Scans forward

Cyclic voltammetry is most commonly used to investigate the polymerization of a new monomer. Polymerization and film deposition are characterized by increasing peak currents for oxidation of the monomer on successive cycles, and the development of redox waves for the polymer at potentials below the onset of monomer oxidation. A nucleation loop, in which the current on the reverse scan is higher than on the corresponding forward scan, is commonly observed during the first cycle.56,57 These features are all illustrated in Fig. 3 for the polymerization of a substituted pyrrole.58... 

Figure 12.16 Potential dependent SFG spectra (a) and the Stark tuning plot (b) from chemisorbed CO on Pt nanoparticles in a CO-saturated 0.1 M H2SO4 electrolyte. Each spectrum was acquired for 10 s (forward scan data only are shown). The potential was scanned from — 0.20 to 0.70 V (vs. Ag/AgCl) at 1 mV/s. Pt nanoparticles were of approximately 7 nm size, and were immobilized on an Au disk.

Despite the asymmetry between the forward and reverse current or charge responses, reversibility may be strictly defined by the transformations depicted in Figure 1.4. The anodic trace is first measured against the prolongation of the forward trace (the trace that would have been obtained if the forward scan had been prolonged beyond the inversion potential), as symbolized by a series of vertical arrows. After symmetry about the horizontal axis, the resulting curve is shifted to the initial potential in the case of the time dependence representation. Alternatively, in the case of the potential dependence representation, another symmetry about E = E° is performed. In both cases, reversibility, in both the chemical and electrochemical senses, is demonstrated by the exact superposition of the hence-transformed reverse trace with the forward trace. 

The equation of the forward scan voltammogram is thus given by the following integral equation (see Section 6.1.4 for the proof). 

In this section we establish the equation of the forward scan current potential curve in dimensionless form (equation 1.3), justify the construction of the reverse trace depicted in Figure 1.4, and derive the charge-potential forward and reverse curves, also in dimensionless form. Linear and semi-infinite diffusion is described by means of the one-dimensional first and second Fick s laws applied to the reactant concentrations. This does not imply necessarily that their activity coefficients are unity but merely that they are constant within the diffusion layer. In this case, the activity coefficient is integrated in the diffusion coefficient. The latter is assumed to be the same for A and B (D). 

Comparing this equation with the equation characterizing the forward scan,... 

During the forward scan, the Laplace expression of equation (1.10) is written... 

The equations of the reverse traces are thus derived straightforwardly from those of the forward scan in the same three cases ... 

Since the waves are irreversible, the forward scan response,... 

Combining equations (6.68) and (6.69) finally leads to the total forward scan response. Examples are given in Figure 6.4. 

Diagnostic criteria to identify a chemical reaction preceding a reversible electron transfer. Granted that when the chemical reaction has a large K value, or its rate is slow, the response looks like an unperturbed reversible process, the simplest and most wished-for opportunity to detect the presence of a preceding equilibrium reaction lies on the possible appearance of an S-shaped curve in the forward scan. Otherwise, other criteria are the following. 

The simplest and most useful case that one can study by single potential step chronoamperometry is that in which " /2> 0/i (z.e. AEor = E2 - E 5 180 mV). This means that the primarily electrogenerated species Red converts to a new species Ox, which is more easily reducible than the initial species Ox. As seen in Section 1.4.3, in cyclic voltammetry such a system exhibits a single reversible process in the forward scan. 

Confirming the assignment, an authentic sample of [PtIV(C6Cl5)4] exhibits in the forward scan only the apparently irreversible reduction PtIV/Ptin, which in the backscan generates the reversible Ptn/Ptm peak-system. 

Fig. 5 Linear sweep and cyclic voltammetry (a) dotted lines five profiles respectively at various typical excitation signal (b) current response times, increasing time shown by arrows] for a and concentration profiles [(c) forward scan cyclic voltammetric experiment.

In these techniques, the concentrations at the electrode do not immediately attain their extreme values after the start of the experiment. Rather, they change with E ox t according to equation (1). While the steepness of the concentration profiles increases with E (forward scan), simultaneously 8 increases in the quiet solution. The latter effect slows down the increase of i with E, and finally (close to the limiting current region) leads to the formation of a peak with a characterishc asymmetric shape. On the reverse scan (after switching the scan direction ad. Ef), products formed in the forward scan can be detected (B, in the case discussed). 

In the second forward scan, at the negative potential, there appear two new anode peaks which correspond to the two inverse reactions above. 

The second anode peak at the higher potential still corresponds to the reaction in which the element S is oxidized to SO. A new cathode peak appears in the inverse scan, which may correspond to the inverse reaction of reaction (6-4). It can probably be said that the Zn(OH)2, which can not be moved away completely from the electrode surface, reacts with reduced element S to form ZnS. The third cathode peak at the lower potential represents the reaction in which the ZnS is reduced to Zn. So there appears a new anode peak in the second forward scan. 

At pH = 9.5, the cyclic voltammogram of the sphalerite activated by CUSO4 is relatively complex. At the upper limit of the scan potential shown in Fig. 6.1, a series of anode peaks appearing in forward scan sequentially correspond to reaction (6-9) and other reactions as follows ... 

For the third-generation dendrimer 26 the first reduction wave is symmetrically bell-shaped and the peak current is proportional to the scan rate (for v < 400 V/s), demonstrating that 26 is adsorbed on the electrode surface. From the area under the peak and the dendrimer area, a bilayer is estimated to be deposited onto the electrode. Upon increasing scan rate (v > 104 V/s), only a fraction of the units are reduced during the forward scan because only part of the fullerene moieties are close enough to the surface to allow direct electron transfer. On the other hand, at lower scan rates, all the fullerene redox sites are accessible and thus they can store charges. 

The first derivative of an LSV wave, for example the forward scan in Fig. 8, has an important feature relevant to measurement precision. The... 

Fig. 12. Cyclic voltammograms showing the effect of increasing rate constant for the eC mechanism. Curves with increasing current on the forward scan correspond to increasing rate constant.

Simply stated, in the forward scan, R is electrochemically generated as indicated by the cathodic current. In the reverse scan, this R is oxidized back to O as indicated by the anodic current. Thus CV is capable of rapidly generating a new species during the forward scan and then probing its fate on the... 

The solid line of Figure 23.1 gives the calculated trace for Equation 23.1 with a moderate heterogeneous charge-transfer rate (a quasi-reversible E system). The circles give the calculated response on the basis of Equations 23.2-23.4, in which the initial product B reacts rapidly to give X, which is oxidized on the return sweep. The anodic wave therefore arises from the electroactive decomposition product (X) of B. The cathodic (forward) scan is therefore an EC process and the reverse scan an E process, so that the overall cyclic mechanism might be referred to as EC,E (Eq. 23.2-23.4). 

If the product, Z, is reducible at a potential more positive than that required to reduce the reactant, A, a single wave is observed on the forward scan, near the... 

The Effect of Illumination. In an alkaline solution, an n-GaP electrode, (111) surface, under illumination shows an anodic photocurrent, accompanied by quantitative dissolution of the electrode. The current-potential curve shows considerable hysterisis as seen in Fig. 2 the anodic current, scanned backward, (toward less positive potential) begins to decrease at a potential much more positive than the onset potential of the anodic current for the forward scanning, the latter being slightly more positive than the Ug value in the dark, Us(dark). 

The electrochemical step is reversible. In this case, it depends on the rate of the chemical step. If the chemical step is much faster, then no return peak will be observed as R will react further to B. If the chemical step is not as fast, a return peak will be observed. However, with a smaller peak current as for the peak in the forward scan, because a fraction of R will be transformed into B, the other fraction is oxidised to O, resulting in the return peak with smaller peak current. 

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