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(December 2010)
Maxwell Technologies "MC" and "BC" series super capacitors (up to 3000 farad capacitance)
ultracapacitor, is an electrochemical capacitor with relatively high energy density. Their energy
density is typically hundreds of times greater than conventional electrolytic capacitors.[1].
A typical D-cell-sized 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.
As of 2011 EDLCs had a maximum working voltage of a few volts (standard electrolytics can
work at hundreds of volts) and capacities of up to 5,000 farads.[2] 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 (0.1 MJ/kg).[3] Up to 85 W⋅h/kg has been achieved at room temperature in
the lab,[4] which is still lower than rapid-charging lithium-titanate batteries.[5]
Much research is being carried out to improve performance; for example an order of magnitude
energy density improvement was achieved in the laboratory in mid-2011.[6] Prices are dropping:
a 3 kF capacitor that cost US$5,000 in 2000 cost $50 in 2011.
EDLCs are used for energy storage rather than as general-purpose circuit components. They have
a variety of commercial applications, notably in "energy smoothing" and momentary-load
devices. They have applications as energy-storage and KERS devices used in vehicles, and for
smaller applications like home solar energy systems where extremely fast charging is a valuable
feature
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 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 of capacitor.
EDLCs do not have a conventional dielectric. Rather than two separate plates separated by an
intervening insulator, these capacitors use virtual plates that are in fact 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 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 seriesconnected 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
[7]
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 1/10 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 one another.
History
General Electric engineers experimenting with devices using porous carbon electrodes first
observed the EDLC effect in 1957.[8] 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 devices, after they accidentally re-discovered the
effect while working on experimental fuel cell designs.[9] 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.
Standard Oil did not commercialize their invention, licensing the technology to NEC, who
finally marketed the results as “super capacitors” in 1978, to provide backup power for
maintaining computer memory.[9] The market expanded slowly for a time, but starting around the
mid-1990s various advances in materials science and refinement of the existing systems led to
rapidly improving performance and an equally rapid reduction in cost.
The first trials of super capacitors in industrial applications were carried out for supporting the
energy supply to robots.
In 2005 aerospace systems and controls company Diehl Luftfahrt Elektronik GmbH chose super
capacitors to power emergency actuation systems for doors and evacuation slides in airliners,
including the new Airbus 380 jumbo jet.[10] In 2005, the ultra capacitor market was between US
$272 million and $400 million, depending on the source.
As of 2007 all solid state micrometer-scale electric double-layer capacitors based on advanced
superionic conductors had been for low-voltage electronics such as deep-sub-voltage
nanoelectronics and related technologies (the 22 nm technological node of CMOS and
beyond).[11]
Comparisons
Super capacitors have several disadvantages and advantages relative to batteries:
Disadvantages

The amount of energy stored per unit weight is generally lower than that of an
electrochemical battery (3–5 W·h/kg for a standard ultracapacitor, although 85 W.h/kg
has been achieved in the lab[4] as of 2010 compared to 30–40 W·h/kg for a lead acid
battery, 100-250 W·h/kg for a lithium-ion battery and about 1/1,000th the volumetric
energy density of gasoline.

Has the highest dielectric absorption 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.

Unlike practical batteries, the voltage across any capacitor, including EDLCs, drops
significantly as it discharges. Effective storage and recovery of energy requires complex
electronic control and switching equipment, with consequent energy loss. A detailed
paper on a multi-voltage 5.3 W EDLC power supply for medical equipment discusses
design principles in detail. It uses a total of 55 F of capacitance, charges in about 150
seconds, and runs for about 60 seconds. The circuit uses switch-mode voltage regulators
followed by linear regulators for clean and stable power, reducing efficiency to about
70%. The authors discuss the types of switching regulator available, buck, boost, and
buck-boost, and conclude that for the widely varying voltage across an EDLC buck-boost
is best, boost second-best, and buck unsuitable[12].

Very low internal resistance allows extremely rapid discharge when shorted, resulting in
a spark hazard similar to any other capacitor of similar voltage and capacitance (generally
much higher than electrochemical cells).
Advantages

Long life, with little degradation over hundreds of thousands of charge 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
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 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[13]

Improved safety, no corrosive electrolyte and low toxicity of materials.

Simple charge methods—no full-charge detection is needed; no danger of overcharging.

When used in conjunction with rechargeable batteries, in some applications the EDLC
can supply energy for a short time, reducing battery cycling duty and extending life
Materials
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 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 m2 — 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 even a thin layer 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 2–3 V, and
thus must be "stacked" (connected in series), just as conventional battery cells must be, to supply
higher voltages.
Activated charcoal is not the "perfect" material for this application. The charge carriers are
actually (in effect) quite large—especially when surrounded by molecules—and are often larger
than the holes left in the charcoal, which are too small to accept them, limiting the storage.
As of 2010 virtually all commercial super capacitors use powdered activated carbon made from
coconut shells.[citation
needed]
Higher performance devices are available, at a significant cost
increase, based on synthetic carbon precursors that are activated with potassium hydroxide
(KOH).
Research in EDLCs focuses on improved materials that offer higher usable surface areas.

Graphene has excellent surface area per unit of gravimetric or volumetric densities, is
highly conductive and can now be produced in various labs, but is not available in
production quantities. Specific energy density of 85.6 Wh/kg at room temperature and
136 Wh/kg at 80 °C (all based on the total electrode weight), measured at a current
density of 1 A/g have been observed. These energy density values are comparable to that
of the Nickel metal hydride battery. The device makes full utilization of the highest
intrinsic surface capacitance and specific surface area of single-layer graphene by
preparing 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 >4 V.[14]

Carbon nanotubes have excellent nanoporosity properties, allowing tiny spaces for the
polymer to sit in the tube and act as a dielectric.[15] 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.[16] The addition of carbon nanotubes in capacitors can greatly improve and enhance
the performance of electric double-layer capacitors. Due to the high surface area and high
conductivity of single-wall carbon nanotubes, the addition of these nanotubes allows
optimization for these capacitors.[17] Multi-walled carbon nanotubes have a presence of
mesopores that allow for easy access of ions at the electrode/electrolyte interface. The
thin walls of a carbon nanotube allow for high capacitance in an electric double-layer
capacitor.[18] By adding multi-walled nanotubes to these capacitors, the resistance of the
electrodes can be decreased. The capacitor cells with multi-walled nanotube fibers had
higher electron and electrolyte-ion conductivities as compared to cells that did not have
these nanotubes. These nanotubes also improved the power capabilities of the
capacitors.[19]
Ragone chart showing energy density vs.power density for various energy-storage devices

Some polymers (e.g. polyacenes and conducting polymers) have a redox (reductionoxidation) storage mechanism along with a high surface area.

Carbon aerogel provides extremely high surface area gravimetric densities of about 400–
1000 m²/g. The electrodes of aerogel super capacitors 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 required large surface area. Small aerogel super capacitors are being
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. Carbon
aerogel capacitors have achieved 325 J/g (90 W·h/kg) energy density and 20 W/g power
density.[20]

Solid activated carbon, also termed consolidated amorphous carbon (CAC). It can have a
surface area exceeding 2800 m2/g and may be cheaper to produce than aerogel carbon.[21]

Tunable nanoporous carbon exhibits systematic pore size control. H2 adsorption
treatment can be used to increase the energy density by as much as 75% over what was
commercially available as of 2005.[22][23]

Mineral-based carbon is a nonactivated carbon, synthesised from metal or metalloid
carbides, e.g. SiC, TiC, Al4C3.[24] 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 this
material was used in a super capacitor 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.[25]

In August 2007 researchers combined a biodegradable paper battery with aligned carbon
nanotubes, designed to function as both a lithium-ion battery and a super capacitor (called
bacitor). 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 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.[26]

Other teams are experimenting with custom materials made of activated polypyrrole, and
nanotube-impregnated papers.
Properties
The properties of EDLCs are being improved as new research progresses.
Capacitance
The capacitance of EDLCs was up to several thousands of farads as of 2011.
Voltage
As of 2011 EDLCs rated up to a maximum working voltage of about 5 V were available.
Specific energy
The specific energy of existing commercial EDLCs ranges from around 0.5 to 30 W·h/kg[27][28]
including lithium ion capacitors, known also as a "hybrid capacitor". Experimental electric
double-layer capacitors have demonstrated specific energies of 30 W·h/kg and have been shown
to be scalable to at least 136 W·h/kg.[29][30] 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 specific energy of 2,400 W·h/kg.
Electrically driven automobiles run at a much higher efficiency. For example, the Tesla Roadster
runs at an average battery-to-wheel efficiency of 88%. This implies that the effective specific
energy of ultra capacitors in an automotive application could be close to 25 W·h/kg, 2 orders of
magnitude less than gasoline.
Power density
EDLCs have energy densities perhaps one tenth that of a rechargeable battery, but power
densities typically 10 to 100 times greater.
Self-discharge
An EDLC which is charged and stored loses its charge (self-discharge) much faster than a typical
electrolytic capacitor, and somewhat faster than a rechargeable battery.
Price
Research and development bring rapid improvements in price as well as physical properties.
Costs have fallen quickly, with cost per kilojoule dropping faster than cost per farad. By 2006 the
cost of super capacitors was 1 cent per farad and $2.85 per kilojoule and dropping.[31] A 3 kF
capacitor that was US$5,000 ten years before was $50 in 2011.[32]
Applications
Vehicles
Heavy and public transport
See also: Capa vehicle
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.[33][34] 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 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 without powerlines
using large 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 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.[35]
In 2001 and 2002 VAG, the public transport operator in Nuremberg, Germany tested an hybrid
bus that uses a diesel-electric battery drive system with electric double-layer capacitors.[36] Since
2003 Mannheim Stadtbahn in Mannheim, Germany has operated a light-rail vehicle (LRV) that
uses EDLCs to store braking energy.[37][38]
Other public transport manufacturers are developing EDLC technology, including mobile
storage[39] and a stationary trackside power supply.[40][41]
A triple hybrid forklift truck uses fuel cells and batteries as primary energy storage and EDLCs
to supplement this energy storage solution.[42]
Automotive
Ultracapacitors are used in some concept prototype vehicles, in order to keep batteries within
resistive heating limits and extend battery life.[43][44] The ultrabattery combines a super capacitor
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).[45][46]
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 super capacitors.[47]
The Toyota TS030 HYBRID LMP1 car uses a hybrid drivetrain with energy storage provided
through the use of super capacitors.[48]
Complementing batteries
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 the cycling duty and extending their life[49]
Low-power applications
EDLCs can be used to operate low-power equipment such as PC Cards, photographic flash,
flashlights, portable media players, and automated meter reading equipment.[50] 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; charges in 150 seconds, runs for 60 seconds).[12]
In 2007 a cordless electric screwdriver that uses an EDLC for energy storage was produced.[51] 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.[52]
Alternative energy
The idea of replacing batteries with capacitors in conjunction with novel energy sources became
a conceptual umbrella of the Green Electricity (GEL) Initiative.[53] One successful GEL Initiative
concept was a muscle-driven autonomous solution that employs a multi-farad EDLC as 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.[54]
Ultra capacitors
Call them ultracapacitors. Or super capacitors. Whatever the name, they exhibit vastly greater
capacitance than conventional caps. Singly, you can buy radiallead board-mount devices rated
for 5 to 10 F at 2.5 V, flashlight-battery size units rated for 120 to 150 F at 5 V, and larger
single-capacitor cans good for 650 to 3000 F at 2.7 V. Note that all of those capacitance values
are in farads. Not so long ago, a couple of thousand microfarads were a lot of capacitance.
Need more? You can buy off-the-shelf modules spec’d for 20 to 500 F, with voltage ratings from
15 to 390 V. If you understand how to balance them in series/parallel combinations, you can
drive a bus with them—no, not two traces on a circuit board, but a passenger-hauling bus.
(Although not very far, as hybrid propulsion systems, chemical batteries, and fuel cells are still in
the picture. More on that shortly.)
What happened? In developing ultracaps, nobody discovered new laws of physics. In fact, the
theory behind them goes back to Helmholtz. Like all capacitors, ultracaps are still about storing
power in the form of an electrical charge between two “plates.” The capacitance is directly
related to the area of the plates and the permittivity of the material between the plates, and it’s
inversely related to the distance between them. After that, the story gets interesting.
Before we had ultracaps to provide astonishingly high values of capacitance, we had
electrolytics. ultra capacitors aren’t electrolytics, but understanding the older tech is helpful in
understanding the new tech.
Electrolytics are so named because one (or both) of the “plates” is a nonmetallic electrolyte on
top of a metallic backing. During manufacturing, a voltage drives a current from the anode metal
through a conductive bath to the cathode. That produces an insulating metal oxide on the surface
of the anode—the dielectric.
One of the phenomena that happens inside electrolytics is the charge accumulation and charge
separation that occurs at the interface when any electrode is immersed into an electrolyte
solution. An accumulation of oppositely charged ions in the solution compensates for excess
charge on the electrode surface. The interface is called the Helmholtz layer.
To understand ultracaps, stop thinking about flat plates (or flat plates rolled up into tubes) with a
dielectric between them, much like peanut butter in a sandwich. In an ultracap,
charging/discharging takes place on the interfaces between porous carbon materials or porous
oxides of certain metals in an electrolyte.
The Helmholtz layers give rise to an effect called doublelayer capacitance. When a dc voltage is
applied across the porous carbon electrodes in an ultracap, compensating accumulations of
cations or anions develop in the solution around the charged electrodes. If no electron transfer
can occur across the interface, a “double layer” of separated charges (electrons or electron
deficiency at the metal side and cations or anions at the solution side of the interface boundary)
exists across the interface (Fig. 1).
The Helmholtz-region capacitance depends on the area of those porous carbon electrodes and the
size of the ions in solution. The capacitance per square centimeter of electrode double layers is
on the order of 10,000 times larger than those of ordinary dielectric capacitors. That’s because
the separation of charges in double layers is about 0.3 to 0.5 nm, instead of 10 to 100 nm in
electrolytics and 1000 nm in mica or polystyrene caps.
There’s a catch to this “double-layer” characteristic, though. The double-layer configuration
reduces the potential capacitance of a practical device because the ultracap consists of a pair of
electrodes, each with half the total mass. In addition, the ultra capacitor is effectively two
capacitors in series. Taken together, that means the ultracap achieves one quarter of the
theoretical capacitance based on electrode area and ion size.
If you want to read the theory behind ultra capacitors in more depth, check out an article from
the Electrochemistry Encyclopedia called “Electrochemical Capacitors, Their Nature, Function,
and Applications” (http://electrochem.cwru.edu/ed/encycl/artc03-elchem-cap.htm) by the late
Brian E. Conway of the University of Ottawa’s chemistry department. Conway was an important
contributor to ultra capacitor research for several decades.
BATTERIES AND ULTRA CAPACITORS
The popular press likes to lump batteries and ultra capacitors together, obscuring a number of
important differences:

Batteries store watt-hours of energy. Capacitors store watts of power.

Batteries depend on chemical reactions with long time constants. They take a
relatively long time to charge, and they’re fussy about the profile of the current that
charges them. Conversely, capacitors are charged by applying a voltage across
their terminals, and their charge rate depends mostly on external resistance.

Batteries deliver power in the form of a more or less constant voltage over long
time periods. Capacitors discharge rapidly, and their output voltage decays
exponentially.

Batteries are good for only a limited number of charge/discharge cycles, and the
number of cycles depends on how deeply they are discharged. Capacitors,
especially ultracapacitors, can be charged and discharged repeatedly for tens of
millions of cycles. (This is an important way that ultracaps differ from
electrolytics—they aren’t cycle-limited by the electrode plating that accompanies
electrolytics’ operation.)

Batteries are big and heavy. Capacitors are small and light.
Many of these differences can be heuristically illustrated in a Ragone plot (Fig. 2). Ragone plots
have more analytical uses, but essentially, they’re log-log graphs of energy density (in this case
in Wh/kg) on the Y axis versus power density (in W/kg) on the X axis. Because they’re log-log
plots, discharge time can be represented as straight-line diagonal parameters.
The Ragone plot helps illustrate the differences among different kinds of battery chemistry,
clustered on the left, and capacitors on the right. Taken together as illustrated on the Ragone plot,
those characteristics make batteries and ultra capacitors complementary to each other, rather than
antagonists. In fact, that’s how they’re often used.
Super capacitor
The super capacitor, also known as ultra capacitor or double-layer capacitor, differs from a
regular capacitor in that it has a very high capacitance. A capacitor stores energy by means of a
static charge as opposed to an electrochemical reaction. Applying a voltage differential on the
positive and negative plates charges the capacitor. This is similar to the buildup of electrical
charge when walking on a carpet. Touching an object releases the energy through the finger.
We group capacitors into three family types and the most basic is the electrostatic capacitor,
with a dry separator. This capacitor has a very low capacitance and is used to filter signals and
tune radio frequencies. The size ranges from a few pico-farad (pf) to low microfarad (uF). The
next member is the electrolytic capacitor, which is used for power filtering, buffering and
coupling. Rated in microfarads (uF), this capacitor has several thousand times the storage
capacity of the electrostatic capacitor and uses a moist separator. The third type is the super
capacitor, rated in farads, which is again thousands of times higher than the electrolytic
capacitor. The super capacitor is ideal for energy storage that undergoes frequent charge and
discharge cycles at high current and short duration.
Faradis a unit of capacitance named after the English physicist Michael Faraday. One farad
stores one coulomb of electrical charge when applying one volt. One microfaradis one million
times smaller than a farad, and one pico-farad is again one million times smaller than the
microfarad.
Engineers at General Electric first experimented with the electric double-layer capacitor, which
led to the development of an early type of super capacitor in 1957. There were no known
commercial applications then. In 1966, Standard Oil rediscovered the effect of the double-layer
capacitor by accident while working on experimental fuel cell designs. The company did not
commercialize the invention but licensed it to NEC, which in 1978 marketed the technology as
“super capacitor” for computer memory backup. It was not until the 1990s that advances in
materials and manufacturing methods led to improved performance and lower cost.
The modern super capacitor is not a battery per se but crosses the boundary into battery
technology by using special electrodes and electrolyte. Several types of electrodes have been
tried and we focuse on the double-layer capacitor (DLC) concept. It is carbon-based, has an
organic electrolyte that is easy to manufacture and is the most common system in use today.
All capacitors have voltage limits. While the electrostatic capacitor can be made to withstand
high volts, the super capacitor is confined to 2.5–2.7V. Voltages of 2.8V and higher are possible
but they would reduce the service life. To achieve higher voltages, several super capacitors are
connected in series. This has disadvantages. Serial connection reduces the total capacitance, and
strings of more than three capacitors require voltage balancing to prevent any cell from going
into over-voltage. This is similar to the protection circuit in lithium-ion batteries.
The specific energy of the super capacitor is low and ranges from 1 to 30Wh/kg. Although high
compared to a regular capacitor, 30Wh/kg is one-fifth that of a consumer Li-ion battery. The
discharge curve is another disadvantage. Whereas the electrochemical battery delivers a steady
voltage in the usable power band, the voltage of the super capacitor decreases on a linear scale
from full to zero voltage. This reduces the usable power spectrum and much of the stored energy
is left behind. Consider the following example.
Take a 6V power source that is allowed to discharge to 4.5V before the equipment cuts off. With
the linear discharge, the super capacitor reaches this voltage threshold within the first quarter of
the cycle and the remaining three-quarters of the energy reserve become unusable. A DC-to-DC
converter could utilize some of the residual energy, but this would add to the cost and introduce a
10 to 15 percent energy loss. A battery with a flat discharge curve, on the other hand, would
deliver 90 to 95 percent of its energy reserve before reaching the voltage threshold. Table 1
compares the super capacitor with a typical Li-ion.
Function
Super capacitor
1–10 seconds
Charge time
10–60 minutes
1 million or 30,000h
Cycle life
500 and higher
2.3 to 2.75V
Cell voltage
Specific
Lithium-ion (general)
3.6 to 3.7V
energy
5 (typical)
100–200
(Wh/kg)
Up to 10,000
Specific power (W/kg)
$20(typical)
Cost per Wh
Service life (in vehicle)
Charge temperature
Discharge temperature
1,000 to 3,000
$2 (typical)
10 to 15 years
5 to 10 years
–40 to 65°C (–40 to
149°F)
0 to 45°C (32°to 113°F)
–40 to 65°C (–40 to –20 to 60°C (–4 to 140°F)
149°F)
Table 1: Performance comparison between super capacitor and Li-ion
Courtesy of Maxwell Technologies, Inc.
Rather than operating as a stand-alone energy storage device, super capacitors work well as lowmaintenance memory backup to bridge short power interruptions. Super capacitors have also
made critical inroads into electric powertrains. The virtue of ultra-rapid charging and delivery of
high current on demand makes the super capacitor an ideal candidate as a peak-load enhancer for
hybrid vehicles, as well as fuel cell applications.
The charge time of a super capacitor is about 10 seconds. The charge characteristic is similar to
an electrochemical battery and the charge current is, to a large extent, limited by the charger. The
initial charge can be made very fast, and the topping charge will take extra time. Provision must
be made to limit the initial current inrush when charging an empty super capacitor. The super
capacitor cannot go into overcharge and does not require full-charge detection; the current
simply stops flowing when the capacitor is full.
The super capacitor can be charged and discharged virtually an unlimited number of times.
Unlike the electrochemical battery, which has a defined cycle life, there is little wear and tear by
cycling a super capacitor. Nor does age affect the device, as it would a battery. Under normal
conditions, a super capacitor fades from the original 100 percent capacity to 80 percent in 10
years. Applying higher voltages than specified shortens the life. The super capacitor functions
well at hot and cold temperatures.
The self-discharge of a super capacitor is substantially higher than that of an electrostatic
capacitor and somewhat higher than the electrochemical battery. The organic electrolyte
contributes to this. The stored energy of a super capacitor decreases from 100 to 50 percent in 30
to 40 days. A nickel-based battery self-discharges 10 to 15 percent per month. Li-ion discharges
only five percent per month.
Super capacitors are expensive in terms of cost per watt. Some design engineers argue that the
money for the super capacitor would better be spent on a larger battery. We need to realize that
the super capacitor and chemical battery are not in competition; rather they are different products
serving unique applications.Table 2 summarizes the advantages and limitations of the super
capacitor.
Virtually unlimited cycle life; can be cycled millions of time
High specific power; low resistance enables high load currents
Charges in seconds; no end-of-charge termination required
Advantages
Simple charging; draws only what it needs; not subject to
overcharge
Safe; forgiving if abused
Excellent low-temperature charge and discharge performance
Low specific energy; holds a fraction of a regular battery
Linear discharge voltage prevents using the full energy spectrum
Limitations
High self-discharge; higher than most batteries
Low cell voltage; requires serial connections with voltage
balancing
High cost pe
How An Ultra Capacitor Works
Ultra capacitors & Super Capacitors store electricity by physically separating positive and
negative charges— different from batteries which do so chemically. The charge they hold is like
the static electricity that can build up on a balloon, but is much greater thanks to the extremely
high surface area of their interior materials.
An advantage of the ultra capacitor is their
super fast rate of charge and discharge... which
is
determined
solely
by
their
physical
properties. A battery relies on a slower
chemical reaction for energy.
A disadvantage of an ultra capacitor is that
currently they store a smaller amount of energy
than a battery does.
Ultracapacitors are very good at efficiently
capturing electricity from regenerative braking,
and can deliver power for acceleration just as
quickly. With no moving parts, they also have a very long lifespan - 500,000 plus
charge/recharge cycles. ultra capacitors are currently used for wind energy, solar energy, and
hydro energy storage.
An ultra capacitor, also known as a double-layer capacitor, polarizes an electrolytic solution to
store energy electro statically. Though it is an electrochemical device, no chemical reactions are
involved in its energy storage mechanism. This mechanism is highly reversible, and allows the
ultra capacitor to be charged and discharged hundreds of thousands of times.
Once the ultra capacitor is charged and energy stored, a load (the electric vehicle's motor) can
use this energy. The amount of energy stored is very large compared to a standard capacitor
because of the enormous surface area created by the porous carbon electrodes and the small
charge separation created by the dielectric separator.
Here is a very basic example of how an ultra capacitor works by using a circuit that uses a
dc motor.
TECHNICAL DESCRIPTION: An ultra capacitor can be viewed as two non reactive porous
plates, or collectors, suspended within an electrolyte, with a voltage potential applied across the
collectors. In an individual ultra-capacitor cell, the applied potential on the positive electrode
attracts the negative ions in the electrolyte, while the potential on the negative electrode attracts
the positive ions. A dielectric separator between the two electrodes prevents the charge from
moving between the two electrodes.
Electrical energy storage devices, such as capacitors, store electrical charge on an electrode.
Other devices, such as electrochemical cells or batteries, utilize the electrode to create, by
chemical reaction, an electrical charge at the electrodes. In both of these, the ability to store or
create electrical charge is a function of the surface area of the electrode. For example, in
capacitors, greater electrode surface area increases the capacitance or energy storage capability
of the device.
As a storage device, the ultracapacitor, relies on the microscopic charge separation at an
electrochemical interface to store energy. Since the capacitance of these devices is proportional
to the active electrode area, increasing the electrode surface area will increase the capacitance,
hence increasing the amount of energy that can be stored. This achievement of high surface area
utilizes materials such as activated carbon or sintered metal powders. However, in both
situations, there is an intrinsic limit to the porosity of these materials, that is, there is an upper
limit to the amount of surface area that can be attained simply by making smaller and smaller
particles. An alternative method must be developed to increase the active electrode surface area
without increasing the size of the device. A much more highly efficient electrode for electrical
energy storage devices could be realized if the surface area could be significantly increased.
Energy Storage
Energy Storage: Solar Energy - Wind Energy - Hydro Energy
Using the ultra capacitor as an energy storage device has been making a lot of ground lately.
ultra capacitors have close to 100 percent efficiency and can be recycled up to 500,000 times.
The introduction of standard battery-sized ultra capacitors is a move that has the potential to
significantly improve market acceptance of ultra capacitors in a variety of applications and
hybrid electric vehicles (HEVs).
Large back up power users such like
manufacturers and utility providers
have been reluctant to move from
their traditional lead-acid batteries
because they are unfamiliar with the
new ultra capacitor technology. All
of this in spite of of the significant
advantages like greater reliability and
efficiency.
The
tide
is
turning
though... more and more companies
are becoming aware.
It was recently noted by Miriam Nagel: "Environmental issues are now coming into play in the
selection of advanced energy storage technologies." "Environmentally friendly technologies
such as flywheels and ultra capacitors – also called super capacitors – may soon get a lot more
consideration in the energy storage markets."
One of the issues that has slowed the ultra capacitor market is the high cost of integrating them
into new designs. It's just now becoming known that ultra capacitors can now be produced at half
the cost of its earlier earlier versions and the savings are likely to be passed on to original
equipment manufacturers (OEM).
Reference
1. ^ Garthwaite, Josie (12 July 2011). "How ultra capacitors work (and why they fall
short)". GigaOM's Earth2Tech. http://gigaom.com/cleantech/how-ultracapacitors-workand-why-they-fall-short/. Retrieved 13 July 2011.
2. ^ 5000F, Nesscap Products
3. ^ A 30 Wh/kg Super capacitor for Solar Energy and a New Battery. Jeol.com (3 October
2007). Retrieved on 13 September 2011.
4. ^ a b Graphene super capacitor breaks storage record. physicsworld.com. Retrieved on 13
September 2011.
5. ^ Note: all references to batteries in this article should be taken to refer to rechargeable,
not primary (aka disposable), batteries.
6. ^ Chemistry World: New carbon material boosts super capacitors. Rsc.org. 13 May 2011.
Retrieved on 13 September 2011.
7. ^ Garthwaite, Josie (12 July). "How ultra capacitors work (and why they fall short)".
Earth2Tech. GigaOM Network. http://gigaom.com/cleantech/how-ultracapacitors-workand-why-they-fall-short/. Retrieved 13 July 2011.
8. ^ US 2800616, Becker, H.I., "Low voltage electrolytic capacitor", issued 1957-07-23
9. ^ a b The Charge of the Ultra – Capacitors. IEEE Spectrum, November 2007
10. ^ Boostcap (of Maxwell Technologies)
11. ^ Высокоёмкие конденсаторы для 0,5 вольтовой наноэлектроники будущего.
Nanometer.ru. 17 October 2007. Retrieved on 13 September 2011.
External links

Super Capacitor Seminar

Article on ultra capacitors at electronicdesign.com

Article on ultra capacitors 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 & Super capacitors 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 super capacitors can help to solve power problems in portable
products.

A web that describes the development of solid-state and hybrid super capacitors from
CNR-ITAE (Messina) Italy