First-principles study of hydrogen adsorption on carbon nanotube surfaces

Based on first-principles theoretical calculations, we find that on pristine single-wall carbon nanotubes (SWNT's), the dissociative adsorption of H₂ molecules is severely suppressed due to very high energy barriers of about 3 eV, while H atoms have low-energy barriers less than 0.3 eV. On Li-doped SWNT's, the energy barriers for the dissociative adsorption of H₂ are lowered by about 0.3--0.5 eV due to the charge transfer to the tube, as compared to the pristine tube. However, these energy barriers are still too high for H₂ to be directly adsorbed, indicating that Li doping itself does not enhance significantly the H adsorption. On the other hand, LiH molecules are easily adsorbed and dissociated with no energy barrier.