Investigations On The Thermodynamics And Kinetics Of Mg-based Hydrogen Storage Materials

Magnesium alloy is one of potential hydrogen storage materials in the hydrogen energy field. However, due to its kinetic limitations and unmanageable desorption temperatures, pure magnesium is unsuitable as a hydrogen storage material. To overcome these disadvantages, tremendous studies in both experiments and theories, such as element substitute and heat treatment, have been done to improve its thermodynamics, kinetics, electrochemistry and circle life to some extent. However, the interaction between metal and hydrogen and the mechanism of thermodynamics and kinetics from the microcosmic point view were still not clear. Therefore, a better understanding of its reaction mechanism is very urgent and also provides the base for further building a theoretical model.In this thesis, firstly, the dissociation/diffusion processes of H2/H on the Mg, MgH2 and MgO surfaces and the charge transfer were investigated. Secondly, the experiment data of ball milling and Chou model were used to validate our calculation results. Then, the pressure-composition-temperature model was deduced with the statistical mechanics. Finally, the mechanism of mixing rate controlling was analyzed by Chou model and the related models were derived.(1) The hydriding/dehydriding processes of pure and Fe3O4/La doped Mg/MgH2 surfaces were investigated with the Density Functional Theory and Nudged Elastic Band methods. Our results show:For the hydrogenation process, the dissociation of H2 was the rate-limiting step for pure MgH2 system. When the MgH2 mixed with catalyst Fe3O4 and La was considered, it was found that the doping of Fe atom has higher catalytic activities than the doping of La atom and vacancy for the H2 dissociation. However, the catalyst has the negative effect on the H diffusion and could eventually make the diffusion become the rate-limiting step. For the dehydrogenation process, the decomposition of H2 was the rate-limiting step for pure MgH2 system. When the MgH2 mixed with catalysts Fe3O4 and La was considered, it was found that Fe atom has higher catalytic effect on the desorption of H atom nearby the Mg atom. Meanwhile, the doping of La has a positive effect on the H atom of next-nearby Mg while the vacancy on the Mg site has a negative effect on the H desorption. Doping of Fe and La together with vacancy could improve the kinetics of H diffusion in the bulk and did not change the rate-limiting step until the end.According to the model calculation, MgH2, MgH2+1.9mol%Fe3O4 and MgH2+1.9mol%La were prepared by mechanical alloy method. The modified Chou model was then used to analyze the experimental results. The fitted results show that the experimental results could be well described by Chou model.(2) The dissociation/diffusion processes of H2/H on the MgO(001) surface were investigated. Our results show shat the activation energies of dissociation and diffusion for H2/H on the MgO(001) surface were much larger than that of H2/H on the Mg(0001) surface. Thus, MgO layer could prevent the hydrogen molecules from penetrating into the material.The diffusion coefficient was then obtained according to the Arrhenius equation and generally agreed with the experiments result.(3) Using the Bragg-Williams model in the statistical mechanics, we got the pressure-composition-temperature model of hydrogen storage alloys:In this new model, compressibility factorαwas used to correct the high pressure and improve the fitting of data. Then, the thermodynamic model was used to describe MgH2, catalyst-MgH2, Mg2Ni and La2Mg17 systems and the enthalpy and entropy as a function of composition were obtained.(4) On the basis of Chou model, a new model for predicting the reaction time has been expressed as an analytic formula using the reaction fraction of hydrogen in hydrogen storage materials, temperature and the granule radii: This model has been used in different systems, which showed that it was easy and flexible to be applied. Meanwhile, the model can describe reaction mechanism of the single and mixed rate-controlling steps.In conclusion, the reaction mechanism of hydriding/dehydriding of Mg-based hydrogen storage alloys was investigated. The close relationships among microstructure, thermodynamics and kinetics were analyzed and a theoretical system has been built for studying the reaction mechanism.