Mechanical alloying of Mg0.8-XTi0.2 and study the effect of adding (x = 0.2 wt%) transition metal like Sc, Zr, or Nb on their phase transitions, activation energy, and hydrogen storage properties

Abstract

Till now Mg-based alloys have attracted much attention due to the high storage capacity of hydrogen. An effort was made to evaluate the apparent activation energy and electrochemical behavior of transition metals such as scandium (Sc), zirconium (Zr), and niobium (Nb) alloyed with Mg–Ti. Mg_0.8Ti_0.2, Mg_0.6Ti_0.2Sc_0.2, Mg_0.6Ti_0.2Zr_0.2, and Mg_0.6Ti_0.2Nb_0.2 alloy powders were synthesized using high-energy ball milling. Ballmilled powders were subjected to structural and morphological characterization using X-ray diffraction and scanning electron microscopy respectively. A strong shift in the inter-planar spacing value of milled powders confirmed supersaturated solid solution of Ti and transition metals in Mg. The inter-planar spacing values before and after milling are found to be 0.24 and 0.21 nm, respectively. Mg_0.8Ti_0.2, Mg_0.6Ti_0.2Sc_0.2, and Mg_0.6Ti_0.2Zr_0.2 alloy powders result in the FCC phase while Mg_0.6Ti_0.2Nb_0.2 powders result in BCC phase, however, the entire powders have an amorphous background. SEM-EDS analysis of the milled powders confirmed the presence of Mg, Ti, Sc, Zr, and Nb elements with a small amount of oxygen. Selected area electron diffraction (SAED) pattern of Mg_0.8Ti_0.2 alloy powders exhibits a nanocrystalline nature owing to their polycrystalline ring pattern. Exothermic peak broadening increases after the substitution of Nb and Zr in Mg_0.8Ti_0.2 alloy powder, which exhibits a lower activation energy (188 kJ mol⁻¹) than others. In cyclic voltammetry, a drenched cathodic peak is observed for Mg_0.8Ti_0.2 at a potential around −0.83 V. In electrochemical impedance spectroscopy, the charge transfer resistance of Mg_0.6Ti_0.2Sc_0.2 is lower than that of Mg_0.6Ti_0.2Zr_0.2 and Mg_0.6Ti_0.2Nb_0.2 alloy but higher than Mg_0.8Ti_0.2 electrode materials, and charge–discharge studies were done on the developed electrode materials. It shows that Mg_0.8Ti_0.2 electrode material delivers a maximum discharge capacity of 535 mA h g⁻¹.