Potential range, practical

The surface concentration Cq Ajc in general depends on the electrode potential, and this can affect significantly the form of the i E) curves. In some situations this dependence can be eliminated and the potential dependence of the probability of the elementary reaction act can be studied (called corrected Tafel plots). This is, for example, in the presence of excess concentration of supporting electrolyte when the /i potential is very small and the surface concentration is practically independent of E. However, the current is then rather high and the measurements in a broad potential range are impossible due to diffusion limitations. One of the possibilities to overcome this difficulty consists of the attachment of the reactants to a spacer film adsorbed at the electrode surface. The measurements in a broad potential range give dependences of the type shown in Fig. 34.4. 

In the meantime, it is of great practical importance to establish how the redox potential ranges can vary from one solvent to another without the occurrence of solvent decomposition. Therefore, we shall first consider the redox potential range in water. Here at the surface of the hydrogen platinized Pt electrode (cf., p. 31) the following reversible reduction can take place ... 

In addition to the thermodynamic quantity E°, the electrode reaction is characterized by two kinetic quantities the charge transfer coefficient a and the conditional rate constant k°. These quantities are often sufficient for a complete description of an electrode reaction, assuming that they are constant over the given potential range. Table 5.1 lists some examples of the constant k. If the constant k° is small, then the electrode reaction occurs only at potentials considerably removed from the standard potential. At these potential values practically only one of the pair of electrode reactions proceeds which is the case of an irreversible or one-way electrode reaction. 

In addition to chemical compositions, another important aspect about SEI formation that is of practical significance to the forming of lithium ion cells is the potential range in which the above reactions occur leading to the formation of the SEI. Because of the earlier observation of the plateau near 0.80 and the concurrent gas evolution, it... 

Derivatization is usually accomplished by linear regression of the experimental data within small E intervals (20 to 30 mV, depending on the quality of the nk — E plots). It should be noted that, upon derivatization, the contribution from the diffusion coefficient disappears. The significance and consequences of the plots of a versus E will be discussed in detail in the following sections. However, it is worth mentioning another practical aspect at this point. By using scan rates of 0.1-50 V s , the potential range in which... 

As a result of that reductive process, a deposit of copper metal (denoted in Eq. 2.2 by s for solid ) is formed on the carbon electrode surface. The prominent anodic peak recorded in the reverse scan corresponds to the oxidative dissolution of the deposit of copper metal previously formed. The reason for the very intense anodic peak current is that the copper deposit is dissolved in a very small time range (i.e., potential range) because, in the dissolution of the thin copper layer, practically no diffusion limitations are involved, whereas in the deposition process (i.e., the cathodic peak), the copper ions have to diffuse through the expanding diffusion layer from the solution to the electrode surface. These processes, labeled as stripping processes, are typical of electrochemically deposited metals such as cadmium, copper, lead, mercury, zinc, etc., and are used for trace analysis in solution [84]. Remarkably, the peak profile is rather symmetrical because no solution-like diffusive behavior is observed. 

Further Observations on the Technique of Steady-State Electrochemical Kinetic Measurements 1. In potentiostatic measurements, the appropriate interval of potential between each measurement depends on the total range of potential variation. It may be between 10 and 50 mV and can be automated and computer controlled (Buck and Kang, 1994). It is helpful to observe a series of steady-state currents at, say, 20 potentials taken from least cathodic to most cathodic, and the same series taken from most cathodic to the least cathodic. The two sets of current densities should be equal at each of the chosen constant potentials. In practice, with reactions involving electrocatalysis, a degree of disagreement up to 25% in the current density at constant potential is to be tolerated. 

Other electrochemical oscillators are known (Wojtowicz, 1972), but none have as yet received practical development. It is not necessary to have specifically iron or aluminum in the system. Any system that undergoes electrochemical oxidation in a potential range so that it can drive the reduction of a substance onto Hg will produce... 

Table 1 gives a short list of approximate potential ranges available in various aqueous solutions and for different electrodes. The values of the potentials quoted are, for practical convenience, relative to an aqueous saturated calomel electrode (SCE) as this is one of the most common and convenient reference electrodes used in aqueous electrochemical studies. 

An electrochemical stability window is defined as the potential range in which an electrode can be polarized in a solution without the passage of substantial Faradaic currents. This definition is only a practical one, as it is impossible to define a precise value for the term substantial. ... 

A practical illustration of the application of more negative (cathodic) potential to carbon steel in seawater to reduce the corrosion rate is provided in Figure 1.69. The figure shows that the more cathodic the applied potential, the lower is the corrosion rate. In the figure rp is the maximum acceptable (or allowed) corrosion rate with a corrosion current density of ip and protection potential of Ep. For this particular case the protection potential range is —800 to —900 mV. The corrosion rate iv may be written as 75... 

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