Physics Letters A 356 (2006) 152–155 www.elsevier.com/locate/pla Direct measurement of hydrogen adsorption in carbon nanotubes/nanofibers by elastic recoil detection F.U. Naab ∗ , M. Dhoubhadel, J.R. Gilbert, M.C. Gilbert, L.K. Savage, O.W. Holland, J.L. Duggan, F.D. McDaniel Ion Beam Modification and Analysis Laboratory, Department of Physics, PO Box 311427, University of North Texas, Denton, TX 76203-1427, USA Received 3 February 2006; received in revised form 12 March 2006; accepted 14 March 2006 Available online 20 March 2006 Communicated by R. Wu Abstract Physi- or chemi-sorption of hydrogen in solid materials offers a viable medium for hydrogen storage since the concentration of hydrogen can exceed that in its gaseous form at high compression. Due to their unique architecture, carbon nanotubes are potentially an excellent carbon- based adsorbent for hydrogen. In this work, we report direct measurements of hydrogen adsorption using elastic recoil detection analysis in single-walled, double-walled, and multi-walled nanotubes, as well as carbon nanofibers. Results are presented for hydrogen adsorption treatment at ambient temperature and above, where chemical rather than physical adsorption is anticipated. The results show that the concentration of hydrogen in all samples over the range of conditions investigated is below 1 wt.%, which is well below that required for a viable storage media.  2006 Elsevier B.V. All rights reserved. PACS: 61.18.Bn; 61.80.Jh; 81.07.De Keywords: Elastic recoil detection (ERD); Carbon nanotubes/nanofibers (CNTs/CNFs); Hydrogen storage 1. Introduction The use of hydrogen as a fuel source provides a viable, relatively clean alternative to fossil fuels. The United States Department of Energy (DOE) has a targeted requirement for a usable H 2 storage capacity of 6.5 wt.%, and operation near am- bient temperature and pressure [1–3]. Early publications, which showed a high storage capacity of hydrogen in single-walled carbon nanotubes [4], prompted great excitement in the field, but further research yielded conflicting results. Reported results of the storage capacity in carbon nanostructures range from less than 1 wt.% [3,5] to ∼ 70 wt.% [6] over a wide range of pres- sure (0.04 to 7 MPa) and temperature (80 to 700 K), which include room temperature (RT) and above [7]. * Corresponding author. E-mail address: fun001@unt.edu (F.U. Naab). It is believed that some of these conflicting results come from inappropriate methods or experimental conditions to measure the hydrogen stored in carbon nanotubes/nanofibers (CNTs/CNFs) [3,5]. In 2002, Hirscher et al. [5] quoted: “up to the present day, none of the promising experiments on hydrogen storage in CNFs and single-walled carbon nanotubes (SWNTs) could be repeated by an independent group in another labo- ratory”. Mainly, three different techniques [5] are applied to study the hydrogen storage in solids. The volumetric method [8,9], the gravimetric method [10,11], and thermal desorption spectroscopy [4,12]. Each of these techniques has there own problems [5,13,14]. Nevertheless, Züttel et al. [15] claimed to have found en- tirely self consistent measurements, where the amount of hy- drogen physisorbed is proportional to the surface area of the SWNTs and equal to 1.5 wt.%/1000 mg/cm 2 . Experimental results using neutron scattering [16,17] to measure the hydrogen adsorption in SWNTs showed that hy- drogen physisorption (∼ 0.5 wt.%) at cryogenic temperature 0375-9601/$ – see front matter  2006 Elsevier B.V. All rights reserved. doi:10.1016/j.physleta.2006.03.031