The Principle of Supercapacitors

　　The electric double-layer capacitor is a novel type of capacitor based on the interfacial double-layer theory proposed by the physicist Hermann von Helmholtz. It is well known that when a metal electrode is immersed in an electrolyte solution, excess charges of opposite sign accumulate on the electrode surface and at the liquid–electrolyte interface, thereby creating a potential difference across the interface. If two electrodes are simultaneously inserted into the electrolyte and a voltage lower than the decomposition voltage of the electrolyte is applied between them, the positive and negative ions in the electrolyte will rapidly migrate toward the respective electrodes under the influence of the electric field, forming closely packed charge layers—known as the electric double layer—on the surfaces of both electrodes. This double layer behaves similarly to the polarization charges generated in the dielectric of a conventional capacitor under an electric field, thus giving rise to a capacitive effect. The tightly packed double layer can be approximated as a parallel-plate capacitor; however, because the spacing between the closely packed charge layers is much smaller than that in ordinary capacitors, the resulting capacitance is significantly higher.

　　Compared with aluminum electrolytic capacitors, electric double-layer capacitors have a higher internal resistance; therefore, they can be charged directly without a series load resistor. Moreover, in the event of overvoltage charging, the electric double-layer capacitor will simply open-circuit rather than suffer device damage—a characteristic that contrasts with the overvoltage breakdown observed in aluminum electrolytic capacitors. At the same time, unlike rechargeable batteries, electric double-layer capacitors can be charged without current limiting, and their cycle life can exceed 10^6 charge–discharge cycles. Consequently, electric double-layer capacitors not only exhibit capacitive behavior but also display battery-like characteristics, making them a novel and unique component that occupies an intermediate position between batteries and conventional capacitors.

　　The underlying principle is as follows: when an electrode is charged, the surface charge on an ideally polarized electrode attracts oppositely charged ions from the surrounding electrolyte solution, causing these ions to adsorb onto the electrode surface and form a double-layer of charge, thereby constituting a double-layer capacitor. Because the distance between the two charge layers is extremely small—typically less than 0.5 nm—and because a specially designed electrode structure is employed to increase the electrode’s surface area by a factor of tens of thousands, a very large capacitance is achieved.