Polarizable vs. Non-Polarizable Electrodes

We may categorize electrodes into two possible types albeit theoretically: those that are perfectly polarizable and those that are perfectly non-polarizable. This categorization is based on what happens to an electrode when a current passes between it and the electrolyte. Perfectly polarizable electrodes are those in which no actual charge crosses the electrode-electrolyte interface when current is applied. Obviously, there has to be current across the interface, but this current is a displacement current, and the electrode behaves as though it were a capacitor. On the other hand, perfectly non-polarizable electrodes are those in which current passes freely across the electrode-electrolyte interface, requiring no energy to make the transition. Hence, for perfectly non-polarizable electrodes there are no over-potentials.

None of these two types of electrodes can be manufactured; nevertheless, some practical electrodes can come close to acquiring their characteristics. Electrodes made of noble metals like platinum come closest to behaving as perfectly polarizable electrodes. Since the materials of these electrodes are relatively inert, it is hard for them to oxidize and dissolve. Hence, current passing between the electrode and the electrolyte changes the concentration primarily of ions at the interface; therefore a majority of the over-potential observed from this type of electrode is a result of V_c (the concentration over-potential). The electrical characteristics of such an electrode show a strong capacitive effect.

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