Mechanism of Current Oscillation during Simultaneous Reduction of Tin(II) Ions, Nitrate, and Nitrobenzene at Au Electrode
Authors: Kurohagi Masayuki , Mukouyama Yoshiharu , Yae Shinji 
Published: February 2020

It has been reported [1] that the interfacial energy, or the interfacial tension, at Au electrode surface in H2SO4 solution increases when Sn2+ 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 (namely, 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 work [2], we showed that one of the factors creating the imbalance was the occurrence of hydrogen evolution reaction near the rear side of the droplet.
When the reduction of Sn2+ was conducted in HNO3 solution, the NB 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 [3]. The flow, known as the Marangoni effect, was attributed to the simultaneous reduction of NO3
- and NB at the electrode. The nitrate reduction occurred on the Sn atoms deposited on the Au surface, whereas the NB reduction (C6H5NO2 + 6H+ + 6 e- → C6H5NH2 + 2H2O), i.e., the formation of aniline (AN), occurred in the vicinty of the NB droplet. Although the NB reduction was not the main current-producing reaction, it played an important role in the Marangoni effect because the AN produced by the reduction decreased the interfacial tension at the NB droplet/HNO3 solution interface.
Interestingly, the reduction current oscillated spontaneously during the simultaneous reduction of Sn2+, NO3
-, and NB. Figure 1b shows a waveform of the current oscillation which was observed when the electrolyte solution was 1.1 M HNO3 + 5 mM SnSO4. In general, most of electrochemical oscillations require the existence of an N-shaped negative differential resistance (N-NDR) characteristic. The mechanism of oscillation, or its oscillatory instability, can be explained by a combination of positive and negative feedback mechanisms. 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 present work, we study the mechanism of the current oscillation observed during the simultaneous reduction. An N-NDR characteristic was observed in a current (I) – potential (E) curve when the 1.1 M HNO3 + 5 mM SnSO4 solution contained 1mM NB (the blue curve in Figure 1c). We thus propose that the adsorption of NB (and/or AN) induced the N-NDR characteristic. The reduction current was mainly produced by the nitrate reduction, and hence we can suggest that the concentration of nitrate ions at the electrode surface is the slow process. This electrochemical system, which shows the droplet motions as well as the spontaneous current oscillation, is of considerable intereset 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. Ishibashia, Y. Fukuda, T. Kuge, Y. Yamada, S. Nakanishi, S. Yae, J. Electrochem. Soc., 165 (2018) H473-H480.

FIGURE CAPTION

Figure 1. (a) Schematic of Au film electrode. The film was produced on a Si wafer by electroless plating. A nitrobenzene (NB) droplet with a volume of 5 mL was put on the film surface before electrolysis. (b) Time course of current at -0.66 V vs. SHE. The working electrode was the Au film electrode. (c) Current (I) – potential (E) curves for an Au-wire electrode. The black curve was obtained in the absence of NB, whereas the blue curve, which shows an N-NDR characteristic, was obtained in the presence of NB.

Figure 1