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Posted: June 18, 2007
Carbon nanohorns for the storage of hydrogen
(Nanowerk News) Hydrogen would be the ideal candidate to replace fossil fuels if only it wasn't so difficult to store it safely. Researchers at the French Centre National de la Recherche Scientifique (CNRS) have discovered a storage solution which is both efficient and cheap: carbon nanohorns. With such structures, the hydrogen-carbon bond is far more stable than with nanotubes. This study removes the obstacles which prevented any possibility of carbon-based nanomaterials finding industrial applications.
Hydrogen, which is the most abundant element in the Universe, is a renewable energy source which could replace fossil fuels. It is non-polluting: the only by-product during its extraction is water. However, the fact that it is difficult to store both safely and cheaply has until now meant that its use has remained marginal.
These carbon nanohorns, which assemble to form dahlia-shaped structures 80 to 100 nanometers in diameter, are promising candidates for the storage of hydrogen. They are not only cheap and light, but the hydrogen-carbon complex possesses greater stability than in other carbon-based materials, in particular nanotubes. (Image: CNRS)
Among the storage methods currently in existence, storing it by adsorption in metals appears too expensive. However, trapping hydrogen in porous materials is not only efficient (all the hydrogen adsorbed can be recovered) but cheap. What's more, the cycle of hydrogen storage and release does not require any reactivation or regeneration of the material. Because of their low mass and high adsorption capacity, carbon-based nanostructures (nanotubes or nanohorns)turn out to be excellent candidates as porous materials. However, carbon nanotubes have a major drawback: using them for storage is only possible at extremely low temperatures (below -196oC), due to the weak interaction between hydrogen and carbon, and this restricts commercial applications. So the future possibility of storing hydrogen inside carbon-based porous materials, as part of a clean energy scheme, closely depends on the force of the interaction between hydrogen and carbon, and on how easily this force can be increased.
Researchers at the Centre de recherche sur la matière divisée (CNRS/University of Orléans), working in collaboration with their colleagues at the Rutherford Appleton Laboratory (UK), the University of the Basque Country in Bilbao and the Consejo Superior de Investigaciones Científicas (Spain), have been studying the bonds between hydrogen and carbon nanohorns (" Nature of the Bound States of Molecular Hydrogen in Carbon Nanohorns").
Nanohorns are materials which are two to three nanometers long. They are cone-shaped, and aggregate to form dahlia-shaped structures 80 to 100 nanometers in diameter which contain no metallic impurities. Since the tips of the cones are pointed, the researchers suspected that there might be a stronger hydrogen-substrate interaction. By using high-resolution neutron spectroscopy, they obtained information about the interaction (mobility of the hydrogen, and the energies and geometry that characterize the hydrogen-nanohorn complex). Their results show that the interaction between hydrogen and nanohorns is far stronger than between carbon nanotubes and hydrogen. The findings suggest that carbon nanohorns are promising materials for the storage of hydrogen. The only snag is that the cost of manufacturing them is currently relatively high.