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of SWNTs was measured by Nutzenadel et al. [30] using carbon nanotube samples from worldwide research groups or companies. In each case, 20 mg of SWNTs were finely ground, mixed with 80 mg of Au powder (which serves as a compacting additive, and does not participate in the electrochemical reaction), and pressed at a pressure of 500 MPa to form a pellet. The SWNT/Au pellet served as the negative electrode, and an Ni plate was used to form the coun-terelectrode. Experiments were conducted in a half cell, using a solution of 6 M KOH as the electrolyte; potentials were referred to an Hg/HgO/OH- reference electrode. The hydrogen weight density was deduced from the amount of discharged current. The obtained hydrogen storage capacity was -0.40 wt%. Nevertheless, the effect of chemical posttreatment and/or metallic impurities on the absorptive capacity of the SWNT samples was not quantified in this work. MWNT samples exhibited a similar hydrogen storage capacity of 0.37 wt%. The same group presented further investigations on a variety of carbon nanotubes [31, 70]. The storage densities range from 0.04 up to 2 wt%, and correlate with the specific surface area of the sample [71]. Nijkamp et al. [72] also observed a similar correlation in low-temperature volumetric measurements on high-surface-area carbon materials.

Lee et al. [29] conducted electrochemical chargedischarge cycling experiments recently. CNT-based composite electrodes were fabricated by mixing and grinding first with conductive Ni powders (99.8%) for 50 min, and later with the organic binder of polytetrafluoroethylene (PTFE) for 20 min with a mixing composition ratio of CNT:Ni:PTFE = 40:50:10. The pellet was made by pressing the mixture into a mold of 10 mm diameter at 200 MPa. This pellet was inserted into Ni metal mesh, and used for a working electrode. The counterelectrode Ni was separated by a polymer separator. The charge-discharge cycles were carried out in a solution of 6 M KOH solution. An Ag/AgCl standard electrode was used as a reference electrode in this measurement. A charge/discharge capacity of 160 mA • h/g was demonstrated, which corresponds to a gravimetric capacity of -0.57 wt%.

Qin et al. [73] investigated the electrochemical properties of an MWNT-Ni electrode. The MWNT-Ni electrode was found to have a high discharge capacity of 200 mA • h/g (-0.7 wt%) and a long charge/discharge cycle life. Recently, electrodes made of purified and open SWNTs behaved like metal hydride electrodes in Ni-MH batteries, showing high electrochemical reversible charging capacity up to 800 mA • h/g corresponding to a hydrogen storage capacity of 2.9 wt% [74]. Most recently, Zuttel et al. [75] studied the electrochemical hydrogen storage capacity of a series of CNTs, and found the highest value of capacity up to 0.9 wt%.

Kibria et al. [76] investigated the electrochemical hydrogen storage behaviors of undoped and alkaline-metal-doped CNTs produced by various preparation techniques. Laser-ablation-grown CNTs produce the highest hydrogen storage capacity of 1.6 wt%. The alkali metal-doped CNTs showed higher hydrogen storage capacities than undoped CNTs. Although the hydrogen storage capacities of Li-doped CVD and AD (arc discharge)-grown CNTs were six times higher than that of undoped CNTs, it is only 0.6 wt%.

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