Synthesis and hydrogen storage properties of aluminum hydrides and boron nitride nanotubes

Abstract

Aluminum hydride (AlH3 or alane) is known to store maximum 10.1 wt. % of hydrogen at relatively low temperature (<100 oC), which partially fulfills the U.S. Department of Energy requirements for gravimetric loading capacity. However, its detailed mechanisms of appearing of different phases, structural stability, and dynamics of hydrogen desorption are still not clear. To understand the desorption properties of hydrogen in alane, thermodynamically stable α-AlH3 was synthesized by employing an ethereal reaction method. The dependence of pathways on phase formation and the properties of hydrogen evolution were investigated, and the results were compared with the ones for γ-AlH3. It was found that α-AlH3 requires 10 oC higher than that of γ-AlH3 to form, and its decomposition rate demonstrated enhanced endothermic stabilities. For desorption, all hydrogen atoms of alane evolved under an isothermal condition at 138 oC in less than 1 hour, and the sample completely transformed to pure aluminum. The results show that the total amount of desorbed hydrogen from α-AlH3 exceeded 9.05 wt. %, with the possibility of a further increase. Easy synthesis, thermal stability, and a large amount of hydrogen desorption of alane fulfill the requirements for light-weight hydrogen storage materials once the pathway of hydrogen cycling is provided. Uniform shape of boron nitride nanotubes (BNNTs) were successfully synthesized by pulverization of boron powder with Fe catalysts in a stainless steel vessel followed by annealing the powder at 1200 °C for 12 hours with flowing N2 and NH3 in a furnace. During the pulverization process, the Fe particles, used as catalysts, were coated on amorphous boron powders. The shape and type of the BNNTs were critically changed by synthesis conditions such as pulverization and annealing time suggesting that ball milling time is an important factor to produce high qulity of homogeneous BNNTs. For the BNNTs produced with 6 hours of ball milling follwed by 12 hours of annealing, the results of transmission electron microscope (TEM) measurements show that the BNNTs have a typical multi-walled tubular shape with a diameter of 30 - 50 nm with a specific surface area of 17.65 m2/g. The magnetization values measured by a superconducting quantum interference device (SQUID) magnetometer exhibited that BNNTs have a typical trend of paramagnetic properties at 2 and 300 K.