  We investigate the hydrogen adsorption capacity of Na-coated carbon nanotubes
(Na-SWCNTs) using first-principles electronic structure calculations at
absolute temperature and pressure. A single Na atom is always found to occupy
the hollow site of a hexagonal carbon ring in all the six different SWCNTs
considered, with a nearly uniform Na-C bond length of 2.5 A. Semiconducting
zigzag nanotubes, (8,0) and (5,0), show stronger binding energies for the Na
atom (-2.1 eV and -2.6 eV respectively), as compared to metallic SWCNTs with
armchair and chiral geometries. The single Na atom can further adsorb up to six
hydrogen molecules with a relatively constant binding energy of -0.26
eV/H$_{2}$. Mulliken population analysis shows that positively charged Na atoms
with 0.82$e$ charge transfer to nearest carbon atoms which polarizes the SWCNT
leading to local dipole moments. This charge-induced dipole interaction is
responsible for the higher hydrogen uptake of Na-coated SWCNTs. The transition
state search shows that diffusion barrier of Na-atom on the SWCNT between two
adjoining C-C rings is 0.35 eV. We also investigate the clustering of Na atoms
to find out the maximum weight percentage adsorption of H$_{2}$ molecules. At
high Na coverage, we show that Na-coated SWCNTs can adsorb 9.2-11.28 wt %
hydrogen. Our analysis shows that, although indeed Na-coated SWCNTs present
potential material for the hydrogen storage, care should be taken to avoid Na
atoms clustering on support material at elevated temperature and pressure, to
achieve higher hydrogen capacity.
