Electroanalytical Techniques 1 Cyclic Voltammetry

Of the electroanalytical techniques, cyclic voltammetry (or linear sweep voltammetry as it is sometimes known) is probably one of the more versatile techniques available to the electrochemist. The derivation of the various forms of cychc voltammetry can be traced to the initial studies of Matheson and Nicols and Randles." Essentially the technique appUes a linearly changing voltage (ramp voltage) to an electrode. The scan of voltage might be 2 V from an appropriate rest potential such that most electrode reactions would be encompassed. Commercially available instrumentation provides voltage scans as wide as 5 V. 

Another application of electrochemistry to heterogeneous catalysis is cyclic voltammetry, which is an important electroanalytical technique. Cyclic voltammograms (CV) trace the transfer of electrons during an oxidation-reduction (redox) reaction (Figure 7.11). 

Linear sweep voltammetry (LSV), with the corresponding reversal technique, cyclic voltammetry (CV), are definitely the most frequently used voltammetric techniques. This is true when a qualitative approach to the study of the redox characteristics of a solution is pursued, as well as when a quantitative study of the electrode mechanism and even the evaluation of the relevant thermodynamic and kinetic parameters are faced, and also when electroanalytical quantitative information are sought. The potential waveforms for LSV and for CV, with the relevant equations expressing the time dependence of E, are shown in Fig. 10.12. 

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. 

Appropriate electroanalytical procedures to verify the one or other case have been given in the references of this section. The main techniques are cyclic voltammetry, chronoamperometry, chronocoulometry, and rotating disk voltammetry. The last one appears to be best suited since constant mass transport in the film is a very important feature as outlined aixive Table 2 gives examples for... 

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). 

In principle, E° can be determined by the widely used electroanalytical techniques (e.g. polarography, cyclic voltammetry [25]). The combination of the techniques is also useful. It has been demonstrated recently where potentiom-etry, coulometry, and spectrophotometry have been applied [26]. The case of the cyclic voltammetry is examined below. 

Cyclic voltammetry has perhaps become the most popular electroanalytical, electrochemical technique [23, 27], and many reports have appeared in which E° values were determined in this way. However, reliable formal potentials can be determined only for electrochemically reversible systems [28]. For any reversible redox system - provided that the electrode applied is perfectly inert, that is, there are... 

Nevertheless, the mid-peak potentials determined by cyclic voltammetry and other characteristic potentials obtained by different electroanalytical techniques (such as pulse, alternating current, or square wave voltammetries) supply valuable information on the behavior of the redox systems. In fact, for the majority of redox reactions, especially for the novel systems, we have only these values. (The cyclic voltammetry almost entirely replaced the polarography which has been used for six decades from 1920. However, the abundant data, especially the half-wave potentials, 1/2, are still very useful sources for providing information on the redox properties of different systems.)... 

Two voltammetric techniques, stationary-electrode voltammetry (SEV) and cyclic voltammetry (CV), are among the most effective electroanalytical methods available for the mechanistic probing of redox systems. In part, the basis for their effectiveness is the capability for rapidly observing redox behavior over the entire potential range available. Since CV is an extension of SEV, many points pertinent to CV are discussed in the SEV section. 

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. 

With the advent of digital implementation of electroanalytical experiments came the technique called staircase cyclic voltammetry, wherein the triangular wave is approximated by a series of small potential steps. The reason for such an approximation is partly due to the impossibility of digitally generating a pure ramp and more importantly to the realization that substantial improvements accrue from sampling the current at the end of each step, where double-layer charging has decayed away. If the steps are small, the data will fit theory based on pure ramps quite well. 

Many of the electroanalytical techniques that are routinely employed in conventional solvents, such as, chronoamperometry, chronocoulometry, chronopotentiometry, coulometry, cyclic (stationary electrode) voltammetry, rotating electrode voltammetry, and pulse voltammetry, have also been applied to molten salts. Some of these techniques are discussed next with special attention to their employment in molten salts. References to noteworthy examples appearing in the literature are included. Background information about these techniques is available elsewhere in this book. 

The most popular electroanalytical technique used at solid electrodes is Cyclic Voltammetry (CV). In this technique, the applied potential is linearly cycled between two potentials, one below the standard potential of the species of interest and one above it (Fig. 7.12). In one half of the cycle the oxidized form of the species is reduced in the other half, it is reoxidized to its original form. The resulting current-voltage relationship (cyclic voltammogram) has a characteristic shape that depends on the kinetics of the electrochemical process, on the coupled chemical reactions, and on diffusion. The one shown in Fig. 7.12 corresponds to the reversible reduction of a soluble redox couple taking place at an electrode modified with a thick porous layer (Hurrell and Abruna, 1988). The peak current ip is directly proportional to the concentration of the electroactive species C (mM), to the volume V (pL) of the accumulation layer, and to the sweep rate v (mVs 1). 

Basically, experimental approaches to ion transfer kinetics rely on classical galvanostatic [152] or potentiostatic [146] techniques, such as chronopotentiometry [118, 138], chronocoulometry [124], cyclic voltammetry [146], convolution potential sweep voltammetry [147], phase selective ac voltammetry [142], or equilibrium impedance measurements [148]. These techniques were applied mostly to liquid-liquid interfaces with a macroscopic area (typically around 0.1 cm ). However, microelectrode methodology has been successfully introduced into liquid-liquid electrochemistry as a novel electroanalytical tool by Senda and coworkers [153] and... 

Before embarking on an electrosynthesis it is desirable to know the potential at which the desired reaction is expected to occur. That information can be obtained from electroanalytical techniques, such as polarography and cyclic voltammetry. ... 

Cyclic voltammetry in supercritical water-0.2 M NaHS04 [88] and ammonia-0.14 M CF3SO3K [88,332] of some organic compounds shows that this electroanalytical technique was applicable under these conditions. The behavior of phenazine in NH3 at —40°C and under supercritical conditions, for example, was analogous two reversible reductions were found in both cases [88]. Dimethyl carbonate has been prepared from CO and MeOH on anodic oxidation in a supercritical mixture of CO2 and MeOH [89]. 

Cyclic voltammetry (CV) is an important and widely used electroanalytical technique. Although CV is infrequently used for quantitative analysis, it finds wide applicability in the study of oxidation/reduction reactions, the detection of reaction intermediates, and the observation of follow-up reactions of products formed at electrodes. In CV, the applied potential is swept in first one direction and then the other while the cunent is measured. A CV experiment may use one full cycle, a partial cycle, or several cycles. 

The first group consists of conventional electroanalytical techniques such as cyclic voltammetry (CV), chronoamperometry, chronopotentiometry, coulometry, and electrochemical impedance spectroscopy (EIS), all of which provide general information about the doping process (see also Chapters 4 and 5). Below are listed some typical questions that can be answered using the above group of techniques ... 

The potential of electroanalytical techniques such as cyclic voltammetry and impedance spectroscopy is also not fully explored in photocatalyt-ic studies which is primarily used for the rapid screening and high throughput evaluation of photocatalysts. The use of in-situ analytical techniques used for heterogeneous catalysis is extensively reviewed elsewhere [213-218]. 

Each electroanalytical technique has certain characteristic potentials, which can be derived from the measured curves. These are the half-wave potential in direct current polarography (DCP), the peak potentials in cyclic voltammetry (CV), the mid-peak potential in cyclic voltammetry, and the peak potential in differential pulse voltammetry (DPV) and square-wave voltammetry. In the case of electrochemical reversibility (see Chap. 1.3) all these characteristic potentials are interrelated and it is important to know their relationship to the standard and formal potential of the redox system. Here follows a brief summary of the most important characteristic potentials. 

In spite of the instrumental simplicity of cyclic voltammetry, the method is surprisingly versatile and an experienced researcher can obtain a considerable amount of information on the basis of just recording a single CV. A large number of different electroanalytical techniques have been applied to the research on conducting polymers, from the chronoamperometric method to AC impedance methods, However, the methods rest only on the electrochemical characteristics of the polymer and naturally the information obtained is rather restricted. 

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