Showing papers in "Russian Journal of Electrochemistry in 2004"

Electrochemical machining of metals: fundamentals of electrochemical shaping

Abstract: Basic problems of metal electrochemical shaping theory are considered. Exact and approximate (quasi-steady-state, locally one-dimensional) methods for solving direct and inverse problems are analyzed. Experimental methods for improving the electrochemical shaping accuracy are considered.

Interpretation of AC Conductivity of Lightly Doped Conducting Polymers in Terms of Hopping Conduction

Abstract: The ac conduction of most disordered materials shows a regime of constant dc conductivity σ(0) at low frequencies and a crossover to a frequency-dependent regime of the type Aωs at high frequencies. We discuss the physical origin of this behavior and the interpretation of the onset frequency of excess ac conduction, ωc, in terms of models for hopping conduction of polaronic carriers in a disordered medium. We emphasize the central role of the critical hopping rate in the percolation lattice for determining both σ(0) and ωc. We show the behavior of σ(ω) for lightly doped polypyrrole samples at different stages of doping, and we discuss the significance of frequency dispersion effects for electrochemical measurements.

Dynamic Characteristic of an Electrochemical Cell with Gauze Electrodes in Convective Diffusion Conditions

Abstract: A frequency dependence of the transfer function of an electrochemical cell is studied experimentally in the frequency span 0.02 to 40 Hz under conditions of controlled convective diffusion. Above the diffusion frequency, experimental data nicely conform to theoretical calculations, but below it the function's decay cannot be explained within existing theoretical notions.

Preparation and Properties of Composite Polypyrrole/Pt Catalyst Systems

Abstract: Three different methods for the preparation and modification of conducting polymer/noble metal catalyst systems consisting of polypyrrole (PPy) and platinum (Pt) are described for the anodic oxidation of methanol. The first method consists of the electrochemical deposition of a thin PPy film on glassy carbon substrate, which is modified with Pt either by electroreduction of hexachloroplatinate, codeposition from a nanodispersed Pt solution, or incorporation of tetrachloroplatinate as counterion followed by cathodic reduction. A second method is based on the preparation of nanoscale PPy(PSS) particles by chemical polymerization with polystyrenesulfonate PSS– as the counterion. This material is a favorable catalyst support for nanodispersed Pt due to its mixed electronic and cationic conductivity. To study the electrochemical properties, the particulate system PPy(PSS)/Pt is fixed in a carbon fiber electrode. A third method was developed which brings the polypyrrole in close contact to a proton exchanger membrane (Nafion) using a special chemical deposition procedure. This method is useful for preparing a membrane electrode assembly (MEA) consisting of Nafion/PPy/Pt. The structural, morphological, and electrocatalytic properties for methanol oxidation were studied depending on the preparation method applied using surface analytical techniques (TEM, SEM, and EDX) and electrochemical measurements (cyclic voltammetry and transient techniques).

Template Synthesis of Polyaniline in the Presence of Poly-(2-acrylamido-2-methyl-1-propanesulfonic Acid)

Abstract: A template oxidative polymerization of aniline in aqueous solutions containing poly-(2-acrylamido-2-methyl-1-propanesulfonic acid) and ammonium persulfate yields a polyaniline complex with the polyacid soluble in water. According to the data of spectral measurements, the process of polymerization is of a pronounced autocatalytic nature. The consequences of this are both the formation of a nonuniform distribution of oxidized and nonoxidized fragments of the polymer chain and the formation of a macroscopic scale of redox nonuniformities in the reaction volume during the process of polymerization. The spectra of absorption of the polyaniline/polyacid films on transparent glass electrodes with a conductive layer of SnO2 and the data of potentiodynamic measurements in the potential region 0–1.0 V (Ag/AgCl) are typical for polyaniline with moderate electroconduction.

Diffusion Sensor of Mechanical Signals: Frequency Response at High Frequencies

Abstract: The frequency response of a diffusion transducer with gauze electrodes, studied theoretically at high frequencies, is proportional to ∼ω-3/2 starting with ω = D/R 2 at ω < ν/R 2 and to ∼ω-1 at ω > ν/R 2. Here, ω = 2πf, f is the signal frequency, ν the electrolyte viscosity, D a diffusion coefficient, and R the radius of the gauze filaments. The calculation is compared with experiment.

Electrochemical Properties of Carbon Black AD-100 and AD-100 Promoted with Pyropolymer of Cobalt Tetra(p-methoxyphenyl)porphyrin

Abstract: Carbon black AD-100: initial, activated, and promoted with pyropolymer of cobalt tetra(p-methoxyphenyl)porphyrin is characterized by a complex of electrochemical (floating electrode, rotating disk electrode, rotating ring-disk electrode, electrochemical impedance) and structural (standard porosimetry, BET) methods of investigation. Procedures for the AD-100 activation and promotion and the preparation of thin layers of the material to be studied and deposited on disk electrodes are described. The effect of the activation and promotion of carbon black on the surface and electrocatalytic activity of materials under study in the reduction of oxygen and hydrogen peroxide is shown. The ratio of constants of oxygen reduction directly to water and through intermediate formation of hydrogen peroxide is determined. A path for the oxygen reduction is discussed.

Composition, Structure, and Corrosion–Electrochemical Properties of Chromium Coatings Deposited from Chromium(III) Electrolytes Containing Formic Acid and Its Derivatives

Abstract: To study electrocatalytic properties of chromium, its deposits are obtained from Cr(III) electrolytes containing formic acid and its four derivatives (formaldehyde, methyl alcohol, formamide, dimethylformamide) and comprehensively examined. Amorphous structure of the deposits is established with an x-ray diffraction analysis. The valence state of chromium and major extrinsic elements is studied by x-ray photoelectron spectroscopy. The carbon reduction degree and the organization of carbon particles in deep layers of the deposits (chainlike or graphite-like structures) is shown to depend on the nature of organic compounds added into the chromium-plating electrolyte. The corrosion–electrochemical behavior of the obtained deposits in 0.5 M H2SO4 is compared to that of polycrystalline chromium and deposits plated from sulfate–oxalate Cr(III) electrolytes.

Effect of Surface Structure on Behavior of RuO2 Electrodes in Sulfuric Acid Aqueous Solution

Abstract: The electrochemical properties of RuO2 electrodes are studied by means of cyclic voltammetry, potential step, and impedance measurements in aqueous 05 mol/dm3 H2SO4 as a supporting electrolyte solution and applying the hanging electrolyte meniscus technique Two types of the electrodes are used: bulk “as-grown” single-crystal having (101) exposed interface and 500 nm film produced by reactive RF magnetron sputtering at 450°C The surface structure of the RuO2 electrodes prepared by different techniques is defined from X-ray LAUE backscattering Experimental data show that, for both RuO2 electrodes, the charging components include a slow diffusion-controlled contribution, due to proton injection–ejection The diffusion characteristics and the diffusion coefficients for the same electrodes are estimated and reported

Electropolymerization of Pyrrole on Oxidizable Metals: Role of Salicylate Ions in the Anodic Behavior of Copper*

Abstract: The behavior of copper electrodes in aqueous salicylate solutions is studied by cyclic voltammetry. The results suggest that copper undergoes partial anodic passivation through the deposition of a copper(II) salicylate compound. Evidence for the formation of this layer is also given by the data obtained from surface enhanced Raman spectroscopy (SERS). Such a film appears to be responsible for the successful growth of polypyrrole films on copper in salicylate media.

Synthetic Semiconductor Diamond Electrodes: Electrochemical Characteristics of Homoepitaxial Boron-doped Films Grown at the (111), (110), and (100) Faces of Diamond Crystals

Abstract: Electrode behavior of homoepitaxial (single-crystal) boron-doped diamond films deposited onto differently orientated faces of dielectric diamond single crystals is studied by the electrochemical impedance and potentiodynamic curve methods. It is shown that the acceptor concentration determined from the slope of Mott–Schottky plots decreases, in the epitaxial films grown under the same conditions, in the series: (111) > (110) > (100). This is explained by different intensity of boron incorporation, from gas phase, into differently orientated faces of the diamond crystals during their growth. The rate of electrode reactions in the Fe(CN)6 3–/4– and Ru(NH3)6 2+/3+ redox systems decreases in the above series, which obeys the earlier found interrelationship between the electrochemical kinetics at diamond electrodes and their doping level.

Oxidation of Methanol on Pt, Pt–Ru, and Pt–Ru–Mo Electrocatalysts Dispersed in Polyaniline: An in situ Infrared Reflectance Spectroscopy Study

Abstract: The electrochemical oxidation of methanol was investigated on a Pt–Ru–Mo catalyst with an in situ infrared reflectance spectroscopy. The electrocatalysts were prepared by an electrochemical deposition and dispersed in a conducting three-dimensional matrix of polyaniline (PAni). We observed that CO2 is produced from methanol oxidation at 350 mV vs. RHE on PAni/Pt–Ru–Mo, which is 100 mV less negative than on PAni/Pt–Ru and 200 mV less than on PAni/Pt. The results suggest that Pt–Ru–Mo is less sensitive to COADS poisoning than Pt–Ru and much more sensitive than Pt. Large differences are observed concerning the average wavenumber of СОADS between Pt–Ru–Mo, Pt–Ru, and Pt.

Study of the Properties of Electrodeposited Polypyrrole Films

Abstract: Experimental and theoretical methods have been used for characterization of the properties of polypyrrole films. The AFM studies show that the morphology of polypyrrole (PPy) films on polycrystalline gold electrodes at the first stages of synthesis depends on the structure of the metal surface. It was established that mobility of anions depends remarkably on the rate of electrodeposition of the polymer film. If PPy film was deposited at relatively low current density, mobility of ClO-4 anion was not high enough to guarantee electroneutrality during redox cycling and cations take part in this process especially when Li+ cations were replaced by more mobile К+ cations. Semiempirical (AM1 and PM3) quantum-chemical methods were used for theoretical studies. It was established that different size and charge of the anions together with the variation in doping levels give rise to a different optimal conformation of oligopyrrole cations which, in turn, define the resulting polymer to be either all-anti (common linear chains) or all-syn (formation of helical structures) or a combination of the two.

Competition between Polymerization and Dissolution of Poly(3-methylthiophene) Films

Abstract: Films of electrically conducting polymer, poly(3-methylthiophene), are dissolved in monomer-free solutions at positive potentials to become thin, whereas they are polymerized in monomer-rich solutions at the same potentials as for the dissolution. A question arises whether they are dissolved or polymerized in solutions including a given concentration of the monomer when a positive potential is applied to the film. Conditions of the competition between the polymerization and the dissolution were searched at potentiostatic experiments for poly(3-methylthiophene) films at various concentrations of monomers and potentials. The dissolution prevailed over the polymerization as the concentration decreased and the potential was less positive. Chronoamperometric currents exhibited oscillation under the competition conditions. The oscillation was explained in terms of the Lotka–Volterra model for a simple oscillation reaction, in which competitive species were the conducting polymer and the monomer.

Importance of Metastable States in Electrocatalytic Processes at Metal Surfaces in Aqueous Media

Abstract: Solid metals and their surfaces can trap or store energy; it appears, therefore, that, for any solid metal in aqueous media, there are two limiting types of surface electrochemistry, one relating to the equilibrated (low-energy) metal atoms, and the other, to the metastable (high-energy) metal atoms. With real metal surfaces, the low-energy state is usually dominant, but the metastable state behavior is often vital with regard to low coverage surface active site behavior. The most significant effects of surface metastability are that it lowers the oxidation potential of the atoms in this state to values within the double layer region and forms the basis of facile interfacial redox couples which often function as mediators in electrocatalytic processes. This rather novel approach was explored with regard to the electrocatalytic behavior of copper in base; the applicability of the approach to the three Group 11 metals (Cu, Ag, and Au) was illustrated, and its relevance to the use of platinum in fuel cell electrocatalysis was outlined.

Pyrite Oxidation Mechanism in Aqueous Solutions: An in situ FTIR Study

Abstract: FTIR spectra for the pyrite electrode/electrolyte interface at polarizations of –0.5 to 0.9 V (NHE) and pH 9.2 are obtained in situ for the first time and interpreted. A fundamentally new model for the pyrite oxidation in aqueous solutions is proposed on the basis of these data. According to the model, the oxidation occurs via an electrochemical (corrosion) mechanism. The cathodic half-reaction proceeds either on sulfur-deficient pyrite areas or FeS defects. The anodic half-reaction proceeds via a “thiosulfate” path, on areas of “common” pyrite characterized by the fixing of the Fermi level in a top portion of the valence band. The basic role of the oxidant consists of maintaining a high potential on anodic areas. The direct (chemical) oxidation of pyrite is an indirect effect and consists of an interaction with products of the anodic reaction. It is shown that the corrosion model, being free of contradictions inherent in previous models, explains some experimental facts left previously without any explanation. From this model follows an indirect mechanism of pyrite bioleaching.

Surface and Subsurface Oxygen on Platinum in a Perchloric Acid Solution

Abstract: The oxidation of polycrystalline platinum in perchloric acid is studied by cyclic voltammetry at a potential scan rate of 0.1 V s–1 in various potential cycling ranges. The earlier model for the formation of a barrier layer of strong complexes consisting of subsurface oxygen Oss, platinum atoms, and anions adsorbed on the latter is shown to correctly describe experimental results on the platinum oxidation in sulfuric and perchloric acids. The regularities in these acids are on the whole similar. A weaker adsorption of perchlorate anions as compared with bisulfate facilitates chemisorption of oxygen at 0.7–0.85 V and hinders exchange by sites Pt ⇔ O at 0.85–1.35 V. A prolonged potential cycling with a cathodic limit of 0.27 V and low anodic limits leads to the accumulation of surface complexes Oss–Ptn–ClO4, which hinder both the oxygen chemisorption and the exchange Pt ⇔ O below 1 V. At more positive potentials, the complexes are destroyed and oxygen penetrates into subsurface platinum layers.

Effect of Cation Nonstoichiometry on the Properties of Solid Electrolyte Ba x Ce0.97Nd0.03O3 - δ (0.90 ≤ x ≤ 1.10)

Abstract: The effect of the cation nonstoichiometry on the electroconduction, electrotransfer, and stability in humid air of solid proton-conducting electrolyte Ba x Ce0.97Nd0.03O3 - δ (0.90 ≤ x ≤ 1.10) is studied. The electroconduction is found to decrease with decreasing content of BaO (x ≤ 1.0) and weakly depend on the BaO excess (x> 1.0). Ceramics of a stoichiometric composition (x = 1) and with lack of barium is stable when stored in humid air at room temperature, as opposed to the ceramics with barium excess, which rapidly decomposes in these conditions. The conduction of materials under study in oxidizing environment is ion-hole and in reducing media, ionic (proton + oxygen). The cation nonstoichiometry barely affects the nature of the ion transfer. An analysis of possible models for the formation of defects shows oxygen vacancies to be always present in Ba x Ce0.97Nd0.03O3 - δ, even at the CeO2 excess exceeding the content of neodymium.

Catalytic Activity of Polyaniline in the Molecular Oxygen Reduction: Its Nature and Mechanism

Abstract: By comparing experiments in air and an inert atmosphere, additional evidence for the polyaniline catalytic action on the air oxygen electroreduction is obtained. The action is compared with that of graphite substrates under the same conditions. The effect of the oxygen supply method and the polyaniline film thickness (weight) on the oxygen reduction rate is evaluated. To elucidate the nature of the polyaniline catalytic activity, quantum-chemical modeling of polyaniline and its adsorption complexes with oxygen is carried out. According to calculations, molecular oxygen can be reversibly chemisorbed at the polyaniline surface that serves as a donor of electron density. The bonding order for the O2*molecules chemisorbed at polyaniline decreases by ∼30% and the bond length increases by 24%. Thus, oxygen molecules acquire higher activity in a chemisorbed state and can then participate in the reduction more actively.

Effect of Ammonium Ions on the Electroreduction of Anions at a Mercury Electrode

Abstract: The effect of ammonium ions on the electroreduction of peroxodisulfate (S2O2- 8) and perbromate (BrO- 4) anions is found to be commensurate with that of potassium ions. Ammonium ions accelerate the reduction of iodate (IO- 3), bromate (BrO- 3), and chromate (CrO2- 4) anions in nonbuffered solutions and does not, in buffered ones. In the former case, the effect is connected with the pH change in the near-electrode layer during the reaction and with the participation of ammonium ions in hydrolytic equilibriums, rather than with a simultaneous transfer of ammonium ions and electrons in an elementary act. The conclusions in the literature in favor of a simultaneous transfer of the proton and electron in the proton-consuming reactions of reduction of anions is shown to be ambiguous.

Oxygen Transport in the SOFC Cathode

Abstract: The impedance of the La0.75Sr0.2MnO3-cathode/electrolyte interface for cathodes with different porosity is measured. The impedance spectra are fitted using a developed model of the oxygen transport at this interface. After the measurements, the cathode is removed from the electrolyte. The contact area and the three-phase boundary length (TPBL) at the interface are estimated from SEM images of the electrolyte surface. The dependence of the interfacial electrical resistance on the microstructure is discussed. It is shown that the bulk diffusion of oxygen vacancies at the interface at 950°C is high enough to use the whole La0.75Sr0.2MnO3/YSZ contact area F for the oxygen transport into the electrolyte for microstructures with 2F/TPBL ≤ 2 μm. The impact of the surface diffusion of oxygen species on polarization resistance at operation temperatures <900°C is discussed. The polarization resistance and the morphology of composite cathodes made from La0.75Sr0.2MnO3/YSZ and yttria- or scandia-stabilized zirconia powders (3YSZ, 8YSZ, 10ScSZ) are investigated by impedance spectroscopy at 800–950°C. The polarization (interfacial) resistance decreases gradually with addition of electrolyte powder in the uLSM cathode material independent of the electrolyte powder used. The interfacial resistance of the uLSM/3YSZ, uLSM/8YSZ, and uLSM/10ScSZ composite cathodes is almost the same. The interaction between uLSM and doped zirconia particles is discussed on the basis of the interfacial resistance, activation energies, and high-frequency impedance.

Regression Analysis of Two Dimensional Pressure Isotherms for Individual Adsorption of Surface Active Molecules and Ions

Abstract: A new version of the regressive technique is proposed for calculating adsorption parameters from two-dimensional pressure isotherms. The method assumes that the Frumkin isotherm is applicable to individual adsorption of surface-active neutral molecules and single-charged ions. Based on literature data, the adsorption parameters are assessed for tetraethylammonium, tetrapropylammonium, and tetrabutylammonium cations and for saccharose molecules for their adsorption from aqueous solutions on the uncharged mercury surface.

Lithium Intercalation into Titanium Dioxide Films from a Propylene Carbonate Solution

Abstract: Intercalation of lithium from an LiClO4 propylene carbonate solution into thin-film TiO2 (rutile) electrodes produced by thermal oxidation of a titanium substrate are studied using cyclic voltammetry and impedance measurements at 0.01 to 105 Hz. An equivalent circuit adequately modeling the impedance spectra of TiO2- and Li x TiO2 electrodes throughout the frequency range studied is proposed. The electrochemical characteristics of film electrodes, the reversibility of intercalation-deintercalation process, the effect of surface passivation on the lithium transfer rate, and the dependence of electric, kinetic, and diffusion parameters on the electrode potential (composition) are discussed. The diffusion coefficient of lithium in Li x TiO2 is 10–12 cm2/s, as estimated by the impedance method.

Electrochemical Halogenation of the Single-Charged Anion of Dodecahydro-7,8-dicarba-nido-undecaborate

Abstract: By means of electrochemical iodination and bromination of potassium dodecahydro-7,8-dicarba-nido-undecaborate, extracted are monoiodide and monobromide derivatives in the form of alkylammonium salts with preparative yields. Their structure is confirmed by the data of elemental analysis and IR and 11B NMR spectroscopy.

Conductivity of Aqueous Solutions of Low-Molecular Chitosan

Abstract: Solutions of chitosan with molecular weight (MW) of ∼20000, ∼9600, and ∼3700 Da are studied conductimetrically and viscosimetrically. The dependence of solution conductivity on the chitosan concentration begins to deviate from linearity simultaneously with an abrupt increase in the solution viscosity starting from concentrations of 20–30 g l–1. The fraction of free counterions (Cl–, CH3COO–) in the 0.1 g-equiv l–1 chitosan solutions significantly depends on the sample's MW. The charge is transferred in solutions predominantly by chloride and acetate ions, with the high-MW cation barely contributing to conduction.

Composite Polyaniline/MF-4SK Membranes: A Chemical Template Synthesis and the Sorption and Conduction Properties

Abstract: The sorption and hydrophilic properties of composite membranes PAN/MF-4SK, obtained by a chemical template synthesis, are studied. The synthesis is performed in static and dynamic conditions for perfluorinated sulfo-cationite membranes MF-4SK in 0.01 M aniline solutions in 0.5 M H2SO4 and 0.01 M FeCl3 in 0.5 M H2SO4. The aniline sorption and polymerization kinetics is studied by conductimetry, radioisotope, and optical methods. The polymerization rate is found to depend on the oxidant concentration and the sequence of the membrane saturation with aniline. An increase in the intensity of the color of a composite film, which depends on the balance between the quinoimine, amine, and radical-cation fragments in the perfluorinated matrix, does not lead to any substantial changes in the moisture content or the ac electroconductivity. Implanting polyaniline chains into the perfluorinated matrix presumably leads to a molecular reorganization of water at the expense of a change in its association at the point where redox fragments of polyaniline converge with side segments of the template matrix.

Indirect Electrooxidation of Organic Substrates by Hydrogen Peroxide Generated in an Oxygen Gas-Diffusion Electrode

Abstract: Indirect electrooxidation of phenol, formaldehyde, and maleic acid in cells with and without a cation-exchange membrane, with a platinum anode and a gas-diffusion carbon black cathode, which generates hydrogen peroxide from molecular oxygen, proceeds with high efficiency and various oxidation depths, which depend on the intermediate nature: the process involving HO 2 - occurs selectively and yields target products, while the formation of HO2 · and HO· leads to the destruction of organic compounds to CO2 and H2O.

Cathodic Hydrogen Evolution on Nickel in Acidic Environment

Abstract: Cathodic hydrogen evolution on polycrystalline nickel with differently prepared surfaces and on individual faces of single-crystal nickel is studied. It is established that the hydrogen overvoltage on nickel in acid media considerably depends on the surface preparation technique. In some cases, the difference in the hydrogen overvoltage cannot be explained by different roughness of electrodes. The absorption of atomic hydrogen in a surface layer may substantially alter electrocatalytic properties of both polycrystalline and single-crystal nickel.

Effect of Lead Ions on the Kinetics of Gold Deposition from Cyanide Electrolytes

Abstract: The depolarization of the gold electrodeposition in the presence of lead ions depends on their concentration and the duration of electrode contact with solution preceding a potential scan in an extremum fashion. At constant coverages of the gold surface by lead adatoms θ, the process rate depends on the overvoltage in accord with the Tafel equation. Effective values of the exchange current i 0 and transfer coefficient α increase with θ from i 0 ≅ 3 × 10–5 A cm–2 and α = 0.23 in pure solutions to 3 × 10–4 A cm–2 and 0.53 at θ ≅ 0.4. The reaction order by cyanide ions is independent of θ and equals nearly –0.9. Effects of lead adatoms on the kinetics of cathodic and anodic processes are compared and the obtained data may be brought to conformance given that their mechanisms in pure solutions differ and converge in the presence of lead adatoms.

Potassium Aluminate Crystalline Structure and Electroconduction

Abstract: The crystalline structure of low- and high-temperature modifications of KAlO2 is studied by a neutron diffraction method combined with a full-profile Rietveld analysis. At low temperatures KAlO2 has an orthorhombic structure (spatial group Pbca), which turns tetragonal (spatial group P41212) at 540°C. During a phase transition along the c axis there open new conduction channels with a large cross-section of voids, which is a probable reason for the conduction jump. The presumed anisotropy of conduction in the low-temperature modification disappears during the phase transition.