Supercapacitor

Maxwell Technologies supercapacitor products

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

The ratio of the storage resulting from each principle can vary greatly, depending on electrode design and electrolyte composition. 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 and asymmetric 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 conventional capacitors and rechargeable batteries. They have the highest available capacitance values per unit volume and the greatest energy density of all capacitors. They support up to 12,000 farads/1.2 volt,[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 greater. Power density is defined as the product of energy density, multiplied by the speed at which the energy is delivered to the load. The greater power density results in much shorter charge/discharge cycles than a battery is capable, and a greater tolerance for numerous charge/discharge cycles.

Within electrochemical capacitors, the electrolyte is the conductive connection between the two electrodes, distinguishing them from electrolytic capacitors, in which the electrolyte only forms the cathode, the second electrode.

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

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

• Low supply current during longer times 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 for vehicles and elevators

Exceptional for electronic components like capacitors 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 making it difficult for users to classify these capacitors.

History

Development of the double layer and pseudocapacitance model

Helmholtz

When a metal (or an electronic conductor) is brought in contact with a solid or liquid ionic-conductor (electrolyte), a common boundary (interface) among the two different phases originates. Helmholtz [7] was the first to realize that charged electrodes immersed in electrolytic solutions 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 in 1853 he showed that an electrical double layer (DL) that is essentially a molecular dielectric achieved electrostatic charge storage.[8] Below the electrolyte's decomposition voltage the stored charge is linearly dependent on the voltage applied.

This early Helmholtz model predicted a constant differential capacitance independent from the charge density depending on the dielectric constant of the solvent and the thickness of the double-layer.[5] [9][10] But this model, while a good foundation for the description of the interface, does not consider important factors including diffusion/mixing of ions in solution, the possibility of adsorption onto the surface and the interaction between solvent dipole moments and the electrode.

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

Gouy\Chapman

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. The "Gouy-Chapman model" made significant improvements by introducing a diffuse model of the DL. In this model the charge distribution of ions as a function of distance from the metal surface allows Maxwell–Boltzmann statistics to be applied. Thus the electric potential decreases exponentially away from the surface of the fluid bulk.[5][11]

Stern

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. In Stern's model, some of the ions adhere to the electrode as suggested by Helmholtz, giving an internal Stern layer and some form a Gouy-Chapman diffuse layer.[12]

The Stern layer accounted for ions' finite size and consequently ions have a closest approach to the electrode on the order of the ionic radius. The Stern model too had limitations, effectively modeling ions as point charges, assuming all significant interactions in the diffuse layer are Coulombic, assuming dielectric permittivity to be constant throughout the double layer, and that fluid viscosity is constant above the slipping plane.[13]

Grahame

Thus, D. C. Grahame modified Stern in 1947.[14] He proposed that some ionic or uncharged species can penetrate the Stern layer, although the closest approach to the electrode is normally occupied by solvent molecules. This could occur if ions lost their solvation shell when the ion approached the electrode. Ions in direct contact with the electrode were called "specifically adsorbed ions". This model proposed the existence of three regions. The inner Helmholtz plane (IHP) plane passing through the centres of the specifically adsorbed ions. The outer Helmholtz plane (OHP) passes through the centres of solvated ions at their distance of closest approach to the electrode. Finally the diffuse layer is the region 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

Bockris/Devanthan/Müller

In 1963 J. O'M. Bockris, M. A. V Devanthan, and K. Alex Müller[15] 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 electrode surface. This first layer of solvent molecules display a strong orientation to the electric field depending on the charge. This orientation has great influence on the permittivity of the solvent which varies with the field strength. The inner Helmholtz plane (IHP) passes through the centers of these molecules. Specifically adsorbed, partially solvated ions appear in this layer. The solvated ions of the electrolyte are outside the IHP. Through the centers of these ions pass a second plane, the outer Helmholtz plane (OHP). The region beyond the OHP is called the diffuse layer. The BDM model now is most commonly used.

Trasatti/Buzzanca

Further research with double layers on ruthenium dioxide films in 1971 by Sergio Trasatti and Giovanni Buzzanca 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.[9]

Conway

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

Between 1975 and 1980 Brian Evans Conway conducted extensive fundamental and development work on the ruthenium oxide type of electrochemical capacitor. In 1991 he described the transition from ‘Supercapacitor’ to ‘Battery’ behavior in electrochemical energy storage and in 1999 he coined the term supercapacitor as explanation for increased capacitance by surface redox reactions with faradaic charge transfer between electrodes and ions.[1][16][17]

His "supercapacitor" stored electrical charge partially in the Helmholtz double-layer and partially was the result of faradaic reactions with "pseudocapacitance" charge transfer of electron and protons between electrode and electrolyte. The working mechanisms of pseudocapacitors are electrosorption, redox reactions and intercalation.

Marcus

The physical and mathematical basics of electron charge transfer without making chemical bonds leading to pseudocapacitance was developed by Rudolph A. Marcus. Marcus Theory is a theory to explain the rates of electron transfer reactions – the rate at which an electron can move or jump from one chemical species to another. It was originally formulated to address outer sphere electron transfer reactions, in which the two chemical species only change in their charge with an electron jumping. For redox reactions without making or breaking bonds Marcus theory takes the place of Henry Eyring's transition state theory which has been derived for reactions with structural changes. R.A. Marcus received the Nobel Prize in Chemistry in 1992 for this theory.

Development of electrochemical capacitors

In the early 1950s, General Electric engineers began experimenting with devices 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, providing a useful electrode material. In 1957 H. Becker developed a "Low voltage electrolytic capacitor with porous carbon electrodes".[18][19][20] 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 devices as "electrical energy storage apparatus", while working on experimental fuel cell designs.[21][22] 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.[23]

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.

These early electrochemical capacitors used a cell design of 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, which was significantly higher than for electrolytic capacitors of the same dimensions. This basic mechanical design remains the basis of most electrochemical capacitors.

SOHIO did not commercialize their invention, licensing the technology to NEC, who finally marketed the results as "supercapacitors" in 1971, to provide backup power for computer memory.[22]

The market expanded slowly for a time. That changes around 1978 as Panasonic marketed its "Goldcaps” brand.[24] This product became a successful back-up energy source for memory backup applications.[22] The competition started some years later. In 1987 ELNA "Dynacap"s entered the market.[25] All these first generation of EDLC's had relatively high internal resistance, which limited the discharge current. They were used for low current applications like powering SRAM chips or for data backup.

At the end of the 1980s improved electrode materials led to higher capacitance values. At the same time the development of electrolytes with better conductivity lowered the Equivalent Series Resistance (ESR) of the capacitors increasing the 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"[5] to underline their use for power applications.

Since the energy content of a capacitor increases with the square of the voltage, researchers were looking for a way to increase the breakdown voltage. Using an anode of a 200V high voltage tantalum electrolytic capacitor in 1994 David A. Evans developed an "Electrolytic-Hybrid Electrochemical Capacitor".[26][27]

These capacitors combine 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, yielding in a hybrid electrochemical capacitor. Evans' capacitors coined Capattery[28] had an energy content about a factor of 5 higher than a comparable tantalum electrolytic capacitor of the same size.[29] Their high costs limited them to specific military applications.

Recent developments in terms of time in the field of supercapacitors are lithium-ion capacitors. This capacitors are hybrid capacitors and were pioneered by FDK in 2007.[30] They combine a high capacitive electrostatic carbon double-layer electrode with a pre-doped lithium-ion battery electrode with much higher pseudocapacitance than the DL electrode. These combination increases the total capacitance value. Additionally, the pre-doping process lowers the anode potential and results in a high cell output voltage which increases the energy density of this hybrids.

Further improvement of supercapacitors is going on. A great number of research and development departments in many companies and universities [31] are working to improve characteristics, such as

• increase of the energy density by developing new nanostructured electrodes, tailored pore sized electrodes, and new pseudocapacitive coating or doping materials
• increase the power density by improving the electrolyte
• increasing of the cycle stability of pseudocapacitive electrodes
• reduction of production cost

Storage principles

Conventional electrostatic vs 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 of a loaded capacitor is stored in a static electric field permeates the dielectric between two metallic conducting plates, 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. The total energy stored in this arrangement increases with the amount of stored charge and the potential between the plates. The amount of charge stored per unit voltage is essentially a function of the electrode size, the reciprocal value of the dielectric thickness, and the material properties (permittivity) of the dielectric. The maximum potential between the plates, the maximal capacitors 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 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.

Conventional capacitors are also called electrostatic capacitors. The potential (voltage) of a charged capacitor correlates linearly with the stored charge.

Electrochemical capacitors now are a new type of capacitors with complete different energy storage principles. They do not have a conventional solid dielectric that separates the charge. The capacitance value of a supercapacitor is determined by electrostatic and electrochemical charge storage principles:

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

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

Because each supercapacitor has 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 are linear with respect to the stored charge just as in conventional capacitors. This linear behavior implies that the voltage across the capacitor is linear with respect to the amount of stored energy. This linear voltage gradient differs from electrochemical rechargeable batteries, in which the voltage across the terminals remains independent of the charged energy, providing a relatively constant voltage.

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.

An electrical double layer (EDL) in an electro-chemical supercapacitor is a structure that appears between the surface of a metallic conductive electrode and an adjacent liquid electrolyte.[8] Applying a voltage to this arrangement causes two layers of ions to be generated; the electrical double-layer. One layer in the surface lattice structure of the electrode. The other layer, with opposite polarity, out of dissolved and solvated ions in the adjacent liquid electrolyte. Both layers of opposite ions are separated from each other by a monolayer of isolating molecules of the solvent, such as water. The monolayer of isolating molecules forms the inner Helmholtz plane (IHP). They adhere firmly by physical adsorption on the surface of the electrode and separate the opposite polarized ions from each other, building a molecular dielectric. The amount of charge in the electrode is matched by the same magnitude of counter-charges in the outer Helmholtz plane (OHP). These phenomena can be used to store electrical charges like in a conventional capacitor. The stored charge in the double-layer 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 "thickness" of a charged layer in the metallic electrode, i.e., 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 stationary. 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 ranges from 0.1 to 10 nm, and is described by the Debye length. The sum of the thicknesses is the total thickness of a double layer.

The very small thickness of the IHP creates an extremely strong electric field E over the separating solvent molecules. At a potential difference of, for example, U = 2 V and a molecular thickness of d = 0.4 nm, the electric field strength will be

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

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 electric field is 6.3 V/8.8 nm = 716 kV/mm, around 7 times lower than in the double-layer. These extremely high double-layer's field strength of about 5000 kV/mm is unrealizable in conventional capacitors with conventional dielectrics. No dielectric material could prevent charge carrier breakthrough. In a double-layer capacitor the chemical stability of the molecular bonds of the solvent molecules prevents breakthrough.[32]

The forces that cause the adhesion are physical, not chemical, forces. Chemical bonds exist within the adsorbed molecules, but they are polarized. The magnitude of the electrical charge that can accumulate in the layers corresponds to the concentration of the adsorbed ions. Up to the electrolyte's decomposition voltage, 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 double-layer is like the dielectric layer in a conventional capacitor, but with the thickness of a single molecule. The early Helmholtz model to calculate the capacitance predicts a constant differential capacitance Cd independent from the charge density, even depending on the dielectric constant ε and the charge layer separation δ.

$\ C_d = \frac{\epsilon}{4 \pi \delta}$

If the solvent of the electrolyte is water then with the influence of the high field strength, the permittivity ε is 6 (instead of 80 without an applied electric field) and the layer separation δ ca. 0.3 nm the value of differential capacitance predicted by the Helmholtz model is about 18 F/cm2.[9] This value can be used to calculate capacitance using the standard formula for conventional plate capacitors if only the surface of the electrodes is known. This capacitance can be calculated with:[33]

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

The capacitance C is therefore greatest in devices made from materials with a high permittivity ε, large electrode plate surface areas A and a small distance d between plates. Because now the activated carbon electrodes have an extraordinary 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), already the older double-layer capacitors 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 connection. If both capacitances of the electrodes have approximately the same value, like in symmetrical supercapacitors, the total capacitance value 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. The ions in the electrolyte may also act as electron donors transferring with a charge-transfer electrons to the atoms of the electrode resulting in a faradaic current. This faradaic charge transfer originates by redox reactions, electrosorptions or intercalations processes between electrolyte and the surface of an electrode is called pseudocapacitance.[34]

Redox reactions in batteries with faradaic charge-transfer between an electrolyte and the 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. Additionally, the chemical processes are relatively slow, extending the charge and discharge time of the batteries.

A fundamental difference between faradaic reactions in batteries and in supercapacitors is that in the latter, the reactions are very a fast sequence of reversible redox processes, or electrosorption or intercalation processes with electron transfer without any phase changes of the electrode molecules. The de-solvated ions or atoms are adsorbed to the electrode and simply cling to the atomic structure of the electrode without making or breaking of chemical bonds.[35] This behavior is the essence of the new class of capacitance, termed "pseudocapacitance". Pseudocapacitive processes lead to a charge dependend linear capacitive behavior as well as the accomplishing non-faradaic double-layer capacitance. The distribution of the amounts of both capacitances depends on the surface area, material and structure of the electrodes. Pseudocapacitance and double-layer capacitance both contribute indivisible to the total capacitance value of these electrochemical capacitors. Capacitors with a high amount of pseudocapacitance are called pseudocapacitors.[2][3]

Interkalated metal atoms between planar graphite layers

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[35] 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.[36][37][38]

Several examples of pseudocapacitance can arise.[34][39] 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

Description of the system type give rise to pseudocapacitance:[34]

• Redox system: Ox + ze‾ ⇌ Red und O2‾ + Hˡ ⇌ in lattice
• Intercalation system: Liˡ in "Ma2"
• Electrosorption, underpotential deposition of metal adatoms: Mˡ + S + ze‾ ⇌ SM (S = surface lattice sites) or Hˡ e‾ + S ⇌ SH

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:

$\mathrm{RuO_2 + xH^+ + xe^- \leftrightarrow RuO_{2-x}(OH)_x}$ where $0 \le x \le 2$[40]

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][41]

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 resulting supercapacitors

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

Supercapacitors are classified 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 in which the static double-layer capacitance is significant higher than the faradaic pseudocapacitance use activated carbon or derivates, carbon aerogels, carbid-derived carbons or carbon nanotubes as electrode material
• Pseudocapacitors – Electrochemical capacitors in which the faradaic pseudocapacitance is significant higher than the static double-layer capacitance use conductive polymer or transition metal oxides with faradaic charge transfer as electrode material
• Hybrid capacitors – Capacitors with asymmetric electrodes exhibits significant high value of double-layer capacitance on one electrode as well as significant high value of pseudocapacitance on the other, coupling a double-layer electrode with a second pseudocapacitance or composite electrode, e.g. lithium-ion capacitors.

In an ideal double-layer capacitor with ideal double-layer electrodes does not exhibit any electrochemical pseudocapacitance. Conversely, an ideal pseudocapacitor exhibits no double-layer capacitance. In actual components, however, double-layer capacitance and pseudocapacitance both contribute indivisible to the total capacitance value of an electrochemical capacitor.[2][3] But the relative contributions of each capacitance type depends on the material and structure the of the electrodes with a widely range of varying the amounts of the both capacitance types.

Construction

Construction details

Supercapacitor cells consist of two collectors, two electrodes, a separator and an electrolyte. The electrolyte contains dissolved and solvated ions that migrate to and from the electrodes during charge and discharge respectively. The electrolyte electrically connects the electrodes to each other. They are mechanically separated from each other by an ion-permeable membrane (separator) that prevents short circuits. The electrodes are via a metallic collector electrically connected to the terminals that access the outside world.

This sandwich-like mechanical cpacitor cell of collector/electrode/separator/electrode/collector is rolled into a cylindrical or folded into a rectangular shape will be mounted into an aluminum can or an adaptable rectangular housing, and impregnated with electrolyte. The electrolyte mostly consists of a liquid or viscous conductive mixture of an organic or aqueous solvent. 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 their components.

The requirements for electrodes are manifold. They have to exhibit good conductivity, high temperature stability, chemical stability and corrosion resistance and high surface areas per unit volume and mass. Broader requirements include environmental friendliness and low cost.

Electrodes

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, 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. Additionally, 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 ångströms (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.

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

Additionally, the ability of the electrode material to perform faradaic charge transfers gives 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).[42] 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]

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

$C_\text{total} = \frac{C_1 \cdot C_2}{C_1 + C_2}$

Supercapacitors can be constructed in two versions, with symmetric or asymmetric electrodes. Symmetry implies that 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 equal to half the value of a single electrode.

For asymmetric constructed capacitors like hybrid supercapacitors, one of the electrode has higher capacitance value than the other.

If C1 >> C2 than Ctotal ≈ C2

Asymmetric electrodes imply that total capacitance can equal that of a single electrode, potentially doubling the capacitance value.

Electrodes for EDLCs

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][36][43]

Activated carbon

Activated carbon (AC) was the first material chosen for EDLC electrodes. Activated carbon has an electrical conductivity of 1,250 to 3,000 S/m, that is only appr. 0.003 percent of metallic conductivity, but it is quite good enough for use as electrodes in supercapacitors.[2][5]

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 (0.035 oz) (a pencil-eraser-sized amount) has a surface area of roughly 1000 to 3000 m2 (11,000 to 32,000 sq ft)[36][42] — about the size of 4 to 12 tennis courts. It is typically a fine powder made 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 and may be cheaper to produce than other carbon derivates.[44] 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 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.[45] Coconut shells produce activated carbon with more micropores than with wood activated carbon (charcoal).[42]

The disadvantage of an AC electrode is that compared to carbon nanotube electrodes, less than 1/3 of the surface area is available for the formation of the double layer.[46] 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 exhibit predominant static double-layer capacitance but also contribute to pseudocapacitance. Pores with diameters <2 nm are accessible only to de-solvated ions and enable faradaic reactions. 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 (ACF) are produced from activated carbon and have a typical diameter of 10 µm. They can have micropores with diameters of <2 nm with a very narrow pore-size distribution that can be readily controlled. The surface area of AFC woven into a textile form is about 2,500 m2/g. Advantages of AFC electrodes are the low electrical resistance along their fibre axis and good contact to the metal collector.[36]

AFC electrodes exhibit predominant double-layer capacitance and add a small amount of pseudocapacitance due to their micropores, which are only accessible for de-solvated ions.

Carbon aerogel

A block of aerogel in hand

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

Aerogel electrodes are made via pyrolysis of resorcinol formaldehyde aerogels.[47] 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. Its mechanical and vibration stability expand the range of supercapacitor applications.

Standard aerogel electrodes exhibit predominantly double-layer capacitance. Aerogel electrodes that incorporate composite material also provide a high amount of pseudocapacitance.[48]

Researchers have created carbon aerogel electrode with gravimetric densities of about 400–1200 m2/g and specific capacitance of 104 F/cm3, yielding in a comparable high energy density of 325 J/g (90 W•h/kg) and power density of 20 W/g.[49][50]

As of 2013, a graphene aerogel with a volumetric density of 0.16 mg/cm3 was synthetized, becoming the lightest material.[51]

Carbide-derived carbon

Pore size distributions for different carbide precursors.

Carbide-derived carbons (CDCs), also known as tunable nanoporous carbons, are a family of carbon materials derived from carbide precursors, such as binary silicon carbide and titanium carbide,[52] that are transformed into pure carbon via physical (e.g., thermal decomposition) or chemical (e.g., halogenation) processes.[53][54][55]

Carbide-derived carbons can exhibit high surface area and tunable pore diameters to maximize ion confinement, increasing pseudocapacitance by faradaic H2 adsorption treatment. Structurally, CDC pore sizes ranges from micropores to mesopores but below macropores.[42] The capacitance of CDC electrodes may be increased by tailoring pore design in the range of micropores. Pores smaller than 1 nm greatly contribute to capacitance even if the solvated ions are larger. This capacitance increase for smaller pores was explained by the distortion of the ion-solvating shell. As pore size approaches the solvation shell size, the solvent molecules are excluded and the de-solvated ions enter into fitting pores, increasing ionic packing density, hence increasing charge storage capability by H2 intercalation with faradaic charge transfer from de-solvated ions additional to the double- layer capacitance. CDC electrodes with tailored pore design, increase 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 a million charge/discharge cycles.[56]

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[57] as paper-like sheets called „nanocomposite paper”.[58]

Graphene has a very high surface area of 2630 m2/g which can lead theoretically to a capacitor of 550 F/g. But the most important advantage of graphene is practical realized very high conductivity with >1700 S/m compared to activated carbon (10 to 100 S/m). As of 2012 a new development use this graphene sheets directly as supercapacitor electrodes without collectors for extreme thin supercapacitors for portable applications.[59]

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 at room temperature equaling that of a conventional nickel metal hydride battery, but can be charged and discharged one hundred to one thousand times faster.[60][61]

The two-dimensional structure of the graphene nanosheet improves charging and discharging. The charge carriers in vertically oriented sheets can quickly migrate into or out of the deeper structures of the electrode, thus speeding up current delivery/acceptance. Such capacitors may be suitable for 100/120 Hz filter applications which is unable to reach for supercapacitors with other carbons as electrode material.[62]

As of 2013 graphene can be produced in various labs, but is not available in production quantities.[63]

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.[64]

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.[65][66]

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.[67]

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.[39][65][68]

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.[69]

Electrodes for pseudocapacitors

Pseudocapacitance with faradaic charge transfer is always a feature occur in double-layers. Pseudocapacitance without double-layer doesn't exist. Hence double-layer capacitance is always present, also in so called pseudocapacitors. The amount of pseudocapacitance on the total capacitance value coins a pseudocapacitor. So the electrodes used in pseudocapacitors have to be able to achieve predominant faradaic processes to have a predominate amount of pseudocapacitance against the double-layer capacitance in a pseudocapacitor.

Metal Oxides

Best understood by the research of B. E. Conway [1][16] are electrodes out of transition metal oxides for high amount of pseudocapacitance. Many oxides of transition metals like ruthenium (RuO
2
), iridium (IrO
2
), iron (Fe
3
O
4
), manganese (MnO
2
) or sulfides such as titanium sulfide (TiS
2
) or their combinations are able to generate many faradaic electron–transferring reactions combined with low conducting resistance.[70] Ruthenium dioxide in combination with H
2
SO
4
electrolyte provides one of the best examples of pseudocapacitance, providing a very high specific capacitance of 720 F/g and a high an energy density of 26.7 Wh/kg compared with EDLC electrodes.[71]

Charge/discharge takes place with electron charge transfer or removal and occurs 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. In addition for these transition metal electrodes, its reversibility is excellent, with a cycle life over several hundred-thousand cycles. But ruthenium is expensive and the 2,4 V voltage window for this capacitors limits their applications to military and space applications.

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 electrode materials are in commercial use, they are still in lab-scale research.[40]

Conductive polymers

Another type of supercapacitors with a high amount of pseudocapacitance use electron-conducting polymers as pseudocapacitive material for their electrodes. Conductive polymers have high conductivity, resulting in a low ESR and a relatively high capacitance. Such conducting polymers include polyaniline, polythiophene, polypyrrole and polyacetylene. They are cost comparable to carbon electrodes.

Supercapacitors with conducting polymer electrodes suffer from a limited stability during cycling.[4] However, supercapacitor using polyacene electrodes provide up to 10,000 cycles with stable electrical behavior,[72] and therefore they are much better than batteries.

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 pseudocapacitive active 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 research for supercapacitors explores composite electrodes.

Carbon nanotubes (CNTs) is one of the new electrode materials research is focussed on. CNTs give a backbone for a homogeneous distribution of metal oxide or electrically conducting polymers (ECPs), producing a high amount of pseudocapacitance and a relatively high amount of 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' 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 combines structural integrity and protects the deposited ECP from mechanical stress caused by the insertion and removal of ions in the ECP during cycling. These composites offer 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.[73] Supercapacitors with composite electrodes with cell voltages up to 4 V can reach energy and power densities of 50 Wh/kg and 22 kW/kg respectively.[74]

Another class of composite electrodes includes activated carbon-based materials like carbon nanotubes modified with deposited metal oxides such as RuO
2
, IrO
2
, MnO
2
or nitrides of molybdenum, titanium and iron. These composite electrodes yield capacities of the order of 150–250 μF/cm2, several times larger than activated carbon electrodes. One team managed am energy density of 188 Wh/kg and a power density of 200 kW/kg using this method. [75]

Another way to enhance CNT composite electrodes is by doping with a pseudocapacitative dopant as in lithium-ion capacitors. 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.[22]

Battery-type electrodes

Battery-type electrodes in hybrid supercapacitors couple a rechargeable battery-type electrode with a carbon electrode in an asymmetric construction. This configuration offers higher energy density than typical supercapacitors with higher power density, longer cycle life and faster charging and recharging times than batteries.

Candidates for battery-type electrodes in hybrid supercapacitors include nickel hydroxide, lead dioxide and LTO as the anode and activated carbon as the cathode with a lithium salt (Li4Ti5O12)in an organic solvent electrolyte. In 1985, Akira Yoshino assembled a prototype cell using lithium-doped carbon as one electrode and lithium cobalt oxide (LiCoO
2
), which is stable in air, as the other.[76] Avoiding metallic lithium improved safety. Meanwhile the metallic electrode was replaced by activated carbon doped with lithium atoms. In this case the relatively small lithium atoms intercalate between the layers of carbon.[77] Battery electrodes influenced the development of electrodes for lithium-ion capacitors, which As of 2013 provided the highest energy density of commercial supercapacitors.[78]

While the structure of such electrodes qualifies them as composites electrodes for supercapacitors, they are typically placed in a separate category.

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 to a symmetrical EDLC is their higher voltage and correspondingly their higher specific energy (up to 10-20 Wh/kg).[79]

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

Electrolytes

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

Beneath this general functionality the electrolyte determines the capacitor's characteristics, the operating voltage, temperature range, the internal resistance (ESR) and the electrodes capacitance. With the same electrode material out of activated carbon f. e. an aqueous electrolyte achieves capacitance values of 160 F/g. An organic electrolyte achieves only a capacitance value of 100 F/g.[38]

The electrolyte must be chemically inert and not chemically attack the materials of the capacitor to ensure over its chemical stability the long time stable behavior of the capacitor’s electrical parameters. Furthermore the electrolyte's viscosity must be low enough to be usable in a production process to wet the porous, sponge-like structure of the electrodes. An ideal electrolyte does not exist, the properties of an electrolyte are always a compromise between performance and other requirements.

Water is a relatively good solvent for inorganic chemicals. Treated with acids such as sulfuric acid (H
2
SO
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, 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 tetraethylammonium tetrafluoroborate (N(Et)
4
BF
4
[80]) or triethyl (metyl) tetrafluoroborate (NMe(Et)
3
BF
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 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.

Separators

Separators have to physically separate the two electrodes to prevent a short circuit occur 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, they must be chemically inert to protect the electrolyte's long term stability and conductivity. Inexpensive devices use open capacitor papers. More sophisticated designs use nonwoven porous polymeric films like polyacrylonitrile or Kapton, woven glass fibers or porous woven ceramic fibres.[81][82]

Collectors and housing

The current collectors serve the electrical contact of the electrode material and are connected to the capacitor’s terminals. The collector is either sprayed onto the electrode or consists of a metal foil to which the electrode is attached. They must be able to easily distribute peak currents of up to 100 A to and from the capacitor cell.

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

Electrical parameters

Capacitance

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

The capacitance values of supercapacitors are specified as "rated capacitance CR". This is the capacitance value for which the capacitor has been designed. The real capacitance value has to be within the limits given by the capacitance tolerances. Typical capacitance values of supercapacitors are in the range of farads (F), a capacitance improvement of three to six orders of magnitude compared with electrolytic capacitors.

The capacitance value of a supercapacitor CDC results out of the energy W of a loaded capacitor, loaded via a DC voltage VDC.

$W=\frac{1}{2}\cdot C_\text{DC} \cdot V_\text{DC}^2$

This capacitance is also called the "DC capacitance".

Conventional capacitors normally are 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. The AC capacitance measurement offers fast results which is important for industrial production lines. But the capacitance value of a supercapacitor depends extreme strongly on the measurement frequency, related to porous electrode structure and the limited ion mobility in the electrolyte. Even at a very low frequency of 10 Hz, the measured capacitance value drops from 100% DC capacitance value down to about 20% of the DC value.

This extraordinary strong frequency depending capacitance characteristic can be explained by the different long distances the ions have to move in the pores of the electrode. The first area at the beginning of the pores could be easily acquired by the ions. Short distance is accompanied by low electrical resistance. For the area behind the distance the ions have to cover are longer, the resistance is higher. This arrangement with different distances the ions have to move in the porous structure of the electrode can be described with a series circuit of cascaded RC (resistor/capacitor) elements with serial RC time constants, see "equivalent circuit". These results in a delayed current flow, reducing the total electrode surface area can be covered with ions if polarity changes – the capacitance decreases with increasing AC frequency. Thus, the total capacitance is only achieved after longer measuring times.

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 standard 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, where the voltage between capacitor terminals drops from 80% (V1) to 40% (V2) of the rated voltage is measured. The capacitance value can be calculated as:

$C_\text{total} = I_\text{discharge} \cdot \frac{t_2-t_1}{V_1-V_2}$

The value of the discharge current is determined by the application. 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 measurement methods that are specified by the individual manufacturers are mainly comparable to the standardized methods.[83][84]

The standardized measurement method is time consuming. Industrial manufacturers cannot use this method during production for each individual component. The capacitance value is instead measured with a faster low frequency AC voltage and applying a correlation factor to compute the specified rated capacitance.

The frequency dependence affects capacitor operation. If supercapacitors operate with rapid charge and discharge cycles, neither the rated capacitance value nor energy density are available. In this case the required capacitance value has to be recalculated for every application condition.

Operating voltage

Supercapacitors are low voltage components, for which safe operation requires that the voltage remains within specified limits. The rated voltage UR for supercapacitors is the maximum DC voltage or peak pulse voltage that may be applied continuously to a capacitor within the specified temperature range. The capacitor should never be subjected to voltages continuously in excess of the rated voltage.

The rated voltage has a safety margin against the chemical limiting condition, the electrolyte's breakdown voltage at which the electrolyte decomposes. The breakdown voltage in a Helmholtz layer separates the solvent water into hydrogen and oxide at a voltage above 1.2 V, than the solvent molecules break and cannot separate the electrical charges from each other anymore. Because the capacitor has two electrodes, the breakdown voltage lies above 2.4 V, the rated voltage of a supercapacitor with aquaeus electrolyte is limited at 2.4 V. For electrolytes with organic solvents the breakdown voltage is above 1.8 V per electrode, the rated voltage of this supercapacitors is limited at 3.6 V. Higher voltages cause hydrogen gas formation and may destroy supercapacitors.

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 also have a lower voltage limit of about 2.2 V.

Operating supercapacitors below the rated voltage positively affects electrical parameters. Changes of capacitance and internal resistance during cycling are lower and life time and charge/discharge cycles may be extended.[84]

Because supercapacitors operate with low voltages, the rated voltage is generally less than the required application voltage. To achieve the required application voltage, it is necessary to connect supercapacitor cells in series connection. Since each supercapacitor has a slight difference in capacitance value and ESR, it is necessary to actively or passively balance the capacitors. Passive balancing employs resistors in parallel with the supercapacitors and stabilizes the voltage. Active balancing may include an electronic voltage management above a threshold that varies the current.

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 or discharging of a supercapacitor 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. During this movement of ions in the electrolyte to the electrodes and back losses occur, which can be measured as the internal DC resistance of the capacitor.

With the electrical model of cascaded, series-connected RC (resistor/capacitor) elements in the electrode pores, the internal resistance of a supercapacitor increases with increasing penetration depth of the charge carriers into the pores. That means, that the internal DC resistance is time dependent and increases during charging or discharging. In supercapacitor 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:

$R_\text{i}= \frac{\Delta V_2}{I_\text{discharge}}$

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

The internal DC resistance Ri determines several supercapacitor properties. It limits the charge and discharge peak currents as well as charge/discharge times. The internal resistance Ri and the capacitance C of the capacitor results in the time constant $\tau$

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

This time constant determines the time over 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. 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 constant internal resistance fully charge during about 5 τ. Since internal resistance increases with charge/discharge, times cannot be calculated with this formula. Thus, charge/discharge time depends on specific individual construction details.

Because the storage of electrical energy in Supercapacitors occur without forming any chemical bonds the current loads, including charge and discharge currents and peak currents are not limited by any chemical reactions. The current load and cycle stability can be very much higher than for batteries.

The current loads only are limited by the internal resistance of a supercapacitor, which may be clearly lower than for batteries. Via this internal DC resistance Ri charge/discharge currents or peak currents generate internal heat losses Ploss

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

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

Heat generally defines the life time of the capacitors by diffusion of the electrolyte. The permissible 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 life time. Out of that reason the specified charge and discharge current for frequent cycling are determined by the internal resistance.

The maximal specified cycle parameters include charge and discharge current, pulse duration and frequency. They are specified within the specified temperature range and over the full voltage range for a defined life time. 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 capacitor life and extend consequently the number of possible cycles.[84]

Supercapacitors, except types with polymer electrodes, can potentially withstand more than one million charge/discharge cycles without substantial capacity drops or internal resistance increases significantly caused by the cycling. Beneath the higher current load this is the second great advantage of supercapacitors compared with batteries. The stability results from the both unique charge storage principles in supercapacitors, the electrostatic storage in Helmholtz double-layers and the electrochemical storage with pseudocapacitance. Both storage principles are chemically stable - no chemical bond appears.

The specified charge and discharge current for stable cycling can be significantly exceeded at infrequent intervals or single peak current pulses. In this case the heat generated by a single pulse may be distributed over the time until the next pulse occurs to reach the relatively small heat increase as an average value. 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.[85] Such currents may, however, not be considered as a frequent permanent value. Such currents produce not only internal heating, adding thermal expansion as a stress factor. They also generate strong electromagnetic forces that affect the electrode-collector connection. Great cycle stability of supercapacitors with up to 1,000,000 cycles with high current charges/discharge requires robust design and construction to prevent damage to the device.

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.

Energy density and power density

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

Supercapacitors in point of stored energy occupy or bridge the gap between electrolytic capacitors and 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

$W_\text{max}=\frac{1}{2}\cdot C_\text{total} \cdot V_\text{loaded}^2$

This formula describes the total amount of energy stored in a capacitor and is often used in science publications to describe new research successes. But in reality only a part of the stored energy is available, the voltage drop and the time constant over the internal resistance reduce the practical available energy specified in datasheets of commercial available components. This effective realized amount of energy Weff a supercapacitor can deliver is reduced by the used voltage difference between Vmax and Vmin and can be represented as:[86]

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

This formula represent also the energy of supercapacitors with asymmetric voltages like lithium ion capacitors.

Energy density is either measured gravimetrically (per unit of mass) in watt-hours per kilogram (Wh/kg) or volumetrically (per unit of volume) in watt-hours per litre (Wh/l).

As of 2013 the commercial available effective gravimetrically energy densities of supercapacitors 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 about 100 to 265 Wh/kg. That means, supercapacitors can store 10 to 100 times more energy than electrolytic capacitors but only one tenth of batteries.

Although the energy densities of supercapacitors are insufficient compared with batteries the capacitors have an important advantage, the power density. Power density combines the energy density with the speed at which the energy can be delivered to the load or can be absorbed. The maximum power Pmax is given by the formula:[86]

$P_\text{max}=\frac{1}{4}\cdot\frac{V^2}{R_i}$

with V = voltage applied and Ri, the internal DC resistance calculated as described in paragraph "Internal resistance".

Power density the time rate of energy transfer is either measured gravimetrically (per unit of mass) in kilowatt per kilogram (kW/kg) or volumetrically (per unit of volume) in kilowatt per litre (kW/l).

The described maximum power Pmax specify the power of a rectangular single maximum current peak of a given voltage. In reality the current peak is not rectangular caused by time constants and the voltage is smaller caused by the voltage drop. The IEC standard 62391–2 therefore proposed a formula to calculate a more reality oriented effective power Peff for supercapacitors for power applications

$P_\text{eff}=\frac{1}{8}\cdot\frac{V^2}{R_i}$

The power density of supercapacitors is typically 10 to 100 times greater than for batteries and can reach values up to 15 kW/kg for industrial produced types. Special development of a tailored composite electrode have achieved a maximum power rating of 990 kW/kg.[87]

Ragone charts are a valuable tool for characterizing supercapacitors and understanding performance as a function of operating conditions. Ragone charts relate energy and power.[88] These log-log plots encompass a wide range of energy storage devices.[89] Energy and power densities were initially only calculated for packaged supercapacitors. However, research into electrode materials requires measurement of individual components, such as an electrode or half-cell, such as the method described by Raut et al.[90] By using a counterelectrode that does not affect the measurements, the characteristics of only the working electrode can be found.

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 don’t result in chemical changes of the electrode material, except for those with polymer electrodes, its lifetimes depend mostly on the evaporation of the liquid electrolyte over time. This evaporation in general is a function of temperature, of current load and current cycle frequency and voltage applied. Current load and cycle frequency generate internal heat, so that the evaporation determining temperature is the sum out of ambient and internal generated. This temperature is measurable as core temperature in the center of a capacitor body. As higher is the core temperature of the capacitor as faster the evaporation as shorter the lifetime.

The voltage applied is responsible for a small amount of gas development over the application time which will reduce the liquid electrolyte and also shorten the lifetime. The higher the voltage the higher the gas development, and the shorter the lifetime.

The evaporation generally results in a decreasing capacitance value and an 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 device has reached "end-of-life". The capacitors are still operable, but only 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 applications with high current loads. Manufacturers, whose supercapacitors are provided for high current loads, use much smaller limits, e.g., 20% loss of capacitance or double the internal resistance.[91] The narrower definition is important for high current load applications, since heat increases linearly with increasing internal resistance. This heat in a borderline case could 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. This long time could not 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 special formula the lifetime of the capacitors could be estimated for lower conditions.

The lifetime specification given in datasheets has to be tested by the manufactures by an accelerated aging test called "endurance test" with maximum temperature and voltage as long as the 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 is the expression of 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.

$L_x =L_0\cdot 2^\frac{T_0-T_x}{10}$

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, have an estimated lifetime of 20,000 h at 45 °C.

The lifetime of supercapacitors is also dependent on the operating voltage, because the development of gas in the liquid electrolyte depends on the voltage applied. 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 beside, has to be seen as an empirical result from one manufacturer.

Life expectancy of supercapacitors for power applications may be also limited by high current loads or number of cycles. This limitation has to be specified by the relevant manufacturer, because it is strongly type dependent.

Self-discharge

Storing electrical energy in the double-layers separates the charge carriers by distances in the range of molecules. Over this short distance some irregularities can occur, leading to a small exchange of charge carriers and gradual discharge the capacitor. This self-discharge is a very small current called leakage current. Leakage depends on capacitance, voltage, temperature and the chemical stability of electrode and 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.[92]

The self-discharge rate is, for most applications, 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 of the same material, theoretically supercapacitors have no true polarity. Normally catastrophic failure does not occur, however lifetime is reduced if a supercapacitor is reverse-charged for some reason. It is recommended practice to maintain the polarity resulting from a formation of the electrodes during production of the capacitors. Asymmetric supercapacitors are inherently polar.

Supercapacitors may not be operated with reverse polarity. This precludes AC operation.

A bar in the insulating sleeve identifies the cathode terminal.

Describing polarity using 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.

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

Comparison of technical parameters

Comparison of supercapacitor parameters

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

Combining different electrodes with different electrolytes yields a large number of varieties of components suitable for diverse applications. Particularly the development of low-ohmic electrolyte systems, in combination with electrodes having a high pseudocapacitance, a large number of technical solutions have been originated, which are reflected many different characteristics of supercapacitors.

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 device 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 12,000 F with a significantly lower ESR value, with ESR under 1 milliohm. 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.[36]

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
APowerCap[93] APowerCap 4…550 2.7 - - 4.5 -
AVX[94] BestCap 0.068…0.56 3.6 - 0.13 - Modules up to 16 V
Cap-XX[95] Cap-XX 0.16…2.4 2.75…2.75 14 1.45 1.36 -
CDE[96] Ultracapacitor 0,1…3000 2.7 0.29 7.7 6.0 -
Cooper[97] PowerStor 0.1…400 2.5…2.7 4.5 5.7 - -
Elna [98] DYNACAP
POWERCAP
0.047…300
2.5...3.6
2.5
8.0
3.0
5.4
5.3
-
-
-
-
Elton[6] Supercapacitor 1800…12000 1.5 0.5 6.8 4.2 Modules up to 29 V
Evans[99] Capattery 0.001…10 125 200 - - Hybrid capacitors
HCC[100] HCAP 0.22…5000 2.7 15 10.6 - Modules up to 45 V
FDK[101][102] EneCapTen 2000 4.0 - 25 14 LI-Ion-capacitors
Illinois[103] Supercapacitor 1…3500 2.3…2.7 0.29 7.6 5.9 -
Ioxus[104] Ultracapacitor 100…3000
220…1000
2.7
2.3
0.26 7.8
8.7
6.0
6.4
-
JSR Micro[105] Ultimo 1100…3300 3.8 1.2 20 12 Li-Ion-capacitors
Korchip[106] STARCAP 0.01…400 2.7 12 7.0 6.1 Modules up to 50 V
Liyuan[107] Supercapacitor 1…400 2.5 10 4.4 4.6 -
LS Mtron[108] Ultracapacitor 100…3000 2.8 0.25 6.0 5.9 Modules up to 84 V
Maxwell[109] Boostcap 10…3000 2..2…2.7 0.29 7.8 6.0 Modules up to 125 V
Murata[110] EDLC 0.35…0.7 2.1 30 0.8 - -
NEC[111] Supercapacitor
LIC Capacitor
0.01…100
1100…1200
2.7
3.8
30,000
1.0
5.3-
-
4.2
-
-
Li-Ion-capacitors
Nesscap[112] 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[113] EVerCAP 0,47…6000 2.5…2.7 2.2 6.9 4.0 -
NCC, ECC[114] DLCCAP 350…2300 2.5 1.2 5.9 4.1 Modules up to 15 V
Panasonic[115] Goldcap 0.015…70 2.1…2.3 100 3.4 - -
Samwha[116] Green-Cap 3…3000 2.7 0.28 7.7 5.6 Modules up to 125 V
Skeleton [117] SkelCap 900…3500 2.85 0.2 14.1 10.1 -
Taiyo Yuden[118] 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[119] Hy-Cap 1.5…800 2.3…3.0 10 8.7 6.3 -
WIMA[120] SuperCap 12…6500 2.5…2.7 0.18 5.2 4.3 Modules up to 112 V
YEC[121] Kapton capacitor 0.5…400 2.7 12 7.0 5.5 -
Yunasko[122] 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

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.

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)
Efficiency (%) 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 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 are not provided for AC applications.

Advantages of supercapacitors compared with rechargeable batteries include:

• Longer lifetime, typically > 10 years, limited by temperature dependent evaporation of liquid electrolyte and current load heating
• Much higher power density
• Extremely high peak currents due to very low internal resistance
• Much higher number of charge/discharge cycles, more than 1,000,000 cycles vs. hundreds for batteries
• Low cost per cycle
• Much lower capacitance loss during charge cycles
• Simple charge methods—no danger of overcharging
• Good reversibility
• High cycle efficiency (95% or more)
• Environmentally friendly, no corrosive electrolyte and low material toxicity.

• Higher price
• Much lower energy density
• Higher self-discharge
• Voltage drops significantly with discharge. Effective storage and recovery of energy requires complex electronic control and switching equipment, with consequent energy loss.
• Spark hazard given a short

Standards

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

Supercapacitors vary sufficiently that they mostly are not interchangeable, especially those with higher energy densities. Applications range from low to high peak currents, requiring standardized test protocols.[123]

Test specifications and parameter requirements are specified in the generic specification

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

The standard defines four application classes, according to capacitor discharge currents:

• 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

Beneath this generic specification three further standards describe 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 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 milliamps current or milliwatts 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:

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

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:

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

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

General applications

Consumer applications

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

For homeworkers 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 of unuse.[125]

Supercapacitors deliver power for photographic flashes in digital cameras and for LED life flashlights that can be charged in e.g. 90 seconds.[126]

In 2013, a television remote control using a carbon on aluminum supercapacitor[127] and portable speakers powered by Maxwell BCAP0350 capacitors[128] (rated at 5.62 Wh/kg)[129] were released.

Industrial applications

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)[130] equipment or for event notification in industrial electronics.

In conjunction with rechargeable batteries, supercapacitors buffer power to and from the batteries, mitigating the effects of short power interruptions and high current peaks. Batteries with the greater energy storage capability are used only during extended interruptions, e.g. if the mains power or a fuel cell fails, which lengthens the lifetime of the battery.

Typical industrial applications for circuits like that are uninterruptible power supplies (UPS), where supercapacitors have replaced banks of electrolytic capacitors that required much more space. This combination reduces the cost per cycle, saves on replacement and maintenance costs, enables the battery to be downsized, and extends battery life.[131][132][133] A disadvantage is the need for a special electronic circuit to reconcile the different behavior of the two storage media.

Public sector applications

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

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.[134]

Aerial lift in Zell am See, Austria

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, comprises 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.[135][136]

Renewable energy applications

Rotor with wind turbine pitch system

Supercapacitors provide backup power for actuators in wind turbine pitch systems, so that blade pitch can be adjusted even if the main supply fails.[137]

Supercapacitors can stabilize voltage for powerlines. Photovoltaic systems exhibit fluctuating loads evoked by clouds that supercapacitors can buffer within milliseconds. This reduces the need to stabilize grid voltage and frequency, balance supply and demand of power and manage real or reactive power.[138][139][140]

Medical

Supercapacitors are advantageous when extremely fast discharging is required. They are used in defibrillators where they can deliver a lethal 500 joules of energy providing the burst of power required to shock the heart.[141]

Heavy and public transport

Supercapacitors can be used to supplement batteries as a starter battery in diesel trucks and railroad railroad locomotives.[142][143] Supercapacitors are used in some pit trains in China to replace conventional trolleys. They bring coal to the surface in coal mines. This approach removes a fire and safety hazard. The capacitors can be charged at the surface in less than 30 minutes.[144]

Aviation

In 2005, aerospace systems and controls company Diehl Luftfahrt Elektronik GmbH chose supercapacitors to power emergency actuators of doors and evacuation slides used in airliners, including the Airbus 380.[137]

Military

The very low internal resistance of advanced supercapacitors allows support for applications that require short-term peak currents. Some of the earliest uses were motor startup (cold diesel engine start) for large engines in tanks and submarines.[145] 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.[146][147]

Energy recovery

A primary challenge of all transport of goods and peoples is reducing energy consumption and reducing CO
2
emissions. Recovery of braking energy (recuperation) helps with both. This requires devices that can quickly store and release energy, such as supercapacitors.

Energy recovery in rails, cranes, lifts and trucks

Green Cargo operates TRAXX locomotives from Bombardier Transportation

Supercapacitors in locomotives can capture the braking energy of a full stop. The recaptured energy can start the engine, accelerate the train and then cruise. Low maintenance operation and environmentally friendly materials encouraged the use of supercapacitors.[142][143]

Container Yard with Rubber Tyre Gantry Crane

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 by using supercapacitors while lowering the load.[148]

A triple hybrid forklift truck uses fuel cells and batteries as primary energy storage and supercapacitors to buffer power peaks. The energy recovered during braking is stored in the supercapacitors. They are able to provide the fork lift with a peak power over 30 kW which is much more than batteries can provide. The triple-hybrid system offers over 50% energy savings compared with diesel or fuel-cell systems.[149]

Supercapacitor terminal tractors transport containers to warehouses. They provide an economical, quiet and pollution-free alternative to diesel terminal tractors.

Supercapacitors are currently in use or under consideration in trash trucks, which can experience as many as a thousand start/stop cycles during a day; and in delivery trucks, which operate at similar cycle rates.

Energy recovery in light-rail and trams

Light rail vehicle in Mannheim

Since 2003 in Mannheim a prototype light-rail vehicle (LRV) using the MITRAC Energy Saver system from Bombardier Transportation stores mechanical braking energy.[150] Compared to conventional modern LRVs or Metro vehicles that return energy into the powerline, onboard energy storage saves up to 30% and reduces peak grid demand by up to 50%.

Light rail vehicle in Hong Kong

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

In April 2011 Rhein-Neckar a German regional transport operator ordered a further 11 light rail vehicles equipped with supercapacitor energy recovering systems.[151]

In 2012 tram operator Geneva Public Transport began tests of an LRV equipped with a prototype supercapacitor energy storage unit mounted on the roof to recover braking energy.[152]

Other public transport manufacturers are delivering light-rail transport systems with supercapacitor energy storing technology, including mobile storage.[153]

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%.[154]

Wireless operation in light-rail and trams

The Paris T3 line runs without overhead wires in some sections

Supercapacitors advances have enabled running where there are no overhead wires or third-rail pickup. The supercapacitors are charged at the stations when the train or tram is at a scheduled stop. The vehicle uses the electricity provided at the station to initially move, with the supercapacitors taking over once moving and away from the station. This advance may electrify many urban rail lines that were previously considered too expensive to fully electrify along the route.

In 2009 in Paris a tram on route T3 was fitted with 48 supercapacitors mounted on the roof to store braking energy and to enable the tram to operate with overhead power only on parts of its route, running on stored energy between electrified segments and recharging quickly during powered segments.[155]

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 regeneration of braking energy and the ability to operate without overhead wires. The train can travel up 2 km with recharging of the supercapacitor bank in 30 seconds at each station via a ground mounted pickup. The supplier claimed the trains could be used in 100 small and medium-sized Chinese cities.[156]

In 2012, in Lyon (France), the SYTRAL (Lyon public transportation administration) starts 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.[157]

Energy recovery in 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 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 the new Ultracap-technology. The Ultracap-system was supplied with 8 Ultracap-modules of 80 V, each containing 36 devices. 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 were used to recapture braking energy and deliver starting energy. Fuel consumption was reduced by currently 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 a reduction in maintenance costs.[158][159]

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 conducted. The supercapacitors could be recharged via an inductive contactless high-speed power charger after every transportation cycle, within 3 to 4 minutes.[160][161]

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