Capacitor, electrochemical leaky

F r d ic Current. The double layer is a leaky capacitor because Faradaic current flows around it. This leaky nature can be represented by a voltage-dependent resistance placed in parallel and called the charge-transfer resistance. Basically, the electrochemical reaction at the electrode surface consists of four thermodynamically defined states, two each on either side of a transition state. These are (11) (/) oxidized species beyond the diffuse double layer and n electrons in the electrode and (2) oxidized species within the outer Helmholtz plane and n electrons in the electrode, on one side of the transition state and (J) reduced species within the outer Helmholtz plane and (4) reduced species beyond the diffuse double layer, on the other. 

Of the numerous equivalent circuits that have been proposed to describe electrochemical interfaces, only a few really apply in the context of a freely corroding interface at or close to kinetic equilibrium. The first circuit (Fig. 7.25a) corresponds to Eq. (7.2) and to the simplest equivalent circuit that can describe a metal/electrolyte interface. Following Boukamp,25 the term Q has been adopted here to describe the leaky capacitor behavior corresponding to the presence of a constant phase element explained by a fundamental dispersion effect. The admittance representation Y of the CPE behavior with frequency w can be described by Eq. (7.5). For n = (1 - p), Eq. (7.5) describes the behavior of a resistor with R = Yq" and for n = P, that of a capacitor with C = Yo- For n = 0.5, Eq. (7.5) becomes the expression of a Warburg (W) component, and when n = -p, it emulates an inductance with L = Yq-1.25... 

---