Overpotential of a Single Electrode

The cell consists of three electrodes a WE, a counter electrode (CE), and a reference electrode (RE). Each of the electrodes should have some special features. The WE surface should carefully be prepared, usually polished. CE should have a large surface and should not degrade, so CE is usually made by a Pt mesh or coil. The RE should have Nemstian behavior and a stable potential. 

FIGURE 6.1 Schematic of a three-electrode electrochemical cell (1) reference electrode (RE), (2) working electrode (WE), and (3) counter electrode (CE). 

FIGURE 6.2 A three-electrode cell for overpotential measurement (1) WE, (2) CE, (3) Ag/AgCl RE, (4) thermometer, (5) and (6) argon gas flow ports for purging, blanketing, and venting. 

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It can be seen that with this interpretation, overpotential of a cell is always a negative value. Furthermore, the cell overpotential consists of contributions from both electrodes, and individual electrode contributions cannot be separated without using a third electrode. We will discuss, in the following section, how the overpotential of a single electrode can be defined. 

Electrode overpotential is generally an additive function of all processes mentioned earlier, and each of them is described by its own overpotential, i),. The total overpotential of a single electrode is a sum of all contributions ... 

In order to measure the overpotential of a single electrode, a three-electrode electrochemical cell should be used. Each of the electrodes has a particular purpose and design. 

If the potential of an electrode deviates from the reversible or equilibrium value, a current flows in either the anodic or cathodic direction. The deviation of the potential from its equilibrium value is the anodic or cathodic overpotential of the electrode. The terms emf and voltage are used here to refer to a cell, whereas the term potential refers to a single electrode (Section 12-1). Overvoltage represents the additional voltage above the reversible cell emf required to permit the passage of a finite current, and overpotential refers to the deviation of the potential of a single electrode from its reversible value. In both cases the ohmic voltage drop iR is first subtracted, as seen below. 

To cause the passage of a net finite current at an electrode, it is necessary to shift the potential from its equilibrium value. This shift in potential, if no changes in concentration occur in the vicinity of the electrode, is the activation overpotential. The qualitative behavior of a single electrode is shown in Figure 14-2. In general, the net... 

It would make sense to use overvoltage for the whole cell and overpotential for a single electrode. However, the authors mostly use both terms as synonyms and not as a means of discriminating two different quantities. In the following work, we will keep the term overpotential. ... 

If current passes through an electrolytic cell, then the potential of each of the electrodes attains a value different from the equilibrium value that the electrode should have in the same system in the absence of current flow. This phenomenon is termed electrode polarization. When a single electrode reaction occurs at a given current density at the electrode, then the degree of polarization can be defined in terms of the over potential. The overpotential r) is equal to the electrode potential E under the given conditions minus the equilibrium electrode potential corresponding to the considered electrode reaction Ec ... 

Mass transport in an electrochemical reactor occurs by three mechanisms migration in the electrical field, film diffusion, and convection. The first of these is a special feature of electrochemical reactions, whereas the other two are common to all reactions that have a solid phase. However, where an inertsupporting electrolyte is used, the effect of migration can be neglected. With this assumption, let us consider a single electrode reaction given by reaction 21.3. When a finite current is passed through the cell and conditions are perfectly reversible, the concentration overpotential can be expressed as (Pickett, 1979)... 

According to Erdey-Griiz and Vol-mer [15], the supersaturation c/cqo, where c is the concentration of ions in solution in equilibrium with a surface of radius of curvature r, and Cqo is the corresponding concentration of ions in equilibrium with a planar surface, is directly determined by the overpotential, r] = E — Erev. where E is the electrode potential and iirev is the equilibrium potential, in the absence of net current flow for a single electrode reaction. 

Plots of Equation (9.55) for different values of overpotential are shown in Fig. 9.22, which also illustrates an experimental transient obtained for the deposition of a single mercury centre on a carbon micro-electrode [23]. 

Looking for experimental data to support a is 0])-rj dependence, as given by Eqs. (25) and presented schematically in Fig. 2, led to the realization that such data are missing in the literature. Most of the available data on w-EACOP-c and w-EACOP-a for a single electrode reaction are referred to overpotentials where diffusion significantly affects the rate of the reaction in both directions (Refs. 48,50,51,54,69-71,122,211-213, 215,220-222,224,228,231, and 241). There are data on w-EACOP at a few activation overpotentials for certain reactions, but only in one direction (Refs. 75,197,217,218,223,227,229, and 232-236). Closer analysis of this evidence indicated that the majority of these data have been determined either in nonisothermal WE-RE cells or within a rather narrow... 

Shortcomings of the choice of the equilibrium state as the electrical reference point in the evaluation of the temperature effect on the rate of electrode reactions, and consequently of the overpotential as an experimental substitute for A(A0) in the WE-RE cell at various temperatures, have been discussed in the previous section. Hence, another reference point should be sought. From a theoretical point of view, the choice is unambiguous—it is the zero point on the relative electrode potential scale, defined by the SHE convention. Basically, this is also an equilibrium state, but of a single reaction selected by convention, namely, the reduction of two hydrogen ions to molecular hydrogen. The value of A0 at the interface when this reaction is held at equilibrium, assuming all species involved are in standard thermodynamic states, is fixed by the SHE convention as zero. The same convention associates additional properties with this reference state temperature, solvent, and solute Independence. Formally, the properties of the SHE satisfy the principal theoretical requirements for the electrical reference point in the evaluation of the effect of temperature on the rate of electrode reactions. 

In the electrochemical kinetics, the concept of the overpotential of a cell and a single electrode should be well understood. 

Corresponding to the charge in the potential of single electrodes which is related to their different overpotentials, a shift in the overall cell voltage is observed. Moreover, an increasing cell temperature can be noticed. Besides Joule-effect heat losses Wj, caused by voltage drops due to the internal resistance Rt (electrolyte, contact to the electrodes, etc.) of the cell, thermal losses WK (related to overpotentials) are the reason for this phenomenon. 

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