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The reduction of H2O2 on Pt electrodes in H2SO4 solution shows various electrochemical oscillations, named sequentially from A to J, as reported previously. The present work demonstrates that this electrochemical system also shows a new type of oscillation, called oscillation K, when an alkali metal sulfate is added to the H2SO4 solution containing H2O2. In the presence of alkali metal ions, the local pH at the electrode surface (pHs) becomes basic during H2O2 reduction, and, consequently, reduction and oxidation reactions of HO2 −, which is formed by dissociation of H2O2, take place at the electrode. Detailed studies reveal that oscillation K is caused by an N-shaped negative differential resistance (N-NDR) that is induced by the autocatalytic reduction of HO2 − and that the oscillation can be classified as a “hidden” N-NDR (HN-NDR) type. In this sense, oscillation K is similar to oscillation H which is induced by the autocatalytic reduction of H2O2 and is an HN-NDR oscillator. Besides, a numerical simulation was conducted to understand how HO2 − reduction and oxidation cause oscillation K. To estimate the change in pHs, our simulation employs a newly developed model in which the dissociation of water and that of H2O2 at the electrode are considered.

Electrochemical oscillations are attractive phenomena from the viewpoint of dynamic self-organization of molecular systems. In general, an N-shaped negative differential resistance (N-NDR) plays a crucial role in the appearance of oscillations because it gives rise to oscillatory instability [1]. Most of the oscillations can be classified into an N-NDR type or “hidden” N-NDR (HN-NDR) type oscillator. The former shows current oscillations under potential controlled conditions and hysteresis loops under current-controlled conditions, whereas the latter shows not only current oscillations but also potential oscillations. Thus, the electrochemical system that shows potential oscillations falls into an HN-NDR type oscillator. However, it is not always true because there are different types of oscillators that show potential oscillations, namely, strictly potentiostatic type, S-shaped NDR (S-NDR) type, coupled N-NDR (CN-NDR) type, and electrochemical reactions and diffusion–convection (ERDC) type oscillators.When iodate (IO3 -) is reduced on an Ag, Au, or Pt electrode in alkaline media, potential oscillations appear in conjunction with hydrogen evolution reaction [2-4]. When the potential of an electrode (E) is scanned at a fast rate, an N-shaped relationship between current (I) and E (i.e., a negative slope in an I-E curve) is observed in the potential range of the oscillations [3]. Thus, the oscillations seem to be attributable to N-NDR characteristics and are likely to be an HN-NDR type. It is highly possible that a Frumkin-type coulomb interaction between IO3 - ions within the diffusive double-layer plays an important role in the N-NDR characteristics, as suggested by Strasser [3]. On the other hand, another reasonable explanation for the potential oscillations was proposed by Li [2, 4]. The negative slope can be attributed to the limiting depletion of IO3 - ions and the oscillations are an ERDC type. That is, the alternative diffusion-limited depletion and convection-enhanced replenishment of the ions by hydrogen evolution causes the oscillations. It should be noted that details concerning these two interpretations are described in literature [1, 5].In this present work, to obtain greater insight into the mechanism of the oscillations, we study the potential oscillations using Au, Ag, and Cu electrodes (see Figure 1) and compare them with the oscillations that appear during the reduction of nitrate (NO3 -) ions. The NO3 - reduction on Ag and Cu electrodes shows two types of potential oscillations in the potential region of hydrogen evolution reaction (called oscillations II and III), as we have reported previously [6, 7]. Recent studies revealed that oscillations II and III can be explained in the framework of ERDC and HN-NDR types, respectively. By considering that the iodate system has common features as the nitrate system, it is reasonable that the iodate system also shows N-NDR characteristics in addition to ERDC. This idea is in line with all the previous studies including the results from electrochemical i...

Electrochemical oscillations are attractive phenomena from the viewpoint of dynamic self-organization of molecular systems. In general, an N-shaped negative differential resistance (N-NDR) plays a crucial role in the appearance of oscillations because it gives rise to oscillatory instability [1]. Most of the oscillations can be classified into an N-NDR type or “hidden” N-NDR (HN-NDR) type oscillator. The former shows current oscillations under potential controlled conditions and hysteresis loops under current-controlled conditions, whereas the latter shows not only current oscillations but also potential oscillations.Iron dissolves in aqueous HNO3 solutions, accompanied by hydrogen evolution reaction. This dissolution reaction is written as follows:Fe → Fe2+ + 2e- (1)2H+ + 2e- → H2. (2)On the other hand, the dissolution does not take place in concentrated HNO3 solutions (i.e., 13 M HNO3) because of the formation of a passivation film on the surface. However, when the concentration of HNO3 is approximately 10 M, the dissolution and passivation of iron occur alternatively. In other words, only when an iron wire is immersed in a high concentration of HNO3, the rest potential of the iron wire oscillates spontaneously. This phenomenon is of interest because its appearance does not require any external devices.In this work, to clarify the mechanism of the spontaneous oscillation, we observe the surface of an iron wire during the oscillation using a high-speed camera (Figure 1) and measure electrochemical impedance spectra. From these studies, we can say that the oscillation is an HN-NDR type, which will be discussed in the presentation. Reference M. Orlik, Self-Organization in Electrochemical Systems I, Springer-Verlag Berlin Heidelberg, Berlin (2012). FIGURE CAPTION Figure 1 Time course of rest potential of an iron electrode in 10 M nitric acid and snapshots of the electrode. Figure 1