Molecular Design of a Reversible Hydrogen Storage Device Composed of the Graphene Nanoflake–Magnesium–H(2) System

[Image: see text] 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 H(2) storage density. In the present study, the binding energies of H(2) to Mg species on GRs, GR–Mg(m+) (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 H(2) molecule is close to 0, whereas those for m = 1 (Mg(+)) and 2 (Mg(2+)) are 0.23 and 13.2 kcal/mol (n = 1), respectively. These features suggest that GR–Mg(2+) has a strong binding affinity toward H(2), whereas GR–Mg(+) has a weak binding energy. In addition, it was found that the first coordination shell is saturated by four H(2) molecules, GR–Mg(2+)–(H(2))(n) (n = 4). Next, direct ab initio molecular dynamics calculations were carried out for the electron-capture process of GR–Mg(2+)–(H(2))(n) and a hole-capture process of GR–Mg(+)–(H(2))(n) (n = 4). After electron capture, the H(2) molecules left and dissociated from GR–Mg(+): GR–Mg(2+)–(H(2))(n) + e(–) → GR–Mg(+) + (H(2))(n) (H(2) is released into the gas phase). In contrast, the H(2) molecules were bound again to GR–Mg(2+) after the hole capture of GR–Mg(+): GR–Mg(+) + (H(2))(n) (gas phase) + hole → GR–Mg(2+)–(H(2))(n). On the basis of these calculations, a model device with reversible H(2) adsorption–desorption properties was designed. These results strongly suggest that the GR–Mg system is capable of H(2) adsorption–desorption reversible storage.