It has been reported [1] that the interfacial energy (or the interfacial tension) at Au electrode surface in H₂SO₄ solution increases when Sn²⁺ ions are reduced (Sn²⁺ + 2e^- → Sn), i.e., when Sn is electrodeposited, on the electrode surface. The increase in the interfacial energy was attributed to the reaction intermediates of the electrodeposition (i.e., the Sn adatoms on the surface), and induced a lateral motion of a nitrobenzene (NB) droplet that was put on the electrode (see Figure 1a). The driving force inducing the droplet motion was an imbalance of the interfacial tension acting on the droplet, and the droplet spontaneously moved toward the area where the interfacial tension was relatively high. In our previous works [2,3], we showed that one of the factors creating the imbalance was the occurrence of hydrogen evolution reaction (HER) near the rear side of the droplet.

When the reduction of Sn²⁺ ions was conducted in HNO₃ solution, the droplet moved on the surface in a different manner, namely, it moved like an amoeba because a liquid flow was induced by a decrease in the interfacial tension at nitrobenzene-electrolyte interface [4]. The flow, known as the Marangoni effect, was attributed to the reductions of nitrate and NB at the electrode. Recently, we found that the reduction current oscillated spontaneously during the simultaneous reductions of Sn²⁺ ions, nitrate, and NB. The waveform of the current oscillation and the photo images of the Au electrode surface are shown in Figure 1. The electrolyte solution used was 1.1 M HNO₃ + 5 mM SnSO₄. The current oscillation was accompanied with a change in the amount of the deposited Sn on the surface (Figure 1c), indicating that the deposited Sn dissolved during the oscillation.

In general, most of electrochemical oscillations require the existence of an N-shaped negative differential resistance (N-NDR). The appearance of oscillation or the oscillatory instability can be explained by the combination of a positive feedback mechanism and a negative one. The positive one is atrributed to an N-NDR and the negative one involves a slow process such as the surface concentration of an electroactive species and the surface coverage of an adsorbed species. In this work, we study the condition for the appearnace of the current oscillation observed during the simultaneous reduction, and also study the origin of its oscillatory instability. This electrochemical system, which shows the droplet motions and the oscillation, is of considerable interset from the viewpoint of dynamic self-organization of molecular systems.

REFERENCES

[1] S. Nakanishi, T. Nagai, D. Ihara, Y. Nakato, Chemphyschem, 9 (2008) 2302-2304.

[2] Y. Mukouyama, T. Shiono, J. Electrochem. Soc., 163 (2016) H36-H41.

[3] Y. Mukouyama, Y. Ishibashi, Y. Fukuda, Y. Yamada, S. Nakanishi, S. Yae, ECS Trans., 80 (2017) 1433-1440.

[4] Y. Mukouyama, Y. Ishibashia, Y. Fukuda, T. Kuge, Y. Yamada, S. Nakanishi, S. Yae, J. Electrochem. Soc., submitted.