Pressure Isotherms of Hydrogen Adsorption in Carbon Nanostructures
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Pressure Isotherms of Hydrogen Adsorption in Carbon Nanostructures Xiaohong Chen, Urszula Dettlaff-Weglikowska, Miroslav Haluska, Martin Hulman, Siegmar Roth, Max Planck Institute für Festkörperforschung, Heisenbergstraße 1, D-70569 Stuttgart, Germany Michael Hirscher and Marion Becher, Max Planck Institute für Metallforschung, Heisenbergstraße 1, D-70569 Stuttgart, Germany ABSTRACT The hydrogen adsorption capacity of various carbon nanostructures including single-wall carbon nanotubes, graphitic nanofibers, activated carbon, and graphite has been measured as a function of pressure and temperature. Our results show that at room temperature and a pressure of 80 bar the hydrogen storage capacity is less than 1 wt.% for all samples. Upon cooling, the capacity of hydrogen adsorption increases with decreasing temperature and the highest value was observed to be 2.9 wt. % at 50 bar and 77 K. The correlation between hydrogen storage capacity and specific surface area is discussed. INTRODUCTION Hydrogen storage in carbon nanostructures has recently attracted great interest because of the large storage capacities indicated by some investigations [1-7]. However, these promising results (the highest is 67.55 wt. % reported by Chamber, A. et. al [1]) have not been independently reproduced anywhere. Further studies have shown that these ‘exciting’ results are questionable. The high values Chen et. al [5] measured on Li- and K-doped carbon nanotubes have been attributed to water adsorption and desorption by Yang [8]. A storage capacity of 6 to 8 wt. % in high-purity single-wall nanotubes opened by sonicate, which has been reported by Heben et al [7], was taken into doubt by Hirscher et al [9]. They found that the majority of hydrogen storage can be ascribed to Ti-alloy particles which are incorporated in the carbon nanotubes during the sonicate treatment. More recently, Tibbetts et al. [10] showed large scepticism on all results claiming more than 1 wt. % storage in different carbon materials at room temperature. These controversial results encouraged us to investigate the hydrogen storage capacity in various nanostructured carbon materials at different pressures and temperatures. Furthermore, the hydrogen uptake was correlated to the specific surface area of the carbon nanostructures. EXPERIMENTAL Single walled carbon nanotubes (SWNTs) and graphitic nanofibers (GNFs) were chemically purified to remove residual catalysts. Therefore, sample was immersed in 3M HNO3, reflux for 16 hours and followed by a rinse with deionised water several times, and then dried at 110 °C for 6 hours. Subsequently, the samples were heated to 300 °C and degassed under vacuum (3 × 10-5 mbar) for 12 hours and then cooled under hydrogen flow to room temperature. All samples investigated were listed in Table 1 including the abbreviations used in the figures. Z9.11.1
Table 1. Nanostructured carbon samples investigated, production method, source and abbreviation. Nanostructure SWNTs
Production Method Laser ablation
SWNTs SWNTs GNFs Activated Carbon Graph
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