Chaotic Oscillations in H2O2 - H2SO4 - Pt Electrochemical System

INTRODUCTION

Electrochemical oscillations are of interest from the viewpoint of dynamic self-organization of molecular systems. Extensive studies have revealed that an N-shaped negative differential resistance (N-NDR) plays an essential role in the majority of the oscillations. Simple periodic oscillations are explained by the interaction between a positive feedback loop due to an N-NDR characteristic and a negative one where a slow process is involved [1].

Electrochemical oscillations are also of interest from the viewpoint of nonlinear dynamics because they sometimes exhibit chaotic oscillations. Our group has reported that chaotic oscillations appear when two kinds of simple periodic oscillations, named oscillation A and oscillation α, occur simultaneously [2]. Oscillation A (or α) appeared during the reduction of H₂O₂ (or S₂O₈^2- dissociated from Na₂S₂O₈) at Pt electrodes in strong acidic solutions such as H₂SO₄ and HClO₄ electrolytes. We concluded that the chaotic oscillations were caused by the interplay among a positive feedback loop due to an N-NDR characteristic, a negative feedback loop where the surface concentration of H₂O₂ was involved, i.e., that for oscillation A, and a negative feedback loop where the surface concentration of S₂O₈²⁻was involved, i.e., that for oscillation α.

On the other hand, the H₂O₂ reduction at Pt electrodes in H₂SO₄ solutions was affected by the salts such as Na₂SO₄ and K₂SO₄ because the alkali metal ions, Na⁺ and K⁺, decreased the electromigration transport of H⁺ ions from the solution bulk to the electrode surface [3]. Recently, oscillation A was found to appear as a chaotic oscillation in the presence of Na₂SO₄, indicating that Na⁺ ions caused a negative feedback loop that induced the chaotic oscillation. In this presentation, the conditions for the appearance of the chaotic oscillation will be discussed. Although the chaotic oscillations due to the simultaneous occurrence of oscillations A and α were also observed in the solutions that contained Na⁺ ions, they were not induced by Na⁺ions, which will also be discussed.

RESULTS and DISCUSSION

Figure 1 shows the current (I) – potential (E) curves for a Pt-disc electrode. When the solution was 0.1 M H₂SO₄ + 0.2 M H₂O₂ (Fig. 1a), the H₂O₂ reduction occurred in the potential region below ca. 0.8 V, and oscillation A appeared at potentials around –0.075 V (vs. SHE) where an N-NDR characteristic appeared. The hydrogen evolution reaction occurred in the potential region below ca. –0.1 V. Oscillation A was simple and periodic as shown in the inset of Fig. 1a.

When the solution contained 0.2 M Na₂SO₄ (Fig. 1b), an aperiodic oscillation, which could be judged as a chaotic oscillation by plotting a next-minimum return map, appeared as shown in the inset. The current values of the higher turning points were almost constant, whereas those of the lower ones were irregular. This indicates that the negative feedback loop for oscillation A played a role in the higher turning points, and also that not only the loop but also another negative feedback loop which was induced by Na⁺ ions worked in the lower turning points. The chaotic oscillation appeared only when the concentration of Na₂SO₄ was approximately 0.2 M, and thus the negative feedback loop was induced only when the concentration of Na⁺was approximately 0.4 M.

When the solution contained 0.1 M Na₂S₂O₈ (Fig. 1c), the reduction of S₂O₈²⁻ occurred below ca. 0.5 V though the current due to the reduction was too small to affect the I – E curve, and a chaotic oscillation also appeared as shown in the inset [2]. The values of the higher turning points and those of the lower ones were irregular, indicating that two negative feedback loops played roles both in the higher turning points and the lower ones. The loop induced by Na⁺ ions did not work in higher ones, nor in the lower ones when the Na⁺ concentration was 0.2 M. We can thus say that the chaotic oscillation was not caused by Na⁺ions.

REFERENCES

[1] M. Orlik, Self-Organization in Electrochemical Systems I, Springer, Berlin (2012).

[2] Y. Mukouyama, H. Kawasaki, D. Hara, S. Nakanishi, J. Solid State Electrochem., DOI 10.1007/s10008-015-2813-z.

[3] Y. Mukouyama, M. Hasegawa, S, Nakanishi, ECS Trans., 64 (2014) accepted.

FIGURE CAPTION

Figure 1.   The I – E curves for a Pt-disc electrode in 0.1 M H₂SO₄ + 0.2 M H₂O₂ (a) without Na₂SO₄ nor Na₂S₂O₈, (b) with 0.2 M Na₂SO₄, and (c) with 0.1 M Na₂S₂O₈, measured under potential-controlled conditions. The insets in panels a, b, and c are the waveforms of oscillations observed at −0.065, −0.085, and −0.070 V, respectively.