Future Perspectives

Research reports aimed at optimizing the carbon nanotubes for hydrogen storage appear frequently in the literature. The current reliable values for hydrogen uptake in carbon nanostructures [3, 32] need to be greatly increased to meet the DOE target. The feasibility of a viable carbon-nanostructure-based storage medium for hydrogen remains undetermined from the results reviewed here, with the current state of the art not providing a solution. This field of study is complicated by the publication of inaccurate storage capacities. It has been shown that it is easy to make large errors in determining hydrogen storage capacities. In this quickly moving field, it becomes critical for every experimental result to be confirmed and corroborated by other groups, to avoid the continued dissemination of unreliable results. In this regard, we would like to advocate that systematic studies of the complementary measurement methods should be carried out, and standard procedures for measuring the H2 uptake within the limitations of each method should be established. International conferences and workshops on gas adsorption measurement in carbon nanotubes should be called to discuss this challenging topic.

The computational chemistry studies provide a useful model of hydrogen physisorption on carbon nanotubes. At room temperature, a single dense monolayer of molecular hydrogen forms on the inside and outside of the tube. Due to the geometry of the system, this implies that a limiting arrangement for maximum storage exists. A second mono-layer, less dense than the first, can form at low temperature and when the nanotubes are charged. This will further increase the storage capacity, but will also have a limiting geometric arrangement. This second charge-induced mono-layer may be one explanation for the enhanced storage in nanotubes with intercalated metals. Alternately, a mechanism that involves the hydrogen molecule dissociating on a metal catalyst followed by atomic hydrogen moving across the tube surface before adsorption is proposed. However, it remains to be shown experimentally or theoretically that these metals can directly promote hydrogen chemisorption and, more importantly for hydrogen storage, whether such a process is reversible.

The temperature and pressure requirements for molecular hydrogen adsorption and desorption, and the kinetics for charging and discharging are also expected to be a function of nanotube diameter and aspect ratio, and still need to be addressed. More work needs to be done to refine these theoretical models, and to develop a model compatible with the requirements of the DOE Hydrogen Plan. Highly accurate experimental results could provide more realistic modeling parameters for the theoretical simulations of hydrogen storage.

It may be a combination of experimental and theoretical efforts that will lead to a full understanding of the adsorption process, so that the uptake can be rationally optimized to commercially attractive levels. It is scientifically interesting and challenging to continue research on the sophisticated experimental techniques and interactions of hydrogen with different and well-characterized carbon nanostructures. On the basis of a thorough understanding of the hydrogen adsorption in carbon nanotubes, fine adjustments and optimization of the production conditions can be conducted to obtain carbon nanotubes with specific structures which facilitate the hydrogen storage in carbon nanotubes. After optimization is complete, it will be necessary to scale up the synthetic and purification techniques that generate the ideal adsorbent nanotubes. These issues represent significant technological and theoretical challenges in the years to come.

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