Reaction polarization, overpotentials

There are several mathematical expressions for polarization overpotential for various cases. In non steady-state electrochemical systems the mathematical manipulation of polarization overpotential is not trivial, so as in systems exhibiting chemical reactions preceding the electrochemical ones. For the relatively simple case of steady-state mass transfer, in which insoluble R is produced in the reaction O + ne < R (e.g., O is a metal ion being electrodeposited) the polarization overpotential is expressed as iyc = fp In ( ) where t]c is the concentra-... 

If net cathodic current flows then this potential is shifted negatively. Concentration polarization (alternatively called -> mass-transport polarization or - concentration overpotential) is encountered if the rate of transport of the redox reactant to the electrode surface is lower than that of the -> charge-transfer reaction. Together with the charge-transfer or -> activation polarization (overpotential), q3, and the polarization (overpotential) due to a preceding chemical reaction, qrxn> (see... 

Upon cathodic and anodic polarization with noble metal electrodes, disregarding dissolved gas species from the atmosphere, the following reduction and oxidation reactions in a pure Na2S04 melt are possible cathodic reactions (increasing overpotential) ... 

Because all electrochemical reactions involve anodic and cathodic reactions, polarization will have components for both reactions. As will be explained later, the electrode potentials have two terms for each electrode surface overpotential ija or ijc and concentration overpotential Apart from these overpotentials, electrical energy will also be expended due to the electrical resistance of the cell components such as electrolyte, diaphragm, busbar, etc. Thus the practical cell voltage (, when a net current is flowing through the cell, is the sum... 

Typical polarization curves for SOFds are shown in Fig. 27-67. As discussed earlier, the open-circuit potential of SOFds is less than 1 because of the high temperature, but the reaction overpotentials are... 

The extent to which anode polarization affects the catalytic properties of the Ni surface for the methane-steam reforming reaction via NEMCA is of considerable practical interest. In a recent investigation62 a 70 wt% Ni-YSZ cermet was used at temperatures 800° to 900°C with low steam to methane ratios, i.e., 0.2 to 0.35. At 900°C the anode characteristics were i<>=0.2 mA/cm2, Oa=2 and ac=1.5. Under these conditions spontaneously generated currents were of the order of 60 mA/cm2 and catalyst overpotentials were as high as 250 mV. It was found that the rate of CH4 consumption due to the reforming reaction increases with increasing catalyst potential, i.e., the reaction exhibits overall electrophobic NEMCA behaviour with a 0.13. Measured A and p values were of the order of 12 and 2 respectively.62 These results show that NEMCA can play an important role in anode performance even when the anode-solid electrolyte interface is non-polarizable (high Io values) as is the case in fuel cell applications. 

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 ... 

Plotting the overpotential against the decadic logarithm of the absolute value of the current density yields the Tafel plot (see Fig. 5.3). Both branches of the resultant curve approach the asymptotes for r RT/F. When this condition is fulfilled, either the first or second exponential term on the right-hand side of Eq. (5.2.28) can be neglected. The electrode reaction then becomes irreversible (cf. page 257) and the polarization curve is given by the Tafel equation... 

Polarization in the cathodic direction accelerates the cathodic reaction and is called cathodic polarization polarization in the anodic direction accelerates the anodic reaction and is called anodic polarization. In Fig. 7-4 the polarization curve is cathodic at potentials more negative and is anodic at potentials more positive than the equilibrium potential E. In electrode reaction kinetics the magnitude of polarization (the potential change in polarization) is called the overvoltage or overpotential and conventionally expressed by symbol ii, which is negative in cathodic polarization and positive in anodic polarization. 

For perovskite electrodes, the earliest kinetic study of hysteretic effects appears to come from Ham-mouche and co-workers, who showed that the i—rj characteristics of porous LSM/YSZ in air at 960 °C exhibit a potentiodynamic hysteresis when scanned slowly (1 mV/s) between 0 and —1200 mV cathodic polarization. " A clearer demonstration of this effect, more recently provided by Jiang and co-workers, is shown in Figure 41.232,233 Hammouche and co-workers attributed this hysteresis to the formation of oxygen vacancies in LSM at high overpotential, which (as discussed in sections 5.2 and 5.3) appears to open a parallel bulk-transport-mediated reaction pathway. However, if this was the only explanation. 

Figure 48. Kenjo s ID macrohomogeneous model for polarization and ohmic losses in a composite electrode, (a) Sketch of the composite microstructure, (b) Description of ionic conduction in the ionic subphase and reaction at the TPB s in terms of interpenetrating thin films following the approach of ref 302. (c) Predicted overpotential profile in the electrode near the electrode/electrolyte interface, (d) Predicted admittance as a function of the electrode thickness as used to fit the data in Figure 47. (Reprinted with permission from refs 300 and 301. Copyright 1991 and 1992 Electrochemical Society, Inc. and Elsevier, reepectively.)...

Steady-State Kinetics, There are two electrochemical methods for determination of the steady-state rate of an electrochemical reaction at the mixed potential. In the first method (the intercept method) the rate is determined as the current coordinate of the intersection of the high overpotential polarization curves for the partial cathodic and anodic processes, measured from the rest potential. In the second method (the low-overpotential method) the rate is determined from the low-overpotential polarization data for partial cathodic and anodic processes, measured from the mixed potential. The first method was illustrated in Figures 8.3 and 8.4. The second method is discussed briefly here. Typical current—potential curves in the vicinity of the mixed potential for the electroless copper deposition (average of six trials) are shown in Figure 8.13. The rate of deposition may be calculated from these curves using the Le Roy equation (29,30) ... 

SO that the concentration of [Zn ] under the same conditions will be 10 g-molecule/L. With these ionic concentrations, the deposition potentials of copper and zinc in the absence of any polarization can each be calculated from Eq. (11.1) to be about —1.30 V. It should be mentioned here again that in practice, Eq. (11.1) refers to reversible equilibrium, a condition in which no net reaction takes place. In practice, electrode reactions are irreversible to an extent. This makes the potential of the anode more noble and the cathode potential less noble than their static potentials calculated from (11.1). The overvoltage is a measure of the degree of the irreversibility, and the electrode is said to be polarized or to exhibit overpotential hence, Eq. (11.2). 

Equation 1.7 for the reduction of protons at a mercury surface in dilute sulphuric add is followed with a high degree of accuracy over the range -9 Tafel plot i.s shown in Figure 1.5. At large values of the overpotential, one reaction dominates and the polarization curve shows linear behaviour. At low values of the overpotential, both the forward and back reactions are important in determining the overall current density and the polarization curve is no longer linear. 

Given that the rates of oxidation and reduction of the half-reactions are controlled by activation polarization only, that = 4-0.07 and = —0.08, and that the exchange current densities for both the oxidation of Fe and reduction of hydrogen in acidic solution are identical, use the data in Tables 3.3 and 3.4 to determine the following quantities. Recall that the potential for each half-cell is the sum of the equilibrium potential and the corresponding overpotential, in this case, r]a-... 

Since in cathodic reactions is always smaller than c°, the concentration polarization has a negative sign, which adds to the activation overpotential in causing the electrode to depart from the equilibrium potential in the negative direction for an electronation reaction. 

Overpotential, ohmic potential, and concentration polarization make electrolysis more difficult. They drive the cell voltage more negative, requiring more voltage from the power supply in Figure 17-1 to drive the reaction forward. 

---