Electric double-layer capacitors have a variety of commercial applications, notably in "energy smoothing" and momentary-load devices. 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. More recently they have become a topic of some interest in the green energy world, where their ability to soak up energy quickly makes them particularly suitable for regenerative braking applications, whereas batteries have difficulty in this application due to slow charging times. If the LEES or EEStor devices can be commercialized, they will make an excellent replacement for batteries in all-electric cars and plug-in hybrids, as they combine quick charging, temperature stability and excellent safety properties.
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 a combination of the number of charges stored and the potential between the plates. The former is essentially a function of size and the material properties of the plates, while the latter is limited by the dielectric breakdown between the plates. Various materials can be inserted between the plates to allow higher voltages to be stored, leading to higher energy densities for any given size.
In contrast with traditional capacitors, electric double-layer capacitors do not have a conventional dielectric, as such. They are based on a structure that contains an electrical double layer. In a double layer, the effective thickness of the "dielectric" is exceedingly thin—on the order of nanometers—and that, combined with the very large surface area, is responsible for their extraordinarily high capacitances in practical sizes.
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 electric double-layer capacitors, much like series-connected cells in higher-voltage batteries.
In general, electric double-layer capacitors 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 electrons to be stored in any given volume. The downside is that the charcoal is taking the place of the improved insulators used in conventional devices, so in general electric double-layer capacitors use low potentials on the order of 2 to 3 V.
Activated charcoal is not the "perfect" material for this application. Free electrons are actually (in effect) quite large, often larger than the holes left in the charcoal, which are too small to accept them, limiting the storage. Recent research in electric double-layer capacitors has generally focused on improved materials that offer even higher usable surface areas. Experimental devices developed at MIT replace the charcoal with carbon nanotubes, which have similar charge storage capability 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. Other teams are experimenting with custom materials made of activated polypyrrole, and even nanotube-impregnated papers.
A completely different approach is being pioneered by EEStor, who claim to have developed a dramatically improved insulator based on barium titanate that improves the permissivity of the insulator by several orders of magnitude, improving energy density not through electron capacity but via much higher potentials. EEStor claims that their capacitors can operate at extremely high voltages, on the order of several thousand volts.
In terms of energy density, existing commercial electric double-layer capacitors range around 0.5 to 10 W·h/kg, with the standardized cells available from Maxwell Technologies rated at 6 W·h/kg. 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, while EEStor claims their examples will offer capacities on the order of 200 to 300 W·h/kg. For comparison, a conventional lead-acid battery is typically 30 to 40 W·h/kg, modern lithium-ion batteries are about 120 W·h/kg, and in an automobile applications gasoline has a net calorific value (NCV) of around 12,000 W·h/kg operating at a 20% tank-to-wheel efficiency.
Additionally, electric double-layer capacitors offer much higher power density than batteries. Power density combines the energy density with the speed that the energy can be drawn out of the device. 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 electric double-layer capacitors have energy densities that are perhaps 1/10th that of a conventional battery, their power density is generally ten to one-hundred times as great (see diagram, right).
The electric double-layer capacitor effect was first noticed in 1957 by General Electric engineers experimenting with devices using porous carbon electrode. 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 were 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. 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. 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. In 2005, the ultracapacitor market was between US $272 million and $400 million, depending on the source. It is rapidly growing, especially in the automotive sector.
Recently , all solid state micrometer-scale electric double-layer capacitors based on advanced superionic conductors had been recognized as critical electron component of future sub-voltage and deep-sub-voltage nanoelectronics and related technologies (22 nm technological node of CMOS and beyond).
Due to the capacitor's high number of charge-discharge cycles (millions or more compared to 200–1000 for most commercially available rechargeable batteries) there are no disposable parts during the whole operating life of the device, which makes the device environmentally friendly. Batteries wear out on the order of a few years, and their highly reactive chemical electrolytes present a serious disposal and safety hazard. This can be improved by only charging under favorable conditions, at an ideal rate, and, for some chemistries, as rarely as possible. Electric double-layer capacitors can help in this regard, acting as a charge conditioner, storing energy from other sources for load balancing purposes and then using any excess energy to charge the batteries only at opportune times.
Other advantages of electric double-layer capacitors compared with rechargeable batteries are extremely low internal resistance or ESR, high efficiency (up to 97-98%), high output power, extremely low heating levels, and improved safety. According to ITS (Institute of Transportation Studies, Davis, CA) test results, the specific power of electric double-layer capacitors can exceed 6 kW/kg at 95% efficiency
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 which employs a multi-farad electric double-layer capacitor (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. As the energy density of electric double-layer capacitors is bridging the gap with batteries, it is hoped that in the near future the automotive industry will start to deploy ultracapacitors as a replacement for chemical batteries.
China is experimenting with a new form of electric bus (capabus) that runs without powerlines using power stored in large onboard electric double-layer capacitors, which are quickly recharged whenever the electric bus stops 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.
Since 2003 Mannheim Stadtbahn in Mannheim, Germany has operated an LRV (light-rail vehicle) which uses electric double-layer capacitors to store braking energy.
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 and also Sitras SES, a stationary version integrated into the trackside power supply . The company Cegelec is also developing a electric double-layer capacitor-based energy storage system. Proton Power Systems has created the world's first triple hybrid Forklift Truck, which uses fuel cells and battery as primary energy storage and electric double-layer capacitors to supplement this overall energy efficient storage solution.
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. As well, 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. In addition, they are biodegradable.
Researchers Submit Patent Application, "Method for Determining Pore Size and Pore Size Distribution of Porous Material Using Phase of Electro-Acoustic Signal", for Approval
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