If you remember basic chemistry, you know that covalent bonds are very strong. You can bind hydrogen, but you cannot take it out. And on the other extreme is weak physisorption. The molecules don’t form chemical bonds. They’re just exhibiting a weak attraction through the van der Waals force. Kubas interaction is in the middle and gives the right kind of binding so hydrogen can be stored and, if you change conditions—heat it up a little or reduce pressure—it can be taken out. You want the framework to be like a fuel tank.

—Boris Yakobson

Kubas allows for reversible storage of hydrogen in ambient conditions and that would make metallacarborane materials highly attractive for everyday use, Yakobson said. Physisorption of hydrogen by the carbon matrix, already demonstrated, would also occur at a much lower percentage, which would be a bit of a bonus, he said.

Between strong chemisorption and weak physisorption [for hydrogen storage], there exists a Kubas type of interaction, a “non-classical” form of binding of H₂ to metal with a binding energy of ~0.4 eV/H₂, which is ideal for the reversible storage at ambient conditions. A single metal atom can bind multiple H₂ molecules via the Kubas interaction, leading to high gravimetric and volumetric density.

The possibility of storing hydrogen via the Kubas interaction has been explored extensively for the case of transition metal (TM) decorated graphitic nanostructures (nanotubes, fullerenes, and graphene). Depending upon the type of metal atom, such complexes can store hydrogen up to 8 wt %. TM-ethylene complexes are once shown to store as much hydrogen as 14 wt %. The idea has been extended to lighter metal decorated carbon materials as well, where significant storage has been predicted. These materials are promising and, if experimentally realized, can easily meet the material-based DOE targets for 2015.

The biggest hurdle on the way to success of such materials is the tendency of metal atoms to aggregate.

—Singh et al.

Doping of carbon nanostructures by boron—which acts as an anchor to the metal atom due to stronger B-TM binding, has been proposed as a means to prevent the aggregation that has challenged earlier attempts, the authors note. However, the practical difficulty of doping carbon nanostructures with boron remains a challenge for successful synthesis of such materials, they say.

The key to the success of hydrogen storage via Kubas interaction may lie in finding nanomaterials where the metal atoms are among the constituent elements (and thus cannot aggregate), yet retain their H-binding ability. One such class is metallacarboranes, derived from the carboranes, one of the most studied classes of boron clusters. Carboranes are essentially borane clusters containing one or more carbon atoms. Replacing one or more BH units of carboranes by metal atoms leads to the formation of metallacarboranes.

—Singh et al.

Metallacarboranes thus combine boron, carbon and metal atoms in a cage-like structure. The team investigated the hydrogen storage capacity of metallacarborane-based MOF. Their study shows that metal in metallacarboranes can bind multiple hydrogen molecules, while carbon can link the clusters to form three-dimensional frameworks. They found that replacing carboranes in MOF by metallacarboranes enhances the wt % due to adsorption of additional H₂ on metal atoms via Kubas interaction. This leads to storage of up to 8.8 wt % in metallacarboranes.

Moving from a pure physisorption to Kubas type of H₂ binding increases the binding strength, which can ensure room temperature storage. The binding energies lie within the reversible adsorption range at ambient conditions. Sc and Ti are recognized as the most optimal metals in maximizing the storage capacity.

—Singh et al.

The Robert Welch Foundation and the Department of Energy supported the project.

Resources

• Abhishek K. Singh, Arta Sadrzadeh, and Boris I. Yakobson (2010) Metallacarboranes: Toward Promising Hydrogen Storage Metal Organic Frameworks. J. Am. Chem. Soc., Article ASAP doi: 10.1021/ja104544s