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.     

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 hydrogen storage in zirconium decorated zeolite templated carbon: Predictions from DFT simulations

The hydrogen (H₂) storage capacity of Zirconium (Zr) decorated zeolite templated carbon (ZTC) has been investigated using sophisticated density functional theory (DFT) simulations. The analysis shows that the Zr atom gets bonded with ZTC strongly with binding energy (BE) of ?3.92 eV due to electron transfer from Zr 4d orbital to C 2p orbital of ZTC. Each Zr atom on ZTC can attach 7H₂ molecules with average binding energy of ?0.433 eV/H₂ providing gravimetric wt% of 9.24, substantially above the limit of 6.5 wt% set by the DoE of the United States of America. The H₂ molecules are involved via Kubas interaction with Zr atom, which involves the charge transfer between Zr 4d orbital and H 1s orbital with interaction energy higher than physisorption but lower than chemisorption. The structural integrity of the system is confirmed via molecular dynamics (MD) simulations at room temperature and at highest desorption temperature of 500 K. We have investigated the chances of metal clustering by computing diffusion energy (E_D) barrier for the movement of Zr atom, and we obtained via calculation, we can infer that the presence of E_D barrier of ～2.36 eV may prevent the possibility. As the system ZTC has been synthesized, Zr doped ZTC is stable, existence of sufficient diffusion barrier prevents the clustering and adsorption energy and wt% of H₂ are within the range prescribed by DoE, we feel that Zr decorated ZTC can be fabricated as promising hydrogen storage material for fuel cell applications.     

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.     

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.     

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.     

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.     

Ab initio study on hydrogen interaction with calcium decorated silicon carbide nanotube

Ab initio study on the viability of calcium decorated silicon carbide nanotube as a hydrogen storage material was conducted. Calcium strongly adsorbs on silicon carbide nanotube (SiCNT) with a significant binding energy of ?2.83 eV, thus calcium's low cohesive energy and strong binding with SiCNT may prevent Ca to form clusters with other adsorbates. Bader charge analysis also revealed a charge transfer of 1.45e from Ca to SiCNT resulting to calcium's cationic state, which may induce charge polarization to a nearby molecule such as hydrogen. Hydrogen molecule was then allowed to interact with the calcium adatom where it exhibited charge polarization, induced by the electric field from calcium's positive charge. This resulted to a significant binding energy of ?0.22 eV for the first hydrogen molecule. Results reveal that Ca on SiCNT can hold up to 7 hydrogen molecules and can be a promising candidate for a hydrogen storage material.     

Sc/Ti decorated novel C24N24 cage: Promising hydrogen storage materials

Inspired by the TM?N₄ coordination environment and the promising hydrogen adsorption property of the Ti–C₄ unit, four novel TM₆C₂₄N₂₄ (TM = Sc, Ti) cages with six TM?N₄/TM?C₄ units are designed simultaneously for the first time in this paper. Their stabilities are studied by using DFT calculations and confirmed by MD simulations. Sc₆C₂₄N₂₄(N₄) and Ti₆C₂₄N₂₄(N₄) can absorb 30 hydrogen molecules and keep the original structures intact. The average hydrogen adsorption energy is 0.13–0.26 eV for Sc₆C₂₄N₂₄(N₄)-6nH₂ (n = 1–5), and 0.09–0.23 eV for Ti₆C₂₄N₂₄(N₄)-6nH₂ (n = 1–5). The hydrogen storage capacities are 6.30 wt % and 6.20 wt %, respectively. Besides, the 30H₂ can be readily adsorbed on Sc₆C₂₄N₂₄(N₄) under 160 K and 100% desorbed at 270 K under 0.1 MPa. The desorption temperature could increase if the pressure becomes higher. Sc₆C₂₄N₂₄(C₄)/Ti₆C₂₄N₂₄(C₄) complexes are higher in energy than corresponding Sc₆C₂₄N₂₄(N₄)/Ti₆C₂₄N₂₄(N₄). They are not suitable as room-temperature hydrogen storage materials due to the structural deformation in the hydrogen storage process.     

A single tiny Tin(O2)n cluster decorated an ultra-small boron nitride nanotube for hydrogen storage material: A density functional theory study

The adsorption of hydrogen (H₂) on tiny titanium dioxide Ti_n(O₂)_n clusters where n = 1, 2 and 3 decorated a (5, 5) ultra-small boron nitride nanotube (BNNT) is studied theoretically using the density functional theory calculation. Ti_n(O₂)_n/BNNT is very stable and it can hold a large number of H₂ molecules while maintaining its stability. That H₂ adsorption on Ti_n(O₂)_n/BNNT/BNNT shifts the geometry of Ti_n(O₂)_n/BNNT as the number of H₂ molecules adsorbed on its surface increased. For example, the bond between N–O increases while the bond between the H atoms in the H₂ molecules shortens. Furthermore, the local density of states (LDOS), crystal orbital overlaps population (COOP), and charge distribution analysis all confirm that H₂ formed a bond with TiO₂/BNNT. Thus, we can conclude that Ti_n(O₂)_n/BNNT is a promising material for hydrogen storage.     

Enhancing hydrogen storage capacity of pyridine-based metal organic framework

Using first principles calculations, we show that the storage capacity as well as desorption temperature of MOFs can be significantly enhanced by decorating pyridine (a common linker in MOFs) by metal atoms. The storage capacity of metal-pyridine complexes are found to be dependent on the type of decorating metal atom. Among the 3d transition metal atoms, Sc turns out to be the most efficient storing unto four H₂ molecules. Most importantly, Sc does not suffer dimerisation on the surface of pyridine, keeping the storage capacity of every metal atom intact. Based on these findings, we propose a metal-decorated pyridine-based MOFs, which has potential to meet the required H₂ storage capacity for vehicular usage.     

Geometric structures and hydrogen storage properties of M2B7 (M=Be,Mg, Ca) clusters

Based on the density functional theory, we investigate the electronic properties of the clusters M₂B₇ (M = Be, Mg, Ca) and their hydrogen storage properties systematically in this paper. Extensive global search results show that the global minimal structures of the three systems (Be₂B₇, Mg₂B₇ and Ca₂B₇) are heptagonal biconical structure, and the two alkaline earth metals are located at the top of the biconical. Chemical bonding analyses show that M₂B₇ clusters have 6σ and 6π delocalized electrons, which are doubly aromatic. At the wB97XD level, the three systems have good hydrogen storage capabilities. The hydrogen storage density of Be₂B₇ is as high as 23.03 wt%, while Mg₂B₇ and Ca₂B₇ also far exceed the hydrogen storage target set by the U.S. Department of Energy in 2017. Their average adsorption energies of H₂ molecules all ranged from 0.1 eV/H₂ to 0.48 eV/H₂, which is fall in between physisorption and chemisorption. Extensive Born Oppenheimer molecular dynamics (BOMD) simulations show that the H₂ molecules of the three systems can be completely released at a certain temperature. Therefore, M₂B₇ systems can achieve reversible adsorption of H₂ molecules at normal temperature and pressure. It can be seen that the B₇ clusters modified by alkaline earth metals may become a promising new nano-hydrogen storage material.     

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.     

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.     

First principles study on hydrogen storage in yttrium doped graphyne: Role of acetylene linkage in enhancing hydrogen storage

Through Density Functional Theory Simulations we predict that a Ytrrium atom attached on graphyne surface can adsorb up to a maximum of 9 molecular hydrogens (H₂), with a uniform binding energy of ∼0.3 eV/H₂ and an average desorption temperature of around 400 K (ideal for fuel cell applications), leading to 10 wt% of hydrogen, substantially higher than the requirement by DoE. The higher hydrogen wt% in Y doped graphyne compared to Y doped Single Walled Carbon Nanotubes (SWNT) and graphene is due to the presence of sp hybridized C atoms (in the acetylene linkage) supplying additional in-plane p_x-p_y orbitals leading to π (π*) bonding (antibonding) states. Charge transfer from metal to carbon nanostructure results in a redistribution of s, p, d orbitals of the metal leading to a non - spin polarized ground state in Y doped graphyne, due to the presence of the acetylene linkage, whereas Y doped SWNT and graphene remain magnetic like the isolated metal atom. In the non-magnetic graphyne + Y system, the net charge transfer from Y to successive H₂ molecules is less than in magnetic Y + graphene and Y + SWNT systems, enabling Y + graphyne to store a larger number of H₂ molecules. Furthermore, our ab initio MD simulations show that the system is stable even at room temperature and there is no dissociation of H₂ molecules, enabling the system to achieve 100% desorption. So Y doped graphyne is found to be a promising hydrogen storage device with high wt%, 100% recyclability and desirable desorption temperature.     

Li decorated heteroborospherene C4B32 as high capacity and reversible hydrogen storage media: A DFT study

In this work, adsorption of H₂ molecules on heteroborospherene C_2v C₄B₃₂ decorated by alkali atoms (Li) is studied by density functional theory calculations. The interaction between Li atoms and C₄B₃₂ is found to be strong, so that it prevents agglomeration of the former. An introduced hydrogen molecule tilts toward the Li atoms and is stably adsorbed on C₄B₃₂. It is obtained that Li₄C₄B₃₂ can store up to 12H₂ molecules with hydrogen uptake capacity of 5.425 wt% and average adsorption energy of ?0.240 eV per H₂. Dynamics simulation results show that 6H₂ molecules can be successfully released at 300 K. Obtained results demonstrate that Li decorated C₄B₃₂ is a promising material for reversible hydrogen storage.     

First-principles study of potassium-doped PC61BM for hydrogen storage

It is known that metal-doped C₆₀ molecule has suitable adsorption energy for H₂ adsorption/release around room temperature. However, the gravimetric storage capacity achieved on solid samples is too low (<1 wt%). In this work we study the hydrogen storage property of a metal-doped C₆₀ derivative, K₇PC₆₁BM, with density functional theory calculations. The complex of K₇PC₆₁BM is selected because it has been observed experimentally. The calculations are performed for an isolated K₇PC₆₁BM molecule, but the limited size of interstitial space for solid phase is taken into consideration in finding the adsorption structure. It is found that K₇PC₆₁BM can at least adsorb 31H₂ molecules (5.07 wt%) due to a compact adsorption structure. The averaged adsorption energy is 0.119 eV/H₂, indicating a desorption temperature near room temperature.     

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.     

Co-doped graphene sheets as a novel adsorbent for hydrogen storage: DFT and DFT-D3 correction dispersion study

In this study, transition metals (TM) such as palladium (Pd) have been introduced on co-doped graphene and defect graphene sheets with nitrogen and boron ad-atoms to investigate the potentials of new adsorbents for hydrogen storage. The first principle studies using density functional theory (DFT) and DFT-D3 correction dispersion were undertaken to calculate the adsorption energy of hydrogen molecule on the graphene sheet. The results showed that applying Pd transition metal could enhance adsorption energy of hydrogen molecules towards pristine sheet. The main problem in applying transition metal on graphene sheets was concerned with clustering. However, the current defects in graphene sheets prevent clustering event. Our simulation results suggested that these defects reduced hydrogen adsorption and substitute dopants such as nitrogen and boron together on graphene sheets could improve the adsorption energy. Thus, two various forms of Pd decorated NB co-doped as hexagonal and double carbon vacancy (DCV) were introduced as new structures for hydrogen storage. A physical adsorption, which is appropriate for reversible hydrogen storage, was implemented for both novel adsorbents. In the two various forms of NB co-doped structures, DCV had the optimum adsorption behavior as adsorption energy level and density of state (DOS) phenomena. Moreover, the results of adsorption energy using DFT method were consistent with that of DFT-D3 correction dispersion and higher amounts of adsorption energy in DFT-D3 method were obtained. Finally, results introduced Pd decorated NB co-doped graphene sheets as a novel material for hydrogen storage.