Electrodes Ideally nonpolarizable

An -> ideal nonpolarizable electrode is one whose potential does not change as current flows in the cell. Much more useful in electrochemistry are the electrodes that change their potential in a wide potential window (in the absence of a - depolarizer) without the passage of significant current. They are called -> ideally polarized electrodes. Current-potential curves, particularly those obtained under steady-state conditions (see -> Tafel plot) are often called polarization curves. In the -> corrosion measurements the ratio of AE/AI in the polarization curve is called the polarization resistance. If during the -> electrode processes the overpotential is related to the -> diffusional transport of the depolarizer we talk about the concentration polarization. If the electrode process requires an -> activation energy, the appropriate overpotential and activation polarization appear. 

For the ideally polarizable interphase, they are all independent. For the ideally nonpolarizable interphase, only two can be controlled independently. We recall that an ideally nonpolarizable electrode is a reversible electrode. By setting the concentrations (more accurately, the activities) of ions in the two phases, we determine the potential. Alternatively, by selling the potential, we determine the ratio of concentrations of this ion in the two phases. We conclude that the electrocapillary equation for the nonpolarizable interphase must have one less degree of freedom. 

Consider a cell composed of two ideal nonpolarizable electrodes, for example, two SCEs immersed in a potassium chloride solution SCE/KCl/SCE. The i-E characteristic of this cell would look like that of a pure resistance (Figure 1.3.8), because the only limitation on current flow is imposed by the resistance of the solution. In fact, these conditions (i.e., paired, nonpolarizable electrodes) are exactly those sought in measurements of solution conductivity. For any real electrodes (e.g., actual SCEs), mass-transfer and charge-transfer overpotentials would also become important at high enough current densities. 

Ideally Polarizable Electrodes and Ideally Nonpolarizable Electrodes. 101... 

Two limiting cases for the description of an electrode are the ideally polarizable electrode and the ideally nonpolarizable electrode [8, 9, 14], The ideally polarizable electrode corresponds to an electrode for which the Zfaiadaic element has infinite resistance (i.e., this element is absent). Such an electrode is modeled as a pure capacitor, with Cdi = Aq 6V (equation 26), in series with the solution resistance. In an ideally polarizable electrode, no electron transfer occurs across the electrode/electrolyte interface at any potential when current is passed rather all current is through capacitive action. No sustained current flow is required to support a large voltage change across the electrode interface. An ideally polarizable electrode is not used as a reference electrode, since the electrode potential is easily perturbed... 

Consider a metal electrode consisting of a silver wire placed inside the body, with a solution of silver ions between the wire and ECF, supporting the reaction Ag" + e <— Ag. This is an example of an electrode of the first kind, which is defined as a metal electrode directly immersed into an electrolyte of ions of the metal s salt. As the concentration of silver ions [Ag" ] decreases, the resistance of the interface increases. At very low silver ion concentrations, the Faradaic impedance Zfaradaic becomes very large, and the interface model shown in Fig. 3(a) reduces to a solution resistance in series with the capacitance C. Such an electrode is an ideally polarizable electrode. At very high silver concentrations, the Faradaic impedance approaches zero and the interface model of Fig. 3(a) reduces to a solution resistance in series with the Faradaic impedance Zfaradaic. which is approximated by the solution resistance only. Such an electrode is an ideally nonpolarizable electrode. 

In the case of an ideally polarizable electrode, the transfer resistance of the charge carrier through the interface is infinitely high (the classical example from electrochemistry is the interface between Hg and an inert aqueous electrolyte such as KCl or sulfuric acid) in the case of the ideally nonpolarizable electrode, in contrast, it is zero (approximately for Ag/AgCl) (cf. Chapter 7). 

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