In the absence of current, an electrochemical system can often be considered as an equilibrium system. A potential shift from the equilibrium value causes electric current to flow in the external circuit. There are two possibilities. In the case of an ideal, or completely polarized electrode 1, the relaxation current ceases to flow after a certain amount of electricity has passed, and the electrochemical system comes into a new equilibrium state. The second possibility is associated with the presence of faradaic current. In this case the relaxation current in time arrives at a certain steady value, which later does not change provided that the composition of the electrolyte solution is kept constant.
The a.c. methods of studying the electrode-solution interface, introduced into electrochemical practice by Dolin et al. 2, are leading to rapid advances and acquiring an ever increasing importance. Of all the a.c. methods, the most widely used is that of electrode impedance 3,4 measurements. The interpretation of impedance measurements data is impossible without a knowledge of the electric double layer properties.
In the case of an ideal, or completely polarized, electrode, in addition to the differential capacity measurements 5,6, the double layer properties can be studied by means of the thermodynamic method with the use of electrocapillary curves and the Gibbs adsorption equation 7.
If the introduction into the solution of substances reacting on the electrode does not change the structure and properties of the double layer, the separation of faradaic impedance from the electrode impedance is achieved in a very simple manner. In this case the total admittance of the electrode is equal to the sum of faradaic admittance and the electric double layer admittance known from independent experiments. The situation is much more complex if the adsorption of the reacting substance is involved 8, which results in change of the electric double layer structure and properties. In this case the impedance of the double layer can no longer be obtained from independent experiments. The situation has proved to be so involved that until recently it has not been possible to find an equivalent electric circuit of the electrode even in the simplest case--a redox reaction with adsorption only of the oxidized (or reduced) form. It should be pointed out here that Barker 8 developed an intuitive method of constructing equivalent circuits of complex electrode processes. However, he did not prove the correctness of these intuitive circuits by analytical calculations, corresponding to a chosen process scheme.