Structure and Hydrogen Storage Properties of Calcium Borohydride Diammoniate

A new type of hydrogen storage material—namely, calcium borohydride diammoniate (Ca(BH 4 ) 2 ·2NH 3 ), is synthesized by reacting calcium borohydride and 2 equiv of ammonia. Structural analyses show that this complex has an orthorhombic structure (space group Pbcn ) with unit-cell parameters of a = 6.4160 Å, b = 8.3900 Å, c = 12.7020 Å, and V = 683.75 Å 3, in which Ca 2+ coordinates with four −BH 4 groups two −NH 3 groups. The presence of NH 3 in the crystal lattice facilitates the formation of B−H···H−N dihydrogen bonding. As a consequence, the bond lengths of B−H and N−H are increased with comparison to Ca(BH 4 ) 2 and NH 3, respectively. Our experimental results show that more than 11.3 wt % hydrogen can be released exothermically from Ca(BH 4 ) 2 ·2NH 3 in a closed vessel at a temperature as low as 250 °C.