An electric double-layer capacitor (EDLC) also known as a supercapacitor or ultracapacitor is a specially designed charge storage device that offers higher capacitance and higher energy density than an electrolytic capacitor. These capacitors can be recharged very quickly and release large amounts of power. Unlike a battery that stores electric charge through chemical reactions, an ELDC stores charge using an electric double layer formed by ions adhering to the surface of an activated carbon electrode.

Working Principle

The capacitance of a capacitor is directly proportional to the area of the conducting plates and inversely proportional to the distance between the plates. To build a supercapacitor with a large capacitance value, the area of the conducting plate is increased and the distance between plates is decreased. 

An ELDC consists of- 

• Two carbon electrodes 
• One electrolyte
• One separator

The electrodes (conducting plates) are made up of metal coated with porous substances such as powdered activated charcoal. The porous nature of the surface increases the surface area making more area available to store more charges. These plates are immersed in an electrolyte and separated by a very thin insulator. This separator is used to provide insulation to the electrodes and prevent from short circuit. The electrolyte contains positively charged ions and negatively charged ions. When voltage is applied across the plates, one of the electrodes becomes positively charged and the other one becomes negatively charged. Due to electrostatic attraction, the negatively charged ions in the electrolyte get attracted towards the positively charged electrode and the positively charged ions in the electrolyte get attracted towards the negatively charged electrode.

These ions form a very thin layer having a thickness of one layer of ions over the electrode. This results in the formation of an electrostatic double layer with a series circuit of two capacitors C₁ and C₂. The upper layer of charges may be considered as C₁ with a small value of separation, d and the lower layer of charges may be considered as C₂ with a small value of separation, d. For both the capacitances, the high surface area of electrodes is large and the separation distance between the charges is small. Hence the total capacitance, which is the series combination of C₁ and C₂ is quite large.

Advantages

• Very high efficiency and reliability
• Fast transient response
• High energy density
• High charging rate 
• Longer life than batteries
• Can withstand a wider range of temperatures compared to batteries
• Lightweight structure
• Can be installed in small areas as it is compact.

Disadvantages

• Difficult to make high-voltage EDLC
• Cannot be used in AC applications
• Discharges more quickly than a battery when not connected to a load.
• Costlier than batteries of similar capacity.

Applications

• Used as a memory backup in electronic devices due to its high efficiency and long life cycles
• Used as a sub-power source for hybrid electric vehicles due to its higher power density
• Energy harvesting 
• Battery load leveling in mobile devices
• Energy regeneration in automobiles.

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