Specific features of the behavior of an electrochemical system in the case of the Hopf instability for a spherical electrode

Model approximations are developed that allow establishing a quantitative relationship between the geometrical parameters of a spherical electrode, the faradaic impedance, and instabilities of the electrochemical system for an electrode reaction under potentiostatic conditions for the adsorption of species preceding their discharge. It is shown that the control parameter ω_H in the Hopf bifurcation point depends on the electrode size. The effect of the Nernst diffusion layer is observed at low frequencies in the range of negative faradaic impedance values.     

Advantage of Fractional Calculus Based Hybrid‐Theoretical‐Computational‐Experimental Approach for Alternating Current Voltammetry

The dynamic electrochemical behavior of electroactive species is believed to be represented better by the fractional calculus, because it can consider the history of mass‐transfers of that species near the electrode surface. The elucidation of mathematical fundamentals of fractional calculus has been recently introduced for batteries, supercapacitors and a few voltammetry studies. The working equations for faradaic fundamental and second‐harmonic (SHac) components of alternating current (ac) for ac voltammetry of an electrochemically reversible redox reaction on an electrode of macroscopic diameter have been derived here by using generalized formulae of the fractional calculus. A computation code is written in Python language with a matrix based algorithm developed based on latest, accurate, efficient and stable Grunwald‐Letnikov‐Improved fractional‐order differentiation equation. That computational code is used to find the concealed faradaic fundamental, SHac components of the total current and other double‐layer parameters of experimentally recorded voltammograms of ruthenium(III/II) redox reaction on gold‐disc electrode by a common electrochemical workstation without having inbuilt Fourier transformation features. The amplitude of the computed faradaic current concealed in the experimental data gets enhanced through this hybrid theoretical‐computational‐experimental approach and thus it keeps scope of application and further improvement in electroanalysis.     

Amperometric detection of simple alcohols in aqueous solutions by application of a triple-pulse potential waveform at platinum electrodes

Application of a triple-pulse waveform is described for the anodic detection of methanol, ethanol, and ethylene glycol. The execution of the waveform incorporates the cleaning and reactivation of the platinum electrode, by alternate anodic and cathodic polarization, with measurement of the faradaic signal for the analyte at the reduced electrode surface. Some results for formic acid are also presented. The waveform is completed within approximately 2 s and the faradaic signal exhibits no decay with time as would be the case for amperometric detection at a constant applied potential. Calibration curves made by plotting —1/I vs. 1/C are linear. This is consistent with a reaction mechanism in which the analyte is adsorbed prior to anodic detection. The technique is applicable for detection in chromatographic and flow-injection systems.     

Switching “on and off” faradaic electrochemistry at an otherwise passivated electrode using gold-coated magnetic nanoparticles

Herein we combined the use of self-assembled monolayer (SAM) modified electrodes and gold coated magnetic nanoparticles (Au@MNPs) to modulate faradaic electrochemistry when nanoparticles are absorbed on and removed from, a passivated electrode using an external magnetic field. Substantial faradaic electrochemistry of potassium ferricyanide in solution is first prevented by modifying gold electrodes with 11-mercaptoundecanoic acid to form a passivating SAM. Restoration of the faradaic electrochemistry is then achieved by introducing Au@MNPs which are brought to the surface using an external magnetic field. The faradaic electrochemistry can again be suppressed by removing Au@MNPs from the SAM using another external magnetic field.     

An immitance study of the mechanism of hydrogen reactions on a tungsten carbide electrode: Part I. Experiment and analysis in terms of a linear model

The ac admittance spectra of a smooth tungsten carbide (WC) electrode in a stirred H₂-saturated 0.5 M H₂SO₄ solution were measured at hydrogen overpotentials from −0.15 to +0.25 V in the frequency range from 0.1 to 900 Hz, at a temperature of 80°C.An extended analysis of immitance spectra in diagrams of various coordinates led to an interpretation of the system under study at any potential in terms of a linear model which took into account the constant phase element behaviour of the electrode double layer and the presence of two parallel, activation-controlled faradaic processes: the hydrogen ionization/evolution process and an additional electrochemical adsorption reaction passivating the electrode surface for the hydrogen process.Computerized fitting of a rational equivalent circuit to the immitance spectra for all the potentials gave the dependences of the equivalent circuit elements on the potential. The fits proved that the mechanism of the faradaic processes taking place on the electrode is probably the same in the entire range of potentials.     

提高电容性电化学储能装置能量容量的一些尝试

本文从作者所在的课题组在超级电容器和超级电容电池方向的研究内容为基础,在电极材料和装置层面综述了电容性电化学储能装置的发展. 导电聚合物和过渡金属氧化物分别与碳纳米管复合后的复合物能显著提高前两者作为电容性法拉第储能电极的电容性能. 活性炭和碳黑等一类碳材料则可作为非法拉第储能的电极材料. 通过对超级电容器正负极电容做相应的匹配调整可以提高超级电容器的最大充电电压,从而提高超级电容器的能量容量. 此外,为了与实际设备相匹配,超级电容可以以双极板的方式串联堆积,满足高电压的需求. 超级电容电池作为新一代的电容性电化学储能装置,分别由具有电容性和法拉第电荷储存原理的电极组成,具有高比功率和高比能量的特点,也是近年来的研究热点.     

Electrolysis with electrolyte dropping electrode: II. Basic properties of the system

In a theoretical discussion the conditions have been pointed out where an interface of two immiscible electrolyte solution behaves as an equilibrium system metal ion-metallic electrode, as an ideally polarized electrode and as an electrode under faradaic current flow. The basic equations for current-electrical potential difference across the interface have been deduced for the cases of ion as well as electron transfer.Experimentally, various base electrolyte systems were studied, the most advantageous among these are LiCl in water+tetrabutylammonium tetraphenylborate in nitrobenzene and MgCl₂ in water+tetrabutylammonium dicarbollyl cobaltate in nitrobenzene. S-shaped polarographic curves were observed with the tetramethylammonium ion. The limiting current is directly proportional to concentration. The limiting currents are somewhat higher than those predicted by the Ilkovi? equation which has been ascribed to the tangential movement of the interface.     

Application of convolution procedures to chronopotentiometry

Convolution procedures are used to extract the faradaic information from chronopotentiometric data, in conditions where significant distortion by double layer charging occurs. The faradaic component of the imposed current is obtained, after measurement of the double layer capacitance, by differentiation of the initial chronopotentiogram. Convolution of this current with the function (πt)^?1/2 leads to a potential-convoluted current relationship freed from the effect of double layer charging. The kinetic characterization of the system using a combined analysis of this relationship and that relating the faradaic current to the electrode potential is discussed for the various types of reaction mechanism. The efficiency of the proposed procedure is tested on the galvanostatic reduction of fluorenone in DMF.     

On second order effects in a galvanic cell: Part I. Polarization by a sine wave modulated high frequency current

A theoretical study is presented concerning the application of a high-frequency alternating current, amplitude modulated by a low-frequency sine wave, to a galvanic cell. Based on the correlation with the faradaic rectification technique, expressions are given for the low-frequency demodulation voltages of the faradaic process and for the double-layer capacity. Both responses, which are in-phase with the modulation signal, are combined by considering them as voltage sources, having respectively the faradaic impedance and the double-layer impedance as internal impedances. The resulting demodulation voltage appears to possess an in-phase and a quadrature component, from which information can be obtained of the high-frequency as well as the low-frequency behaviour of the cell.     

Square-wave Voltammetry and Electrochemical Faradaic Spectroscopy of a Reversible Electrode Reaction: Determination of the Concentration Fraction of the Redox Couple

A comparative study conducted with square-wave voltammetry (SWV) and electrochemical faradaic spectroscopy (EFS) is presented for a reversible electrode reaction of dissolved redox couple in the presence of both Ox and Red components. In agreement with previous studies, the net peak current ΔΨ_p of the theoretical SW voltammograms is positioned at the formal potential E^0′ and does not depend on the concentration ratio c(Ox)/c(Red). However the forward-to-backward peak current ratio is sensitive to the redox state. For very low SW amplitudes, theoretical data imply superior features of EFS over SWV. Theoretical and experimental calibration lines are in good agreement within the interval 0.2≤x(Ox)≤0.8.     

Effect of infiltration material on a LSM15/CGO10 electrochemical reactor in the electrochemical oxidation of propene

The effect of infiltrating on a La_0.85Sr_0.15MnO₃/Ce_0.9Gd_0.1O_1.95 11-layer electrochemical reactor with CeO₂ and Ce_0.8Pr_0.2O_2?δ was studied in propene oxidation at open-circuit voltage and under polarization as a function of reaction temperature. This work outlined the importance of catalytic and electrochemical properties of infiltrated material on the ability to increase propene conversion under polarization with good faradaic efficiency. Electrochemical impedance spectroscopy was used to study the effect of infiltration material on electrode properties. The infiltration of a mixed ionic and electronic conductor, like Ce_0.8Pr_0.2O_2?δ , increased the electrode performance at low temperature but decreased the lifetime of the oxygen ion promoters on the catalyst/electrode surface, reducing the faradaic efficiency of the reaction. The infiltration of CeO₂ provided high propene conversion at open circuit and high effect of polarization associated with good faradaic efficiency, especially at low temperature.     

Photoelectrochemical Reduction of Carbon Dioxide with a Copper Graphitic Carbon Nitride Photocathode

Research on the photoreduction of CO₂ often has been dominated by the use of sacrificial reducing agents. A pathway that avoids this problem would be the development of photocathodes for CO₂ reduction that could then be coupled to a photoanodic oxygen evolution reaction. Here, we present the use of copper-substituted graphitic carbon nitride (Cu−CN) on a fluorinated tin oxide (FTO) electrode for the photoelectrochemical two-electron reduction of CO₂ to CO as a major product (>95 %) and formic acid (<5 %). The results show that at a potential of −2.5 V versus Fc\Fc⁺ the CO₂ reduction activity of Cu−CN on FTO electrode improves by 25 % upon illumination by visible light with a faradaic efficiency of nearly 100 %. Independently, X-ray photoelectron spectroscopy conclusively shows a pronounced increase in the electrical conductivity of the Cu−CN upon white light illumination under vacuum and a contactless measuring configuration. This photo-assisted charge mobility is shown to play a key role in the increased reactivity and faradaic efficiency for the reduction of CO₂.     

Voltammetry at partially covered electrodes: Part III. Faradaic impedance measurements at model electrodes

Faradaic, impedances at model electrodes partially covered with a photoresist layer have been studied theoretically and experimentally. Equations for the faradaic impedance are derived based on the theoretical model and approach described in Part I of this series of papers. Experimental data for the hexacyanoferrate system at various model electrodes give excellent agreement, with theoretical predictions for the diffusion impedance behavior, and the applicability of the derived equations to the estimation of the degree of coverage and the size of the active regions is confirmed. Furthermore, the application of such model electrodes to the kinetic study of electrode reactions with high heterogeneous charge transfer rates is suggested.     

Adsorption of urea on a polycrystalline copper electrode

The adsorption of urea on a polycrystalline copper electrode from 0.01 M NaClO₄ solution has been studied by impedance spectroscopy and radiometric method. The dependence of the surface concentration of urea on the electrode potential and the bulk concentration was determined. From radiometric data, it follows that the adsorption of urea on the copper electrode takes place in the entire range of studied potentials where no faradaic processes occur. In this range, the process of adsorption is practically reversible with respect to the potential and the bulk concentration of urea. The experimental data were described by the Langmuir and the virial isotherms and the Gibbs energy of adsorption were calculated. The data of the urea adsorption on different electrodes have been compared and the role of the kind of the metal on the adsorption process was discussed.     

Ac polarography and faradaic impedance of strongly adsorbed electroactive species: Part IV. Experimental study of the faradaic impedance of the redox systems benzo(c)cinnoline-dihydrobenzo(c) cinnoline

The faradaic impedance of the surface redox system benzo(c)cinnoline-dihydrobenzo(c)-cinnoline is studied experimentally in aqueous medium between pH 5 and 13. The variations of the impedance components are in good accord with the theoretical predictions. A V-shaped curve is found for log k_s=f(pH) (k_s=rate constant of the surface electrochemical reaction). It is estimated that the determination of rate constant values up to 2×10⁴ s^?1 on a mercury electrode is possible by this method.     

Differential pulse polarography and voltammetry with a microprocessor-controlled polarograph and a pressurized mercury electrode

The performance of a microprocessor-controlled polarograph with a pressurized mercury electrode system has been evaluated. For the technique of differential pulse polarography, the theory applying to the pressurized mercury electrode in the dropping mercury format is shown to be the same as for a conventional gravity-controlled mercury electrode system. At the short drop times used (0.2–0.4 s), faradaic distortion terms are shown to influence the shape of the observed differential pulse polarograms. A substantial decrease in sensitivity is also incurred in using these short drop times, compared with the longer ones generally employed in differential pulse polarography. Results for differential pulse anodic stripping conform to the usual expectations.     

Minimization of Electrode Adsorption Effects: The Cadmium–Humic Acid System Studied by Phase Sensitive Alternating Current Polarography

The use of phase‐sensitive Alternating Current Polarography (ACP) is investigated for the minimization of adsorption effects in metal‐ligand systems with induced metal adsorption onto the electrode. When ACP is applied to obtain information on metal complexation in bulk, some problems arise from the faradaic contribution of adsorbed species. This effect can be corrected by using the capacitive current measured in the potential region of the faradaic peak. Using this correction, ACP produces similar results to those from Reverse Pulse Polarography (RPP), a technique that minimizes electrode adsorption effects on the measured currents. The method proposed is applied to the study of the Cd–humic acid system that has been investigated by ACP, RPP and Differential Pulse Polarography (DPP).     

The admittance of a simple electrocatalytic process coupled with an additional electrochemical adsorption reaction

The admittance of the process which is under activation control, is analysed in stationary conditions for the case of coupling in parallel with an additional electrochemical adsorption reaction which influences the electrode activity for the main process.The whole system has two time constants. As its equivalent circuit, a two-time constant electrical RC circuit of a particular structure is proposed; this structure consists of two subcircuits coupled in parallel.The use of a multi-time constant RC equivalent circuit for a faradaic process results in most cases in an ambiguous interpretation of the process, since the majority of structures of such circuits have multiple solutions. The choice of the correct interpretation needs reference to arguments outside the scope of ac measurements.It is indicated that the above ambiguity does not appear if one relinquishes the use of equivalent circuits in the modelling of complex faradaic systems.     

Pressure-driven bipolar electrochemistry

Here we report that pressure-driven flow alone (no external electrical energy) can be used to drive faradaic electrochemical reactions in microchannels with charged walls. Specifically, we show that solution flow can generate streaming potentials on the order of volts and that this is sufficient to carry out reactions on the anodic and cathodic poles of a bipolar electrode (BPE). The existence of faradaic reactions is proven by electrodissolution of Ag from the anodic end of the BPE.     

An electrochemical and optical study of the rupture and restoration of the passivating HgO multilayer on a dropping mercury electrode in aqueous 1 M NaOH solution at anodic potentials

It is shown that the passivating layer on the surface of a dropping mercury electrode in 1 M NaOH at anodic potentials due to drop growth continuously is being mechanically ruptured and electrochemically repaired. This phenomenon is studied by means of microscopy, reflectometry and polarography. The latter technique allows current spikes to be measured. The relation between faradaic current during such a spike and time is discussed.