The interplay of electron transfer and mass transport control

3 THE INTERPLAY OF ELECTRON TRANSFER AND MASS TRANSPORT CONTROL 

It was noted in the introduction to this chapter that the reduction of O R is at least a three-step process, equations (1.20)-(1.22) involving both mass transport and electron transfer, and that the rate of the overall sequence, and therefore current density, depends on the slowest step. Having considered electron transfer and mass transport independently, we can now consider the shape of a 

At the equilibrium potential, no net current flows. As the potential is made negative to , a reduction current is observed. Initially, it will be very small and the surface concentration of O remains close to its bulk concentration this potential region will lead to a linear Tafel plot (Fig. 1.15(b)). As the potential is made more negative, the rate of reduction increases rapidly, in fact exponentially 

In Fig. 1.15, the data are shown as both I-E and log I-rj plots and the correlation between Tafel, mixed and mass transport controlled regions on the two figures should be noted. It is also important to note that a linear Tafel region can sometimes be observed over several orders of magnitude of current density but the range is limited by  

Below this current the back-reaction is significant (equation (1.36)). 

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