Nanoscale tubular vessels for storage of methane at ambient temperatures

Novel carbon nanostructures can serve as effective storage media for methane, a source of clean energy for the future. We have used Grand Canonical Monte Carlo Simulation for the modeling of methane storage at 293 K and pressures up to 80 MPa in idealized bundles of (10,10) armchair-type single-walled carbon nanotubes and wormlike carbon pores. We have found that these carbon nanomaterials can be treated as the world's smallest high-capacity methane storage vessels. Our simulation results indicate that such novel carbon nanostructures can reach a high volumetric energy storage, exceeding the US FreedomCAR Partnership target of 2010 (5.4 MJ dm(-3)), at low to moderate pressures ranging from 1 to 7 MPa at 293 K. On the contrary, in the absence of these nanomaterials, methane needs to be compressed to approximately 13 MPa at 293 K to achieve the same target. The light carbon membranes composed of bundles of single-walled carbon nanotubes or wormlike pores efficiently physisorb methane at low to moderate pressures at 293 K, which we believe should be particularly important for automobiles and stationary devices. However, above 15-20 MPa at 293 K, all investigated samples of novel carbon nanomaterials are not as effective when compared with compression alone since the stored volumetric energy and power saturate at values below those of the bulk, compressed fluid.

Enhancement of hydrogen physisorption on single-walled carbon nanotubes resulting from defects created by carbon bombardment

The defect effect on hydrogen adsorption on single-walled carbon nanotubes (SWNTs) has been studied by using extensive molecular dynamics simulations and density functional theory (DFT) calculations. It indicates that the defects created on the exterior wall of the SWNTs by bombarding the tube wall with carbon atoms and C-2 dimers at a collision energy of 20 eV can enhance the hydrogen adsorption potential of the SWNTs substantially. The average adsorption energy for a H-2 molecule adsorbed on the exterior wall of a defected (10,10) SWNT is similar to 150 meV, while that for a H-2 molecule adsorbed on the exterior wall of a perfect (10,10) SWNT is similar to 104 meV. The H-2 sticking coefficient is very sensitive to temperature, and has a maximum value around 70 to 90 K. The electron density contours, the local density of states, and the electron transfers obtained from the DFT calculations clearly indicate that the H-2 molecules are all physisorbed on the SWNTs. At temperatures above 200 K, most of the H-2 molecules adsorbed on the perfect SWNT are soon desorbed, but the H-2 molecules can still remain on the defected SWNTs at 300 K. The detailed processes of H-2 molecules adsorbing on and desorbing from the (10,10) SWNTs are demonstrated.

Effects of carbon nanotubes and metal catalysts on hydrogen storage in magnesium nanocomposites

This paper reports a study on nanostructured magnesium composites with carbon nanotubes (CNTs) and catalytic transition metals with high H-2 adsorption capacity and fast adsorption kinetics at reduced hydrogenation temperatures. Nanostructures in such a composite are shown to be responsible for improvements in both adsorption capacity and kinetics. It is found that the carbon nanotubes significantly increase the hydrogen storage capacity, and the catalytic transition metals (Fe and Ti) greatly improve the kinetics. This could be understood from the enhancement of diffusion by CNTs and decrease in energy barrier of hydrogen dissociation at the magnesium surface.