Potential step techniques

Although the mechanisms discussed above are still topics of debate, it is now firmly established that the electrodeposition of conducting polymers proceeds via some kind of nucleation and phase-growth mechanism, akin to the electrodeposition of metals.56,72-74 Both cyclic voltammetry and potential step techniques have been widely used to investigate these processes, and the electrochemical observations have been supported by various types of spectroscopy62,75-78 and microscopy.78-80... 

Kontturi et al. studied TEA ion transfer across water-1,2-DCE microinterfaces covered by different PCs using short potential step techniques [12]. The enhancement in the forward rate constant was observed for all lipids and increased with the surface coverage (Fig. 6). 

The kinetics of CO oxidation from HClOi, solutions on the (100), (111) and (311) single crystal planes of platinum has been investigated. Electrochemical oxidation of CO involves a surface reaction between adsorbed CO molecules and a surface oxide of Pt. To determine the rate of this reaction the electrode was first covered by a monolayer of CO and subsequently exposed to anodic potentials at which Pt oxide is formed. Under these conditions the rate of CO oxidation is controlled by the rate of nucleation and growth of the oxide islands in the CO monolayer. By combination of the single and double potential step techniques the rates of the nucleation and the island growth have been determined independently. The results show that the rate of the two processes significantly depend on the crystallography of the Pt surfaces. 

Whereas with potential step techniques, 2 = ktm, with alternating-current methods, 2 = k/v, and with RDEV, 2 = kb1 jD, where the symbols are as defined in Chapter E) Full treatment of the diffusion-reaction problem is provided in Section 6.2.1. 

The kinetics of following chemical reactions cannot be studied by the single potential step technique in that the response would simply obey the Cottrell equation. In contrast, the double potential step technique, that measures the response exhibited by either the reagent Ox or the product Red, is sensitive to the chemical fate of Red. The cathodic response before the inversion of the applied potential (t < x) is expressed by the Cottrell equation ... 

The responses of the forward and reverse steps are both affected by the chemical complication in fact, the reduction current will be greater than that predicted by the Cottrell equation, whereas the reoxidation current will be lower than that predicted by the Cottrell equation. This implies that to gain information on the regeneration of the reagent it is sufficient to use the single potential step technique. 

What precedes is true for any other electrochemical technique, using in each case the appropriate experimental parameter for varying the diffusion rate (the frequency in impedance methods, the measurement time in potential-step techniques, and so on). 

In the following sections, this condition will be assumed to be fulfilled, in order to discuss the principles of the most important potential step techniques. 

Figure 3.2 Generalized excitation signal for potential-step techniques. Step from initial potential (Ej) to step potential (Es) to final potential (Ef). x is the duration of the potential step at Es.

The simulation of the ECE mechanism may also employ the double-potential-step technique, but a working curve can be constructed from single-potential-step data also. This is because some of the current that passes, as A is converted to B, is due to the electrolysis of C, the decomposition product of B. The greater the decomposition rate of B, the more current flows, approaching the rate of... 

A complete comprehension of Single Pulse electrochemical techniques is fundamental for the study of more complex techniques that will be analyzed in the following chapters. Hence, the concept of half-wave potential, for example, will be defined here and then characterized in all electrochemical techniques [1, 3, 8]. Moreover, when very small electrodes are used, a stationary current-potential response is reached. This is independent of the conditions of the system prior to each potential step and even of the way the current-potential was obtained (i.e., by means of a controlled potential technique or a controlled current one) [9, 10]. So, the stationary solutions deduced in this chapter for the current-potential curves for single potential step techniques are applicable to any multipotential step or sweep technique such as Staircase Voltammetry or Cyclic Voltammetry. Moreover, many of the functional dependences shown in this chapter for different diffusion fields are maintained in the following chapters when multipulse techniques are described if the superposition principle can be applied. 

In this technique, the DME is kept at an initial potential Ei during a time /, at which the electrode reaction cannot occur and then it is polarized by a potential pulse of increasing amplitude, E2. The measurement of the current during a short pulse time t2 at the end of the drop life confers important advantages on NPP over dcP in relation to the elimination of double-layer effects. In any case, this technique is actually a single potential step technique in which the perturbation shown in... 

As for the permeability measurements, most techniques based on the analysis of transient behavior of a mixed conducting material [iii, iv, vii, viii] make it possible to determine the ambipolar diffusion coefficients (- ambipolar conductivity). The transient methods analyze the kinetics of weight relaxation (gravimetry), composition (e.g. coulometric -> titration), or electrical response (e.g. conductivity -> relaxation or potential step techniques) after a definite change in the - chemical potential of a component or/and an -> electrical potential difference between electrodes. In selected cases, the use of blocking electrodes is possible, with the limitations similar to steady-state methods. See also - relaxation techniques. 

Step potential — is the value the potential is stepped in various - potential step techniques, including multistep versions (- chronoamperometry), as well as for the potential ramp in - voltammetry. 

The use of a potential-step technique such as cyclic staircase voltammetry represents a simple alternative to Ichise s method (j0 of obtaining information on both adsorption and electron transfer kinetics. The current decay immediately after a step is primarily capacitive while current at later times is almost totally due to electron transfer reactions. Thus, by measuring the current at several times during each step and by changing the scan rate, information on both the kinetics of the electrode process and the differential capacity can be obtained with a single sweep. 

An attempt was made by Doblhofer et al. [210] to separate surface from bulk charging processes for thermally prepared Ru02 using the potential step technique. These authors [210] concluded that some bulk diffusion was involved, presumably involving protons, and estimated a diffusion coefficient of 10 19 cm2 s1. Weston and Steele [213] deduced a diffusion coefficient value for protons in porous powder electrodes of Ru02 which is approximately similar to the value of Doblhofer et al. [210]. Iwakura and co-workers [214], on the other hand, employed cyclic voltammetry in deduc-... 

Fig. 9. Variation of potential, current and photon flux vs. time in an ECL experiment performed using the triple-potential-step technique.

Theoretical specific capacity 307 Triple-potential-step technique 23 Triplet-triplet annihilation 4, 20,25, 26, 30 33... 

The applications of the RDE presented so far all relate to operation under steady-state conditions. However, it has been shown that the possibility of discriminating between closely related mechanisms, such as the eCe and the eCeh, may be improved considerably by using a potential-step technique together with the RDE. The reader interested in the details is referred to the original literature [256,262,263]. 

Potential-step techniques can be used to study a variety of types of coupled chemical reactions. In these cases the experiment is performed under diffusion control, and each system is solved with the appropriate initial and boundary conditions. 

Cydodextrins (CyDs) with their largely hydrophobic cavities of variable size and numerous ways of chemical modification are the subject of intensive electrochemical research induding both their behavior in homogeneous solutions and in thin films attached to the electrode surfaces [1-8]. Electroanalytical methods measuring the current response to the potential applied, linear scan, staircase, and pulse voltammetries, and potential-step techniques such as chronoamperometry and... 

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