Cathode/electrolyte interface overvoltage

The electron transfer reactions at the semiconductor/electrolyte interface occur either via the conduction band or the valence band. The total current is therefore given by the sum of four partial currents, denoted as represent electron transfer via the conduction anc valence bands, respectively, and the superscripts, a and c, indicate anodic anc cathodic processes, respectively. Let us assume nereafter that the electron transfer occurs only via the conduction band. In a simple case where the concentration of the electrolyte is sufficiently high and only the overvoltages at the Helmholtz layer (tjh) and in the space charge layer (rjsc) are important, the ica and cc can be given as follows4)... 

To develop any electrochemical process, a voltage should be applied between anodes and cathodes of the cell. This voltage is the addition of several contributions, such as the reversible cell voltage, the overvoltages, and the ohmic drops, that are related to the current in different ways. One of these contributions, the overvoltage, controls the rate of the transfer of electrons to the electrochemically active species through the electrode-electrolyte interface when there is no limitation in the availability of these active species on the interface (no mass-transfer control and no control by a preceding reaction). In this case, the relationship between the current that flows between the anodes and the cathodes of a cell and the overpotential is... 

P unoccupied states of the metal and the occupied states of the reduced species is larger and thus leads to f+> f hence to an anodic total current density i > 0. Figure 1.21c depicts the situation for a negative overvoltage T) with a larger overlap between the occupied states of the metal and the empty states of the oxidized species, which leads to i >i+ hence to a cathodic total current density i < 0. These concepts of charge transfer by tuimel processes are used for the semiconductor/electrolyte interface and are presented in Section 1.18. 

---