Mechanism of Potential Oscillation (Named Oscillation K) Induced By pH Increase at Electrode Surface in “H2O2-H2so4-Pt Electrode” System

Abstract

Electrochemical oscillations are attractive phenomena from the viewpoint of dynamic self-organization of molecular systems. In general, an N-shaped negative differential resistance (N-NDR) plays a crucial role for the appearance of oscillations because it gives rise to an oscillatory instability. Most of the oscillations can be classified into an N-NDR type or “hidden” N-NDR (HN-NDR) type oscillator. The former shows current oscillations under potential controlled conditions and hysteresis loops under current controlled conditions, whereas the latter shows not only current oscillations but also potential oscillations. Thus, an electrochemical system that shows potential oscillations mostly falls into an HN-NDR type oscillator.Interestingly, the reduction of H2O2 at Pt electrodes in H2SO4 solution shows eleven types of electrochemical oscillations, named sequentially from A to K [1-3]. Oscillations H, I, J, and K appear when a salt such as Na2SO4 and K2SO4 is added to the solution. In the presence of the alkali metal ions originated from the salt, the transport rate of H+ to the electrode surface decreases because the electromigration of H+ is inhibited by the metal ions. Thus, the local pH at the electrode surface becomes strongly basic during the H2O2 reduction even though the electrolyte solution is strongly acidic (e.g., 0.1 M H2SO4). In the vicinity of the electrode surface, H2O2, which is a weak acid with a pK a of 11.7, dissociates to form HO2 - ion. The ion is electrochemically reduced to OH- or oxidized to O2, leading to the appearance of the oscillations.In this work, the extent of the local pH increase is estimated by using a numerical simulation and the mechanisms of oscillation K is studied based on the simulation. REFERENCES [1] Y. Mukouyama, H. Kawasaki, D. Hara, Y. Yamada, S. Nakanishi, J. Electrochem. Soc., 164 (2017) H1.[2] Y. Mukouyama, H. Kawasaki, D. Hara, Y. Yamada, S. Nakanishi, J. Electrochem. Soc., 164 (2017) H675.[3] R. Mizuochi, M. Kikuchi, Y. Yamada, Y. Mukouyama, S. Nakanishi, ECS Meeting s, MA2018-02, 2122 (2018). FIGURE CAPTION Figure 1 Current (j) - potential (U) curves for 0.1 M H2SO4 + 0.3 M H2O2 + x M Na2SO4 where x = (top) 0 and (bottom) 0.1, measured (left) under potential controlled conditions and (right) under current controlled ones. Figure 1