Cyclic voltammetry scan rate effects

Crev-Erev diagnostics in cyclic voltammetry — Figure. The effect of the scan rate on the current components of the simulated cyclic voltammograms of a CreyEvev reaction... 

The first example [15] shows how cyclic voltammetry may be used to demonstrate the rapid isomerisation of the anion radical of diethylmaleate (DEM). Fig. 6.17 shows the voltammograms for DEM and diethylfumarate (DEF), the CIS and trans isomers respecively. The reduction of DEF shows a well formed reduction peak, Ep = —1.41 V vs SCE, and a coupled anodic peak above 0.1 Vs the curve has all the properties of a reversible le process. Below this scan rate, there is evidence for a slow chemical reaction, and more detailed investigations showed this to be dimerisation. The reduction peak for DEM, below 5 Vs is much broader, Ep = —1.61V, with no anodic peak corresponding to the reverse process. There is, however, an anodic peak at —1.35 V which is the same potential required for the oxidation of DEF. Also on the second and subsequent cycles a new reduction peak is seen to grow at —1.41V. On increasing the potential scan rate above 50Vs a small anodic peak is seen where the oxidation of DEM is expected and the anodic peak at —1.35 V becomes relatively smaller (i.e. when corrected for scan rate effects). Finally, the current functions, for the main reduction peaks for DEF and DEM are almost independent of v and correspond to reasonable values for a reversible and irreversible le reduction... 

Figure 3.96 The effect of increasing time of exposure (as indicated) of a gold electrode once-modified with SSBipy to thiophenol on the cyclic voltammetry of horse heart cytochrome t (0.4mM). 20 mM sodium phosphate/0.1 M NaCI04 pH 7.0. Scan rate 20mVs l. From Hill...

In the case when the preceding chemical reaction occurs at a rate of the same order as the intervention time scale of cyclic voltammetry, the repercussions of the chemical complication on the potential of the electrode process are virtually negligible, whereas there is a significant effect on the current. In particular, it is characteristic of this mechanism that the forward current decreases with the scan rate much more than the reverse current. This implies that the current ratio ipr/ipf is always greater than 1, increasing as scan rates are increased. 

Like DPV, OSWV is very effective in solving almost overlapping processes. With respect to DPV, OSWV can use higher scan rates (from a few hundreds of mV s-1 to a few V s-1). The typical scan rate (which is given by the product SWfrequency X A.Ebase) is around 0.2 V s-1 (which is the typical scan rate of cyclic voltammetry). This allows one to compete with the eventual presence of chemical complications coupled to the electron transfers. 

SEV is an effective means of probing homogeneous chemical reactions that are coupled to electrode reactions, especially when it is extended to cyclic voltammetry as described in the next section. Considerable information can be obtained from the dependence of ip and Ep on the rate of potential scan. Figure 3.20 illustrates the behavior of ip and Ep with variation in scan rate for a reversible heterogeneous electron transfer reaction that is coupled to various types of homogeneous chemical reactions. The current function j/p is proportional to ip according to the equation... 

Cyclic stationary-electrode voltammetry, usually called cyclic voltammetry (CV), is perhaps the most effective and versatile electroanalytical technique available for the mechanistic study of redox systems [37,39,41-44]. It enables the electrode potential to be scanned rapidly in search of redox couples. Once located, a couple can then be characterized from the potentials of peaks on the cyclic voltammogram and from changes caused by variation of the scan rate. CV is often the first experiment performed in an electrochemical study. Since cyclic voltammetry is a logical extension of stationary-electrode voltammetry (SEV), some important aspects of CV were treated in the preceding section. 

Figure 4.11 Effect of SAM formation on the cyclic voltammetry of ferrocenylmethyltrimethylam-monium on a polycrystalline gold electrode the supporting electrolyte is 0.5 M H2SO4, with a scan rate of 0.1 V s-1. Curve (a) is the reversible cyclic voltammogram obtained on bare gold, while curves (b)-(d) are obtained on the same electrode with different monolayers (for details see text). The symbols represent theoretical fits to a microarray electrode model. Reprinted with permission from H.O. Finklea, D.A. Snider, J. Fedyk, E. Sabatani, Y. Gafni and I. Rubinstein, Langmuir, 9,3660 (1993). Copyright (1993) American Chemical Society...

Obviously, therefore there must be an intermediate case in which the kinetics of both the forward and reverse electron-transfer processes have to be taken account of. Such systems are described as being quasi-reversible and as would be expected, the scan rate can have a considerable effect on the nature of the cyclic voltammetry. At sufficiently slow scan rates, quasi-reversible processes appear to be fully reversible. However, as the scan rate is increased, the kinetics of the electron transfer are not fast enough to maintain (Nernstian) equilibrium. In the scan-rate region when the process is quasi-reversible, the following observations are made. 

The thermochemical cycle in Scheme 4 can be used to estimate the effect of one-electron oxidation on metal-hydride acidities. The method is analogous to one that has been extensively used to investigate organic cation radicals [10c]. Eq. 29 shows that measurements of °ox(MH) and °ox(M ) provide relative p a data for metal hydrides and their cation radicals. Absolute values for p a(MH +) are obtained if the acidities of the neutral hydrides are known. The oxidation potentials of 18-electron hydrides can be readily obtained by cyclic voltammetry. In our experience, the waves that are obtained are frequently chemically irreversible, even at rather high scan rates. Consequently, the oxidation peak potentials will be kinetically shifted and represent minimum values for the true °ox(MH) data, the estimates for p a(MH +) represent maximum values, and calculated Ap a are minimum values. 

For some of the less complex ec processes, cyclic voltammetry (i-E) waves have been used to show, qualitatively, the effects of the follow-up reaction (reaction 2). The model first developed for this scheme consider only pseudo first-order conditions where the heterogeneous process was either reversible (6,7) or irreversible (7,8). Accordingly, when the homogeneous rate constant was very large, the i-E scan appeared similar to a conventional polargraphic wave without any peaks. The complications of Nicholson and Shain (7) provide cyclic i-E data that can be compared directly to experimental results. While these data can be used to fit the case represented by equations 1 and 2 (n - 1), more complex schemes... 

Similar effects can be observed at single layer interfaces when they are exposed to appropriate redox couples in an external solution (40, 45, 46). With a solution couple the possibility exists for electron transfer either by diffusion through the film or by electron hopping from the external solution to the electrode using the conductivity properties of the film. In general, both pathways exist and, as shown by a series of cyclic voltammetry experiments, they can coexist. The relative importance of the two pathways is controllable by making variations in the film thickness or in the potential-time electrochemical scan rate (45). 

Cyclic-voltammetry measurements of quasireversible systems yield more easily to interpretation. Both the cathodic and anodic peak potentials shift as a function of scan rate, resulting in an increasing AE as v increases. This dependence of AEp on electron-transfer rate is used to measure the k value of the system, but AE also increases monotonically with v from the effects of uncompensated resistance, and the two effects are difficult to separate. The absence of appreciable resistance effects must be insured when making these measurements. Many reported rate constants are erroneous because of improper attention to this problem ... 

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