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

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 (R_ct) 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.