On the hydrogen storage performance of Cu-doped and Cu-decorated graphene quantum dots: a computational study

Hydrogen gas is a promising renewable energy source. The hydrogen storage performance of two differently modified graphene surfaces, particularly Cu-doped and Cu-decorated circumcoronene (CC), is investigated using density functional theory, 6-311G* basis set and Bader's quantum theory of atoms in molecules (QTAIM). It is found that the Cu-doped CC is able to bind three H2 molecules on one Cu atom, while the Cu-decorated CC is able to bind up to five H2 molecules on one Cu atom. Changes in the topology of charge density upon the H2 adsorption are evaluated under the formalism of QTAIM analysis. The QTAIM analysis of bond critical points as well as the density of states analysis show that the interaction between Cu and adsorbed H2 molecules can be considered as a physisorption (a van der Waals type interaction). Overall, the results presented in this study point out that the Cu-decorated graphene surfaces are more suitable potential candidates for hydrogen storage than the Cu-doped ones. Furthermore, the inclusion of diffuse functions in the basis set is critically considered.