First principles investigation of hydrogen physical adsorption on graphynes' layers

Graphynes are 2D porous structures deriving from graphene featuring triangular and regularly distributed subnanometer pores, which may be exploited to host small gaseous species. First principles adsorption energies of molecular hydrogen (H-2) on graphene, graphdiyne and graphtriyne molecular prototypes are obtained at the MP2C level of theory. First, a single layer is investigated and it is found that graphynes are more suited than graphene for H-2 physical adsorption since they provide larger binding energies at equilibrium distances much closer to the 2D plane. In particular, for graphtriyne a flat minimum located right in the geometric center of the pore is identified. A novel graphite composed of graphtriyne stacked sheets is then proposed and an estimation of its 3D arrangement is obtained at the DFT level of theory. In contrast to pristine graphite this new carbon material allow both H-2 intercalation and out-of-plane diffusion by exploiting the larger volume provided by its nanopores. Related H-2 binding energies for intercalation and in-pore adsorption are around 0.1 eV and they could lead to high storage capacities. The proposed carbon-based layered material may represent a safer and potentially cheaper alternative for hydrogen on-board storage than conventional solutions based on cryogenic liquefaction and/or high compression. (C) 2015 Elsevier Ltd. All rights reserved.