermost edge position) nanoclusters. Moreover, compared with the Ti dopant, the Nb will generate a lower activation barrier for H₂ dissociation on TM-doped Mg₅₅ nanoclusters. We also suggest that the subsurface and surface positions (Mg₅₄TM2, Mg₅₄TM3, Mg₅₄TM4) are the ideal substitutional sites for TMs.     

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.     

The structural and electronic properties of Li-doped fluorinated graphene and its application to hydrogen storage

By using first-principles density functional theory, a theoretical investigation of Li-doped fluorinated graphene and its application as a hydrogen storage media is performed. It is found that a mixture between sp³ and a higher degree of sp² of the carbon orbitals after doping with Li would restore the distorted fluorinated graphene, and a fluorinated graphene layer with Li adsorbed on single or double-sides could store hydrogen up to 9 or 16.2 wt%. Regarding the H₂ adsorption mechanism, it has been demonstrated that the enhanced electrostatic field around the Li atom originates from the increased charge transfer from Li to graphene and F atoms with more electronegativity. Hybridization interaction between Li and graphene is also responsible for the adsorption of H₂ molecules.