Microstructure And Electrochemical Properties Of The AB₅-type Co-free Hydrogen Storage Electrode Alloys

In this thesis, based on the review of the research and development of AB5 type low-Co and Co-free hydrogen storage electrode alloys, some ways for improving the overall properties of AB5 type Co-free electrode alloy were proposed as the objects of this study. By means of XRD , SEM,EDS analyses and electrochemical measurements, the effects of the alloy preparation methods (including heat-treatment and rapidly quenching) and the alloy stoichiometry on the phase structure and electrochemical properties of some Co-free alloys were studied systemically.For the Co-free MlNi4.0Al0.3Si0.1Fe0.6 alloys, prepared by conventional casting (as-cast) and melt-spinning with different cooling rates(5,10,15,20m/s), the phase structure and electrochemical properties of these alloys were comparatively studied. It is found that all of the alloys are single CaCu5 phase structure.After melt-spinning treatment, the microstructure of the alloy change from the coarse dendritic structure (as-cast) to the fine cellular structure and the melt-spun alloy have a more homogenenous composition. Compared to the as-cast alloy, the melt-spun alloys have a lower discharge capacity and inferior high-rate dischargeability, but a much improved cycling stability as the cooling rate increases. Among the alloys studied, the melt-spun alloy (5m/s) has the best overall electrochemical properties: its maximum discharge capacity Cmax=292.58mAh/g, activation process needs only 2 cycles, the high-rate dischargeability HRD300=86.64%, and the capacity retention rate after 300 charge-discharge cycles reaches 74.15%.The microstructure and electrochemical properties of the Co-free MlNi4.0Al0.3Si0.1Fe0.6 alloy heat-treated under different conditions (1173K X 3h, 1223K X3h, 1273KX3h,1373KX3h) were also investigated. It is found all the heat-treated alloys prepared with all these methods are all single CaCu5 phase structure and have a more homogeneous composition. After heat-treatment, most of the alloys show a equiaxedstructure except the alloy(1173KX3h) still has the coarse dendritic structure. Compare to the as-cast alloys, the heat-treated alloys have a higher discharge capacity and poorer high-rate dischargeability, but a much better cycling stability .Among the alloys studied, the heat-treated alloy(1173KX3h)has the best overall electrochemical properties: its maximum discharge capacity Cmax=304.67mAh/g, activation process needs only 2 cycles, the high-rate dischargeability HRD300=84.18%, and the capacity retention rate after 300 charge-discharge cycles is 71.91%.For the as-cast Ml(Ni0.80Al0.06Si0.02Fe0.12)x(x=4.6-5.6) alloys ,the effect of alloy stoichiometry on the structure and electrochemical properties of the alloys were studied systematically. It is found that all the alloys have a typical coarse dendritic structure are single CaCu5 phase structure, except the alloys (x<5.0) contain the Ce2Ni7 secondary phase. The electrochemical tests show that the maximum discharge capacity and high-rate dischargeability are improved when the stoichiometry x increases. However,with increasing of x, the cycling stability of the alloys increases first and then decreases after passing a maximum at x=5.2.Among the alloys studied, the as-cast alloy(x=5.2) has the best overall electrochemical properties: its maximumdischarge capacity Cmax =293.86mAh/g, activation process needs only 2 cycles, the high-rate dischargeability HRD300=89.34%, and the capacity retention rate after 300 charge-discharge cycles is 74.51%.On the bases of above investigation, the Ml(Ni0.80Al0.06 Si0.02Fe0.12)x(x=4.6-5.6) alloys were heat-treated at the temperature 1173K for 3h, and the effects of heat-treatment on the phase structure and electrochemical properties of alloys with different stoichiometry were studied. It is found that the heat-treatment does not change the phase structure of the alloys apparently, the alloys (x<5.0) still contain some Ce2Ni7 secondary phase and others consisted of single CaCu5 phase, and all of the heated-treated alloys still have a typical coarse dendrit...