| Due to the high hydrogen storage capacity, light metal hydrides have gained much attention over the past decades for on-board hydrogen storage. However, these hydrides generally suffer from sluggish hydrogen sorption properties, thus restricting their practical applications. In this dissertation, the Mg-Al-H system is selected as the subject of the study based on a comprehensive review of the research and development of hydrogen storage materials.Firstly, the synthesis of AlH3 was explored to prepare AlH3 as the Al source for the Mg-Al-H system. Pure α-AlH3 and γ-AlH3 were successfully prepared with hydrogen desorption capacity of 8.3-8.5 wt% and desorption temperature range of 120-160℃ with a heating rate of 2℃ min-1 Peak desorption temperatures of α-AlH3 and γ-AlH3 were determined by DSC with a heating rate of 5℃ min-1 to be 171.4 and 163.2℃, respectively. Apparent activation energies for the desorptions of α-AlH3 and γ-AlH3 were estimated by Kissinger method to be 94.6 and 86.3 kJ mol-1, respectively. Both the desorption temperature and the activation energy of γ-AlH3 are bwer than α-AlH3; this is ascribed to the phase transition of γ-AlH3 to α-AlH3 before decomposition, which leads to the activation of the AlH3 crystal lattice.Secondly, the impact of ball milling on the microstructures and hydrogen desorption properties of the pure γ-AlH3 was studied. A 28℃ reduction of the desorption temperature for 10 h-milled γ-AlH3 was observed by DSC. The time needed to release 90% of hydrogen for 10 h-milled γ-AlH3 is only 28.6% that for un-milled γ-AlH3. The desorption of γ-AlH3 is kinetically controlled by three dimensional nucleation and growth of Al. It was found that ball milling changes the decomposition path of γ-AlH3; the ball-milled γ-AlH3 prefers to decompose directly without firstly transforming to α-AlH3, which was attributed to the different decomposition mechanisms of the particle surface (γ-AlH3→ Al+H2) and the particle interior (γ-AlH3→α-AlH3→ Al+H2) of γ-AlH3.Then, AlH3 was employed as Al source replacing the metallic Al to improve the hydrogen sorption properties of MgH2 in the Mg-Al-H system. The MgH2 desorption temperature reduction by AlH3 is 30℃ more than that by Al. Isothermal hydrogen desorption investigation results indicated that in the Mg-Al-H system, Al just slightly improves the hydrogen desorption kinetics of MgH2, while AlH3 improves significantly both the hydrogen desorption kinetics and the hydrogen desorption extent of MgH2. Under conditions of 300℃ and 5 MPa H2, MgH2 in the Mg-Al-H system can be fully recovered and the MgH2-AlH3 composite showed better absorption kinetics. It was suggested that AIH3 is better in improving the hydrogen sorption properties of MgH2 than the metallic Al for the fact that Al* formed in situ from the decomposition of AlH3 is oxide-free on the particle surfaces, which effectively increases the chemical activity of Al*. Furthermore, the brittleness of AIH3 makes it easier to mix MgH2 with AlH3, which would result in a uniform distribution of Mg and Al and shortening of the diffusion length.The MgH2-AlH3 composites were further studied to gain an insight into the hydrogen sorption properties and reaction mechanisms of this system. Hydrogen of over 7.51 wt% is released from the MgH2+xAlH3 (x= 0.25,0.5,1) composites and the onset desorption temperatures of MgH2 in the composites are reduced by over 68℃ compared with that of the pure MgH2. The decomposition of MgH2 in the composite is composed of two steps with the reaction between MgH2 and Al forming Mg17Al12 as the first step and the self-decomposition of the residual MgH2 as the second step. The hydrogen desorption kinetics of MgH2 in the composites is significantly improved and the hydrogen desorption activation energy of MgH2 in the MgH2+0.25 AIH3 composite is reduced by 17% compared with the pure MgH2. JMA kinetic studies showed that the hydrogen desorption kinetic model of MgH2 in the composite is different from the pure MgH2. Under conditions of 300℃ and 5 MPa H2, MgH2 can be fully recovered in the composite, while it is just partial recovery for the pure MgH2. The MgH2+0.25AlH3 composite exhibits the highest reversible hydrogen storage capacity of 6.10 wt%(H/M). However, this composite also suffers from severe kinetic decline upon cycling.NbF5 was introduced to further improve the hydrogen sorption properties of the MgH2+0.25AlH3 composite. It was found that addition of 1 moP% NbF5 remarkably destabilizes AIH3, which leads to almost full decomposition of AIH3 during ball milling the MgH2-AlH3-NbF5 composite. Moreover, NbF5 reduces the hydrogen desorption temperature of MgH2 in the composite by 44℃ and improves the hydrogen desorption kinetics by 2 times at 300℃ within 1 h. In addition, the cycling sorption property of the NbF5-doped composite is much better than the undoped one. Microstructure analysis indicated that the grain growth and particle agglomeration of MgH2 leads to the decline of the kinetics of the composite, however, NbFs addition can suppress the grain growth of MgH2 during sorption cycling, thus improving the hydrogen sorption properties of the composite. Finally, it was suggested that the decomposition of AIH3 taking place during the heating process, instead of the preparation process, is crucial in improving the hydrogen desorption properties of MgH2.CeF3 was employed to improve the MgH2-AlH3 composite taking into account that CeF3 do not significantly destabilize AlH3 like NbF5. It was found that an 86℃ reduction of the hydrogen desorption temperature of MgH2 was achieved for the MgH2 with co-addition of 0.25AIH3 and 0.01CeF3. Isothermal hydrogen desorption investigations demonstrated that the co-addition of AIH3 and CeF3 significantly enhances the hydrogen desorption kinetics of MgH2, with the absence of the induction period in the initial stage and the acceleration of hydrogen desorption process. In addition, this co-doped MgH2 shows very good cycling stability at 300 ℃, with a 1-h capacity of 3.5 wt% and a 3-h capacity of 4.5 wt%. Microstructure analysis indicated that during the hydrogen desorption process, MgH2 may react with Al (generated from the in situ decomposition of AIH3) to form Mg solid solution and Mg14Al12, which contributes to the thermodynamic improvement of the Mg-based material. Besides, MgH2 may also react with CeF3 to form MgF2 and CeH2-3, which act both as the hydrogen diffusion gateways and as the impediment to the grain growth of MgH2 during hydrogen sorption cycling, thus improving the hydrogen desorption kinetics and the cycling stability of MgH2. Finally, it was found that the presence of AIH3 will kinetically help CeF3 to take its positive effect on the hydrogen desorption properties of MgH2. All in all, synergistic addition of AIH3 and CeF3 simultaneously tailors the hydrogen desorption thermodynamic and kinetic properties of MgH2... |
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