# User:Elcap/Supercapacitor

Supercapacitor (SC),[1] former called electric double-layer capacitor (EDLC), is the generic term for a family of electrochemical capacitors. Supercapacitors, sometimes also called ultracapacitors, don't have a conventional solid dielectric. The capacitance value of an electrochemical capacitor is determined by two storage principles, which both contribute indivisible to the total capacitance:[2][3][4]

The ratio of the two storage principles 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 an order of magnitude over that of the double-layer by itself.[1]

Supercapacitors are divided into three families, based on the design of the electrodes:

• Double-layer capacitors – with carbon electrodes or derivates with much higher static double-layer capacitance than the faradaic pseudocapacitance
• Pseudocapacitors – with electrodes out of metal oxides or conducting polymers with a high amount of faradaic pseudocapacitance
• Hybrid capacitors – capacitors with special electrodes that exhibit both significant double-layer capacitance and pseudocapacitance, such as lithium-ion capacitors
Hierarchical classification of supercapacitors and related types

Supercapacitors bridge the gap between capacitors and rechargeable batteries. They have the highest available capacitance values per unit volume and the greatest energy densityof all capacitors. They support up to 10.000 F/1.2 V,[6] with capacitance values up to 10.000 times that of electrolytic capacitors.[1] While existing supercapacitors have energy densities that are approximately 10% of a conventional battery, their power density is generally 10 to 100 times as great. Power density combines energy density with the speed at which the energy can be delivered to the load. This makes charge and discharge cycles of supercapacitors much faster than batteries. Additionally, they will tolerate much more charge and discharge cycles than batteries.

In these electrochemical capacitors, the electrolyte is the conductive connection between the two electrodes. This distinguishes them from electrolytic capacitors, in which the electrolyte is the cathode and thus forms the second electrode.

Supercapacitors are polarized and must operate with the correct polarity. Polarity is controlled by design with asymmetric electrodes, or, for symmetric electrodes, by a potential applied during manufacture.

Supercapacitors support a broad spectrum of applications for power and energy requirements, including:

• Long duration low current for memory backup in (SRAMs)
• Power electronics that require very short, high current, as in the KERSsystem in Formula 1 cars
• Recovery of braking energy in vehicles

Exceptional are the manifold different trade or series names used for supercapacitors like: APowerCap, BestCap, BoostCap, CAP-XX, DLCAP, EneCapTen, EVerCAP, DynaCap, Faradcap, GreenCap, Goldcap, HY-CAP, Kapton capacitor, Super capacitor, SuperCap, PAS Capacitor, PowerStor, PseudoCap, Ultracapacitor.

## History

### Development of electrochemical capacitors

In the early 1950s General Electric engineers experimenting with devices using porous carbon electrodes for fuel cells and novel rechargeable batteries. Activated charcoal on the one hand is an electrical conductor and on the other hand it is an extremely porous, "spongy" form of carbon with an extraordinarily high specific surface area so this material seems to be ideal for electrodes. Out of these reasons H. Becker of General Electric developed in 1957 a "Low voltage electrolytic capacitor with porous carbon electrodes".[7][8][9] He believed that the energy was stored as charge in the carbon pores like in the pores in the etched foils of electrolytic capacitors. Because the double layer mechanism in capacitors was unknown at that 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 follow up on this work. In 1966 researchers at Standard Oil of Ohio (SOHIO) developed another version of the devices as “Electrical energy storage apparatus”, while working on experimental fuel cell designs.[10][11]

The true nature of electrochemical energy storage has not been named in this patent. Even in 1970, the electrochemical capacitor patented by Donald L. Boos was registered as an electrolytic capacitor with activated carbon electrodes.[12]

Principle construction of a supercapacitor; 1. power source, 2. collector, 3.polarized electrode, 4. Helmholtz double layer, 5. electrolyte having positive and negative ions, 6. Separator. By applying a voltage to the capacitor at both electrodes a respective Helmholtz double layer is formed, which has a positive or negative layer of ions from the electrolyte deposited in a mirror image on the respective opposite electrode.

All this first electrochemical capacitors used a cell design of two aluminum foils covered with activated carbon coins the electrodes which are soaked with an electrolyte and separated by a thin porous insulator implemented in a common housing. This design gave a capacitor with a capacitance value in the "farad" area, which was significantly higher than for electrolytic capacitors in the same dimensions. This basic mechanical design remains the basis of most electrochemical capacitors up to now.

Due to poor sales figures SOHIO did not commercialize their invention, licensing the technology to NEC, who finally marketed the results as “supercapacitors” in 1971, to provide backup power for maintaining computer memory.[11]

Other manufacturers followed from the end of the 1970s, each with their own brand name because the term Electrical Double Layer Capacitor (EDLC), which was at that time the term used in the publications, was too bulky and the trading name Supercapacitor was already occupied by NEC.

The market expanded slowly for a time. That changes around 1978 as Panasonic marketed and sold on a worldwide basis its "Goldcaps”.[13] This product gives a breakthrough as back-up energy source for replacement of batteries memory backup applications.[11] The competition started some years later. 1987 ELNA with its "Dynacap" ",[14] new EDLC's came on the market. All these first EDLC’s had a relatively high internal resistance, which limited the discharge current, so they only were used for low current applications like battery for SRAM for data backup etc.

At the end of 1980s various developments in electrode materials led to higher capacitance values and in low ohmic electrolytes to lower the ESR in order to increase the charge/discharge currents. This led to rapidly improving performance and an equally rapid reduction in cost.

The first supercapacitor with low internal resistance for power applications was developed in 1982 for military applications through the Pinnacle Research Institute (PRI), and were established in the market under the brand name "PRI Ultracapacitor". In 1992, the Maxwell Laboratories, later Maxwell Technologies took over this development. Maxwell kept the term Ultracapacitor from PRI and called them "Boost Caps" [5] to underline the qualification of this electrochemical capacitors for power applications.

Since the energy content of a capacitor is increasing with the square of the voltage researchers were looking for a way to increase the breakdown voltage of electrochemical capacitors. Using an anode of a 200V high voltage tantalum electrolytic capacitor David A. Evans developed 1994 an "Electrolytic-Hybrid Electrochemical Capacitor". [15][16]

These capacitors take advantage of features of electrolytic and electrochemical capacitors. They combine the high dielectric strength of an anode from an electrolytic capacitor, and the high capacitance with a pseudocapacitive metal oxide (ruthenium (IV) oxide) cathode from an electrochemical capacitor building a hybrid electrochemical capacitor. This hybrid capacitors Evans coined Capattery® series[17], had an energy content by about a factor of 5 higher than a comparable tantalum electrolytic capacitor of the same size.[18] But these capacitors are quite expensive, so they are currently used only in very specific military applications.

The recent developments in terms of time in the field of supercapacitors are the lithium-ion capacitors, which also belong to the hybrid capacitors. They were launched first time on the market by FDK in 2007.[19] These supercapacitors use a combination of an electrostatic double-layer electrode with a doped lithium-ion electrochemical battery electrode to generate a very high pseudocapacitance additional to a high static double-layer capacitance. Hence, thus use of the special properties of two technologies increase the energy density of the capacitor, wherein the ability of rapid charging and discharging with a high number of cycles is maintained.

### Development of the double layer and pseudocapacitance model

When a metal (or an electronic conductor) is brought in contact with a solid or liquid ionic-conductor (electrolyte), a common boundary (interface) among this two different phases originates. Helmholtz [20] was the first realizing, that charged electrodes immersed in electrolytic solutions will repel the coions of the charge while attracting counterions to their surfaces. With the two layers of opposite polarity formed at the interface between electrode and electrolyte he propose 1853 the model of an electrical double layer (DL) which is essentially a moleculear dielectric achieved an electrostatically storage of the charge.[21] Below a decomposition voltage of the electrolyte this arrangement behaves like a capacitor in which the stored charge is linearly dependent on the voltage applied.

This early Helmholtz model predicts a constant differential capacitance independent from the charge density even depending on the dielectric constant of the solvent and the thickness of the double-layer.[22] [5] But this model, while a good foundation for the description of the interface does not take into account several important factors: diffusion/mixing of ions in solution, the possibility of adsorption on to the surface and the interaction between solvent dipole moments and the electrode. This leads to the second model.

Simplyfied illustration of the potential development in the area and in the further course of a Helmholtz double layer.

Louis Georges Gouy in 1910 and David Leonard Chapman in 1913 both observed that capacitance was not a constant and that it depended on the applied potential and the ionic concentration. They made significant improvements by introducing a diffuse model of the electrical DL later on called “Gouy-Chapman model”. In this model the charge distribution of ions as a function of distance away from the metal surface allows the Maxwell–Boltzmann statistics to be applied. That means the electric potential decreases exponentially away from the surface to the fluid bulk. [5][23]

The Gouy-Chapman model fails for highly charged DLs. In order to resolve this problem Otto Stern in 1924 suggested the combination of the Helmholtz and Gouy-Chapman models. Stern combined the two previous models, with some of the ions adhering to the electrode as suggested by Helmholtz and giving an internal Stern layer and some forming a Gouy-Chapman type diffuse layer. [24]

The Stern layer take into account the fact that ions have finite size, and consequently have a closest approach to the electrode of the order of the ionic radius. But the Stern model still has some limitations, such as ions are effectively modeled as point charges, the only significant interactions in the diffuse layer are Coulombic, dielectric permittivity is assumed constant throughout the double layer, and the viscosity of fluid is constant above slipping plane. [25]

Thus, D. C. Grahame modify the Stern model in 1947.[26] He proposed, that some ionic or uncharged species can penetrate into the Stern layer, although the closest approach to the electrode is normally occupied by solvent molecules. This could come about if ions losses their solvation shell when the ion came close to the electrode. Ions in direct contact with the metallic electrode were called “specifically adsorbed ions”. This model characterize the existence of three regions. First, the inner Helmholtz plane (IHP) plane passing through the centres of the specifically adsorbed ions, second, the outer Helmholtz plane (OHP) passes through the centres of solvated ions at their distance of closest approach to the electrode and, third, the diffuse layer i.e. the region that lies beyond the OHP.

Schematic representation of a double layer on an electrode (BMD) model. 1. Inner Helmholtz plane, (IHP), 2. Outer Helmholtz plane (OHP), 3. Diffuse layer, 4. Solvated ions (cations) 5. Specifically adsorbed ions (redox ion, which contributes to the pseudocapacitance), 6. Molecules of the electrolyte solvent

In 1963 J. O'M. Bockris, M. A. V Devanthan, and K. Alex Müller[27] proposed a model (BDM model) of the double-layer that included the action of the solvent in the interface. They suggested that the attached molecules of the solvent, such as water, would have a fixed alignment to the surface of the electrode. This first layer of solvent molecules has a strong orientation to the electric field depending on the charge, and this orientation has great influence to the permittivity of the solvent which vary with the field strength. Through the centers of these molecules the inner Helmholtz plane (IHP) is passing. In this layer could also be some specifically adsorbed ions which are only partially solvated. The solvated ions of the electrolyte are located outside the IHP layer. Through the centers of these ions pass a second plane, the outer Helmholtz plane (OHP). The region that lies beyond the OHP is called the diffuse layer. The BDM model now is most commonly used.

Further research with double layers on ruthenium dioxide films in 1971 by Sergio Trasatti and Giovanni Buzzanca has been demonstrated, that the electrochemical behavior of these electrodes at low voltages with specific adsorbed ions was like that of capacitors. The specific adsorption of the ions in this region of potential could also involve a partial charge transfer between the ion and the electrode. It was the first step towards pseudo-capacitors.[22]

Ph.D., Brian Evans Conway within the John Bockris Group At Imperical College, London 1947

Between 1975 and 1980, by Brian Evans Conway extensive fundamental and development work was carried out on the ruthenium oxide type of electrochemical capacitor. 1991 he described the transition from ‘Supercapacitor’ to ‘Battery’ behavior in electrochemical energy storage and 1999 he coined the term supercapacitor as explanation for increased capacitance by surface redox reactions with faradaic charge transfer between electrodes and ions.[28][1][29]

The type of capacitor he coined supercapacitor store the electrical charge partially in the Helmholtz double-layer and partially it was the result of faradaic reactions with charge transfer of electron and protons between electrode and electrolyte coined “pseudocapacitance”. The working mechanisms of pseudocapacitors are electrosorption, redox reactions and intercalation.

## Storage principles

### Differences between "electrostatic" and "electrochemical" energy storage

Charge storage principles of different capacitor types and their inherent voltage progression
The voltage behavior of supercapacitors and batteries during charging/discharging differs clearly

In conventional capacitors such as ceramic capacitors, and film capacitors the electric energy is stored in a static electric field permeate the dielectric between two metallic conducting plates, the electrodes. The electric field originates by the removal of charge carriers, typically electrons, from one metal electrode and depositing them on the opposite electrode. This charge separation creates a potential between the two electrodes, which can be harnessed in an external circuit. The total energy stored in this fashion increases with both the amount of charge stored and the potential between the plates. The amount of charge stored per unit voltage is essentially a function of the size, the reciprocal value of the distance, and the material properties of the material in between the plates (the dielectric), while the potential between the plates is limited by the breakdown field strength of the dielectric. The dielectric controls the capacitor's voltage.

Therefore, they are called static capacitors. The potential of a charged capacitor decreases linearly between the electrodes. This static storage also applies for electrolytic capacitors in which most of the potential decreases over the thin oxide layer of the anode. The electrolyte as cathode may be a little bit resistive so that for “wet” electrolytic capacitors a small amount of the potential decreases over the electrolyte. For electrolytic capacitors with high conductive solid polymer electrolyte this voltage drop is negligible.

Electrochemical capacitors now are a new type of capacitors. They do not have a conventional solid dielectric which separates the electrical charge. The capacitance value of an electrochemical capacitor is determined by two high-capacity different storage principles:

• 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. This capacitance is called double-layer capacitance and is static in origin.
• Electrochemical storage of the electrical energy, achieved by redox reactions with specifically adsorbed ions from the electrolyte, intercalation of atoms in the layer lattice or electrosorption, underpotential deposition of hydrogen or metal adatoms in surface lattice sites that results in a reversible faradaic charge-transfer on the electrode. This capacitance is called pseudocapacitance and is faradaic in origin.[5]

Double-layer capacitance and pseudocapacitance both contribute to the total capacitance value of a supercapacitor.[3][2]

Because each supercapacitor have two electrodes the potential of the capacitor decreases symmetrically over both Helmholtz layers whereby a little voltage drop across the ESR of the electrolyte achieved.

Both the electrostatic storage of energy in the Helmholtz double layer and the storage of electrochemical energy with the faradaic charge transfer and redox reactions are linear to the total charge stored in the capacitor. Due to this linear behavior of storage of electric energy in a supercapacitor the voltage across the capacitor corresponds linear to the amount of stored energy. This linear voltage gradient related to the stored energy in electrochemical capacitors differs from the behavior of electrochemical batteries, in which the voltage across the terminals remains independent of the charged energy nearly on a constant voltage level.

### Electrostatic double-layer capacitance

Simplified view of a double-layer of negative ions in the electrode and solvated positive ions in the liquid electrolyte, detached from each other through a layer of polarized molecules of the solvent.

Through the description of the electrical phenomena at an interface between a metallic conductive electrode and a liquid electrolyte, Helmholtz laid the theoretical foundations of an electrical double layer.[21] A double-layer will be generated by applying a voltage to an arrangement of an electrode and an electrolyte. According to the polarity of the voltage the dissolved and solvated ions in the electrolyte will move in direction to the electrodes. Than two layers of ions will be generated, one in the surface of the electrode and the other with opposite polarity with the dissolved ions in the adjacent liquid electrolyte. These layers of opposite ions are disconnected by a monolayer of isolating molecules of the solvent, f. e. water. These layer of isolating molecule, the inner Helmholtz plane, adhere firmly by physical adsorption on the surface of the electrode and disconnect the opposite ions from each other, building a molecular dielectric. The amount of charge in the electrode will be compensated by the same magnitude of counter-charges in the outer Helmholtz plane. These phenomena can be used to store electrical charges like in a capacitor. The stored charge in the inner Helmholtz plane forms an electric field which corresponds to the strength of the applied voltage. It is only effective in the molecular layer of the solvent molecules, and is static in origin.

The "thickness" of a charged layer in the metallic electrode, that is the average extension perpendicular to the surface, is about 0.1 nm. It mainly depends on the electron density because the atoms in solid electrodes are not movable. In the electrolyte, the thickness depends on the size of the molecules of the solvent and of the movement and concentration of ions in the solvent. It is about 0.1 to 10 nm, and is described by the Debye length. Both thicknesses results in the total thickness of a double layer.

Due to very low thickness of the molecular layer of solvent molecules in the inner Helmholtz plane a very strong electric field E will be formed. At a potential difference of, for example, U = 2V and a molecular thickness of d = 0.4nm, the electric field strength will be

${\displaystyle E={\frac {U}{d}}={\frac {2\ {\text{V}}}{0{,}4\ {\text{nm}}}}=5000\ {\text{kV/mm}}}$

To compare this figure with values from other capacitor types an estimation for electrolytic capacitors should be done. The voltage proof of aluminum oxide, the dielectric layer of aluminum electrolytic capacitors is approximately 1.4 nm/V. For a 6.3 V capacitor therefore the layer is 8.8 nm. The field strength in the dielectric of an electrolytic capacitor than could be calculated with 6.3 V/8.8 nm = 716 kV/mm.

The extremely high field strength of about 5000 kV/mm in a double-layer is unrealizable in conventional capacitors with a conventional dielectrics. No dielectric material would have prevented a breakthrough of the charge carriers. In a double-layer capacitor the chemical stability of the molecular bonds of the solvent molecules prevents a breakthrough.[30]

The forces that cause the adhesion are not chemical bonds but physical forces. Chemical bonds persist within of the adsorbed molecules, but they are polarized. The magnitude of the electrical charge that can accumulate in the layers correspond to the concentration of the adsorbed ions. Up to a decomposition voltage of the electrolyte this arrangement behaves like a capacitor in which the stored electrical charge is linearly dependent on the voltage applied.

Structure and function of an ideal double-layer capacitor. Applying a voltage to the capacitor at both electrodes a Helmholtz double-layer will be formed separating the adhered ions in the electrolyte in a mirror charge distribution of opposite polarity

The Helmholtz double-layer is effective like a dielectric layer in a conventional capacitor but with the thickness of a single molecule. Using the early Helmholtz model to calculate the capacitance this model predicts a constant differential capacitance Cd independent from the charge density even depending on the dielectric constant ε and the charge layer separation δ.

${\displaystyle \ C_{d}={\frac {\epsilon }{4\pi \delta }}}$

If the solvent of the electrolyte is water than with the influence of the high field strength the permittivity ε is 6 (instead of 80 at normal conditions) and the layer separation δ ca. 3 Angstrom (0.3 nm) the value of differential capacitance predicted by the Helmholtz model can be expected to be about 18 F/cm2.[22] This value can be used for the calculation of the capacitance of a double-layer capacitor using the standard formula for conventional plate capacitors if only the surface of the electrodes is known. This capacitance can be calculated with:

${\displaystyle C={\frac {\varepsilon A}{d}}}$.

The capacitance C is therefore greatest in devices made from materials with a high permittivity ε, large electrode plate surfaces A, and small distance d between plates. Hence, the extremely thin Helmholtz double-layer being of the order of a few Angstroms (0.3-0.8 nm) and the activated carbon material used for electrodes, which have an extremely large surface in the range of 10 to 40 µF/cm2 it is understandable why supercapacitors have the highest capacitance values among the capacitors.[2][5]

Because an electrochemical capacitor is composed out of two electrodes the charge distribution in the Helmholtz layer at one electrode can also be found in mirror image of opposite polarity in the second Helmholtz layer at the second electrode. Therefore, the total capacitance value of a double-layer capacitor is the result of two capacitors connected in series. Because both capacitances of the electrodes have approximately the same value, the total capacitance is roughly half the capacitance of one electrode.

### Electrochemical pseudocapacitance

Simplified view of a double-layer with specifically adsorbed ions which have submitted their charge to the electrode to explain the faradaic charge-transfer of the pseudocapacitance.

In a Helmholtz double-layer not only a static double-layer capacitance originates. Specifically adsorbed ions with redox reactions, electrosorption and intercalation results in faradaic charge-transfer between electrolyte and surface of an electrode called pseudocapacitance. Double-layer capacitance and pseudocapacitance both contribute to the total capacitance value of an electrochemical capacitor.[2][3] The distribution of the amounts of both capacitances depends on the surface area, material and structure the of the electrodes.

Redox reactions in batteries with faradaic charge-transfer between an electrolyte and a surface of an electrode are well known since decades. But these chemical processes are associated with chemical reactions of the electrode materials usually with attendant phase changes. Although these chemical processes are relatively reversible, the charge and discharge of batteries often results in irreversibility reaction products of the chemical electrode-reagents. Accordingly, the cycle-life of rechargeable batteries is usually limited, and varies with the battery type. Additional the chemical processes are relatively slow extending the charge and discharge time of the batteries.

An essential fundamental difference from redox reactions in batteries arises in supercapacitors, were a fast sequence of reversible redox processes with a linear function of degree of faradaic charge transfers take place. This behavior is the basic function of a new class of capacitance, the pseudocapacitance. Pseudocapacitance comprise fast and reversible faradaic processes with charge transfer between electrolyte and the electrode and is accomplished through reduction-oxidation reactions (redox reactions), electrosorption and intercalation processes in combination with the nonfaradaic formation of an electric double-layer. Capacitors with a high amount of pseudocapacitance are called pseudocapacitors.

Applying a voltage at the capacitor terminals the polarized ions or charged atoms in the electrolyte are moving to the opposite polarized electrode forms a double-layer. Depending on the structure or the surface material of the electrode a pseudocapacitance can originate when specifically adsorbed cations pervades the double-layer proceeding in several one-electron stages an excess of electrons. The electrons involved in the faradaic processes are transferred to or from valence-electron states (orbitals) of the redox electrode reagent. The electrons enter the negative electrode and flow through the external circuit to the positive electrode were a second double-layer with an equal number of anions has formed. But these anions will not take the electrons back. They are present on the surface of the electrode in the charged state, and the electrons remain in the quite strongly ionized and "electron hungry" transition-metal ions of the electrode. This kind of pseudocapacitance has a linear function within narrow limits and is determined by the potential-dependent degree of coverage of surface with the adsorbed anions from the electrolyte. The storage capacity of the pseudocapacitance with an electrochemical charge transfer takes place to an extent limited by a finite quantity of reagent or of available surface.

Discharging the pseudocapacitance the reaction of charge transfer is reversed and the ions or atoms leave the double-layer and move into the electrolyte distributing randomly in the space between both electrodes.

Unlike in batteries in pseudocapacitors the redox reactions or intercalation processes with faradaic charge-transfer do not result in slow chemical processes with chemical reactions or phase changes of the electrode materials between charge and discharge. The atoms or ions contribute to the pseudocapacitance simply cling[31] to the atomic structure of the electrode and charges are distributed on surfaces by physical adsorption processes that do not involve the making or breaking of chemical bonds. These faradaic charge transfer processes for charge storing or discharging employed in pseudocapacitors are very fast, much faster than the chemical processes in batteries.

Confinement of solvated ions in pores, such as those present in carbide-derived carbon (CDC). As the pore size approaches the size of the solvation shell, the solvent molecules are removed, resulting in larger ionic packing density and increased charge storage capability.

The ability of electrodes, to accomplish pseudocapacitance effects like redox reactions of electroactive species, electrosorption of H or metal ad-atoms or intercalation, which leads to a pseudocapacitance, strongly depend on the chemical affinity of electrode materials to the ions sorbed 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 inserted by doping in the conductive electrode material like active carbon as well as conducting polymers such as polyaniline or derivatives of polythiophene covering the surface of conductive electrode material.

Pseudocapacitance may also originates by the structure and especially by the pore size of the electrodes. The use of carbide-derived carbons (CDCs) or carbon nanotubes /CNTs) for electrodes provides a network of very small pores formed by nanotube entanglement. These carbon nanoporous with diameters in the range of <2 nm can be referred to as intercalated pores. Solvated ions in the electrolyte can’t enter these small pores but de-solvated ions which have reduced their ion dimensions are able to enter resulting in larger ionic packing density and increase charge storage capability. The tailored sizes of pores in nano-structured carbon electrodes can maximize ion confinement, increasing specific capacitance by faradaic H2 adsorption treatment. Occupation of these pores by de-solvated ions from the electrolyte solution occurs according to the intercalation mechanism, which is Faradaic in nature facilitate electro-adsorption.[32][33])[34]

Several examples of pseudocapacitance can arise.[35][36] Three types of electrochemical processes giving rise to pseudocapacitance have been utilized in supercapacitors. These are

• redox reactions involving ions from the electrolyte
• intercalation of atoms out of the electrolyte in the layer lattice, and the
• electrosorption, underpotential deposition of hydrogen or metal adatoms in surface lattice sites

Best researched and understood is the pseudocapacitance at ruthenium oxide (RuO2).[1] Here the pseudocapacitance originates out of a coupled reversible redox reactions with several oxidation steps with overlapping potential. The electrons mostly come from the valence orbitals of the electrode material. The electron transfer reaction is very fast, and can be accompanied with high currents.

The electron transfer reaction take place according to the following equation:

${\displaystyle \mathrm {RuO_{2}+xH^{+}+xe^{-}\leftrightarrow RuO_{2}-_{x}(OH)_{x}} }$[37]

During charging and discharging in this charge-transfer transition H+ protons are incorporated into the crystal lattice of ruthenium or removed from it. This originates an electrochemical faradaic storage of electrical energy without any chemical transformation of the electrode material. The OH groups are deposited as a molecular layer on the electrode surface and remain in the region of the Helmholtz layer. Since the measurable voltage from the redox reaction is proportional to the charged state, the behavior of the reaction is like a capacitor and not like a battery, wherein the voltage is largely independent of the state of charge.

A cyclic voltammogram shows the fundamental difference of the current curves between static capacitors and pseudocapacitors

The properties of pseudocapacitance can be expressed in a cyclic voltammogram. For an ideal double-layer capacitor the sign of the current changes immediately after reversing the potential and the shape of the voltammetry is rectangular. For this electrostatic type of energy storage the current is independent on potential of the electrode. For double-layer capacitors with resistive losses the shape changes into a parallelogram. For electrodes with faradaic pseudocapacitance the electrical charge stored in the capacitor is strongly dependent on the potential of the electrode. Therefore, the voltammetry characteristic deviate from the parallelogram form caused by a delay of potential during reversing the potential coming from kinetically processes during charging the pseudocapacitance.[3][38]

In real existing pseudocapacitors the pseudocapacitance and double-layer capacitance both contribute to the total capacitance value of this capacitor. However, if the electrode materials consist of transition metal oxides, then the pseudocapacitance enhance the value of specific capacitance ca. 10 -100 times depending on the nature of oxides at the same electrode area.[3]

The electrochemical redox reactions or intercalation processes are very fast and most of them don’t result in chemical reactions with chemical bonds. So the electrochemical capacitors, which have a high pseudocapacitance, do have two major advantages over batteries: Charge-storing and discharging of electrochemical capacitors is much faster than in batteries, almost so fast as in conventional capacitors, and the cycle lifetime is much higher than for batteries. But batteries can generally store significantly more energy per unit mass than electrochemical capacitors. So although electrochemical capacitors have lower energy densities than batteries, they have higher power densities and much longer cycle lifetime.

## Types of supercapacitors

Flow chart of the types of supercapacitors. Double-layer capacitors and pseudocapacitors as well as hybrid capacitors are defined over their electrode designs.

Supercapacitor is the generic term for the family of electrochemical capacitors. They store, as described above, its electric energy with the two different storage principles, the

• static double-layer capacitance and
• electrochemical pseudocapacitance.

In an ideal double-layer capacitor with ideal double-layer electrodes does not occur any electrochemical pseudocapacitance. The same applies for pseudocapacitors. In an ideal pseudocapacitor no double-layer capacitance exist. In reality, however, double-layer capacitance and pseudocapacitance both contribute to the total capacitance value of an electrochemical capacitor.[2][3] But the distribution of the amounts of both capacitances depends on the material and structure the of the electrodes with a widely range of varying the amounts of the both storage principles. Out of this knowledge supercapacitors are divided into three different types, based on the design of the electrodes, which determined the amount of double-layer and pseudocapacitance on the total capacitance value of the supercapacitor.[2]

• Double-layer capacitors– Electrochemical capacitors with electrodes out of activated carbon or derivates in which the static double-layer capacitance is higher than the faradaic pseudocapacitance
• Pseudocapacitors – Electrochemical capacitors with special designed electrodes in which the faradaic pseudocapacitance is higher than the static double-layer capacitance
• Hybrid capacitors – Capacitors with special electrodes that exhibit significant double-layer capacitance and pseudocapacitance. These capacitors include some new developments with special electrodes (e.g. lithium-ion capacitors).

Double-layer capacitors with static charge storage in Helmholtz double-layers have electrodes made of activated carbon, of carbon aerogels, of carbid-derived carbons or of carbon nanotubes.

Pseudocapacitors with faradaic electrochemical redox reactions, electrosorption or intercalation use electrodes out of conductive polymer or of transition metal oxides.

The combination of one double-layer electrode coupled with a second electrode out of a composite electrode with a high amount of pseudocapacitance provides a hybrid capacitor. Hybrid capacitors have both a high amount of double-layer capacitance as well as pseudocapacitance.

## Construction

A supercapacitor cell basically consists out of two electrodes, a separator, and an electrolyte connecting electrically the both electrodes. The both electrodes each are electrically connected to the terminals of the outside world with a metallic current collector foil.

The both electrodes mostly consist out of activated carbon. This material is electrically conductive and thus can be used as an electrode, and have a very large surface to increase the capacitance. The both electrodes are separated by an ion permeable membrane (separator) used as insulator to protect the electrodes against direct contact and short circuits. This composite is subsequently rolled or folded into a cylindrical or rectangular shape and can be stacked in an aluminum can or an adaptable rectangular housing. Then the cell is impregnated with an electrolyte. The electrolyte mostly consists out of a liquid or viscous electrolyte, organic or aqueous type, or may be of solid state. Beneath the electrode structure the electrolyte defines the characteristics of the capacitor, the power or peak current capability, the operating voltage range, and the allowable temperature range. Last but not least the housing will be hermetically closed to ensure stable behavior over the specified life time.

## Materials

The properties of supercapacitors come from the interaction of all parts of the capacitors. Especially the combination of electrode material and kind of electrolyte determine the functionality and the electrical characteristics of the capacitors.

### Electrodes for EDLCs

A micrograph of activated charcoal under bright field illumination on a light microscope. Notice the fractal-like shape of the particles hinting at their enormous surface area. Each particle in this image, despite being only around 0.1 mm across, has a surface area of several square metres.

The electrodes in supercapacitors typically are made out of an extremely porous, "spongy" material like activated carbon with an extraordinarily high specific surface area. These electrodes are in general thin coatings applied to a metallic conducting current collector. The requirements for electrodes are manifold. They have to have a good conductivity and high temperature stability, as well as to be chemically inert and resistant against corrosion. Moreover, they should be environmentally friendly and have to be produced at the lowest cost. Like for conventional capacitors also for supercapacitors the amount of charge stored per unit voltage is a function of the electrode surface (size), and the reciprocal value of the double-layer thickness. Additional the ability of the electrode material to perform faradaic charge transfers give rise to the total capacitance value of an electrochemical capacitor.

Because the extremely thin Helmholtz double-layer being of the order of a few Angstroms (0.3-0.8 nm) the distance of the disconnected charge carrier is the smallest and gives rise for a high capacitance value. Generally, however, to realize the extreme high capacitance values of supercapacitors the electrodes have to have the largest possible surface at the smallest volume, because the double-layer capacitance as well as the pseudocapacitance is surface area dependent.

Generally it could be said, that the smaller the pores in the electrode, the greater the capacitance and the energy density of the capacitor. But smaller pores increase the internal resistance (ESR) and decreases charging and the discharging of the capacitor. The power density decreases. That means, for applications with high peak currents an electrode material with large pores and low internal losses is required, while for applications with high energy density an electrode material with small pores is required.

Additional the ability of the electrode material to perform faradaic charge transfers give rise to the total capacitance value of an electrochemical capacitor. Structurally, pore sizes in carbons ranges from micropores (referred to pores less than 2 nm) to mesopores (referred to pores between 2 and 50 nm) but below macropores (referred to pores greater than 50 nm).[39] Pores gives rise to pseudocapacitance are related to the pore diameter in the range of nanometer (<2 nm), which are accessible only for de-solvated ions and is connected with faradaic reactions between ion and carbon electrode surface. Hence, at the carbon surface apart from the electrostatic double-layer capacitance, a significant pseudocapacitance is often manifested.[3]

Basic illustration of the functionality of a supercapacitor, the voltage distribution inside of the capacitor and its simplified equivalent DC circuit

In all supercapacitors the two electrodes of the capacitor forms a series circuit of two individual capacitors C1 and C2. The total capacitance Ctotal therefore is given by the formula

${\displaystyle C_{\text{total}}={\frac {C_{1}\cdot C_{2}}{C_{1}+C_{2}}}}$

Supercapacitors can be constructed in two versions, with symmetric or asymmetric electrodes. In symmetric supercapacitors both electrodes have the same capacitance value.

That means, if C1 = C2 than Ctotal = 0.5 • C1

For symmetric constructed supercapacitors the total capacitance value of a supercapacitor is equal to half the value of a single electrode.

For asymmetric constructed capacitors, normally hybrid capacitors are asymmetric, one of the electrodes can have a much higher capacitance value.

If C1 >> C2 than Ctotal ≈ C2

For asymmetric supercapacitors the total capacitance value can increase up to the value of a single electrode. Compared with a symmetric capacitor the total capacitance value can be doubled.

The most commonly used electrode material for supercapacitors is carbon in its various manifestations like activated carbon (AC), carbon fibre-cloth (AFC), carbide-derived carbon (CDC), carbon aerogel, graphite (graphene), and carbon nanotubes (CNTs). [3][32][40]

#### Activated carbon

File:Activated-carbon.jpg
Nanopores in activated carbon, as viewed by an electron microscope.

The first used material for electric double-layer capacitor electrodes was (AC). Activated carbon has an electrical conductivity of 1.25 to 3×105 S/m, this conductivity is 100 to 10.000 times lower than metallic conductivity but it is quite good enough for use as electrodes in supercapacitors.[2][5]

Otherwise activated carbon like activated charcoal is an extremely porous, "spongy" form of carbon with an extraordinarily high specific surface area — a common approximation is that 1 gram (a pencil-eraser-sized amount) has a surface area of roughly 1000 to 3000 m2[32] — about the size of 4 to 12 tennis courts. It is typically a powder made up of extremely fine but very "rough" particles, which, in bulk, form a low-density heap with many holes. The distribution of the extremely fine but very "rough" particles, all electrical connected to each other, gives an electrode body with an extremely high surface per volume leading to a very high double-layer capacitance.

Solid activated carbon, also termed consolidated amorphous carbon (CAC) is the most used electrode material for supercapacitors. Activated carbon may have a surface area exceeding 2800 m2/g and may be cheaper to produce than other classifications.[41] It will be produced out of activated carbon powder pressed into the desired design forming a block of high porosity with a wide distribution of pore sizes. An electrode made of activated carbon with a surface area of about 1000 m2/g results in a typical double-layer capacitance of about 10 μF/cm2 respectively a specific capacitance of 100 F/g.

As of 2010 virtually all commercial supercapacitors use powdered activated carbon made environmentally friendly from coconut shells. [42] Coconut shell activated carbon has a larger quantity of micropores compared with wood activated carbon (charcoal) increasing the surface area. [39] Disadvantage of an activated carbon electrode is, that In contrast of f. e. nanotube electrodes only less than 1/3 of the surface area is available for the formation of an ionic double layer.[43] Higher performance devices are available, at a significant cost increase, based on synthetic carbon precursors that are activated with potassium hydroxide (KOH).

Activated carbon electrodes exhibits predominant static double-layer capacitance but contribute also to a pseudocapacitance of supercapacitors. That is related to the pore diameter in the range of nanometer (<2 nm), which are accessible only for de-solvated ions and is connected with faradaic reactions between ion and carbon electrode surface. Hence, at the carbon surface apart from the electrostatic double-layer capacitance, a significant pseudocapacitance is often manifested.[3].

#### Activated carbon fibres

SEM image of carbon nanotube bundles with a surface of about 1500 m2/g

Activated carbon fibres are produced out of activated carbon with a typical diameter of 10 µm. This activated carbon fibres (ACF) have microporos with diameters of <2 nm with a very narrow pore-size distribution which can be more readily controlled. The surface area of AFC woven into a textile form is about 2500 m2/g. Advantages of AFC electrodes are the low electrical resistance along their fibre axis and the good contact to the metal collector. [32]

AFC electrodes also exhibits predominant static double-layer capacitance but contribute with a small amount of pseudocapacitance on the total capacitance value of supercapacitors with such electrodes.

#### Carbon aerogel

A block of aerogel in hand

Carbon aerogel is a synthetic very high porous and ultralight material derived from an organic gel, in which the liquid component of the gel has been replaced by pyrolysis with a gas. It is also called “frozen smoke”.

The electrodes of aerogel supercapacitors are made via pyrolysis of resorcinol formaldehyde aerogels.[44] Carbon aerogel are more electrical conductive than most activated carbons. They enables thin and mechanical stable electrodes with a thickness in the range of several hundred microns and with uniform pore size. Its mechanically stability can be used as electrode material for supercapacitors with high vibration stability.

Standard aerogel capacitors exhibits a predominant static double-layer capacitance and are offered as relatively small backup capacitors in microelectronics.

Researchers have achieved carbon aerogel electrodes provides a high surface area with gravimetric densities of about 400–1200 m2/g, a very high specific capacitance as 104 F/cm3 which results in a high energy density of 325 J/g (90 W•h/kg) and high power density of 20 W/g.[45][46]

New developments of aerogel electrodes for supercapacitors working with composite material providing a high amount of pseudocapacitance. [47]

As of 2013, a graphene aerogel with a density of 0.16 mg/cm3 was synthetized, becoming the new lightest material.[48]

#### Carbid-derived carbon

Pore size distributions for different carbide precursors.

Carbide-derived carbons (CDCs), also known as tunable nanoporous carbons, are a large family of carbon materials derived from carbide precursors, such as binary SiC, TiC,[49] (e.g. SiC, TiC) that are transformed into pure carbon via physical (e.g., thermal decomposition) or chemical (e.g., halogenation) processes. [50][51]

Carbide-derived carbons have been synthesized to exhibit high surface areas and tunable pore diameters to maximize ion confinement, increasing specific capacitance by faradaic H
2
adsorption treatment (pseudocapacitance). Structurally, CDC pore sizes ranges from micropores to mesopores but below macropores.[39] The capacitance of CDC electrodes may be increased by tailored design of the pores in the range of micropores. Pores smaller than 1 nm greatly contribute on the capacitance even if the size of the solvated ions is larger. This capacitance increase for smaller pores was explained by the distortion of the ion solvating shell. As the pore size approaches the size of the solvation shell of the ions, the solvent molecules are removed, and the de-solvated ions enter into fitting pore sizes resulting in larger ionic packing density and increase charge storage capability by intercalation with charge transfer from de-solvated ions. Hence, tailored sizes of pores in nano-structured carbon electrodes can maximize ion confinement, increasing pseudocapacitance by faradaic H2 adsorption treatment additional to the double- layer capacitance. This CDC electrodes with tailored pore design increase the energy density by as much as 75 % over conventional activated carbons.

As of 2013 this material was used in a supercapacitor with an energy density of 8.3 Wh/kg having 4,000 F capacitance and can withstand 1.000.000 charge/discharge cycles.[52]

#### Graphene

Graphene is an atomic-scale honeycomb lattice made of carbon atoms.

Graphene is a one-atom thick sheet of pure carbon, with atoms arranged in a regular hexagonal pattern similar to graphite, which has more than one layer. It can be produced in ultrathin planar layers[53] as sheets like paper called „nanocomposite paper”.[54]

The practical realized conductivity of graphene is with >1700 S/m very high compared to activated carbon (10 – 100 S/g). [55] Additionally graphene has a very high surface area of 2630 m2/g which can lead theoretically to a capacitor of 550 F/g.

Graphene can now be produced in various labs, but is not available in production quantities. Graphene supercapacitors perhaps paving the way for the massive installation of renewable energies such as wind and solar power as well as recuperation of brake energy in cars because they have the promise of significantly improving the amount of electrical power in fast capacitors.[56]

One of the new developed graphene-based supercapacitor use curved graphene sheets that will not restack face-to-face forming mesopores which are accessible to and wettable by environmentally friendly ionic electrolyte working at a voltage up to 4 V. They have a specific energy density of 85.6 W•h/kg at room temperature and can store as much charge as a conventional nickel metal hydride battery, but can be charged and discharged a hundred to a thousand times faster than these batteries. [57][58]

Another development use graphene sheets directly as supercapacitor electrodes without the need for binders or current collectors as is the case for conventional supercapacitors. Additionally the liquid electrolyte is replaced with a polymer gelled electrolyte that also acts as a separator reducing the device thickness and weight and simplifying the fabrication process.[55]

The two-dimensional structure of the graphene sheet also improves the charging and discharging of graphene supercapacitors. The charge carriers in vertically oriented graphene nanosheets can quickly migrate into or out of the deeper structures of the electrode and thus speeding up the charge and discharging time. Such capacitors may be suitable even for 100/120 Hz filter applications which are unable to reach for supercapacitors with active carbons as electrode material.

#### Carbon nanotubes

A scanning tunneling microscopy image of single-walled carbon nanotube

Carbon nanotubes (CNTs), also called buckytubes, are carbon molecules with a cylindrical nanostructure. They have a hollow structure with the walls formed by one-atom-thick sheets of graphene. These sheets are rolled at specific and discrete ("chiral") angles, and the combination of the rolling angle and radius decides the nanotube properties as electrical conductivity, wettability with electrolyte as well as the accessibility of ions. Nanotubes are categorized as single-walled nanotubes (SWNTs) and multi-walled nanotubes (MWNTs), that are additional graphene tubes around the core of a SWNTmuch like the Russian matroyska dolls. Single-walled carbon nanotubes have cylindrical walls with diameters ranging between 1 and 3 nm. Multiwalled carbon nanotubes have thicker and nested coaxial walls, consisting of several coaxial graphene cylinders separated by spacing (0.34 nm) that is close to the interlayer distance in graphene. The nanotubes can grow directly onto the collector substrate, f. e. on a silicon wafer. A typical height of a nanotube electrode is appr. 20 to 100 µm.[59]

Carbon nanotubes in supercapacitors can greatly improve and enhance the performance of these electro-chemical capacitors. Due to the high wettable surface area and high conductivity of single-wall carbon or multiwall nanotubes, the addition of these nanotubes allows optimization for these capacitors.[60][61]

CNTs as electrode material for supercapacitors can store about the same charge as activated carbon per unit surface area but the surface of the nanotubes arranged in a more regular pattern or highly aligned provide a greater wettable surface area. Comparing with the possible surface area of about 3000 m2/g of activated carbons, CNTs possess a moderate specific surface area. SWNTs have a high theoretical specific surface area of 1315 m2/g, while that of MWNTs would be lower and is determined by the diameter of the tubes and the number of the graphene walls. Nevertheless, CNTs do have higher capacitance than activated carbon electrodes, e.g., 102 F/g for MWNTs and 180 F/g for SWNTs.[62]

Multi-walled carbon nanotubes have a presence of 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 in order to occupy the carbon pores, resulting in larger ionic packing density and increased charge storage capability with faradaic pseudocapacitance. However, the considerable volume change during the repeated intercalation and depletion of ions in the charge and discharge process has largely decreased their mechanical stability in the use. To this end, research on carbon nanotubes which show a high surface area and high mechanical strength, electrical conductivity and chemical stability is going on to improve the performance of supercapacitors.[36][63][60]

Supercapacitors with carbon nanotubes as well as graphene are considered as the potentially revolutionary energy storage materials due to their excellent properties. The referred review gives an overview regarding the basic mechanism, design, fabrication and achievement of latest research progresses.[64]

### Electrodes for pseudocapacitors

In pseudocapacitors double-layer capacitance and pseudocapacitance both contribute to the total capacitance value of a supercapacitor but with a predominate amount of pseudocapacitance. The electrical charges are accumulated mainly as the result of fairly reversible faradaic redox reactions, electrosorbtion or intercalation.

#### Metal Oxides

The electrodes used in pseudocapacitors have to be able to achieve faradaic processes. Many such electron–conducting reactions are known in which oxides of transition metals like oxides of ruthenium (RuO2), iridium (IrO2, iron (Fe3O4), manganese (MnO2) or sulfides like titanium sulfide (TiS2) or their combinations take part. Supercapacitors with electrodes out of ruthenium dioxide provides one of the best examples of electrochemical pseudocapacitance. [1] Very high specific capacitance of up to 720 F/g and an energy density of 26.7 Wh/kg, based on metal oxide electrode material and H2SO4 electrolytefor pseudocapacitors using ruthenium dioxide electrodes could be realized.[65]

The charge or discharge take place with proton insertion or de-insertion and occurs over a window of about 1.2 V. This pseudocapacitance is roughly 100 times higher than for a standard double-layer capacitance using activated carbon electrodes. In addition for these electrodes out of transition metals, its reversibility is excellent, with a cycle life over several hundred-thousand cycles. But ruthenium based aqueous electrochemical capacitors are expensive, and the 1 V voltage window limits their applications are limited to small electronic devices.

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, these electrode materials are far from being commercially used in pseudocapacitors.[37]

#### Conducting polymers

Another type pseudocapacitors with a high amount of pseudocapacitance use electron-conducting polymers as pseudocapacitive material for their electrodes. Conducting polymers have a high conductivity resulting in a low ESR and a relatively high capacitance. Such conducting polymers are polyaniline, polythiophene, polypyrrole, polyacethylene and others. They are cost comparable to carbon electrodes. The use of conducting polymers as electrodes is based on the relatively high reversibility of the redox reaction processes with faradaic charge transfer. Such processes are called electrochemical doping or dedoping the polymers with anions and cations. The greatest capacitance and power density have the n/p-type polymer configuration, with one negatively charged (n-doped) and one positively charged (p-doped) conducting polymer electrode. But conducting polymers suffer from a limited stability during cycling that reduces the initial performance.[4] However, supercapacitor using polyacene electrodes are specified up to 10,000 cycles with stable electrical behavior.[66]

A new conductive polymer material for pseudocapacitor electrodes is a conjugated microporous polymers (CMP) These material is a sub-class of porous materials, related to conductive polymers and allows a large number of faradaic reactions additional to the high double-layer capacitance. With this Aza-Fused π-Conjugated Microporous Framework they developed a supercapacitor with an energy density of about 50 Wh/kg and a charge/discharge cycle stability of 10,000 cycles.[67][68]

### Electrodes for hybrid capacitors

All market successful hybrid supercapacitors are asymmetric. They combine faradaic and non-faradaic processes by coupling a conventional electrostatic double-layer electrode with an electrode with high or very high pseudocapacitance. In such systems the pseudocapacitance electrode provides high energy density while the EDLC electrode out of carbon in one of its various manifestations enables high power capability.

While pseudocapacitance electrodes generally have higher capacitances and lower resistances than EDLC electrodes, they also have lower maximum voltages and especially those with conducting polymers have less cycling stability. Asymmetric hybrid supercapacitors that couple these two electrodes mitigate this disadvantage to achieve higher energy and power densities and better cycling stability.

#### Composite electrodes

Composite electrodes are constructed from carbon-based material with incorporated or deposited pseudocapacitve activ materials like metal oxides and conducting polymers. The carbon-based material provides the capacitor with a certain amount of static double-layer capacitance and the implemented pseudocapacitive material facilitate the mostly higher amount of faradaic pseudocapacitance. As of 2013 most of research and new developed supercapacitors take place with composite electrodes.

Carbon nanotubes (CNTs) is one of the new electrode materials research is focused on. Carbon nanotubes gives a backbone for a homogenous distribution of metal oxide or electrically conducting polymers (ECPs) in a composite electrode, having a high amount of pseudocapacitance and add a relatively high amount of double-layer capacitance. These electrodes are able to achieve higher capacitances than either a pure carbon or pure metal oxide or polymer-based electrode. This is attributed to the accessibility of the entangled mat structure, which allows a uniform coating of pseudocapacitive materials and a three-dimensional distribution of charge.

Pure conducting polymers as the pseudocapacitance material are mechanically weak. A composite CNT electrode coated with ECP deliver the structural integrity and preserves the deposited ECP pseudocapacitive material from mechanical changes to limit the mechanical stress caused by the insertion and removal of ions in the ECP during cycling. These composites have been able to achieve a cycling stability comparable to that of EDLCs. The resultant composites show significantly improved charge storage with capacitance values ranging from 100 to 330 F/g for different asymmetric configurations. [69] Developments of supercapacitors with composite electrodes with cell voltages up to 4 V can reach a superior energy and power densities of 50 Wh/kg and 22 kW/kg respectively.[70]

Another class of composite electrodes includes activated carbon-based materials like carbon nanotubes modified with deposited conducting metal oxides such as RuO2 and IrO2 and manganese dioxide, or nitrides of molybdenum, titanium and iron. These composite electrodes yield capacities of the order of 150–250 μF/cm2, which happen to be several times larger than the standard AC-based electrodes.

Composite electrodes often based on battery developments. One interesting example is shown by a battery-capacitor hybrid devices called “Nanohybrid capacitor”. Composed of deposited Li4Ti5O12 (LTO) on carbon nano fibres (CNF) as anode and an activated carbon as cathode this supercapacitor has demonstrated to deliver 3 to 4.5 times higher energy density of the conventional EDLCs with the same level of power capability cycled up to a few 10,000 cycles.[71]

Another way to enhance the capacitance of CNTs is doping the tubes with a pseudocapacive dopant like lithium atoms used in lithium-ion capacitors. The negative electrode is made of lithium doped carbon, which enables lower negative potential of the electrode while the positive electrode is made of activated carbon. This results in the larger voltage of 3.8 - 4 V that prevents electrolyte oxidation. As of 2007 researcher on the MIT have reached a capacitance of 550 F/g with a lithium doped CNT composite electrode.[11]

But developments not only with CNTs take place. As of 2012 a new composite electrode with carbon aerogel as basis material deposited with nickel cobaltite nanocrystals was developed.[47] The success is made possible by the fuller utilization of NiCo2O4 for pseudocapacitance generation, enabled by using highly conductive mesoporous carbon aerogels as the hosting matrix and an ultrathin NiCo2O4 nanostructure on the backbone of the matrix, as well as an easy transport of charge carriers, ions, and electrons, within the composite electrode. This nickel cobaltite/carbon aerogel composite shows very high specific capacitance values of around 800 to 1700 F/g.

#### Battery-type electrodes

Battery-type electrodes in hybrid supercapacitors couple always a rechargeable battery-type electrode with a carbon electrode in an asymmetric construction. This specialized configuration reflects the demand for higher energy density supercapacitors combining with the higher power density, the longer cycle life, and faster charging and recharging times of supercapacitors.

Research on rechargeable batteries has focused first on using nickel hydroxide, lead dioxide, and Lithium titanate oxide (LTO) (Li4Ti5O12) as one electrode and activated carbon as the other with a lithium salt in an organic solvent electrolyte. In 1985, Akira Yoshino assembled a prototype cell using carbonaceous material into which lithium ions could be inserted as one electrode, and lithium cobalt oxide (LiCoO2), which is stable in air, as the other.[72] By using composite materials without metallic lithium, safety was dramatically improved over batteries which used lithium metal. Meanwhile, the metallic electrode is replaced by activated carbon electrochemical doped with lithium atoms. In this case the relatively small lithium atoms intercalate between the layers of the carbon.[73] This development of battery electrodes have influenced the development of electrodes for lithium-ion capacitors, which As of 2013 reach the highest energy density of commercial offered supercapacitors.

While the development of rechargeable batteries is going on the development of new hybrid supercapacitors with battery-type electrodes is going on, too.[71]

The relation over the common development between battery and capacitor electrodes is clear. But the classification of battery-type electrodes is ambiguous. If not the source defines the classification but the construction, all battery-type electrodes may be classified as composite electrodes.

#### Asymmetric electrodes (Pseudo/EDLC)

Recently some asymmetric hybrid supercapacitors were developed in which the positive electrode were based on a real pseudocapacitive metal oxides electrode, and the negative electrode on an EDLC activated carbon electrode.

An advantage of the hybride-type supercapacitors coupling a real pseudocapacitance electrode with a double-layer electrode compared with a symmetrical EDLC is their higher voltage and correspondingly their higher specific energy (up to 10-20 Wh/kg).[74]

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

### Electrolytes

The electrolyte in supercapacitors is the electrically conductive connection between the two electrodes. It determine the operating voltage with which the capacitor can be used, its temperature range, the internal resistance (ESR) and over its chemical stability the long term behavior of the electrical parameters of the capacitor. An electrolyte always consists out of a solvent with dissolved chemicals, which dissociate into positive cations and negative anions making the electrolyte electrical conductive. The more ions the electrolyte contains, the better is its conductivity. The viscosity of the electrolyte must be low enough to wet the porous, sponge-like structure of the electrodes. It must also be chemically inert and do not attack the materials of the capacitor chemically. An ideal electrolyte does not exist, the properties of an electrolyte are always a compromise between performance and requirements.

The electrolyte also has influence on the capacitance of an electrode. With the same electrode material out of activated carbon, for example, with an aqueous electrolyte a capacitance of 160 F/g is achieved. With an electrolyte based on organic solvents, however, only a capacitance of 100 F/g is obtained.[34]

Water is a relatively good and low cost 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 hexafluorido arsenate (LiAsF6), with water as solvent relatively high conductivity values of about 100 to 1000 mS/cm can be achieved. Aqueous electrolytes have a dissociation voltage of 1.2 V and a relatively low operating temperature range. They are used in supercapacitors with low energy density, but a high power density.

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 tetra ethyl ammonium tetrafluoroborate (N(Et)4BF4,[75]) or triethyl (metyl) tetrafluoroborate (NMe(Et)3BF4) are more expensive than aqueous electrolytes, but they have a higher dissociation voltage of typically 2.5 V to about 4 V, and a higher temperature range. The lower electrical conductivity of organic solvents from about 10 to 60 S/cm leads to a lower power density, but since the energy density increases with the square of the voltage, supercapacitors with organic solvent electrolytes have a higher energy density due to the higher operating voltage of the capacitors.

### Separators

Separators have to separate the two electrodes mechanically from each other, in order to prevent a short circuit between the electrodes. It can be very thin (a few hundredths of a millimeter) and must be very porous to minimize the internal resistance (ESR) of the capacitor. Furthermore, they must be chemically inert in order to minimize the influence on the long term stability and the conductivity of the electrolyte. Inexpensive solutions use open capacitor papers as separators, professional supercapacitors using nonwoven porous polymeric films like polyacrylonitrile or Kapton, woven glass fibers or porous woven ceramic fibres as separators.[76][77]

### Collectors and housing

The current collectors serve the electrical contact of the electrode material, and connect them to the terminals of the capacitor. They must have very good conductivity, after all, they should easily distribute peak currents of up to 100 A to the capacitor cell or remove them from it.

If the housing is made out of a metal, usually aluminum, the collectors should be made out of the same material as the housing, because otherwise two different metals and the presence of an electrolyte forms a galvanic cell which could lead to corrosion. The collectors are either sprayed in a spraying process to the electrodes or consist of a metal foil, on which the electrode is attached.

## Electrical parameters

### Capacitance

Schematic illustration of the capacitance behavior resulting out of the porous structure of the electrodes
Equivalent circuit of a supercapacitor
Frequency depending of the capacitance value of a 50 F supercapacitor

A typical conventional electrolytic capacitor may have capacitance of up to tens of millifarads. The same size supercapacitor might reach several farads, an improvement of two up to three orders of magnitude.

Here an excample out of the practice. A typical conventional electrolytic capacitor of dimensions dxl = 35x50 mm have a capacitance of 82,000 µF at 10 V resp. 680 µF at 420 V.[78] The same sized supercapacitor reach 300 F at 2.5 V[79], a difference of three orders of magnitude.

The maximum capacitance of a commercial deliverable supercapacitor was in 2013 up to 12,000 F with 1.5 V in a round can of 84x32x210 mm.[80] Supercapacitors with several thousands of farads are deliverable from many manufacturers. The capacitance values of supercapacitors are specified as „rated capacitance“. This is a capacitance value within the limits given by the tolerance of the capacitor.

The capacitance value of conventional capacitors normally would be measured with a small AC voltage (f.e. 0.5 V) and a frequency of 100 Hz or 1 kHz depending on the capacitor type. But the capacitance value of a supercapacitor depends strongly on the frequency. These frequency dependence of the capacitance is related to the porous structure of the electrodes and the limited mobility of the ions in the electrolyte. The resulting characteristics can be described quite well with an electric series circuit of cascaded RC elements. To exploit the entire capacity of all pores up to the end of the electrode, all individual pore capacitances have to be achieved via the several serial RC time constants. These result in a delayed current flow, which makes the time or frequency dependent value of the total capacitance. Or, with other words, thus, the total capacitance of supercapacitors only achieved after longer measuring times.

Measuring the capacitance with an AC voltage, even with a very low frequency, only the greatly reduced capacitances at the first pores gives rise to the capacitance value. Even at a frequency of 10 Hz, the measured capacitance value drops to only about 20 % of the DC value. The frequency dependence of the capacitance will affect the operation of the capacitors. If supercapacitors operate with rapid charge and discharge cycles neither the rated capacitance value nor the theoretical maximum energy stored in the capacitor is available. In this case for every application condition the required capacitance value has to be calculated individual.

Illustration of the measurement conditions for measuring the capacitance of supercapacitors

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

The total capacitance value Ctotal results out of the energy W of a loaded capacitor, loaded via a voltage Vloaded.

${\displaystyle W={\frac {1}{2}}\cdot C_{\text{total}}\cdot V_{\text{loaded}}^{2}}$

This capacitance is also called the "DC capacitance".

The measuring method start 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. After a charge for 30 min has finished, the capacitor has to be discharged with a constant discharge current Idischarge. Than the time t1 and t2 , where the voltage between capacitor terminals drop during discharging from of 80 % (V1) to 40 % (V2) of the rated voltage has to be measured. The capacitance value than can be calculated by the following formula:

${\displaystyle C_{\text{total}}=I_{\text{discharge}}\cdot {\frac {t_{2}-t_{1}}{V_{1}-V_{2}}}}$

The value of the discharge current is determined by the application for which the supercapacitors are provided. The IEC standard defines four classes here:

• Class 1, Memory backup, discharge current in mA = 1 • C (F)
• Class 2, Energy storage, discharge current in mA = 0,4 • C (F) • V (V)
• Class 3, Power, discharge current in mA = 4 • C (F) • V (V)
• Class 4, Instantaneous power, discharge current in mA = 40 • C (F) • V (V)

The measurement methods that are specified by the individual manufacturers are mainly comparable to the standardized methods.[81][82].

These standardized measurement method is very time consuming. Industrial manufacturers can not use this method during production for each component individual. The capacitance value therefore will be measured with a more faster AC voltage measurement with a small measurement frequency and use a correlation factor to determine the rated capacitance.

### Operating voltage

Supercapacitors are low voltage electronic components, for which the safe operation requires that the voltage applied always is within specified limits. The limit for the operating voltage is the rated voltage. The rated voltage for supercapacitors is the maximum direct voltage (DC) or peak value of pulse voltage which may be applied continuously to a capacitor within the specified temperature range. The rated voltage is the voltage in which the performance data of the capacitors are measured. At no time the capacitor should be subjected to voltages continuously in excess of the rated voltage.

The rated voltage has a safety margin against the chemical limiting condition, the decomposition voltage of the electrolyte. A chemical decomposition is the separation of a chemical compound into its elements when exposed f. e. a voltage. In this case the decomposition voltage in a Helmholtz layer separate the solvent water in hydrogen and oxide at a voltage of 1.2 V. Because the capacitor has two electrodes than the rated voltage of a supercapacitor with aquaeus electrolyte is 2.4 V. For electrolytes with organic solvents the decomposition voltage is a little bit higher and can reach 1.8 V per electrode or 3.6 V respectively. Applying higher voltages as the rated voltage to the capacitor an electrolytic decomposition take place which is associated with gas formation and thus may destroy the capacitor.

Standard supercapacitors with aquaeus electrolyte normally are specified with a rated voltage of 2.1 to 2.3 V, capacitors with organic solvents with 2,5 to 2,7 V. Lithium-ion capacitors with doped electrodes may reach a rated voltage of about 3.8 to 4 V, but, however, due to the doping, a lower voltage limit of about 2.2 V may not be dropped.

Operating the supercapacitors with a voltage lower than the rated voltage has positive effects to the electrical parameter. Changes of capacitance and internal resistance during cycling are lower, life time and charge/discharge cycles may be extended.[82]

Because supercapacitors are operating only with low voltages, the rated voltage is generally less than the application voltage required. To achieve the required application voltage, it is necessary to connect supercapacitor cells in series connection. Since each supercapacitor will have a slight difference in capacitance value and internal resistance it is necessary to balance the series connected capacitors either by a passive balancing or by an active balancing. The concept of passive balancing employs resistors in parallel with the supercapacitors and stabilize the voltage over the capacitor. Active balancing additional may adapt a voltage management at a level above a given threshold varies the maximum current during balancing by product.

### Internal resistance

The internal DC resistance of a supercapacitor can be calculated out of the voltage drop obtained from the intersection of the auxiliary line extended from the straight part and the time base at the time of discharge start

Charging of a supercapacitor over a DC voltage applied is connected to the movement of the charge carriers (ions) in the electrolyte through the separator as far as deep into the pores of the electrode. Discharging the capacitor the movement has to go the same way out. During this movement of the ions in the electrolyte losses occur, which can be measured as the internal DC resistance of the capacitor.

With the electrical model of cascaded series connected RC elements in the pores of the electrode, see #Capacitance, it is easily explained, that the internal resistance of a supercapacitor increases with increasing penetration depth of the charge carriers into the pores of the electrodes. That means, that the internal DC resistance is time dependent and increases during charging or discharging. But since in the applications of supercapacitors often only the switch-on and switch-off range is interesting, the internal resistance Ri can be calculated out of the voltage drop ΔV2 at the time of discharge start with a constant discharge current Idischarge. It is obtained from the intersection of the auxiliary line extended from the straight part and the time base at the time of discharge start (see picture right). The resistance can be calculated by the formula:

${\displaystyle R_{\text{i}}={\frac {\Delta V_{2}}{I_{\text{discharge}}}}}$

The discharge current Idischarge for the measurement of the internal resistance can be taken from the classification classes according to the standard IEC 62391-1, see #Capacitance.

This internal DC resistance should not be confused with the internal AC resistance called Equivalent Series Resistance (ESR) normally specified for capacitors. It is measured at 1 kHz, and has a compared with the DC resistance a much smaller resistance value. The AC ESR should not be used to calculate inrush currents or other peak currents.

The internal DC resistance Ri determined several properties of supercapacitors. It limits the charge and discharge peak currents as well as the time a supercapacitor can be charged or discharged. The internal resistance Ri and the capacitance C of the capacitor results in the time constant ${\displaystyle \tau }$

${\displaystyle \tau =R_{\text{i}}\cdot C}$

This time constant determines the time at which a capacitor can be charged or discharged. A 100 F capacitor with an internal resistance of 30 mΩ for example, has a time constant of 0.03 • 100 = 3 s, that means, after 3 seconds charging with a current limited only by the internal resistance, the capacitor has 62.3  % of full charge or is discharged to 36.8 % of full charge.

Standard capacitors with a constant internal resistance normally now reach the full charging of the capacitor during a period of about 5 τ. Since the internal resistance of a supercapacitor is not a constant value but increases the full charging or discharging cannot be calculated with the formula above. The time for full charging or discharging depends on individual construction details of used the supercapacitor type.

Compared with batteries the current loads, especially charge and discharge currents and peak currents of supercapacitors can be 10 to 100 times higher than for batteries. That depend on the internal resistance which may be clearly lower for supercapacitors than for batteries.

The internal DC resistance Ri determine the limits of the current loads as charge and discharge currents or peak currents in power applications of supercapacitors. The partly very high charge and discharge currents generate via the internal resistance internal heat losses Ploss .

${\displaystyle P_{\text{loss}}=R_{\text{i}}\cdot I^{2}}$

These internal generated heat have to be distributed to the ambient in order to keep the capacitor temperature lower than the maximal allowed temperature. Therefore, the current is limited.

Depending on the combination of electrode porosity, the size of the pores and the electrolyte used the permissible charge and discharge currents can differ enormous between different series of the different manufacturers.

### Cycle stability

Supercapacitors are distinguished from batteries by a much larger cycle stability. They can withstand 1,000,000 charging and discharging cycles without the capacity drops appreciably or the internal resistance increases significantly. The large cycle stability of supercapacitors results from the principles of the static storage of electrical charges with Helmholtz double-layers and the electrochemical storage with pseudocapacitances. Both storage principles don’t result in a permanent chemical change of the electrode material.

The cycle mode for a supercapacitor includes a permissible permanent charging and discharging of the capacitor within the specified temperature range and over the full voltage range. The magnitude of the current flowing in this operating and the frequency of charge and discharge cycles is limited by the internal heating of the capacitor. Since this internal generated heat together with the ambient temperature determine the capacitor temperature, the conditions of the cycle mode determine the life of the capacitor. Because the slow changes in the electrical parameters during the life of the capacitors, resulting from the diffusion of the electrolyte and possible chemical processes in the capacitor depend directly with the capacitor temperature.

It is a general rule that a lower current load, which can be achieved either by a lower voltage range or slower charging and discharging, increases the number of possible cycles and extend the capacitor life. [82]

The specified charge and discharge current for permanent cycling can be significantly exceeded for infrequent applications. Such a "peak power current" for power applications for huge supercapacitors with a capacity of more than 1000 F can reach a maximum peak current value of about 1000 A.[83] Such currents may, however, not be considered as a frequent permanent value. Because with such high currents, occurs not only a strong internal heating of the capacitors, wherein the thermal expansion is an additional stress factor, but it also caused strong electromagnetic forces with effect on the strength of the electrode-collector connection. A great cycle strability of supercapacitors with up to 1,000,000 cycles coupled with sporadic occurring peak power load therefore is not only a question of the chemical stability of the capacitor materials but also a result of a mechanically robust and stable construction.

### Energy density and power density

Ragone plot showing power density vs. energy density of various capacitors and batteries

Supercapacitors in point of stored energy are bridging the gap between electrolytic capacitors and rechargeable batteries.

The amount of energy stored in a supercapacitor is called specific energy. The energy W of a capacitor is given by the formula

${\displaystyle W={\frac {1}{2}}\cdot C_{\text{total}}\cdot V_{\text{loaded}}^{2}}$

For a supercapacitor with asymmetric voltages or if not the total voltage spread is used the energy W can be represented as[84]:

${\displaystyle W={\frac {1}{2}}\ C\cdot \ (V_{\text{max}}^{2}-V_{\text{min}}^{2})}$

It is either measured as gravimetric energy density per unit of mass in watt-hours per kilogram (W⋅h/kg) or as volumetric energy density per unit of volume in watt-hours per litre (W⋅h/l).

As of 2013 the range of commercial available energy densities of supercapacitors including lithium ion capacitors reach from around 0.5 to 15 W•h/kg[85] (typical value) or maximal 30 W•h/kg[86] (maximum value of the same type). For comparison, an aluminum electrolytic capacitors stores typically 0,01 to 0.3 W•h/kg while a conventional lead-acid battery stores typically 30 to 40 W•h/kg and modern lithium-ion batteries about 100 to 265 W•h/kg. That means, supercapacitors can store about 10 to 100 times more energy than electrolytic capacitors but only one tenth of rechargeable lithium ion batteries.

The energy density depends strongly on the measurement conditions. Because the charge as well as the discharge of supercapacitors is related to the internal time constant result from ESR the maximum energy density in practice only is a theoretical value. In applications the available energy density is coupled on current and cycle speed.

A special feature of supercapacitors is the power of the capacitor, the ability of very fast high current charge and discharge cycles at a rate comparable with conventional electrolytic capacitors that is typically limited by internal heating. The maximum power P a capacitor can withstand is given by the formula[87]:

${\displaystyle P={\frac {1}{4}}\cdot {\frac {V^{2}}{ESR}}}$

The measure of this fast charge/discharge rate is power density, that is the amount of power (time rate of energy transfer) per unit weight or volume stated in kilowatt per kilogram (kW/kg) or kilowatt per litre (kW/l). Power density combines the energy density with the speed at which the energy can be delivered to the load. Because batteries store energy in a chemical reaction which makes charge and discharge relatively slowly the power density of supercapacitors is typically 10 to 100 times greater. Or expressed shorter, supercapacitors can store lesser energy than rechargeable batteries but can deliver the energy much faster.

Power density and energy density are usually displayed in a so-called Ragone plot. With such a diagram, the position of a particular storage technology compared with other technologies, is visually clearly represented.

Supercapacitors can store lesser energy than rechargeable batteries but can deliver the energy much faster. By their ability of rapid charge and discharge cycles combined with high energy densities supercapacitors are therefore caught in the public eye, because they fulfill the requirement for energy recovering the brake energy in new concepts of electric mobility as well as the requirements for supplying cars, buses and trains with peak energy loads.

The lifetime of supercapacitors depends mainly on the capacitor temperature and the voltage applied

Supercapacitors also are distinguished from batteries by a much longer lifetime. Since the storage principles of supercapacitors to all intents and purposes don’t result in a permanent chemical change of the electrode material, the life time of the capacitors mostly only depend on the evaporation of the non solid electrolyte over the time. This evaporation is as faster as higher is the temperature of the capacitor and the voltage applied. The evaporation generally results in a decreasing capacitance value and an increasing internal resistance. Related to the slow changes of the electrical parameters, after a time the capacitors reduce their proper functionality. It depends on the application of the capacitors, whether the aberration of the parameters have any influence on the proper functionality or not. To harmonize this, change limits are defined whose exceeding are seen as so-called "wear-out failures". These limits are specified in the IEC/EN standard 62391-2.

According to IEC/EN 62391-2 the capacitance value can be reduced by 30 % compared to its initial value. If these limit is exceeded this capacitor is counted as “wear-out failure” reaching the “end-of-life” point. For the internal resistance this point has reached, if the resistance value exceeded four times the value of its data sheet specifications. If any one of these limits exceeded, the end of the life of the capacitor is reached. The capacitors are then even further operable, but only with reduced electrical properties.

These changes, however, permitted according to the standard, are usually too high for applications with high charge and discharge current loads. Therefore, some manufacturers, whose supercapacitors are provided for high current loads, define the maximum changing parameter within the specified lifetime of the capacitors with significantly narrower percentages, f.e. with only 20 % change of the capacitance value combined with the maximum change of the internal resistance to the double value specified in the data sheet.[88]. In particular for the internal resistance this narrower definition is important for capacitors with high current load applications, since the heat development in the capacitor increases linear with the increasing internal resistance. This heat increasing could possibly result to an unacceptable gas development in the capacitor may destroy the capacitor.

The real lifetime, also called “service life”, “life expectancy” or “load life”, of supercapacitors in their applications can reach 10 to 15 years or more. This long time could not be tested by manufacturers. Hence, the manufacturers are testing their products with an accelerated aging only at the maximum temperature and voltage conditions as long as no or a limited number of wear-out failures occur. This test is defined in the responsible standard and is called “endurance test”. The tested lifetime at maximum condition will be specified in datasheets in the notation “testet time/max. temperature” like "5000 h/65 °C". This value is strongly dependent on the particular series of the different manufacturers.

This lifetime, tested with maximum conditions and specified by the manufacturers in their datasheets, can be used for calculation of the expected lifetime at conditions coming from the application. Similar to electrolytic capacitors with non solid electrolyte in generally the so-called “10-degrees-rule” will be used for those estimations. This rule is the expression of the Arrhenius equation, a simple formula for the temperature dependence of reaction rates. After that doubles the estimatable life for every 10 °C lower operating temperature, because the changes in the electrical parameters run correspondingly slower.

${\displaystyle L_{x}=L_{0}\cdot 2^{\frac {T_{0}-T_{x}}{10}}}$

With

• 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, may have an estimated operation life time of 20,000 h at 45 h.

The lifetime of supercapacitors is in contrast to the electrolytic capacitors, also dependent on the operating voltage because the development of gas in the liquid electrolyte depends on the voltage applied. As lower the voltage as smaller the gas development, as longer the lifetime. But for the voltage depending of the lifetime no general formula is given. The curves of the voltage depending lifetime, which are shown from the picture beside, therefore only has to be seen as an empirical value of one manufacturer.

In supercapacitors which are used for very high charge and discharge current loads also the higher currents can influence the lifetime. Hence, for supercapacitors for power applications the lifetime is also limited by the number of switching cycles. But also as for the voltage depending of the lifetime no general formula for the influence of higher currents to the lifetime is given.

### Self-discharge rate and leakage current

Storing electrical energy in the static Helmholtz double-layers separates the charge carriers with distances in the range of molecules to each other. Over this short distance some “durty” effects can occur, leading to the exchange of charge carriers discharging the capacitor. This self-discharge is a very small current called leakage current. It depends on the capacitance value, the voltage applied, the temperature of the capacitor, and type dependent on the chemical behavior of the combination of electrode material and electrolyte. The leakage current at room temperature, based on the amount of charges stored, is so low, that typically the current is specified as self-discharge time of the capacitor. Usually the self-discharge time for supercapcitors will be specified in hours, days or weeks. As an example here is listed a "5.5 V/1 F “Goldcapacitor" produced by Panasonic, which is specified with a voltage drop at 20 °C from 5.5 V down to 3 V in 600 hours (25 days or 3.6 weeks) for a double cell capacitor.[89]

The self-discharge rate is, for most applications of supercapacitors, sufficiently low enough, but it is higher than in accumulators.

### Polarity

A negative bar on the insulating sleeve indicates the cathode terminal of the capacitor

Although the anode and cathode of symmetrical supercapacitors consist out of the same material, theoretically supercapacitors have no true polarity. Normally catastrophic failure will not occur, only the life will be reduced if a supercapacitor is reversed charged for some reason. However, it is recommended practice to maintain the polarity resulting by a formation of the electrodes during production of the capacitors.

For supercapacitors with asymmetric electrode design the polarity additional results out of the asymmetric design.

Out of these reasons supercapacitors has to operate as polarized capacitors. They may not be operated in "wrong" polarity opposite to the polarity identification. This also excludes operation with AC voltages.

The polarity marking of supercapacitors is made like for aluminum electrolytic capacitors with non solid electrolyte with a bar in the insulating sleeve for the minus pol to identify the cathode terminal.

The description of the polarity of components by the terms "anode" and "cathode" can lead to a confusion, because depending on whether a component is considered as a generator or as a consumer the polarity changes. For an accumulator or a battery the cathode has a positive polarity (+) and the anode has negative polarity (-). For capacitors as consumer components the cathode has negative polarity (-) and the anode has positive polarity (+). This requires special attention if batteries should be substituted by supercapacitors or switched in parallel with batteries.

Supercapacitors can be built also in a bipolar design and thus can be suitable for small AC voltages in the very low frequency range.

## Comparisation of technical parameters

### Comparisation of supercapacitor parameters

1 Farad supercapacitor with 5.5 volts constructed out of two single cells in series connection

The technology of supercapacitors is relatively young. But by combining different electrodes with many different electrolytes, a large number of different solutions have been developed, which are reflected in the different characteristics of supercapacitors. Particularly with the development of low-ohmic electrolyte systems, in combination with electrodes having a high pseudocapacitance, a large number of technical solutions is originated. Correspondingly diverse is the range of supercapacitors on the market

As can be seen from the following table, therefore, the capacitors of the various manufacturers are significantly different not only in the capacitance range but also in the values for the specific capacitance (F/cm3), the cell voltage, the internal resistance (ESR) and the energy density.

In the table, the ESR in each case refers to the largest capacitance value of the respective manufacturer. Throughout rough estimation, the supercapacitors are thereby divided into two groups. The first group comprises capacitors with greater ESR values of about 20 milliohms and relatively small capacitance values of 0.1 to 470 F. These are the typical "double-layer capacitors" for memory back-up or similar applications. The second group with capacitance values of about 100 to 12,000 F has a significantly lower ESR values, which partly go down under 1 milliohms. These supercapacitors are suitable for power applications. A correlation of some supercapacitor series of different manufacturers to the various construction features is provided in the report of Pandolfo and Hollenkamp.[32]

Electrical parameter of supercapacitor series of different manufacturers
Manufacturer Series
name
Capacitance
range
( F)
Cell
voltage
(V)
ESR-
at Cmax
(mΩ)
Volumetric
energy-
density
(Wh/dm3)
Gravimetric
energy-
density
(Wh/kg)
Remarks
ACT, [90] Premlis® 2000 4 6.5 - 15.0 Li-Ion-capacitors
APowerCap[91] APowerCap 4…550 2.7 - - 4.5 -
AVX [92] BestCap® 0.068…0.56 3.6 - 0.13 - Modules up to 16 V
Cap-XX [93] Cap-XX 0.16…2.4 2.75…2.75 14 1.45 1.36 -
CDE [94] Ultracapacitor 0,1…3000 2.7 0.29 7.7 6.0 -
Cooper [95] PowerStor 0.1…400 2.5…2.7 4.5 5.7 - -
Elna [96] DYNACAP
POWERCAP
0,047…300
2.5...3.6
2.5
8.0
3.0
5.4
5.3
-
-
-
-
Elton [97] Supercapacitor 1800…12000 1.5 0.5 6.8 4.2 Modules up to 29 V
Evans [98] Capattery 0,001…10 125 200 - - Hybrid capacitors
HCC [99] HCAP 0.22…5000 2.7 15 10.6 - Modules up to 45 V
FDK [100] EneCapTen 2000 4.0 - 25 14 LI-Ion-capacitors
Illinois [101] Supercapacitor 1…3500 2.3…2.7 0.29 7.6 5.9 -
Ioxus [102] Ultracapacitor 100…3000
220…1000
2.7
2.3
0.26 7.8
8.7
6.0
6.4
-
JSR Micro [103] Ultimo 1100…3300 3.8 1.2 20 12 Li-Ion-capacitors
Korchip [104] STARCAP 0.01…400 2.7 12 7.0 6.1 Modules up to 50 V
Liyuan [105] Supercapacitor 1…400 2.5 10 4.4 4.6 -
LS Mtron [106] Ultracapacitor 100…3000 2.8 0.25 6.0 5.9 Modules up to 84 V
Maxwell [107] Boostcap® 10…3000 2..2…2.7 0.29 7.8 6.0 Modules up to 125 V
Murata [108] EDLC 0.35…0.7 2.1 30 0.8 - -
NEC [109] Supercapacitor
LIC Capacitor
0.01…100
1100…1200
2.7
3.8
30,000
1.0
5.3-
-
4.2
-
-
Li-Ion-capacitors
Nesscap [110] EDLC,
Pseudocapacitor
3…60
50…300
2.3
2.3
35
18
4.3
12.9
3,3
8.7
Modules up to 125 V
Nichicon [111] EVerCAP® 0,47…6000 2.5…2.7 2.2 6.9 4.0 -
NCC, ECC [112] DLCCAP 350…2300 2.5 1.2 5.9 4.1 Modules up to 15 V
Panasonic [113] Goldcap 0.015…70 2.1…2.3 100 3.4 - -
Samwha [114] Green-Cap® 3…3000 2.7 0.28 7.7 5.6 Modules up to 125 V
Skeleton [115] SkelCap 900…3500 2.85 0.2 14.1 10.1 -
SPEL[116] Supercapacitor 25…6000 2.9 0.28 8.0 6.0 Modules up to 190 V
Taiyo Yuden [117] PAS Capacitor
LIC Capacitor
0.03…50
0.25…200
2.5…3.0
3.8
70
50
6.1
-
-
-
Pseudo capacitors
Li-Ion-capacitors
VinaTech [118] Hy-Cap 1.5…800 2.3…3.0 10 8.7 6.3 -
WIMA [119] SuperCap 12…6500 2.5…2.7 0.18 5.2 4.3 Modules up to 112 V
YEC [120] Kapton capacitor 0.5…400 2.7 12 7.0 5.5 -
Yunasko [121] Ultracapacitor 480…1700 2.7 0.17 6.1 5.8 -
Footnote: Volumetric and gravimetric energy density calculated by maximum capacitance, related voltage and dimensions if not specified in the datasheet

### Parameter comparisation of technologies

Supercapacitors are in competition with electrolytic capacitors and rechargeable batteries especially lithium-ion batteries. The following table shows the generalized major parameters of the three different supercapacitor families in comparison with electrolytic capacitors and batteries.

Parameters of supercapacitors
compared with electrolytic capacitors and lithium-ion batteries
Parameter Aluminum
electrolytic
capacitors
Supercapacitors Lithium-
ion-
batteries
Double-layer
capacitors
for
memory backup
Super-
capacitors
for power
applications
Pseudo and
Hybrid
capacitors
(Li-Ion
capacitors)
Temperature
range (°C)
−40 to 125 −20 to +70 −20 to +70 −20 to +70 −20 to +60
Cell
voltage (V)
4 to 550 1.2 to 3.3 2.2 to 3.3 2.2 to 3.8 2.5 to 4.2
Charge/discharge
cycles
unlimited 105 to 106 105 to 106 2 • 104 to 105 500 to 104
Capacitance range
(F)
≤ 1 0.1 to 470 100 to 12000 300 to 3300
Energy density
(Wh/kg)
0.01 to 0.3 1.5 to 3.9 4 to 9 10 to 15 100 to 265
Power density
(kW/kg)
> 100 2 to 10 3 to 10 3 to 14 0,3 to 1.5
Self discharge time
at room temperature
short
(days)
middle
(weeks)
middle
(weeks)
long
(month)
long
(month)
Effciency (%) 99 95 95 90 90
Life time
at room temperature
(Years)
> 20 5 to 10 5 to 10 5 to 10 3 to 5

Electrolytic capacitors have, due to their design, which allows unlimited charge/discharge cycles, their high dielectric strength up to 550 V and their good frequency response as AC resistance in the lower frequency range very different properties from supercapacitors.

Supercapacitors are not provided for AC applications. They can store about 10 to 100 times more energy than electrolytic capacitors but only one tenth of rechargeable lithium ion batteries.

Advantages of supercapacitors compared with rechargeable batteries include:

• Longer lifetime, typically > 10 years, limited by temperature dependent evaporation of liquid electrolyte and internal heating by current loads,
• Much higher number of charge/discharge cycles
• Charge and discharge with small degradation of capacitance over hundreds of thousands of charge cycles.
• Very high rates of charge and discharge cycles
• Low cost per cycle
• Simple charge methods—no full-charge detection is needed; no danger of overcharging
• Good reversibility
• Extremely low internal resistance (ESR)
• High cycle efficiency (95% or more)
• Much higher power density
• environmentally friendly, no corrosive electrolyte and low toxicity of materials.
• Supercapacitors can help in conjunction with batteries by acting as a charge conditioner, storing energy from other sources for load balancing purposes and then using any excess energy to charge the batteries at a suitable time reducing battery cycling duty and extending battery life

• Higher price than rechargeable batteries
• The amount of energy stored per unit weight or volume is generally lower than that of a rechargeable battery
• Higher self-discharge – the rate is considerably higher than that of an electrochemical battery.
• Low maximum voltage – series connections are needed to obtain higher voltages, and voltage balancing may be required.
• Unlike practical batteries, the voltage across any capacitor, including supercapacitors, drops significantly as it discharges. Effective storage and recovery of energy requires complex electronic control and switching equipment, with consequent energy loss.
• Very low internal resistance allows extremely rapid discharge when shorted, resulting in a spark hazard similar to any other capacitor of similar voltage and capacitance (generally much higher than electrochemical cells).

## Standards

Classification of supercapacitors into classes regarding to IEC 62391-1, IEC 62567and BS EN 61881-3 standards

Supercapacitors have very specific electrical parameters. They mostly are not interchangeable at all, especially the types for higher energy densities. Out of reason of their different applications from low current up to high peak current demand the conditions for testing their electrical parameters has to be standardized.[122]

The test specifications and the requirements of the electrical parameters are specified in the generic specification

• IEC/EN 62391–1, Fixed electric double layer capacitors for use in electronic equipment

Within the generic specification four application classes depending on the application related discharge currents are defined:

• Class 1 mainly used for RAM memory backup with discharge current units in mA = 1 • C (F) ranging from nA to μA,
• Class 2 for energy storage, mainly used for driving motors require a short time operation, with discharge current units in mA = 0,4 • C (F) • V (V) ranging from mA to A,
• Class 3 for energy storage with higher power demand for a long time operation, with discharge current units in mA = 4 • C (F) • V (V) ranging from mA to A,
• Class 4 for energy storage for applications that requires instantaneous power with relatively high current units or peak currents even with a short operating time in mA = 40 • C (F) • V (V) ranging up to several hundreds of amperes

(C = rated capacitance in farad, V = rated voltage in V)

Beneath this generic specification three further standards for special applications exists:

• 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 are not provided for AC applications. They can store smaller amount of energy than rechargeable lithium ion batteries, but with very fast charge and discharge cycles, much higher number of charge/discharge cycles, high cycle efficiency and longer lifetime. The ideal applications for supercapacitors are all those demanding energy for a duration in a time range from millisecond to app. some minutes. The energy needed for these applications and have to be stored in the capacitors span from milliamps current or milliwatt power in the longer time range up to several hundred amps current or several hundred kilowatts power needs for short times.

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

${\displaystyle t={\frac {C\cdot (U_{\text{charge}}-U_{\text{min}})}{I}}}$

wherein 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:

${\displaystyle t={\frac {1}{2P}}\cdot C\cdot (U_{\text{charge}}^{2}-U_{\text{min}}^{2}).}$

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

### General applications

#### Consumer applications

For consumer applications in flashlights for digital cameras or in professional photographic flash devices supercapacitors deliver the fast energy demand for flashes. Two LED flashlights using supercapacitors were released in 2009. They charge in 90 seconds.[123]

In applications with fluctuating loads supercapacitors stabilize the power supply of laptop computers, PDA’s, GPS, portable media players , hand-held devices[124], and photovoltaic systems.

For homeworkers in 2007 a cordless electric screwdriver that uses a supercapacitor for energy storage was produced.[125] It charges in 90 seconds, retains 85% of the charge after 3 months, and holds enough charge for about half the screws (22) a comparable screwdriver with a rechargeable battery will handle (37).

#### Public applications

Street light combining solar cell power source with LED lamps and supercapacitors for energy storing

Relatively new is the application for supercapacitors in street lighting. In Sado City, Niigata Prefecture, Japan, now the first street lights combining a stand-alone power source with solar cells with energy saving LEDs. Supercapacitors store the solar generated energy and supply 2 LED lamps with 15 W power consumption over night. The Supercapacitors ensures long life (> 10 years) and stable characteristics under various climates, from areas that exceeds 40 °C in summer down to temperatures below -20 °C in winter.[126]

#### Medical applications

Supercapacitors are advantageous when extremely fast charging or discharging is required. In professional medical applications, supercapacitors have been used to power a handheld, laser-based breast cancer detector (55 F to provide 5.3 W at multiple voltages; charges in 150 seconds, runs for 60 seconds)[127].

Implantable defibrillators are designed to restore a normal heart rhythm by providing life-saving electrical pulse therapies upon detection of arrhythmias. A strong pulse transmitting about 10 – 40 J of energy at a potential difference of 600 to 800 V will be applied to the heart tissue if severe arrhythmia is detected. Supercapacitors can deliver this energy to a high-voltage circuit for enabling this life-preserving therapy. [128]

#### Industrial applications

Supercapacitors can be used in industrial applications to operate low-power equipment such as memory backup for RAMs and SRAMs, micro-controller power backup, PC Cards, for shutdown operations, for automated meter reading (AMR)[129] in advanced metering equipment or for event notification in industrial electronics.

Rotor with wind turbine pitch system

In many applications, supercapacitors are used in conjunction with rechargeable batteries. These combination connects the power density of the capacitor with the greater energy storage capability of the battery. Buffering the power of the battery, the supercapacitor can handle short interruptions, when the input power source, f f. e. the mains power or a fuel cell fails, and provides high power ratings from the capacitor to a load. The capacitor response in delivering the power is very fast, buffering the inrush current peaks. That reduce the battery cycles duty and extende battery life because the batteries are used only during long interruptions. Typical industrial applications for circuits like that are uninterruptible power supplies UPS) where supercapacitors have replaced batteries of electrolytic capacitors which need much more space. These combination of supercapacitor and battery reduce the cost per cycle, save on replacement and maintenance costs, and enable a battery to be downsized, and can extend the life of batteries.[130] Implementing a supercapacitor solution like described provides a long-life, reliable, and very fast UPS[131][132][133] Disadvantage of this battery/capacitor combination is the need of a special electronic circuit to be able to compensate the different storage behavior of both types.

Supercapacitors also used in UPS for actuators in wind turbine pitch systems, turning the blades even if the main supply fails.[134] So it is possible to turn the blades out of the wind even at storm is blowing protecting the wind turbine.

In 2005 aerospace systems and controls company Diehl Luftfahrt Elektronik GmbH chose supercapacitors to power emergency actuation systems for doors and evacuation slides in airliners, including the new Airbus 380 jumbo jet.[134]

#### Stabilizing renewable energy sources

Supercapacitors also can be used to stabilize the mains voltage. For applications with fluctuating loads like photovoltaic systems the electrical output of photovoltaic arrays doesn't only fluctuate with the seasons or between night and day. It's also affected by local weather. Power production breaks down for a short time if clouds drift over the modules. Consequently, the grid voltage drops. Supercapacitors are able to act as a buffer with these fluctuations from renewable energy sources to stabilize the grid voltage within milliseconds in the power grid itself. As a result, there's no need to adjust costs effective controls at power stations to provide stabilization the line voltage and frequency, balance supply and demand of power and manage real or reactive power.[135][136][137]

#### Military applications

With lower internal resistance of advanced supercapacitors, these components are suitable for applications in which short-term peak currents were needed. Some of the earliest uses of supercapacitors were motor startup (cold Diesel engine start) for large engines in tanks and submarines.[138] In these applications, buffering the power of the battery, the supercapacitor can handle short current peaks, buffer the batteries to be used only during long activities, and reducing the cycling duty. Further military applications are using the high power density are phased array radar antenna, laser power supply, military radio communications, avionics display and instrumentation, backup power for airbag deployment, and GPS guided missiles and projectiles. [139][140]

### Heavy and public transport

The primary challenges of any public transport is reducing the fuel consumption or more generally reducing energy consumption and reduction of CO2 emissions. Regeneration of braking energy (recuperation) is the most effective way to reduce the above-mentioned points. This generate the demand for devices can store very fast enough energy over long times with a high cycle rate. Electrochemical capacitors fulfill these requirements and are therefore used in a lot of applications in all kinds of transportation.

#### Trucks and rails

Green Cargo operates TRAXX locomotives from Bombardier Transportation

Supercapacitors can be used as supplement to the battery as a starter battery in diesel trucks and railroad railroad locomotives. The use of supercapacitors as energy storage device in the locomotives with electric drive expands the regenerative braking range to full stopping. This creates the conditions for full use of kinetic energy of the train. It is possible to use the regenerative braking power in diesel electric locomotives for starting engine, acceleration, and operation mode. Maintenance-free operation and environmentally friendly materials were further arguments for the use of supercapacitors in this application. [141][142]

Container Yard with Rubber Tyre Gantry Crane

Mobile hybrid diesel/electric rubber tyred gantry crane for moving and stacking containers within the stacking areas of a container terminal needs a lot of energy for moving and lifting the boxes. This energy could regenerate partly by using a supercapacitor energy storage system captures and stores regenerative energy during braking the crane and dropping the load resulting in improved efficiency.[143]

A triple hybrid forklift truck uses fuel cells and batteries as primary energy storage and supercapacitors to bridge power peaks to supplement this energy storage solution. The triple-hybrid system offers over 50 % energy savings compared with diesel engine forklifts or fuel-cell-only systems.[144]

Supercapacitor terminal tractors are used in shipping docks to transport containers to warehouses. They provide an economical, quiet, and pollution free alternative to diesel terminal tractors. Supercapacitors are used in some pit trains in China to replace conventional trolley pit train. They are used in coal mines to bring coal to the surface. The supercapacitor pit train remove a fire and safety hazard from coal mines. The capacitors can be charged at the surface terminal in less than 30 minutes.[145]

Supercapacitors are also currently in use or being considered for use in heavy hybrid vehicles in trash trucks, which can experience as many as a thousand start/stop cycles during a day; and in delivery trucks, which operate on similar drive cycles.

#### Public transportation light-rails and trams

and

Light rail vehicle in Mannheim

Since 2003 in Mannheim a prototype of a light-rail vehicle (LRV) using the MITRAC Energy Saver system from Bombardier Transportation have the ability to convert the mechanical braking energy of the train into electrical energy and to feed it back into the catenary respectively can store the braking energy on the train in supercapacitors and use it during the next acceleration of the vehicle. [146] Compared to conventional modern LRVs or Metro vehicles, which are already using regeneration into the line, a train using a propulsion system with onboard energy storage results in further energy savings up to 30 % and a reduction of the peak power demand by up to 50 %.

In 2009 in Paris a light-rail vehicle (LRV) was fitted with a bank of 48 supercapacitors mounted on the roof both to store braking energy, and to enable the LRV to operate with overhead catenary only on parts of its route, running on stored energy between electrified segments and recharging quickly at segments equipped with catenary.[147]

Light rail vehicle inHong Kong

In addition, the supercapacitors enabled the LRV's to operate in an area of Heidelberg without overhead wires. The EDLC equipment cost an additional €270,000 per vehicle, which is expected to be recovered in the first 15 years of operation.

In April 2011 Rhein-Neckar Verkehr ordered 11 more LRVs equipped with EDLCs.[148]

In 2012 the tram operator TPG in Geneva began tests of a light-rail vehicle (LRV) equipped with a prototype supercapacitor energy storage unit mounted on the roof to recover braking energy.[149]

In August 2012 the CSR Zhouzhou Electric Locomotive corporation of China presented a prototype two-car light metro train equipped with a roof-mounted supercapacitor unit providing both regeneration of braking energy, and the ability to operate without overhead wires while charging the supercapacitors at stations, which the supplier believes could potentially be used in 100 smaller and medium-sized Chinese cities.[150]

Other public transport manufacturers are delivering modern light-rail transport systems with supercapacitor energy storing technology, including mobile storage.[151]

Hong Kong's South Island metro line is to be equipped with two 2 MW energy storage units using supercapacitors, which are expected to reduce energy consumption by 10%.[152]

#### Aerial lift

Aerial lift in Zell am See, Austria

In Zell am See, Austria, a aerial lift is frequently employed to connect the city with the "house" mountain Schmittenhöhe. The gondolas sometimes runs over 24 hours per day needing electricity for lights, door opening and communication. Because only the time at the stations can be used for loading, and time for loading at the stations was too short for full loading of the batteries and additional the life time of the batteries was insufficient a solution with supercapacitors was found. They can be charged much faster with a higher number of cycles and longer life time.

#### Public transport buses

MAN Ultracapbus in Nuremberg, Germany

The first hybrid bus with supercapacitors in Europe was presented to the public in 2001 in Nuremberg, Germany. It was the so-called "Ultracapbus" of MAN, and was tested in the real line operation in 2001/2002 by VAG, the public transport operator in Nuremberg. The test vehicle was equipped with a diesel-electric drive in combination with the new Ultracp-technology. The Ultracap-system was supplied with 8 Ultracap-modules of 80 V containing 36 supercapacitors each, the system worked with 640 V, could be charged and discharged with 400 A maximum current and had an energy content of 0,4 kWh with a weight of 400 kg. The supercapacitors were used to accumulate braking-energy and deliver starting energy. The advantages of the system were a significant reduction in fuel consumption (currently 10 to 15% compared to conventional diesel vehicles), reduction of CO2 emissions, starting the bus without disturbing noise and exhaust pollution, low-vibration drive, and a reduction in maintenance costs.[153][154]

Electric bus at EXPO 2010 in Shanghai (Capabus) recharging at the bus stop

As of 2002 in Luzern, Switzerland a test with electric bus fleet called TOHYCO-Rider was accomplished. The supercapacitors of the buses can be recharged via an inductive non-contact high-speed power charger after every transportation cycle within 3 to 4 minutes. [155]

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