Optimization of single-walled carbon nanotube arrays for methane storage at room temperature

The adsorption storage of methane on triangular arrays of single-walled carbon nanotubes (SWNT) at room temperature was investigated by the grand canonical Monte Carlo (GCMC) method. In the simulation, carbon atoms on the tubular wall were structured according to the (m, m) armchair arrangement, and the site-to-site method was used to calculate the interaction between a methane molecule inside the tube and a carbon atom on the tubular wall. Excess volumetric and gravimetric endohedral adsorptions of methane in the (15,15), (20, 20), (25, 25), and (30, 30) SWNT arrays were obtained. For every armchair SWNT array above, the van der Waals (VDW) gap has been varied from 0.335 to 1.0 nm to optimize methane storage in the interstices of SWNT arrays. The usable capacity ratio (UCR), which is defined as the mass of available fuel in an adsorbent-loaded vessel divided by the mass of available fuel in a vessel without adsorbent, was used as the criterion to judge the adsorption performance of SWNT arrays with different parameters. Results indicate that the (15, 15) SWNT arrays with a VDW gap of Delta = 0.8 nm is the optimal adsorbent among all of the cases studied for methane storage at room temperature. At p = 4.1 M Pa, the total volumetric and gravimetric capacities (including endohedral and exohedral adsorption) of methane on the SWNT arrays with the optimal parameters reach 216 V/V and 215 g CH4/kg C, respectively. It not only greatly exceeds the target (150 V/V) of the Department of Energy (U.S.A.) but also is slightly greater than the capacity (200 V/V) of compressed natural gas at 20 M Pa. The attainment of the exciting result attributes to the optimization of the VDW gap parameter of SWNT arrays, because the adsorption of methane in the interstices surpasses 60% of the total amount at the optimal condition, while its value is less than 15% of the total amount at a VDW gap of Delta = 0.335 and p = 4.1 M Pa.