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Figure 8. TGA profile of Li-doped carbon nanotubes in 0.1 MPa wet H2. (a) Increasing temperature from 20 to 500 °C at 5 °C/min. (b) Cooling at slower rates. The total weight of Li/C was 10 mg. 2.3% (vol.) water vapor was added in H2. Reprinted with permission from [65], R. T. Yang, Carbon 38, 623 (2000). © 2000, Elsevier Science.

TGA cycling features, which are strikingly similar to those reported by Chen [64] and Yang [65]. This result is obtained for Li-containing samples measured in Ar without H2 and in LiOH samples measured in either H2 or Ar. To elucidate the species responsible for weight cycling, additional gases (N2, O2, H2O, or air) are deliberately introduced into the TGA furnace (Fig. 9). Adding 0.5 sccm of dry air to the TGA gas stream has spectacular effects on the cycling features, and the magnitude grows as the O2 concentration increases with continued exposure. The amplification derives from the catalytic reaction between H2 and O2 under the specific conditions with the Pt pan as a catalyst. Therefore, these cycling features arise from atmospheric impurities in the TGA which finally react with Li or K to form LiOH or KOH. The authors conclude that the cycling species responsible for the weight changes is, in fact, H2O, in agreement with Yang [65]. The H2 storage results vary from 0.72 to 4.2 wt%, depending on the initial weight and scan rate under 46% H2 in Ar on Li-doped CNTs.

Zidan and Rao [67] believed that the introduction of nanoparticles (dopants) to the nanotubes can enhance hydrogen bonding at the edge and inside the tubes, allowing for hydrogen storage to occur. The success of making doped carbon nanotubes with transition metals and alloys can allow for a weak covalent bond similar to cases of a dihydrogen bond that is not restricted to a pure physisorp-tion or chemisorption bond [68, 69]. Controlling the type and size of tubes and dopant is expected to tune the product for hydrogen sorption to occur at a desired temperature and pressure. Preliminary results showed an indication of 0.51% by weight hydrogen uptake and release from samples. A thermogravimetric system capable of operating at a wide

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