Hydrogen is an energy carrier which holds tremendous promise as a new renewable and cleans energy option [1]

Hydrogen is an energy carrier which holds tremendous promise as a new renewable and cleans energy option 1. The U.S. Department of Energy (DOE) set a target for the ideal hydrogen storage materials should reach 5.5 wt% gravimetric density by the year 2020 2. Hydrogen is a convenient, safe, versatile fuel source that can be easily converted to a desired form of energy without releasing harmful emissions. Hydrogen is recognized as the ideal fuel due to its high utilization efficiency and environmental friendliness 3, 4. However, it is very difficult to store hydrogen under ambient conditions due to very weak intermolecular interactions among hydrogen molecules. Hence, the development of a safe, effective, stable, and cheap hydrogen storage medium has attracted increasing attention in the scientific community. An efficient hydrogen storage material must possess fast sorption kinetics, high volumetric/gravimetric density, relatively low dehydrogenation temperatures for chemical hydrides, 5-8 and the hydrogen adsorption energy should be in the range of -0.2 to -0.7 eV at room temperature, 8. Carbon nanotubes (CNT) and boron nitride nanotubes (BNNT) have attracted much attention as candidates for a hydrogen storage media 5, 9. During the past decade, the carbon-based materials have been considered as promising media for hydrogen storage 10, 11. However, due to the very weak physical adsorption of H2 for most materials including carbon-based materials, attention has been directed at non-carbon nanosystems composed of light elements such as B and N 9. Boron nitride nanotubes (BNNTs) are inorganic analogues of carbon nanotubes (CNTs) 12 and theoretically predicted 13 and then successfully synthesized in 1995 14. BNNTs have attracted considerable attention due to the undisputed fact that in contrast to metallic or semiconducting CNTs: BNNTs are wide-gap semiconductors with almost same band gaps of 5.5 eV, independent of the tube diameter, helicity, and the number of tube walls 15, and they are chemically and thermally more stable 13,16–18. Furthermore, the interaction of hydrogen molecules with material surfaces can be enhanced by heteropolar bonds at surfaces, a feature that is present in BNNTs but absent in CNTs. Given these unique properties, as one of the most interesting non-carbon nanotubes 19, BNNT has high potential practical application in hydrogen storage. In recent years especially, Ma et al. 20 measured the hydrogen storage ability of BN nanotubes and found that multiwall BN nanotubes can uptake 1.8–2.6 wt % hydrogen under about 10 MPa at room temperature and 70 % of adsorbed hydrogen is chemisorbed. In theoretical studies, Yuan and Liew 21 reported that boron nitride impurities will cause a decrease in Young’s moduli of SWCNTs. Moreover, the effect of these impurities in zigzag SWCNTs is more significant because of the linking characteristics of an increase in electrons. In addition, some methods have been shown to improve the efficiency of storage. An increase in the diameter of BNNT can increase the efficiency of hydrogen storage 22. Thus far, several hydrogen storage methods have been suggested. Further, Tang et al. 23 improved the concentration of hydrogen storage to 4.6 wt% by bending the BNNTs.
BNNTs have excellent mechanical properties, thermal conductivity, and resistance to oxidation at high temperatures, which makes them most valuable in nanodevices working in hazardous and high-temperature environments 24-26. Ju et al. 27 have investigated the effect of uniaxial strain on the electronic properties of (8, 0) zigzag and (5, 5) armchair BNNT. They have found that the two different types of BNNTs show very similar mechanical properties and variations in HOMO–LUMO gaps at different strains. Li et al. 28 demonstrated that the transport property of CNT with a double vacancy is reduced under external force. The stress-strain curve of armchair CNTs shows a step-by-step increasing behaviour, and the C-C bond length varies significantly at specific strain during the tensile process.
Metal-functionalization has been found to be a very useful scheme to improve or induce some unique properties of nanotubes 29. Both experimental 30-33 and theoretical 34, 35 studies were reported on the interaction of transition metal atoms with “perfect” BNNTs (BNNTs without intrinsic defects). The defective 36-39, carbon-doped 40, 41, Si-doped 38, Ti-doped 42, Ni-doped 43. Pt-doped 35 BNNTs have been examined, and these results show that the hydrogen storage capacity is significantly enhanced with respect to pristine BNNT and BN clusters 20,44-46. The adsorption of Ni onto single-walled BNNTs with intrinsic defects has been studied using DFT calculations, and the results of that study were reported by Zhao et al. 47. They found that the existence of defects in BNNTs clearly improves the chemical reactivity associated with Ni adsorption. In addition, the charge transfer, band gap, and density of states of Ni adsorbed onto BNNTs have been reported. Y. Liu et al. 48 investigated the hydrogen storage of Na-decorated single- and double-sided graphyne and their BN analogues. They found that the Na decorated double-sided graphyne and BN analogue the hydrogen storage capacities could reach to 5.98 and 5.84 wt%, with the average adsorption energies of 0.25 and 0.17 eV/H2, respectively. W. Lei et al. 49 measured the hydrogen storage ability of oxygen doped boron nitride (BN) nanosheets with 2–6 atomic layers, synthesized by a facile sol-gel method and found that a storage capacity of 5.7 wt% under 5 MPa at room temperature and 89% of the stored hydrogen can be released when the hydrogen pressure is reduced to ambient conditions.

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