Effect of ball milling nanocrystallization on hydrogen storage performance of Mg95Ni2Pr3 hydrogen storage alloy

Ball milling nanocrystallization is considered to be an effective method to improve the hydrogen absorption and hydrogen release kinetics of Mg-based hydrogen storage alloys. In order to obtain amorphous and nanocrystalline structures, ball milling technology was used to treat as-cast Mg95Ni2Pr3 alloy. The effects of ball milling time (0 h, 5 h, 10 h, 15 h, 20 h) on the microstructure, hydrogen absorption and hydrogen release dynamics and thermodynamics of the alloy were studied. The microstructure of the alloy was observed by X-ray diffractometer, scanning electron microscope, and transmission electron microscope, and the results were analyzed. The hydrogen absorption and hydrogen release properties of the alloy were studied by PCT hydrogen storage tester. The hydrogen desorption activation energy of hydrogen storage alloy was calculated using the Arrhenius method. The results show that ball milling produces a large number of crystal defects, which gives the alloy a high surface activity and better hydrogen absorption and hydrogen release properties. With the prolongation of ball milling time, the organization of Mg95Ni2Pr3 alloy gradually evolved from polycrystalline to nanocrystalline and amorphous phases. With the increase of ball milling time, the content of nanocrystalline phases increases and then decreases. At 10 h of ball milling, the alloy was mainly composed of nanocrystalline phases. Further extending the ball milling time, the nanocrystalline content decreases, and the amorphous phase content increases. As the ball milling time reaches 20 h, the alloy has the most amorphous tissue, but the hydrogen storage capacity of the alloy is recovered with the cycle of hydrogen absorption and hydrogen release reaction because of the crystallization of the amorphous tissue of the alloy. The alloy with 10 h of ball milling has the best overall hydrogen storage performance. Under the conditions of 553 K and 3.5 MPa, the 4.499 wt.% H2 can be absorbed by alloys ball-milled for 10 h under complete activation conditions for 10 min. The enthalpy and entropy of hydrogen formation of hydrogen storage alloy with a ball milling time of 10 h are -75.60 kJ/mol and -134.60 J/(mol·k), respectively. In addition, the alloy milled for 10 h has the lowest hydrogen desorption activation energy of 59.66 kJ/mol, which is the reason why the alloy has the best hydrogen storage performance.