Diffusion Coefficient Of Electroactive Species Research Articles

Electrochemical Measurement of Interfacial Distribution and Diffusion Coefficients of Electroactive Species for Ion-Exchange Membranes: Application to Br2/Br- Redox Couple.

A novel method has been proposed for rapid determination of principal transmembrane transport parameters for solute electroactive co-ions/molecules, in relation to the crossover problem in power sources. It is based on direct measurements of current for the electrode, separated from solution by an ion-exchange membrane, under voltammetric and chronoamperometric regimes. An electroactive reagent is initially distributed within the membrane/solution space under equilibrium. Then, potential change induces its transformation into the product at the electrode under the diffusion-limited regime. For the chronoamperometric experiment, the electrode potential steps backward after the current stabilization, thus inducing an opposite redox transformation. Novel analytical solutions for nonstationary concentrations and current have been derived for such two-stage regime. The comparison of theoretical predictions with experimental data for the Br2/Br- redox couple (where only Br- is initially present) has provided the diffusion coefficients of the Br- and Br2 species inside the membrane, D(Br-) = (2.98 ± 0.27) 10-6 cm2/s and D(Br2) = (1.10 ± 0.07) 10-6 cm2/s, and the distribution coefficient of the Br- species at the membrane/solution boundary, K(Br-) = 0.190 ± 0.005, for various HBr additions (0.125-0.75 M) to aqueous 2 M H2SO4 solution. This possibility to determine transport characteristics of two electroactive species, the initial solute component and its redox product, within a single experiment, represents a unique feature of this study.

In Situ Electrochemical Impedance Spectroscopic Method for Determination of Surface Roughness and Morphological Convexity.

A novel in situ electrochemical impedance spectroscopy (EIS) method is developed for the determination of RMS roughness (h), electroactive roughness factor (Rc*), and morphological convexity (H̅*) of the electrode surface. Our method uses the angular frequency of maximum phase (ωM) in anomalous Warburg impedance to extract in situ RMS roughness (h). The compact electric double layer (C-EDL) formation frequency (ωH) is used to extract the electroactive roughness factor and morphological convexity. The theory unravels the inverse square root dependence of h on ωM through an elegant equation, , where D is the diffusion coefficient of electroactive species. Similarly, the equation for the electroactive roughness factor is Rc* = ωH0/ωH and ωH0 is the smooth electrode C-EDL formation frequency. These equations are validated for the nanoparticles deposited and mechanically roughened Pt electrodes. Finally, this in situ method is applicable for both low and high roughness electrodes which transcend the limitations of contemporary methods.

Towards Understanding the Cathode Process Mechanism and Kinetics in Molten LiF–AlF3 during the Treatment of Spent Pt/Al2O3 Catalysts

Electrochemical decomposition of spent catalyst dissolved in molten salts is a promising approach for the extraction of precious metals from them. This article reports the results of the study of aluminum electrowinning from the xLiF–(1-x)AlF3 melt (x = 0.64; 0.85) containing 0–5 wt.% of spent petroleum Pt/γ-Al2O3 catalyst on a tungsten electrode at 740–800 °C through cyclic voltammetry and chronoamperometry. The results evidence that the aluminum reduction in the LiF–AlF3 melts is a diffusion-controlled two-step process. Both one-electron and two-electron steps occur simultaneously at close (or same) potentials, which affect the cyclic voltammograms. The diffusion coefficients of electroactive species for the one-electron process were (2.20–6.50)∙10−6 cm2·s–1, and for the two-electron process, they were (0.15–2.20)−6 cm2·s−1. The numbers of electrons found from the chronoamperometry data were in the range from 1.06 to 1.90, indicating the variations of the partial current densities of the one- and two-electron processes. The 64LiF–36AlF3 melt with about 2.5 wt.% of the spent catalysts seems a better electrolyte for the catalyst treatment in terms of cathodic process and alumina solubility, and the range of temperatures from 780 to 800 °C is applicable. The mechanism of aluminum reduction from the studied melts seems complicated and deserves further study to find the optimal process parameters for aluminum reduction during the spent catalyst treatment and the primary metal production as well.

Single Potential Step Chronoamperometry for EC’ Reaction at Rough Electrodes: Theory and Experiment

The theory is developed for the chronoamperometry of first order homogeneous catalytic chemical reaction coupled with electron transfer ($\rm{EC^{\prime}}$) on a rough electrode. We obtained a solution for the diffusion problem on an arbitrary and random topography of the electrode to generalize Danckwerts' equation. Derived equation of random electrode roughness is used for developing theory of (statistically isotropic) self-affine fractal on limited length scales. The characteristics fractal morphological parameters, viz. $D_{H}$, $\ell$ and $\ell_{\tau}$, are varied for detailed analysis of current transient. The current transient shows anomalously enhanced response for a rough electrode. Our theoretical results explain both transient and steady-state behavior of the current response under the influence of roughness. Steady-state current response depends on the rate constant, the diffusion coefficient of electroactive species and width of roughness. Finally, the theory is compared and validated with experimental data of ferrocyanide and ascorbic acid coupled system on rough Pt electrodes.

Voltammetric Characterization of Grafted Polymer Modified with ZnO Nanoparticles on Glassy Carbon Electrode

In this study, a grafted polymer (GP) with ZnO nanoparticles (GP/ZnO NPs) was attached on the surface of glassy carbon electrode (GCE), in order to produce a new modified electrode (GP/ZnO NPs-GCE). The gamma irradiation method was used to grafted polystyrene (polymer) with acrylonitrile (monomer), while slow evaporation process was used to prepare the new modified electrode. The cyclic voltammetry (CV) of K4[Fe(CN)6] was used to study the electrochemical properties GP/ZnO NPs-GCE. The peak separation (ΔEpa-c) was 500 mV between the redox peaks of Fe(II)/Fe(III) in an aqueous solution of 1 M KCl and the current ratio of redox current peaks (Ipa/Ipc) was ≈ 1 for the modified electrode. This indicated that the modified electrode has s good reversibility and conductivity, wherefore; it was applied in the voltammetric filed. It was found that the modified electrode GP/ZnO NPs-GCE have a reasonable solubility and stability at various pH medium. Additionally, the sensitivity of the electrochemical analysis by cyclic voltammetric (CV) method is extensively subjected to the pH medium and the scan rate (SR). A couple of redox current peaks of K4[Fe(CN)6] in KCl solution was observed with a reversible process: Fe3+/Fe2+. Finally a good diffusion coefficient of electroactive species (D) for the new modified electrode was found in this study by chronoamperometry method using Cottrell equation.

In Ukrainian

A review of recent literature data on appearance of spontaneous oscillations in electrochemical systems of practical relevance is presented. They include electrocatalytic reactions of electrooxidation of hydrogen and small organic molecules on noble metals, oscillatory processes during preparation of different nanostructured materials such as nanotubes and multilayered structures, electrochemical systems with electrodissolution – passivation -electrodeposition of metals and alloys. Many of these systems belong to N-NDR or HN-NDR type of oscillator. They are investigated by different experimental techniques including electrochemical impedance spectroscopy, differential electrochemical mass spectroscopy, surface-enhanced infrared absorption spectroscopy, electrochemical quartz crystal microbalance/nanobalance, Auger electron analysis etc. It has been shown that electrochemical oscillations and more complex spatio-temporal surface structures in these systems are very sensitive to variation of experimental parameters such as applied potential, current density, solution concentration, structure and composition of electrode/electrolyte interface etc. The influence of these parameters on non-linear behavior of a model N-NDR electrochemical system was presented based on the theory of electrochemical impedance spectroscopy. An electrochemical reaction in the chosen model is related to the potential-dependent adsorption/desorption of electroactive particles on a planar, cylindrical or spherical interface and a preceding chemical reaction in the Nernst diffusion layer under potentiostatic conditions. The obtained results indicate that the range of Hopf instability giving rise to spontaneous oscillations in the system can be regulated by variation of electrode form and radius, thickness of the Nernst diffusion layer, diffusion coefficient of electroactive species, ohmic loses, bulk concentration of electroactive species. The role of mass transfer function in appearance of dynamic instabilities in the non-equilibrium system was determined.

Experimental corroboration of the theory of chronoamperometry at high roughness electrode for reversible charge transfer

An asymptotic theory for the single potential step chronoamperometry is validated for ferrocene in 1-Butyl-3-methylimidazolium tetrafluoroborate ([bmim] [BF4]) room temperature ionic liquid (RTIL) at nano-particles deposited mesoporous gold wires. The elegance of asymptotic equation arises due to minimal need of (only two) morphological surface characteristics, viz. roughness factor (R*) and width of roughness of mesoporous layer (h). It also accounts for the contribution from unequal diffusion coefficient of electro-active species and uncompensated solution resistance. Theory is also corrected for electric double layer contribution. Mesoporous electrodes are created (with R* >10) over mechanically roughened gold wires by depositing electrochemically synthesized gold nano-particles. Morphological parameters, R* and h, are determined using cyclic voltammetry (CV) and scanning electron microscopy (SEM), viz. CV and SEM method (see S.Dhillon and R.Kant, 2013). Asymptotic equation shows quantitative agreement with the experimental data over a broad time scale (between 0.2ms and 100s). Finally, this asymptotic equation can be used for the extraction of statistical roughness along with usual electrochemical quantities.

Influence of viscosity on chronoamperometry of reversible redox system on rough and nanoparticles deposited Pt electrode: Aqueous/glycerol and RTIL medium

Recently developed theories allow us to predict expected electrochemical response from quantitative surface texture information obtained from various microscopies. We generalized power spectrum based chronoamperometry theory involving species with unequal diffusion coefficient of electroactive species in presence of uncompensated solution resistance. Chronoamperometric response in two viscous media, like ionic liquid and glycerol, is studied and it shows strong influence of electrode roughness up to longer time scales. Measurements were carried out for two reversible charge transfer systems: (i) solution of Fe(CN)63− and Fe(CN)64− in 3M NaNO3 at various compositions of glycerol and (ii) ferrocene in [bmim][BF4] RTIL. Viscous medium induces suppression in growth of diffusion length which allows us to see enhanced and prolonged roughness effect in chronoamperometry. It is found that the enhanced viscosity: (i) induces delay, through enhanced ohmic contribution, in the onset of anomalous regime and (ii) causes dilation in roughness influenced time regime (the enlargement of roughness influenced region). These two effects allow determination of microscopic area from short time chronoamperometric data which is not possible in non-viscous aqueous medium. The corrections in the chronoamperometric current for the electric double layer and the random convective flow are also accomplished.

Electroactive Ionic Liquids Based on Anions Modified with 1,4-Dimethoxybenzene As Redox Shuttle for Overcharge of Lithium-Ion Batteries

Lithium-ion technology currently answers the need for energy storage in many electronic devices due to their good cycle life and high energy density. Larger lithium-ion batteries (LIB) are also developed to meet the requirements for the transportation sector, but the performance and security still need to be improved. For instance, secondary reactions can occur during an overcharge which initiate when the positive electrode is over-oxidized. The over-oxidation of the positive electrode leads to an irreversible degradation of electrode material, generates radical and other reactive species, and oxygen evolution which can cause a fast thermal runaway into the LIB leading to its possible explosion. Several measures can be taken to avoid thermal runaway of the battery when an overcharge event occur. Amongst them, the use of a redox shuttle is particularly efficient since it will prevent the electrode from reaching an unstable oxidation state above the one obtained during normal charging. Redox shuttles (R-S) operate by carrying the excess charge from the positive to the negative electrode to prevent reaching higher oxidation states. To be used efficiently, R-S must be stable in both oxidized and reduced states to maintain protection for a large number of cycles and should have an oxidation potential of at least 300 mV above that of the electrode to avoid self-discharge during normal cell operation. In addition, to achieve protection at high charging rates, it should diffuse rapidly and be solubilized at a high concentration in the electrolyte. We demonstrated recently that these conditions can be met by modifying the structure of ionic liquids with an electroactive group and that these R-S can be applied to prevent the overcharge of LiFePO4 electrodes for more than 200 cycles without affecting its charge storage properties. The modification of ionic liquids with redox moieties offers numerous possibilities to modulate the redox potential and the solubility of redox shuttles. In this study, we propose to use anion redox ionic liquids (ARILs) as R-S. We present the physicochemical and electrochemical characterizations of ARIL electrolytes such as viscosity and ionic conductivity measurements of LIB electrolyte and diffusion coefficient of electroactive species using cyclic voltammetry and chronoamperometry. Ionic liquid redox shuttles based on ferrocene and 1,4-dimethoxybenzene have been tested in LIB, using different electrode systems such as Li/LiTi2(PO4)3, and Li4Ti5O12/LiFePO4. We present charge–discharge curves with a 100% overcharge and long term overcharge curves for each R-S with the appropriate electrode at C/10. Charge-discharge curves with overcharge at different C rates were also done to obtain the Ragone plot. The importance of the structure of the ionic liquid redox shuttles on the protection and stability upon cycling will be discussed. This presentation highlights the modulated potential of 1,4-dimethoxybenzene derivatives following the anion structure of ARIL. This study demonstrates the –(SO2)N(SO2)CF3 group can play a similar role than the bulky tert-butyl group and in addition, R-S potential is increased due to the electron-withdrawing effect. Increased operating voltage of LIB requires striking a balance between a high redox potential and electrochemical stability of electrolyte containing the R-S. Figure 1

Redox Ionic Liquid and Electroactive Lithium Salt As Redox Shuttle for Overcharge of Lithium-Ion Batteries

Lithium-ion technology currently answers the need for energy storage in many electronic devices due to their good cycle life and high energy density. Larger lithium-ion batteries (LIB) are also developed to meet the requirements for the transportation sector, but the performance and security still need to be improved. For instance, secondary reactions can occur during an overcharge which initiate when the positive electrode is over-oxidized. The over-oxidation of the positive electrode leads to an irreversible degradation of electrode material, generates radical and other reactive species, and oxygen evolution which can cause a fast thermal runaway into the LIB leading to its possible explosion. Several measures can be taken to avoid thermal runaway of the battery when an overcharge event occur. Amongst them, the use of a redox shuttle is particularly efficient since it will prevent the electrode from reaching an unstable oxidation state above the one obtained during normal charging. Redox shuttles (R-S) operate by carrying the excess charge from the positive to the negative electrode to prevent reaching higher oxidation states. To be used efficiently, R-S must be stable in both oxidized and reduced states to maintain protection for a large number of cycles and should have an oxidation potential of at least 300 mV above that of the electrode to avoid self-discharge during normal cell operation. In addition, to achieve protection at high charging rates, it should diffuse rapidly and be solubilized at a high concentration in the electrolyte. We demonstrated recently that these conditions can be met by modifying the structure of ionic liquids with an electroactive group and that these R-S can be applied to prevent the overcharge of LiFePO4 electrodes for more than 200 cycles without affecting its charge storage properties. The modification of ionic liquids with redox moieties offers numerous possibilities to modulate the redox potential and the solubility of redox shuttles. In this study, we propose to use redox ionic liquids (RILs) or electroactive lithium salts (ELi) as R-S. We present the physicochemical and electrochemical characterizations of RIL or ELi electrolytes such as viscosity and ionic conductivity measurements of LIB electrolyte and diffusion coefficient of electroactive species using cyclic voltammetry. Ionic liquid redox shuttles based on ferrocene and 2,5-di-tert-butyl-1,4-dimethoxybenzene have been tested in LIB, using different positive electrodes (e.g. LiTi2(PO4)3, and LiFePO4). We present charge–discharge curves with a 100% overcharge and long term overcharge curves for each R-S with the appropriate electrode at C/10. Charge-discharge curves with overcharge at different C rates were also done to obtain the Ragone plot and the results suggest a better protection above C/4 for the anion RIL and ELi than cation RIL because of migration effect favoring the modification of anion. The importance of the structure of the ionic liquid redox shuttles on the protection and stability upon cycling will be discussed. Figure 1

Electrochemical Behaviors of Ce(III) in Molten AlCl3-NaCl under Various Contents of Fluoride

The electrochemical behavior of CeCl3 was investigated in molten AlCl3-NaCl melt with various concentrations of fluoride anions using molybdenum electrode at a temperature of 473 K. Transient electrochemical techniques, such as cyclic voltammetry and square wave voltammetry, were performed to reveal the diffusion coefficients of the electroactive species and reduction steps. Results suggest that there are two steps for Ce(III) ion reduction in molten AlCl3-NaCl: Ce(III)→Ce(II) and Ce(II)→Ce. The reduction processes shifted to Ce(III)→Ce when NaF was added in the melt in increasing amount. The molar ratio of fluoride ions and cerium (F/Ce ratio) was employed as a parameter to illustrate the influence of fluoride anions. The diffusion coefficient of electroactive species were determined in AlCl3-NaCl melt at 473 K by using square wave voltammetry.

Photo-electrochemical Studies by AC Impedance Spectroscopy on Electrodeposited n-Type PbS

Abstract Electrochemical deposition and characterization of PbS nanocrystals on Pb electrodes in alkaline solution containing thiourea are carried out by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) methods respectively. The diffusion coefficient of electroactive species (Pb+2) at the synthetic potential −0.36 V/SCE is investigated in different alkaline media. The analysis of PbS crystals using X-ray diffraction (XRD) shows that the average of crystalline size and the lattice constant parameter are approximately 39.04 and 0.5948 nm respectively. Also, the morphology of surface was studied by scanning electron microscope (SEM). This work focuses on EIS extraction of Mott–Schottky plots and calculation of several important parameters including Debye length (LD), carrier density (ND), and flat band potential (Efb) for PbS. The parameters are calculated as 3.40 × 10−8 cm, 8.65 × 1019 cm−3, and 0.035 V/NHE respectively in the dark. The comparison of these values in the dark and under light provides some insights into the charge transfer. The increase in the charge transfer resistance (Rct) in the dark with respect to under illumination and the confirmation of the presence of surface states in the n-type PbS are some of the obtained results.

Diffusion and Migration of Redox Active Molecules in Nano-Electrochemical Systems

The absence of supporting electrolyte leads to great variety in electrode responses when the charge numbers and diffusion coefficients of electroactive species vary. The variability in the voltammetric behavior of the redox species may be further increased after diminishing the size of- or the distance between the working electrodes to the nanometer range.In a system containing no added inert electrolyte, both diffusion and migration contribute to the total flux of the substance transported to the electrode. The flux for each of the three solutes (i.e. S, P, and Y- the substrate, the product, and the counterion of the substrate (if it is charged), respectively) can be represented by the Nernst-Planck equation.If the solution containing a redox species is placed between two closely-spaced plane electrodes (separated by a distance l) oriented parallel to each other, and both electrodes are independently polarized, then the mass transport will take place in the limited linear space and the steady-state conditions will be easily attainable. In such the electrochemical cell the product of the electrooxidation at one electrode is transported to- and re-electroreduced at the other electrode (communicating electrodes).The obtained analytical solutions of the transport equations [1] allowed derivation of the analytical expressions for the limiting electrostatic potential and the limiting faradaic current recorded in a thin layer dual electrode system [2]. Interestingly, the expression for the limiting electrostatic potential at the interface is identical to that derived for the semi-infinite diffusional-migrational transport to a hemispherical microelectrode in the absence of supporting electrolyte. The corresponding expression for the limiting current, iL , normalized with respect to the purely diffusional response, id L , is identical to its equivalent derived for the semi-infinite diffusional-migrational transport to a hemispherical microelectrode when either θ (DP / DS ) is equal to 1 (equal diffusion coefficients of electroactive species, any zS , zP , zS ≠ zP ) or zS is zero (uncharged substrate, any θ and zP , and zS ≠ zP ) or zP is zero (uncharged product, any θ and zS , and zP ≠ zS ). If both electroactive species bear charge a specific relation between iL / id L and θ is established for various zS , zP , zY values.The theoretical modeling revealed high sensitivity of the thin-layer electrode system response to the diffusion coefficients diversity. It is particularly pronounced for the charge reversal processes: the θ - dependence of the limiting current “blows up” at θ = 1 where the limiting conditions can not be reached and the transport is purely migrational.The theory allowed one to examine quantitatively the limitation of the derived expressions due to the application of the electroneutrality condition at any point in the system. The electroneutrality approximation in the mass transport modeling associated with the electron exchange implies that the electric field resulting from the flow of current through a solution is not big enough to induce the charge accumulation at any point in the solution. This assumption is generally valid if the electrical double layer occupies an insignificant fraction of the transport depletion layer. This may become questionable when the distance between the electrodes tends to nanometer range. The electrode system modeled in this work gave one the opportunity to test the validity of the electroneutrality condition. It was found that the departure from the electroneutrality diminishes with the increase in the distance between the electrodes. For a 100-nm separation of electrodes and comparable diffusion coefficients of the redox species, the error (expressed by the local net charge density in the solution, ρ exc ) does not exceed 1% of the initial substrate concentration (CS 0). For the electrode processes with charge sign retention (for which the limiting conditions can be reached) the error ρ exc does not exceed 10% of CS 0 down to 60-nm gap between the electrodes for the entire θ range examined (0.2 – 5.0). The calculations indicated also a strong dependence of ρ exc with respect to θ. However, the direction of this dependence is a function of the type of the electrode process.The theoretical predictions derived for ultra-thin layer dual electrode system provide quantitative characteristics of the redox cycling process affected by the migrational contribution. The results may be of great significance for the extension of applicability of many devices utilizing redox cycling, including the electrochemical scanning microscopy in low-supported media. Additionally, by decreasing the distance between the electrodes to the nanometer range it may be possible to perform electrochemical detection of individual redox-active molecules affected by migration [3].[1] W. Hyk, Z. Stojek Anal. Chem.74 (2002) 4805.[2] W. Hyk, Z. Stojek Electrochemistry Communications34 (2013) 192.[3] M.A.G. Zevenbergen, P.S. Singh, E.D. Goluch, B.L. Wolfrum, S.G. Lemay, Nano Lett. 11 (2011) 2881.

Probes Based on Silica Sol-Gels for Voltammetry in the Absence of Electrolyte in the Sample Phase

Silica sol-gels that are processed at room temperature contain residual solute. By selection of salts that have the properties of a humidistat in the sol, the resulting gel will contain a controlled amount of water, thereby making the resistance of the sol-gel independent of humidity of a contacting gas phase. Further, when the processing conditions result in a mesoporous sol-gel, diffusion coefficients of electroactive species that are encapsulated therein are in the 10-6 cm2s-1 range. The latter is illustrated by the cyclic voltammetry of hexacyanoferrate when a solid-state cell is fabricated to incorporate three electrodes in the silica matrix. This design is extended to the voltammetry of gas-phase species by extending the working electrode beyond the sol-gel so that a three-phase boundary is formed. In this manner the detection of carbon monoxide, hydrazine, and hydrogen peroxide by amperometry is achieved. This cell design was evaluated as an amperometric detector for gas chromatography. Initial experiments were in the flow injection mode with He flowed at 0.63 mL min-1 as the carrier gas and100- µL injections of H2 comprised the sample. Wirh a Pt working electrode at 0.5 V vs. a Pt quasi-reference electrode, linear response to concentration over range 1.7 x 10-6 g to 7.6 x 10-6 g was observed. Extensions to 1,2-ethandithiol, phenol, p-cresol, and thioanisole yielded detection limits of 4, 1, 3, and 70 ppmv (k = 3 criterion), respectively, when operating the working electrode at 800 mV. Application to aqueous samples in the absence of supporting electrolyte also were achieved; however, a problem is that leaching of the electrolyte from the sol-gel cell was a limitation. To address this problem, the sol-gel matrix is being modified to contain a macromolar cation (anion) of a size that is greater than the ca. 0.5 nm pore size of the silica. An example is to immobilize 3,4- ethyelenedioxythiophene (EDOT) in the sol-gel and oxidize it to the polymeric form, PEDOT. Results on the polymerization process will be shown, and the efficacy of the resulting composite as a matrix for a probe to determine various species in water in the absence of supporting electrolyte will be presented.

Pulse electrodeposition of Prussian Blue thin films

The effects of pulse electrodeposition parameters like peak current density and frequency on the electrochemical properties of Prussian Blue thin films were investigated. Electrochemical Impedance Spectroscopy, Cyclic Voltammetry and Chronoamperometry tests were carried out on Prussian Blue thin films which were pulse electrodeposited on Indium Tin Oxide coated glass substrates. The results showed that increase in the peak current densities and using higher pulsating frequencies during electrodeposition decreases the charge transfer resistance of the thin films while the diffusion coefficient of electroactive species in the films is increased as a consequence of using the same pulsating parameters. In addition, pulse electrodeposition technique does not alter deposition mechanism and morphology of the Prussian Blue thin films.

Electrochemical Behaviour Of Ferrocene/Ferricinium Redox System In Pure And CO2 Saturated [C6MIM][PF6

The electrochemical behaviour of ferrocene has been studied in green solvent combining pure ionic liquid and CO2. Experimental results were achieved for the Fc/Fc+ (ferrocene/ferricinium ion) oxidation/reduction in pure and CO2 saturated 1-hexyl-3-methylimidazolium hexafluorophosphate ([C6MIM][PF6]) over the temperature range from 298.15 K to 323.15 K. Peak shaped voltammograms were obtained and usual electrochemical parameters were calculated in order to classify electron process at the electrode. Diffusion coefficients of electroactive species were determined in the order of 10-11 m2 s-1 using the Randles–Ševcík relationship. Mass transport was also favoured by both increases in temperature and dilution of CO2 in the media.

Charge neutralization process of mobile species developed during potentiodynamic conditions. Part 2: Simulation and fit of probe beam deflection experiments

The theoretical model presented in Part 1 is employed to simulate and fit experimental probe beam deflection (PBD) data of Fe(CN) 6 3 - / Fe(CN) 6 4 - and Fe 3+/Fe 2+ couples collected under voltammetric conditions. The dependence of beam deviation on the diffusion coefficients and on the charges of involved species is described. The fit of voltammetric curves was performed to get the values for the standard potential of each system as well as diffusion coefficients of electroactive species. The data were then employed to fit a set of experimental voltadeflectometric profiles collected at different beam-electrode distances. The least-squares Simplex algorithm was employed to fit proposed functions to experimental data. The data obtained from the fitted voltadeflectometric curves were consistent with values previously reported for chronodeflectometry profiles, supporting the hypothesis that the refraction index not only should be considered a function of the concentration of the neutral salts dissolved, but also depends on the concentration gradient of each soluble species. The effects of the scan rate and the beam-electrode distance are discussed in detail. In relation to this, simple relationships between these adjustable parameters and the angle of the deflected laser beam are also provided. Thus, it is now possible to perform a quantitative analysis on the response of techniques such as PBD when the heterogeneous charge transfer of electroactive species is studied with a cyclic linear potential scan.

Charge Neutralization Process of Mobile Species at Any Distance from the Electrode/Solution Interface. 2. Concentration Gradients during Potential Pulse Experiments

The theoretical model presented in part 1 of this work is employed to simulate and fit experimental probe beam deflection (PBD) data of Fe(CN)6(3-)/Fe(CN)6(4-) and Fe3+/Fe2+ couples. Current and beam deviation dependency on time at constant potential (chronoamperometry and chronodeflectometry) is analyzed via a new treatment based on the migration and diffusion properties of all the species involved. The diffusion coefficients of electroactive species are obtained by fitting chronoamperometric curves. Those coefficients are then employed to simulate the respective chronodeflectometric profiles. The experimental data and the theoretical function are fitted by the minimum squares Simplex algorithm. The effect of working with systems in which both electroactive species are charged is discussed in detail. Specifically, the possibility of quantitative analysis of nonspecific techniques data is analyzed when a relative high concentration of supporting electrolyte is used. Such analysis widens the scope of techniques as PBD since in many cases the effect of supporting electrolyte species could be negligible as compared to the response of electroactive species. The variation of the refraction index with the concentration gradient of each soluble species is also discussed.

Determination of diffusion coefficients of electroactive species in time-of-flight experiments using a microdispenser and microelectrodes.

Two novel methods for the determination of diffusion coefficients of redox species combining the special properties of microdispensing devices and microelectrodes are presented. Both are based on the local application of tiny volumes of the redox-active species by means of a dispenser nozzle at a defined distance from the surface of a microelectrode. The microelectrode, which is inserted through the bottom into an electrochemical cell, is held at a constant potential sufficient to oxidize or reduce the electro-active species under diffusional control. The dispenser, which is filled with the electro-active species, can be positioned by means of micrometer screws over the microelectrode. After dispensing a defined number of droplets near the microelectrode surface, the current through the microelectrode is recorded, usually yielding a peak-shaped curve having a defined time delay between the shooting of the droplets and the maximum current. The time that is necessary to attain maximum current, together with the known distance between two dispensing points, can be used to determine the diffusion coefficient of the electroactive species without knowledge of any system parameters, such as concentration of the redox species, diameter of the electroactive surface or number of transferred electrons. A similar method for the determination of diffusion coefficient of redox species involves a second redox species for calibration purposes. A mixture of both species is shot close to the microelectrode surface. Due to the different formal potentials of the redox species that are used, they can be distinguished in sequential experiments by variation of the potentials that are applied to the microelectrode, and it is thus possible to determine the individual transit times of the redox species independently. The difference in the transit times, together with the known diffusion coefficient of one of the redox species, can be used to calculate the unknown diffusion coefficient of the second one.

Structural and thermodynamic aspects of niobium deposition in the system LiF-KF-K2NbF7

Abstract The complex physico-chemical analysis of the system LiF-KF-K2NbF7, based on the phase diagram, density, surface tension, and viscosity measurements, was performed. The results proved the presence of the congruently melting compound K3NbF8 even in the ternary melts. The calculated co-ordinates of the two ternary eutectic points are: e1: 22.3 mole % LiF, 9.4 mole % KF; 68.3 mole % K2NbF7; te = 649 °C; e2: 45.5 mole % LiF, 51.7 mole % KF; 2.8 mole % K2NbF7; te = 486 °C. The probable inaccuracy in the calculated ternary phase diagram is 4.7 °C. The degree of thermal dissociation of the additive compound K3NbF8 α0 = 0.55 at 1100 K, calculated from the density measurement agrees well with the value determined from the analysis of the phase diagram (α0 = 0.44) as well as from the viscosity measurement (α0 = 0.45 at 1100 K) and refers to the pronounced thermal dissociation of this compound at melting. The formation of K3NbF8 was confirmed by all of the followed physico-chemical parameters and it is the general feature of this system. The deposition of Nb from the LiF-KF-K2NbF7 melts takes place via the reversible, diffusion-controlled two-step reduction mechanism [ NbF 7 ] 2− + e − = [ NbF 6 ] 2− + F − [ NbF 6 ] 2− + 4e − = Nb (0) + 6 F − In the presence of oxygen in the melt the formation of the [NbOF5]2− or/and [NbO2F]− oxyfluorocomplexes takes place, depending on the concentration of O2− anions. Good experimental conditions for niobium deposition process can be expected when nO/nNb(V) 1 inhomogeneous niobium deposits containing niobium oxide solid solution, even a non-metal deposit could be obtained. The diffusion coefficients of electroactive species were calculated from the results of electrochemical measurements.