Deviations of capacitive and inductive loops in the electrochemical impedance of a dissolving iron electrode.

The electrochemical impedance of an iron electrode often shows the capacitive and inductive loops on the complex plane. The capacitive loop originates from the time constant of the charge transfer resistance and the electric double layer capacitance. The inductive loop is explained by Faradaic processes involving the reaction intermediate. In some cases, these loops deviate from a true semicircle. In this paper, the origins and curve-fitting methods for the deviated loops of electrochemical impedance are discussed. The constant phase element (CPE) was used to present the deviation of the capacitive loop instead of electric double layer capacitance. The reaction rate constants, which are a function of the frequency, are proposed for the Faradaic impedance to present the deviated inductive loop.     

氯化硝基四氮唑蓝对钢在盐酸溶液中的缓蚀作用

采用失重法、动电位极化曲线、电化学阻抗谱(EIS)和扫描电子显微镜(SEM)研究了氯化硝基四氮唑蓝(NTBC)在1.0-5.0mo·lL-1HCl溶液中对冷轧钢(CRS)的缓蚀作用.结果表明:NTBC在1.0mo·lL-1HCl溶液中对冷轧钢具有良好的缓蚀作用,且在钢表面的吸附符合Langmuir吸附等温式.缓蚀率随缓蚀剂浓度的增加而增大,但随盐酸浓度和温度的增加而减小.求出了相应的吸附热力学(吸附自由能ΔG0,吸附焓ΔH0,吸附熵ΔS0)和腐蚀动力学参数(腐蚀速率常数k,动力学常数B),并根据这些参数讨论了缓蚀作用机理.动电位极化曲线表明:NTBC为混合抑制型缓蚀剂;EIS谱在高频区呈容抗弧,在低频区出现感抗弧,电荷转移电阻随缓蚀剂浓度的增加而增大.SEM再次表明NTBC对钢在盐酸介质中的腐蚀产生了明显的抑制作用.     

Electrochemical impedance spectroscopy of tethered bilayer membranes

The electrochemical impedance spectra (EIS) of tethered bilayer membranes (tBLMs) were analyzed, and the analytical solution for the spectral response of membranes containing natural or artificially introduced defects was derived. The analysis carried out in this work shows that the EIS features of an individual membrane defect cannot be modeled by conventional electrical elements. The primary reason for this is the complex nature of impedance of the submembrane ionic reservoir separating the phospholipid layer and the solid support. We demonstrate that its EIS response, in the case of radially symmetric defects, is described by the Hankel functions of a complex variable. Therefore, neither the impedance of the submembrane reservoir nor the total impedance of tBLMs can be modeled using the conventional elements of the equivalent electrical circuits of interfaces. There are, however, some limiting cases in which the complexity of the EIS response of the submembrane space reduces. In the high frequency limit, the EIS response of a submembrane space that surrounds the defect transforms into a response of a constant phase element (CPE) with the exponent (α) value of 0.5. The onset of this transformation is, beside other parameters, dependent on the defect size. Large-sized defects push the frequency limit lower, therefore, the EIS spectra exhibiting CPE behavior with α ≈ 0.5, can serve as a diagnostic criterion for the presence of such defects. In the low frequency limit, the response is dependent on the density of the defects, and it transforms into the capacitive impedance if the area occupied by a defect is finite. The higher the defect density, the higher the frequency edge at which the onset of the capacitive behavior is observed. Consequently, the presented analysis provides practical tools to evaluate the defect density in tBLMs, which could be utilized in tBLM-based biosensor applications. Alternatively, if the parameters of the defects, e.g., ion channels, such as the diameter and the conductance are known, the EIS data analysis provides a possibility to estimate other physical parameters of the system, such as thickness of the submembrane reservoir and its conductance. Finally, current analysis demonstrates a possibility to discriminate between the situations, in which the membrane defects are evenly distributed or clustered on the surface of tBLMs. Such sensitivity of EIS could be used for elucidation of the mechanisms of interaction between the proteins and the membranes.     

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．     

干湿循环下混凝土中钢筋腐蚀的电化学检测

应用电化学噪声技术,结合电化学阻抗谱研究了干湿循环条件下3种不同pH值的3.5%NaCl溶液中混凝土钢筋的腐蚀过程.结果表明,钢筋的腐蚀分为3个阶段:钝化膜的溶解期、腐蚀活化期和腐蚀产物的累积期.在pH1的溶液中经过30个干湿循环后主要以均匀腐蚀为主,而对pH3和pH7溶液,则主要以点蚀为主,散粒噪声分析证实了混凝土中钢筋在强酸性溶液中更容易发生腐蚀.电化学阻抗谱分析也进一步说明了混凝土中钢筋的腐蚀经历了3个阶段:第1阶段Nyquist谱图中出现两个时间常数,高频区为混凝土层的容抗性质,低频区为钢筋与混凝土界面的电荷转移电阻;第2阶段,中频区增加一个时间常数,这与腐蚀产物的累积有关;第3阶段,随着腐蚀产物向混凝土中的扩散,中频区容抗弧有所减小,特别是pH1体系,中频弧几乎消失.SEM形貌表征观察到随着溶液酸度的增加钢筋表面的锈层明显增多.     

Impedance spectroscopic study on ionic transport in a pH sensitive membrane

The characteristics of ionic transport under the influence of an AC electric field in a pH sensitive membrane made of poly(acrylamidocaproic acid) were studied using impedance spectroscopy. The dielectric spectra were obtained at various temperatures and for a range of pH values of HCl aqueous solutions in which the membrane was equilibrated. The contribution from the ionic transport which can be identified by a power dependency on frequency (i.e. ω⁻ⁿ; where 0 < n < 1) in the dielectric loss spectra, provided a means of determining the tortuosity of the long range ionic transport pathways.     

An impedance study of electron transport and electron transfer in composite polypyrrole + polystyrenesulphonate films

At low potentials, the impedance of polypyrrole + polystyrenesulphonate changes from the simple transmission line response observed at high potentials to a more complex response including a high frequency semicircle in the complex plane representation. There is also a shift in the high frequency limiting real impedance as the electronic resistance R_e of the polymer becomes comparable with, and then greater than, the solution and film ionic resistances. The ionic conductivity of the polymer composite increases with decreasing potential, and becomes constant at its maximum value in the potential range where the changes in impedance behaviour occur. This greatly simplifies interpretation of the low potential impedance results, and allows unambiguous assignment of circuit elements. The charge transfer resistance R_ct, which causes the high frequency semicircles, is due to electron transfer at the polymer|electrode interface. Both R_ct and R_e decrease exponentially with increasing potential at 60 mV per decade. This observation validates the use, at low potentials, of theoretical models in which conducting polymers are treated as redox polymers despite their failure to follow the Nemst equation at higher potentials.     

Thermoelectrochemical Impedance of an Electrode System Metal/Conducting Polymer/Solution

For a conducting-polymer-based electrode, the temperature coefficient of potential (thermoelectrochemical impedance) is calculated for a wide range of frequencies at which the electrode temperature relative to the solution bulk varies. The obtained frequency spectra depend on the potential distribution between the metal/polymer and polymer/solution interfaces, as well as on the heat transfer conditions in the polymer film and solution.     

Adsorption from a Layer of Finite Thickness on a Planar Electrode

Within the Nernst diffusion model, the effect the convection has on the adsorption at a planar electrode is studied for the case of a diffusion-controlled stage of adsorption and the behavior of the Frumkin–Melik–Gaikazyan finite adsorption impedance is analyzed. Allowing for the convection leads to new functional frequency dependences of constituents of this impedance at low frequencies, where the active constituent depends on the diffusion layer thickness more heavily than the capacitive one. During adsorption of neutral molecules from a layer of finite thickness, an additional relaxation time emerges in an electrochemical system, which results from a finite rate of motion of species in the near-electrode layer. Ignoring the convection may lead to erroneous interpretation of the adsorption mechanism.     

Electrochemical impedance spectroscopy fingerprints the ion selectivity of microgel functionalized ion-exchange membranes

Surface modification methods are applied to alter interfacial phenomena and improve ion transport through membranes. In this work we present a novel method for tailoring the surface of cation-exchange membranes based on the deposition of thin microgel monolayers. The charge of such layers exerts a strong influence on the monovalent-ion-selectivity, and this is reflected in the electrochemical impedance responses. Membranes coated with uncharged microgels show similar behavior to that of unmodified ones, with impedance spectra dominated by low-frequency diffusional arcs. However, membranes modified with positively charged microgels exhibit an increased resistance due to the hindered transport of cations through the modification. An additional high-frequency capacitive arc is obtained with the monovalent-ion-selective membranes, which is attributed to concentration polarization effects at the membrane/modification interface. The characteristic frequency of this arc decreases with the valency of the ion, thus proving that multivalent ions pass through the modification layer at rates much slower than monovalent ones. Accordingly, electrochemical impedance spectroscopy has been used to feature monovalent-ion-selective properties of layered membranes.     

Impedance simulation of a solid oxide fuel cell anode in time domain

The purpose of the current study is to simulate the behavior of a solid oxide fuel cell (SOFC) anode under sinusoidal excitation. The obtained harmonic response is used as a base for electrochemical impedance spectra simulation. The electrochemical impedance spectroscopy (EIS) is a powerful non-destructive tool for SOFC researches. In order to evaluate the EIS experimental results, efforts are devoted to develop EIS numerical simulation tools. In this study, a planar SOFC is modeled, and the steady state behavior and frequency response, as well as the electrochemical spectra of the anode, are obtained. The developed model couples the electrochemical kinetics with mass transport. The Butler–Volmer equation is used for the anode electrochemistry, and the species equations are used for gas transport in the anode channel. In order to solve the system of the nonlinear equations, an in-house code based on finite difference method is developed and utilized. A parametric study is also carried out, and the results are discussed. The simulation results are in good agreement with published data. Results show a capacitive semicircle in the Nyquist plot, which is identical to the gas diffusion impedance as reported in literatures.     

Modeling of finite diffusion impedance by <Emphasis Type="Italic">RC</Emphasis> two-terminal circuit

The Maxwell model consisting of three series capacitive circuits (SCCs) was employed for constructing an equivalent circuit of finite diffusion impedance (BW). The parameters of a low-frequency SCC were determined using a power series expansion of frequency dependences of BW capacitance and conductivity. All the further SCCs are calculated on the basis of the coincidence between BW and Warburg impedance at high-frequencies. Exact formulas are presented that allow extending an equivalent circuit by adding new SCCs, which is necessary for extension of the frequency range, in which the model and BW yield similar results.     

An in situ STM/AFM and impedance spectroscopy study of the extremely pure 1-butyl-1-methylpyrrolidinium tris(pentafluoroethyl)trifluorophosphate/Au(111) interface: potential dependent solvation layers and the herringbone reconstruction

The structure and dynamics of the interfacial layers between the extremely pure air- and water-stable ionic liquid 1-butyl-1-methylpyrrolidinium tris(pentafluoroethyl)trifluorophosphate and Au(111) has been investigated using in situ scanning tunneling microscopy, cyclic voltammetry, electrochemical impedance spectroscopy, and atomic force microscopy measurements. The in situ scanning tunnelling microscopy measurements reveal that the Au(111) surface undergoes a reconstruction, and at -1.2 V versus Pt quasi-reference the famous (22 × √3) herringbone superstructure is probed. Atomic force microscopy measurements show that multiple ion pair layers are present at the ionic liquid/Au interface which are dependent on the electrode potential. Upon applying cathodic electrode potentials, stronger ionic liquid near surface structure is detected: both the number of near surface layers and the force required to rupture these layers increases. The electrochemical impedance spectroscopy results reveal that three distinct processes take place at the interface. The fastest process is capacitive in its low-frequency limit and is identified with electrochemical double layer formation. The differential electrochemical double layer capacitance exhibits a local maximum at -0.2 V versus Pt quasi-reference, which is most likely caused by changes in the orientation of cations in the innermost layer. In the potential range between -0.84 V and -1.04 V, a second capacitive process is observed which is slower than electrochemical double layer formation. This process seems to be related to the herringbone reconstruction. In the frequency range below 1 Hz, the onset of an ultraslow faradaic process is found. This process becomes faster when the electrode potential is shifted to more negative potentials.     

Influence of nanoporous carbon electrode thickness on the electrochemical characteristics of a nanoporous carbon|tetraethylammonium tetrafluoroborate in acetonitrile solution interface

Electrochemical characteristics for the nanoporous carbon|Et₄NBF₄+acetonitrile interface have been studied by cyclic voltammetry and impedance spectroscopy methods. The influence of the electrolyte concentration and thickness of the nanoporous electrode material on the shape of the cyclic voltammetry and impedance curves has been established and the reasons for these phenomena are discussed. A value of zero charge potential, depending slightly on the structure and concentration of the electrolyte, the region of ideal polarizability and other characteristics have been established. The nanoporous nature of the carbon electrodes introduces a distribution of resistive and capacitive elements, giving rise to complicated electrochemical behaviour. Analysis of the complex plane plots shows that the nanoporous carbon|Et₄NBF₄+acetonitrile electrolyte interface can be simulated by an equivalent circuit, in which two parallel conduction paths in the solid and liquid phases are interconnected by the double-layer capacitance in parallel with the complex admittance of the hindered reaction of the charge transfer or of the partial charge transfer (i.e. adsorption stage limited) process. The values of the characteristic frequency depend on the electrolyte concentration and electrode potential, i.e. on the nature of the ions adsorbed at the surface of the nanoporous carbon electrode. The value of the solid state phase resistance established is independent of the thickness of the electrode material.     

Analysis of electrochemical impedance spectra of anodised pure aluminium foil with various etch tunnel length distributions used for electrolytic capacitors

The present work analyses the electrochemical impedance spectra of anodized pure Al foil with various etch tunnel length distributions, used for electrolytic capacitors. The lengths of the etch tunnels formed on the Al foil specimen were more widely distributed with increasing etching time. The etch tunnel length distributions were quantitatively measured by scanning electron microscopy. All the measured impedance spectra hardly exhibited the ideal capacitive behaviour in the high-frequency region and they deviated more significantly from the ideal capacitive behaviour with increasing etching time. For the numerical calculation of the total impedance of the electrode, the transmission line model was modified to reflect the etch tunnel length distribution, which represents the respective contributions of etch tunnels with different lengths to the total impedance. The total impedance was calculated by the integration of the impedance of one etch tunnel with respect to the etch tunnel length by taking the value of the etch tunnel length distribution determined experimentally. From the coincidence of the impedance spectra calculated numerically with those spectra measured experimentally, it is concluded that the deviation of the impedance spectra from the ideal capacitive behaviour in the high-frequency region originates from the characteristic frequency dispersion that depends upon the etch tunnel length distribution.     

A novel pH sensing membrane based on an ionic liquid-polymer composite

A novel pH sensing membrane was developed that consists of the ionic liquid n-cetylpyridinium hexafluorophosphate (CPFP), poly(vinyl chloride), and quinhydrone. The membrane is stable and flexible and can be easily deposited on the electrode. Electrochemical impedance spectroscopy was used to study the interfacial charge transfer of this membrane. Compared to a traditional plasticizer-based membrane electrode, the new electrode possesses excellent potentiometric characteristics for monitoring pH, such as a response time of less than 10 s, high sensitivity, stability, and reproducibility. The response is almost Nernstian, with a slope of ?57.5?±?0.2 mV pH^?1 in the pH range from 2 to 9.5. The new electrode was used for direct monitoring of pH in real food samples.

Electrochemical elucidation on the mechanism of uncoupling caused by hydrophobic weak acids

Uncoupler-mediated cation transport has been investigated by cyclic voltammetry for the ion transfer from one aqueous phase (W1) to another (W2) across a bilayer lipid membrane (BLM) in the presence of typical uncouplers, 3,5-di(tert-butyl)-4-hydroxybenzylidenemalononitril (SF6847) and 2,4-dinitrophenol (DNP). The voltammograms for the ion transfer were in a steady state and exhibited a rotated sigmoidal shape that was almost symmetrical about the origin (0 V, 0 A). The plot of the ion transfer current against pH was a bell-type curve centered on pH approximately = pK(a) + 1, K(a) being the dissociation constant of the uncouplers in the aqueous phase. Taking into account the ion transfer reactions at the W1|BLM and the BLM|W2 interfaces, these properties were well explained by our proposed model which considers that the ion transfer current is attributable to the facilitated transfers of H(+) and Na(+). The buffer action in the aqueous phase was found to play an important role in the facilitated H(+)-transfer across the BLM. The nature of the pH-dependence of the ion transfer current was reasonably explained from an electrochemical viewpoint based on the distribution coefficient of the anionic and neutral forms of SF6847, as estimated from its absorption spectra in liposomal membrane. The proposed model is also valuable for understanding the pH-dependence of uncoupling activity in mitochondria in the literature.     

Poly(styrene sulfonic acid)–poly(vinylidene fluoride) interpolymer ion-exchange membranes. II. Ultrafiltration properties

Highly porous interpolymer ion-exchange membranes of poly(styrene sulfonic acid) and poly(vinylidene fluoride) have been investigated under pressure filtration with KCI, Na₂SO₄, erythrosin, and bovine serum albumin as solutes in the feed solution. The rejection of the ionic solutes is governed by a Donnan exclusion of electrolyte from the membrane phase. A model for the transport behavior is proposed that includes both diffusive and convective salt transport. The calculated rejections agree adequately with the observed data.     

导电聚苯胺／MnO2空气阴极氧还原动力学

采用动电位扫描、交流阻抗技术研究了导电聚苯胺/MnO2复合阴极上氧还原反应动力学.动电位扫描表明氧在该复合阴极上还原的极化曲线服从Butler-Volmer公式,表观标准活化能为184.9 kJ/mol,反应为电化学步骤控制;交流阻抗谱观察到氧阴极还原由3个明显的线圈组成,表明氧阴极还原分3步进行,第1个圆弧随过电位的增大而显著减小,表明第1步电荷转移过程的确为氧还原反应的速率控制步骤;导电聚苯胺的高比表面积与MnO2的多微毛细管结构使氧在该复合电极上还原变得容易.     

Impedance spectroscopy of the electrode-tissue interface of living heart with isoösmotic conductivity perturbation

Impedance spectroscopy was used to solve the Pt electrode interface with metabolically active perfused living heart. Three impedance spectra were observed: the Warburg impedance (Z_W∞), a single high angle constant-phase-element, and a thin-film impedance (Z_D). When characterized again after cyclic change of ionic strength (and hence conductivity κ) each interface had one of only two spectra, with exclusion of Z_W∞. The in vivo interfacial impedance spectrum is thus neither single-valued nor stable in time. Because metal|living tissue interfaces are obligatory circuit elements in biosensors and electrodes in heart and brain, the multiple-valued and thin-film character of its impedance are significant.