Synthesis, Hvdrogen Storage Properties And Corresponding Mechanisms Of Maqnesium Alanate-based Materials

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. Mg(AlH4)2attracts a lot of attention due to its relatively high hydrogen capacity, but the relatively high hydrogenation/dehydrogenation temperatures and poor reversibility cannot fulfill the requirements of practical application. Furthermore, the synthesis of Mg(AlH4)2is so difficult that Mg(AlH4)2is not commercially available until now, which severely hinders the development of Mg(AlH4)2-based hydrogen storage materials. In this paper, aiming at these problems, the controllable synthesis, high-energy ball milling, catalyst-doping, composite and nanosizing of Mg(AlH4)2were systematically investigated and the corresponding mechanisms were also revealed.First, a controllable synthesis method was developed to prepare high-purity Mg(AlH4)2submicron rods, and the hydrogen storage thermodynamics and kinetics of the as-prepared Mg(AlH4)2submicron rods were also systematically investigated. The diameter of the as-prepared Mg(AlH4)2submicron rods are0.5μm, and its purity is96.1%. The as-prepared Mg(AlH4)2release9.0wt%of hydrogen through a three-step reaction. MgH2and Al are formed after the first dehydrogenation step, and then Al(Mg) solid solution and MgH2are formed after the second dehydrogenation step, and finally Al3Mg2and Al(Mg) solid solution are formed after the third dehydrogenation step. The first dehydrogenation step is not reversible due to its exothermic nature, and the other two steps are reversible. Kinetic investigations indicate that the first dehydrogenation step of the as-prepared Mg(AlH4)2is a diffusion-controlled reaction with an relatively high apparent activation energy of123.0kJ mol-1, which is mainly responsible for its high on-set dehydrogenation temperature.Second, the effect of high-energy ball milling on the hydrogen storage properties of Mg(AlH4)2and its mechanism were systematically investigated. After milling for12h, the hydrogen desorption temperatures of Mg(AlH4)2is lowered by40℃. From macro-to micro-scale, high-energy ball milling changes the particle size, grain size, microstrain and lattice distortion of Mg(AlH4)2, specifically decreases the particle size and grain size and increases the microstrain and lattice distortion with prolonging the milling time. The decreases in particle size and grain size can shorten the diffusion distance of the species involved in the dehydrogenation reaction, and the increases in the microstrain and lattice distortion can enhance the diffusivity of the species involved in the dehydrogenation reaction, therefore synergically improve the dehydrogenation kinetics of Mg(AlH4)2.Moreover, the increases in the microstrain and lattice distortion can also raise the Gibbs free energy of Mg(AlH4)2, consequently changes the dehydrogenation thermodynamics of Mg(AlH4)2.Third, the effect of titanium fluoride on the hydrogen storage properties and its mechanism were investigated. It was found that doping with TiF3and TiF4can significantly reduce the dehydrogenation temperatures of Mg(AlH4)2, and the catalytic activity of TiF4is superior to that of TiF3.2.5mol%TiF4-doped Mg(AlH4)2starts to desorb hydrogen at40℃, and can release4.0wt%of hydrogen at82℃in100min. These is because TiF4can react with Mg(AlH4)2during milling, to in situ form the active catalyst Ti and increase defects, which change the nucleation and growth mode of the first-step dehydrogenation products of Mg(AlH4)2, and consequently lower the activation barrier of the first dehydrogenation step of Mg(AlH4)2.However, the reversibility of Mg(AlH4)2is not improved by doping with TiF4, as2.5mol%TiF4-doped Mg(AlH4)2is still partial reversible.Fourth, the hydrogen storage properties of Mg(AlH4)2/LiBH4composites and its mechanism were investigated. The research on the Mg(AlH4)2-xLiBH4composites with different molar ratio (x=2,4,6) revealed that with increasing the content of LiBH4, the dehydrogenation temperature of the second step of the Mg(AlH4)2-xLiBH4composite (the reaction of MgH2and Al) is lowered, and the dehydrogenation temperature of the third step (the reaction of Al3Mg2, Al(Mg) and LiBH4) is raised. The reason for the decrease in the reaction temperature of MgH2and Al is:LiBH4is a catalyst for the reaction of MgH2and Al, and the increased content of LiBH4lowers its kinetic barrier; the reason for the increase in the reaction temperature of Al3Mg2, Al(Mg) and LiBH4is:the increased content of LiBH4causes the change of the reaction path and raises the enthalpy change. Furthermore, the effect of byproduct NaCl or LiCl from the synthesis of Mg(AlH4)2on Mg(AlH4)2-6LiBH4composite was then investigated. It is found that NaCl can react with LiBH4to form NaBH4and LiCl, changing the chemical composition of the composite; Cl" ion in LiCl promotes the second dehydrogenation step of Mg(AlH4)2-6LiBH4composite (the reaction of and Al), and LiCl restrains the third dehydrogenation step of Mg(AlH4)2-6LiBH4composite by hindering the contact between Al3Mg2, Al(Mg) and LiBH4Finally, a mechanical-force-driven physical vapour deposition (MFPVD) method was developed to prepare Mg(AlH4)2nanorods; the formation mechanism was discussed; the hydrogen storage properties were also investigated systematically. It is found that the intense physical force from high-energy ball milling can vaporize the coordination polymer [Mg(AlH4)2(Et2O)]n which possesses a one-dimensional chain-like structure. Then the vaporized material can deposit onto the substrate and self-assemble one-dimensionally to form [Mg(AlH4)2(Et2O)]n nanorods. During successive heat treatment, the Et2O molecules were removed to form Mg(AlH4)2nanorods. The diameter of the as-prepared Mg(AlH4)2nanorods is20-40nm. The as-prepared Mg(AlH4)2nanorods exhibit superior hydrogen storage properties to the Mg(AlH4)2microrods, especially, the as-prepared Mg(AlH4)2nanorods can maintain their nanorod-like morphology during dehydrogenation and hydrogenation processes, consequently show excellent cycling stability. Moreover, LiBH4nanobelts with width of10-40nm were also successfully prepared by a similar process, indicating that the MFPVD method is general enough that can be extended to other complex hydrides which possess organic coordination polymers with unique morphology.