First-Principle Study of Hydrogen Adsorption on Na-Coated Carbon Nanotubes

We investigate the hydrogen adsorption capacity of Na-coated carbon nanotube using first-principles plane wave method. A single Na atom always occupies the hollow site of a hexagonal carbon ring in all, with a nearly uniform Na-C bond length of 2.5 A. Semiconducting zigzag nanotubes, (8,0) show stronger binding energies of the Na atom of -2.1 eV. The single Na atom can further adsorb up to six hydrogen molecules with a relatively constant binding energy of -0.26 eV/H₂. Mulliken population analysis shows that positively charged Na atoms with 0.82e charge transfer to nearest carbon atoms which polarizes the CNT leading to local dipole moments. This charge- induced dipole interaction is responsible for the higher hydrogen uptake of Na-coated CNT. We also investigate the clustering of Na atoms to find out the maximum weight percentage adsorption of H₂ molecules. At high Na coverage, we show that Na-coated CNTs can adsorb 9.2 wt % hydrogen. Our analysis shows that, although indeed Na-coated CNT present potential material for the hydrogen storage, care should be taken to avoid Na atoms clustering on support material, to achieve higher hydrogen capacity.