Electric double-layer (BMD model) NT-int

Pseudocapacitors store electrical energy faradaically by electron charge transfer between

electrode An electrode is an electrical conductor used to make contact with a nonmetallic part of a circuit (e.g. a semiconductor, an electrolyte, a vacuum or air). Electrodes are essential parts of batteries that can consist of a variety of materials de ...

and

electrolyte An electrolyte is a medium containing ions that is electrically conducting through the movement of those ions, but not conducting electrons. This includes most soluble salts, acids, and bases dissolved in a polar solvent, such as water. Upon dis ...

. This is accomplished through electrosorption, reduction-oxidation reactions (

redox reactions Redox (reduction–oxidation, , ) is a type of chemical reaction in which the oxidation states of substrate change. Oxidation is the loss of electrons or an increase in the oxidation state, while reduction is the gain of electrons or a d ...

), and intercalation processes, termed ''

pseudocapacitance Pseudocapacitance is the Electrochemistry, electrochemical storage of electricity in an Supercapacitor, electrochemical capacitor (Pseudocapacitor). This faradaic charge transfer originates by a very fast sequence of reversible Faradaic current, fa ...

''. A pseudocapacitor is part of an

electrochemical Electrochemistry is the branch of physical chemistry concerned with the relationship between electrical potential difference, as a measurable and quantitative phenomenon, and identifiable chemical change, with the potential difference as an outco ...

capacitor A capacitor is a device that stores electrical energy in an electric field by virtue of accumulating electric charges on two close surfaces insulated from each other. It is a passive electronic component with two terminals. The effect of ...

, and forms together with an

electric double-layer capacitor A supercapacitor (SC), also called an ultracapacitor, is a high-capacity capacitor, with a capacitance value much higher than other capacitors but with lower voltage limits. It bridges the gap between electrolytic capacitors and rechargeable b ...

(EDLC) to create a

supercapacitor A supercapacitor (SC), also called an ultracapacitor, is a high-capacity capacitor, with a capacitance value much higher than other capacitors but with lower voltage limits. It bridges the gap between electrolytic capacitors and rechargeable ba ...

. Pseudocapacitance and

double-layer capacitance Double-layer capacitance is the important characteristic of the electrical double layer which appears, for example, at the interface between a conductive electrode and an adjacent liquid electrolyte. At this boundary two layers of charge with oppo ...

add up to a common inseparable capacitance value of a supercapacitor. However, they can be effective with very different parts of the total capacitance value depending on the design of the electrodes. A pseudocapacitance may be higher by a factor of 100 as a double-layer capacitance with the same electrode surface. A pseudocapacitor has a chemical reaction at the electrode, unlike EDLCs where the electrical charge storage is stored electrostatically with no interaction between the electrode and the ions. Pseudocapacitance is accompanied by an

electron The electron ( or ) is a subatomic particle with a negative one elementary electric charge. Electrons belong to the first generation of the lepton particle family, and are generally thought to be elementary particles because they have no kn ...

charge-transfer between

and electrode coming from a de-solvated and

adsorbed Adsorption is the adhesion of atoms, ions or molecules from a gas, liquid or dissolved solid to a surface. This process creates a film of the ''adsorbate'' on the surface of the ''adsorbent''. This process differs from absorption, in which a ...

ion An ion () is an atom or molecule with a net electrical charge. The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by conve ...

. One electron per charge unit is involved. The adsorbed ion has no

chemical reaction A chemical reaction is a process that leads to the IUPAC nomenclature for organic transformations, chemical transformation of one set of chemical substances to another. Classically, chemical reactions encompass changes that only involve the pos ...

with the

atom Every atom is composed of a nucleus and one or more electrons bound to the nucleus. The nucleus is made of one or more protons and a number of neutrons. Only the most common variety of hydrogen has no neutrons. Every solid, liquid, gas, and ...

s of the electrode (no

chemical bond A chemical bond is a lasting attraction between atoms or ions that enables the formation of molecules and crystals. The bond may result from the electrostatic force between oppositely charged ions as in ionic bonds, or through the sharing of ...

s arise) since only a charge-transfer takes place. An example is a redox reaction where the ion is O²⁺ and during charging, one electrode hosts a reduction reaction and the other an oxidation reaction. Under discharge the reactions are reversed. Unlike batteries, in faradaic electron charge-transfer ions simply cling to the atomic structure of an electrode. This faradaic energy storage with only fast redox reactions makes charging and discharging much faster than batteries. Electrochemical pseudocapacitors use

metal oxide An oxide () is a chemical compound that contains at least one oxygen atom and one other element in its chemical formula. "Oxide" itself is the dianion of oxygen, an O2– (molecular) ion. with oxygen in the oxidation state of −2. Most of the E ...

conductive polymer Conductive polymers or, more precisely, intrinsically conducting polymers (ICPs) are organic polymers that conduct electricity. Such compounds may have metallic conductivity or can be semiconductors. The biggest advantage of conductive polymer ...

electrodes with a high amount of electrochemical pseudocapacitance. The amount of

electric charge Electric charge is the physical property of matter that causes charged matter to experience a force when placed in an electromagnetic field. Electric charge can be ''positive'' or ''negative'' (commonly carried by protons and electrons respe ...

stored in a pseudocapacitance is linearly proportional to the applied

voltage Voltage, also known as electric pressure, electric tension, or (electric) potential difference, is the difference in electric potential between two points. In a static electric field, it corresponds to the work needed per unit of charge to m ...

. The unit of pseudocapacitance is the

farad The farad (symbol: F) is the unit of electrical capacitance, the ability of a body to store an electrical charge, in the International System of Units (SI). It is named after the English physicist Michael Faraday (1791–1867). In SI base unit ...

Examples of Pseudocapacitors

Brezesinki et al. showed that mesoporous films of ''α''-MoO₃ have improved charge storage due to lithium ions inserting into the gaps of ''α''-MoO₃. They claim this intercalation pseudocapacitance takes place on the same timescale as redox pseudocapacitance and gives better charge-storage capacity without changing kinetics in mesoporous MoO₃. This approach is promising for batteries with rapid charging ability, comparable to that of lithium batteries, and is promising for efficient energy materials. Other groups have used vanadium oxide thin films on carbon nanotubes for pseudocapacitors. Kim et al. electrochemically deposited amorphous V₂O₅·H₂O onto a carbon nanotube film. The three-dimensional structure of the carbon nanotubes substrate facilitates high specific lithium-ion capacitance and shows three times higher capacitance than vanadium oxide deposited on a typical Pt substrate. These studies demonstrate the capability of deposited oxides to effectively store charge in pseudocapacitors. Conducting polymers, such as polypyrrole (PPy) and poly(3,4-ethylenedioxythiophene) (PEDOT), have tunable electronic conductivity and can achieve high doping levels with the proper counterion. A high-performing conducting polymer pseudocapacitor has high cycling stability after undergoing charge/discharge cycles. Successful approaches include embedding the redox polymer in a host phase (e.g. titanium carbide) for stability and depositing a carbonaceous shell onto the conducting polymer electrode. These techniques improve cyclability and stability of the pseudocapacitor device.{{Cite journal , author-last1=Bryan , author-first1=Aimee M. , author-last2=Santino , author-first2=Luciano M. , author-last3=Lu , author-first3=Yang , author-last4=Acharya , author-first4=Shinjita , author-last5=D’Arcy , author-first5=Julio M. , date=2016-09-13 , title=Conducting Polymers for Pseudocapacitive Energy Storage , journal=Chemistry of Materials , volume=28 , issue=17 , pages=5989–5998 , doi=10.1021/acs.chemmater.6b01762 , issn=0897-4756

References

Capacitors