Mg_(2-x)Ti_xNi (x = 0, 0.5) alloys prepared by mechanical alloying for electrochemical hydrogen storage: Experiments and first-principles calculations

摘要

Mg_(2-x)Ti_xNi (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 Mg_2Ni phase have been investigated by first-principles density functional theory calculations. XRD analysis results indicate that Ti substitution for Mg in Mg_2Ni-type alloys results in the formation of TiNi (Pm-3m) and TiNi_3 intermetallics. With the increase of milling time, the TiNi phase captures Ni from Mg_2Ni to further form TiNi_3 phase and the MgO phase increases. The calculated results of enthalpy of formation indicate that the most preferable site of Ti substitution in Mg_2Ni lattice is Mg(6i) position and the stability of phase gradually decreases along the sequence TiNi_3 phase > TiNi phase > Mg_9Ti_3Mg(6i)Ni_6 Ti-doped phase > Mg_2Ni phase. SEM observations show that the average particle sizes of Mg_2Ni and Mg_(1.5)Ti_(0.5)Ni milled alloys decrease and increase, respectively with increasing the milling time. The TEM analysis results reveal that TiNi and Mg_2Ni coexist as nanocrystallites in the Mg_(1.5)Ti_(0.5)Ni alloy milled for 20 h. Electrochemical measurements indicate that the maximum discharge capacities of Mg_2Ni and Mg_(1.5)Ti_(0.5)Ni alloys rise and decline, respec tively with the prolongation of milling time. The Mg_(1.5)Ti_(0.5)Ni alloy milled for 20 h shows the highest discharge capacity among all milled alloys. The capacity retaining rate of Mg_(1.5)Ti_(0.5)Ni milled alloys is better than that of Mg_2Ni milled alloys.