A synergistic strategy established by the combination of two H-enriched B–N based hydrides towards superior dehydrogenation

Abstract

A strategy for establishing H^δ+⋯^−δH interactions by the combination of two kinds of H-enriched B–N based hydrides, ammine metal borohydrides (AMBs) and ammonia borane (AB), to achieve superior dehydrogenation properties is reported. Two novel combined complexes: Al(BH₄)₃·6NH₃–4AB and Li₂Al(BH₄)₅(NH₃BH₃)₃·6NH₃ were successfully synthesized. Structural analysis revealed that a partial NH₃ unit transferred from Al(BH₄)₃·6NH₃ to AB, resulting in the formation of two new phases of Al(BH₄)₃·5.4NH₃ and NH₃BH₃·0.15NH₃ in the Al(BH₄)₃·6NH₃–4AB composite. In contrast, Li₂Al(BH₄)₅(NH₃BH₃)₃·6NH₃ formed with a single-phase that was indexed to a cubic unit cell with a refined lattice parameter, a = 23.1220(3) Å. The structure of Li₂Al(BH₄)₅(NH₃BH₃)₃·6NH₃ is composed of alternate Li⁺, [Al(NH₃)₆]³⁺ and AB layers stacked along the b-axis as a 3D framework. Compared to the unitary compound, the H-enriched complex system presented a mutual dehydrogenation improvement in terms of a considerable decrease in the dehydrogenation temperature and the preferable suppression of the simultaneous release of by-products; for example, over 11 wt% of hydrogen, with a purity of >98 mol%, can be released from both Al(BH₄)₃·6NH₃–4AB and Li₂Al(BH₄)₅(NH₃BH₃)₃·6NH₃ below 120 °C. The significantly improved dehydrogenation in the H-enriched complex system can be attributed to the initial interaction between the AB and an NH₃ group (from the AMBs), which results in the balanced B–H and N–H units in the AMBs, thereby leading to a more activated and thorough H^δ+⋯^−δH interaction in the composite. Moreover, an ammonia-liquification technique was employed to impregnate the complex system into a hypercrosslinked nano-porous polymer (PSDB) template, resulting in the average particle size of the Al(BH₄)₃·6NH₃–4AB composite to be <5 nm, which enables it to release more than 10 wt% high-pure hydrogen (>99.8 mol%) below 110 °C. These advanced dehydrogenation properties affirm Al(BH₄)₃·6NH₃–4AB and Li₂Al(BH₄)₅(NH₃BH₃)₃·6NH₃ as strong candidates for potential hydrogen storage materials.