Theoretical characterization of axial deformation effects on hydrogen storage of Ti decorated armchair (5,5) SWCNT

Theoretical calculations have been performed in the framework of density functional theory to characterize the effect of axial deformation on hydrogen storage of Ti decorated armchair (5,5) SWCNT. The theoretical characterization has been carried out in terms of H₂ adsorption energies that are lying in the desirable energy window (?0.2 to ?0.6?eV) recommended by DOE, as well as a variety of physicochemical properties. A remarkable and significant change in H₂ adsorption energy is observed under the effect of only (1%) axial strain. Axial relaxation leads to H₂ adsorption energies within the recommended energy range for hydrogen storage, in contrast to axial compression. Simultaneous weakening of π and σ interactions, due to the effect of axial relaxation and loss of spatial orbital overlap, is in favor of hydrogen adsorption in the recommended energy range, and dominates the effect of charge transfer from Ti 3d to C 2p of the SWCNT. The calculated pairwise and non pairwise additive components confirm that the role of the SWCNT is not restricted to supporting the metal. Polarizability and hperpolarizabilty calculations as well as spectral analysis characterize the relaxed structure (Z?=?1.02), for which H₂ adsorption energy (?0.34?eV) is in the recommended energy range for hydrogen storage, to be energetically more preferable than the compressed structure (Z?=?0.99). The results offer a way to control and characterize the hydrogenation process of metal functionalized SWCNTs by strain loading.     

Substitution effects on the hydrogen storage behavior of AB2 alloys by first principles

The hydrogen storage behavior of the TiCr₂ and ZrCr₂ alloys substituted with the third components (Zr, V, Fe, Ni) have been studied using first-principles calculations. The change of the hydrogen absorption energies caused by metal doping is arising from the charge transfer among the doped alloys interior. Zr and V atoms devoted abundant electrons, leading to a great enhancement of the H absorption energy, while Fe and Ni atoms always accepted electrons, yielding a remarkable decrease of the H absorption energy. The hydrogen diffusion energy barrier is closely correlated with the geometry effect rather than the electronic structure.     

Structural, vibrational and thermodynamic properties of Mg2 FeH6 complex hydride

Mg₂FeH₆, which has one of the highest hydrogen storage capacities among Mg based 3d-transitional metal hydrides, is considered as an attractive material for hydrogen storage. Within density-functional perturbation theory (DFPT), we have investigated the structural, vibrational and thermodynamic properties of Mg₂FeH₆. The band structure calculation shows that this compound is a semiconductor with a direct X-X energy gap of 1.96 eV. The calculated phonon frequencies for the Raman-active and the infrared-active modes are assigned. The phonon dispersion curves together with the corresponding phonon density of states and longitudinal-transverse optical (LO-TO) splitting are also calculated. Findings are also presented for the temperature-dependent behaviors of some thermodynamic properties such as free energy, internal energy, entropy and heat capacity within the quasi-harmonic approximation based on the calculated phonon density of states.     

Hydrogen dissociation on oxide covered MgH2 by catalytically active vacancies

MgH₂ is an important ingredient in modern reactive hydride composites to be used as hydrogen storage materials. The surface composition and chemical state of ball-milled MgH₂ is studied during hydrogen desorption by means of X-ray photoelectron spectroscopy. Simultaneously, the desorption rate of hydrogen is monitored, which is compared to dissociative properties of the surface investigated by hydrogen-deuterium exchange experiments. It is found that MgH₂ is also oxide covered during desorption demonstrating that MgO is able to recombine atomic hydrogen. The corresponding catalytic sites are associated with low coordinated surface vacancies on the oxide. The maximum surface concentration of these vacancies is very small, which is countered by a very high turnover frequency due to a small activation energy for dissociation of hydrogen of 0.1 eV on the single vacancy. The study provides insight into the catalytic role played by the oxide additives in MgH₂, which are superior catalysts for hydrogen sorption even when compared to 3d-metals.     

Transition metal-metal oxide hybrids as versatile materials for hydrogen storage

Metal atom located on metal oxide (MMO) is a promising material with various applications such as hydrogen storage. As one of the metal oxides, niobium oxide (NbO) presents fascinating properties that make it a possibly applicable in MMOs. Here, we investigated the feasibility of transition metal-NbO hybrids as MMO materials for application in the hydrogen storage technology. In this respect, the hydrogen adsorption of transition metals (Fe, Ni, Cu, Pd, Ag, and Pt) decorated on the NbO nanocluster has been explored using density functional theory calculations. We found that the adsorption energy of the H₂ molecule on the NbO adsorbent is remarkably increased by locating the transition metals on the NbO metal oxide. Our results reveal that the transition metals decorated on the NbO nanocluster can act as active sites for hydrogen adsorption. Among the studied transition metals, Pt shows the highest hydrogen capacity up to 6.52 wt%.     

Hydrogen storage using carbon adsorbents: past, present and future

Interest in hydrogen as a fuel has grown dramatically since 1990, and many advances in hydrogen production and utilization technologies have been made. However, hydrogen storage technologies must be significantly advanced if a hydrogen based energy system, particularly in the transportation sector, is to be established. Hydrogen can be made available on-board vehicles in containers of compressed or liquefied H₂, in metal hydrides, via chemical storage or by gas-on-solid adsorption. Although each method possesses desirable characteristics, no approach satisfies all of the efficiency, size, weight, cost and safety requirements for transportation or utility use. Gas-on-solid adsorption is an inherently safe and potentially high energy density hydrogen storage method that could be extremely energy efficient. Consequently, the hydrogen storage properties of high surface area “activated” carbons have been extensively studied. However, activated carbons are ineffective in storing hydrogen because only a small fraction of the pores in the typically wide pore-size distribution are small enough to interact strongly with hydrogen molecules at room temperatures and moderate pressures. Recently, many new carbon nanostructured absorbents have been produced including graphite nanofibers and carbon multi-wall and single-wall nanotubes. The following review provides a brief history of the hydrogen adsorption studies on activated carbons and comments on the recent experimental and theoretical investigations of the hydrogen adsorption properties of the new nanostructured carbon materials. Received: 16 October 2000 / Accepted: 15 November 2000 / Published online: 9 February 2001     

Proton magnetic resonance studies of TiCuH

The TiCuH structure can be described as alternating layers of TiH₂ and Cu atoms and, thus, corresponds to a two-dimensional analog of metal hydrides with CaF₂- type structures. Measurements of the proton relaxation times for TiCuH yield an activation energy of 0.79 eV for hydrogen diffusion. This result strongly suggests the H-atoms in TiCuH jump directly between the tetrahedral sites rather than through the vacant octahedral sites as proposed for H-atom jumps in f.c.c. TiH_x. In addition, the Korringa constant and hydrogen diffusion rate for TiCuH are compared with similar parameters for TiH_x and several ternary titanium hydrides.     

Li and Na Co-decorated carbon nitride nanotubes as promising new hydrogen storage media

The capacity of Li and Na co-decorated carbon nitride nanotube (CNNT) for hydrogen storage is studied using first-principles density functional theory. The results show that with two H₂ molecules attached to per Li and four H₂ molecules per Na the Li and Na co-decorated CNNT gains a gravimetric density of H₂ as high as 9.09 wt% via electrostatic interaction without the clustering of the deposited metal atoms (at T=0 K). The average adsorption energy of hydrogen molecule is in the range of 0.09-0.22 eV/H₂, which is suitable for practical hydrogen storage at ambient temperatures.     

Structural and H2 sorption properties of MgH2�C10?wt%ZrCrM (M?=?Cu, Ni) nano-composites

Magnesium and its hydride MgH₂ are widely regarded as promising candidates for hydrogen storage materials due to its benefits of high gravimetric and volumetric capacity, excellent reversibility, abundance in the earth and a low cost. Much attention has been paid to improve its absorption/desorption kinetics, trying to make it useful for practical applications. To make composite of MgH₂ with other hydrogen storage compounds is an effective method to improve the hydrogen storage properties. In this study nano-composite of MgH₂ with ZrCrCu alloy was prepared using high energy ball-milling for 5 h under Ar atmosphere. Microstructure and morphology of the composites were characterized by means of X-ray diffraction (XRD), scanning electron microscopy (SEM). XRD patterns show that no alloy formation between MgH₂ and elements of the alloys takes place during milling. Different morphology of the powders as-milled and after cycling was observed by SEM. Pressure?Ccomposition isotherms of these composites have been obtained in the pressure range 0.1?C10 bar at 275 and 300 °C. The absorption/desorption kinetics data have been analyzed using pressure 0.1?C5.0 bar at 275 and 300 °C to understand the mechanism of the hydriding/dehydriding reaction processes. A comparison of these results has been attempted with our previous published results of MgH₂?C10 wt%ZrCrNi in order to find the better composite for storage applications. It is observed by DSC curves that the onset temperature of hydrogen desorption is decreased for MgH₂?C10%ZrCrNi in comparison to MgH₂ which further decreased for MgH₂?C10%ZrCrCu. However, little loss in hydrogen absorption/desorption capacity is also observed for ZrCrCu composite in comparison to that of ZrCrNi composite.     

Energy of hydrogen dissolution in FCC hydrides of disordered Ti–V–Cr alloys according to density functional theory data

Ti–V–Cr alloys are hydrogen storage materials, but their characteristics, which are important for practical applications, depend strongly on composition. The search for an optimal composition with given characteristics requires the support of theoretical calculations of the electronic structure of alloys and their hydrides. In this paper, the interstitial energy and energy of hydrogen dissolution in the hydride of a ternary disordered Ti_0.33V_0.27Cr_0.4H_1.75 alloy with a face-centered cubic lattice were calculated within the framework of the density functional theory using the pseudopotential method. The deviation of the dissolution energy distribution from the Gaussian distribution is shown. Based on the data obtained for a particular hydride, the energy distributions of hydrogen dissolution in a number of hydrides of alloys (Ti_0.8Cr)_1–xV_x with x = 0.9, 0.8, 0.7, and 0.6 have been derived. A correlation was found between the theoretically calculated width of the energy distribution of hydrogen dissolution and the experimental slope of the pressure “plateau.”     

First-principle study of interaction of molecular hydrogen with BC<Subscript>3</Subscript> composite single-walled nanotube

The physisorption of molecular hydrogen in BC₃ composite single-walled nanotube, investigated using density functional theory, was compared with single-walled carbon nanotube. Both external and internal adsorption sites of these two nanotubes have been studied with the hydrogen molecular axis oriented parallel to the nanotube wall. The calculated results show that: ([see full textsee full text]) the physisorption energies of a H₂ molecule are larger for BC₃(8,0) composite nanotube than for C(8,0) nanotube at all adsorption sites examined. ([see full textsee full text]) For these two nanotubes, the physisorption energies are larger for hydrogen bound inside the nanotubes than for adsorption outside the nanotubes. The different behavior between these two nanotubes is explained by the contour plots of electron density and charge-density difference of them. The present computations suggest that BC₃ nanotube may be a better candidate for hydrogen storage than carbon nanotube.     

Review of hydrogen storage in inorganic fullerene-like nanotubes

Following the discovery of carbon nanotubes, inorganic fullerene-like nanotubes such as WS₂–MoS₂, NbS₂, TiS₂, and BN were reported. Inorganic (non-carbon) nanotubes constitute an important class of nanomaterials with interesting properties and potential applications. As known, efficient hydrogen storage is one key problem in the development of a hydrogen energy system. Hydrogen storage using carbon nanostructures is scientifically interesting and challenging. It thus would be worthwhile to look into hydrogen storage in inorganic nanotubes because the van der Waals gaps between the nanotube layers are potential candidates for hydrogen uptake. Furthermore, the inorganic nanotubes combine two elements, which is different from the pure carbon nanotubes. These may show a novel hydrogen adsorption–desorption mechanism. The present review provides a brief study of hydrogen adsorption on MoS₂, TiS₂, and BN nanotubes. PACS 81.07.De; 81.07.-b; 81.05.Tp; 68.43.-h     

Hydrogen storage in BC3 composite single-walled nanotube:a combined density functional theory and Monte Carlo investigation

This paper applies a density functional theory(DFT) and grand canonical Monte Carlo simulations(GCMC) to investigate the physisorptions of molecular hydrogen in single-walled BC 3 nanotubes and carbon nanotubes.The DFT calculations may provide useful information about the nature of hydrogen adsorption and physisorption energies in selected adsorption sites of these two nanotubes.Furthermore,the GCMC simulations can reproduce their storage capacity by calculating the weight percentage of the adsorbed molecular hydrogen under different conditions.The present results have shown that with both computational methods,the hydrogen storage capacity of BC 3 nanotubes is superior to that of carbon nanotubes.The reasons causing different behaviour of hydrogen storage in these two nanotubes are explained by using their contour plots of electron density and charge-density difference.     

Theoretical investigation of the adsorption and diffusion of hydrogen on the surface and in the bulk of the intermetallic compound Mg2Ni

The intermetallic compound Mg₂Ni as a potential material for hydrogen storage has been investigated theoretically. The sorption and diffusion of a hydrogen atom in the bulk and on the surface of this material, as well as the step-by-step process of dissociative chemisorption of a H₂ molecule on the surface, have been considered. The dependence of the sorption energy of atomic hydrogen on the structural characteristics of the intermetallic compound Mg₂Ni has been analyzed.     

Hydrogen storage of beryllium adsorbed on graphene doping with boron: First-principles calculations

Based on density-functional theory, we find that B-doped graphene significantly enhances the Be adsorption energy and prevent Be atoms from clustering. The complex of Be adsorbed on B-doped graphene can serve as a high-capacity hydrogen storage medium: the hydrogen storage capacity (HSC) can reach up to 15.1 wt% with average adsorption energy ?0.298 eV/H₂ for double-sided adsorption. It has exceeded the target specified by US Department of Energy with HSC of 9 wt% and a binding energy of ?0.2 to ?0.6 eV/H₂ at near-ambient conditions. By analyzing the projected electronic density of states of the adsorbed system, we show that the high HSC is due to the change of electron distribution of H₂ molecules and a graphene system decorated with B and Be atoms.     

三明治结构graphene-2Li-graphene的储氢性能

本文使用密度泛函理论中的广义梯度近似对扩展三明治结构graphene-2Li-graphene的几何结构、电子性质和储氢性能进行计算研究.计算得知:位于单层石墨烯中六元环面心位上方的单个Li原子与基底之间的结合能最大(1.19 eV),但小于固体Li的实验内聚能(1.63 eV),然而,在双层石墨烯之间的单个Li原子与基底的结合能增加到3.41 eV,远大于固体Li的实验内聚能,因此位于双层石墨烯之间的多个Li原子不会成簇,有利于进一步储氢.扩展三明治结构graphene-2Li-graphene中每个Li原子最多可以吸附3个H_2分子,储氢密度高达10.20 wt.%,超过美国能源部制定的5.5 wt.%的目标.该体系对1—3个H_2分子的平均吸附能分别为0.37,0.17和0.12 eV,介于物理吸附和化学吸附(0.1—0.8 eV)之间,因此该体系可以实现常温常压下对H_2的可逆吸附.通过对态密度分析可知,每个Li原子主要通过电场极化作用吸附多个H_2分子.动力学和巨配分函数计算表明graphene-2Li-graphene结构对H_2分子具有良好的可逆吸附性能.该研究可以为开发良好的储氢材料提供一个好的研究思路,为实验工作提供理论依据.     

Computational investigation of hydrogen storage on B5V3

Based on density functional theory method with 6-311+G(d,p) basis set, the structures, stability and hydrogen storage capacity of B₅V₃ have been theoretically investigated. It is found that a maximum of seven hydrogen molecules can be adsorbed on B₅V₃ with gravimetric uptake capacity of 6.39 wt%. The uptake capacity exceeds the target set by the US Department of Energy for vehicular application. Moreover, the average adsorption energy of B₅V₃ 01 (7H₂) is 0.60 eV/H₂ in the desirable range of reversible hydrogen storage. The kinetic stability of H₂ adsorbed on B₅V₃ 01 is confirmed by using gap between highest occupied molecular orbital (HOMO)and the lowest unoccupied molecular orbital (LUMO). The gap value of B₅V₃ 01 (7H₂) is 2.81 eV, which indicates the compound with high stability. In addition, the thermochemistry calculation (Gibbs free energy corrected adsorption energy) is used to analyse if the adsorption is favourable or not at different temperatures. It can be found that the Gibbs corrected adsorption energy of B₅V₃ 01 (7H₂) is still positive at 400 K at 1 atm. It means that the adsorption of seven hydrogen molecules on B₅V₃ 01 is energetically favourable in a fairly wide temperature range. All the results show that B₅V₃ 01 can be considered as a promising material for hydrogen storage.     

High pressure phase transition in super-cell LiBH4: An ab initio prediction

LiBH₄ which have attracted considerable attention from researchers due to the crystal structure characteristics is a metal hydride that can bind four hydrogen atoms. LiBH₄ which has high gravimetric and volumetric hydrogen density shows phase transitions at high pressures. In this regard, we created and analyzed LiBH₄ structure based on the first principles calculations, and then obtained the super-cell LiBH₄ structure. We achieved the phase transitions up to 20 GPa pressure with 2 GPa regular intervals for the super-cell LiBH₄. We observed the Pnma to Pnma*, Pnma* to P2₁/c, and P2₁/c to C2/c phase transitions and calculated the volume contractions accompanying these phase transitions. According to the obtained volumetric values, one can conclude that LiBH₄ can minimize the volumetric requirements of the hydrogen storage for systems that can be used at high pressures. Thus, the hydrogen storage capacity of LiBH₄ may increase at particular phases.     

MgH2(110)表面Pd单原子催化氢脱附反应的第一性原理研究

本文采用第一性原理密度泛函理论计算研究了MgH₂(110)表面吸附单原子Pd后的氢脱附反应. 计算发现,在吸附一个Pd单原子后,MgH₂(110)表面氢脱附反应的能垒可以从1.802 eV显著地降低到1.154 eV,表明Pd单原子对于氢脱附具有很强的催化效应. 并且,Pd单原子催化还可以将氢脱附的温度从573 K显著地降低到了367 K,从而使MgH₂(110)表面的氢脱附反应更加容易和快速地发生. 此外,通过MgH₂(110)表面氢溢出机制的反向过程来讨论了氢脱附反应的微观过程. 该研究表明Pd/MgH₂薄膜在未来的实验中可作为良好的储氢材料.     

Interaction between nanobuds and hydrogen molecules: A first-principles study

Hydrogen storage materials are crucial for the wide application of hydrogen in fuel cells. In this Letter, the interaction between hydrogen molecules and nanobuds has been studied using the Dmol3 package. The results show that the adsorption energies of hydrogen molecules onto nanobuds range from 0.069 eV to 0.115 eV, and that the adsorption energies are not sensitive to the nanobuds' structures but closely related to the number of carbon atoms around H₂ molecules. The energy barrier of a hydrogen molecule entering C176 is 2.38 eV. Each C176 nanobud can accommodate four H₂ molecules. The stress existing in nanobuds induces alterative charge distribution, which can improve the hydrogen storage performance of nanobuds to a certain extent.