Polarization activation overpotential

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.

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. 

Fig. 13.7. Graphical representation of the influence of the internal resistance of an electrochemical energy converter on the cell potential when mass-transfer polarization is negligible. The early nonlinear part of the curve represents the effect of the activation overpotential on the cell potential before ohmic polarization has become important.

Figure 3.3.7 Theoretical (dashed dotted) and real (solid) cell voltage (V) - current density (I) performance characteristics of a fuel cell. Overpotentials are responsible for the difference between theoretical and real performance and cause efficiency losses. They split into (i) activation polarization overpotentials at anode and cathode due to slow chemical kinetics, (ii) ohmic polarization overpotential due to ohmic voltage losses along the circuit, and (iii) concentration polarization overpotentials due to mass-transport limitations. The activation overpotentials of the cathode are typically the largest contribution to the total overvoltage.

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. 

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)... 

Chemical reactions can happen before or after the charge-transfer step. Any step can be rate determining, that is, the slowest one determines the total reaction rate. As the electrode polarizes, the resulting overpotential consists of several factors. The most important ones are activation, concentration, and resistance overpotentials. The activation overpotential results from the limited rate of a charge-transfer step, concentration overpotential from the mass-transfer step, and resistance overpotential is the result of ohmic resistances such as solution resistance. Depending on the nature of the slowest step, the reaction is activation, mass transfer, or resistance controlled. 

The thickness distribution of electrodeposits depends on the current distribution over the cathode, which determines the local current density on the surface. The current distribution is determined by the geometrical characteristics of the electrodes and the cell, the polarization at the electrode surface, and the mass transfer in the electrolyte. The primary current distribution depends only on the current and resistance of the electrolyte on the path from anode to cathode. The reaction overpotential (activation overpotential) and the concentration overpotential (diffusion overpotential) are neglected. The secondary... 

These relations are based on the fact that the potential loss due to charge-transfer reactions are negligible (the activation overpotential approaches zero). The diffusion overpotential is usually negative during cathodic processes and positive during anodic processes. The effects of concentration polarization are usually pronounced at high current densities, when a reaction species at the interface is consumed at a faster rate than it can be... 

Concentration Polarization (Diffusion Overpotential). If copper is made cathode in a solution of dilute CUSO4 in which the activity of cupric ion is represented by (Cu ), then the potential ( )i, in absence of external current, is given by the Nernst equation... 

The activation overpotential is the potential loss to drive the electrochemical reactions from equilibrimn state. Therefore, it is the potential loss when there is a net current production from the electrode, i.e. a net reaction rate. In PEM fuel cell, the activation overpotential at the anode is negligible compared to that of the cathode. Activation polarization depends on factors such as the properties of the electrode material, ion-ion interactions, ion-solvent interactions and characteristics of the electric double l er at the electrode-electrolyte interface. Activation polarization may be reduced by increasing operating temperature and by increasing the active surface area of the catalyst. 

This model was used primarily to predict the polarization effects (due to ohmic and activation overpotentials) and the water management requirements. The model computed the required water input at the anode side and required water removal rate at the cathode side necessary to maintain full hydration of the Nafion membrane at all times. 

Figure 6.2 Typical polarization curves that can be recorded from a BES. OCV, open cell voltage OCV, open cell voltage used to determine current-dependent internal resistance. Sections A, activation overpotentials are the dominant voltage loss in this region B, current-dependent voltage loss...

On closing the circuit and allowing a small current to ran through the system, the activation overpotentials are the greatest source of voltage loss. Activation polarization losses can be determined by the Tafel equation [20] when concentration polarization is not taken into account (Figure 6.2, section A, and Figure 6.3) [2, 36, 65]. 

When the electrode is polarized, the overpotential n is composed of the activation overpotential n and the concentration overpotential n ... 

In electrochemical studies, voltage loss due to a transport of reactants to the catalyst sites is usually marginal and g is called overpotential, or activation overpotential. Polarization voltage is a more general term, which includes transport loss (see below). In the following we will use the two terms as synonyms. 

We will consider only the influence of activation overpotential or overvoltage on secondary current distribution. It is useful to regard the slope of the polarization curve dE /di (if any effect of concentration overpotential can be ignored) as a polarization resistance R. This represents the slowness of charge transfer across the interface and is based on the electrode kinetics of the reaction. If acts in series with R, the resistance of the electrolyte, we can distinguish between two situations. If R R, then the kinetics of charge transfer and not electrolyte resistance determine the current distribution, i.e., secondary current distribution dominates. Conversely, if R R, primary current distribution dominates. Secondary current distributions tend to smooth out the severe nonlinear variations of current associated with primary distributions and they eliminate infinite currents associated with electrode edges. 

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