AMMONIA-BORANE BASED HYDROGEN STORAGE MATERIALS - A COMPUTATIONAL STUDY

Abstract

Ammonia-borane is a promising hydrogen storage material. However, it exhibits slow thermal kinetic behaviour below 100oC and produces several detrimental volatile by-products including borazine (HNBH)3 and diborane (B2H6) upon dehydrogenation. Consequently, various approaches have been investigated to further improve the dehydrogenation properties of ammonia-borane. Two current outstanding approaches are: (i) the integration of metals such as Li and Mg to NH3BH3, leading to the formation of metal amidoboranes such as LiNH2BH3, Mg(NH2BH3)2.NH3 and LiBH4.MgCl2.NH3; and (ii) the utilisation of polymers to confine ammonia-borane. Both of these approaches have been reported to significantly improve the dehydrogenation properties of ammonia-borane. However, the detailed dehydrogenation mechanisms have not been fully understood, as such, the aim of this thesis is to comprehensively investigate the dehydrogenation mechanisms of these novel compounds, and to elucidate the critical roles of metals and polymers in facilitating the hydrogen release using quantum chemical calculations. We have found two different roles of metals and polymers in the dehydrogenation of these compounds. In the metal amidoboranes, metal atoms play an important catalytic role in carrying the hydride from the BH3 group to combine with the proton of the NH3 group and facilitate the H2 release. In the case of polymer-confined ammonia-borane, the carbonyl functional group is of extreme importance. It initially facilitates the breaking of the B-N bond of ammonia-borane, and reassembles NH3 and BH3 into a zwitterionic intermediate. This intermediate has a particular structure that accommodates the facile interaction between the proton of the NH3 group and the hydride of the BH3 group, leading to the facile H2 elimination.