Pergamon hr. J. Hydrogen Etqqy, Vol. 21,No. IO,pp. 887 890,1996 Copyright(K” 1996 International Association for Hydrogen Energy Elsevler Science Ltd PII: SO360-3199(96)00021--3 Printed in Great Britain. All rightsreserved 0360.-3199,,96 $15.00+0.00 THE CHARACTERISTICS OF THE NEGATIVE ELECTRODE OF A NICKEL- METAL HYDRIDE BATTERY MINGMING GENG and DEREK 0. NORTHWOOD Department of Mechanical and Materials Engineering, University of Windsor, Windsor, Ontario. Canada N9B 3P4 (Receiredfor puhlicution 16 February 1996) Abstract-The negative electrodes of an experimental sealed battery (AA size, 1Ah capacity) were made from a multicomponent alloy (Mm,, ,,Ti, osNi,85 Co, ,,Mn, ,SAI, 35) powder coated with 10wt% nickel, polytetrafluoroethylene (PTFE) and nickel powder. The Ohmic resistivity of the electrode increases with increasing PTFE content. The average discharge voltage of the battery decreases with increasing discharge current and increases marginally with increasing temperature. The overpotential of the negative electrode decreases with increasing temperature. Copyright C 1996International Association for Hydrogen Energy I. INTRODUCTION Hydrogen storage alloys, which were discovered about 20 years ago, have been widely studied for their appli- cation as energy storage media and battery electrodes. The hydriding alloys (metal hydrides: MH) are used as the negative electrode of a rechargeable battery, essen- tially replacing the cadmium electrode in the widely used Ni-Cd battery. The main driving force for replacing the cadmium is environmental, Cd being a relatively toxic material. And as a by-product in the production of zinc, there is a shortage of cadmium on the world market. There are, of course, other advantages of the Ni-MH battery over the Ni-Cd battery, including potentially higher energy density, higher charge/discharge rates, low temperature capability and an absence of a memory effect [I, 21. However, significant improvements are required in terms of longer cycle lifetime of the alloys, and in the cell construction for rapid charge/discharge, in order to develop a completely sealed cell. The capacity decay of LaNi,-based alloys upon charge/ discharge cycling has been ascribed to the decomposition and oxidation of the alloys thus forming La(OH), [3]. Microencapsulation of the alloy powder with various kinds of electrodeless coatings such as Cu, Ni-P, Ni-B and Ni-Pd has been shown to be effective in improving the cycle lifetime and in generating a high dischargeability [4-71. In a previous work the electrochemical charac- terization of Mm, ,Ti, ,Ni, ,Co, ,Mn, ,A& 3 alloy powder coated with 10 wt% palladium and nickel was reported [8] and it was found that the Ni-Pd-coated alloy powder had a higher rate of discharge than the uncoated alloy powder. In this paper, the manufacture of a Ni-MH battery is described m detail. The electrochemical characteristics of the MH electrode and, in particular, the high-rate charger/discharge capability of a Ni-coated Mm,,,Ti, 05 Ni, &o. ,,Mn, 15AI,,,, alloy powder for the battery were studied. 2. EXPERIMENTAL DETAILS The hydrogen storage alloy of nominal composition Mm, gsTi,,,Ni, &o,,,Mn,,.,,Al, 35 (where Mm denotes Misch metal, which comprised 43.1 wt% La, 3.5 wt% Ce, 13.3 wt% Pr and 38.9 wt% Nd) was prepared by induction melting and rapid cooling. The cast alloy was annealed at 900°C for 10 hours in vacuum. The crystal structure of the alloy was identified by X-ray diffraction using Co Ka radiation. The cast alloy was pulverized mechanically to a 40-50 pm particle size. The alloy pow- der was activated by immersing in 60 ml drn-’ hydro- chloric acid and 40 g dmmi stannous chloride aqueous solution for 5-7 min. at room temperature and then in 5 ml dmm3 hydrochloric acid and 0.3 g dm-3 palladium chloride aqueous solution for 4-6 min. at room tem- perature. The activated alloy powder was immersed in a chemical plating solution containing 25 g dmm3 nickel sulphate, 25-30 g dm-’ sodium hypophosphite, 10 g drnm3 sodium citrate and 30 g dm-’ ammonium chloride (35-5O”C, pH 9-10) aqueous solution to achieve micro- encapsulation of 10 wt% nickel. The composition of the negative electrode in our experiments was 100 g Ni-coated MH alloy powder, 10 g nickel powder and 3 wt% polytetrafluoroethylene (PTFE) dispersion (60%). The mixture of MH alloy pow- der, nickel powder and PTFE dispersion was covered 887