Highly-effective Regeneration Of NaBH₄ And Its Utilization For Synthesis Of Perovskite-type Hydrogen Storage Material

Metal borohydrides,especially the light metal borohydrides,are of the character of higher hydrogen storage capacity,which is far more than the DOE targets for volumetric and gravimetric hydrogen densities of hydrogen storage materials used for on-board or portable applications.Therefore,metal borohydride is one of the most promising hydrogen storage materials,which is now a key and hot research area.However,there are some crucial issues need to be solved before using borohydrides as hydrogen storage materials,such as sluggish kinetics,high hydrogen desorption temperature and possible toxic gas emission.Hydrolysis of metal borohydride is an attractive method for hydrogen production.Unfortunately,hydrogen production by hydrolysis is a typical irreversible process.The one-off usage of borohydrides leads to a high cost,which is unfordable for end users.This issue has to be addressed,therefore,this thesis is aiming to investigate the thermal decomposition of metal borohydride and,particularly,develop an efficient way for the cost-effective regeneration of the comsumed borohydrides.The high-energy ball milling method was selected to regenerate the borohydride from its hydrolysis byproduct due to the merits of the easiness,mild synthesis conditions,low cost and scalability.The synthesis of perovskite-type borohydride is also included in this thesis,aiming to provide a new category hydrogen storage materials with the advantages of low desorption temperature and high hydroge capacity.NaBH₄ was regenerated from its hydrolysis product,NaBO2,by a high energy ball milling method at room temperature.In order to maximize the yield of NaBH₄,four groups of experiments including the variation of ball milling time,the molar ratio of MgH₂/NaBO2,H2 pressure and methanol additives were systematically studied.By optimizing the experimental parameters and additives,the maximum yield of NaBH₄?89%?was achieved at an optimum condition of molar ratio of MgH₂/NaBO2,milling time,inter gas?H2?pressure,the amount of methanol being 2.7:1,12 h,3MPa and 0.15 ml,respectively.Furthermore,hydrolysis experiment illustrates that the regenerated NaBH₄ can be directly hydrolysed with excellent hydrolysis performance?comparable to the purchased NaBH₄?.The mechanism of Na BH4 regeneration has also been investigated and indicates that the NaBH₄ formation involves a two-step H transfer in which NaBOH2 is an intermediate characterized by the solid-state nuclear magnetic resonance?NMR?.Synthesis of complexes by a simple and cost-effective method is promising to optimize the dehydrogenation of borohydride.In this thesis,a perovskite-type hydrogen-rich material,NH₄Ca?BH₄?₃,was synthesized by planetary ball milling.It was found that temperature of hydrogen desorption is very low as an initial dehydrogenation temperature of NH₄Ca?BH₄?₃ is about 84 °C that falls into the DOE target for hydrogen release.The kinetics is also very good,with activation energy of 286 kJ/mol according to the Kissinger equation.Furthermore,the toxic gas emission was greatly inhibited during the heating process.The FTIR and NMR results show that the dehydrogenation process of NH₄Ca?BH₄?₃ can be divided into two separate decompositions of NH4BH4 and Ca?BH4?2,respectively.