Physisorption (physical adsorption in which the adsorbate adheres to the surface only through weak intermolecular interactions) and chemisorption (adsorption in which a molecule adheres to a surface through the formation of a chemical bond) are both possible mechanisms for hydrogen storage in carbon nanotubes.

Although most previous studies have focused on the hydrogen storage through physisorption, according to the SSRL research team, recent analysis has indicated the potential for up to 7.5 wt. % hydrogen storage in nanotubes through chemisorption by saturating the C-C double bonds in the nanotube walls and forming C-H (Carbon-Hydrogen) bonds.

The DOE FreedomCAR program roadmap for on-board hydrogen storage system specifies storage capacity of 6 wt. % of hydrogen by 2010.

SEM image of as grown SWCN film. White “ropes” are nanotubes and nanotube bundles. White “spots” are silica catalyst support particles. Source: SSRL

The researchers bombarded a film of carbon nanotubes with hydrogen. The incoming hydrogens break the C-C double bonds, allowing a hydrogen to attach to a carbon while the carbon atoms renew their grip on each other with single bonds.

Assessment with different x-ray spectroscopy techniques revealed that about 65% (±15 %) of the carbon atoms had bonded to hydrogen atoms. Assuming that each hydrogenated carbon atom bonds to one hydrogen atom, the team estimated the hydrogen capacity of the studied SWCN to be 5.1±1.2 wt. %.

It was a surprise that we could get so many carbon-hydrogen bonds. It gives us hope it can be used as a material for storing hydrogen.

—Anders Nilsson (Materials Research)

Further investigation demonstrated that the hydrogenation takes place not only on the surface, but also inside of the nanotube bundles.

Reversibility (releasing the hydrogen for use) poses more of an issue. The results showed than the nanotube film preserved its shape and structure for at least two cycles of hydrogenation/dehydrogenation but the number of defects in the walls of SWCN increased significantly. The team found that all C-H bonds break at the temperature above 600 °C.

For this approach to be viable, it requires a means to dissociate hydrogen and to shape the energetics of the C-H bonds to allow for hydrogen release at 50-100 °C. The team thinks that the former can be solved using an appropriate metal catalyst for hydrogen dissociation and the latter can be accomplished by using SWCN with a well-defined radius.

This was the first experiment conducted on the new SPEAR3 beamline 5-1. The work was supported by the Global Climate Energy Project as well as the DOE.

Resources:

• Hydrogenation of Single-Walled Nanotubes