Application of a wall jet disk electrode combined with an electrochemical quartz crystal microbalance to the study of the dissolution of copper in acidic chloride media

An electrochemical quartz crystal microbalance (EQCM) system combined with a wall jet disk electrode (WJDE) was applied to the investigation of copper dissolution in acidic chloride media. The average valence of dissolved ions, n, was calculated from the resonant frequency of the quartz crystal and the applied current. The value of n between 1 and 2 indicates that both Cu(I) and Cu(II) dissolved from the copper electrode at the same time. The formation rate of Cu(II) increased with decreasing chloride concentration. The influence of the hydrodynamic conditions on the dissolution behaviour was discussed.     

Conditions for the onset of current oscillations at the limiting current of the iron electrodissolution in sulfuric acid solutions

The experimental conditions and parameters that control the transition from a steady to an nonsteady limiting current and vice versa in the anodic Fe dissolution are studied in sulfuric acid solutions by using a downward-facing Fe electrode. Periodic and chaotic current oscillations are observed at the beginning of the limiting current region within a fixed potential region by decreasing the sulfuric acid concentration, by increasing the Fe-disk rotation rate and by increasing a series external resistance. It is found that current oscillations emerge when theIR drop exceeds a critical value (IR)_crit. The latter depends on the hydrodynamic conditions pertaining to the system. This is realized by the differences appeared in the value of (IR)_crit on using a stationary or a rotating Fe electrode. The effect of hydrodynamic conditions is shown by additional experimental results obtained by using an upward-facing Fe electrode, by increasing the density and viscosity of the solution on addition of glycerol, and by altering the solution properties near the Fe surface on addition of halide ions. Dedicated to the ninetieth anniversary of Ya.M. Kolotyrkin’s birth. This article was submitted by the authors in English.     

Radial diffusion and entrance effects in porous flow-through electrodes

Radial diffusion has two effects on the performance of a flow-through electrode: (a) it may limit the maximum experimentally attainable degree of conversion, and/or (b) the polarization at a certain current output may be lower for smaller than for larger pores. A dimensionless groupwas developed as a criterion of reactant conversion under certain conditions of flow rate, pore diameter and diffusion coefficient. The lengths of both diffusional and hydrodynamic entrance regions are calculated. The ratio between the length of the diffusional entrance region and the electrode thickness is related to the criterion of reactant conversion . It was shown that in all conditions of fully developed laminar flow inside the pores, radial diffusion must be fast with respect to axial convection: hence complete reactant conversion is possible. The effect of radial hydrodynamic dispersion is briefly outlined. It increases the rate of radial mass transfer, hence the degree of conversion and decreases the length of the diffusional entrance region. The length of the hydrodynamic entrance region would normally be much shorter than the diffusional entrance length. They become comparable only when the radial dispersion coefficient is of the same order as the kinematic viscosity of the electrolyte.     

Three-dimensional porous metal electrodes: Fabrication,characterisation and use

Diverse three-dimensional (3D) porous metal electrodes, including meshes, foams and felts, are used in electrochemical flow reactors for a wide range of industrial applications, such as energy storage, electrosynthesis and degradation of pollutants. Recent work centres on the hierarchical decoration and coating of 3D electrodes with catalysts, although the study of their performance in a controlled and reproducible flow and mass transfer environment ought to receive more attention. New advances have considered metal nanofelts and nanomesh porous electrodes with superior electrode surface area. Opportunities are found in additive manufacturing, advanced structural characterisation by, for example, X-ray computed tomography, and in the modelling of hydrodynamic characteristics, current distribution and mass transfer coefficient of these electrode materials.     

Ellipsometric and microwave reflectivity studies of current oscillations during anodic dissolution of p-Si in fluoride solutions

Periodic current oscillations during anodic dissolution of monocrystalline p-Si(100) in buffered ammonium fluoride solutions (0.1 mol dm⁻³ fluoride, pH 4.5) were investigated using a flow cell in order to eliminate mass transport limitations. The flow cell was designed to permit simultaneous in-situ ellipsometry, impedance and potential modulated microwave reflectivity measurements. Analysis of the ellipsometric response showed that the current oscillations are accompanied by a synchronous variation of the overall oxide thickness with an amplitude of 4.5±0.1 nm. Analysis of the relationship between the total oxide thickness and the current during the oscillation cycle shows that to a first approximation the rate of chemical dissolution of anodic oxide remains constant. Oscillations of the electrode admittance and potential modulated microwave reflectivity were also measured. The imaginary component of the admittance is related to the oscillation in thickness of a narrow inner region of ‘dry’ oxide and to changes in the accumulation capacitance. The oscillation in the potential modulated microwave reflectivity is interpreted in terms of the changes in the density of holes accumulated at the p-Si SiO₂ interface.     

Hydrodynamic Modulation Voltammetry with a Dual Disk Chopped Flow‐Microjet Electrode (CF‐MJE)

A novel form of hydrodynamic modulation voltammetry (HMV) is described, based on the periodic variation of mass transport in a microjet electrode (MJE) system, in combination with phase‐sensitive detection techniques. In the configuration developed, a jet of solution is fired from a nozzle that is aligned directly over the surface of a dual disk Pt‐Pt ultramicroelectrode (UME). The potential at each electrode is controlled separately. A rotating blade, positioned between the nozzle and the UME probe, is used to periodically interrupt flow to the electrode surface, resulting in modulation of the overall mass transfer rate between two defined extremes. The use of a dual disk UME enables two transport‐limited current signals to be recorded simultaneously, one for the analyte of interest, and the other for a ‘reference species’ (oxygen for the studies described herein). The latter current response corresponds to the variation in mass transport rate in the chopped flow (CF) arrangement and is used as the signal for phase sensitive detection of the analyte current. Studies of potassium hexachloroiridate (III) [IrCl ] oxidation in aqueous solution are used to demonstrate the capabilities of the technique. HMV in the CF‐MJE arrangement allows quantitative concentration measurements, down to at least 5×10^?7 M.     

Electrochemical Monitor of Collapse of a Solitary Cavitation Bubble

The change in the current in an electrochemical cell is studied simultaneously with the dynamics and luminescence of a solitary cavitation bubble produced by focusing a shock wave near a disk anode 0.6 mm in diameter in an NaCl solution. The current is maximum at the instant of the bubble collapse. The increase in the current is due to the mass transfer near the electrode intensified by the hydrodynamic flow around the bubble.     

Oxygen reduction on rotating porous cylinder of modified reticulated vitreous carbon

A rotating cylinder porous electrode (RCPE) of reticulated vitreous carbon (RVC) matrix was used for oxygen reduction reaction (ORR) in H₂SO₄ solutions. Cyclic voltammetry and hydrodynamic voltammetric techniques were used for electrochemical characterization of the ORR. Cyclic voltammograms in stationary solutions showed better performance of the anodically oxidized RVC (for periods of 1 and 5 min) for the ORR than the untreated RVC in which the first scan (ORR) after the surface treatment was of no utility, and the second scan was presented here. The hydrodynamic voltammograms obtained at the treated RCPE gave well-defined limiting current plateau with positively shifted onset potential as compared with the untreated (plain) RVC electrode. The analysis of the limiting current data on RCPE and the determination of a limiting current enhancement factor α enabled us to quantify the enhancement extent exerted by the anodic oxidation treatment. An enhancement factor of up to ∼3 was obtained at the RCPE electrode anodically oxidized for 5 min. It was found that the α slightly decreased with the rotation speed depending on the extent of anodic oxidation of RVC. This was attributed to the different mode of mass transfer (diffusion) to the interior of the micropores with different microstructure resulting from different extent of anodic oxidation. X-ray photoelectron spectroscopic and scanning electron microscopic measurements helped us to characterize the anodically oxidized RVC surface.     

Mass transfer enhancement for mesh electrode in a tubular electrochemical reactor using experimental and numerical simulation method

The effect of electrode configuration on mass transfer rate in a tubular electrochemical reactor was investigated with the limiting-current method. The expression of mass transfer enhancement factor was derived from the dimensionless equations for different electrode configurations. Furthermore, the local velocity distribution in the tubular electrochemical reactor was simulated by computational fluid dynamics technology. These simulation results explained the reason of mass transfer enhancement for mesh electrode: the rotation of the solution is induced by high rates of shear, and small eddies are formed. The local velocity distributions in bulk for X-dimension get flatter because of the lateral momentum transfer. Due to the impact and obstruction of electrode holes, the turbulence of the electrolyte gets drastic and the mass transfer performance in the tubular electrochemical reactor is improved.     

Parameters of electrolysis in 3D flow electrode in limiting diffusion current mode

The analytical expressions were derived to calculate thickness of a three-dimensional flow electrode (TFE) working in limiting diffusion current mode in the cases of unidirectional and multidirectional electric field and solution flow for the given degree of metal extraction. The algorithm was suggested on the basis on the derived formulas and earlier published mathematical models of nonstationary electrolysis at TFE to calculate the whole electrode working conditions at the limiting diffusion current with parallel electrochemical reactions. The calculations and experimental studies were carried out on copper electrodeposition from sulfate electrolyte on TFE working in the limiting diffusion current mode. The effect of major electrochemical reactions to the distribution of potential and partial current densities by the electrode thickness was demonstrated. The process potential distribution was given for various electrode conductivities. A good compliance was shown between the results of calculations and experimental studies.     

Computational analysis of a molten-salt electrochemical system for nuclear waste treatment

This paper describes ongoing research into the multi-physics model development of an electrorefining process for the treatment of spent nuclear fuel. A forced convection of molten eutectic (LiCl–KCl) electrolyte in an electrorefining cell is considered to establish an appropriate electro-fluid model within the 3-dimensional framework of a conventional computational fluid dynamic model. This computational platform includes the electrochemical reaction rate of charge transfer kinetics which is described by a Butler–Volmer equation, while mass transport is considered using an ionic transport equation. The coupling of the local overpotential distribution and uranium concentration gradient makes it possible to predict the local current density distribution at the electrode surfaces.     

Mn3+/Mn2+氧化还原体系的旋转圆盘电极法性能

氧化还原液流电池的独特性能受到关注[1～4]。本文综合运用循环伏安法,旋转圆盘电极法研究锰离子浓度较高(0 25～0 35mol·L-1、转速范围较宽(400～4200rpm)、同时考虑过电位的条件下Mn(Ⅲ)/Mn(Ⅱ)电对在RDE铂盘电极上的电极过程动力学并确定有关的动力学参数,了解其影响因素,为该电对作为氧化还原液流电池正极活性材料提供动力学依据。1　实验部分铂电极使用前在铬酸洗液中浸10min,水冲洗干净,蒸馏水淋洗,然后在6 3mol·L-1H2SO4中超声清洗10min,再用CHI660电化学工作站(美国CH仪器公司)控制,以0 05V/s的扫速在-1 0～1 2V之间扫…     

Design and Performance of a New Thin‐Layer Radial‐Flow Holder for a Quartz Crystal Resonator of an Electrochemical Quartz Crystal Microbalance

A new compact holder for either 5‐ or 10‐MHz AT‐cut quartz crystal resonator of an electrochemical quartz crystal microbalance was designed, fabricated and characterized. The holder is a hydrodynamically controlled thin‐layer radial‐flow microelectrochemical cell. Its unique feature consists of (i) a micrometer‐screw adjustable distance between the movable coaxial assembly of the Ag/Ag⁺ pseudoreference electrode and the inlet capillary nozzle with respect to the metal‐film working electrode of the quartz crystal resonator, and (ii) a U‐clamp mountable resonator, easily accessible for change without using any tools. The inlet solution stream is centered axially against the working electrode. The holder performance was tested under different flow conditions. These include hydrodynamic voltammetry measurements on the Fe(CN) /Fe(CN) couple, i.e., a redox system with no mass transfer across the solution–electrode interface, as well as simultaneous chronoamperometry and chronoelectrogravimetry measurements under flow injection analysis (FIA) conditions on the Ag/Ag⁺ couple, i.e., a system with electrodeposition of a rigid metallic film. Moreover, simultaneous changes of resonant frequency and dynamic resistance were measured under FIA conditions for a glycerol solution, i.e., an electroinactive viscous medium. For the 30<F_m<180 μL min^?1 volume flow rate of solution and 50<d<250 μm nozzle‐to‐resonator distance, the holder operates in a thin‐layer radial‐flow regime at a fully developed laminar flow. For F_m=30 μL min^?1 and d=100 μm, both mass and charge conversion accompanying silver electrodeposition is appreciably high and close to 35%. Simultaneous measurements of the resonant frequency change and current‐potential or current‐time transients allowed investigations of electrochemical processes involving mass changes of rigid deposits while those of the frequency change and dynamic resistance change involve changes of viscoelastic properties of medium.     

Influence de la distribution de potentiel électrochimique sur l'interprétation de la courbe courant-tension anodique d'une électrode tournante d'alliage Fe-31 % Ni en milieu sulfurique

Influence of the electrochemical potential distribution on the interpretation of the anodic current-voltage curve of a rotating electrode of a Fe-31 % Ni alloy in normal sulfuric acid solution. The anodic steady-state polarization curve of a Fe-31 % Ni rotating-ring electrode in normal sulfuric acid was studied, and the dissolution profiles obtained at different points of the curve were surveyed. We showed that the use of a rotating-ring electrode allowed to obtain the entire J(E) relationship between the current density J and the electrochemical potential E, whereas this relationship cannot be obtained from the polarization curve of the classical rotating-disk electrode because of the non-uniform potential distribution over the disk surface. With the ring electrode, additional information on the transitions between three different states of the metal (homogeneous dissolution, intergranular dissolution and passivity) could be obtained.     

Electrochemical approaches for the fabrication and/or characterization of pure and hybrid templated mesoporous oxide thin films: a review

Electrochemistry can be used for fabrication and characterization of mesoporous oxide films. First, this review provides insight into the methods used to prepare templated mesoporous thin films on an electrode surface, i.e., evaporation-induced self-assembly (EISA) and electrochemically assisted self-assembly (EASA). Electrochemical characterization of mass transport processes in pure and organically functionalized mesoporous oxide films is then discussed. The electrochemical response can be basically restricted by the electron/mass transfer reaction at the electrode–film interface and diffusion through mesopore channels. The contributions of cyclic voltammetry, hydrodynamic voltammetry, electrochemical impedance spectroscopy, and scanning electrochemical microscopy to the characterization of films with distinct mesostructures are finally described, with special emphasis on identification of conditions that can affect the electrochemical response recorded with such modified electrodes.

Insights into analyte electrolysis in an electrospray emitter from chronopotentiometry experiments and mass transport calculations

Insights into the electrolysis of analytes at the electrode surface of an electrospray (ES) emitter capillary are realized through an examination of the results from off-line chronopotentiometry experiments and from mass transport calculations for flow through tubular electrodes. The expected magnitudes and trends in the interfacial potential in an ES emitter under different solution conditions and current densities, using different metal electrodes, are revealed by the chronopotentiometry data. The mass transport calculations reveal the electrode area required for complete analyte electrolysis at a given volumetric flow rate. On the basis of these two pieces of information, the design of ES emitters that may maximize and those that may minimize analyte electrolysis during ES mass spectrometry are discussed.     

Hot-wire amperometric monitoring of flowing streams

This paper describes the design of a hot-wire electrochemical flow detector, and the advantages accrued from the effects of locally increased temperature, mainly thermally induced convection, upon the amperometric monitoring of flowing streams. A new hydrodynamic modulation voltammetric approach is presented, in which the solution flow rate remains constant while the temperature of the working electrode is modulated. Factors influencing the response, including the flow rate, temperature pulse, or applied potential, have been investigated. The hot-wire operation results also in a significant enhancement of the flow injection amperometric response. The minimal flow rate dependence observed with the heated electrode should benefit the on-line monitoring of streams with fluctuated natural convection, as well as various in-situ remote sensing applications.     

表面增强拉曼光谱研究吸附电位对CO在Pt电极上氧化行为的影响

预吸附电势对CO在Pt上的氧化行为有重要影响. 在电势低于0.3 V（vs. RHE）下,预吸附的CO单层在Pt电极上0.4 V就开始氧化,并会在0.6 V左右出现1个较小的氧化峰,即所谓的“氧化前峰”,而在双电层电位区间饱和吸附的CO只有当电势正于0.6 V才氧化     

Electrolytic Recovery of Cadmium from Cyanide Washing Solutions on Filtering Carbon-Graphite Electrodes

Electrorecovery of cadmium(II) on filtering carbon-graphite electrodes from washing cyanide solutions formed in cadmium-plating of articles in a cyanide electrolyte was studied. The effect of the dilution factor of the supporting electrolyte on the run of cathodic polarization curves, pH value, and solution resistivity was analyzed. The influence exerted by the efficiency of cadmium(II) recovery on the carbon-graphite electrodes was studied in relation to current density, solution flow rate, cadmium(II) concentration in solution, dilution factor of the supporting electrolyte, and material of the carbon-graphite electrode. The possibility of dissolution of cadmium deposited onto a filtering carbon-graphite electrode via anodic dissolution or because of the operation of a short-circuited electrochemical system constituted by the carbon-graphite electrode, cadmium, and cyanide solution was considered.