Hydrogen sorption in defective hexagonal BN sheets and BN nanotubes

We perform ab initio simulations on the interaction of molecular hydrogen with the native and substitutional defects of single hexagonal boron-nitride sheets and small-diameter (8,0) nanotubes. We find that the adsorption of molecular hydrogen on both types of structure is endothermic with respect to dissociation, with the small-diameter nanotube possessing the smaller barrier. Although chemisorption along the tube axis is energetically preferred, the barrier for dissociation is lower for chemisorption across the tube axis, implying that chemisorbed hydrogen can be kinetically trapped in a higher energy state. Dopants that maximize the localization of the highest occupied molecular orbital and lowest unoccupied molecular orbital states maximize hydrogen binding energies. Carbon dopants do not enhance H-2 binding in contrast to the literature, whereas silicon dopants for nitrogen provide H-2 binding energies of 0.8 eV, at the upper end of the range required to meet DOE targets for hydrogen storage. The formation energy of most defects is reduced with increasing curvature except for the carbon substitutionals. Vacancies reduce the barriers for H-2 dissociation for the planar sheets but not for strongly curved nanotubes. The surface stress induced by nanotube curvature boosts the hydrogen storage capabilities of vacancies compared to the sheet, with the nitrogen vacancy chemisorbing 4H and allowing a H-2 molecule to enter the interior of the tube. The hydrogen binding properties of boron-nitride systems are strongly dependent on the defects and dopants present. Pretreating of these systems so as to partially remove nitrogen should enhance H-2 adsorption properties.