Influence On The Dynamics Of Hydrogen Diffusion Of Transition Mentals Doped Mg₂Ni Alloys

In this thesis, we have studied the influence on the dynamics of hydrogen diffusion of different transition metals doped Mg2Ni from the electronic structure calculation by using LST/QST method, which can provide theoretic supports to the experimental results.1. The adsorption performance and diffusivity of hydrogen on the Mg2Ni (100) surface are studied in the thesis. The hollow site (K site) is the most stable site for hydrogen absorption. The adsorption energy is 0.85 eV. After optimizing the structure with hydrogen on the hollow site, we regard it as the initial state. After optimizing the structure with hydrogen on the Mg-Ni bridge site at the subsurface, we regard it as the final state. Then, we search the transition state by LST/QST method. Our calculated energy barrier for hydrogen diffusion is 0.91 eV, which is consistent with the experimental results that Mg2Ni hydride is very stable and its sorption kinetics is poor.2. On the AgMg11Ni6(100) surface and TiMg11Ni6(100) surface, the Ni top site is the most stable site for hydrogen absorption. The energy barrier for hydrogen diffusion from Ni site on AgMg11Ni6(100) surface to the subsurface is 0.24 eV, while it is 0.72 eV for TiMg11Ni6(100). The energy barrier decreases when Mg2Ni is doped with Ag and Ti. Our calculated results are in line with the experimental results. The experimental results show that the hydrogen absorption enthalpy of Mg2Ni is-65.14 kJ·mol-1, while the hydrogen absorption enthalpy of Mg2-xAgxNi (x=0.16667) is-53.63 kJ·mol-1. It is reasonable to add a little Ag in Mg2Ni so that the kinetics of the Mg2-xAgxNi(x=0.05,0.1) hydrogen storage alloys can be improved. Experimental results also show that the kinetic property improves after Ti doping. The Mg1.5Ti0.5Ni alloy reaches to the maximum of its discharge capacity after 20 hours. After doping Ag and Ti, hydrogen can diffuse more easily in the alloys, improving its kinetic property.3. The energy barrier for hydrogen diffusion from the hollow site on Mg11Ni6Pd(100) surface to the Mg-Ni bridge site on the subsurface is 0.47 eV. The energy barrier for hydrogen diffusion from Mg11Ni6Zn(100) surface to the Mg-Ni bridge site on the subsurface is 0.07 eV, which is much less than that on the clean surface. The distance between Mg and Pd/Zn is closer than the distance between Mg and Ni, indicating that Mg has strong interaction with Pd/Zn. It weakens the interaction of H and Mg, making it easier for hydrogen to diffuse in the alloys. The experiment show that the hydrogen diffusion coefficient increases with the increasing the Pd content in the doped alloy. Doping Pd can increase the desorption rate in the Pd doped Mg2Ni alloy. The dynamic performance of Zn doped Mg2Ni also improved from the experimental results. Our calculated results are in accordance with the experimental results.On the Mg12Ni5Zr(100) and Mg12Ni5Mn(100) surface, the Mg top site and the Mn top site are the most stable site for hydrogen absorption. The adsorption energy is 1.68 eV and 0.96 eV, respectively. The adsorption energy increases when Mg2Ni is doped with Zr and Mn, demonstrating that the adsorption of hydrogen is stable. The distance between Mg and Zr increases, illustrating that the interaction of Zr and Mg is weakened. After doping Mn, the distance between each atom decreases, strengthening the interaction of H and Mg/Ni. According to the experimental reports, substituting Zr on Ni can improve the discharge property, while substituting Mn on Ni can increase the hydrogen storage capacity and raise the discharge capacity. The calculated result is consistent with the experimental results.