Enhanced electrochemical hydrogen storage properties of Mg2NiH4 by coating with nano-nickel

Abstract

In this paper, we succeed to modify the Mg2NiH4 with nano-nickel coating via mechanical milling, denoted by Mg2NiH4 – x nano-nickel (x = 0, 1, 2, 3). Effect of different nano-nickel coating amount on the structural and electrochemical properties of Mg2NiH4 has been investigated in detail. X-ray diffraction (XRD) analyses show that the crystalline Mg2NiH4 transforms into the nanocrystalline and amorphous state by mechanical milling with nano-nickel. High resolution transmission electron microscopy (HRTEM) and corresponding selected area electron diffraction (SAED) analyses confirm the existence of nanocrystalline and amorphous structure. Electrochemical measurements reveal that the maximum discharge capacity of Mg2NiH4 first increases to 896 mAh/g when x = 2 then decreases with increasing the nano-nickel coating amount. Furthermore, the positive shift of the corrosion potential demonstrates the improvement of anti-corrosion ability during charging/discharging cycles. High rate dischargeability (HRD) test shows that the kinetic property of Mg2NiH4 is obviously enhanced with increasing the nano-nickel coating amount. The exchange current density (I0) increases and the charge-transfer resistance (Rct) decreases with the increase of nano-nickel content, indicating that the charge-transfer rate on the electrode surface is increased. Meanwhile, the hydrogen diffusion ability inside the hydride bulk is also accelerated by nano-nickel coating.