Effect of mechanical grinding on the electrochemical hydrogen storage properties of Mg–Ni–Y alloy

Abstract

The nanocrystalline and amorphous Mg2Ni-type electrode alloys with a composition of Mg20 − xYxNi10 (x = 1 and 4) were fabricated by mechanical milling, and the effects of milling time on the structures and electrochemical hydrogen storage performances of the alloys were investigated in detail. Test results of X-ray diffraction (XRD), transmission electron microscopy (TEM), and scanning electron microscopy (SEM) reveal that a nanocrystalline structure can be obtained through mechanical milling, and the amorphization degree of the alloy visibly increases with milling time prolonging. When Y content is x = 1, the substitution of Y for Mg results in the formation of secondary YMgNi4 + YMg3 phases without altering the major phase Mg2Ni, but when Y content is x = 4, the major phase of the alloys changes into YMgNi4 + YMg3 phases. The capacity retaining rate of 20th cycle is reduced from 48.3 to 37.7 % for the x = 1 alloy and from 89.8 to 74.7 % for the x = 4 alloy by increasing milling time from 0 (as-cast is defined as milling time of 0 h) to 70 h. The x = 1 alloy milled for 30 h obtains the maximum discharge capacity of 418.5 mA h g−1, and for x = 4 alloy milled for 10 h, it is 341.5 mA h g−1. The effect of milling time on the electrochemical kinetics of the alloys is related to Y content. When Y content is x = 1, the high rate discharge ability (HRD), diffusion coefficient of hydrogen atom (D), and charge transfer rate all increase with milling time prolonging. However, for x = 4 alloy, the electrochemical kinetics is getting steadily worse with ball milling time increasing.