Superhalogens as building blocks of complex hydrides for hydrogen storage

Abstract

Superhalogens are species whose electron affinity (EA) or vertical detachment energy (VDE) exceeds those of halogens. These species typically consist of a central electropositive atom with electronegative ligands. The EA or VDE of species can be further increased by using superhalogens as ligands, which are termed as hyperhalogens. Having established BH4− as a superhalogen, we have studied BH4−x(BH4)x− (x=1–4) hyperhalogen anions and their Li-complexes LiBH4−x(BH4)x using density functional theory. The VDE of these anions is larger than that of BH4−, which increases with the increase in number of peripheral BH4 moieties (x). The hydrogen storage capacity of LiBH4−x(BH4)x complexes is higher but binding energy is smaller than that of LiBH4, a typical complex hydride. The linear correlation between the dehydrogenation energy of LiBH4−x(BH4)x complexes and the VDE of BH4−x(BH4)x− anions is established. These complexes are found to be thermodynamically stable against dissociation into LiBH4 and borane. This study demonstrates the role of superhalogens in designing new materials for hydrogen storage and should also motivate experimentalists to synthesize LiBH4−x(BH4)x (x=1–4) complexes.