Function of Double-Layer Capacitors based on the Helmholtz Principle

The construction of a double-layer capacitor can be described as a plate capacitor where the main focus is to obtain electrodes with an extremely large surface. For this purpose the ideal medium is activated carbon allowing capacitances of up to 100 F/g of the electrode's active mass to be achieved.

The electrolyte, the conductive liquid between the electrodes, is a conducting salt dissolved in an aqueous or organic solvent which permits to apply voltages of 2.5 V. The actual double-layer consists of ions which, when voltage is applied, attach to the positive or negative electrode corresponding to their opposite poles and thus create a dielectric gauge of a few Angstrom* only. This results in a very high capacitance yield caused by the very huge surface of the electrode. To visualise this, the internal surface of a double-layer capacitor would cover a football pitch.

Due to the high conductivity of the electrolyte and the small distance between the electrodes a serial equivalent resistance in the mΩ range is achieved with the entire electrolyte being absorbed in the activated carbon layer.

Charge or discharge of the double-layer capacitor is combined with the transformation of the layers in the electrical field and thus with the movement of the charge carriers in the solvent - even through the separator film. This phenomenon represents the main reason for the limited AC voltage capability and the steep decrease of capacitance versus frequency.

* 1 Å = 0.000 000 1 mm

Construction principle of
WIMA double-layer capacitors