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 H-2 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 similar to 6 angstrom. 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 similar to 6.1 wt % hydrogen with a binding energy of 22 similar to 26 kJ/mol.