ELECTROCHEMICAL STUDIES ON THE EFFECT OF COBALT SUBSTITUTION BY IRON IN LaNi3.55Mn0.4Al0.3Co0.75 ALLOYS

C. Khaldi1*, H. Mathlouthi1, J. Lamloumi1, A. Percheron-Guégan2

1L.M.M.P, E.S.S.T.T, 5 Avenue Taha Hussein 1008 Tunis, Tunisie

2L.C.M.T.R, G.L.V.T, C.N.R.S, 2-8 Rue Henri Dunant, 94320, Thiais Cedex, France

* Corresponding author. E-mail: chokri.khaldi@esstt.rnu.tn

Received: 07 February 2005; revised version accepted: 02 September 2005

Abstract

The effect of cobalt substitution by iron on the electrochemical hydriding and dehydriding behaviour of LaNi3.55Mn0.4Al0.3Co0.75 type hydrogen storage alloys in 1M aqueous KOH solution was investigated. The electrode characteristics of LaNi3.55Mn0.4Al0.3Co0.75-xFex (x = 0, 0.15, 0.55), such as the activation process, the discharge capacity and the cycle life, indicate that the partial substitution of cobalt in the LaNi3.55Mn0.4Al0.3Co0.75 alloy decreases the discharge capacity but improves the cycle life characteristics of this alloy. The electrochemical kinetics were evaluated using electrochemical impedance spectroscopy technique (EIS), and it was found that both the diffusion and the charge transfer of hydrogen play a role in the electrolytic absorption of hydrogen. The hydrogen diffusivity in the alloys and the exchange current density were estimated by the EIS and the constant potential discharge (CPD) techniques.

Keywords: Hydrogen absorbing materials, Metal hydrides, Electrode materials, Electrochemical reaction.

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