The world's supply of petroleum is running out, and its combustion is destroying the environment. Such problems have motivated the development of alternative sources of energy.
Hydrogen is considered a clean energy storage material, because (1) it can be obtained from renewable sources and (2) its only combustion product is water. Challenges that must be met include efficiency (to lower the energy input required for energy release) and the ability to safely store a large amount of hydrogen in a small space.
Carter Kittrell, James Tour, and coworkers of Rice University (Houston) and the National Renewable Energy Laboratory (Golden, Colorado) have tackled these challenges with carbon nanotubes. Compared to common carbon materials, their nanotubes physically absorb almost twice as much hydrogen, even at low pressures.
The nanotubes.
Carbon nanotubes can be spun into well-aligned fibers (bundles of nanotubes), but are packed too tightly for optimum hydrogen storage. The scientists increased the spacing between the fibers (by roughly 30%) by incorporating small molecule linkers (methylenedianiline) between the nanotubes.
This roughly 0.8 nanometer spacing between the nanotubes is optimum for enhanced hydrogen adsorption (storage), without creating excess void space between the nanotubes. Additionally, the highly aligned fibers enable fast and efficient hydrogen passage through the tubes, rendering them ideal for fast hydrogen uptake and release.
Hydrogen storage.
At -321ºF, the carbon nanotubes adsorb 1.85 weight percent of hydrogen for every 500 square meters per gram of nanotubes, at 2 atmospheres of pressure. This is roughly twice the amount of hydrogen (1 weight percent) of typical carbon materials at 30 to 50 atmospheres of pressure.
For hydrogen storage, it is also important to maintain one of the unique chemical properties of the nanotubes, the ability of the electrons to shuttle around more easily than in most carbon materials. Eliminating this property reduces nanotube hydrogen storage capability down to those typically observed with carbon materials.
Future directions.
It is noteworthy that the scientists' single-walled carbon nanotubes are typically very difficult and expensive to prepare. However, due to the wide utility of these nanotubes in a broad range of scientific and engineering disciplines, this is a challenge being successfully met by many scientists in industry.
The pore size, surface area, and chemical properties of the nanotubes are all important for optimum hydrogen storage. The scientists intend to optimize these properties for even more efficient hydrogen storage, with the long-term goal of developing a realistic material that works at room temperature.
for more information:
Leonard, A. D.; Hudson, J. L.; Fan, H.; Booker, R.; Simpson, L. J.;
O'Neill, K. J.; Parilla, P. A.; Heben, M. J.; Pasquall, M.;
Kittrell, C.; Tour, J. M.
Nanoengineered carbon scaffolds for hydrogen storage.
J. Am. Chem. Soc. 2009, 131, 723-728.