Hidden Negative Differential Resistance Research Articles

Enhanced Efficiency of CO-Containing Hydrogen Electroxidation with Autonomous Oscillations

Electrochemical reactions exhibit autosustained oscillatory states once the appropriate parameters are applied. As shown herein, the main impact of operating a reaction in an oscillatory state is the increased efficiency. The electroxidation of CO-containing hydrogen is analyzed because its oscillating mechanism mimics the oscillator known as hidden negative differential resistance (HNDR). With that, experimental results and the thermodynamic calculations of this contribution could be extended for other HNDR oscillators such as ethanol and methanol electroxidation. The enlarged efficiency results from a smaller chemical dissipation.

Complex kinetics, high frequency oscillations and temperature compensation in the electro-oxidation of ethylene glycol on platinum

Despite the fact that the majority of the catalytic electro-oxidation of small organic molecules presents oscillatory kinetics under certain conditions, there are few systematic studies concerning the influence of experimental parameters on the oscillatory dynamics. Of the studies available, most are devoted to C1 molecules and just some scattered data are available for C2 molecules. We present in this work a comprehensive study of the electro-oxidation of ethylene glycol on polycrystalline platinum surfaces and in alkaline media. The system was studied by means of electrochemical impedance spectroscopy, cyclic voltammetry, and chronoamperometry, and the impact of parameters such as applied current, ethylene glycol concentration, and temperature were investigated. As in the case of other parent systems, the instabilities in this system were associated with a hidden negative differential resistance, as identified by impedance data. Very rich and robust dynamics were observed, including the presence of harmonic and mixed mode oscillations and chaotic states, in some parameter region. Oscillation frequencies of about 16 Hz characterized the fastest oscillations ever reported for the electro-oxidation of small organic molecules. Those high frequencies were strongly influenced by the electrolyte pH and far less affected by the EG concentration. The system was regularly dependent on temperature under voltammetric conditions but rather independent within the oscillatory regime.

Cationic catalysis and hidden negative differential resistance in reduction of radical anion of nitrobenzene

Well-known reduction of nitrobenzene in acetonitrile yields an anion radical, which is further reduced at more negative potentials. The reduction of anion radical is strongly influenced by the nature of tetraalkylammonium salts (tetramethyl- to tetraheptylammonium) used as supporting electrolytes. At low concentration of tetrahexylammonium hexafluorophosphate the anion radical reduction occurs at extremely negative potential, however, traces of alkali metal cations yield a remarkable acceleration effect. At potentials where alkali metal cations are reduced the acceleration ceases. As a result the current–potential curves show negative or hidden negative slope. Electrochemical impedance spectroscopy sensitively indicates the presence of the negative differential resistance (NDR) or the hidden negative differential resistance (HNDR). Oscillatory behavior based on this type of cationic catalysis is currently under investigation.

Nonlinear phenomena in the electrochemical oxidation of sulfide

The electro-oxidation of sulfide on a Pt electrode is found to exhibit both N-NDR (negative differential resistance) and HN-NDR (hidden negative differential resistance) types of oscillations, making it the first system known to be capable of supporting both types of nonlinear instabilities without changing electrolyte compositions. Six distinct oscillation windows are observed when the external current is adjusted as the control parameter. In addition, temperature also exhibits subtle influences on the observed reaction behavior, where varying the temperature from 30.0 °C results in more complicated oscillatory phenomena.

Oscillations and period-doubling bifurcations in the electrochemical oxidation of thiourea

The electrochemical oxidation of thiourea on a Pt electrode was found to exhibit both simple and period-doubled oscillations. Measurements of the potentiostatic I/Ø behavior and impedance spectra suggest that the studied system belongs to the hidden negative differential resistance (HNDR) oscillator. The dependence of complex oscillations on initial compositions of the reaction mixture and temperature has also been characterized, which showed that: (1) lowering the reaction temperature facilitates the occurrence of complex oscillations; (2) the frequency of oscillation increases linearly with respect to the temperature; and (3) hydrochloric acid exhibits stronger influence on the reaction behavior than nitric acid.

Electrochemical oscillations in the methanol oxidation on Pt

Experimental observations of temporal dynamics in the electrocatalytic oxidation of methanol (CH 3OH) on a polycrystalline platinum electrode are reported. Hidden negative differential resistance (HNDR) and instabilities of the system were investigated by means of an electrochemical impedance spectrum analysis and potential oscillations under galvanostatic control. This result could be applied for a direct methanol fuel cell (DMFC) with higher energy efficiency.

Oscillatory Peroxodisulfate Reduction on Pt and Au Electrodes under High Ionic Strength Conditions, Caused by the Catalytic Effect of Adsorbed OH

Four electrochemical oscillations of different types (named oscillations α, β, γ, and δ), which are different from previously reported Frumkin-type oscillations in low ionic strength electrolytes, appear for peroxodisulfate (S2O82-) reduction on Pt and Au electrodes under high ionic strength conditions. Impedance measurements for oscillation γ that appears in a region of the most positive potentials have shown that it is classified as hidden negative differential resistance (hidden-NDR or HNDR) oscillators. Detailed electrochemical measurements have suggested that the NDR arises from a decrease (with a negative potential shift) in the surface coverage of adsorbed OH, which acts as a catalyst for the dissociative adsorption of S2O82- (the first step of its reduction). The NDR is hidden by the desorption (with the negative potential shift) of adsorbed sulfate (SO42-) produced by the dissociative adsorption and reduction of S2O82-, which works as a site-blocking agent. Mathematical simulation based on this m...

New-Type Electrochemical Oscillation Caused by Electrode−Surface Inhomogeneity and Electrical Coupling as Well as Solution Stirring through Electrochemical Gas Evolution Reaction

The appearance mechanism of an electrochemical oscillation (previously called oscillation B), observed for Pt electrodes in H2O2-containing acidic electrolytes in a potential region of hydrogen evolution, has been investigated. Though it is reported that electrochemical oscillations in general appear in potential regions of negative differential resistance (NDR) or hidden NDR (HNDR), impedance analyses have shown that no NDR or HNDR is present in the potential region of oscillation B. Besides, oscillation B is observed only for Pt electrodes with atomically roughened surfaces, not for electrodes with atomically flat surfaces. A possible explanation is proposed by assuming the presence of small local “active” areas in the electrode surface, in which the H2O2 reduction is not prevented by a full coverage of under-potential deposited hydrogen, contrary to the other normal “nonactive” areas occupying the major part of the electrode surface. The appearance of oscillation B is reproduced by mathematical simulation based on the model, together with the consideration of electrical coupling between the active and nonactive areas as well as solution stirring by hydrogen-gas evolution. Oscillation B can be classified into a new category of oscillators, which may be called “coupled NDR” oscillators.

Spatio-temporal interfacial potential patterns during the electrocatalyzed oxidation of formic acid on Bi-modified Pt

We report experimental observations of the spatio-temporal dynamics in the electro-oxidation of formic acid on a Pt ring electrode modified by bismuth adatoms. Bismuth modification significantly enhanced the current density and it was found to considerably increase the existence range of oscillations and spatio-temporal self-organization. Hidden negative differential resistance and the existence of a Hopf bifurcation were deduced from the electrochemical impedance spectra and the occurrence of galvanostatic oscillations. The pattern formation resulted from hybrid effects of the nonlinear chemistry during formic acid oxidation and the long-range coupling of the interfacial potential induced by the chosen geometry (ring type) of the working electrode. Reversible transitions between traveling pulses and oscillating standing waves were observed when the outer potential or the formic acid concentration near the electrode were used as control parameters. Experimental results were compared with computer simulations of a reaction-migration system. The role of electrode inhomogeneities in pattern formation and the transform between patterns were discussed.

Mechanisms of Two Electrochemical Oscillations of Different Types, Observed for H2O2 Reduction on a Pt Electrode in the Presence of a Small Amount of Halide Ions

Two electrochemical oscillations, previously called oscillation C and D, are observed for H2O2 reduction on a Pt electrode in an acidic solution when a small amount of halide ions is added to the solution. Detailed studies, including impedance analyses and in-situ light reflectance measurements as well as mathematical simulations, have revealed that both oscillations C and D fall into hidden negative differential resistance (HNDR) oscillators, though oscillation D can be classified into a new-type HNDR oscillator not reported thus far. The H2O2 reduction on Pt has two-type NDR’s: One arises from a decrease in the coverage of adsorbed OH (acting as an autocatalyst for the H2O2 reduction) with a negative potential shift, and the other arises from suppression of the H2O2 reduction by formation of under-potential deposited hydrogen (upd-H) in further negative potentials. Oscillation C appears from hiding of the former NDR by a decrease in the coverage (θX) of adsorbed halide ions (acting as a site blocking a...