Clustering of Sc on SWNT and reduction of hydrogen uptake:: Ab-initio all-electron calculations

Energies and kinetic barriers associated with transition metal (Sc) clustering on a single-walled carbon nanotube (SWNT) and graphene were studied by the all-electron density functional method. The analysis shows that the binding energy of Sc atom on SWNT is highly sensitive to the tube diameter and chirality. The metal atoms clustering on common SWNT, with diameters similar to 1-2 nm, is energetically favorable and kinetically permitted. Although well-separated, lone Sc atoms on SWNT can enhance the hydrogen storage capacity, their aggregation into clusters significantly reduces the hydrogen uptake; e.g., a Sc-4 cluster has the same hydrogen uptake as a single Sc atom. Our analysis shows that, although indeed light transition metal decorated SWNT present potential material for. the hydrogen storage, utter care should be taken to avoid the metal clustering on support material, to achieve and maintain higher hydrogen capacity.