Regeneration of Ammonia-Borane Complex for Hydrogen Storage
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Regeneration of Ammonia-Borane Complex for Hydrogen Storage Nahid Mohajeri and Ali T-Raissi Florida Solar Energy Center, University of Central Florida Cocoa, FL 32922, U.S.A. ABSTRACT At the Florida Solar Energy Center (FSEC), a research program is underway for developing a high-density hydrogen storage system based on amine-borane (AB) complexes. Due to their high hydrogen capacity, these hydrides have been employed, in the past, as disposable hydrogen sources for fuel cell applications. However, to meet the requirements for hydrogen storage onboard vehicles, it is essential that cost effective and energy efficient methods for the regeneration (i.e. hydrogenation) of the spent (dehydrogenated) AB complexes can be found that utilize only hydrogen and/or electricity (i.e. the only plausible hydrogen economy energy carriers). We are studying two ammoniaborane (NH3BH3)-based systems with high hydrogen storage capacity. The first system employs a borazine-cyclotriborazane cycle. Borazine is a product of NH3BH3 thermolysis. Cyclotriborazane is the inorganic analog of cyclohexane. The second system employs polymeric AB complexes such as poly-(aminoborane) and polyborazylene. Poly-(aminoborane), an inorganic analog of polyethylene, is also a product of amoniaborane thermolysis whilepolyborazylene is the product of borazine thermolysis. For the two systems above, we are developing regeneration (i.e. reduction of borazine, poly-(aminoborane) and polyborazylene) schemes based on: 1) catalytic hydrogenation and 2) indirect (multi-step) synthesis techniques. INTRODUCTION Species with empirical formula BH2NH2 and BxNxHy are considered group IIIB chemical hydrides. These species have been the focus of investigation as a means of high capacity hydrogen (H2) storage material for a long time. Among these compounds, ammoniaborane, BH3NH3, has the highest hydrogen content (19.6 wt%) and used in the disposable hydrogen storage systems for fuel cell applications and as a solid propellant for generating H2/D2 gas [1ai]. Its volumetric energy density (4.94 kWh/L) is superior to that of liquid hydrogen (2.36 kWh/L). At ambient conditions, ammoniaborane is a white crystalline solid, which is stable in water and air. Although amine-boranes (ABs) have very high hydrogen storage capacity, their regeneration is the main obstacle to their use as a viable transportation fuel. The release of hydrogen gas from ammoniaborane is accomplished via thermolysis. This process begins at temperatures below 140°C but a temperature of about 1200°C is needed to liberate the last mole of H2 and to form boron nitride (BN). The overall process is exothermic but heat must be supplied to activate the material [2a-f]. Since high temperatures are required for complete dehydrogenation of BH3NH3 and the chemical inertness of BN, it is preferable to end the dehydrogenation reaction at an intermediate stage. In that case, poly-(aminoborane), borazine (B3N3H6), and hydrogen are the main intermediate decomposition products. Borazine (B3N3H6) 1 is a planar boron-ni
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