The properties of supercapacitors come from the interaction of their internal materials. Especially, the combination of electrode material and type of electrolyte determine the functionality and thermal and electrical characteristics of the capacitors.
Electrodes of activated carbon 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 centimeters. Supercapacitor electrodes are generally thin coatings applied and electrically connected to a conductive, metallic
current collector. Electrodes must have good conductivity, high temperature stability, long-term chemical stability (
inertness), high corrosion resistance and high surface areas per unit volume and mass. Other requirements include environmental friendliness and low cost. The amount of double-layer as well as pseudocapacitance stored per unit voltage in a supercapacitor is predominantly a function of the electrode surface area. Therefore, supercapacitor electrodes are typically made of porous,
spongy material with an extraordinarily high
specific surface area, such as
activated carbon. Additionally, the ability of the electrode material to perform faradaic charge transfers enhances the total capacitance. Generally the smaller the electrode's pores, the greater the capacitance and
specific energy. However, smaller pores increase
equivalent series resistance (ESR) and decrease
specific power. Applications with high peak currents require larger pores and low internal losses, while applications requiring high specific energy need small pores.
Electrodes for EDLCs The most commonly used electrode material for supercapacitors is carbon in various manifestations such as
activated carbon (AC), carbon fibre-cloth (AFC),
carbide-derived carbon (CDC), carbon
aerogel,
graphite (
graphene),
graphane and
carbon nanotubes (CNTs). Carbon-based electrodes exhibit predominantly static double-layer capacitance, even though a small amount of pseudocapacitance may also be present depending on the pore size distribution. Pore sizes in carbons typically range from
micropores (less than 2 nm) to
mesopores (2-50 nm), — about the size of 4 to 12
tennis courts. The bulk form used in electrodes is low-density with many pores, giving high double-layer capacitance. Solid activated carbon, also termed
consolidated amorphous carbon (CAC) is the most used electrode material for supercapacitors and may be cheaper than other carbon derivatives. It is produced from activated carbon powder pressed into the desired shape, forming a block with a wide distribution of pore sizes. An electrode with a surface area of about 1000 m2/g results in a typical double-layer capacitance of about 10 μF/cm2 and a specific capacitance of 100 F/g. virtually all commercial supercapacitors use powdered activated carbon made from coconut shells. Coconut shells produce activated carbon with more micropores than does charcoal made from wood. and are more conductive than most activated carbons. They enable thin and mechanically stable electrodes with a thickness in the range of several hundred
micrometres (μm) and with uniform pore size. Aerogel electrodes also provide mechanical and vibration stability for supercapacitors used in high-vibration environments. Researchers have created a carbon aerogel electrode with
gravimetric densities of about 400–1200 m2/g and volumetric capacitance of 104 F/cm3, yielding a specific energy of () and specific power of . Standard aerogel electrodes exhibit predominantly double-layer capacitance. Aerogel electrodes that incorporate
composite material can add a high amount of pseudocapacitance.
Carbide-derived carbon Carbide-derived carbon (CDC), also known as tunable nanoporous carbon, is a family of carbon materials derived from
carbide precursors, such as binary
silicon carbide and
titanium carbide, that are transformed into pure carbon via physical,
e.g.,
thermal decomposition or chemical,
e.g.,
halogenation) processes. Carbide-derived carbons can exhibit high surface area and tunable pore diameters (from micropores to mesopores) to maximize ion confinement, increasing pseudocapacitance by faradaic adsorption treatment. CDC electrodes with tailored pore design offer as much as 75% greater specific energy than conventional activated carbons. , a CDC supercapacitor offered a specific energy of 10.1 Wh/kg, 3,500 F capacitance and over one million charge-discharge cycles.
Graphene made of carbon atoms
Graphene is a one-atom thick sheet of
graphite, with atoms arranged in a regular hexagonal pattern, also called "nanocomposite paper". Graphene has a theoretical specific surface area of 2630 m2/g which can theoretically lead to a capacitance of 550 F/g. In addition, an advantage of graphene over activated carbon is its higher electrical conductivity. , a new development used graphene sheets directly as electrodes without collectors for portable applications. In one embodiment, a graphene-based supercapacitor uses curved graphene sheets that do not stack face-to-face, forming mesopores that are accessible to and wettable by ionic electrolytes at voltages up to 4 V. A specific energy of () is obtained at room temperature equaling that of a conventional
nickel–metal hydride battery, but with 100–1000 times greater specific power. The two-dimensional structure of graphene improves charging and discharging. Charge carriers in vertically oriented sheets can quickly migrate into or out of the deeper structures of the electrode, thus increasing currents. Such capacitors may be suitable for 100/120 Hz filter applications, which are unreachable for supercapacitors using other carbon materials.
Graphene Fabrication Techniques for Supercapacitors Chemical vapor deposition is a popular fabrication method for graphene utilized in supercapacitors, producing a high quality monolayer or few-layer graphene. The process begins by introducing a
Hydrocarbon (most commonly methane, CH4) into a reaction chamber. A metal catalyst such as copper (Cu) or nickel (Ni) are then placed in the reaction chamber to act as a substrate. These metals are frequently used due to their ability to decompose methane into free carbon atoms that will be used in the graphene formation. The chamber is then heated to temperatures between 700 and 1000 degrees Celsius in order to decompose the methane molecules at the surface of the
substrate. The methane is decomposed into hydrogen gas (H2) that is vented from the chamber, and Carbon (C) atoms are released onto the substrate. As Carbon atoms are absorbed onto the substrate, they begin to diffuse along the surface and nucleate. The Carbon atoms will naturally arrange themselves in the
honeycomb lattice of graphene. Depending on the temperature, pressure, and concentration of methane, the number of graphene layers produced will vary. Once the graphene layer is formed, the chamber is cooled down and the now coated substrate is removed. The graphene is then transferred from its metal substrate onto a new surface, depending on the application. The method to perform this transfer is typically a PMMA-mediated approach (poly methyl-methacrylate). Graphene is especially valuable in Supercapacitors due to its low-resistance pathways for electron flow, which is an essential part of the high power output of supercapacitors.
Mechanical exfoliation is another fabrication method for graphene sheets used in supercapacitors. The process begins by selecting a high quality
graphite, with high purity single-crystal graphite preferred. Next, select a piece of tape that can peel off thin layers of the graphite without taking off large chunks of the material,
scotch tape is frequently used in this method. Repeatedly press the tape onto the graphite and gently peel it off. As the process is repeated, thinner and thinner
graphene sheets will be transferred onto the tape. Once a thin graphene layer has been pressed onto the tape, the tape is positioned on a clean substrate such as a silicon wafer or film. The tape with the graphene is pressed gently onto the substrate to transfer the graphene layers to the surface of the substrate and then slowly peeled off.
Electrodes for pseudocapacitors MnO2 and
RuO2 are typical materials used as
electrodes for pseudocapacitors, since they have the electrochemical signature of a capacitive electrode (linear dependence on current versus voltage curve) as well as exhibiting
aic behavior. Additionally, the charge storage originates from electron-transfer mechanisms rather than accumulation of ions in the
electrochemical double layer. Pseudocapacitors were created through faradaic
redox reactions that occur within the active electrode materials. More research was focused on
transition-metal oxides such as MnO2 since transition-metal oxides have a lower cost compared to noble metal oxides such as RuO2. Moreover, the charge storage mechanisms of transition-metal oxides are based predominantly on pseudocapacitance. Two mechanisms of MnO2 charge storage behavior were introduced. The first mechanism implies the intercalation of
protons (H+) or
alkali metal cations (C+) in the bulk of the material upon reduction followed by deintercalation upon
oxidation. : MnO2 + H+ (C+) + e− MnOOH(C) The second mechanism is based on the surface
adsorption of
electrolyte cations on MnO2. : (MnO2)surface + C+ + e− (MnO2− C+)surface Not every material that exhibits faradaic behavior can be used as an electrode for pseudocapacitors, such as
Ni(OH)2 since it is a battery type electrode (non-linear dependence on current versus voltage curve).
Metal oxides Brian Evans Conway's research Charge/discharge takes place over a window of about 1.2 V per electrode. This pseudocapacitance of about 720 F/g is roughly 100 times higher than for double-layer capacitance using
activated carbon electrodes. These transition metal electrodes offer excellent reversibility, with several hundred-thousand cycles. However, ruthenium is expensive and the 2.4 V voltage window for this capacitor limits their applications to military and space applications. Das et al. reported highest capacitance value (1715 F/g) for
ruthenium oxide based supercapacitor with electrodeposited ruthenium oxide onto porous single wall
carbon nanotube film electrode. A high specific
capacitance of 1715 F/g has been reported which closely approaches the predicted theoretical maximum capacitance of 2000 F/g. In 2014, a supercapacitor anchored on a
graphene foam electrode delivered specific capacitance of 502.78 F/g and areal capacitance of 1.11 F/cm2) leading to a specific energy of 39.28 Wh/kg and specific power of 128.01 kW/kg over 8,000 cycles with constant performance. The device was a three-dimensional (3D) sub-5 nm hydrous ruthenium-anchored
graphene and
carbon nanotube (CNT) hybrid foam (RGM) architecture. The graphene foam was conformally covered with hybrid networks of
nanoparticles and anchored CNTs.
Conductive polymers Another approach uses
electron-conducting polymers as pseudocapacitive material. Although mechanically weak,
conductive polymers have high
conductivity, resulting in a low ESR and a relatively high capacitance. Such conducting polymers include
polyaniline,
polythiophene,
polypyrrole and
polyacetylene. Such electrodes also employ electrochemical doping or dedoping of the polymers with
anions and
cations. Electrodes made from, or coated with,
conductive polymers have costs comparable to
carbon electrodes. Conducting polymer electrodes generally suffer from limited cycling stability. However,
polyacene electrodes provide up to 10,000 cycles, much better than batteries.
Electrodes for hybrid capacitors All commercial hybrid supercapacitors are asymmetric. They combine an electrode with high amount of
pseudocapacitance with an electrode with a high amount of double-layer capacitance. In such systems the faradaic pseudocapacitance electrode with their higher capacitance provides high
specific energy while the non-faradaic EDLC electrode enables high
specific power. An advantage of the hybrid-type supercapacitors compared with symmetrical EDLC's is their higher specific capacitance value as well as their higher rated voltage and correspondingly their higher specific energy.
Advanced electrode materials Nickel-cobalt oxides (NiCo2O4): NiCo2O4 spinel structures synthesized via hydrothermal methods exhibit a theoretical capacitance of ~3,500 F/g due to synergistic redox contributions from nickel (Ni2+/Ni3+) and cobalt (Co2+/Co3+) ions. Asymmetric configurations pairing NiCo2O4 cathodes with activated carbon anodes achieve energy densities of 89.6 Wh/kg at 796 W/kg, retaining 93% capacitance after 10,000 cycles.
Graphene-metal oxide hybrids: Graphene-MnO2 nanocomposites leverage graphene's high electrical conductivity (106 S/m) and MnO2's pseudocapacitance. Atomic layer deposition (ALD) creates uniform MnO2 coatings on graphene nanosheets, achieving 1,100 F/g with 95% cycle stability over 5,000 cycles. These hybrids are scalable for grid storage applications.
High-temperature designs: ALD-coated barium titanate (BaTiO3) ceramics sintered at 1,100 °C exhibit permittivity >8,000 and breakdown voltages exceeding 500 V, enabling ultracapacitors for aerospace energy systems.
Composite electrodes Composite electrodes for hybrid-type supercapacitors are constructed from carbon-based material with incorporated or deposited pseudocapacitive active materials like metal oxides and conducting polymers. most research for supercapacitors explores composite electrodes. CNTs give a backbone for a homogeneous distribution of metal oxide or electrically conducting polymers (ECPs), producing good pseudocapacitance and good double-layer capacitance. These electrodes achieve higher capacitances than either pure carbon or pure metal oxide or polymer-based electrodes. This is attributed to the accessibility of the nanotubes' tangled mat structure, which allows a uniform coating of pseudocapacitive materials and three-dimensional charge distribution. The process to anchor pseudocapactive materials usually uses a hydrothermal process. However, a recent researcher, Li et al., from the University of Delaware found a facile and scalable approach to precipitate MnO2 on a SWNT film to make an organic-electrolyte based supercapacitor. Another way to enhance CNT electrodes is by doping with a pseudocapacitive dopant as in
lithium-ion capacitors. In this case the relatively small lithium atoms intercalate between the layers of carbon. The anode is made of lithium-doped carbon, which enables lower negative potential with a cathode made of activated carbon. This results in a larger voltage of 3.8-4 V that prevents electrolyte oxidation. As of 2007 they had achieved capacitance of 550 F/g.
Battery-type electrodes Rechargeable battery electrodes influenced the development of electrodes for new hybrid-type supercapacitor electrodes as for
lithium-ion capacitors. Together with a carbon EDLC electrode in an asymmetric construction offers this configuration higher specific energy than typical supercapacitors with higher specific power, longer cycle life and faster charging and recharging times than batteries.
Asymmetric electrodes (pseudo/EDLC) Recently some asymmetric hybrid supercapacitors were developed in which the positive electrode were based on a real pseudocapacitive metal oxide electrode (not a composite electrode), and the negative electrode on an EDLC activated carbon electrode. Asymmetric supercapacitors (ASC) have shown a great potential candidate for high-performance supercapacitor due to their wide operating potential which can remarkably enhance the capacitive behavior. An advantage of this type of supercapacitors is their higher voltage and correspondingly their higher specific energy (up to 10-20 Wh/kg (36-72 kJ/kg)).And they also have good cycling stability. For example, researchers use a kind of novel skutterudite Ni–CoP3 nanosheets and use it as positive electrodes with activated carbon (AC) as negative electrodes to fabricate asymmetric supercapacitor (ASC). It exhibits high energy density of 89.6 Wh/kg at 796 W/kg and stability of 93% after 10,000 cycles, which can be a great potential to be an excellent next-generation electrode candidate. The electrolyte must be chemically inert and not chemically attack the other materials in the capacitor to ensure long time stable behavior of the capacitor's electrical parameters. The electrolyte's viscosity must be low enough to wet the porous, sponge-like structure of the electrodes. An ideal electrolyte does not exist, forcing a compromise between performance and other requirements.
Water is a relatively good solvent for
inorganic chemicals. Treated with
acids such as
sulfuric acid (),
alkalis such as
potassium hydroxide (KOH), or
salts such as quaternary
phosphonium salts,
sodium perchlorate (),
lithium perchlorate () or lithium hexafluoride
arsenate (), water offers relatively high conductivity values of about 100 to 1000 m
S/cm. Aqueous electrolytes have a dissociation voltage of 1.15 V per electrode (2.3 V capacitor voltage) and a relatively low
operating temperature range. They are used in supercapacitors with low specific energy and high specific power. 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 () or triethyl (metyl) tetrafluoroborate () are more expensive than aqueous electrolytes, but they have a higher dissociation voltage of typically 1.35 V per electrode (2.7 V capacitor voltage), and a higher temperature range. The lower electrical conductivity of organic solvents (10 to 60 mS/cm) leads to a lower specific power, but since the specific energy increases with the square of the voltage, a higher specific energy.
Ionic electrolytes consists of liquid salts that can be stable in a wider
electrochemical window, enabling capacitor voltages above 3.5 V. Ionic electrolytes typically have an ionic conductivity of a few mS/cm, lower than aqueous or organic electrolytes.
Separators Separators have to physically separate the two electrodes to prevent a short circuit by direct contact. It can be very thin (a few hundredths of a millimeter) and must be very porous to the conducting ions to minimize ESR. Furthermore, separators must be chemically inert to protect the electrolyte's stability and conductivity. Inexpensive components use open capacitor papers. More sophisticated designs use nonwoven porous polymeric films like
polyacrylonitrile or
Kapton, woven glass fibers or porous woven ceramic fibres.
Collectors and housing Current collectors connect the electrodes to the capacitor's terminals. The collector is either sprayed onto the electrode or is a metal foil. They must be able to distribute peak currents of up to 100 A. If the housing is made of a metal (typically aluminum), the collectors should be made from the same material to avoid forming a corrosive
galvanic cell. == Electrical parameters ==