Rerouting Pathways of Solid-State Ammonia Borane Energy Release

Ammonia borane (NH3BH3, AB) represents a promising energy-dense material for hydrogen storage and propulsion; however, its energy release mechanisms on oxidation by solid-state oxidizers are not well understood. In this study, through in situ time-of-flight mass spectrometry supported by attenuated total reflection-Fourier transform infrared spectroscopy and density functional theory calculations, we investigate the fundamental reaction mechanisms involved in the energy release from solid-state AB with different chemical oxidizers. We show that the reaction of AB with oxidizers like KClO4 is mediated by [NH3BH2NH3]+[BH4]− (DADB) formation, resulting in its kinetic entrapment into low-energy BNHx clusters that are resistant to further oxidation, thus limiting complete energy extraction. In contrast, with an ammonium-based oxidizer such as NH4ClO4, the presence of NH4+ ions enables AB to follow an alternative reaction pathway forming [NH3BH2NH3]+[ClO4]− rather than DADB, thus inhibiting the formation of BNHx species and facilitating its complete oxidation. This alternative reaction route causes the AB/NH4ClO4 system to exhibit remarkably higher energy release rates over that of AB/KClO4 (∼27x) and the standard Al/NH4ClO4 propellant (∼7x).