Aug 25, 2011
Understanding how buckyballs assist the release of hydrogen from lithium borohydride
Complex light metal hydrides possess many properties that make them attractive as a storage medium for hydrogen, but typically catalysts are required to lower the hydrogen desorption temperature and to facilitate hydrogen uptake in the form of a reversible reaction. The overwhelming focus in the search for catalysing agents has been on compounds containing titanium, but the precise mechanism of their actions remains somewhat obscure. A recent experiment has now shown that fullerenes (C60) can also act as catalysts for both hydrogen uptake and release in lithium borohydride (LiBH4). In an effort to understand the involved mechanism, researchers have employed density functional theory to carry out a detailed study of the interaction between this complex metal hydride and the carbon nanomaterial.
Working in a close collaboration with the experimental group headed by Ragaiy Zidan at Savannah River National Laboratory, US, Ralph Scheicher and co-workers in Sweden had previously explored theoretically how carbon nanostructures can act as catalysts for hydrogen release in the complex metal hydride sodium alanate (NaAlH4). The stability of the AlH4 moiety is due to the charge transfer from Na, resulting in an ionic bond between Na+ and AlH4– and a strengthening of the covalent bond between Al and H. The interaction of NaAlH4 with any electronegative substrate, such as a carbon nanotube or fullerene, affects the amount of charge donated from Na to AlH4 and thus weakens the Al-H bonds, causing hydrogen to desorb at lower temperatures.
Focus on fullerenes
Further experiments by Zidan's group concentrated on the catalysing effects of fullerenes (C60) on hydrogen uptake and release in lithium borohydride (LiBH4). In an effort to better understand the involved mechanism, Scheicher and co-workers have carried out a detailed computational study of the interaction between LiBH4 and C60. Considering a stepwise removal of the hydrogen from LiBH4, they found that the presence of C60 can lead to a substantial reduction in the involved hydrogen removal energies. This effect is explained as a consequence of the interaction between the negatively charged borohydride complex and the C60 entity.
Full details can be found in the journal Nanotechnology.
About the author
Dr Ralph H Scheicher is an assistant professor at Uppsala University, Sweden, in the Condensed Matter Theory (CMT) group headed by Prof. Rajeev Ahuja who also holds an appointment at the Royal Institute of Technology (KTH) in Stockholm. Dr Sa Li, a graduate from the CMT group, is a research assistant professor at Virginia Commonwealth University in Richmond where she works in the group of Distinguished Professor Puru Jena. Dr C Moyses Araujo and Dr Andreas Blomqvist both graduated from the CMT group in Uppsala. Araujo has since moved to Yale University and Blomqvist is now based at Sandvik Tooling R&D in Stockholm.