Synthesis of ammonia borane for hydrogen storage applications

A new synthetic procedure to make the condensed phase hydrogen storage material, ammonia borane (NH₃BH₃, abbreviated as AB), is described and compared with previous literature procedures. Ammonia borane with a gravimetric density ca. 194 gm H₂ kg⁻¹ and a volumetric density ca. 146 H₂ litre⁻¹, is a promising chemical hydrogen storage material for fuel cell powered applications. The work shows that ammonium borohydride, NH₄BH₄, formed in situ by the metathesis of NH₄X and MBH₄ salts (M = Na, Li; X = Cl, F) in liquid NH₃, can be induced to decompose in an organic ether to yield AB in near quantitative yield. The purity of the AB prepared by this one-pot synthetic strategy is sufficient to meet the thermal stability requirements for on-board hydrogen storage.