Large cryogenic storage of hydrogen in carbon nanotubes at low pressures
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J.C. Grossman Lawrence Livermore National Laboratory, 7000 East Avenue, Livermore, California 94550
P. Zhang, M.W. Cole, and V. Crespi Department of Physics, The Pennsylvania State University, University Park, Pennsylvania 16802
H. Goto and J. Fujiwara Honda R&D Co., Ltd., Wako Research Center, Saitama 351-0193, Japan
P.C. Eklundb) Department of Physics, The Pennsylvania State University, University Park, Pennsylvania 16802 (Received 4 January 2002; accepted 15 May 2002)
We report up to 6 wt% storage of H2 at 2 atm and T ⳱ 77 K in processed bundles of single-walled carbon nanotubes. The hydrogen storage isotherms are completely reversible; D2 isotherms confirmed this anomalous low-pressure adsorption and also revealed the effects of quantum mechanical zero point motion. We propose that our postsynthesis treatment of the sample improves access for hydrogen to the central pores within individual nanotubes and may also create a roughened tube surface with an increased binding energy for hydrogen. Such an enhancement may be needed to understand the strong adsorption at low pressure. We obtained an experimental isosteric heat qst ⳱ 125 ± 5 meV. Calculations are also presented that indicate disorder in the tube wall enhances the binding energy of H2.
I. INTRODUCTION
For at least a decade, fuel-cell-powered ground transportation has been recognized as a much-needed technology to ameliorate environmental problems associated with hydrocarbon combustion.1 Currently, the technology for powering vehicles (i.e., fuel cells) is significantly more advanced than that for storing the fuel (hydrogen). One figure of merit for hydrogen storage is the weight percent (wt%) hydrogen stored relative to the weight of the storage medium. The United States Department of Energy has recently estimated that approximately 6–7 wt% storage is a benchmark for a viable ground transportation technology.2 Hydrogen storage in a lightweight carbon material would therefore be particularly attractive. The physical adsorption of gases within the micropores (diameter
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