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

We studied ammonia borane (AB), NH(3)BH(3), in the presence of excess hydrogen (H(2)) pressure and discovered a solid phase, AB(H(2))(x), where x approximately 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 (nu(1)) at frequency 70 cm(-1) below the free H(2) vibron, and the other (nu(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 dnu(1)/dP = 4 cm(-1)/GPa and dnu(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.