Electrochemical Behavior of Cobalt Hydroxide Used as Additive in the Nickel Hydroxide Electrode

Abstract

As an attempt to understand better how cobalt hydroxide additives improve the nickel electrode performance, the Co ( OH ) 2 / CoOOH redox system has been investigated through electrochemical cycling starting from a commercial Co ( OH ) 2 sample. A study of the influence of texture and morphology as well as cycling parameters was performed. For charge rates greater than C/5, relative to the amount of Co ( OH ) 2 , the electrochemical oxidation was found to be a solid‐state process. This process led to a nonstoichiometric Co x 4 + Co 1 − x 3 + OOH 1 − x phase having a mosaic texture with enhanced electronic conductivity due to the presence of Co 4 + ions. For lower charge rates (C/100), the reaction rate is slower, and Co 2 + can dissolve in the electrolyte, leading to a less conductive phase having a stoichiometric composition (CoOOH) and a monolithic texture. When present, the Co 4 + ions are reduced to Co 3 + , at 1.05 V while other reductions Co 3 + → Co 2 + and Co 2 + → Co ° take place at a lower potential, 0.67 and 0.0 V, respectively. These two reactions are both associated with a dissolution of Co(II) species, followed by a migration of cobalt toward the current collector, with the overall result being an electrode degradation. © 2000 The Electrochemical Society. All rights reserved.