Mechanism of Potential Oscillation Induced by Na2so4 and K2so4 in H2O2-H2so4-Pt System
Electrochemical oscillations have been reported in a variety of systems, and most of them can be classified into an NDR (negative differential resistance) oscillator or an HNDR (hidden-NDR) oscillator. The former shows only current oscillations under potential controlled conditions. The latter, however, shows not only current oscillations but also potential oscillations under current controlled conditions.
The H2O2 reduction at Pt electrodes in H2SO4 solutions (H2O2 + 2H+ + 2e- → 2H2O) has a unique feature where various kinds of oscillations, named oscillations A, B, C, D, E, F and G, are observed, as we reported more than a decade ago. The mechanisms of the oscillations except for oscillation G have been investigated and the appearance of such a various types of oscillations has been explained by the following three kinds of NDRs:
1. The NDR induced by the formation of upd-H (oscillations A, B and D).
2. The NDR induced by the formation of adsorbed OH (oscillations C and E).
3. The NDR induced by the formation of adsorbed Br (oscillation F).
We call them NDR-H, NDR-OH and NDR-Br. We have also reported that another oscillation, named oscillation H, appears as a potential oscillation under current controlled conditions in the presence of Na2SO4 or K2SO4 in the H2O2-H2SO4-Pt electrochemical system . The potential range of oscillation H overlapped with the potential region of NDR-OH. Therefore, we supposed that oscillation H was induced by NDR-OH. In this work, we performed electrochemical impedance measurements in order to study the mechanism of oscillations H.
RESULTS and DISCUSSION
Figures 1a and 1b show the current (I) – potential (E) curves for a Pt-disc electrode in 0.10 M H2SO4 + 0.25 M H2O2 with or without Na2SO4, measured under current controlled conditions. The H2O2-reduction current started to flow at ca. 0.8 V. In the absence of Na2SO4, oscillation B appeared as a potential oscillation, which was accompanied by the hydrogen evolution reaction. However, in the presence of Na2SO4, oscillation B did not appear but oscillation H did in the potential range between ca. 0.1 and 0.6 V. Very similar behavior was observed by the addition of K2SO4. Figure 1c shows the Nyquist plot obtained at 0.46 V. It indicates that oscillation H was induced by NDR-OH because NDR-OH was observed at 0.46 V without Na2SO4. The plot also indicates that oscillation H can be classified into the HNDR-oscillator.
In the presence of the Na+ or K+ ions originating from the salts, the transport rate of H+ to the electrode surface by the electromigration decreases  and consequently the local pH at the electrode surface becomes basic . When the solution is basic, H2O2 dissociates to form HO2- ion (H2O2 ⇌ HO2- + H+) because H2O2 is a weak acid. During oscillation H, oxygen bubbles evolved from the electrode surface due to the presence of HO2-. Therefore, the appearance of oscillation H is probably attributed to an interaction between the oxygen evolution reaction (HO2- + OH- → O2 + H2O + 2e-) and NDR-OH.
This work is partially supported by the Research Institute for Science and Technology of Tokyo Denki University under Grants Q12E-02.
 Y. Mukouyama, M. Hasegawa, S. Yamamoto, S, Nakanishi and H. Okamoto, ECS Trans., 58 (2013) in press.