Electrochemical Oscillations (Named Oscillations H and K) during H2O2 Reduction on Pt Electrodes Induced by a Local pH Increase at the Electrode Surface

Abstract

The reduction of H2O2 on Pt electrodes in H2SO4 solution shows various electrochemical oscillations, named sequentially from A to J, as reported previously. The present work demonstrates that this electrochemical system also shows a new type of oscillation, called oscillation K, when an alkali metal sulfate is added to the H2SO4 solution containing H2O2. In the presence of alkali metal ions, the local pH at the electrode surface (pHs) becomes basic during H2O2 reduction, and, consequently, reduction and oxidation reactions of HO2 −, which is formed by dissociation of H2O2, take place at the electrode. Detailed studies reveal that oscillation K is caused by an N-shaped negative differential resistance (N-NDR) that is induced by the autocatalytic reduction of HO2 − and that the oscillation can be classified as a “hidden” N-NDR (HN-NDR) type. In this sense, oscillation K is similar to oscillation H which is induced by the autocatalytic reduction of H2O2 and is an HN-NDR oscillator. Besides, a numerical simulation was conducted to understand how HO2 − reduction and oxidation cause oscillation K. To estimate the change in pHs, our simulation employs a newly developed model in which the dissociation of water and that of H2O2 at the electrode are considered.