Types of Overpotential

The various types of overpotentials are dealt with in more detail in Section 20.1 but it is appropriate here to outline the significant factors in relation to their importance in controlling the rate of corrosion reaction. 

The first two terms on the right-hand side of this equation express the proper overpotential of the electrode reaction rjr (also called the activation overpotential) while the last term, r)c, is the EMF of the concentration cell without transport, if the components of the redox system in one cell compartment have concentrations (cOx)x=0 and (cRed)x=0 and, in the other compartment, Cqx and cRcd. The overpotential given by this expression includes the excess work carried out as a result of concentration changes at the electrode. This type of overpotential was called the concentration overpotential by Nernst. The expression for a concentration cell without transport can be used here under the assumption that a sufficiently high concentration of the indifferent electrolyte suppresses migration. 

Figure 33 Typical plot of cell potential vs. current for fuel cells illustrating regions of control by various types of overpotentials [317].

Crystallization overpotential is treated as a special type of overpotential. In the PEVD system, any barrier to the process by which atoms are incorporated into or removed from the crystal lattice of the PEVD product (D) leads to crystallization overpotential r. ... 

The third important type of overpotential is caused by mass-transport limitation. It is called concentration overpotential r because, when the reaction is influenced by the rate of mass transport, the concentration at the surface is different from its bulk value (cf. Eq. 13D). Since the reaction rate depends only on the concentration at the surface, it is necessary to apply a higher overpotential to maintain the same rate when the concentration at the surface is lower than in the bulk. 

In proper kinetic studies one needs to isolate this type of overpotential. A mechanistic study is then based on determining the dependence of on current density over a wide range of experimental conditions. 

The so-called polarization of a fuel cell can be attributed to different phenomena, each one being, in general, quantified by an overpotential. Three main types of overpotentials can be considered ... 

Let us now turn to detailed discussion of the various types of overpotential. When doing so it is necessary to keep in mind the assumption that it is only the discussed step which limits the reaction rate, i.e. when discussing reaction overpotential we assume that all other steps, diffusion, electron-transfer etc., are virtually at equilibrium and derive the equations accordingly. In practice this assumption is not always correct. There may be two steps of the same rate, both of which limit the electrode reaction. For example the reduction of Cd(CN)4 " at the dropping mercury electrode proceeds via the mechanism... 

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