Ca coated boron nanostructures: Energetics and possible usage for hydrogen storage

We investigate the adsorption of calcium atoms on the surface of novel boron fullerenes and the alpha boron sheet using density functional theory within the generalized gradient approximation. Recent theoretical works show that the B$_{80}$ fullerene coated with one Ca atom on each pentagonal ring is capable of storing up to 60 H$_2$ molecules, yielding a gravimetric density of 8.2 wt.\%. We have performed a detailed investigation of Ca adsorption on the B$_{80}$ fullerene. In contrast to recent results, our calculations show that a single Ca atom prefers to occupy a hexagonal site rather than a pentagonal site. The calculations on Ca$_{12}$B$_{80}$, Ca$_{20}$B$_{80}$ and Ca$_{32}$B$_{80}$ indicate that Ca atoms prefer to uniformly coat the whole surface over selectively occupying the pentagonal or hexagonal sites. We have examined the feasibility of H$_2$ storage on Ca$_{32}$H$_{80}$ in the range 7.2 - 8.2 wt\%. Our calculations indicate that the uniformly coated Ca$_{32}$B$_{80}$ fullerene is not capable of achieving a hydrogen storage capacity of 7.2 wt. \% or higher. It is necessary to somehow reduce the Ca concentration on the B$_{80}$ surface to achieve a 8.2 wt. \% of hydrogen.