Abstract:
Recent progress in the production, purification, and experimental and theoretical investigations of
carbon nanotubes for hydrogen storage are reviewed. From the industrial point of view, the chemical
vapor deposition process has shown advantages over laser ablation and electric-arc-discharge
methods. The ultimate goal in nanotube synthesis should be to gain control over geometrical
aspects of nanotubes, such as location and orientation, and the atomic structure of nanotubes,
including helicity and diameter. There is currently no effective and simple purification procedure
that fulfills all requirements for processing carbon nanotubes. Purification is still the bottleneck for
technical applications, especially where large amounts of material are required. Although the alkalimetal-
doped carbon nanotubes showed high H2 weight uptake, further investigations indicated that
some of this uptake was due to water rather than hydrogen. This discovery indicates a potential
source of error in evaluation of the storage capacity of doped carbon nanotubes. Nevertheless, currently
available single-wall nanotubes yield a hydrogen uptake value near 4 wt% under moderate
pressure and room temperature. A further 50% increase is needed to meet U.S. Department of
Energy targets for commercial exploitation. Meeting this target will require combining experimental
and theoretical efforts to achieve a full understanding of the adsorption process, so that the uptake
can be rationally optimized to commercially attractive levels. Large-scale production and purification
of carbon nanotubes and remarkable improvement of H2 storage capacity in carbon nanotubes
represent significant technological and theoretical challenges in the years to come.
