Interplay among Hydrogen Chemisorption, Intercalation, and Bulk Diffusion at the Graphene-Covered Ni(111) Crystal

The interaction between a Ni(111) substrate covered by a complete Gr monolayer and H atoms occurs through two parallel routes leading to the hydrogen chemisorption on graphene and, at much lower rate but still with some ease, to the intercalation of H atoms below it. This latter reaction determines a direct interaction of the H atoms with the metal surface and eventually the H diffusion into the Ni bulk under the Gr cover. In this study we have combined high-resolution X-ray photoelectron spectroscopy, thermal programmed desorption, and density functional theory calculations to establish how the chemisorption and intercalation yields and their interplay depend on temperature and to find out how graphene affects the amount and the evolution of the hydrogen diffused in the Ni bulk. We found that between 150 and 320 K, hydrogen chemisorption on Gr is independent of temperature and that Gr lifting, which signals the H intercalation below it, does not occur below 180-200 K, being limited by an energy barrier of the order of 150 meV. For the heavily hydrogenated samples, when H atoms diffuse also into the Ni bulk, the Gr cover plays a key role for H storage because it strongly enhances the amount of H loaded in the interface with respect to the bare Ni(111) substrate. This behavior, possibly exhibited also by other graphene/metal interfaces provided that intercalation of H below graphene can readily occur, might foster the design of innovative materials to be applied for H storage.