On the basis of the first principle density functional theory (DFT) the stability, the electronic structure and the hydrogen adsorption of a metal functionalized monolayer of AGNR-O (armchair graphene nanoribbons) have been studied. The AGNR-O and AGNR-OH structures were decorated with different metal atoms, M = Ca, Mg, Ti. Ca and Mg as decorating metals were found to be superior to the transition metals due to small cohesive energies (no clustering) and the capability of binding multiple hydrogen molecules. The binding of up to four hydrogen molecules to the alkaline earth and transition metal atoms has been investigated. For the alkaline earth metal atoms the binding energy per H2 molecule is almost independent from the number of adsorbed H2 molecules. An increase in the adsorption energy by a factor of six or even more, from 0.07 eV/H2 for “naked” nanoribbons to 0.35 eV/H2 or to 0.69 eV/H2 for AGNR-O structures has been observed. This different hydrogen binding energy is due to different construction of the AGNR-O edges. The AGNR edge terminations influence the binding energies of the M/AGNR and consequently the H2 binding energies on M/AGNR. If the binding energy between the metal atoms and AGNR is not very strong, then H2 could bind much stronger to the M/AGNR. This could be a possible route to 10 wt % hydrogen storage capacity.
This work was financially supported by the European Union via Graphene Flagship grant 604391. The simulations were performed by using the HPC resources on the Juropa supercomputer at the Forschungszentrum Jülich (Germany). One of the authors (D. C. T) would like to thank for useful discussions to Kai Trepte.
©2016 Walter de Gruyter Berlin/Boston