Ab initio investigation of molecular hydrogen physisorption on graphene and carbon nanotubes

Density-functional theory is used to investigate hydrogen physisorption on a graphene layer and on single wall carbon nanotubes. Both external and internal adsorption sites of (9, 0) and (10, 0) carbon nanotubes have been studied with the hydrogen molecular axis oriented parallel or perpendicular to the nanotube wall. A range of hydrogen molecule binding sites has been examined and it is found that hydrogen binds weakly to each of the graphitic structures and at all adsorption sites examined. Calculations using different functionals reveal that the binding energies are a factor of 2 larger for hydrogen bound inside the nanotubes than for adsorption outside the nanotubes or on the graphene layer. Furthermore, configurations of the hydrogen molecular axis parallel to the nanotube wall or graphene layer bind more effectively than configurations where the axis is normal to the carbon nanostructures. The differing behavior between the carbon nanostructures is attributed to the curvature of the structure and the hydrogen-carbon electron interactions, where analysis of the electron density reveals evidence of charge redistribution with little charge transfer. The potential of hydrogen physisorption to carbon nanostructures for hydrogen storage and delivery is also discussed.