Reversible and Irreversible Interfaces

Although it is quite reasonable to discuss the bulk properties of homogeneous phases in isolation, it is seldom possible in electrochemical situations to neglect the interfaces, since potentials and fluxes are usually measured or defined at junctions between ionicaUy and electronically conducting phases. In general, two extreme 

The first type is an interface which is reversible to the species under consideration. The term reversible implies certain thermodynamic and kinetic properties. Thermodynamically, it means that an equilibrium relation of the type 

KineticaUy, the term is less well defined and depends more explicitly on the nature of the experiment. In practice, it means that the exchange current density (the microscopic flux crossing the interface equally in both directions at equilibrium) is very much greater than the net current density crossing the interface during the experiment or the measuring process. At appreciable current densities, however, the net current density may eventually exceed the exchange current density interface kinetics then become important. 

In the electrochemical literature it is useful to refer to a reversible interface or interfacial reaction as one whose potential is determined only by the thermodynamic potentials of the various electroactive species at the electrode surface. In other words, it is only necessary to take into account mass transport to and from the interface, and not the inherent heterogeneous kinetics of the interfacial reaction itself, when discussing the rate of the charge transfer reaction. This nomenclature has two principal disadvantages. First, it neglects the fact that mass transport to the interface, whether migration or diffusion, is inherently an irreversible or dissipative 

An interface may be reversible to one species, but blocking to others. In addition, in multicomponent systems, a reversible electrode may not necessarily define the thermodynamic potentials of all components present at the interface. 

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