Aluminum-Doping Effects on the Electronic States of Graphene Nanoflake: Diffusion and Hydrogen Storage Mechanism.

Graphene nanoflakes are widely utilized as high-performance molecular devices due to their chemical stability and light weight. In the present study, the interaction of aluminum species with graphene nanoflake (denoted as GR-Al) has been investigated using the density functional theory (DFT) method to elucidate the doping effects of Al metal on the electronic states of GR. The mechanisms of the diffusion of Al on GR surface and the hydrogen storage of GR-Al were also investigated in detail. The neutral, mono-, di-, and trivalent Al ions (expressed as Al, Al+, Al2+, and Al3+, respectively) were examined as the Al species. The DFT calculations showed that the charge transfer interaction between Al and GR plays an important role in the binding of Al species to GR. The diffusion path of Al on GR surface was determined: the barrier heights of Al diffusion were calculated to be 2.1-2.8 kcal mol-1, which are lower than Li+ on GR (7.2 kcal/mol). The possibility of using GR-Al for hydrogen storage was also discussed on the basis of the theoretical results.

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+ (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 (n = 4). Next, direct ab initio molecular dynamics calculations were carried out for the electron-capture process of GR-Mg2+-(H2) n and a hole-capture process of GR-Mg+-(H2) n (n = 4). After electron capture, the H2 molecules left and dissociated from GR-Mg+: GR-Mg2+-(H2) n + e- → GR-Mg+ + (H2) n (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) n (gas phase) + hole → GR-Mg2+-(H2) n . 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.