Phase structures and electrochemical properties of Ca0.4Mg0.6(Ni0.9Al0.05M0.05)2(M=Cu,Mn,Cr or Co) alloys

The phase structures, hydrogen storage and electrochemical properties of the Ca0.4Mg0.6(Ni0.9Al0.05M0.05)2 (M=CuM=Cu, Mn, Cr or Co) alloys were investigated. The Cu-substituted alloy contains a single C15 phase. The partial substitution of Al by Mn strongly decreases the abundance of the C15 phase and leads to the formation of the C36-type MgNi2 phase. The partial substitutions of Al by Cr or Co can form a secondary phase Ni3Al and a minor C14 phase CaMg2 besides the main C15 phase. Moreover, the C36 phase MgNi2 also appears in the Ca0.4Mg0.6 (Ni0.9Al0.05Co0.05)2 alloy. The hydrogen absorption capacities of the Ca0.4Mg0.6(Ni0.9Al0.05M0.05)2 (M=CuM=Cu, Cr or Co) alloys are 1.44, 1.34 and 1.36 wt%, respectively. The hydrogen desorption capacities are, respectively, 0.79, 0.64 and 0.82 wt%. The alloy electrodes do not need activation during electrochemical measurements. Although the discharge capacities of the alloys decrease to some extent, the cyclic stabilities can be improved because of the suitable substitutions.