The hydrogen storage mechanism of SWNTs was studied through molecular dynamics simulations. Assuming the simple physicla adsorption of hydrogen to the surface of SWNTs, potential forms between H_2-H_2 and C-H_2 were both expressed by Lennard-Jones functions. Fixing the relative coordinates of by Lennard-Jones functions. Fixing the relative coordinates of carbon atoms to the center of mass of each SWNT, the center of mass motion was modeled with the van der Waals force between SWNTs. Hydrogen absorption in small bundles of (8.8), (10,10) and (12,12) SWNTs were tested. In order to realize the adsorption between SWNTs within reasonable simulation period, the van der Waals energy between tubes was once decreased and recovered. While the amount of hydrogen adsorption per unit carbon mass inside SWNTs increased with increasing diameter, adsorption between tubes was almost constant. Total amount of hydrogen adsorption for 77 K and 15 MPa system was predicted as 6.9, 7.7, and 8.1, (10,10), and (12,12) bundles, respectively.
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