Hydrazine borane-induced destabilization of ammonia borane, and vice versa

Abstract

In the field of solid-state chemical hydrogen storage, ammonia borane NH3BH3 has been widely studied while hydrazine borane N2H4BH3 can be considered as a "novel" material. In the present work, we investigated the behaviour of these boranes when mixed together in a mole ratio of 1:1. Hydrazine borane and ammonia borane destabilize each other. Though stable at 20-25 °C, the mixture melts at ∼ 30 °C and then undergoes significant decomposition, with desorption of hydrogen H2 and hydrazine N2H4 from 67 °C. This is explained by the fact that the presence of hydrazine borane disrupts the H(δ+)⋯ H(δ-) network of ammonia borane, and vice versa; the mixture is then much less stable than the pristine boranes. The mixture can nevertheless be stabilized (by heat- or vacuum-treatment and thus extraction of evolving hydrogen and hydrazine), making the as-obtained solid a potential chemical hydrogen storage material. Over the range 25-300 °C, it is able to release ca. 11.4 wt% of almost pure H2. Furthermore forms boron nitride as the solid residue, at temperatures as low as 300 °C.

Keywords: Ammonia borane; Boron nitride; Hydrazine; Hydrazine borane; Hydrogen; Polyborazylene.