Studies On Electron Structures And Properties Of Hydrogen Storage Alloys

As solid carriers of hydrogen, hydrogen storage alloys have received much attention for their high reliability and excellent energy conversion functions. Ideal hydrogen storage alloys should have following characters: outstanding kinetics property, easiness to be activated, high resistance to toxicity, high mass and volume hydrogen storage density, good cycle stability and low cost. But by far, there is no material satisfying all the requirements. The development of super hydrogen storage alloys relies on the essential understanding of hydrogen storage properties in the electronic level. In this paper, the Empirical Electron Theory of Solids and Molecules(EET) is utilized to comprehensively analyze valence electron structures of hydrogen storage alloys and alloy hydrides. In the mean time, the connections between structures and properties are built, which helps to enrich fundamental theories of hydrogen storage alloys and provide the theoretical foundation for the alloy design.Valence electron structures of LaNi5-xRx series alloys are calculated and the effects of different element substitution on the hardness and equilibrium hydrogen pressure of LaNi5 are analyzed from the aspect of the electronic structure. The hardness coefficient to describe the hardness of the alloy and the equilibrium hydrogen pressure index to reveal hydrogen pressure are put forward together with their respective mathematical expressions. These two parameters have good correspondence with the results from first-principles and the experiments.Aiming at La-Ni, Mg-Ni, Ni-Ti and Fe-Ti alloy systems all having typical hydrogen storage alloys, valence electron structures of thirteen alloys are calculated. Results show that in typical hydrogen storage alloys, chemical bonds between different atoms A-B prevail over bonds between the same atoms A-A or B-B, while the situation is reversed for non-hydrogen storage alloys. On this basis, the hydrogen storage ability parameter is put forward to reveal the difference among different alloys.Valence electron structures of more than ten kinds of AmBnHk alloy hydrides are calculated. Results demonstrate that valence electron structures of these hydrides take on similar traits. In these hydrides, non-hydride forming element B and H comprise stronger chemical bond net, by contrast, hydride forming element A and H constitute weaker bond net. Such bond net makes it possible for hydrogen to be reversibly absorbed and desorbed. These results consist with those from first-principles. In addition, the strongest bond energy in B-H strong bond nets corresponding to per H atom has positive correspondence with the decomposition temperature of hydrides.The cell volume of the hydrogen storage alloy is closely connected with its mass hydrogen storage density, hydrogen storage capacity, and equilibrium hydrogen pressure, so the cell volume is one of the most important parameters in the alloy design. In this paper, empirical atom model of Vegard’s law is established, based on TFDC model. The model can be represented by the numerical method and the graphic method. The application of the empirical model to equiatomic continuous solid solutions obtains 86% accuracy rate by the numerical method and 80% accuracy rate by the graphic method. By comparison, for limited solid solutions and hydrogen storage alloys, the prediction accuracy rate drops to about 60%. Furthermore, in hydrogen storage alloys, the situation that the shortest bond comprises the same kind of atoms cannot be handled by the empirical atom model. With consideration of large size and electronic difference between two components in limited solid solutions and hydrogen storage alloys, new atom parameters are required to be introduced into the empirical atom model so as to reflect complex multi-atom actions.