Adsorption of Hydrogen on Carbon Single-Walled Nanotubes as Measured by the Volumetric Technique
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Adsorption Of Hydrogen On Carbon Single-Walled Nanotubes As Measured By The Volumetric Technique Philip A. Parilla, Anne C. Dillon, Thomas Gennett1, Jeff L. Alleman, Kim M. Jones, and Michael J. Heben Basic Sciences Center, National Renewable Energy Laboratory Golden, CO, 80401, U.S.A. 1 Department of Chemistry and Center for Materials Science, Rochester Institute of Technology, Rochester, NY, 14623, U.S.A. ABSTRACT We report on hydrogen adsorption onto carbon single-walled nanotube (SWNT) material as measured by the volumetric technique. This method determines the amount adsorbed by monitoring the pressure in a known volume at a known temperature with a given number of moles of hydrogen introduced. The raw material for the samples is produced by pulsed laser vaporization of a graphite target containing Ni (0.6 at%) and Co (0.6 at%) dopants. This is followed with a purification procedure and then by a cutting procedure. Finally, the sample is degassed before the hydrogen adsorption is measured. The cutting procedure introduces a titanium alloy which participates in the overall hydrogen adsorption of the SWNT material. Importantly though, the amount of hydrogen adsorbed in the SWNT material cannot be explained by absorption from the alloy alone. A description of the apparatus that has allowed us to measure these samples is given and we discuss crucial experimental procedures needed to activate the SWNT samples for hydrogen adsorption. INTRODUCTION Of the many possible applications for carbon SWNTs, one of the most intriguing is for the storage of hydrogen. As originally reported, [1] hydrogen adsorption onto samples containing SWNTs have gravimetric storage density per SWNT estimated between 5 and 10 wt%. This would provide high energy storage efficiencies for applications requiring lightweight storage capabilities such as for vehicles. Since this original report, we have worked toward developing the growth and purification of SWNTs as well as understanding the nature of their hydrogen adsorption. This has resulted in highly pure samples capable of adsorbing up to ~7 wt% hydrogen as measured by temperature programmed desorption (TPD). Additionally, other groups have performed experiments to measure the hydrogen adsorption on a variety of carbon nanotube materials and have resulted in a wide range of results at various temperatures and pressures. [2-8] While the lack of agreement among the measurements is, at first glance, puzzling, it may not be so surprising given the many different types of nanotube materials used as samples in the experiments. These samples range from the as-produced raw soot from a variety of production techniques to processed samples from a variety of purification schemes and perhaps with additional treatments to "open" or cut the tubes. Besides containing some fraction of carbon nanotubes, the raw soots usually contain some degree of metal catalyst and non-nanotube carbon components that are either mixed with the tubes and/or are coating the tubes. The types and relative fractions of the compon
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