Hydrogen Storage by Carbon Nanotubes

Abstract

Safe, lightweight, and cost-effective materials are required to practically store hydrogen for use in portable fuel cell applications. Compressed hydrogen and on-board hydrocarbon reforming present certain advantages, but their limitations must ultimately render them insufficient. Storage in hydrides and adsorption systems show promise in modeling and experimentation, but a practical medium remains unavailable. Since the earliest report of adsorption on single-walled carbon nanotubes (SWNT) in 1997, a number of controversial publications have claimed the hydrogen capacity of these materials to be between 0.1 to 10 wt. %. However, no study has yet demonstrated a plateau of adsorption with pressure that would verify the reported capacity. A volumetric adsorption measurement instrument was designed and constructed to resolve this controversy. The instrument is capable of degassing samples under high vacuum and offers unprecedented measurements of hydrogen storage up to a pressure of 300 atm and a broad range of temperatures. In addition, an electrical probe within the sample cell was designed to study the mechanism of adsorption in situ. The best hydrogen storage observed on bundles of purified SWNT was 1.6 wt. % at 264 atm and 200 K. At room temperature, a high-pressure plateau was found corresponding to an adsorption of 0.9 wt. % at a pressure of 300 atm, which equates to an adsorption to surface area ratio of 1.14 wt. %/l 000 m2/g. Contrary to the claim by the Caltech Group [Ye et al., 1999], resistance measurements of purified SWNT bundles revealed that bundles do not separate under high pressure. Instead, the bundles were found to compress under the action of external pressure, leading to an increase in conductivity with pressure. A simple geometrical model suggests that without this bundle separation the volume displaced by the sample may counteract the benefit gained by adsorption because of the increase in gas density at high pressure. The isosteric heat of adsorption on SWNT bundles was measured to be between 3.9 and 5.0 kJ/mol at low levels of adsorption, and the activation energy for adsorption determined by the Langmuir model was found to be 1.9 kJ/mol. These low energy parameters are indicative of weak physisorption.