Charge-transfer processes, scanning

In the case of an irreversible charge-transfer process the rate of electron transfer is insufficient to maintain the charge-transfer process at equilibrium. The shape of the cyclic voltammogram is modified and peak positions shift as a function of scan rate (unlike the reversible case). A more detailed discussion can be found elsewhere.93... 

In order to confirm this behavior, the cyclic voltammograms obtained at a planar electrode in CV and SCV (for A = 5 mV) for a Nemstian charge transfer process at different values of the scan rate are shown in Fig. 5.11. The effect of the ohmic drop and charging current has been considered by including an uncompensated resistance Ru = 0.1 K 2 and a double-layer capacitance Cdi = 20pFcm 2. 

As can be seen in this figure, the combined effect of ohmic drop and double-layer capacitance is much more serious in the case of CV. The increase of the scan rate (and therefore of the current) causes a shift of the peak potentials which is 50 mV for the direct peak in the case of the CV with v = 100 V s 1 with respect to a situation with Ru = 0 (this shift can be erroneously attributed to a non-reversible character of the charge transfer process see Sect. 5.3.1). Under the same conditions the shift in the peak potential observed in SCV is 25 mV. Concerning the increase of the current observed, in the case of CV the peak current has a value 26 % higher than that in the absence of the charging current for v = 100 Vs 1, whereas in SCV this increase is 11 %. In view of these results, it is evident that these undesirable effects in the current are much less severe in the case of multipulse techniques, due to the discrete nature of the recorded current. The CV response can be greatly distorted by the charging and double-layer contributions (see the CV response for v = 500 V s-1) and their minimization is advisable where possible. 

Fig. 5.11 Current-potential response of CV and SCV (for A = 5mV) for a Nemstian charge transfer process taking place at a planar electrode for different values of the scan rate (shown in the figure). Dashed-dotted lines Pure faradaic component (SCV and CV) calculated by using the numerical procedure proposed in [21, 22]. Dashed lines Charging current calculated from Eqs. (5.77) (SCV) and (5.76) (CV). Solid lines total current calculated as indicated in Eq. (5.75). /JU = 0.1K 2, C,u 20pFcm 2, Area = 0.05 cm2, cj, = ImM, = 0, Do = Dr = 10 5cm2s 1...

Equations (5.83) and (5.84) and the curves in Fig. 5.12 indicate that both peak current and potential of the CV curves change with the scan rate, a feature which is not observed for the peak potential of reversible processes (see Eq. (5.57)). However, the experimental evidence that for a given system the potential peak of the cathodic CV curves shifts to more negative values with increasing scan rate should be used with caution when assigning a non-reversible behavior to the system since, similar displacements can be observed for Nemstian systems when the ohmic drop has an important effect (see Fig. 5.11). Thus, the shift of the CV peak potential with the scan rate is not always a guarantee of a non-reversible charge transfer process. 

Fig. 6.7 Variation of (a) the peak potential and (b) the peak current of the cyclic voltarrunograms with the scan rate (through the parameter Qa) at disc (solid line) and spherical (dashed line) electrodes. The value of the electrode radius is fixed fulfilling that r = rs = 0.01 cm. K = 0. The values of the peak potential and peak current for a simple fast charge transfer process at planar electrodes are indicated (dashed-dotted line). Reproduced with permission from [21]...

In both types of membrane systems, the current-potential curves corresponding to the first and second scans must be mirror images, which indicates that the ion transfer processes taking place at both the outer and inner interfaces are reversible. Thus, CSWV can be used as an excellent tool for analyzing the reversibility of charge transfer processes. 

The kinetics of the fac lmer isomerization step can be determined quantitatively from the scan-rate dependence of the oxidation process. Both theory and experiment show that the peak potential corresponding to the oxidation of the /ac° species ( p ) shifts to less positive potentials as the scan rate is increased. This occurs because the oxidation charge-transfer process is electrochemically reversible. Under these circumstances, the isomerization step following the charge transfer removes the product and causes the equilibrium position to move to the right in (41), which effectively facilitates the oxidation step. Consequently, at low scan rates, when the isomerization step is important, the oxidation process requires a lower thermodynamic driving force in order to occur and hence a less positive potential is observed. If the electron-transfer (E) step had been irreversible, the isomerization reaction would have no effect on the voltammetric response since the C step would not be rate determining and no kinetic data could be obtained. 

A few years ago Bard and his group developed the technique called scanning electrochemical microscopy (SECM) which makes possible a spatial analysi,s of charge transfer processes [9]. In this method an additional tip electrode of a diameter of about 2 pm is used as well as the three other electrodes (semiconductor, counter and reference electrode). Assuming that a redox system is reduced at the semiconductor, then the reduced species can be re-oxidized at the tip electrode, the latter being polarized positively with respect to the redox potential. The corresponding tip current / [ is proportional to the local concentration of the product formed at the semiconductor surface and therefore also to the corresponding local semiconductor current, provided... 

C Wei, AJ Bard, MV Mirkin. Scanning electrochemical microscopy. 31. Application of SECM to the study of charge-transfer processes at the liquid-liquid interface. J Phys Chem 99 16033-16042, 1995. 

Conventional approaches based on electrochemical techniques, surface tension, and extraction methods have allowed the estabhshment of thermodynamic and kinetic information concerning partition equilibrium, rate of charge transfer, and adsorption of surfactant and ionic species at the hquid/Uquid interface [4—6]. In particular, electrochemical methods are tremendously sensitive to charge transfer processes at this interface. For instance, conventional instm-mentation allowed the monitoring of ion transfer across a hquid/hquid interface supported on a single micron-sized hole [7, 8]. On the other hand, the concentration profile of species reacting at the interface can be accurately monitored by scanning electrochemical microscopy [9, 10]. However, a detailed picture of the chemical environment at the junction between the two immiscible liquids caimot be directly accessed by purely electrochemical means. The implementation of in-situ spectroscopic techniques has allowed access to key information such as ... 

At low scan rate, peak currents vary linearly with the scan rate, which is characteristic of the behavior of a thin layer material deposited on the electrode. Conversely, at higher scan rates, linear variation with the square root of the scan rate is frequently observed, corresponding to diffusion limited currents, arising either from charge transfer process (hopping mechanism) or from an ionic contribution (diffusion of counterions). The response is also strongly dependent upon the nature and concentration of electrolyte (especially the anion [13]). 

Fig. 12.5 (a) Illustration of charge transfer process and oxygen reduction reaction on PDDA-CNT (b) LSV curves of ORR in an 02-saturated 0.1 MKOH solution at a scan rate of 10 mV s . The rotation rate is 1,600 rpm (reproduced with permission [51])... 

We ascribe [9] both processes B/E and C/D (Fig. 1) to nickel-centered reversible charge transfer processes. The former is observed only at high scan rates when the dimerization reaction is hampered and is thus assigned to the reduction of the Ni(ll) anion radical [Ni(ll)L] —, whereas the latter corresponds to the reduction of the dimer... 

However, there remains a possibility that the appearance of two pairs of voltammetric peaks may arise from particle adsorption onto the electrode surface. As in solid films, the ferrocene moieties might exhibit different energetic states and accessibility to counterions because of spatial effects. This hypothesis is discounted by results from two additional experiments. First, the cathodic and anodic current density of the redox peaks was found to be linearly proportional to the square root of potential scan rates, suggesting that the charge-transfer processes were under diffusion control. Second, after the electrochanical measurements in the Ru=CH-Fc particle solution, the Au electrode was taken out and rinsed with a copious amount of DMF and then immersed into a same electrolyte solution without the nanoparticles. Only featureless voltammetric responses were observed, as shown in Figure 3.13 (long dashed curve). In short, both measurements signify minimal surface adsorption of the particles. 

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