Effect of substituting Ni with Cu on the cycle stability of La0.7Mg0.3Ni2.55−xCo0.45Cux (x = 0-0.4) electrode alloy prepared by casting and rapid quenching

In order to improve the cycle stability of the La-Mg-Ni system (PuNi3-type) hydrogen storage alloy, Ni in the alloy was partly substituted by Cu. Hydrogen storage alloys La0.7Mg0.3Ni2.55−xCo0.45Cux (x = 0, 0.1, 0.2, 0.3, 0.4) were prepared by casting and rapid quenching. The effects of substituting Ni with Cu and the quenching rate on the microstructures and the cycle stability of the as-cast and quenched alloys were investigated in detail. The results obtained by X-ray diffraction show that the as-cast and quenched alloys have a multiphase structure, including the (La, Mg)Ni3 phase, the LaNi5 phase and the LaNi2 phase, and the amount of the LaNi2 phase increased with the increase of the Cu content. The substitution and rapid quenching have an inappreciable influence on the phase compositions of the alloys, but both obviously changed the phase abundances of the alloys. The results derived by transmission electron microscopy confirm that the substitution of Cu for Ni is favourable for the formation of an amorphous phase in the as-quenched alloys. The results obtained by the electrochemical measurement indicate that substituting Ni with Cu improved the cycle stability. When the Cu content increases from 0 to 0.4, the cycle lives of the as-cast and rapidly solidified alloys increased from 72 cycles to 88 cycles and from 100 cycles to 122 cycles, respectively.