Hydrogen Storage Properties And Electrochemical Characteristics Of Mm-Mg-Ni Based Alloys

The RE-Mg-Ni-based alloy electrodes, as negative electrode materials of Nickel-metal hydride (Ni/MH) rechargeable batteries, have been paid much extensively attention due to their high charge/discharge capacity, long cycle life, and environmental advantage. In this paper, the RE-Mg-Ni-based hydrogen storage alloys are prepared by induction melting under argon atmosphere protection. The influence of add elements and heat treatment on the phase composition and the electrochemical properties of them are investiaged by PCT, X-Ray Powder Diffraction (XRD) and electrochemical measurement methods.The result shows that the alloy phases mainly consist of LaNi5phase. With the increasing of Al, the cell volume of the LaNi5phase gradually decreases, Owing to the spatial position of hydrogen atom occupied becomes smaller, the hydrogen storage capacity drops. Eectrochemical studies indicates the maximal discharge capacity of the Mm0.75Mg0.25Ni3.5Coo.2Alt(x=0.0-0.4) alloy electrodes are:385mA-h/g (x=0.0),378mA-h/g (x=0.1),337mA·h/g (x=0.2),334mA·h/g (x=0.3) and323mA·h/g (x=0.4), which is well agreement with the hydrogen storage capacity. After100charge/discharge cycles, the discharge capacity retention rate of the electrodes increases from52.6%to84.6%when the x value varys from0.0to0.4.Mm0.75Mg0.25Ni3.5Co0.2Al0.3annealed at different temperatures is studied systematically on account of the electrochemical kinetics and cyclic stability performance of Mm0.75Mg0.25Ni3.5Co0.2Alo.3alloy electrodes are optimized.The results find that he discharge capacity of annealed alloy electrodes are almost unchanged in the range of temperature650℃-850℃, and the maximum discharge capacity of alloy electrodes are:346mA·h/g (As-cast),346mA·h/g(650℃),340mA·h/g(750℃),343mA-h/g (850℃) and343mA·h/g (950℃).However, after150charge/discharge cycle, the cyclic stability (Si50) of the alloy electrodes are55.75%(As-cast),59.54%(650℃),57.44%(750℃),58.31%(850℃),54.51%(950℃), respectively. It could conclude that the cyclic stability of the electrodes would be improved at the annealing temperature range of50℃-850℃.To further investigate the hydrogen storage property and the electrochemical performance of the Mm0.75Mg0.25Ni35Co0.2Al.3alloy electrodes, changing the ratio Pr/Mm of Mm0.75Mg0.25Ni35Co0.2Al.3alloy reveals that the alloy phases mainly composed of LaNis phase, LaAlNi4phase and diminishes La2Ni7phase when Pr/Mm ratio is within5%-25%. The cell volume of the LaNi5phase reduces as the Pr/Mm ratio increases from5%to25%, while the diffraction peak intensity of La2Ni7phase and LaAlNi4phase are weakened. The maximum discharge capacity of the alloy electrodes decreases from336mA·h/g (Pr/Mm=5%) to316mA-h/g (Pr/Mm=25%). However, the cyclic stability (S100) of alloy electrodes is enhanced, the discharge capacity retention rate of the alloy electrodes raise from66.37%to73.73%as the ratio of Pr/Mm changes from5%to25%after100charge/discharge cycles. Meanwhile, the high-rate discharge performance has also been significantly improved, and it rises from66.2%(Pr/Mm=5%) to75.6%(Pr/Mm=25%) at the discharge current density of1200mA/g. The electrochemical kinetics tests find that the anodic peal current density Ip of the electrodes are1087.0,1530.3,1559.3,2020.3and1407.6mA/g, the limiting current density IL are869.0,1302.3,1798.7,1788.0and1525.7mA/g, respectively, which indicates that the ratio of Pr/Mm in the range of15%-25%is contribute to enhancing the the electrochemical kinetics performance of the Mmo.75Mgo.25Ni35Co0.2Al0.3alloy.In the reaction, methods of temperature subsection reaction are adopted to investigate the electrochemical property of Mmo.75Mgo.25Ni3.5Co0.2Al0.3alloy which is annealled under different temperatures. The results show that the maximal discharge capacity and the cyclic stability of it are greatly improved remarkable in the temperature of700℃-800℃, and especially its comprehensive electrochemical properties of it is the best as the anneal temperature is800℃. Comparing to as-cast alloy electrode, the maximum discharge capacity of it adds up from343mA·h/g to351mA·h/g, the capacity retention rate grows from66.29%o82.27%fter130charge/discharge cycles, the high rate discharge ability HRD1200goes up from74.5%o76.60%, the anodic oxidation peak current density ascends from1119.3mA/g to2184.3mA/g and the limiting current density increases from945.3mA/g to2072.3mA/g when the alloy is annealed at800℃.