Electrochemical Impregnation of Nickel Hydroxide: Flow‐Through vs. Stagnant Electrodes

Abstract

The electrochemical impregnation of nickel hydroxide in porous electrodes has been investigated both experimentally and theoretically. The loading level and plaque expansion were the most important parameters to be considered. The effects of flow, counterelectrode placement, and pH have been identified. A novel flow-through electrochemical impregnation is proposed in which the electrolyte is forced through the porous nickel plaque. The thickening of the plaque can be reduced while maintaining high loading capacity. A mathematical model for flow-through electrochemical impregnation is presented which describes the transport of the nitrate, nickel, and hydroxyl ions and the consecutive heterogeneous electrochemical reduction of nitrate and the homogeneous precipitation reaction of nickel hydroxide. The distributions of rate of precipitation and active material within the porous electrodes are obtained. Deviation of loading curve from Faraday's law, in the case of stagnant electrodes, is believed to be the dissolution of surface-deposited Ni(OH)2; while the main loss in loading, in the case of flow-through electrodes, is attributed to the hydrodynamic washout.Early in the research and development of nickel hydroxide electrodes, effort was focused on achieving high loading of active material into sintered plaques in order to increase the specific energy of battery systems. Higher specific energies were obtained by increasing the sinter porosity, varying the powder type, addition of lightweight inert filler material, and increasing the impregnation rate by raising the solution temperature to its boiling point. However, higher porosity results in a structurally weak material which cannot endure the stresses involved in impregnation and cycling. In addition, Seiger et al. (1) have shown that as the plaque becomes more porous, the tendency to blister and shed increases which leads to a decrease in utilization efficiency, i.e., the amount of active material that is electrochemically available during discharge (the measured capacity) divided by the amount of active material impregnated into the porous sintered plaque (the theoretical capacity). They found experimentally that the utilization efficiency depends linearly upon sinter porosity over a wide range of practical porosities used. Ford and Baer (2) have also shown that utilization efficiency begins to decrease markedly at a loading of 2.1 g/cm ~.In recent years the trend has been to somewhat sacrifice loading in order to achieve longer design life. Lim (3) found that increasing the loading level of the nickel electrode could result in a better initial performance, which is not necessarily an indication of good life performance. This is because several physical changes occur inside the porous structures during cycling, especially at high loading, and adversely shorten the cycle life. These structural changes can be related to the dimensional expansion of the electrode. One may therefore infer that there must exist an optimum loading level for a long nickel electrode's ...

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