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] "MC" and "BC" series supercapacitors (up to 3000 ] capacitance)]]
An '''Electric double-layer capacitor''', also known as '''supercapacitor''', '''supercondenser''', '''pseudocapacitor''', '''] capacitor''' ('''EDLC'''), or '''ultracapacitor''', is an electrochemical ] that has an unusually high ] when compared to common capacitors, typically on the order of thousands of times greater than a high capacity ]. For instance, a typical ] sized electrolytic capacitor will have a ] in the range of tens of ]s. The same size electric double-layer capacitor would have a capacitance of several ]s, an improvement of about two or three ] in capacitance, but usually at a lower working voltage. Larger double-layer capacitors have capacities up to 5,000 farads {{As of|2010|lc=on}}.<ref>, Nesscap Products</ref> The highest energy density in production is 30 ],<ref name="30whkg">http://www.jeol.com/NEWSEVENTS/PressReleases/tabid/521/articleType/ArticleView/articleId/112/A-30-Whkg-Supercapacitor-for-Solar-Energy-and-a-New-Battery.aspx</ref> below rapid-charging ].


{{R from merge|Supercapacitor}}
EDLCs have a variety of commercial applications, notably in "energy smoothing" and momentary-load devices. They have applications as energy-storage devices used in ], and for smaller applications like home solar energy systems where extremely fast charging is a valuable feature.

''Note: all references to batteries in this article should be taken to refer to rechargeable, not primary (aka disposable), batteries''.

==Concept==
], middle: ], right: electric double-layer capacitor]]

In a conventional ], energy is stored by the removal of ]s, typically ]s, from one metal plate and depositing them on another. This charge separation creates a ] between the two plates, which can be harnessed in an external ]. The total energy stored in this fashion is proportional to both the amount of charge stored and the potential between the plates. The amount of charge stored per unit voltage is essentially a function of the size, the distance, and the material properties of the plates and the ] (i.e. the material in between the plates), while the potential between the plates is limited by ] of the substance separating the plates. Different materials sandwiched between the plates to separate them result in different voltages to be stored. Optimizing the material leads to higher energy densities for any given size of capacitor.

EDLCs do not have a conventional ]. Rather than two separate plates separated by an intervening substance, these capacitors use "plates" that are in fact two layers of the same substrate, and their electrical properties, the so-called "]", result in the effective separation of charge despite the vanishingly thin (on the order of nanometers) physical separation of the layers. The lack of need for a bulky layer of dielectric permits the packing of "plates" with much larger surface area into a given size, resulting in extraordinarily high capacitances in practical-sized packages.

In an electrical double layer, each layer by itself is quite ], but the physics at the interface where the layers are effectively in contact means that no significant current can flow between the layers. However, the double layer can withstand only a low ], which means that electric double-layer capacitors rated for higher voltages must be made of matched ] individual EDLCs, much like series-connected cells in higher-voltage batteries.

In general, EDLCs improve storage density through the use of a ] material, typically ], in place of the conventional ] barrier. Activated charcoal is a powder made up of extremely small and very "rough" particles, which, in bulk, form a low-density volume of particles with holes between them that resembles a ]. The overall surface area of even a thin layer of such a material is many times greater than a traditional material like aluminum, allowing many more charge carriers (]s or ] from the ]) to be stored in any given volume. The charcoal, which is not a good insulator, is taking the place of the excellent insulators used in conventional devices, so in general EDLCs can only use low potentials on the order of 2 to 3 V.

Activated charcoal is not the "perfect" material for this application. The charge carriers are actually (in effect) quite large—especially when surrounded by solvent molecules—and are often larger than the holes left in the charcoal, which are too small to accept them, limiting the storage. Most recent research in EDLCs has focused on improved materials that offer even higher ''usable'' surface areas. Experimental devices developed at ] replace the charcoal with ]s, which can store about the same charge as charcoal (which is almost pure carbon) but are mechanically arranged in a much more regular pattern that exposes a much greater suitable surface area.<ref name=mit>, Deborah Halber, MIT News Office, February 8, 2006</ref> Other teams are experimenting with custom materials made of activated ], and nanotube-impregnated papers.
] showing ] vs. ] for various energy-storage devices]]

The ] of existing commercial EDLCs ranges from around 0.5 to 30 W·h/kg, with the standardized cells available from ] rated at 6 W·h/kg and ACT in production of 30 W·h/kg.<ref></ref>

ACT's capacitor is actually a ], known also as a "hybrid capacitor". Experimental electric double-layer capacitors from the have demonstrated densities of 30 W·h/kg and appear to be scalable to 60 W·h/kg in the short term,<ref>Carbon Nanotube Enhanced Ultracapacitors, MIT LEES ultracapacitor project</ref> while ] claims their examples will offer energy densities of about 400 W·h/kg. For comparison, a conventional ] stores typically 30 to 40 W·h/kg and modern ] about 160 W·h/kg. ] has a ] (NCV) of around 12,000 W·h/kg; automobile applications operate at about 20% tank-to-wheel efficiency, giving an effective energy density of 2,400 W·h/kg.

EDLCs have much higher ] than batteries. Power density combines the energy density with the speed that the energy can be delivered to the ]. Batteries, which are based on the movement of charge carriers in a liquid electrolyte, have relatively slow charge and discharge times. Capacitors, on the other hand, can be charged or discharged at a rate that is typically limited by current heating of the electrodes. So while existing EDLCs have ''energy'' densities that are perhaps 1/10th that of a conventional battery, their ''power'' density is generally 10 to 100 times as great (see diagram, right).

==History==

The EDLC effect was first noticed in 1957 by ] engineers experimenting with devices using porous carbon electrodes.<ref>{{Ref patent|country=US|number=2800616|title=Low voltage electrolytic capacitor|gdate=1957-07-23|invent1=Becker, H.I.}}</ref> It was believed that the energy was stored in the carbon pores and it exhibited "exceptionally high capacitance", although the mechanism was unknown at that time.

General Electric did not immediately follow up on this work, and the modern version of the devices was eventually developed by researchers at ] in 1966, after they accidentally re-discovered the effect while working on experimental ] designs.<ref name=ieee>. IEEE Spectrum, November 2007</ref> Their cell design used two layers of ] separated by a thin porous insulator, and this basic mechanical design remains the basis of most electric double-layer capacitors to this day.

Standard Oil also failed to commercialize their invention, licensing the technology to ], who finally marketed the results as “supercapacitors” in 1978, to provide backup power for maintaining computer memory.<ref name=ieee/> The market expanded slowly for a time, but starting around the mid-1990s various advances in ] and simple development of the existing systems led to rapidly improving performance and an equally rapid reduction in cost.

The first trials of supercapacitors in industrial applications were carried out for supporting the energy supply to robots.<ref>http://rgn.hr/~dkuhinek/nids_daliborkuhinek/1%20OEE-RN/5Seminari/2006_2007/13%20Superkondenzator.ppt</ref>

In 2005 aerospace systems and controls company ] GmbH chose ultracapacitors Boostcap (of ]) to power emergency actuation systems for doors and ]s in ]s, including the new ] jumbo jet. Also in 2005, the ultracapacitor market was between US $272 million and $400 million, depending on the source.

In 2006 ] and his team at MIT began working on a "super battery", using ] technology to improve upon capacitors. They hope to put them on the market within five years. <ref>http://www.technologyreview.com/nanotech/wtr_16326,303,p1.html</ref><ref>http://web.mit.edu/erc/spotlights/ultracapacitor.html</ref>

In 2007<ref></ref> all ] micrometer-scale electric double-layer capacitors based on ] have been for future low-voltage electronics such as ] and related technologies (the 22 nm technological node of CMOS and beyond).

== Technology ==
Supercapacitors have several disadvantages and advantages relative to batteries, as described below.<ref>http://rgn.hr/~dkuhinek/nids_daliborkuhinek/1%20OEE-RN/5Seminari/2006_2007/13%20Superkondenzator.ppt</ref>

=== Disadvantages ===
* The amount of energy stored per unit weight is considerably lower than that of an electrochemical battery (3&ndash;5 W·h/kg for an ultracapacitor {{As of|2010|lc=on}} compared to 30-40 W·h/kg for a ]), and about 1/1,000th the volumetric energy density of gasoline.
*As with any capacitor, the voltage varies with the energy stored. Effective storage and recovery of energy requires complex electronic control and switching equipment, with consequent energy loss
*Has the highest ] of any type of capacitor.
*High ] - the rate is considerably higher than that of an electrochemical battery.
*Cells have low voltages - serial connections are needed to obtain higher voltages. Voltage balancing is required if more than three capacitors are connected in series.
*Linear discharge voltage prevents use of the full energy spectrum.
*Due to rapid and large release of energy (albeit over short times), EDLC's have the potential to be deadly to humans. One example is the case of rescue workers accidentally discharging an ultracap in hybrids electrics during ].

=== Advantages ===
*Long life, with little degradation over hundreds of thousands of ]s. Due to the capacitor's high number of charge-discharge cycles (millions or more compared to 200 to 1000 for most commercially available rechargeable batteries) it will last for the entire lifetime of most devices, which makes the device environmentally friendly. Rechargeable batteries wear out typically over a few years, and their highly reactive chemical electrolytes present a disposal and safety hazard. Battery lifetime can be optimised by only charging under favorable conditions, at an ideal rate and, for some chemistries, as infrequently as possible. EDLCs can help in conjunction with batteries by acting as a charge conditioner, storing energy from other sources for load balancing purposes and then using any excess energy to charge the batteries at a suitable time.
*Low cost ''per cycle''
*Good reversibility
*Very high ] and discharge.
*Extremely low internal resistance (]) and consequent high cycle efficiency (95% or more) and extremely low heating levels
*High output power
*High specific power. According to ITS (Institute of Transportation Studies, Davis, California) test results, the ] of electric double-layer capacitors can exceed 6 ]/] at 95% efficiency<ref> . APowerCap press release, 2006.</ref>
*Improved safety, no corrosive electrolyte and low toxicity of materials.
*Rapid charging—supercapacitors charge in seconds.
*Simple charge methods—no full-charge detection is needed; no danger of ].

=== Materials ===

], ], ]s and certain ]s, or carbon ]s, are practical for supercapacitors:

Virtually all commercial supercapacitors manufactured by ], ], ], ], ], and others use powdered activated carbon made from ] shells.<ref name = "Caltech"></ref> Some companies also build higher performance devices, at a significant cost increase, based on synthetic carbon precursors that are activated with ] (KOH).<ref name = "Caltech"></ref>

*] has excellent surface area per unit of gravimetric or volumetric densities, is highly conductive and can now be produced in various labs. It will not be long before large volumes of Graphene are produced for use in supercapacitors.

*Carbon nanotubes have excellent nanoporosity properties, allowing tiny spaces for the polymer to sit in the tube and act as a dielectric. MIT's Laboratory of Electromagnetic and Electronic Systems (LEES) is researching using carbon nanotubes.<ref name = "MIT">. MIT press release, 2006.</ref>

*Some polymers (eg. ]s) have a ] (reduction-oxidation) storage mechanism along with a high surface area.

*Supercapacitors are also being made of carbon ]. This is a unique material providing extremely high surface area of about 400-1000 m²/g. The electrodes of aerogel supercapacitors are usually made of non-] paper made from ]s and ] with organic aerogel, which then undergoes ]. The paper is a ] where the carbon fibers provide structural integrity and the aerogel provides the required large surface. Small aerogel supercapacitors are being used as backup electricity storage in microelectronics, but applications for ]s are expected. Aerogel capacitors can only work at a few volts; higher voltages would ionize the carbon and damage the capacitor. Carbon aerogel capacitors have achieved 325 J/g (90 W·h/kg) energy density and 20 W/g power density.<ref name = "AIP">Lerner EJ, . ''The Industrial Physicist'' (2004)</ref>

*The company claims to be able to make supercapacitors from solid ], which they call ''consolidated ]'' (CAC). It can have a surface area exceeding 2800 m<sup>2</sup>/g and according to {{patent|US|6787235}} may be cheaper to produce than aerogel carbon.

*Systematic pore size control and H<sub>2</sub> adsorption treatment showed by Y-Carbon<ref></ref> to produce ] can be used to increase the energy density by as much as 75% over what is commercially available {{As of|2005|lc=on}}.<ref name=CarbideDerivedCarbons>Yushin, G., Dash, R.K., Jagiello, J., Fischer, J.E., & Gogotsi, Y. (2006). Carbide derived carbons: effect of pore size on hydrogen storage and heat of adsorption. Advanced Functional Materials, 16(17), 2288-2293, Retrieved from http://nano.materials.drexel.edu/Papers/200500830.pdf</ref>

*The company developed supercapacitors from mineral-based carbon. This nonactivated carbon is synthesised from metal or metalloid carbides, e.g. SiC, TiC, Al4C3, etc. as claimed in {{patent|US|6602742}} and {{patent|WO|2005118471}}. The synthesised nanostructured porous carbon, often called Carbide Derived Carbon (CDC), has a surface area of about 400 m²/g to 2000 m²/g with a specific capacitance of up to 100 F/mL (in organic electrolyte). {{As of|2006}} they claimed a supercapacitor with a volume of 135 mL and 200 g weight having 1.6 kF capacitance. The energy density is more than 47 kJ/L at 2.85 V and power density of over 20 W/g.<ref> (2006)</ref>

*In August 2007 a research team at ] developed a paper ] with aligned carbon nanotubes, designed to function as both a lithium-ion battery and a supercapacitor (called ''bacitor''), using an ], essentially a liquid ], as the ]. The sheets can be rolled, twisted, folded, or cut into numerous shapes with no loss of integrity or efficiency, or stacked, like printer paper (or a ]), to boost total output. Further, they can be made in a variety of sizes, from ] to ]. Their light weight and low cost make them attractive for portable electronics, ], ]s, and toys (such as ]), while their ability to use electrolytes in blood make them potentially useful for medical devices such as ]. They are ].<ref>. Rensselaer Polytechnic Institute press release (13 August 2007)</ref>

== Applications ==

===Vehicles===
====Heavy and public transport====
{{See also|Capa vehicle}}
Some of the earliest uses were motor startup capacitors for large engines in ]s and ]s, and as the cost has fallen they have started to appear on diesel trucks and railroad locomotives.<ref>, US DoE overview</ref> More recently they have become a topic of some interest in the ] world, where their ability to store energy much faster than batteries makes them particularly suitable for ] applications. New technology {{As of|2010|alt=in development}} could potentially make EDLCs with high enough energy density to be an attractive replacement for batteries in ] and ]s, as EDLCs charge quickly and are stable with temperature.

China is experimenting with a new form of ] (capabus) that runs without powerlines using power stored in large onboard EDLCs, which are quickly recharged whenever the bus is at any ] (under so-called '''electric umbrellas'''), and fully charged in the ]. A few prototypes were being tested in Shanghai in early 2005. In 2006, two commercial bus routes began to use electric double-layer capacitor buses; one of them is route 11 in ].<ref> (in Chinese, archived page)</ref>

In 2001 and 2002 VAG, the public transport operator in ], Germany tested an ] that uses a ] battery drive system with electric double-layer capacitors.<ref></ref>

Since 2003 Mannheim Stadtbahn in Mannheim, Germany has operated an LRV (]) that uses electric double-layer capacitors to store braking energy.<ref>. Richard Hope, '']'' July 2006</ref><ref>M. Steiner. . Bombardier presentation (2006).</ref>

Other companies from the public transport manufacturing sector are developing electric double-layer capacitor technology: The Transportation Systems division of ] is developing a mobile energy storage based on double-layer capacitors called Sibac Energy Storage<ref> Siemens AG Sibac ES Product Page (as of November 2007)</ref> and also Sitras SES, a stationary version integrated into the trackside power supply.<ref>Siemens AG Sitras SES Product Page (as of November 2007)</ref> The company Cegelec is also developing an electric double-layer capacitor-based energy storage system.<ref>http://www.cegelec.cz/20-zarizeni-na-vyuziti-rekuperovane-energie.html</ref>

Proton Power Systems has created the world's first triple hybrid Forklift Truck, which uses ]s and ] as primary energy storage and EDLCs to supplement this energy storage solution.<ref> Fuel Cell Works press release (2007).</ref>

====Private vehicles====
Ultracapacitors are used in some electric vehicles, such as ]'s concept prototype, to store rapidly available energy with their high ], in order to keep batteries within safe resistive heating limits and extend battery life.<ref>{{cite news| url=http://www.nytimes.com/2008/01/13/automobiles/13ULTRA.html | work=The New York Times | title=Closing the Power Gap Between a Hybrid's Supply and Demand | first=Matthew L. | last=Wald | date=2008-01-13 | accessdate=2010-05-01}}</ref><ref>http://www.afstrinity.net/afstrinity-xh150-pressrelease.pdf</ref> The ] combines a supercapacitor and a battery in a single unit, creating an electric vehicle battery that lasts longer, costs less and is more powerful than current technologies used in ] electric vehicles (PHEVs).<ref>http://www6.lexisnexis.com/publisher/EndUser?Action=UserDisplayFullDocument&orgId=101846&topicId=103840033&docId=l:732161238</ref>

==== Motor racing====
The ], the governing body for many motor racing events, proposed in the ''Power-Train Regulation Framework for ]'' version 1.3 of 23 May 2007 that a new set of ] regulations be issued that includes a hybrid drive of up to 200 kW input and output power using "superbatteries" made with both batteries and supercapacitors.<ref>http://paddocktalk.com/news/html/modules/ew_filemanager/07images/f1/fia/332668895__2011_Power_Train_Regulation_Framework.pdf</ref>

=== Consumer electronics ===
EDLCs can be used in ]s, ] devices in ], ], ]s, and in ],<ref>http://fplreflib.findlay.co.uk/articles/6610/if-the-cap-fits.pdf</ref> particularly where extremely fast charging is desirable.

In 2007 a cordless ] that uses an EDLC for energy storage was produced.<ref></ref> It charges in 90 seconds, retains 85% of the charge after 3 months, and holds enough charge for about half the screws (22) a comparable screwdriver with a rechargeable battery will handle (37). Two LED flashlights using EDLCs were released in 2009. They charge in 90 seconds<ref></ref>

===Alternative energy sources===
The idea of replacing batteries with capacitors in conjunction with novel alternative energy sources became a conceptual umbrella of the Green Electricity (GEL) Initiative , , introduced by Dr. Alexander Bell. One particular successful implementation of the GEL Initiative concept was a muscle-driven autonomous solution that employs a multi-farad EDLC (hecto- and kilofarad range capacitors are now available) as an intermediate energy storage to power a variety of portable electrical and electronic devices such as MP3 players, AM/FM radios, flashlights, cell phones, and emergency kits.<ref> (Alexander Bell, EDN, 11/21/2005)</ref> As the energy density of EDLCs is bridging the gap with batteries, the ] is deploying ultracapacitors as a replacement for chemical batteries.

Several companies have begun capitalizing on this maturing technology, which can provide significant power and energy from a small component. Companies that have been conducting research and technology for this emerging industry are listed below:

*CAP-XX Ltd
*EnerG2
*Fluidic Energy Inc.
*Graphene Energy Inc.
*Maxwell Technologies
*Ioxus, Inc.

== Price ==
Costs have fallen quickly, with cost per ] dropping faster than cost per farad. {{As of|2006}} the cost of supercapacitors was 1 cent per farad and $2.85 per kilojoule, and was expected to drop further.<ref>http://www.electronicsweekly.com/Articles/2006/03/03/37810/Supercapacitors-see-growth-as-costs-fall.htm</ref>

== Market ==

According to ], ultracapacitor market growth will continue during 2009 to 2014. Worldwide business, over US$275 million in 2009, will continue to grow at an AAGR of 21.4% through 2014.<ref>http://www.innoresearch.net/report_summary.aspx?id=71&pg=171&rcd=ET-111&pd=2/1/2010</ref>

== See also ==
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== References ==
{{Reflist|colwidth=35em}}

== External links ==
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