Electric double-layer capacitor
Electrical double-layer capacitors (EDLC) are, together with pseudocapacitors, part of a new type of electrochemical capacitors called supercapacitors, also known as ultracapacitors. Supercapacitors do not have a conventional solid dielectric. The capacitance value of an electrochemical capacitor is determined by two storage principles:
- Double-layer capacitance – electrostatic storage of the electrical energy achieved by separation of charge in a Helmholtz double layer at the interface between the surface of a conductor electrode and an electrolytic solution electrolyte. The separation of charge distance in a double-layer is on the order of a few Angstroms (0.3–0.8 nm) and is static in origin.
- Pseudocapacitance – Electrochemical storage of the electrical energy, achieved by redox reactions electrosorbtion or intercalation on the surface of the electrode by specifically adsorbed ions that results in a reversible faradaic charge-transfer on the electrode.
Double-layer capacitance and pseudocapacitance both contribute to the total capacitance value of a supercapacitor. However, the ratio of the two can vary greatly, depending on the design of the electrodes and the composition of the electrolyte. Pseudocapacitance can increase the capacitance value by as much as an order of magnitude over that of the double-layer by itself.
Supercapacitors are divided into three families, based on the design of the electrodes:
- Double-layer capacitors – with carbon electrodes or derivates with much higher static double-layer capacitance than the faradaic pseudocapacitance
- Pseudocapacitors – with electrodes out of metal oxides or conducting polymers with much higher faradaic pseudocapacitance than than the static double-layer capacitance
- Hybrid capacitors – capacitors with special electrodes that exhibit both significant double-layer capacitance and pseudocapacitance, such as lithium-ion capacitors
Supercapacitors have the highest available capacitance values per unit volume and the greatest energy density of all capacitors. They support up to 12,000 F/1.2 V, with capacitance values up to 10,000 times that of electrolytic capacitors. Supercapacitors bridge the gap between capacitors and rechargeable batteries. In terms of specific energy, as well as in terms of specific power, this gap covers several orders of magnitude. However, batteries still have about ten times the capacity of supercapacitors. While existing supercapacitors have energy densities that are approximately 10% of a conventional battery, their power density is generally 10 to 100 times as great. Power density combines energy density with the speed at which the energy can be delivered to the load. This makes charge and discharge cycles of supercapacitors much faster than batteries. Additionally, they will tolerate many more charge and discharge cycles than batteries.
In these electrochemical capacitors, the electrolyte is the conductive connection between the two electrodes. This distinguishes them from electrolytic capacitors, in which the electrolyte is the cathode and thus forms the second electrode.
Supercapacitors are polarized and must operate with the correct polarity. Polarity is controlled by design with asymmetric electrodes, or, for symmetric electrodes, by a potential applied during manufacture.
Supercapacitors support a broad spectrum of applications for power and energy requirements, including:
- Long duration low current for memory back up in (SRAMs)
- Power electronics that require very short, high current, as in the KERS system in Formula 1 cars
- Recovery of braking energy in vehicles
Exceptional are the manifold different trade or series names used for supercapacitors like: APowerCap, BestCap, BoostCap, CAP-XX, DLCAP, EneCapTen, EVerCAP, DynaCap, Faradcap, GreenCap, Goldcap, HY-CAP, Kapton capacitor, Super capacitor, SuperCap, PAS Capacitor, PowerStor, PseudoCap, Ultracapacitor.
In a conventional capacitor, energy is stored by moving charge carriers, typically electrons, from one metal plate to 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 increases with 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 material in between the plates (the dielectric), while the potential between the plates is limited by the breakdown field strength of the dielectric. The dielectric controls the capacitor's voltage. Optimizing the material leads to higher energy density for a given size.
EDLCs do not have a conventional dielectric. Rather than two separate plates separated by an intervening insulator, these capacitors use virtual plates made of two layers of the same substrate. Their electrochemical 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 and the porosity of the material used, permits the packing of plates with much larger surface area into a given volume, resulting in high capacitances in uniquely small packages.
In an electrical double layer, each layer by itself is quite conductive, but the physics at the interface between them means that no significant current can flow between the layers. The double layer can withstand only a low voltage, which means that higher voltages are achieved by matched series-connected individual EDLCs, much like series-connected cells in higher-voltage batteries.
EDLCs have much higher power density than batteries. Power density combines the energy density with the speed at which 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 can be charged or discharged at a rate that is typically limited by the heat tolerance of the electrodes.
While existing EDLCs have energy densities that are perhaps 1/10 that of a conventional battery, their power density is generally 10 to 100 times as great. This makes them most suited to an intermediary role between electrochemical batteries and electrostatic capacitors, where neither sustained energy release nor immediate power demands dominate.
|This section requires expansion. (November 2012)|
General Electric engineers experimenting with porous carbon electrodes first observed the EDLC effect in 1957. They believed that the energy was stored in the carbon pores and the device exhibited "exceptionally high capacitance", although the mechanism was unknown at that time.
General Electric did not immediately follow up on this work. In 1966 researchers at Standard Oil of Ohio developed the modern version of the device, after they accidentally re-discovered the effect while working on experimental fuel cell designs. Their cell design used two layers of activated charcoal separated by a thin porous insulator. This basic mechanical design remains the basis of most electric double-layer capacitors.
Standard Oil did not 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. The market expanded slowly for a time, but starting around the mid-1990s various advances in materials science and refinement of existing systems led to rapidly improving performance and rapid cost reduction.
The first trials of supercapacitors in industrial applications supported the energy supply to robots.
In 2005 aerospace systems and controls company Diehl Luftfahrt Elektronik GmbH chose supercapacitors to power emergency actuation systems for doors and evacuation slides in airliners, including the Airbus 380 jumbo jet. In 2005, the market reached between US $272 million and $400 million.
As of 2007 solid state micrometer-scale electric double-layer capacitors based on advanced superionic conductors were employed for low-voltage electronics such as deep-sub-voltage nanoelectronics and related technologies (the 22 nm technological node of CMOS and beyond).
As of 2010 multi-voltage 5.3 W EDLC power supply for medical equipment produced a total of 55 F of capacitance, charged in about 150 seconds and ran for about 60 seconds. The circuit used switch-mode voltage regulators followed by linear regulators for clean and stable power, reducing efficiency to about 70%. The developers recommended a buck-boost best handles the widely varying voltage across an EDLC buck-boost.
Each EDLC cell consists of two electrodes, a separator and an electrolyte. The two electrodes are often electrically connected to their terminals via a metallic collector foil. The electrodes are usually made from activated carbon since this material is electrically conductive and has a very large surface area to increase the capacitance. The electrodes are separated by an ion permeable membrane (separator) used as an insulator to prevent short circuits between the electrodes. This composite is rolled or folded into a cylindrical or rectangular shape and can be stacked in an aluminium can or a rectangular housing. The cell is typically impregnated with a liquid or viscous electrolyte, either organic or aqueous, although some are solid state. The electrolyte depends on the application, the power requirement or peak current demand, the operating voltage and the allowable temperature range. The outer housing is hermetically sealed.
||This article contains a pro and con list. (November 2012)|
Advantages of supercapacitors include:
- Long life, with little degradation over hundreds of thousands of charge cycles. Due to the capacitor's high number of charge-discharge cycles (compared to 200 to 1000 for most 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 charging only 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 optimally charge batteries.
- Low cost per cycle
- Good reversibility
- Fast charge and discharge.
- Low internal resistance—Low ESR and consequent high cycle efficiency (95% or more)
- Low heating levels during charge and discharge
- High output power
- High specific power/power density—According to the Institute of Transportation Studies, the specific power of electric double-layer capacitors can exceed 6 kW/kg at 95% efficiency.
- Improved safety—Uses non-corrosive electrolytes and low material toxicity.
- Simple charge methods—no danger of overcharging, thus no need for full-charge detection.
- In conjunction with rechargeable batteries, some applications use EDLC to supply energy directly, reducing battery cycling and extending life.
- Low energy density—The amount of energy stored per unit weight is generally lower than that of electrochemical batteries (3 to 5 W·h/kg, although 85 W·h/kg has been achieved in the lab as of 2010[update] compared to 30 to 40 W·h/kg for a lead acid battery, 100 to 250 W·h/kg for a lithium-ion battery and about 0.1% of the volumetric energy density of gasoline).
- High dielectric absorption—highest of any type of capacitor.
- High self-discharge—the rate is considerably higher than that of an electrochemical battery.
- Low maximum voltage—series connections are needed to obtain higher voltages and voltage balancing may be required.
- Rapid voltage drop—Unlike batteries, the voltage across any capacitor drops significantly as it discharges. Effective energy recovery requires complex electronic control and switching equipment, with consequent energy loss.
- Spark hazard—Low internal resistance allows extremely rapid discharge when shorted, resulting in a spark hazard generally much greater than with batteries.
In general, EDLCs improve storage density through the use of a nanoporous material, typically activated charcoal, in place of the conventional insulating dielectric barrier. Activated charcoal is an extremely porous, "spongy" form of carbon with an extraordinarily high specific surface area—a common approximation is that 1 gram (a pencil-eraser-sized amount) has a surface area of roughly 250 square metres (2,700 sq ft)—about the size of a tennis court. It is typically a powder made up of extremely fine but very "rough" particles, which, in bulk, form a low-density heap with many holes. As the surface area of such a material is many times greater than a traditional material like aluminum, many more charge carriers (ions or radicals from the electrolyte) can be stored in a given volume. As carbon is not a good insulator (vs. the excellent insulators used in conventional devices), in general EDLCs are limited to low potentials on the order of 2 to 3 V and thus are "stacked" (connected in series) to supply higher voltages.
Activated charcoal is not the "perfect" material for this application. The charge carriers it provides are far larger than the holes left in the charcoal, which are too small to accept them, limiting the storage. The mismatch is exacerbated when the carbon is surrounded by solvent molecules.
As of 2010[update] virtually all commercial supercapacitors use powdered activated carbon made from coconut shells. Higher performance devices are available, at a significant cost increase, based on synthetic carbon precursors that are activated with potassium hydroxide (KOH).
Research materials 
|Material||Energy density/power density||Notes|
|Graphene||85.6 W·h/kg at room temperature and 136 W·h/kg at 80 °C at a current density of 1 A/g comparable to that of nickel metal hydride batteries||The device uses curved graphene sheets that do not restack face-to-face. The curved shape enables the formation of mesopores accessible to and wettable by environmentally benign ionic liquids capable of operating at a voltage over 4 V. These devices fully use the surface capacitance and specific surface area of single-layer graphene.|
|Carbon nanotubes||?||Allow polymer to sit in the tube and act as a dielectric. Carbon nanotubes can store about the same charge as charcoal (which is almost pure carbon) per unit surface area, but nanotubes can be arranged in a more regular pattern that exposes greater suitable surface area. The high surface area and conductivity of single-wall carbon nanotubes further increase energy density. Multi-walled carbon nanotubes have mesopores that allow for easy access of ions at the electrode/electrolyte interface. Adding multi-walled nanotubes lowers resistance. Capacitors with multi-walled nanotube fibers had higher electron and electrolyte-ion conductivities than others. Improved power density.|
|Polyacenes and conducting polymers||?||redox (reduction-oxidation) storage mechanism along with a high surface area|
|Tunable nanoporous carbon (carbide-derived carbon)||?||exhibit high surface areas and tunable pore diameters to maximize ion confinement, increasing specific capacitance and energy densities above those offered by similar endohedral carbon allotropes. H2 adsorption treatment can increase energy density by as much as 75% over 2005-era commercial products.|
|Carbon aerogel||90 W·h/kg energy density and 20 W/g power density||Gravimetric densities of about 400–1000 m²/g. Electrodes are a composite material usually made of non-woven paper made from carbon fibers and coated with organic aerogel, which then undergoes pyrolysis. The carbon fibers provide structural integrity and the aerogel provides the surface area. Small aerogel supercapacitors are used as backup electricity storage in microelectronics. Aerogel capacitors can only work at a few volts; higher voltages ionize the carbon and damage the capacitor.|
|Solid activated carbon or consolidated amorphous carbon (CAC)||?||Surface area exceeding 2,800 m2/g|
|Mineral-based carbon||10 Wh/kg energy density and 50 kW/kg power density of devices||nonactivated carbon, synthesised from metal or metalloid carbides, e.g. SiC, TiC,Al4C3. The synthesised nanostructured porous carbon, often called Carbide Derived Carbon (CDC), has a surface area of about400 m2/g to 2,000 m2/g with a specific capacitance of up to100 F/mL (in organic electrolyte). As of 2006[update] this material was used in a supercapacitor with a volume of 135 mL and 200 g weight having 1.6 kF capacitance.|
|Bacitor||?||biodegradable paper battery with aligned carbon nanotubes, designed to function as both a lithium-ion battery and capacitor. The device employed an ionic liquid, essentially a liquid salt, as the electrolyte. The paper sheets can be rolled, twisted, folded, or cut with no loss of integrity or efficiency, or stacked, like ordinary paper (or a voltaic pile), to boost total output. They can be made in a variety of sizes, from postage stamp to broadsheet. Their light weight and low cost.|
|Polypyrrole and nanotube-impregnated papers.||Research|
A typical D-cell-sized conventional electrolytic capacitor may have capacitance of up to tens of millifarads. The same size EDLC might reach several farads, an improvement of two orders of magnitude. EDLCs maximum working voltage of a few volts was two orders of magnitude less than standard electrolytics. The amount of energy stored per unit of mass is called specific energy, which is often measured in watt-hours per kilogram (W⋅h/kg) or megajoules per kilogram (MJ/kg). In 2010 the highest available EDLC specific energy was 30 W⋅h/kg (approximately 0.01 MJ/kg).
The specific energy of existing commercial EDLCs ranges from around 0.5 to 30 W·h/kg including lithium ion capacitors, known also as a "hybrid capacitor". Experimental electric double-layer capacitors demonstrate specific energies of 30 W·h/kg and scale to at least 136 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; automobiles operate at about 20% tank-to-wheel efficiency, giving an effective specific energy of 2,400 W·h/kg. Electric automobiles run at a much higher efficiency. For example, the Tesla Roadster runs at an average battery-to-wheel efficiency of 88%. This is also a factor to be taken into account when dealing with first approximate comparisons.
Up to 85 W·h/kg has been achieved at room temperature in the lab, which is still lower than rapid-charging lithium-titanate batteries. As of 2012[update] commercially available EDLCs typically have mass-to-volume ratio between 0.33 and 3.89 kg/l.
Research is ongoing to improve performance. For example, an order of magnitude power density improvement was achieved in the laboratory in mid-2011.
A charged EDLC loses its charge (self-discharge) much faster than a typical electrolytic capacitor and somewhat faster than a rechargeable battery.
Costs per kilojoule has dropped faster than cost per farad. As of 2006 the cost of supercapacitors was 1 cent per farad and $2.85 per kilojoule and dropping. In 2001, a 3 kF capacitor cost US$5,000; by 2011, its cost had dropped to $50.
General automotive 
EDLCs are used in some concept vehicles, to keep batteries within resistive heating limits and extend battery life. An "ultrabattery" combines a supercapacitor and a battery in one unit, creating an electric vehicle battery that lasts longer, costs less and is more powerful than current plug-in hybrid electric vehicles (PHEVs).
Heavy transport 
Some of the earliest uses of EDLCs 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. In the 2000s they attracted attention in the electric car industry, where their ability to charge 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 respect to temperature.
China is experimenting with a new form of electric bus (capabus) that runs uses onboard EDLCs, which quickly recharge whenever the bus is at any bus stop (under so-called electric umbrellas) and fully charge in the terminus. A few prototypes were tested in Shanghai in early 2005. In 2006, two commercial bus routes converted, including Route 11 in Shanghai.
In 2001 and 2002 VAG, the public transport operator in Nuremberg, Germany tested a hybrid bus that uses a diesel-electric battery drive system with EDLCs. From 2003 to 2008 Mannheim Stadtbahn in Mannheim, Germany operated a light-rail vehicle (LRV) using EDLCs for regenerative braking. In October 2007 Rhein-Neckar Verkehr in Germany ordered 19 light-rail vehicles (LRVs) equipped with EDLCs to store braking energy, using Bombardier MITRAC equipment as tested in Mannheim. The tests in Mannheim showed 30% energy savings. In addition, the EDLCs enabled the LRV's to operate in an area of Heidelberg without overhead wires. The EDLC equipment cost an additional €270,000 per vehicle, which is expected to be recovered in the first 15 years of operation. In April 2011 Rhein-Neckar Verkehr ordered 11 more LRVs equipped with EDLCs.
In 2009 in Paris a light-rail vehicle (LRV) was fitted with a bank of 48 supercapacitors mounted on the roof both to store braking energy and operate without an overhead line on parts of its route, running on stored energy between electrified segments and recharging quickly at segments equipped with catenary. In 2012 tram operator TPG in Geneva began tests of a light-rail vehicle (LRV) equipped with a prototype supercapacitor mounted on the roof to recover braking energy. In August 2012 the CSR Zhouzhou Electric Locomotive corporation of China presented a prototype two-car light metro train equipped with a roof-mounted EDLC providing regenerative braking and the ability to operate without overhead wires while charging at stations.
Other public transport manufacturers are developing EDLC technology, including mobile storage and a stationary trackside power supply. Hong Kong's South Island metro line is to be equipped with two 2 MW energy storage EDLC units, which are expected to reduce energy consumption by 10%. Adetel Group has developed its own energy saver named ″NeoGreen″ for LRV and metros 
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 power train regulations be extended to include a hybrid drive of up to 200 kW input and output power using battery/EDLC hybrids.
The Toyota TS030 HYBRID LMP1 car uses a hybrid drivetrain with EDLCs. In the 2012 24 Hours of Le Mans race a TS030 using EDLCs qualified with a fastest lap only 1.055 seconds slower (3:24.842 versus 3:23.787) than the fastest car, an Audi R18 e-tron quattro with flywheel energy storage. The energy storage devices made the Audi and Toyota hybrids the fastest cars in the race. In the 2012 Le Mans race the two competing TS030s, one of which was in the lead for part of the race, both retired for reasons unrelated to the EDLCs. TS030s then won three of the other races in the 8-race 2012 FIA World Endurance Championship season.
Personal car 
The Max Planck Institute claimed that their EDLC prototype offered power density (0.47 kW/kg) and energy density (300 Wh/kg) greater than many batteries and other EDLCs. A 200 kg EDLC in an electric vehicle, would produce 60 kWh. A car that needed 10-15 kW of power at 100 km/h, would have a theoretical range of 400–600 km assuming 100% efficient usage of the energy and a range of 360–540 km at 10% combined losses in ELDC, power electronics and drive train.
Battery complement 
When used in conjunction with rechargeable batteries in uninterruptible power supplies and similar applications, the EDLC can handle short interruptions, requiring the batteries to be used only during long interruptions, reducing cycles and extending battery life.
Low-power applications 
EDLCs can drive low-power equipment such as PC Cards, photographic flash, flashlights, portable media players and automated meter reading equipment. They are advantageous when extremely fast charging is required. In professional medical applications, EDLCs have been used to power a handheld, laser-based breast cancer detector (55 F to provide 5.3 W at multiple voltages) that charges in 150 seconds and runs for 60 seconds).
In 2007 a cordless electric screwdriver that uses an EDLC for energy storage was produced. 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.
According to Innovative Research and Products (iRAP), ultracapacitor market growth will continue during 2009 to 2014. They forecast that worldwide business, over US$275 million in 2009, would continue to grow at an average annual growth rate of 21.4% through 2014.
See also 
- B. E. Conway (1999) (in German), [ at Google Books Electrochemical Supercapacitors: Scientific Fundamentals and Technological Applications], Berlin: Springer, pp. 1–8, ISBN 0306457369,  at Google Books See also Brian E. Conway in Electrochemistry Encyclopedia: Electrochemical Capacitors — Their Nature, Function and Applications
- Adam Marcus Namisnyk. "A SURVEY OF ELECTROCHEMICAL SUPERCAPACITOR TECHNOLOG" (in German). Retrieved 2011-06-24.
- Elzbieta Frackowiak, Francois Beguin, PERGAMON, Carbon 39 (2001) 937–950, Carbon materials for the electrochemical storage of energy in Capacitors PDF
- Kötz, R.; Carlen, M. (2000). "Principles and applications of electrochemical capacitors". Electrochimica Acta 45: 2483–2498.
- Garthwaite, Josie (12 July 2011). "How ultracapacitors work (and why they fall short)". Earth2Tech. GigaOM Network. Retrieved 13 July 2011. More than one of
- US 2800616, Becker, H.I., "Low voltage electrolytic capacitor", issued 1957-07-23
- The Charge of the Ultra – Capacitors. IEEE Spectrum, November 2007
- Boostcap (of Maxwell Technologies)
- "Высокоёмкие конденсаторы для 0,5 вольтовой наноэлектроники будущего". Nanometer.ru. 17 October 2007. Retrieved 2013-03-14.
- Park, Chulsung; No, Keunsik; Chou, Pai H. TurboCap: A Batteryless, Supercapacitor-based Power Supply. University of California, USA and National Tsing Hua University, Taiwan. CiteSeerX: 10.1.1.143.2621.
- Prototype Test APowerCap press release: Results highly appreciated by Ultracapacitor Experts, 2006.
- Laine, Jorge; Simon Yunes (1992). "Effect of the preparation method on the pore size distribution of activated carbon from coconut shell". Carbon 30 (4): 601–604. doi:10.1016/0008-6223(92)90178-Y.
- Liu, Chenguang; Yu, Zhenning; Jang, Bor Z.; Zhamu, Aruna; Jang, Bor Z. (2010). "Graphene-Based Supercapacitor with an Ultrahigh Energy Density". Nano Letters (American Chemical Society) 10 (12): 4863–4868. doi:10.1021/nl102661q.
- MIT LEES on Batteries. MIT press release, 2006.
- Researchers fired up over new battery, Deborah Halber, MIT News Office, 8 February 2006
- Arepalli, S.; H. Fireman, C. Huffman, P. Moloney, P. Nikolaev, L. Yowell, C.D. Higgins, K. Kim, P.A. Kohl, S.P. Turano and W.J. Ready (2005). "Carbon-Nanotube-Based Electrochemical Double-Layer Capacitor Technologies for Spaceﬂight Applications". JOM: 24–31.
- Du, C. S.; Pan N. J. Power Sources 2006, 160, 1487–1494
- Dillon, A.C. (2010). "Carbon Nanotubes for Photoconversion and Electrical Energy Storage". Chem. Rev. 110 (11): 6856–6872. doi:10.1021/cr9003314. PMID 20839769.
- 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 fromhttp://nano.materials.drexel.edu/Papers/200500830.pdf
- Y-Carbon. Y-carbon.us. Retrieved on 13 September 2011.
- Lerner EJ, "Less is more with aerogels: A laboratory curiosity develops practical uses". The Industrial Physicist(2004)
- Reticle US 6787235
- US 6602742 and WO 2005118471
- developments in carbide derived carbon (2006)
- . Skeleton Technologies, Estonia.
- batteries: storing power in a sheet of paper. Rensselaer Polytechnic Institute press release (13 August 2007)
- 5000F, Nesscap Products
- A 30 W·h/kg Supercapacitor for Solar Energy and a New Battery. Jeol.com (3 October 2007). Retrieved on 13 September 2011.
- Advanced Capacitor Technologies, Inc. ( ACT ). Act.jp. Retrieved on 13 September 2011.
- Ultracapacitor – Google Video – une vidéo Techniek en wetenschap. Dailymotion. Retrieved on 13 September 2011.
- Liu, Chenguang; Yu, Zhenning; Neff, David; Zhamu, Aruna; Jang, Bor Z. (2010). "Graphene-Based Supercapacitor with an Ultrahigh Energy Density". Nano Letters 10 (12): 4863. doi:10.1021/nl102661q.
- Carbon Nanotube Enhanced Ultracapacitors, MIT LEES ultracapacitor project
- Graphene supercapacitor breaks storage record. physicsworld.com. Retrieved on 13 September 2011.
- Note: all references to batteries in this article should be taken to refer to rechargeable, not primary (aka disposable), batteries.
- "Super Capacitor Products". Illinoiscapacitor.com. Retrieved 2013-03-14.
- Chemistry World: New carbon material boosts supercapacitors. Rsc.org. 13 May 2011. Retrieved on 13 September 2011.
- Supercapacitors see growth as costs fall. Electronics Weekly. 3 March 2006. Retrieved on 13 September 2011.
- Advent of Ultracapacitors Signals Change in Wind Turbine Capabilities. renewableenergyworld.com. March 2011. Retrieved on 13 September 2011.
- AFS Trinity Wald, Matthew L. (13 January 2008). "Closing the Power Gap Between a Hybrid's Supply and Demand". The New York Times. Retrieved 1 May 2010.
- AFS TRINITY UNVEILS 150 MPG EXTREME HYBRID (XH™) SUV. (PDF) . AFS Trinity Power Corporation. 13 January 2008. Retrieved on 13 September 2011.
- http://www.csiro.au/science/Ultra-Battery.html. Missing or empty
- "Ultracapacitors". Office of Energy Efficiency and Renewable Energy. 15 April 2009. Retrieved 10 January 2011. "Many applications can benefit from ultracapacitors, especially HEVs and PHEVs. Ultracapacitors can be the primary energy source during acceleration and hill climbing, as well as for recovery of braking energy because they are excellent at providing quick bursts of energy."
- Supercapacitors, US DoE overview[dead link]
- "Ultracapacitors". Office of Energy Efficiency and Renewable Energy. 15 April 2009. Retrieved 10 January 2011. "Using an ultracapacitor in conjunction with a battery combines the power performance of the former with the greater energy storage capability of the latter. It can extend the life of a battery, save on replacement and maintenance costs and enable a battery to be downsized."
-  (in Chinese, archived page)
- VAG Verkehrs-AG Nürnberg. En.vag.de. Retrieved on 13 September 2011.
- UltraCaps win out in energy storage. Richard Hope, Railway Gazette International July 2006
- M. Steiner. MITRAC Energy Saver. Bombardier presentation (2006).
- Bombardier MITRAC Energy Saver, http://www.bombardier.com/files/en/supporting_docs/Mitrac_Energy_Saver.pdf
- Super cap tests complete, http://www.railwaygazette.com/nc/news/single-view/view/supercap-tests-complete.html Railway Gazette International, 18 May 2008
- Rhein Neckar Verkehr orders more supercapacitor trams, http://www.railwaygazette.com/nc/news/single-view/view/rhein-neckar-verkehr-orders-more-supercapacitor-trams.html Railway Gazette International, April 2011
- 'Supercapacitors to be tested on Paris STEEM tram,'http://www.railwaygazette.com/nc/news/single-view/view/supercapacitors-to-be-tested-on-paris-steem-tram.html Railway Gazette International, July 2009
- 'Geneve tram trial assesses supercapacitor performance,' Railway Gazette International, 7 August 2012, http://www.railwaygazette.com/news/industry-technology/single-view/view/geneve-tram-trial-assesses-supercapacitor-performance.html
- 'Supercapacitor light metro train unveiled, Railway Gazette International, 23 August 2012, http://www.railwaygazette.com/news/single-view/view/supercapacitor-light-metro-train-unveiled.html
- The Transportation Systems division of Siemens AG is developing mobile energy storage called Sibac Energy Storage Siemens AG Sibac ES[dead link] Sibac ES Product Page (as of November 2007)
- Sitras SES Sitras SES Sitras SES Product Page (as of November 2007)
- Cegelec a.s. | Electrical equipment for municipal mass transit | utilization of regenerated energy | transport. Cegelec.cz. Retrieved on 13 September 2011.
- 'Supercapacitor Energy Storage for South Island line,' Railway Gazette International, August 3, 2012,http://www.railwaygazette.com/news/single-view/view/supercapacitor-energy-storage-for-south-island-line.html
- Proton Power Systems Unveils the World’s First Triple-hybrid Forklift Truck. Fuel Cell Works press release (2007).
- Formula One 2011: Power-Train Regulation Framework. (PDF) . 24 May 2007. Retrieved on 13 September 2011.
- Racecar Engineering: Toyota TS030 LMP1 Hybrid revealed. (HTML). 24 January 2012. Retrieved on 24 January 2012.
- TOYOTA Racing Impresses in Le Mans Qualifying, http://www.toyotahybridracing.com/toyota-racing-impresses-in-le-mans-qualifying/?myvar=News
- Parrish, Alton (2010-01-18). "Ideas, Inventions And Innovations: Polyaniline Nanoporous Carbon Electrode Materials Yield Most Powerful Supercapacitors to Date Say Max Planck Researchers". Nanopatentsandinnovations.blogspot.se. Retrieved 2013-03-14.
- "United States Patent Application: 0100008021". Appft1.uspto.gov. Retrieved 2013-03-14.
- Using SuperCapacitors for Energy Storage, 2010. discoversolarenergy.com. Retrieved on 13 September 2011.
- Graham Pitcher If the cap fits ... New Electronics. 26 March 2006
- Coleman FlashCell Cordless Screwdriver. Ohgizmo.com (1 October 2007). Retrieved on 13 September 2011.
- Ultracapacitor LED Flashlight Charges In 90 Seconds
- Ultracapacitors For Stationary, Industrial, Consumer And Transport Energy Storage – An Industry, Technology And Market Analysis
- 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