Electronic charges, transport across interface

There is need to distinguish more generally between those processes in which charge transport across interfaces is driven by electron exchange and those in which ions and charge-bearing particles are transferred. 

This chapter will be concerned with the kinetics of charge transfer across an electrically charged interface and the transport and chemical processes accompanying this phenomenon. Processes at membranes that often have analogous features will be considered in Chapter 6. The interface that is most often studied is that between an electronically conductive phase (mostly a metal electrode) and an electrolyte, and thus these systems will be dealt with first. 

The observed current is proportional to the rate of such a transfer of electronic charge. This rate of the electrode reaction can depend on the rate of electron conduction through the electrode, mass transport of electroanalyte through the solution, or electron transfer across the electrode-solution interface. At low voltages, the observed rate may be due to either mass transport or the rate of electron transfer. However, at more extreme potentials, the observed current is proportional to the rate of mass transport, which is easier to treat. 

The preoccupation with the interface that has characterized the discussion so far is based on an important assumption The transport aspects of ionics are playing their supply role so well that one has not been aware of the logistic problems of charge transfer. Except for some preliminary indications (cf. Section 7.3.1), the interface has been assumed never to fall short of its needs (of electron acceptors and donors). But there are situations where the charge-transfer reaction is inadequately supplied with its material requirements (e.g., of electron acceptors). Here, a supply problem arises. The transport of electron acceptors and donors in the solution becomes the important event. Ionic transport begins to control the rate of charge transfer across the interface then the viewpoint has to become electrolyte centered. 

The two blue arrows, marked as NO3- (nitrate ions) and as c (electrons), point to the continuous flow of negative electric charge across the entire electric circuit, consisting both of the cell and the external load. Ions are the charge carriers in the electrolyte, while electrons transport the charge in the metal and the external load. The transition from electronic to ionic charge transport occurs at the electrode/ electrolyte interface upon electron transfer between the electrode and an electron acceptor or donor in the electrolyte. 

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. 

Figure 6.2 Possible pathways of the charge transfer reactions and the charge transport processes proceeding at a three-phase electrode consisting of an electrochemically active Phase II, and electrolyte solution (Phase III), and an electron conductor (Phase I). The electron flux shows the direction in which electrons can be transferred across the interface I/II and...

Metal deposition and dissolution (34) In the electrodeposition of solid metals such as silver and zinc, the cation is transported across the electrochemical interface to sites on the electrode surface (Figure 6-4). The positive charge of the cation is offset by electrons from the metal, and the adsorbed species becomes an adatom. These species have surface mobility and migrate along the electrode surface to an imperfection such as a step dislocation, where they enter into the crystal lattice. In the absence of sufficient step dislocations to accommodate the rate of deposition, the adatom surface concentration increases until two- or three-dimensional nucleation occurs. The rate of such nucleation and surface migration strongly influences the morphology of the electrocrystalhzation process. The reverse of this process is involved with electrodissolution of crystalline electro-deposits. 

The charges that we are concerned with here are the electronic charges (electrons and holes). For charges of this type to be transported across the interface, electrochemical reactions must take place. In the presence of a membrane that is impermeable to ions, what will happen then Here, the membrane must serve at least two functions (i) a pathway for electronic charges and (ii) an electrode surface for chemical transformation (reduction and oxidation or redox reactions). 

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