An Investigation On The Mechanical Amorphization And Electrochemical Properties Of The Ln-Mg-based Hydrogen Storage Alloys

Based on the review of the research and development of the Mg-based hydrogen storage alloys, La₂Mg₁₇ hydrogen storage alloy was selected as the study object of this work due to its higher storage capacity than Mg-Ni system alloy. However, the application of La₂Mg₁₇ alloy in the electrochemical hydrogen storage can hardly be performed because of its poor kinetic properties and high hydride stability. The purpose in this thesis is to make La₂Mg₁₇ alloy charge/discharge reversibly by ball-milling with certain amounts of Ni or Co powders. By means of XRD, SEM, XPS and AES analyses and the electrochemical test methods including the galvanostatic charge-discharge, EIS, linear polarization, anodic polarization and cyclic voltammogram etc., the microstructure and electrochemical properties of the ball-milled composites were systematically studied. The study includes the followings: the effect of Ni/Co amounts and ball-milling time on the microstructure and electrochemical properties of the ball-milledLa₂ Mg₁₇ composite alloys, the effect of small addition of TiO₂, Bi₂O₃, MnO₂, Fe₂O₃ and mechanical microencapsulation with Ti, graphite, Al, Co, Ni on the microstructure and electrochemical properties of amorphousLa₂Mg₁₇+200wt.%Ni composites, the influence of Ni substitution for Mg in La₂Mg_17-xNi_x (x=1, 3, 5), Al substitution for Ni in La₂Mg₁₄Ni_3-xAl_x(x=0.5, 1, 1.5, 2) and Ti substitution for Mg in La₂Mg_14-xTi_xNi₃ (x=1, 2) on the phase structure of the as-cast alloy and electrochemical properties of the ball-milled composites with 200wt.%Ni, and the cycling capacity degradation mechanism of the amorphousLa₂Mg₁₇-Ni alloy.The study of the influence of Ni/Co amounts and ball-milling time on the microstructure and electrochemical properties of La₂Mg₁₇ composite alloys revealed that ball-milling with certain amount of Ni/Co can make La₂Mg₁₇ alloy charge/dishcharge reversibly at room temperature. The maximum discharge capacity of amorphous La₂Mg₁₇+200wt.%Ni and La₂Mg₁₇+200wt.%Co composites is 929.5 and 748.2mAh/g, respectively. It is found that ball-milling with adequate amount ofNi/Co powders is advantageous for the formation of amorphous structure, thus leading to the improvement of thermodynamic and kinetic properties of the ball-milled composites. The discharge capacity of the amorphous composite is influnced by both the amorphization degree and particle size. There exist an optimal Ni amount and ball-milling time for the achievement of the highest discharge capacity. Before the optimal value, the discharge capacity is mainly influenced by the amorphization degree, and beyond the optimal one, the particle size becomes the dominating factor.In order to improve the overall electrochemical properties of the amorphous La₂Mg₁₇+200wt.%Ni composite, the effects of adding small amounts of TiO₂, Bi₂O₃, Fe₂O₃, MnO₂ and mechanical microencapsulation with Ti, graphite, Al, Co, Ni were investigated. The results indicate that the addition of small amounts of metal oxides can improve the amorphization degreee of the composites, thus leading to the improvement of discharge capacity and high-rate dischargeability. The maximum discharge capacity of the amorphous La₂Mg₁₇+200wt.%Ni composite with addition of 3wt.%TiO₂ is 979.7mAh/g. The improvement of discharge capacity and high-rate dischargeability can be attributed to the following two factors. On the one hand, metal oxides have certain catalytic effect on the electrochemical hydriding/dehydriding of the Ln-Mg-based amorphous alloys, thus leading to the decrease of activation energy and electrochemical reaction resistance and the increase of exchange current density. On the other hand, metal oxides are very bittle and helpful for the refinement of alloy particle and the augmentation of specific surface areas, which make the hydrogen adsorption and oxidation much easier. But the addition of metal oxides has little effect on the cycling stability. The effect of mechanical microencapsulation with different elements on the electrochemical properties of the amorphous La₂Mg₁₇ + 200wt.%Ni composite was thoroughly invetigated. The results show that Ti and Al microencapsulation decreases the discharge capacity and high-rate dischargeability but improves the cycling stability. While Co and graphite microencapsulation improves the discharge capacity and high-rate dischargeability but degrades the cycling stability. Ni element is an exception, which displays the above twofoldfuntions. The maximum discharge capacity C_max and cycling retention rate after 60 cycles S₆₀ of the amorphous La₂Mg₁₇+200wt.%Ni composite are 861.9mAh/g and 34.4%, respectively. While the C_max and S₆₀ of the composites microencapsulated with 5wt.% Ti, Al, Co, graphite, Ni are 690.4, 823.7, 936.4, 918.9, 921.6mAh/g and 53.6, 38.0, 34.3,32.9,35.5%, respectively.In order to improve the overall electrochemical properties of the amorphous alloy, Mg in the La₂Mg₁₇ alloy was first partly substituted by Ni, and the phase structure of the as-cast La₂Mg_17-xNi_x(x=1, 3, 5) alloy and electrochemical properties of the ball-milled composites with 200wt.%Ni were investigated. The XRD results reveal that as-cast La₂Mg_17-xNi_x(x=1, 3, 5) alloy has a multiphase structure, including La₂Mg₁₇ and Mg₂Ni. And with the increase of Ni amount, the amount of Mg₂Ni increases but that of La₂Mg₁₇ decreases. The Ni substitution can also accelerate the amorphization process of the ball-milled composites. Electrochemical studies reveal that Ni substitution leads to the improvement of high-rate dischargeability and cycling stability, but to the decrease of discharge capacity. The ball-milled La₂Mg₁₄Ni₃+200wt.%Ni amorphous composite displays better overall electrochemical properties, e.g., its maximum discharge capacity is 704.8mAh/g, the HRD₁₄₄₀ value reaches 72.4%, S20 is as high as 63.0%. Based on these results, Al and Ti were selected in addition as the substitution elements for Ni and Mg in La₂Mg₁₄Ni₃ alloy, and the phase structure of the as-cast La₂Mg₁₄Ni_3-xAl_x(x=0.5, 1, 1.5, 2) and La₂Mg_14-xTi,Ni₃(x=1, 2) alloys and the electrochemical properties of the ball-milled composites with 200wt.%Ni were investigated. The XRD results indicate that the substitution of Ni by Al and Mg by Ti leads to the appearance of AINi and TiNi phase besides La₂Mg₁₇ and Mg₂Ni. Electrochemical results reveal that substitution of Al for Ni and Ti for Mg leads to the improvement of discharge capacity, among which the maximum discharge capacities of La₂Mg₁₄Ni_1.5Al_1.5+200wt.%Ni and La₂Mg₁₂Ti₂Ni₃ + 200wt.%Ni amorphuos alloys are 827.2 and 843.1mAh/g, respectively. This is mainly due to the existence of catalytic phase AINi or TiNi, which plays an important role in improving the electrocatalytic activity and increasing the discharge capacity. But the substitution of Al or Ti leads to the worsening of high-rate dischargeability andcycling performance.Based on the study of the cycling capacity degradation mechanism of the amorphous La₂Mg₁₇-Ni system alloys, it is found that the structure stability of the alloys is good during cycling and has no effect on the cycling capacity degradation of the alloys. The main reason of the fast cycling capacity degradation of the alloys is the corrosion of La and Mg on the alloy surface, especially the corrosion of Mg, which decreases the amounts of hydrogen absorption elements. During the initial cycling period, the pulverization of the alloy particles accelerates the corrosion process, thus leading to the fast degradation of discharge capacity at initial period. And afterwards, the particle pulverization tends to terminate and the surface corrosion layer becomes thicker, leading to the decrease of capacity degradation rate.