The Synthesis And Property Control Of Borohydride

Metal borohydrides,especially the light metal borohydrides have especially high volumetric and gravimetric hydrogen densities,which is far more than the DOE targets for hydrogen storage materials used for on-board and portable applications.Therefore,as one of the most promising hydrogen storage materials,metal borohydride is now a key and hot research area.However,there are some crucial issues during the themal decomposition process of borohydrides,such as sluggish kinetics,high hydrogen desorption temperature and possible toxic gas emission.Then the hydrolysis process of borohydride is irreversible,the cost of borohydride preparation and regeneration is a key problem that restricts its large-scale application.Therefore,this thesis is aiming to develop an efficient way for the cost-effective regeneration of the comsumed borohydrides,particularly investigate the thermal decomposition of metal borohydride.Fistly,Na BH₄ is regenerated from its hydrolysis product?NaBO₂?2H₂O?using TiH₂ or titanium alloys as reducing agent through a high energy ball milling method.The effects of ball milling time,the molar ratio of TiAl₉/NaBO₂?2H₂O,H₂ pressure and reducing agent on the yield of Na BH₄ were systematically studied.By optimizing the experimental parameters and additives,the maximum yield of Na BH₄?38.95%?was achieved at an optimum condition of milling time,inter gas?H₂?pressure,the overdoes percent of TiAl₉ being 10h,2MPa and 30%respectively.Furthermore,according to the phenomenon that the longer milling time the lower yield of NaBH₄,this experiment is designed to ball-mill Na BH₄ with TiH₂ and titanium aluminum alloys to improve the thermal dehydrogenation performance of NaBH₄.By the MS and DSC test,this work reveals the effects of titanium droping on the thermal decomposition performance of NaBH₄ after 3h ball-millng.At last,based on a cationic substitution reaction,a perovskite-type hydrogen-rich material,NH₄Mg?BH₄?₃,was synthesized by cryogenic ball milling.Furthermore,by analysising the MS and DSC results,it is demonstrated that the cationic subsitution plays a vital role on surpressing inpurity gas during the themal decomposition of NH₄Mg?BH₄?₃.The initial dehydrogenation temperature of NH₄Mg?BH₄?₃ is about 60°C and the dehydrogenation kinetics is remarkable with activation energy of 64.94 kJ/mol calculated from the Kissinger equation.The FTIR and NMR results show that the dehydrogenation process of NH₄Mg?BH₄?₃ can be divided into two separate decomposition steps of NH₄BH₄ and Mg?BH₄?₂,respectively.