Single-Walled Carbon Nanotubes:  Efficient Nanomaterials for Separation and On-Board Vehicle Storage of Hydrogen and Methane Mixture at Room Temperature?

Abstract

We investigate possible usage of single-walled carbon nanotubes (SWNTs) as an efficient storage and separation device of hydrogen−methane mixtures at room temperature. The study has been done using Grand Canonical Monte Carlo simulations for modeling storage and separation of hydrogen−methane mixtures in idealized SWNTs bundles. These mixtures have been studied at several pressures, up to 12 MPa. We have found that the values of the stored volumetric energy and equilibrium selectivity greatly depend on the chiral vector (i.e., pore diameter) of the nanotubes. The bundle formed by [5,4] SWNTs (nanotube diameter of 6.2 Å) can be regarded as a threshold value. Below that value the densification of hydrogen or methane is negligible. Bundles with wider nanotube diameter (i.e., 12.2, 13.6, 24.4 Å) seem to be promising nanomaterials for hydrogen−methane storage and separation at 293 K. SWNTs with pore diameters greater than 24.4 Å (i.e., [18,18]) are less efficient for both on-board vehicle energy storage and separation of hydrogen−methane mixture at 293 K with pressures up to 12 MPa. SWNTs comprised of cylindrical pores of 8.2 and 6.8 Å in diameter (equivalent chiral vector [6,6] and [5,5], respectively) are the most promising for separation of the hydrogen−methane mixture at room temperature, with the former selectively adsorbing methane and the latter selectively adsorbing hydrogen. We observed that inside the pores of [6,6] nanotubes absorbed methane forms a quasi-one-dimensional crystal when the system has thermalized. The average intermolecular distance of such a crystal is smaller than the one of liquid methane in bulk at 111.5 K, therefore exhibiting the quasi-one-dimensional system clear compression characteristics. On the other hand, for a smaller nanotube diameter of 6.8 Å the hydrogen can enter into the tubes and methane remaining in bulk. We found that in the interior of [5,5] nanotubes adsorbed/compressed hydrogen forms a quasi-one-dimensional crystal.

Hydrogen adsorption capacities of multi-walled boron nitride nanotubes and nanotube arrays: a grand canonical Monte Carlo study

“…However, the percentage by weight of adsorbed hydrogen increases more rapidly within SWBNNTs, and hydrogen storage decreases with the number of nanotube walls; in other words, (15,15) and (22,22) SWBNNTs show enhanced hydrogen physisorption compared to that of (8,8)@ (11,11)@(15,15) and (7,7)@(15,15)@(22,22) TWBNNTs under the same thermodynamic conditions. The results also show that the TWBNNTs all have similar hydrogen storage values, while more variation in the hydrogen storage values is observed for the SWBNNTs.…”

“…We first considered two SWBNNTs [ (15,15) Fixing the tube length and varying the pressure, we plotted the percentage by weight of adsorbed hydrogen (H 2 /system wt%) against pressure; the results can be seen in Fig. 1a.…”

“…Such applications of nanotubes would enable a new system of delivering energy, called the "hydrogen economy" [1], as well as novel advanced interdisciplinary science associated with it. Therefore, studying and observing the abilities of MWBNNTs and MWBNNTAs to act as hydrogen adsorbents-as we have done in the work described in the present paper-can help us to determine whether they are good candidates for practical gas/hydrogen storage materials [12][13][14][15][16].…”

“…In each case, the contents of the first set of parentheses relate to the innermost nanotube; the contents of the second set of parentheses relate to the middle nanotube, and (only in the case of TWBNNTs) the contents of the last set of parentheses relate to the outermost nanotube. For example, an (11,11)@ (15,15) DWBNNT is a double-walled nanotube with an inner (11,11) nanotube and an outer (15,15) nanotube, while an (8,8)@(11,11)@ (15,15) TWBNNT is a triplewalled nanotube with an innermost (8,8) nanotube, a middle (11,11) nanotube, and an outermost (15,15) nanotube. Furthermore, it should be noted that the major diameter of a DW/TW BNNT is equal to the diameter of its outermost nanotube, and only the diameter of the inner nanotube was varied for the DW/TW BNNTs examined in the simulations performed in this work.…”

Advances in the application of nanotechnology in enabling a ‘hydrogen economy’

“…The hydrogen capacity increased with increasing Pd content. Kowalczyk et al [193] investigated the suitability of different pore size nanotubes for storage and separation of hydrogen-methane mixtures. The pore size depends on chirality.…”