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A novel non-linear phenomenon, namely simultaneous electrochemical oscillations both on the cathode with Fe(CN) 6 3− reduction and on the anode with Fe(CN) 6 4− oxidation, is described. Complicated quasi-periodic behavior occurred in the simultaneous potential oscillations by the coupling of the simple period-one oscillations with two incommensurate oscillating frequencies, and complex periodic behavior appeared in the simultaneous current oscillations with the frequency locking.

We have discovered a new kind of current oscillation during the reduction or oxidation of some anions, which appeared over the limiting reduction or oxidation current of the anions and was loaded on the current of hydrogen or oxygen evolution. The overlap of a positive feedback (convection mass transfer of the anions induced by hydrogen or oxygen evolution, and the increase of the potential difference in the cell when the current is smaller) and a negative feedback (depletion of the surface concentration of the anions by reduction or oxidation, and the decrease of the potential difference in the cell when the current is larger) between the bistable states accounts for the oscillations, and a ‘hidden’ negative impedance in the second ascending branch of the polarization curve was observed for the first time. Our model has also shed some light on the other systems reported in the literature, in which convection mass transfer also plays an important role in the current oscillations. In addition, the potential oscillations during the oxidation of Fe(CN) 6 4− are reported here for the first time, and are similar to the potential oscillations during the reduction of Fe(CN) 6 3− reported in our previous paper [Z. Li, J. Cai, S. Zhou, J. Electroanal. Chem., in press].

Potential oscillations, which were observed during Fe(CN)63− reduction under galvanostatic conditions in 1 mol dm−3 NaOH solution on a platinum electrode, are described. The bifurcation boundaries were measured in the parameter plane of applied current vs. Fe(CN)63− concentration. A broad limiting current plateau (zero slope) occurring in the current-potential curve in combination with a negative real impedance is a typical characteristics for this kind of oscillation, and the oscillation amplitudes are within that region. The bistability, i.e., Fe(CN)63− reduction and hydrogen evolution at different potentials, coupled with convection feedback induced by hydrogen evolution, accounts for the oscillations. The sequence of chaotic-quasiperiodic-periodic-quasiperiodic oscillations, which appeared when the applied current was increased monotonously, was characterized by time series, power spectra, phase portraits and one-dimensional maps.