Multiple Li+- and Mg2+-decorated PAHs: potential systems for reversible hydrogen storage

The hydrogen binding efficiency of multiple metal-ion-(Li+, Mg2+)-decorated small Polycyclic Aromatic Hydrocarbons (PAHs) has been investigated using Density Functional Theory (DFT). The CAM-B3LYP method and 6-311+G(d, p) basis set have been used for the systematic calculation of the interaction energy of the metal ion with the aromatic system (Delta E) and the average binding energy (Delta BE) of the hydrogen molecules to these metal-ion-decorated PAHs. Our results show that the aromatic ring current associated with a PAH can bind at most one pair of metal ions, with the metal ions ensconcing themselves with appreciable values of Delta E both on the same and opposite faces of a PAH with certain preferences. The Delta BE values associated with H-2 binding on Li+-decorated systems are found to be almost similar for anthracene and phenanthrene, which are higher in comparison to naphthacene. The Mg2+-decorated counterparts, however, exhibit Delta BE values of around four to five times higher than the Li+-decorated ones. The nature of interaction between hydrogen molecules and metal ions is predicted by the topological analysis of the atoms in molecules formalism (AIM), employing an AIMAll program. The Natural Population Analysis (NPA) method is used for the evaluation of charge distribution between donor hydrogen molecules and acceptor metal ions. The value of charge transfer from H-2 to metal ions (Delta E-CT) is evaluated employing Natural Bond Orbital (NBO) analysis. The charge transfer from the bonding orbital of the hydrogen molecule to the antibonding lone pair orbital of the metal ion is taken into account for stability of the complexes. All the complexes possess high gravimetric storage capacity, and are found to be maximum for Mg2+-decorated anthracene (9.6 wt% H-2).