The binary boron lithium clusters B12Lin with n = 1-14: in search for hydrogen storage materials.

Abstract

Molecular structures and properties of the binary clusters containing twelve boron atoms mixed with n lithium atoms, B12Lin with n = 1-14, were investigated using density functional theory with the TPSSh functional and the 6-311+G(d) basis set. Energetic parameters including relative energies, average binding energies and second-order energies of the entire series were predicted using the coupled-cluster theory (U)CCSD(T) in conjunction with the cc-pVTZ basis set. Several lowest-lying isomers were determined for each size B12Lin whose energies differ from each other by <3 kcal mol-1, except for n = 1, 2 and 4 (≤5 kcal mol-1), and particularly n = 8 (∼13 kcal mol-1). Electronic structure and chemical bonding in some specific sizes such as B12Li4, B12Li8 and B12Li14 were analyzed in detail. We established the electron shells of some magic clusters such as the B12Li4 cone for which we proposed a mixed cone-disk electron shell model. Thanks to both the phenomenological shell and Clemenger-Nilsson models, B12Li8 which contains a specific set of shells of 44 valence electrons is a high stability species. The arrangement of Li atoms around a fullerene B12 framework shows that the mixed B12Li8 emerges as the most suitable of this cluster series to adsorb molecular hydrogen. Up to 32 H2 molecules can strongly be attached to the B12Li8 cluster which is thus predicted to be a realistic candidate for hydrogen storage material with gravimetric density reaching up to a theoritical limit of 26 wt%. Attachment of the fifth H2 molecule to each Li atom of B12Li8 results in weaker average bonds but can give rise to a total of 40 H2 molecules, corresponding to 30 wt% of hydrogen.