The interaction of hydrogen with Li-coated C70 fullerene: A DFT study

We have applied density functional calculations to study the structure, stability and hydrogen storage properties of Li-coated C₇₀ fullerenes. Our results show that among different possibilities for the geometry of Li-coated C₇₀ fullerenes, Li atoms prefer to occupy exohedral and endohedral positions on top of the pentagons. Among isomers of Li₂C₇₀ and Li₆C₇₀, those in which one of the Li atoms occupy the endohedral position of one of the polar pentagonal rings and the other ones situate on the top of the pentagons around the opposite polar pentagon, have higher binding energies. Charge transfer is occurred from Li to C₇₀, putting extra electrons in the vicinity of the Li atom. This results in the binding of hydrogen in atomic form to the on-top C site that is nearest to the Li atom in the preferred configuration of hydrogenated Li-coated C₇₀ fullerene. Therefore, there are two types of hydrogen binding in Li₆C₇₀. One group of hydrogen bind to Li atoms in quasi-molecular form, which will desorb at a lower temperature, and the other group of H atoms bind to C in atomic form, which will desorb at higher temperatures.     

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

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

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.     

First principles of Si-doped BC2N single layer for hydrogen evolution reaction (HER)

Understanding the Hydrogen Evolution Reaction (HER) process is fundamental to use hydrogen as a sustainable (clean and renewable) energy source. Using first-principles calculations, we study the HER process when Si-doped a h-BC₂N single layer. The pristine BC₂N presents semiconducting properties with a band gap of 1.6–2.0 eV, being appropriate as a catalytic in the water splitting process. When Si is incorporated into the BC₂N monolayer, we obtain that the most stable site (lower formation energy) occurs when the Si atom replaces a C atom (Si_C). The Si atom moves out of the plane forming a buckling structure and the semiconducting properties are maintaining without spin effects. However, Si_B and Si_N give rise to two unpaired spin electronic levels inside the band gap and a magnetic moment of 1 μ_B. The adsorption energies of an H₂ molecule on the top of the Si atom are in the range of 50–100 meV, which are greater than the calculated ones for H₂ adsorbed on graphene and h-BN nanosystems but still low to be considered as an optimized medium for hydrogen storage. In addition, we observe that dispersive forces (van der Waals interactions) are responsible for half part of the adsorption energies. Strain due to the difference between the atomic radius of Si and C as well as the less stability of the Si–H bonds compared to the C–H ones leads to the Gibbs free energy (ΔG1) for hydrogen adsorbed on Si_C near zero, showing that Si-doped h-BC₂N is a potential system for HER.     

Hydrogen storage in Ca-decorated carbyne C10-ring on either Dnh or D(n/2)h symmetry. DFT study

We computationally investigate the hydrogen storage properties of carbyne C₁₀-ring structure on either D_nh or D_(n/2)h symmetry decorated with calcium (Ca) atoms adsorbed on its outer surface. The calculations are carried out on DFT-GGA-PW91 and DFT-GGA-PBE levels of theory as implemented in Biovia Materials Studio modeling and simulation software. To account for van der Waals interactions we also carried out calculations using DFT-D method of Grimme. Dmol³ is used to calculate total energies, HOMO-LUMO electronic charge density, Mulliken population analysis, and electrostatic potential fitting charges (ESP). Based on these results: i) the average binding energy of Ca atom doping to C₁₀-ring is ~2.3 eV (PW91) and ~2.1 eV (PBE). ii) Up to seven H₂ molecules per Ca atom can be physically adsorbed with an average energy of ~0.2 eV per H₂ molecule. iii) This physisorption leads to 8.09 wt percentage (wt. %) for the gravimetric storage capacity. According to these results, calcium-decorated carbyne C₁₀-ring structure is excellent candidate for hydrogen storage at ambient conditions with application to fuel cells.     

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

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

Ti decorated heterocyclic rings for hydrogen storage

This study uses first-principles calculations to investigate and compare the hydrogen storage properties of Ti doped benzene (C₆H₆Ti) and Ti doped borazine (B₃N₃H₆Ti) complexes. C₆H₆Ti and B₃N₃H₆Ti complex each can adsorb four H₂ molecules, but the former has a 0.11 wt% higher H₂ uptake capacity than the latter. Ti atoms bind to C₆H₆ more strongly than B₃N₃H₆. The hydrogen adsorption energies with Gibbs free energy correction for C₆H₆Ti and B₃N₃H₆Ti complexes are 0.17 and 0.45 eV, respectively, indicating reversible hydrogen adsorption. The hydrogen adsorption properties of C₆H₆Ti have also been studied after boron (B) and nitrogen (N) atom substitutions. Several B and N substituted structures between C₆H₆Ti and B₃N₃H₆Ti with different boron and nitrogen concentration and at different positions were considered. Initially, one boron and one nitrogen atom is substituted for two carbon atoms of benzene at three different positions and three different structures are obtained. Seven structures are possible when four carbon atoms of benzene are replaced by two boron and two nitrogen atoms at different positions. The hydrogen storage capacity of the C₆H₆Ti complex increases as boron and nitrogen atom concentrations increases. The positions of substituted boron and nitrogen atoms have less impact on H₂ uptake capacity for the same B and N concentration. The position and concentration of B and N affects the H₂ adsorption energy as well as the temperature and pressure range for thermodynamically favorable H₂ adsorption. The H₂ desorption temperature for all the complexes is found to be higher than 250 K indicates the stronger binding of H₂ molecules with these complexes.     

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.     

First-principles predictions of potential hydrogen storage materials: Novel sandwich-type ethylene dimetallocene complexes

The hydrogen storage capacities of a sandwich-type ethylene dimetallocene complex (Cp₂Ti₂C₂H₄) are studied using first-principles calculations. It is found that the TiC₂H₄Ti molecule can intercalate into the two cyclopentadienyl (Cp) rings and form a stable sandwich-type complex. Each Ti atom can adsorb a maximum of three H₂ molecules, which corresponds to a gravimetric storage capacity of 4.73 wt%. This hydrogen storage capacity is close to the 2015 target of 5.5% set by the US Department of Energy (DOE) in 2009. Furthermore, the Cp₂Ti₂C₂H₄ molecule proposed in this paper is favorable for both adsorption and desorption of hydrogen molecules at room temperature and ambient pressure because its average binding energy of 0.34 eV/H₂.     

Theoretical studies in the stability of vacancies in zeolite templated carbon for hydrogen storage

In this work, we report DFT calculations of the energy formation and stability of multi-vacancies in a unit of Zeolite Template Carbon (C₃₉H₉). We label as V_n the respective vacancy where n carbon atoms have been removed from the pristine C₃₉H₉ structure. The results show that V₂, V₄, V₆ and V₉ are the most stable vacancies on the ZTC structure. This result agrees with many other studies. Besides, the most stable vacancy of ZTC structure is when nine carbon atoms are removed (V₉) from the ZTC structure. The formation of pentagon rings in the reconstruction of the ZTC vacancy give drastic effect on the energetics stability. Therefore, the formation of pentagon rings eliminates the dangling bonds thus lowering the energy formation. It is also carried out the decoration of ZTC vacancy with Lithium and Calcium atoms, this is the way to use de ZTC vacancy decorated as a medium for hydrogen storage. The results show that the ZTC vacancy decorated with 3 Lithium atoms can adsorb a maximum of nine hydrogen molecules (3 hydrogen molecules per Lithium atom). This gives a gravimetric storage capacity of 4.44 wt percent (wt. %), which is not enough for meeting DOE gravimetric target. On the other hand, to reach DOE gravimetric target, the study of ZTC vacancy decorated with 3 Calcium atoms is carried out, which can adsorb maximum of fifteen hydrogen molecules (5 hydrogen molecules per Calcium atom), this gives gravimetric storage capacity of 5.81 wt %, which meet DOE gravimetric targets, besides the binding energy of hydrogen molecules on ZTC vacancy decorated with 3 Calcium is calculated. These energies are in the range 0.2453–0.2053 eV/H₂, which are desirable energies for hydrogen adsorption. This is demonstrated by building isotherm adsorption path. The results show that forming vacancies on ZTC structure decorated with three Calcium atoms (3CaC₃₀H₉) is good candidate as medium for hydrogen storage.     

A first-principle study of calcium-decorated BC2N sheet doped by boron or carbon for high hydrogen storage

Hydrogen adsorption and storage on calcium-decorated BC₂N sheets doped by Boron or Carbon were investigated using the first-principles calculations. Unlike the weak bond between Ca atoms and pristine BC₂N, doping boron or carbon atoms on BC₂N sheet can significantly strengthen the Ca atoms on the BC₂N, especially for BC₂NB_C and BC₂NC_N. It is observed that Ca decorated BC₂NB_C and BC₂NC_N possess strong donation and back-donation of Ca with the sheets, which is responsible for enhanced binding energy to eliminate the clustering problem. Up to four hydrogen molecules can stably attach to a Ca atom with an average adsorption energy of ∼0.3 eV, which is in the range that permits hydrogen recycling at ambient conditions. The Ca decorated BC₂NB_C and BC₂NC_N complexes can work as high-capacity hydrogen storage materials with the practical usable capacities of 8.36 wt% and 8.38 wt%, respectively.     

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.     

Boron‐ and nitrogen‐doped penta‐graphene as a promising material for hydrogen storage: A computational study

We fulfill a comprehensive study based on density functional theory (DFT) computations to cast insight into the dissociation mechanism of hydrogen molecule on pristine, B‐, and N‐doped penta‐graphene. The doping effect has been also illustrated by varying the concentration of dopant from 4.2 at% (one doping atom in 24 host atoms) to 8.3 at% (two doping atoms in 24 host atoms) and by contemplating different doping sites. Our theoretical investigation shows that the adsorption energy of H₂ molecule and H atom on the substrate can be substantially enhanced by incorporating boron or nitrogen into penta‐graphene sheet. The B‐ and N‐doped penta‐graphene can effectively decompose H₂ molecule into two H atoms. Our results demonstrate that activation energies for H₂ dissociation and H diffusion on the B‐ and N‐doped penta‐graphene are much smaller than the pristine penta‐graphene. Further investigation of increasing concentration dopants of the penta‐graphene sheet gives sufficiently low activation barrier for H₂ dissociation process. This investigation reveals that the boron and nitrogen dopants can act as effective active site for H₂ dissociation and storage.     

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.     

MgScH15: A highly stable cluster for hydrogen storage

Mg-Sc-H systems exhibit superior stability and high hydrogen storage capacity among the large class of magnesium-based hydrogen storage materials, but the underlying mechanisms for their outstanding hydrogen storage properties remain largely unexplored and require further investigation. Here, we have performed a comprehensive investigation on the structural evolution of MgScH_n (n = 10–20) clusters by unbiased Crystal structure AnaLYsis by Particle Swarm Optimization (CALYPSO) method combined with density functional theory (DFT) optimizations at the B3PW91/6-311 + G(d) level. Our results show that the MgScH₁₅ cluster with C_s symmetry is found to be the most stable cluster with good hydrogen storage capacity of 17.8 wt% due to the strong interaction among the 1s orbitals of the H atoms, the 2p orbitals of the Mg atom and the 3d orbitals of the Sc atom. The present findings are promising for further exploring novel hydrogen storage nanomaterials.     

Aluminum-silicon hydride clusters for prospective hydrogen storage

The structures and bonding properties of Al₄Si₂H_2n (n = 0–10) clusters are systematically studied by using the evolutionary algorithm combined with ab initio computations. While the H atoms are bond on the terminal sites of the clusters at low H contents, the Al atoms are combined together by double H-bridges and the Al/Si atoms are tetrahedrally coordinated at high H contents. The Al₄Si₂H_2n clusters break into a few fragments for n = 9,10. Analysis on the bonding natures shows that the Al–H bonds are strongly polarized and the Si atoms balance the charge states of the Al/H atoms according to the hydrogen concentrations. The hydrogen storage capacity in Al₄Si₂H₁₆ cluster reaches 8.9 wt%, and the estimated strength of the hydrogen bonding is about −0.55 eV per H₂, which falls in the ideal window for reversible hydrogen storage at ambient temperatures. The high hydrogen capacity and moderate bonding strength suggest that Al–Si hydrides can be promising candidates for hydrogen storage.     

Hydrogen storage on Ti decorated SiC nanostructures: A first principles study

In the present study we report the hydrogen adsorption behavior of two SiC nanostructures; a planar sheet and a nanotube (10, 0) of 1 nm diameter decorated by Ti atoms on it. All calculations have been performed using a plane-wave based pseudopotential method. The lowest energy structure of the Ti adsorbed SiC sheet shows that Ti atom distorts the sheet in such a way that one of the Si atoms goes down the plane and the Ti atom bind with nearest three C atoms. The interaction of this Ti decorated sheet with hydrogen suggests that each Ti atom can bind up to four hydrogen molecules (all hydrogens are adsorbed in the molecular form) with an average binding energy of 0.37 eV. For SiC nanotube, the adsorption of Ti favors the hexagonal hollow site. Moreover, on interaction of this Ti decorated tube with hydrogen leads to dissociation of the first hydrogen molecule in the atomic form and thereafter adsorbs hydrogen in the molecular form. The average binding energy of hydrogen molecules on this Ti decorated tube is estimated to be 0.65 eV. Based on these results we infer that the Ti decorated SiC nanostructures moderately bind with hydrogen molecules (within the energy window for hydrogen storage materials) and therefore, can be considered as one of the potential hydrogen storage material.     

A reversible hydrogen storage material of Li-decorated two-dimensional (2D) C4N monolayer: First principles calculations

Two-dimensional (2D) materials can be regarded as potential hydrogen storage candidates because of their splendid chemical stability and high specific surface area. Recently, a new dumbbell-like carbon nitride (C₄N) monolayer with orbital hybridization of sp³ is reported. Motivated from the above exploration, the hydrogen adsorption properties of Li-decorated C₄N monolayer are comprehensively investigated via first principles calculations based on the density functional theory (DFT). It is found that the Dirac points and Dirac cones exists in the Brillouin zone (BZ) from the calculated electronic structure and indicates the C₄N can be used as an excellent topological material. Also, the calculated phonon spectra demonstrate that the C₄N monolayer owns a strong stability. Moreover, the calculated binding energy of decorated Li atom is bigger than its cohesive energy and results in Li atoms disperse over the surface of C₄N monolayer uniformly without clustering. In addition, the Li₈C₄N complex can capture up to 24H₂ molecules with an optimal hydrogen adsorption energy of −0.281 eV/H₂ and achieves the hydrogen storage density of 8.0 wt%. The ab initio molecular dynamics (AIMD) simulations suggest that the H₂ molecules can be desorbed quickly at 300 K. This study reveals that Li-decorated C₄N monolayer can be served as a promising hydrogen storage material.     

Storage and permeation of hydrogen molecule,atom and ions (H+ and H−) through silicon carbide nanotube; a DFT approach

The barriers for the encapsulation and decapsulation of hydrogen ions (cationic hydrogen and hydride), atom, and molecule through silicon carbide nanotube are thoroughly studied. DFT method is selected to measure the kinetic barriers for the passage of hydrogen atom, ions and molecule through nanotube via scanning potential energy surface. The kinetic barriers for the passage (encapsulation and decapsulation) of hydrogen are very important to understand the mechanism of hydrogen storage and release. The barriers for the permeation of H, H⁺ and H^? across SiC nanosheet are lower compared to hydrogen molecule (H₂). The exohedral and endohedral adsorption of hydrogen ions (cation and anion), atom and exohedral hydrogen molecule on silicon carbide are exothermic in nature. Whereas the encapsulation of hydrogen molecule in silicon carbide is endothermic. Electronic properties are analyzed through measurement of energy gap between highest occupied and lowest unoccupied molecular orbitals gap (G_H-L) and the density of state (DOS) spectra. The G_H-L analysis reveals that endohedral complexes have more pronounced effect on electronic properties compared to exohedral complexes. The SiC nanotube has highly favorable properties for storage and release of hydrogen ions, and atom.