Polarization curves of redox electron transfers

For p-type electrodes, the cathodic current is carried at low overvoltages by the minority carriers (electrons) in the conduction band and is controlled at high overvoltages by the limiting current of electron diffusion the anodic current is carried by the mtqority carriers (holes) in the valence band and the concentration of interfacial holes increases with increasing anodic overvoltage until the Fermi level is pinned in the valence band at the electrode interface, where the anodic current finally becomes an electron injection current into the electrode. 

The polarization curves shown in Fig. 8-27 correspond to Eqns. 8-62 to 8-65 while the electrode is in the state of band edge level pinning. As examples, Fig. 8-28 shows the cathodic polarization curves of several redox reactions at an 

6 Redca Fermi level and band edge level 

As discussed in Sec. 8.3.5, a redox reaction current due to electron or hole transfer depends not only on the concentration of interfadal electrons or holes at the electrode but also on the state density of the redox electrons or redox holes in the range of energy where the electron transfer takes place. Hence, it is important in the kinetics of electron or hole transfer to realize the level of the band edge Cc or Ev of the electrode relative to the most probable level cred or cox of redox electrons or redox holes in the hydrated redox particles. 

For redox reactions due to cathodic transfer of electrons via the conduction band, the cathodic current is expected to be maximum when the most probable vacant level eqx of the oxidant particle is in the same level as the conduction band edge e this cathodic current gradually decreases with increasing separation of eox firom ej. as shown in Fig. 8-29. The same conclusion may also be drown fh m Eqn. 8-61. 

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