A Theoretical Study of The Effect of Ball Milling Technique on Hydrogen Storage of Alkhali Borohydrids

The ground-state crystal structures of the alkali borohydrides MBH4 (M= Li, Na, K, Rb) have been established from first principle calculations by comparison of boron charge densities. Alkali borohydrides are potential hydrogen storage material because of their high capacity of H2. However, their high thermodynamic stability is unfavorable for dehydrogenation processes. Understanding the bonding nature of M-B is essential for predicting their dehydrogenation mechanism and then improving their dehydrogenation performance. Quadrupolar parameters of nuclei can be used as a tool to understand the electronic structure of compounds. In this work the charge density distribution in MBH4 (M= Li, Na, K, Rb) was compared. Thus using calculated Nuclear Quadrupole Coupling Constants (NQCC) of 11B nuclei, their electronic structure was studied. The results show that11B-NQCCs of borohydrides is a criterion of their thermodynamically stability and ball milled alkali metal zinc borohydrides demonstrates better catalytic properties with small 11B-NQCC. Therefore, boron atoms in the studied mixed metal borohydrids have weaker bond with metal and easier condition for dehydrogenation in mixed metal borohydrids are seen. The electric field gradient (EFG) of quadrupolar nuclei were calculated to obtain NQCC parameters. All calculations performed using Gaussian 03 at B3LYP/6-311G* GGGGG GG GGGGGG.