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

Abstract

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 °C to set the baseline. Solid-state 11B-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 sp3 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.

Metal-free, polyether-mediated H2-release from ammonia borane: roles of hydrogen bonding interactions in promoting dehydrogenation

“…The previously proposed mechanism 12,42,43,53,54 involving the reactive DADB intermediate for the thermally-induced dehydrogenation of AB can thus be extended with the polyetheral promoters (Fig. 11).…”

“…The formation of DADB ([H 3 NBH 2 NH 3 ] + [BH 4 ] À ) has previously been proposed to be an initial, key process for enhancing AB dehydrogenation. 12,[41][42][43]53,54 In the proposed mechanism of AB dehydrogenation, two AB molecules interacted with each other to generate DADB following the formation of the new phase AB. 55 More recently, the influence of ammonium functionality (NH 4 + ) on the rate of AB dehydrogenation was demonstrated by the addition of NH 4 Cl to AB, which increased the rate of H 2 -release.…”

“…12 Moreover, the addition of trace amounts of DADB to pristine AB has been reported to reduce the induction period or onset temperature for H 2 -release. 12 In addition, Sneddon and coworkers found that an ionic liquid, 1-butyl-3-methyl imidazolium chloride (bmimCl), significantly enhanced the rate of H 2 -release from a mixture of AB and bmimCl (50 : 50, wt%). 34 The authors found that the ionic liquid facilitated the formation of DADB in the highly polar medium, which improved the rate of H 2 -release from AB.…”

“…11 In general, when compared to AB hydrolysis, the thermal decomposition of AB provides much higher gravimetric hydrogen storage density and is thus particularly desirable for long-term and/or transportation applications. Thermolysis of pristine AB, however, has exhibited slow dehydrogenation kinetics at 90 1C with an induction period 12 for potential fuel cell applications. In this regard, the development of an efficient procedure for AB dehydrogenation under mild conditions has attracted significant attention for its potential to enable chemical hydrogen storage with high energy density.…”

Amineborane Based Chemical Hydrogen Storage - Final Report

“…These rates are also faster than DADB H 2 -release from the solid state. 49 As can be seen in the NMR studies in Figure 3.16, the initial 11 B NMR spectrum obtained from a 10-wt% DADB/bmimOTF sample showed only the broad resonances expected for the DADB (NH 3 ) 2 BH 2 + (-13.3 ppm) and BH 4 -(-37.6 ppm) components. However, after heating for only 10 min at 85 o C, most of the DADB had been converted to PAB.…”