International Journal of Chemistry, Mathematics and Physics (IJCMP) [Vol-5, Issue-5, Sep-Oct, 2021] https://dx.doi.org/10.22161/ijcmp.5.5.1 ISSN: 2456-866X http://www.aipublications.com/ijcmp/ Page | 1 Determination of the hydrogen coefficient diffusion D H in the MmNi 3.55 Mn 0.4 Al 0.3 Co 0.75-x Fe x (0 ≤ x ≤0.75) electrodes alloys by cyclic voltammetry M. Ben Moussa 1,2,* , M. Abdellaoui 1, , J. Lamloumi 3 1 Physics Department, College of Applied Science, Umm AL-Qura University, Makkah, Saudi Arabia. 2 Institut National de Recherche et d’Analyse Physico-chimique, Pôle Technologique de Sidi Thabet, 2020 Sidi Thabet, Tunisie. 3 Laboratoire de Mécanique, Matériaux et Procédés, ESSTT, 5 Avenue Taha Hussein 1008, Tunisie. *Corresponding author Received: 10 Sep 2021; Received in revised form: 12 Oct 2021; Accepted: 20 Oct 2021; Available online: 31 Oct 2021 ©2020 The Author(s). Published by AI Publications. This is an open access article under the CC BY license (https://creativecommons.org/licenses/by/4.0/) Abstract— The hydrogen storage alloys MmNi3.55Mn0.4Al0.3Co0.75-xFex (0 ≤ x ≤0.75) were used as negative electrodes in the Ni-MH accumulators. The chronopotentiommetry and the cyclic voltammetry were applied to characterize the electrochemical properties of these alloys. The obtained results showed that the substitution of the cobalt atoms by iron atoms has a good effect on the life cycle of the electrode. The discharge capacity reaches its maximum in MmNi3.55Mn0.4Al0.3Co0.75-x Fex ( x = 0, 0.15, 0.35, 0.55 and 0.75) are, respectively, equal to 270, 266, 260, 210 and 200 mAh/g after 12 charge-discharge cycles. The diffusion behaviour of hydrogen in the negative electrodes made from these alloys was characterized by cyclic voltammetry after few activation cycles. The values of the hydrogen coefficient in MmNi3.55Mn0.4Al0.3Co0.75-x Fex ( x = 0, 0.15, 0.35, 0.55 and 0.75) are, respectively, equal to 5.86 10 -10 , 1.95 10 -9 , 3.44 10 -9 , 2.96 10 -9 and 4.98 10 -10 cm 2 s -1 . However, the values of the charge transfer coefficients are respectively equal to 0.35, 0.6, 0.5, 0.33 and 0.3. These results showed that the substitution of cobalt by iron decreases the reversibility and the kinetic of the electrochemical reaction in these alloys. Keywords— life cycle, discharge capacity, hydrogen diffusion coefficient. I. INTRODUCTION The AB5-type hydrogen storage alloys have been used as negative electrodes for Nickel-metal hydride (Ni- MH) batteries in order to replace the conventional Ni/Cd accumulators. In fact, the Ni-MH batteries, as compared to the Ni-Cd ones, exhibit a high energy density, a long life cycle, a stronger resistance to over charge-discharge and a good environmental compatibility [1-3]. The LaNi5-based alloy, is the most promising candidate because it can absorb and desorb a large amount of hydrogen in alkaline solution [4]. Neverthless, it exhibits a high equilibrium pressure of 1.7 bar at room temperature and its electrochemical capacity decreases rapidly when increasing the charge discharge cycles. This such latte phenomena was attributed to the oxidation and the formation of the La(OH)3 film at the surface [1]. To decrease the plateau pressure and inhibit the oxidation phenomena, some works have been carried out to replace the lanthanum by mischmetal (a mixture of different rare earth elements such as: Ce, Nd and Pr) and a part of nickel atoms with different transition elements such as Mn, Al and Co [3, 5-8]. Xianxia et al. [9] used the cyclic voltammetry to determine the hydrogen diffusion coefficient in the MmNi3.65Al0.2Mn0.4Co0.75 hydride. They [9] found that the value of the hydrogen diffusion coefficient increases with increasing the depth of discharge (DOD). They [9] assume that at low DOD, the hydrogen concentration is higher than that at high DOD. In other words, the number of vacant sites available for hydrogen occupation increases by increasing the DOD leading to an easier diffusion of the hydrogen. Qin Li et al. [10] investigated the hydriding properties of the MmNi5-xSnx (x = 0-0.5) aloys by electrochemical measurements and pressure-composition isotherms. They found that the