  Two-dimensional porous carbon nanomaterials are proven to be promising
hydrogen storage substrates as they possess high surface area, large number of
active sites, low molecular mass, and hydrogen molecules can be adsorbed on
both sides of these materials. By performing first-principles density
functional theory-based calculations, we report ultrahigh reversible hydrogen
uptake in lithium decorated 2D carbon allotrope PAI-graphene, which is formed
of a regular pattern of polymerized as-indacenes (PAI). We found that a single
unit cell of PAI-graphene can be decorated by 8 Li atoms, in which each Li atom
can reversibly adsorb 4 hydrogen molecules, leading to 15.7 % of H uptake,
remarkably higher than the DOE demand of 6.5 %. Li atom donates its valence
2s-electron to PAI-graphene and gets ionized. The adsorption energies of the
various H2 attached to Li-atom are found to be suitable for reversible use
during practical applications. Hydrogen molecules get attached to the ionized
metal atom by electrostatic interactions. An energy barrier of 1.48 eV is
present for the diffusion of Li atoms between the two most stable adsorption
sites which justifies the absence of the clustering of Li atoms.
