Cyclic voltammetry and AC impedance of MmNi_3.55Co_0.75Mn_0.4Al_0.3 alloy single-particle electrode for rechargeable Ni/MH battery

Voltammetry and ac impedance techniques were run on a single particle of MmNi_3.55Co_0.75Mn_0.4Al_0.3 alloy electrode with the aim of refining the investigating of the kinetics of the hydrogen adsorption-absorption processes into mischmetal alloys. The experimental impedance spectra for the hydrogen adsorption/absorption are similar in shape and in accordance with the theoretical predictions. Data were analyzed using an equivalent circuit based on the knowledge of the different processes taking place in the metal hydride electrodes. The model fitted the experimental data remarkably well. The equivalent circuit considers the charge-transfer reaction, the hydrogen transition between the adsorbed and absorbed states, and the impedance of finite hydrogen diffusion into the metal hydride electrode particle with spherical symmetry. The values of the obtained diffusion coefficient of hydrogen in the solid phase were 8.7-9 × 10^-8 cm² s^-1 for the hydrogenation and dehydrogenation processes, respectively.