AB–MH (Ammonia Borane–Metal Hydride) composites: systematic understanding of dehydrogenation properties

Abstract

Dehydrogenation properties of AB–MH (Ammonia Borane–Metal Hydride, M = K, Na, Li, Ca, Mg, Al) composites were systematically investigated by thermal and mass analyses. The results suggest that the Pauling electronegativity of M, χ_p, is a good indicator to predict the phases of composites, the dehydrogenation temperature and the amount of by-product gases (NH₃ and B₂H₆). The phases of composites were classified by χ_p as follows. MBH₄ was formed for M = K, Na (χ_p ≤ 0.9), MNH₂BH₃ was formed for M = Na, Li (0.9 ≤ χ_p ≤ 1.0) and no new compounds were formed for M = Ca, Mg, Al (1.0 ≤ χ_p). The 1st dehydrogenation temperatures of the samples (M = Na, Li, Ca, Mg) were 10–20 °C lower than that of AB itself (χ_p ≤ 1.2). The amount of NH₃ was decreased as χ_p increased. On the other hand, the amount of B₂H₆ was decreased as χ_p decreased. The emission of B₃H₆N₃ could occur by the reaction of NH₃ and B₂H₆. Finally, AB–MAlH₄ (M = Na, Li) composites, which were prepared based on the indicator, showed superior potential as hydrogen storage materials because they did not desorb any by-products NH₃, B₂H₆ and B₃H₆N₃.