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 commerical 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 Cox4+Co1-x3+OOH1-x phase having a mosaic texture with enhanced electronic conductivity due to the presence of Co4+ ions. For lower charge rates (C/100), the reaction rate is slower, and Co2+ can dissolve in the electrolyte, leading to a less conductive phase having a stoichiometric composition (CoOOH) and a monolithic texture. When present, the Co4+ ions are reduced to Co3+, at 1.05 V while other reductions Co3+ --> Co2+ and Co2+ --> Co degrees take place at a lower potential, 0.67 and 0.0 V, respectively. These two reactions an 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 (C) 2000 The Electrochemical Society. S0013-4651(99)07-010-X. All rights reserved.