Cyclic voltammetry heterogeneous kinetics

Cyclic voltammetry (adsorption, monolayers) Potentiodynamic polarisation (passivation, activation) Cathodic reduction (thickness) Frequency response analysis (electrical properties, heterogeneity) Chronopotentiometry (kinetics)... 

Cyclic voltammetry is the most widely used technique for acquiring qualitative information about electrochemical reactions. The power of cyclic voltammetry results from its ability to rapidly provide considerable information on the thermodynamics of redox processes, on the kinetics of heterogeneous electron-transfer reactions, and on coupled chemical reactions or adsorption processes. Cyclic voltammetry is often the first experiment performed in an electroanalytical study. In particular, it offers a rapid location of redox potentials of the electroactive species, and convenient evaluation of the effect of media upon the redox process. 

Early studies of ET dynamics at externally biased interfaces were based on conventional cyclic voltammetry employing four-electrode potentiostats [62,67 70,79]. The formal pseudo-first-order electron-transfer rate constants [ket(cms )] were measured on the basis of the Nicholson method [99] and convolution potential sweep voltammetry [79,100] in the presence of an excess of one of the reactant species. The constant composition approximation allows expression of the ET rate constant with the same units as in heterogeneous reaction on solid electrodes. However, any comparison with the expression described in Section II.B requires the transformation to bimolecular units, i.e., M cms . Values of of the order of 1-2 x lO cms (0.05 to O.IM cms ) were reported for Fe(CN)g in the aqueous phase and the redox species Lu(PC)2, Sn(PC)2, TCNQ, and RuTPP(Py)2 in DCE [62,70]. Despite the fact that large potential perturbations across the interface introduce interferences in kinetic analysis [101], these early estimations allowed some preliminary comparisons to established ET models in heterogeneous media. 

Cyclic voltammetry is generally considered to be of limited use in ultratrace electrochemical analysis. This is because the high double layercharging currents observed at a macroelectrode make the signal-to-back-ground ratio low. The voltammograms in Eig. 9B clearly show that at the NEEs, cyclic voltammetry can be a very powerful electroanalytical technique. There is, however, a caveat. Because the NEEs are more sensitive to electron transfer kinetics, the enhancement in detection limit that is, in principle, possible could be lost for couples with low values of the heterogeneous rate constant. This is because one effect of slow electron transfer kinetics at the NEE is to lower the measured Faradaic currents (e.g.. Fig. 8). 

Equation (25) is general in that it does not depend on the electrochemical method employed to obtain the i-E data. Moreover, unlike conventional electrochemical methods such as cyclic or linear scan voltammetry, all of the experimental i-E data are used in kinetic analysis (as opposed to using limited information such as the peak potentials and half-widths when using cyclic voltammetry). Finally, and of particular importance, the convolution analysis has the great advantage that the heterogeneous ET kinetics can be analyzed without the need of defining a priori the ET rate law. By contrast, in conventional voltammetric analyses, a specific ET rate law (as a rule, the Butler-Volmer rate law) must be used to extract the relevant kinetic information. 

Variation of cyclic voltammetry peak potential separation with the heterogeneous kinetic parameter i//... 

High-speed cyclic voltammetry in the 105 V/s range has recently been applied to kinetic investigations of extremely rapid heterogeneous reactions and... 

Electron transfer properties of polyhalogenated biphenyls were investigated by cyclic voltammetry. The primary reduction peak of 4,4 -dichlorobiphenyl, involving replacement of halide with hydrogen in an irreversible ECE- type reaction, are under kinetic control of the initial ET step. Electrochemical transfer coefficients, standard potentials and standard heterogeneous rate constants were also estimated from the voltammetric data230. 

Eirrev diagnostics in cyclic voltammetry — For electrochemical systems with kinetic constraints in the heterogeneous electron transfer reaction (- irreversibility)... 

The time domain on a window accessed by a given experiment or technique, e.g., femtosecond, picosecond, microsecond, millisecond. The time scale (or domain) is often characterized by a set of physical parameters associated with a given experiment or technique, e.g., r2 ]/1) (for - ultramicroelectrode experiments) - thus if the electrode radius is 10-7 cm and the - diffusion coefficient D = 1 x 10-5 cm2/s-1 the time scale would be 10 9s. Closely related to the operative kinetic term, e.g., the time domain that must be accessed to measure a first-order -> rate constant k (s-1) will be l//ci the time domain that must be accessed to measure a given heterogeneous rate constant, k willbe /)/k2. In - cyclic voltammetry this time domain will be achieved when RT/F v = D/k2 with an ultramicroelectrode this time domain will be achieved (in a steady-state measurement when r /D = D/k2 or ro = D/k at a microelectrode [i-ii]. 

The development of ultramicroelectrodes with characteristic physical dimensions below 25 pm has allowed the implementation of faster transients in recent years, as discussed in Section 2.4. For CA and DPSC this means that a smaller step time x can be employed, while there is no advantage to a larger t. Rather, steady-state currents are attained here, owing to the contribution from spherical diffusion for the small electrodes. However, by combination of the use of ultramicroelectrodes and microelectrodes, the useful time window of the techniques is widened considerably. Compared to scanning techniques such as linear sweep voltammetry and cyclic voltammetry, described in the following, the step techniques have the advantage that the responses are independent of heterogeneous kinetics if the potential is properly adjusted. The result is that fewer parameters need to be adjusted for the determination of rate constants. 

DO Wipf, EW Kristensen, MR Deakin, RM Wightman. Fast-scan cyclic voltammetry as a method to measure rapid heterogeneous electron-transfer kinetics. Anal Chem 60 306-310, 1988. 

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