Adsorption and dissociation of molecular hydrogen on the edges of graphene nanoribbons

The dissociation and adsorption of molecular hydrogen on the edges of graphene nanoribbons of widths of 1.14 and 1.36 nm, is investigated within the density functional formalism. Here, graphene nanoribbons are used as models for the pore walls of some nanoporous carbons (carbide-derived carbons among others) which have been shown to be formed by one-atom thick graphene layers interconnected among them and exhibiting exposed edges (Lopez et al. in J Chem Phys 135: 104706, 2011). The aim of this study is to shed some light on the contribution of the edges of the pore walls to the hydrogen storage capacity of nanoporous carbons. Nanoribbons with zigzag and armchair edge terminations have been considered. Molecular hydrogen dissociates and adsorbs atomically at the nanoribbon edges with no or small activation barrier. The adsorption energies per hydrogen molecule are quite large, 2.5 and 5.7 eV for armchair and zigzag edges, respectively. This indicates that the graphene edges are very reactive and will be saturated with hydrogen whenever available. However, under mild conditions of pressure and temperature hydrogen cannot be desorbed from the edges and, therefore, the edges do not contribute to the reversible storage capacity of the material. The magnetic properties of saturated and unsaturated ribbons are also discussed.