Cyclic Voltammetry at Macroelectrodes

For the forward scan, three distinct regions are observed  

Between +0.3 V and = + 0.15 V, no current is passed as there is insufficient driving force (overpotential) for A to be reduced to B. 

As the potential is further decreased the rate of reduction of A to B increases. The measured current increases approximately exponentially. Since the electron transfer kinetics are fast in nature (i.e. the species is electro-chemicaUyreversible) the concentrations of species A and B at the electrode surface obey the Nernst equation (Eq. 4.1). 

Accordingly, at zero overpotential, as marked on Fig. 4.1 by the dashed line, the concentrations of species A and B at the electrode surface are equal. 

On reversing the potential, the concentrations of species A and B continue to obey the Nernst equation and hence scanning in the positive direction, a peak 

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The concentration of the supporting electrolyte must be more than 100 times that of the eiectroactive species to ensure purely diffusional conditions in cyclic voltammetry at macroelectrodes [8]. 

In the next chapter the simulation of cyclic voltammetry at macroelectrodes will be introduced. The main aspects of the most widely used... 

Fig. 7.4. Cyclic voltammetry at macroelectrodes (left) and under near steady-state conditions (right) for the one-electron reduction of species at different concentrations...

Preparation and Electrochemical Behavior of Macroelectrodes Modified with Polv(I). The electrochemical behavior of 2 was investigated in CH3CN/0.I 1 [11-BU4N]PF via cyclic voltammetry at a Pt electrode. 

Figure 4. Cyclic voltammetry at three scan rates for Au macroelectrodes in 1.0 M NaC104 at pH 1.5 buffer (phosphate) derivatized with only acyl ferrocene thiol, 4c, (5.2 x 10 10 mol/cm2) (top) only quinone thiol. 5, (5.6 x 10 10 mol/cm2) (middle) and a mixture of 4c and 5 at 2.8 x 10 1( mol/cm2 and 2.8 x 10 1( mol/cm, respectively (bottom). Reproduced with permission from ref. 1. Copyright 1991 American Association for the Advancment of Science.

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). 

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). 

The introduction of ultramicroelectrodes in the field of voltammetric analysis offers access to cyclic voltammetry experiments that are impossible with conventionally sized macroelectrodes. In addition to analyses in small volumes or at microscopic locations, microelectrodes allow measurements in resistive media and make it possible to perform high scan rate voltammetry [9,10]. 

The effects of the dimensionless kinetic parameter Kq on the cyclic voltammetry of an E mechanism at a macroelectrode are shown in Figure 4.4 based on the BV model. As the Kq value decreases and the process is less reversible, the peak current decreases and the peak potentials move away from the formal potential, giving rise to an increase in the peak-to-peak separation (which is larger than the value of 2.218 T T/F mV for reversible processes). 

Fig. 5.3. Effect of the chemical kinetics on the cyclic voltammetry of the first-order catalytic mechanism at a macroelectrode.

Between the extremes of linear diffusion at macroelectrodes and steady-state voltammetry, intermediate situations can be found where the overall effect on the electrochemical response is the balance of ohmic drop and migration effects. As shown above, in cyclic voltammetry the former increases the peak-to-peak separation and decreases the magnitude of the peaks whereas electromigration can result in the increase or decrease in the current depending on the charge of the electroactive species. 

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. 

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