Adsorption of Molecular Hydrogen in a Graphene-Carbon Nanotube System

In this theoretical study, the H2 adsorption is considered in a novel system formed by a (6,6) carbon nanotube fragment and a planar graphene layer portion, when they are separated 4.72 a.u., with both subsystems inside a cubic supercell box of 25 a.u. side. There is greater H2 adsorption inside the carbon nanotube than in the interstitial space between the graphene layer and the carbon nanotube. The results are compared with the way the H2 molecule is adsorbed upon a lonely graphene layer and inside or outside the (6,6) carbon nanotube. It is studied if in the interstitial space close to the middle point between the wall and the graphene layer the hydrogen molecule could be adsorbed with a greater binding energy that in any other case. This was not possible for the selected supercell, but there are given some suggestions to be explored (using a nanotube with smaller radii or increasing the size of the supercell), that perhaps can optimize the binding energy for H2 adsorption. A general result is that the size of the cubic supercell can be selected to confine not only hydrogen inside the carbon nanotube but also in the interstitial space between the carbon nanotube wall and the graphene layer. It is believed that the studied system or a modification of this could sooner or later be used in a competitive way in comparison with other H2 storage materials respect to the hydrogen adsorption and desorption process.