Study of the contribution of cobalt additive to the behavior of the nickel oxy-hydroxide electrode by potentiodynamic techniques

Using stepwise potentiodynamic cycling with observations of the chronoamperometric responses, we demonstrated different behaviors for Co additives in nickel oxy-hydroxides electrodes (NOE), depending on their form: either co-precipitated or postadded as a coating. First, we show that for pure CO(OH)(2), oxidation occurs at 0.9 V directly in a solid-state process or through a dissolution re-crystallization process, depending on the oxidation rate. Then, we also show that Co(OH)(2) post-added in Ni(OH)(2) behaves as pure Co(OH)(2) whereas when Co is co-precipitated in the Ni(OH)(2) synthesis, it has no specific redox signature of its own but it improves the nickel oxy-hydroxide electrode chargeability-making it possible to clearly observe the various redox domains of the NOE around 1.3 V vs. Cd-Cd(OH)(2). Then, such a technique could be used as a probe to determine-depending on the potential (V,,) at which the CO(H) to CO(HI) oxidation occurs-whether cobalt was simply added, coated (V-ox = 0.9 V) or co-precipitated (V-ox = 1.3 V) into the nickel oxy-hydroxide electrode. Finally, it is shown that using appropriate cut-off potential upon reduction, one can follow possible changes in the Co additive situation upon long-term cycling of NOE. (C) 2002 Elsevier Science B.V. All rights reserved.