PNNL

Abstract

Ammonia borane (AB) is under significant investigation as a possible hydrogen storage material. While many chemical additives have been demonstrated to have a significant positive effect on hydrogen release from ammonia borane, many provide additional complications in the regeneration cycle. Mechanically alloyed hexagonal BN (nano-BN) has been shown to facilitate the release of hydrogen from AB at lower temperature, with minimal induction time, less exothermically, and inert nano-BN may be easily removed during any regeneration of the spent AB. The samples were prepared by mechanically alloying AB with nano-BN. Raman spectroscopy indicates that the AB:nano-BN samples are physical mixtures of AB and h-BN. The release of hydrogen from AB:nano-BN mixtures as well as the decomposition products were characterized by 11B magic angle spinning (MAS) solid state NMR, TGA/DSC/MS with 15N labeled AB, and solution 11B NMR spectroscopy. The 11B MAS solid state NMR spectrum shows that diammonate of diborane (DADB) is present in the mechanically alloyed mixture, which drastically shortens the induction period for hydrogen release from AB. Analysis of the TGA/DSC/MS spectra using 15N labeled AB shows that all the borazine (BZ) produced in the reaction comes from AB and that increasing nano-BN surface area results in increased amounts of BZ. However, under high temperature, 150°C, isothermal conditions, the amount of BZ released was the same as for neat AB. High resolution transmission electron microscopy (HRTEM), selected area diffraction (SAD), and electron energy loss spectroscopy (EELS) of the initial and final nano-BN additive provide evidence for crystallinity loss but not significant chemical changes. The higher concentration of BZ observed for low temperature dehydrogenation of AB:nano-BN mixtures versus neat AB is attributed to a surface interaction that favors the formation of precursors which ultimately result in BZ. This pathway can be avoided through isothermal heating at temperatures >150°C.