Storing hydrogen in graphene layers with tunable interlayer spacing*

Carbon nanostructures are being studied for hydrogen storage. However the nature of H-C interactions in such structures is unclear. We use first-principles simulation to model H adsorption between graphene layers. The adsorption of H$_{2}$ between layers is evaluated, particularly with respect to inter-layer variation thus simulating the effect of stressing the graphite for H storage. We note that H$_{2}$ dissociates when the inter-graphene distance is reduced (the graphite is compressed). When the ratio of H:C=1:1, the graphene changes from a planar to a diamond-like structure. The H-C interaction changes from weak physisorption to strong chemisorption. When the pressure is reduced, H atoms can recombine to form H$_{2}$ by overcoming a small energy barrier. Based on this work, we propose a new scheme for H storage in C nanostructures: by way of altering the inter-graphene distance, the C structure can effectively ``inhale,'' store and release hydrogen in a controlled manner.