Carbon Nanotubes: Energetics of Hydrogen Chemisorption
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Among the potential storage materials for use in portable hydrogen-containing devices, carbon nanotubes have received a relatively large amount of attention. Their adsorptive properties and relatively light weight make carbon nanotubes ideal candidates for the development of efficient H2-powered vehicular fuel cells. Evidence for hydrogen adsorption levels large enough to be used in such devices was first reported by Dillon et al. in 1997. Since that time, experiments attempting to reproduce these results and to optimize the levels of hydrogen adsorption have been inconclusive as to the applicability of carbon nanotubes toward this technology. The difficulty in reproducing high adsorption levels has led to a number of computational studies concerning hydrogen interactions with carbon nanotubes. Many computational studies have attempted to model the process of hydrogen adsorption to nanotubes to quantify the relationship between environmental parameters and hydrogen uptake amounts. In an attempt to clarify the experimental results, adsorption models have been designed and simulations based on these models have been performed. Yet the results of many of these simulations have not corresponded with the observed experimental levels of hydrogen adsorption. This has led to questions concerning both the validity of the experimental results and the physical reality of the models. A central uncertainty is the nature of the molecular-level mechanism through which adsorption occurs. To resolve this critical uncertainty, researchers turned to atomistic investigations of interactions between hydrogen and nanotube surfaces. Molecular dynamics simulations, kinetic studies, and first principles calculations have all been performed recently in an attempt to elucidate more clearly the adsorption mechanism. Among the significant conclusions derived from the results of the atomistic studies are that the deformation of the carbon nanotubes should be considered during the adsorption process and that localized, chemical bonds must be formed between adsorbed hydrogen and the nanotube walls to explain the experimental findings. In addition, the atomistic studies make it clear that the energetics of potential adsorption mechanisms strongly depend on the nanotube geometry. The purpose of this article is to review the current understanding of the adsorption process and to establish the necessity for a further understanding of the adsorption mechanism. The dependence of hydrogen adsorption on the geometry of the nanotubes will be demonstrated. Because of the atomistic nature of the adsorption process, first principles calculations should play a vital role in resolving uncertainties regarding the applicability of carbon nanotubes to hydrogen storage.