Study On The Modification And Corresponding Mechanisms Of Lithium Borohydride-based Materials For Hydrogen Storage

Hydrogen energy is an ideal choice that can solve the problems of the depletion of fossil fuels and the contamination of environment. However, the technique of safe and efficient storage of hydrogen is the key barrier that prevents hydrogen energy from widespread utilization. LiBH4 has been attracting a great deal of attention as one of the promising materials for hydrogen storage due to its high gravimetric and volumetric hydrogen capacities of 18.5 wt% and 121 kg Hb/m3, respectively. Unfortunately, its practical applications are hampered by unfavorable high thermal stability, sluggish kinetics, and relatively high operating temperature, etc. Based on an overview of the research and development of LiBH4 for hydrogen storage, the methods of nanoconfinement, catalytic doping and reactive destabilization are selected to modify LiBH4 alone or synergistically in this dissertation. The matter evolution during the de-/rehydrogenation and their corresponding mechanisms are also investigated systematically.The dissertation first reports a remarkable enhancement of the hydrogen storage capacity and reversibility of LiBH4 nanoconfined in a new type of scaffold with high porosity and excellent mechanical stability, densified zeolite-templated carbon (ZTC), which was successfully synthesized by template method. The results show that, the desorption behavior of LiBH4 nanoconfined system barely changed after its densification due to the high mechanical stability of ZTC. When the loading rate of LiBH4 is 53 vol% to the total pore volume of ZTC, the LiBH4 can be completely confined into the ZTC matrix and presents the best hydrogen storage properties. That is, the onset dehydrogenation temperature reduces to 194 ℃ plus the desorption activation energy dramatically decreases to 129.0 kJ/mol, and the rehydrogenation of LiBH4 is achieved under mild conditions (260 ℃ and 120 bar H2) with an improved cycle stability. By ultra-high pressure densification and high uploading amount of LiBH4, it can greatly improve not only the gravimetric capacity (6.92 wt%) but also the volumetric capacities (75.43 g/L). These findings lay the foundations for further application of the LiBH4 nanoconfined system.Secondly, the low-temperature reversible hydrogen storage properties of LiBH4 were further modified by the synergistic effect of nanoconfinement and catalytic doping of NbF5. A careful study shows that the onset dehydrogenation temperature for nanoconfined LiBH4@CMK3-NbF5 system is reduced to 150 ℃ plus the desorption activation energy dramatically decreases to 97.8 kJ/mol. Furthermore, the rehydrogenation of L1BH4 is achieved under mild conditions (200 ℃ and 60 bar H2) with an improved cycle stability. These results are attributed to the active Nb-containing species (NbHx and NbB2) and the function of F anions (LiBH4xFx), as well as the nanosized particles of LiBH4 and high specific surface area of the MC scaffold.Then, the reactive destabilization of MgH2 and catalytic doping of transition metal chloride MCl2 (M=Fe, Co, Ni) were used to further modify the reversible hydrogen storage properties of LiBH4 synergistically. The results show that, all these chlorides can significantly enhance the de-/rehydrogenation kinetics and cycle stability of 2LiBH4-MgH2 system, and their overall modification effect as follows: NiCl2、CoCl2、FeCl2. Wherein, NiCl2-doped sample shows no incubation period for generating MgB2 during dehydrogenation and exhibits the lowest two-step desorption activation energy (19.5 ± 6.9 kJ/mol and 116.1 ± 3.8 kJ/mol, respectively). During the milling process, the MCl2 will be transformed into small transition metal borides Mxby, which plays an important role for the subsequent enhanced de-/rehydrogenation kinetics. Among them, Ni3B can be further in-situ transformed into stable and highly dispersed nano-MgNi3B2, which may act as the nucleation site for MgB2 formation, and thus the incubation period shrinks.In order to avoid the by-product of LiCl in the NiCl2-doped 2LiBH4-MgH2 system, direct doping of Ni-B nanoparticles were chosen to further improve the reversible hydrogen storage properties of the system. Three kinds of Ni-B nanoparticles with different crystalline states and particle sizes were prepared by wet-chemical reduction and mechanochemical methods, respectively. The study found that, all of these as-synthesized Ni-B nanoparticles can significantly enhance the re-/dehydrogenation kinetics of system, resulting in no incubation period for the formation of MgB2 during dehydrogenation. The more disordered amorphous structure and smaller size the Ni-B particles are, the better catalytic effect is obtained. The overall modification effect as follows:amorphous NiB-AM> amorphous NiB-AR> crystalline N13B-CR> NiCl2. The micro structure analyses clearly reveal the in-situ formation of MgNi3B2 phase in dehydrogenation process, which acts as the nucleation agents for MgB2 formation determined by edge-to-edge model.Finally, by a new method of direct carbonization for MOFs, the highly dispersed and nanosized carbon supported Ni nanoparticles (Ni/C) were successfully synthesized. Direct doping of the Ni/C nanoparticles into 2LiBH4-MgH2 system can significantly improve the de-/rehydrogenation kinetics and cycle stability, resulting in no incubation period after the first cycle. The two-step desorption activation energy is also greatly reduced. The microstructure analyses reveal that, the Ni/C will be transformed into nanosized Ni4B3 by an exothermic reaction with LiBE4. Then the in-situ formed Ni4B3 can be further transformed into stable and highly dispersed nano-MgNi3B2 in the subsequent desorption process. The indirect evidence of MgNi3B2 as nucleating agent for the formation of MgB2 is also observed. The way of Ni/C-induced formation of highly dispersed nano-Ni-B can avoid the agglomeration phenomenon when doping Ni-B nanoparticles directly, so as to more effectively improve the efficiency of the catalyst.