Hidden N-Shaped Negative Differential Resistance Induced By Hydrogen Bubble Evolution during Reduction of Nitrate Ions on Cu Electrode
Authors: Kuge Terumasa , Mukouyama Yoshiharu 
Published: December 2020

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 an 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 [2].
Electrochemical reduction reactions are accompanied with hydrogen bubble evolution when the electrode potential is much more negative than the onset potential of hydrogen evolution reaction (HER). The bubble evolution often causes a disturbance in the electrochemical measurements and analysis. Thus, electrochemical studies are mostly conducted in the potential region where the HER does not occur or where bubbles do not affect the electrochemical reactions. Probably for this reason, little has been studied on the electrochemical reduction of nitrate ions in the potential region of the HER, though the reduction has been extensively studied for years.
We recently found that three types of oscillations (named oscillations I, II, and III) occurred during the reduction on Cu electrode [3, 4]. Interestingly, as shown in Figure 1, they appeared as current and potential oscillations in the potential region of the HER. The appearance of oscillations I and III depended on the acidity of solutions: oscillation I appeared when the nitrate reduction occurred in acidic solutions (the left panels) whereas oscillation III did in basic ones (the right panels). On the other hand, oscillation II appeared irrespective of the acidity.
This work focuses on studying the mechanism of oscillation II. A high-speed video recording showed that hydrogen bubbles evolved synchronously with oscillation II. More importantly, electrochemical impedance measurements indicated that oscillation II was an HN-NDR type oscillator (the panels d1 and d2). These led to a clear conclusion that the bubble evolution induced positive and negative feedback effects during the nitrate reduction. Interestingly, a detailed study showed that the potential oscillation that was observed when the solution was acidic (e.g., oscillation II shown in b1 and c1) behaved like an ERCD type oscillator. In the presentation, we will discuss the difference between the HN-NDR and ERDC oscillators and explain the mechanism of oscillation II.

Figure 1