电化学阻抗法研究环氧膜的吸水性能

电化学阻抗法研究环氧膜的吸水性能罗小雯，陈月辉，李善君，周伟舫（复旦大学高分子科学系，化学系，上海，２００４３３）关键词电化学阻抗谱，环氧膜，交联密度，吸水性能水在环氧树脂膜中有较强的吸收和扩散能力，例如ＸＤ７３４２／ＴＭＡＢ于１９５℃固化２４ｈ，在...     

电化学阻抗谱法研究环氧膜的渗水机理

本工作测量了覆盖环氧膜的铝电极在NaCl溶液中的电化学阻抗谱, 并求得了该膜的膜电阻、膜电容和环氧膜与底金属间的腐蚀电阻及双电层电容。由膜电容求得该膜的介电常数为5.2。此种膜未浸入NaCl溶液前的介电常数为4.9。将上述数据与未覆盖环氧膜的铝电极的电化学阻抗数据比较, 可认为溶液是经上述膜中的化学通道而渗入膜内的。     

Interfacial behaviour of trideoxyribonucleotide d(CpGpG) and its complex with the antitumour drug cis platin at the mercury/electrolyte solution interface

The adsorption of the trideoxyribonucleotide diphosphate d(CpGpG) and its complex with cis-DDP at the dropping mercury electrode was investigated by measurement of differential capacitance of the electric double layer. It was found that non-platinated trimer yields an anomalous pit on the capacitance curves at higher bulk concentrations. On the other hand, the platinated trinucleotide does not yield the pit under the same experimental conditions. It was suggested that the compact and perpendicular adsorption film, responsible for the formation of the anomalous pit on the capacitance curves, does not appear if the stacking interactions between parallel base rings are disturbed.     

Electrochemical impedance spectroscopy analysis of chalcopyrite CuFeS2 electrodes

A chalcopyrite CuFeS₂ electrode obtained from the “El Teniente” mine has been studied by Electrochemical Impedance Spectroscopy (EIS) in an alkaline solution for different oxidation potentials. The experimental results can be interpreted from a Randles equivalent circuit, V_dc<0.4 V vs. saturated calomel electrode (SCE), and a surface layer model for V_dc>0.4 V vs. SCE. From these results, the variation with the d.c. applied potentials of charge transfer electrical resistance of the redox reaction, the double layer capacitance and other characteristic parameters are considered.     

Electrochemistry,ion adsorption and dynamics in the double layer: a study of NaCl(aq) on graphite

Graphite and related sp² carbons are ubiquitous electrode materials with particular promise for use in e.g., energy storage and desalination devices, but very little is known about the properties of the carbon–electrolyte double layer at technologically relevant concentrations. Here, the (electrified) graphite–NaCl(aq) interface was examined using constant chemical potential molecular dynamics (CμMD) simulations; this approach avoids ion depletion (due to surface adsorption) and maintains a constant concentration, electroneutral bulk solution beyond the surface. Specific Na⁺ adsorption at the graphite basal surface causes charging of the interface in the absence of an applied potential. At moderate bulk concentrations, this leads to accumulation of counter-ions in a diffuse layer to balance the effective surface charge, consistent with established models of the electrical double layer. Beyond ∼0.6 M, however, a combination of over-screening and ion crowding in the double layer results in alternating compact layers of charge density perpendicular to the interface. The transition to this regime is marked by an increasing double layer size and anomalous negative shifts to the potential of zero charge with incremental changes to the bulk concentration. Our observations are supported by changes to the position of the differential capacitance minimum measured by electrochemical impedance spectroscopy, and are explained in terms of the screening behaviour and asymmetric ion adsorption. Furthermore, a striking level of agreement between the differential capacitance from solution evaluated in simulations and measured in experiments allows us to critically assess electrochemical capacitance measurements which have previously been considered to report simply on the density of states of the graphite material at the potential of zero charge. Our work shows that the solution side of the double layer provides the more dominant contribution to the overall measured capacitance. Finally, ion crowding at the highest concentrations (beyond ∼5 M) leads to the formation of liquid-like NaCl clusters confined to highly non-ideal regions of the double layer, where ion diffusion is up to five times slower than in the bulk. The implications of changes to the speciation of ions on reactive events in the double layer are discussed.

CμMD reveals multi-layer electrolyte screening in the double layer beyond 0.6 M, which affects ion activities, speciation and mobility; asymmetric charge screening explains concentration dependent changes to electrochemical properties.     

Effect of Nb alloying additions on the characteristics of anodic oxide films on zirconium and their stability in NaOH solution

The characteristics of oxide films on Zr and Zr–Nb alloys (with Nb content of 2.5, 5, and 10 at.%) galvanostatically formed (at a current density of 100 μA cm^?2) in 0.5 M H₂SO₄ solution were investigated by means of electrochemical impedance spectroscopy. Electrochemical impedance spectroscopy spectra were interpreted in terms of an “equivalent circuit” with the circuit elements representing the electrochemical properties of a single layer oxide. The resistance of the oxide films was found to increase with increased Nb content in the alloy while the capacitance showed an opposite trend. The stability of the anodic oxide films grown in the sulfuric acid solution on Zr and Zr–Nb alloys was investigated by simultaneously measuring the electrode capacitance and resistance at a working frequency of 1 kHz as a function of exposure time to naturally aerated 3 M NaOH solution. Analyses of the electrode capacitance and resistance values indicated a decrease in chemical dissolution rate of the oxide films with the increase of Nb content in the alloy.     

Adsorption behavior of bis (2-ethylhexyl) sodium sulfosuccinate (AOT) at the mercury-electrolyte solution interface as a function of electrode potential and time

The dependence of the differential capacitance (C) of the electrode double layer of a hanging mercury drop electrode in bis (2-ethylhexyl) sodium sulfosuccinate (AOT) solutions on electrode potential (E) and time is measured using three-dimensional phase sensitive ac voltammetry. This methodology, possessing a very wide time window that permits a detailed study of the adsorption phenomena, is based on the reconstruction of C vs E curves, sampled after many phase-sensitive ac chronoamperometric experiments. The shape of these curves allows an estimation of the structure of the layer of AOT molecules absorbed at the electrode surface. AOT molecules form micelles in bulk solutions and they also associate in the charged interface under the strong influence of the electric field into surface aggregates which depend on their concentration and applied potential. The presence of AOT micelles in the bulk solution can be linked with the appearance of a surface film at potentials more negative than those corresponding to a condensed film linked with a capacitance value slightly higher than that normally attributed to a compact layer. The whole phenomenon is proved to be very dependant upon time.     

Effect of the diffuse double layer on the interpretation of EQCM results in dilute NaClO4 solutions

The response of the electrochemical quartz crystal microbalance (EQCM) in dilute NaClO₄ solutions was studied with gold and iron electrodes during a stepwise increase of the perchlorate concentration. In the range from 10⁻⁴ M to 7.8×10⁻² M, the quartz resonant frequency of the 10 MHz AT cut crystals increased by about 700 Hz, indicating a mass loss on the electrode. A model was developed in which the diffuse double layer and the oscillating bulk electrolyte layer, characterised by the velocity decay length of the damped shear wave in solution, are treated as two independent, superimposed sheets. By assuming a characteristic thickness of the diffuse double layer according to the Gouy–Chapman theory and by treating the diffuse double layer as a rigid sheet, the measured mass loss could be simulated qualitatively. The viscosity changes in the diffuse double layer as well as in the sensed electrolyte bulk layer were found to be negligible in the concentration range investigated. In dilute solutions, the frequency shift following a concentration change is entirely due to thinning of the diffuse double layer with increasing concentration. The results demonstrate the importance of diffuse double layer effects for EQCM measurements in dilute electrolytes.     

Electrochemical impedance spectroscopy of polynucleotide adsorption

The dependence of the impedance of the electrode double layer of mercury electrode on frequency around the potentials of the tensammetric peaks of single-stranded and double-helical polynucleotides and DNA was studied. From the frequency dependence of the impedance of the electrode double layer represented in a complex plane impedance plot, the electric equivalent circuit of the electrode covered with adsorbed DNA layer was determined. It was concluded that the desorption of denatured ssDNA is accompanied by higher dielectric losses than the desorption of native dsDNA. This can be explained by the higher flexibility of ssDNA compared to the dsDNA. The capacitance peak of single-stranded polyadenylic acid (poly A) observed at pH 8 around -1.3 V splits at low frequencies in two peaks.     

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.     

The adsorption structure of nonionic surfactant on the sessile mercury electrode in a thin liquid film

In relation to a colloid stability, the adsorption structure of the Stern layer on a sessile mercury electrode in a thin liquid film of nonionic surfactant was investigated by measuring the double layer capacitance. The Stern capacitance on the electrode in the film could be detected when the measuring frequency used was low, for the resistance of the film was not extremely high but of the order of several thousand ohm. It was found that the adsorption structure of nonionic surfactant in the thin liquid film shows a stratification different from that of bulk.     

Analysis of Impedance Spectra of Vacuum-Annealed Undoped Polycrystalline Diamond Electrodes

Impedance spectra for a new electrode material, undoped polycrystalline diamond films vacuum-annealed at 1775 to 1915 K, taken in indifferent electrolytes are analyzed. Regular changing of parameters of the equivalent circuit (constant-phase element, ohmic and faradaic resistances) with increasing anneal temperature is demonstrated. It is concluded that both the amount of a nondiamond conducting phase in the electrode bulk increased and its properties (first and foremost, the conductivity) changed with increasing anneal temperature.     

Kinetics of electroreduction of S<Subscript>2</Subscript>O<Stack><Subscript>8</Subscript><Superscript>2−</Superscript></Stack> anions on mechanically renewable silver electrode

The kinetics of electroreduction of peroxodisulfate anions on a mechanically renewed silver electrode is studied by voltammetric and impedance methods. Impedance diagrams obtained in the region of negatively charged metal surface are successfully modeled by the equivalent circuit formed by the solution resistance and also the reaction resistance connected in parallel with the constant-phase element substituted for the double layer capacitance. The analysis of experimental data carried out in terms of this circuit and their comparison with the literature data makes it possible to assume that the reduction kinetics of S₂O ₈ ^2? anions on this electrode can be adequately described in the framework of the phenomenological theory of slow discharge. The reasons for deviation of the literature data on the kinetics of this reaction on polycrystalline Ag electrode from the relationships following from the slow discharge theory are put forward and substantiated.     

粉末多孔镍电极电化学阻抗谱及其数学模型

镍电极反应动力学在大多情况下是受固态质子扩散过程控制的，以此为出发点建立了具有明确物理意义的镍电极电阻抗谱（ＥＩＳ）的数学模型，并以该模型为基础，讨论了一些模型参数如双电层电容Ｃｄ１，质子扩散系数Ｄ及活性物质粒子半径ｒ０等改变，电极的不同荷电状态及多孔镍电极中的传质过程对镍电阻阻抗谱的影响，理论模型较好地解释了一些实验结果。     

Electrochemical Properties of Carbon Nanoparticles Entrapped in Silica Matrix

Carbon-based electrode materials have been widely used for many years for electrochemical charge storage, energy generation, and catalysis. We have developed an electrode material with high specific capacitance by entrapping graphite nanoparticles into a sol-gel network. Films from the resulting colloidal suspensions were highly porous due to the removal of the entrapped organic solvents from sol-gel matrix giving rise to high Brunauer-Emmett-Teller (BET) specific surface areas (654 m(2)/g) and a high capacitance density ( approximately 37 F/g). An exponential increase of capacitance was observed with decreasing scan rates in cyclic voltammetry studies on these films suggesting the presence of pores ranging from micro (< 2 nm) to mesopores. BET surface analysis and scanning electron microscope images of these films also confirmed the presence of the micropores as well as mesopores. A steep drop in the double layer capacitance with polar electrolytes was observed when the films were rendered hydrophilic upon exposure to a mild oxygen plasma. We propose a model whereby the microporous hydrophobic sol-gel matrix perturbs the hydration of ions which moves ions closer to the graphite nanoparticles and consequently increase the capacitance of the film.     

Characterization of Electrode/Electrolyte Interface of ECIS Devices

Electric cell‐substrate impedance sensing requires low electrode/electrolyte interface impedance for effective biomedical and biophysical applications. Thus a complete understanding of physical processes involved in the formation of an electric double layer is required to design a low interface impedance device. This paper presents the numerical simulation of the impedance for the electrode/electrolyte interface of three‐electrode devices along with the practical realization for the effective workout of impedance sensing devices. The three‐electrode based impedance sensing devices along with phosphate buffered saline as electrolyte is simulated using COMSOL Multiphysics to evaluate the impedance of the electrode/electrolyte interface. Microfabrication technology is used to realize three‐electrode impedance sensing devices with diverse configuration which are used to measure the electrode/electrolyte interface impedance. The measured impedance data were then compared with the COMSOL simulated results and it is found that both the data sets fitted well with less than 5 % RSE. The results obtained from simulation and experiments indicate that the impedance due to double layer diffusion dominates in the low frequency region up to few kHz whereas electrolytic bulk resistance plays a major role in the higher frequency range. The experimental impedance data were further interpreted by electrochemical impedance spectroscopy analysis software to model the equivalent circuit of the electrochemical system.     

Highly conductive nanostructured C-TiO2 electrodes with enhanced electrochemical stability and double layer charge storage capacitance

The present work reports the structural and electrochemical properties of carbon-modified nanostructured TiO(2) electrodes (C-TiO(2)) prepared by anodizing titanium in a fluoride-based electrolyte followed by thermal annealing in an atmosphere of methane and hydrogen in the presence of Fe precursors. The C-TiO(2) nanostructured electrodes are highly conductive and contain more than 1 × 10(10) /cm(2) of nanowires or nanotubes to enhance their double layer charge capacitance and electrochemical stability. Electrogenerated chemiluminescence (ECL) study shows that a C-TiO(2) electrode can replace noble metal electrodes for ultrasensitive ECL detection. Dynamic potential control experiments of redox reactions show that the C-TiO(2) electrode has a broad potential window for a redox reaction. Double layer charging capacitance of the C-TiO(2) electrode is found to be 3 orders of magnitude higher than an ideal planar electrode because of its high surface area and efficient charge collection capability from the nanowire structured surface. The effect of anodization voltage, surface treatment with Fe precursors for carbon modification, the barrier layer between the Ti substrate, and anodized layer on the double layer charging capacitance is studied. Ferrocene carboxylic acid binds covalently to the anodized Ti surface forming a self-assembled monolayer, serving as an ideal precursor layer to yield C-TiO(2) electrodes with better double layer charging performance than the other precursors.     

Salient features of the electrical double layer structure on mechanically renewed gold-silver electrodes in aqueous solutions of a surface-inactive electrolyte

The behavior of electrodes, which are made of binary Au-Ag alloys (Ag content 1–15 at %) and renewed by mechanical cutting in aqueous solutions of sodium fluoride, is studied with the aid of cyclic voltammetry and impedance methods. It is established that, in the region of potentials corresponding to ideal polarizability, the differential capacitance of the electrical double layer rapidly changes with time elapsed after the renewal of the surface of the electrodes. The change in the capacitance is brought about by the exit of silver atoms into a surface layer. The implication is that silver is the surface-active component in these alloys. The rate of the surface segregation of silver atoms depends on the electrode potential. The segregation rate substantially increases upon going into the region that corresponds to positive charges of the silver electrode surface and to the beginning of adsorption of atomic oxygen on the electrode. Based on phenomenological models, a method for processing capacitance curves is realized, which links experimentally observed time effects to variations that occur in the surface composition, and assumptions concerning the mechanism of relaxation processes that are responsible for the observed time effects are put forth. Explicit data on the effect, which is exerted by mechanical renewal on the composition of the surface layer of Au-Ag alloys at different distances from the interface with a vacuum, are obtained with the aid of an x-ray photoelectron spectroscopy method. It is established that the surface layer (～0.5 nm) is enriched by silver atoms as compared with the alloy’s bulk.     

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.     

D.C. voltammetry of ionic liquid-based capacitors: Effects of Faradaic reactions, electrolyte resistance and voltage scan speed investigated using an electrode of carbon nanotubes in EMIM-EtSO4

Carbon nanotube (CNT) electrodes in combination with ionic liquid (IL) electrolytes are potentially important for energy storage systems. We report electrochemical investigation of such a system involving a paper-electrode of multi-wall CNT (MWCNT) in the IL of 1-ethyl-3-methyl imidazolium ethylsulfate (EMIM-EtSO₄). Our study concentrates on the analytical aspects of cyclic voltammetry (CV) to probe the double layer capacitance of these relatively unconventional systems (that involve rather large charge-discharge time constants). Both theoretical and experimental aspects of CV for such systems are discussed, focusing in particular, on the effects of Faradaic side-reactions, electrolyte resistance and voltage scan speeds. The results are analyzed using an electrode equivalent circuit (EEC) model, demonstrating a method to account for the typical artifacts expected in CV of CNT-IL interfaces.