The Research On Adsorption/Desorption Properties And Electronic Mechanism Of Magnesium And Its Alloy Hydrides

Magnesium and its alloys have been considered to be one of the most promising materials for hydrogen storage because of their high storage capacity, low cost and light weight. However, the high desorption temperature and slow sorption kinetics limit their pratical applications. In order to improve the hydrogenation/dehydrogenation thermodynamic and kinetics properties of Mg and its alloy hydrides, many modifying studies have been experimentally performed, and some remarkable academic achievements have been obtained. However, the theoretical mechanisms related to hydrogenation/dehydrogenation properties of Mg-based hydrogen storage alloys are scarce up to now. In order to design the adavanced practical magnesium-based hydrogen storage materials, Mg, Mg₂Ni as well as their respective hydrides are systematically investigated in this dissertation. Based on theoretical calculations and experimental investigations, the microphysical processes of H adsorption and desorption in Mg and its hydride systems, and the corresponding catalytic mechanisms are studied. Besides, the microstructures, dehydrogenation properties and micromechanisms of Mg₂Ni and its hydrides are also investigated.Based on the simulations of microphysical processes for the hydrogenation of Mg, the adsorption, dissociation and diffusion properties of H₂ on clean Mg (0001) surface are systematically investigated. It is found that the weak dissociation ability of H₂ and slow diffusion velocity of H from surface into bulk are the main rate-limiting steps for the adsorption kinetics of MgH₂ system.The adsorption, dissociation and diffusion of H₂ on vacancy defective and Pd atom coadsorption Mg (0001) surface are investigated systematically. The results show that vacancy defects benefit enhancing the physisorption interaction between H₂ and Mg surface, and the dissociation energy barrier of H₂ reduces to some extent. Whereas, for the Mg (0001) surface with Pd atom coadsorption, it is noted that there is a strong chemisorption interaction between the Pd atom and H₂ adsorbed, and the dissociation energy barrier of H₂ reduces remarkably. The vacancy defect on Mg (0001) surface not only benefits H atom diffusion in Mg bulk with relatively more diffusion paths compared with that of clean surface, but also decreases significantly the energy barrier of H penetrating the topmost layer Mg atoms. Based on the simulations of microphysical prosesses for the dehydrogenation of MgH₂, the hydrogen desorption properties from clean, Mg atom vavancy defective and Pd atom doping MgH₂(001)/(110) surfaces are investigated systematically. It is found that H recombination and desorption are the main rate-limiting steps for the dehydrogenation kinetics of MgH₂ system. Comparatively, Mg atom vacancy and doping Pd atom both benefit decreasing the energy barriers of H recombination on MgH₂ surface.It is put forward that the catalytic mechanisms of transition metal oxides improving the dehydrogenation properties of MgH₂ system mainly come from the catalytic effects of transition metal Tm and O element. TiO₂,V₂O₅ and Nb₂O₅, well-known experimentally as the most excellent catalysts, are chosen and the influences of Tm(Tm=Ti, V, Nb) doping atoms as well as Mg atom vacacny on the dehydrogenation properties and electronic structures of MgH₂ system are investigated. It is shown that Ti, V and Nb doping can weaken the structural stability of MgH₂ phase and is favorable for improvement of the dehydrogenation properties of MgH₂ system. Mg atom vacancy further decreases the structural stability of MgH₂ system. Comparatively, the substitutions of Mg for Ti, V and Nb play the major role in improving the dehydrogenation properities of MgH₂ system.The influences of second phase hydride NbH_x on the dehydrogenation properties of MgH₂ system are investigated by devising a supercell model of NbH_x/MgH₂ interface. The results show that NbH_x phase makes Mg and H atoms in MgH₂ phase move to the inferface, and the NbH_x/MgH₂ interface presents evident"interface effect", enhancing the drive force of H diffusion in MgH₂ bulk and nucleation ofα-Mg phase. The NbH_x/MgH₂ interface weakens the structural stability of MgH₂ phase and improves the dehydrogenating properties of MgH₂ system.The structural characteristics and dehydrogenation properties of Mg₂Ni phase as well as its high/low temperature (HT/LT-) Mg₂NiH₄ hydrides are systematically investigated. It is found that HT-Mg₂NiH₄ presents a lower structural stability and higher dehydrogenation ability than LT phase. There is a mixed ionic-covalent bonding between Ni and H in [NiH₄]^4- anions embedded in the matrix of Mg²⁺ cations in both hydrides.By means of component substitution, the influences of alloying elements on the dehydrogenation properties of Mg₂NiH₄ hydride are investigated systematically. Following the theoretical calculations, 2Mg-Ni and 2Mg-0.75Ni-0.25Cu mixture powder are ball-milled at hydrogen atmosphere, and their microstructures and dehydrogenation properties are studied. It is found that Al, Ti, Fe, Co and Cu doping atoms are all found to weaken the stabilities of Mg₂NiH₄ hydrides. Al, Ti and Cu doping atoms improve the dehydrogenation abilities of Mg₂NiH₄ hydrides, whereas, the roles of Fe and Co doping atoms are reverse. Mg₂CoH₅ and Mg₂FeH₆ hydrides, which are more stable than Mg₂NiH₄, can easily form in the Mg₂NiH₄ systems with Fe or Co doping. This may be the main reason that Fe and Co doping Mg₂NiH₄ systems have worse dehydrogenation properties. The following experimental researches show that Cu doping not only decreases the desorption temperature of Mg₂NiH₄ system, but also elevates its dehydrogenation velocity. The experimental results further confirm the reliability of theoretical calculations as well as the accuracy of forecasting.