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

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.