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

Abstract

We studied ammonia borane (AB), NH₃BH₃, in the presence of excess hydrogen (H₂) pressure and discovered a solid phase, AB(H₂)_x, where x ≈1.3–2. The new AB–H₂ compound can store an estimated 8–12 wt % molecular H₂ in addition to the chemically bonded H₂ 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 H₂. The compound has 2 Raman H₂ vibron peaks from the absorbed H₂ in this phase: one (ν₁) at frequency 70 cm⁻¹ below the free H₂ vibron, and the other (ν₂) at higher frequency overlapping with the free H₂ vibron at 6 GPa. The peaks shift linearly over the pressure interval of 6–16 GPa with average pressure coefficients of dν₁/dP = 4 cm⁻¹/GPa and dν₂/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 H₂ which indicate the structural complexity and low symmetry of this phase. Storage of significant amounts of additional molecular H₂ in AB increases the already high hydrogen content of AB, and may provide guidance for developing improved hydrogen storage materials.