Improvement of the electrochemical hydrogen storage performance of Mg2Ni by the partial replacements of Mg by Al, Ti and Zr

Mg2Ni, Mg1.5Al0.5Ni, Mg1.5Zr0.5Ni, Mg1.5Ti0.5Ni, Mg1.5Zr0.25Al0.25Ni, Mg1.5Zr0.25Ti0.25Ni and Mg1.5Ti0.25Al0.25Ni alloys were synthesized by mechanical alloying and their electrochemical hydrogen storage characteristics were investigated. X-ray diffraction studies showed that while Al was retarding, Zr and Ti were facilitating the amorphization of Mg2Ni phase. The initial discharge capacities of Mg1.5Ti0.5Ni, Mg1.5Zr0.5Ni and Mg1.5Al0.5Ni alloys were 414, 322 and 166 mA h g−1, respectively. Although Mg1.5Al0.5Ni alloy had very low initial discharge capacity, the capacity retaining rate of this alloy was much better than those of Ti- and Zr-including alloys. The potentiodyanamic polarization experiments in 6 M KOH solution presented that Mg was passive and Ni was immune in the charge/discharge potential range (−1.0 VHg/HgO and −0.5 VHg/HgO). At the same conditions Ti and Zr had moderate, and Al had extremely higher dissolution rates. The analysis by the electrochemical impedance spectroscopy revealed that the increase in the charge transfer resistance of Mg1.5Al0.5Ni alloy was relatively low with the increase in depth of discharge. This observation was attributed to the formation of the porous unstable Mg(OH)2 layer due to the high rate dissolution of the disseminated Al2O3 and thus the exposition of the underlying electro-catalytically active Ni sites. The charge transfer resistance of Mg1.5Ti0.5Ni alloy increased sharply with the increase in depth of discharge possibly due to the stabilizing effect of Ti-oxide on Mg(OH)2. The presence of Ti-oxide, however, was predicted to make Mg(OH)2 barrier layer more penetrable by hydrogen atoms, since the increased stability of the surface layer the cyclic stability of Mg1.5Ti0.5Ni alloy was relatively satisfactory.