Mechanism of Potential Oscillation Induced by Na2so4 and K2so4 in H2O2-H2so4-Pt System

INTRODUCTION

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 [1]. 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 [2] and consequently the local pH at the electrode surface becomes basic [1]. 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.

REFERENCES

[1] Y. Mukouyama, M. Hasegawa, S. Yamamoto, S, Nakanishi and H. Okamoto, ECS Trans., 58 (2013) in press.

[2] Y. Mukouyama, M. Kikuchi and H. Okamoto, J. Electroanal. Chem., 617, 179 (2008).