Modeling of electrocatalysis at conducting polymer modified electrodes: nonlinear current-concentration profiles

A model of electrocatalytic processes taking place at electrodes modified with a layer of a conducting polymer presented earlier has been expanded to a wider variety of parameters related to electric conductivity of a modifier layer. Relatively low values of charge carrier diffusion coefficients within the modifier film have been used in calculations, thus modeling a semiconducting behaviour of conducting polymer film, typical for a wide range of these materials. As a result, nonlinear hyperbolic dependencies of current on concentration have been obtained and discussed for definite combinations of a linear term of chemical kinetics related to a simple second-order chemical redox reaction, and two diffusion processes, related to reactant and charge carriers. Concerning biosensor application of electrocatalysis, the performed analysis showed that nonlinear current-concentration profiles can be observed even in the absence of a hyperbolic Michaelis-Menten type kinetics.     

Coupled Electron‐Proton Transport in Electropolymerized Methylene Blue and the Influences of Its Protonation Level on the Rate of Electron Exchange with β‐Nicotinamide Adenine Dinucleotide

This paper describes electrochemical characteristics of poly(methylene blue) electrolytically deposited on glassy carbon and examines the electrocatalytic activity of the polymer toward oxidation of the coenzyme NADH. Redox‐active properties of the cationic polyelectrolyte arose from both electron self‐exchange between electroactive sites and a high ionic film‐conductivity. The diffusion coefficient of charge carriers in the film increased with decreasing solution pH, indicating the pH dependence of the electron diffusion coefficient. The electrocatalytic oxidation of NADH at the polymer‐modified electrode proceeded via an intermediate charge‐transfer complex of the reduced polymer with the oxidized coenzyme. The complex dissociated more rapidly into the oxidation products as the reduced polymer protonated. Thus, the rate constant for the cross‐exchange reaction rose with a decrease in pH. For NADH oxidation, the polyelectrolyte exhibited an electrocatalytic activity higher than the monomeric dye because of a stronger oxidizing power of the second oxidized form of the polymer.     

In situ Raman spectroelectrochemical study of electrocatalytic processes at polyaniline modified electrodes: Redox vs. metal-like catalysis

Two processes of electrocatalytic oxidation of solution species at conducting polymer modified electrodes have been studied for the first time with in situ Resonance Raman spectroelectrochemical technique at a red laser excitation (λ = 632.8 nm), namely electrooxidation of hydroquinone at a sulfonated polyaniline modified electrode in an acidic solution and electrooxidation of ascorbic acid at polyaniline modified electrode in a pH-neutral solution. In both cases, characteristic Raman features have been identified for different redox forms of conducting polymers and changes in the net redox state of a polymer layer during electrooxidation of solution species have been studied. It has been shown that an increase in the concentration of oxidizable species causes an increase of the net content of a reduced form of polyaniline in the modifying layer. From this, the redox (vs. metal-like) mechanism of electrocatalysis at conducting polymer modified electrodes has been deduced.     

Reactions of solute species at an electrode modified with titanocene functionalized polypyrrole film: ferrocene and titanocene dichloride

We have studied for the first time the ability of a conducting polymer film, p(Tc3Py), representing a polypyrrole matrix with covalently attached titanocene dichloride (TcCl₂) centers, to serve as an intermediator for the electron charge transport between the electrode and the reaction sites of solute reactants. The standard potential of the first of these electroactive species, ferrocene (Fc), is in the range where the polymer matrix is in its slightly oxidized state so that solute Fc species give a reversible response at the surface of this modified electrode. Another solute reactant, TcCl₂, was studied in solutions in which it demonstrates a (quasi)reversible behavior at bare electrode surfaces, THF+TBAPF₆ and AN+TEACl. The standard redox potential of this species belongs to the range of the electroactivity of immobilized TcCl₂ centers (where the matrix is in its non-conducting state) so that the electron charge has to be transported via stepwise redox reactions between neighboring centers inside the film. The combination, solute reactant+film, results in a greater CV current compared to the response of the film in background solution or of the solute species at the bare electrode surface. This current for THF solution even exceeds the sum of separate currents for the film and the reactant. This finding is attributed to a catalytic effect of solute species as redox intermediators for the transformation of immobilized electroactive centers leading to a greater degree of the film reduction. The presence of solute TcCl₂ species results in a much greater stability of immobilized centers (compared to the corresponding reactant-free solution), both in the course of CV with the passage of the range of their response and in experiments with the film holding at the potential within this range. This holding leads to an almost constant current related to the reaction of solute species at the film/solution interface. Our estimate shows that immobilized centers undergo above 10,000 reversible transformations (without an observed tendency to the degradation) to ensure the passage of this current. The conclusion has been drawn that immobilized TcCl₂ centers are able to serve as sufficiently stable redox intermediators for the electron charge transport across the film, a prerequisite for the catalytic applications of such films.Abbreviations AN acetonitrile - THF tetrahydrofuran - Cp cyclopentadienyl, C₅H₅ - Cp cyclopentadienyl radical, C₅H₄ - Fc ferrocene, Cp₂Fe - TcCl₂ titanocene=bis(cyclopentadienyl)titanium dichloride, Cp₂TiCl₂ or its radical CpCpTiCl₂ - PPy polypyrrole - Tc3Py titanocene-propyl-pyrrole, Cl₂TiCpCp(CH₂)₃NC₄H₄ - p(Tc3Py) polymer obtained from Tc3Py - TBAPF₆ tetrabuthylammonium hexafluorophosphate - TEACl tetraethylammonium chlorideDedicated to Zbigniew Galus on the occasion of his 70th birthday.     

甲醇在铂微粒修饰的聚硫堇电极上的电催化氧化

利用电化学循环伏安和现场ＦＴＩＲ反射光谱等技术研究了甲醇在铂微粒修饰的聚硫堇电极上的电催化氧化。结果表明，循环伏安法制备的铂微粒均匀分散于聚合物膜上，其粒径大小约为３０－１３０ｎｍ；复合修饰电极对甲醇电化学氧化呈现了较高的催化活性，其催化活性的大小依赖于Ｐｔ载量。现场ＦＴＩＲ光谱实验揭示了线性吸附的ＣＯ物种是甲醇在复合电极上氧化的唯一中间体，这种吸附的ＣＯ物种在复合修饰电极上更容易被氧化为最终产物     

Electrocatalytic oxidation of some biologically important compounds at conducting polymer electrodes modified by metal complexes

Conducting poly(3-methylthiophene) electrodes were electrochemically prepared. The resulting polymer films were modified with an inorganic complex, ferrocene. The incorporation of the ferrocene/ferrocenium moiety into the polymer film resulted in enhanced charge transfer towards the oxidation of some organic molecules of biological interest. The electrochemical response of the complex-containing polymer electrode was compared to that of the unmodified polymer electrode and that of the substrate. Apparent diffusion coefficients of the redox species were estimated from the cyclic voltammetric data for different biological molecules at the ferrocene-containing polymer electrode. Infra-red spectroscopic measurements for the “as-grown” films revealed the presence of the inorganic complex within the polymer. The modified polymer electrode showed noticeable enhancement for the charge transfer across the film interface and can be used as an electrochemical sensor for biological compounds. Received: 3 June 1997 / Accepted: 7 July 1997     

Modeling of electrocatalysis at chemically modified electrodes: a combination of second-order and Michaelis-type chemical kinetics

The model presented accounts for the diffusion of a reactant and of charge carriers within the modifier layer placed at electrode surface, and redox interaction between reactant and an active center bearing charge carriers. The study extends our previous model by the use of a combination of two kinds of redox interaction—a simple chemical second-order reaction, and Michaelis-type redox reaction. Depending on relative increments from these two kinetic models, either linear, or hyperbolic dependencies of electric current on reactant concentration were obtained. The results obtained have been analyzed in terms of current-concentration interdependencies.     

二茂铁-磷钼酸电荷转移配合物修饰电极的制备及电化学性能的研究

采用电化学方法在异电基体玻碳（GC）电极上制备了二茂铁 （Fc）－磷钼酸（PMo12）电荷转移配合 物修饰电极（Fe5PMo12/GC)。研究了该电极的电化学行为,发现其在硫酸溶液中进行伏安扫描时具有良好的稳定性,而且对酸性水溶液中的过氧化氢具有较好的电催化学原作用, 初步探讨了电催化还原机理。     

Electrocatalytic oxidation of hydrazine at cobalt hexacyanoferrate- modified glassy carbon, Pt and Au electrodes

The electrocatalytic oxidation of hydrazine has been studied on glassy carbon, Pt and Au electrodes modified by cobalt hexacyanoferrate (CoHCF) using cyclic voltammetry and rotating disc techniques. It has been shown that the oxidation of hydrazine to nitrogen occurs at the potential coinciding with that of Co(II) to Co(III) transformation in a CoHCF film, where no oxidation signal is observed at a bare glassy carbon electrode. A Tafel plot, derived from RDE voltammograms, exhibits a slope of 150 mV, indicating a one-electron charge transfer process to be the rate-limiting step. The electrocatalytic efficiency of the modified electrode towards hydrazine oxidation depends on solution pH, and the optimum range was found to be located between pH 5 and pH 7. The kinetic behaviour and location of the electrocatalytic process were examined using the W.J. Albery diagnosis table, and it was concluded that the reaction has either a “surface” or a “layer” reaction mechanism. Pt- and Au-CoHCF-modified electrodes show no significant electrocatalytic activity towards hydrazine oxidation. Received: 25 April 1997 / Accepted: 12 August 1997     

Electrochemical response of ascorbic acid at conducting and electrogenerated polymer modified electrodes for electroanalytical applications: a review

The present short review deals with electroanalytical aspects of electrochemical response of ascorbic acid (Vitamin C) at conducting and electrogenerated polymer modified electrodes. Two main topics are considered: (i) electrocatalytic oxidation of ascorbate at conducting polymer modified electrodes, leading to electroanalytical techniques for ascorbate assay, and (ii) retardation of ascorbate penetration through a layer of electrogenerated polymers, leading to permselective coatings and their diverse uses, especially for biosensing devices.     

Simulation of Electrochemical Process for Glucose Oxidase Immobilized Conducting Polymer Electrodes

Abstract

Computer simulation of electrochemical processes that govern the operation of conducting polymer modified electrodes (CPME) have been reported in this paper. Comparison of the behaviour of a biocatalyst (GOX) in free solution and in the immobilized phase in conducting polymer modified electrodes (CPME) has been provided The output has been obtained using the Runga Kutta numerical method solved by programming in FORTRAN 77. The results point out that the catalytic current generated by an immobilized enzyme in layer is larger as compared to that for the enzyme in solution, and that it varies with the thickness of the diffusion layer.     

聚苯胺薄膜修饰电极对抗坏血酸的电催化氧化

本文表明聚苯胺(PAn)薄膜修饰电极对水溶液中的抗坏血酸(AH_2)在较宽的pH范围和较宽的浓度范围内均有良好的电催化氧化作用, 为EC平行催化过程。利用旋转圆盘电极(RDE)进行了催化过程动力学分析, 求出了催化反应动力学参数。在抗坏血酸浓度10~(-2)～10~(-6) mol·L~(-1)范围内, 催化峰电流与AH_2浓度均成良好的线性关系, 且PAn薄膜修饰电极具有很好的稳定性, 有应用分析抗坏血酸的意义。     

Redox switching and oxygen evolution at oxidized metal and metal oxide electrodes: iron in base

Outstanding issues regarding the film formation, redox switching characteristics and the oxygen evolution reaction (OER) electrocatalytic behaviour of multicycled iron oxyhydroxide films in aqueous alkaline solution have been revisited. The oxide is grown using a repetitive potential multicycling technique, and the mechanism of the latter hydrous oxide formation process has been discussed. A duplex layer model of the oxide/solution interphase region is proposed. The acid/base behaviour of the hydrous oxide and the microdispersed nature of the latter material has been emphasised. The hydrous oxide is considered as a porous assembly of interlinked octahedrally coordinated anionic metal oxyhydroxide surfaquo complexes which form an open network structure. The latter contains considerable quantities of water molecules which facilitate hydroxide ion discharge at the metal site during active oxygen evolution, and also charge compensating cations. The dynamics of redox switching has been quantified via analysis of the cyclic voltammetry response as a function of potential sweep rate using the Laviron-Aoki electron hopping diffusion model by analogy with redox polymer modified electrodes. Steady state Tafel plot analysis has been used to elucidate the kinetics and mechanism of oxygen evolution. Tafel slope values of ca. 60 mV dec(-1) and ca. 120 mV dec(-1) are found at low and high overpotentials respectively, whereas the reaction order with respect to hydroxide ion activity changes from ca. 3/2 to ca. 1 as the potential is increased. These observations are rationalised in terms of a kinetic scheme involving Temkin adsorption and the rate determining formation of a physisorbed hydrogen peroxide intermediate on the oxide surface. The dual Tafel slope behaviour is ascribed to the potential dependence of the surface coverage of adsorbed intermediates.     

Permselectivity of neurotransmitters at overoxidized polypyrrole-film-coated glassy carbon electrodes

The permselectivity of neurotransmitters such as dopamine, epinephrine, and norepinephrine at overoxidized polypyrrole (OPPY)-film-coated glassy carbon electrodes has been investigated. The chemically-modified electrodes exhibit attractive permselectivity and antifouling properties of rejecting anionic species, e.g. ascorbate, etc. Compared with the response of neurotransmitters at modified electrodes overoxidized in phosphate buffer solution (pH 7.4), higher sensitivity and reversibility response can be obtained at modified electrodes overoxidized in sodium hydroxide solution. The effect of film thickness on the permselective response was tested. Rotating disk electrode experiments were used to determine the apparent diffusion coefficients of several electroactive solutes in the OPPY films. The influence of the hydrophobicity of the organic ions on the permeability within the polymer films was discussed. Dopamine and epinephrine were determined at the 1 x 10(-6)-1 x 10(-4) M level by means of voltammetry after an exposure period of 2 min in 0.1 M phosphate buffer (pH 7.4) with detection limits of 8 x 10(-7) M and 6 x 10(-7) M respectively.     

Charge trapping reactions in bilayer electrodes

The cyclic voltammetric peaks for charging trapping and untrapping reactions between the inner and outer redox polymer films of five bilayer electrodes are compared to a theory for control of the rate of charge trapping by electron diffusion rates in the inner polymer film. The five bilayer electrodes use various different redox polymer films (electropolymerized poly-pyridine complexes of Fe, Ru, and Os, and polyvinylferrocene) arranged in different orders. The currents on the rising edge of the bilayer trapping and untrapping peaks follow the electron diffusion theory up to ca. 80% of the peak current; currents thereafter are controlled by another process(es). The analysis yields values for the electron diffusion constants in the inner bilayer polymer films, which agree with one another for different bilayers having the same inner film polymer films and which also agree with independent determinations by other methods. Two of the bilayers are made from the same two polymers, arranged in different inner-outer order. These bilayers also illustrate the occurrence of a “leak reaction”, in which charge trapped in the outer film is discharged via a thermodynamically unfavorable electron transfer reaction with the inner polymer film.     

Characterization of stainless steel electrode modified by a thin film of polyaniline containing pt particles and its electrocatalytic activity for methanol oxidation

The catalytic behavior of stainless steel (SS) electrode modified by a thin film of polyaniline (PANI) containing platinum particles was studied for electrooxidation of methanol and compared with a platinated Pt/PANI electrode in acidic aqueous solution. Cyclic voltammetry (CV), chronoamperometry, CO stripping techniques were used to investigate electrochemical properties and electrocatalytic activity of SS/PANI/Pt and Pt/PANI/Pt electrodes. The morphology and particle size of Pt catalysts were characterized by Transmission Electron Microscopy (TEM) measurement. The effects of various parameters such as thickness of polymer film, medium temperature and stability of the modified electrodes on methanol oxidation were also investigated. The results indicated that the modified SS electrode exhibited a considerably high electrocatalytic activity on the methanol oxidation as well as the modified Pt electrode.     

Electrocatalytic oxidation of NADH at glassy carbon electrodes modified with an electropolymerized film of nile blue A

The kinetic parameters for the electrocatalytic oxidation of di-hydronicotiamide adenine dinucleotide (NADH) at glassy carbon electrodes modified with an electropolymerized film of nile blue A (PNB) have been determined based on rotating disk electrode measurements. The rate constant for the chemical reaction between NADH and PNB is strongly influenced by NADH concentration and the pH value of solution, and it decreases with increasing NADH concentration, indicating that the electrocatalytic process proceeds via the formation of an intermediate of charge-transfer complex between NADH and PNB.     

Electrocatalysis of 2,5-dimercapto-1,3,5-thiadiazole by 3,4-ethylenedioxy-substituted conducting polymers

The electronic properties of electropolymerized films of the 3,4-ethylenedioxy-substituted conducting polymers (CP) poly(3,4-ethylenedioxythiophene) (PEDOT), poly(3,4-ethylenedioxypyrrole) (PEDOP) and poly(3,4-ethylenedioxyselenophene) (PEDOS) have been investigated, along with their electrocatalytic activity toward 2,5-dimercapto-1,3,4-thiadiazole (DMcT). For the electropolymerized films, the conductivity onset potential was most negative for PEDOP (-1.50 V), followed by PEDOS (-1.35 V) and with PEDOT possessing the most positive onset (-1.15 V). The heterogeneous charge transfer rate constant for DMcT in solution at polymer-film-modified glassy carbon electrodes (GCEs) was studied. It was found that compared to PEDOP, both PEDOS and PEDOT performed better as electrocatalysts, with PEDOS having a heterogeneous charge transfer rate constant of 1.8 × 10(-3) cm/s. The film morphology of the electropolymerized films was investigated via SEM, and some film characteristics could be correlated with electrocatalytic activity. The potential use of CP/DMcT composites for lithium ion batteries (LIBs) is discussed.