Double-layer capacitance is the important characteristic of the electrical double layer[1][2] which appears at the interface between a surface and a fluid (for example, between a conductive electrode and an adjacent liquid electrolyte). At this boundary two layers of electric charge with opposing polarity form, one at the surface of the electrode, and one in the electrolyte. These two layers, electrons on the electrode and ions in the electrolyte, are typically separated by a single layer of solvent molecules that adhere to the surface of the electrode and act like a dielectric in a conventional capacitor. The amount of charge stored in double-layer capacitor depends on the applied voltage.
The double-layer capacitance is the physical principle behind the electrostatic double-layer type of supercapacitors.