Preparation Of Magnesium-based Hydrogen Storage Alloy And Its Electrochemical Performance Study

Mg-based hydrogen storage alloys are considered to be one of the most promising negative materials for Ni-MH batteries because they have many merits, such as large hydrogen storage capacity, low density, rich resources and low price. But its practical application is limited by the harsh hydrogenation/dehydrogenation kinetics and poor cyclic life. To overcome these drawbacks, mechanical alloying was used to prepare amorphous alloys in this paper. However, too poor cyclic stability of MgNi-based alloys with amorphous structure prevented their practical application. Based on this, the effects of element doping, thermal treating and elaboration of alloy composites on electrochemical properties of MgNi alloy were studied in this paper. X-ray diffraction (XRD) and scanning electron microscope (SEM) were employed to analyze the phase structure and surface configuration of the alloys, respectively. Charge-discharge test, cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS) and Tafel polarization test were carried out to explore the electrochemical properties of the alloys, and the mechanism of the discharge capacity decline was studied.The results show that:(1) Milling duration is an important factor to affect the electrochemical performance of Mg-Ti-Ni alloy electrode. It is found that 90 h is the most suitable milling time with a ball to powder weight ratio of 30:1 at a speed of 200 rpm, and the sample after 90 h milling has an amorphous structure.(2) These alloys prepared by MA have good electrochemical activation characteristics, reaching the maximum discharge capacity in the first cycle.(3) Though the initial discharge capacity fades with increasing Ti content, the cyclic performance is improved to a certain extent. The maximum discharge capacity of MgTi0.2 Ni alloy is 401.1 mAh·g-1 while MgNi is 438.4 mAh·g-1. The higher the Ti content is, the better the cycle performance of the alloy will present. After 30 charge-discharge cycles, the discharge capacity of MgTio.2Ni alloy electrode is 124.2 mAh·g-1, retaining 31.0% of its maximum capacity, which is 13.7% higher than that of MgNi. The reason for the improvement of cycle performance of alloy electrodes is that a layer of dense TiO2 film formed on the alloy surface could inhibit the oxidation of Mg and prevent further corrosion of the powder.(4) A little amount of Al and Zn is added to Mg-Ti-Ni to prepare alloy Mg0.9Ti0.1Al0.1Ni0.99Zn0.01, and the doping of these elements improves the cyclic stability of the alloys. After 30 charge and discharge cycles, this alloy remains 27.8% of its maximum discharge capacity (402.1 mAh·g-1).(5) Among MgTi0.2-xZrxNi0.9Al0.1 (x=0.00,0.05,0.10,0.15) alloys, MgTi0.15Zr0.05Ni0.9Al0.1 alloy electrode shows the best cyclic stability and anti-corrosion performance, with an adding ratio of Ti and Zr 3:1.(6) The influence of coating time of Cu and addition amount of some metallic oxide such as La2O3 and Co3O4 were invesgated. It shows that the alloy electrodes of Cu-coated for 10 h and 20%Co3O4 added exhibit the best cyclic stability. After 30 charge-discharge cycles, the retention rates are 37.8% and 28.5%, respectively.