Quasi-Fermi levels and electrode reactions

Charge transfer reactions on semiconductor electrodes proceed under the condition of anodic and cathodic polarization in which the Fermi level epfsc) is different either from the Fermi level Eputicox) of redox electron transfer reactions or from the equivalent Fermi level ep,ioN) of ion transfer reactions. For redox electron transfer reactions, thermodynamic requirement for the anodic and cathodic reactions to proceed is given by the following inequalities  

Under the condition of photoexcitation, the quasi-Fermi level, instead of the original Fermi level, determines the possibility of redox electron transfer reactions. The thermodynamic requirement is then given, for the transfer of cathodic electrons to proceed from the conduction band to oxidant particles, by the inequality of Eqn. 10-7  

Similarly, the thermodynamic requirement for the transfer of anodic holes to proceed from the valence band to reductant particles is given by Eqn. 10-8  

The cathodic current of electron transfer is proportional to the concentration of interfadal electrons, n and the anodic current of hole transfer is proportional to the concentration of interfacial holes, p., in semiconductor electrodes as described in Sec. 8.3. Since the concentration of interfacial electrons or holes depends on the quasi-Fermi level of interfacial electrons or holes in the electrode as shown in Eqn. 10-3 or 10—4 (n, = n + dra and p, =p + 4P ), the transfer current of cathodic electrons or anodic holes under the condition of photoexdtation depends on the quasi-Fermi level of interfadal electrons, nCp, or the quasi-Fermi level of interfadal holes, pEp It also follows from Sec. 8.3 that the anodic current of electron transfer (the ipjection of electrons into the conduction hand) or the cathodic current of hole transfer (the ipjection of holes into the valence band) does not depend on the 

photoexcitation permits those electron transfer reactions to occur which are thermodynamicadly impossible in the dark, provided that the photon irradiation shifts the energy level of interfacial electrons (or holes) in the anodic (or cathodic) direction and produces the thermodynamic affinity for the reactions. 

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