Diffusion-overpotential reduction curve

As the rate of the electron transfer process increases it must eventually be fast compared with the maximum rate of mass transport and the surface concentration will then become zero. Diffusion is then the rate-determining step (of sequence (1.16)—(1.18)) and the current becomes independent of potential with the value given by equation (1.53). A similar argument applies to the oxidation reaction although the limiting current is about one-third of the plateau reduction current because of the ratio Cp /Cr employed. Figure 1.10 shows the I-E curves for both the reversible and the irreversible cases. In the former, the I-E response arises directly from equation (1.20) while for an irreversible couple we need an overpotential to drive both the oxidation and reduction processes, see equations (1.35) and (1.37). 

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