Simulations of the pure pillared structure (a) and the structure doped with lithium (b). Hydrogen molecules are green; lithium atoms are purple. Click to enlarge. Credit: ACS

Light nanoporous materials that can store hydrogen by physisorption are attractive due to their capability for fast loading and unloading. Their downside, however, is that only a small amount of hydrogen can be stored at room temperature, because the interaction between hydrogen and host material is dominated by weak van der Waals forces.

To address this limitation, high surface area and appropriate pore size are key parameters for achieving high hydrogen storage, the researchers note, and as such, nanoporous carbon structures have been of long interest as hydrogen storage media. While new materials such as metal organic frameworks (MOFs) offer great potential, carbon-based materials possess a superior structural stability and amenability to a wide range of processing conditions, keeping them in the race for commercial applications, the researchers write in their paper.

The only thing that is missing is a way to increase their storage capacity. A possible route to achieve this is by synthesizing novel carbon-based architectures of large surface area, suitable for storage pores, like carbon nanoscrolls (CNSs) and fullerene intercalated graphite sheets...despite the enhancement, the total amount of hydrogen stored in those structures remains far from the DOE targets. What is needed is to find a way to further increase the amount of adsorbed molecules. It has been shown that hydrogen’s adsorption depends on the porosity of the material. Thus, in order to increase the amount of adsorbed molecules, we should have high micropore volume and narrow micropore size distribution. Evidently, tailored porosity is the key aspect in the development of new promising carbon based materials, as the efficient use of space plays a significant role.

—Dimitrakakis et al. (2008

Other research has shown that doping carbon materials with alkali metals (such as lithium) increases of the hydrogen storage capacity because of the charge of the alkali metal that polarizes the H2 molecules. The physisorption of hydrogen is then dominated by this charge-induced dipole interaction.

...a multiscale theoretical investigation proved that CNTs and graphene sheets can be combined to form novel 3-D nanostructures, capable of enhancing hydrogen storage. Ab initio calculations revealed that, even on the junction of this material, hydrogen’s interaction remains weak, comparable with already known hydrogen-carbon interaction values. The importance of the “charge induced dipole” in this type of interaction was verified, since hydrogen bonds with a difference of more than 1 order of magnitude when a lithium cation is present. Those findings were also supported by GCMC [grand canonical Monte Carlo] calculations. It was further proven that this novel material, when doped with lithium cations, can reach DOE’s volumetric target for mobile applications, under ambient conditions.

Experimentalists are challenged to fabricate this material and validate its storage capacity.

—Dimitrakakis et al. (2008)

Resources

• Georgios K. Dimitrakakis, Emmanuel Tylianakis, and George E. Froudakis (2008) Pillared Graphene: A New 3-D Network Nanostructure for Enhanced Hydrogen Storage. ASAP Nano Lett., doi: 10.1021/nl801417w