Large Cryogenic Storage of Hydrogen in Carbon Nanotubes at Low Pressures
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Large Cryogenic Storage of Hydrogen in Carbon Nanotubes at Low Pressures B. K. Pradhan1, A. Harutyunyan1, D. Stojkovic1, P. Zhang1, M. W.Cole1, V. Crespi1, H. Goto3, J. Fujiwara3 and P. C. Eklund1 1 Department of Physics, The Pennsylvania State University, University Park, PA16802, USA 2 Honda R&D Co., Ltd. Wako Research Center, Saitama 351-0193, Japan ABSTRACT We report (6 wt %) storage of H 2 at T=77 K in processed bundles of single-walled carbon nanotubes at P=2 atmospheres. The hydrogen storage isotherms are completely reversible. D 2 isotherms confirm this anomalous low-pressure adsorption and further reveal the effects of quantum mechanical zero point motion. We propose that our post-synthesis treatment of the sample not only improves access for hydrogen to the central pores within individual nanotubes, but also may create a roughened tube surface with an enhanced binding energy for hydrogen. Such an enhancement is needed to understand the strong adsorption at low pressure. We obtain an experimental isosteric heat qst=125 ± 5 meV for processed SWNT materials. INTRODUCTION The ever-growing demand for energy, mediated by the 1990 Clean Air Act and 1992 Energy Policy Act in the United States has pointed out the need for the developments of cleaner fuels and more efficient engines [1]. Because of a limited supply and adverse environmental problems, fossil fuels must be replaced with pollution-free fuels derived from renewable resources. For these reason, Hydrogen is an ideal candidate providing more energy than fossil fuel on a weight basis. Hydrogen storage in a light weight carbon material would therefore be particularly attractive. The physical adsorption of gases within the micropores (diameter < 2 nm) of carbon materials has been actively studied for more than 50 years [2]. Many such carbon materials have been developed with very high experimental specific surface areas of As ~ 1000 to 3000 m2/g [3]. These materials tend to be disordered, with convoluted surfaces and predominately sp2 C-C bonding. Despite these impressively large surface areas, these materials have not shown promise for hydrogen storage [2,3]. Recently reports of very high, reversible adsorption of molecular hydrogen in nano-carbon materials, i.e., pure carbon nanotubes, alkali-doped graphites, and pure and alkali-doped graphite nanofibers (GNFs) have generated tremendous interest in the research community, stimulating much experimental works and many theoretical studies worldwide[4-9]. In this study, we show that the wt% hydrogen adsorption depends on the post-synthesis treatment of the materials. SWNTs were carefully characterized with HRTEM, Raman scattering, N2 adsorption isotherms. EXPERIMENTAL The SWNTs materials (arc-discharge derived: Ni-Y catalyzed) were obtained from Carbolex. This raw material was processed to remove amorphous and multi-shell carbon and residual catalyst. To accomplish this, the materials were first subjected to a high temperature oxidation under flow of dry air, then refluxed with mild mineral acid (HCl) in order
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