Semi-infinite Diffusion Conditions Research Articles

Revisiting Thin-Layer Electrochemistry in a Chip-Type Cell for the Study of Electro-organic Reactions.

It is important but challenging to elucidate the electrochemical reaction mechanisms of organic compounds using electroanalytical methods. Particularly, a rapid and straightforward method that provides information on reaction intermediates or other key electrochemical parameters may be useful. In this work, we exploited the advantages of classic thin-layer electrochemistry to develop a thin-layer electroanalysis microchip (TEAM). The TEAM provided better-resolved voltammetric peaks than under semi-infinite diffusion conditions owing to its small height. Importantly, rapid and accurate determination of the number of electrons transferred, n, was enabled by mechanically confining the microliter-scale volume analyte at the electrode, while securing ionic conduction using polyelectrolyte gels. The performance of the TEAM was validated using voltammetry and coulometry of standard redox couples. Utilizing the TEAM, a (spectro)electrochemical analysis of FM 1-43, an organic dye widely used in neuroscience, was successfully performed. Moreover, the TEAM was applied to study the electrochemical oxidation mechanism of pivanilides and alkyltrifluoroborate salts with different substituents and solvents. This work suggests that TEAM is a promising tool to provide invaluable mechanistic information and promote the rational design of electrosynthetic strategies.

Kinetics of Redox Processes in the Polymer Films of Ni(II) Salen Type Complexes

The voltammetry and chronocoulometry methods were used in the research of kinetics of redox processes occurring on electrodes modified with conductive polymers, derived from the following complexes: (±)-trans- N,N’-bis(salicylidene)-1,2-cyclohexanediaminenickel(II) (poly[Ni(salcn)]), ((±)-trans-N,N’-bis(3,-methylsalicylidene)-1,2-cyclohexanediaminenickel(II) (poly[Ni(salcn(Me))]) and ((±)-trans-N,N’-bis(3,-tert-butylsalicylidene)-1,2-cyclohexanediaminenickel(II) (poly[Ni(salcn(Bu))]). Spectroelectrochemical method and chronopotentiometry were used to determine the mechanisms of redox processes. In poly[Ni(salcn)] and poly[Ni(salcn(Bu))] the ligand is oxidized, whereas in poly[Ni(salcn(Me))] both the ligand and the central ion are oxidized. The processes take place according to the EE mechanism. The poly[Ni(salcn)] and poly[Ni(salcn(Me))] films of thickness obtained as a result of 1–30 electropolymerization cycles, as well as poly[Ni(salcn(Bu))] of thickness obtained as a result of up to 150 cycles were studied. The investigated films of each complex are limited by surface processes, diffusion and surface-diffusion processes, depending on the film thickness. For the films that generate surface-diffusion processes, the boundaries of such processes have been determined. For the diffusion processes, the boundaries of semi-infinite diffusion regime and finite diffusion regime have been defined. On this basis, the cD1/2 value for anode and cathode processes providing semi-infinite diffusion conditions has been determined as a criterion for selection of the optimal conditions for modification of the electrodes.

Lithium transport processes in electrodes on the basis of Li3V2(PO4)3 by constant current chronopotentiometry, cyclic voltammetry and pulse chronoamperometry

Lithium transport in electrode material on the basis of lithium vanadium phosphate (Li3V2(PO4)3) with the NASICON structure was studied by the constant current chronopotentiometry (also called as galvanostatic charge-discharge technique), cyclic voltammetry (CV) and pulse chronoamperometry (PITT). The shape of the cyclic voltammograms of the electrode under study at various potential scan rates and galvanostatic charge-discharge curves at various currents as well as the character of their change during cycling were established. PITT current transients of the Li3V2(PO4)3 electrode had a characteristic and non-trivial shape, splitting into short, middle, and long-time fragments. The limiting current obtained by means of the zero-moment extrapolation of the short-time j,t fragment does not depend on the lithium concentration in the intercalate, which pointed to its relation to the solid electrolyte interphase (SEI), whereas the shape of the current transients over the whole temporal scale was strongly influenced by the lithium content in the electrode. By integration of current for each PITT step and subsequent summation of charges the intercalation isotherm of the Li3V2(PO4)3 electrode was determined. The differential form of the intercalation isotherm is in agreement with the cyclic voltammogram shape at low potential scan rates (≤ 0.05 mV s−1). Within the whole j,t-curves of the Li3V2(PO4)3 electrode the short-time j,t fragments can be located where semi-infinite diffusion conditions are preserved so that the mathematical treatment in the framework of such diffusion type approximation with surface control is justified. The values of the lithium diffusion coefficients (D) were found and the dependence of D on the electrode potential was demonstrated. The lithium diffusion coefficient ranges found by CV and PITT overlap and are within the limits of 10−15–10−12 cm2 s−1.

Digital Simulation of Cyclic Chronopotentiometry and Cyclic Reciprocal Derivative Chronopotentiometry for Linear Adsorption Systems

Digital simulations were applied to generate potential-time response curves for cyclic chronopotentiometry under a constant current in order to gain insight into a reversible charge transfer process at a planar electrode under semi-infinite diffusion conditions. These simulations were carried out for two-species systems in which either both or just one component undergoes adsorption, and this process can be accurately described using the Henry isotherm. From the chronopotentiometric curves obtained, and following their numerical differentiation, the corresponding cyclic reciprocal derivative chronopotentiometric curves were generated and compared with the previously reported analytical solutions. The results obtained according to the proposed simulation scheme were shown to be in perfect agreement with the analytical solutions. It was also demonstrated that digital simulations are a powerful and versatile tool for studying and modeling two-species adsorption systems.

EChem++ – An object-oriented problem solving environment for electrochemistry: Part 4. Adaptive multilevel finite elements applied to electrochemical models Algorithm and benchmark calculations

We present an approach for the modeling and simulation of electrochemical processes based on a general mathematical formulation, for which we expect future extensibility. For the numerical solution we propose the Rothe method with adaptive time integration performed by Rosenbrock schemes. For spatial integration we choose an adaptive finite element algorithm, controlled by a hierarchical a posteriori error estimator. The algorithm is designed to simulate models of potential-controlled transient experiments in 1D cell geometries and diffusional transport. It is tested for cyclic voltammetry with a fast electron transfer under semi-infinite diffusion conditions. Results of selected Rosenbrock methods are compared. We discuss the software design of the algorithm, as well as its coupling to the electrochemical compiler Ecco within an object-oriented framework. The software developed within this study is part of EChem++ and can be obtained freely as open-source code.

Fundamental features of metal ion determination by stripping chronopotentiometry

A study of the features of stripping chronopotentiometry (SCP) as a function of the stripping current magnitude is presented. Transformation of the E versus t traces into d t/d E versus E curves allows the charging current to be eliminated from the analytical signal (the transition time, τ). Manipulation of the applied stripping current allows the stripping time regime to be varied from conditions of semi-infinite linear diffusion ( Iτ 1/2 constant; τ∝ c 2) to conditions approaching complete depletion ( Iτ constant; τ∝ c). The sensitivity and resolution of SCP are greatest under conditions of semi-infinite diffusion, and are comparable with those of the more widely exploited DP-SV technique

Use of binary metal deposits on a cylindrical carbon-fiber microelectrode in the voltammetric determination of metal ions

The effect of binary metal deposits on a cylindrical carbon-fiber microelectrode on the determination of metals by direct and stripping voltammetry was studied. The electrolytic deposition of a binary system of copper and thallium, cadmium, lead, or mercury on the electrode in an alkaline solution resulted in the disappearance of the electroreduction peak of dissolved oxygen in the potential range from -0.8 to -1.4 V and in a decrease in the background current. Under the conditions of limited diffusion, the peak currents of Ni(II), Co(II), and Zn(II) in differential pulse voltammograms were 3–7 times higher than those calculated for a reversible electrode process under the conditions of semi-infinite diffusion. Because of this, the determination limit for metal ions in direct voltammetry was lowered to 1 X 10-6 M. With a binary copper-thallium system, the peak current of zinc(II) reduction can be be detected in the presence of 5000-fold molar amounts of copper(II). The deposition of binary copper-lead and copper-thallium systems under the conditions of limited diffusion reduced the effect of negative interaction between the components of these systems and made possible the determination of lead(II) and thallium(I) by stripping voltammetry using additional peaks.

Effects of different anions on the electrochemical behaviour of In

The presence of In in aluminium promotes its activation, making it suitable for use as a sacrificial anode. The participation of Cl − ions in the activation effect that In exerts on Al is still a matter of discussion. Therefore, the electrochemical behaviour of In in aqueous and aprotic media containing different anions was studied. The potentiodynamic technique was used in the potential region where it is known activation of Al–In alloys occurs. Metal oxidation in the neighbourhood of the pzc in the presence of Cl −, SO 4 2− and ClO 4 − ions was observed. The anodic charges involved in aprotic media were greater than in aqueous media where hydrogen evolution reaction (HER) and a metal interaction with OH − ions can take place. These results were explained in terms of specific anion adsorption inducing oxidation of surface metal atoms and leading to the formation of a salt film through which anions migrate. Aspects of salt film formation were analysed using voltammetric equations for semiinfinite diffusion conditions. Diffusion coefficient values of an order of 10 −11 cm 2 s −1 were obtained, their variation with the potential sweep rate being interpreted as a film aging process. The findings presented in this work demonstrate that the effect exerted by In in aluminium alloys is related to the specific Cl − ions adsorption, at a potential more electronegative than that at which Al activates, avoiding its repassivation.

Airtight in Situ Thin-Layer Reflection−Absorption FT-IR Microspectroelectrochemical Cell for the Study of Nonaqueous Systems

An airtight thin-layer reflection FT-IR microspectroelectrochemical cell suitable for the study of both aqueous and nonaqueous electrochemical systems is described. Due to its design, the cell can be easily assembled using disk microelectrodes. This variable-thickness cell can be used for the voltammetric study of both aqueous and nonaqueous systems under semi-infinite diffusion and thin-layer conditions. Dichloromethane solutions of ferrocene were used to test the in situ FT-IR microspectroelectrochemical performance of the cell. The cyclic voltammetric behavior of an oxygen- and water-free CH(2)Cl(2) solution of Ru(3)(CO)(12), under semi-infinite diffusion conditions, provides evidence of the airtightness of this cell.

Differential linear sweep and differential cyclic voltabsorptometry in a long-path-length spectroelectrochemical cell: Eqr, Eir and ErCi′

The theory of differential linear sweep voltabsorptometry (DLSVA) and differential cyclic voltabsorptometry (DCVA) in long-path-length spectroelectrochemistry is presented for irreversible (Eir), quasi-reversible (Eqr) and ErCi′ mechanisms under conditions of semi-infinite diffusion. It is concluded that this method may be suitable for estimating electrochemical parameters because it is more sensitive than the approach using an optically transparent electrode. All the theoretical results re experimentally verified. The standard heterogeneous rate constant k s and the transfer coefficient α of two electrochemical quasi-reversible systems (methylene blue in 1.0 M HCl and indigo carmine in 0.1 M HCl) were estimated using the simplex method and good fits between theory and experiment were obtained. A system comprising K 4Fe(CN) 6 + ascorbic acid in 0.5 M HNO 3 + 0.5 M glycine buffer was utilized to test the theory for the ErCi′ mechanism. The rate constant of the pseudo-first-order catalytic reaction was also estimated by simplex fitting. The estimated results are in agreement with experimental values.

An electrochemical cell for simultaneous electrochemical and spectroelectrochemical measurements under semi-infinite diffusion conditions and thin-layer conditions

A combination cell to accomplish simultaneous electrochemical and spectroelectrochemical measurements under both thin-layer and semi-infinite diffusion conditions is described and characterized. Fibre optics and a reflective electrode are used to couple the cell to the spectrophotometer. This allows different electrode materials to be used. Moreover, the cell is thermostatically controlled and equipped with a magnetic stirrer, and can be used for temperatures down to −40°C. The application of electrochemical pulse and sweep techniques are demonstrated. Dynamic spectroelectrochemical techniques such as linear sweep and cyclic voltabsorptometry (LSVA and CVA) as well as the corresponding derivative voltabsorptometric techniques derivative linear voltabsorptometry (DLSVA) and derivative cyclic voltabsorptometry (DCVA) are also applicable under thin-layer conditions. DLSVA and DCVA are the optical analogues of linear sweep and cyclic voltammetry. No epoxy cement or other sealing compounds are required and the solution comes in contact with only quartz, Teflon and the electrodes. Under aprotic conditions the cell response is in accordance with theory down to a solution thickness of 15 μm, where rapid exhaustive electrolysis intrinsic to thin-layer electrochemistry can be achieved in less than 1 s. The electrochemical and spectroelectrochemical characterization of the cell demonstrated that this design is very well suited to different electrochemical pulse and sweep techniques with simultaneous spectroscopic characterization of reaction products under finite and semi-infinite diffusion conditions in organic solvents. This offers the opportunity for cross-correlations of the electrochemical and spectroscopic information, which should lead to more reliable results. The adjustment of different thin-layer thicknesses is highly reproducible and the exchange of the solution inside the thin-layer cavity with the bulk solution after each thin-layer experiment can be easily performed. The electrochemical behaviour of the cell is in accordance with theory for cyclovoltammetric measurements under both bulk and thin-layer conditions. Derivative voltabsorptometric techniques are applicable and the response of the cell is in accordance with theory, particularly under finite diffusion conditions. The use of a bifurcated optical fibre bundle allows a more flexible experimental arrangement, and the application of a triple split bundle for the investigation of light-sensitive electron-transfer compounds [34,35] is in progress. The third end of the optical fibre bundle will be used to apply additional selective irradiation to convert electron-transfer-active photochromic compounds inside the thin-layer cavity depending on the redox state. An additional aspect of the current investigations is the application of the present cell for electrogenerated chemiluminescence (ECL) [36,37]. The highly reflecting electrode and the integrated stirrer are advantageous for this type of measurement. Finally, further work is in progress to utilize the integrated temperature control of the cell for spectroelectrochemistry at low temperatures, particularly with more unusual solvents like liquid sulphur dioxide [38] and liquid dimethylamine [39].

Cyclic and steady-state voltammetric studies of the mechanism and kinetics of some solute inorganic redox-species reactions at a gold electrode covered with poly- o-phenylenediamine

Application of both cyclic voltammetry and rotating disc electrode technique to the study of the mechanism and kinetics of dissolved redox-species reactions (such as Fe 2+/3+, Fe(CN) 4−/3− 6 and ce 3+/4+) at a Au electrode coated with poly- o-phenylenediamine (PPD) are discussed. The results show that the half-wave potential values for the reduction of these species at polymer modified electrode are distributed near the standard (formal) potential of PPD. In this case the reactions occur at the polymer/solution interface. One extra wave is observed for negatively charged redox-species due to occurrence of the reaction at the metal/polymer interface under semi-infinite diffusion conditions. The difference between the mechanism of electrochemical reactions at electrodes coated with electronically conducting and conventional redox-polymer films is connected with the difference of the equilibrium potential distribution in these two systems.

Chalmers J. Lyons (1874–1935)

The presence of In in aluminium promotes its activation, making it suitable for use as a sacrificial anode. The participation of Cl− ions in the activation effect that In exerts on Al is still a matter of discussion. Therefore, the electrochemical behaviour of In in aqueous and aprotic media containing different anions was studied. The potentiodynamic technique was used in the potential region where it is known activation of Al–In alloys occurs. Metal oxidation in the neighbourhood of the pzc in the presence of Cl−, SO42− and ClO4− ions was observed. The anodic charges involved in aprotic media were greater than in aqueous media where hydrogen evolution reaction (HER) and a metal interaction with OH− ions can take place. These results were explained in terms of specific anion adsorption inducing oxidation of surface metal atoms and leading to the formation of a salt film through which anions migrate. Aspects of salt film formation were analysed using voltammetric equations for semiinfinite diffusion conditions. Diffusion coefficient values of an order of 10−11 cm2 s−1 were obtained, their variation with the potential sweep rate being interpreted as a film aging process. The findings presented in this work demonstrate that the effect exerted by In in aluminium alloys is related to the specific Cl− ions adsorption, at a potential more electronegative than that at which Al activates, avoiding its repassivation.