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.     

Mass transport and effective diffusion coefficient in the reduction of hydrogen ions from aqueous sulfuric acid solutions: Numerical modeling

This work presents results of the numerical solution to a system of equations of material balance and the movement of particles in solution under the influence of the forces of diffusion, migration, and convection, which describe the process of mass transport during the reduction of hydrogen ions at a rotating disk electrode from aqueous sulfuric acid solutions with and without excess supporting electrolyte. Results of the calculations show that the diffusion kinetics of hydrogen ion reduction can be observed only with measurements in dilute (≤10^?3 M) sulfuric acid solutions with an excess of indifferent supporting electrolyte. For more acidic solutions it is necessary to take into account the simultaneous diffusion of hydrogen and bisulfate ions. In the study of the regularities of hydrogen ion reduction in sulfuric acid solutions with a sulfate supporting electrolyte, it is necessary to take into account that with excess supporting electrolyte, the limiting current of hydrogen reduction is caused solely by the diffusion of bisulfate ions, but for small concentration ratios of the supporting electrolyte to acid, the influence of migration effects is significant.     

Gas-Generating Porous Electrodes: The Nature of the Low-Polarizability Portion in the Polarization Curves

The phenomenon of a low-polarizability portion (LPP) is discussed in the paper. The phenomenon is responsible for the possible rapid increase in the current in polarization curves (PC) for the gas generation in gas-generating porous electrodes (GGPE). A fresh viewpoint on the nature of LPP is propounded. The inflection in PC, which follows the linear Tafel portion, owes its inception to the emergence, in the electrode pores, of a network of gas pores that are freed of electrolyte and are connected with each other. The effective diffusion coefficient for gas molecules in gas pores filled with water vapor is larger than the effective diffusion coefficient for the same gas molecules in liquid pores filled with an electrolyte solution by several orders of magnitude. The emergence of a gas phase in a GGPE leads to a rapid increase in the effective diffusion coefficient. This circumstance in turn is capable of rapidly intensifying processes of the formation and removal of gas, which leads to a considerable increase in the overall current. A method for obtaining an approximate solution of the problem (the notion on ideal porous electrode) is suggested. A system of equations is derived, which can sufficiently accurately, qualitatively and quantitatively describe the character of variations in a polarization curve in the initial part of a low-polarizability portion. Theoretical and experimental polarization curves for the chlorine evolution on a dimensionally stable anode are compared quantitatively.     

Anodic grounding electrodes made of electroconducting elastomers: Estimating their electrochemical parameters and reliability by the operational impedance method

The electrochemical behavior of electrodes made of electroconducting elastomers and intended for anodically grounding metal pipelines, to protect them electrochemically, is considered. The electrodes’ transition resistance R_tr is selected as the main parameter characterizing their operational state. Dependences of R_tr on the chargeq passed through the electrodes are obtained for different anodic polarization current densities. The dependences suggest that predicting the electrodes’ performance at elevated polarization current densities is well justified. The performance and lifetime of the electrodes can be predicted by comparing the measured value of R_tr with a calibration R_tr vs.q curve for a given electrode type. Dedicated to the ninetieth anniversary of Ya.M. Kolotyrkin’s birth.     

Single solid-oxide fuel cells with supporting ni-cermet anode

Single solid-oxide fuel cells (SOFCs) with a porous (36-41%) supporting Ni-cermet anode are manufactured and tested. The effect of the thickness of the supporting Ni-cermet anode on the electrochemical characteristics of single SOFCs is studied. It is shown that polarization losses on electrodes at the current density of 1.2 A/cm² increase by about 2 times from 0.13 to 0.25 V at an increase in the thickness of the supporting Ni-cermet anode from 0.40 to 1.27 mm. The impedance spectroscopy method is used to identify relaxation processes responsible for the behavior of the fuel cell anode and cathode. It is found that a significant percentage of polarization losses on the anode is due to transport limitations in fuel supply to the three-phase nickel/electrolyte/gas phase interface and removal of the reaction products away from it.     

Electrochemical oxidation of organic pollutants in low conductive solutions

This mini-review supplies current opinion about the most recent works, which have been carried out toward the electrochemical treatment of organic compounds spike in low conductive solution. In particular, the first section is focused on the use of a solid polymer electrolyte in order to allow current flux with a low cell voltage even in a solution without supporting electrolyte. Meanwhile, the second section describes the microfluidic cells that are characterized by very small distances between electrodes (tens or few hundreds μm) that reduce the ohmic resistances and increase the mass transport of the pollutants to electrodes’ surfaces.     

Electrochemistry of nanopore electrodes in low ionic strength solutions

The steady-state voltammetric behavior of truncated conical nanopore electrodes (20-200 nm orifice radii) has been investigated in low ionic strength solutions. Voltammetric currents at the nanopore electrode reflect both diffusive and migrational fluxes of the redox molecule and, thus, are strongly dependent on the charge of the redox molecule and the relative concentrations of the supporting electrolyte and redox molecule. In acetonitrile solutions, the limiting current for the oxidation of the positively charged ferrocenylmethyltrimethylammonium ion is suppressed at low supporting electrolyte concentrations, while the limiting current for the oxidation of the neutral species ferrocene is unaffected by the ionic strength. The dependence of the limiting current on the relative concentrations of the supporting electrolyte and redox molecule is accurately predicted by theory previously developed for microdisk electrodes. Anomalous values of the voltammetric half-wave potential observed at very small nanopore electrodes (<50 nm radius orifice radii) are ascribed to a boundary potential between the pore interior and bulk solution (i.e., a Donnan-type potential).     

Zur kinetik der metallionenadsorption: II. Galvanostatische einschaltmessungen an silber-, bleiund thalliumadsorbaten auf goldelektroden

The kinetics of the metal ion adsorption of Ag⁺, Tl⁺ and Pb²⁺ on polycrystalline and (111) orientated monocycrystalline gold electrodes were investigated using a low-level short-time polarization technique. In the present case the systems were potentiostatically prepolarized under steady state equilibrium conditions. The perturbation was realized by galvanostatic square pulses. The observed overpotential—time transients were analyzed by polynomial curve fitting. The results can be interpreted in terms of the developed polarization model [1]: (1) the process of metal ion adsorption is not reversible; (2) the hindrance is caused by charge transfer and can also be influenced by surface diffusion of the adsorbed metal ion species. In the case of Ag⁺ and Pb²⁺ the calculated mean values of the exchange current densities of the sorption processes are lower than the exchange current densities of the corresponding metal ion electrodes and they do not depend markedly on the potential. The data obtained are discussed.     

Studying the O2, Metal/O2– (Solid Electrolyte) Electrode System with Use of Model Electrodes: The Exchange Current Density Determination

Cells with model gold and platinum electrodes are studied by an impedance spectroscopy method. Distinguishing features of these electrodes are a large characteristic size of the metal/electrolyte contact (up to 100 m) and such well definable geometrical parameters as the length of triple-phase boundary (TPB) and the contact area. E. Shouler discovered that the electrolyte resistance, measured in an electrochemical cell, varies with the oxygen activity. Modern view is that this is connected with the TPB expansion. The expansion parameters determined here depend on the electrolyte prehistory and reach a few tens of micrometers. Specific (referred to a unit TPB length) values of polarization conductivities of both electrodes are obtained. Depending on the electrode prehistory, they amount to (2–6.5) × 10^–4 and (3–22)× 10^–4 S cm^–1 for Pt and Au, respectively, at 977°C in an oxygen atmosphere. The polarization conductivity, calculated per unit active area of electrolyte, is independent of the Pt electrode prehistory. Calculated exchange current densities are compared with radioisotope assay results.     

多孔与平面硅电极界面的电化学行为

研究了重掺杂n-型单晶硅(CSi)在氢氟酸体系中生成多孔硅(PSi)的电化学行为,根据线性极化曲线,选取不同的电流密度,采用恒电流阳极极化法,制备了一系列多孔硅层。利用扫描电子显微镜对其进行了表面和断面形貌的表征,通过线性扫描极化技术和计时电位法,比较了单晶硅电极和多孔硅电极的电化学行为,分析了多孔硅形成前后的塔菲尔曲线和计时电位曲线,给出了多孔硅形成过程中的重要电化学参数,如腐蚀电流、开路电位、塔菲尔斜率等。并对其进行深入分析,根据实验结果,提出了单晶硅电极/电解质界面和多孔硅电极/电解质界面的结构模型,并利用该模型讨论了两种电极界面的电化学特性。     

Peculiarities of anodic behavior of gold electrode in thiosulfate electrolytes

Using the methods of quartz microgravimetry and voltammetry, the anodic behavior of gold electrode in thiosulfate electrolytes is studied in the pH range of 7 to 11. It is found that, in the potential range from 0.15 to 1.0 V (NHE), the anodic current is associated predominantly with the oxidation of thiosulfate ions, and the gold dissolution rate in this electrolyte is negligibly low (< 0.02 mA/cm²). It is shown that the study of anodic processes in the neutral thiosulfate electrolytes requires stabilization of solution acidity, because the near-anode layer can be acidified to the pH values, which are sufficient for the formation of elemental sulfur. It is found that the use of Britten-Robinson buffer solution with pH 7 as the supporting electrolyte changes significantly the polarization curve of thiosulfate ion oxidation, but does not raise the gold dissolution rate. An increase in the solution pH to 11 and an exposure of electrode at various potentials (−0.5 and 0.15 V) prior to the onset of potential scanning also do not accelerate considerably the gold dissolution in the thiosulfate electrolyte. A comparison between the regularities of gold anodic behavior in the thiosulfate solutions and earlier studied gold dissolution in the cyanide and thiocarbamide electrolytes showed that they are similar. It is supposed that the specific features of anodic processes in these cases are of a similar nature: the metal dissolution proceeds with the formation of two-ligand complexes with linear structure, which is typical for all aforementioned ligands.     

Reduction of metol on mechanically renewed metal electrodes

The electroreduction of metol on mechanically renewed metallic electrodes is studied by direct voltammetry with linear potential sweep. Reduction peaks of metol are found in a neutral supporting electrolyte (0.02–0.5 M Na₂SO₄) on nickel, silver, and mercury electrodes before the potential of hydrogen liberation from the supporting electrolyte. The shape and parameters of the cathodic peak depend on an electrode material, and also on the composition and pH of the supporting solution. A probable mechanism of the electroreduction of metol is proposed. The regeneration of nickel and silver electrodes by in situ mechanical cutting of a 0.5-μm surface layer provides good reproducibility of the value of peak current; it is proportional to the concentration of metol in the range 2 × 10⁻³–1.8 × 10⁻² M.     

Numeric Simulation of Natural Convection in Horizontal Layer of Binary Electrolyte at Constant Cell Voltage

Space dissipative structures resulting from convective instability of a binary electrolyte solution between two horizontal plane electrodes at a constant cell voltage are discussed. A system of hydrodynamic nonlinear equations is numerically integrated on a computer by the finite-difference method with spatial grid of 20 × 20. A relation between the applied voltage and Raleigh number is presented. The time dependences of current and electrode polarization during system relaxation towards steady state are obtained. The dependence of the electrode process acceleration due to natural convection on the applied voltage is found. The steady-state isolines of functions of current and concentration are constructed for some voltages. At too high Raleigh numbers, transition is observed to a turbulent mode.     

Self-accelerating chlorine evolution on porous anodes of finite thickness

Operation of a finite-thickness porous electrode under the chlorine evolution conditions is analyzed in the framework of the convective diffusion model. According to calculations, the gas evolution in the anode’s porous coating is a necessary but insufficient condition for the self-acceleration of the electrode process. Despite the gas evolution process in the anode pores, the anodic process on the low-activity electrodes decelerates, which is externally manifested in an increased Tafel slope of the polarization curve as compared with that for a smooth electrode. Self-acceleration of the anodic chlorine evolution takes place only on electrodes with true exchange currents in excess of 10^-4 A cm^-2. Externally, the self-acceleration effect manifests itself in the emergence of a low-polarizability portion in the high-current region of the polarization curve. Such a different effect of the gas evolution process on the chlorine reaction kinetics at porous electrodes of different catalytic activity is due to an altered balance between the diffusive and convective current constituents in the overall process rate following a change in the exchange current for either electrode.     

The role of the electrical double layer and ion pairing on the electrochemical oxidation of hexachloroiridate(III) at Pt electrodes of nanometer dimensions

The steady-state voltammetric oxidation of hexachloroiridate(III), IrCl6(3-) (1-5 mM), in the presence and absence of an excess supporting electrolyte was investigated at disk- and hemispherical-shaped Pt electrodes with radii ranging from 48 nm to 12.5 microm. Thermodynamic, kinetic, and transport parameters that define the shape and magnitude of the voltammetric wave exhibit a complex dependence on whether a supporting electrolyte is present in the solution. First, the half-wave potential, E1/2, for oxidation of IrCl6(3-) shifts to more positive potentials in the presence of a supporting electrolyte, a consequence of the relative difference in the strength of ion pairing of IrCl6(3-) and IrCl6(2-) by the supporting electrolyte cation. E1/2 increases in the order no electrolyte < n-tetrabutylammonium < Na+ approximately K+ approximately Ca2+, but is independent of the supporting electrolyte anion (Cl-, NO3-, PF6-). Second, the heterogeneous electron-transfer rate constant for oxidation of IrCl6(3-) increases by approximately an order of magnitude in the presence of a supporting electrolyte. Third, in the absence of electrolyte, mass transport limited currents deviate significantly from predicted values based on the Nernst-Planck equation, but only when the electrode radius is smaller than ca. 1 microm. The latter two effects (Frumkin and dynamic diffuse layer effects) result from the dependence of interfacial electrical fields and, thus, the rates of electron-transfer and ion migration, on the supporting electrolyte concentration. We also demonstrate that the theoretical shape of the voltammetric response for oxidation or reduction of a highly charged redox species (e.g., IrCl6(3-)) is essentially independent of whether a supporting electrolyte is present in the solution. This finding can greatly simplify the analysis of heterogeneous electron-transfer rates using steady-state voltammetry in low ionic strength solutions.     

Effect of Supporting Electrolyte Composition on the Intensity of Metal Lines in Electrolyte-Cathode Discharge Spectra

The effect of the supporting electrolyte (SE) concentration on the intensity of metal lines in electrolyte-cathode drop-spark and glow discharge spectra was studied. It was shown that the signal attains a maximum for both discharges in the range of supporting electrolyte concentrations from 0.2 to 0.8 M. A model that qualitatively explains such curve shapes by the effect of the ionic strength of the solution on the catholyte atomization is proposed.     

New accelerated method of impregnation by aqueous electrolyte of carbon black gas-diffusion electrodes to study their structural and electrochemical characteristics

A new method of fast impregnation of carbon black gas-diffusion hydrophobized electrodes is suggested under their cathodic polarization in an alkaline aqueous solution of tetrabutylammonium bromide (TBAB) at the potentials of hydrogen evolution. As dependent on the quantitative content of polytetrafluoroethylene (PTFE) in the electrode, current density, time, TBAB concentration, various degree of electrode wetting is observed, up to nearly complete electrode flooding in just several hours. When electrodes are stored, their original electrolyte porosity is not recovered. the electrode with 8 wt % of PTFE was used to show the effect of the electrode flooding degree on the double layer capacity, average diameter of electrolyte pores, their surface area and activity in the case of oxygen reduction. This method may be used for simulation of the process of flooding of gas-diffusion electrodes by electrolyte in the course of their prolonged operation.     

反渗透膜UTC-70在水溶液中的介电谱及其解析

根据平面层状体系介电弛豫理论研究了反渗透膜UTC-70在各种浓度氯化钠和氯化钾溶液中的介电弛豫行为.利用计算机拟合的方法得到膜/溶液体系的介电参数,并由此计算得到了UTC-70膜相和水溶液相的相参数,获得了反映反渗透膜UTC-70荷电情况的信息及其与电解质溶液浓度的关系,介电解析的结果解释了介电弛豫的产生机制.     

Electrochemical detection of Cu(I) and Cu(II) in styrene media

A method is described to detect Cu(II) and Cu(I) added as bromide simultaneously in styrene solution containing tetrahexylammoniumperchloraat (THAP) as supporting electrolyte. It was found that Cu(II) and Cu(I) behave similarly in styrene and in aqueous solution. Reduction of Cu(II) and Cu(I) to metallic copper, as well as oxidation of Cu(I) to Cu(II) and the dissolution of a deposited metallic copper layer are observed. Ohmic drop problems were circumvented by adding THAP to the styrene solution and using ultramicro electrodes. The simultaneous detection of Cu(II) and Cu(I) is based on recording a cyclic voltammetric curve in a mixture of these compounds and calculating their concentration from the cathodic limiting current obtained at −0.80 V vs. RE and the anodic stripping peak corresponding to the dissolution of metallic copper. A detection limit of 2.0×10⁻⁴ mol l⁻¹ was obtained for both Cu(II) and Cu(I) and reproducible results were obtained concerning sensitivity and stability of the calibration curves.     

Synthesis and Conductivity of CaZr<Subscript>0.9</Subscript>Y<Subscript>0.1</Subscript>O<Subscript>3–δ</Subscript> Electrolyte Films on Supporting Composite Electrodes

Conditions of the chemical solution deposition of CaZrO₃-based electrolyte films on supporting composite electrodes are studied. The films are formed on the composites of CaZrO3 with metal oxides СuO, Fe₂O₃, and NiO. The morphology and the phase and elemental composition of supports and films are studied as well as the gas permeability and conductivity of films. It is concluded that composites of calcium zirconate with nickel can be recommended as the supporting anodes for the proton-conducting CaZr_0.9Y_0.1O_3–δ film electrolyte.