Al enhances the H${}_2$ storage capacity of graphene at nanoribbon borders but not at central sites: A study using nonlocal van der Waals density functionals

We performed ab initio density-functional-theory calculations to investigate the adsorption of molecular hydrogen on pristine and Al-doped hydrogen-passivated zigzag graphene nanoribbons (GNRs) using nonlocal van der Waals functionals that have recently been proposed for an accurate description of exchange and correlation effects in weakly bound systems. Our results, which are compared with those obtained using the standard local density and generalized gradient approximations, show that neither pristine GNRs nor substitutionally Al-doped GNRs with an Al atom occupying a central, lateral, or subedge site satisfy the binding-energy criterion specified by the U.S. Department of Energy for novel hydrogen-storage materials. However, the criterion is satisfied by a GNR doped with an Al atom at an edge site and also by zigzag GNRs with adsorbed Al at lateral hole sites.

Figure 1

Schematic representation of the (initial) positions of the ${\mathrm{H}}_2$ molecule on the 10-zigzag GNR from which the passivating hydrogens have been omitted for clarity: CHS (central hole site), LHS (lateral hole site), and EBS (edge bay site). Al doping was performed by replacing a C atom with an Al atom at a CS (central site), an LS (lateral site), an SES (subedge site), or an ES (edge site). Also shown are the two starting orientations of the ${\mathrm{H}}_2$ molecule: parallel and vertical to the GNR plane.

Figure 2

Optimized structure of the system ${\mathrm{H}}_2$–Al-doped GNR in the CHS(P)(Ferro-A) configuration as obtained using the optB88-vdW functional.

Figure 3

Upper two panels: DOS of the system ${\mathrm{H}}_2$-GNR in the CHS(P)(Ferro-A) configuration, projected on the H atom closer to the GNR plane and on the C atom closest to this H atom as obtained using the optB88-vdW functional (left) and the LDA (right). (The DOS $\delta$ functions have been replaced by Gaussian functions of finite breadth.) Lower three panels: As above but for GNR doped with Al at a CS and including the projection on the Al atom.

Figure 4

Upper two panels: DOS of the system ${\mathrm{H}}_2$-GNR in the EBS(V)(Ferro-A) configuration, projected on the H atom closer to the GNR plane and on the C atom closest to this H atom as obtained using the optB88-vdW functional (left) and the LDA (right). (The DOS $\delta$ functions have been replaced by Gaussian functions of finite breadth.) Lower three panels: As above but for GNR doped with Al at an ES and including the projection on the Al atom.

Figure 5

Projection on an energy window containing the bonding state of the ${\mathrm{H}}_2$ molecule of the charge densities of the systems ${\mathrm{H}}_2$-GNR [(a) and (b)] and ${\mathrm{H}}_2$–Al-doped GNR [(c) and (d)] in the EBS(V)(Ferro-A) configuration as obtained using the optB88-vdW functional [(a) and (c)] and the LDA [(b) and (d)]. Only the relevant part of each system is shown.