Double-Layer Potential Ranges or Windows

Electrochemical cell containing two electrode-electrolyte interfaces. M = metal lead I. M = metal electrode 11. M = metal electrode III. M = metal lead I. S = electrolyte solution. 

In electrochemistry, when no electrode reactions can occur within a fairly wide electrode potential range, the result is called an ideal polarizable electrode, completely polarizable electrode, or totally polarized electrode. 

Consequently, the electrode behaves like a capacitor and only capacitive current (no faradic current) flows upon a change of potential in a certain electrode potential range. The electrode [carbon/1.0 M NaCl (aq) interface] mentioned above can behave as an ideal polarized elecfrode buf only within the double-layer range from -0.6 to 0.2 V versus NHE. 

It is important to choose an electrolyte with a wide electrochemically stable range. For a solvent, the selection seems difficult due to its intrinsic electrochemical stability. For example, for an aqueous solution, the electrochemical disassociation window of water is around 1.23 V at room temperature. If water is used as a supercapacitor electrolyte solvent, the maximum cell voltage will be around 1.23 V if acetonitrile is the solvent, the electrode potential window is around 2.0 V with an ion liquid, the electrode potential window can be as high as 4.0 V. Therefore, different solvents have different potential windows. Table 2.2 lists several common solvents and their potential windows for supercapacitors. 

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