The binary boron lithium clusters B₁₂Li with n = 1-14: in search for hydrogen storage materials

Molecular structures and properties of the binary clusters containing twelve boron atoms mixed with n lithium atoms, B₁₂Li 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 B₁₂Li 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.