Principle of energy storage

Electrochemical capacitors store the electric energy in an electrochemical double layer (Helmholtz Layer) formed at a solid:electrolyte interface.

Positive and negative ionic charges within the electrolyte accumulate at the surface of the solid electrode and compensate for the electronic charge at the electrode surface.
The thickness of the double layer depends on the concentration of the electrolyte and on the size of the ions and is in the order of 5–10 A, for concentrated electrolytes. The double layer capacitance is about 10–20 mF:cm2 for a smooth electrode in concentrated electrolyte solution and can be estimated according to equation

 Eq. (1)

C/A=Є*oЄt/d

assuming a relative dielectric constant or of 10 for water in the double layer [5]. d being the thickness of the double-layer with surface area A. The corresponding electric field in the electrochemical double layer is very high and assumes values of up to 106 V:cm easily.

Compared to conventional capacitors where a total capacitance of pF and mF is typical, the capacitance of and the energy density stored in the electrochemical double layer is rather high per se and the idea to build a capacitor based on this effect is tempting. In order to achieve a higher capacitance the electrode surface area is additionally increased by using porous electrodes with an extremely large internal effective surface. Combination of two such electrodes gives an

electrochemical capacitor of rather high capacitance. a schematic diagram of an electrochemical double-layer capacitor consisting of a single cell with a high surface-area electrode material, which is loaded with electrolyte. The electrodes are separated by a porous separator, containing the same electrolyte as the active material.

those obtained for available batteries but much higher than for conventional capacitors. It should be mentioned

that the above values depend on the double layer capacitance, the specific surface area of the respective

electrode material, the wetting behavior of the pores, and on the nominal cell voltage.