Alkali Metal Ions Induced Electrochemical Oscillations in H2O2 - H2SO4 - Pt System

INTRODUCTION

Electrochemical oscillations, which are attractive phenomena from the viewpoint of dynamic self-organization of molecular systems, have been reported in a variety of electrochemical reactions [1]. Among them, the H₂O₂ reduction at Pt electrodes in H₂SO₄ solutions (H₂O₂ + 2H⁺ + 2e^- → 2H₂O) has an especially unique feature where nine kinds of oscillations, named oscillations A, B, C, D, E, F, G, H, and I are observed [2-4]. Oscillation A appeared in the potential region of the formation of under-potential deposited H (upd-H) which suppressed the H₂O₂ reduction, whereas oscillation B appeared in the region of hydrogen evolution reaction (HER). Oscillations C and D appeared when a small amount of Cl^- or Br^- ions, which adsorbed on a Pt surface, were added to electrolysis solutions. Oscillation E appeared when a single-crystal Pt(111) electrode was used because H₂O₂ was autocatalytically reduced especially on the Pt(111) surface. Oscillations F and G appeared in the presence of a small amount of Br^- ions when a Pt(111) or Pt(100) electrode was used.

Oscillations H and I appeared when the cations such as Na⁺ and K⁺ were added to electrolysis solutions [2-4]. In the presence of the cations, the transport rate of H⁺ to the electrode surface by the electromigration decreased and consequently the local pH at the electrode surface became basic during the H₂O₂ reduction. In the vicinity of the surface where the pH was basic, H₂O₂ dissociated to form HO₂^- ion, which was electrochemically reduced to OH^- or oxidized to O₂. Oscillation H was tentatively caused by the interaction between the autocatalytic H₂O₂ reduction and the electrooxidation of HO₂^- [2, 3]. On the other hand, oscillation I was probably caused by the suppression of the reduction of HO₂^- due to the formation of upd-H [4]. Recently, a new oscillation, which was named oscillation J, was found to appear during the H₂O₂ reduction in the presence of the cations. In this presentation, the conditions for the appearance of oscillation J and its mechanism will 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 + 0.1 M H₂O₂ (Fig. 1a), the H₂O₂ reduction occurred in the potential region below ca. 0.8 V under potential controlled conditions. The reduction was suppressed, that is, an N-shaped negative differential resistance (N-NDR) appeared, at around ca. –0.1 V due the formation of upd-H. Oscillation A appeared as a current oscillation at the potentials of the N-NDR and the HER due to the H⁺ reduction occurred in the potential region below ca. –0.15 V. Oscillation A did not appeared under current controlled conditions (Fig. 1b). When the concentration of H₂O₂ was 0.3 M, oscillation B appeared both under potential controlled conditions and under current controlled ones (Figs. 1c and 1d).

The I – E curves were strongly affected by adding Na₂SO₄ or K₂SO₄ to the solution because the local pH at the electrode surface became basic during the H₂O₂ reduction in the presence of Na⁺ or K⁺ [2-4]. When the solution was 0.1 M H₂SO₄ + 0.3 M H₂O₂ + 0.1 M Na₂SO₄ (Figs. 1e and 1f), oscillations A and B did not appear. However, oscillation I appeared at around –0.8 V under potential controlled conditions. The HER due to the water reduction occurred in the potential region below ca. –1.0 V. When the H₂O₂ concentration was high, e.g., 0.9 M (Figs. 1g and h), oscillations I and H appeared under potential controlled conditions and under current controlled conditions, respectively. The new oscillation, oscillation J, appeared at around –1.15 V under potential controlled conditions and also appeared in the potential range between –0.4 and –1.5 V under current controlled conditions. Oscillation J synchronized with the behavior of hydrogen bubbles formed by the HER. Thus, the bubble evolution probably played a role in the mechanism of oscillation J.

REFERENCES

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

[2] Y. Mukouyama, M. Hasegawa, S. Yamamoto, S. Nakanishi, H. Okamoto, ECS Trans., 58 (25) 99-107, 2014.

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

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

FIGURE CAPTION

Figure 1.   The I – E curves for 0.1 M H₂SO₄ + x M H₂O₂ + y M Na₂SO₄, where (x, y) = (0.1, 0), (0.3, 0), (0.3, 0.1), and (0.9, 0.1) from top to bottom, measured (left) under current controlled conditions and (right) under current controlled conditions.