Cyclic voltammetry experimental response

Useful experimental parameters in cyclic voltammetry are (i) the value of the separation of the potentials at which the anodic and cathodic peak currents occur, A = Pia — PiC, and (ii) the half wave potential, 1/2, the potential mid-way between the peak potentials. A value of AE of c. 0.057 V at 25°C is diagnostic of a Nernstian response, such as that shown in Figure 2.87. More generally, if n electrons are transferred from R, then the separation will be 0.057/n V. It should be noted that the expected value for AE of 0.57/nV has no relationship to the usual Nernstian slope of RT/nF = 0.059/n V at 25UC. 

As mentioned in the introduction to controlled potential electrolysis (Section 2.3), there are various indirect methods to calculate the number of electrons transferred in a redox process. One method which can be rapidly carried out, but can only be used for electrochemically reversible processes (or for processes not complicated by chemical reactions), compares the cyclic voltammetric response exhibited by a species with its chronoamperometric response obtained under the same experimental conditions.23 This is based on the fact that in cyclic voltammetry the peak current is given by the Randles-Sevcik equation ... 

As is well known in the field of electrochemistry in general, electrode kinetics may be conveniently examined by cyclic voltammetry (CV) and by frequency response analysis (ac impedance). The kinetics of the various polymer electrodes considered so far in this chapter will be discussed in terms of results obtained by these two experimental techniques. 

These studies have been mainly carried out using cyclic voltammetry and frequency response analysis as experimental tools. As a typical example. Fig. 9.12 illustrates the voltammogram related to the p-doping process of a polypyrrole film electrode in the LiClQ -propylene carbonate electrolyte, i.e. the reaction already indicated by (9.16). 

Cyclic voltammetry has gained widespread usage as a probe of molecular redox properties. I have indicated how this technique is typically employed to study the mechanisms and rates of some electrode processes. It must be emphasized that adherence of the CV responses to the criteria diagnostic of a certain mechanism demonstrates consistency between theory and experiment, rather than proof of the mechanism, since the fit to one mechanism may not be unique. It is incumbent upon the experimenter to bring other possible experimental probes to bear on the question. These will often include coulometry, product identification, and spectroelectrochemistry. 

The main inconvenience of the ERDs construction is the lack of reproducibility. Due to the tiny electrode surfaces, small variations imply big changes. The sealing between the electrode surface and the insulator material is very crucial for obtaining a well-defined electrode surface and low noise. Their characterization can be achieved by different techniques [17]. Scanning electron microscopy (SEM) is suitable for UMEs but not for smaller ERDs. Information about ERD dimensions can be obtained from the experimental (by chronoamperometry or cyclic voltammetry) and theoretical response in well-defined electrochemical systems [5]. Moreover, this electrochemical characterization shows several limitations when ERDs approach the low nanometric scale [8,14,36]. 

SECM SG/TC experiments were carried out to prove that the product of the initial two-electron oxidation process diffused into the solution, where it would react homogeneously and irreversibly. For these measurements, a 10 /xm diameter Au tip UME was stationed 1 /xm above a 100 /xm diameter Au substrate electrode. With the tip held at a potential of —1.3 V versus saturated mercurous sulfate electrode (SMSE), to collect substrategenerated species by reduction, the substrate electrode was scanned through the range of potentials to effect the oxidation of borohydride. The substrate and tip electrode responses for this experiment are shown in Figure 16. The fact that a cathodic current flowed at the tip, when the substrate was at a potential where borohydride oxidation occurred, proved that the intermediate formed in the initial two-electron transfer process (presumed to be mono-borane), diffused into the solution. An upper limit of 500 s 1 was estimated for the rate constant describing the reaction of this species (with water or OH ), based on the diffusion time in the experimental configuration. This was consistent with the results of the cyclic voltammetry experiments (11). 

A great deal of effort has been spent in studying the mechanisms of complex electrode reactions. One general approach is based on steady-state current-potential curves. Theoretical responses are derived on the basis of mechanistic alternatives, then one compares predicted behavior, such as the variation of exchange current with reactant concentration, with the behavior found experimentally. A number of excellent expositions of this approach are available in the literature (8-14, 25, 26, 35). We will not delve into specific cases in this chapter, except in Problems 3.7 and 3.10. More commonly, complex behavior is elucidated by studies of transient responses, such as cyclic voltammetry at different scan rates. The experimental study of multistep reactions by such techniques is covered in Chapter 12. 

B. A. Brookes, T. J. Davies, A. C. Fisher, R. G. Evans, S. J. Wilkins, K. Yunus, J. D. Wadhawan, and R. G. Compton. Computational and experimental study of the cyclic voltammetry response of partially blocked electrodes. Part 1. Nonoverlapping, uniformly distributed blocking systems,... 

Figure 5A shows the sigmoidal-shaped responses that characterize steady-state mass transfer in slow scan-rate cyclic voltammetry. In contrast, as illustrated in Figure 5B, at short experimental timescales (high scan rates), peaked responses similar to those observed at conventional macroelectrodes are seen. 

The uncertainty does not stem only from the way the data are interpreted. It also depends heavily on the experimental method employed to obtain these data. In Fig. 17 we show an example of data obtained during the deposition of Ag on An, in both the upd and the opd regions, measured galvanostatically j = constant) and by cyclic voltammetry at different scan rates. The cathodic parts of the CV curves were used to construct the plot of A/ as a function of the charge passed. In measurements conducted under galvanostatic conditions, all values of the frequency shift were above the calculated line, as shown also in Fig. 16, practically independent of the applied current density. The response always had the same shape a delay until a few atomic layers have been deposited, followed by a line having the theoretical slope, but shifted upwards (i.e., to higher frequencies). Cyclic voltammetry leads... 

In derivative cyclic voltammetry [23, 37-39], the experimental I-E response is differentiated electronically, and the result is presented as a plot of dl/dE vs E, dljdE crosses the E axis at the potential of the peak on the cyclic voltammogram and this potential can be measured with a much higher precision, maybe 0.1 mV. In addition the ratio of forward to reverse peaks on the dljdE vs E curve can be measured accurately, in contrast to conventional cyclic voltammetry where uncertainty over the base line always causes difficulty in the measurement of the reverse peak and hence / //. Hence derivative cyclic voltammetry represents simply an improvement in presentation. 

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