Current-potential curves characterized

The film electrodeposition process was studied by means of linear sweep voltammetry. The rate of electrochemical reaction was determined from current density (current-potential curves). The film deposits were characterized by chemical analysis, IR - spectroscopy, XRD, TG, TGA and SEM methods. 

The current-potential curve in cyclic voltammetry is as depicted in Figure 1.1c and c, and characterized by4... 

The normalized current-potential curves are thus a function of the two parameters A and oc. An example corresponding to a = 0.5 is shown in Figure 1.19. Decreasing the parameter A as a result of a decrease in the rate constant and/or an increase in scan rate triggers a shift of the cathodic potential toward negative values and of the anodic potential in the reverse direction, thus increasing the irreversibility of the cyclic voltammetric response. When complete irreversibility is reached (i.e., when there is no anodic current underneath the cathodic current, and vice versa), a limiting situation is reached, characterized by... 

Voltammetric current-potential curves are important in elucidating electrode processes. However, if the electrode process is complicated, they cannot provide enough information to interpret the process definitely. Moreover, they cannot give direct insight into what is happening on a microscopic or molecular level at the electrode surface. In order to overcome these problems, many characterization methods that combine voltammetry and non-electrochemical techniques have appeared in the last 20 years. Many review articles are available on combined characterization methods [10]. Only four examples are described below. For applications of these combined methods in non-aqueous solutions, see Chapter 9. 

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 single step voltammetry, the existence of chemical reactions coupled to the charge transfer can affect the half-wave potential Ey2 and the limiting current l. For an in-depth characterization of these processes, we will study them more extensively under planar diffusion and, then, under spherical diffusion and so their characteristic steady state current potential curves. These are applicable to any electrochemical technique as previously discussed (see Sect. 2.7). In order to distinguish the different behavior of catalytic, CE, and EC mechanisms (the ECE process will be analyzed later), the boundary conditions of the three processes will be given first in a comparative way to facilitate the understanding of their similarities and differences, and then they will be analyzed and solved one by one. The first-order catalytic mechanism will be described first, because its particular reaction scheme makes it easier to study. 

Because the flow of electric current always involves the transport of matter in solution and chemical transformations at the solution-electrode interface, local behavior can only be approached. It can be approximated, however, by a reference electrode whose potential is controlled by a well-defined electron-transfer process in which the essential solid phases are present in an adequate amount and the solution constituents are present at sufficiently high concentrations. The electron transfer is a dynamic process, occurring even when no net current flows and the larger the anodic and cathodic components of this exchange current, the more nearly reversible and nonpolarizable the reference electrode will be. A large exchange current increases the slope of the current-potential curve so that the potential of the electrode is more nearly independent of the current. The current-potential curves (polarization curves) are frequently used to characterize the reversibility of reference electrodes. 

The Half-Wave Potential.—In the preceding description of the analytical applications of the dropping mercury cathode it has been supposed that the nature of the reducible substance has been determined and that the position of the corresponding wave on the current-potential curv c is known. If the substance has not been previously identified, however, it is possible to do so by means of the polarographic curve. The reducible material is characterized by its half-wave potential this is the potential... 

In the pore formation regime, the dissolution current increases exponentially with applied potential for p-type silicon and heavily doped n-type silicon [61, 63-66]. The potential range over which this exponential behavior is observed is dependent on dopant concentration and HF concentration. The exponential current-potential curves are characterized by a slope of 60 mV (kT/q) on a plot of the logarithm of the etching current versus applied potential, as can be seen in Fig. 8 [65]. 

Results of the fractionation of the mixture are presented in Table 2A. Rough characterization of the fractions was made by comparing their capacity current potential curves with those of the model mixtures. The shape of the curve obtained in effluent samples after separation of the hydrophobic neutral component corresponds to that of the mixture of fulvic acid and Dextran. After removal of (he hydrophobic acid fraction, the remaining fraction resembles the... 

Current-potential curves, particularly those obtained under steady-state conditions, are sometimes called polarization curves. We have seen that an ideal polarized electrode (Section 1.2.1) shows a very large change in potential upon the passage of an infinitesimal current thus ideal polarizability is characterized by a horizontal region of an i-E curve (Figure 1.3.5a). A substance that tends to cause the potential of an electrode to be nearer to its equilibrium value by virtue of being oxidized or reduced is called a depolarizer An... 

Basing logic on the current-potential curves (including the kinetic and thermodynamic aspects) and not only on a thermodynamic potential scale A zero current in a non-equilibrium state is characterized by a mixed potential at least two different half-reactions occur at the interface (oxidation and reduction)... 

In the case of monomerization, the form of the current-potential curve and the values of peak current and potential as a function of sweep rate provide a useful means of characterizing this mechanism. In the case of the dimerization type of reaction, however, the i-E curves are not appreciably affected by the chemical reaction. Also, unless the dimerization is relatively slow, the detection of an intermediate in the anodic direction corresponding to a reversed scan is difficult. With acetophenone reduction, for example, reoxidation of the ketyl radical intermediate formed in the cathodic direction of sweep in acid solutions (CHs-COH -c -h e CH3 COH < ) is not observed. 

In the Rotating Disk Electrode (RDE) technique, the current-potential curves on smooth platinum exhibit an anodic limiting current density, which depends on rotation rate in both acidic and alkaline media [46]. These plots are well described by equation (19), which holds for a diffusion overpotential alone. Similar relationships have been observed in acidic solutions for Ir, Rh, and Pd, and well-characterized Pt-Ru, Pt-Rh, Pt-Sn [53], and Pt-Au [51] alloys, and also for Ni in alkaline solutions. In the case of platinum, a evolution of the limiting diffusion current density to a limiting reaction current density ( x) independent of rotation rate, is observed as a consequence of the rate-determining H2 adsorption. 

K is small (the equilibrium is shifted to the left) and X is large. The supply of the species A inside the time window is provided by the forward chemical reaction. This situation (characterized, for example, by 1 < 2 < 10 at K = lO " values) is called pure kinetic zone. The current-potential curves are perturbed by the coupled chemical reaction so that kinetic parameters can be evaluated (zone KZ) ... 

Fig. 33. Normalized DPP current-potential curves for a reduction process characterized by the C E mechanism, i/ ppp = AIpp/Id vs. n(E - E,/2) dependence at dEqp = T50 mV, pulse starting at x = 0.952 s, pulse width tp = 40ms K= 10 . Forward rate constants, kf (s ) (1) 00, (2) 10 (3) 10 (4) 10 (5) 10-. Curves 1-5 with dEop<0, curves l -5 with dEj5p>0. Adapted from [121].

Figure 63JS.2 Characterization of a 25 im Hg/Pt hemispherial UME. (a) In-situ micrographs of mercury deposition (0-300 sec) from a 10 mM Hg2(NOj)2 solution with 0.1 M KNOj supporting electrolyte acidified to 0.5% with HNOj. (h) The deposition curve recorded during a 300 sec potential step of —0.1 V vs. Ag/AgCl. A 1 mm Pt wire served as the counter electrode and a fritted Ag/AgCl electrode served as the reference electrode, (c) Current potential curves at Pt and Hg/Pt UMEs in 0.1 M KNOj. Reprinted with permission from reference (3). Copyright, the American Chemical Society.

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