Electrochemical properties of the MmNi3.55Mn0.4Al0.3Co0.4Fe0.35 compound

In this paper, the electrochemical properties of the MmNi3.55Mn0.4Al0.3Co0.4Fe0.35 alloy used as a negative electrode in Ni-MH accumulators, have been investigated by different electrochemical methods such as cyclic voltammetry, chronopotentiometry, chronoamperometry and electrochemical impedance spectroscopy. The experimental results indicate that the discharge capacity reaches a maximum value of 260 mAh g−1 after 12 cycles and then decreases to about 200 mAh g−1 after 70 cycles. The value of the mean diffusion coefficient DH, determined by cyclic voltammetry, is about 3.44 × 10−9 cm2 s−1, whereas the charge transfer coefficient α, determined by the same method, is about 0.5 which allows us to conclude that the electrochemical reaction is reversible. The hydrogen diffusion coefficients in this compound, corresponding to 10 and 100% of the charge state, determined by electrochemical impedance spectroscopy, are, respectively, equal to 4.15 × 10−9 cm2 s−1 (α phase) and 2.15 × 10−9 cm2 s−1 (β phase). These values are higher, for the α phase and less, for the β phase, than the mean value determined by cyclic voltammetry. We assume that this is related to the number of interstitial sites susceptible to accept the hydrogen atom, which are more numerous in the α phase than in the β phase. The chronoamperometry shows that the average size of the particles involved in the electrochemical reaction is about 12 μm.