Storage of molecular hydrogen in an ammonia borane compound at high pressure

We studied ammonia borane ( AB), NH3BH3, in the presence of excess hydrogen ( H-2) pressure and discovered a solid phase, AB( H-2)(x), where x approximate to 1.3-2. The new AB-H-2 compound can store an estimated 8-12 wt% molecular H-2 in addition to the chemically bonded H-2 in AB. This phase formed slowly at 6.2 GPa, but the reaction rate could be enhanced by crushing the AB sample to increase its contact area with H-2. The compound has 2 Raman H-2 vibron peaks from the absorbed H-2 in this phase: one (v(1)) at frequency 70 cm(-1) below the free H-2 vibron, and the other (v(2)) at higher frequency overlapping with the free H-2 vibron at 6 GPa. The peaks shift linearly over the pressure interval of 6-16 GPa with average pressure coefficients of dv(1)/dP = 4 cm(-1)/GPa and dv(2)/dP = 6 cm(-1)/GPa. The formation of the compound is accompanied by changes in the N-H and B-H stretching Raman peaks resulting from the AB interactions with H-2 which indicate the structural complexity and low symmetry of this phase. Storage of significant amounts of additional molecular H-2 in AB increases the already high hydrogen content of AB, and may provide guidance for developing improved hydrogen storage materials.