Investigation on electrochemical performances of melt-spun nanocrystalline and amorphous Mg 2Ni 1− xMn x ( x = 0–0.4) electrode alloys

Abstract

In order to enhance the glass forming ability of the Mn 2Ni-type electrode alloy, Ni in the Mg 2Ni compound is partially substituted by Mn. 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) are fabricated 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 successfully obtained. The microstructures of the as-spun alloy ribbons are characterized by XRD, SEM and TEM. The electrochemical hydrogen storage characteristics of the as-spun alloy ribbons were tested by an automatic galvanostatic system. The electrochemical impedance spectra (EIS) are plotted by an electrochemical workstation (PARSTAT 2273). The hydrogen diffusion coefficients in the alloys are calculated by virtue of potential-step method. The results show that no amorphous structure is detected in the as-spun Mn-free alloy, whereas the as-spun alloys containing Mn display a nanocrystalline and amorphous structure. The amorphization degree of the alloy increases with rising spinning rate, suggesting that the substitution of Mn for Ni facilitates the glass formation in the Mg 2Ni-type alloy. The substitution of Mn for Ni markedly improves the electrochemical hydrogen storage performances of the Mg 2Ni-type alloy, enhancing both the discharge capacity and the electrochemical cycle stability. 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 increasing amount of Mn substitution.