Vibrating Zinc Electrodes in Ni/Zn Batteries

Abstract

In electrochemical processes where diffusion is rate controlling, the use of vibrating electrodes may result in an increase in the average mass transfer rate. The mechanism for the increase is that vibration changes the pattern of free convective flow developed during the process into a different flow pattern. Four models for the mass transfer enhancement are reviewed and modified. Vibrating zinc electrodes in Ni/Zn batteries (Vibrocel TM) are analyzed. The analyses include fluid dynamics, mass transfer, and electrochemical deposition phenomena. Three types of fluid flows are considered. The first, free-convective flow, is induced by the difference in density of the electrolyte near the electrode and in the bulk. The other two types, induced by the vibration of the zinc electrodes, are a periodic flow consisting of an oscillatory bulk flow and an oscillatory boundary flow, and a steady streaming near the upper and lower edges of the electrodes. The mass-transfer coefficients of zincate and hydroxide ions. are calculated. The morphology of electrochemical deposition of zinc from alkaline zincate solution is related to ion-surface interactions. The observed morphologies of zinc deposition in a cell with vibrating electrodes are explained. The limiting current density at the vibrating zinc electrode is calculated. Ni/Zn Cells with Vibrating ElectrodesTypically, Ni/Zn batteries have problems of poor cycle life caused by zinc electrode-related failures. Two major causes of failure are zinc dendrite growth and electrode shape change. During battery charging cycles, zinc dendrites form, grow, and eventually penetrate through the separator, thereby causing the formation of short circuits to the nickel hydroxide electrode. Shape change refers to the redistribution of active zinc material over the surface of the zinc electrode as a result of cell cycling. It has been experimentally demonstrated that shape change is caused by a combination of nonuniform current distribution (1) and convective flow driven by osmotic pumping of membrane separators (2). As a result of shape change, certain parts of the electrode receive a dense zinc deposit while other parts have no deposit. This reduces the effective surface area and capacity of the cell. Research efforts to minimize these two effects include the development of (i) low-resistance, high strength separators, (it) stable inorganic or organic additives to improve zinc deposition, and (iii) batteries with vibrating zinc electrodes.The vibrating electrode concept is based on a discovery made by yon Krusenstierna (3,4). He observed that if the zinc electrode was vibrated during charging in a small cell with an excess KOH electrolyte containing zincate, the problems of dendrite formation and shape change were effectively suppressed. Exide Management and Technology Company has implemented the concept of the vibrating zinc electrode in their Ni/Zn Vibrocel TM (5). Table I gives the design specification of a Vibrocel TM. The substrate of the Vibrocel TM zinc electrode is 32% perforated,...

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