ABSTRACT
The addition of catalysts to complex hydrides is aimed at enhancing the hydrogen absorption desorption properties. Here we show that the addition of carbon nanostructure C60 to LiBH4 has a remarkable catalytic effect, enhancing the uptake and release of hydrogen. A fullerene-LiBH4 composite demonstrates catalytic properties with not only lowered hydrogen desorption temperatures but also regenerative rehydrogenation at a relatively low temperature of 350 degrees C. This catalytic effect probably originates from C60 interfering with the charge transfer from Li to the BH4 moiety, resulting in a minimized ionic bond between Li+ and BH4(-), and a weakened covalent bond between B and H. Interaction of LiBH4 with an electronegative substrate such as carbon fullerene affects the ability of Li to donate its charge to BH4, consequently weakening the B-H bond and causing hydrogen to desorb at lower temperatures as well as facilitating the absorption of H2. Degradation of cycling capacity is observed and is probably due to the formation of diboranes or other irreversible intermediates.
Subject(s)
Borohydrides/chemistry , Crystallization/methods , Fullerenes/chemistry , Hydrogen/chemistry , Lithium Compounds/chemistry , Nanostructures/chemistry , Nanostructures/ultrastructure , Nanotechnology/methods , Catalysis , Hydrogen/isolation & purification , Macromolecular Substances/chemistry , Materials Testing , Molecular Conformation , Particle Size , Surface PropertiesABSTRACT
A synergistic approach involving experiment and first-principles theory not only shows that carbon nanostructures can be used as catalysts for hydrogen uptake and release in complex metal hydrides such as sodium alanate, NaAlH(4), but also provides an unambiguous understanding of how the catalysts work. Here we show that the stability of NaAlH(4) originates with the charge transfer from Na to the AlH(4) moiety, resulting in an ionic bond between Na(+) and AlH(4)(-) and a covalent bond between Al and H. Interaction of NaAlH(4) with an electronegative substrate such as carbon fullerene or nanotube affects the ability of Na to donate its charge to AlH(4), consequently weakening the Al-H bond and causing hydrogen to desorb at lower temperatures as well as facilitating the absorption of H(2) to reverse the dehydrogenation reaction. In addition, based on our experimental observations and theoretical calculations it appears the curvature of the carbon nanostructure plays a role in the catalytic process. Ab initio molecular dynamics simulation further reveals the time evolution of the charge transfer process.
