Potential oscillations during the electrocatalytic oxidation of sulfide on a microstructured Ti/Ta2O5-IrO2 electrode

The electrooxidation of sulfide on a microstructured oxide electrode Ti/Ta2O5-IrO2 was studied using electrochemical methods such as cyclic voltammetry, differential capacity, galvanostatic measurements and electrochemical impedance spectroscopy (EIS). Sulfide and hydrosulfide can be oxidized to sulfur, polysulfides, and sulfate depending upon the electrode potential. Our surface analysis illustrates that the Ti/Ta2O5-IrO2 electrode prepared in this study has a "cracked mud" structure with oxide particles sitting on the top of the electrode surface which have a particle size of around 100 nm. For the first time, two distinct galvanostatic potential oscillations, named as Oscillation A and Oscillation B, respectively, are observed during the electrooxidation of sulfide on an oxide electrode. The features of the potential oscillations strongly depend on the applied current densities. Oscillation A, located in the low current region, has larger amplitudes and much smaller frequencies than Oscillation B, which occurs in the high current region. Our EIS studies show that both Oscillation A and Oscillation B can be classified into HNDR (hidden negative differential resistance) oscillators with oxygen evolution involved. Oscillation A is caused by the variation of the S2-/HS- surface concentration from the diffusion-limited depletion by oxidation and from the convection-induced replenishment by periodic oxygen evolution, while Oscillation B is due to the synergic effect of sulfur formation/removal and constant oxygen evolution.