Supercapacitor

A supercapacitor (SC), also called an

ultracapacitor, is a high-capacity capacitor with a
capacitance value much higher than other
capacitors, but with lower voltage limits, that
bridges the gap between electrolytic capacitors and
rechargeable batteries. It typically stores 10 to 100
times more energy per unit volume or mass than
electrolytic capacitors, can accept and deliver
charge much faster than batteries, and tolerates
many more charge and discharge cycles than
rechargeable batteries.

Supercapacitors are used in applications requiring

many rapid charge/discharge cycles, rather than Schematic illustration of a supercapacitor[1]
long term compact energy storage — in
automobiles, buses, trains, cranes and elevators,
where they are used for regenerative braking, short-
term energy storage, or burst-mode power
delivery.[2] Smaller units are used as power 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,[3] 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 A diagram that shows a hierarchical classification of
double-layer capacitance than supercapacitors and capacitors of related types.
electrochemical pseudocapacitance,
achieving separation of charge in a
Helmholtz double layer at the interface between the surface of a conductive electrode and an
electrolyte. The separation of charge is of the order of a few 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
Electrostatic double-layer capacitance
Potential distribution
Comparison with other storage technologies
Styles
Construction details
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".[4][5][6]
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.[7][8] 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.[9]

Early electrochemical capacitors used two aluminum foils covered with activated carbon — the
electrodes — that 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 1978, to provide backup power for computer memory.[8]

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" behaviour in electrochemical energy storage. In 1999 he defined the term "supercapacitor" to
make reference to the increase in observed capacitance by surface redox reactions with faradaic charge
transfer between electrodes and ions.[10][11] 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.[12] This
product became a successful energy source for memory backup applications.[8] 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. Typical construction of a
supercapacitor: (1) power source,
Additionally, depending on electrode material and surface shape, (2) collector, (3) polarized electrode,
some ions may permeate the double layer becoming specifically (4) Helmholtz double layer,
(5) electrolyte having positive and
adsorbed ions and contribute with pseudocapacitance to the total
negative ions, (6) separator.
capacitance of the supercapacitor.

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] Simplified view of a double-layer of
negative ions in the electrode and
Applying a voltage to an electrochemical capacitor causes both solvated positive ions in the liquid
electrodes in the capacitor to generate electrical double-layers. These electrolyte, separated by a layer of
double-layers consist of two layers of charges: one electronic layer is polarized solvent molecules.
in the surface lattice structure of the electrode, and the other, with
opposite polarity, emerges from dissolved and 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
Structure and function of an ideal
areas A and small distance between plates d. As a result, double-
double-layer capacitor. Applying a
layer capacitors have much higher capacitance values than
voltage to the capacitor at both
conventional capacitors, arising from the extremely large surface electrodes a Helmholtz double-layer
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 the electrolyte in a mirror charge
of the 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.[10][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 originates by a very fast sequence of reversible redox, intercalation or
electrosorption processes. The adsorbed ion has no chemical reaction with the atoms of the electrode (no
chemical bonds arise[29]) since only a charge-transfer take place.

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 surface. As
such, the storage capacity of
faradaic pseudocapacitance
is limited by the finite
quantity of reagent in the
available surface.
A cyclic voltammogram shows the
fundamental differences between A faradaic
static capacitance (rectangular) and
pseudocapacitance only Simplified view of a double-layer with
pseudocapacitance (curved)
occurs together with a static specifically adsorbed ions which
double-layer capacitance, have submitted their charge to the
and its magnitude may electrode to explain the faradaic
exceed the value of double-layer capacitance for the same surface charge-transfer of the
area by factor 100, depending on the nature and the structure of the pseudocapacitance.
electrode, because all the pseudocapacitance reactions take place
only with de-solvated ions, which are much smaller than solvated
ion with their solvating shell.[10][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 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 The voltage behavior of
decreases over the anode's thin oxide layer. The somewhat resistive supercapacitors and batteries during
liquid electrolyte (cathode) accounts for a small decrease of potential charging/discharging differs clearly
for "wet" electrolytic capacitors, while electrolytic capacitors with
solid conductive polymer electrolyte this voltage drop is 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 Double-layer Lithium-ion
Parameter electrolytic capacitors Supercapacitors Pseudocapacitors batteries
capacitors (memory & hybrid (Li-Ion)
backup) (high power) (long-term)

Temperature range, −40 ... −20 ...

−40 ... +70 °C −20 ... +70 °C −20 ... +70 °C
Celsius (°C) +125 °C +60 °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,
unlimited 100 k ... 1 000 k 100 k ... 1 000 k 20 k ... 100 k 0.5 k ... 10 k
thousands (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
Watt hours
Wh/kg Wh/kg Wh/kg Wh/kg Wh/kg
per kilogram (Wh/kg)
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 alternating current (AC) reactance 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 Radial style of a
supercapacitor used for supercapacitor for
mobile components PCB mounting used
for industrial
applications

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

Schematic construction Schematic construction of a

of a wound supercapacitor with stacked
supercapacitor electrodes
1. terminals, 2. safety 1. positive electrode, 2. negative
vent, 3. sealing disc, 4. electrode, 3. separator
aluminum 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 area of the material is
about 100,000 times greater 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. The cell is then 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.
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
Family tree of supercapacitor types. Double-layer
Pseudocapacitors — with transition metal
capacitors and pseudocapacitors as well as hybrid
oxide or conducting polymer electrodes with a
capacitors are defined over their electrode designs.
high electrochemical pseudocapacitance
Hybrid capacitors — with asymmetric
electrodes, one of which exhibits mostly
electrostatic and the other mostly electrochemical capacitance, such as lithium-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.[10][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[10][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 A micrograph of activated carbon
unit voltage in a supercapacitor is predominantly a function of the under bright field illumination on a
electrode surface area. Therefore, supercapacitor electrodes are light microscope. Notice the fractal-
typically made of porous, spongy material with an extraordinarily like shape of the particles hinting at
high specific surface area, such as activated carbon. Additionally, the their enormous surface area. Each
ability of the electrode material to perform faradaic charge transfers particle in this image, despite being
enhances the total capacitance. only around 0.1 mm across, has a
surface area of several square
Generally the smaller the electrode's pores, the greater the meters.
capacitance and specific energy. However, smaller pores increase
equivalent series resistance (ESR) and decrease specific power.
Applications with high peak currents require 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 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 A block of silica aerogel in hand
also provide mechanical and vibration stability for supercapacitors
used in high-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.
As of 2015, a CDC supercapacitor offered a specific energy of
10.1 Wh/kg, 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 Pore size distributions for different

surface area of 2630 m 2/g which can carbide precursors.
Graphene is an atomic-
scale honeycomb lattice theoretically lead to a capacitance of
made of carbon atoms. 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 at voltages up to 4 V. A
specific energy of 85.6 Wh/kg (308 kJ/kg) is obtained at room temperature equaling that of a conventional
nickel metal hydride battery, but with 100-1000 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.
A scanning tunneling microscopy
They have a hollow structure
image of single-walled carbon
with walls formed by one-
nanotube
atom-thick sheets of
graphite. These sheets are
rolled at specific and discrete
("chiral") angles, and the combination of chiral angle and radius
SEM image of carbon nanotube
controls properties such as electrical conductivity, electrolyte
bundles with a surface of about
wettability and ion access. Nanotubes are categorized as single-
1500 m2/g
walled nanotubes (SWNTs) or multi-walled nanotubes (MWNTs).
The latter have one or more outer tubes successively 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.

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][56][57]

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.[58]

MnO2 + H+(C+) +e− ⇌ MnOOH(C)[59]

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).[60]

Metal oxides

Brian Evans Conway's research[10][11] 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.[61] 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).[62]

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.[63] 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.[64][65]

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.[66]

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. However, polyacene
electrodes provide up to 10,000 cycles, much better than batteries.[67]

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.

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.[68]

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.[69] 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.[8] and reach a specific energy up to 14 Wh/kg (50.4 kJ/kg).[70]

Battery-type electrodes

Rechargeable battery electrodes influenced the development of electrodes for new hybrid-type
supercapacitor electrodes as for lithium-ion capacitors.[71] 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)).

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.[72]

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[73]) 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.

Ionic

Ionic electrolytes consists of liquid salts that can be stable in a wider electrochemical window, enabling
capacitor voltages above 3.5 V. Ionic electrolytes typically have an ionic conductivity of a few mS/cm,
lower than aqueous or organic electrolytes.[74]

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.[75][76]

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.

This value is also called the "DC capacitance".

Schematic illustration of the
Measurement capacitance behavior resulting out of
the porous structure of the electrodes
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 capacitance measurement offers fast
results, important for industrial production 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, Equivalent circuit with cascaded RC
the measured capacitance value drops from 100 to 20 percent of the elements
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
constants. These result in delayed current flow, reducing the total
electrode 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 Frequency depending of the
times. capacitance value of a 50 F
supercapacitor
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 and t2,
Illustration of the measurement
for the voltage to drop from 80% (V1) to 40% (V2) of the rated conditions for measuring the
voltage is 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.[77][78]

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
oxygen. The solvent molecules then cannot separate the electrical
A 5.5 volt supercapacitor is
charges from each other. Higher voltages than rated voltage cause
constructed out of two single cells,
hydrogen gas formation or a short circuit.
each rated to at least 2.75 volts, in
series connection
Standard supercapacitors with aqueous electrolyte normally are
specified with a rated voltage of 2.1 to 2.3 V and capacitors with
organic solvents with 2.5 to 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.
Supercapacitors with ionic electrolytes can exceed an operating voltage of 3.5 V.[74]

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.[78]

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
The internal DC resistance can be
dependent and increases during charge/discharge. In applications
calculated out of the voltage drop
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
auxiliary line extended from the
the time of discharge, starting with a constant discharge current straight part and the time base at the
Idischarge. It is obtained from the intersection of the auxiliary line time of discharge start
extended from the straight part and the time 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 due to 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.[78]

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.[79] 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). [80] Measured device capacitance across
an EDLC's operating voltage
Device properties can also be seen to be dependent on device
temperature. As 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.[80]

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 applications, because the voltage drop and the
time constant over the internal resistance mean that some of the
stored charge is inaccessible. The effective realized amount of
energy Weff is reduced by the used voltage difference between Vmax
and Vmin and can be represented as:

Ragone chart showing specific power

vs. specific energy of various
capacitors and batteries
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:

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.[81][82]

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 is generally a function of temperature, 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 The lifetime of supercapacitors
capacitor body. The higher the core temperature the faster the depends mainly on the capacitor
evaporation and the shorter the lifetime. temperature and the voltage applied

Evaporation generally results in decreasing capacitance and

increasing internal 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.[83] 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.[84]

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. [80]

Polarity A graph plotting voltage over time,

after the application of a charge
Since the positive and negative electrodes (or simply positrode and
negatrode, respectively) of symmetric supercapacitors consist of the
same material, 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 A negative bar on the insulating
cathode to describe the electrodes in supercapacitors (and also sleeve indicates the cathode terminal
rechargeable batteries including lithium ion batteries) can lead to of the capacitor
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 two electrodes. Therefore, the concepts of cathode and anode do not apply.

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.[85]

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.[86]

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,

Classification of supercapacitors into classes regarding
according to discharge current levels:
to IEC 62391-1, IEC 62567and BS EN 61881-3
standards
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
detail specification - Electric double-layer capacitors for power application
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 alternating current (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,[87] 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.[88]

Some portable speakers are powered by supercapacitors.[89]

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.[90]

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.[91][92] These current differences create
unwanted voltage fluctuations and therefore power oscillations on the grid.[91] 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.[93][94]

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)[95] 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.[96][97][98]

Supercapacitors provide backup power for actuators in wind turbine pitch systems, so that blade pitch can be
adjusted even if the main supply fails.[99]
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 lines to
consume reactive power and improve the AC power factor in a
lagging power flow circuit. This would allow for a better used real
power to produced power and make the grid overall more Rotor with wind turbine pitch system
efficient.[100][101][102][103]

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.[104] 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.[105][106]

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.[107]

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.[108]

Medical

Supercapacitors are used in defibrillators where they can deliver 500 joules to shock
the heart back into sinus rhythm.[109]

Transport

Street light
combining a solar Aviation
cell power source
with LED lamps In 2005, aerospace systems and controls company Diehl Luftfahrt Elektronik GmbH
and chose supercapacitors to power emergency actuators for doors and evacuation slides
supercapacitors used in airliners, including the Airbus 380.[99]
for energy storage

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.[110] 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.[111][112]

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.[113] 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.[113]

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
Green Cargo operates TRAXX
supercapacitors.[114]
locomotives from Bombardier
Transportation
Cranes, forklifts and tractors

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.[115]

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
Container yard with rubber tyre
compared with Diesel or fuel-cell systems.[116]
gantry crane
Supercapacitor-powered terminal tractors transport containers to
warehouses. They provide an economical, quiet and pollution-free
alternative to Diesel terminal tractors.[117]

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 light rail city lines
to replace overhead wires that are too expensive to fully route.

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.[118][119] 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 drive the vehicle
up to 1 km without overhead line supply, thus better integrating the Light rail vehicle in Mannheim
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%.[120]

In 2009 supercapacitors enabled LRVs 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.[121]

In 2009, Alstom and RATP equipped a Citadis tram with an

experimental energy recovery system called "STEEM".[122] The
system is fitted with 48 roof-mounted supercapacitors to store
braking energy, which provides tramways with a high level of
energy autonomy by enabling them to run without overhead power
lines on parts of its route, recharging while traveling on powered
Supercapacitors are used to power
the Paris T3 tram line on sections
stop-over stations. During the tests, which took place between the
without overhead wires and to
Porte d’Italie and Porte de Choisy stops on line T3 of the tramway
recover energy during braking. network in Paris, the tramset used an average of approximately 16%
less energy.[123]

In 2012 tram operator Geneva Public Transport began tests of an

LRV equipped with a prototype roof-mounted supercapacitor unit to
recover braking energy.[124]

Siemens is delivering supercapacitor-enhanced light-rail transport

systems that include mobile storage.[125]

Hong Kong's South Island metro line is to be equipped with two

2 MW energy storage units that are expected to reduce energy
A supercapacitor-equipped tram on
consumption by 10%.[126]
the Rio de Janeiro Light Rail
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.[127] 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 to 4 kilometres
(2.5 mi).[128] As of 2017, Zhuzhou's supercapacitor vehicles are also used on the new Nanjing streetcar
system, and are undergoing trials in Wuhan.[129]

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.[130]

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.[131] 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.[132]

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.

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
MAN Ultracapbus in Nuremberg,
Germany
vibration and reduced maintenance costs.[133][134]

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.[135]

In early 2005 Shanghai tested a new form of electric bus called

capabus that runs without powerlines (catenary free operation) using
Electric bus at EXPO 2010 in
large onboard supercapacitors that partially recharge whenever the
Shanghai (Capabus) recharging at
bus is at a stop (under so-called electric umbrellas), and fully charge the bus stop
in the terminus. In 2006, two commercial bus routes began to use the
capabuses; one of them is route 11 in Shanghai. It was estimated 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.[136]

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.[137][138]

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
Toyota TS030 Hybrid at 2012 24
issued that includes a hybrid World champion Sebastian Vettel in
Hours of Le Mans motor race
drive of up to 200 kW input Malaysia 2010
and output power using
"superbatteries" made with
batteries and supercapacitors connected in parallel (KERS).[139][140] 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.[141][142] 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)[143] 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.[128]

Hybrid electric vehicles

Supercapacitor/battery combinations in electric vehicles (EV) and

hybrid electric vehicles (HEV) are well investigated.[86][144][145] 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.[146] 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.[147]

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%.[148]

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.[149]

Toyota's Yaris Hybrid-R concept car uses a supercapacitor to provide quick bursts of power.[128]

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.[128]

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.[150][151]

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, expanding temperature range, increasing lifetimes and reducing costs.[20] Projects
include tailored-pore-size electrodes, pseudocapacitive coating or
doping materials and improved electrolytes.

Aerial lift in Zell am See, Austria

 Announcements
Specific Specific
Development Date Cycles Capacitance Notes
energy[A] power

Subnanometer
scale electrolyte
Graphene sheets compressed by integration
capillary compression of a 2013 60 Wh/L created a
volatile liquid[152] continuous ion
transport
network.

Vertically aligned carbon 2007 First

2009 13.50 Wh/kg 37.12 W/g 300,000
nanotubes electrodes[8][54] realization[153]
2013
Single-layers of
curved graphene
sheets that do
not restack face-
to-face, forming
mesopores that
Curved graphene sheets[49][50] 2010 85.6 Wh/kg 550 F/g are accessible to
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[154][155]
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[156] 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[157][158] microporous
framework
A tailored meso-
macro pore
structure held
SWNT composite electrode[159] 2011 990 W/kg
more electrolyte,
ensuring facile
ion transport
Nickel hydroxide nanoflake on 2012 50.6 Wh/kg 3300 F/g Asymmetric
CNT composite electrode[160] supercapacitor
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
carbon
Battery-electrode nanohybrid[71] 2012 40 Wh/l 7.5 W/l 10,000 nanofibres
(CNF) anode
and an activated
carbon cathode
Nickel cobaltite,
a low cost and
Nickel cobaltite deposited on an
2012 53 Wh/kg 2.25 W/kg 1700 F/g environmentally
mesoporous carbon aerogel[161]
friendly
supercapacitive
material
Wet
electrochemical
process
intercalated
Na(+) ions into
Manganese dioxide intercalated MnO2
2013 110 Wh/kg 1000 F/g interlayers. The
nanoflakes[162]
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[163]
size, with at
least some
covalent bonds.
Graphene-based planar micro-
On chip line
supercapacitors for on-chip 2013 2.42 Wh/l
filtering
energy storage[164]
Electrodes:
Ru0.95O20.2–
Dielectric:
Ca2Nb3O10–.
27.5 μF cm Room-
Nanosheet capacitors[165][166] 2014 −2 temperature
solution-based
manufacturing
processes. Total
thickness less
than 30 nm.

LSG/manganese dioxide[167] 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 graphene/solid- 0.02 Survives
2015 9 mF/cm2
state electrolyte[168][169] mA/cm2 repeated flexing.

Tungsten trioxide (WO3) nano-

wires and two-dimensional 2D shells
enveloped by shells of a 2016 ~100 Wh/l 1 kW/l 30,000 surrounding
transition-metal dichalcogenide, nanowires
tungsten disulfide (WS2)[170][171]

A Research into electrode materials requires measurement of individual components, such as an electrode or
half-cell.[172] 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.[173]

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.[174] 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%.[175]

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.[176]

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,[12] 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

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External links
ELECTRIC DOUBLE LAYER AND CAPACITANCE RESPONSE, The Bockris, Devanathan
and Muller model (http://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/20111009120
757/http://ocw.mit.edu/courses/chemical-engineering/10-626-electrochemical-energy-systems-
spring-2011/lecture-notes/)
Namisnyk, A SURVEY OF ELECTROCHEMICAL SUPERCAPACITOR TECHNOLOGY (http
s://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://servic
es.eng.uts.edu.au/cempe/subjects_JGZ/eet/Capstone%20thesis_AN.pdf)
Simple Capacitors to Supercapacitors - An Overview (http://www.electrochemsci.org/papers/vo
l3/3111196.pdf)
Technologies and applications of Supercapacitors, University of Mondragon (https://web.archiv
e.org/web/20140130133838/http://www.mondragon.edu/en/phs/research/research-lines/electri
cal-energy/news-folder/workshop/Mondragon%202012_06_22_Gallay.pdf)
 Properties and applications of supercapacitors from the state-of-the-art to future trends (http://
www.garmanage.com/atelier/root/public/Contacting/biblio.cache/PCIM2000.pdf)
Perspectives on supercapacitors, pseudocapacitors and batteries (http://www.icevirtuallibrary.c
om/docserver/fulltext/nme3-0136.html?expires=1367075349&id=id&accname=guest&checksu
m=4EA72F169F4580857DB1E60C24B4A1B7)
Standardization challenges for electricity storage devices (https://web.archive.org/web/201905
18200244/http://www.cars21.com/files/news/EVS-24-10439%2520Bossche.pdf)
False capacitance of supercapacitors (https://arxiv.org/abs/1604.08154)
ENHANCED PHYSICS-BASED REDUCED-ORDER MODEL OF NON-FARADAIC
ELECTRICAL DOUBLE-LAYER CAPACITOR DYNAMICS (https://dspace.library.colostate.edu/
handle/10976/166930)
Article on ultracapacitors at electronicdesign.com (https://web.archive.org/web/200805212050
53/http://electronicdesign.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/scie
nce/displaystory.cfm?story_id=10601407) (The Economist).
An Encyclopedia Article (https://web.archive.org/web/20120813020215/http://electrochem.cwr
u.edu/encycl/art-c03-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.or
g/dl/interface/spr/spr08/if_spr08.htm)
Nanoflowers Improve Ultracapacitors: A novel design could boost energy storage (http://www.t
echnologyreview.com/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 (ht
tps://web.archive.org/web/20150113004449/http://fplreflib.findlay.co.uk/articles/6610/if-the-cap-
fits.pdf).
A Web site that describes the development of solid-state and hybrid supercapacitors from
CNR-ITAE (Messina) Italy (https://web.archive.org/web/20110725001344/http://www.nanocapa
citors.altervista.org/)

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