The Effects of Chemical Additives on the Induction Phase in Solid-State Thermal Decomposition of Ammonia Borane

The thermal decomposition of ammonia borane (AB) in the absence and presence of chemical additives was investigated to develop an approach for reducing the induction period for hydrogen release in the solid state. Gas chromatography techniques were used to measure the yield of hydrogen as a function of time under isothermal conditions between 75 and 90 degrees C to set the baseline. Solid-state B-11-NMR spectroscopy of the products produced after 1 mol equiv of hydrogen had been desorbed from AB (i.e., polyaminoborane) showed a complex mixture of sp(3) boron species. Raman microscopy was used to follow the transformation of crystalline AB to amorphous AB upon heating and the subsequent formation of the diammoniate of diborane (DADB). A gas buret was used to monitor the time-dependent release of hydrogen from AB in the presence of chemical additives. The combination of these approaches provides insight into the mechanism of hydrogen release from solid AB. The release of molecular hydrogen is described by a process involving sequential induction (disruption of dihydrogen bonds), nucleation (formation of DADB), and growth (hydrogen release through dehydrocoupling). Addition of DADB or ammonium chloride to neat AB significantly reduces the induction time for hydrogen release. These results provide approaches to improve the hydrogen storage properties of AB.