An Ab Initio Study of Molecular Hydrogen Interaction with SiC Nanotube-A Precursor to Hydrogen Storage

First principles calculations using both density functional theory (DFT) and hybrid density functional theory (HDFT) and the finite cluster approximation have been performed to study the adsorption of molecular hydrogen on three types of armchair (9, 9) silicon carbide nanotubes. The distances of molecular hydrogen from the outer wall of the nanotubes have been optimized using the B3LYP and PW91 functionals. For the PW91 functional, the carbon top site is the most preferred site for type 1 nanotube with adsorption energy of 1.134 kcal/mol, whereas the second hollow site is the most preferred site for type 2 with adsorption energy of 1.423 kcal/mol and the C-C bridge site for type 3 with adsorption energy of 1.265 kcal/mol. The corresponding optimized distances of the hydrogen molecule are 3.1 angstrom, 2.7 angstrom and 2.9 angstrom. For the B3LYP functional, the C-Si normal bridge site has been found to be the most preferred adsorption site in case of type 1 with adsorption energy of 0.277 kcal/mol while the C-C bridge site is the most preferred site for type 2 and type 3 nanotubes with adsorption energies of 0.303 kcal/mol. The corresponding optimized distances of the hydrogen molecule are 3.3 angstrom, 3.1 angstrom, and 3.1 angstrom. Thus, the adsorption energies using the PW91 functional are found to be consistently higher than those using the B3LYP functional; however, the adsorption distances using the B3LYP functional are normally higher than the corresponding distances using the PW91 functional. This clearly indicates the different nature of the exchange-correlation functionals inherent in PW91 and B3LYP, given that PW91 is a pure DFT functional and B3LYP is a HDFT functional. Current studies indicate that silicon carbide nanotubes can possibly be used as a proper media for hydrogen storage at ambient conditions.