Double-layer capacitance

Double-layer capacitance is the storing of electrical energy by means of the electrical double layer effect. This electrical phenomenon appears at the interface between a conductive electrode and an adjacent liquid electrolyte as observed, for example, in a supercapacitor. At this boundary two layers of ions with opposing polarity form if a voltage is applied. The two layers of ions are separated by a single layer of solvent molecules that adheres to the surface of the electrode and acts like a dielectric in a conventional capacitor.

The amount of electric charge stored in double-layer capacitance is linearly proportional to the applied voltage and depends primarily on the electrode surface. The unit of capacitance is the farad.

Helmholtz laid the theoretical foundations of the double layer phenomenon. It is used in every electrochemical capacitor to store electrical energy.

Every capacitor has two electrodes, mechanically separated by a separator. They are electrically connected via the electrolyte, a mixture of positive and negative ions dissolved in a solvent such as water. An area originates at the electrode surfaces where the liquid electrolyte contacts the electrode's conductive metallic surface. This interface forms a common boundary between two phases of matter, such as an insoluble solid electrode surface and an adjacent liquid electrolyte. In this interface occurs a special phenomenon of the double layer effect.

By applying a voltage the electrodes generate two layers of polarized ions. One layer is in the surface lattice structure of the electrode. The other layer, with opposite polarity, originates from dissolved and solvated ions distributed in the electrolyte that has moved in the direction of the polarized electrode. These two layers of polarized ions are separated by a monolayer of solvent molecules. The molecular monolayer forms the inner Helmholtz plane (IHP). It adheres by physical adsorption on the electrode surface and separates the oppositely polarized ions from each other, forming a molecular dielectric.

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