Overpotential effects

A key issue in the redox switching of electroactive polymers relates to the value of the potential at which coupled electron/ion transfer occurs. The importance of this issue is illustrated in Fig. 13.3. The six cubes in 

First we consider application of a large instantaneous high overpotential step. As seen in Fig. 13.3, there are two possible high overpotential paths, 

Next we consider the case where a low overpotential exists at some point in the electrochemical switching process, as is necessarily the case in cyclic voltammetric or chronopotentiometric experiments. Now, four additional mechanisms become possible on the time scale of the electron/ion transfer process. This is so because the electron/ion transfer step, E, is no longer constrained to be the first. Consequently, either one or both of the chemical steps may precede the coupled electron/ion transfer. During the oxidation of R to Of, if solvation of Ra is more rapid than its reconfiguration, the two mechanisms, CEC and CC E may occur. If reconfiguration of R is more rapid than its solvation, two additional mechanisms, C EC and C CE, become possible. 

Analogous to the oxidation process discussed above, the conversion of Of back to Ra may follow six different reduction paths, depending on the overpotential. Consequently, a redox cycle may involve any of 36 possible mechanisms for the hypothetical case of a single start state and a single end state. 

Quite generally, large overpotential techniques, followed by instantaneously open circuiting the filmed electrode, will greatly simplify solving 

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For the electrolysis of a solution to be maintained, the potential applied to the electrodes of the cell (Eapp ) must overcome the decomposition potential of the electrolyte (ED) (which as shown above includes the back e.m.f. and also any overpotential effects), as well as the electrical resistance of the solution. Thus, Eapp must be equal to or greater than (ED + IR), where / is the electrolysis current, and R the cell resistance. As electrolysis proceeds, the concentration of the cation which is being deposited decreases, and consequently the cathode potential changes. 

It must be emphasised that in evaluating the limiting cathode potential to be applied in the separation of two given metals, simple calculation of the equilbrium potentials from the Nernst Equation is insufficient due account must be taken of any overpotential effects. If we carry out, for each metal, the procedure described in Section 12.2 for determination of decomposition potentials, but include a reference electrode (calomel electrode) in the circuit, then we can ascertain the value of the cathode potential for each current setting and plot the current-potential curves. Schematic current-cathode potential... 

To minimize overpotential effects, cathodes axe usually made of finely divided platinum on a porous support, for aqueous electrolytes. The catalytic surfaces of the anodes are particularly susceptible to poisoning by CO, olefins, sulfur compounds, and other impurities in the fuel. These lie above H2 in the chemisorption series (Eq. 6.3). 

In general, the susceptibility of metals M to aqueous corrosion is expected to correlate inversely with the E° values for the reduction of Mm+(aq) to M(s) the less positive E° is, the greater is the tendency of M to corrode in aerated water. Factors that can upset predictions based on E° include the presence of protective films (passivation), overpotential effects (Sections 15.4 and 16.6), effects of complexing agents, and incursion of a cathode reaction other than O2 reduction or H2 evolution. 

The tertiary current distribution Ohmic factors, charge transfer controlled overpotential effects, and mass transport are considered. Concentration gradients can produce concentration overpotentials. The potential across the electrochemical interface can vary with position on the electrode. 

Perhaps the first numerical investigation of lithographically patterned electrodeposition was published by Alkire et al. [46]. In this work, the finite-element method was used to calculate the secondary current distribution at an electrode patterned with negligibly thin insulating stripes. (This is classified as a secondary current distribution problem because surface overpotential effects are included but concentration effects are not.) Growth of the electrodeposit was simulated in a series of pseudosteady time steps, where each node on the electrode boundary was moved at each... 

Maggio et al. [34] studied the water transport in a fuel cell using a semi-empirical approach. They modeled the concentration overpotential effect using an empirical function between the cathode gas porosity and current density (since current density is related to water production). The effective gas porosity was assumed to decrease linearly with increasing current density. This is due to the increasing percentage of gas pores occupied by liquid water. Their results indicate that dehydration of the membrane is likely to occur on the anode side rather than the cathode side. 

Overpotential and Current-Voltage Relationship. The observed overpotential for chlorine evolution at 2-10 kA/m is in the range of 80-110 mV [159]-[162], about 70-100 mV of which is due to diffusion overpotential effects [162]. The overpotential for the generation of oxygen under similar pH and temperature conditions lies is ca. 300 mV more anodic than that of chlorine generation. Other than oxygen evolution, the only other side reaction is formation of chlorate. 

Overvoltages for various types of chlor—alkali cells are given in Table 8. A typical example of the overvoltage effect is in the operation of a mercury cell where Hg is used as the cathode material. The overpotential of the H2 evolution reaction on Hg is high hence it is possible to form sodium amalgam without H2 generation, thereby eliminating the need for a separator in the cell. 

Seconday Current Distribution. When activation overvoltage alone is superimposed on the primary current distribution, the effect of secondary current distribution occurs. High overpotentials would be required for the primary current distribution to be achieved at the edge of the electrode. Because the electrode is essentially unipotential, this requires a redistribution of electrolyte potential. This, ia turn, redistributes the current. Therefore, the result of the influence of the activation overvoltage is that the primary current distribution tends to be evened out. The activation overpotential is exponential with current density. Thus the overall cell voltages are not ohmic, especially at low currents. 

Tertiay Current Distribution. The current distribution is again impacted when the overpotential influence is that of concentration. As the limiting current density takes effect, this impact occurs. The result is that the higher current density is distorted toward the entrance of the cell. Because of the nonuniform electrolyte resistance, secondary and tertiary current distribution are further compHcated when there is gas evolution along the cell track. Examples of iavestigations ia this area are available (50—52). 

Fig. 1.31 Shape of cathodic polarisation curve when transport overpotential is rate controlling, (a) Effect of velocity on ( l and corrosion rate, (b) effect of concentration on tY and corrosion rate and (c) effect of position and slope of anodic curve (after Stern...

The nature and the physical state of the metal employed for the electrodes. The fact that reactions involving gas evolution usually require less overpotential at platinised than at polished platinum electrodes is due to the much larger effective area of the platinised electrode and thus the smaller current density at a given electrolysis current. 

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. 

Electrochemical analytical techniques are a class of titration methods which in turn can be subdivided into potentiometric titrations using ion-selective electrodes and polarographic methods. Polarographic methods are based on the suppression of the overpotential associated with oxygen or other species in the polarographic cell caused by surfactants or on the effect of surfactants on the capacitance of the electrode. One example of this latter case is the method based on the interference of anionic surfactants with cationic surfactants, or vice versa, on the capacitance of a mercury drop electrode. This interference can be used in the one-phase titration of sulfates without indicator to determine the endpoint... 

Assuming the formation of N0 nuclei at the first stages of oxidation, the effective relaxed area (taking into account the overlap between neighboring expanding conductive regions) at every overpotential tj can be estimated by means of the Avrami equation.177 We arrive at... 

Figure 5.13. Effect of catalyst overpotential, AUWR, on catalytic rate and on catalyst work function changes, AO, during ethylene oxidation on Pt/YSZ at 400°C.34Reprinted with permission from Elsevier Science.

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