Oxygen reduction reaction Butler-Volmer equation

In the electrochemical mechanism of corrosion, the metal dissolution—which involves the loss of electrons vis- -vis oxidation—must be accompanied by a cathodic reaction that consumes electrons, which is typically oxygen or proton or water reduction. According to the Butler-Volmer equation, the current density for the anodic reaction varies according to... 

The oxygen reduction reaction is considered as very slow in this case Jo -> 0 and J7c, °o and the Butler-Volmer equation becomes ... 

Equations (4.91) to (4.93) can be applied to any cathodic partial reaction for which the charge-transfer step obeys the Butler-Volmer equation. In corrosion, oxygen reduction is often under mixed control. Figure 4.25 shows the cathodic polarization curve for oxygen reduction, measured on a platinum electrode [6]. The shape of the curve suggests a relatively low value for the ratio I o/ltil-... 

The oxygen reduction reaction is notoriously sluggish. The faradaic current density for this reaction is given by the cathodic branch of the Butler-Volmer equation ... 

For many reactions, the charge transfer is only one elementary step in a sequence of many others. Some substances break chemical bonds and form new ones. Oxygen reduction is a relatively complicated process with several intermediate species corresponding to a sequence of reaction steps. Nevertheless, a corrosion reaction is often ruled by the Butler-Volmer equation, although reaction steps other than the charge transfer may be rate determining as well. 

Each reaction may be subdivided into further detailed reaction steps. The overall reaction follows a Butler-Volmer equation. For large cathodic overvoltages, reduction may become diffusion limited and thus might reach the potentially independent maximum current density j d as described in Equation 1.119 and in Figure 1.23 and as discussed again for metal dissolution with oxygen reduction in Section 1.14. 

In this notation, anodic current is positive, while cathodic current is negative. As the later section on oxygen reduction will show, the Tafel slope can change with overpotential. This is because the Butler-Volmer law only applies to outer-sphere reactions. Although it can describe electrode reactions, the equation does not account for repulsive interactions of the adsorbates or changes in the reaction mechanism as potential is changed. 

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