Stable calcium adsorbates on carbon nanostructures: Applications for high-capacity hydrogen storage

We have investigated the stability of calcium adsorbates on carbon nanotubes and defective graphene using first-principles calculations. For ultranarrow carbon nanotubes, we find that the effect of chirality is more important to the adsorption as compared to the diameter $(D)$. The binding energy for a calcium atom absorbed on the (5,0) tube is about 1.4 eV higher than that on the (3,3) tube. We also find that calcium atoms on the octagon defect of graphene are also stable. The binding energies of calcium adsorbates on narrow tubes (3.2 <D<5.6 ) and defective graphene are high, which indicates that the calcium monolayer should be stable without clustering. The results show that every calcium atom can uptake four to five H₂ and the hydrogen storage can reach 9 wt %.