A theoretical deduction of the shape and size of nanocarbons suitable for hydrogen storage

We evaluated the adsorption energy of a hydrogen molecule in nanocarbons consisting of graphene sheets. The nanocarbon shapes were a pair of disks with separation 2 d , a cylinder with radius d , and a truncated sphere with radius d . We obtained the adsorption energy in the form of a 10–4 Lennard–Jones function with respect to 1/ d . The values of the potential depth ( D ) and equilibrium distance ( d e ), respectively, were 94 meV and 2.89 Å for the disk pair, 158 meV and 3.14 Å for the cylinder, and 203 meV and 3.37 Å for the sphere. When d = d e , the adsorption energy of the disk pair (cylinder) became deeper than −0.9 D , and it approached − D when the radius (length) increased to more than twice its separation (radius). The adsorption energy of the sphere was increased from − D to −0.5 D when the radius of the opening increased from 0 to d e . These results suggest that porous carbon materials can increase the adsorption energy by up to ∼200 meV if the carbon atoms are arranged on a spherical-like surface with ∼7 Å separation. This may lead to practical hydrogen storage for fuel cells.