Hydrogen adsorption at the graphene surface: A vdW-DF perspective

We use the recently developed van der Waals density functional (vdW-DF) along with the C09x exchange functional to explore the interactions of molecular hydrogen with a single sheet of graphene. Our calculations demonstrate the importance of the inclusion of dispersion interactions for evaluating the adsorption capacity of graphitic, sparsely packed materials. In particular we show that, although the commonly used local density approximation (LDA) exchane-correlation functional gives reasonable H-2-graphene separation distances, these interactions are in fact too strong and decrease too rapidly as the H-2-graphene seperation distances increases. On the other hand, calculations employing the generalized gradient approximation (GGA) for exchange and correlation exhibit very little binding. The vdW-DFC09x functional, however, gives binding very similar to bechmark second-order Moller-Plesset (MP2) theory results for the H-2-coronesne interaction and has an appropriate long range interaction with the graphene shett. This shortcoming of the LDA and GGA functionals is shown to have significant consequences on the overall adsorption densities of H-2 near the graphene, thus emphasizing the fundamental importance of properly characterizing the interactions of adsorbed molecules in porous media.