N80油套管钢应力状态下的CO2腐蚀行为研究

利用交流阻抗技术研究了拉伸、压缩和弯曲三种应力状态下N80油套管钢在饱和CO₂地层水中的腐蚀过程. 结果表明, 三种状态下的阳极电化学阻抗谱均出现了高频容抗弧、中低频感抗弧和低频容抗弧三个时间常数, 其中高频容抗弧与双电层电容和传递电阻有关, 感抗弧对应膜的活性溶解, 低频容抗弧与试样表面膜的生成有关. 随着三种状态下应变的增加, 感抗弧逐渐扩大, 而低频容抗弧则逐渐缩小, 表明应力状态下N80钢的CO₂腐蚀会加剧. 三种状态下的阴极阻抗谱高频均由容抗弧组成, 中低频由Warburg阻抗和容抗弧共同组成, 三种状态下随着应变程度的增加, Warburg阻抗增加, 反应电阻减小, 双电层电容减小, 说明应力会加速阴极反应过程的进行, 从而增加N80钢的CO₂腐蚀速率.     

Complex rate constant for an electrochemical system involving an adsorbed intermediate

Two different methods of obtaining the impedance of a system involving an electrochemical reaction with an adsorbed intermediate are compared. The earlier method combines in parallel an impedance derived separately for the overall reaction with that for the rest of the system. It is appropriate for a fully supported electrolyte system but seems much less applicable to the unsupported situation. In contrast, the method derived and discussed herein involves complex reaction rate constants which may be directly incorporated in expressions for the total system impedance. This method may, therefore, be applied to either supported or unsupported situations. Several specific cases of the general complex rate constant results are discussed, particularly, various frequency dependencies possible. The overall small-signal impedance may be inductive in some parts of the frequency range and the low frequency limiting differential resistance negative, zero, or positive under various conditions.     

尖晶石锂锰氧化物锂离子嵌脱过程的交流阻抗谱研究

用粉末微电极研究了尖晶石锂锰氧化物在不同嵌锂状态下嵌脱锂过程的交流阻 抗图谱，提出了新的等效电路模型。描述锂离子在固体中扩散的Warburg阻抗与累 积或消耗的嵌入电容在Lix MnO4中的x≤0．5时串联，0．5＜x≤1时并联。用提出 的等效电路模型分阶段拟合了实验所得的交流阻抗谱，拟合值与实验值相当吻合， 由拟合结果得到不同电位下锂离子在表面膜中的迁移电阻和电容，界面电荷传递电 阻，双层电容，锂离子在固体中扩散系数和累积或消耗的嵌入电容。     

Impedance measurements for determination of pore texture of a carbon membrane

The pore texture of a carbon membrane was determined by impedance measurements carried out over a wide frequency range. The impedance obtained could be characterised by a resistance Ω and a double layer capacitance Cn. Ω is the third of electrolyte resistance in pores and Cn is the double layer capacitance corresponding to the developed pore surface. Membrane samples of different length but with the same thickness were studied. As expected, it was shown that the Cn value is proportional to the weight of the membrane whereas the Ω value is inversely proportional. Cn value allowed us to evaluate the specific area of the porous membrane from the value of the double layer capacitance per unit area determined for a plain carbon electrode. Moreover, impedance diagrams obtained were found to be very similar to those of cylindrical pore even though the pore texture is very intricate. Thus, impedance measurements can be applied to porous electrodes of more intricate pore texture and evaluate the radius, depth and pore number of its equivalent cylindrical pore electrode.     

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.     

Electrochemical impedance spectroscopy at single-walled carbon nanotube network ultramicroelectrodes

Electrochemical impedance spectroscopy (EIS), coupled with chemical vapour deposition (CVD) grown single-walled carbon nanotube (SWNT) network disk-shaped ultramicroelectrodes (UMEs), gives stable, very well-defined and highly reproducible EIS responses for electrolysis of a simple outer sphere redox couple (FcTMA^+/2+). The resulting EIS data can be fitted accurately using a simple electrical circuit model, enabling information on double-layer capacitance, diffusion coefficient of the electroactive species and the rate constant of ET (k⁰) to be extracted in a single EIS experiment. These values are replicated for a range of mediator concentrations and UME sizes (in the range 25–100 μm diameter) demonstrating the robustness of the method. These initial studies bode well for impedance based electroanalysis using SWNT network UMEs.     

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.     

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.     

Study of the adsorption of a hydrophobic peptide onto carbon surfaces by capacitance measurements

The adsorption of a hydrophobic peptide (β-CN (193–209)) onto glassy carbon and a carbon support of membrane was studied by double-layer capacitance measurements using electrochemical impedance spectroscopy (SIE). The kinetics of adsorption was investigated by recording the changes that occurred in the double-layer capacitance after adding the peptide. The change in capacitance was interpreted in terms of the number of close-contact areas between the peptide and the surface. A two-consecutive reaction model was used to describe peptide adsorption. The first step was attributed to the adsorption reaction itself and the second to a change in the conformation of the adsorbed peptide molecules. The corresponding theoretical equation of the variation of capacitance with time fits our results. Moreover, it allowed determination of the time constants for both reactions and three double-layer capacitances: the bare surface and the surfaces covered with monolayers of the peptide in states 1 and 2. For glassy carbon, the activation energies are 4.7 and 3.5 kJ mol⁻¹ for the first and second step of the reaction and the reaction orders are 0.64 and 0.35, respectively. These values are different for each step. This confirms the model of two separate reactions to describe adsorption. The first step comprises two elementary mechanisms. The first may correspond to the adsorption of the peptide onto the surface. The second may be consistent with a modification of the adsorbed molecules induced by the adsorption of other molecules over them. The second step of the process requires the contribution of peptides of the solution for it to occur. The changes in the double-layer capacitance depend on the peptide bulk concentration. This suggests that the peptide adsorbs in a different conformation depending on its bulk concentration. Peptide polymerisation could be responsible for this phenomenon.     

Generalized equivalent circuits for mixed conductors: silver sulfide as a model system

Abstract  

The generalized equivalent circuit for Hebb–Wagner polarization in the frequency domain proposed by Jamnik and Maier (J Electrochem Soc 146:4183, 1999) includes the space-charge polarization that was previously neglected. In the present work, using a self-coded Fortran program, the completely generalized equivalent circuit is successfully applied to a mixed conducting silver sulfide with an AgI electrode that suppresses the electronic flow. A whole set of fit parameters, such as geometric capacitance, partial conductivities, chemical capacitance or diffusivity, and the blocking and shunting characteristics of electrodes are independently but self-consistently obtained over a range of silver activities, as controlled by a galvanic cell. The interfacial capacitance was found to be much larger than the diffuse space-charge double-layer capacitance and was thus ascribed to the adsorption capacitance at the core of the interface, which should be connected in parallel with the space-charge double-layer polarization. Two simplified equivalent circuits were shown to be good approximations for the spectra at the extreme low and high silver activity, respectively.     

Compact and diffuse double layer interaction in unsupported system small-signal response

Previous exact results for the small-signal impedance of an unsupported electrode/material/electrode system which include effects of the finite size of charge carriers are simplified and discussed. The material contains non-recombining charges of opposite sign with the positive one immobile and uniformly distributed. General boundary conditions which encompass the range from no electrode reaction to ohmic electrode behavior are employed. In the presence of an electrode reaction, the interaction of the compact and diffuse double layers leads to considerably more complexity in the equivalent circuit than might appear in simple treatments of the supported case, in which the diffuse double layer capacitance is neglected or the compact double layer and diffuse double layer capacitances are placed in series. Two different approximate equivalent circuits made up of frequency-independent elements are found which yield remarkable agreement with the exact results over the entire frequency range of interest. The first involves the ordinary approximate circuit (OAC) previously found in the absence of compact layer effects plus a series compact layer contribution involving a parallel resonant circuit with quality factor at resonance which may approach unity. Pseudo-inductance effects are found to be extremely significant in this representation. The second approximate equivalent circuit, simpler and almost as accurate as the first, has the same form as the original OAC but with its reaction element values altered by the presence of the compact layer. For non-Butler-Volmer electrode kinetics an upper limit is found for the experimentally determinable apparent reaction rate constant, a feature of practical importance for thin films or membranes. The response of thin films and membranes, including compact layer effects, can very readily be erroneously confused with pure bulk response, yielding entirely incorrect values for the geometrical capacitance and bulk resistance of the material.     

Modeling the surface phenomena in carbon paste electrodes by low frequency impedance and double-layer capacitance measurements

Impedance and capacitance studies have been performed with covalently coupled Glucose oxidase (GOD) enzyme, covalently coupled flavin adenine dinucleotide (FAD), reconstituted GOD enzyme and blank carbon paste electrodes to study the changes in the electrochemical interfacial properties. Impedance studies were performed using a low frequency impedance technique and the electrochemical surface capacitance was measured by a pulse technique. We have attempted to fit the experimental values to an equivalent circuit model. The Randles' cell circuit with Warburg impedance modeled well the experimental values and the behavior of the enzyme electrodes. The individual components of the model were calculated and the parameters were explained. The blank paste electrode showed a constant phase element behavior.     

Dynamics of a nanowire in twisted nematic liquid crystal

The dynamics of a nanowire immersed in a nematic liquid crystal sandwiched between two parallel plates are studied by applying an equivalent capacitance approach used in electrostatics. A lower cut-off eigenfrequency for the oscillation of the nanowire is found and the lighter the mass, the smaller the critical cell separation at which the cut-off frequency occurs is needed. A simulation of the dynamical process of the metal wires shows that the relaxation time is proportional to η/m in the small mass region.     

In situ monitoring of gold-surface adsorption and acidic denaturation of human serum albumin by an isolation-capacitance-adopted electrochemical quartz crystal impedance system

Combined measurements of piezoelectric quartz crystal impedance (PQCI) and electrochemical impedance spectrum (EIS) using a suitable isolation capacitance is reported for the first time to monitor in situ adsorption and acidic denaturation of human serum albumin (HSA) on gold electrodes in Britton-Robinson (B-R) buffers. This method provides simultaneously mutual-interference-free and accurate parameters of EIS and PQCI. Effects of surface thiol-modification, electrode-potential and solution pH on HSA adsorption were examined and discussed. Comparative experiments of HSA adsorption in a B-R buffer of pH 6.42 on bare, cysteine- and 1-dodecanethiol-modified gold electrodes revealed that HSA adsorption is more significant on a hydrophobic (1-dodecanethiol-modified) surface. Insignificant electrode-potential effect implied minor electrostatic effects on HSA adsorption. The adsorption amount of HSA at pH 3.28 was found to be notably greater than those at pH 4.84 and 6.42. To characterize HSA adsorption, electrode standard rate constants (k_s) of the Fe(CN)₆³⁻/Fe(CN)₆⁴⁻ couple were measured before and after HSA adsorption. The k_s-pH curves on an HSA-modified Au electrode revealed that k_s increased abruptly with the decrease of solution pH below pH ∼4. Moreover, pH-dependent responses of the resonant frequency, the motional resistance, the double-layer capacitance, the capacitance of adsorbed HSA layer and the peak absorbance of HSA solutions at 278 nm all exhibited an inflexion change at pH ∼4, and these findings have been explained on the basis of acidic denaturation of HSA and electrical charges carried by HSA molecules.     

Electrochemical behavior of nickel in electrolytes based on 1-n-butyl-3-methylimidazolium tetrafluoroborate ionic liquid for capacitor applications

Electrochemical studies were performed using Ni electrodes in solutions of a mixture of ethylene glycol or of γ-butyrolactone with 1-n-butyl-3-methylimidazolium tetrafluoroborate ionic liquid. The aim of the study was to evaluate the use of these systems in electrochemical double-layer capacitor. Cyclic voltammetry experiments showed a potential range at which the Ni electrode behaved as a polarizable electrode. Ni oxidizes at high anodic potentials. Inside the potential range without electrochemical activity, the capacitance and the solution resistance, which were evaluated by impedance electrochemical spectroscopy, were compared for the two solutions tested. Conductivity measurements of the electrolytes with different compositions were also acquired. The results of cyclic voltammetry indicated that the Ni has a wide electrochemical window and low current peak densities of oxidation in the γ-butyrolactone medium than in ethylene glycol medium. The γ-butyrolactone and 1-n-butyl-3-methylimidazolium tetrafluoroborate ionic liquid solutions had the highest conductivity values. Decreased 1-n-butyl-3-methylimidazolium tetrafluoroborate ionic liquid content in different solvent mixtures resulted in an increase in the capacitance value at the Ni/electrolyte interface. The highest capacitance values were obtained for Ni in ethylene glycol and 1-n-butyl-3-methylimidazolium tetrafluoroborate medium.     

Hysteresis in the measurement of double-layer capacitance at the gold–ionic liquid interface

The capacitance of the polycrystalline gold electrode–ionic liquid BMIMTf [1-butyl-3- methyl imidazolium trifluoromethanesulfonate (triflate)] interface has been studied using ac voltammetry and electrochemical impedance spectroscopy (EIS). Slow potential scanning reveals a massive hysteresis between potential scans toward negative or positive directions and provides a distorted C(E) curve. Kinetic studies indicate that 10 min wait times are required after each potential step for system relaxation. EIS resolves the overall series capacitance into contributions from ion adsorption and double-layer capacitance, C_DL. Isolation of C_DL reveals that even 10 min of equilibration time is insufficient to completely remove the hysteresis. The shape of the negative-going C_DL(E) curve is camel-shaped whereas the shape of the positive-going curve is bell-shaped.     

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。     

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.     

Theory of electrode reactions of redox couples confined to electrode surfaces at monolayer levels: Part II. Cyclic voltammetry and ac impedance measurements

The expression of the current-potential relationship derived in Part I for simple one-step surface redox-electrode reactions of the species confined to electrode surfaces is applied to cyclic voltammetry and the method of faradaic impedance measurements. A method to obtain cyclic voltammograms is presented for a quasi-reversible or general case and equations for reversible and irreversible cyclic voltammograms are derived. The effect of the interaction parameters, W/RT and ΔW^≠/RT, kinetic parameters, Λ and α, and coordination number z on the waveform of the cyclic voltammograms is discussed. An interesting feature in the voltammograms, i.e. the appearance of two peaks, is predicted when W/RT < −2.19 for z = 6 in spite of the simple one-step redox process. Furthermore, equations for the faradaic resistance and capacitance are derived and it is shown that the faradaic impedance is independent of the frequency of ac perturbation.