Electrochemical hydrogen storage characteristics of nanocrystalline and amorphous Mg 2Ni-type alloys prepared by melt-spinning

Abstract

The nanocrystalline and amorphous Mg 2Ni-type alloys with nominal compositions of Mg 2Ni 1- x Mn x ( x=0, 0.1, 0.2, 0.3, 0.4) were synthesized by melt-spinning technique. The spun alloy ribbons with a continuous length, a thickness of about 30 μm and a width of about 25 mm are obtained. The structures of the as-spun alloy ribbons were characterized by XRD and HRTEM. The electrochemical hydrogen storage characteristics of the as-spun alloy ribbons were measured by an automatic galvanostatic system. The electrochemical impedance spectrums (EIS) were plotted by an electrochemical workstation. The hydrogen diffusion coefficients ( D) in the alloys were calculated by virtue of potential-step measurement. The results show that all the as-spun ( x=0) alloys hold a typical nanocrystalline structure, whereas the as-spun ( x=0.4) alloy displays a nanocrystalline and amorphous structure, confirming that the substitution of Mn for Ni facilitates the glass formation in the Mg 2Ni-type alloy. The substitution of Mn for Ni significantly improves the electrochemical hydrogen storage performances of the alloys, involving the discharge capacity and the electrochemical cycle stability. With an increase in the amount of Mn substitution from 0 to 0.4, the discharge capacity of the as-spun (20 m/s) alloy increases from 96.5 to 265.3 mA·h/g, and its capacity retaining rate ( S 20) at the 20th cycle increases from 31.3% to 70.2%. Furthermore, the high rate dischargeability (HRD), electrochemical impedance spectrum and potential-step measurements all indicate that the electrochemical kinetics of the alloy electrodes first increases then decreases with raising the amount of Mn substitution.