“…Carbon materials in multiple forms, like porous carbons, activated carbon flakes, graphene, fullerenes, and carbon nanotubes, are outlined as candidates for storage . Doping these materials with metal atoms, clusters, and nanoparticles significantly improves their hydrogen storage capacity. − One proposal to explain this improvement has been to assume a mechanism of spillover, in which the hydrogen molecules first adsorb on the surface of the metallic nanoparticle and dissociate, and then, the hydrogen atoms migrate toward the carbon substrate . However, this mechanism has been analyzed in detail for the case of Pd nanoparticles supported on graphene by performing accurate atomistic dynamical simulations, and its validity has been questioned, because of the existence of large spillover barriers. , Sizable barriers have also been predicted from calculations for other supported transition metal clusters. − Palladium is, in fact, a promising metal for applications related to hydrogen storage because it can absorb large amounts of hydrogen in the form of palladium hydrides .…”