Hybrid Solid-State Hydrogen Storage Materials

This thesis investigates the chemistry of ammonia borane (NH₃BH₃) relevant to the development of hydrogen storage systems for vehicular applications. Because of its high hydrogen content and low molecular weight ammonia borane has the potential to meet stringent gravimetric hydrogen storage targets of >9 wt%. Two of the three moles of H₂ in ammonia borane can be released under relatively mild conditions, with the highest gravimetric yield obtained in the solid-state. However, ammonia borane does not deliver sufficient H₂ at practical temperatures and the products formed upon H₂ loss are not amenable to regeneration back to the parent compound. The literature synthesis of ammonia borane was modified to facilitate large scale synthesis, and the deuterated analogues ND₃BH₃ and NH₃BD₃ were prepared for the purpose of mechanistic studies. The effect of lithium amide on the kinetics of dehydrogenation of ammonia borane was assessed by means of solid-state reaction in a series of specific molar ratios. Upon mixing lithium amide and ammonia borane, an exothermic reaction ensued resulting in the formation of a weakly bound adduct with an H₂N...BH₃-NH₃ environment. Thermal decomposition at or above temperatures of 50◦C of this phase was shown to liberate >9 wt% H₂. The mechanism of hydrogen evolution was investigated by means of reacting lithium amide and deuterated ammonia borane isotopologues, followed by analysis of the isotopic composition of evolved gaseous products by mass spectrometry. From these results, an intermolecular multi-step reaction mechanism was proposed, with the rates of the first stage strongly dependent on the concentration of lithium amide present. Compounds exhibiting a BN₃ environment (identi-fied by means of solid-state ¹¹B NMR spectroscopy) were formed during the first stage, and subsequently cross link to form a non-volatile solid. Further heating of this non-volatile solid phase ultimately resulted in the formation of crystalline Li₃BN₂ - identified by means of powder X-ray diffractometry. This compound has been identified as a potential hydrogen storage material due to its lightweight and theoretically high hydrogen content. It may also be amenable to hydrogen re-absorption. The LiNH₂/CH₃NH₂BH₃ system was also investigated. Thermal decomposition occurred through the same mechanism described for the LiNH₂/NH₃BH₃ system to theoretically evolve >8 wt% hydrogen. The gases evolved on thermal decomposition were predominantly H₂ with traces of methane detected by mass spectrometry