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

Abstract

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 H 2 loading and nanotube type, what is the H 2 distribution in the nanotube bundle? 2. For a given nanotube type, what is the maximal loading (H 2 coverage)? 3. What is the diameter range and chirality for which H 2 adsorption is most energetically favorable? Our simulation results suggest strong dependence of H 2 adsorption energies on the nanotube diameter but less dependence on the chirality. Substantial lattice expansion upon H 2 adsorption was found. The average adsorption energy increases with the lowering of nanotube diameter (higher curvature) and decreases with higher H 2 loading. The calculated H 2 vibrational power spectra and radial distribution functions indicate a strong attractive interaction between H 2 and nanotube walls. The calculated diffusion coefficients are much higher than what has been reported for H 2 in microporous materials such as zeolites, indicating that diffusivity does not present a problem for hydrogen storage in carbon nanotubes.

Pore size distribution and supercritical hydrogen adsorption in activated carbon fibers

“…above T c = 33 K) the H 2 -pore interactions are more dominant than the H 2 -H 2 interactions and condensation within pores does not occur. Characteristic H 2 adsorption enthalpies associated with large slit pores therefore approach the enthalpy of H 2 adsorption on carbon, typically measured between 4 and 6 kJ mol −1 [3,4]. With smaller pores, the adsorption enthalpy increases due to the overlap of potentials fields from each face of the pore.…”

“…The total volume of the Sieverts instrument, including both the reference volume and the reactor volume, is less than 68 cm 3 , permitting accurate measurements using the volumetric method. The true sample volume was subtracted by assuming a skeletal density of 2.1 g ml −1 , which is known to be accurate for carbon samples.…”

“…The packing density begins to decrease, though, when the pore is no longer wide enough to accommodate two H 2 monolayers. Ab initio calculations of hydrogen adsorption in ideal slit pore carbons are largely 3 Present address: UCCS, Equipe Chimie du Solide, CNRS UMR 8181, ENSC Lille-UST Lille, BP 90108, 59652 Villeneuve d'Ascq cedex, France. consistent with this picture [2,5]. Experimental work on the fundamental limitations of hydrogen delivery from carbon sorbents is rare, however, because uniform slit pore structures do not occur in nature.…”

Review on carbon nanotubes and carbon nanotube bundles for gas/ion separation and water purification studied by molecular dynamics simulation

“…The earlier reports (Cheng et al 2005;Zolfaghari et al 2007) revealed that CNTs curvature may directly impact the adsorption. Adsorption isotherms, selfdiffusion coefficient, heat of adsorption, and structural properties of neon gas were calculated and evaluated in more detail.…”

Description of Adsorbed Phases on Carbon Surfaces: A Comparative Study of Several Graphene Models

“…The level of curvature of valence orbital depends on the material radius: great radius lead to hybridization near sp 2 pure, while small radius leads to sp 3 (Niyogi, 2002). To describe phenomena that occur at molecular level one employs simulations, Monte Carlo grand canonical to study adsorption at equilibrium, or Molecular Dynamics (Cheng et al, 2005) when one is interested transport properties far from the equilibrium. However, the accuracy of these descriptions depend on the potential models adopted, choosing generally the 12-6 Lennard Jones potential, or the Crowell -Brown potential that don't take into account the surface curvature (Wang & Johnson, 1999).…”