Microstructure and electrochemical performance of Zn-doped of Mg2Ni1-xZnx hydrogen storage alloys

Abstract

The microstructures, electrochemical, thermodynamic properties and desorption kinetics of as-cast Mg2Ni1-xZnx (x = 0, 0.08, 0.17, 0.25, 0.33, or 0.41) hydrogen storage alloys are investigated in this study. The X-ray diffraction (XRD) and scanning electron microscopy (SEM) results demonstrated that the Mg2Ni1-xZnx alloys are comprised of multiphase structure, thereinto, the diffraction peak of the major phase Mg2Ni is shifted to a small angle, and its unit cell volume is increased obviously. It was found that the maximum discharge capacity of the alloy electrode firstly increases and then decreases with the increase of the Zn content, and is 52.22 mAh/g when the x = 0.25. The PCT curves showed that the equilibrium hydrogen pressure of alloys is increased with increasing Zn content, which is mainly caused by the reducing of thermodynamic stability for the metal hydride by addition of Zn. Mg2Ni0.67Zn0.33 alloy has the lowest value of hydrogen desorption enthalpy (ΔH) and entropy (ΔS), which are 66.5 kJ/mol and 111.5 J/K/mol, respectively. It is also found that the addition of Zn in the Mg2Ni alloys significantly reduced their dehydrogenation activation energy (Ea). The Mg2Ni0.75Zn0.25 alloy has the lowest Ea value (17.01 kJ/mol), which is much lower than that (46.07 kJ/mol) of the free Zn Mg2Ni. This result is confirmed and in good agreement with the hydrogen diffusion rate (5.3068 cm2/s). However, the surface of the Zn-doped alloy is more easily corroded, leading to reduced capacity retention rate.