Showing papers in "Russian Journal of Electrochemistry in 2007"

Coupled convection of solution near the surface of ion-exchange membranes in intensive current regimes

Abstract: Mechanisms responsible for the overlimiting ion transfer in membranes systems are discussed. The overlimiting transfer is shown to be due largely to the action of four effects coupled with the concentration polarization of the system. Two of these are connected with the water dissociation near the membrane/solution interface: the emergence of additional charge carriers (ions H+ or OH−) in the depleted solution layer and the exaltation of transfer of salt counterions. The latter effect is connected with the perturbation of electric field caused by the water dissociation products. The other two effects are two versions of coupled convection, which leads to partial destruction of the depleted diffusion layer. These include gravitational convection and electroconvection. The former is caused by the emergence of the solution’s density gradient. The latter develops via a mechanism of electroosmotic slip. In this work, methods of voltammetry and chronopotentiometry and pH measurements are used to study the transfer of ions through homogeneous membranes Nafion-117 and AMX as a function of the concentration of sodium chloride solutions in the underlimiting and overlimiting current regimes. In a 0.1 M NaCl solution, gravitational convection makes a considerable contribution to the transfer of salt ions near the membrane surface in intensive current regimes. The influence of this effect on the electrochemical behavior of membrane systems weakens with the solution dilution and with increasing relative transfer of the H+ and OH− ions that are generated at the membrane/solution interface. In conditions where gravitational convection is suppressed and the water dissociation near the membrane/solution interface is not great, the major contribution to the overlimiting growth of current is made by electroconvection. Topics for discussion in the paper include the mutual influence of effects on one another, in particular, the effect the rate of generation of the H+ and OH− ions exerts on the gravitational convection and electroconvection and the reasons for the different behavior of cation-and anion-exchange membranes in intensive current regimes.

Effect of oxygen vacancies in anodic titanium oxide films on the kinetics of the oxygen electrode reaction

Abstract: The effect of oxygen vacancies in the anodic oxide film on passive titanium on the kinetics of the oxygen electrode reaction has been studied by potentiodynamic polarization and electrochemical impedance spectroscopy (EIS). Oxide films of different donor density were prepared galvanostatically at various current densities until a potential of 20.0 VSHE was achieved. The semiconductive properties of the oxide films were characterized using EIS and Mott-Schottky analysis, and the thickness was measured using ellipsometry. The film thickness was found to be almost constant at ∼44.7 ± 2.0 nm, but Mott-Schottky analysis of the measured high frequency interracial capacitance showed that the donor (oxygen vacancy) density in the n-type passive film decreased sharply with increasing oxide film formation rate (current density). Passive titanium surfaces covering a wide range of donor density were used as substrates for ascertaining relationships between the rates of oxygen reduction/evolution and the donor density. These studies show that the rates of both reactions are higher for passive films having higher donor densities. Possible explanations include enhancement of the conductivity of the film due to the vacancies facilitating charge transfer and the surface oxygen vacancies acting as catalytic sites for the reactions. The possible involvement of surface oxygen vacancies in the oxygen electrode reaction was explored by determining the kinetic order of the OER with respect to the donor concentration. The kinetic orders were found to be greater than zero, indicating that oxygen vacancies are involved as electrocatalytic reaction centers in both the oxygen evolution and reduction reactions.

Voltammetric determination of ciprofloxacin based on the enhancement effect of cetyltrimethylammonium bromide (CTAB) at carbon paste electrode

Abstract: Herein, an electrochemical method was developed for the determination of ciprofloxacin based on the enhancement effect of cetyltrimethylammonium bromide (CTAB). In pH 7.0 phosphate buffer, a poorly-defined oxidation peak is observed at carbon paste electrode (CPE) for ciprofloxacin. However, the oxidation peak current remarkably increases in the presence of low concentration of CTAB, suggesting that CTAB exhibits obvious enhancement effect to the determination of ciprofloxacin. All the experimental parameters, such as supporting electrolyte, pH value, concentration of CTAB, and accumulation time, were optimized for ciprofloxacin analysis. This new method possesses high sensitivity (detection limit is 5.0 × 10−8 mol l−1), wide linearity (1.0 × 10−7−2.0 × 10−5 mol l−1), rapid response, low cost and simplicity. Finally, this method was successfully employed to detect ciprofloxacin in drugs.

The electrochemical properties of dopamine, epinephrine and their simultaneous determination at a poly(L-methionine) modified electrode

Abstract: A poly(L-methionine) modified electrode, fabricated by electrochemical immobilization of the L-methionine on a glassy carbon electrode, was used for simultaneous determination of dopamine and epinephrine through cyclic voltammetry. The electrochemical properties of dopamine and epinephrine have been investigated. This sensor gave two separated cathodic peaks at −0.282 and 0.112 V for EP and DA, respectively. A linear response was obtained in the range of 5.0 × 10−7 to 1.0 × 10−4 mol l−1 for epinephrine, and 1.0 × 10−6 to 5.0 × 10−4 mol l−1 for dopamine. The detection limits were 3.6 × 10−7 mol l−1 and 4.2 × 10−7 mol l−1 for epinephrine and dopamine, respectively. This method was successfully applied for simultaneous determination of dopamine and epinephrine in human urines.

Nanocomposites with Mixed Electronic and Protonic Conduction for Electrocatalysis

Abstract: Novel multipurpose electrode materials that have various catalytic applications are developed. The materials are composites based on multiwalled carbon nanotubes and electroconducting polymer polyaniline, organized as “fiber in a jacket.” Nanoparticles of metallic platinum and compounds of transition metals are additionally immobilized in the polymer layer. It is demonstrated experimentally that the materials possess high electronic and protonic conductance, thermal stability, hydrophilicity, large specific surface area, and considerable porosity.

Nafion effect on the lead dioxide electrodeposition kinetics

Abstract: Electrodeposition of lead dioxide in the presence of Nafion in solution is described by a four-stage kinetic scheme The limiting stage at low polarizations is that of the second-electron transfer, and at high polarizations, the stage of delivery of lead compounds to the electrode surface Nafion adsorption at the oxide profoundly affects the process kinetics

Acid-base chemistry of montmorillonitic and beidellitic-montmorillonitic smectite

Abstract: Suspension of a Tunisian purified smectite and American montmorillonite are studied by acid-base potentiometric and mass titrations. These experimental methods are used to determine the point of zero net proton charge (PZNPC). A very good agreement is observed between the two kinds of experiments. The two Namontmorillonites, studied at different ionic strengths, present proton adsorption vs. pH curves with a common crossing point. The PZNPC of the edge sites are 8.02 for Tunisian purified smectite and 8.11 for pure American montmorillonite. By analyzing the proton adsorption or desorption (H+ vs. pH) curves, one may assume the presence of four active sites at the surface. The montmorillonite surface undergoes two successive protonations and two successive deprotonations. Below pH A1OH groups) occur simultaneously. For pH > PZNPC and in alkaline pH range, deprotonation of surface hydroxyl groups exposed at the edge sites (>SiOH, and >A1OH at high pH) of montmorillonite platelets causes an overall negative charge.

Achievements in the field of proton-conductive polymer electrolyte membranes

Abstract: The 2000–2006 achievements in the field of synthesis, property examination, and application of proton-exchange membranes are reviewed on the basis of more than 120 papers.

Corrosion behavior of magnesium and its alloy in NaCl solution

Abstract: The electrochemical behavior of cast Mg, AZ91, and cast AZ91 in 0.1 M NaCl solution is investigated by measuring open-circuit potential (OCP), steady-state current-potential, and electrochemical impedance spectra (EIS). The similar electrochemical impedance behavior is found of three corrosion electrodes. There are two capacitances in high-and medium-frequency domains and one inductive loop or component in low-frequency domain. From equivalent circuit simulation, cast AZ91 has the worst corrosion resistance. The EIS results are in good agreement with those obtained by OCP and polarization curves. Based on the Cao theory, a simple corrosion mechanism is put forward, supplying a possible explanation for low-frequency inductive behavior for Mg and its alloy in NaCl solution at OCP.

Nanostructured catalysts for cathodes of oxygen-hydrogen fuel cells

Abstract: Bimetallic catalysts platinum-cobalt, platinum-chromium, and platinum-tungsten, deposited onto highly dispersed carbon black from complex cluster-type compounds of corresponding metals with a 1: 1 atomic ratio of metals are developed. The catalysts are characterized by methods of x-ray diffraction analysis and energy dispersive analysis of x-rays. The procedure involving use of a thin-film rotating disk electrode is employed to probe kinetic parameters of the oxygen reduction reaction on the catalysts developed. The investigated binary catalysts exhibit specific electrochemical characteristics that are not inferior and, in some cases, are superior to the characteristics intrinsic to the commercial platinum catalyst E-TEK, when tested in the composition of a gas-diffusion electrode under conditions that are close to real conditions in which cathodes of oxygen-hydrogen fuel cells operate.

Hydrogen bonding association in the electroreduced intermediates of benzoquinones and naphthoquinones

Abstract: Keeping in view the importance of chemical and biological functions of quinone based couples; two different series of quinones, namely benzoquinones and naphthoquinones, are investigated electrochemically. Five compounds of each series are studied systematically in dichloromethane, acetonitrile, and propylene carbonate and from there the half-wave potentials of first and second reductions are obtained through cyclicoltammetry measurements. Four different alcohols are used with increasing concentrations as hydrogen bond donors on the basis of their increasing acidity. The hydrogen-bonding power is analyzed from the positive shifts in both the waves with increasing concentrations of alcohols. The quantitative comparison is made while calculating the thermodynamic association constants and number of alcohol molecules bonded to both anion and dianion of quinones. The qualitative behavior and quantitative data both indicate the quinone-alcohol interaction as hydrogen bonding and the strength of hydrogen bond is dependant upon the nature of species involved in this couple. From the cyclic voltammetric data the relative effects of hydroxylic additives and different substituted quinones on equilibrium constant are also observed. Solvent effect is rationalized in favor of hydrogen bonding in terms of solvent polarity parameters.

Redox-switchable binding of the Mg2+ ions by 21,31-diphenyl-12,42-dioxo-7,10,13-trioxa-1,4(3,1)-diquinoxaline-2(2,3),3(3,2)-diindolysine-cyclopentadecaphane

Abstract: The binding of the Li+, Na+, K+, Mg2+, and Co2+ ions by 21,31-diphenyl-12,42-dioxo-7,10,13-trioxa-1,4(3,1)-diquinoxaline-2(2,3),3(3,2)-diindolysine-cyclopentadecaphane containing two indolysine fragments, two quinoxaline fragments, and 3,6,9-trioxyundecane spacer in the acetonitrile/01 M Bu4NBF4 environment is studied by the method of cyclic voltammetry It is demonstrated that the Li+, Na+, K+, and Co2+ ions are not bound by this macrocycle, whereas selective redox-switchable binding is observed for the Mg2+ ions The macrocycle binds the Mg2+ ions way more efficiently as compared with its radical cation and dication The indolysinequinoxaline fragments play the determining role in the binding

Anionic high-temperature conduction in single crystals of nonstoichiometric phases R 1 − y M y F 3 − y (R = La-Lu, M = Ca, Sr, Ba) with the tysonite (LaF 3 ) structure

Abstract: Electrophysical properties of single crystals of nonstoichiometric phases R1 − y M y F3 − y , where R = La-Lu, M = Ca, Sr, or Ba, with the tysonite (LaF3) structure, which are present in a metastable state after being grown and cooled, are measured in the temperature interval extending from 300 to 1073 K. It is discovered that, during a sufficiently long high-temperature investigation, solid solutions R1 − y Ca y F3 − y , where R = Tb, Dy, or Ho, undergo irreversible variations in the phase composition in the temperature region 723 to 823 K. This level of temperatures, which correspond to partial decomposition of phases R1 − y Ca y F3 − y with the rare-earth elements of the end of the period, lies above the temperatures to which the fluoride solid electrolytes are usually heated when used in solid-state electrochemical devices. The temperature and concentration dependences of the phases’ electroconduction are explained in the framework of the vacancy mechanism of anionic transport.

Analyzing electrochemical noise with Chebyshev’s discrete polynomials

Abstract: The principles underlying a novel method intended for analyzing experimental data obtained when studying fluctuation processes are considered. The method in question is Chebyshev’s spectroscopy. The application of this method allows one to determine statistic characteristics of steady-state electrochemical noise against the background of severe deterministic interference without invoking the procedure of the fitting of the initial data. The potentialities of this novel method, which is intended for treating noise experiment, are demonstrated by examining model examples and analyzing the electrochemical noise generated by a lithium electrode placed in an aprotic organic electrolyte.

Electrochemical synthesis of rare-earth metal (Eu, Nd) borides in molten salts

Abstract: Rare-earth metal borides are widely used in different fields of modern techniques. Electrochemical synthesis at moderate temperatures (973–1023 K) is a cost-effective alternative to direct reaction techniques. The present work reports the mechanism and kinetics of boron and europium, boron and neodymium joint electrodischarge in chloride-fluoride molten systems. The optimum regimes of europium and neodymium borides electrodeposition are worked out on the base of voltammetric experiments. Europium compound is synthesized as a single-phase EuB6 product, while neodymium compounds is co-deposited as NdB4 and NdB6.

Improving corrosion protection performance of polypyrrole coating by tungstate ion dopants

Abstract: Polypyrrole ( PPy ) and polypyrrole-tungstate ( PPy - ) coatings were electrodeposited on mild steel (MS) electrodes by cyclic voltammetry (CV) technique. Aqueous oxalic acid solution was used as sup- porting electrolyte for these processes. For the electrodeposition of PPy - , tungstate anion was added to the supporting electrolyte. The surface morphologies of the two types of coated-samples were characterized by scanning electron microscopy (SEM) and line-scan EDX analysis. Furthermore, open circuit potential (OCP) monitoring, polarization and electrochemical impedance spectroscopy methods were performed for comparing the corrosion protection performances. Corrosion studies indicate that dopants can improve corrosion protection properties by taking part in: (i) the passivation process occurred prior to electrodeposition process, (ii) the electrosynthesis process and incorporation into polymer chain.

Surface and bulk conduction and thermodynamic properties of ionic salt CsH5(PO4)2

Abstract: Transport properties of ionic salt CsH5(PO4)2 are studied by the impedance method. The salt’s bulk conductivity ranges from 10−8 to 10−4 S cm−1 in the temperature interval 90 to 145°C. The apparent activation energy is high (1.6–2.0 eV). The conductivity is slightly anisotropic: it is maximum in the [001] direction and minimum in the [100] direction (∼5.6 and 1 times × 10−6 S cm−1, respectively, at 130°C). The conductivity of polycrystalline samples is higher by 1–2 orders of magnitude, and the activation energy drops to 1.05 eV due to the formation of a pseudoliquid layer with a high proton mobility at the intercrystallite boundary. The salt’s thermodynamic properties are examined by differential scanning calorimetry and thermogravimetry. No phase transitions are discovered in the salt up to the melting point (151.6°C), with the melting enthalpy equal to ∼34 kJ mol−1. The crystallization occurs at lower temperatures (107°C) and the crystallization enthalpy (−18 kJ mol−1) is lower than the melting enthalpy. The melting is accompanied by slow decomposition of the salt. Factors affecting the proton transport in the salt are analyzed.

Conductivity of low-temperature KF-AlF3 electrolytes containing lithium fluoride and alumina

Abstract: Electroconductance of molten electrolytes KF-AlF3 (cryolite ratio 1.3) containing LiF or/and Al2O3 is evaluated using empirical equations. The electroconductivity of molten mixtures KF-AlF3, KF-AlF3-Al2O3, KF-AlF3-LiF, and KF-AlF3-LiF-Al2O3 is measured in the temperature interval 687 to 797°C. The electroconductivity is determined in cells with a capillary of pyrolytic boron nitride and parallel molybdenum electrodes. Comparing calculated and experimental data on the electroconductivity of cryolite-alumina melts confirms that the model proposed for the calculations reflects the temperature and concentration dependences adequately enough and makes it possible to forecast variations in the electroconductivity upon adding different admixtures into the electrolyte.

Conductivity and thermal expansion of the Ce0.8Gd0.2O1.9 solid electrolyte in the oxidizing and reducing atmospheres

Abstract: Highly compact (99%) solid electrolyte Ce0.8Gd0.2O1.9 with submicron (0.3 μm) grains is synthesized. The dilatometric (20–850°C) and conductivity (180–350°C) measurements are performed on the electrolyte in air and as a function of the partial oxygen pressure $$p_{O_2 } $$ (0.21−1×10−25 atm) at 600, 700, and 800°C. An inflection is found in the temperature dependences of the thermal coefficient of linear expansion and conductivity (impedance measurements) at ∼230°C, which is the evidence for a phase transition. The activation energies for conduction in the grain bulk and boundaries differ only slightly, indicating that the grain boundaries’ resistance is caused not by the precipitation of the second phase at the boundaries, but most probably by the presence of intergranular nanopores. The dilatometric measurements confirm a significant increase in the linear dimensions of Ce0.8Gd0.2O1.9 in the reducing atmospheres with a parallel increase in its electron conductivity. The electron conductivity and specific elongation increase proportionally to $$p_{O_2 }^{ - 1/4} $$ at all temperatures. The $$p_{O_2 } $$ values, at which the transport numbers of ions t i = 0.5, are determined. They are 10−22.5, 10−20, and 10−18 atm at 600, 700, and 800°C, respectively.

Electrochemical behavior of MF-4SK/polyaniline composites as investigated by membrane voltammetry

Abstract: Membrane voltammetry was used to investigate composites consisting of perfluorinated MF-4SK membranes and polyaniline (PANI) and synthesized under various conditions. A theoretical analysis of the influence of transport-structure parameters of the ion-exchange membranes on their selectivity and limiting-current value is carried out. For MF-4SK/PANI composites, the increase in the exchange potential at the overlimiting state is found to be more than 2V compared to the original membrane. An analysis of how various factors influence the parameters of a current-voltage curve shows that the presence of anilinium and Fe3+ ions in the electromembrane system has no effect on the value of the potential at the onset of the overlimiting state. A phenomenological model is proposed to account for the increased plateau length at the limiting current through a change in the energetic state of water in the perfluorinated-membrane matrix resulting from the template synthesis of polyaniline.

Electrodeposition of a Cobalt-Molybdenum Alloy from an Ammonia-Citrate Electrolyte

Abstract: Kinetics of processes of electrochemical production of a cobalt-molybdenum alloy out of an ammonia-citrate electrolyte is studied. The electrolyte’s composition is similar to that used for depositing a nickel-molybdenum alloy. It is established that the cobalt-molybdenum alloy undergoes deposition at smaller values of pH (5.0–6.0) than the nickel-molybdenum alloy (7.0–9.0). The current efficiency for the cobalt-molybdenum alloy is substantially dependent on the electrolyte pH, whereas the chemical composition of the obtained deposits is practically independent of the electrolyte pH in the pH interval 5.0–8.0 at current densities of 0.025 to 0.100 A cm−2. On the other hand, a change in the electrolyte pH produces a considerable effect on the morphology of the obtained deposits. At large values of pH (pH 8.0), one can obtain a powder-like deposit of the cobalt-molybdenum alloy with a small value of the current efficiency. The deposits that are obtained in the pH region 5.0–6.0 have some cracks, with the number of cracks increasing with the electrolyte pH.

Studies in the field of electrochemistry of organic compounds in 2000–2006

Abstract: This review presents the author’s views on the state-of-art in the electrochemistry of organic compounds, based on the analysis of the data published from 2000 to 2006. The number of publications that consider to one or another extent the electrochemical reactions involving organic compounds constantly increases. The range of problems studied is intimately related with the demands of new technologies. The largest number of publications are devoted to the electrochemical polymerization and the properties of polymeric films on the electrode surface and also to the mechanism of electrode reactions of metal-complex compounds and their properties. The directions such as electrochemistry of nanomaterials and the methods of modification of electrodes are largely associated with the use of electroactive polymers. Fine organic electrosynthesis including the technological developments in the destruction of organic compounds, e. g., for cleaning of waste water, fade to the background. A new direction in this field is the development of amperometric sensors that employ modified electrodes based on electrochemically activated polymeric films.

Chemical and electrochemical processes in low-temperature superionic hydrogen sulfide sensors

Abstract: Effect the morphology of the surface of the working electrode (PbS) exerts on the sensitivity of a low-temperature potentiometric hydrogen sulfide sensor is studied. The sensor, which is based on electrochemical cell Na x WO3/NASICON/PbS, may be used for fast selective detection of hydrogen sulfide in air in natural conditions. It is demonstrated that the sensors with PbS that are deposited out of solution have a faster response than the pressed-to ones. The dependence of EMF on the hydrogen sulfide concentration for the former is linear in semilogarithmic coordinates. Thus difference is explained by the microstructure of the lead sulfide layer. It is shown that the lead sulfide interaction with hydrogen sulfide involves a reversible partial reduction of sulfur and lead at the surface. The species that form in so doing contain sulfur atoms in lower oxidation degrees (poly-and oligo sulfides, sulfite). A mechanism of the sensor operation is proposed on the basis of data yielded by experiment and quantum-chemical simulation. The mechanism includes reversible transport of hydrogen from sulfur atoms to oxygen atoms.

Study of the mechanism of redox transformations of sterically hindered N-aryl-o-iminoquinones

Abstract: The mechanisms of redox transformations of sterically hindered [1IBQ]-[3IBQ], 9,10-iminophenanthraquinone [4IFQ], and o-aminophenol [5AP] have been studied by cyclic voltammetry. It has been shown that the reduction process in tetrahydrofuran consists of two consecutive reversible steps leading to the formation of a radical anion and a dianion. In the case of acetonitrile, only the first redox process is detected, which involves the steps of protonation and electron addition resulting in the formation of a monoanion. Quantum-chemical calculations indicate a lower degree of participation of the six-membered nonaromatic carbon ring in the spin density delocalization in the o-iminosemiquinonate radical anions as compared to o-benzosemiquinonate radical anions. The oxidation of iminoquinones results in the formation of an unstable radical cation, which undergoes further chemical transformations. An increase in the acidity of a medium leads to the formation of protonated forms with the reduction potential being shifted anodically. The introduction of screening groups makes it possible to suppress side reactions of hydrolysis and cyclization of oxidized o-aminophenol [5AP] forms. Therefore, the major product of complete electrolysis is o-iminobenzoquinone [2IBQ].

Electrochemical behavior of nitrogenated nanocrystalline diamond electrodes

Abstract: Nitrogenated nanocrystalline diamond films with controlled conductivity are deposited from microwave plasma in CH4-Ar-H2-N2 gas mixtures. They are characterized using atomic force microscopy, Raman spectroscopy, and electrophysical measurements. Their electrochemical properties are studied by cyclic voltammetry and electrochemical impedance spectroscopy. Kinetic parameters of reactions in [Fe(CN)6]3-/4- redox system are determined. The character of electrode behavior is controlled by the degree on nitrogenation. With the increasing of the nitrogen content in the reaction gas mixture (from 0 to 25%), the potential window somewhat narrows, the background current increases, the reversibility of reactions in the [Fe(CN)6]3-/4- redox system increases. By and large, the transition occurs from the electrochemical behavior of a “poor conductor” to that of a metal-like electrode.

Ionic conduction and chemical diffusion in solid solutions of superionic conductors Cu2X-Me2X (Me = Ag, Li; X = S, Se)

Abstract: Ionic conduction and chemical diffusion in silver-and lithium-substituted superionic copper selenides and sulfides are studied. Upon substitution of lithium for copper, a strong increase in the activation energy of ionic conduction in copper sulfide and selenide and the concomitant several-fold decrease in the ionic conductivity are observed. The reasons for the deterioration of ionic transport conditions are elucidated. In contrast to the substitution of lithium for copper, the substitution of silver leads to higher ionic conductivity and the appearance of superionic conduction by two types of cations. Structural aspects of diffusion are considered. Different consequences of substitution of atoms into the cationic sublattice are associated with different electronic sheaths of lithium (alkali metal), silver, and copper (noble metals), which give rise to different types and degrees of hybridization of valence electrons of atoms in the disordered sublattice and the anionic frame.

Tantalum oxide effect on the surface structure and morphology of the IrO2 and IrO2 + RuO2 + TiO2 coatings and on the corrosion and electrochemical properties of anodes prepared from these

Abstract: Alterations in the phase composition, porosity, and surface morphology of coatings are examined following the insertion of a quantity of Ta2O5 into active coatings prepared from IrO2 or IrO2 + RuO2 + TiO2 (OIRTA). It is shown that even an insignificant concentration of Ta2O5 in a coating renders it substantially amorphous and leads to the appearance of a large number of wide protracted cracks in the coating. The latter extends the surface of anodes and boosts their apparent catalytic activity in the chlorine evolution reaction. In addition, this accelerates the diffusion of chloride ions toward the front surface of anodes, which noticeably reduces the overvoltage of the chlorine evolution reaction when manufacturing sodium chlorate. The coatings’ amorphization and the development of their surface substantially reduce the corrosion resistance of these anodes as compared with OIRTA.

Cysteine adsorption on the Au(111) surface and the electron transfer in configuration of a scanning tunneling microscope: A quantum-chemical approach

Abstract: Adsorption of two forms, molecule and radical, of amino acid L-cysteine (Cys) on the Au12 cluster that simulates the (111) face of single-crystal gold is studied in the framework of the density functional theory. Effects of solvation of adsorbed Cys particles and lateral interaction in a monolayer are analyzed. The simulation predicts a commensurate adsorption energetics of the molecule and radical, with a difference between the “on-top,” “hollow,” and “bridge” positions. An analysis of lateral electrostatic interactions points to the stability of a cluster comprising six Cys particles, which conforms to the size of a fragment observed experimentally. Adsorption calculations are used to build three-dimensional isosurfaces (STM images), where the tungsten needle of the scanning tunneling microscope is simulated by a tungsten atom or by small clusters. The calculated images are sensitive to both the Cys shape and the orientation of adsorbed Cys particles. Calculation results are compared with fresh in situ submolecular-resolution STM data. Simulated images (with commensurate contributions made by sulfur atom and amino group) built for Cys radical adsorbed in the “on-top” position give best conformance to experiment.

Electrochemical study of the thionine dye incorporated into ZSM-5 and HZSM-5 zeolites

Abstract: The electrochemical properties of thionine dye adsorbed into ZSM-5 and HZSM-5 zeolites (TH/ZSM-5, TH/HZSM-5) are studied in 0.5 M KCl solution. The dye is strongly retained and not easily leached from the zeolites matrix. The samples are incorporated into the carbon paste electrode (TH/ZSM-5/P, TH/HZSM-5/P) for cyclic voltammetric measurements. The redox reactions of thionine incorporated into ZSM-5 zeolite contain a quasi-reversible, two-electron one proton in the pH range 1 to 10, but thionine-loaded HZSM-5 zeolite undergoes a quasi-reversible two-electron two-protons redox reaction under acidic conditions and a one proton two-electron redox reaction takes place under basic conditions. The separation of the anodic and cathodic potentials (E p) is high in thionine-loaded zeolites (>100) with respect to the solution of thionine (E p = 34 for ZSM-5/P and 36 mV for HZSM-5/P), indicating that there are strong interaction between thionine molecules and the zeolites. The midpoint potentials (E m) for TH/ZSM-5/P and TH/HZSM-5/P are −0.203 and −0.381 V, respectively. However, the midpoint potentials for the solution of thionine for the electrode system of ZSM-5/P and HZSM-5/P are −0.335 and −0.407 V, respectively. Thus, thionine dye molecules incorporated into the zeolites can be reduced more easily with respect to solution of thionine. In various electrolyte solutions, the midpoint potentials remains constant, but the midpoint potential of the thionine-zeolite electrodes depends on the solution pH. Influence of the pH of the solution on the midpoint potential of an immobilized dye reveals that thionine molecules are accessible to protons. This property is ascribed to the formation of mesopores in the structure of our zeolites suffering from a calcination step.

Interaction of activated carbon electrodes with sulfuric acid solutions

Abstract: Impedance and potentiodynamic studies have been carried out to investigate reversible processes on the surface of electrodes based on activated carbon (AC). Relationships have been found between the reversible capacity and true resistance, on the one hand, and the potential, on the other. Stoichiometric characteristics of AC charging in sulfuric acid have been obtained. A mechanism of the reversible surface process is suggested. It is concluded that Faraday pseudocapacity makes a considerable contribution to charging of AC electrolytes. The amount of gas liberated in the course of AC wetting in electrolyte has been measured. It is assumed that the amount of liberated gas depends on the amount of active oxygen involved in the current-forming reaction. Based on the data obtained, a redox surface reaction is substantiated in general form.