Integrated description of electrode/electrolyte interfaces based on equivalent circuits and its verification using impedance measurements

An integrated theory describing both faradaic and nonfaradaic currents obtained upon potential step at an electrified electrode/electrolyte interface has been developed based on equivalent circuits that had been used to explain electrochemical reactions and experimentally verified. The faradaic current is shown to consist of the mass transport-dependent and -independent parts, which is in general agreement with the expression previously derived from the diffusion equations. The decay of the capacitive current is determined by the time constant represented by the product of the resistance obtained from the parallel connection of the solution and polarization resistances and the double layer capacitance; this is not consistent with the current understanding of the capacitive current decay, which takes into account the double layer capacitance and the solution resistance only. Many insights into the electron-transfer reactions are discussed based on the interpretation of impedance representation of the system, which would not have been possible without the present theory.     

Scanning electrochemical microscopy 50. Kinetic study of electrode reactions by the tip generation-substrate collection mode

A scanning electrochemical microscopy (SECM) methodology for localized quantitative kinetic studies of electrode reactions based on the tip generation-substrate collection (TG-SC) operation mode is presented. This approach does not use the mediator feedback required in typical kinetic SECM experiments. The reactant is galvanostatically electrogenerated on a tip placed in proximity to the substrate. It diffuses through the tip-substrate gap and undergoes the reaction of interest on the substrate surface. The substrate current is monitored with time until it reaches an apparent steady-state value. The process was digitally simulated using an explicit finite difference method, for an irreversible first-order electrode reaction at the substrate. Transient responses, steady-state polarization curves, and TG-SC approach curves can be used to obtain substrate kinetics. The effects of the experimental parameters were analyzed. The possibility of easily changing the experimental conditions with the SECM is an attractive approach to obtain independent evidence that can be used for a strict test of reaction mechanisms. The technique was applied for a preliminary simplified kinetic examination of the oxygen reduction reaction in phosphoric acid.     

Limitations of the potential step technique to impedance measurements using discrete time Fourier transform

Recently, a new method of measuring impedance of electrochemical systems was proposed in the literature by Yoo and Park (Yoo, J.-S.; Park, S.-M. Anal. Chem. 2000, 72, 2035). It is based on the analysis of system response to a potential step. Differentiation of the applied potential step and the current response in the time domain followed by applying Fourier transform to both signals allows for determination of the system's impedance. It has been proposed that the measurements carried out in a short time period permit the determination of the system's impedance in the whole frequency range. The aim of the present work was to verify the validity of the impedance spectra obtained using this method, as well as to establish the conditions for which the method may be used. This method was tested using simulated data for a simple ideally polarized electrode and a simple one-electron redox system in the solution. The results show that the reliable impedance spectra may be obtained only for frequencies between 1/(NDeltat) and 1/(2Deltat), where Deltat denotes the sampling time and N is the number of points acquired during the experiment. However, the artifacts are generated when the experimental data are extrapolated to lower frequencies.     

Information on the mechanism of mechanochemical reaction from detailed studies of the reaction kinetics

Mechanochemical and mechanical alloying processes take place between colliding surfaces in the heavy container of a ball mill, where the in situ examination of the reaction mechanism is extremely challenging. As shown in this paper, useful indirect information can be obtained from detailed analysis of the reaction kinetics. A shaker mill with a single ball was used, so that time could be replaced with the number of collisions as the variable of kinetics. A simple stochastic model was developed that is capable of describing the kinetics of gradual mechanochemical reactions and the variation of physical properties such as grain size. The kinetic constant is directly related to the fraction of powder processed in a single collision, and its value indicates that only a few micrograms of powder are processed in a single collision. Measuring the kinetic constant as a function of impact energy revealed that a minimum impact energy, on the order of a few hundreds of a Joule, is needed to initiate chemical change. The model was also applied to the self-sustaining reaction between Ti and graphite. In that case, the critical number of collisions required for ignition characterizes the speed of mechanical activation.     

Chronoamperometry of surface-confined redox couples for irreversible two-step and three-step consecutive reaction mechanisms

Chronoamperometry has been widely employed to determine kinetic rate constants for electron-transfer reactions of surface-confined redox couples. When the mechanism for the transformation is more complicated than a one-electron transfer, deviations can be expected from an exponential decay of the current transient. Theory is presented in this paper for irreversible, two-step and three-step consecutive reaction mechanisms in the form of analytical solutions of the corresponding differential equations. The theory should find application to surface electrode reactions with two-electron "n-values".     

Method for determination of association and dissociation rate constants of reversible bimolecular reactions by isothermal titration calorimeters

The rate law equation for reversible bimolecular reactions, which are describable by association and dissociation rate constants (k1 and k-1), is not solvable to a plain formula under stoichiometric reaction conditions. Therefore, it is a general technique to observe such reactions under pseudo first-order conditions, which make the reactions a single-exponential process, and enable us to determine k1 and k-1 without any complicated iterative computations needed to analyze the same reactions under stoichiometric reaction conditions. However, the accelerated reaction rates under pseudo first-order conditions are not always favorable to the physicochemical tools employing a slow or medium response time, such as thermal analysis instruments. In this study, we have developed a simple non-iterative analytical method to determine k1 and k-1 of reversible bimolecular reactions under stoichiometric conditions on the basis of experimental data of isothermal titration calorimetry (ITC), which is generally used to determine thermodynamic parameters rather than kinetic constants. Our method is principally based on the general principle of chemical bindings caused along with the titration processes, that is, the chemical relaxation kinetics, which had been hitherto considered in the analysis on the ITC data.     

Characterizing homogeneous chemistry using well-mixed microeddies

Well-mixed reaction volumes are often sought in engineered microchemical devices and can be an important feature of naturally occurring physicochemical processes such as pitting corrosion. Steady streaming eddies can serve as well-mixed, easily controlled microliter chemical reactors for characterizing homogeneous chemical reactions. Here, steady streaming eddies are produced by oscillating a liquid-filled cuvette around a stationary cylindrical electrode (radius 406 microm, length 1.6 cm) at audible frequencies (75 Hz). Oxidant (ferricyanide) electrochemically dosed at small rates (相似文献   

Pulse voltammetric and ac impedance spectroscopic studies on lithium ion transfer at an electrolyte/Li4/3Ti5/3O4 electrode interface

Pulse voltammetry and ac impedance spectroscopy were used to study the lithium ion kinetics at a lithium ion insertion electrode consisting of Li4/3Ti5/3O4 thin films in an organic electrolyte. In the cyclic voltammogram, two redox peaks appeared at around 1.56 V vs Li/Li+ due to the insertion and extraction of lithium ion at the electrode. Differential pulse voltammetry gave a large reduction current at approximately 1.56 V during a cathodic scan due to lithium ion insertion into the electrode. From the peak current and potential, the charge-transfer resistance was evaluated by quantitative analysis using approximate equations for irreversible reactions. In the Nyquist plot, one semicircle was observed at 1.56 V, which was assigned to the charge-transfer resistance due to lithium ion transfer at the electrode/electrolyte interface. The value of the charge-transfer resistance at 1.56 V was almost identical to that evaluated by differential pulse voltammetry with an identical characteristic relaxation time. This result shows that both dc differential pulse voltammetry and ac impedance spectroscopy are useful for elucidating the phase transfer kinetics of lithium ion at insertion electrodes.     

A strategy for the determination of enzyme kinetics using electrospray ionization with an ion trap mass spectrometer.

A simple and rapid means of enzyme kinetic analysis was achieved using electrospray ionization mass spectrometry and a one-point normalization factor. The model system used, glutathione S-transferase from porcine liver, is a two-substrate enzyme catalyzing the conjugation of glutathione with a variety of compounds containing an electrophilic center. An internal standard that is structurally similar to the product was added to the reaction quench solution, and a single-point normalization factor was used to determine the product concentration without the need of a calibration curve. Kinetic parameters, such as Km, Vmax and Ki (for thyroxine), obtained by electrospray mass spectrometry agreed with those obtained from traditional UV-vis spectroscopy, and competitive vs noncompetitive inhibition reactions could be delineated via mass spectrometry. These results suggest that our method can be applied to enzymatic processes in which spectrophotometric or spectrofluorometric assays are not feasible or when the relevant substrates do not incorporate chromophores or fluorophores. This new method is competitive with traditional UV assays in that it is facile and it involves very little analysis time.     

Direct electrochemical probing of DNA hybridization on oligonucleotide-functionalized polypyrrole

We here report a new approach for the direct label of a free detection of DNA hybridization using electrochemical techniques applied to a system made of a polypyrrole matrix supported at the surface of an electrode and modified with DNA probes by covalent grafting. The binding occurs at sites where the polypyrrole is functionalized with activated ester groups. The hybridization reactions with the DNA complementary target and non-complementary target were investigated by both amperometric analysis and non-faradaic electrochemical impedance spectroscopy. The results show a significant modification in the electrochemical response of the layer upon addition of the complementary target. A variation of the redox activity of polypyrrole was demonstrated and was shown to correlate with the result obtained with impedance measurement which was related to the modification of conjugation of the polypyrrole backbone.

The Nyquist plots obtained upon hybridization reaction were analysed by the Randles circuit. Electrical parameters such as capacity and transfer charge resistance were analysed and it was shown that the hybridization reaction caused a decrease of charge transfer and an increase of capacity. The charge transfer resistance R₂ showed a linear variation as a function of the complementary target concentration. The sensitivity and the detection limit were determined and raised 1 pmol which constituted a substantial enhancement when compared to the results obtained by other systems.     

Highly sensitive voltammetric and impedimetric sensor based on an ionic liquid/cobalt hexacyanoferrate nanoparticle modified multi-walled carbon nanotubes electrode for diclofenac analysis

This paper describes the development of 1-methyl-3-butylimidazolium chloride ionic liquid/cobalt hexacyanoferrate nanoparticle modified multi-walled carbon nanotubes nanocomposite paste electrode for the electrocatalytic and adsorptive stripping voltammetric and impedimetric determination of diclofenac (DIC) in real samples. The nanocomposite was prepared by a simple chemical method and was characterised by scanning electron microscopy, Fourier transform infrared spectroscopy and atomic absorption spectroscopy. Also, the electrochemical behaviours of the modified electrode and the electrocatalytic oxidation of DIC were investigated in detail by cyclic voltammetry, chronoamperometry and electrochemical impedance spectroscopy techniques. The kinetic parameters such as electron transfer coefficient, and apparent rate constant for the redox reaction between DIC and the modified electrode were also determined using electrochemical approaches. It was found that the modified electrode exhibited excellent electrocatalytic activity toward the oxidation of DIC, and under the optimised conditions, the linear response range and detection limit were found to be 1.0–100.0 and 0.3 μM, respectively using the differential pulse voltammetry method. The proposed method was applied for the sensitive and selective determination of diclofenac in the urine samples and pharmaceutical formulations with satisfactory results.     

Real-time impedance measurements during electrochemical experiments and their application to aniline oxidation

Development of an in situ technique for measuring electrochemical impedance spectra in real time during an electrochemical experiment is described. The technique is based on staircase voltammetry with relatively large step heights, in which a series of increasing potential steps are applied to an electrochemical system, and the resulting currents are sampled. The first derivatives of the currents thus obtained are then converted to ac current signals in frequency domain, and impedances are computed from them. To demonstrate the technique as a tool for studying the electrode/electrolyte interface during the electrochemical reaction, we chose an electrochemical oxidation reaction of aniline, whose reaction products have been known to continuously change the electrode surface due to the polymer film growth on its surface, and report a number of observations that would not have been obtained without such in situ experiments. A suggestion is also made on the use of staircase voltammetry for mechanistic studies on complex electrochemical reactions by simply varying the sampling time.     

Linear systems with polynomials of filtered Poisson processes

Moment equations are calculated exactly for the response of linear systems subjected polynomials of filtered Poisson processes. The Itô formula for stochastic differential equations driven by Poisson white noise is applied to derive moment equations. It is shown that the set of moment equations is closed. The proposed method is used to calculate moments up to the fourth order for the response of two linear systems subjected to quadratic forms of filtered Poisson processes. Results by Monte Carlo simulations are also presented for comparison.     

Ultra high-performance liquid chromatography-tandem mass spectrometry for the simultaneous analysis of asparagine, sugars, and acrylamide in Maillard reactions

We developed an automated microwave digestion labstation (MDL) combined with ultra high-performance liquid chromatography-tandem mass spectrometry (UHPLC-MS/MS) method under the control of positive-negative ion switching as a robust kinetic study tool for rapid and simultaneous quantification of asparagine, glucose, fructose, and acrylamide in Maillard reaction products. Maillard reactions were conducted in a potato model via MDL. The two-step simple pretreatment procedures included the addition of isotope internal standards (15)N(2)-asparagine, (13)C(6)-glucose, and D(3)-acrylamide, followed by appropriate dilution with the mobile phase and filtration. Analytes were separated on a Hypercarb column and monitored by MS/MS. Study of matrix effects indicated Maillard reaction products induce an ionization suppression of both positive and negative precursor ions, but quantitative results are corrected through the use of isotopically labeled internal standards. Using this method, the limit of detection (LOD) and limit of quantification (LOQ) ranges of all analytes were calculated as 0.04-0.6 and 0.1-1.1 μM, respectively. Excellent repeatability (RSD < 9.6%) and acceptable within-laboratory reproducibility (RSD < 9.2%) substantially supported the use of this method for sample analysis. The present kinetic tools, with 10-50 min mimic of Maillard reactions and short instrumental run time (5.5 min per sample), were successfully validated and applied to simultaneous determination of acrylamide and its precursors and intermediates during Maillard reactions and kinetic elucidation. Furthermore, current tools of MDL combined with simple sample treatment procedures and UHPLC-MS/MS analysis reduce sample analysis time and labor in the kinetic study.     

Kinetic model of aluminum oxidation by water vapor in heterogeneous plasma: Gas-phase kinetics

A kinetic model of gas-phase and heterophase plasma-chemical processes taking place in a gas-discharge aluminum-water reactor with a pumping system is proposed. The model leads to a relatively simple system of equations that covers a great variety of reactions and offers an analytical solution for the composition of plasma with microparticles using experimental data. Along with the cycle of reactions typical for these heterogeneous mixtures, two more closed cycles of aluminum transformations are revealed in the plasma: atomic and molecular, generated by a stepwise ionization of Al and AlO by plasma electrons. The rate of aluminum oxidation and hydrogen formation through the molecular channel under the experimental conditions was comparable to the rate of heterogeneous processes. The inefficiency of the equilibrium oxidation mode is demonstrated.     

Steady-state limiting currents at finite conical microelectrodes

The investigation of steady-state diffusion-limiting currents at finite conical microelectrodes is reported. Such electrodes are of particular interest as probes in studies of kinetic reactions, measurements in microenvironments, and high-resolution electrochemical imaging. The diffusion-limiting currents were calculated using numerical (finite element) analysis and compared to those obtained at inlaid disk and hemispheroidal microelectrodes. Time-dependent simulations demonstrating the approach of the diffusion current to a steady-state value are also reported. The steady-state diffusion-limiting currents obtained were found to be a strong function of the electrode geometry, including the aspect ratio of the cone and the thickness of the insulating sheath. Thus, simple, approximate analytical expressions which account for these geometrical dependencies in the simulated steady-state diffusion-limited currents are also reported. As a limiting case, an analytical approximation was obtained for the steady-state current to a microdisk as a function of the insulator thickness. The use of these approximate equations in calculating electrode radii from steady-state diffusion-limiting currents is demonstrated, and good agreement was found with previously reported experimental studies. Use of the frequently implemented hemispherical theory to analyze steady-state diffusion-limiting currents obtained with finite conical microelectrodes is shown to result in an experimentally acceptable underestimation of the electrode radius.     

A phase field model of electrochemical impedance spectroscopy

A general phase field model for electrochemistry is presented in the context of simulating electrochemical impedance spectroscopy (EIS) experiments. The model tracks species, phases, and charge distribution. Governing partial differential equations are derived from a simple set of assumptions including ideal bulk solution thermodynamics, a method of interpolating between bulk phases, physical balance laws, and the second law of thermodynamics. A source term in the conservation of mass expression captures the behavior of the chemical reactions through the interface and in the bulk phases. Three example systems are given to demonstrate the variety of EIS behavior that can be simulated by this general model. These example systems show two-time constant transport, finite-length diffusion, and constant phase element behavior. Future versions of this model can be used to study the link between EIS and microstructure.     

A new alternative representation of impedance data using the derivative of the tangent of the phase angle: Application to the YSZ system and composites

The aim of this work is to propose a new alternative representation of impedance data using the derivative of the tangent of the phase angle, which allows enhanced discrimination between processes with relaxation frequencies that are very close. The new representation allows discrimination between overlapped processes within a factor of 2 in their relaxation frequencies for process with similar strength. Equations for the simplified behaviour of the impedance data have been proposed to obtain all the parameters of the processes involved in the impedance spectrum. This new alternative representation has been applied to bulk and grain boundary responses of YSZ with very satisfactory results. It has also been applied to the qualitative study of impedance data of a CuO composite showing the usefulness of this representation to discriminate different electrode processes. This approach provides an ab initio method of identify the contributing components to an electrochemical impedance spectrum with quite remarkable resolution. It is suggested that if this method is applied to provide starting parameters for non-linear least squares fitting using constant phase elements, then problems due to correlation of parameters and identification of components can be minimised.     

Diagnostics of anodic stripping mechanisms under square-wave voltammetry conditions using bismuth film substrates

A mechanistic study to provide diagnostics of anodic stripping electrode processes at bismuth-film electrodes is presented from both theoretical and experimental points of view. Theoretical models for three types of electrode mechanisms are developed under conditions of square-wave voltammetry, combining rigorous modeling based on integral equations and the step function method, resulting in derivation of a single numerical recurrent formula to predict the outcome of the voltammetric experiment. In the course of the deposition step, it has been assumed that a uniform film of the metal analyte is formed on the bismuth substrate, in situ deposited onto a glassy carbon electrode surface, without considering mass transfer within either the bismuth or the metal analyte film. Theoretical data are analyzed in terms of dimensionless critical parameters related with electrode kinetics, mass transfer, adsorption equilibria, and possible lateral interactions within the deposited metal particles. Theoretical analysis enables definition of simple criteria for differentiation and characterization of electrode processes. Comparing theoretical and experimental data, anodic stripping processes of zinc(II), cadmium(II), and lead(II) are successfully characterized, revealing significant differences in their reaction pathways. The proposed easy-to-perform diagnostic route is considered to be of a general use while the bismuth film exploited in this study served as a convenient nonmercury model substrate surface.     

Applications of sensitivity analysis to combustion chemistry

Combustion chemical models usually contain several hundred or thousand kinetic rate parameters. Most simulation packages calculate local concentration sensitivities, but it is frequently not easy to extract meaningful information from large sensitivity matrices. Principal component analysis is a simple post-processing technique that summarizes sensitivity information and also reveals the effect of simultaneously changing parameters.A new program package, called KINALC, has been created for the analysis of gas-phase reaction systems. This program is an extension to CHEMKIN based simulation programs. KINALC processes the concentration sensitivity information in four different ways and allows a comparison of the sensitivity information to other methods, based on the study of reaction rates and stoichiometry, for the analysis of complex mechanisms. KINALC is available through the World Wide Web.The various methods are illustrated by the analysis of a detailed chemical model for hydrogen combustion. Local sensitivity analysis of models of homogeneous hydrogen explosion and of premixed laminar hydrogen-air flame has been carried out and the sensitivity results reveal that the chemical processes are very similar in these physically different systems at the corresponding temperatures.