Development of Regenerable High Capacity Boron Nitrogen Hydrides as Hydrogen Storage Materials

Abstract

The objective of this three-phase project is to develop synthesis and hydrogen extraction processes for nitrogen/boron hydride compounds that will permit exploitation of the high hydrogen content of these materials. The primary compound of interest in this project is ammonia-borane (NH{sub 3}BH{sub 3}), a white solid, stable at ambient conditions, containing 19.6% of its weight as hydrogen. With a low-pressure on-board storage and an efficient heating system to release hydrogen, ammonia-borane has a potential to meet DOE's year 2015 specific energy and energy density targets. If the ammonia-borane synthesis process could use the ammonia-borane decomposition products as the starting raw material, an efficient recycle loop could be set up for converting the decomposition products back into the starting boron-nitrogen hydride. This project is addressing two key challenges facing the exploitation of the boron/nitrogen hydrides (ammonia-borane), as hydrogen storage material: (1) Development of a simple, efficient, and controllable system for extracting most of the available hydrogen, realizing the high hydrogen density on a system weight/volume basis, and (2) Development of a large-capacity, inexpensive, ammonia-borane regeneration process starting from its decomposition products (BNHx) for recycle. During Phase I of the program both catalytic and non-catalytic decomposition of ammonia borane are being investigated to determine optimum decomposition conditions in terms of temperature for decomposition, rate of hydrogen release, purity of hydrogen produced, thermal efficiency of decomposition, and regenerability of the decomposition products. The non-catalytic studies provide a base-line performance to evaluate catalytic decomposition. Utilization of solid phase catalysts mixed with ammonia-borane was explored for its potential to lower the decomposition temperature, to increase the rate of hydrogen release at a given temperature, to lead to decomposition products amenable for regeneration, and direct catalytic hydrogenation of the decomposition products. Two different approaches of heating ammonia-borane are being investigated: (a) 'heat to material approach' in which a fixed compartmentalized ammonia-borane is heated by a carefully controlled heating pattern, and (b) 'material to heat approach' in which a small amount of ammonia-borane is dispensed at a time in a fixed hot zone. All stages of AB decomposition are exothermic which should allow the small 'hot zone' used in the second approach for heating to be self-sustaining. During the past year hydrogen release efforts focused on the second approach determining the amount of hydrogen released, kinetics of hydrogen release, and the amounts of impurities released as a function of AB decomposition temperature in the 'hot zone.'