Ines Sahli, Mouna Elghali, Marwa Ayari, Ouassim Ghodbane and Mohieddine Abdellaoui
The electrochemical behavior of LaZr2Cr4Ni5-based alloys applied as negative electrode materials for Ni–metal hydride (Ni–MH) batteries was investigated by cyclic voltammetry (CV), chronoamperometry and electrochemical impedance spectroscopy (EIS) techniques. The hydrogen diffusion coefficient, determined by CV, was equal to 1.28 × 10−8 cm2s−1 reflecting an appropriate electrochemical hydrogenation kinetic of the LaZr2Cr4Ni5-based compound. The evolution of the hydrogen diffusion coefficient was also investigated by EIS first at different state of charge (10% and 100%) and second as function of charge/discharge cycling. Upon the first cycle, the diffusion coefficient increases from 31.72 × 10-8 cm2s-1 to reach a maximum value of 13.14 × 10-6 cm2s-1 at the fifth cycle. A further cycling leads to a sharp decrease of the diffusion coefficient to 82.14 × 10-8 cm2s-1 after 30 cycles. The hydrogen diffusion coefficient values determined by electrochemical impedance spectroscopy after 50 charge-discharge cycles are equal to 4.41.10-8cm2s−1 for the α phase (10% state of charge) and 1.12 × 10-8 cm2 s−1 for the β phase (100% state of charge). As compared to the mean value determined by cyclic voltammetry, these values are higher for α phase and less for the β phase.
The exchange current densities of the electrodes were estimated as a function of the charge/discharge cycling by EIS. The charge transfer kinetic is faster at the beginning of cycling. The chronoamperometry measurements indicate that the size of the cluster of particles involved in the electrochemical reaction (the depth or the degree of the material impregnation by the electrolyte) decreases from 63 to 6.2 μm after 50 cycles.
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