Effects of Mg Partial Substitution for Pr on the Electrochemical Performances of Pr-Mg-Ni-based Alloys

The influences of Mg substitution for Pr on the phase structures, hydrogen desorption characteristics and electrochemical properties of Pr1-xMgxNi3.1Co0.3Al0.1 (x = 0.0, 0.1, 0.15, 0.17, 0.2, 0.25, 0.3) alloys have been investigated. Partial replacement of Mg for Pr promotes the formation of super-stacking phases, refines crystal grains and decreases cell volumes. When x <= 0.2, the alloys consist of a main Ce(2)Ni(7)type (Pr,Mg)(2)Ni-7 phase and a few CaCu5-type PrNi5 phase. As Mg content further increases, the alloys mainly contain Ce2Ni7-type (Pr,Mg)(2)Ni-7, PuNi3-type Pr2MgNi9 and CaCu5-type PrNi5 phases. The plateau pressure of hydrogen desorption is initially increased and then decreased by the substitution and the reversibility is enhanced. Electrochemical analysis shows that the cycling stability for the alloy with x = 0.15 similar to 0.17 reaches 87.8% after 100 charge/discharge cycles. Meanwhile the maximum discharge capacity is improved. Further study shows that Mg increases the ratio of super-stacking structures so that the discharge capacity is enhanced. Mg optimize P-eq by adjusting the cell volumes of the phases so as to ameliorate the electrochemical performances. Alloys with x = 0.15 similar to 0.17 shows the optimal electrochemical performances.