||The lead section of this article may need to be rewritten. (March 2014)|
Supercapacitor (SC), formerly electric double-layer capacitor (EDLC) or ultracapacitor, is the generic term for a family of electrochemical capacitors. As opposed to nanoscale dielectric capacitors which also have high capacitance values, supercapacitors don't have a conventional solid dielectric. The capacitance value of an electrochemical capacitor is determined by two storage principles, which both contribute indivisibly to the total capacitance:
- Double-layer capacitance – Electrostatic storage achieved by separation of charge in a Helmholtz double layer at the interface between the surface of a conductive electrode and an electrolyte. The separation of charge is of the order of a few ångströms (0.3–0.8 nm), much smaller than in a conventional capacitor.
- Pseudocapacitance – Faradaic electrochemical storage with electron charge-transfer, achieved by redox reactions, intercalation or electrosorption.
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 be as much as 10× that of double-layer capacitance.
Supercapacitors are divided into three families, based on electrode design:
- Double-layer capacitors – with carbon electrodes or derivates with much higher electrostatic double-layer capacitance than electrochemical pseudocapacitance
- Pseudocapacitors – with metal oxide or conducting polymer electrodes with a high amount of electrochemical pseudocapacitance
- Hybrid capacitors – capacitors with asymmetric electrodes, one of which exhibits mostly electrostatic and the other mostly electrochemical capacitance, such as lithium-ion capacitors
Supercapacitors bridge the gap between conventional capacitors and rechargeable batteries. They store the most energy per unit volume or mass (energy density) among capacitors. They support up to 10,000 farads/1.2 volt, up to 10,000 times that of electrolytic capacitors, but deliver or accept less than half as much energy per unit time (power density).
By contrast, while supercapacitors have energy densities that are approximately 10% of conventional batteries, their power density is generally 10 to 100 times greater. This results in much shorter charge/discharge cycles than batteries. Additionally, they will tolerate many 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, including:
- Low supply current for memory backup in static random-access memory (SRAM)
- Power for cars, buses, trains, cranes and elevators, including energy recovery from braking, short-term energy storage and burst-mode power delivery
There are many different trade or series names used for supercapacitors, such as: APowerCap, BestCap, BoostCap, CAP-XX, DLCAP, EneCapTen, EVerCAP, DynaCap, Faradcap, GreenCap, Goldcap, HY-CAP, Kapton capacitor, Super capacitor, SuperCap, PAS Capacitor, PowerStor, PseudoCap, Ultracapacitor.
- 1 History
- 2 Basics
- 3 Construction
- 4 Materials
- 4.1 Supercapacitor types
- 4.2 Electrodes
- 4.3 Electrodes for EDLCs
- 4.4 Electrodes for pseudocapacitors
- 4.5 Electrodes for hybrid capacitors
- 4.6 Electrolytes
- 4.7 Separators
- 4.8 Collectors and housing
- 5 Electrical parameters
- 6 Comparison of technical parameters
- 7 Standards
- 8 Applications
- 8.1 General applications
- 8.2 Energy recovery
- 9 New developments
- 10 Market
- 11 See also
- 12 Literature
- 13 References
- 14 External links
Development of the double layer and pseudocapacitance model see Double layer (interfacial)
Evolution of components
In the early 1950s, General Electric engineers began experimenting with components using porous carbon electrodes for fuel cells and rechargeable batteries. Activated charcoal is an electrical conductor that is an extremely porous "spongy" form of carbon with a high specific surface area. In 1957 H. Becker developed a "Low voltage electrolytic capacitor with porous carbon electrodes". He believed that the energy was stored as a charge in the carbon pores as in the pores of the etched foils of electrolytic capacitors. Because the double layer mechanism was not known at the time, he wrote in the patent: "It is not known exactly what is taking place in the component if it is used for energy storage, but it leads to an extremely high capacity."
General Electric did not immediately pursue this work. In 1966 researchers at Standard Oil of Ohio (SOHIO) developed another version of the component as "electrical energy storage apparatus", while working on experimental fuel cell designs. The nature of electrochemical energy storage was not described in this patent. Even in 1970, the electrochemical capacitor patented by Donald L. Boos was registered as an electrolytic capacitor with activated carbon electrodes.
Early electrochemical capacitors used two aluminum foils covered with activated carbon - the electrodes - which were soaked in an electrolyte and separated by a thin porous insulator. This design gave a capacitor with a capacitance value in the one farad area, significantly higher than electrolytic capacitors of the same dimensions. This basic mechanical design remains the basis of most electrochemical capacitors.
Between 1975 and 1980 Brian Evans Conway conducted extensive fundamental and development work on ruthenium oxide electrochemical capacitors. In 1991 he described the difference between ‘Supercapacitor’ and ‘Battery’ behavior in electrochemical energy storage. In 1999 he coined the term supercapacitor to explain the increased capacitance by surface redox reactions with faradaic charge transfer between electrodes and ions.
His "supercapacitor" stored electrical charge partially in the Helmholtz double-layer and partially as the result of faradaic reactions with "pseudocapacitance" charge transfer of electrons and protons between electrode and electrolyte. The working mechanisms of pseudocapacitors are redox reactions, intercalation and electrosorption. With his research, Conway greatly expanded the knowledge of electrochemical capacitors.
The market expanded slowly. That changed around 1978 as Panasonic marketed its "Goldcaps” brand. This product became a successful energy source for memory backup applications. Competition started only years later. In 1987 ELNA "Dynacap"s entered the market. First generation EDLC's had relatively high internal resistance that limited the discharge current. They were used for low current applications such as powering SRAM chips or for data backup.
At the end of the 1980s improved electrode materials increased capacitance values. At the same time the development of electrolytes with better conductivity lowered the Equivalent Series Resistance (ESR) increasing charge/discharge currents. The first supercapacitor with low internal resistance was developed in 1982 for military applications through the Pinnacle Research Institute (PRI), and were marketed under the brand name "PRI Ultracapacitor". In 1992, Maxwell Laboratories, (later Maxwell Technologies) took over this development. Maxwell adopted the term Ultracapacitor from PRI and called them "Boost Caps" to underline their use for power applications.
Since capacitors' energy content increases with the square of the voltage, researchers were looking for a way to increase the electrolyte's breakdown voltage. In 1994 using the anode of a 200V high voltage tantalum electrolytic capacitor, David A. Evans developed an "Electrolytic-Hybrid Electrochemical Capacitor".
These capacitors combine features of electrolytic and electrochemical capacitors. They combine the high dielectric strength of an anode from an electrolytic capacitor with the high capacitance of a pseudocapacitive metal oxide (ruthenium (IV) oxide) cathode from an electrochemical capacitor, yielding a hybrid electrochemical capacitor. Evans' capacitors, coined Capattery, had an energy content about a factor of 5 higher than a comparable tantalum electrolytic capacitor of the same size. Their high costs limited them to specific military applications.
Recent developments include lithium-ion capacitors. These hybrid capacitors were pioneered by FDK in 2007. They combine an electrostatic carbon electrode with a pre-doped lithium-ion electrochemical electrode. This combination increases the capacitance value. Additionally, the pre-doping process lowers the anode potential and results in a high cell output voltage, further increasing energy density.
Research departments are active in many companies and universities are working to improve characteristics, such as energy density, power density, cycle stability and reduce production costs.
Electrochemical capacitors (supercapacitors) consist of two electrodes separated by an ion permeable membrane (separator), and an electrolyte connecting electrically the both electrodes. By applying a voltage to the capacitor an electric double layer at both electrodes is formed, which has a positive or negative layer of ions deposited in a mirror image on the opposite electrode.
The two electrodes form a series circuit of two individual capacitors C1 and C2. The total capacitance Ctotal is given by the formula
Supercapacitors may have either symmetric or asymmetric electrodes. Symmetry implies that both electrodes have the same capacitance value. If C1 = C2, then Ctotal = 0.5 ⋅ C1. For symmetric capacitors the total capacitance value equals half the value of a single electrode.
For asymmetric capacitors one of the electrodes typically has a higher capacitance value than the other. If C1 >> C2, then Ctotal ≈ C2. Thus, with asymmetric electrodes the total capacitance may be approximately equal to the smaller electrode.
Electrochemical capacitors uses the double-layer effect to store electric energy. This double-layer has no conventional solid dielectric which separates the charges. The capacitance values of electrochemical capacitors are determined by two new and different high-capacity storage principles in the electric double-layer on their electrodes:
- Double-layer capacitance, electrostatic storage of the electrical energy achieved by separation of charge in a Helmholtz double layer.
- Pseudocapacitance, electrochemical storage of the electrical energy achieved by faradaic redox reactions with charge-transfer.
The amount of charge stored per unit voltage in an electrochemical capacitor is primarily a function of the electrode size but the amount of capacitance of each storage principle can vary extremely. Double-layer capacitance and pseudocapacitance both contribute to the total capacitance value of an electrochemical capacitor. Both capacitances are only separable by measurement techniques.
Electrostatic double-layer capacitance
Every electrochemical capacitor has two electrodes, mechanically separated by a separator, which are electrically connected to each other via the electrolyte. The electrolyte is a mixture of positive and negative ions dissolved in a solvent such as water. At each of the two electrodes surfaces originates an area in which the liquid electrolyte contacts the conductive metallic surface of the electrode. This interface forming a common boundary among two different phases of matter, such as an insoluble solid electrode surface and an adjacent liquid electrolyte. In this interface occurs a very special phenomenon of the double layer effect.
Applying a voltage to an electro-chemical capacitor both electrodes in the capacitor generates electrical double-layers. These double layers consist out of two layers of ions. One layer is in the surface lattice structure of the electrode. The other layer, with opposite polarity, emerges from dissolved and solvated ions in the electrolyte. The two layers are separated by a monolayer of solvent molecules, e. g. for water as solvent by water molecules. The monolayer forms the inner Helmholtz plane (IHP). It adheres by physical adsorption on the surface of the electrode and separates the oppositely polarized ions from each other, becoming a molecular dielectric. 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 amount of charge in the electrode is matched by the magnitude of counter-charges in outer Helmholtz plane (OHP). This double-layer phenomena store electrical charges as in a conventional capacitor. The double-layer charge forms a static electric field in the molecular layer of the solvent molecules in the IHP that corresponds to the strength of the applied voltage.
The double-layer serves approximately like the dielectric layer in a conventional capacitor, but with the thickness of a single molecule. Therefore to calculate the capacitance the standard formula for conventional plate capacitors can be used. This capacitance can be calculated with:
The capacitance C is greatest in capacitors made from materials with a high permittivity ε, large electrode plate surface areas A and reciprocal to the distance d between plates.
If the electrolyte solvent is water then the influence of the high field strength creates permittivity ε of 6 (instead of 80 without an applied electric field). Because activated carbon electrodes have an extremely large surface area in the range of 10 to 40 µF/cm2 and the extremely thin double-layer distance is on the order of a few ångströms (0.3-0.8 nm), the double-layer capacitors have much higher capacitance values than conventional capacitors.
The amount of charge stored per unit voltage in an electrochemical capacitor is primarily a function of the electrode size. The electrostatic storage of energy in the double-layers is linear with respect to the stored charge, and correspond to the concentration of the adsorbed ions. But deviating from conventional capacitors, where usually the charge is transferred via electrons, the capacitance of the double-layer capacitors depends from the limited moving speed of ions in the electrolyte and the resistive porous structure of the electrodes. Capacitance values of supercapacitors depends strongly on the measuring time. Charging and discharging electric double-layers in principle is unlimited. No chemical changes take place. Lifetimes of real supercapacitors only are limited by electrolyte evaporation effects.
Applying a voltage at the electrochemical capacitor terminals moves the polarized ions or charged atoms in the electrolyte to the opposite polarized electrode and forms a double-layer, separated by a single layer of solvent molecules. Pseudocapacitance can originate when specifically adsorbed cations out of the electrolyte pervade the double-layer. This pseudocapacitance stores electrical energy by means of reversible faradaic redox reactions on the surface of suitable electrodes in an electrochemical capacitor with a electric double-layer. Pseudocapacitance is accompanied with an electron charge-transfer between electrolyte and electrode coming from a de-solvated and adsorbed ion whereby only one electron per charge unit is participating. This faradaic charge transfer originates by a very fast sequence of reversible redox, intercalation or electrosorption processes. The adsorbed ion has no chemical reaction with the atoms of the electrode. No chemical bonds arise. Only a charge-transfer take place.
The electrons involved in the faradaic processes are transferred to or from valence electron states (orbitals) of the redox electrode reagent. They enter the negative electrode and flow through the external circuit to the positive electrode where a second double-layer with an equal number of anions has formed. But these anions don’t accept the electrons. They remain on the electrode's surface in the charged state, and the electrons remain in the 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 surface coverage of the adsorbed anions. The storage capacity of the pseudocapacitance is limited by the finite quantity of reagent of available surface. A faradaic pseudocapacitance still only occurs together with a static double-layer capacitance. Pseudocapacitance and double-layer capacitance both contribute indivisible to the total capacitance value of the chemical capacitor. The amount of pseudocapacitance depends on the surface area, material and structure of the electrodes.
Since all the pseudocapacitance reactions take place only with de-solvated ions, which are much smaller than solvated ion with their solvating shell, the pseudocapacitance can be much higher than the double-layer capacitance for the same electrode surface. Therefore the pseudocapacitance may exceed the value of double-layer capacitance for the same surface area by factor 100, depending on the nature and the structure of the electrode.
The ability of electrodes to accomplish pseudocapacitance effects by redox reactions, intercalation or electrosorption strongly depends on the chemical affinity of electrode materials to the ions adsorbed on the electrode surface as well as on the structure and dimension of the electrode pores. Materials exhibiting redox behavior for use as electrodes in pseudocapacitors are transition-metal oxides like ruthenium (RuO2), iridium (IrO2), or manganese (MnO2) inserted by doping in the conductive electrode material such as active carbon, as well as conducting polymers such as polyaniline or derivatives of polythiophene covering the electrode material.
Pseudocapacitance may also originate from the structure and especially from the pore size of the electrodes. The tailored sizes of pores in nano-structured carbon electrodes like carbide-derived carbons (CDCs) or carbon nanotubes (CNTs) for electrodes can be referred to as intercalated pores which can be entered by de-solvated ions from the electrolyte solution and originates pseudocapacitance.
Conventional capacitors consist out of two electrodes which are separated by a dielectric material. In conventional capacitors such as ceramic capacitors and film capacitors, the electric charge of a loaded capacitor is stored in a static electric field that permeates the dielectric between the electrodes. The electric field originates by the separation of charge carriers and the strength of the electric field correlates with the potential between the two electrodes. It drops over the dielectric. The total energy increases with the amount of stored charge and the potential between the plates. The maximum potential between the plates, the maximal voltage, is limited by the dielectric's breakdown field strength.
This static storage also applies for electrolytic capacitors in which most of the potential decreases over the anode's thin oxide layer. The electrolyte as cathode may be somewhat resistive so that for "wet" electrolytic capacitors, a small amount of the potential decreases over the electrolyte. For electrolytic capacitors with solid conductive polymer electrolyte this voltage drop is negligible.
Conventional capacitors are also called electrostatic capacitors. The potential (voltage) of a charged capacitor correlates linearly with the stored charge.
Different from conventional capacitors electrochemical capacitors (supercapacitors) basically consists out of two electrodes separated by an ion permeable membrane (separator), and electrically connected via an electrolyte. In this double-layer electrodes a mixture of a double-layer and pseudocapacitance is stored. If both electrodes have approximately the same resistance (#Internal resistance), the potential of the capacitor decreases symmetrically over both double-layers, whereby a voltage drop across the ESR of the electrolyte is achieved. The maximum potential across the capacitor, the maximal voltage, is limited by the electrolytes decomposition voltage.
Both the electrostatic and electrochemical storage of energy in electrochemical capacitors are linear with respect to the stored charge, just as in conventional capacitors. The voltage between the capacitors terminals is linear with respect to the amount of stored energy. This linear voltage gradient differs from rechargeable electrochemical batteries, in which the voltage between the terminals remains independent of the amount of stored energy, providing a relatively constant voltage.
Supercapacitors are constructed with two metal foils (current collectors), each coated with an electrode material such as activated carbon. The collectors serve as the power connection between the electrode material and the external terminals of the capacitor. Specifically to the electrode material is its very large surface area. In this example the activated carbon is electrochemically etched, so that the surface of the material is about a factor 100,000 larger than the smooth surface. The electrodes are kept apart by an ion-permeable membrane (separator) used as an insulator to protect the electrodes against short circuits. This construction is subsequently rolled or folded into a cylindrical or rectangular shape and can be stacked in an aluminum can or an adaptable rectangular housing. Then the cell is impregnated with a liquid or viscous electrolyte of organic or aqueous type, or may be solid-state. The electrolyte, an ionic conductor, enters the pores of the electrodes and serves as the conductive connection between the electrodes across the separator. Finally the housing is hermetically sealed to ensure stable behavior over the specified lifetime.
Radial style of a lithium-ion capacitor for PCB mounting used for industrial applications
The properties of supercapacitors come from the interaction of their internal materials. Especially, the combination of electrode material and type of electrolyte determine the functionality and the thermal and electrical characteristics of the capacitors.
Additionally, the three members of the supercapacitor family are determined by their electrode material and structure.
As described above, supercapacitors store their electrical energy with two different storage principles: static double-layer capacitance and electrochemical pseudocapacitance. The distribution of the two types of capacitance depends on the material and structure of the electrodes. Based on this, the supercapacitor family is divided into three types:
- Double-layer capacitors – with activated carbon electrodes or derivatives with much higher electrostatic double-layer capacitance than electrochemical pseudocapacitance
- Pseudocapacitors – with transition metal oxide or conducting polymer electrodes with a high electrochemical pseudocapacitance
- Hybrid capacitors – capacitors with asymmetric electrodes, one of which exhibits mostly electrostatic and the other mostly electrochemical capacitance, such as lithium-ion capacitors
Supercapacitor electrodes are generally thin coatings applied and electrically connected to a conductive, metallic current collector. Electrodes must have good conductivity, high temperature stability, long-term chemical stability (inertness), high corrosion resistance and high surface areas per unit volume and mass. Other requirements include environmental friendliness and low cost.
The amount of double-layer as well as pseudocapacitance stored per unit voltage in a supercapacitor is predominantly a function of the electrode surface area. Therefore supercapacitor electrodes are typically made of porous, spongy material with an extraordinarily high specific surface area, such as activated carbon. Additionally, the ability of the electrode material to perform faradaic charge transfers enhances the total capacitance.
Structurally, pore sizes in carbons range from micropores (less than 2 nm) to mesopores (2-50 nm) but below macropores (greater than 50 nm). Pseudocapacitance requires micropores that are accessible only to de-solvated ions.
Generally the smaller the electrode's pores, the greater the capacitance and energy density. However, smaller pores increase (ESR) and decrease power density. Applications with high peak currents require larger pores and low internal losses, while applications requiring high energy density need small pores.
Electrodes for EDLCs
The most commonly used electrode material for supercapacitors is carbon in various manifestations such as activated carbon (AC), carbon fibre-cloth (AFC), carbide-derived carbon (CDC), carbon aerogel, graphite (graphene), and carbon nanotubes (CNTs).
Activated carbon (AC) was the first material chosen for EDLC electrodes. It has an electrical conductivity of 1,250 to 3,000 S/m, approximately 0.003% of metallic conductivity, but sufficient for supercapacitors.
Activated charcoal is an extremely porous form of carbon with a high specific surface area — a common approximation is that 1 gram (0.035 oz) (a pencil-eraser-sized amount) has a surface area of roughly 1,000 to 3,000 square metres (11,000 to 32,000 sq ft) — about the size of 4 to 12 tennis courts. It is typically a powder made of fine but "rough" particles. The bulk form used in electrodes is low-density with many pores, giving high double-layer capacitance.
Solid activated carbon, also termed consolidated amorphous carbon (CAC) is the most used electrode material for supercapacitors and may be cheaper than other carbon derivatives. It is produced from activated carbon powder pressed into the desired shape, forming a block with a wide distribution of pore sizes. An electrode with a surface area of about 1000 m2/g results in a typical double-layer capacitance of about 10 μF/cm2 and a specific capacitance of 100 F/g.
As of 2010[update] virtually all commercial supercapacitors use powdered activated carbon made from environmentally friendly coconut shells. Coconut shells produce activated carbon with more micropores than with charcoal from wood.
Activated carbon electrodes exhibit predominantly static double-layer capacitance, but also exhibit pseudocapacitance. Pores with diameters <2 nm are accessible only to de-solvated ions and enable faradaic reactions.
Carbon nanotubes offer 3x more surface area than activated carbon. Higher performance, higher cost components are available, based on synthetic carbon precursors that are activated with potassium hydroxide (KOH).
Activated carbon fibres
Activated carbon fibres (ACF) are produced from activated carbon and have a typical diameter of 10 µm. They can have micropores with a very narrow pore-size distribution that can be readily controlled. The surface area of AFC woven into a textile is about 2500 m2/g. Advantages of AFC electrodes include low electrical resistance along the fibre axis and good contact to the collector.
AFC electrodes exhibit predominantly double-layer capacitance with a small amount of pseudocapacitance due to their micropores.
Aerogel electrodes are made via pyrolysis of resorcinol formaldehyde aerogels. Carbon aerogels are more conductive than most activated carbons. They enable thin and mechanically stable electrodes with a thickness in the range of several hundred micrometers (µm) and with uniform pore size. It also exhibits mechanical and vibration stability for supercapacitors in high vibration applications.
Researchers have created a carbon aerogel electrode with gravimetric densities of about 400–1200 m2/g and specific capacitance of 104 F/cm3, yielding an energy density of 325 J/g (325 kJ/kg, 90 W•h/kg) and power density of 20 W/g.
Carbide-derived carbon (CDC), also known as tunable nanoporous carbon, is a family of carbon materials derived from carbide precursors, such as binary silicon carbide and titanium carbide, that are transformed into pure carbon via physical (e.g., thermal decomposition) or chemical (e.g., halogenation) processes.
Carbide-derived carbons can exhibit high surface area and tunable pore diameters to maximize ion confinement, increasing pseudocapacitance by faradaic H
2 adsorption treatment. Structurally, CDC pore sizes range from micropores to mesopores. Capacitance may be increased by using micropores. Sub-1 nm pores contribute to capacitance even if the solvated ions are larger. This capacitance increase is explained by the distortion of the ion-solvating shell. As pore size approaches the solvation shell size, solvent molecules are excluded and de-solvated ions fill the pores, increasing ionic packing density and storage capability by faradaic H
2 intercalation. CDC electrodes with tailored pore design offer as much as 75% greater energy density than conventional activated carbons.
In 2013 a CDC supercapacitor offered an energy density of 8.3 Wh/kg (29.88 kJ/kg) having 4,000 F capacitance and one million charge/discharge cycles.
Graphene has a surface area of 2630 m2/g which can lead theoretically to a capacitor of 550 F/g. Its important advantage is high conductivity >1700 S/m compared to activated carbon (10 to 100 S/m). As of 2012[update] a new development used graphene sheets directly as electrodes without collectors for portable applications.
One graphene-based supercapacitor uses curved graphene sheets that do not stack face-to-face, forming mesopores that are accessible to and wettable by environmentally friendly ionic electrolytes at voltages up to 4 V. They have a specific energy density of 85.6 W·h/kg (308 kJ/kg) at room temperature equaling that of a conventional nickel metal hydride battery, but with 100-1000 times greater power density.
The two-dimensional structure of graphene improves charging and discharging. Charge carriers in vertically oriented sheets can quickly migrate into or out of the deeper structures of the electrode, thus increasing currents. Such capacitors may be suitable for 100/120 Hz filter applications, which are unreachable for supercapacitors using other carbons.
Carbon nanotubes (CNTs), also called buckytubes, are carbon molecules with a cylindrical nanostructure. They have a hollow structure with walls formed by one-atom-thick sheets of graphene. These sheets are rolled at specific and discrete ("chiral") angles, and the combination of chiral angle and radius controls properties such as electrical conductivity, electrolyte wettability and ion access. Nanotubes are categorized as single-walled nanotubes (SWNTs) or multi-walled nanotubes (MWNTs). The latter have one or more outer tubes successively enveloping a SWNT, much like the Russian matroyska dolls. SWNTs have diameters ranging between 1 and 3 nm. MWNTs have thicker coaxial walls, separated by spacing (0.34 nm) that is close to graphene's interlayer distance.
Nanotubes can grow vertically on the collector substrate, such as a silicon wafer. Typical lengths are 20 to 100 µm.
CNTs can store about the same charge as activated carbon per unit surface area, but nanotubes' surface is arranged in a regular pattern, providing greater wettability. SWNTs have a high theoretical specific surface area of 1315 m2/g, while MWNTs' SSA is lower and is determined by the diameter of the tubes and degree of nesting, compared with a surface area of about 3000 m2/g of activated carbons. Nevertheless, CNTs have higher capacitance than activated carbon electrodes, e.g., 102 F/g for MWNTs and 180 F/g for SWNTs.
MWNTs have mesopores that allow for easy access of ions at the electrode/electrolyte interface. As the pore size approaches the size of the ion solvation shell, the solvent molecules are partially stripped, resulting in larger ionic packing density and increased faradaic storage capability. However, the considerable volume change during repeated intercalation and depletion decreases their mechanical stability. To this end, research to increase surface area, mechanical strength, electrical conductivity and chemical stability is ongoing.
Electrodes for pseudocapacitors
Pseudocapacitance with faradaic charge transfer also is always present in carbon double-layer electrodes, but the amount of pseudocapacitance in EDLC electrodes is relatively low. Pseudocapacitance electrodes have surfaces able to achieve sufficient faradaic processes to have a majority of pseudocapacitance. Pseudocapacitance electrodes without double-layer capacitance do not exist.
B. E. Conway's research described electrodes of transition metal oxides that exhibited high amounts of pseudocapacitance. Oxides of transition metals including ruthenium (RuO
2), iridium (IrO
2), iron (Fe
4), manganese (MnO
2) or sulfides such as titanium sulfide (TiS
2) alone or in combination generate strong faradaic electron–transferring reactions combined with low resistance. Ruthenium dioxide in combination with H
4 electrolyte provides specific capacitance of 720 F/g and a high energy density of 26.7 Wh/kg (96.12 kJ/kg).
Charge/discharge takes place over a window of about 1.2 V per electrode. This pseudocapacitance of about 720 F/g is roughly 100 times higher than for double-layer capacitance using activated carbon electrodes. These transition metal electrodes offer excellent reversibility, with several hundred-thousand cycles. However, ruthenium is expensive and the 2.4 V voltage window for this capacitor limits their applications to military and space applications.
In 2014 a RuO
2 supercapacitor anchored on a graphene foam electrode delivered specific capacitance of 502.78 F g−1 and areal capacitance of 1.11 F cm−2) leading to an energy density of 39.28 Wh/kg and power density of 128.01 kW/kg over 8,000 cycles with constant performance. The device was a three-dimensional (3D) sub-5 nm hydrous ruthenium-anchored graphene and carbon nanotube (CNT) hybrid foam (RGM) architecture. The graphene foam was conformally covered with hybrid networks of RuO
2 nanoparticles and anchored CNTs.
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 (MnO
2). However, none of these oxides are in commercial use.
Another approach uses electron-conducting polymers as pseudocapacitive material. Although mechanically weak, conductive polymers have high conductivity, resulting in a low ESR and a relatively high capacitance. Such conducting polymers include polyaniline, polythiophene, polypyrrole and polyacetylene. Such electrodes employ also electrochemical doping or dedoping of the polymers with anions and cations. Electrodes out of or coated with conductive polymers are cost comparable to carbon electrodes.
Electrodes for hybrid capacitors
All commercial hybrid supercapacitors are asymmetric. They combine an electrode with high amount of pseudocapacitance with an electrode with a high amount of double-layer capacitance. In such systems the faradaic pseudocapacitance electrode with their higher capacitance provides high energy density while the non-faradaic EDLC electrode enables high power density. An advantage of the hybride-type supercapacitors compared with symmetrical EDLC’s is their higher specific capacitance value as well as their higher rated voltage and correspondingly their higher specific energy.
Composite electrodes for hybrid-type supercapacitors are constructed from carbon-based material with incorporated or deposited pseudocapacitive active materials like metal oxides and conducting polymers. As of 2013[update] most research for supercapacitors explores composite electrodes.
CNTs give a backbone for a homogeneous distribution of metal oxide or electrically conducting polymers (ECPs), producing good pseudocapacitance and good double-layer capacitance. These electrodes achieve higher capacitances than either pure carbon or pure metal oxide or polymer-based electrodes. This is attributed to the accessibility of the nanotubes' tangled mat structure, which allows a uniform coating of pseudocapacitive materials and three-dimensional charge distribution.
Another way to enhance CNT electrodes is by doping with a pseudocapacitive dopant as in lithium-ion capacitors. In this case the relatively small lithium atoms intercalate between the layers of carbon. The anode is made of lithium-doped carbon, which enables lower negative potential with a cathode made of activated carbon. This results in a larger voltage of 3.8-4 V that prevents electrolyte oxidation. As of 2007 they had achieved capacitance of 550 F/g. and reach an energy density up to 14 Wh/kg (50.4 kJ/kg).
Rechargeable battery electrodes influenced the development of electrodes for new hybrid-type supercapacitor electrodes as for lithium-ion capacitors. Together with an carbon EDLC electrode in an asymmetric construction offers this configuration higher energy density than typical supercapacitors with higher power density, longer cycle life and faster charging and recharging times than batteries.
While their structure qualifies them as composite electrodes, they are typically placed in the category of 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 oxide electrode (not a composite electrode), and the negative electrode on an EDLC activated carbon electrode.
An advantage of this type of supercapacitors is their higher voltage and correspondingly their higher specific energy (up to 10-20 Wh/kg (36-72 kJ/kg)).
As far as known no commercial offered supercapacitors with such kind of asymmetric electrodes are on the market.
Electrolytes consist of a solvent and dissolved chemicals that dissociate into positive cations and negative anions, making the electrolyte electrically conductive. The more ions the electrolyte contains, the better its conductivity. In supercapacitors electrolytes are the electrically conductive connection between the two electrodes. Additionally, in supercapacitors the electrolyte provides the molecules for the separating monolayer in the Helmholtz double-layer and delivers the ions for pseudocapacitance.
The electrolyte determines the capacitor's characteristics: its operating voltage, temperature range, ESR and capacitance. With the same activated carbon electrode an aqueous electrolyte achieves capacitance values of 160 F/g, while an organic electrolyte achieves only 100 F/g.
The electrolyte must be chemically inert and not chemically attack the other materials in the capacitor to ensure long time stable behavior of the capacitor’s electrical parameters. The electrolyte's viscosity must be low enough to wet the porous, sponge-like structure of the electrodes. An ideal electrolyte does not exist, forcing a compromise between performance and other requirements.
Water is a relatively good solvent for inorganic chemicals. Treated with acids such as sulfuric acid (H
4), alkalis such as potassium hydroxide (KOH), or salts such as quaternary phosphonium salts, sodium perchlorate (NaClO
4), lithium perchlorate (LiClO
4) or lithium hexafluoride arsenate (LiAsF
6), water offers relatively high conductivity values of about 100 to 1000 mS/cm. Aqueous electrolytes have a dissociation voltage of 1.15 V per electrode (2,3 V capacitor voltage) and a relatively low operating temperature range. They are used in supercapacitors with low energy density and 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 tetraethylammonium tetrafluoroborate (N(Et)
4) or triethyl (metyl) tetrafluoroborate (NMe(Et)
4) are more expensive than aqueous electrolytes, but they have a higher dissociation voltage of typically 1.35 V per electrode (2,7 V capacitor voltage), and a higher temperature range. The lower electrical conductivity of organic solvents (10 to 60 S/cm) leads to a lower power density, but since the energy density increases with the square of the voltage, a higher energy density.
Separators have to physically separate the two electrodes to prevent a short circuit by direct contact. It can be very thin (a few hundredths of a millimeter) and must be very porous to the conducting ions to minimize ESR. Furthermore, separators must be chemically inert to protect the electrolyte's stability and conductivity. Inexpensive components use open capacitor papers. More sophisticated designs use nonwoven porous polymeric films like polyacrylonitrile or Kapton, woven glass fibers or porous woven ceramic fibres.
Collectors and housing
Current collectors connect the electrodes to the capacitor’s terminals. The collector is either sprayed onto the electrode or is a metal foil. They must be able to distribute peak currents of up to 100 A.
If the housing is made out of a metal (typically aluminum) the collectors should be made from the same material to avoid forming a corrosive galvanic cell.
Capacitance values for commercial capacitors are specified as "rated capacitance CR". This is the value for which the capacitor has been designed. The value for an actual component must be within the limits given by the specified tolerance. Typical values are in the range of farads (F), three to six orders of magnitude larger than those of electrolytic capacitors.
The capacitance value results from the energy W of a loaded capacitor loaded via a DC voltage VDC.
This value is also called the "DC capacitance".
Conventional capacitors are normally measured with a small AC voltage (0.5 V) and a frequency of 100 Hz or 1 kHz depending on the capacitor type. The AC capacitance measurement offers fast results, important for industrial production lines. The capacitance value of a supercapacitor depends strongly on the measurement frequency, which is related to the porous electrode structure and thelimited electrolyte's ion mobility. Even at a low frequency of 10 Hz, the measured capacitance value drops from 100 to 20 percent of the DC capacitance value.
This extraordinary strong frequency dependence can be explained by the different distances the ions have to move in the electrode's pores. The area at the beginning of the pores can easily be accessed by the ions. The short distance is accompanied by low electrical resistance. The greater the distance the ions have to cover, the higher the resistance. This phenomenon can be described with a series circuit of cascaded RC (resistor/capacitor) elements with serial RC time constants. These result in delayed current flow, reducing the total electrode surface area that can be covered with ions if polarity changes – capacitance decreases with increasing AC frequency. Thus, the total capacitance is only achieved after longer measuring times.
Out of the reason of the very strong frequency dependence of the capacitance this electrical parameter has to be measured with a special constant current charge and discharge measurement, defined in IEC standards 62391-1 and -2.
Measurement starts with charging the capacitor. The voltage has to be applied and after the constant current/constant voltage power supply has achieved the rated voltage, the capacitor has to be charged for 30 minutes. Next, the capacitor has to be discharged with a constant discharge current Idischarge. Than the time t1 and t2, for the voltage to drop from 80% (V1) to 40% (V2) of the rated voltage is measured. The capacitance value is calculated as:
The value of the discharge current is determined by the application. The IEC standard defines four classes:
- 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 standardized measuring method is too time consuming for manufacturers to use during production for each individual component. For industrial produced capacitors the capacitance value is instead measured with a faster low frequency AC voltage and a correlation factor is used to compute the rated capacitance.
This frequency dependence affects capacitor operation. Rapid charge and discharge cycles mean that neither the rated capacitance value nor energy density are available. In this case the rated capacitance value is recalculated for each application condition.
Supercapacitors are low voltage components. Safe operation requires that the voltage remain within specified limits. The rated voltage UR is the maximum DC voltage or peak pulse voltage that may be applied continuously and remain within the specified temperature range. Capacitors should never be subjected to voltages continuously in excess of the rated voltage.
The rated voltage includes a safety margin against the electrolyte's breakdown voltage at which the electrolyte decomposes. The breakdown voltage decomposes the separating solvent molecules in the Helmholtz double-layer, f. e. water splits into hydrogen and oxide. The solvent molecules then cannot separate the electrical charges from each other. Higher voltages than rated voltage cause hydrogen gas formation or a short circuit.
Standard supercapacitors with aqueous electrolyte normally are specified with a rated voltage of 2.1 to 2.3 V and capacitors with organic solvents with 2.5 to 2.7 V. Lithium-ion capacitors with doped electrodes may reach a rated voltage of 3.8 to 4 V, but have a lower voltage limit of about 2.2 V.
Operating supercapacitors below the rated voltage improves the long-time behavior of the electrical parameters. Capacitance values and internal resistance during cycling are more stable and lifetime and charge/discharge cycles may be extended.
Supercapacitors rated voltages are generally lower than applications require. Higher application voltages require connecting cells in series. Since each component has a slight difference in capacitance value and ESR, it is necessary to actively or passively balance them to stabilize the applied voltage. Passive balancing employs resistors in parallel with the supercapacitors. Active balancing may include electronic voltage management above a threshold that varies the current.
Charging/discharging a supercapacitor is connected to the movement of charge carriers (ions) in the electrolyte across the separator to the electrodes and into their porous structure. Losses occur during this movement that can be measured as the internal DC resistance.
With the electrical model of cascaded, series-connected RC (resistor/capacitor) elements in the electrode pores, the internal resistance increases with the increasing penetration depth of the charge carriers into the pores. The internal DC resistance is time dependent and increases during charge/discharge. In applications often only the switch-on and switch-off range is interesting. The internal resistance Ri can be calculated from the voltage drop ΔV2 at the time of discharge, starting 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). Resistance can be calculated by:
The discharge current Idischarge for the measurement of internal resistance can be taken from the classification according to IEC 62391-1.
This internal DC resistance Ri should not be confused with the internal AC resistance called Equivalent Series Resistance (ESR) normally specified for capacitors. It is measured at 1 kHz. ESR is much smaller than DC resistance. ESR is not relevant for calculating superconductor inrush currents or other peak currents.
Ri determines several supercapacitor properties. It limits the charge and discharge peak currents as well as charge/discharge times. Ri and the capacitance C results in the time constant
This time constant determines the charge/discharge time. A 100 F capacitor with an internal resistance of 30 mΩ for example, has a time constant of 0.03 • 100 = 3 s. After 3 seconds charging with a current limited only by internal resistance, the capacitor has 62.3% of full charge (or is discharged to 36.8% of full charge).
Standard capacitors with constant internal resistance fully charge during about 5 τ. Since internal resistance increases with charge/discharge, actual times cannot be calculated with this formula. Thus, charge/discharge time depends on specific individual construction details.
Current load and cycle stability
Because supercapacitors operate without forming chemical bonds, current loads, including charge, discharge and peak currents are not limited by reaction constraints. Current load and cycle stability can be much higher than for rechargeable batteries. Current loads are limited only by internal resistance, which may be substantially lower than for batteries.
Internal resistance "Ri" and charge/discharge currents or peak currents "I" generate internal heat losses "Ploss" according to:
This heat must be released and distributed to the ambient environment to maintain operating temperatures below the specified maximum temperature.
Heat generally defines capacitor lifetime because of electrolyte diffusion. The heat generation coming from current loads should be smaller than 5 to 10 K at maximum ambient temperature (which has only minor influence on expected lifetime). For that reason the specified charge and discharge currents for frequent cycling are determined by internal resistance.
The specified cycle parameters under maximal conditions include charge and discharge current, pulse duration and frequency. They are specified for a defined temperature range and over the full voltage range for a defined lifetime. They can differ enormously depending on the combination of electrode porosity, pore size and electrolyte. Generally a lower current load increases capacitor life and increases the number of cycles. This can be achieved either by a lower voltage range or slower charging and discharging.
Supercapacitors (except those with polymer electrodes) can potentially support more than one million charge/discharge cycles without substantial capacity drops or internal resistance increases. Beneath the higher current load is this the second great advantage of supercapacitors over batteries. The stability results from the dual electrostatic and electrochemical storage principles.
The specified charge and discharge currents can be significantly exceeded by lowering the frequency or by single pulses. Heat generated by a single pulse may be spread over the time until the next pulse occurs to ensure a relatively small average heat increase. Such a "peak power current" for power applications for supercapacitors of more than 1000 F can provide a maximum peak current of about 1000 A. Such high currents generate high thermal stress and high electromagnetic forces that can damage the electrode-collector connection requiring robust design and construction of the capacitors.
Energy density and power density
Supercapacitors occupy the gap between high power/low energy electrolytic capacitors and low power/high energy rechargeable batteries. The amount of energy stored in a supercapacitor is called specific energy. The energy Wmax of a capacitor is given by the formula
This formula describes the amount of energy stored and is often used to describe new research successes. However, only part of the stored energy is available to applications, because the voltage drop and the time constant over the internal resistance mean that some of the stored charge is inaccessible. The effective realized amount of energy Weff is reduced by the used voltage difference between Vmax and Vmin and can be represented as:
This formula also represents the energy asymmetric voltage components such as lithium ion capacitors.
As of 2013[update] commercial gravimetric energy densities range from around 0.5 to 15 Wh/kg. For comparison, an aluminum electrolytic capacitor stores typically 0.01 to 0.3 Wh/kg, while a conventional lead-acid battery stores typically 30 to 40 Wh/kg and modern lithium-ion batteries 100 to 265 Wh/kg. Supercapacitors can therefore store 10 to 100 times more energy than electrolytic capacitors, but only one tenth as much as batteries.
Although the energy densities of supercapacitors are insufficient compared with batteries the capacitors have an important advantage, the power density. Power density describes the speed at which energy can be delivered to/absorbed from the load. The maximum power Pmax is given by the formula:
with V = voltage applied and Ri, the internal DC resistance.
Power density is measured either gravimetrically in kilowatts per kilogram (kW/kg) or volumetrically in kilowatts per litre (kW/l).
The described maximum power Pmax specifies the power of a theoretical rectangular single maximum current peak of a given voltage. In real circuits the current peak is not rectangular and the voltage is smaller, caused by the voltage drop. IEC 62391–2 established a more realistic effective power Peff for supercapacitors for power applications:
Supercapacitor power density is typically 10 to 100 times greater than for batteries and can reach values up to 15 kW/kg.
Ragone charts relate energy to power and are a valuable tool for characterizing and visualizing energy storage components. With such a diagram, the position of power density and energy density of different storage technologies is easily to compare, see diagram.
Supercapacitors exhibit a much longer lifetime than batteries. Since supercapacitors do not rely on chemical changes in the electrodes (except for those with polymer electrodes) lifetimes depend mostly on the rate of evaporation of the liquid electrolyte. This evaporation in general is a function of temperature, of current load, current cycle frequency and voltage. Current load and cycle frequency generate internal heat, so that the evaporation-determining temperature is the sum of ambient and internal heat. This temperature is measurable as core temperature in the center of a capacitor body. The higher the core temperature the faster the evaporation and the shorter the lifetime.
Evaporation generally results in decreasing capacitance and increasing internal resistance. According to IEC/EN 62391-2 capacitance reductions of over 30% or internal resistance exceeding four times its data sheet specifications are considered "wear-out failures", implying that the component has reached end-of-life. The capacitors are operable, but with reduced capabilities. It depends on the application of the capacitors, whether the aberration of the parameters have any influence on the proper functionality or not.
Such large changes of electrical parameters specified in IEC/EN 62391-2 are usually unacceptable for high current load applications. Components that support high current loads use much smaller limits, e.g., 20% loss of capacitance or double the internal resistance. The narrower definition is important for such applications, since heat increases linearly with increasing internal resistance and the maximum temperature should not be exceeded. Temperatures higher than specified can destroy the capacitor.
The real application lifetime of supercapacitors, also called "service life", "life expectancy" or "load life", can reach 10 to 15 years or more at room temperature. Such long periods cannot be tested by manufacturers. Hence, they specify the expected capacitor lifetime at the maximum temperature and voltage conditions. The results are specified in datasheets using the notation "tested time (hours)/max. temperature (°C)", such as "5000 h/65 °C". With this value and a formula, lifetimes can be estimated for lower conditions.
Datasheet lifetime specification is tested by the manufactures using an accelerated aging test called "endurance test" with maximum temperature and voltage over a specified time. For a "zero defect" product policy during this test no wear out or total failure may occur.
The lifetime specification from datasheets can be used for estimation of expected lifetime according to conditions coming from the application. The "10-degrees-rule" used for electrolytic capacitors with non-solid electrolyte is used for those estimations and can be used for supercapacitors, too. This rule employs the Arrhenius equation, a simple formula for the temperature dependence of reaction rates. For every 10 °C reduction in operating temperature, the estimated life doubles.
- Lx = estimated lifetime
- L0 = specified lifetime
- T0 = upper specified capacitor temperature
- Tx = actual operating temperature of the capacitor cell
Calculated with this formula, capacitors specified with 5000 h at 65 °C, have an estimated lifetime of 20,000 h at 45 °C.
Lifetimes are also dependent on the operating voltage, because the development of gas in the liquid electrolyte depends on the voltage. The lower the voltage the smaller the gas development and the longer the lifetime. No general formula relates voltage to lifetime. The voltage dependent curves shown from the picture are an empirical result from one manufacturer.
Life expectancy for power applications may be also limited by current load or number of cycles. This limitation has to be specified by the relevant manufacturer and is strongly type dependent.
Storing electrical energy in the double-layer separates the charge carriers by distance within the pores by distances in the range of molecules. Over this short distance irregularities can occur, leading to a small exchange of charge carriers and gradual discharge. This self-discharge is called leakage current. Leakage depends on capacitance, voltage, temperature and the chemical stability of the electrode/electrolyte combination. At room temperature leakage is so low that it is specified as time to self-discharge. Supercapacitor self-discharge time is specified in hours, days or weeks. As an example, a 5.5 V/1 F Panasonic "Goldcapacitor" specifies 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.
Although the anode and cathode of symmetric supercapacitors consist of the same material, theoretically supercapacitors have no true polarity. Normally catastrophic failure does not occur, however reverse-charging a supercapacitor lowers its capacity. It is recommended practice to maintain the polarity resulting from a formation of the electrodes during production. Asymmetric supercapacitors are inherently polar.
Supercapacitors may not be operated with reverse polarity, precluding AC operation.
A bar in the insulating sleeve identifies the cathode terminal in a polarized component.
The terms "anode" and "cathode" can lead to confusion, because the polarity changes depending on whether a component is considered as a generator or as a consumer. For an accumulator or a battery the cathode has a positive polarity (+) and the anode has negative polarity (-). For capacitors the cathode has negative polarity (-) and the anode has positive polarity (+). This requires special attention if supercapacitors are substituted or switched in parallel with batteries.
Comparison of technical parameters
Mixing electrodes and electrolytes yields a variety of components suitable for diverse applications. The development of low-ohmic electrolyte systems, in combination with electrodes with high pseudocapacitance, enable many more technical solutions.
The following table shows differences among capacitors of various manufacturers in capacitance range, cell voltage, internal resistance (ESR, DC or AC value) and volumetric and gravimetric energy density.
In the table, ESR refers to the component with the largest capacitance value of the respective manufacturer. Roughly, they divide supercapacitors into two groups. The first group offers greater ESR values of about 20 milliohms and relatively small capacitance of 0.1 to 470 F. These are "double-layer capacitors" for memory back-up or similar applications. The second group offers 100 to 10,000 F with a significantly lower ESR value under 1 milliohm. These components are suitable for power applications. A correlation of some supercapacitor series of different manufacturers to the various construction features is provided in Pandolfo and Hollenkamp.
|AVX||BestCap||0.068…0.56||3.6||-||0.13||-||Modules up to 16 V|
|Elton||Supercapacitor||1800…10000||1.5||0.5||6.8||4.2||Modules up to 29 V|
|HCC||HCAP||0.22…5000||2.7||15||10.6||-||Modules up to 45 V|
|Korchip||STARCAP||0.01…400||2.7||12||7.0||6.1||Modules up to 50 V|
|LS Mtron||Ultracapacitor||100…3000||2.8||0.25||6.0||5.9||Modules up to 84 V|
|Maxwell||Boostcap||10…3000||2..2…2.7||0.29||7.8||6.0||Modules up to 125 V|
|Modules up to 125 V|
|NCC, ECC||DLCCAP||350…2300||2.5||1.2||5.9||4.1||Modules up to 15 V|
|Samwha||Green-Cap||3…3000||2.7||0.28||7.7||5.6||Modules up to 125 V|
|Taiyo Yuden||PAS Capacitor
|WIMA||SuperCap||12…6500||2.5…2.7||0.18||5.2||4.3||Modules up to 112 V|
|Footnote: Volumetric and gravimetric energy density calculated by maximum capacitance, related voltage and dimensions if not specified in the datasheet|
Parametric comparison of technologies
Supercapacitors compete with electrolytic capacitors and rechargeable batteries especially lithium-ion batteries. The following table compares the major parameters of the three main supercapacitor families with electrolytic capacitors and batteries.
|−40 to 125||−20 to +70||−20 to +70||−20 to +70||−20 to +60|
|4 to 550||1.2 to 3.3||2.2 to 3.3||2.2 to 3.8||2.5 to 4.2|
|unlimited||105 to 106||105 to 106||2 • 104 to 105||500 to 104|
|≤ 1||0.1 to 470||100 to 12000||300 to 3300||—|
|0.01 to 0.3||1.5 to 3.9||4 to 9||10 to 15||100 to 265|
|> 100||2 to 10||3 to 10||3 to 14||0.3 to 1.5|
|Self discharge time
at room temperature
at room temperature
|> 20||5 to 10||5 to 10||5 to 10||3 to 5|
Electrolytic capacitors feature unlimited charge/discharge cycles, high dielectric strength (up to 550 V) and good frequency response as AC resistance in the lower frequency range. Supercapacitors can store 10 to 100 times more energy than electrolytic capacitors but they do not support AC applications.
With regards to rechargeable batteries supercapacitors feature higher peak currents, low cost per cycle, no danger of overcharging, good reversibility, non-corrosive electrolyte and low material toxicity, while batteries offer, lower purchase cost, stable voltage under discharge,but they require complex electronic control and switching equipment, with consequent energy loss and spark hazard given a short.
Supercapacitors vary sufficiently that they are rarely interchangeable, especially those with higher energy densities. Applications range from low to high peak currents, requiring standardized test protocols.
Test specifications and parameter requirements are specified in the generic specification
The standard defines four application classes, according to discharge current levels:
- Class 1: Memory backup
- Class 2: Energy storage, mainly used for driving motors require a short time operation,
- Class 3: Power, higher power demand for a long time operation,
- Class 4: Instantaneous power, for applications that requires relatively high current units or peak currents ranging up to several hundreds of amperes even with a short operating time
Three further standards describe special applications:
- IEC 62391–2, Fixed electric double-layer capacitors for use in electronic equipment - Blank detail specification - Electric double-layer capacitors for power application -
- IEC 62576, Electric double-layer capacitors for use in hybrid electric vehicles. Test methods for electrical characteristics
- BS/EN 61881-3, Railway applications. Rolling stock equipment. Capacitors for power electronics. Electric double-layer capacitors
Supercapacitors do not support AC applications.
Supercapacitors have advantages in applications where a large amount of power is needed for a relatively short time, where a very high number of charge/discharge cycles or a longer lifetime is required. Typical applications range from milliamp currents or milliwatts of power for up to a few minutes to several amps current or several hundred kilowatts power for much shorter periods.
The time t a supercapacitor can deliver a constant current I can be calculated as:
as the capacitor voltage decreases from Ucharge down to Umin.
If the application needs a constant power P for a certain time t this can be calculated as:
wherein also the capacitor voltage decreases from Ucharge down to Umin.
A cordless electric screwdriver with supercapacitors for energy storage has about half the run time of a comparable battery model, but can be fully charged in 90 seconds. It retains 85% of its charge after three months left idle.
Supercapacitors provide backup or emergency shutdown power to low-power equipment such as RAM, SRAM, micro-controllers and PC Cards. They are the sole power source for low energy applications such as automated meter reading (AMR) equipment or for event notification in industrial electronics.
Supercapacitors buffer power to and from rechargeable batteries, mitigating the effects of short power interruptions and high current peaks. Batteries kick in only during extended interruptions, e.g., if the mains power or a fuel cell fails, which lengthens battery life.
Typical industrial applications for such circuits are uninterruptible power supplies (UPS), where supercapacitors have replaced much larger banks of electrolytic capacitors. This combination reduces the cost per cycle, saves on replacement and maintenance costs, enables the battery to be downsized and extends battery life. A disadvantage is the need for a special circuit to reconcile the differing behaviors.
Supercapacitors are used as battery replacement in some pit trains in China to substitute conventional trolleys in coal mines. They bring coal to the surface. This approach removes a fire and safety hazard from coal mines coming from the batteries. The capacitors can be charged at the surface in less than 30 minutes.
Supercapacitors can stabilize voltage for powerlines. Wind and photovoltaic systems exhibit fluctuating loads evoked by clouds that supercapacitors can buffer within milliseconds. This helps stabilize grid voltage and frequency, balance supply and demand of power and manage real or reactive power.
Supercapacitors are suitable temporary energy storage devices for energy harvesting systems. In energy harvesting systems the energy is collected from the ambient or renewable sources, e.g. mechanical movement, light or electromagnetic fields, and converted to electrical energy in an energy storage device. For example, it was demonstrated that energy collected from RF (radio frequency) fields (using an RF antenna as an appropriate rectifier circuit) can be stored to a printed supercapacitor. The harvested energy was then used to power an application-specific integrated circuit (ASIC) circuit for over 10 hours. 
Sado City, in Japan's Niigata Prefecture, has street lights that combine a stand-alone power source with solar cells and LEDs. Supercapacitors store the solar energy and supply 2 LED lamps, providing 15 W power consumption overnight. The supercapacitors can last more than 10 years and offer stable performance under various weather conditions, including temperatures from +40 to below -20 °C.
In Zell am See, Austria, an aerial lift connects the city with Schmittenhöhe mountain. The gondolas sometimes run 24 hours per day, using electricity for lights, door opening and communication. The only available time for recharging batteries at the stations is during the brief intervals of guest loading and unloading, which is too short to recharge batteries. Supercapacitors offer a fast charge, higher number of cycles and longer life time than batteries.
Emirates Air Line (cable car), also known as the Thames cable car, is a 1-kilometre (0.62 mi) gondola line that crosses the Thames from the Greenwich Peninsula to the Royal Docks. The cabins are equipped with a modern infotainment system, which is powered by supercapacitors.
In 2005, aerospace systems and controls company Diehl Luftfahrt Elektronik GmbH chose supercapacitors to power emergency actuators for doors and evacuation slides used in airliners, including the Airbus 380.
Supercapacitors' low internal resistance supports applications that require short-term high currents. Among the earliest uses were motor startup (cold diesel engine start) for large engines in tanks and submarines. Supercapacitors buffer the battery, handling short current peaks and reducing cycling. Further military applications that require high power density are phased array radar antennae, laser power supplies, military radio communications, avionics displays and instrumentation, backup power for airbag deployment and GPS-guided missiles and projectiles.
A primary challenge of all transport is reducing energy consumption and reducing CO
2 emissions. Recovery of braking energy (recuperation or regeneration) helps with both. This requires components that can quickly store and release energy over long times with a high cycle rate. Supercapacitors fulfill these requirements and are therefore used in a lot of applications in all kinds of transportation.
Supercapacitors can be used to supplement batteries in starter systems in diesel railroad locomotives with diesel-electric transmission. The capacitors capture the braking energy of a full stop and deliver the peak current for starting the diesel engine and acceleration of the train and ensures the stabilization of catenary voltage. Depending on the driving mode up to 30% energy saving is possible by recovery of braking energy. Low maintenance and environmentally friendly materials encouraged the choice of supercapacitors.
Cranes, forklifts and tractors
Mobile hybrid diesel-electric rubber tyred gantry cranes move and stack containers within a terminal. Lifting the boxes requires large amounts of energy. Some of the energy could be recaptured while lowering the load resulting in improved efficiency.
A triple hybrid forklift truck uses fuel cells and batteries as primary energy storage and supercapacitors to buffer power peaks by storing braking energy. They provide the fork lift with peak power over 30 kW. The triple-hybrid system offers over 50% energy savings compared with diesel or fuel-cell systems.
Light-rails and trams
Supercapacitors make it possible not only to reduce energy but additional to do away catenary overhead lines in historical city areas preserving the city’s architectural heritage. This approach may allow many new LRV city lines to serve catenary overhead wires that are too expensive to fully route installation.
In 2003 Mannheim, adopted a prototype light-rail vehicle (LRV) using the MITRAC Energy Saver system from Bombardier Transportation to store mechanical braking energy with a roof-mounted supercapacitor unit. It contains several units each made of 192 capacitors with 2700 F /2.7 V interconnected in three parallel lines. This circuit results in a 518 V system with an energy content of 1.5 kWh. For acceleration when starting this "on-board-system" can provided the LRV with 600 kW and can drive the vehicle up to 1 km without catenary supply integrating the LRV into the urban environment by driving without catenary lines. Compared to conventional LRVs or Metro vehicles that return energy into the grid, onboard energy storage saves up to 30% and reduces peak grid demand by up to 50%.
In 2009 supercapacitors enabled LRV's to operate in the historical city area of Heidelberg without catenary overhead wires preserving the city’s architectural heritage. The SC equipment cost an additional €270,000 per vehicle, which was expected to be recovered over the first 15 years of operation. The supercapacitors are charged at stop-over stations when the vehicle is at a scheduled stop. This approach may allow many LRV city lines to serve catenary overhead wires that are too expensive to fully route installation. In April 2011 German regional transport operator Rhein-Neckar, responsible for Heidelberg, ordered a further 11 units.
In 2009 in Paris a tram on route T3 operates with an energy recovery system of the manufacturer Alstom called "STEEM". The system is fitted with 48 roof-mounted supercapacitors to store braking energy provides tramways with a high level of energy autonomy by enabling them to run without catenary power on parts of its route, recharging while traveling on powered stop-over stations. During the tests, the tramset used an average of approximately 16% less energy.
Hong Kong's South Island metro line is to be equipped with two 2 MW energy storage units that are expected to reduce energy consumption by 10%.
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. The train can travel up 2 km without wires, recharging in 30 seconds at stations via a ground mounted pickup. The supplier claimed the trains could be used in 100 small and medium-sized Chinese cities.
In 2012, in Lyon (France), the SYTRAL (Lyon public transportation administration) started experiments of a "way side regeneration" system built by Adetel Group which has developed its own energy saver named ″NeoGreen″ for LRV, LRT and metros.
The first hybrid bus with supercapacitors in Europe came in 2001 in Nuremberg, Germany. It was MAN's so-called "Ultracapbus", and was tested in real operation in 2001/2002. The test vehicle was equipped with a diesel-electric drive in combination with supercapacitors. The system was supplied with 8 Ultracap modules of 80 V, each containing 36 components. The system worked with 640 V and could be charged/discharged at 400 A. Its energy content was 0.4 kWh with a weight of 400 kg.
The supercapacitors recaptured braking energy and delivered starting energy. Fuel consumption was reduced by 10 to 15% compared to conventional diesel vehicles. Other advantages included reduction of CO
2 emissions, quiet and emissions-free engine starts, lower vibration and reduced maintenance costs.
As of 2002[update] in Luzern, Switzerland an electric bus fleet called TOHYCO-Rider was tested. The supercapacitors could be recharged via an inductive contactless high-speed power charger after every transportation cycle, within 3 to 4 minutes.
In early 2005 Shanghai tested a new form of electric bus called capabus that runs without powerlines (catenary free operation) using large onboard supercapacitors that partially recharge whenever the bus is at a stop (under so-called electric umbrellas), and fully charge in the terminus. In 2006, two commercial bus routes began to use the capabuses; one of them is route 11 in Shanghai. It was estimated that the supercapacitor bus was cheaper than a lithium-ion battery bus, and one of its buses had one-tenth the energy cost of a diesel bus with lifetime fuel savings of $200,000.
A hybrid electric bus called tribrid was unveiled in 2008 by the University of Glamorgan, Wales, for use as student transport. It is powered by hydrogen fuel or solar cells, batteries and ultracapacitors.
The FIA, a governing body for motor racing events, proposed in the Power-Train Regulation Framework for Formula 1 version 1.3 of 23 May 2007 that a new set of power train regulations be issued that includes a hybrid drive of up to 200 kW input and output power using "superbatteries" made with batteries and supercapacitors connected in parallel (KERS). About 20% tank-to-wheel efficiency could be reached using the KERS system.
The Toyota TS030 Hybrid LMP1 car, a racing car developed under Le Mans Prototype rules, uses a hybrid drivetrain with supercapacitors. In the 2012 24 Hours of Le Mans race a TS030 qualified with a fastest lap only 1.055 seconds slower (3:24.842 versus 3:23.787) than the fastest car, an Audi R18 e-tron quattro with flywheel energy storage. The supercapacitor and flywheel components, whose rapid charge-discharge capabilities help in both braking and acceleration, made the Audi and Toyota hybrids the fastest cars in the race. In the 2012 Le Mans race the two competing TS030s, one of which was in the lead for part of the race, both retired for reasons unrelated to the supercapacitors. The TS030 won three of the 8 races in the 2012 FIA World Endurance Championship season.
Hybrid electric vehicles
Supercapacitor/battery combinations in electric vehicles (EV) and hybrid electric vehicles (HEV) are well investigated. A 20 to 60% fuel reduction has been claimed by recovering brake energy in EVs or HEVs. The ability of supercapacitors to charge much faster than batteries, their stable electrical properties, broader temperature range and longer lifetime are suitable, but weight, volume and especially cost mitigate those advantages.
Supercapacitors lower energy density makes them unsuitable for use as a stand-alone energy source for long distance driving. The fuel economy improvement between a capacitor and a battery solution is about 20% and is available only for shorter trips. For long distance driving the advantage decreases to 6%. Vehicles combining capacitors and batteries run only in experimental vehicles.
As of 2013[update] all automotive manufacturers of EV or HEVs have developed prototypes that uses supercapacitors instead of batteries to store braking energy in order to improve driveline efficiency. The Mazda 6 is the only production car that uses supercapacitors to recover braking energy. Branded as i-eloop, the regenerative braking is claimed to reduce fuel consumption by about 10%.
Russian Yo-cars Ё-mobile series is an ё-concept and ё-crossover hybrid vehicle working with a gas driven Wankel motor and an electric generator for driving. A supercapacitor with relatively low capacitance recovers brake energy to power the electric motor when accelerating from a stop.
As of 2013[update] commercially available lithium-ion supercapacitors offered the highest gravimetric energy density to date, reaching 15 Wh/kg (54 kJ/kg). Research focuses on improving energy density, reducing internal resistance, expanding temperature range, increasing lifetimes and reducing costs. Projects include nanostructured electrodes, tailored-pore-size electrodes, pseudocapacitive coating or doping materials and improved electrolytes.
|Graphene sheets compressed by capillary compression of a volatile liquid||2013||60 Wh/l||Subnanometer scale electrolyte integration created a continuous ion transport network.|
|Vertically aligned carbon nanotubes electrodes||2007
|13.50 Wh/kg||37.12 W/g||300,000||first realization|
|Curved graphene sheets||2010||85.6 Wh/kg||550 F/g||Single-layers of curved graphene sheets that do not restack face-to-face, forming mesopores that are accessible to and wettable by environmentally friendly ionic electrolytes at a voltage up to 4 V.|
|KOH restructured graphite oxide||2011||85 Wh/kg||>10,000||550 F/g||potassium hydroxide restructured the carbon to make a three dimensional porous network|
|Activated Graphene-Based Carbons as Supercapacitor Electrodes with Macro- and Mesopores||2013||74 Wh/kg||three-dimensional pore structures in graphene-derived carbons in which mesopores are integrated into macroporous scaffolds with a surface area of 3290 m2/g|
|Conjugated microporous polymer||2011||53 Wh/kg||10,000||Aza-fused π-conjugated microporous framework|
|SWNT composite electrode||2011||990 kW/kg||A tailored meso-macro pore structure held more electrolyte, ensuring facile ion transport|
|Nickel hydroxide nanoflake on CNT composite electrode||2012||50.6 Wh/kg||3300 F/g||Asymmetric supercapacitor using the Ni(OH)2/CNT/NF electrode as the anode assembled with an activated carbon (AC) cathode achieving a cell voltage of 1.8 V|
|Battery-electrode nanohybrid||2012||40 Wh/l||7.5 kW/l||10,000||Li
12 (LTO) deposited on carbon nanofibres (CNF) anode and an activated carbon cathode
|Nickel cobaltite deposited on mesoporous carbon aerogel||2012||53 Wh/kg||2.25 kW/kg||1700 F/g||Nickel cobaltite, a low cost and an environmentally friendly supercapacitive material|
|Manganese dioxide intercalated nanoflakes||2013||110 Wh/kg||1000 F/g||Wet electrochemical process intercalated Na(+) ions into MnO
2 interlayers. The nanoflake electrodes exhibit faster ionic diffusion with enhanced redox peaks.
|3D porous graphene electrode||2013||98 Wh/kg||231 F/g||Wrinkled single layer graphene sheets a few nanometers in size, with at least some covalent bonds.|
|Graphene-based planar micro-supercapacitors for on-chip energy storage||2013||2.42 Wh/l||On chip line filtering|
|Quantum nanoclusters of dipolar metal oxides in TiO
2 or TAO
Footnote: Research into electrode materials requires measurement of individual components, such as an electrode or half-cell. By using a counterelectrode that does not affect the measurements, the characteristics of only the electrode of interest can be revealed. Energy and power densities for real supercapacitors only have more or less roughly 1/3 of the electrode density.
The market for batteries (estimated by Frost & Sullivan) grew from US$47.5 billion, (76.4% or US$36.3 billion of which was rechargeable batteries) to US$95 billion. The market for supercapacitors is still a small niche market that is not keeping pace with its larger rival.
In 2012, NanoMarkets forecast sales to grow to US$3.5 billion by 2020, an increase of about 900% within 10 years. Assumptions underlying this growth include a rapidly improving price/performance ratio and evolving "green energy" applications, such as energy recovery in electric vehicles. Otherwise the market was forecast to grow about 30% overall between 2013 and 2018 and remain in the hundreds of millions of dollars.
Supercapacitor costs in 2006 were US$0.01 per farad or US$2.85 per kilojoule, moving in 2008 below US$0.01 per farad, and were expected to drop further in the medium term.
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