Electrochemical Oscillations Research Articles

Electron Transfer Mechanism of Substituted Benzimidazoles: Dimer Switching, Oscillations, and Search for Singlet Fission Properties

Reduction and oxidation of 4,7-dimethyl-1,3-dimethoxybenzimidazolium cation (1) and the related dihydroxy analogue (2) were investigated by electrochemical and in situ spectroelectrochemical methods. Quantum chemical methods were applied to UV–vis spectra in native, reduced, and oxidized forms. Compounds were searched for possible formation of a cation radical and a dication radical suitable for the singlet fission effect. Indeed 1 yields by oxidation the target quinoidal structure. However, the reduction step for both compounds is coupled with very fast dimerization and prevents obtaining a stable target form. The complex mechanism of the reduction process yields electrochemical current oscillation. Estimation of the maximum Ljapunov exponent proves characteristics of the deterministic chaos.

Decoding Network Structure in On-Chip Integrated Flow Cells with Synchronization of Electrochemical Oscillators

The analysis of network interactions among dynamical units and the impact of the coupling on self-organized structures is a challenging task with implications in many biological and engineered systems. We explore the coupling topology that arises through the potential drops in a flow channel in a lab-on-chip device that accommodates chemical reactions on electrode arrays. The networks are revealed by analysis of the synchronization patterns with the use of an oscillatory chemical reaction (nickel electrodissolution) and are further confirmed by direct decoding using phase model analysis. In dual electrode configuration, a variety coupling schemes, (uni- or bidirectional positive or negative) were identified depending on the relative placement of the reference and counter electrodes (e.g., placed at the same or the opposite ends of the flow channel). With three electrodes, the network consists of a superposition of a localized (upstream) and global (all-to-all) coupling. With six electrodes, the unique, position dependent coupling topology resulted spatially organized partial synchronization such that there was a synchrony gradient along the quasi-one-dimensional spatial coordinate. The networked, electrode potential (current) spike generating electrochemical reactions hold potential for construction of an in-situ information processing unit to be used in electrochemical devices in sensors and batteries.

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.

Appearance of New Oscillation (Named Oscillation H) Induced by Na2SO4 and K2SO4 in Electroreduction of H2O2 on Platinum

The reduction of H2O2 on Pt electrodes in H2SO4 solutions shows a various types of electrochemical oscillations, named sequentially from A to G, as has been reported previously. This paper shows that salts such as Na2SO4 and K2SO4, which are often used as supporting electrolytes for electrochemical reactions, cause significant effects on the H2O2 reduction. A new potential oscillation, named oscillations H, appears in the H2O2-H2SO4-Pt system when Na2SO4 or K2SO4 is added to the solutions. Oscillation H is induced by the cations originating from the salts, Na+ and K+, though they do not react on Pt electrode. In the presence of Na+ or K+, the local pH at the electrode surface becomes basic, and consequently the oxidation of HO2−, formed via a dissociation of H2O2 (H2O2 → HO2− + H+), occurs on the electrode surface. Oxygen bubbles due to the HO2− oxidation evolve from the electrode synchronously with the potential oscillation. Detailed studies, including electrochemical impedance measurements and numerical simulation, reveal that oscillation H can be explained by an interplay of the HO2− oxidation and an autocatalytic effect involved in the H2O2 reduction.

Nonlinear Instabilities during the Electrochemical Oxidation of Hydroxymethanesulfinate

Electrochemical oxidation of hydroxymethanesulfinate on platinum and gold electrodes was investigated in this study. Scanning electron microscopy measurements show that the electro-oxidation process caused significant pitting of Au electrodes. The continuous dissolution of the Au surface is likely to have played an important role in the sustained simple and complex oscillatory behaviors in both current density and potential observed on Au surface. On a Pt surface, only transient oscillations were observed under potentiostatic conditions, whereas sustained potential oscillations were seen under galvanostatic conditions. Electrochemical impedance spectroscopy (EIS) results indicate that the oxidation of hydroxymethanesulfinate possesses a hidden negative differential resistance, suggesting that this electrochemical oscillator belongs to the HN-NDR class. Mass spectroscopy analysis of the reaction solution suggests the formation of thiosulfate (S2O32−), bisulfate (HSO4−) as well as sulfite (SO32−).

Effects of temperature and current density on the porous structure of InP

The electrochemical oscillations that occur during the etching of InP and the three-dimensional (3D) porous structure of the resulting material were studied by performing electrochemical etching in a 3.5 mol L−1 NaCl solution at different temperatures and current densities. The characteristics of the oscillations were modified by adjusting the electrochemical parameters. The relationship between the oscillations and the porous structure of the etched InP was investigated. In the experiments, porous InP with a gradient-index (GI) structure was formed at relatively low temperatures (10–22 °C) and current densities (100–300 mA cm−2). Furthermore, two types of incomplete pores observed in the bottom layer of the porous structure of InP were found to correspond to incomplete oscillations (one-quarter and one-half of an oscillation, respectively), which provided useful insight into how pore growth proceeds in the etched InP. Based on the results obtained in this work, the growth mechanism of porous InP is discussed in this article.

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

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

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

Restoring oscillatory behavior from amplitude death with anti-phase synchronization patterns in networks of electrochemical oscillations

The dynamical behavior of delay-coupled networks of electrochemical reactions is investigated to explore the formation of amplitude death (AD) and the synchronization states in a parameter region around the amplitude death region. It is shown that difference coupling with odd and even numbered ring and random networks can produce the AD phenomenon. Furthermore, this AD can be restored by changing the coupling type from difference to direct coupling. The restored oscillations tend to create synchronization patterns in which neighboring elements are in nearly anti-phase configuration. The ring networks produce frozen and rotating phase waves, while the random network exhibits a complex synchronization pattern with interwoven frozen and propagating phase waves. The experimental results are interpreted with a coupled Stuart-Landau oscillator model. The experimental and theoretical results reveal that AD behavior is a robust feature of delayed coupled networks of chemical units; if an oscillatory behavior is required again, even a small amount of direct coupling could be sufficient to restore the oscillations. The restored nearly anti-phase oscillatory patterns, which, to a certain extent, reflect the symmetry of the network, represent an effective means to overcome the AD phenomenon.

The effect of temperature on the coupled slow and fast dynamics of an electrochemical oscillator.

The coupling among disparate time-scales is ubiquitous in many chemical and biological systems. We have recently investigated the effect of fast and, long-term, slow dynamics in surface processes underlying some electrocatalytic reactions. Herein we report on the effect of temperature on the coupled slow and fast dynamics of a model system, namely the electro-oxidation of formic acid on platinum studied at five temperatures between 5 and 45 °C. The main result was a turning point found at 25 °C, which clearly defines two regions for the temperature dependency on the overall kinetics. In addition, the long-term evolution allowed us to compare reaction steps related to fast and slow evolutions. Results were discussed in terms of the key role of PtO species, which chemically couple slow and fast dynamics. In summary we were able to: (a) identify the competition between two reaction steps as responsible for the occurrence of two temperature domains; (b) compare the relative activation energies of these two steps; and (c) suggest the role of a given reaction step on the period-increasing set of reactions involved in the oscillatory dynamics. The introduced methodology could be applied to other systems to uncover the temperature dependence of complex chemical networks.

The emergence of synchrony behavior in weakly coupled electrochemical oscillators via a ‘metallic plate’

We have made an effort to investigate thoroughly the coupled behavior of a spatio-temporal system of two limit cycle electrochemical oscillators (Fe/H2SO4) in the absence and presence of a metallic plate (using copper and platinum). The oscillators exchange information via the common electrolyte solution and the magnitude of information exchange varies with the distance (d) between the oscillators, i.e. the lesser the d the more will be the coupling strength and vice-versa. The presence of metallic plate between the working electrodes (Fe) enhances the magnitude of potential distribution in the solution which leads to the increase in entrainment area, i.e. the product of distance between the oscillators d and their parameter mismatch. To explain the experimental findings numerically, the pulse coupling mechanism based on the delta function (Kronecker) has been introduced.

Complex Rotating Waves and Long Transients in a Ring Network of Electrochemical Oscillators with Sparse Random Cross-Connections.

We perform experiments and phase model simulations with a ring network of oscillatory electrochemical reactions to explore the effect of random connections and nonisochronicity of the interactions on the pattern formation. A few additional links facilitate the emergence of the fully synchronized state. With larger nonisochronicity, complex rotating waves or persistent irregular phase dynamics can derail the convergence to global synchronization. The observed long transients of irregular phase dynamics exemplify the possibility of a sudden onset of hypersynchronous behavior without any external stimulus or network reorganization.

SYNCHRONIZATION OF HETEROGENEOUS OSCILLATORS UNDER NETWORK MODIFICATIONS: PERTURBATION AND OPTIMIZATION OF THE SYNCHRONY ALIGNMENT FUNCTION.

Synchronization is central to many complex systems in engineering physics (e.g., the power-grid, Josephson junction circuits, and electro-chemical oscillators) and biology (e.g., neuronal, circadian, and cardiac rhythms). Despite these widespread applications-for which proper functionality depends sensitively on the extent of synchronization-there remains a lack of understanding for how systems can best evolve and adapt to enhance or inhibit synchronization. We study how network modifications affect the synchronization properties of network-coupled dynamical systems that have heterogeneous node dynamics (e.g., phase oscillators with non-identical frequencies), which is often the case for real-world systems. Our approach relies on a synchrony alignment function (SAF) that quantifies the interplay between heterogeneity of the network and of the oscillators and provides an objective measure for a system's ability to synchronize. We conduct a spectral perturbation analysis of the SAF for structural network modifications including the addition and removal of edges, which subsequently ranks the edges according to their importance to synchronization. Based on this analysis, we develop gradient-descent algorithms to efficiently solve optimization problems that aim to maximize phase synchronization via network modifications. We support these and other results with numerical experiments.

Electrochemical Oscillation during Electro-Synthesis of KMnO4 under Highly-Alkaline Condition

Regular electrochemical oscillation (EO) was discovered for the first time on Fe anode under highly-alkaline condition in a simulation process with the industrial electro-synthesis of KMnO4 from K2MnO4. The amplitude of the current oscillation reaches up to 46% of the average current, which is response for up to ∼30% of the total power consumption. Different from traditional EO on Fe anode under acidic condition, it is found to be caused by periodic alternation of soluble hydroxide complex and hydroxide precipitation on the electrode surface. It only occurs at very high [OH–], when ferric hydroxide can be further converted to soluble hydroxide complex. Unlike in conventional weakly alkaline and neutral conditions, MnO42–, MnO4– and H2O2 will form a complicated dynamic loop under the strongly alkaline condition. Related researches would help to provide new ideas for energy-saving and emission-reduction electrochemical engineering process.

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.

Current and potential oscillations during the electro-oxidation of bromide ions

Electrochemical oxidation of bromide ions was investigated in aqueous solution, in which sustained oscillations in both current and potential were observed. Under galvanostatic conditions the electrode potential only oscillates within two narrow regions around 0.9V and 1.6V, respectively. Sustained current oscillations at the high potentials applied were accompanied with bubble formation and are often irregular, implicating that their development may be attributed with convection-driven processes. At the low potentials, the current oscillations exhibit several features associated with a capacitance mediated electrochemical oscillator, such as (1) taking place around a current plateau in a cyclic voltammogram, (2) occurring without the need of an external resistor, (3) showing a decreasing amplitude with the increase of external resistance, and (4) emerging on a positive differential resistance branch. Admittance spectra at a potential close to the bifurcation point also exhibit resonance behavior. The reversible formation and dissolution of tribromide complexes (Br3−) from the reaction between bromide (Br−) and its electro-oxidation product bromine (Br2) play a key role in this system.

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

Chaotic Oscillations in H2O2 - H2so4 - Pt Electrochemical System

INTRODUCTION Electrochemical oscillations are of interest from the viewpoint of dynamic self-organization of molecular systems. Extensive studies have revealed that an N-shaped negative differential resistance (N-NDR) plays an essential role in the majority of the oscillations. Simple periodic oscillations are explained by the interaction between a positive feedback loop due to an N-NDR characteristic and a negative one where a slow process is involved [1]. Electrochemical oscillations are also of interest from the viewpoint of nonlinear dynamics because they sometimes exhibit chaotic oscillations. Our group has reported that chaotic oscillations appear when two kinds of simple periodic oscillations, named oscillation A and oscillation α, occur simultaneously [2]. Oscillation A (or α) appeared during the reduction of H2O2 (or S2O8 2- dissociated from Na2S2O8) at Pt electrodes in strong acidic solutions such as H2SO4 and HClO4 electrolytes. We concluded that the chaotic oscillations were caused by the interplay among a positive feedback loop due to an N-NDR characteristic, a negative feedback loop where the surface concentration of H2O2 was involved, i.e., that for oscillation A, and a negative feedback loop where the surface concentration of S2O8 2−was involved, i.e., that for oscillation α. On the other hand, the H2O2 reduction at Pt electrodes in H2SO4 solutions was affected by the salts such as Na2SO4 and K2SO4 because the alkali metal ions, Na+ and K+, decreased the electromigration transport of H+ ions from the solution bulk to the electrode surface [3]. Recently, oscillation A was found to appear as a chaotic oscillation in the presence of Na2SO4, indicating that Na+ ions caused a negative feedback loop that induced the chaotic oscillation. In this presentation, the conditions for the appearance of the chaotic oscillation will be discussed. Although the chaotic oscillations due to the simultaneous occurrence of oscillations A and α were also observed in the solutions that contained Na+ ions, they were not induced by Na+ions, which will also be discussed. RESULTS and DISCUSSION Figure 1 shows the current (I) – potential (E) curves for a Pt-disc electrode. When the solution was 0.1 M H2SO4 + 0.2 M H2O2 (Fig. 1a), the H2O2 reduction occurred in the potential region below ca. 0.8 V, and oscillation A appeared at potentials around –0.075 V (vs. SHE) where an N-NDR characteristic appeared. The hydrogen evolution reaction occurred in the potential region below ca. –0.1 V. Oscillation A was simple and periodic as shown in the inset of Fig. 1a. When the solution contained 0.2 M Na2SO4 (Fig. 1b), an aperiodic oscillation, which could be judged as a chaotic oscillation by plotting a next-minimum return map, appeared as shown in the inset. The current values of the higher turning points were almost constant, whereas those of the lower ones were irregular. This indicates that the negative feedback loop for oscillation A played a role in the higher turning points, and also that not only the loop but also another negative feedback loop which was induced by Na+ ions worked in the lower turning points. The chaotic oscillation appeared only when the concentration of Na2SO4 was approximately 0.2 M, and thus the negative feedback loop was induced only when the concentration of Na+was approximately 0.4 M. When the solution contained 0.1 M Na2S2O8 (Fig. 1c), the reduction of S2O8 2− occurred below ca. 0.5 V though the current due to the reduction was too small to affect the I – E curve, and a chaotic oscillation also appeared as shown in the inset [2]. The values of the higher turning points and those of the lower ones were irregular, indicating that two negative feedback loops played roles both in the higher turning points and the lower ones. The loop induced by Na+ ions did not work in higher ones, nor in the lower ones when the Na+ concentration was 0.2 M. We can thus say that the chaotic oscillation was not caused by Na+ions. REFERENCES [1] M. Orlik, Self-Organization in Electrochemical Systems I, Springer, Berlin (2012). [2] Y. Mukouyama, H. Kawasaki, D. Hara, S. Nakanishi, J. Solid State Electrochem., DOI 10.1007/s10008-015-2813-z. [3] Y. Mukouyama, M. Hasegawa, S, Nakanishi, ECS Trans., 64 (2014) accepted. FIGURE CAPTION Figure 1. The I – E curves for a Pt-disc electrode in 0.1 M H2SO4 + 0.2 M H2O2 (a) without Na2SO4 nor Na2S2O8, (b) with 0.2 M Na2SO4, and (c) with 0.1 M Na2S2O8, measured under potential-controlled conditions. The insets in panels a, b, and c are the waveforms of oscillations observed at −0.065, −0.085, and −0.070 V, respectively. Figure 1

Alkali Metal Ions Induced Electrochemical Oscillations in H2O2 - H2so4 - Pt System

INTRODUCTION Electrochemical oscillations, which are attractive phenomena from the viewpoint of dynamic self-organization of molecular systems, have been reported in a variety of electrochemical reactions [1]. Among them, the H2O2 reduction at Pt electrodes in H2SO4 solutions (H2O2 + 2H+ + 2e- → 2H2O) has an especially unique feature where nine kinds of oscillations, named oscillations A, B, C, D, E, F, G, H, and I are observed [2-4]. Oscillation A appeared in the potential region of the formation of under-potential deposited H (upd-H) which suppressed the H2O2 reduction, whereas oscillation B appeared in the region of hydrogen evolution reaction (HER). Oscillations C and D appeared when a small amount of Cl- or Br- ions, which adsorbed on a Pt surface, were added to electrolysis solutions. Oscillation E appeared when a single-crystal Pt(111) electrode was used because H2O2 was autocatalytically reduced especially on the Pt(111) surface. Oscillations F and G appeared in the presence of a small amount of Br- ions when a Pt(111) or Pt(100) electrode was used. Oscillations H and I appeared when the cations such as Na+ and K+ were added to electrolysis solutions [2-4]. In the presence of the cations, the transport rate of H+ to the electrode surface by the electromigration decreased and consequently the local pH at the electrode surface became basic during the H2O2 reduction. In the vicinity of the surface where the pH was basic, H2O2 dissociated to form HO2 - ion, which was electrochemically reduced to OH- or oxidized to O2. Oscillation H was tentatively caused by the interaction between the autocatalytic H2O2 reduction and the electrooxidation of HO2 - [2, 3]. On the other hand, oscillation I was probably caused by the suppression of the reduction of HO2 - due to the formation of upd-H [4]. Recently, a new oscillation, which was named oscillation J, was found to appear during the H2O2 reduction in the presence of the cations. In this presentation, the conditions for the appearance of oscillation J and its mechanism will be discussed. RESULTS and DISCUSSION Figure 1 shows the current (I) – potential (E) curves for a Pt-disc electrode. When the solution was 0.1 M H2SO4 + 0.1 M H2O2 (Fig. 1a), the H2O2 reduction occurred in the potential region below ca. 0.8 V under potential controlled conditions. The reduction was suppressed, that is, an N-shaped negative differential resistance (N-NDR) appeared, at around ca. –0.1 V due the formation of upd-H. Oscillation A appeared as a current oscillation at the potentials of the N-NDR and the HER due to the H+ reduction occurred in the potential region below ca. –0.15 V. Oscillation A did not appeared under current controlled conditions (Fig. 1b). When the concentration of H2O2 was 0.3 M, oscillation B appeared both under potential controlled conditions and under current controlled ones (Figs. 1c and 1d). The I – E curves were strongly affected by adding Na2SO4 or K2SO4 to the solution because the local pH at the electrode surface became basic during the H2O2 reduction in the presence of Na+ or K+ [2-4]. When the solution was 0.1 M H2SO4 + 0.3 M H2O2 + 0.1 M Na2SO4 (Figs. 1e and 1f), oscillations A and B did not appear. However, oscillation I appeared at around –0.8 V under potential controlled conditions. The HER due to the water reduction occurred in the potential region below ca. –1.0 V. When the H2O2 concentration was high, e.g., 0.9 M (Figs. 1g and h), oscillations I and H appeared under potential controlled conditions and under current controlled conditions, respectively. The new oscillation, oscillation J, appeared at around –1.15 V under potential controlled conditions and also appeared in the potential range between –0.4 and –1.5 V under current controlled conditions. Oscillation J synchronized with the behavior of hydrogen bubbles formed by the HER. Thus, the bubble evolution probably played a role in the mechanism of oscillation J. REFERENCES [1] M. Orlik, Self-Organization in Electrochemical Systems I, Springer, Berlin (2012). [2] Y. Mukouyama, M. Hasegawa, S. Yamamoto, S. Nakanishi, H. Okamoto, ECS Trans., 58 (25) 99-107, 2014. [3] Y. Mukouyama, M. Hasegawa, S. Nakanishi, ECS Trans., 64 (2014) accepted. [4] Y. Mukouyama, M. Hasegawa, S. Nakanishi, ECS Trans., 64 (2014) accepted. FIGURE CAPTION Figure 1. The I – E curves for 0.1 M H2SO4 + x M H2O2 + y M Na2SO4, where (x, y) = (0.1, 0), (0.3, 0), (0.3, 0.1), and (0.9, 0.1) from top to bottom, measured (left) under current controlled conditions and (right) under current controlled conditions. Figure 1