Fast Electrode Reactions Research Articles

Superior Cycling Performance of SiOx/C Composite with Arrayed Mesoporous Architecture as Anode Material for Lithium-Ion Batteries

The poor cycling performance due to the huge volume change of Si-based anode materials upon insertion and extraction of lithium ions has severely limited their widespread implementation. To address this issue, SiOx/C composite with arrayed mesoporous architecture, narrow pore size distribution (2–4 nm) and high specific surface area is synthesized via a template assisted hydrothermal route and a carbon-coating process. The uniformly arrayed mesoporous and thin pore walls of SiOx/C composite ensure a fast electrode reaction and provide a self-buffering function during the charge/discharge process, which make the SiOx/C anode have excellent electrochemical properties, including high specific capacity (780 mA h g−1), superior cycling stability (0.02% decay per cycle), and excellent rate capability. This unique structure can be promoted to other high energy electrode materials with a huge volume change.

Multi-electron redox reaction of an organic radical cathode induced by a mesopore carbon network with nitroxide polymers

An organic radical based composite cathode comprised of poly(2,2,6,6-tetramethylpiperidinyloxy-4-yl methacrylate) (PTMA)-Ketjenblack was developed by a simple solvent-less electrode fabrication method. The composite cathode demonstrated a two-electron redox reaction of PTMA that is from an aminoxy anion (n-type) via a radical to an oxoammonium cation (p-type) with the corresponding redox potential at 2.8-3.1 V and 3.5-3.7 V vs. Li/Li(+) when evaluated in lithium half cells. Moreover, the PTMA-Ketjenblack composite electrode exhibits fast electrode reaction kinetics and an enhanced solid electrolyte interface by cyclic voltammetry and electrochemical impedance spectroscopy measurements. These improved electrochemical properties contribute to increased capacity (300 mA h g(-1)), a high rate (50% capacity retention after 100 C rate excursions) and a long cycle life in the cell performance evaluations. Morphological and compositional characterization indicates a unique mesopore network of Ketjenblack with the PTMA matrix, which highly facilitates the interaction between the conductive media and the radical species, resulting in the performance enhancement of the PTMA-Ketjenblack composite cathode.

Nanoengineered Polypyrrole‐Coated Fe2O3@C Multifunctional Composites with an Improved Cycle Stability as Lithium‐Ion Anodes

AbstractNovel multifunctional composites composed of highly dispersed nanosized Fe2O3 particles, a tubular mesoporous carbon host, and a conductive polypyrrole (PPy) sealing layer are hierarchically assembled via two facile processes, including bottom‐up introduction of Fe2O3 nanoparticles in tubular mesoporous carbons, followed by in situ surface sealing with the PPy coating. Fe2O3 particles are well‐dispersed within the carbon matrix and PPy is spatially and selectively coated onto the external surface and the pore entrances of the Fe2O3@C composite, thereby bridging the composite particles together into a larger unit. As an anode material for Li‐ion batteries (LIBs), the PPy‐coated Fe2O3@C composite exhibits stable cycle performance. Additionally, the PPy‐coated Fe2O3@C composite also possesses fast electrode reaction kinetics, high Fe2O3 use efficiency, and large volumetric capacity. The excellent electrochemical performance is associated with a synergistic effect of the highly porous carbon matrix and the conducting PPy sealing layer. Such multifunctional configuration prevents the aggregation of NPs and maintains the structural integrity of active materials, in addition to effectively enhancing the electronic conductivity and warranting the stability of as‐formed solid electrolyte interface (SEI) films. This nanoengineering strategy might open new avenues for the design of other multifunctional composite architectures as electrode materials in order to achieve high‐performance LIBs.

Theory of Square-Wave Voltammetry of Two-Electron Reduction with the Adsorption of Intermediate

Thermodynamically unstable intermediate of fast and reversible two-electron electrode reaction can be stabilized by the adsorption to the electrode surface. In square-wave voltammetry of this reaction mechanism, the split response may appear if the electrode surface is not completely covered by the adsorbed intermediate. The dependence of the difference between the net peak potentials of the prepeak and postpeak on the square-wave frequency is analyzed theoretically. This relationship can be used for the estimation of adsorption constant.

Square-wave voltammetry of dissolved redox couple

A theory is developed of square-wave voltammetry of the system in which both the reactant and the product of a simple electrode reaction are initially present in the electrolytic solution. A net response of fast and reversible electrode reaction depends on the sum of concentrations of reactant and product. Under this condition the ratio of maxima and minima of the components of net response can be used for the calculation of the ratio of concentrations of reactant and product. If the electrode reaction is kinetically controlled, the net response may split in two peaks. The potentials of these peaks can be used for the determination of transfer coefficients.

Electrochemical characterization of simvastatin by abrasive stripping and square-wave voltammetry

Abstract The electrochemical characteristics of the lipid-lowering drug simvastatin were investigated on graphite and mercury electrode. Solid microparticles of simvastatin were mechanically immobilized on the surface of graphite electrode and immersed into aqueous electrolyte. The microparticles were oxidized at 1.1 V in totally irreversible electrode reaction. Simvastatin dissolved in acetonitrile and admixed with 0.1 M Na 2 B 4 O 7 –KH 2 PO 4 aqueous buffer solution pH 7 is strongly adsorbed on the surface of mercury electrode. Adsorbed simvastatin was reduced at −1.45 V in the fast and reversible electrode reaction followed by a chemical reaction.

Application of the finite analytical numerical method. Part 3. Digital simulation of charge transfer to a micro-ring electrode interface

The finite analytical numerical method (FAM) combined with a conformal map has been used to study diffusion problems at a micro-ring electrode. The chronoamperometric and voltammetric responses for both reversible and quasi-reversible charge transfer at ring microelectrodes of different thicknesses are simulated. The effect of the thickness of the micro-ring electrode, the potential sweep rate and the heterogeneous electron transfer kinetic parameter on the voltammetric responses is discussed. The results show that the thin micro-ring electrode is more appropriate for the evaluation of fast electrode reaction rate constants than a thick micro-ring or microdisk electrode.

Application of ac impedance in fuel cell research and development

In applying ac impedance to fuel cells and their porous (gas diffusion) electrodes the emphasis lies on different fuel cell components, and their properties, according to the fuel cell type. The focus has been directed at the electrode/electrolyte interface in MCFC and PAFC, whereas in SOFC and PEMFC the ionic/electronic conductivity of the electrolyte or the characteristics of its composite with the electrocatalyst is of primary interest. The limitations of ac impedance in fuel cell application are in part due to difficulties of interpretation and in part due to experimental difficulties because of the generally fast electrode reaction kinetics. Further research directions are indicated.

The voltammetric behaviour of H and O adatoms in the 189–292 K range on platinum in HClO 4 · 5.5 H 2O and H 2SO 4 · 12H 2O

The voltammetric behaviour of HClO 4·5.5H 2O and H 2SO 4·12H 2O on polycrystalline platinum has been studied in the range 189–292 K. The H and OH electrosorption processes appear as fast surface electrode reactions over the entire temperature range. Results are explained taking into account both the clathrate structure of HClO 4 · 5.5 H 2O and the chain structure of H 2SO 4·12H 2O.

Electrochemical NO2 gas sensors: Model and mechanism for the electroreduction of NO2

AbstractTwo types of sensing devices have been studied in order to elucidate the mechanism of response of amperometric gas sensors. A high‐surface‐area electrode (HSAE) sensor produces fast electrode reactions, and the steady‐state sensor signals are controlled either by permeation or by the supply of analyte to the sensor. For low‐surface‐area electrode (LSAE) sensors, a kinetically controlled signal is observed. A mechanism that involves a “chemically” induced changes in the gold electrode surface oxide coverage is proposed, and the activation energy for NO2 reduction at a gold electrocatalyst is calculated. A general model of sensor response is proposed that can describe the behavior of the steady‐state response for both HSAE and LSAE sensors.

Diffusion to fractal surfaces—IV. The case of the rotating disc electrode of fractal surface

The limiting current of a fast electrode reaction on a rotating, inactive disc with an electrochemically active fractal subset was theoretically predicted to be proportional to ω α, with ω being the rotation frequency, and the exponent α was determiend by the fractal dimension, D f, of the active area as α = ( D f−1)/2. The experimental verification is presented.

Convolutive voltammetry with microelectrodes

Hemispherical microelectrodes prepared by electroplating of mercury on platinum microelectrodes were used in studies of fast charge transfer processes under convolutive voltammetry conditions. The algorithm used in the convolutive transformation was modified to account for spherical symmetry of diffusion. The sweep rate was varied from 0.1 to 220 V/s. Low sweep rate experiments were used to test the symmetry of the mass transport. It was found that the mass transport to (and inside) the electrodes can be described with good accuracy by the spherical symmetry diffusion. It was shown that the spherical convolution can be used to determine from cyclic voltammetric data the diffusion coefficient of a depolarizer and the number of electrons transfered to (or from) the depolarizer molecule in the electrode process.At high sweep rates the technique was found useful in studies of fast electrode reactions. The technique was tested in the determination of kinetic parameters for Cd2+ reduction in aqueous solutions and p-dicyanobenzene reduction in DMF solutions. In both cases good agreement with values reported in literature was found. Properties of Hg/Pt microelectrodes were also studied using the ac impedance techniques. The ac impedance measurements were carried out at various frequencies (0.1 Hz to 100 kHz) in solutions containing 0.1 to 10−5 M of the supporting electrolyte.

High-frequency impedance spectroscopy of fast electrode reactions

A new technique of high-frequency impedance spectroscopy working in the frequency range between 1 kHz and 5 MHz has been developed. The method is characterized by a special electrochemical cell with two working electrodes, a newly designed dc control being completely ac-decoupled, a highly precise impedance measurement technique (ICEP = Impedance Conversion and Error Processing) and quantitative calculation of the electrodynamic field distribution in the cell. The accuracy of the measurements is better than 0.05%. The method was tested on the fast Ag/Ag + system. The results obtained are in quantitative agreement with a theoretical transfer function based on a 2-D inhomogeneous surface model.

Extension of DC Transient Techniques to the Nonlinear Current Density‐Overpotential Range and the Limitations of Linearized Approximations

The applicability of the dc relaxation techniques for the measurement of kinetics of fast electrode reactions has been limited generally to the linear current density‐overpotential range by the mathematical difficulties in obtaining a closed‐form solution of the differential equations describing the system, confining the measurements to the vicinity of the equilibrium potential. Evaluation of the experimental data with numerical curve‐fitting procedures offers a possibility for the extension of these techniques into the nonlinear range. The feasibility of such extension was demonstrated by a curve‐fitting data‐evaluation method with a nonlinear multidimensional least squares minimization package coupled to a software package designed to solve systems of nonlinear partial differential equations. The main advantage of this approach is that measurements can be made at high enough overpotentials to avoid signal‐to‐noise ratio problems without introducing linearization errors in the data evaluation. The error of the linearized relation is not negligible under many experimental conditions. A quantitative evaluation of this error has been made as a function of three dimensionless parameters: , , and . The error in the exchange current density determination, with the assumption of linear relation, will be at least as large as the error of the linearized equation.

Metal Deposition‐Dissolution in Molten Halides: On the Question of Measurability of Very Fast Electrode Reaction Rates

Metal deposition‐dissolution reactions in molten salts are very fast and difficult to investigate, as evidenced by large discrepancies in the reported exchange current densities. An error analysis of the dc relaxation techniques was carried out, and it was determined that the measurement of the exchange current density can have a systematic error if the surface reaction rate is much faster than the diffusion rate. This systematic error will result in measured exchange current densities approximately equal to the largest exchange current density measurable under the given conditions. This systematic error can also result in fortuitously linear vs. and vs. plots, even when the results are grossly in error. A computer curve fitting data evaluation coupled with a statistical sensitivity analysis is suggested to avoid these problems. The exchange current densities of iron, nickel, and molybdenum were measured in eutectic melt at 450°C using an improved double‐pulse galvanostatic technique. Only that of iron could be measured reliably ( at ion concentration). The nickel reaction is too fast to be measured (at least 5 A‐cm−2 at ion concentration), and the molybdenum results are unreliable because of melt decomposition. The large discrepancies between these results and some results reported by earlier workers can be explained on the basis of the above‐described error analysis. It is concluded that past experiments resulting in large exchange current densities should be reexamined with improved measuring and data‐evaluation techniques because it is possible that the reported values represent only the applicability limit of the measuring technique rather than the true exchange current density.

Effects of fluidization states on polarization characteristics of flow-through particulate electrode.

To study the factors influencing the polarization characteristics of a flow-through electrode with electro-conductive particles, theoretical analyses have been made by the one-dimensional expression of a two-phase model with combinations of the general electro-chemical reaction rate and the fundamental equation of fluidization state. Potential, overpotential and effectiveness factor of the bed were studied for two systems, fast electrode reaction and slow electrode reaction. To indicate the performance of polarization characteristics of the particulate electrode, bed overpotential was introduced. Experimentally, potential profiles and polarization curves were measured in the particulate electrode with graphite particles by using the reduction of ferricyanide in alkaline solution. The variation of polarization curves with changing bed expansion were well explained by the calculated results with an available fraction of surface area of electrode of f= 0.38.

Cell Design Principles for the Measurement of Kinetics of Fast Electrode Reactions

The rates of electrode processes in high temperature molten salts are often very fast and special precautions are needed in their measurement. The measurement methods are invariably some sort of relaxation technique where a fast perturbation of the current (or the potential) is applied and the resulting potential (or current) relaxation is recorded. The measuring cells used in these experiments have to be carefully designed so that the distortion of the fast electrical signals and the introduction of spurious signals are avoided. The two requirements of a good cell design can be simply stated as (i) the assurance of uniform current distribution on the working electrode surface and (ii) small impedance. The latter requirement includes small stray capacitances and small inductances, which is necessary to avoid detrimental phase shifts and inductive spikes in the signal. In addition, one requires a small ohmic resistance of the solution between the reference and the working electrodes. Three cell designs, aimed at fulfilling these requirements, are described.

Mediating effects of ferric chelate compound in microbial fuel cells

The performance of bio-fuel cells containing Escherichia coli, glucose and a series of ferric chelate reagents was studies. The measured coulombic outputs indicate that the most of the compounds work effectively as electron-transfer mediators in the fuel cells. These outputs were compared with measured rate constants for the reduction of ferric chelates by E. coli and the electrochemical reaction of these compounds at a carbon electrode. The results suggest that a good mediator for microbial fuel cells is one which shows fast reduction by E. coli, together with a fast electrode reaction. In regenerative fuel cells which were run for 5 days, coulombic yields over 70 % were obtained on the basis of complete oxidation of added glucose.

Mediating effects of ferric chelate compounds in microbial fuel cells

The performance of bio-fuel cells containing Escherichia coli , glucose and a series of ferric chelate reagents was studied. The measured coulombic outputs indicate that most of the compounds work effectively as electron-transfer mediators in the fuel cells. These outputs were compared with measured rate constants for the reduction of ferric chelates by E. coli and the electrochemical reaction of these compounds at a carbon electrode. The results suggest that a good mediator for microbial fuel cells is one which shows fast reduction by E. coli , together with a fast electrode reaction. In regenerative fuel cells which were run for five days, coulombic yields over 70% were obtained on the basis of complete oxidation of added glucose.

Evaluation of the Single and Double Pulse Galvanostatic Relaxation Techniques for the Measurement of the Kinetics of Fast Electrode Reactions in Molten Salts

Evaluation of the Single and Double Pulse Galvanostatic Relaxation Techniques for the Measurement of the Kinetics of Fast Electrode Reactions in Molten Salts