Potential Oscillations during Electroreduction of Iodate and Nitrate

Abstract

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 impedance spectroscopy [3], electrochemical quartz crystal microbalance, and in situ Raman spectroscopy [4], which will be discussed in this presentation. REFERENCES M. Orlik, Self-Organization in Electrochemical Systems I, Springer-Verlag Berlin Heidelberg, Berlin (2012). Z. Li, J. Cai and S. Zhou, J. Chem. Soc., Faraday Trans., 93, 3519 (1997). P. Strasser, M. Lübke, C. Eickes and M. Eiswirth, J. Electroanal. Chem., 462, 19 (1999). Z. L. Li, Z. J. Niu, T. H. Wu, H. D. Nie and X. M. Xiao, Electrochem. Commun., 5, 297 (2003). M. Orlik, Self-Organization in Electrochemical Systems II, Springer-Verlag Berlin Heidelberg, Berlin (2012). Y. Mukouyama, S. Yamamoto, R. Nakazato, S. Nakanishi and H. Okamoto, ECS Trans, 58, 61 (2013). Y. Mukouyama, S. Yamamoto, S. Nakanishi and H. Okamoto, ECS Trans, 58, 85 (2014). FIGURE CAPTION Figure 1 I–E curves obtained using (top) an Au-wire electrode, (middle) an Ag-wire electrode, and (bottom) a Cu-wire electrode, under (left) potential-controlled conditions at a scan rate of 10 mV s−1 and (right) current-controlled conditions at a scan rate of 0.05 mA s−1. The electrolyte: 0.1 M NaOH + 0.1 M KIO3. Figure 1