Oscillatory Peroxodisulfate Reduction on Pt and Au Electrodes under High Ionic Strength Conditions, Caused by the Catalytic Effect of Adsorbed OH

Four electrochemical oscillations of different types (named oscillations α, β, γ, and δ), which are different from previously reported Frumkin-type oscillations in low ionic strength electrolytes, appear for peroxodisulfate (S 2 O 8 2- ) reduction on Pt and Au electrodes under high ionic strength conditions. Impedance measurements for oscillation γ that appears in a region of the most positive potentials have shown that it is classified as hidden negative differential resistance (hidden-NDR or HNDR) oscillators. Detailed electrochemical measurements have suggested that the NDR arises from a decrease (with a negative potential shift) in the surface coverage of adsorbed OH, which acts as a catalyst for the dissociative adsorption of S 2 O 8 2- (the first step of its reduction). The NDR is hidden by the desorption (with the negative potential shift) of adsorbed sulfate (SO 4 2- ) produced by the dissociative adsorption and reduction of S 2 O 8 2-, which works as a site-blocking agent. Mathematical simulation based on this model has reproduced the appearance of oscillation γ. The result supports our previously proposed concept of autocatalytic effect of adsorbed OH and catalytic effect of adsorbed halogen on the dissociative adsorption of H 2 O 2, showing the generality of the electrocatalytic effect of adsorbed OH and halogen.