Molecular Dynamics Simulation of Hydrogen Storage in Single-walled Carbon Nanotubes

The hydrogen storage mechanism of SWNTs was studied through molecular dynamics simulations. Assuming the simple physical adsorption of hydrogen to the surface of SWNTs, potential forms between H2-H2 and C-H2 were both expressed 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 wt % for (8,8), (10,10), and (12,12) bundles, respectively. INTRODUCTION Dillon et al. [1] suggested the possibility of achieving very high hydrogen storage capacity per unit mass by using singlewalled carbon nanotubes (SWNTs), as hydrogen supply source of the fuel cell for automobiles. SWNTs, originally discovered by Iijima [2], can now be efficiently produced by such techniques as laser-vaporization in high temperature oven [3] and the arc-discharge method [4]. By experiments using relatively high-purity SWNTs, hydrogen storage capacity of about 8 % at 80 K and 120 atm was reported by Ye et al. [5], and about 4.2 % at room temperature and 100 atm was reported by Liu et al. [6]. One the other hand, the hydrogen adsorption characteristics were examined by molecular simulations using the Monte Carlo method [Wang and Johnson [7] and Darkrim and Levesque [8]]. However, there are still many unknown questions on the mechanism of such efficient hydrogen storage. It is still argued whether the adsorption to the interior of SWNT or outside is more important, and whether it is simply physical adsorption or some chemical reaction is being involved. In this study, the physical adsorption of hydrogen molecules to a small bundle of SWNTs was simulated by the molecular dynamics method. A special attention was paid to the intrusion of hydrogen molecules in-between the SWNTs by broadening the spacing of each SWNT. The relaxation of the energy parameter of van der Waals potential between SWNTs was necessary for the simulation of the hydrogen intrusion within the reasonable simulation period. Through a separate simpler simulation, the amount of hydrogen absorption interior of each SWNT was calculated. Finally, the predicted capacity of total hydrogen storage was compared with experiments. NOMENCLATURE A: Unit crystal vector d0: Diameter of a SWNT Ec: Cohesive energy per carbon atom m: An integer n: An integer N: Number of molecules p: Pressure R: Distance between two SWNTs r: Distance between two molecules T: Temperature