Palladium Clusters Anchored on Graphene Vacancies and Their Effect on the Reversible Adsorption of Hydrogen

The hydrogen storage capacity of nanoporous carbons can be enhanced through metal doping, for instance doping with palladium. However, there are two problems that may limit the positive effect of metal doping on the reversible storage capacity. First, clustering of the metal atoms decreases its effectiveness, which is largest for maximum dispersion. A second problem that is often overlooked is that the desorption of metal hydrogen complexes may compete favorably with the desorption of hydrogen molecules. Desorption of complexes would spoil the reversible storage of hydrogen in the material. Both problems can be avoided by firmly anchoring the metal atoms and clusters to defects of the carbon substrate, for instance vacancies. With this goal in mind, we have performed density functional calculations to investigate the desorption of hydrogen and of Pd-hydrogen complexes from Pd atoms and clusters supported on pristine graphene and on graphene layers with vacancies. We show that palladium atoms bind much stronger to graphene vacancies, binding energy E-b = 5.13 eV, than to pristine graphene, E-b = 1.09 eV. The Pd atoms tend to nucleate and form clusters around the vacancies and the small Pd-n clusters (n = 2-6) also bind strongly, E-b approximate to 5 eV, to the vacancies. However, the Pd-Pd interaction is much smaller than the Pd-vacancy interaction, and therefore, the vacancies favor the dispersion of palladium on the graphene layer. For hydrogen adsobed on Pd atoms and clusters supported on pristine graphene, desorption of Pd-hydrogen complexes competes with desorption of molecular hydrogen. However, for hydrogen adsobed on a Pd atom anchored on a graphene vacancy, the desorption of the PdH2 complex costs 4.2 eV, and therefore, it does not compete with the desorption of molecular hydrogen, which takes place with an energy cost of only 0.2 eV. This shows the beneficial effect that anchoring Pd atoms and clusters to graphene vacancies has on the reversible adsorption/desorption of hydrogen.