Redox polymer poly-N,N′-2,3-dimethylbutane-2,3-diyl-bis(salicylideniminato)nickel: An impedance spectroscopy study

Films of poly-N,N-2,3-dimethylbutane-2,3-diyl-bis(salicylideniminato)nickel are studied by cyclic voltammetry and faradaic impedance methods in acetonitrile solutions containing 0.1 M tetraethylammonium tetrafluoroborate. The resistance to charge transfer and the double-layer capacitance, calculated from the semicircle in the high-frequency spectrum portion, are referred to the film/electrolyte interface. From the low- and mid-frequency impedance spectra portions, the low-frequency capacitance and the effective diffusion coefficient of charge carriers are determined. The assumption about a noticeable film porosity is shored up by direct scanning electron microscopy examination of film surfaces.__________Translated from Elektrokhimiya, Vol. 41, No. 4, 2005, pp. 433–441.Original Russian Text Copyright © 2005 by Dmitrieva, Logvinov, Kurdakova, Kondratev, Malev, Timonov.     

Impedance Response of a Thin Film on an Electrode: Deciphering the Influence of the Double Layer Capacitance

The different contributions of the interfacial capacitance are identified in the case of passive materials or thin protective coatings deposited on the electrode surface. The method is based on a graphical analysis of the EIS results to determine both the passive-film capacitance in the high-frequency domain and the double-layer capacitance in the low-frequency domain. The proposed analysis is shown to be independent of the physicochemical origins of the frequency dispersion of the interfacial capacitances which results, from an analysis point of view of the experimental results, in the use of a constant-phase element However, for a correct evaluation of the thin-film properties such as its thickness, the high-frequency data must be corrected for the double-layer contribution. In particular, it is shown that if the double-layer capacitance gives a frequency-dispersed response, it is necessary to correct the high-frequency part for the double-layer constant-phase elements. This is first demonstrated on synthetic data and then used for the determination of the thickness of thin oxide film formed on Al in neutral pH solution.     

Demodulation of a sinusoidal signal by a reversible electrode and its application to polarography

A theoretical analysis of the “faradaic demodulation signal” caused by the non-linearity of the faradaic admittance is given for a plane, stationary electrode with a single step electrode process. It is assumed that mass transfer of the substances involved occurs by diffusion only. The theoretical conclusions are tested and confirmed by polarographic measurements with a dropping mercury electrode (DME).     

A simple Maxwell-Wagner-Butler-Volmer approach to the impedance of the hydrogen electrode in a nafion fuel cell

A simple one-dimensional model of the impedance of a hydrogen/Nafion electrode is set up combining the usual Maxwell-Wagner approach for linear, homogeneous, and isotropic media with the linearized Butler-Volmer equation for the interfacial, electrochemical reaction. Only one relaxation semicircle is normally seen in the Nyquist diagram, but a low-frequency arc may appear at high overvoltages. The model is described by only two dimensionless parameters (in addition to the dimensionless frequency). These parameters are related to the double-layer capacitance and to the interfacial electrochemical reaction rate, respectively. With some adjustments, the model can be used to explain the observed equilibrium impedance from 40 to 70 degrees C of a symmetric cell of the type C/Pt/H(2)|Nafion 117/H(2)/Pt/C. The hydrogen electrodes in this cell were built up as a disperse multiphase region (carbon, platinum grains, Nafion 117, and hydrogen gas) as commonly done in solid polymer fuel cells.     

An ac voltammetry study of Pt oxide growth

AC voltammetry of polycrystalline Pt in sulfuric acid solutions has been used to study the growth kinetics of the thin anodic Pt oxide film. Data were collected from 2 Hz to 50 kHz, one frequency per cycle, and were analyzed in the complex impedance plane. The faradaic process was modeled as a resistance parallel to the double-layer impedance, with a value approximately independent of potential in the do voltammetry plateau region. The equivalent circuit for the known growth law is derived and is shown to be a series RC combination. The capacitance was not detected but is expected to have a negligible effect in the measured frequency range. The value of the resistance found was consistent with the growth law found in other experiments. Evidence for additional faradaic elements in the equivalent circuit was inconclusive. We found no additional features in the impedance spectra at higher frequencies that could be associated with the fast electrosorption of OH suggested by other workers. The reversibility of the early stages of growth is therefore associated with structural reversibility rather than a fast process.     

电催化界面和反应的电化学阻抗谱研究：经典永不褪色

本文章综述了电催化领域电化学阻抗谱（EIS）的相关研究. 首先概述了从二十世纪初到现在这一专业领域的发展历史. 然后介绍了电催化阻抗理论的几个里程碑. 其中,着重介绍了目前分析电催化EIS数据的主流模型——Dolin-Ershler模型. 之后,具体讨论了铂金单晶的双电层电容,特别是围绕这一基础课题的实验和理论上的挑战. 我们质疑采用Dolin-Ershler模型获取稀溶液中双电层电容的合理性. 因为在稀溶液中,双电层效应使得双电层电容具有频散特性,因而双电层电容的低频部分在分析过程中可能被遗失了. 未来,我们期待看到新的实验去证明或反驳一个最近的理论预测,即铂电极在氧化物生成电位区域中具有非单调表面电荷关系和负双电层电容.     

On-line FFT faradaic admittance measurements application to A.C. cyclic voltammetry

A measurement approach is described and data are presented which demonstrate the ability to effect a.c. cyclic voltammetric measurements with the on-line digital FFT approach to faradaic admittance data acquisition. The equipment utilized enables complete faradaic admittance spectra to be obtained at an effective spectrum acquisition rate of 10 s^?1, so that the d.c. potential range encompassed by a typical cyclic wave can be encompassed with adequate resolution in the E_dc dimension in ≥6 s, approximately. The instrument features dynamic, computerized measurement and compensation of the non-faradaic ohmic resistance and double-layer capacitance contributions to the acquired total cell admittance. Measurements with quasi-reversible systems yield the expected faradaic admittance and phase angle responses over a quite generous bandwidth. Applications to mercury and platinum electrodes are illustrated.     

Anodic stripping voltammetry with a symmetric double-step waveform

The high sensitivity of differential pulse anodic stripping voltammetry (d.p.a.s.v.) is based on discrimination against the charging current by using short current-sampling periods near the end of the applied pulses. In this paper, the capacitive current is eliminated by using a symmetric double-step waveform superimposed on the voltage ramp. The areas under the adjacent current peaks are subtracted from each other which effectively cancels the contribution of the double-layer charging current but preserves the changes in the faradaic current. By using a mercury thin-film electrode the base line is reasonably flat down to nanomolar levels of Cd, Cu and Pb with a deposition time of 3–5 min. Compared with d.p.a.s.v., the proposed method has the advantages of a higher scan rate and simple electronic circuitry.     

Electrochemical Capacitors

The current literature sources on the electrochemical capacitors, which are divided into the film (dielectric), electrolytic, and supercapacitors, are reviewed. The supercapacitors are in turn subdivided into the double-layer capacitors, which use the EDL recharge on a highly-developed interfacial surface of electrodes; pseudocapacitors, where the charge is stored in a faradaic pseudocapacitance of sufficiently reversible redox reactions and the EDL capacitance; and hybrid capacitors, which employ a variety of electrodes. A macrokinetic theory of operation of double-layer capacitors is considered. Effect of various factors on the properties of electrodes utilized in supercapacitors is analyzed. A novel type of hybrid capacitor, which has a negative electrode of activated carbon cloth and a PbSO₄/PbO₂ positive electrode, is proposed. A theory of capillary equilibrium in hermetically sealed electrochemical capacitors is considered. Specific features of the application of voltammetric and impedance methods to studying electrochemical processes in supercapacitors are revealed. Characteristics of electrochemical capacitors and batteries are compared.     

Interpretation of the impedance comprising negative capacitance and constant phase elements on iron electrode in weakly acidic media

The negative low-frequency capacitance that appears in interpretations of impedance of the iron electrode in weakly acidic solutions is shown to arise in the case of interaction of two consecutive nonequilibrium flows that constitute a two-stage anodic faradaic process of intermediate adsorption in the prepassivation potential range. The low-frequency capacitance is negative throughout a potential range where the logarithm of rate constant vs. potential (logk vs. E) dependence has the higher slope for the limiting stage. The low-frequency capacitance becomes positive at higher anodic potentials and for the other limiting stage.     

Study of the underlying electrochemistry of polycrystalline gold electrodes in aqueous solution and electrocatalysis by large amplitude fourier transformed alternating current voltammetry

Polycrystalline gold electrodes of the kind that are routinely used in analysis and catalysis in aqueous media are often regarded as exhibiting relatively simple double-layer charging/discharging and monolayer oxide formation/removal in the positive potential region. Application of the large amplitude Fourier transformed alternating current (FT-ac) voltammetric technique that allows the faradaic current contribution of fast electron-transfer processes to be emphasized in the higher harmonic components has revealed the presence of well-defined faradaic (premonolayer oxidation) processes at positive potentials in the double-layer region in acidic and basic media which are enhanced by electrochemical activation. These underlying quasi-reversible interfacial electron-transfer processes may mediate the course of electrocatalytic oxidation reactions of hydrazine, ethylene glycol, and glucose on gold electrodes in aqueous media. The observed responses support key assumptions associated with the incipient hydrous oxide adatom mediator (IHOAM) model of electrocatalysis.     

Double-layer and pseudocapacitance types of electrochemical capacitors and their applications to the development of hybrid devices

The basis of the complementary use of electrochemical capacitors (so-called supercapacitors) in hybrid electric power generation by rechargeable batteries and fuel cells is explored. Electrochemical capacitors are of two types: one where the interfacial double-layer capacitance of high specific area carbon materials is the basis of electric charge storage (as ions and electrons); and the other where pseudocapacitance, associated with electrosorption and surface redox processes at high-area electrode materials, e.g. RuO₂, or at conducting polymers, provides the basis of charge storage. The former, double-layer, type of capacitance stores charge non-faradaically while the latter type, pseudocapacitance, stores charge indirectly through faradaic chemical processes but its electrical behaviour is like that of a capacitor. Two types of hybrid battery/capacitor system are recognized: one based on combination of an electrochemical capacitor cell with a rechargeable battery or a fuel cell in a load-leveling function, e.g. in an electric vehicle power train; and the other based on combination of a faradaic battery-type electrode coupled internally with a capacitative electrode in a two-electrode hybrid module (termed an asymmetric capacitor). Optimization of operation of such systems in terms of balancing of active masses, of power and charge densities, and choice of maximum but limited states-of-discharge, is treated.     

Electrochemical noise of a hydrogen-air polymer electrolyte fuel cell operating at different loads

The electrochemical noise of a polymer membrane hydrogen-air fuel cell operating at different load currents was measured in serial experiments. Spectral power densities of the noise are shown to be divided into three regions. At frequencies greater than 3–10 Hz, the spectrum dependence has a constant slope of ??2 in the bilogarithmic coordinates. At frequencies 0.3–5 Hz, there is a horizontal plateau in which length is determined by the value of a load. At frequencies less than 0.3 Hz, the dependence of spectral power density has a slope of ??2. Medium-frequency plateau and high-frequency slope of spectral power densities of the noise were approximated by model RC circuits. The values of Faradic resistance and double-layer capacitance connected in parallel were obtained from the electrochemical impedance data. At load voltages higher 0.5 V, the height of the plateau was shown to be proportional to the 2.68 power of the load current value.     

On second-order effects in a galvanic cell: Part III. Application to modulation polarography

The simultaneous application to a galvanic cell of two sinusoidal voltages, superimposed on a linear ramp, causes a sinusoidally amplitude-modulated sine wave current to flow as a result of the non-linear behaviour of the double layer and the faradaic processes. Phase-sensitive detection of this current with two lock-in amplifiers in tandem allows a high rejection of the charging current, enabling a sensitive quantitative analysis down to concentrations well below 10^?6M.The technique also seems suitable for the study of electrode kinetics; in the absence of electroactive species it can be used in double-layer studies.     

Kinetics of the Zn(II)/Zn(Hg) electrode reactions in DMSO solutions of halide ions

The electrode reaction Zn(II)/Zn(Hg) in complex chloride, bromide, and iodide solutions with DMSO as solvent and ammonium perchlorate as supporting electrolyte has been studied at the equilibrium potential by the faradaic impedance method and a square-wave method. Furthermore, double-layer data have been determined by electrocapillary measurements. The results indicate that the zinc chloride and bromide complexes do not contribute noticeably to the exchange current density, while in the iodide system both the solvated zinc ion and the first complex take part in the charge transfer. From the dissimilar results valid for water and DMSO solutions the conclusion is made that probably ligand bridging at the amalgam by the halide ions is operative in water solutions, whereas in DMSO the larger solvent molecules adsorbed can form a steric hindrance to ligand bridging by chloride or bromide ions.     

交流示波极谱法中i~f-E曲线的研究 8:"电极/溶液"界面动态双电层微分电容的快速测量

"电极/溶液"界面动在记双电层微分电容是电极过程动力学中一个重要的物理量,但其测量方法,多数仍停留在对逐一测定的数据点进行作图分析上.本文以i~f-E曲线上电流峰的理论公式为基础,提出了一个快速测量"电极/溶液"界面动态双层微分电容的新方法.     

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.     

Staircase pulse voltammetry

The faradaic current of interest can often be obscured by some extraneous current. A new method called staircase pulse voltammetry (SPV), with which involving a potential pulse is superimposed on a staircase, is used for removing the double-layer charging current. This paper presents the theoretical consideration and the experimental evidence of this method for a simple reversible system and a catalytic process. The parameters affecting currents are also discussed. Experimental verifications are in agreement with the theories.     

连续电荷传递反应的阻抗谱与电极反应机制的关系

对于含有一个吸附中间物的不可逆连续电行传递反应，在较低的过电位下，电极过程的Faraday阻抗通常在复平面上显示一个低频感抗弧．在较高的过电位下，这个感抗弧将转化为容抗弧，这一转变预示着速控步骤的变化．相反，当反应速率一直被其中的一个步骤控制时，它的法拉第弛豫始终表现出容抗行为．     

玻碳电极界面的阻抗谱数学表达及定量分析

玻碳电极（GCE）是各类电化学传感器常用的基础电极,其界面特征直接影响检测性能。本文详细考察了电极体系的电化学过程,针对GCE传感界面,探讨了一个等效电路中电解质电阻、电荷输运电阻、扩散阻抗、电化学（氧化/还原）反应阻抗、表面吸附阻抗和双电层电容等电学元件的物理意义,并给出了对应的数学模型。通过改变模型中5个参数值,模拟了不同状态下的阻抗谱,分析了电极系统各参数对GCE阻抗谱的贡献规律。最后,采用该数学模型对裸GCE和修饰GCE在铁氰化钾溶液中的阻抗谱进行分析,拟合结果与实验数据吻合度高;基于拟合获得参数,定量对比分析了修饰前后电极表面的特征变化。