Ammonium borohydride: Solid hydrogen storage material with highest gravimetric hydrogen content

Various options for hydrogen storage are being studied ranging gases (hydrogen), liquids (e.g., methanol) to solids (e.g., lithium borohydride, boranes and amides). However, no single material today meets all the required technical targets. Here we report the synthesis, structure and hydrogen desorption properties of a unique hydrogen storage material, i.e., one that stores hydrogen on both the cation and anion, ammonium borohydride. Further we show the compound releases greater than 20 wt% hydrogen in three steps at temperatures less than 180 °C and provide evidence that NH4BH4 crystallizes at room temperature and standard pressure in a rock-salt structure. The experimental lattice parameters provide an estimate of the volumetric density, ca. 154 g Hydrogen/liter of NH4BH4. The high volumetric capacity is comparable to many metal hydrides, however, as NH4BH4 has hydrogen stored on both the cationic and anionic sites which leads to a significantly greater gravimetric density (244 gm Hydrogen/kg).