Supercapacitors are electrochemical capacitors that store a large amount of charge at a given voltage when compared to normal capacitors. They can charge and discharge by reversible adsorption and desorption of ions at the interface between electrode material and electrolyte. Supercapacitors may have a high capacitance range of up to 15000 Farad. They are different from conventional capacitors as they have high power, high energy density, longer life cycle, and faster charge-discharge rate.

Basic Design of Supercapacitors

Two electrodes are separated from one another by an ion-permeable membrane (separator), and an electrolyte connects the two electrodes ionically in electrochemical capacitors (supercapacitors). Ions in the electrolyte create electric double layers with polarities opposite to those of the electrodes when a voltage is applied to polarise the electrodes. The positively polarised electrodes have a layer of negative ions at the electrode/electrolyte interface and a layer of positive ions that adsorb onto the negative layer to balance the charge. The negatively polarised electrode exhibits the opposite behavior.

Additionally, some ions may penetrate the double layer and become particularly adsorbed ions, contributing with pseudo capacitance to the total capacitance of the supercapacitor depending on the electrode material and surface shape.

Types of Supercapacitors

Supercapacitors are classified into various types:

The distribution of the two types of capacitances, static double-layer capacitance and electrochemical pseudo capacitance, which are used to store electrical energy in supercapacitors, relies on the composition and design of the electrodes. Based on the storage principle, there are three different types of supercapacitors:

• Electrostatic double-layer capacitance in double-layer capacitors (EDLCs) with activated carbon electrodes or their derivatives is substantially larger than electrochemical pseudo capacitance.
• Pseudo capacitors with high electrochemical pseudo capacitance and transition metal oxide or conducting polymer electrodes.
• Lithium-ion capacitors are examples of hybrid capacitors having asymmetric electrodes, one of which mostly displays electrostatic capacitance while the other primarily displays electrochemical capacitance.

The separation of charge in a Helmholtz double layer at the interface between the surface of a conductor electrode and an electrolytic solution electrolyte results in double-layer capacitance, an electrostatic method of storing electrical energy. In a double-layer, the charge separation distance is on the order of a few angströms (0.3–0.8 nm) and is static.

Pseudocapacitance: A reversible faradaic charge transfer on the electrode is produced by electrochemical storage of the electrical energy by redox processes, electrosorption, or intercalation on the surface of the electrode by ions that have been selectively adsorbed.

Key applications of Supercapacitors

Supercapacitors can stabilize the power supply in applications with varying loads, such as laptop computers, PDAs, GPS, portable media players, handheld devices, and solar systems. They can provide energy for LED torches that can be charged in considerably less time.

Current variations and harmonics are caused by a variety of non-linear loads, including EV chargers, HEVs, air conditioners, and sophisticated power conversion devices. Unwanted voltage fluctuations and consequent power oscillations on the grid are caused by these current discrepancies. Supercapacitors can be used as a load-grid interface to solve this issue by acting as a buffer between the grid and the high pulse power coming from the charging station.

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