Rate Constant Of Electrode Reaction Research Articles

Determination of Effective Surface Area and Reaction Rate Constant By Cyclic Voltammetry Considering Ohmic Resistance and CPE Effects

One of the most commonly used electroanalytical techniques for determining the effective surface area and reaction rate constant in a redox system is linear sweep voltammetry due to its sensitive response to a reaction and easy implementation. In linear sweep voltammetry, a linear potential scan is applied to an immobile electrode submerged in an unstirred analyte. The responses to the applied potential is the total current which consists of the faradaic and non-faradaic currents. The faradaic current is the current generated by the reaction of interest, while the non-faradaic current is generated by an electrical double layer. As the faradaic processes received high attention in modeling [1-3], the non-faradaic current, which can be described by the constant phase element (CPE), received less attention. The conventional approach to evaluate the effective surface area and reaction rate constant is to assume that the non-faradaic current is linear and the ohmic drop is negligible. For a quasireversible reaction, by extrapolating the baseline, the peak current and potential are obtained and applied to the relation of Matsuda and Ayabe [4] and Nicholson’s method [5] to determine for the effective surface area and the reaction rate constant, respectively. However, this conventional approach could lead to an inaccurately estimated peak current and potential since the non-faradaic current is not always linear and the ohmic drop, which results in the peak potential shifting, occasionally cannot be negligible. In addition, the faradaic and non-faradaic currents are coupled through ohmic drop effects. Our research group has successfully developed numerical modeling of voltammetry, including the combined effects of ohmic resistance, CPE, mass transfer, and faradaic processes [6]. In this model, the CPE parameters, i.e. the pseudo-capacitance and the dispersion coefficient, are required as inputs. Generally, the CPE parameters were evaluated by electrochemical impedance spectroscopy (EIS). However, the EIS analysis is time-consuming and requires prior knowledge about the equivalent circuit of a system to obtain the CPE parameters. Furthermore, it measures only impedance of a system at one specific potential in frequency domain, while a voltammetry method is a potentiodynamic measurement, which operates under a wide potential window. Therefore, the semi-theoretical approach is introduced to evaluate the CPE parameters under voltammetry condition. This approach possesses several advantages for evaluating the CPE parameters, such as time-saving, simple, and straightforward. In this study, by considering the effects of ohmic resistance and CPE, an approach for determining the electrode effective surface area and reaction rate constant using linear sweep voltammetry was developed. By fitting the peak current and potential of the numerical simulation to those of the experimental data (see Fig. 1a), the electrode effective surface area and reaction rate constant can be determined. As the non-faradaic current becomes relatively more important at faster scan rates, it is difficult to extrapolate the baseline, as depicted in Fig. 1b. This difficulty could lead to underestimation of the effective surface area since the relation of Matsuda and Ayabe, needs the faradaic peak current as an input. Furthermore, Nicholson’s method, which evaluates the reaction rate constant directly from the peak separation, would underestimate the reaction rate constant if the ohmic drop effect was appreciable. Therefore, owing to the problem that the non-faradaic current and ohmic drop effect occasionally could not be avoided, using our approach that takes them into consideration would lead to more accurate prediction of such parameters. Acknowledgment The authors would also like to express their gratitude to the Ministry of Education, Culture, Sports, Science and Technology, Japan, for providing financial support under the scholarship program for foreign students.

On the Limitations of the Doyle-Fuller-Newman Model across Operating Temperatures in Predicting Lithium-Ion Battery Dynamics

Despite their promising potential, lithium-ion batteries have not witnessed wide adoption in large-scale applications such as electrified propulsion. This is attributed to: a) safety concerns and performance degradation due to aging, and b) lack of a comprehensive understanding of battery behavior. Lithium-ion transport processes are non-linear and span multiple length scales. As such, ion transport can be modeled on a multiplicity of scales. Macroscale battery models describing mass and charge transport are well suited as a reference model for developing model-based control and estimation strategies. The battery research community, for long, has considered the Doyle-Fuller-Newman (DFN) macroscale model [1, 2] as the benchmark to evaluate the predictive ability of reduced-order and simplified electrochemical models. However, macroscale models are approximate representations of micro-scale battery dynamics, and are vulnerable as predictive tools under specific operating conditions. The motivation to pursue this work is driven by the need to develop tools to guide battery researchers on the use of the most appropriate models for predicting battery dynamics. This work specifically identifies the operating temperature conditions that violate the underlying assumptions of the continuum equations of the DFN model, and leads to loss of model predictiveness. Starting with the Poisson-Nernst-Planck (PNP) micro-scale equations, a multiple-scale expansion is applied to rigorously derive macroscopic equations of mass and charge transport [3]. Physics-based conditions are identified under which classical macroscale models accurately describe lithium-ion micro-scale dynamics with an accuracy prescribed by the homogenization technique. These conditions are represented schematically in the form of phase diagrams. The temperature-dependent dynamics of lithium-ion battery electrodes are examined using the electrolyte phase diagram [4], and the results obtained indicate that standard macroscopic models fail to describe micro-scale processes in batteries that are operated above critical temperature conditions. The results predicted by analytical studies in previous work [4] are confirmed through numerical simulations in the present work. The equations of the full-homogenized macroscale (FHM) model developed in [3] are resolved using the finite element modeling software COMSOL Multiphysics®. Numerical simulations are performed using the FHM model and the DFN macroscale model developed in COMSOL by Plett [5]. The performance of both models is assessed against data from experiments conducted on 18650 cylindrical lithium-ion cells with nickel manganese cobalt oxide cathode at the Battery Aging and Characterization Laboratory at Clemson University. 2A (1 C-rate) constant current discharge experiments are conducted at 5°C, 23°C, 45°C, and 52°C. The measured battery voltage responses are used to assess the predictive ability of the FHM and DFN models. To prevent any bias, the same geometrical and stoichiometric parameters values are used in both models, and kept constant across different temperatures. For each data set, five parameters are identified for both the models as a function of temperature: electrode diffusion coefficients and reaction rate constants, and contact resistance. The results indicate that the DFN model, which predicts battery dynamics accurately at 5°C and 23°C, fails to replicate the same at 45°C and 52°C towards the end of cell discharge. Simulations results for the performance of both models at these temperatures are illustrated in Fig. 1. The FHM model accurately predicts battery response under all temperature conditions. The lack of accuracy of the DFN model at higher temperatures is due to the inability of the electrolyte equations to capture micro-scale dynamics, indicated by a phase diagram study [4]. The outcome of this work will enable the development of physics-based control strategies to prolong battery useful life for battery management system applications. References Doyle, T. F. Fuller, and J. Newman, “Modeling of galvanostatic charge and discharge of the lithium/polymer/insertion cell”, J. Electrochem. Soc., vol. 140, no. 6, pp. 1526-1533, (1993).Doyle and J. Newman, “The use of mathematical modeling in the design of lithium/polymer battery systems”, Electrochimica Acta, vol. 40, no. 13-14, pp. 2191-2196, (1995).Arunachalam, S. Onori, and I. Battiato, “On veracity of macroscopic Lithium-ion battery models”, J. Electrochem. Soc ., vol. 162, no.10, pp. A1940-A1951, (2015).Arunachalam, S. Onori, and I. Battiato, “Temperature-dependent multiscale-dynamics in Lithium-Ion battery electrochemical models”, in Proceedings of the 2015 American Control Conference, pp. 305-310, IEEE, (2015).L. Plett, “Battery Management Systems, Volume I: Battery Modeling”, Artech House, (2015). Figure 1

Direct electrochemistry and electrocatalysis of lobetyolin via magnetic functionalized reduced graphene oxide film fabricated electrochemical sensor

A novel lobetyolin electrochemical sensor based on a magnetic functionalized reduced graphene oxide/Nafion nanohybrid film has been introduced in this work. The magnetic functionalized reduced graphene oxide was characterized by fourier transform infrared spectroscopy, atomic force microscope, X-ray diffraction, transmission electron microscopy and thermogravimetric analysis. The scanning electron microscopy characterized the morphology and microstructure of the prepared sensors, and the electrochemical effective surface areas of the prepared sensors were also calculated by chronocoulometry method. The electrochemical behavior of lobetyolin on the magnetic functionalized reduced graphene oxide/Nafion nanohybrid modified glassy carbon electrode was investigated by cyclic voltammetry and differential pulse voltammetry in a phosphate buffer solution of pH6.0. The electron-transfer coefficient (α), electron transfer number (n), and electrode reaction rate constant (Κs) were calculated as 0.78, 0.73, and 4.63s−1, respectively. Under the optimized conditions, the sensor based on magnetic functionalized reduced graphene oxide/Nafion showed a linear voltammetric response to the lobetyolin concentration at 1.0×10−7 to 1.0×10−4mol/L with detection limit (S/N=3)of 4.3×10−8mol/L. The proposed sensor also displayed acceptable reproducibility, long-term stability, and high selectivity, and performs well for analysis of lobetyolin in real samples. The voltammetric sensor was successfully applied to detect lobetyolin in Codonopsis pilosula with recovery values in the range of 96.12%–102.66%.

Specific Response of Quinone Redox at 2-Aminoethanethiol Modified Electrode

Self-assembled monolayers of alkanethiols on electrode surface have been the subjects of intensive studies in both fundamental interests of surface chemistry and their technological significance. The self-assembly is one of the most widely used techniques to construct ordered molecular structures and to control surface characteristics, due to its facile procedure. 2-aminoethanethiol has been used as an anchor to introduce functional molecules to electrode surfaces as it has a reactive amino group and its carbon chain is short. We have found that a 2-aminotehanethiol modified Au electrode shows a specific electrochemical response to p-hydroquinone in aqueous solution. That is, a peak separation (ΔE p) in the CV appeared to be very large at the first potential cycle, but it drastically decreased upon successive potential cycling. We have been studying condition where the specific response takes place and its mechanism. Figure 1A shows typical CVs of an aminoethanethiol modified Au disk electrode in a 0.1 M sodium perchlorate solution containing 5 mM hydroquinone at 100 mV s-1. The ΔE p for quinone redox was found to be 530 mV for the first cycle, indicating a slow electron transfer (Fig. 1A curve a). After application of +0.9 V vs. Ag/AgCl for 30 min at which hydroquinone is oxidized, the ΔE p largely decreased to 120 mV (Fig. 1A curve b) and the CV overlapped with that of a bare electrode. On the other hand, application if –0.5 V was applied, the CV remained virtually constant. Interestingly, when the electrode was left in the solution at open circuit or –0.5 V for 3 h, the ΔE p increased to the original value (530 mV). These results indicate that the changes in the ΔE p is reversible and not originating from peeling of the film, and the increase can be attributed to an interaction between aminoethanethiol and hydroquinone, whereas the decrease is due to oxidation of hydroquinone. A Au electrode modified with 2-dimethylanimoethanethiol showed similar behavior (Fig. 1B), indicating that Schiff base formation from amino group of 2-aminoethanethiol and p-quinone is not the dominant cause of the changes in the electrochemical response. At the current stage, we speculate two possible mechanisms for the changes in the electrochemical response. The first one is changes in structure of aminoethanethiol due to changes in an interaction with quinone, although it is known that short alkyl chains on the electrode surface do not block electron transfer. The second one is a chemical reaction between hydroquinone and aminoethanethiol, which produces derivatives with slow electrode reaction rate constant. We currently investigate those two possibilities with STM measurements for an aminoethanethiol modified electrode before and after the electrolysis and with NMR measurements for the reaction. Figure 1

Electrochemical behavior and voltammetric determination of p-methylaminophenol sulfate using LiCoO2 nanosphere modified electrode

In this study, LiCoO2 nanoparticles were synthesized by the hydrothermal method and characterized by scanning electron microscopy, transmission electron microscopy, and X-ray diffraction analyses, which exhibited flower-like nanospheres composed of uniform nanosheets. The synthesized LiCoO2 nanospheres were further used for the modification of 1-hexylpyridinium hexafluorophosphate based carbon ionic liquid electrode (CILE). The electrochemical performance of the modified electrode, LiCoO2/CILE, was studied by cyclic voltammetry and electrochemical impedance spectroscopy. Electrooxidation of p-methylaminophenol sulfate (metol) on the modified electrode was further tested and electrochemical responses of metol were significantly enhanced owing to the presence of LiCoO2 nanospheres with large surface area and fast electron transfer rate. The electrochemical parameters such as the charge transfer coefficient, the number of electron transferred, the standard electrode reaction rate constant, and the apparent diffusion coefficient for metol electrooxidation were calculated as 0.51, 0.91, 0.92s−1 and 7.06×10−5cm2/s, respectively. Under the optimized conditions, metol can be linearly detected in the concentration range from 0.4 to 400.0μmol/L with a detection limit of 2.46×10−7mol/L (3σ). The optimized method was further applied to the detection of metol content in artificial wastewater samples with satisfactory results.

Theory of square-wave voltammetry of two-step electrode reaction with kinetically stabilized intermediate

Square-wave voltammograms of quasireversible EE mechanism with thermodynamically unstable intermediate may consist either of a single peak, or of two peaks if the second electron transfer appears quasireversible at the lowest frequency and irreversible at the highest frequency. The development of the second peak depends on the critical dimensionless kinetic parameter which was calculated in this paper. This parameter can be used for the estimation of standard rate constant of the second electrode reaction.

Electrocatalytic oxidation of quinine sulfate at a multiwall carbon nanotubes-ionic liquid modified glassy carbon electrode and its electrochemical determination

The electrocatalytic oxidation of quinine sulfate (QS) was investigated at a glassy carbon electrode, modified by a gel containing multiwall carbon nanotubes (MWCNTs) and room-temperature ionic liquid of 1-Butyl-3-methylimidazolium hexafluorophate (BMIMPF6) in 0.10 M of phosphate buffer solution (PBS, pH 6.8). It was found that an irreversible anodic oxidation peak of QS with Epa as 0.99 V appeared at MWCNTs-RTIL/glassy carbon electrode (GCE). The electrode reaction process was a diffusion-controlled one and the electrochemical oxidation involved two electrons transferring and two protons participation. Furthermore, the charge-transfer coefficient (α), diffusion coefficient (D), and electrode reaction rate constant (kf) of QS were found to be 0.87, 7.89 × 10−3 cm2⋅s−1 and 3.43 × 10−2 s−1, respectively. Under optimized conditions, linear calibration curves were obtained over the QS concentration range 3.0 × 10−6 to 1.0 × 10−4 M by square wave voltammetry, and the detection limit was found to be 0.44 μM based on the signal-to-noise ratio of 3. In addition, the novel MWCNTs-RTIL/GCE was characterized by the electrochemical impedance spectroscopy and the proposed method has been successfully applied in the electrochemical quantitative determination of quinine content in commercial injection samples and the determination results could meet the requirement.

Synthesis and electroreduction of 2-[(8-hydroxyquinoline-5-yl)azo]benzo[c]cinnoline in DMSO–H2O (1:1) medium

2-[(8-Hydroxyquinoline-5-yl)azo]benzo[c]cinnoline (HQAB) was prepared and characterized by elemental analysis, MS, FTIR and 1H NMR techniques. The electrochemical reduction of HQAB has been investigated by cyclic voltammetry, chronoamperometry and controlled potential electrolysis at mercury pool electrode in the pH range 3.5–9.4. The number of electrons transferred in the electrode reaction, diffusion coefficients and standart rate constants were calculated. In acidic medium, cyclic voltammograms display four cathodic peaks, with the total exchange of 6 e– and 6 H+. By contrast, the reverse scan displays two anodic peaks. Constant potential electrolysis at –1.0 V and TLC analysis of the product reveals that the reduction of azo group (in the bridge) in HQAB does not stop at the hydrazo stage but goes further through the cleavage of –NH–NH– linkage to give amino compounds as the final products. The voltammograms recorded in basic medium exhibit two cathodic peaks corresponding to 4 e–, 4 H+ and two reverse anodic peaks, and thus the reduction stopped at hydrazo stage. A tentative mechanism for the reduction has been suggested.

Voltammetric determination of sericin based on its interaction with carmine

A simple yet sensitive method is developed for the determination of sericin using voltammetry based on the interaction between sericin and carmine for the first time. In the absence of sericin, carmine has a pair of well-defined redox peaks in a pH 1.81 Britton–Robinson buffer solution. Although no new redox peaks appear upon the addition of sericin into a carmine solution, the peak currents of the old peaks reduce while the peak potentials shift positively. This observation is attributed to the decrease in the diffusion coefficient and electrode reaction rate constant of carmine in the presence of sericin. A binding mechanism is proposed and discussed, and the binding constant and binding ratio are calculated as 2.32 × 10 6 L mol −1 and 1:1, respectively. Furthermore, the decrease in the peak currents is found proportional to the sericin concentration in the range of 32.0–800.0 μg mL −1 with a detection limit of 13.52 μg mL −1. The method is further applied to the determination of sericin in degumming wastewater with satisfied average recoveries from 96.7 to 103.3%. The results are in good agreement with those obtained by the conventional Coomassie brilliant blue G-250 spectrophotometric method.

Electrochemical Study on the Interaction of Protein with Bromothymol Blue and Its Analytical Application

Abstract The interaction of bromothymol blue(BB) with human serum albumin(HSA) was studied by electrochemical techniques and a sensitive method for proteins assay was developed. When BB interacted with HSA, the voltammetric peak current value of BB decreased linearly with the concentration of HSA in a range of 1.0—40.0 mg/L, and the peak potential shifted negatively. Based on the results, a sensitive assay method for proteins, such as HSA, bovine serum albumin(BSA), and egg albumin etc. was established. This method was further applied to determining the HSA in healthy human blood samples, and the results are not significantly different from those obtained by the classic Coomassie Brilliant Blue G-250 spectrophotometic method. The detecting conditions of this method were optimized and the interaction mechanism was discussed. The results show that the electrochemical parameters(formal potential E 0 , standard rate constant of the electrode reaction k s , parameter of kinetic nα ) of BB have no obvious changes before and after the interaction, which indicate that BB can interact with HSA, forming an electrochemical non-active complex. The equilibrium constant(β s ) and the binding ratio( m ) for this complex were calculated. The m is 4 and β s is 1.41×10 19 . This method is fast, simple, highly sensitive, and has good selectivity, which can be used in clinical measurements.

Electrochemical behavior and determination of rutin on a pyridinium-based ionic liquid modified carbon paste electrode

In this paper the electrochemical behavior of rutin on a pyridinium-typed ionic liquid modified carbon paste electrode (IL-CPE) was investigated and further used for rutin sample determination. The IL-CPE showed strong electrocatalytic effects to the oxidation of rutin. In phosphate buffer solution (PBS, pH 2.5; 0.1 M) a pair of well-defined cyclic voltammetric redox peaks of rutin appeared with the redox peak located at 512 mV (Epa) and 448 mV (Epc) ( vs. SCE), respectively. The redox peak current was increased about 27.5 times more than that on traditional carbon paste electrode (CPE). The electrochemical parameters of rutin on the IL-CPE were calculated with the results of the charge transfer coefficient ( α), the number of electron transfer ( n) and the electrode reaction rate constant ( k s ) as 0.53, 1.80 and 2.39 s −1, respectively. The cathodic peak currents increased linearly with the concentration of rutin in the range from 5.0 × 10 −7 to 1.0 × 10 −4 M with the detection limit as 3.58 × 10 −7 M (3 σ). The relative standard deviation (RSD) of 10 successive detection of 5.0 × 10 −5 M rutin was 4.2%. The method was successfully applied to the determination of rutin content in tablets samples with good recovery. The modified electrode showed good stability and reproducibility without the influence of the coexisting substances.

Theory of irreversible electrode reactions coupled to ion transfer across the liquid/liquid interface

Kinetically controlled electro-oxidation of a redox probe dissolved in the organic solvent, which is interposed between an electrode surface and an aqueous solution as a thin layer, is analyzed theoretically. It is demonstrated that the electrode reaction rate constant can be measured by the variation of scan rate in linear scan voltammetry both in the absence and in the superfluity of the supporting electrolyte dissolved in the film.

Redox kinetics of adriamycin adsorbed on the surface of graphite and mercury electrodes

Kinetics of the surface redox reactions of adriamycin (doxorubicin hydrochloride) adsorbed on paraffin-impregnated graphite electrode (PIGE) and on mercury electrode is measured by square-wave voltammetry. In 0.9 mol/L KNO 3 buffered to pH 4.65, the standard electrode reaction rate constants of the first quinone/hydroquinone redox couple (see Scheme 2) on PIGE and mercury are k s1 = 49 ± 12 s − 1 and k s1 = 147 ± 36 s − 1 , respectively. Under the same conditions, the standard rate constant of the second redox couple on the PIGE is smaller than 4 s − 1 and the electron transfer coefficient of the reduction is α 2 = 0.35.

Electron-Transfer Kinetics of Np3 + ∕ Np4 + , NpO2 + ∕ NpO22 + , V2 + ∕ V3 + , and VO2 + ∕ VO2 + at Carbon Electrodes

Like vanadium redox couples ( and ), neptunium is a good candidate for the active materials of a redox-flow battery because it possesses two isostructural and reversible redox couples ( and ). The standard rate constants of the neptunium electrode reactions were first determined at four types of carbon electrodes. The values for and are and , respectively, at the plane of pyrolytic graphite [PG( plane)] and also and , respectively, at plastic formed carbon (PFC). The standard rate constants for the and electrode reactions were also first determined at PG( plane) and PFC electrodes. The determined values are larger at the PFC electrode, and for and , respectively, than at the PG( plane) electrode. There is about a order of difference between the values of neptunium and vanadium ions, and this difference was discussed in reference to the homogeneous electron-exchange rate constants of these redox couples.

Mass transport in gas channels of electrochemical cells with a solid-oxide electrolyte: Allowing for the electrode reaction rates in numerical calculations

The concentration fields, Nusselt number distributions along the electrode, and ratios of concentrations of electrochemically active components at the entrance into and exit out of an electrochemical cell are determined by numerical integration of the convective-diffusion equation for a broad range of rate constants of electrode reactions, Peclet numbers, and geometrical parameters of channels. Three regions of parameter values are revealed. At low reaction rates (K l 20), it is completely determined by the diffusion in the gas phase; and in the intermediate region of values of K l it is necessary to allow for both the diffusion processes and the electrode reactions.

Chronocoulometric studies of the kinetics of the electro-reduction of Eu 3+ in dimethylsulfoxide: Experiment and simulation

Standard electrode reaction rate constants for the one-electron reduction of Eu(CF 3SO 3) 3 on mercury in different concentrations of the supporting electrolyte tetrabutylammonium perchlorate in dimethylsulfoxide have been measured by single and double potential step chronocoulometry. Simulation programs were developed to extract the standard reaction rate constants from chronocoulometric experiments. The k s-values agreed well with data obtained from cyclic voltammetry and from ac techniques. The reaction rate constants depended on the concentration of the supporting electrolyte. Correlations of the k s-values with electrolyte properties are discussed.

Influence of the supporting electrolyte on standard electrode reaction rate constants on mercury

The influence of the nature and the concentration of the supporting electrolyte on standard reaction rate constants in non-aqueous solvents is discussed. Reductions of the monoatomic cations Zn2+, Cd2+ and Eu3+ as well as the reduction of bis(biphenyl)chromium(I) serve as models to evaluate the electrolyte contributions to standard reaction rate constants. The limitations of the Marcus theory and the Frumkin model to account for the effect of the electrolyte on standard reaction rate constants are described. The dependence of the apparent standard reaction rates on electrolyte properties such as the supporting electrolyte concentration, the viscosity and the resistivity are evaluated. Differences in the activities of the electroactive species at the reaction site and in the bulk of the solution are considered for qualitative descriptions of the electrolyte effect on standard reaction rates of Eu3+ on the one hand and of bis(biphenyl)chromium(I) on the other.

The Decamethylferrocene(+/0) Electrode Reaction in Organic Solvents at Variable Pressure and Temperature

Half-wave potentials E(1/2) relative to a Ag/Ag(+) electrode, mean diffusion coefficients D, and standard electrode reaction rate constants k(el) are reported for the decamethylferrocene(+/0) couple (DmFc(+/0)) in nine organic solvents at variable pressure and (for five solvents) temperature. Limited data on the ferrocene(+/0) (Fc(+/0)) and Fe(phen)(3)(3+/2+) electrode reactions are included for comparison. Although E(1/2) for DmFc(+/0) correlates only loosely with the reciprocal of the solvent dielectric constant epsilon at ambient pressure, its pressure dependence expressed as the volume of reaction Delta V(cell) is a linear function of Phi = (1/epsilon)( partial differential ln epsilon/ partial differential P)(T) (the Drude-Nernst relation). Interpretation of the temperature dependence data is made difficult by enthalpy-entropy compensation. Measurements of D for solutions containing 0.5 mol L(-1) tetrabutylammonium perchlorate (TBAP) at 25 degrees C and ambient pressure are inversely proportional to the viscosities eta of the pure solvents as expected from the Stokes-Einstein relation, despite the fact that increasing [TBAP] results in increased eta. The activation volume Delta V(diff)(++) for diffusion of DmFc(+/0) ranges from 7 to 17 cm(3) mol(-1) and generally increases with increasing eta and thus with increasing [TBAP]. The activation volumes Delta V(el)(++) for the electrode reactions of DmFc(+/0) and Fc(+/0) are all positive, equaling the corresponding Delta V(diff)(++) values within the experimental uncertainty and contrast sharply with the negative Delta V(ex)(++) values characteristic of the corresponding self-exchange reactions in homogeneous solution. These facts, together with the thermal activation parameters, point to solvent dynamical control of the electrode (but not the homogeneous self-exchange) reactions. The apparent radii of the electroactive species according to the Drude-Nernst and Stokes-Einstein relations cannot be satisfactorily related to their crystallographic radii and are better regarded as adjustable parameters with limited physical significance.

Electrode kinetics of Eu 3+ in sodium perchlorate solutions in dimethylsulfoxide

The influence of the concentration of the sodium perchlorate supporting electrolyte on the standard electrode reaction rate constants for the one-electron reduction of Eu(CF 3SO 3) 3 on mercury in dimethylsulfoxide has been studied. The k s app-values were derived from both ac polarography on the dropping mercury electrode and from cyclic voltammetry on a hanging mercury drop in a two-electrode system. The ac data was evaluated by Randles [Trans. Faraday Soc. 48 (1952) 937] and by Sluyters [Rec. Trav. Chim. 79 (1960) 1092] methods. The standard reaction rates from cyclic voltammetry were extracted via a simulation program. Densities, viscosities, specific resistances (resistivities) and double layer capacitances of the electrolytes were measured. Correlations between electrolyte properties and the k s app-values were investigated. The standard electrode reaction rates were also treated within the Frumkin theory.

Electrochemistry and electrocatalysis of binuclear cobalt phthalocyaninehexasulfonate–surfactant film modified electrode

Electrochemistry and electrocatalysis of binuclear cobalt phthalocyaninehexasulfonate sodium salt (bi-CoPc) were investigated successfully in liquid crystal films of didodecyldimethylammonium bromide (DDAB) on pyrolytic graphite (PG) electrodes in aqueous solution. This new bi-CoPc–surfactant film modified electrode could reduce aggregation of bi-CoPc in aqueous solution, which was confirmed by the electronic absorption spectra in the visible region and images of transmission electron microscopy (TEM). The charge transport diffusion coefficient ( D ct) and apparent heterogeneous electrode reaction rate constant ( k s) for this modified electrode were estimated. The reduction of bi-CoPc ligand showed excellent electrocatalytic ability for the reduction of trichloroacetic acid (TCA) and catalytic current had a linear relationship with the concentration of TCA in the range of 3×10 −5–7.5×10 −3 M. The reduction of bi-Co(III)Pc/bi-Co(II)Pc couple exhibited new pattern of catalytic reactivity in the reduction of oxygen. Peak current for the cathodic reduction of oxygen is proportion to the square root of the scan rate in the range of 5–1000 mV s −1 in oxygen-saturated solution.