Molecular Dynamics Simulations on the Effects of Diameter and Chirality on Hydrogen Adsorption in Single Walled Carbon Nanotubes

We present systematic molecular dynamics simulation studies of hydrogen storage in single walled carbon nanotubes of various diameters and chiralities using a recently developed curvature-dependent force field. Our main objective is to address the following fundamental issues:  1. For a given H₂ loading and nanotube type, what is the H₂ distribution in the nanotube bundle? 2. For a given nanotube type, what is the maximal loading (H₂ coverage)? 3. What is the diameter range and chirality for which H₂ adsorption is most energetically favorable? Our simulation results suggest strong dependence of H₂ adsorption energies on the nanotube diameter but less dependence on the chirality. Substantial lattice expansion upon H₂ adsorption was found. The average adsorption energy increases with the lowering of nanotube diameter (higher curvature) and decreases with higher H₂ loading. The calculated H₂ vibrational power spectra and radial distribution functions indicate a strong attractive interaction between H₂ and nanotube walls. The calculated diffusion coefficients are much higher than what has been reported for H₂ in microporous materials such as zeolites, indicating that diffusivity does not present a problem for hydrogen storage in carbon nanotubes.