Molecular Design of a Reversible Hydrogen Storage Device Composed of the Graphene Nanoflake-Magnesium-H₂ System

Carbon materials such as graphene nanoflakes (GRs), carbon nanotubes, and fullerene can be widely used for hydrogen storage. In general, metal doping of these materials leads to an increase in their H₂ storage density. In the present study, the binding energies of H₂ to Mg species on GRs, GR-Mg (m = 0-2), were calculated using density functional theory calculations. Mg has a wide range of atomic charges. In the case of GR-Mg (m = 0, Mg atom), the binding energy of one H₂ molecule is close to 0, whereas those for m = 1 (Mg⁺) and 2 (Mg²⁺) are 0.23 and 13.2 kcal/mol (n = 1), respectively. These features suggest that GR-Mg²⁺ has a strong binding affinity toward H₂, whereas GR-Mg⁺ has a weak binding energy. In addition, it was found that the first coordination shell is saturated by four H₂ molecules, GR-Mg²⁺-(H₂) (n = 4). Next, direct ab initio molecular dynamics calculations were carried out for the electron-capture process of GR-Mg²⁺-(H₂) and a hole-capture process of GR-Mg⁺-(H₂) (n = 4). After electron capture, the H₂ molecules left and dissociated from GR-Mg⁺: GR-Mg²⁺-(H₂) + e^- → GR-Mg⁺ + (H₂) (H₂ is released into the gas phase). In contrast, the H₂ molecules were bound again to GR-Mg²⁺ after the hole capture of GR-Mg⁺: GR-Mg⁺ + (H₂) (gas phase) + hole → GR-Mg²⁺-(H₂) . On the basis of these calculations, a model device with reversible H₂ adsorption-desorption properties was designed. These results strongly suggest that the GR-Mg system is capable of H₂ adsorption-desorption reversible storage.