Optimization of Single-Wall Nanotube Synthesis For Hydrogen Storage

Carbon single-wall nanotubes (SWNTs) are capable of adsorbing hydrogen quickly, to high density, at ambient temperatures and pressures. Last year, we showed that hydrogen storage densities on SWNTs made by laser vaporization ranged from 3.5 to 4.5 wt% after a cutting procedure was performed. We present details of the cutting procedure here and show that, when optimized, hydrogen storage densities up to 7 wt% can be achieved. Infrared absorption spectroscopy measurements on pristine and H2-charged samples indicate that no C-H bonds are formed in the process. These experiments are in agreement with an earlier temperature programmed desorption analysis that showed that hydrogen molecules are not dissociated when bound to the SWNT surfaces. All in all, we find that the interaction between H2 and single-wall nanotubes is mid-way between conventional van der Waals adsorption and chemical bond formation. A detailed understanding of the mechanism coupled with a high degree of control during synthesis should allow useful hydrogen adsorbents to be designed and constructed. Currently Available Hydrogen Storage Technologies Hydrogen can be made available on-board vehicles in containers of compressed or liquefied H2, in metal hydrides, or by gas-on-solid adsorption. Hydrogen can also be generated on-board by reaction or decomposition of a hydrogen-containing molecular species [1]. Although each method possesses desirable characteristics, no approach satisfies all of the efficiency, size, weight, cost and safety requirements for transportation or utility use. The U.S. Department of Energy energy density goals for vehicular hydrogen storage call for systems with 6.5 wt % H2 and 62 kg H2/m . Gas-on-solid adsorption is an inherently safe and potentially high energy density hydrogen storage method that should be more energy efficient than either chemical or metal hydrides, and compressed gas storage. Consequently, the hydrogen storage properties of high surface area "activated" carbons have been extensively studied [2-4]. However, activated carbons are ineffective in hydrogen storage systems because only a small fraction of the pores in the typically wide pore-size distribution are small enough to interact strongly with gas phase hydrogen molecules. The first measurements of hydrogen adsorption on carbon single-wall nanotubes (SWNTs) were performed with highly impure samples. The room-temperature stabilization that was demonstrated at atmospheric pressures suggested the possibility of 5-10 wt % hydrogen storage in SWNT-based systems [5]. Contradictory results from purified SWNTs indicated that such high storage densities could only be achieved with cryogenic temperatures (80 K) and high pressures (158 atm) [6], consistent with theoretical consideration of van der Waals interactions between H2 and SWNTs [7-9]. However, we showed last year that SWNTs can adsorb between 3.5 and 4.5 wt% at room temperature and room pressure when un-optimized preparation procedures were employed [10], and large-diameter SWNTs were recently shown to adsorb 4.2 wt % hydrogen at room temperature and ~100 atm. [11]. This year we show that hydrogen storage densities can be optimized to values as high as 7 wt%, and also present results from experiments designed to elucidate the mechanisms responsible for the unique hydrogen adsorption properties.