Study On The Effect Mechanism Of Mg-based Additives On The Hydriding And Dehydriding Performance Of LiBH₄Complex Hydride

LiBH4is widely regarded as a promising candidate for mobile applications because of its high gravimetric capacity (18.5wt%) and volumetric density (121kg·H2·m-3). However, the use of LiBH4as an on-board hydrogen storage material is hampered by its unfavorably high thermal stability, relatively slow kinetics and difficulty for rehydrogenation. Based on the overall review of the development about LiBH4-related reactive hydride composites (RHCs), Mg17Al12-hydride and Ce2Mg17-hydride were used as reactants to study the de-/rehydrogenation behaviors of LiBH4-MgH2-Al composite and LiBH4-MgH2-CeH2.5i composite. X-Ray Diffraction (XRD). Fourier Infrared Spectrum (FT-IR). synchronized Differential Scanning Calorimetry-Thermogravimetry-Mass Spectrum (DSC/TG/MS), and Temperature Programmed Desorption (TPD) were employed to characterize the microstructure and de-/rehydrogenation performances of the above composites. In addition, the effect mechanism of Mg-based additives on the hydriding and dehydriding performance of LiBH4complex hydride was also studied in this thesis.The de-/rehydrogenantion behaviors and microstructures of LiBH4-MgH2-Al composite were investigated. It is found that mechanical milling with Mg17Al12-hydride markedly improves the reversible performance of LiBH4. The composite possesses a dehydrogenation capacity of10.3wt%and can be reversibly operated at450℃. About8.6wt%of hydrogen can be restored in its first isothermal rehydrogenation at450℃. This composite presents a two-step release during dehydrogenation, corresponding to the decomposition of MgH2(325℃) and LiBH4(390℃), respectively. The superior reversible hydrogen storage property should be associated with the formation of MgB2and AIB2in the dehydriding process. The melting of Mg17Al12is favorable for nucleation and formation of MgB2and A1B2. During rehydrogenation, MgB2and AlB2enhance hydriding process and improve hydriding kinetics.The study on the de-/rehydrogenation performance and microstructures of LiBH4-MgH2-CeH2.51composite shows that this composite possesses a maximum capacity of8.5wt%hydrogen releasing below400℃. Although its decomposition behavior is quite similar to that of LiBH4-MgH2-Al composite, the starting decomposition temperatures are substantially lower. However, its rehydrogenation capacity at450℃ is only4.2wt%after dehydrogenation under initial vacuum. The microstructure analysis indicates that the self-decomposition of LiBH4to LiH and amorphous boron is the main factor leading to the reduction of rehydriding capacity in the following cycles. Dehydrogenation under initial hydrogen back-pressure of0.4MPa significantly enhances the reversibility of LiBH4-MgH2-CeH2.51composite, mostly because self-decomposition of LiBH4can be inhibited and the formation of MgB2phase is promoted.