Koutecky Levich Plots Research Articles

Electrocatalytic reduction of protons and oxygen at glassy carbon electrodes coated with a cobalt(II)/platinum(II) porphyrin

The electrocatalytic reduction of protons in 1.0 M perchloric acid at glassy carbon electrodes anodically modified with a Co(II)/Pt(II) porphyrin show shifts of 400 mV versus Ag/AgCl when compared to the same electrodes which have not been anodically modified. Anodic cycling of glassy carbon electrodes coated with the Co(II)/Pt(II) porphyrin in this study form stable electroactive films capable of improving both electroreduction of protons to hydrogen and oxygen to both peroxide and water. Electrooxidation of glassy carbon electrodes coated with the free base porphyrin show no improvement in catalytic ability for the reduction of protons in acidic solution or the reduction of molecular oxygen in basic solution. Glassy carbon electrodes coated with the Co(II)/Pt(II) porphyrin indicate, by rotating disk electrochemistry, that the electrocatalysis of oxygen is a two electron process leading to the formation of hydrogen peroxide. Koutecky–Levich plots of the data obtained from the reduction of oxygen at electrode surfaces coated with the Co(II)/Pt(II) porphyrin after oxidation of the surface indicate that 25% of the oxygen is reduced by four electrons directly to water while 75% of the oxygen is reduced by two electrons to hydrogen peroxide.

Oxygen reduction on Cu–Zn alloys

The reduction of oxygen on brass type Cu77Zn21Al2 in contact with 0.1 mol L−1 borax at pH 9.2 with and without chlorides was investigated and compared with the results on spectroscopic Cu. The surface film was characterized in situ by means of reflectance spectroscopy and by electrochemical impedance in the potential range negative to the open circuit potential. The main parameters associated to the kinetics of the oxygen reduction reaction were analyzed with a rotating disc electrode. The results show that within the potentials studied, a porous film was formed on brass while no significant spectral features were observed, except at −1.2 V where zinc oxo-hydroxides grow in chloride-free solutions and dissolve when chlorides are present in the system. The number of electrons exchanged for brass was close to 4 in both solutions, showing that oxygen is predominantly reduced to hydroxyl ions, with a reaction order equal to one. The residual presence of oxides on the surface results in Koutecky–Levich plots with slopes dependent on the applied potential and interfere with the calculation of the Tafel slope.

Pt-based electrocatalysts for polymer electrolyte membrane fuel cells prepared by supercritical deposition technique

Pt-based electrocatalysts were prepared on different carbon supports which are multiwall carbon nanotubes (MWCNTs), Vulcan XC 72R (VXR) and black pearl 2000 (BP2000) using a supercritical carbon dioxide (scCO2) deposition technique. These catalysts were characterized by using X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM) and cyclic voltammetry (CV). XRD and HRTEM results demonstrated that the scCO2 deposition technique enables a high surface area metal phase to be deposited, with the size of the Pt particles ranging from 1 to 2nm. The electrochemical surface areas (ESAs) of the prepared electrocatalysts were compared to the surface areas of commercial ETEK Pt/C (10wt% Pt) and Tanaka Pt/C (46.5wt% Pt) catalysts. The CV data indicate that the ESAs of the prepared Pt/VXR and Pt/MWCNT catalysts are about three times larger than that of the commercial ETEK catalyst for similar (10wt% Pt) loadings. Oxygen reduction activity was investigated by hydrodynamic voltammetry. From the slope of Koutecky–Levich plots, the average number of electrons transferred in the oxygen reduction reaction (ORR) was 3.5, 3.6 and 3.7 for Pt/BP2000, Pt/VXR and Pt/MWCNT, correspondingly, which indicated almost complete reduction of oxygen to water.

Electrocatalytic oxidation and determination of nitrite on carbon paste electrode modified with oxovanadium(IV)-4-methyl salophen

Electrocatalytic oxidation and detection of nitrite at a carbon paste electrode chemically modified with N, N′-bis(salicylaldehyde)-4-methyl-1,2-phenylenediimino oxovanadium(IV) as a Schiff base complex ([VO(SB)]), was thoroughly investigated. Anodic oxidation of nitrite (in pH 4) at the [VO(SB)]-modified carbon paste electrode occurred at low overpotential (0.87 V versus Ag/AgCl), and treatment of the voltammetric data showed that it was a purely diffusion-controlled reaction involving one-electron in the rate-determining step. The mechanism for the interaction of nitrite with the vanadium(IV) Schiff base complex is proposed to involve the V IV(SB) 2+/V III(SB) + redox process. The modified electrode exhibits good catalytic activity for the oxidation of nitrite with good sensitivity over the wide concentration range of 3.90 × 10 −6–4.05 × 10 −3 M nitrite, and a detection limit of 6.13 × 10 −7 M ( S/ N = 3). The heterogeneous rate constant, k′, and the standard heterogeneous rate constant, k 0, for the oxidation of nitrite at the surface of the modified electrode were determined by rotating disk electrode voltammetry using the Koutecky–Levich plot. The transfer coefficient ( α) for electrocatalytic oxidation of nitrite and the diffusion coefficient of this substance under the experimental conditions were also investigated in this study.

Electrocatalytic Oxidation of Hydrazine at a Cobalt(II) Schiff-Base-Modified Carbon Paste Electrode

Electrocatalytic oxidation and detection of hydrazine under pH 6.0 conditions were studied with a carbon paste electrode modified with a Co(II) Schiff base complex [, acid (2-hydroxy-3-methoxy-benzylidene)-hydrazide]. Anodic oxidation of hydrazine at the -modified carbon paste electrode occurred at low overpotential ( vs ), and treatment of the voltammetric data showed that it was a purely diffusion-controlled reaction involving one electron in the rate-determining step. The catalytic oxidation peak current was linearly dependent on the hydrazine concentration over the wide range to . The heterogeneous rate constant for the oxidation of hydrazine at the surface of the modified electrode was determined by rotating disk electrode voltammetry using the Koutecky–Levich plot. A Tafel plot, derived from voltammograms, indicated a one-electron charge-transfer process to be the rate-limiting step, and the overall number of electrons involved in the catalytic oxidation of hydrazine was found to be 4. The mechanism for the interaction of hydrazine with the complex-modified carbon paste electrode is proposed to involve the redox process.

Electrocatalytic Reduction of Dioxygen at Carbon Paste Electrode Modified with a Novel Cobalt(III) Schiff's Base Complex

AbstractA carbon paste modified electrode with a new cobalt(III) Schiff's base complex (CPME) and its application to electrocatalytic activity for dioxygen reduction is developed. The electrochemical behavior and stability of the CPME as well as the two‐electron reduction of O2 at the electrode were investigated using cyclic voltammetry, chronoamperometry and rotating disk electrode methods. At the CPME, the reduction of dioxygen to hydrogen peroxide occurs at potentials where it is not observed at a bare carbon paste electrode. The CPME exhibited potent and persistent electrocatalysis for O2 reduction in acetate buffer solutions of pH 4.0 with an overpotential of about 800 mV lower than unmodified CPE and drastic increase in the peak current. The heterogeneous rate constant for the reduction of O2 at the surface of CPME was determined by hydrodynamic voltammetry using the Koutecky–Levich plot. A possible catalytic mechanism is proposed and discussed.

Redox mediation at electroactive polymer coated electrodes: Mechanistic diagnosis criteria from steady state polarization curves

In this work, we analyze the i–E response of a redox mediator modified electrode in steady state and in the absence of permeation of the reactants into the film. We show that, besides some kinetic criteria existing in the literature based on the behavior of the limiting currents, there are others based on the entire i–E response. This analysis allows to explain several features of the i–E curve, yet unexplained, such as the shift of the half wave potential, electrochemical rectification, the shape of the Koutecky–Levich plots and the shape of the E vs. ln [(iL − i)/i] plots. From them, it is possible to determine, almost unmistakably, the type of kinetic control and to obtain the corresponding kinetic parameters.

Preparation and Electrochemical Characteristics of MnOx – CoTMPP ∕ BP Composite Catalyst for Oxygen Reduction Reaction in Alkaline Solution

–cobalt tetramethoxyphenylporphyrin (CoTMPP)/black pearls 2000 carbon black (BP) composite catalysts were synthesized by depositing CoTMPP onto carbon-supported , followed by heat-treatment from in atmosphere. The electrochemical characteristics of the catalysts for oxygen reduction reaction (ORR) were evaluated by the rotating disk electrode technique in half cells using KOH solution. BP2000 was used as a carbon support, which was pretreated with 30% to enhance the catalytic activities. Koutecky–Levich plots reveal a two-electron process, giving evidence that the ORR is mainly controlled by the formation of . Doping into the carbon support improves the activity of catalysts for ORR. There are two oxygen reduction peaks produced from these catalysts during the negative cyclic voltammetry scan. We obtained the best results using for the heat-treatment. At these conditions the X-ray diffraction results revealed that is mainly cubic MnO. The battery performance of a zinc-air single cell using as cathode catalyst was investigated, and the results show that the battery has good performance; the maximum output current density reached at a potential of .

Electrochemistry of Free Chlorine and Monochloramine and its Relevance to the Presence of Pb in Drinking Water

The commonly used disinfectants in drinking water are free chlorine (in the form of HOCl/OCl-) and monochloramine (NH2Cl). While free chlorine reacts with natural organic matter in water to produce chlorinated hydrocarbon byproducts, there is also concern that NH2Cl may react with Pbto produce soluble Pb(II) products--leading to elevated Pb levels in drinking water. In this study, electrochemical methods are used to compare the thermodynamics and kinetics of the reduction of these two disinfectants. The standard reduction potential for NH2Cl/Cl- was estimated to be +1.45 V in acidic media and +0.74 V in alkaline media versus NHE using thermodynamic cycles. The kinetics of electroreduction of the two disinfectants was studied using an Au rotating disk electrode. The exchange current densities estimated from Koutecky-Levich plots were 8.2 x 10(-5) and 4.1 x 10(-5) A/cm2, and by low overpotential experiments were 7.5 +/- 0.3 x 10(-5) and 3.7 +/- 0.4 x 10(-5) A/cm2 for free chlorine and NH2Cl, respectively. The rate constantforthe electrochemical reduction of free chlorine at equilibrium is approximately twice as large as that for the reduction of NH2Cl. Equilibrium potential measurements show that free chlorine will oxidize Pb to PbO2 above pH 1.7, whereas NH2Cl will oxidize Pb to PbO2 only above about pH 9.5, if the total dissolved inorganic carbon (DIC) is 18 ppm. Hence, NH2Cl is not capable of producing a passivating PbO2 layer on Pb, and could lead to elevated levels of dissolved Pb in drinking water.

Electrocatalytic oxidation of hydroxylamine at a rutin multi-wall carbon nanotubes modified glassy carbon electrode: Improvement of the catalytic activity

A highly sensitive sensor for the determination of hydroxylamine was described. It consists of a rutin multi-wall carbon nanotubes (RMWCNT) modified glassy carbon electrode (GCE). The cyclic voltammograms of the modified electrode indicated that the surface confined rutin were strongly dependent on the solution pH, as anticipated for quinone/hydroquinone functionalities. The RMWCNT modified GCE exhibited catalytic activity toward the electro-oxidation of hydroxylamine. The results show that there is a dramatic enhancement of the anodic peak current of hydroxylamine oxidation at RMWCNT modified GCE relative the value obtained at the rutin modified GCE (RMGCE) or activated GCE (AGCE). In other words the combination of MWCNT and mediator (rutin here) definitely improve the characteristics of hydroxylamine oxidation. The kinetics parameters such as the electron transfer coefficient, α, the heterogeneous rate constant, k′, and the standard heterogeneous rate constant, k 0, for the electro-oxidation of hydroxylamine at the RMWCNT modified GCE were determined using RDE voltammetry. The results show k′ are strongly potential dependent and increase with increasing the potential which considering for drawing the Koutecky–Levich plot. In amperometric measurements at a RMWCNT modified electrode during the addition of even 1.0 μM hydroxylamine, a well-defined response was observed. The plot of electrocatalytic current versus hydroxylamine concentration is constituted from two linear segments, corresponding to two different range of 1.0–33.8 and 33.8–81.7 μM. Finally, it was found that the relative standard deviation of amperometry current of 10.0 μM hydroxylamine for 15 replicate determinations is 2.6%.

Preparation and electrocatalytic properties of the TBO/nafion chemically-modified electrodes

Abstract The glassy carbon electrode modified with nafion and toluidine blue O (TBO) was prepared and their electrochemical properties were investigated. The TBO molecules were strongly and irreversibly adsorbed on the nafion-modified glassy carbon surface. The electrochemical behavior and mechanism for interactions of the TBO molecules with the nafion film were investigated through cyclic voltammetric and UV–vis spectra methods. The electrocatalytic reductions of nitric oxide and oxygen were performed at this modified electrode by cyclic voltammetry, rotating ring disk voltammetry and chronoamperometry as diagnostic techniques. The nafion membrane played a duel role as a matrix for the TBO immobilization and also helped to partitioning the NO from the solution phase. The diffusion coefficients of nitric oxide and oxygen at the TBO/nafion/GC modified electrode were calculated using chronoamperometric technique and rotating ring disk voltammetry, respectively. The ‘ D ’ value for nitric oxide and oxygen obtained by the above methods were found as 1.2798 × 10 −6 and 1.4 × 10 −5 cm 2 s −1 , respectively. From the intercept value of the Koutecky–Levich plot, the catalytic reaction rate constant, k , was calculated for O 2 reduction and found as 6.6 × 10 4 M −1 S −1 .

A Fe3O4-Based Chemical Sensor for Cathodic Determination of Hydrogen Peroxide

A new cathodic scheme for hydrogen peroxide (H2O2) measurement by Fe3O4-based chemical sensor was described. The unique characteristic of electrocatalytic property was firstly investigated by voltammetry. And then the amperometric response of H2O2 was measured at −0.2 V (vs. Ag/AgCl) by Fe3O4 modified glassy carbon rotating disk electrode. The kinetic parameter was also calculated from Koutecky-Levich plot, and the value was 6.4×10−4 cm s−1 in pH 3 citrate buffer. In order to benefit the possible biomedical applications, Fe3O4/chitosan modified electrode was also investigated in this experiment. There were several characteristic enhancements by the coated chitosan thin film for H2O2 sensor. The calibration curves were found to be linear up to 4.0 and 5.0 mM (r=0.999) in pH 3 and 7 with the detection limits of 7.6 and 7.4 μM L−1 (S/N=3). The stability was evaluated by the results of half-life time (t50%) for 9 months at room temperature and 24 months at 4 °C.

Electrocatalytic Reduction of Oxygen in a Novel Catalytic System with Cobalt Phthalocyanines and Manganese Oxide

This article concerns the efficient dioxygen reduction by a novel catalytic system that is comprised of two catalysts: one for the electroreduction of through the two-electron process and the other for the subsequent chemical decomposition of hydrogen peroxide generated. Here we represent the combined catalytic system of cobalt phthalocyanines and manganese oxide (MnOOH). Each catalyst performs well independently, leading to the totally four-electron reduction of in alkaline media. This was confirmed from almost twofold increases of the cathodic current in cyclic voltammograms and the steady-state current in rotating ring disk electrode voltammograms, the collection efficiency and the number of electrons calculated from the Koutecky-Levich plot. As a consequence, it is concluded that this combined catalytic system works efficiently for the reduction. © 2004 The Electrochemical Society. All rights reserved.

Supramolecular tetracluster-cobalt porphyrin: a four-electron transfer catalyst for dioxygen reduction

Electrocatalysis by CoTCP {CoTCP = meso-tetrakis(4-pyridyl)porphynatocobalt(III)} coordinated to four [Ru 3(μ 3-O)(μ 2-CH 3CO 2) 6(py) 2] + complexes in the four-electron reduction of dioxygen, has been unequivocally demonstrated in this work by using two types of electrostatically assembled films of CoTCP and anionic zinc or free-base meso-tetrakis(4-sulfonatophenyl)porphyrins (ZnTPPS or H 2TPPS), as well as, by employing different electrode materials. An enhanced electrocatalytic activity has been observed, in spite of the fact that the typical bis-coordination of dioxygen to two cobalt porphyrin sites is precluded in such CoTCP/ZnTPPS or CoTCP/H 2TPPS bilayered films. In addition, negligible ring currents have been observed in ring-disk voltammetry measurements, yielding straight Levich and Koutecky–Levich plots, whose slopes approached the theoretical 4e − curve in air, or in O 2 saturated solutions (pH 3–5). The use of gold instead of glassy carbon (GC) electrodes has not also significantly perturbed the mechanism. By ruling out any influence from the electrode materials, a well known critical point in the catalysis by cobalt porphyrins, the results have shown that CoTCP is acting as a four-electron transfer catalyst for dioxygen reduction. Also, by excluding the possibility of bis-coordination of dioxygen, it was shown that the electronic and supramolecular effects exerted by the peripheral ruthenium cluster complexes should be triggering the four-electron catalytic activity of the cobalt porphyrin center.

Thermogravimetry/Mass Spectrometry Investigations on the Formation of Oxygen Reduction Catalysts for PEM Fuel Cells on the Basis of Heat‐Treated Iron Phenanthroline Complexes

AbstractOxygen reduction catalysts for PEM fuel cells on the basis of carbon supported, heat‐treated iron phenanthroline complexes have been investigated with rotating disc‐electrode (RDE) measurements and with thermogravimetry/mass spectrometry (TG/MS). It has been found, that the slope of the Koutecky–Levich plot from RDE measurements changes with electrode potential and with catalyst loading of the electrode. The results can be understood in terms of a two times 2‐electron reduction of oxygen at the iron‐based catalysts. TG/MS indicate, that during the heat‐treatment step the phenathroline molecule of the catalyst precursor is completely embedded into the carbon. The results are discussed in terms of the structure of the active centre of these catalysts.

Catalysts for oxygen reduction from heat-treated carbon-supported iron phenantroline complexes

Oxygen reduction catalysts were prepared by heat treatment of carbon supported iron phenantroline complexes in Ar or NH3. The optimum carbon black loading with iron was found to be 2%, the optimum heat treatment temperature was about 800 °C. X-ray diffractogramms and TEM showed the occurrence of crystalline species at higher catalyst loadings; however, these species seem not to contribute significantly to the catalytic activity. From the slope of the Koutecky–Levich plot, an average number of 3.7 electrons transferred per oxygen molecule was calculated, which is consistent with RRDE data. A Tafel slope of about 120 mV (decade)−1 indicates that the first electron transfer is rate determining.

Comparison of Voltammetric Responses of Toluene and Xylenes at Iron(III)-Doped, Bismuth(V)-Doped, and Undoped β-Lead Dioxide Film Electrodes in 0.50 M H 2 SO 4

Voltammetric activities are compared for the designated compounds in 0.50 M at the specified film electrodes configured as rotated disks. The effective number of electrons equiv mol−1) calculated from the Koutecky-Levich plot is for toluene oxidation at the electrode, compared to and at the and electrodes, respectively. Gas chromatography-mass spectrometry data confirm that the primary product of toluene oxidation is benzyl alcohol (2 equiv mol−1) at the electrode with production of small amounts of benzaldehyde (4 equiv mol−1) and benzoic acid (6 equiv mol−1). X-ray photoelectron spectroscopy data indicate the ratio of Fe:Pb is ca. 1:100 (atom:atom) in the films. By comparison, a Bi:Pb ratio of 33:100 (atom:atom) is easily attained in films. Nevertheless, the apparent heterogeneous rate constant for toluene oxidation is larger at the electrode compared to the electrode X-ray absorption near-edge structure data confirm speculation that iron exists in the 3+ oxidation state with octahedral coordination by O atoms in films. The large activity of the electrode is attributed to the benefit of adsorption of aromatic molecules at Fe(III) sites. © 2001 The Electrochemical Society. All rights reserved.

Electroless deposition of Pt on Ti: Part II. Catalytic activity for oxygen reduction

The reduction of oxygen on electroless deposited platinum on a freshly polished titanium electrode was studied in aqueous 0.1 M HClO 4 and 0.2 M NaOH solutions, utilising the rotating-disc electrode technique. TEM studies show that at short deposition times Pt is highly dispersed on titanium. At longer deposition times the degree of dispersion decreases due to coalescence and agglomeration processes and larger clusters are formed. The kinetics of oxygen reduction were affected substantially by the time of Pt deposition on Ti at the open-circuit potential. A kinetic analysis of the current–potential curves was made in the kinetically controlled region in the form of Tafel plots and in the mixed kinetic-diffusion control region in the form of Koutecky–Levich plots. The catalytic activity of the Pt crystals decreases with increasing deposition time. The results were interpreted in terms of the effect of the Pt crystal size. The larger the Pt particles the lower the catalytic activity. The role of titanium oxide formation and the electronic effects were also discussed.

Spectroelectrochemical investigation of a dinuclear cobalt porphyrin complex with ionic groups in DMSO †

The dinuclear cobalt porphyrin of (5,10,15,20-tetrakis(N-methyl-4-pyridinio)porphyrinato)cobalt(II) (CoTMPyP) with (5,10,15,20-tetrakis(4-sulfonatophenyl)porphyrinato)cobaltate(II) (CoTPPS) has allowed spectroelectrochemical analysis of electron transfer processes in DMSO. In the II–II valence state it undergoes stepwise two-electron transfer accompanied by a dissociation equilibrium (Kd = 1.3 × 10−6 M) with redox potentials at −0.37 and −0.17 V vs. Fc–Fc+. The kinetics of the dissociation allows a snapshot of the species present without perturbing the equilibrium distribution. The dissociation in the II–II valence state is promoted by oxidation to the higher valence states due to the increasing charge on the complex and solvation of DMSO at the cobalt. The dissociation constants of the dinuclear porphyrin in the II–III and III–III valence states were determined to be 2.5 × 10−6 and 5.0 × 10−6 M, respectively, using Job plots. EXAFS analysis reveals that the Co ⋯ Co distance in the dimeric porphyrins is 3.37 Å which is not drastically influenced by the valence state in the solid state. The dinuclear cobalt porphyrin complex acts as an excellent catalyst for reduction of oxygen in which the number of electrons transferred was determined to be n = 3.9 based on Koutecky–Levich plots.

Oxygen reduction kinetics at platinum electrodes covered with perfluorinated ionomer in the presence of impurity cations Fe3+, Ni2+ and Cu2+

The effect of impurity cations on the kinetics of oxygen reduction at a platinum surface covered with perfluoro-sulfonated ionomer film was investigated in 0.1 N H2SO4 in the presence of 0.001 N Fe3+, Ni2+ or Cu2+ ions, using rotating disk electrodes. The platinum disk of a rotating disk electrode was spin-coated with Nafion® solution, and after drying the Nafion® the film-covered platinum was tested for oxygen reduction kinetics. Koutecky–Levich plots and linear sweep voltammetry made possible the independent measurement of both charge transfer and diffusion kinetics of oxygen reduction at the same electrode. The rate of the charge transfer of the oxygen reduction reaction at the platinum/ionomer membrane interface was calculated along with the immersion time. It was discovered that the impurity ions hindered enormously the rate of the charge transfer step at platinum covered with perfluorosulfonated ionomer. No suppression effect for oxygen reduction was observed for bare platinum in a solution containing impurity ions, indicating that the hindrance effect is specific to the metal electrode/ionomer membrane interface. Also, both the diffusion coefficient of oxygen and oxygen concentration in the membrane were decreased by the presence of impurity cations. The relevance of the problem to the performance degradation of polymer electrolyte fuel cells is discussed in relation to the occurrence of contamination by these impurity ions during the operation of the cell.