http://www.maxwell.com/products/ultracapacitorshttp://en.wikipedia.org/wiki/Supercapacitor

https://gigaom.com/2011/07/12/how-ultracapacitors-work-and-why-they-fall-short/

Hang around the energy storage crowd long enough, and youll hear chatter about ultracapacitors. Tesla Motors chief executive Elon Musk has said he believes capacitors will even supercede batteries.What is it that makes ultracapacitors such a promising technology?And if ultracapacitors are so great, why have they lost out to batteries, so far, asthe energy storage device of choicefor applications like electric cars and the power grid?Put simply, ultracapacitors are some of the best devices around for delivering a quick surge of power. Because an ultracapacitor stores energy in an electric field, rather than in a chemical reaction, it can survive hundreds of thousands more charge and discharge cycles than a battery can.A more thorough answer, however, looks at how ultracapacitors compare to capacitors and batteries. From there well walk through some of the inherent strengths and weaknesses of ultracaps, how they can enhance (rather than compete with) batteries, and what the opportunities are to advance ultracapacitor technology.Capacitor 101A basic capacitor consists of two metal plates, or conductors (typically aluminum), separated by an insulator, such as air or a filmmade of plastic, or ceramic. During charging, electrons accumulate on one conductor, and depart from the other. In effect, a negative charge builds on one side while a positive charge builds on the other.The negatively charged electronswantto join the depleted (positive) side, but cant cross over that non-conductive insulator (for the most part, anywaythere is some leakage). This separation of positive and negative charges, which want to balance out, or neutralize, each other, creates whats called an electric field. Discharging occurs when the electrons are given a path to flow to the other sidein other words, when balance is restored.Putting the ultra in ultracapacitors

Ultracap diagram courtesy of NRELUltracapacitors also have two metal plates, but they are coated with a sponge-like, porous material known as activated carbon. And theyreimmersed in an electrolytemade of positive and negative ions dissolved in a solvent. One carbon-coated plate, or electrode, is positive, and the other is negative. During charging, ions from the electrolyte accumulate on the surface of eachcarbon-coated plate.Like capacitors, ultracapacitors store energy in an electric field, which is created between two oppositely charged particles when they are separated. Recall that in an ultracapacitor, we have this electrolyte, in which an equal number of positive and negative ions are uniformly dispersed. And remember that in a capacitor, negative charge builds on one side and positive charge builds on the other. Similarly, in an ultracapacitor, when voltage is applied across the two metal plates (i.e. during charging), a charge still builds on the two electrodesone positive, one negative. This then causes each electrode to attract ions of the opposite charge.But for an ultracapacitor, each carbon electrode ends up havingtwo layersof charge coating its surface (thus, ultracaps are also called double layer capacitors), John Kassakian, a professor in MITs Laboratory for Electromagnetic and Electronic Systems (LEES), explained to me: In effect, an ultracapacitor is actually two capacitors in series, one at each electrode.Joel Schindall, another professor in MITs LEES and associate director of the lab, explained that during discharging, the charge on the plates decreases as electrons flow through an external circuit. The ions are no longer attracted to the plate as strongly, he said, so they break off and once again distribute themselves evenly through the electrolyte.The ultracap advantage

Unlike capacitors and ultracapacitors, batteries store energy in a chemical reaction. Ions are actually inserted into the atomic structure of an electrode (in an ultracap, the ions simply cling). This is an important distinction, because storing energywithout chemical reactionsallows ultracapacitors to charge and discharge much faster than batteries, Schindall explained. And because capacitors dont suffer the wear and tear caused by chemical reactions, they can also last much longer. (See previous post:Why lithium-ion batteries die so young)Charge separation is at work in both capacitors and ultracapacitors. But in a capacitor, the separated charges can get no closer than the distance between the two metal plates. Theyre awfully close togetheron the order of tens of micronsbut limited by the thickness of that ceramic or paper film in the middle (one micron is one-thousandth of a millimeter). In an ultracapacitor, the distance between the ions and opposite-charged electrode is so tiny its measured in nanometers (one-thousandth of a micron).Why should we care about such small distances? Turns out the size of the electric field isinverselyproportional to the separation distance. The shorter distance between those separated charges in an ultracapacitor translates to a larger electric fieldand much more energy storage capacity.Thats only part of why ultracapacitors can store more energy than regular capacitors. The activated carbon is also key. See, its so spongy, according to Schindall, that it affords a surface area 10,000 to 100,000 times greater than the linear surface area of the naked metal. Put simply, all those nooks and crannies in the surface allow more ions to cling to the electrode.Measuring capacitancePrincipio del formularioGet all the news you need about Cleantech with the Gigaom newsletterSubscribeFinal del formularioSurface area makes a huge difference for whats calledcapacitance, or the amount of electric charge a device will hold given a certain amount of voltage. Capacitance is the key metric for comparing capacitor performance, and its measured in Farads (named,asLostfans might appreciate, after the chemist and physicist Michael Faraday).Now, the Farad is such a huge unit of measurement, its like measuring distance in light years, said Schindall. So its much more common to see microfarads (one-millionthof a farad) and even picofarads (one-millionth of a microfarad).A capacitor the size of a D-cell battery, for example, has a capacitance of only about 20 microfarads. But a similarly sized ultracapacitor has a capacitance of 300 Farads. That means, at the same voltage, the ultracapacitor could in theory store up to 15 million times more energy than the capacitor.Here is where we run into some of the challenges with ultracapacitors, however. A typical 20-microfarad capacitor would be able to handle as much as 300 volts, while an ultracap would be rated at only 2.7 volts. At a higher voltage, the electrolyte starts to break down. So realistically were talking about an ultracapacitor storing about 1,500 times the energy of a comparably sized capacitor, said Schindall.Ultracaps and batteries as partners

Despite offering a huge leap over regular capacitors, ultracaps still lag behind batteries when it comes to energy storage capacity. Ultracapacitors (which are alsomore expensive per energy unitthan batteries), can store only about 5 percent of the energy of comparable lithium-ion batteries. And that, said Schindall, is a fatal flaw for many applications.It would be technically possible, for example, to use ultracaps instead of lithium-ion batteries in cell phones, with some serious benefits: You would never have to replace the ultracapacitor, said Schindall, and the phone would recharge very quickly. But the phone wouldnt stay charged for very long at all with todays ultracapacitorsperhaps as little as 90 minutes, or five hours max, Schindall said.Ultracapacitors are very effective, however, at accepting or delivering a sudden surge of energy, and that makes them a good partner for lithium-ion batteries, Schindall explained. In an electric car, for example, an ultracapacitor could provide the power needed for acceleration, while a battery provides range and recharges the ultracap between surges.Think of it this way: The ultracapacitor is like a small bucket with a big spout. Water can flow in or out very fast, but theres not very much of it. The battery is like a big bucket with a tiny spout. It can hold much more water, but it takes a long time to fill and drain it. The small bucket can provide a brief power surge (lots of water in this analogy), and then refill gradually from the big bucket, Schindall explained.Putting ultracaps to work

Already, Schindall believes some electric vehicle manufacturers are using ultracapacitors for acceleration. The devices also appear in hundreds of other applications, from cell phone base stations to alarm clocks (as backup power) to audio systems.For most music, Schindall explained, a high-end audio system with big speakers might do just fine with a 1-watt amplifier. But then the kettle drum comes in, demanding a sudden power surge of 1-kilowatt. One solution, Schindall said, is to build a 1-watt supply, plus an ultracapacitor to handle the peak.Ultracapacitors hold promise for a similar job on the electric grid. Today, transmission lines operate below full capacity (often somewhere above 90 percent), said Schindall, in order to leave a buffer for power surges. Banks of ultracapacitors could be set up to absorb power surges, enabling transmission lines to run closer to 100 percent capacity.It might not seem like much, especially considering that it would take warehouse-sized banks for ultracaps to do the job. But installing ultracapacitors to handle the peaks would actually be much cheaper, Schindall said, than adding even 5 percent more capacity with new transmission lines.In cars, ultracapacitorscould play a role in the growing market for microhybrids,which cut the engine during idling. In thesestart-stop systems, Schindall explained in an email, The ultracapacitor would provide power during the stop (lights, radio, air conditioner, etc.). It would also provide power for the restart, and then be recharged during the next interval of travel.How to build better ultracapacitorsThere are two basic ways to improve the performance of ultracapacitors: increase the surface area of the plate coating, and increase the maximum amount of voltage that the device can handle.Recall old Faraday again. Capacitance, measured in Farads, is how much electric energy our device will hold given a certain voltage. Increase the voltage, and you can increase the amount of energy our device holds (energy is equal to half the capacitance, multiplied by voltage squared).Schindall is tackling the surface area challenge using carbon nanotubes (more like a shag carpet orpaintbrushthan the sponge-like activated carbon). Other researchers, he noted, are working with graphene or better activated carbon. In addition to boosting the surface area, carbon nanotubes and graphene can also withstand a somewhat higher voltage than activated carbon, said Schindall.The voltage challenge, meanwhile seems to be a tougher road, he said. Researchers are experimenting with ionic liquid electrolytes (all ion, no solvent, behaves like a liquid), which under the right conditions can operate at up to three times the voltage of conventional electrolytes.But ionic liquids are fussy, Schindall said. They dont like being liquids, and tend to freeze below room temperature. Theyre also expensive, and they have higher resistance than conventional electrolytes, which means you cant get energy out as fast. The maximum powerone of ultracaps key advantagesis decreased. As Schindall put it, Theres always a tradeoff.

CASTELLANOQu es lo que hace que los ultracondensadores una tecnologa tan prometedora? Y si ultracondensadores son tan grandes, por qu tienen que perdieron a bateras, hasta el momento, ya que el dispositivo de almacenamiento de energa de eleccin para aplicaciones como los coches elctricos y la red elctrica?En pocas palabras, los ultracondensadores son algunos de los mejores dispositivos para la entrega de alrededor de un aumento rpido de la energa. Debido a que un almacena energa ultracondensadores en un campo elctrico, en lugar de en una reaccin qumica, que puede sobrevivir a cientos de miles ms ciclos de carga y descarga que una lata de la batera.Una respuesta ms a fondo, sin embargo, analiza cmo los ultracondensadores comparan con condensadores y bateras. A partir de ah vamos a caminar a travs de algunas de las fortalezas y debilidades de ultracaps inherentes, cmo pueden mejorar (en lugar de competir con) las bateras, y cules son las oportunidades para avanzar en la tecnologa ultracondensador.

El condensador 101Un condensador bsico consiste en dos placas de metal, o conductores (normalmente de aluminio), separados por un aislante, como por ejemplo aire o una pelcula hecha de plstico o de cermica. Durante la carga, los electrones se acumulan en un conductor, y salen de la otra. En efecto, una carga negativa se basa en un lado mientras que una carga positiva se basa en el otro.Los electrones cargados negativamente quieren unirse a la parte empobrecido (positivo), pero no puede cruzar que aislante no conductor (en su mayor parte, de todos modos, hay alguna fuga). Esta separacin de cargas positivas y negativas, que quieren equilibrar o neutralizar, s, crea lo que se llama un campo elctrico. La descarga se produce cuando los electrones se les da un camino a fluir hacia el lado de la otra en otras palabras, cuando se restaura el equilibrio.Poner la "ultra" en ultracondensadores

Ultracap diagrama cortesa de NRELUltracondensadores tambin tiene dos placas de metal, pero se recubren con un material poroso de tipo esponja conocido como carbn activado. Y estn inmersos en un electrolito hecho de iones positivos y negativos disueltos en un disolvente. Una placa revestida de carbono, o el electrodo, es positivo, y el otro es negativo. Durante la carga, los iones de electrolito se acumulan en la superficie de cada placa revestida de carbono.Como condensadores, ultracondensadores almacenar energa en un campo elctrico, que se crea entre dos partculas con cargas opuestas cuando estn separados. Recordemos que en un ultracondensador, tenemos este electrolito, en el que un nmero igual de iones positivos y negativos se dispersan uniformemente. Y recuerda que en un condensador, la carga negativa se basa en un lado y carga positiva se basa en el otro. Del mismo modo, en un supercondensador, cuando se aplica voltaje a travs de las dos placas de metal (es decir, durante la carga), una carga todava se basa en los dos electrodos-positivo, uno negativo. Esto provoca que cada electrodo para atraer los iones de carga opuesta.Pero para un ultracondensador, cada electrodo de carbono llega a tener dos capas de carga que recubren su superficie (por lo tanto, ultracaps tambin se llaman "condensadores de doble capa"), John Kassakian, profesor en el Laboratorio del MIT para electromagnticos y sistemas electrnicos (LEES), explic a m: "En efecto, un ultracondensador es en realidad dos condensadores en serie, uno en cada electrodo."Joel Schindall, otro profesor en LEES del MIT y director asociado del laboratorio, explic que durante la descarga, la carga en las placas disminuye a medida que los electrones fluyen a travs de un circuito externo. "Los iones ya no se sienten atrados por la placa con tanta fuerza", dijo, "por lo que se desprenden y una vez ms se distribuyen uniformemente a travs del electrolito."La ventaja Ultracap

A diferencia de los condensadores y supercondensadores, bateras almacenan energa en una reaccin qumica. Los iones son en realidad insertan en la estructura atmica de un electrodo (en una Ultracap, los iones simplemente se aferran). Esta es una distincin importante, ya que el almacenamiento de energa sin reactionsallows qumicos ultracondensadores para la carga y descarga mucho ms rpido que las bateras, explic Schindall. Y debido a que los condensadores no sufren el desgaste provocado por reacciones qumicas, tambin pueden durar mucho ms tiempo. (Ver post anterior: Por qu las bateras de iones de litio mueren tan jvenes)Separacin de la carga est en el trabajo en ambos condensadores y supercondensadores. Pero en un condensador, los cargos separados pueden llegar a no menos de la distancia entre las dos placas de metal. Son tremendamente cerca juntos, del orden de decenas de micras, pero limitado por el espesor de esa pelcula cermica o papel en el medio (una micra es la milsima parte de un milmetro). En un ultracondensador, la distancia entre los iones y electrodos opuesto cargada es tan pequeo que mide en nanmetros (una milsima parte de una micra).Por qu nos preocupamos por distancias tan pequeas? Resulta que el tamao del campo elctrico es inversamente proporcional a la distancia de separacin. La distancia ms corta entre esas cargas separadas en un supercondensador se traduce en un campo y mucha ms capacidad de almacenamiento de energa elctrica ms grande.Eso es slo una parte de la razn por ultracondensadores pueden almacenar ms energa que los condensadores normales. El carbn activado tambin es clave. Ver, es "tan esponjoso", segn Schindall, que proporciona un rea de superficie de 10.000 a 100.000 veces mayor que el rea de superficie lineal del metal desnudo. En pocas palabras, todos los rincones y grietas en la superficie permiten ms iones de aferrarse al electrodo.Capacitancia MedicinRecibe todas las noticias que necesita sobre Cleantech boletn GigaOMSuscribirSuperficie hace una gran diferencia para lo que es calledcapacitance, o la cantidad de carga elctrica de un dispositivo celebrar dada una cierta cantidad de voltaje. La capacitancia es la medida clave para comparar el rendimiento del condensador, y se mide en faradios (con nombre, como los fans de Lost puede apreciar, por el qumico y fsico Michael Faraday).Ahora, el faradio es una gran unidad de medida tal, "es como medir la distancia en aos luz", dijo Schindall. As que es mucho ms comn ver microfaradios (una millonsima parte de un faradio) e incluso picofaradios (una millonsima parte de un microfaradio).Un condensador del tamao de una batera de tipo D, por ejemplo, tiene una capacitancia de slo aproximadamente 20 microfaradios. Sin embargo, un ultracapacitor de tamao similar tiene una capacitancia de 300 faradios. Eso significa que, en el mismo voltaje, el ultracondensador podra en teora almacenar hasta 15 millones de veces ms energa que el condensador.Aqu es donde nos encontramos con algunos de los desafos con los ultracondensadores, sin embargo. Un condensador tpico de 20 microfaradios sera capaz de manejar tanto como 300 voltios, mientras que un Ultracap sera clasificado a slo 2,7 voltios. En un voltaje ms alto, el electrolito empieza a descomponerse. As que de manera realista que estamos hablando de un ultracondensador almacenar aproximadamente 1.500 veces la energa de un condensador de tamao comparable, dijo Schindall.Ultracaps y bateras como socios

A pesar de ofrecer un gran salto sobre los condensadores normales, ultracaps siguen a la zaga bateras cuando se trata de la capacidad de almacenamiento de energa. Los ultracondensadores (que son also more caros por unidad de energa que las bateras), puede almacenar slo el 5 por ciento de la energa de las bateras de iones de litio comparables. Y eso, dijo Schindall, es un "error fatal" para muchas aplicaciones.Sera tcnicamente posible, por ejemplo, para utilizar ultracaps en lugar de bateras de iones de litio en los telfonos celulares, con algunos beneficios graves: Nunca tendra que sustituir el ultracondensador, dijo Schindall, y el telfono sera recargar rpidamente. Pero el telfono no se quedara mucho tiempo cargada en absoluto con los actuales ultracondensadores-quizs tan slo 90 minutos, o cinco horas mximo, dijeron Schindall.Los ultracondensadores son muy efectivos, sin embargo, en la aceptacin o la entrega de un aumento repentino de la energa, y que ellos un buen socio para bateras de iones de litio hace, explic Schindall. En un coche elctrico, por ejemplo, un ultracondensador podra proporcionar la energa necesaria para la aceleracin, mientras que una batera proporciona la gama y se recarga la Ultracap entre oleadas.Pinsalo de esta manera: El ultracondensador es como un pequeo cubo con un gran pico. El agua puede fluir dentro o fuera muy rpido, pero no hay mucho de ella. La batera es como un cubo grande con un pequeo pico. Puede contener mucha ms agua, pero se necesita mucho tiempo para llenar y drenarlo. El pequeo cubo puede proporcionar un breve "subida de tensin" ("mucha agua" en esta analoga), y vulvalo a llenar poco a poco de la gran cubo, explic Schindall.Poner ultracaps para trabajar

Ya, Schindall cree que algunos fabricantes de vehculos elctricos utilizan ultracondensadores para la aceleracin. Los dispositivos tambin aparecen en cientos de otras aplicaciones, de las estaciones de base de telefona celular a los relojes de alarma (como energa de reserva) a sistemas de audio.Para la mayora de la msica, Schindall explic, un sistema de audio de alta gama con grandes altavoces podra hacer muy bien con un amplificador de 1 vatio. "Pero entonces el timbal entra," exigiendo una subida de tensin repentina de 1 kilovatio. Una solucin, Schindall dijo, es la construccin de un suministro de 1 vatio, ms un ultracondensador para manejar el pico.Los ultracondensadores prometedores para un trabajo similar sobre la red elctrica. Hoy en da, las lneas de transmisin operan por debajo de su capacidad total (a menudo en algn lugar por encima del 90 por ciento), dijo Schindall, con el fin de dejar un buffer para subidas de tensin. Bancos de ultracondensadores podran establecerse para absorber las sobrecargas de energa, permitiendo lneas de transmisin para funcionar ms cerca de 100 por ciento de capacidad.Puede que no parezca mucho, especialmente teniendo en cuenta que se necesitaran los bancos de depsito de tamao para ultracaps para hacer el trabajo. Pero la instalacin de ultracondensadores para manejar los picos en realidad sera mucho ms barato, Schindall dijo, que la adicin de incluso un 5 por ciento ms de capacidad con nuevas lneas de transmisin.En coches, los ultracondensadores podran desempear un papel en el creciente mercado de "microhbridos", que cortan el motor al ralent. En estos sistemas de "start-stop", Schindall explic en un correo electrnico, "El ultracondensador proporcionara energa durante la parada (luces, radio, aire acondicionado, etc.)." Tambin sera proporcionar energa para el reinicio, y luego ser " recargado durante el siguiente intervalo de viajes ".Cmo construir mejores ultracondensadoresHay dos formas bsicas para mejorar el desempeo de los ultracondensadores: aumentar la superficie de la capa de placa, y aumentar la cantidad mxima de tensin que el dispositivo puede manejar.Recordemos viejo Faraday nuevo. Capacitancia, medida en faradios, es la cantidad de energa elctrica nuestro dispositivo celebrar dado un cierto voltaje. Aumente la tensin, y puede aumentar la cantidad de energa que nuestro dispositivo tiene (la energa es igual a la mitad de la capacidad, multiplicada por la tensin al cuadrado).Schindall est abordando el desafo superficie utilizando nanotubos de carbono (ms como una alfombra de peluche o un pincel que el carbn activado de tipo esponja). Otros investigadores, seal, estn trabajando con el grafeno o carbn activado mejor. Adems de aumentar el rea de superficie, los nanotubos de carbono y grafeno tambin pueden "soportar una tensin un poco ms alto" que el carbn activado, dijo Schindall.El reto de tensin, por su parte "parece ser un camino ms difcil", dijo. Los investigadores estn experimentando con electrolitos inicos lquidos (todo in, ningn disolvente, se comporta como un lquido), que en las condiciones adecuadas puede operar a un mximo de tres veces la tensin de los electrolitos convencionales.Pero los lquidos inicos son "quisquilloso", dijo Schindall. "No les gusta ser lquidos," y tienden a congelar por debajo de la temperatura ambiente. Tambin son caros, y tienen mayor resistencia que los electrolitos convencionales, lo que significa que no se puede obtener la energa tan rpido. -Se ventajas disminuy el poder, uno de los principales Ultracaps 'mximo. Como Schindall dijo, "Siempre hay una solucin de compromiso."

http://spectrum.ieee.org/transportation/advanced-cars/the-charge-of-the-ultra-capacitors

In 1995, a small fleetof innovative electric buses began running along 15-minute routes through a park at the northern end of Moscow. A decade later, a few dozen seaport cranes in Asia, a couple of light-rail trains in Europe, and a battalion of garbage trucks in the United States have joined their high-tech ranks.A smattering of mass-transit vehicles and industrial machines may seem like one wimpy revolution, but revolutionary they are. Unlike most of their electric relatives, these vehicles all share one key attribute: they don't run on batteries. Instead, they are powered by ultracapacitors, which are souped-up versions of that tried-and-true workhorse of electrical engineering, the capacitor.A bank of ultracapacitors releases a burst of energy to help a crane heave its load aloft; they then capture energy released during the descent to recharge. Buses, trams, and garbage trucks powered by the devices all run for short stretches before stopping, and it's during braking that the ultracapacitors can partially recharge themselves from the energy that's normally wasted, giving the vehicles much of the juice they need to get to their next destinations.Because no chemical reaction is involved, ultracapacitors--also known as supercapacitors and double-layer capacitors--are much more effective at rapid, regenerative energy storage than chemical batteries are. What's more, rechargeable batteries usually degrade within a few thousand charge-discharge cycles. In a given year, a light-rail vehicle might go through as many as 300 000 charging cycles, which is far more than a battery can handle. (Although flywheel energy-storage systems can be used to get around that difficulty, a heavy and complicated transmission system is needed to transfer the energy.)The synergy between batteries and capacitors--two of the sturdiest and oldest components of electrical engineering--has been growing, to the point where ultracapacitors may soon be almost as indispensable to portable electricity as batteries are now.Ultracapacitors are already all over the place. Millions of them provide backup power for the memory used in microcomputers and cellphones. They also supply brief bursts of energy to numerous consumer products containing batteries. In a camera, for example, an ultracapacitor can extend battery life by providing the oomph for power-intensive functions, like zooming in for a close-up.Perhaps most exciting is what ultracapacitors could do for electric cars. They're being explored as replacements for the batteries in hybrid cars. In ordinary cars, they could help level the load on the battery by powering acceleration and recovering energy during braking. Most deadly to the life of a battery are the moments when it is subjected to high-current pulses and charged or discharged too quickly. Conveniently, delivering or accepting power during short-duration events is the ultracapacitor's strongest suit. And because capacitors function well in temperatures as low as 40 C, they can give electric cars a boost in cold weather, when batteries are at their worst.Commercially available ultracapacitors already address those needs to some extent and can provide many times the power of batteries of the same weight or size. But in terms of the amount of energy they can hold, ultracapacitors lag far behind. The major difference is that batteries store energy in the bulk of their material, whereas all forms of capacitors store energy only on the surface of a material. Like a battery, an ultracapacitor is filled with an ionic solution--an electrolyte--and its current collectors attach to the electrodes and conduct current to and from them. The collectors are coated with a thin film of activated carbon that has orders of magnitude more surface area than ordinary capacitors. The amount of surface area in ultracapacitor designs has so far been constrained by the limitations in the porosity of the activated carbon.The innovation that my colleagues John Kassakian and Riccardo Signorelli and I have pursued at MIT is to replace the activated carbon with a dense, microscopic forest of carbon nanotubes that is grown directly on the surface of the current collector. We think--and our work so far supports our theory--that by doing so, we can create a device that can hold up to 50 percent as much electrical energy as a comparably sized battery. This feat would allow ultracapacitors to supplant batteries in a number of mainstream applications.It's almost engineering heresyto suggest that a capacitor could power a car. Indeed, the common capacitor stores a puny amount of energy. At equivalent voltage, a chemical battery can store at least a million times as much energy as a conventional capacitor of the same size. Put two ordinary capacitors the size of a D-cell battery in your flashlight, each charged to 1.5 volts, and the bulb will go out in less than a second, if it lights at all. An ultracapacitor of the same size, however, has a capacitance of about 350 farads and could light the bulb for about 2 minutes.Before delving into our methods, I should explain the basics of capacitors and ultracapacitors. Capacitors have been around since 1745, beating batteries to the scene by half a century. Ultracapacitors are much more recent, but they're not exactly new, either. Engineers at Standard Oil patented ultracapacitor technology in 1966, an unanticipated product of their fuel-cell research. Standard Oil licensed the technology to NEC Corp., of Tokyo, which commercialized the results as supercapacitors in 1978, to provide backup power for maintaining computer memory.A capacitor consists of two electrodes, or plates, separated by a thin insulator. When a voltage is applied to the electrodes, an electric field builds up between the plates. A capacitor's energy is stored in such an electric field, without requiring any sort of chemical reaction. Thus a capacitor has an almost unlimited lifetime. It's also fast. Depending on its physical structure, typical charge and discharge times are on the order of a microsecond; sometimes they are as quick as a picosecond.Three main factors determine how much electrical energy a capacitor can store: the surface area of the electrodes, their distance from each other, and the dielectric constant of the material separating them. However, you can push conventional capacitor designs only so far. What the Standard Oil engineers did was to develop a capacitor that functions differently. They coated two aluminum electrodes with a 100-micrometer-thick layer of carbon. The carbon was first chemically etched to produce many holes that extended through the material, as in a sponge, so that the interior surface area was about 100 000 times as large as the outside. (This process is said to activate the carbon.)They filled the interior with an electrolyte and used a porous insulator, one similar to paper, to keep the electrodes from shorting out. When a voltage is applied, the ions are attracted to the electrode with the opposite charge, where they cling electrostatically to the pores in the carbon. At the low voltages used in ultracapacitors, carbon is inert and does not react chemically with the ions attached to it. Nor do the ions become oxidized or reduced, as they do at the higher voltages used in an electrolytic cell.This approach allowed the engineers at Standard Oil to build a multifarad device. At the time, even large capacitors had nowhere near a farad of capacitance. Today, ultracapacitors can store 5 percent as much energy as a modern lithium-ion battery. Ultracapacitors with a capacitance of up to 5000 farads measure about 5 centimeters by 5 cm by 15 cm, which is an amazingly high capacitance relative to its volume. The D-cell battery is also significantly heavier than the equivalently sized capacitor, which weighs about 60 grams.Hundreds of thousandsof ultracapacitors are manufactured each year, for applications that require rapid recharging, high power output, and repetitive cycling. In 2005, the ultracapacitor market was between US $272 million and $400 million, depending on the source, and it's growing, especially in the automotive sector. Though ultracapacitors have generally remained a niche player, the situation may soon change.My laboratory at MIT--the Laboratory for Electromagnetic and Electronic Systems--works with several automobile manufacturers to investigate ways to improve vehicle performance. About four years ago, I assisted on a project to evaluate commercial ultracapacitors for use in cars. While on a flight from Boston to Detroit, I read an article describing a way to grow vertically aligned carbon nanotubes on a flat surface. This is a truly amazing process. A sheet of silica is covered with a nanometer-thick layer of an iron catalyst. The sheet is placed in a vacuum, heated to 650 C, and exposed to a thin hydrocarbon gas, perhaps ethanol or acetylene. The heat causes the iron to form tiny droplets, which steal carbon molecules from the gas. The carbon molecules then begin to self-assemble into tubes, which grow upward from each of the droplets.By virtue of their dimensions, it struck me that those nanotubes held the promise of even higher porosity than the activated carbon used in commercial ultracapacitors. Together the nanotubes have an enormous surface area, and their dimensions are more uniform than those of the activated-carbon pores, making them more like a paintbrush than a sponge.There are two major limitations to the conductivity of activated carbon--the high porosity means there isn't much carbon material to carry current, and the material must be glued to the aluminum current collector using a binder, which exhibits a somewhat high resistance. If my colleagues and I replaced the activated carbon with billions of nanotubes, we predicted we could make an ultracapacitor that could store at least 25 percent--and perhaps as much as 50 percent--of the energy in a chemical battery of equivalent weight. (To get that much improvement, we'd have to make a number of other changes, as well, such as increasing the number of ions in the electrolyte to reflect that new-found storage space.)Another advantage of nanotubes over activated carbon is that their structure makes them less chemically reactive, so they can operate at a higher voltage. And certain types of nanotubes, depending on their geometry, can be excellent conductors--which means they can supply more power than ultracapacitors outfitted with activated carbon [see illustration, "Piling on The Farads"].Even better, this nanotube-enhanced ultracapacitor would retain all the advantages ordinary ultracapacitors have over batteries: they would deliver energy in quick bursts, they would perform well in cold weather, and they would have much longer life spans. If this ultracapacitor could be developed, it would be revolutionary.It was clear from the outset that a lot of know-how would be needed to make an ultracapacitor according to our design--knowledge of chemical-vapor deposition, electron microscopy, material science, quantum chemistry. And it's a challenge to get people with all those skills together. One of the strengths of a research university is its incredible diversity of expertise and equipment, plus there's the willingness of faculty to collaborate. Nobody in my lab had experience fabricating carbon nanotubes, but much of the early research in that area at MIT was done in the building next door, at a laboratory under the direction of Mildred Dresselhaus. Using those facilities and aided by Dresselhaus and her lab colleagues, we succeeded in synthesizing a nanotube forest on a small piece of silica in only a few months.Nanotubes can vary in size, and the ones we're growing are about 5 nm across, or about 1/10 000th the diameter of a human hair. Each tube is about 100 m long, and they can be spaced as little as 5 nm apart [see image, "Electric Shag," below].

Image: Riccardo Signorelli/MITElectric shag:A cross section of an electrode made with carbon nanotubes.But the sliver of silica was only the start. Silica is an insulator, and we needed a conducting material. After more than a year of false starts, we finally designed and built a custom reactor for chemical-vapor deposition and have used it to grow nanotubes on a conducting substrate. We are now packaging this collection of nanotubes in a prototype ultracapacitor.We believe that within a few months we'll be able to demonstrate results that outperform today's designs by a wide margin. There will still be a big challenge ahead of us at that point: to see whether our devices can be manufactured at prices that make them attractive for mainstream applications. We are optimistic, though, because chemical-vapor deposition is already used on a huge scale in semiconductor manufacturing, and the raw materials that we need are cheap.It's not a straight path from high-density ultracapacitors to practical electric cars, but what my colleagues and I have done may constitute one big step along a tortuous route to making such vehicles more convenient and attractive to consumers. Even if it takes many years before ultracapacitors on their own can power either full battery-electric or hybrid cars, we're already at the point where such devices could easily assist lithium-ion batteries [see illustration, "How to Ultracap A Car"]. When the car's electric motor needs high current for a short time, the ultracapacitor supplies it. After the demand eases, the ultracapacitor recharges from the battery. When the motor, working now as a generator, delivers high current for a brief interval--which is typically what happens with regenerative braking--the same thing happens in reverse. A computer would monitor voltages, the state of charge, load, and demand, and then adjust the current flow accordingly using some additional dc-dc power electronics. The added weight and expense involved might not matter if it improves vehicle performance and makes the battery last longer.Small-cell ultracapacitors can be used in cars for purposes other than in the drivetrain. They can be integrated into air-conditioning, electric power steering, power locks, and window systems--components that demand high peak currents, which typically require large-diameter wiring. The need is intermittent, and the average power is low, so having ultracapacitors provide the high current at strategic points would permit thinner wiring to be installed. With the high price of copper these days, such changes can shave an appreciable amount from the cost of a vehicle.Safety is another motivation. Suppose a car has electrically actuated brakes or door locks and the wiring harness fails because of a defect or an accident. A local ultracapacitor can still provide power for a few precious seconds or minutes.Such devices are by no means limited to vehicles. Society is in the midst of an energy crisis, and many sources of green energy would benefit from regenerative energy storage. Electric power grids could be 10 percent more efficient if there could be simple, inexpensive ways to store energy locally at the point of use. And if renewable energy is ever to displace fossil fuels, engineers will need to devise better ways to store wind power when the wind is not blowing and solar power when the sun is not shining.My colleagues and I are not the only ones researching ultracapacitor technology, of course. All the existing ultracapacitor manufacturers--including Maxwell Technologies, NessCap, Panasonic, Nippon Chemi-Con, and Power Systems Co.--are working on improved activated carbons or devices where one electrode functions as a battery and the other as an ultracapacitor. The Japanese government has provided $25 million for nanotube research, money that has supported a promising joint effort between Nippon Chemi-Con and AIST National Lab to explore nanotube-based techniques. Investigators at Rensselaer Polytechnic Institute, in Troy, N.Y., recently announced, in theProceedings of the National Academy of Sciences of the United States of America, an exciting combined battery-nanotube ultracapacitor fabric to store electrical energy.And nanotube forests are not the only way to provide increased porosity. Power Systems, in Japan, for example, has been getting good results with a type of graphene structure that it calls a nanogate.There's a slightly different approach to modified capacitors that has been generating a lot of buzz lately, developed by a start-up called EEStor, in Cedar Park, Texas. EEStor has focused on improving the dielectric, rather than the capacitor's plates. Its design uses barium titanate, which has a high dielectric constant. High-dielectric-constant substances allow for high-value capacitors that are still small in size. The downside is that such materials generally are unable to withstand electrostatic fields of the same intensity as low-dielectric-constant substances such as air. EEStor claims that the capacitors can operate at extremely high voltages, on the order of several thousand volts, leading to very high storage capacities. One concern is that high voltages can cause a dielectric to break down irreversibly in the presence of even slight imperfections in the material. Only time will tell how its design fares.Improving substantially on the means to store electrical energy would be a welcome development, and high-density capacitive storage is one promising avenue of research. Although batteries and capacitors are old inventions, our particular technique could not have been pursued until recently. Just as semiconductor designers have created smaller and smaller transistors, so have engineers in other areas learned to manipulate objects with ever-more-minuscule dimensions. The ability to sculpt materials at the atomic level is new and evolving. Engineers can use these new techniques to achieve novel properties and, in the case of my lab's research, to move toward a nanoengineered carbon that might usher in the next generation of energy storage.