Ammonia borane and a new ammonia borane-hydrogen compound at high pressure

Ammonia borane (AB) is a novel hydrogen storage material with its high gravimetric and volumetric hydrogen density compared to other candidates. High pressure studies can improve our understanding of structural relationships in hydrogen storage materials. We investigated the effect of pressure on the bonding in AB using Raman spectroscopy up to 22.3 GPa in a diamond anvil cell. Two new transitions were observed at approximately 5 and 12 GPa. We also studied AB in the presence of excess hydrogen (H₂) pressure and discovered a novel solid phase, AB(H₂)_x, where x ∼ 1.3 - 2. The new AB-H₂ compound can store an estimated 8 - 12 wt% molecular H₂ in addition to the chemically bonded H₂ in AB. The complex reaction kinetics, bonding variations, and slow reaction rate give hope for designing alternative chemical paths to synthesize and retain the new compound for practical hydrogen storage application.