Maxwell Technologies "MC" and "BC" series supercapacitors (up to 3000 farad capacitance)

Electric double-layer capacitors, also known as supercapacitors, pseudocapacitors, electrochemical double layer capacitors (EDLCs), or ultracapacitors, are electrochemical capacitors that have an unusually high energy density when compared to common capacitors, typically on the order of thousands of times greater than a high capacity electrolytic capacitor. For instance, a typical D-cell sized electrolytic capacitor will have a capacitance in the range of tens of millifarads. The same size electric double-layer capacitor would have a capacitance of several farads, an improvement of about two or three orders of magnitude in capacitance, but usually at a lower working voltage. Larger double-layer capacitors have capacities up to 5,000 farads as of 2010^[update].^[1] The highest energy density in production is 30 W·h/kg.^[2]

EDLCs have a variety of commercial applications, notably in "energy smoothing" and momentary-load devices. They have applications as energy-storage devices used in vehicles, and for smaller applications like home solar 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, batteries.

[edit] Concept

Comparison of construction diagrams of three capacitors. Left: "normal" capacitor, middle: electrolytic, right: electric double-layer capacitor

In a conventional capacitor energy is stored by the removal of charge carriers, typically electrons, from one metal plate and depositing them on another. This charge separation creates a potential between the two plates, which can be harnessed in an external circuit. 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 is essentially a function of size and the material properties of the plates, while the potential between the plates is limited by dielectric breakdown 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 dielectric. 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 "electrical double layer", 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 conductive, 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 voltage, which means that electric double-layer capacitors rated for higher voltages must be made of matched series-connected individual EDLCs, much like series-connected cells in higher-voltage batteries.

In general, EDLCs improve storage density through the use of a nanoporous material, typically activated charcoal, in place of the conventional insulating 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 sponge. 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 (ions or radicals from the electrolyte) 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 MIT replace the charcoal with carbon nanotubes, 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.^[3] Other teams are experimenting with custom materials made of activated polypyrrole, and nanotube-impregnated papers.

Ragone chart showing energy density vs. power density for various energy-storage devices

The energy density of existing commercial EDLCs ranges from around 0.5 to 30 W·h/kg, with the standardized cells available from Maxwell Technologies rated at 6 W·h/kg and ACT in production of 30 W·h/kg.^[4][5] ACT's capacitor is actually a lithium ion capacitor, known also as a "hybrid capacitor". Experimental electric double-layer capacitors from the MIT LEES project have demonstrated densities of 30 W·h/kg and appear to be scalable to 60 W·h/kg in the short term,^[6] while EEStor claims their examples will offer energy densities of about 400 W·h/kg. For comparison, a conventional lead-acid battery stores typically 30 to 40 W·h/kg and modern lithium-ion batteries about 160 W·h/kg. Gasoline has a net calorific value (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 power density than batteries. Power density combines the energy density with the speed that the energy can be delivered to the load. 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).

[edit] History

The EDLC effect was first noticed in 1957 by General Electric engineers experimenting with devices using porous carbon electrodes.^[7] 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 Standard Oil of Ohio in 1966, after they accidentally re-discovered the effect while working on experimental fuel cell designs.^[8] Their cell design used two layers of activated charcoal 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 NEC, who finally marketed the results as “supercapacitors” in 1978, to provide backup power for maintaining computer memory.^[8] The market expanded slowly for a time, but starting around the mid-1990s various advances in materials science 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.^[9]

In 2005 aerospace systems and controls company Diehl Luftfahrt Elektronik GmbH chose ultracapacitors Boostcap (of Maxwell Technologies) to power emergency actuation systems for doors and evacuation slides in airliners, including the new Airbus 380 jumbo jet. Also in 2005, the ultracapacitor market was between US $272 million and $400 million, depending on the source.

In 2006, Joel Schindall and his team at MIT began working on a "super battery", using nanotube technology to improve upon capacitors. They hope to put them on the market within five years.

In 2007^[10] all solid state micrometer-scale electric double-layer capacitors based on advanced superionic conductors have been for future low-voltage electronics such as deep-sub-voltage nanoelectronics and related technologies (the 22 nm technological node of CMOS and beyond).

[edit] Technology

Supercapacitors have several disadvantages and advantages relative to batteries, as described below.^[11]

[edit] Disadvantages

• The amount of energy stored per unit weight is considerably lower than that of an electrochemical battery (3-5 W·h/kg for an ultracapacitor as of 2010^{[update][citation needed]} compared to 30-40 W·h/kg for a lead acid battery), and about 1/10,000th 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 losses of energy
• Has the highest dielectric absorption of any type of capacitor.
• High self-discharge - 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.

[edit] Advantages

• Long life, with little degradation over hundreds of thousands of cycles. 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 rates of charge and discharge.
• Extremely low internal resistance (ESR) 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 specific power of electric double-layer capacitors can exceed 6 kW/kg at 95% efficiency^[12]
• 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 overcharge.

[edit] Materials

Activated carbon, graphene, carbon nanotubes and certain conductive polymers, or carbon aerogels, are practical for supercapacitors:

Virtually all commercial supercapacitors manufactured by Panasonic, Nesscap, Maxwell Technologies, Nippon Chemi-Con, Axion Power, and others use powdered activated carbon made from coconut shells^{[citation needed]}. Some companies also build higher performance devices, at a significant cost increase, based on synthetic carbon precursors that are activated with potassium hydroxide (KOH)^{[citation needed]}.

• Graphene 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 is produced for 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.^[13]

• Some polymers (eg. polyacenes) have a redox (reduction-oxidation) storage mechanism along with a high surface area.

• Supercapacitors are also being made of carbon aerogel. 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-woven paper made from carbon fibers and coated with organic aerogel, which then undergoes pyrolysis. The paper is a composite material 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 electric vehicles 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.^[14]

• The company Reticle claims to be able to make supercapacitors from solid activated carbon, which they call consolidated amorphous carbon (CAC). It can have a surface area exceeding 2800 m²/g and according to US patent 6787235 may be cheaper to produce than aerogel carbon.

• Systematic pore size control and H₂ adsorption treatment showed by Y-Carbon^[15] to produce tunable nanoporous carbon can be used to increase the energy density by as much as 75% over what is commercially available as of 2005^[update].^[16]

• The company Tartu Technologies 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 US patent 6602742 and WO patent 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^[update] 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.^[17]

• In August 2007 a research team at RPI developed a paper battery with aligned carbon nanotubes, designed to function as both a lithium-ion battery and a supercapacitor (called bacitor), using an ionic liquid, essentially a liquid salt, as the electrolyte. 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 Voltaic pile), to boost total output. Further, they can be made in a variety of sizes, from postage stamp to broadsheet. Their light weight and low cost make them attractive for portable electronics, aircraft, automobiles, and toys (such as model aircraft), while their ability to use electrolytes in blood make them potentially useful for medical devices such as pacemakers. They are biodegradable.^[18]

[edit] Applications

[edit] Vehicles

[edit] Heavy and public transport

Some of the earliest uses were motor startup capacitors for large engines in tanks and submarines, and as the cost has fallen they have started to appear on diesel trucks and railroad locomotives.^[19] More recently they have become a topic of some interest in the green energy world, where their ability to store energy much faster than batteries makes them particularly suitable for regenerative braking applications. New technology in development^[update] could potentially make EDLCs with high enough energy density to be an attractive replacement for batteries in all-electric cars and plug-in hybrids, as EDLCs charge quickly and are stable with temperature.

China is experimenting with a new form of electric bus (capabus) that runs without powerlines using power stored in large onboard EDLCs, which are quickly recharged whenever the bus is at any bus stop (under so-called electric umbrellas), and fully charged in the terminus. 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 Shanghai.^[20]

In 2001 and 2002 VAG, the public transport operator in Nuremberg, Germany tested an hybrid bus which uses a diesel-electric battery drive system with electric double-layer capacitors.^[21]

Since 2003 Mannheim Stadtbahn in Mannheim, Germany has operated an LRV (light-rail vehicle) which uses electric double-layer capacitors to store braking energy.^[22][23]

Other companies from the public transport manufacturing sector are developing electric double-layer capacitor technology: The Transportation Systems division of Siemens AG is developing a mobile energy storage based on double-layer capacitors called Sibac Energy Storage^[24] and also Sitras SES, a stationary version integrated into the trackside power supply.^[25] The company Cegelec is also developing an electric double-layer capacitor-based energy storage system.^[26]

Proton Power Systems has created the world's first triple hybrid Forklift Truck, which uses fuel cells and batteries as primary energy storage and EDLCs to supplement this energy storage solution.^[27]

[edit] Private vehicles

Ultracapacitors are used in some electric vehicles, such as AFS Trinity's concept prototype, to store rapidly available energy with their high power density, in order to keep batteries within safe resistive heating limits and extend battery life.^[28][29] The Ultrabattery 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 plug-in hybrid electric vehicles (PHEVs).^[30]

[edit] Motor racing

The FIA, the governing body for many motor racing events, proposed in the Power-Train Regulation Framework for Formula 1 version 1.3 of 23 May 2007 that a new set of power train 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.^[31]

[edit] Consumer electronics

EDLCs can be used in PC Cards, flash photography devices in digital cameras, flashlights, portable media players, and in automated meter reading^[32], particularly where extremely fast charging is desirable.

• In 2007 Coleman announced in 2007 a cordless electric screwdriver which uses an EDLC for energy storage.^[33] It charges in 90 seconds and retains 85% of the charge after 3 months, but holds less charge than a rechargeable battery.

• Two flashlights released by 5.11 Tactical in 2009 use electric double-layer capacitors rather than batteries, and charge in 90 seconds^[34]

[edit] 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 [2], [3], introduced by Dr. Alexander Bell. One particular successful implementation of the GEL Initiative concept was a muscle-driven autonomous solution which 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.^[35] As the energy density of EDLCs is bridging the gap with batteries, the vehicle industry 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 which have been conducting research and technology for this emerging industry are listed below:

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

[edit] Price

Costs have fallen quickly, with cost per kilojoule dropping faster than cost per farad. As of 2006^[update] the cost of supercapacitors was 1 cent per farad and $2.85 per kilojoule, and was expected to drop further.^[36]

[edit] Market

According to Innovative Research and Products (iRAP), 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.^[37]

[edit] See also

• Electric vehicle battery
• Types of capacitors
• Nanoflower
• Rechargeable electricity storage system
• Flywheel energy storage
• List of emerging technologies
• Lithium ion capacitor
• Self-powered equipment
• Mechanically powered flashlight

[edit] References

[edit] External links

• Super Capacitor Seminar
• Article on ultracapacitors at electronicdesign.com
• Article on ultracapacitors at batteryuniversity.com
• A new version of an old idea is threatening the battery industry (The Economist).
• An Encyclopedia Article From the Yeager center at CWRU.
• Ultracapacitors & Supercapacitors Forum
• Special Issue of Interface magazine on electrochemical capacitors
• Nanoflowers Improve Ultracapacitors: A novel design could boost energy storage (Technology Review) and Can nanoscopic meadows drive electric cars forward? (New Scientist)
• If the cap fits... How supercapacitors can help to solve power problems in portable products.
• A web that describes the development of solid-state and hybrid supercapacitors from CNR-ITAE (Messina) Italy