Electrical double-layer capacitors capacitance values

For the first time, a totally solid-state electric double layer capacitor (EDLC) was fabricated using PEO-KOH-H2O as the SPE and the polymer electrolyte could replace large amount of liquid KOH electrolyte [17,18]. The ideal rectangular shape of cyclic voltammety result for this solid-state EDLC was obtained, and the real value of specific capacitance was 90 F g". It was only slightly lower than that of liquid electrolyte supercapacitor, and it might be related to the electrode material and structure. 

Double-layer properties of porous carbon materials have been widely investigated in relation to the development of the electrochemical capacitors. For detailed information the reader should consult specialized literature. For porous carbons materials, the double-layer capacitance depends on their specific snrface area [82,83], pore stmcture (notably, the pore size distribntion) [84-87], and their crystalline stmctnre and snrface chemistry [83,88,89], Shi [84] measnred the dc capacitance of varions carbons in a KOH electrolyte and noticed that the overall capacitance may reasonably be described as a sum of the capacitance of micro- and mesopores. Assuming that the electrical double layer propagates into micropores accessible for N2 adsorption, the author estimated the differential donble-layer capacitance per unit of micropore surface area as 15 to 20 p,F/cm. Lower values were reported by Vilinskaya... 

Double layer capacitance Excess charge in an electrode surface is compensated by a build-up of opposite-charged ions (Helmholtz layer), creating an electrical double layer. This layer is mathematically treated as a parallel plate capacitor. Typical values are on the order of tens of micro farads per cm. ... 

The electrical response of such a system is comparable to that of a capacitor. Being of faradic origin and non-electrostatic, this capacitance is distinguished from the double-layer one and is called pseudo-capacitance. In summary, the electrical double-layer formation is a universal property of a polarized material surface, and pseudo-capacitance is an additional property which depends both on the type of electrode material and electrolyte. Compared to the double-layer normalized capacitance ( 10 pF cm ), it has generally a high value (100-400 pF cm ), because it involves the bulk of the electrode and not only the surface. From a practical point of view, pseudo-capacitance contributes to enhancing the capacitance of materials and their energy density. 

The distance between the plates of the first capacitor is fixed, whereas that of the second one varies as a function of the electrolyte concentration. At low concentrations, the diffuse layer is extended deeply into the solution and the capacitance of the electric double layer is determined mainly by the value of Q. If has a minimum af 2 = 0. Thus the applied potential equals the PZC. This observation can be used for determining the PZC. 

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