Basics Knowledge of Super Capacitor

Conventional capacitors use solid-state dielectrics to store energy. In contrast, super capacitors use two electrical layers, often referred to as EDLC (electrochemical double-layer capacitors). In EDLC, a physical mechanism is used to generate an electrical double layer of dielectric function. The charge and discharge process is performed by an ion absorbing layer on the surface of the positive and negative active carbon electrodes. The static separation distance of the charge on the EDLC bilayer is very small - between 0.3 and 0.8 nm. The follow figure  shows the ionic activity of the charge (left) and discharge (right) processes.

Electric double layer capacitors typically store charge in activated carbon electrodes

EDLC uses ion migration in thin film activated carbon to store charge. A voltage is applied across the two electrodes of the capacitor to cause ions in the electrolyte to migrate in an attempt to flip the charge on the electrode (charging process). The positively charged ions move to the negative electrode, while the negatively charged ions move to the positive electrode, forming two charge layers within the electrolyte: a positive electrode and a negative electrode. Removing this voltage causes the ions to move in the opposite direction, forming a discharge process.

The capacitance values ​​of supercapacitors are determined by their size, geometry, and composite. The capacitance of aluminum electrolyte devices ranges from 10-6 to 10-2 F (farad). The Panasonic Gold Super Capacitor product line offers capacitance values ​​up to 70F. Rechargeable batteries used in consumer products provide higher capacitance – depending on the size of the battery.

In competition with battery technology, new materials provide new power for supercapacitors. Researchers have created graphene devices that are comparable in energy density to nickel-metal hydride batteries – up to 85 watt-hours per kilogram at room temperature. Like all other supercapacitors, they charge very fast—just a few seconds or a few minutes.

New devices built with conductive polymers can increase the capacitance capacity, which is derived from the charge storage principle known as tantalum capacitors, which are produced by chemical reduction-oxidation (redox) reactions on the electrodes. Typically, the ion is O2+. During the charging process, one of the electrodes undergoes a reduction reaction while the other electrode is an oxidation reaction. During the discharge, these reactions reverse and the ions move in opposite directions between the electrolytes.

The electric double layer capacitor and the tantalum capacitor together determine the capacitance value of the super capacitor. Depending on the specific electrode design and electrolyte composition, the relative contributions of the two vary widely. In some cases, the capacitance of the tantalum capacitor is increased by an order of magnitude higher than the capacitance of the double-layer board itself.

（PS:Tantalum capacitors, or Faraday quasi-capacitors, are two-dimensional or quasi-two-dimensional spaces in the surface or bulk of an electrode. Electroactive materials are underpotentially deposited, highly reversible chemical adsorption, desorption or oxidation, and reduction reactions. , generating a capacitance related to the electrode charging potential.）