Mg2−xTixNi (x = 0, 0.5) alloys prepared by mechanical alloying for electrochemical hydrogen storage: Experiments and first-principles calculations

Abstract

Mg2−xTixNi (x = 0, 0.5) electrode alloys have been prepared by mechanical alloying (MA) under argon atmosphere at room temperature using a planetary high-energy ball mill. The microstructures of synthesized alloys are characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The effects of substitutional doping of Ti in Mg2Ni phase have been investigated by first-principles density functional theory calculations. XRD analysis results indicate that Ti substitution for Mg in Mg2Ni-type alloys results in the formation of TiNi (Pm-3m) and TiNi3 intermetallics. With the increase of milling time, the TiNi phase captures Ni from Mg2Ni to further form TiNi3 phase and the MgO phase increases. The calculated results of enthalpy of formation indicate that the most preferable site of Ti substitution in Mg2Ni lattice is Mg(6i) position and the stability of phase gradually decreases along the sequence TiNi3 phase > TiNi phase > Mg9Ti3Mg(6i)Ni6 Ti-doped phase > Mg2Ni phase. SEM observations show that the average particle sizes of Mg2Ni and Mg1.5Ti0.5Ni milled alloys decrease and increase, respectively with increasing the milling time. The TEM analysis results reveal that TiNi and Mg2Ni coexist as nanocrystallites in the Mg1.5Ti0.5Ni alloy milled for 20 h. Electrochemical measurements indicate that the maximum discharge capacities of Mg2Ni and Mg1.5Ti0.5Ni alloys rise and decline, respectively with the prolongation of milling time. The Mg1.5Ti0.5Ni alloy milled for 20 h shows the highest discharge capacity among all milled alloys. The capacity retaining rate of Mg1.5Ti0.5Ni milled alloys is better than that of Mg2Ni milled alloys.