Oscillatory electro-oxidation of thiosulfate on gold

• Two instability regions of E-j curves were observed in the electro-oxidation reaction of thiosulfate on a gold surface.
• The system displays supercritical Hopf, period-doubling, homoclinic and bursting bifurcations, especially the period-doubling of mixed-mode oscillations.
• The oscillations in both regions have the character of hidden N-shaped negative differential resistance (HN-NDR).
• The main oxidation products were mapped as a function of the applied potential by Capillary Electrophoresis.

We report experimental results on the oscillatory electro-oxidation of thiosulfate on polycrystalline gold electrode in buffered media of pH 6.0. Importantly, we observed oscillations in the current density (potential) when the system was studied under linear sweep of potential (current density). The system displayed supercritical Hopf, period-doubling, homoclinic and bursting bifurcations at different applied currents and potentials. After presenting the dynamics under both potentiostatic and galvanostatic regimes, we characterized the system in terms of its electrical (ohmic drop compensation and electrochemical impedance spectroscopy) and chemical (Capillary Electrophoresis analysis) aspects. Ohmic resistance compensation and electrochemical impedance spectroscopy confirmed the hidden N-shaped negative differential resistance oscillatory character. Capillary electrophoresis analysis revealed the predominance of different oxidation products as a function of the applied potential. Finally, we suggest a tentative mechanism underlying the kinetic instabilities.

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