Charge Transfer Coefficient Research Articles

Electrochemical Determination of Cysteine Using a 2-(2,5-Dihydroxy-Phenyl)-Naphtho[1,2-d] Oxazole-5-Sulfonic Acid (DHPNOS) and Cerium Oxide Nanoparticle (CeO2NP) Composite Carbon Paste Electrode (CPE)

An electrochemical sensor for the electrocatalytic oxidation of cysteine was designed based upon a 2-(2,5-dihydroxy-phenyl)-naphtho[1,2-d] oxazole-5-sulfonic acid (DHPNOS) and cerium oxide nanoparticle (CN) modified carbon paste electrode (CPE). An investigation of the redox properties of this modified electrode indicates a reduction in oxidative overvoltage and an intense increase in the analytical current. The pH, modifier, and nanoparticles were optimized to 7, 4% weight percent, and 5% weight percent, respectively. The apparent charge transfer rate constant (ks) and transfer coefficient for electron transfer between modifier and electrode were 2.776 s−1 and 0.47, respectively. Using differential pulse voltammetry, a linear range from 1 to40 µM with a detection limit of 0.33 µM was observed in phosphate buffer (pH 7.0). This work introduces a simple approach for selective determination of L-cysteine in the presence of uric acid. Also, based on chronoamperometry, the diffusion coefficient was determined to be 2.61 × 10−5 cm2 s−1. The effect of interferences on L-cysteine peak current were studied. The repeatability and reproducibility of the designed sensor were also measured with relative standard deviation values of 1.00% and 4.45%, respectively. The constructed sensor was employed for the determination of L-cysteine in acetylcysteine tablets.

Rotating Disk Electrode Study of Sm(III)/Sm(II) in LiCl-KCl Eutectic Molten Salt

Understanding the redox reactions of fission products in molten salts is crucial for developing pyroprocessing techniques for used nuclear fuel. A rotating disk electrode is useful for investigating the electrochemical reactions with controlled mass transfer conditions, but its application has been limited in high-temperature corrosive molten salts. This study employs a tungsten (W) rotating disk electrode (RDE) to measure the electrochemical and kinetic properties of the Sm(III)/Sm(II) redox reaction in a LiCl-KCl eutectic molten salt. The properties of the Sm(III)/Sm(II) redox reaction, including diffusion coefficients, exchange current densities, charge transfer coefficients, activation energies, and Tafel slopes, were determined over a temperature range of 723–803 K using limiting currents in linear sweep voltammetry at various rotating speeds and mass transfer-corrected Tafel plots. The kinetic parameters obtained using the rotating disk electrode system can be useful for optimizing the design of pyroprocessing techniques.

Eugenol's electrochemical behavior, complexation interaction with copper chloride, antioxidant activity, and potential drug molecular docking application for Covid-19

Electrochemical studies were conducted to analyze the behavior of eugenol, CuCl2, and their complex using cyclic voltammetry. The oxidation mechanisms of eugenol and the redox behavior of copper ions were elucidated, showing differences in reversibility and charge transfer coefficients. Various kinetic and solvation parameters were determined. The redox behavior of CuCl2 was found to be more reversible compared to the copper-eugenol complex. The copper-eugenol complex exhibited enhanced antioxidant activity compared to eugenol and standard ascorbic acid. The eugenol was oxidized to form eugenol quinone methide through two postulated irreversible mechanisms. Molecular docking studies suggested higher potential bioactivity of the copper-eugenol complex towards the target protein of COVID-19 than the eugenol ligand.

(Keynote) Unified Quantum Theory of Electrochemical Kinetics Based on Coupled Ion-Electron Transfer

A general theory of coupled ion–electron transfer (CIET) is presented, which unifies quantum kinetics of electron transfer (ET) with classical kinetics of bond-breaking ion transfer (IT) in a general framework of non-equilibrium thermodynamics [1]. In the limit of large reorganization energy, the theory predicts Marcus kinetics of “electron-coupled ion transfer” (ECIT). In the limit of large ion-transfer energies, the theory predicts generalized Butler–Volmer kinetics of “ion-coupled electron transfer” (ICET), where the charge transfer coefficient and exchange current are connected to microscopic properties of the electrode/electrolyte interface. In the ICET regime, the reductive and oxidative branches of Tafel’s law are predicted to hold over a wide but finite range of overpotentials, bounded by the ion-transfer energies for oxidation and reduction, respectively. The probability distribution of transferring electron energies in CIET smoothly interpolates between a shifted Gaussian distribution for ECIT (as in the Gerischer–Marcus theory of ET) to an asymmetric, fat-tailed Meixner distribution centered at the Fermi level for ICET. The latter may help interpret asymmetric line shapes in x-ray photo-electron spectroscopy (XPS) and Auger electron spectroscopy (AES) for oxidized metal surfaces in terms of shake-up relaxation of the ionized atom and its image polaron by ICET. In the limit of large overpotentials, the theory predicts a transition to inverted Marcus ECIT, leading to a universal reaction-limited current for metal electrodes, dominated by barrierless quantum transitions. Uniformly valid, closed-form asymptotic approximations are derived that smoothly transition between the limiting rate expressions for ICET and ECIT for metal electrodes, using simple but accurate mathematical functions. The theory is applied to lithium intercalation in lithium iron phosphate (LFP) and found to provide a consistent description of the observed current dependence on overpotential, temperature and concentration, learned from x-ray images [2]. CIET theory thus provides a critical bridge between quantum electrochemistry and electrochemical engineering.[1] M. Z. Bazant, Unified quantum theory of electrochemical kinetics by coupled ion-electron transfer, Faraday Discussions 246, 60-124 (2023).[2] H. Zhao, H. D. Deng, A. E. Cohen, J. Lim, Y. Li, D. Fraggedakis, B. Jiang, B. D. Storey, W. C. Chueh, R. D. Braatz, and M. Z. Bazant, Learning heterogeneous reaction kinetics from X-ray movies pixel-by-pixel, Nature 621, 289-294 (2023). Figure 1

Interrogation of Anodic Stripping Voltammetry Peaks of Metals for the Determination of Electrochemical Reaction Parameters

The aim of this study was to develop a method for the analysis of peaks obtained during anodic stripping voltammetry (ASV) at silver-based mercury film electrodes (SBMFE) in the presence of transition and rare earth metals. Such stripping peaks may originate from reversible or irreversible electrochemical processes, with a plethora of unknown parameters, such as number of electrons involved, electrochemical rate constant, formal potential, charge transfer coefficient, and metal concentration (in the film and/or solution). In particular, we were interested in the concentration of transition metals and rare earth elements deposited simultaneously in the mercury film during ASV in order to quantify the electrochemical amalgamation of each species and their rates of deposition.Herein, we studied the concurrent ASV of multiple metals using a mercury film electrode as part of the development of the OREATE (Oxide Reduction by Electrochemical Amalgamation and Thermal Extraction) process. Certain of these species generate electrochemically irreversible processes, while others produce reversible ones. In addition, the reduction of most rare earth metals into the mercury film occurs well below the onset of the hydrogen evolution reaction, preventing us from probing these reduction processes. For example, the reduction of cerium(III) (an electrochemically irreversible process) occurs at a formal potential of −1.64 V vs. Ag/AgCl (3 M KCl). Furthermore, the presence of multiple metals in solution can lead to overlapping stripping peaks and mask these oxidative processes. For instance, the irreversible oxidative stripping of cerium (−0.75 V vs. Ag/AgCl (3 M KCl)) from the mercury film can obscure the reversible stripping of zinc (−0.95 V vs. Ag/AgCl (3 M KCl)).In this talk, we describe the use of thin layer electrochemistry equations to extract electrochemical parameters from the ASV at a mercury film electrode. This electroanalytical method enables investigation of processes that are otherwise obscured by the hydrogen evolution reaction, as well as resolution of overlapping stripping processes. This permits the determination of the concentration of each metal deposited in the film during the electrochemical amalgamation. By summing the thin layer electrochemical equations for reversible and irreversible processes, we were able to extract the number of electrons involved, the electrochemical rate constants, and the charge transfer coefficients for simultaneous oxidative processes. Finally, we were able to obtain the concentrations of multiple species in the mercury film and their rates of deposition.In conclusion, we have established a protocol for the interrogation of electrochemical reaction parameters in anodic stripping voltammetry of mixed-metal systems. Moreover, we demonstrated an electrochemical method to quantitatively separate overlapping peaks for the determination of the species’ concentrations, all based on the principles of thin layer behavior.

Modeling Interfacial Proton-Coupled Electron Transfer Reactions Involving Imidazolium Proton Donors

Proton-coupled electron transfer (PCET) is an elementary reaction that plays a pivotal role in various electrochemical energy conversion and storage processes. PCET reactions involving nonaqueous proton donors are of interest for applications in energy storage such as redox flow batteries, as well as in CO2 and O2 reduction catalysis. Yet, there remains much to be understood about the molecular reactivity of these systems. In this talk, I will describe periodic density functional theory (DFT) studies of interfacial PCET reactions involving different imidazolium proton donors on Pt(111) electrode surfaces.The imidazole conjugate bases adsorb strongly to the electrode surface; however, this adsorption behavior varies with isomers that feature distinct proton positions and with different functional substituent groups. Because the adsorption of imidazole conjugate bases could affect reactivity, these models are used as a foundation to understand the intrinsic PCET kinetics of these systems. The Volmer and Heyrovsky steps of hydrogen evolution reaction are two of the most basic heterogeneous electrochemical PCET reactions and are therefore chosen as model interfacial PCET reactions in this study. Potential-dependent reaction energies and activation barriers are calculated using the charge extrapolation method, where the electrode potential is tuned by modifying proton coverage for different-sized unit cells. This approach is used to develop relationships between PCET reaction thermodynamics and kinetics through Brønsted–Evans–Polanyi (BEP) relationships for different imidazoles, where BEP slopes are analogs to charge transfer coefficients in the Butler–Volmer equation. Investigating reaction parameters influencing charge transfer kinetics at the electrode-electrolyte interface is crucial for designing efficient energy storage systems, impacting overall device performance. These studies show trends in the kinetic behavior of different functionalized imidazoles, which can aid in the design of catalytic and energy storage systems.

Revealing the Role of Redox Reaction Selectivity and Mass Transfer in Current–Voltage Predictions for Ensembles of Photocatalysts

Photocatalysts are conceptually simple reaction units where nanoscale semiconductors integrated with catalysts drive a pair of redox reactions on illumination. However, the proximity of reaction sites performing cathodic and anodic reactions poses dire challenges to realize large light-to-fuel conversion efficiencies. In this study, a powerful, yet straightforward, equivalent-circuit detail-balance modeling framework is developed and applied to evaluate the performance of photocatalytic systems featuring multiple light absorbers. Specifically, low bandgap iridium-doped strontium titanate is modeled a Z-scheme photocatalyst to effect desired hydrogen evolution and iron-based redox shuttle oxidation reactions. Our model has unique capabilities to simulate competing redox reactions and address mass-transfer limitations. In a significant departure from state-of-the-art circuit models, our study develops tools to perform load-line analyses by incorporating a net electrochemical load curve that includes both desired and competing redox reactions. Consequently, reaction selectivity is predicted from equivalent circuit models for photocatalytic and photoelectrochemical systems. Our investigation into ensembles comprised of multiple, semi-transparent light absorbers reveals their potential to outperform a single, optically thick light absorber, particularly when operated under mass-transfer-limited conditions. However, this outcome hinges on minimizing mass-transfer rates of select redox species to prevent undesired reactions of hydrogen oxidation and/or redox shuttle reduction. Our findings demonstrate that reaction selectivity can be achieved by tuning asymmetry in redox species mass-transfer even with perfectly symmetric electrocatalytic charge-transfer coefficients. The influences of various kinetic, mass-transfer, and thermodynamic parameters are explored to offer crucial insights to inform the next-generation of photocatalysts, selective coatings, and reactor designs.

Parameter estimation of PEM fuel cells using metaheuristic algorithms

Developing an accurate model for precise parameter estimation is crucial for the design of polymer electrolyte membrane fuel cells (PEMFC). While many studies can meet the evaluation criteria for parameter estimation, there is a significant physical inconsistency when applying the Generalized Steady State of Electrochemical Model (GSSEM) prediction in the optimization framework. This study aims to address this inconsistency by adding necessary constraints to the optimization model. It employs the Artificial Hummingbird Algorithm (AHA) and Lévy flight improvement to find unknown parameters by minimizing the sum of squared errors (SSE). The proposed method has been rigorously tested with seven PEMFC cases, demonstrating its robustness and superior accuracy compared to alternative approaches. Notably, the cathode charge transfer coefficient exhibits zero relative percent difference (RPD) between the two sources, confirming its physical consistency. Furthermore, the proposed constrained optimization model lays the groundwork for achieving consistent parameter estimation results for PEMFC when utilizing GSSEM.

Modeling and Analysis of Polarization Losses in Solid Oxide Fuel Cells with Siloxane Contamination

In this study, the degradation of the solid-oxide fuel cell (SOFC) nickel-yttria stabilized zirconia anode under decamethyltetrasiloxane (L4) contamination is examined with experiments and modeling. A model is developed for the polarization losses based on the charge transfer coefficient, α, and diffusion layer thickness, δ, and fitted to the experimental data to understand how the siloxane degrades the SOFC performance with time. The results of the model indicate that the total polarization losses increase approximately 44% over the course of the 180 min experiment at 350 mA cm−2. Activation losses dominate the polarization losses initially but decrease in their total contribution while concentration losses increase. Scanning electron microscopy (SEM) with wavelength dispersive X-ray spectroscopy (WDS) elemental mapping indicates that silicon deposition is highest at the outer edge of the anode and forms a barrier layer to fuel diffusion, increasing concentration losses. When the model is applied to other previous D4 and L4 siloxane experiments conducted over a period of 40 h, similar trends in polarization losses are observed. Polarization losses increase more rapidly with D4 compared to L4 siloxane contamination, with concentration losses increasing the fastest with both types of siloxane.

Shungite Paste Electrodes: Basic Characterization and Initial Examples of Applicability in Electroanalysis

This article introduces a new type of carbon paste electrode prepared from black raw shungite. In powdered form, this carbonaceous material was mixed with several nonpolar binders. The resulting shungite pastes were microscopically and electrochemically characterized. Mixtures of several pasting liquids with different contents of shungite powder were tested to select the optimal composition and compared with other types of carbon paste-based electrodes made of graphite and glassy carbon powder. In terms of physical and mechanical properties, shungite paste electrodes (ShPEs) formed a composite mass being like dense pastes from glassy carbon microspheres, having harder consistency than that of traditional graphitic carbon pastes. The respective electrochemical measurements with ShPEs were based on cyclic voltammetry of ferri-/ferro-cyanide redox pairs, allowing us to evaluate some typical parameters such as electrochemically active surface area, double-layer capacitance, potential range in the working media given, heterogeneous rate constant, charge-transfer coefficient, exchange current density, and open-circuit potential. The whole study with ShPEs was then completed with three different examples of possible electroanalytical applications, confirming that the carbon paste-like configuration with powdered shungite represents an environmentally friendly (green) and low-cost electrode material with good stability in mixed aqueous-organic mixtures, and hence with interesting prospects in electroanalysis of biologically active organic compounds. It seems that similar analytical parameters of the already established variants of carbon paste electrodes can also be expected for their shungite analogues.

Integrating multiphasic CuSx/FeSx nanostructured electrocatalyst for enhanced oxygen and hydrogen evolution reactions in saline water splitting

This study employed an electrodeposition approach to synthesize multiphasic CuSx and FeSx on nickel foam (NF) for application in saline water splitting. This multiphasic electrocatalyst exhibits a cauliflower morphology and develops a porous fused-type morphology upon partial oxidation. The NF/CuSx/FeSx electrode with partial oxidation exhibits the lowest overpotential of 181 mV at 10 mA/cm2 and a Tafel slope of 163 mV/decade for the oxygen evolution reaction (OER). The overpotential of 73 mV at 10 mA/cm2 and a Tafel slope of 165 mV/decade were found for the hydrogen evolution reaction (HER). A charge transfer coefficient value of ∼0.5 in OER and HER indicates that the rate-determining step depends on the surface adsorption of reaction species. The presence of an unpaired electron during partial oxidation can create additional active sites and reduce solution resistance (Rs). This can improve the interaction between reactants and intermediates, improving OER and HER performance. NF/CuSx/FeSx composites demonstrated robust stability using real seawater splitting over 80 hours in HER with negligible degradation. However, catalyst breakdown in OER after 10 hours due to prolonged exposure to higher potentials, resulting in oxidative corrosion. This study offers a multiphasic electrode design using the electrodeposition technique to produce green hydrogen energy through seawater splitting.

Green Synthesis of Chlorophyta Seaweed Mediated Mn3O4 Nanoparticles: Electrochemical Applications and Evaluation of Capacitive and Diffusive Contributions

This study focuses on the sustainable synthesis of Mn3O4 nanoparticles using Chlorophyta seaweed extracts for electrochemical capacitor applications. X-ray diffraction (XRD), ultraviolet-visible (UV–Visible) spectroscopy, scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), and Fourier-transform infrared (FTIR) spectroscopy were used to evaluate the produced nanoparticles to determine their crystallinity, structure, shape, and elemental composition. The XRD results showed diffraction peaks that correspond to the Hausmannite Mn3O4 phase, with a 24.4 nm-sized average crystallite. The electrochemical properties were examined through cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). When compared to a bare Ni electrode, the electrochemical analysis showed a voltammetric response that was 17-fold higher. For the kinetics of the electrochemical reaction, the charge transfer coefficient (αa) and apparent electron transfer rate constant (k s) were determined to be 4.0 s−1 and 0.33, respectively. The diffusion coefficient, as evaluated using the Randles-Sevcik equation, was 1.78 × 10−6 cm2/s. The analysis conducted at various scan rates revealed a diffusive contribution of 90.6% at 5 mV/s and 67.7% at 100 mV/s. In contrast, at a scan rate of 100 mV/s, the capacitive contribution increased from 9.4 to 32.3%. The change in the time for the interaction between ions and electrode material is responsible for the reduction in the diffusive component and the increase in the capacitive component.

From Fundamental Interfacial Reaction Kinetics to Macroscopic Current–Voltage Characteristics: Case Study of Solid Acid Fuel Cell Limitations and Possibilities

AbstractThe unique properties of solid acid electrolytes, in particular CsH2PO4, are in many ways ideal for fuel cell operation. However, the technology is constrained by high cathode overpotentials. Here a simplified cathode geometry is employed to obtain the fundamental electrochemical parameters (exchange current density and charge transfer coefficient) describing the oxygen reduction reaction (ORR) at the CsH2PO4‐Pt‐gas interface. The parameters are incorporated into a 1D model of the voltage–current characteristics of realistic SAFC cathodes, which reproduced the measured polarization behavior of such cathodes without recourse to fitting adjustable parameters. Following this validation, the model is utilized to evaluate the impact of changes to cathode properties, microstructure, and operating conditions. Of these, the charge transfer coefficient, measured to have a value of ≈0.6 for ORR on Pt in the SAFC cathode environment, is found to have the greatest impact on power output. Nevertheless, even without material modifications, a combination of microstructural and operational modifications are identified with projected performance metrics meeting Department of Energy targets (0.8 V at 300 mA cm−2, and peak power density of 1 W cm−2), albeit at high Pt loadings. However, the analysis indicates that truly meaningful advances will likely necessitate the discovery of alternative ORR catalysts.

Biocompatible Gold Nanoparticles-Modified Fluorine Doped Tin Oxide Electrode for the Fabrication of Enzyme-Free Glucose Sensor.

This work demonstrates the use of jute stick extract as a reducing and stabilizing agent for the synthesis of spherical gold nanoparticles (AuNPs). In UV-Vis spectroscopy, peak at 550 nm was used to confirm the formation of AuNPs. The spherical surface morphology of AuNPs was determined through SEM and TEM analysis. While XRD investigation revealed the crystallinity of the prepared AuNPs. To ensure the biocompatibility of synthesized AuNPs, a bacterial investigation was conducted with negative results towards bacterial strain. The, modified FTO with AuNPs were able to detect glucose in CV analysis and the constructed sensor displayed a wide linear range of 50 μM to 40 mM with a detection limit of 20 μM. Scan rate analysis was performed to determine the charge transfer coefficient (0.42) and Tafel slope (102 mV/decade). Furthermore, the interfacial surface mechanism is illustrated to understand the interaction of glucose with the electrode surface in an alkaline medium and the product formation through the dehydrogenation and hydrolysis process. The prepared sensor also showed good stability, reproducibility, and anti-interference capabilities. In the case of real sample analysis, we used a blood serum sample. A low RSD value (<10 %) suggests the practical use of AuNPs/FTO in real-life applications.

Integrated model for accurate internal state estimation of polymer electrolyte membrane fuel cells

This study unveils a refined methodology for fuel cell behavior simulation. On the basis of three multiphysics models for the polymer membrane, catalyst layer, and mass transport, a new type of integrated model was developed. Using this model, the performance curves for various fuel cell states, charge transfer coefficient of reaction kinetics, concentration gradient inside the catalyst, and ion conductivity of the separator in the thickness direction for various fuel cell states were estimated. The proposed method can estimate microscale to macroscale fuel cell behavior with minimal computational effort while keeping the error relative to experimental data small by combining the most appropriate models to simulate each part of the fuel cell as needed.

A Liquid Alloy Anode for the Electrolytic Reduction of Uranium Oxide in Molten Lithium Chloride

A cost-effective anode material for uranium oxide electrolytic reduction in lithium chloride is still in deficiency. In this work, the application of liquid lithium-bismuth alloy anode was investigated. In the LiCl electrolyte at 923 K, UO2 was reduced electrochemically in cathode, while Li-Bi alloy served as counter electrode. Partial reduction of UO2 was verified by X-ray powder diffraction when the cathode potential was intentionally controlled above the lithium reduction potential. In contrast, when the precipitation of lithium metal was intentionally controlled, the reduction of UO2 was significantly improved. The charge transfer coefficient of UO2/U reaction was also calculated. Regeneration of Li-Bi alloy in LiCl-Li2O through electrolysis was proposed. Carbon, gold, and platinum had been investigated as anode materials. According to the potential variation curve, lithium was not effectively reduced into bismuth as in pure LiCl when oxygen ion was present in the molten salt. These three materials failed to demonstrate advantage in the regeneration of Li-Bi alloy.

Current Interrupt Technique to Fully Characterize PEMWE Cells

It is common practice to characterize cells in polymer electrolyte membrane water electrolysis (PEMWE) using electrochemical impedance spectroscopy (EIS) and Tafel analysis, which require special equipment and operation procedures. Additionally, these techniques are not suitable for large industrial size cells with very low impedances. We present a simpler approach based on a novel evaluation of the current interrupt (CI) technique. The CI technique utilizes the voltage response after an instantaneous drop of electric current to identify the ohmic resistance R Ω, charge transfer resistance R ct and double-layer capacity C dl in a simplified equivalent circuit (EC) of the cell. A direct link to results of typical EIS and Tafel analysis can be defined by using the improved CI method which considers a non-linear activation resistance instead of a constant charge transfer resistance. Thereby, access to equivalent information as the established standard method is granted, while being applicable to all cell and stack sizes without requiring special equipment (e.g. impedance spectrometer). The agreement with experimental data is significantly improved over the assumption of a constant charge transfer coefficient. Consistency of the proposed interpretation with explicit EIS and Tafel analysis is demonstrated and options for industrial application of the evaluation scheme are discussed.

Electrochemical Study of OH− in Molten MgCl2-NaCl-KCl Salts

The present study determines the equilibrium concentration of OH- in molten MgCl2-NaCl-KCl salts and measures the kinetic properties data of OH- (diffusion coefficient, exchange current density, charge transfer coefficient, limiting current density, and standard rate constant) accordingly at different temperatures. The equilibrium concentration of OH- descends as the temperature increases, which ranges from 4.11 to 2.84 × 10−6 mol cm−3 in the temperature range of 600 °C–800 °C. The diffusion coefficient follows Arrhenius law with a value between 2.46 and 4.48 × 10−4 cm2 s−1. The exchange and limit current densities, and standard rate constant increase with temperature. The calculated standard rate constant k0 follows Arrhenius law, which is in the range from 1.23 to 2.50 × 10−4 cm s−1. The measured charge transfer coefficient is independent of temperature, which validates the present measurements.

Kinetics of electrochemical Eu3+ to Eu2+ reduction in aqueous media

All lanthanides have a stable trivalent oxidation state, but some of them can also occur in the divalent or tetravalent state. Europium is well-known for its divalent oxidation state, which has been investigated in aqueous, organic, and molten salt media. In contrast to other lanthanides, Eu3+ can be easily reduced (Eo=−0.34 V vs. SHE) via chemical, electrochemical or photochemical routes, and Eu2+ is quite stable in a variety of electrolytes, including nitrate salts. To date, the kinetics of the Eu3+/Eu2+ reduction reaction has been investigated only by polarography and chronopotentiometry, most often in perchlorate medium. In this study, the kinetics of the Eu3+/Eu2+ couple were analysed in nitrate, chloride and perchlorate media by cyclic voltammetry and linear sweep voltammetry with a rotating disk electrode (RDE). The diffusion coefficient, charge transfer coefficient and rate constants were calculated based on the Levich and Koutecký-Levich analysis and Tafel plot to gain insight in the electrochemical process and the influence of the electrolyte type on it. Given the pre-existing characterization of this redox couple in perchlorate medium, it is selected as a reference system to both compare the kinetic parameters and to validate whether the methodology can be applied to other electrolytes. The Eu3+ reduction reaction was found to be quasi-reversible in these electrolytes and the calculated kinetic parameters are in line with the previously reported values.

From the Modeling of an Electrochemical YSZ-Based Gas Sensor Used in Electrolysis Mode.

Electrochemical sensors have been used for many decades. However, the modeling of such sensors used in electrolysis mode is poorly documented, especially in the case of multiple gases' parallel actions. These are of great interest since they constitute the first brick to bring information on the natures and concentrations of gaseous mixture compositions, thanks to gray box modeling of sensor arrays, for example. Based on Butler-Volmer's equations, a model assuming parallel reactions at gold cathode has been introduced in this article and confronted with experimental results. The establishment of the model is based on the extraction of three variables: the charge transfer coefficient "α", the reaction order γ, and the reaction constant rate k0. Tests performed without pollutants and with different concentrations of oxygen could be nicely fitted using the model. The influence of the polarization current on the three variables of the model has been evaluated, showing a clear influence on the constant rate and the reaction order. Moreover, increasing the polarization current enabled us to obtain selectivity for oxidant gases. Similarly, the effect of the oxygen concentration was evaluated. Results showed that, in this case, the charge transfer coefficients "α" obtained for oxidant gases are quite different from the ones obtained in the polarization current varying conditions. Therefore, the model will be interesting in situations where polarization current and oxygen content are not varied together. Variation of polarization current can be quite interesting to obtain increased information for multivariate analysis purposes in constant oxygen content situations. Additionally, other parameters have to be considered for applications in which the oxygen content is bound to change, such as exhaust gases or combustion.