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Boron-based organometallic nanostructures: hydrogen storage properties and structure stability.

Transition-metal (TM) boride and carboride nanostructures are studied as model organometallic materials for hydrogen storage. The dispersed TM atoms function as H2 sorption centers on the surface of the boron or carbon-boron substrate. The flexibility offered in the variety of possible structures permits the study of the effect of the TM-TM distance on the storage capacity. When the TMs are too close to one another, TM-TM bonding reduces the capacity. Even when separated by distances larger than the normal TM-TM bond length, delocalization of TM valence electrons can still lower the hydrogen capacity. An optimal TM-TM distance for the structural motifs studied here is approximately 6 A. Our study also permitted the evaluation of new TM boride nanostructures. We predict a low-energy single-walled scandium triboride (ScB3) nanotube that can bind approximately 6.1 wt % hydrogen with a binding energy of 22 approximately 26 kJ/mol.