Molecular Modeling of Adsorptive Energy Storage:  Hydrogen Storage in Single-Walled Carbon Nanotubes

In this paper, density functional theory is used to estimate hydrogen adsorption 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 system of compressed gas. This regime is also a function of pore size. We have calculated the overall hydrogen volumetric and gravimetric density within the framework of a typical high-pressure gas storage system. Within the regime 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 Department of Energy. The computed gravimetric storage densities also fall short of those reported in the literature ( Nature 1997, 386, 377). We discuss several possible mechanisms by which higher gravimetric density could be rationalized, including chemisorption, adsorption at interstitial sites, and swelling of the nanotube array.