Polarization and Overpotential

Overpotential n always reduces theoretical cell potential when current is flowing. Thus, current-voltage curves obtained under steady state are also referred to as polarization curves. 

Basically, there are two types of polarization (1) activation polarization and (2) concentration polarization. Activation polarization is caused by resistance to the passage of potential-determining ions through phase boundary at the electrode-electrolyte interface. For many electrodes, a large polarization is observed at low current density, mainly because of activation polarization. 

The inhibition of the transport of an ion through a layer contiguous to the electrode causes resistance polarization. The presence of foreign substances on the cathode surface may consist of electrolyte anions and cations, oxides or hydroxides, or other organic or inorganic components of the electrolyte. These substances are adsorbed at the electrolyte surface, and when the electrolyte is completely covered by foreign substance, it is passive. This gives rise to resistance polarization. Resistance polarization is also a type of activation polarization. 

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There can be concentration polarization and overpotential at both the cathode and the anode. 

Concentration polarization and overpotential can both occur at the working and auxiliary electrodes. There is an ohmic potential drop between working and auxiliary electrodes. To obtain the best measurement of the working electrode potential, the reference electrode should be placed as close as possible to the working electrode (Figure 17-4). 

It should be noted that voltage loss, polarization, and overpotential are all interchangeable terms and refer to a voltage loss. In general, the operating voltage of a fuel cell can be represented as the departure from ideal voltage caused by the various polarizations ... 

The additional potential required to maintain a current flowing in a cell when the concentration of the electroactive species at the electrode surface is less than that in the bulk solution. In extreme cases, the cell current reaches a limiting value determined by the rate of transport of the electroactive species to the electrode surface from the bulk solution. The current is then independent of cell potential and the electrode or cell is said to be completely polarized. Concentration overpotential decreases with stirring and with increasing electrode area, temperature and ionic strength. 

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. 

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.)...

Polarization. The net current flow produced in a cell results in a deviation of each half-cell potential from the equilibrium value listed in Table 3.3. This deviation from equilibrium is termed polarization, the magnitude of which is given the lowercase greek symbol eta, q and is called the overpotential, E-E°. There are two primary types of polarization activation polarization and concentration polarization. 

Figure 3.9 Evans diagram showing effect of activation polarization on overpotential for a hydrogen electrode. Reprinted, by permission, from W. Callister, Materials Science and Engineering An Introduction, p. 574, 5th ed. Copyright 2000 by John Wiley Sons, Inc.

Except for the need to take concentration overpotential into account in electroanalytical studies, it is an important factor for energy losses in electrochemical power sources (e.g., in -> batteries, fuel cells, etc.) and -> electrolysis (e.g., in electrochemical materials production, -> electroplating, etc.). Concentration overpotential is called also concentration polarization and mass transfer overpotential. 

Resistance overpotential p and activation overpotential p are characteristic of irreversible reactions and are, therefore, termed irreversible overpotentials . Since deviations from the equilibrium potential due to changes in the concentrations of the reactants are largely reversible, concentration overpotential p is known as a reversible polarization . Crystallization overpotential p is more complicated. It can be caused either by reversible polarization or irreversible polarization . The details will be discussed later. 

According to (14-6) the concentration overpotential is zero when the current is zero. ForO < i < the value ofnconc ° g tive, corresponding to cathodic polarization, and increases without limit as i approaches... 

The activation overpotential ti is defined hy E — and therefore has a negative value for cathodic polarization and a positive value for anodic polarization. From (14-13)... 

These examples are based on both electrodes operating in the activation polarization regime, in which the logarithm of the current is proportional to the overpotential. However, there are situations - particularly at low concentrations - in which the electrochemical reaction is limited by mass transport to the electrode surface. This is referred to as concentration polarization, and is illustrated in Figure 13.2d. In this case, above a critical overpotential the current becomes constant, which appears as a vertical line in the plot. A new mixed potential is established at the intersection of this vertical line and the cathode polarization for the oxygen reduction. This potential depends on the gas concentration, and thus can be used for the chemical sensor signal. 

In the derivations of Eq 3.14 and 3.19 for the metal oxidation current density, iox M, and the metal-ion reduction current density, ired M, it was not necessary to restrict the half-cell potential to its equilibrium value. Deviation from E M will occur if the potential of either the metal or the solution is changed, resulting in an overpotential defined in general by Eq 3.1. More specifically, small deviations are associated with charge-transfer polarization, and the overpotential is designated as ... 

Fig. 27. Current distribution relations for a case where all forms of polarization are considered. In curve (a) uniform current distribution in pore, for example, with exchange current density io = amp cm and overpotential (ij) less than 0.1 volt (b)...

The electrochemical (//e) and the phase overpotentials (//ph) are called the activation polarization, rj. The overall overvoltage is the sum of the concentration polarization and the activation polarization ... 

Current flows through the corrosion system (electrochemical cell) only when the redox reaction is not at equihbrium. The difference between the operating electrode potential, E, and the equihbrium potential, e q, is defined as the electrode polarization, AE. Thus, the electrode polarization is a deviation from the equihbrium potential in the presence of current. When a cathodic current is imposed, the potential is displaced to the negative side, causing cathodic polarization to be negative. When an anodic current is apphed, polarization is positive. The electrode polarization and defined nature of the hmiting step is called electrode overpotential or overvoltage. 

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