Palladium Clusters Supported on Graphene Monovacancies for Hydrogen Storage

We give a detailed description of the atomic structure and the energetics of H-2 adsorption on Pd-n (n = 14) clusters supported on graphene monovacancies. The large binding energy of small Pdn clusters on these vacancies is a result of strong hybridization between the unsaturated carbon and the Pd atoms, and it further indicates that anchoring avoids migration of Pd clusters on the graphene surface. We found that the binding energy of a single H-2 is dependent on both the Pd cluster size and the adsorption site. In general, the H-2 bond cleavage is favored by Pd clusters, indicating the formation of metal hydrides. Our calculations predict that the sequential binding energy of H-2 decreases from 1.2 to 0.085 eV as a function of the number of adsorbed molecules and that the graphene surface modulates the metal-hydrogen interaction. The analysis of the absorption energies and H-2 average bond lengths suggest that the supported Pd-4 cluster is a unique species and potential hydrogen storage candidate because it is able to hold up to four molecules covalently with moderate average binding energy within the optimal range for an efficient cyclic adsorption/desorption process at room temperature and moderate pressures. These results show that the interaction between graphene monovacancies and metal nanoparticles can explain the role that defects play in the dramatic enhancement of hydrogen storage in metal-decorated graphene samples.