Ammonia borane (AB) is under significant investigation as a possible hydrogen storage material. While chemical additives have been shown to lower the temperature for hydrogen release from ammonia borane, many provide additional complications in the regeneration cycle. Mechanically alloyed hexagonal boron nitride (nano-BN) has been shown to facilitate the release of hydrogen from AB at lower temperature, with minimal induction time and less exothermicity, 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 was characterized by 11B magic angle spinning (MAS) solid state NMR spectroscopy, TGA/DSC/MS with 15N-labeled AB, and solution 11B NMR spectroscopy. The 11B MAS solid state NMR spectrum shows that diammoniate 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 with 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 significantly decreases. 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 lowtemperature 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 lower than 150 °C.
ASJC Scopus subject areas
- Physical and Theoretical Chemistry
- Electronic, Optical and Magnetic Materials
- Surfaces, Coatings and Films