Adsorption Of Hydrogen Molecule On B_nAl_24-nN₂₄（n=24,12,0） Cages

The energy crisis brings severe challenges for the sustainable development of the worldeconomy with increasing consumption of non-renewable resources, such as oil, gaseous andcoal etc. There is an urgent requirement for finding and developing renewable green energy.Hydrogen is an ideal energy carrier in the future energy systems. However, a safe, efficientand economic storage method is a crucial prerequisite for the widely application of hydrogenin the future energy facilities. While the gaseous and cryoliquid hydrogen storage systems arecurrently an option, it’s impractical for vehicular energy due to safety concerns and thegravimetric/volumetric density constraints. Solid hydrogen storage is potentially superior withits storage capacity, energy efficiency and safety. Studies show that hydrogen storagematerials with special physical/chemical properties can provide many advantages, includinghydrogen storage alloy, coordination hydride materials, metal organic framework compoundsand carbon-based materials, which have made a lot of progress in experiments and theories.However, no hydrogen storage materials have been reported to be able to meet the actuallyrequirements. Improving the electronic structure of the systems will give a good guidance forthe interaction of material and hydrogen molecule.Boron nitride （BN） is composed of lightweight elements in Ⅲ-ⅤGroup, and thesynthetic technology of boron nitride nanostructures has made a lot of progress, such astubular, cage-like, bamboo-like, the capsule-like, etc. Compared with the series of carbon,B/N atoms formed strong covalent bond in the BN systems. The theoretical analyses showthat BN nanotubes or clusters can interact with H₂better, which is possible to becomedesirable hydrogen storage materials. However, the current studies show that the adsorptionenergy is very weak, and don’t reach the hydrogen storage requirements at room temperature.One possible way is that doped BN systems may be able to improve the adsorption strength. Itwill provide a theoretical basis and scientific guidance to find the materials with excellenthydrogen storage properties at room temperature by studying the hydrogen storage propertiesof the doped BN systems, investigating the mechanism of interaction between materials andH₂and exploring the way to improve the adsorption strength at the level of molecular orbitaltheory.In the present work, B₁₂Al₁₂N₂₄cages are generated by replacing twelve boron atoms with aluminum in B₂₄N₂₄, and the stabilities of all isomers are calculated by using the densityfunctional theory B3LYP/6-31G（d, p）. The results show that the aluminum and boron atomstend to segregate in B₁₂Al₁₂N₂₄. In the most stable structure, aluminum and boron atomslocate at the two halves of the cage. The electronic structure of B₂₄N₂₄, B₁₂Al₁₂N₂₄andAl₂₄N₂₄cage is compared. The adsorption of H₂at the different positions of stable structuresurface is also investigated. Analysis of the molecular orbitals show that the orbitals ofhydrogen molecule （both the bonding σ orbital and the antibonding σ*orbital） and theorbitals of the clusters can overlap slightly, and the mixing coefficient is very small.Considering the absolute values of the binding energies, however, the adsorption of hydrogenon these clusters is still weak physisorption.