Cyclic voltammetry voltammograms, shape

Benzo[r]bcnzo[3,4]cinnolino[l,2- ]cinnoline 61 and a series of 5,6-dihydrobenzo[c]cinnoline derivatives, including compounds 62, were studied by cyclic voltammetry. The unusual shape of the voltammograms of the investigated compounds can be explained by an EiE-DISP mechanism with a slow disproportionation reaction <1996JA5020>. 

Cyclic voltammetry at spherical electrodes. As discussed in Chapter 1, Section 4.2.3, diffusion laws at a spherical electrode must take into account the curvature r0 of the electrode. The mathematical treatment of diffusion at a spherical electrode becomes somewhat more complicated6 with respect to the preceding one for planar diffusion and we will not dwell on it. On the basis of what we will see in Chapter 11, Section 2, it is important to consider that, under radial diffusion, the cyclic voltammogram loses its peak-shaped profile to assume a sigmoidal profile, see Figure 6. 

It is conceivable that the presence of such complications must affect the shape of the cyclic voltammograms, and hence perturb to some extent the diagnostic criteria for the above-mentioned fundamental electron transfer processes. As these reactions proceed at their own rates, cyclic voltammetry will be able to detect them only if their rates fall within the time scale of the voltammetric technique (which ranges from a few tens of seconds to a few milliseconds). 

The conclusions drawn from analysis of the chronoamperometric response of sphere and disk electrodes apply equally to other electrochemical techniques, such as cyclic voltammetry. The characteristic time, tc, of a cyclic voltammetry experiment can be conveniently expressed by the reciprocal of the scan rate RT/nFv. When rc (Dtc),/ , the voltammogram will appear as predicted for a macroplanar electrode (Chap. 3), and when rc (Dtc) A, the voltammogram will take on a sigmoidal shape given by ... 

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). 

A reversible one-electron transfer process (19) is initially examined. For all forms of hydrodynamic electrode, material reaches the electrode via diffusion and convection. In the cases of the RDE and ChE under steady-state conditions, solutions to the mass transport equations are combined with the Nernst equation to obtain the reversible response shown in Fig. 26. A sigmoidal-shaped voltammogram is obtained, in contrast to the peak-shaped voltammetric response obtained in cyclic voltammetry. 

According to eq. 2 a constant current appears in the cyclic voltammogram (CV) when Q is plotted versus U. In real systems such as porous carbon electrodes, both load resistances due to the spatial distributed capacitance in the pores (circuit model in fig.l) and surface functional groups cause a deviation from the rectangular CV-shape. While the first induces a finite time constant in the charging process, the latter are identified by current peaks in the CV [14,6]. The voltage range used for cyclic voltammetry was -0.2 to 0.8 Volt vs.. g/, gCl at a scanrate of 5 mV/s, respectively. 

The transition from typical peak-shaped voltammograms at fast sweep rates in the linear diffusion region to steady-state voltammograms at small v is shown for cyclic voltammetry in Figure 6.2.2. In the steady-state region, the voltammograms are... 

If the direction of the potential ramp is reversed at the end of the sweep back to the starting potential, the technique is called cyclic voltammetry. In this case, one scan is due to the reduction of a species (cathodic current) and the other is due to oxidation of that species (anodic current). The two current plots combined make a voltammogram (Figure 5.8). Anodic and cathodic currents are always opposite in sign and sigmoidal in shape. Cyclic voltammetry allows variations in scan rate to study kinetics and can also characterise the oxidation and reduction behaviour of various electroactive compounds. 

Cyclic voltammetry is a method frequently used to measure 7s,i ni. Mediated bioelectrocatalysis yields cyclic voltammograms (CVs) of different shapes as illustrated in Fig. 2, depending on the measuring conditions [11]. Curve (a) is the wave for a reversible electrode reaction of an Mox/Mred redox couple. Bioelectrocatalysis mediated with the Mqx/ Mred redox couple produces a sigmoidal catalytic wave as curve (c) under the conditions [Mred] - M and [S] Ks. When [Mred] is increased to higher concentrations, an anodic peak of the diffusion current of Mred rnay be overlapped on the catalytic current as depicted by curve (d) the current, however, becomes steady state after appropriate periods... 

When microelectrodes are used instead of the normal-sized electrode in cyclic voltammetry experiments, voltammograms with the steady-state wave shape are obtained (Fig. 10.5.2). The half-wave potential, E1/2 is equal to the mid-peak potential of the cyclic voltammetric curve recorded using a large electrode. 

The popularity of the cychc voltammetry (CV) technique has led to its extensive study and numerous simple criteria are available for immediate anal-j sis of electrochemical systems from the shape, position and time-behaviour of the experimental voltammograms [1, 2], For example, a quick inspection of the cyclic voltammograms offers information about the diffusive or adsorptive nature of the electrode process, its kinetic and thermodynamic parameters, as well as the existence and characteristics of coupled homogeneous chemical reactions [2]. This electrochemical method is also very useful for the evaluation of the magnitude of imdesirable effects such as those derived from ohmic drop or double-layer capacitance. Accordingly, cyclic voltammetry is frequently used for the analysis of electroactive species and surfaces, and for the determination of reaction mechanisms and rate constants. 

When the formal potentials of the adsorbed and non-adsorbed redox couples are similar, a single wave is obtained where the contribution of the electron transfer involving the adsorbed species increases with the scan rate. Thus, there is a transition from diffusional-shaped voltammograms at slow scan rates to adsorptive-shaped at fast scan rates. To understand this behaviour and illustrate the characteristics of adsorptive voltammograms let us consider the response in cyclic voltammetry of a monolayer of species A that undergoes a one-electron, fully reversible electron transfer ... 

When cyclic voltammetry is performed with microelectrodes it is possible to record wave-shaped steady-state voltammograms at not too high scan rates, similar to dc polarograms. Ideally, there is almost no hysteresis and the half-wave potential is equal to the mid-peak potential of the cyclic voltammograms at macroelectrodes (see Chap. II. 1). 

With very fast scan rate cyclic voltammetry, an upper limit of the scan rate where standard theory prevails is given by the condition that the diffusion layer becomes equal in size to the diffuse layer (see Fig. II. 1.8). It has been estimated that this limit occurs at a scan rate of 1-2 x 10 V s [45]. In the other extreme, at very slow scan rates, natural convection is known to affect the shape of experimental cyclic voltammograms. 

Staircase voltammetry is in fact a modified, discrete linear scan (or cyclic) voltammetry. The potential scan can be reversed in SV, similarly as it is done in cyclic voltammetry, and then a cyclic staircase voltammogram can be obtained. Staircase voltammograms are peaked-shaped the same as linear scan voltammo-grams. There are some differences between these voltammetries anyway. A linear scan (or cyclic) voltammogram forms a continuous current vs. potential curve, while each staircase voltammogram consists of a number oii-E points. Also, the peak heights obtained under conditions of identical scan rates in linear scan and staircase voltammetries (v = AE/At) may differ considerably. 

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