Electrochemical behavior of cobalt hydroxide used as additive in the nickel hydroxide electrode

As an attempt to understand better how cobalt hydroxide additives improve t he 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 a s cycling parameters was performed. For charge rates greater than C/5, rela tive 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 conductiv ity due to the presence of Co4+ ions. For lower charge rates (C/100), the r eaction rate is slower, and Co2+ can dissolve in the electrolyte, leading t o 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 plac e at a lower potential, 0.67 and 0.0 V, respectively. These two reactions a n both associated with a dissolution of Co(II) species, followed by a migra tion of cobalt toward the current collector, with the overall result being an electrode degradation (C) 2000 The Electrochemical Society. S0013-4651(9 9)07-010-X. All rights reserved.