1. Concept

A supercapacitor or ultracapacitor is a new energy storage device between the traditional capacitor and the rechargeable battery, which has the characteristics of fast charging and discharging of the capacitor, and at the same time has the energy storage characteristics of the battery. This type of capacitor has a high capacity to store the energy charge and has a very high value of capacitance than the normal capacitor (up to 2KF).

2. Principle

A supercapacitor is a new type of component that stores energy through a two-layer interface formed between an electrode and an electrolyte. When the electrode is in contact with the electrolyte, due to the effects of Coulomb force, intermolecular force and interatomic force, the solid-liquid interface exhibits a stable double-layer charge with an opposite sign, which is called an interface double layer. The electric double layer supercapacitor was regarded as two inactive porous plates suspended in an electrolyte, and the voltage was applied to two plates. An electric double-layer capacitor is created on the surface of the two electrodes when the potential provided to the positive electrode plate attracts negative ions in the electrolyte and the potential applied to the negative electrode plate attracts positive ions. The electric double-layer capacitor can be classified into a carbon electrode double-layer supercapacitor, a metal oxide electrode super capacitor, and an organic polymer electrode super capacitor depending on the electrode material.

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3. Features

In comparison to batteries and conventional physical capacitors, supercapacitors’ properties are primarily reflected in:

(1) High power density. It can achieve 102–104 W/kg, which is significantly more than the battery’s power density level.

(2) Long cycle life. After a high-speed deep charge and discharge cycle of 500,000 to 1 million cycles in a few seconds, the characteristics of the supercapacitor change little, and the capacity and internal resistance are only reduced by 10% to 20%.

(3) The working temperature limit is wide. Since the adsorption and desorption rates of ions in the supercapacitor do not change much at low temperatures, their capacity changes are much smaller than those of batteries. Commercial supercapacitors have an operating temperature range of -40°C to +80°C.

(4) Maintenance free. The supercapacitor has high charging and discharging efficiency, has a certain tolerance to overcharge and over-discharge, and can stably charge and discharge repeatedly, and theoretically, maintenance is not required.

(5) Environment friendly. Supercapacitors do not use heavy metals and other harmful chemicals in the production process and have a long life span, so they are a new type of green power source.

4. Classification

For supercapacitors, there are different classification methods depending on the content.

First, supercapacitors can be classified into two groups based on various energy storage mechanisms:

Faraday quasi capacitors
electric double-layer capacitors.

The electric double-layer capacitor, among them, primarily produces adsorption energy by adsorbing on the electrode’s surface with pure electrostatic charge. In order to achieve energy storage and conversion, Faraday quasi-capacitors primarily generate Faraday quasi-capacitance through reversible redox reactions on the surface and near the surface of Faraday quasi-capacitor active electrode materials (such as transition metal oxides and high molecular polymers).

Secondly, depending on the type of electrolyte, it can be divided into two categories:

water-based supercapacitors
organic supercapacitors.

In addition, depending on whether the types of active materials are the same, they can be classified into symmetric supercapacitors and asymmetric supercapacitors.

Secondly, according to the state form of the electrolyte, the supercapacitor can be divided into two categories: a solid electrolyte supercapacitor and a liquid electrolyte supercapacitor.

5. Main parameters of supercapacitor

1) Lifetime: When the internal resistance of the supercapacitor increases, the capacity reduction j is within the specified parameter range, and its effective use time can be extended, generally related to its characteristics as specified in Article 4. The impact on life is active dryness, increased internal resistance, and the ability to store electrical energy drops to 63.2%.

2) Voltage: The supercapacitor has a recommended voltage and a recommended operating voltage. If the used voltage is higher than the recommended voltage, the life of the capacitor will be shortened, but the capacitor can work continuously for a long time under the high voltage state, and the activated carbon inside the capacitor will decompose to form a gas. It is advantageous to store electrical energy, but it can’t exceed 1.3 times the recommended voltage, otherwise, the supercapacitor will be damaged due to the high voltage.

3) Temperature: The normal operating temperature of the supercapacitor is -40 to 70 °C. Temperature and voltage are important factors influencing the life of supercapacitors. For every 5 °C increase in temperature, the life of the capacitor will drop by 10%. At low temperatures, increasing the operating voltage of the capacitor, the internal resistance of the capacitor does not rise, and the efficiency of use of the capacitor can be improved.

4) Discharge: The internal resistance of the capacitor plays a significant role in the pulse charging technology, whereas the capacity plays a significant role in the tiny current discharge.

5) Charging: There are many ways to charge the capacitor, such as constant current charging, constant voltage charging, and pulse charging. During the charging process, connecting a resistor in the capacitor circuit will reduce the charging current and improve the battery life.