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

Abstract

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 post-added 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 ox) at which the Co(II) to Co(III) 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.