What is Electric Double Layer Capacitor (EDLC)?
Electric Double Layer Capacitor (EDLC) is an ultracapacitor (or supercapacitor) based on electrodes made from varieties of carbon. Electrolyte is either an aqueous solution, or an organic solution in liquid form. The electrodes are separated by a permeable separator.
Carbons in these capacitors provide a large area of contact for electrode surface. These are all nanomaterials, highly porous in nature, which allow electrolyte to enter its volume and form contact surface throughout its volume. Effective surface area can go as high as 2600 sq. m. per gram of material.

It must be noted that there is no separate dielectric in ultracapacitor. A double layer of Helmholtz layers is formed at the interface of electrode / electrolyte. This serves as dielectric between the two, and forms a capacitor. Similar capacitor is formed at the second electrode as well. Combination of these two capacitors in series go to form the overall capacitance. This composition of capacitor gives this class of ultracapacitors the name ‘Electric Double Layer Capacitor’, or EDLC for short.
The two capacitors formed at the electrodes, in series with each other, go to form the effective capacitance between electrodes. The voltage either C1 or C2 can withstand depends on the decomposition voltage of electrolyte. This limits the rated voltages of EDLCs between 1.3 V to 3.0 V.
The separator membrane keeps the two halves of capacitor isolated from each other, while allowing ions to pass through. Both electrodes are mounted on metal conductors, which are then connected to external terminals. We may go through the most important material used in EDLCs.
Electrode being highly porous, contact area between electrode material and electrolyte is quite high, and decides the capacitance value and energy stored in capacitor. Initial capacitors were developed using activated carbon, which continue to remain in use even today. Other carbon based materials have been developed over the years, and research continues till date to get further improvement in these materials.
1- Activated Carbon (AC)
Activated carbon is mostly made from coconut shells / husks, coal, wood, and other carbonaceous matter. The grade used for ultracapacitor is mostly derived from coconut shells. These are converted to fine charcoal powder and activated under heat and steam, which gives pure carbon with high surface area.
Researchers are still engaged in finding betterment in surface area of activated carbon so as to increase energy density. Basically, the carbon particles are themselves porous structure, having hollow spaces in them, called pores. These pores are of various sizes as shown in figure.

Large pores greater than 50μ are called macropres, the ones having size between 2μ to 50 μ are mesopores, and those smaller than 2μ are micropores. Combination and size of pores go to form overall structure of activated carbon. This structure and composition of particles plays an important role in capacitor, as the electrolyte has to enter these pores to achieve highest possible electrode contact area. Electrolyte molecules often have size larger than micropores, and full advantage cannot be taken of all available surface for contact. Effective area available from coconut shell based carbon, varies between 800-1400 M2/gm. This limits energy density of EDLC using carbon electrodes to typically 5-6 Wh/Kg, and its power density to 1-2 kW/Kg.
2- Carbon Nanotubes (CNT)
Nanotubes are material structure where nanotube of 100-300micron length are densely packed at about 1012-1013 numbers per sq. cm. They may be considered as sheets of atoms rolled to form tubes, which go to form the volume of material.
Carbon nanotubes (CNT) come in different configurations, and are a good alternative to activated carbon. Their hollow structure allows the electrolyte to enter minute spaces inside, thereby increasing the contact surface area significantly in comparison to AC. They can even be grown on metal directly on metal sheets / foils.

Nanotubes have the advantage of offering much greater electrolyte penetration due to their hollow structure (even with lower surface area than AC), and hence both energy density and power density higher than Activated carbon. Penetration of electrolyte is so thorough that energy storage is several times that of AC . This factor also reduces series resistance (ESR) of EDLC capacitor drastically, and increases energy storage. Capacitance can as high as 102-140 F/g due to perfect penetration of electrolyte. Energy densities of 10 Wh/Kg are achieved, with specific power of 30 kW/Kg. Life of these capacitors will be between 300.000 to 500,000 charge / discharge cycles.
3- Carbo Aerogel
Aerogels are similar in nature to a gel material except that internal liquid is replaced by air. They are nanometer sized particles are bonded in three–dimensional highly porous structure. They are almost 90-99.5% air, with a density of the order of 1.9 mg/cc (water has density of 1 g/cc). Being almost as light as air, it is also called by alternative names ‘blue smoke’, ‘frozen smoke’, or solid smoke’. A cubic inch of aerogel has a surface area of about 1500-2500 s . m. /cc, even reaching to 3000 sq. m. / cc, which is larger than a football field. Aerogels are known to have very high thermal stability of over 400℃, because they are composed of thermally insulating (but electrically conductive) gas (no conduction), and also have no convection (being solid).

Theoretical energy density of capacitors made from aerogels can go up to 325 kJ/Kg, about 70% of Li-ion battery. Working voltage of aerogel capacitors can be 2.5-32 V, and the capacitors have very low ESR because of its structure. Energy density can be about 10 kW/kg.
4- Graphene
Graphene is sheet of one atom thickness. It is the material being researched most all over the world for ultracapacitors because of its unique properties. Being one atom thick, the sheets are fully open for contact with electrolyte from all sides, giving largest possible effective surface area for formation of dielectric.
To understand graphene thickness, a regular graphite pencil may be used to make black surface on a small area of paper, and then the graphite layer, if separated from paper, is graphene (this is not easy though). Old lab process for graphene was to deposit graphite oxide on a computer DVD, and then expose the disc to laser beam. Graphene layer can then be separated from disc.

A graphene capacitor can store energy comparable to Li-ion battery, charge or discharge in seconds, and has life of about a million operations. Structure of graphene allows flexible capacitors, and thin flat capacitors. However, graphene is not easy to manufacture, and pure graphene is all the more difficult, and is very costly today because of complicated process with low yield, though costs are coming down over time. Lot of research is directed towards making graphene capacitors with higher densities viable, and few manufacturers have come out with graphene capacitors.
Energy density of graphene capacitors can go to 70-85 Wh/Kg, comparable to Li-ion battery. Capacitance can go as high as 300 F/gm or more, with specific power 16 kW / Kg, Thus, graphene capacitors create a possibility of an ultracapacitor with energy density comparable to Li-ion battery, with high power capability of EDLC.