Supercapacitor

A supercapacitor (SC) (also called a supercap,

ultracapacitor or Goldcap[2]) is a high-capacity capacitor
with capacitance values much higher than other capacitors
(but lower voltage limits) that bridge the gap between
electrolytic capacitors and rechargeable batteries. They
typically store 10 to 100 times more energy per unit volume
or mass than electrolytic capacitors, can accept and deliver
charge much faster than batteries, and tolerate many more
charge and discharge cycles than rechargeable batteries.

Supercapacitors are used in applications requiring many

rapid charge/discharge cycles rather than long term
compact energy storage: within cars, buses, trains, cranes
Schematic illustration of a supercapacitor[1]
and elevators, where they are used for regenerative braking,
short-term energy storage or burst-mode power delivery.[3]
Smaller units are used as memory backup for static
random-access memory (SRAM).

Unlike ordinary capacitors, supercapacitors do not use the

conventional solid dielectric, but rather, they use
electrostatic double-layer capacitance and electrochemical
pseudocapacitance,[4] both of which contribute to the total
capacitance of the capacitor, with a few differences:

Electrostatic double-layer capacitors (EDLCs)

use carbon electrodes or derivatives with much
higher electrostatic double-layer capacitance than
electrochemical pseudocapacitance, achieving
separation of charge in a Helmholtz double layer
at the interface between the surface of a
A diagram that shows a hierarchical classification of
conductive electrode and an electrolyte. The
separation of charge is of the order of a few supercapacitors and capacitors of related types.
ångströms (0.3–0.8 nm), much smaller than in a
conventional capacitor.
Electrochemical pseudocapacitors use metal oxide or conducting polymer electrodes with a high amount of
electrochemical pseudocapacitance additional to the double-layer capacitance. Pseudocapacitance is achieved
by Faradaic electron charge-transfer with redox reactions, intercalation or electrosorption.
Hybrid capacitors, such as the lithium-ion capacitor, use electrodes with differing characteristics: one exhibiting
mostly electrostatic capacitance and the other mostly electrochemical capacitance.
The electrolyte forms an ionic conductive connection between the two electrodes which distinguishes them from conventional
electrolytic capacitors where a dielectric layer always exists, and the so-called electrolyte (e.g., MnO2 or conducting polymer) is
in fact part of the second electrode (the cathode, or more correctly the positive electrode). Supercapacitors are polarized by design
with asymmetric electrodes, or, for symmetric electrodes, by a potential applied during manufacture.

Contents
 History
Evolution of components
Design
Basic design
Capacitance distribution
Storage principles
Potential distribution
Comparison with other storage technologies
Styles
Construction details
Supercapacitor types
Materials
Electrodes
Electrolytes
Separators
Collectors and housing
Electrical parameters
Capacitance
Operating voltage
Internal resistance
Current load and cycle stability
Device capacitance and resistance dependence on operating voltage and temperature
Energy capacity
Specific energy and specific power
Lifetime
Self-discharge
Post charge voltage relaxation
Polarity
Comparison of selected commercial supercapacitors
Standards
Applications
General
Transport
Energy recovery
Developments
Market
Trade or series names
See also
Literature
References
External links

History
Development of the double layer and pseudocapacitance models (see Double layer (interfacial)).

Evolution of components
In the early 1950s, General Electric engineers began experimenting with porous carbon electrodes, in the design of capacitors,
from the design of fuel cells and rechargeable batteries. Activated charcoal is an electrical conductor that is an extremely porous
"spongy" form of carbon with a high specific surface area. In 1957 H. Becker developed a "Low voltage electrolytic capacitor
with porous carbon electrodes".[5][6][7] He believed that the energy was stored as a charge in the carbon pores as in the pores of
the etched foils of electrolytic capacitors. Because the double layer mechanism was not known by him at the time, he wrote in the
patent: "It is not known exactly what is taking place in the component if it is used for energy storage, but it leads to an extremely
high capacity."

General Electric did not immediately pursue this work. In 1966 researchers at Standard Oil of Ohio (SOHIO) developed another
version of the component as "electrical energy storage apparatus", while working on experimental fuel cell designs.[8][9] The
nature of electrochemical energy storage was not described in this patent. Even in 1970, the electrochemical capacitor patented by
Donald L. Boos was registered as an electrolytic capacitor with activated carbon electrodes.[10]

Early electrochemical capacitors used two aluminum foils covered with activated carbon—the electrodes—which were soaked in
an electrolyte and separated by a thin porous insulator. This design gave a capacitor with a capacitance on the order of one farad,
significantly higher than electrolytic capacitors of the same dimensions. This basic mechanical design remains the basis of most
electrochemical capacitors.

SOHIO did not commercialize their invention, licensing the technology to NEC, who finally marketed the results as
"supercapacitors" in 1971, to provide backup power for computer memory.[9]

Between 1975 and 1980 Brian Evans Conway conducted extensive fundamental and development work on ruthenium oxide
electrochemical capacitors. In 1991 he described the difference between "Supercapacitor" and "Battery" behavior in
electrochemical energy storage. In 1999 he coined the term supercapacitor to explain the increased capacitance by surface redox
reactions with faradaic charge transfer between electrodes and ions.[11][12] His "supercapacitor" stored electrical charge partially
in the Helmholtz double-layer and partially as result of faradaic reactions with "pseudocapacitance" charge transfer of electrons
and protons between electrode and electrolyte. The working mechanisms of pseudocapacitors are redox reactions, intercalation
and electrosorption (adsorption onto a surface). With his research, Conway greatly expanded the knowledge of electrochemical
capacitors.

The market expanded slowly. That changed around 1978 as Panasonic marketed its Goldcaps brand.[2] This product became a
successful energy source for memory backup applications.[9] Competition started only years later. In 1987 ELNA "Dynacap"s
entered the market.[13] First generation EDLC's had relatively high internal resistance that limited the discharge current. They
were used for low current applications such as powering SRAM chips or for data backup.

At the end of the 1980s, improved electrode materials increased capacitance values. At the same time, the development of
electrolytes with better conductivity lowered the equivalent series resistance (ESR) increasing charge/discharge currents. The first
supercapacitor with low internal resistance was developed in 1982 for military applications through the Pinnacle Research
Institute (PRI), and were marketed under the brand name "PRI Ultracapacitor". In 1992, Maxwell Laboratories (later Maxwell
Technologies) took over this development. Maxwell adopted the term Ultracapacitor from PRI and called them "Boost Caps"[14]
to underline their use for power applications.

Since capacitors' energy content increases with the square of the voltage, researchers were looking for a way to increase the
electrolyte's breakdown voltage. In 1994 using the anode of a 200V high voltage tantalum electrolytic capacitor, David A. Evans
developed an "Electrolytic-Hybrid Electrochemical Capacitor".[15][16] These capacitors combine features of electrolytic and
electrochemical capacitors. They combine the high dielectric strength of an anode from an electrolytic capacitor with the high
capacitance of a pseudocapacitive metal oxide (ruthenium (IV) oxide) cathode from an electrochemical capacitor, yielding a
hybrid electrochemical capacitor. Evans' capacitors, coined Capattery,[17] had an energy content about a factor of 5 higher than a
comparable tantalum electrolytic capacitor of the same size.[18] Their high costs limited them to specific military applications.
Recent developments include lithium-ion capacitors. These hybrid capacitors were pioneered by FDK in 2007.[19] They combine
an electrostatic carbon electrode with a pre-doped lithium-ion electrochemical electrode. This combination increases the
capacitance value. Additionally, the pre-doping process lowers the anode potential and results in a high cell output voltage,
further increasing specific energy.

Research departments active in many companies and universities[20] are working to improve characteristics such as specific
energy, specific power, and cycle stability and to reduce production costs.

Design

Basic design
Electrochemical capacitors (supercapacitors) consist of two electrodes separated
by an ion-permeable membrane (separator), and an electrolyte ionically
connecting both electrodes. When the electrodes are polarized by an applied
voltage, ions in the electrolyte form electric double layers of opposite polarity to
the electrode's polarity. For example, positively polarized electrodes will have a
layer of negative ions at the electrode/electrolyte interface along with a charge-
balancing layer of positive ions adsorbing onto the negative layer. The opposite
is true for the negatively polarized electrode.

Additionally, depending on electrode material and surface shape, some ions may Typical construction of a
supercapacitor: (1) power source,
permeate the double layer becoming specifically adsorbed ions and contribute
(2) collector, (3) polarized electrode,
with pseudocapacitance to the total capacitance of the supercapacitor. (4) Helmholtz double layer,
(5) electrolyte having positive and
negative ions, (6) separator.
Capacitance distribution
The two electrodes form a series circuit of two individual capacitors C1 and C2.
The total capacitance Ctotal is given by the formula

Supercapacitors may have either symmetric or asymmetric electrodes. Symmetry implies that both electrodes have the same
capacitance value, yielding a total capacitance of half the value of each single electrode (if C1 = C2, then Ctotal = ½ C1). For
asymmetric capacitors, the total capacitance can be taken as that of the electrode with the smaller capacitance (if C1 >> C2, then
Ctotal ≈ C2).

Storage principles
Electrochemical capacitors use the double-layer effect to store electric energy; however, this double-layer has no conventional
solid dielectric to separate the charges. There are two storage principles in the electric double-layer of the electrodes that
contribute to the total capacitance of an electrochemical capacitor:[21]

Double-layer capacitance, electrostatic storage of the electrical energy achieved by separation of charge in a
Helmholtz double layer.[22]
Pseudocapacitance, electrochemical storage of the electrical energy achieved by faradaic redox reactions with
charge-transfer.[14]
Both capacitances are only separable by measurement techniques. The amount of charge stored per unit voltage in an
electrochemical capacitor is primarily a function of the electrode size, although the amount of capacitance of each storage
principle can vary extremely.

Practically, these storage principles yield a capacitor with a capacitance value in the order of 1 to 100 farad.

Electrostatic double-layer capacitance

Every electrochemical capacitor has two electrodes, mechanically separated by a
separator, which are ionically connected to each other via the electrolyte. The
electrolyte is a mixture of positive and negative ions dissolved in a solvent such
as water. At each of the two electrode surfaces originates an area in which the
liquid electrolyte contacts the conductive metallic surface of the electrode. This
interface forms a common boundary among two different phases of matter, such
as an insoluble solid electrode surface and an adjacent liquid electrolyte. In this
interface occurs a very special phenomenon of the double layer effect.[23]

Applying a voltage to an electrochemical capacitor causes both electrodes in the Simplified view of a double-layer of
capacitor to generate electrical double-layers. These double-layers consist of two negative ions in the electrode and
layers of charges: one electronic layer is in the surface lattice structure of the solvated positive ions in the liquid
electrode, and the other, with opposite polarity, emerges from dissolved and electrolyte, separated by a layer of
polarized solvent molecules.
solvated ions in the electrolyte. The two layers are separated by a monolayer of
solvent molecules, e. g. for water as solvent by water molecules, called inner
Helmholtz plane (IHP). Solvent molecules adhere by physical adsorption on the surface of the electrode and separate the
oppositely polarized ions from each other, and can be idealised as a molecular dielectric. In the process, there is no transfer of
charge between electrode and electrolyte, so the forces that cause the adhesion are not chemical bonds but physical forces (e.g.
electrostatic forces). The adsorbed molecules are polarized but, due to the lack of transfer of charge between electrolyte and
electrode, suffered no chemical changes.

The amount of charge in the electrode is matched by the magnitude of counter-charges in outer Helmholtz plane (OHP). This
double-layer phenomena stores electrical charges as in a conventional capacitor. The double-layer charge forms a static electric
field in the molecular layer of the solvent molecules in the IHP that corresponds to the strength of the applied voltage.

The double-layer serves approximately as the dielectric layer in a conventional

capacitor, albeit with the thickness of a single molecule. Thus, the standard
formula for conventional plate capacitors can be used to calculate their
capacitance:[24]

Accordingly, capacitance C is greatest in capacitors made from materials with a

high permittivity ε, large electrode plate surface areas A and small distance Structure and function of an ideal
between plates d. As a result, double-layer capacitors have much higher double-layer capacitor. Applying a
capacitance values than conventional capacitors, arising from the extremely voltage to the capacitor at both
electrodes a Helmholtz double-layer
large surface area of activated carbon electrodes and the extremely thin double-
will be formed separating the ions in
layer distance on the order of a few ångströms (0.3-0.8 nm), of order of the
the electrolyte in a mirror charge
Debye length.[14][22] distribution of opposite polarity
The main drawback of carbon electrodes of double-layer SCs is small values of quantum capacitance[25] which act in series[26]
with capacitance of ionic space charge. Therefore, further increase of density of capacitance in SCs can be connected with
increasing of quantum capacitance of carbon electrode nanostructures.[25]

The amount of charge stored per unit voltage in an electrochemical capacitor is primarily a function of the electrode size. The
electrostatic storage of energy in the double-layers is linear with respect to the stored charge, and correspond to the concentration
of the adsorbed ions. Also, while charge in conventional capacitors is transferred via electrons, capacitance in double-layer
capacitors is related to the limited moving speed of ions in the electrolyte and the resistive porous structure of the electrodes.
Since no chemical changes take place within the electrode or electrolyte, charging and discharging electric double-layers in
principle is unlimited. Real supercapacitors lifetimes are only limited by electrolyte evaporation effects.

Electrochemical pseudocapacitance
Applying a voltage at the electrochemical capacitor terminals moves electrolyte
ions to the opposite polarized electrode and forms a double-layer in which a
single layer of solvent molecules acts as separator. Pseudocapacitance can
originate when specifically adsorbed ions out of the electrolyte pervade the
double-layer. This pseudocapacitance stores electrical energy by means of
reversible faradaic redox reactions on the surface of suitable electrodes in an
electrochemical capacitor with an electric double-layer.[11][21][22][27][28]
Pseudocapacitance is accompanied with an electron charge-transfer between
electrolyte and electrode coming from a de-solvated and adsorbed ion whereby
only one electron per charge unit is participating. This faradaic charge transfer
Simplified view of a double-layer with
originates by a very fast sequence of reversible redox, intercalation or
specifically adsorbed ions which
electrosorption processes. The adsorbed ion has no chemical reaction with the have submitted their charge to the
atoms of the electrode (no chemical bonds arise[29]) since only a charge-transfer electrode to explain the faradaic
take place. charge-transfer of the
pseudocapacitance.
The electrons involved in the
faradaic processes are
transferred to or from valence electron states (orbitals) of the redox electrode
reagent. They enter the negative electrode and flow through the external circuit
to the positive electrode where a second double-layer with an equal number of
anions has formed. The electrons reaching the positive electrode are not
transferred to the anions forming the double-layer, instead they remain in the
strongly ionized and "electron hungry" transition-metal ions of the electrode's
A cyclic voltammogram shows the surface. As such, the storage capacity of faradaic pseudocapacitance is limited
fundamental differences between
by the finite quantity of reagent in the available surface.
static capacitance (rectangular) and
pseudocapacitance (curved) A faradaic pseudocapacitance only occurs together with a static double-layer
capacitance, and its magnitude may exceed the value of double-layer capacitance
for the same surface area by factor 100, depending on the nature and the
structure of the electrode because all the pseudocapacitance reactions take place only with de-solvated ions, which are much
smaller than solvated ion with their solvating shell.[11][27] The amount of pseudocapacitance has a linear function within narrow
limits determined by the potential-dependent degree of surface coverage of the adsorbed anions.

The ability of electrodes to accomplish pseudocapacitance effects by redox reactions, intercalation or electrosorption strongly
depends on the chemical affinity of electrode materials to the ions adsorbed on the electrode surface as well as on the structure
and dimension of the electrode pores. Materials exhibiting redox behavior for use as electrodes in pseudocapacitors are transition-
metal oxides like RuO2, IrO2, or MnO2 inserted by doping in the conductive electrode material such as active carbon, as well as
conducting polymers such as polyaniline or derivatives of polythiophene covering the electrode material.

The amount of electric charge stored in a pseudocapacitance is linearly proportional to the applied voltage. The unit of
pseudocapacitance is farad.

Potential distribution

Basic illustration of the functionality of

Charge storage principles of different capacitor types
a supercapacitor, the voltage
and their internal potential distribution
distribution inside of the capacitor and
its simplified equivalent DC circuit
Conventional capacitors (also known as electrostatic capacitors), such as
ceramic capacitors and film capacitors, consist of two electrodes which are
separated by a dielectric material. When charged, the energy is stored in a static
electric field that permeates the dielectric between the electrodes. The total
energy increases with the amount of stored charge, which in turn correlates
linearly with the potential (voltage) between the plates. The maximum potential
difference between the plates (the maximal voltage) is limited by the dielectric's
breakdown field strength. The same static storage also applies for electrolytic
capacitors in which most of the potential decreases over the anode's thin oxide
layer. The somewhat resistive liquid electrolyte (cathode) accounts for a small
The voltage behavior of
decrease of potential for "wet" electrolytic capacitors, while electrolytic supercapacitors and batteries during
capacitors with solid conductive polymer electrolyte this voltage drop is charging/discharging differs clearly
negligible.

In contrast, electrochemical capacitors (supercapacitors) consists of two electrodes separated by an ion-permeable membrane
(separator) and electrically connected via an electrolyte. Energy storage occurs within the double-layers of both electrodes as a
mixture of a double-layer capacitance and pseudocapacitance. When both electrodes have approximately the same resistance
(internal resistance), the potential of the capacitor decreases symmetrically over both double-layers, whereby a voltage drop
across the equivalent series resistance (ESR) of the electrolyte is achieved. For asymmetrical supercapacitors like hybrid
capacitors the voltage drop between the electrodes could be asymmetrical. The maximum potential across the capacitor (the
maximal voltage) is limited by the electrolyte decomposition voltage.

Both electrostatic and electrochemical energy storage in supercapacitors are linear with respect to the stored charge, just as in
conventional capacitors. The voltage between the capacitor terminals is linear with respect to the amount of stored energy. Such
linear voltage gradient differs from rechargeable electrochemical batteries, in which the voltage between the terminals remains
independent of the amount of stored energy, providing a relatively constant voltage.

Comparison with other storage technologies

Supercapacitors compete with electrolytic capacitors and rechargeable batteries, especially lithium-ion batteries. The following
table compares the major parameters of the three main supercapacitor families with electrolytic capacitors and batteries.

Performance parameters of supercapacitors

compared with electrolytic capacitors and lithium-ion batteries
— Supercapacitors —
Aluminium Lithium-
Parameter electrolytic Double-layer Supercapacitors Pseudocapacitors ion
capacitors capacitors & hybrid (Li-Ion) batteries
(memory backup) (high power) (long-term)
Temperature
−40 ... −20 ...
range, −40 ... +70 °C −20 ... +70 °C −20 ... +70 °C
+125 °C +60 °C
Celsius (°C)
Maximum charge,
4 ... 630 V 1.2 ... 3.3 V 2.2 ... 3.3 V 2.2 ... 3.8 V 2.5 ... 4.2 V
Volts (V)

Recharge cycles, 0.5 k ...

unlimited 100 k ... 1 000 k 100 k ... 1 000 k 20 k ... 100 k
thousands (k) 10 k
Capacitance,
≤ 2.7 F 0.1 ... 470 F 100 ... 12 000 F 300 ... 3 300 F —
Farads (F)
Specific energy,
0.01 ... 0.3 1.5 ... 3.9 4 ... 9 10 ... 15 100 ... 265
milli-Watt hours
mW·h/g mW·h/g mW·h/g mW·h/g mW·h/g
per gram (mW·h/g)
Specific power,
0.3 ...
Watts per > 100 W/g 2 ... 10 W/g 3 ... 10 W/g 3 ... 14 W/g
1.5 W/g
gram (W/g)
Self-discharge short middle middle long long
time at room temp. (days) (weeks) (weeks) (month) (month)
Efficiency (%) 99% 95% 95% 90% 90%
Working life at
room > 20 y 5 ... 10 y 5 ... 10 y 5 ... 10 y 3 ... 5 y
temp., in years (y)

Electrolytic capacitors feature unlimited charge/discharge cycles, high dielectric strength (up to 550 V) and good frequency
response as AC resistance in the lower frequency range. Supercapacitors can store 10 to 100 times more energy than electrolytic
capacitors but they do not support AC applications.

With regards to rechargeable batteries supercapacitors feature higher peak currents, low cost per cycle, no danger of
overcharging, good reversibility, non-corrosive electrolyte and low material toxicity. Batteries offer lower purchase cost and
stable voltage under discharge, but require complex electronic control and switching equipment, with consequent energy loss and
spark hazard given a short.

Styles
Supercapacitors are made in different styles such as flat with a single pair of electrodes, wound in a cylindrical case or stacked in
a rectangular case. Because they cover a broad range of capacitance values the size of the cases can vary.

Different styles of supercapacitors

 Flat style of a supercapacitor used Radial style of a
for mobile components supercapacitor for PCB
mounting used for
industrial applications

Construction details
Construction details of wound and stacked supercapacitors with activated carbon electrodes

Schematic construction of a Schematic construction of a

wound supercapacitor supercapacitor with stacked electrodes
1. terminals, 2. safety vent, 1. positive electrode, 2. negative
3. sealing disc, 4. aluminum electrode, 3. separator
can, 5. positive pole, 6.
separator, 7. carbon
electrode, 8. collector, 9.
carbon electrode, 10.
negative pole

Supercapacitors are constructed with two metal foils (current collectors), each coated with an electrode material such as activated
carbon, which serve as the power connection between the electrode material and the external terminals of the capacitor.
Specifically to the electrode material is a very large surface area. In this example the activated carbon is electrochemically etched,
so that the surface of the material is about a factor 100,000 larger than the smooth surface. The electrodes are kept apart by an
ion-permeable membrane (separator) used as an insulator to protect the electrodes against short circuits. This construction is
subsequently rolled or folded into a cylindrical or rectangular shape and can be stacked in an aluminum can or an adaptable
rectangular housing. Then the cell is impregnated with a liquid or viscous electrolyte of organic or aqueous type. The electrolyte,
an ionic conductor, enters the pores of the electrodes and serves as the conductive connection between the electrodes across the
separator. Finally the housing is hermetically sealed to ensure stable behavior over the specified lifetime.

Supercapacitor types
 Electrical energy is stored in supercapacitors via two
storage principles: static double-layer capacitance and
electrochemical pseudocapacitance; and the distribution of
the two types of capacitance depends on the material and
structure of the electrodes. There are three types of
supercapacitors based on storage principle:[14][22]

Double-layer capacitors (EDLCs) – with activated

carbon electrodes or derivatives with much higher
electrostatic double-layer capacitance than
electrochemical pseudocapacitance
Pseudocapacitors – with transition metal oxide or
conducting polymer electrodes with a high
electrochemical pseudocapacitance
Hybrid capacitors – with asymmetric electrodes, one
Family tree of supercapacitor types. Double-layer of which exhibits mostly electrostatic and the other
capacitors and pseudocapacitors as well as hybrid mostly electrochemical capacitance, such as lithium-
capacitors are defined over their electrode designs. ion capacitors
Because double-layer capacitance and pseudocapacitance
both contribute inseparably to the total capacitance value of
an electrochemical capacitor, a correct description of these capacitors only can be given under the generic term. The concepts of
supercapattery and supercabattery have been recently proposed to better represent those hybrid devices that behave more like the
supercapacitor and the rechargeable battery, respectively.[30]

The capacitance value of a supercapacitor is determined by two storage principles:

Double-layer capacitance – electrostatic storage of the electrical energy achieved by separation of charge in a
Helmholtz double layer at the interface between the surface of a conductor electrode and an electrolytic solution
electrolyte. The separation of charge distance in a double-layer is on the order of a few ångströms (0.3–0.8 nm)
and is static in origin.[14]
Pseudocapacitance – Electrochemical storage of the electrical energy, achieved by redox reactions,
electrosorption or intercalation on the surface of the electrode by specifically adsorbed ions, that results in a
reversible faradaic charge-transfer on the electrode.[14]
Double-layer capacitance and pseudocapacitance both contribute inseparably to the total capacitance value of a
supercapacitor.[21] However, the ratio of the two can vary greatly, depending on the design of the electrodes and the composition
of the electrolyte. Pseudocapacitance can increase the capacitance value by as much as a factor of ten over that of the double-
layer by itself.[11][27]

Electric double-layer capacitors (EDLC) are electrochemical capacitors in which energy storage predominantly is achieved by
double-layer capacitance. In the past, all electrochemical capacitors were called "double-layer capacitors". Contemporary usage
sees double-layer capacitors, together with pseudocapacitors, as part of a larger family of electrochemical capacitors[11][27] called
supercapacitors. They are also known as ultracapacitors.

Materials
The properties of supercapacitors come from the interaction of their internal materials. Especially, the combination of electrode
material and type of electrolyte determine the functionality and thermal and electrical characteristics of the capacitors.

Electrodes
Supercapacitor electrodes are generally thin coatings applied and electrically
connected to a conductive, metallic current collector. Electrodes must have good
conductivity, high temperature stability, long-term chemical stability (inertness),
high corrosion resistance and high surface areas per unit volume and mass. Other
requirements include environmental friendliness and low cost.

The amount of double-layer as well as pseudocapacitance stored per unit voltage

in a supercapacitor is predominantly a function of the electrode surface area.
A micrograph of activated carbon
Therefore, supercapacitor electrodes are typically made of porous, spongy
under bright field illumination on a
material with an extraordinarily high specific surface area, such as activated light microscope. Notice the fractal-
carbon. Additionally, the ability of the electrode material to perform faradaic like shape of the particles hinting at
charge transfers enhances the total capacitance. their enormous surface area. Each
particle in this image, despite being
Generally the smaller the electrode's pores, the greater the capacitance and only around 0.1 mm across, has a
specific energy. However, smaller pores increase equivalent series resistance surface area of several square
(ESR) and decrease specific power. Applications with high peak currents require meters.

larger pores and low internal losses, while applications requiring high specific
energy need small pores.

Electrodes for EDLCs

The most commonly used electrode material for supercapacitors is carbon in various manifestations such as activated carbon
(AC), carbon fibre-cloth (AFC), carbide-derived carbon (CDC), carbon aerogel, graphite (graphene), graphane[31] and carbon
nanotubes (CNTs).[21][32][33]

Carbon-based electrodes exhibit predominantly static double-layer capacitance, even though a small amount of
pseudocapacitance may also be present depending on the pore size distribution. Pore sizes in carbons typically range from
micropores (less than 2 nm) to mesopores (2-50 nm),[34] but only micropores (<2 nm) contribute to pseudocapacitance. As pore
size approaches the solvation shell size, solvent molecules are excluded and only unsolvated ions fill the pores (even for large
ions), increasing ionic packing density and storage capability by faradaic H2 intercalation.[21]

Activated carbon
Activated carbon (AC) was the first material chosen for EDLC electrodes. Even though its electrical conductivity is
approximately 0.003% that of metals (1,250 to 2,000 S/m), it is sufficient for supercapacitors.[22][14]

Activated carbon is an extremely porous form of carbon with a high specific surface area — a common approximation is that 1
gram (0.035 oz) (a pencil-eraser-sized amount) has a surface area of roughly 1,000 to 3,000 square metres (11,000 to
32,000 sq ft)[32][34] — about the size of 4 to 12 tennis courts. The bulk form used in electrodes is low-density with many pores,
giving high double-layer capacitance.

Solid activated carbon, also termed consolidated amorphous carbon (CAC) is the most used electrode material for
supercapacitors and may be cheaper than other carbon derivatives.[35] It is produced from activated carbon powder pressed into
the desired shape, forming a block with a wide distribution of pore sizes. An electrode with a surface area of about 1000 m2/g
results in a typical double-layer capacitance of about 10 μF/cm2 and a specific capacitance of 100 F/g.

As of 2010 virtually all commercial supercapacitors use powdered activated carbon made from coconut shells.[36] Coconut shells
produce activated carbon with more micropores than does charcoal made from wood.[34]

Activated carbon fibres

Activated carbon fibres (ACF) are produced from activated carbon and have a typical diameter of 10 µm. They can have
micropores with a very narrow pore-size distribution that can be readily controlled. The surface area of ACF woven into a textile
is about 2500 m2/g. Advantages of ACF electrodes include low electrical resistance along the fibre axis and good contact to the
collector.[32]

As for activated carbon, ACF electrodes exhibit predominantly double-layer capacitance with a small amount of
pseudocapacitance due to their micropores.

Carbon aerogel
Carbon aerogel is a highly porous, synthetic, ultralight material derived from an
organic gel in which the liquid component of the gel has been replaced with a
gas.

Aerogel electrodes are made via pyrolysis of resorcinol-formaldehyde

aerogels[37] and are more conductive than most activated carbons. They enable
thin and mechanically stable electrodes with a thickness in the range of several
hundred micrometres (µm) and with uniform pore size. Aerogel electrodes also
provide mechanical and vibration stability for supercapacitors used in high- A block of silica aerogel in hand
vibration environments.

Researchers have created a carbon aerogel electrode with gravimetric densities of about 400–1200 m2/g and volumetric
capacitance of 104 F/cm3, yielding a specific energy of 325 kJ/kg (90 Wh/kg) and specific power of 20 W/g.[38][39]

Standard aerogel electrodes exhibit predominantly double-layer capacitance. Aerogel electrodes that incorporate composite
material can add a high amount of pseudocapacitance.[40]

Carbide-derived carbon
Carbide-derived carbon (CDC), also known as tunable nanoporous carbon, is a
family of carbon materials derived from carbide precursors, such as binary
silicon carbide and titanium carbide, that are transformed into pure carbon via
physical (e.g., thermal decomposition) or chemical (e.g., halogenation)
processes.[41][42]

Carbide-derived carbons can exhibit high surface area and tunable pore
diameters (from micropores to mesopores) to maximize ion confinement,
increasing pseudocapacitance by faradaic H2 adsorption treatment. CDC
electrodes with tailored pore design offer as much as 75% greater specific energy
than conventional activated carbons.
Pore size distributions for different
As of 2015, a CDC supercapacitor offered a specific energy of 10.1 Wh/kg, carbide precursors.
3,500 F capacitance and over one million charge-discharge cycles.[43]

Graphene
Graphene is a one-atom thick sheet of graphite, with atoms arranged in a regular hexagonal pattern,[44][45] also called
"nanocomposite paper".[46]
 Graphene has a theoretical specific surface area of 2630 m2/g which can theoretically lead to a
capacitance of 550 F/g. In addition, an advantage of graphene over activated carbon is its
higher electrical conductivity. As of 2012 a new development used graphene sheets directly as
electrodes without collectors for portable applications.[47][48]

In one embodiment, a graphene-based supercapacitor uses curved graphene sheets that do not
stack face-to-face, forming mesopores that are accessible to and wettable by ionic electrolytes
Graphene is an atomic- at voltages up to 4 V. A specific energy of 85.6 Wh/kg (308 kJ/kg) is obtained at room
scale honeycomb lattice
temperature equaling that of a conventional nickel metal hydride battery, but with 100-1000
made of carbon atoms.
times greater specific power.[49][50]

The two-dimensional structure of graphene improves charging and discharging. Charge

carriers in vertically oriented sheets can quickly migrate into or out of the deeper structures of the electrode, thus increasing
currents. Such capacitors may be suitable for 100/120 Hz filter applications, which are unreachable for supercapacitors using
other carbon materials.[51]

Carbon nanotubes
Carbon nanotubes (CNTs), also
called buckytubes, are carbon
molecules with a cylindrical
nanostructure. They have a
A scanning tunneling microscopy hollow structure with walls
image of single-walled carbon
formed by one-atom-thick
nanotube
sheets of graphite. These sheets
are rolled at specific and
discrete ("chiral") angles, and the combination of chiral angle and radius controls
properties such as electrical conductivity, electrolyte wettability and ion access. SEM image of carbon nanotube
Nanotubes are categorized as single-walled nanotubes (SWNTs) or multi-walled bundles with a surface of about
nanotubes (MWNTs). The latter have one or more outer tubes successively 1500 m2/g
enveloping a SWNT, much like the Russian matryoshka dolls. SWNTs have
diameters ranging between 1 and 3 nm. MWNTs have thicker coaxial walls,
separated by spacing (0.34 nm) that is close to graphene's interlayer distance.

Nanotubes can grow vertically on the collector substrate, such as a silicon wafer. Typical lengths are 20 to 100 µm.[52]

Carbon nanotubes can greatly improve capacitor performance, due to the highly wettable surface area and high
conductivity.[53][54]

A SWNT-based supercapacitor with aqueous electrolyte was systematically studied at University of Delaware in Prof. Bingqing
Wei's group. Li et al., for the first time, discovered that the ion-size effect and the electrode-electrolyte wettability are the
dominant factors affecting the electrochemical behavior of flexible SWCNTs-supercapacitors in different 1 molar aqueous
electrolytes with different anions and cations. The experimental results also showed for flexible supercapacitor that it is suggested
to put enough pressure between the two electrodes to improve the aqueous electrolyte CNT supercapacitor.[55]

CNTs can store about the same charge as activated carbon per unit surface area, but nanotubes' surface is arranged in a regular
pattern, providing greater wettability. SWNTs have a high theoretical specific surface area of 1315 m2/g, while that for MWNTs
is lower and is determined by the diameter of the tubes and degree of nesting, compared with a surface area of about 3000 m2/g
of activated carbons. Nevertheless, CNTs have higher capacitance than activated carbon electrodes, e.g., 102 F/g for MWNTs and
180 F/g for SWNTs.[56]
MWNTs have mesopores that allow for easy access of ions at the electrode–electrolyte interface. As the pore size approaches the
size of the ion solvation shell, the solvent molecules are partially stripped, resulting in larger ionic packing density and increased
faradaic storage capability. However, the considerable volume change during repeated intercalation and depletion decreases their
mechanical stability. To this end, research to increase surface area, mechanical strength, electrical conductivity and chemical
stability is ongoing.[53][57][58]

Electrodes for pseudocapacitors

MnO2 and RuO2 are typical materials used as electrodes for pseudocapacitors, since they have the electrochemical signature of a
capacitive electrode (linear dependence on current versus voltage curve) as well as exhibiting faradaic behavior. Additionally, the
charge storage originates from electron-transfer mechanisms rather than accumulation of ions in the electrochemical double layer.
Pseudocapacitors were created through faradaic redox reactions that occur within the active electrode materials. More research
was focused on transition-metal oxides such as MnO2 since transition-metal oxides have a lower cost compared to noble metal
oxides such as RuO2. Moreover, the charge storage mechanisms of transition-metal oxides are based predominantly on
pseudocapacitance. Two mechanisms of MnO2 charge storage behavior were introduced. The first mechanism implies the
intercalation of protons (H+) or alkali metal cations (C+) in the bulk of the material upon reduction followed by deintercalation
upon oxidation.[59]

MnO2 + H+(C+) +e− ⇌ MnOOH(C)[60]

The second mechanism is based on the surface adsorption of electrolyte cations on MnO2.

(MnO2)surface + C+ +e− ⇌ (MnO2− C+)surface

Not every material that exhibits faradaic behavior can be used as an electrode for pseudocapacitors, such as Ni(OH)2 since it is a
battery type electrode (non-linear dependence on current versus voltage curve).[61]

Metal oxides
Brian Evans Conway's research[11][12] described electrodes of transition metal oxides that exhibited high amounts of
pseudocapacitance. Oxides of transition metals including ruthenium (RuO2), iridium (IrO2), iron (Fe3O4), manganese (MnO2) or
sulfides such as titanium sulfide (TiS2) alone or in combination generate strong faradaic electron–transferring reactions combined
with low resistance.[62] Ruthenium dioxide in combination with H2SO4 electrolyte provides specific capacitance of 720 F/g and a
high specific energy of 26.7 Wh/kg (96.12 kJ/kg).[63]

Charge/discharge takes place over a window of about 1.2 V per electrode. This pseudocapacitance of about 720 F/g is roughly
100 times higher than for double-layer capacitance using activated carbon electrodes. These transition metal electrodes offer
excellent reversibility, with several hundred-thousand cycles. However, ruthenium is expensive and the 2.4 V voltage window for
this capacitor limits their applications to military and space applications. Das et al. reported highest capacitance value (1715 F/g)
for ruthenium oxide based supercapacitor with electrodeposited ruthenium oxide onto porous single wall carbon nanotube film
electrode.[64] A high specific capacitance of 1715 F/g has been reported which closely approaches the predicted theoretical
maximum RuO2 capacitance of 2000 F/g.

In 2014 a RuO2 supercapacitor anchored on a graphene foam electrode delivered specific capacitance of 502.78 F/g and areal
capacitance of 1.11 F/cm2) leading to a specific energy of 39.28 Wh/kg and specific power of 128.01 kW/kg over 8,000 cycles
with constant performance. The device was a three-dimensional (3D) sub-5 nm hydrous ruthenium-anchored graphene and
carbon nanotube (CNT) hybrid foam (RGM) architecture. The graphene foam was conformally covered with hybrid networks of
RuO2 nanoparticles and anchored CNTs.[65][66]
Less expensive oxides of iron, vanadium, nickel and cobalt have been tested in aqueous electrolytes, but none has been
investigated as much as manganese dioxide (MnO2). However, none of these oxides are in commercial use.[67]

Conductive polymers
Another approach uses electron-conducting polymers as pseudocapacitive material. Although mechanically weak, conductive
polymers have high conductivity, resulting in a low ESR and a relatively high capacitance. Such conducting polymers include
polyaniline, polythiophene, polypyrrole and polyacetylene. Such electrodes also employ electrochemical doping or dedoping of
the polymers with anions and cations. Electrodes made from or coated with conductive polymers have costs comparable to carbon
electrodes.

Conducting polymer electrodes generally suffer from limited cycling stability.[68] However, polyacene electrodes provide up to
10,000 cycles, much better than batteries.[69]

Electrodes for hybrid capacitors

All commercial hybrid supercapacitors are asymmetric. They combine an electrode with high amount of pseudocapacitance with
an electrode with a high amount of double-layer capacitance. In such systems the faradaic pseudocapacitance electrode with their
higher capacitance provides high specific energy while the non-faradaic EDLC electrode enables high specific power. An
advantage of the hybrid-type supercapacitors compared with symmetrical EDLC's is their higher specific capacitance value as
well as their higher rated voltage and correspondingly their higher specific energy.[70]

Composite electrodes
Composite electrodes for hybrid-type supercapacitors are constructed from carbon-based material with incorporated or deposited
pseudocapacitive active materials like metal oxides and conducting polymers. As of 2013 most research for supercapacitors
explores composite electrodes.

CNTs give a backbone for a homogeneous distribution of metal oxide or electrically conducting polymers (ECPs), producing
good pseudocapacitance and good double-layer capacitance. These electrodes achieve higher capacitances than either pure carbon
or pure metal oxide or polymer-based electrodes. This is attributed to the accessibility of the nanotubes' tangled mat structure,
which allows a uniform coating of pseudocapacitive materials and three-dimensional charge distribution. The process to anchor
pseudocapacitve materials usually uses a hydrothermal process. However, a recent researcher, Li et al., from the University of
Delaware found a facile and scalable approach to precipitate MnO2 on a SWNT film to make an organic-electrolyte based
supercapacitor.[71]

Another way to enhance CNT electrodes is by doping with a pseudocapacitive dopant as in lithium-ion capacitors. In this case the
relatively small lithium atoms intercalate between the layers of carbon.[72] The anode is made of lithium-doped carbon, which
enables lower negative potential with a cathode made of activated carbon. This results in a larger voltage of 3.8-4 V that prevents
electrolyte oxidation. As of 2007 they had achieved capacitance of 550 F/g.[9] and reach a specific energy up to 14 Wh/kg
(50.4 kJ/kg).[73]

Battery-type electrodes
Rechargeable battery electrodes influenced the development of electrodes for new hybrid-type supercapacitor electrodes as for
lithium-ion capacitors.[74] Together with a carbon EDLC electrode in an asymmetric construction offers this configuration higher
specific energy than typical supercapacitors with higher specific power, longer cycle life and faster charging and recharging times
than batteries.
Asymmetric electrodes (pseudo/EDLC)
Recently some asymmetric hybrid supercapacitors were developed in which the positive electrode were based on a real
pseudocapacitive metal oxide electrode (not a composite electrode), and the negative electrode on an EDLC activated carbon
electrode.

An advantage of this type of supercapacitors is their higher voltage and correspondingly their higher specific energy (up to 10-
20 Wh/kg (36-72 kJ/kg)).[75]

As far as known no commercial offered supercapacitors with such kind of asymmetric electrodes are on the market.

Electrolytes
Electrolytes consist of a solvent and dissolved chemicals that dissociate into positive cations and negative anions, making the
electrolyte electrically conductive. The more ions the electrolyte contains, the better its conductivity. In supercapacitors
electrolytes are the electrically conductive connection between the two electrodes. Additionally, in supercapacitors the electrolyte
provides the molecules for the separating monolayer in the Helmholtz double-layer and delivers the ions for pseudocapacitance.

The electrolyte determines the capacitor's characteristics: its operating voltage, temperature range, ESR and capacitance. With the
same activated carbon electrode an aqueous electrolyte achieves capacitance values of 160 F/g, while an organic electrolyte
achieves only 100 F/g.[76]

The electrolyte must be chemically inert and not chemically attack the other materials in the capacitor to ensure long time stable
behavior of the capacitor's electrical parameters. The electrolyte's viscosity must be low enough to wet the porous, sponge-like
structure of the electrodes. An ideal electrolyte does not exist, forcing a compromise between performance and other
requirements.

Aqueous
Water is a relatively good solvent for inorganic chemicals. Treated with acids such as sulfuric acid (H2SO4), alkalis such as
potassium hydroxide (KOH), or salts such as quaternary phosphonium salts, sodium perchlorate (NaClO4), lithium perchlorate
(LiClO4) or lithium hexafluoride arsenate (LiAsF6), water offers relatively high conductivity values of about 100 to 1000 mS/cm.
Aqueous electrolytes have a dissociation voltage of 1.15 V per electrode (2.3 V capacitor voltage) and a relatively low operating
temperature range. They are used in supercapacitors with low specific energy and high specific power.

Organic
Electrolytes with organic solvents such as acetonitrile, propylene carbonate, tetrahydrofuran, diethyl carbonate, γ-butyrolactone
and solutions with quaternary ammonium salts or alkyl ammonium salts such as tetraethylammonium tetrafluoroborate
(N(Et)4BF4[77]) or triethyl (metyl) tetrafluoroborate (NMe(Et)3BF4) are more expensive than aqueous electrolytes, but they have
a higher dissociation voltage of typically 1.35 V per electrode (2.7 V capacitor voltage), and a higher temperature range. The
lower electrical conductivity of organic solvents (10 to 60 mS/cm) leads to a lower specific power, but since the specific energy
increases with the square of the voltage, a higher specific energy.

Separators
Separators have to physically separate the two electrodes to prevent a short circuit by direct contact. It can be very thin (a few
hundredths of a millimeter) and must be very porous to the conducting ions to minimize ESR. Furthermore, separators must be
chemically inert to protect the electrolyte's stability and conductivity. Inexpensive components use open capacitor papers. More
sophisticated designs use nonwoven porous polymeric films like polyacrylonitrile or Kapton, woven glass fibers or porous woven
ceramic fibres.[78][79]

Collectors and housing

Current collectors connect the electrodes to the capacitor's terminals. The collector is either sprayed onto the electrode or is a
metal foil. They must be able to distribute peak currents of up to 100 A.

If the housing is made out of a metal (typically aluminum) the collectors should be made from the same material to avoid forming
a corrosive galvanic cell.

Electrical parameters

Capacitance
Capacitance values for commercial capacitors are specified as "rated capacitance
CR". This is the value for which the capacitor has been designed. The value for
an actual component must be within the limits given by the specified tolerance.
Typical values are in the range of farads (F), three to six orders of magnitude
larger than those of electrolytic capacitors.

The capacitance value results from the energy (expressed in Joule) of a

loaded capacitor loaded via a DC voltage VDC.
Schematic illustration of the
capacitance behavior resulting out of
the porous structure of the electrodes

This value is also called the "DC capacitance".

Measurement
Conventional capacitors are normally measured with a small AC voltage (0.5 V)
and a frequency of 100 Hz or 1 kHz depending on the capacitor type. The AC
Equivalent circuit with cascaded RC
capacitance measurement offers fast results, important for industrial production
elements
lines. The capacitance value of a supercapacitor depends strongly on the
measurement frequency, which is related to the porous electrode structure and
the limited electrolyte's ion mobility. Even at a low frequency of 10 Hz, the
measured capacitance value drops from 100 to 20 percent of the DC capacitance
value.

This extraordinary strong frequency dependence can be explained by the

different distances the ions have to move in the electrode's pores. The area at the
beginning of the pores can easily be accessed by the ions. The short distance is
accompanied by low electrical resistance. The greater the distance the ions have
to cover, the higher the resistance. This phenomenon can be described with a
series circuit of cascaded RC (resistor/capacitor) elements with serial RC time Frequency depending of the
constants. These result in delayed current flow, reducing the total electrode capacitance value of a 50 F
supercapacitor
surface area that can be covered with ions if polarity changes – capacitance decreases with increasing AC frequency. Thus, the
total capacitance is only achieved after longer measuring times.

Out of the reason of the very strong frequency dependence of the capacitance
this electrical parameter has to be measured with a special constant current
charge and discharge measurement, defined in IEC standards 62391-1 and -2.

Measurement starts with charging the capacitor. The voltage has to be applied
and after the constant current/constant voltage power supply has achieved the
rated voltage, the capacitor has to be charged for 30 minutes. Next, the capacitor
has to be discharged with a constant discharge current Idischarge. Then the time t1
Illustration of the measurement
and t2, for the voltage to drop from 80% (V1) to 40% (V2) of the rated voltage is
conditions for measuring the
measured. The capacitance value is calculated as:
capacitance of supercapacitors

The value of the discharge current is determined by the application. The IEC standard defines four classes:

1. Memory backup, discharge current in mA = 1 • C (F)

2. Energy storage, discharge current in mA = 0,4 • C (F) • V (V)
3. Power, discharge current in mA = 4 • C (F) • V (V)
4. Instantaneous power, discharge current in mA = 40 • C (F) • V (V)
The measurement methods employed by individual manufacturers are mainly comparable to the standardized methods.[80][81]

The standardized measuring method is too time consuming for manufacturers to use during production for each individual
component. For industrial produced capacitors the capacitance value is instead measured with a faster low frequency AC voltage
and a correlation factor is used to compute the rated capacitance.

This frequency dependence affects capacitor operation. Rapid charge and discharge cycles mean that neither the rated capacitance
value nor specific energy are available. In this case the rated capacitance value is recalculated for each application condition.

Operating voltage
Supercapacitors are low voltage components. Safe operation requires that the
voltage remain within specified limits. The rated voltage UR is the maximum
DC voltage or peak pulse voltage that may be applied continuously and remain
within the specified temperature range. Capacitors should never be subjected to
voltages continuously in excess of the rated voltage.

The rated voltage includes a safety margin against the electrolyte's breakdown
voltage at which the electrolyte decomposes. The breakdown voltage
decomposes the separating solvent molecules in the Helmholtz double-layer, f. e.
water splits into hydrogen and oxide. The solvent molecules then cannot separate
the electrical charges from each other. Higher voltages than rated voltage cause
hydrogen gas formation or a short circuit. A 5.5 volt supercapacitor is
constructed out of two single cells,
Standard supercapacitors with aqueous electrolyte normally are specified with a
each rated to at least 2.75 volts, in
rated voltage of 2.1 to 2.3 V and capacitors with organic solvents with 2.5 to series connection
2.7 V. Lithium-ion capacitors with doped electrodes may reach a rated voltage of
3.8 to 4 V, but have a lower voltage limit of about 2.2 V.
Operating supercapacitors below the rated voltage improves the long-time behavior of the electrical parameters. Capacitance
values and internal resistance during cycling are more stable and lifetime and charge/discharge cycles may be extended.[81]

Higher application voltages require connecting cells in series. Since each component has a slight difference in capacitance value
and ESR, it is necessary to actively or passively balance them to stabilize the applied voltage. Passive balancing employs resistors
in parallel with the supercapacitors. Active balancing may include electronic voltage management above a threshold that varies
the current.

Internal resistance
Charging/discharging a supercapacitor is connected to the movement of charge
carriers (ions) in the electrolyte across the separator to the electrodes and into
their porous structure. Losses occur during this movement that can be measured
as the internal DC resistance.

With the electrical model of cascaded, series-connected RC (resistor/capacitor)

elements in the electrode pores, the internal resistance increases with the
increasing penetration depth of the charge carriers into the pores. The internal
DC resistance is time dependent and increases during charge/discharge. In The internal DC resistance can be
calculated out of the voltage drop
applications often only the switch-on and switch-off range is interesting. The
obtained from the intersection of the
internal resistance Ri can be calculated from the voltage drop ΔV2 at the time of
auxiliary line extended from the
discharge, starting with a constant discharge current Idischarge. It is obtained from straight part and the time base at the
the intersection of the auxiliary line extended from the straight part and the time time of discharge start
base at the time of discharge start (see picture right). Resistance can be
calculated by:

The discharge current Idischarge for the measurement of internal resistance can be taken from the classification according to IEC
62391-1.

This internal DC resistance Ri should not be confused with the internal AC resistance called equivalent series resistance (ESR)
normally specified for capacitors. It is measured at 1 kHz. ESR is much smaller than DC resistance. ESR is not relevant for
calculating superconductor inrush currents or other peak currents.

Ri determines several supercapacitor properties. It limits the charge and discharge peak currents as well as charge/discharge
times. Ri and the capacitance C results in the time constant

This time constant determines the charge/discharge time. A 100 F capacitor with an internal resistance of 30 mΩ for example, has
a time constant of 0.03 • 100 = 3 s. After 3 seconds charging with a current limited only by internal resistance, the capacitor has
63.2% of full charge (or is discharged to 36.8% of full charge).

Standard capacitors with constant internal resistance fully charge during about 5 τ. Since internal resistance increases with
charge/discharge, actual times cannot be calculated with this formula. Thus, charge/discharge time depends on specific individual
construction details.
Current load and cycle stability
Because supercapacitors operate without forming chemical bonds, current loads, including charge, discharge and peak currents
are not limited by reaction constraints. Current load and cycle stability can be much higher than for rechargeable batteries.
Current loads are limited only by internal resistance, which may be substantially lower than for batteries.

Internal resistance "Ri" and charge/discharge currents or peak currents "I" generate internal heat losses "Ploss" according to:

This heat must be released and distributed to the ambient environment to maintain operating temperatures below the specified
maximum temperature.

Heat generally defines capacitor lifetime because of electrolyte diffusion. The heat generation coming from current loads should
be smaller than 5 to 10 K at maximum ambient temperature (which has only minor influence on expected lifetime). For that
reason the specified charge and discharge currents for frequent cycling are determined by internal resistance.

The specified cycle parameters under maximal conditions include charge and discharge current, pulse duration and frequency.
They are specified for a defined temperature range and over the full voltage range for a defined lifetime. They can differ
enormously depending on the combination of electrode porosity, pore size and electrolyte. Generally a lower current load
increases capacitor life and increases the number of cycles. This can be achieved either by a lower voltage range or slower
charging and discharging.[81]

Supercapacitors (except those with polymer electrodes) can potentially support more than one million charge/discharge cycles
without substantial capacity drops or internal resistance increases. Beneath the higher current load is this the second great
advantage of supercapacitors over batteries. The stability results from the dual electrostatic and electrochemical storage
principles.

The specified charge and discharge currents can be significantly exceeded by lowering the frequency or by single pulses. Heat
generated by a single pulse may be spread over the time until the next pulse occurs to ensure a relatively small average heat
increase. Such a "peak power current" for power applications for supercapacitors of more than 1000 F can provide a maximum
peak current of about 1000 A.[82] Such high currents generate high thermal stress and high electromagnetic forces that can
damage the electrode-collector connection requiring robust design and construction of the capacitors.

Device capacitance and resistance dependence on operating voltage and

temperature
Device parameters such as capacitance initial resistance and steady state
resistance are not constant but are variable and dependent on the device's
operating voltage. Device capacitance will have a measurable increase as the
operating voltage increases. For example: a 100F device can be seen to vary
26% from its maximum capacitance over its entire operational voltage range.
Similar dependence on operating voltage is seen in steady state resistance (Rss)
and initial resistance (Ri). [83]
Measured device capacitance across
Device properties can also be seen to be dependent on device temperature. As
an EDLC's operating voltage
the temperature of the device changes either through operation of varying
ambient temperature, the internal properties such as capacitance and resistance
will vary as well. Device capacitance is seen to increase as the operating temperature increases.[83]
Energy capacity
Supercapacitors occupy the gap between high power/low energy electrolytic
capacitors and low power/high energy rechargeable batteries. The energy Wmax
(expressed in Joule) that can be stored in a capacitor is given by the formula

This formula describes the amount of energy stored and is often used to describe
new research successes. However, only part of the stored energy is available to Ragone chart showing specific power
applications, because the voltage drop and the time constant over the internal vs. specific energy of various
resistance mean that some of the stored charge is inaccessible. The effective capacitors and batteries
realized amount of energy Weff is reduced by the used voltage difference
between Vmax and Vmin and can be represented as:[84]

This formula also represents the energy asymmetric voltage components such as lithium ion capacitors.

Specific energy and specific power

The amount of energy that can be stored in a capacitor per mass of that capacitor is called its specific energy. Specific energy is
measured gravimetrically (per unit of mass) in watt-hours per kilogram (Wh/kg).

The amount of energy can be stored in a capacitor per volume of that capacitor is called its energy density. Energy density is
measured volumetrically (per unit of volume) in watt-hours per litre (Wh/l).

As of 2013 commercial specific energies range from around 0.5 to 15 Wh/kg. For comparison, an aluminum electrolytic capacitor
stores typically 0.01 to 0.3 Wh/kg, while a conventional lead-acid battery stores typically 30 to 40 Wh/kg and modern lithium-ion
batteries 100 to 265 Wh/kg. Supercapacitors can therefore store 10 to 100 times more energy than electrolytic capacitors, but only
one tenth as much as batteries. For reference, petrol fuel has a specific energy of 44.4 MJ/kg or 12 300 Wh/kg (in vehicle
propulsion, the efficiency of energy conversions should be considered resulting in 3700 Wh/kg considering a typical 30% internal
combustion engine efficiency).

Commercial energy density (also called volumetric specific energy in some literature) varies widely but in general range from
around 5 to 8 Wh/l. Units of liters and dm3 can be used interchangeably. In comparison, petrol fuel has an energy density of 32.4
MJ/l or 9000 Wh/l.

Although the specific energy of supercapacitors is insufficient compared with batteries, capacitors have the important advantage
of the specific power. Specific power describes the speed at which energy can be delivered to/absorbed from the load. The
maximum power is given by the formula:[84]

with V = voltage applied and Ri, the internal DC resistance of the capacitor.

Specific power is measured either gravimetrically in kilowatts per kilogram (kW/kg, specific power) or volumetrically in
kilowatts per litre (kW/l, power density).
The described maximum power Pmax specifies the power of a theoretical rectangular single maximum current peak of a given
voltage. In real circuits the current peak is not rectangular and the voltage is smaller, caused by the voltage drop. IEC 62391–2
established a more realistic effective power Peff for supercapacitors for power applications:

Supercapacitor specific power is typically 10 to 100 times greater than for batteries and can reach values up to 15 kW/kg.

Ragone charts relate energy to power and are a valuable tool for characterizing and visualizing energy storage components. With
such a diagram, the position of specific power and specific energy of different storage technologies is easily to compare, see
diagram.[85][86]

Lifetime
Since supercapacitors do not rely on chemical changes in the electrodes (except
for those with polymer electrodes), lifetimes depend mostly on the rate of
evaporation of the liquid electrolyte. This evaporation in general is a function of
temperature, of current load, current cycle frequency and voltage. Current load
and cycle frequency generate internal heat, so that the evaporation-determining
temperature is the sum of ambient and internal heat. This temperature is
measurable as core temperature in the center of a capacitor body. The higher the
core temperature the faster the evaporation and the shorter the lifetime. The lifetime of supercapacitors
depends mainly on the capacitor
Evaporation generally results in decreasing capacitance and increasing internal temperature and the voltage applied
resistance. According to IEC/EN 62391-2 capacitance reductions of over 30% or
internal resistance exceeding four times its data sheet specifications are
considered "wear-out failures", implying that the component has reached end-of-life. The capacitors are operable, but with
reduced capabilities. Whether the aberration of the parameters have any influence on the proper functionality or not depends on
the application of the capacitors.

Such large changes of electrical parameters specified in IEC/EN 62391-2 are usually unacceptable for high current load
applications. Components that support high current loads use much smaller limits, e.g., 20% loss of capacitance or double the
internal resistance.[87] The narrower definition is important for such applications, since heat increases linearly with increasing
internal resistance and the maximum temperature should not be exceeded. Temperatures higher than specified can destroy the
capacitor.

The real application lifetime of supercapacitors, also called "service life", "life expectancy" or "load life", can reach 10 to 15
years or more at room temperature. Such long periods cannot be tested by manufacturers. Hence, they specify the expected
capacitor lifetime at the maximum temperature and voltage conditions. The results are specified in datasheets using the notation
"tested time (hours)/max. temperature (°C)", such as "5000 h/65 °C". With this value and expressions derived from historical
data, lifetimes can be estimated for lower temperature conditions.

Datasheet lifetime specification is tested by the manufactures using an accelerated aging test called "endurance test" with
maximum temperature and voltage over a specified time. For a "zero defect" product policy during this test no wear out or total
failure may occur.

The lifetime specification from datasheets can be used to estimate the expected lifetime for a given design. The "10-degrees-rule"
used for electrolytic capacitors with non-solid electrolyte is used in those estimations and can be used for supercapacitors. This
rule employs the Arrhenius equation, a simple formula for the temperature dependence of reaction rates. For every 10 °C
reduction in operating temperature, the estimated life doubles.

With

Lx = estimated lifetime
L0 = specified lifetime
T0 = upper specified capacitor temperature
Tx = actual operating temperature of the capacitor cell
Calculated with this formula, capacitors specified with 5000 h at 65 °C, have an estimated lifetime of 20,000 h at 45 °C.

Lifetimes are also dependent on the operating voltage, because the development of gas in the liquid electrolyte depends on the
voltage. The lower the voltage the smaller the gas development and the longer the lifetime. No general formula relates voltage to
lifetime. The voltage dependent curves shown from the picture are an empirical result from one manufacturer.

Life expectancy for power applications may be also limited by current load or number of cycles. This limitation has to be
specified by the relevant manufacturer and is strongly type dependent.

Self-discharge
Storing electrical energy in the double-layer separates the charge carriers within the pores by distances in the range of molecules.
Over this short distance irregularities can occur, leading to a small exchange of charge carriers and gradual discharge. This self-
discharge is called leakage current. Leakage depends on capacitance, voltage, temperature and the chemical stability of the
electrode/electrolyte combination. At room temperature leakage is so low that it is specified as time to self-discharge.
Supercapacitor self-discharge time is specified in hours, days or weeks. As an example, a 5.5 V/F Panasonic "Goldcapacitor"
specifies a voltage drop at 20 °C from 5.5 V to 3 V in 600 hours (25 days or 3.6 weeks) for a double cell capacitor.[88]

Post charge voltage relaxation

It has been noticed that after the EDLC experiences a charge or discharge, the
voltage will drift over time, relaxing toward its previous voltage level. The
observed relaxation can occur over several hours and is likely due to long
diffusion time constants of the porous electrodes within the EDLC. [83]

Polarity
Since the positive and negative electrodes (or simply positrode and negatrode, A graph plotting voltage over time,
respectively) of symmetric supercapacitors consist of the same material, after the application of a charge
theoretically supercapacitors have no true polarity and catastrophic failure does
not normally occur. However reverse-charging a supercapacitor lowers its
capacity, so it is recommended practice to maintain the polarity resulting from the formation of the electrodes during production.
Asymmetric supercapacitors are inherently polar.

Pseudocapacitor and hybrid supercapacitors which have electrochemical charge properties may not be operated with reverse
polarity, precluding their use in AC operation. However, this limitation does not apply to EDLC supercapacitors

A bar in the insulating sleeve identifies the negative terminal in a polarized component.
In some literature, the terms "anode" and "cathode" are used in place of negative
electrode and positive electrode. Using anode and cathode to describe the
electrodes in supercapacitors (and also rechargeable batteries including lithium
ion batteries) can lead to confusion, because the polarity changes depending on
whether a component is considered as a generator or as a consumer of current. In
electrochemistry, cathode and anode are related to reduction and oxidation
reactions, respectively. However, in supercapacitors based on electric double
layer capacitance, there is no oxidation and/or reduction reactions on any of the
A negative bar on the insulating
two electrodes. Therefore, the concepts of cathode and anode do not apply.
sleeve indicates the cathode terminal
of the capacitor
Comparison of selected commercial
supercapacitors
The range of electrodes and electrolytes available yields a variety of components suitable for diverse applications. The
development of low-ohmic electrolyte systems, in combination with electrodes with high pseudocapacitance, enable many more
technical solutions.

The following table shows differences among capacitors of various manufacturers in capacitance range, cell voltage, internal
resistance (ESR, DC or AC value) and volumetric and gravimetric specific energy.

In the table, ESR refers to the component with the largest capacitance value of the respective manufacturer. Roughly, they divide
supercapacitors into two groups. The first group offers greater ESR values of about 20 milliohms and relatively small capacitance
of 0.1 to 470 F. These are "double-layer capacitors" for memory back-up or similar applications. The second group offers 100 to
10,000 F with a significantly lower ESR value under 1 milliohm. These components are suitable for power applications. A
correlation of some supercapacitor series of different manufacturers to the various construction features is provided in Pandolfo
and Hollenkamp.[32]

In commercial double-layer capacitors, or, more specifically, EDLCs in which energy storage is predominantly achieved by
double-layer capacitance, energy is stored by forming an electrical double layer of electrolyte ions on the surface of conductive
electrodes. Since EDLCs are not limited by the electrochemical charge transfer kinetics of batteries, they can charge and
discharge at a much higher rate, with lifetimes of more than 1 million cycles. The EDLC energy density is determined by
operating voltage and the specific capacitance (farad/gram or farad/cm3) of the electrode/electrolyte system. The specific
capacitance is related to the Specific Surface Area (SSA) accessible by the electrolyte, its interfacial double-layer capacitance,
and the electrode material density.

Commercial EDLCs are based on two symmetric electrodes impregnated with electrolytes comprising tetraethylammonium
tetrafluoroborate salts in organic solvents. Current EDLCs containing organic electrolytes operate at 2.7 V and reach energy
densities around 5-8 Wh/kg and 7 to 10 Wh/l. The specific capacitance is related to the specific surface area (SSA) accessible by
the electrolyte, its interfacial double-layer capacitance, and the electrode material density. Graphene-based platelets with
mesoporous spacer material is a promising structure for increasing the SSA of the electrolyte.[89]

Standards
Supercapacitors vary sufficiently that they are rarely interchangeable, especially those with higher specific energy. Applications
range from low to high peak currents, requiring standardized test protocols.[90]

Test specifications and parameter requirements are specified in the generic specification

IEC/EN 62391–1, Fixed electric double layer capacitors for use in electronic equipment.
The standard defines four application classes, according to
discharge current levels:

1. Memory backup
2. Energy storage, mainly used for driving motors
require a short time operation,
3. Power, higher power demand for a long time
operation,
4. Instantaneous power, for applications that
requires relatively high current units or peak
currents ranging up to several hundreds of
amperes even with a short operating time
Three further standards describe special applications:

IEC 62391–2, Fixed electric double-layer

capacitors for use in electronic equipment - Blank Classification of supercapacitors into classes regarding
detail specification - Electric double-layer to IEC 62391-1, IEC 62567and BS EN 61881-3
capacitors for power application
standards
IEC 62576, Electric double-layer capacitors for
use in hybrid electric vehicles. Test methods for
electrical characteristics
BS/EN 61881-3, Railway applications. Rolling stock equipment. Capacitors for power electronics. Electric double-
layer capacitors

Applications
Supercapacitors do not support AC applications.

Supercapacitors have advantages in applications where a large amount of power is needed for a relatively short time, where a
very high number of charge/discharge cycles or a longer lifetime is required. Typical applications range from milliamp currents or
milliwatts of power for up to a few minutes to several amps current or several hundred kilowatts power for much shorter periods.

The time t a supercapacitor can deliver a constant current I can be calculated as:

as the capacitor voltage decreases from Ucharge down to Umin.

If the application needs a constant power P for a certain time t this can be calculated as:

wherein also the capacitor voltage decreases from Ucharge down to Umin.

General

Consumer electronics
In applications with fluctuating loads, such as laptop computers, PDA's, GPS, portable media players, hand-held devices,[91] and
photovoltaic systems, supercapacitors can stabilize the power supply.
Supercapacitors deliver power for photographic flashes in digital cameras and for LED flashlights that can be charged in much
shorter periods of time, e.g., 90 seconds.[92]

Some portable speakers are powered by supercapacitors.[93]

Tools
A cordless electric screwdriver with supercapacitors for energy storage has about half the run time of a comparable battery model,
but can be fully charged in 90 seconds. It retains 85% of its charge after three months left idle.[94]

Grid power buffer

Numerous non-linear loads, such as EV chargers, HEVs, air conditioning systems, and advanced power conversion systems cause
current fluctuations and harmonics.[95][96] These current differences create unwanted voltage fluctuations and therefore power
oscillations on the grid.[95] Power oscillations not only reduce the efficiency of the grid, but can cause voltage drops in the
common coupling bus, and considerable frequency fluctuations throughout the entire system. To overcome this problem,
supercapacitors can be implemented as an interface between the load and the grid to act as a buffer between the grid and the high
pulse power drawn from the charging station.

[97][98]

Low-power equipment power buffer

Supercapacitors provide backup or emergency shutdown power to low-power equipment such as RAM, SRAM, micro-controllers
and PC Cards. They are the sole power source for low energy applications such as automated meter reading (AMR)[99]
equipment or for event notification in industrial electronics.

Supercapacitors buffer power to and from rechargeable batteries, mitigating the effects of short power interruptions and high
current peaks. Batteries kick in only during extended interruptions, e.g., if the mains power or a fuel cell fails, which lengthens
battery life.

Uninterruptible power supplies (UPS) may be powered by supercapacitors, which can replace much larger banks of electrolytic
capacitors. This combination reduces the cost per cycle, saves on replacement and maintenance costs, enables the battery to be
downsized and extends battery life.[100][101][102]

Supercapacitors provide backup power for actuators in wind turbine pitch

systems, so that blade pitch can be adjusted even if the main supply fails.[103]

Voltage stabilizer
Supercapacitors can stabilize voltage fluctuations for powerlines by acting as
dampeners. Wind and photovoltaic systems exhibit fluctuating supply evoked by
gusting or clouds that supercapacitors can buffer within milliseconds. Also,
similar to electrolytic capacitors, supercapacitors are also placed along the power
Rotor with wind turbine pitch system
lines to consume reactive power and improve the AC power factor in a leading
power flow circuit. This would allow for a better used real power to produced
power and make the grid overall more efficient.[104][105][106][107]

Micro Grids
Micro Grids are usually powered by clean and renewable energy. Most of this energy generation, however, is not constant
throughout the day and does not usually match demand. Supercapacitors can be used for micro grid storage to instantaneously
inject power when the demand is high and the production dips momentarily, and to store energy in the reverse conditions. They
are useful in this scenario, because micro grids are increasingly producing power in DC, and capacitors can be utilized in both
DC and AC applications. Supercapacitors work best in conjunction with chemical batteries.They provide an immediate voltage
buffer to compensate for quick changing power loads due to their high charge and discharge rate through an active control
system.[108] Once the voltage is buffered, it is put through an inverter to supply AC power to the grid. It is important to note that
supercapacitors cannot provide frequency correction in this form directly in the AC grid.[109][110]

Energy harvesting
Supercapacitors are suitable temporary energy storage devices for energy harvesting systems. In energy harvesting systems, the
energy is collected from the ambient or renewable sources, e.g. mechanical movement, light or electromagnetic fields, and
converted to electrical energy in an energy storage device. For example, it was demonstrated that energy collected from RF (radio
frequency) fields (using an RF antenna as an appropriate rectifier circuit) can be stored to a printed supercapacitor. The harvested
energy was then used to power an application-specific integrated circuit (ASIC) circuit for over 10 hours.[111]

Incorporation into batteries

The UltraBattery is a hybrid rechargeable lead-acid battery and a supercapacitor. Its cell construction contains a standard lead-
acid battery positive electrode, standard sulphuric acid electrolyte and a specially prepared negative carbon-based electrode that
store electrical energy with double-layer capacitance. The presence of the supercapacitor electrode alters the chemistry of the
battery and affords it significant protection from sulfation in high rate partial state of charge use, which is the typical failure mode
of valve regulated lead-acid cells used this way. The resulting cell performs with characteristics beyond either a lead-acid cell or a
supercapacitor, with charge and discharge rates, cycle life, efficiency and performance all enhanced.

Street lights
Sado City, in Japan's Niigata Prefecture, has street lights that combine a stand-alone power source
with solar cells and LEDs. Supercapacitors store the solar energy and supply 2 LED lamps, providing
15 W power consumption overnight. The supercapacitors can last more than 10 years and offer stable
performance under various weather conditions, including temperatures from +40 to below
-20 °C.[112]

Medical
Supercapacitors are used in defibrillators where they can deliver 500 joules to shock the heart back
into sinus rhythm.[113]
Street light
combining a solar
cell power source Transport
with LED lamps
and
supercapacitors Aviation
for energy storage
In 2005, aerospace systems and controls company Diehl Luftfahrt Elektronik GmbH chose
supercapacitors to power emergency actuators for doors and evacuation slides used in airliners,
including the Airbus 380.[103]

Military
Supercapacitors' low internal resistance supports applications that require short-term high currents. Among the earliest uses were
motor startup (cold engine starts, particularly with diesels) for large engines in tanks and submarines.[114] Supercapacitors buffer
the battery, handling short current peaks, reducing cycling and extending battery life.

Further military applications that require high specific power are phased array radar antennae, laser power supplies, military radio
communications, avionics displays and instrumentation, backup power for airbag deployment and GPS-guided missiles and
projectiles.[115][116]

Automotive
Toyota's Yaris Hybrid-R concept car uses a supercapacitor to provide bursts of power. PSA Peugeot Citroën has started using
supercapacitors as part of its stop-start fuel-saving system, which permits faster initial acceleration.[117] Mazda's i-ELOOP
system stores energy in a supercapacitor during deceleration and uses it to power on-board electrical systems while the engine is
stopped by the stop-start system.

Bus/tram
Maxwell Technologies, an American supercapacitor-maker, claimed that more than 20,000 hybrid buses use the devices to
increase acceleration, particularly in China. Guangzhou, In 2014 China began using trams powered with supercapacitors that are
recharged in 30 seconds by a device positioned between the rails, storing power to run the tram for up to 4 km — more than
enough to reach the next stop, where the cycle can be repeated.[117]

Energy recovery
A primary challenge of all transport is reducing energy consumption and reducing CO2 emissions. Recovery of braking energy
(recuperation or regeneration) helps with both. This requires components that can quickly store and release energy over long
times with a high cycle rate. Supercapacitors fulfill these requirements and are therefore used in a lot of applications in all kinds
of transportation.

Railway
Supercapacitors can be used to supplement batteries in starter systems in diesel
railroad locomotives with diesel-electric transmission. The capacitors capture the
braking energy of a full stop and deliver the peak current for starting the diesel
engine and acceleration of the train and ensures the stabilization of line voltage.
Depending on the driving mode up to 30% energy saving is possible by recovery
of braking energy. Low maintenance and environmentally friendly materials
encouraged the choice of supercapacitors.[118][119]

Green Cargo operates TRAXX

Cranes, forklifts and tractors locomotives from Bombardier
Transportation
Mobile hybrid diesel-electric rubber tyred gantry cranes move and stack
containers within a terminal. Lifting the boxes requires large amounts of energy.
Some of the energy could be recaptured while lowering the load resulting in improved efficiency.[120]

A triple hybrid forklift truck uses fuel cells and batteries as primary energy storage and supercapacitors to buffer power peaks by
storing braking energy. They provide the fork lift with peak power over 30 kW. The triple-hybrid system offers over 50% energy
savings compared with diesel or fuel-cell systems.[121]
 Supercapacitor-powered terminal tractors transport containers to warehouses.
They provide an economical, quiet and pollution-free alternative to diesel
terminal tractors.[122]

Light-rails and trams

Supercapacitors make it possible not only to reduce energy but to replace
overhead lines in historical city areas, so preserving the city's architectural
heritage. This approach may allow many new LRV city lines to replace overhead
Container yard with rubber tyre wires that are too expensive to fully route.
gantry crane
In 2003 Mannheim adopted a
prototype light-rail vehicle
(LRV) using the MITRAC Energy Saver system from Bombardier
Transportation to store mechanical braking energy with a roof-mounted
supercapacitor unit.[123][124] It contains several units each made of 192
capacitors with 2700 F / 2.7 V interconnected in three parallel lines. This circuit
results in a 518 V system with an energy content of 1.5 kWh. For acceleration
when starting this "on-board-system" can provide the LRV with 600 kW and can Light rail vehicle in Mannheim
drive the vehicle up to 1 km without overhead line supply, thus better integrating
the LRV into the urban environment. Compared to conventional LRVs or Metro
vehicles that return energy into the grid, onboard energy storage saves up to 30% and reduces peak grid demand by up to
50%.[125]

In 2009 supercapacitors enabled LRV's to operate in the historical city area of

Heidelberg without overhead wires, thus preserving the city's architectural
heritage. The SC equipment cost an additional €270,000 per vehicle, which was
expected to be recovered over the first 15 years of operation. The
supercapacitors are charged at stop-over stations when the vehicle is at a
scheduled stop. In April 2011 German regional transport operator Rhein-Neckar,
responsible for Heidelberg, ordered a further 11 units.[126]

In 2009, Alstom and RATP equipped a Citadis tram with an experimental energy
Supercapacitors are used to power recovery system called "STEEM".[127] The system is fitted with 48 roof-
the Paris T3 tram line on sections mounted supercapacitors to store braking energy which provides tramways with
without overhead wires and to
a high level of energy autonomy by enabling them to run without overhead
recover energy during braking.
power lines on parts of its route, recharging while traveling on powered stop-
over stations. During the tests, which took place between the Porte d’Italie and
Porte de Choisy stops on line T3 of the tramway network in Paris, the tramset used an average of approximately 16% less
energy.[128]

In 2012 tram operator Geneva Public Transport began tests of an LRV equipped with a prototype roof-mounted supercapacitor
unit to recover braking energy.[129]

Siemens is delivering supercapacitor-enhanced light-rail transport systems that include mobile storage.[130]

Hong Kong's South Island metro line is to be equipped with two 2 MW energy storage units that are expected to reduce energy
consumption by 10%.[131]
In August 2012 the CSR Zhuzhou Electric Locomotive corporation of China
presented a prototype two-car light metro train equipped with a roof-mounted
supercapacitor unit. The train can travel up 2 km without wires, recharging in
30 seconds at stations via a ground mounted pickup. The supplier claimed the
trains could be used in 100 small and medium-sized Chinese cities.[132] Seven
trams (street cars) powered by supercapacitors were scheduled to go into
operation in 2014 in Guangzhou, China. The supercapacitors are recharged in
30 seconds by a device positioned between the rails. That powers the tram for up
A supercapacitor-equipped tram on
to 4 kilometres (2.5 mi).[133] As of 2017, Zhuzhou's supercapacitor vehicles are
the Rio de Janeiro Light Rail
also used on the new Nanjing streetcar system, and are undergoing trials in
Wuhan.[134]

In 2012, in Lyon (France), the SYTRAL (Lyon public transportation administration) started experiments of a "way side
regeneration" system built by Adetel Group which has developed its own energy saver named ″NeoGreen″ for LRV, LRT and
metros.[135]

In 2015, Alstom announced SRS, an energy storage system that charges supercapacitors on board a tram by means of ground-
level conductor rails located at tram stops. This allows trams to operate without overhead lines for short distances.[136] The
system has been touted as an alternative to the company's ground-level power supply (APS) system, or can be used in conjunction
with it, as in the case of the VLT network in Rio de Janeiro, Brazil, which opened in 2016.[137]

Buses
The first hybrid bus with supercapacitors in Europe came in 2001 in Nuremberg,
Germany. It was MAN's so-called "Ultracapbus", and was tested in real
operation in 2001/2002. The test vehicle was equipped with a diesel-electric
drive in combination with supercapacitors. The system was supplied with 8
Ultracap modules of 80 V, each containing 36 components. The system worked
with 640 V and could be charged/discharged at 400 A. Its energy content was
0.4 kWh with a weight of 400 kg.
MAN Ultracapbus in Nuremberg,
Germany The supercapacitors recaptured braking energy and delivered starting energy.
Fuel consumption was reduced by 10 to 15% compared to conventional diesel
vehicles. Other advantages included reduction of CO2 emissions, quiet and
emissions-free engine starts, lower vibration and reduced maintenance costs.[138][139]

As of 2002 in Luzern, Switzerland an electric bus fleet called TOHYCO-Rider

was tested. The supercapacitors could be recharged via an inductive contactless
high-speed power charger after every transportation cycle, within 3 to 4
minutes.[140]

In early 2005 Shanghai tested a new form of electric bus called capabus that runs
without powerlines (catenary free operation) using large onboard supercapacitors
that partially recharge whenever the bus is at a stop (under so-called electric Electric bus at EXPO 2010 in
umbrellas), and fully charge in the terminus. In 2006, two commercial bus routes Shanghai (Capabus) recharging at
began to use the capabuses; one of them is route 11 in Shanghai. It was estimated the bus stop

that the supercapacitor bus was cheaper than a lithium-ion battery bus, and one
of its buses had one-tenth the energy cost of a diesel bus with lifetime fuel
savings of $200,000.[141]
A hybrid electric bus called tribrid was unveiled in 2008 by the University of Glamorgan, Wales, for use as student transport. It is
powered by hydrogen fuel or solar cells, batteries and ultracapacitors.[142][143]

Motor racing
The FIA, a governing body for
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
Toyota TS030 Hybrid at 2012 24
Hours of Le Mans motor race includes a hybrid drive of up to World champion Sebastian Vettel in
200 kW input and output power Malaysia 2010
using "superbatteries" made
with batteries and supercapacitors connected in parallel (KERS).[144][145] About
20% tank-to-wheel efficiency could be reached using the KERS system.

The Toyota TS030 Hybrid LMP1 car, a racing car developed under Le Mans Prototype rules, uses a hybrid drivetrain with
supercapacitors.[146][147] In the 2012 24 Hours of Le Mans race a TS030 qualified with a fastest lap only 1.055 seconds slower
(3:24.842 versus 3:23.787)[148] than the fastest car, an Audi R18 e-tron quattro with flywheel energy storage. The supercapacitor
and flywheel components, whose rapid charge-discharge capabilities help in both braking and acceleration, made the Audi and
Toyota hybrids the fastest cars in the race. In the 2012 Le Mans race the two competing TS030s, one of which was in the lead for
part of the race, both retired for reasons unrelated to the supercapacitors. The TS030 won three of the 8 races in the 2012 FIA
World Endurance Championship season. In 2014 the Toyota TS040 Hybrid used a supercapacitor to add 480 horsepower from
two electric motors.[133]

Hybrid electric vehicles

Supercapacitor/battery combinations in electric vehicles (EV) and hybrid electric
vehicles (HEV) are well investigated.[90][149][150] A 20 to 60% fuel reduction
has been claimed by recovering brake energy in EVs or HEVs. The ability of
supercapacitors to charge much faster than batteries, their stable electrical
properties, broader temperature range and longer lifetime are suitable, but
weight, volume and especially cost mitigate those advantages.
RAV4 HEV
Supercapacitors' lower specific energy makes them unsuitable for use as a stand-
alone energy source for long distance driving.[151] The fuel economy
improvement between a capacitor and a battery solution is about 20% and is available only for shorter trips. For long distance
driving the advantage decreases to 6%. Vehicles combining capacitors and batteries run only in experimental vehicles.[152]

As of 2013 all automotive manufacturers of EV or HEVs have developed prototypes that uses supercapacitors instead of batteries
to store braking energy in order to improve driveline efficiency. The Mazda 6 is the only production car that uses supercapacitors
to recover braking energy. Branded as i-eloop, the regenerative braking is claimed to reduce fuel consumption by about 10%.[153]

Russian Yo-cars Ё-mobile series was a concept and crossover hybrid vehicle working with a gasoline driven rotary vane type and
an electric generator for driving the traction motors. A supercapacitor with relatively low capacitance recovers brake energy to
power the electric motor when accelerating from a stop.[154]

Toyota's Yaris Hybrid-R concept car uses a supercapacitor to provide quick bursts of power.[133]
PSA Peugeot Citroën fit supercapacitors to some of its cars as part of its stop-start fuel-saving system, as this permits faster start-
ups when the traffic lights turn green.[133]

Gondolas
In Zell am See, Austria, an aerial lift connects the city with Schmittenhöhe
mountain. The gondolas sometimes run 24 hours per day, using electricity for
lights, door opening and communication. The only available time for recharging
batteries at the stations is during the brief intervals of guest loading and
unloading, which is too short to recharge batteries. Supercapacitors offer a fast
charge, higher number of cycles and longer life time than batteries.

Emirates Air Line (cable car), also known as the Thames cable car, is a 1-
kilometre (0.62 mi) gondola line that crosses the Thames from the Greenwich
Peninsula to the Royal Docks. The cabins are equipped with a modern
infotainment system, which is powered by supercapacitors.[155][156]

Developments
As of 2013 commercially available lithium-ion supercapacitors offered the
highest gravimetric specific energy to date, reaching 15 Wh/kg (54 kJ/kg).
Research focuses on improving specific energy, reducing internal resistance,
Aerial lift in Zell am See, Austria
expanding temperature range, increasing lifetimes and reducing costs.[20]
Projects include tailored-pore-size electrodes, pseudocapacitive coating or
doping materials and improved electrolytes.
 Announcements
Specific Specific
Development Date Cycles Capacitance Notes
energy[A] power

Subnanometer
Graphene sheets scale electrolyte
compressed by capillary integration
compression of a volatile 2013 60 Wh/L created a
continuous ion
liquid[157]
transport
network.

Vertically aligned carbon 2007 First

2009 13.50 Wh/kg 37.12 W/g 300,000
nanotubes electrodes[9][54] realization[158]
2013
Single-layers of
curved graphene
sheets that do
not restack face-
to-face, forming
Curved graphene mesopores that
2010 85.6 Wh/kg 550 F/g are accessible to
sheets[49][50]
and wettable by
environmentally
friendly ionic
electrolytes at a
voltage up to
4 V.
Potassium
hydroxide
KOH restructured graphite restructured the
2011 85 Wh/kg >10,000 200 F/g carbon to make
oxide[159][160]
a three
dimensional
porous network
Three-
dimensional
pore structures
in graphene-
Activated graphene-based derived carbons
carbons as supercapacitor in which
2013 74 Wh/kg
electrodes with macro- and mesopores are
mesopores[161] integrated into
macroporous
scaffolds with a
surface area of
3290 m2/g
Aza-fused π-
Conjugated microporous conjugated
2011 53 Wh/kg 10,000
polymer[162][163] microporous
framework
A tailored meso-
macro pore
SWNT composite structure held
2011 990 W/kg
electrode[164] more electrolyte,
ensuring facile
ion transport
Nickel hydroxide nanoflake 2012 50.6 Wh/kg 3300 F/g Asymmetric
on CNT composite supercapacitor
electrode[165] using the
Ni(OH)2/CNT/NF
 electrode as the
anode
assembled with
an activated
carbon (AC)
cathode
achieving a cell
voltage of 1.8 V

Li4Ti5O12 (LTO)
deposited on
Battery-electrode carbon
2012 40 Wh/l 7.5 W/l 10,000 nanofibres
nanohybrid[74]
(CNF) anode
and an activated
carbon cathode
Nickel cobaltite,
a low cost and
Nickel cobaltite deposited on an
mesoporous carbon 2012 53 Wh/kg 2.25 W/kg 1700 F/g environmentally
aerogel[166] friendly
supercapacitive
material
Wet
electrochemical
process
intercalated
Na(+) ions into
Manganese dioxide MnO2
2013 110 Wh/kg 1000 F/g interlayers. The
intercalated nanoflakes[167]
nanoflake
electrodes
exhibit faster
ionic diffusion
with enhanced
redox peaks.
Wrinkled single
layer graphene
3D porous graphene sheets a few
2013 98 Wh/kg 231 F/g nanometers in
electrode[168]
size, with at
least some
covalent bonds.
Graphene-based planar
On chip line
micro-supercapacitors for on- 2013 2.42 Wh/l
filtering
chip energy storage[169]
Electrodes:
Ru0.95O20.2–
Dielectric:
Ca2Nb3O10–.
Nanosheet 27.5 μF cm Room-
2014 temperature
capacitors[170][171] −2
solution-based
manufacturing
processes. Total
thickness less
than 30 nm.

LSG/manganese dioxide[172] 2015 42 Wh/l 10 kW/l 10,000 Three-

dimensional
laser-scribed
graphene (LSG)
 structure for
conductivity,
porosity and
surface area.
Electrodes are
around 15
microns thick.
Laser-induced
0.02 Survives
graphene/solid-state 2015 9 mF/cm2
mA/cm2 repeated flexing.
electrolyte[173][174]
Tungsten trioxide (WO3)
nano-wires and two-
2D shells
dimensional enveloped by
2016 ~100 Wh/l 1 kW/l 30,000 surrounding
shells of a transition-metal
nanowires
dichalcogenide, tungsten
disulfide (WS2)[175][176]

A Research into electrode materials requires measurement of individual components, such as an electrode or half-cell.[177] By
using a counterelectrode that does not affect the measurements, the characteristics of only the electrode of interest can be
revealed. Specific energy and power for real supercapacitors only have more or less roughly 1/3 of the electrode density.

Market
As of 2016 worldwide sales of supercapacitors is about US$400 million.[178]

The market for batteries (estimated by Frost & Sullivan) grew from US$47.5 billion, (76.4% or US$36.3 billion of which was
rechargeable batteries) to US$95 billion.[179] The market for supercapacitors is still a small niche market that is not keeping pace
with its larger rival.

In 2016, IDTechEx forecast sales to grow from $240 million to $2 billion by 2026, an annual increase of about 24%.[180]

Supercapacitor costs in 2006 were US$0.01 per farad or US$2.85 per kilojoule, moving in 2008 below US$0.01 per farad, and
were expected to drop further in the medium term.[181]

Trade or series names

Exceptional for electronic components like capacitors are the manifold different trade or series names used for supercapacitors
like: APowerCap, BestCap, BoostCap, CAP-XX, C-SECH, DLCAP, EneCapTen, EVerCAP, DynaCap, Faradcap, GreenCap,
Goldcap, HY-CAP, Kapton capacitor, Super capacitor, SuperCap, PAS Capacitor, PowerStor, PseudoCap, Ultracapacitor making
it difficult for users to classify these capacitors. (Compare with #Comparison of technical parameters)

See also
Capa vehicle, including capabus
Capacitor types
Conjugated microporous polymer
Electric vehicle battery
Flywheel energy storage
Human power, also known as self-powered equipment
List of emerging technologies
Lithium-ion capacitor
Mechanically powered flashlight
Nanoflower – A compound that results in formations which in microscopic view resemble flowers
Literature
Abruña, H. D.; Kiya, Y.; Henderson, J. C. (2008). "Batteries and Electrochemical Capacitors" (http://ecee.colorad
o.edu/~ecen4555/SourceMaterial/ElectricalEnerStor1208.pdf) (PDF). Phys. Today. 61 (12): 43–47.
doi:10.1063/1.3047681 (https://doi.org/10.1063%2F1.3047681).
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External links
ELECTRIC DOUBLE LAYER AND CAPACITANCE RESPONSE, The Bockris, Devanathan and Muller model (htt
p://uqu.edu.sa/files2/tiny_mce/plugins/filemanager/files/27/08_Appendix.pdf)
MIT OPEN COURSEWARE, Lecture 37 and others (https://web.archive.org/web/20111009120757/http://ocw.mit.
edu/courses/chemical-engineering/10-626-electrochemical-energy-systems-spring-2011/lecture-notes/)
Namisnyk, A SURVEY OF ELECTROCHEMICAL SUPERCAPACITOR TECHNOLOGY (https://web.archive.org/
web/20141222044332/http://services.eng.uts.edu.au/cempe/subjects_JGZ/eet/Capstone%20thesis_AN.pdf)
Supercapacitors: A Brief Overview (https://web.archive.org/web/20141222044332/http://services.eng.uts.edu.au/
cempe/subjects_JGZ/eet/Capstone%20thesis_AN.pdf)
Simple Capacitors to Supercapacitors - An Overview (http://www.electrochemsci.org/papers/vol3/3111196.pdf)
Technologies and applications of Supercapacitors, University of Mondragon (https://web.archive.org/web/201401
30133838/http://www.mondragon.edu/en/phs/research/research-lines/electrical-energy/news-folder/workshop/Mo
ndragon%202012_06_22_Gallay.pdf)
Properties and applications of supercapacitors from the state-of-the-art to future trends (http://www.garmanage.c
om/atelier/root/public/Contacting/biblio.cache/PCIM2000.pdf)
Perspectives on supercapacitors, pseudocapacitors and batteries (http://www.icevirtuallibrary.com/docserver/fullt
ext/nme3-0136.html?expires=1367075349&id=id&accname=guest&checksum=4EA72F169F4580857DB1E60C2
 4B4A1B7)
Standardization challenges for electricity storage devices (https://web.archive.org/web/20190518200244/http://w
ww.cars21.com/files/news/EVS-24-10439%2520Bossche.pdf)
False capacitance of supercapacitors (https://arxiv.org/abs/1604.08154)
[4] (https://dspace.library.colostate.edu/handle/10976/166930)
Article on ultracapacitors at electronicdesign.com (https://web.archive.org/web/20080521205053/http://electronic
design.com/Articles/Index.cfm?AD=1&AD=1&ArticleID=17465)
A new version of an old idea is threatening the battery industry (http://www.economist.com/science/displaystory.cf
m?story_id=10601407) (The Economist).
An Encyclopedia Article (https://web.archive.org/web/20120813020215/http://electrochem.cwru.edu/encycl/art-c0
3-elchem-cap.htm) From the Yeager center at CWRU.
Ultracapacitors & Supercapacitors Forum (http://www.ultracapacitors.org/)
Special Issue of Interface magazine on electrochemical capacitors (http://www.electrochem.org/dl/interface/spr/s
pr08/if_spr08.htm)
Nanoflowers Improve Ultracapacitors: A novel design could boost energy storage (http://www.technologyreview.c
om/Energy/21375/?a=f) (Technology Review) and Can nanoscopic meadows drive electric cars forward? (https://
www.newscientist.com/article/dn14753) (New Scientist)
If the cap fits... How supercapacitors can help to solve power problems in portable products (https://web.archive.
org/web/20150113004449/http://fplreflib.findlay.co.uk/articles/6610/if-the-cap-fits.pdf).
A web that describes the development of solid-state and hybrid supercapacitors from CNR-ITAE (Messina) Italy
(https://web.archive.org/web/20110725001344/http://www.nanocapacitors.altervista.org/)

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