| Mg-Ni-based amorphous alloys are considered as one of the most promising negative electrode materials due to the good dehydrogenation kinetics, large hydrogen capacity and so on. In this paper, a brief review of the research and development of the Mg-Ni-based hydrogen storage alloys was introduced. On this basis, the Mg-Ni-Pr bulk amorphous alloy with high glass forming ability (GFA) was selected. The Mg-Ni-Pr amorphous alloy was prepared by copper-mold injection casting considering the specific situation of the laboratory. The microstructure evolution, the electrochemical properties and the corresponding electrochemical hydrogen storage properties during charge/discharge cycling were investigated systematically by means of XRD, SEM, DSC analysis and the electrochemical test methods which included land test system and electrochemical desorption P-C isotherm test system. In addition, the effects of substitution Ag for Mg of Mg-Ni-Pr amorphous hydrogen storage alloys on the electrochemical properties were investigated systematically.The Mg-Ni-Pr amorphous alloy after milling was used for the hydrogen storage alloy material. The effect of ball-milling time on the electrochemical properties of the Mg-Ni-Pr amorphous hydrogen storage alloys was studied. It was found that80h ball-milled alloy electrode exhibited the discharge capacity of418.4mAh/g, and the capacity retention rate reached90.37%after20charge-discharge cycles. The amorphous structures were important factors to achieve a high discharge capacity and to improve the cycle stability during charge/discharge cycling. Results of XRD and SEM analyses showed that alloy particles of Mg65Ni21Pr14kept amorphous after long time ball-milling, and the shape of alloy particles was polygonal or irregular and gradually become spherical or spherical-alike. The amorphous structure crystallized step by step, and gradually formed Mg2NiH4, Mg2Ni, Mg(OH)2phases as hydrogenation/dehydrogenation proceeded, and this also was the main reason for the degradation of the discharge capacity of the amorphous Mg-Ni-Pr hydrogen storage alloys.The pressure-composition-isotherms (PCT) curve was tested by a simple electrochemical method. It showed that the main discharge voltage was between-1.0V to-0.85V and there was a major discharge potential plateau form0～300mAh/g. The electrode showed a wide plateau pressure between0.01MPa and0.1MPa at the highest discharge capacity and the maximum amount of hydrogen desorption reached1.6148%at298K.DSC curves showed that partial substitution of Ag for Mg decreases the liquids temperature and melting temperature. In the10K/min,20K/min,40K/min,60K/min,80K/min heating rates with continuous heating process, Mg62Ni21Pr14Ag3amorphous alloy had three distinct exothermic peaks and it showed that the crystallization procedure of the amorphous alloy finished for3steps. At heating rates in the range10-80K/min, Tg,Tx,Tp1,Tp2,Tp3shifted to higher temperatures with increasing heating rate and it showed the super cooled liquid region gradually become much wider.The electrochemical study results showed that the electrochemical properties of alloy electrodes were obviously improved by appropriate Ag additive. The charge voltage of the alloy electrodes decreased with substitution Ag for partly Mg; the charging resistance of the alloy electrodes reduced with Ag addition; the discharge voltage of the alloy electrodes went up and reduced the stability of the metal hydride. The maximum discharge capacity and the cycle stability first increased and then decreased with the increase of Mg contents. The high rate discharge discharge-ability of alloy electrodes was also improved. Among them, the Mg62Ni21Pr14Ag3amorphous alloy electrode had the maximum discharge capacity reaching430.5mAh/g and the best cycle stability reaching90.97%. |
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