Electrochemical Oscillation during H2O2 Reduction in Basic Solutions

INTRODUCTION

Chemical and electrochemical oscillations have been studied extensively because they are attractive phenomena from the viewpoint of dynamic self-organization of molecular systems.

The H₂O₂ reduction at Pt electrodes in acidic 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. Oscillation A is observed in the potential region of formation of under-potential deposited H (upd-H), whereas oscillation B is observed in the region of hydrogen evolution reaction (HER) (see. Fig. 1b). Oscillations C and D are observed in the presence of a small amount of Cl^- or Br^-ions which adsorb on Pt surface. Oscillations E, F and G are observed when a single-crystal Pt(111) or Pt(100) electrodes are used.

We have also reported that another oscillation, named oscillation H, appears under current controlled conditions in the presence of Na₂SO₄ or K₂SO₄ in the H₂O₂-H₂SO₄-Pt electrochemical system [1]. 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 during the reduction of H₂O₂. When the solution is basic, H₂O₂ dissociates to form HO₂^- ion (H₂O₂ ⇌ HO₂^- + H⁺) because H₂O₂ is a weak acid. The presence of HO₂^-ion probably attributes to the appearance of oscillation H [1].

Recently, we have found another current oscillation appears during the reduction of H₂O₂when the solution is basic. In this work, we studied the conditions for the appearance of the oscillation and the mechanism of the oscillation itself.

RESULTS and DISCUSSION

Figure 1 shows the current (I) – potential (E) curves for a Pt-disc electrode in 0.14 M NaOH + 0.25 M H₂O₂ and 0.05 M H₂SO₄ + 0.25 M H₂O₂ with or without Na₂SO₄, measured under current controlled conditions. In the basic solution (Fig. 1a), the H₂O₂-reduction current started to flow at ca. 0.1 V vs. SHE. A current oscillation appeared at around -0.85 V. The HER occurred in the potential region below ca. -0.95 V.

In the acidic solution without Na₂SO₄ (Fig. 1b), oscillations A and B appeared. However, in the presence of Na₂SO₄ (Fig. 1c), both oscillations A and B did not appear. The current oscillation did at around -0.85 V and the HER occurred in the potential region below ca. -0.95 V, similarly to the basic solution. This was because the local pH at the electrode surface was basic in the presence of Na⁺ ions. The waveform of the oscillation was periodic, as shown in Figure 2. The oscillation period was around 0.1 s and it increased as the potential decreased. Very similar behavior was observed by the addition of K₂SO₄.

These results show that the current oscillation appeared during the H₂O₂ reduction when the solution was basic. That is, the current oscillation appeared during the electrochemical reduction of HO₂^- ion (HO₂^- + H₂O + 2e^- → 3OH^-). It appeared in the potential region where a negative differential resistance (NDR) was formed. Therefore, the NDR played an important role for the appearance of the oscillation.

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).