Preparation And Reversible Hydrogen Sorption Mechanisms Of Sodium Borohydride-Scandium Fluoride Hydrogen Storage Systems

“Hydrogen Economy”has promoted the development of modern society and it is one of the best way to realize the sustainable ultilization of energy in the future.As an important step,the use of hydrogen storage materials is essential for the hydrogen economy.Sodium borohydride?NaBH₄?is a solid-state hydrogen storage material with high hydrogen storage capacity and volumetric hydrogen storage density,low cost,safety and stability.It has broad application prospects in the field of hydrogen storage and generation in the future.However,the desorption temperature of NaBH₄ is above 565°C and the rehydrogenation cannot be realized for dehydrogenated products,which means they do not have reversibility.Therefore,the application of NaBH₄ in the field of hydrogen storage is limited.In this dissertation,the NaBH₄-based binary and multi-component hydrogen storage systems were constructed by adding 3d transition metal fluorides to overcome the disadvantages of pure NaBH₄ in hydrogen storage.The hydrogen sorption thermodynamic performance of NaBH₄-based composite systems,the phase composition of each process and reaction mechanisms were fully analyzed and discussed based on the XRD,DSC/TGA,FTIR,and PCT measurements.The main results can be summarized as follows:In this study,3NaBH₄/ScF₃ composite hydrogen storage system was prepared by ball milling method.Furthermore,XRD analysis shows that the milling process was just mechanical mixing and no chemical reaction occurrs.The initial hydrogen desorption temperature is around 300°C,the second and third decomposition processes starts at 420°C and 520°C,respectively,and the total hydrogen release amount is 5.54 wt.%.The F^-in the ScF₃ additive can partially replace H^-in NaBH₄,and the hydrogenation temperature can be reduced by forming the compound NaBH_xF_4-x,and the dehydrogenation thermodynamic properties can be improved accordingly.The formation of ScB₂ by Sc³⁺and B contributes to the improved hydrogen reversibility in the composite system.The test results showed that the complete hydrogenation product of 3NaBH₄/ScF₃ system?NaF,B and ScB₂?did not have hydrogen reversibility,while the product after the second dehydrogenation step?Na₃ScF₆,NaBF₄ and ScB₂?can absorb 1.59wt.%hydrogen under the circumstance of 420°C,3.2 MPa hydrogen pressure for 8 hours,which means the 3NaBH₄/ScF₃ composite system has partial hydrogen sorption reversibility.Based on the study of 3NaBH₄/ScF₃,the rare earth fluoride YF₃ was further introduced into the composite.The 3NaBH₄/0.5ScF₃/0.5YF₃composite hydrogen storage system was prepared by ball milling process.The study of hydrogen storage performance of 3NaBH₄/0.5ScF₃/0.5YF₃composite shows that the initial hydrogenation occurrs at 130°C,and the desorption process can be divided into three consecutive stages.By way of isothermal hydrogenation test,it can be found that the activation energy for hydrogen absorption process of this ternary hydrogen storage system is233.45 kJ·mol^-1 H₂,and the complete hydrogen release product can achieve 2.15 wt.%reversible hydrogen absorption at 380°C.Moreover,XRD and FTIR results show that the rare earth fluoride YF₃ plays a major role in the improvement of system's reversibility,which leads to the regeneration of NaBH₄ and also results in a partial hydrogen absorption reversiblity in the complete hydrogenation product of NaBH₄-ScF₃.