Molecular modeling of adsorptive energy storage: Hydrogen storage in single-walled carbon nanotubes

In this paper, density functional theory is used to estimate hydrogen adsor ption in a novel carbonaceous material, single-walled carbon nanotubes. An idealized adsorbent structure for the nanotubes is assumed. We have mapped out the regime of operating pressures and temperatures where an adsorption- based storage system is expected to deliver more hydrogen than a similar sy stem of compressed gas. This regime is also a function of pore size. We hav e calculated the overall hydrogen volumetric and gravimetric density within the framework of a typical high-pressure gas storage system. Within the re gime of operating conditions where adsorptive storage seems attractive, the storage properties of hydrogen in a carbon nanotube system appear to fall far short of the targets of 62 kg of H-2/m(3) and 6.5 wt % H-2 set by the D epartment of Energy. The computed gravimetric storage densities also fall s hort of those reported in the literature (Nature 1997, 386, 377). We discus s several possible mechanisms by which higher gravimetric density could be rationalized, including chemisorption, adsorption at interstitial sites, an d swelling of the nanotube array.