Effect of substitution of Mm for La on the electrochemical properties of the LaNi3.55Mn0.4Al0.3Co0.75 compound

In this paper, the electrochemical properties of the MmNi3.55Mn0.4Al0.3Co0.75 compound, used as negative electrode of Ni-MH accumulator, have been studied and compared to those of the LaNi3.55Mn0.4Al0.3Co0.75 electrode. Several methods, such as, the chronopotentiometry, the cyclic voltammetry (CV), the chronoamperometry and the electrochemical impedance spectroscopy (EIS) were applied to characterize these electrochemical properties. The discharge capacity of the MmNi3.55Mn0.4Al0.3Co0.75 compound increases to reach 270 mAh/g after 12 cycles and then decreases to 200 mAh/g after 20 cycles. However, the electrochemical discharge capacity of the LaNi3.55Mn0.4Al0.3Co0.75 compound reaches its maximum value of 335 mAh/g after 7 cycles and then decreases to 180 mAh/g after 50 cycles. The decrease of the discharge capacity for the two alloys was attributed to the corrosion and the deprecipitation of the alloys in the KOH aqueous solution. A good stability of the discharge capacity can be observed after a number of cycles bigger than 20 and 50, respectively, for the Mm and the La alloys. The charge transfer coefficient and the average hydrogen diffusion coefficient determined by the cyclic voltammetry are 0.35, 0.221, 5.86 × 10−10 and 2.1 × 10−10 cm2 s−1, respectively, for the MmNi3.55Mn0.4Al0.3Co0.75 and LaNi3.55Mn0.4Al0.3Co0.75 alloys. The obtained values of the hydrogen diffusion coefficient DH, at 10% of charge of MmNi3.55Mn0.4Al0.3Co0.75 and LaNi3.55Mn0.4Al0.3Co0.75 compounds are 6.64 × 10−10 and 2.7 × 10−11 cm2 s−1, respectively. The chronoamperometry method shows that the size of the particles participating in the electrochemical reaction is about 12 and 31 μm, respectively, for the MmNi3.55Mn0.4Al0.3Co0.75 and LaNi3.55Mn0.4Al0.3Co0.75 alloys.