Effect of external pressure on the hydrogen storage capacity of a graphene flake: molecular dynamics

The hydrogenation process of a crumpled graphene flake under the temperature and pressure is calculated by molecular dynamics. A graphene flake is placed in a hydrogen atmosphere containing both atomic and molecular hydrogen and exposed at finite temperature and pressure in an isothermal-isobaric (NPT) ensemble. Results show that the best gravimetric density is achieved at 77 K and external pressure of 140 atm. However, the increase in the gravimetric density at 77 K is due to the physical adsorption of hydrogen molecules, i.e., van der Waals forces are formed between the carbon surface and H2 molecules. At room temperature, the number of H atoms that formed a covalent bond with the edge C atoms increases during exposure at this temperature. The molecular dynamics simulation demonstrates that different types of hydrogen adsorption by a graphene flake predominate at two temperatures: at 77 K, physical adsorption plays the main role, and at 300 K, chemical adsorption. And the combination of high external pressure and low temperature makes it possible to achieve high values of a hydrogen sorption.