Putting hydrogen into the framework

Metallacarboranes can be used as viable natural hydrogen storage materials, say researchers from the US and India [Singh et al., J. Am. Chem. Soc. (2010) doi: 10.1021/ja104544s].

 

Metallacarboranes are clusters consisting of boron and carbon atoms in which one or several BH units are replaced by a transition metal atom to form a cage containing boron, metal and carbon. The remarkable thing is that they are capable of binding hydrogen in a rather unusual way- via the Kubas interaction. While most chemical bonding relies on strong chemisorption involving covalent bonds which are difficult to break or weak physisorption involving van der Waals interactions, the Kubas type of interaction lies somewhere between the two in strength which could potentially change the future of hydrogen storage. Hydrogen storage has long been the focus of many research laboratories but the metal organic frameworks and metal hyrides investigated so far suffer from binding too strongly or too weakly to hydrogen. The Kubas interaction however, provides the right level of binding strength.

 

The team from Rice University in the US and the Indian Institute of Science in India have used first principle calculations to show that metallacarboranes containing Sc or Ti achieve an average binding energy of around 0.4 eV/H₂. This means that they are capable of reversible storage at ambient conditions. Each metal atom can bind up to five H₂ molecules and the cages can store around 8 wt % of hydrogen on the metallacarborane cluster, a value which exceeds the US department of energy goals for set for 2015.

 

Boris Yakobson of Rice University explains to Materials Today why the system is so effective. “The metal organic framework provides a scaffold, and some of its links include transition metal atoms capable of catching and holding H₂, yet not too strongly, so it can be released when needed for fuel. Now, to prevent transition metals from aggregating into a blob together, each metal atom must be held, like a gem held in a casing. This is accomplished by placing transition metal atoms in the carborane casings-part of the scaffold of the entire metal organic framework. High porosity of the metal organic framework is important is it allows H₂ to enter and leave freely.”

 

In addition to binding up to 5 molecules of H₂ via the Kubas interaction, metallacarboranes can also physisorb more hydrogen within the pores of the frameworks.

 

As Yakobson says, “If chemists can synthesize this particular framework with metallacarborane as an element, this (hydrogen storage) may become a reality."