N-Shaped Negative Differential Resistance in Nitrate Reduction Reactions on Cu Electrode
Authors: Kuge Terumasa , Yamada Yuri , Nakanishi Shuji , Mukouyama Yoshiharu 
Published: February 2020

Nitrate ions, NO3
-, cause environmental problems such as water pollution and health problems including cancer and diabetes. The removal of nitrate ions from water by electrochemical reactions has been researched for many years, because the electrochemical treatment, which does not require any chemicals, is an environmentally friendly method of removing the nitrate ions from water. Thus, the electrochemical reduction of NO3
- has been extensively studied as summarized in literatures [1, 2]. The studies have revealed that Cu is an excellent catalyst for the electrochemical reduction [2].
We have reported [3, 4] that the nitrate reduction on Cu exhibits three types of electrochemical oscillations, named oscillations I, II and III, both under potential and current controlled conditions and also that the oscillations are accompanied by the hydrogen evolution reaction (HER). As can be seen in Figure 1 (which shows the current (I) – potential (E) curves for a Cu-wire electrode), which oscillation appeared depended on the solution acidity: oscillation I appeared when the nitrate reduction occurred in acidic solutions (panels a and b), oscillation II appeared independently of the solution acidity (a, b, and d), and oscillation III appeared when the solution was basic (c and d).
It is well accepted that N-shaped negative differential resistance (N-NDR) plays an important role in most of electrochemical oscillations. This is because an N-NDR causes a positive feedback mechanism that is essential for the appearance of an electrochemical oscillation. The nitrate reduction was thought to be suprressed by adsorbed hydrogen atoms on Cu surface, i.e., by the reaction intermediates for the HER, and thus the N-NDR that induced oscillation I was ascribed to the suppression of the nitrate reduction [3]. On the other hand, although an N-NDR was observed at aroud -1.4 V for basic solutions (panel c), i.e., at potentials within the potential range of oscillation II (compare panel c with d), the factor causing the N-NDR was not identified.
Recently, in order to identify the factor, we studied the reduction of nitrite ions, NO2
-, in basic solutions. Similarly to the nitrate reduction, an N-NDR was observed under potential controlled conditions (panel e) and an oscillation appeared under current controlled conditions (f). The oscillation was identified as oscillation II because it had the typical feature of oscillation II: the electrode potential oscillated synchronousely with the behavior of hydrogen bubble evolution. Thus, the appearance of oscillation II was thought to be independent of the kind of reactive species, which led to the conclusion that the bubble evolution played an essential role in oscillation II. In this presentation, we will show how and why the bubble evolution induces the N-NDR that causes oscillation II, and also discuss the mechasnim of oscillation II.

REFERENCES

[1] C. Milhano and D. Pletcher, in Modern Aspects of Electrochemistry 45, R. E. White, Editor, p. 1, Springer, New York (2009).
[2] V. Rosca, M. Duca, M. T. de Groot and M. T. M. Koper, Chem. Rev. 109, 2209 (2009).
[3] Y. Mukouyama, S. Yamamoto, R. Nakazato, S. Nakanishi and H. Okamoto, ECS Trans., 50 (48) 61-70, (2013).
[4] Y. Mukouyama, S. Yamamoto, S. Nakanishi and H. Okamoto, ECS Trans., 58 (25) 85-97, (2014).

FIGURE CAPTION

Figure 1. The I – E curves for (a, b) 0.1 M H2SO4 + 0.1 M KNO3, (c, d) 0.2 M NaOH + 0.2 M KNO3, and (e, f) 0.2 M NaOH + 0.2 M NaNO2, measured (left) under potential controlled conditions and (right) under current controlled conditions.

Figure 1