Interaction Between Hydrogen And Cnts

Current theoretical studies of simple physical adsorption on pure carbon nanotubes predict a maximum of 14 wt% under low temperature and ideal conditions. The real adsorption process is likely to be more complicated, and the theoretical simulations have the potential for further refinement. In particular, as more quantitative and microscopically in-depth experimental results become available, the model parameters and assumptions will become more realistic.

Only some of the experimental results agree well with the theoretical calculations. It is not difficult to understand this because experiments are performed on real-world materials, whereas calculations are based on idealized models. The carbon nanotubes made from various preparation methods under varied conditions will have different structural features. It is also probable that the carbon nanotubes are synthesized with defects or damaged, disordered, and functionalized during purification and cutting treatments. Molecular simulations which consider the effects of packing disorder, diameter polydispersity, functionalization, and nanotube wall effects are currently underway [143].

Little is known of the effects of nanotube stereochemistry on hydrogen uptake. SWNTs may, in the future, be prepared in bulk quantities with a variety of diameters and stereochemistries, depending on the nature of the rolling process of the graphitic sheet. Zigzag, armchair, and chiral nanotubes have been identified by Raman spectroscopy [144], but methods of forming them in pure high yield have not been discovered. The structure of the nanotube affects its properties, including conductance, density, and lattice structure, and possibly hydrogen adsorption. For example, an SWNT is metallic if the value (m — n) is divisible by 3. Consequently, when tubes are formed with random values of m and n, we would expect that two-thirds of nanotubes would be semiconducting, and the other third metallic. Raman studies at multiple wavelengths which enable the differentiation between semiconducting and metallic nanotubes may enable an answer which is better suited for hydrogen adsorption [145]. If a preference is determined, the aim of current preparative carbon nanotube research should be directed to producing specific metallic or semiconducting nanotubes. Recently, diameter tuning of SWNTs has been reported through variation in laser pulse power [146]. This advance is important in furthering progress toward the DOE goals since there is theoretical and experimental evidence that the diameter of SWNTs can affect the capacity, thermodynamics, and kinetics of hydrogen storage. In addition to controlling nanotube size distributions, Heben's group also learned how to detangle and order nanotubes on a large scale [147]. This capability may eventually be important for achieving high packing densities, and therefore high volumetric hydrogen storage densities.

There is considerable debate over the issue of how SWNTs interact with hydrogen in the scientific community [50]. Is it a purely physical or chemical interaction or is it perhaps somewhere in between? It is important to obtain a deeper understanding of this process so that accurate theoretical models and predictions may be developed. Factors such as tube diameter and chirality may need to be targeted for synthesis, and so that their interaction with storage capacity and performance characteristics can be understood and optimized. In a previous study, Dillon et al. [26] showed that hydrogen is not dissociated when adsorbed on arc-generated SWNTs, even though the binding energy is 19.62 kJ • mol-1. Recently, they used TPD and FTIR techniques to show that hydrogen is nondissociatively adsorbed on laser-generated nanotubes [50]. They found that the interaction between hydrogen and SWNTs was midway between conventional van der Waals adsorption and chemical bond formation. Both infrared and Raman investigations on hydrogen-charged high-capacity purified samples of SWNTs are presently underway in an effort to better understand the nature of the carbon-hydrogen interaction.

None of the experiments has so far addressed the local structure of the binding sites, further complicating comparisons with theoretical predictions. Binding energies of physisorbed hydrogen can be obtained by thermal analysis, but these do not provide atomic-scale geometric information about the binding sites. Furthermore, experimental binding energies are generally much larger than values calculated for the idealized structure. Inelastic scattering of thermal neutrons, a spectroscopic technique that is sensitive to the local environment in which the hydrogen molecules are trapped, is now being used to probe the nature of the hydrogen adsorption site in carbon nanotubes [148]. A proof-of-principle experiment was recently published, and Figure 10 shows the temperature dependence of the neutron energy-loss spectra from this work. In principle, one should be able to determine how much hydrogen is trapped in multiple sites of different symmetry. Furthermore, the temperature dependence of the energy-loss peaks associated with different sites is a direct measure of the binding energy, a quantity that can be directly compared with theory. The shift in peak position and the decrease in intensity with temperature signify physisorption on a tube surface, with a binding energy slightly larger than would be obtained on a flat graphite surface. This experiment demonstrates that rotational spectroscopy using thermal neutrons provides important microscopic information about hydrogen-binding sites in nanotubes, although the experimental conditions are of no practical interest (0.6 g as-grown SWNTs, 25 K, 11 MPa).

As shown in Table 1, there is a large range of published values for reversible hydrogen storage in carbon nanotubes. One reason for this range may be insufficient characterization of the carbon nanotubes as mixtures of opened and unopened, single-walled and multiwalled, various diameter and helicities have been tested together with other unknown carbonaceous species. Another reason may be insufficient rigor in making hydrogen adsorption measurements on samples on the order of several milligrams where minute parasitic effects can easily and erroneously be attributed to

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