N-shaped Negative Differential Resistance Research Articles

Corrosion Potential Oscillation of Iron Electrodes in Nitric Acid

Although iron dissolves in diluted nitric acid solutions, it does not dissolve in concentrated solutions because of surface passivation. When a small amount of water is added to a concentrated solution, the dissolution and passivation of iron occur alternately, and the amount of gaseous products generated by nitric acid reduction varies in an oscillatory manner. During the corrosion process, the corrosion potential of iron oscillates spontaneously. In this study, we investigated the factors that cause oscillations in corrosion potential through electrochemical measurements using a three-electrode system and numerical simulations. The study revealed that an N-shaped negative differential resistance characteristic of iron oxidation plays a vital role in the oscillation of the corrosion potential and, simultaneously, the reduction of nitric acid results in oscillation. We considered metal corrosion has essential factors resulting in oscillatory instability. Thus, the corrosion potentials of various metals are expected to oscillate spontaneously under the appropriate conditions.

Potential Oscillations during Electroreduction of Iodate and Nitrate

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

Electrochemical Oscillation during Dissolution of Iron in High Concentration of Nitric Acid

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

Electrochemical Oscillations (Named Oscillations H and K) during H2O2 Reduction on Pt Electrodes Induced by a Local pH Increase at the Electrode Surface

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.

Influence of Alkali Metal Ions on Electrochemical Oscillations Via an Increase in the Electrode Surface pH

The local pH of an electrochemical interface (pHs) determines both the kinetics and thermodynamics of various electrochemical reactions in aqueous solutions, and might, therefore, have a strong impact on the activity and selectivity of such reactions. pHs is different from the bulk pH of the electrolyte solution (pHb) in electrochemical reactions that consume H+ or OH- ions. This difference is particularly pronounced when the electrolyte solution contains a salt like Na2SO4 or K2SO4, because the transport of H+ in the solution is suppressed, owing to the alkali metal ion (Na+ or K+) that originates from the salt. For example, pHs significantly increases during the hydrogen evolution reaction (HER) on various metal electrodes in 0.1 M H2SO4 solutions, when an appropriate amount of Na2SO4 or K2SO4 is added to these solutions, as reported by our previous studies [1, 2]. In this case, even though pHb is ~1, pHs increases up to 13 at high overpotentials [2]. pHs also increases significantly during the hydrogen peroxide reduction reaction (HPRR) on Pt electrodes in H2SO4 solutions containing Na2SO4 or K2SO4 [3].Salts like Na2SO4 and K2SO4 are often chosen as supporting electrolytes in electrochemical reactions because the ions constituting the salts, such as Na+ and SO4 2-, are nonreactive in aqueous solutions. Typically, such supporting electrolytes are used to reduce the solution resistance, which presumably increases the reaction efficiency. However, as mentioned above, salts with alkali metal ions have a strong impact especially on electrochemical reactions that consume H+. In other words, alkali metal ions suppress such electrochemical reactions.Interestingly, as will be shown in this presentation, alkali metal ions can lead to oscillatory instability [1, 3]. For example, HER shows a potential oscillation under current-controlled conditions (HER oscillation). Detailed studies using a high-pressure apparatus, which facilitates the study of the HER in a high overpotential region where hydrogen bubbles evolve vigorously, have revealed that the HER oscillation can be explained in a framework based on the electrochemical reactions and diffusion–convection (ERDC) mechanism [4]. HPRR exhibits four types of oscillations that are associated with the increase in pHs because the perhydroxyl ion formed at the electrode surface (H2O2 ⇌ HO2 - + H+, pK a = 11.7) electrochemically reduces to a hydroxyl ion or oxidizes to oxygen. Various studies including electrochemical impedance measurements have revealed that the four types of oscillations in HPRR can be explained by considering the N-shaped negative differential resistance (N-NDR) characteristics. REFERENCES [1] a) Y. Mukouyama, M. Kikuchi, H. Okamoto, J. Electroanal. Chem., 617, 179 (2008); b) Y. Mukouyama, R. Nakazato, T. Shiono, S. Nakanishi, H. Okamoto, J. Electroanal. Chem., 713, 39 (2014).[2] Y. Mukouyama, S. Nakanishi, Front. Energy Res., available online at doi: 10.3389/fenrg.2020.582284.[3] a) Y. Mukouyama, H. Kawasaki, D. Hara, Y. Yamada, S. Nakanishi, J. Electrochem. Soc., 164 (2017) H1; b) Y. Mukouyama, H. Kawasaki, D. Hara, Y. Yamada, S. Nakanishi, J. Electrochem. Soc., 164 (2017) H675; c) R. Mizuochi, M. Kikuchi, Y. Yamada, Y. Mukouyama, S. Nakanishi, ECS Meeting s, MA2018-02, 2122 (2018).[4] T. Kuge, T. Nishimoto, M. Kurohagi, K. Maeda, S. Yae, Y. Mukouyama, Electrochemistry, 88, 157 (2020).

Hidden N-Shaped Negative Differential Resistance Induced By Hydrogen Bubble Evolution during Reduction of Nitrate Ions on Cu Electrode

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

Mechanism of Potential Oscillation (Named Oscillation K) Induced By pH Increase at Electrode Surface in “H2O2-H2so4-Pt Electrode” System

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 for the appearance of oscillations because it gives rise to an oscillatory instability. 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, an electrochemical system that shows potential oscillations mostly falls into an HN-NDR type oscillator.Interestingly, the reduction of H2O2 at Pt electrodes in H2SO4 solution shows eleven types of electrochemical oscillations, named sequentially from A to K [1-3]. Oscillations H, I, J, and K appear when a salt such as Na2SO4 and K2SO4 is added to the solution. In the presence of the alkali metal ions originated from the salt, the transport rate of H+ to the electrode surface decreases because the electromigration of H+ is inhibited by the metal ions. Thus, the local pH at the electrode surface becomes strongly basic during the H2O2 reduction even though the electrolyte solution is strongly acidic (e.g., 0.1 M H2SO4). In the vicinity of the electrode surface, H2O2, which is a weak acid with a pK a of 11.7, dissociates to form HO2 - ion. The ion is electrochemically reduced to OH- or oxidized to O2, leading to the appearance of the oscillations.In this work, the extent of the local pH increase is estimated by using a numerical simulation and the mechanisms of oscillation K is studied based on the simulation. REFERENCES [1] Y. Mukouyama, H. Kawasaki, D. Hara, Y. Yamada, S. Nakanishi, J. Electrochem. Soc., 164 (2017) H1.[2] Y. Mukouyama, H. Kawasaki, D. Hara, Y. Yamada, S. Nakanishi, J. Electrochem. Soc., 164 (2017) H675.[3] R. Mizuochi, M. Kikuchi, Y. Yamada, Y. Mukouyama, S. Nakanishi, ECS Meeting s, MA2018-02, 2122 (2018). FIGURE CAPTION Figure 1 Current (j) - potential (U) curves for 0.1 M H2SO4 + 0.3 M H2O2 + x M Na2SO4 where x = (top) 0 and (bottom) 0.1, measured (left) under potential controlled conditions and (right) under current controlled ones. Figure 1

Potential Oscillation during Hydrogen Evolution Reaction: Hidden N-Shaped Negative Differential Resistance Due to Hydrogen Bubbles

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. Hydrogen evolution reaction (HER), which is the cathode reaction in water electrolysis, has been extensively studied using various kinds of electrodes because it is a fundamental electrochemical reaction and also because it is an important reaction for the production of H2. When the electrode potential is much negative than the onset potential of HER, a large number of hydrogen bubbles are formed on electrode surface. This often causes a disturbance in electrochemical reactions, leading to difficulties in electrochemical measurements, controls, and analysis. Thus, the HER has been mostly studied in a low overpotential region where no bubbles are formed or in an intermediate region where bubbles do not affect electrochemical measurements. We have reported [2] that the HER on Pt and Au in 0.10 M (M = mol/dm3) H2SO4 solution is anomalously affected by salts such as Na2SO4 and K2SO4 in a high overpotential region, e.g., more negative than -0.5 V vs. SHE, as shown in Figure 1a. The reduction current due to the HER became low (in absolute value) when Na2SO4 or K2SO4 was added to the solution. Interestingly, a potential oscillation, named HER oscillation, appeared under current controlled conditions (Figures 1b and 1c). As reported recently [3], the oscillation also appeared in the absence of the salts when the concentration of H2SO4 was as low as 0.03 M. The amount of hydrogen bubbles oscillated synchronously with the potential oscillation: the bubbles evolved more vigorously in the low potential region where the reduction of water occurred than in the high potential region where the reduction of H+ occurred. Therefore, we proposed that the bubble evolution played role in both the positive and negative feedback mechanisms, the combination of which caused an oscillatory instability. The proposed mechanism, however, cannot explain N-NDR characteristics though the oscillation seems to be an HN-NDR type oscillator because it appears as potential oscillations. In the present study, to clarify how the bubble evolution is related to N-NDR characteristics and an NDR-hiding factor, the bubble evolution is studied using Pt electrodes under atmospheric pressure (0.1 M Pa) conditions and also under high-pressure (up to 1 M Pa) conditions. Furthermore, the bubble behaviour is carefully observed by using a high-speed camera. In this presentation, we will show how hydrogen bubbles concurrently induce and hide the N-NDR characteristics. REFERENCES [1] M. Orlik, Self-Organization in Electrochemical Systems I, Springer, Berlin (2012). [2] Y. Mukouyama, M. Kikuchi, H. Okamoto, J. Electroanal. Chem., 617 (2008) 179. [3] Y. Mukouyama, R. Nakazato, T. Shiono, S. Nakanishi, H. Okamoto, J. Electroanal. Chem., 713 (2014) 39. FIGURE CAPTION Figure 1. Current (I) - potential (E) curves for a Pt electrode in 0.10 M H2SO4 solution with or without 0.05 M Na2SO4, measured (a) under potential controlled conditions and (b) under current controlled conditions. (c) Time course of E measured at I = -16 mA. Figure 1

Mechanism of Current Oscillation during Simultaneous Reduction of Tin(II) Ions, Nitrate, and Nitrobenzene at Au Electrode

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

Sulfate-induced electrochemical instability in the transpassive region during the electrooxidation of Na2S on Pt

The effects of chemisorbed sulfate on the spatiotemporal dynamics of Na2S in strongly alkaline solutions were investigated via electrochemical measurements and simultaneous imaging with a charge-coupled device (CCD) camera. Sulfate adsorption on platinum was found to generate positive feedback, promoting the electrooxidation of sulfide and introducing new hidden N-shaped negative differential resistance (HN-NDR) oscillations at approximately 2.3–2.6 V. In terms of patterns at the electrode/electrolyte interface, sulfate addition inhibited oxygen formation and promoted the emergence of rich patterns in the transpassive potential region due to synchronous sulfur deposition and dissolution, fronts, spirals, and multiarm spirals. A mechanism for HN-NDR oscillations induced by the competitive adsorption of sulfate and hydroxide was proposed, and the effects of sulfate adsorption on Pt oxides in a pure sodium hydroxide solution were elucidated: chemisorbed sulfate ions significantly inhibited the formation of high-valence Pt oxides and delayed the oxygen evolution potential by 0.55 V.

Current Oscillation during Simultaneous Reduction of Tin(II) Ions, Nitrate, and Nitrobenzene at Au Electrode

It has been reported [1] that the interfacial energy (or the interfacial tension) at Au electrode surface in H2SO4 solution increases when Sn2+ ions 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 (i.e., the 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 works [2,3], we showed that one of the factors creating the imbalance was the occurrence of hydrogen evolution reaction (HER) near the rear side of the droplet. When the reduction of Sn2+ ions was conducted in HNO3 solution, the 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 [4]. The flow, known as the Marangoni effect, was attributed to the reductions of nitrate and NB at the electrode. Recently, we found that the reduction current oscillated spontaneously during the simultaneous reductions of Sn2+ ions, nitrate, and NB. The waveform of the current oscillation and the photo images of the Au electrode surface are shown in Figure 1. The electrolyte solution used was 1.1 M HNO3 + 5 mM SnSO4. The current oscillation was accompanied with a change in the amount of the deposited Sn on the surface (Figure 1c), indicating that the deposited Sn dissolved during the oscillation. In general, most of electrochemical oscillations require the existence of an N-shaped negative differential resistance (N-NDR). The appearance of oscillation or the oscillatory instability can be explained by the combination of a positive feedback mechanism and a negative one. 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 work, we study the condition for the appearnace of the current oscillation observed during the simultaneous reduction, and also study the origin of its oscillatory instability. This electrochemical system, which shows the droplet motions and the oscillation, is of considerable interset 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. Ishibashi, Y. Fukuda, Y. Yamada, S. Nakanishi, S. Yae, ECS Trans., 80 (2017) 1433-1440. [4] Y. Mukouyama, Y. Ishibashia, Y. Fukuda, T. Kuge, Y. Yamada, S. Nakanishi, S. Yae, J. Electrochem. Soc., submitted. Figure 1

N-Shaped Negative Differential Resistance in the Oxidation of Methanol on Platinum in the Absence of Water

We have reported that the methanol oxidation on Pt electrode produces a large current in the absence of water (Electrochim. Acta, 136, 385 (2014)). The amount of adsorbed CO decreases at a high potential, leading us to postulate that adsorbed CO reacts with methanol to form methyl formate to produce vacant sites, where methanol is oxidized to form methyl formate or formaldehyde. The methanol oxidation exhibits potential oscillations in the absence of water. This indicates that there exists an N-shaped negative differential resistance (N-NDR), leading us to postulate that the adsorbed methanol acts both as a site blocker and as a reactant. In this present work, to verify these postulations, we study the methanol oxidation by using liquid chromatography and measuring electrochemical impedance spectroscopy. Furthermore, we suggest that the adsorption of methanol on Pt, which is the first elementary step of the oxidation, is responsible for the N-NDR.

N-Shaped Negative Differential Resistance in the Oxidation of Methanol on Platinum in the Absence of Water

The electrochemical oxidation of methanol on platinum has been extensively studied because it is one of the fundamental electrochemical reactions and also because it is an important reaction in fuel cell technology. At the anode of the direct methanol fuel cell, the oxidation reaction occurs in the presence of water: CH3OH + H2O → CO2 + 6H+ + 6e-. (1) It is well known that the oxidation reaction proceeds via a dual path mechanism consisting of indirect and direct paths. The indirect path involves adsorbed carbon monoxide (COad) as a poisoning intermediate, and the direct path involves a non-CO intermediate as a reactive one. It is also well known that the electrode potential spontaneously oscillates when the methanol oxidation is carried out under galvanostatic conditions (see Figure 1c). In general, an N-shaped negative differential resistance (N-NDR) plays an important role in an electrochemical oscillation. On the analogy of the oxidation of formic acid in aqueous solutions [1], it is most likely that the methanol oxidation is suppressed by the adsorbed water. In other words, an N-NDR, which is involved in the direct path of the methanol oxidation in aqueous solution, is attributed to the adsorption of water.Although water plays important roles not only in the N-NDR but also in removal of COad, little studies have been carried out with respect to the methanol oxidation in the absence of water. We then have studied the effect of water content on the methanol oxidation [2]. The study showed that the oxidation produced current without water, and also that the current was larger than that in the presence of water, e.g., 0.1 and 1 M methanol aqueous solutions (see left panels in Figure 1). The removal of COad was observed at high potentials, which was directly confirmed by surface-enhanced infrared absorption spectroscopy. Accordingly, we proposed that the CO desorption was via the reaction: COad + CH3OH → HCOOCH3, (2) and also that the current producing reactions was: CH3OH → HCHO + 2H+ + 2e- (3) or 2CH3OH → HCOOCH3 + 4H+ + 4e-. (4) Surprisingly, the potential oscillation occurred in the absence of water (see Figure 1d), clearly indicating that there existed an N-NDR that was not attributed to the water adsorption. In the presentation, we will provide evidence that reaction (4) preferentially occurs when water is absent and also will propose an NDR-inducing factor other than the adsorbed water. REFERENCES [1] T. Gojuki, Y. Numata, Y. Mukouyama, H. Okamoto, Electrochimica Acta, 129, 142-151 (2014). [2] H. Okamoto, T. Gojuki, N. Okano, T. Kuge, M. Morita, A. Maruyama, Y. Mukouyama, Electrochim . Acta, 136, 385-395 (2014). This work is partially supported by Research Institute for Science and Technology of Tokyo Denki University under Grant Q16K-02. FIGURE CAPTION Figure 1 (a and b) Voltammograms for the oxidation of methanol at a sweep rate of 0.1 V/s, and (c and d) time course of electrode potential measured under galvanostatic conditions. The concentration of methanol is (top) 0.1 and (bottom) 25 M. Figure 1

Porous Silicon NDR-Based High Power RF Oscillator Diode

A high power oscillation occurring on a porous silicon diode with an obvious dc N-shaped negative differential resistance (NDR) at room temperature is presented for the first time. The voltage oscillation swing was up to 12.4 V and oscillation frequency was 101.3 kHz, which belongs to the radio frequency range. The oscillation was based on a porous silicon superlattice diode structure, which was observed to provide a stable NDR with a high peak-to-valley current ratio as high as 7.9 for dc current–voltage characteristics at room temperature. A superlattice diode structure was formed by an electrochemical etching method on an n-type (100) silicon wafer with an alternating etching current. In addition, a dc constant current source was used as the power supply for the diode oscillation. In particular, two steady operating points were observed for the dc NDR current–voltage characteristics of the circuit load line. The system produced a steady oscillation between these two operating points in a harmonic form.

Appearance of New Oscillations (Named Oscillations I and J) during Reduction of H2O2 on Platinum Electrode

The reduction of H2O2 on Pt electrodes in H2SO4 solution shows various types of electrochemical oscillations, named sequentially from A to H, as has been reported previously. One of them, oscillation H, appears when Na2SO4 or K2SO4 is added to the acidic solution. In the presence of the cation dissociated from the salt, Na+ or K+, the pH at the electrode surface becomes basic during the H2O2 reduction, leading to the appearance of oscillation H. This present work demonstrates that new types of oscillations, named oscillations I and J, are also induced by the pH increase due to the cation. In contrast to oscillation H, oscillations I and J also appear when the H2O2 reduction occurs on Pt electrodes in NaOH solution. They are caused by an N-shaped negative differential resistance (N-NDR) induced by the formation of under-potential deposited H. Oscillation I appears in the potential region of the N-NDR, whereas oscillation J appears at potentials more negative than the N-NDR region, i.e., at potentials where hydrogen evolution reaction occurs. Detailed studies reveal that oscillations I and J are classified into an N-NDR and a coupled N-NDR (CN-NDR) oscillator, respectively.

N-Shaped Negative Differential Resistance in Nitrate Reduction Reactions on Cu Electrode

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

Potential Oscillations during Oxidation of Methanol on Pt Electrode in the Absence of Water

The electrochemical oxidation of methanol on Pt has been extensively studied in order to develop direct methanol fuel cells. On the anode of the fuel cells, the oxidation reaction occurs in the presence of water: CH3OH + H2O → CO2 + 6H+ + 6e-. (1) It is well accepted that the oxidation reaction proceeds via a dual path mechanism consisting of indirect and direct paths. The indirect path involves adsorbed carbon monoxide (COad) as a poisoning intermediate, and the direct path involves a non-CO intermediate as a reactive one. Although water, which adsorbs and produces hydroxide and oxide on Pt, plays an important role in oxidative removal of COad, little studies have been carried out with respect to the methanol oxidation in the absence of water. We, therefore, have studied if the methanol oxidation proceeds without water. As reported in our previous study [1], the oxidation produced current in the absence of water, and the current was larger than that in the presence of water, e.g., 0.1 and 1 M methanol aqueous solutions (see Figure 1 for 0.1 M solution). The ability of producing current for the methanol oxidation without water is due to desorption of COad, which has been directly confirmed by surface-enhanced infrared absorption spectroscopy. Accordingly, we have proposed that the CO desorption is via the reaction: COad + CH3OH → HCOOCH3, (2) and also that the current producing reactions is: CH3OH → HCHO + 2H+ + 2e- (3) or 2CH3OH → HCOOCH3 + 4H+ + 4e-. (4) It is well known that the electrode potential spontaneously oscillates when the methanol oxidation in aqueous solutions is performed under galvanostatic conditions. In general, an N-shaped negative differential resistance (N-NDR) plays an important role in an electrochemical oscillation, because it causes a positive feedback mechanism. On the analogy of an N-NDR that is observed for the oxidation of formic acid in aqueous solutions [2], it is most likely that an N-NDR, which is involved in the direct path of the methanol oxidation in aqueous solutions, is attributed to the adsorption of water on the electrode surface, that is, the methanol oxidation is suppressed by the water adsorption. Surprisingly, the potential oscillation has been found to occur in the absence of water [1]. This clearly indicates that there exists an N-NDR that is not attributed to the water adsorption; hence we have proposed that the adsorbed methanol acts as a site blocker at the same time as a reactant. In this work, to verify this, the methanol oxidation without water will be studied in detail. REFERENCES [1] H. Okamoto, T. Gojuki, N. Okano, T. Kuge, M. Morita, A. Maruyama, Y. Mukouyama, Electrochim . Acta, 136, 385-395 (2014). [2] T. Gojuki, Y. Numata, Y. Mukouyama, H. Okamoto, Electrochimica Acta, 129, 142-151 (2014). This work was partially supported by Research Institute for Science and Technology of Tokyo Denki University under Grant Q16K-02. FIGURE CAPTION Figure 1 (left) Voltammograms for the oxidation of methanol at a sweep rate of 0.1 V/s, and (right) waveforms of potential oscillations observed at (top) 0.3 and (bottom) 0.8 mA. The concentration of methanol is (top) 0.1 and (bottom) 25 M. Figure 1

Bifurcation Behavior in Oscillations during H2O2 Reduction at Pt Electrodes

Electrochemical oscillations are of interest from the viewpoint of dynamic self-organization of molecular systems. Extensive studies have revealed that N-shaped negative differential resistance (N-NDR) plays an essential role in the majority of the oscillations because an N-NDR induces a positive feedback mechanism. Simple periodic oscillations are explained by the combination of a positive feedback loop and a negative one, latter of which involves a slow process such as the surface concentration of an electroactive species and the surface coverage of an adsorbed species [1]. Among the electrochemical oscillations, the reduction of H2O2 at Pt electrodes in H2SO4 solutions (H2O2 + 2H+ + 2e- → 2H2O) has an especially unique feature because it exhibits ten kinds of oscillations, as we have reported before [2]. One of the oscillations, named oscillation A, appeared in the potential region where under-potential deposited H (upd-H), which suppressed the H2O2 reduction, was formed on Pt surface (see Figure 1a). Another oscillation, named oscillation D, appeared when a small amount of Br- ions, which adsorbed on Pt surface, was added to the solutions. The reduction of S2O8 2- at Pt electrodes in acidic solutions (S2O8 2- + 2e- → 2 SO4 2-) was also unique because it showed several kinds of oscillations. The formation of upd-H also suppressed the S2O8 2- reduction; hence one of the oscillations, named oscillation a, appeared in the potential region of upd-H [3]. The electrochemical oscillations are also of interest from the viewpoint of nonlinear dynamics because they often exhibit transitions among non-oscillatory state, periodic oscillations, quasi-periodic ones, mixed-mode ones, and chaotic ones via period-adding bifurcation, period-doubling bifurcation, and so on. We have been studying bifurcation behavior in the oscillations during the H2O2 and S2O8 2- reductions. The studies showed [3,4] that the complex oscillations such as period-n, mixed-mode, and chaotic oscillations were observed when oscillation A appeared simultaneously with oscillation a (or D) though each was period-1 oscillation (see Figure 1b for the chaotic oscillation induced by the simultaneous occurrence of oscillations A and a). Furthermore, the studies led us to the conclusion that the complex oscillations appeared when two kinds of simple periodic oscillations occurred at the same time, and also that they were caused by the interplay among a positive feedback loop and two negative feedback loops. Recently, we found [5] that oscillation A changed into the complex oscillations when inorganic salts such as Na2SO4 and K2SO4 are added to the solutions (see Figure 1c for the chaotic oscillation). The solutions originally contained (bi)sulfate ions dissociated from H2SO4, and hence we explained that alkali metal ions, i.e., Na+ and K+ ions, caused a negative feedback loop that induced the complex oscillations. This presentation will show the bifurcation behavior in detail and also provide a definitive factor for the appearance of the complex oscillations. REFERENCES [1] M. Orlik, Self-Organization in Electrochemical Systems I, Springer, Berlin, 2012. [2] Y. Mukouyama, H. Kawasaki, D. Hara, M. Kikuchi, Y. Yamada, S, Nakanishi, ECS Trans., 69 (39) 37-45, 2015. [3] Y. Mukouyama, H. Kawasaki, D. Hara, S. Nakanishi, J. Solid State Electrochem., 19, 3253-3263, 2015. [4] Y. Mukouyama, M. Kikuchi, H. Okamoto, J. Solid State Electrochem., 9, 290-295, 2005. [5] Y. Mukouyama, D. Hara, H. Kawasaki, M. Kikuchi, Y. Yamada, S, Nakanishi, ECS Trans., 69 (39) 47-57, 2015. FIGURE CAPTION Figure 1. The I – E curves for a Pt-disc electrode in 0.1 M H2SO4 + 0.2 M H2O2 (a) without Na2S2O8 nor Na2SO4, (b) with 0.1 M Na2S2O8, and (c) with 0.2 M Na2SO4, measured under potential-controlled conditions. The insets in panels a, b, and c are the waveforms of oscillations observed at −0.065, −0.070, and −0.085 V, respectively. Figure 1

Alkali Metal Ions Induced Electrochemical Oscillations in H2O2 - H2SO4 - Pt System

A new electrochemical oscillation, named oscillations J, has been found to appear during the H2O2 reduction on Pt in strongly acidic solutions. It appears both as current and potential oscillations when the salts such as Na2SO4 and K2SO4 are added to the acidic solutions containing H2O2. The appearance of oscillation J is attributed to the presence of alkali metal ions such as Na+ and K+ ions, which increases the local pH at the electrode surface from acidic to basic during H2O2 reduction. Furthermore, oscillation J also appears during H2O2 reduction in strongly basic solutions. Although the role of an N-shaped negative differential resistance (N-NDR) for oscillation J is not clarified yet, oscillation J is probably classified into a coupled N-NDR (CN-NDR) oscillator.

Electrochemical Oscillations during Reduction of Nitrate and Nitrite Ions at High Overpotential

INTRODUCTION The electrochemical reduction of nitrate ions, NO3 -, has been extensively studied concerning the mechanism of the nitrate reduction, the influence of electrolysis conditions, and the applications of the nitrate reduction, etc. [1, 2]. The studies have revealed that Cu is a catalyst for the electrochemical reduction [2]. Moreover, the nitrate reduction on Cu in acidic solutions was found to exhibit two types of electrochemical oscillations when it was accompanied by the hydrogen evolution reaction (HER) [3]. The oscillations appeared both under potential controlled conditions and under current controlled conditions. One of the oscillations, named oscillation I, appeared at a higher potential or a lower current (in absolute value) region where hydrogen bubbles due to the HER evolved slightly. Another oscillation, named oscillation II, appeared at a lower potential or a higher current region where hydrogen bubbles evolved vigorously (see Figs. 1a and 1b). An N-shaped negative differential resistance (N-NDR) plays an important role in most of the electrochemical oscillations, because an N-NDR characteristic causes a positive feedback mechanism. It was reported that the NO3 - reduction was suppressed by adsorbed hydrogen atoms on a Cu surface, i.e., by reaction intermediates for the HER. Therefore, the N-NDR that caused oscillation I was ascribed to the suppression of the NO3 -reduction, and the mechanism of oscillation I could be explained by the N-NDR [3]. On the other hand, the mechanism of oscillation II was unclear because the factor causing an N-NDR characteristic for oscillation II was not detected. Then, we have been investigating oscillation II, which yielded the following novel findings. Oscillation II also appeared during the nitrate reduction in non-acidic solutions (see Figs. 1c and 1d) [4].Recently, oscillation II was found to appear during the nitrite ions, NO2 -(see Figs. 1e and 1f).The hydrogen bubbles evolved synchronously with oscillation II [4]. In this presentation, the mechanism of oscillation II will be discussed on the basis of the findings. RESULTS and DISCUSSION Figure 1 shows the current (I) – potential (E) curves for a Cu-wire electrode. When the solution was 0.1 M H2SO2 + 0.1 M KNO3 (Figs. 1a and 1b), the NO3 - reduction occurred in the potential region below ca. 0.2 V, and the reduction of water occurred in the potential region below ca. -1.5 V. Oscillations I appeared as a current oscillation at around -0.6 V and as a potential oscillation in the potential range between -0.4 and -0.7 V, that is, it was accompanied by the HER due to the H+reduction. On the other hand, oscillation II appeared as a current oscillation in the potential region below ca. -1.7 V and as a potential oscillation which ranged lower than -1.8 V, that is, it was accompanied by the HER due to the water reduction. These results lead to the idea that oscillation I appeared when the local pH at the electrode surface was acidic whereas oscillation II did when it was basic. When the solution was non-acidic, e.g., 0.2 M KNO3 (Figs 1c and 1d) and 0.2 M NaOH + 0.2 M KNO3 (data not shown), oscillation I did not appear whereas oscillation II appeared accompanied by the HER due to the water reduction [4], which supported the above idea. Furthermore, an oscillation, which was thought to be oscillation II because it synchronized with the behavior of the hydrogen bubble evolution, appeared when the solution was 0.2 M NaNO2(Figs. 1e and 1f). This indicates that the appearance of oscillation II was independent of the kind of reactive species. Thus, it can be concluded that the bubble evolution probably played an essential role in the appearance of oscillation II. REFERENCES [1] C. Milhano and D. Pletcher, in Modern Aspects of Electrochsmistry45, 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 KNO3, and (e, f) 0.2 M NaNO2, measured (left) under potential controlled conditions and (right) under current controlled conditions. Figure 1