Vibrating Zinc Electrodes in Ni/Zn Batteries

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 Zn electrodes in Ni/Zn batteries (Vibrocel) 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 Zn 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 Zn from alkaline zincate solution is related to ion/surface interactions. The observed morphologies of Zn deposition in a cell with vibrating electrodes are explained. The limiting current density at the vibrating Zn electrode is calculated. 48 ref.--AA