A Theoretical Study On Hydrogen Storage Of Boron Nitride Clusters

Hydrogen energy is a reproducible and clean energy source, which has attracted extensive attentions in recent years. The use of hydrogen requires an effective, safe, and stable storage medium. However, hydrogen storage is the bottleneck for developing hydrogen economy in on-board vehicle applications. Among various methods to store hydrogen, none is recognized as an adequate method for practical applications. Non-carbon nanosystem comprised of light elements such as boron and nitrogen atoms also have been noticed. Boron nitride nanostructures have a wide range of attractive properties, such as high-temperature stability, a low dielectric constant, large thermal conductivity, and oxidation resistance, leading to a number of potential applications as a structural or electronic material. The hetero polar nature in the BN nanostructures offers higher binding energy for hydrogen storage compared to the carbon based materials.Currently, a large number of the studies on hydrogen storage capacity and mechanism of BN nanotubes and sheets have been reported. The DFT investigation of hydrogenation of the small B12N12 shows that B12N12 may be a proper candidate material for hydrogen storage. Considering that it is difficult to obtain unique sized BnNn cage, it is necessary to explore the hydrogen storage ability of larger BnNn cage and to conclude how the size of BnNn cage affect its hydrogen storage ability. In addition, endohedral atoms and ions on hydrocarbon fullerene-like cage and perhydrogenated silicon cage have been studied at various levels of theory, and these studies have proved that some properties of cages could be revised by endohedral species. Some studies on endohedral boron nitride complexes have shown that the endohedral species could increase the binding energy of hydrogen storage. In this thesis, we present a systematic study of the effects of endohedral atoms and ions including metal and nonmetal on hydrogenation of B16N16 and B12N12 based on DFT calculations. The main contents and results are as follows.1. The hydrogenation of B16N16 cage has been studied using ab initio molecular orbital theory with B3LYP/6-31G(d) method and compared with the results of hydrogenation of B16N16 cage. The structure characters of the most stable B16N16Hn (n= 2-32) isomers are discussed in detail. The results show that H atoms prefer to adsorb on the R46 bond in pairs form and favor the four-member rings when n<14; when n=14 and 16, the flat structures with broken B-N bond present special stability; and when n>16, the structure were dominated by non-hydrogenated planar six-membered rings. The average binding energies of hydrogenated B16N16 cage are smaller than that of B12N12 cage, especially in high hydrogen coverage. The smaller angle distortion and shorter average B-N bond length of B16N16 are the main reason for the smaller average binding energy per H2 of B16N16Hn comparing with B12N12Hn. Calculation of the Gibbs free energy of the reaction of B16N16+16H2→B16N16H32 as a founction of temperature shows that this reaction will reverse at about 110 K, which is lower than the reversing temperature 410 K for the reaction of B12N12+12H2→B12N12H24.2. The structures and properties of the most stable X@B16N16 and X@B16N16H32(X= Li+, Na+, K+, Mg2+, Ne, O2-,S2-, F-, and Cl-) complexes along with the size and charge of the endohedral species are discussed comprehensively from the aspects of the average B-N bond lengths, average binding energy per H2 molecule and inclusion energy. On the basis of computational results, it is found that the small and highly charged guest species is favorable for endohedral, the average binding energies of hydrogenated X@B16N16 complexes are larger than that of pristine B16N16 cage on the whole, and all endohedral ions in this work could raise the reversing temperature of perhydrogenation reaction except for O2- and F-. Especially, Mg2+ encapsulation is favorable thermodynamically and it could obviously reduce the hydrogen adsorption barriers by 12.05 kcal/mol. Also, the reaction of Mg2+@B16N16+16H2→Mg2+@B16N16H32 will reverse at 260 K, which is higher than the reversing temperature 110 K for the reaction of B16N16+16H2→B16N16H32.3. The hydrogenation of endohedral X@B12N12(X= Li0/+, Na0/+, K+, Mg2+, O2-, S2-,F-, Cl-, H, Ne) complexes have been studied at the B3LYP/6-31G* level of density functional theory. The structures and properties of them are compared with the results of hydrogenation of endohedral X@B16N16 (X= Li+, Na+, K+, Mg2+, Ne, O2-, S2-, F-, and Cl-) complexes. The results show that the host cages have more compact geometries when metal atoms, rather than cations, are inside. The average binding energies of hydrogenated X@B12N12 complexes are larger than that of pristine B12N12 cage on the whole, and the magnitude of increase is larger than that of X@B16N16 corresponding to B16N16 cage. The endohedral cage complexes with low parent cage strain energies, large cage internal cavity volumes, and a small, highly charged guest species are the most stable. Similarly, Mg2+ encapsulation is favorable thermodynamically and it could obviously reduce the hydrogen adsorption barriers by 14.6 kcal/mol. While except for O2-and Li, all of these endohedral species will increase the reversing temperature 410 K for the reaction of B12N12+12H2→B12N12H24.4. The hydrogenation of endohedral X@B16N16(X= Cu, Ag, Au, Ni, Pd, Pt) complexes also have been studied at the density functional theory at the B3LYP/6-31G* level. The structures and properties of the most stable X@B16N16 and X@B16N16H32 complexes are discussed detailed and compared with the results of foregoing statement. It is found that the average binding energies of all hydrogenated X@B16N16 complexes are larger than that of pristine B16N16 cage. The average B-N bond length of X@B16N16 complexes increase with the increasing of endohedral atomic radius. All of these endohedral species could reduce the hydrogen adsorption barriers and raise the reversing temperature of the perhydrogenated B16N16 cage. Especially, the reaction of Ag@B16N16+16H2→Ag@B16N16H32 will reverse at 290 K, which is close to room temperature.