Ionic currents across interfaces between

The first major observation of ionic current across the interface between two immiscible solutions was reported by Nernst and Riesenfeld," who in 1902, studied the transport of colored electrolytes across water-phenol-water concentration cells. However, it was only in 1974 that Gavach et al applied what we could call... 

In case of semiconductor electrodes the properties of the interface between a semiconductor and a solution are similar to those of the interface between a semiconductor and a metal (see - Schottky barrier). There are, however, some particularities. At this interface the semiconductor presents electronic conduction whereas the liquid presents ionic conduction. In the semiconductor, the density of electronic states at the chemical potential can be equal to zero, imposing constraints to charge transport through the interface, but even in this case it is still the chemical potential that determines the magnitude of the equilibrium current across the interface, which is achieved by electron-ion exchange. The equilibrium signifies the absence of any net currents through the interface. 

As long as the potential difference between the two phases, A, lies between the values of A< cath and A< anod (either A< bo(B o w) or A0aw(A w- o), whichever has smaller magnitude), no appreciable ionic current will be observed across the interface (such an interface is called an unpolarized interface ). but for the values of A outside the range, A< cath to A< anod there will be an appreciable ionic current flow observed, as shown in Figure 24. 

The theories of the electronic and ionic currents have some features in common. One may formulate models in which the current is limited by the injection into the film from the contacts of positively or negatively charged carriers, or one may consider an equilibrium state to exist across either or both interfaces. One may postulate space-charge limited currents, trapping, and recombination processes. One of the chief differences between the ionic and the electronic currents is that the average velocity of the ions is approximately exponentially dependent on the field for fields which produce experimentally observable ionic currents, whereas the average velocity of electrons is linearly dependent on the field at low fields with different types of nonlinearity at high fields. 

The WE and CE combination represents a driven electrochemical cell. The presence of the RE allows the separation of the applied potential into a controlled portion (between the RE and the WE) and a controlling portion (between the RE and the CE). The voltage between the RE and the CE is changed by the potentio-stat in order the keep the controlled portion at the desired value. Consider the application of a potential Vin to the WE that is more positive than its rest potential, VffiSt, with respect to RE. By definition, polarization of the WE anodically (i.e., in a positive direction) would lead to an anodic current through the WE-solution interface and a release of electrons to the external circuit. These electrons would be transported by the potentiostat to the CE. A reduction reaction would occur at the CE-solution interface facilitated by a more negative potential across it. The circuit would be completed by ionic conduction through the solution. 

The electric potential difference between interfaces (III) and (tl) depends on the electronic conductivity of deposit (D). If the deposit is an exclusive electronic conductor, the difference is close to zero and the current is limited by ionic conduction in the deposit on the other hand, if it is an exclusive ionic conductor, an internal potential close to the open circuit emf value is built up. Ionic conduction relies on the leakage electronic current. In either case, the electric potential cfiflerence across (D) can be expressed as... 

More recently, this topic has been revisited by Daikhin and Urbakh [88] who presented a kinetic description of ionic surfactant transfer across an ITIES that includes the charging of the interface, adsorption, and transfer as well as characteristics of the electrical circuit. This model showed that the irregular current oscillations are due to a dynamic instability induced by the interplay between the potential-dependent adsorption and direct transfer across the interface. In particular, this model showed that current anomalies occur in a potential range close to the standard ion-transfer potential. 

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