| Mg2Ni alloy has been proposed as one of the most promising candidates to be used as on-board hydrogen storage materials in fuel cells and negative electrode of Ni-MH rechargeable battery because of its high hydrogen storage capacity and low cost. However, the metal hydride Mg2NiH4showed sluggish hydriding and dehydriding kinetics due to its high thermodynamic stability. Moreover, Mg2Ni alloy was easily oxidized in K.OH solutions during the electrochemical charging/discharging cycles, which would lead to the discharge capacity decline sharply. The high energy ball milling and additives coating methods were usually adopted to improve the hydrogen storage properties of Mg2Ni alloy. In this thesis, the as-cast Mg2Ni alloy was melted in medium frequency furnace, and then Mg2Ni-Ni-RExOy composites were further prepared by ball milling method. The structure and the elements distribution condition of the composites were analysed by XRD, SEM and EDS techniques, the electrochemical properties and dynamic performances were tested systematically. The reaction mechanisms of the additives were explored. The main research work was as follows:The effects of different content of Ni powder and the milling time on the structure and hydrogen storage properties of Mg2Ni alloy were studied. It was showed that by extending the milling time and increasing the Ni content, the hydrogen storage electrochemical and dynamic properties of Mg2Ni alloy could be improved evidently.The influence of different kinds of rare earth oxides (CeO2、Fu2O3、La2O3) on Mg2Ni alloy were explored. The structure analysis showed that samples with rare earth oxides additives exhibited higher degree of amorphization. The hydrogen storage properties of Mg2Ni alloy were significantly improved. The catalysis effects were closely associated with the type of additives, the order of catalysis effects was CeO2>Eu2O3>La2O3.Nanosized Ce1-x(La0.5Eu0.5)xO2-δ and Ce1-x(Fe0.5La0.5)x02-δ solid solutions were synthesized via hydrothermal method. The overall performances of Mg2Ni-Ni-Ce1-x(La0.5Eu0.5)xO2-δ composites under different ball milling conditions were studied systematically. Comparing with undoped CeO2additive, it was found that the maximum discharge capacity, electrochemical cycle stability and electrochemical kinetics of the milled composites with doped solid solutions can be improved more obviously. The catalysis effects on the dynamic properties of Mg2Ni alloy between Ce1-x(Fe0.5La0.5)xO2-δ and Ce1-x(La0.5Eu0.5)xO2-δ were compared. The results showed that Ce1-x(Fe0.5La0.5)xO2-δ exhibited better catalytic activity on improving the dynamic properties of Mg2Ni alloy. It can be explained that Fe3+ions doped into Ce1-x(Fe0.5La0.5)xO2-δ solid solutions could adjust the structure and the stress state of lattices more obviously, which improve the ability of transmission capacity. Meanwhile, the nano-particles with the smaller size would create many more active sites on the surface of Mg2Ni alloys, which could enhance the hydrogen storage properties of the composites.By researching on the reaction mechanisms of the additives, it was found that the additives could increase the proportion of amorphous and nanocrystalline in the alloys, and accelerate the behavior of H atoms transmission on the surface or in the bulk of alloys. CeO2had a special open type crystal structure and Ce ions could change its valence easily, thus CeO2could act as the medium to accommodate and transfer H atom. The doped CeO2based solid solutions showed better catalysis effects, which should be attributed to that the dopant changed the cell lattice structure and the stress state to enhance their abilities of transfer atoms. Meanwhile, the size effects of nano-particles could make them show better performances. |
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