|The procedure expands the geometry of a SWCNT fiber (upper left) and then locks the expanded form into a stable shape with cross linkers (bottom). Click to enlarge. Credit: ACS|
Researchers at Rice University and the National Renewable Energy Laboratory (NREL) have engineered single-walled carbon nanotube (SWCNT) fibers to become a scaffold for the storage of hydrogen. The 3-D nanoengineered fibers physisorb twice as much hydrogen per unit surface area as do typical macroporous carbon materials.
These fiber-based systems can have high density, and combined with the outstanding thermal conductivity of carbon nanotubes, can point a way toward solving the volumetric and heat-transfer constraints that limit some other hydrogen-storage supports, the team writes in a paper published online 22 December in the Journal of the American Chemical Society.
Devising cost-effective and practical on-board hydrogen storage systems with sufficient storage density to support targeted driving ranges is an on-going problem for the development of hydrogen-fueled vehicles. The authors note that using high pressure storage tanks in consumer automobiles lessens their attractiveness. An alternative to large volume, weight and pressure tanks is reversibly binding hydrogen to a lightweight solid-phase support. This approach falls into two categories: covalent binding (chemisorption) of the dissociated hydrogen, usually as a metal hydride; and physical adsorption (physisorption) of the hydrogen molecule.
The use of physisorption rather than chemisorption of hydrogen atoms to a surface eliminates the need for high heating to desorb the hydrogen from the solid phase, thereby providing faster kinetics of release. This also provides high energy efficiency and essentially complete availability of all stored hydrogen at lower pressures, generally in the 1-100 atm range. In addition, it is desirable to have a storage medium that is stable upon cycling, that can provide good thermal conductivity to dissipate the heat of adsorption, and that has paths with minimal tortuosity for fast kinetics of uptake.
Although many solid-phase supports have been prepared, they are neither high density nor do they have high thermal conductivity for heat removal during the adsorption step. We show here the fabrication of chemically cross-linked 3-dimensional (3-D) frameworks of single-walled carbon nanotube (SWCNT) fibers. These fibers physisorb twice as much hydrogen, at low pressures, with respect to their surface areas, than typical macroporous carbon materials.—Leonard et al. 2008
SWCNT fibers themselves do not have sufficient surface area for the storage of hydrogen because they are bundled together tightly. Swelling the fibers in oleum (20% free SO3, fuming sulfuric acid) causes them to swell. The expanded geometry can be locked in by inserting cross-links that are stable in oleum, resulting in an enlarged and stable structure even after removal of the solvent.
It is the interstitial spaces between nanotubes that could be tuned by the choice of intercalating acid and cross-linker, providing higher surface area for hydrogen adsorption and also a multifaced environment for the hydrogen to assume several points of physisorption contact.—Leonard et al. 2008
Extrapolating from their experimental results, the team concluded that their scaffolded SWCNT structure could support 3.7 wt % hydrogen uptake with a surface area of 1,000 m2/g and 7.4 wt % hydrogen uptake at 2,000 m2/g using only 2 bar of pressure.
The research team is working to synthesize the SWCNT fibers with better pore sizes for the storage of hydrogen, with the ultimate goal of developing a hydrogen vehicle fuel tank that works near ambient temperature and pressure. As part of this, they are investigating other methods than using oleum (which results in some challenges in removing trapped sulfuric acid) to achieve higher surface areas, and are also using the scaffolds as platforms for supporting metals to enhance the hydrogen uptake at ambient temperature.
Ashley D. Leonard, Jared L. Hudson, Hua Fan, Richard Booker, Lin J. Simpson, Kevin J. O’Neill, Philip A. Parilla, Michael J. Heben, Matteo Pasquali, Carter Kittrell, and James M. Tour (2008) Nanoengineered Carbon Scaffolds for Hydrogen Storage. J. Am. Chem. Soc., Article ASAP doi: 10.1021/ja806633p