Ab initio study of lithium decoration of popgraphene and hydrogen storage capacity of the hybrid nanostructure

The successful realization of extended ‘5-5-8’ line defects in graphene in a controlled way has suggested the possible formation of a new 2D carbon allotrope consisting in pentagonal-octagonal-pentagonal carbon rings. The mechanical and thermal stability of this nanostructure, called popgraphene, has recently been confirmed on the basis of first-principles calculations. Moreover, it has been proposed as a promising anode material for use in Li-ion batteries with fast charge/discharge rates. In the present paper we perform density functional theory calculations to investigate the hydrogen storage ability of popgraphene. Our calculations show that popgraphene can be decorated in a very stable way by placing Li atoms on the pentagonal carbon rings of both sides of the sheet, leaving free the octagonal carbon rings. In that condition, the Li-decorated popgraphene sheet can bind up to four H2 molecules per unit cell, with an average adsorption energy in a range between physisorption and atomic chemisorption, which would allow for reversible hydrogen storage at moderate temperatures and pressures. Moreover, the gravimetric density of the Li-decorated popgraphene is 4.24 wt%, which is almost equal to the threshold specified by the U.S. Department of Energy for novel hydrogen-storage materials. All these results show that Li-decorated popgraphene nanostructures could be good materials for hydrogen storage.