It has been suggested that Pseudocapacitor be merged into this article. (Discuss) Proposed since January 2024. |
Pseudocapacitance is the electrochemical storage of electricity in an electrochemical capacitor known as a pseudocapacitor . This faradaic charge transfer originates by a very fast sequence of reversible faradaic redox, electrosorption or intercalation processes on the surface of suitable electrodes. [1] [2] [3] Pseudocapacitance is accompanied by an electron charge-transfer between electrolyte and electrode coming from a de-solvated and adsorbed ion. One electron per charge unit is involved. The adsorbed ion has no chemical reaction with the atoms of the electrode (no chemical bonds arise [4] ) since only a charge-transfer takes place.
Faradaic pseudocapacitance only occurs together with static double-layer capacitance. Pseudocapacitance and double-layer capacitance both contribute inseparably to the total capacitance value.
The amount of pseudocapacitance depends on the surface area, material and structure of the electrodes. Pseudocapacitance may contribute more capacitance than double-layer capacitance for the same surface area by 100x. [1]
The amount of electric charge stored in a pseudocapacitance is linearly proportional to the applied voltage. The unit of pseudocapacitance is farad.
Redox reactions in batteries with faradaic charge-transfer between an electrolyte and the surface of an electrode were characterized decades ago. These chemical processes are associated with chemical reactions of the electrode materials usually with attendant phase changes. Although these chemical processes are relatively reversible, battery charge/discharge cycles often irreversibly produce unreversed chemical reaction products of the reagents. Accordingly, the cycle-life of rechargeable batteries is usually limited. Further, the reaction products lower power density. Additionally, the chemical processes are relatively slow, extending charge/discharge times.
A fundamental difference between redox reactions in batteries and in electrochemical capacitors (supercapacitors) is that in the latter, the reactions are a very fast sequence of reversible processes with electron transfer without any phase changes of the electrode molecules. They do not involve making or breaking chemical bonds. The de-solvated atoms or ions contributing the pseudocapacitance simply cling [4] to the atomic structure of the electrode and charges are distributed on surfaces by physical adsorption processes. Compared with batteries, supercapacitor faradaic processes are much faster and more stable over time, because they leave only traces of reaction products. Despite the reduced amount of these products, they cause capacitance degradation. This behavior is the essence of pseudocapacitance.
Pseudocapacitive processes lead to a charge-dependent, linear capacitive behavior, as well as the accomplishment of non-faradaic double-layer capacitance in contrast to batteries, which have a nearly charge-independent behavior. The amount of pseudocapacitance depends on the surface area, material and structure of the electrodes. The pseudocapacitance may exceed the value of double-layer capacitance for the same surface area by 100x. [1]
Applying a voltage at the capacitor terminals moves the polarized ions or charged atoms in the electrolyte to the opposite polarized electrode. Between the surfaces of the electrodes and the adjacent electrolyte an electric double-layer forms. One layer of ions on the electrode surface and the second layer of adjacent polarized and solvated ions in the electrolyte move to the opposite polarized electrode. The two ion layers are separated by a single layer of electrolyte molecules. Between the two layers, a static electric field forms that results in double-layer capacitance. Accompanied by the electric double-layer, some de-solvated electrolyte ions pervade the separating solvent layer and are adsorbed by the electrode's surface atoms. They are specifically adsorbed and deliver their charge to the electrode. In other words, the ions in the electrolyte within the Helmholtz double-layer also act as electron donors and transfer electrons to the electrode atoms, resulting in a faradaic current. This faradaic charge transfer, originated by a fast sequence of reversible redox reactions, electrosorptions or intercalation processes between electrolyte and the electrode surface is called pseudocapacitance. [5]
Depending on the electrode's structure or surface material, pseudocapacitance can originate when specifically adsorbed ions pervade the double-layer, proceeding in several one-electron stages. The electrons involved in the faradaic processes are transferred to or from the electrode's valence-electron states (orbitals) and flow through the external circuit to the opposite electrode where a second double-layer with an equal number of opposite-charged ions forms. The electrons remain in the strongly ionized and electrode surface's "electron hungry" transition-metal ions and are not transferred to the adsorbed ions. This kind of pseudocapacitance has a linear function within narrow limits and is determined by the potential-dependent degree of surface coverage of the adsorbed anions. The storage capacity of the pseudocapacitance is limited by the finite quantity of reagent or of available surface.
Systems that give rise to pseudocapacitance: [5]
All three types of electrochemical processes have appeared in supercapacitors. [5] [6]
When discharging pseudocapacitance, the charge transfer is reversed and the ions or atoms leave the double-layer and spread throughout the electrolyte.
Electrodes' ability to produce pseudocapacitance strongly depends on the electrode materials' chemical affinity to the ions adsorbed on the electrode surface as well as on the electrode pore structure and dimension. Materials exhibiting redox behavior for use as pseudocapacitor electrodes are transition-metal oxides inserted by doping in the conductive electrode material such as active carbon, as well as conducting polymers such as polyaniline or derivatives of polythiophene covering the electrode material.
These materials provide high pseudocapacitance and were thoroughly studied by Conway. [1] [7] Many oxides of transition metals like ruthenium (RuO
2), iridium (IrO
2), iron (Fe
3O
4), manganese (MnO
2) or sulfides such as titanium sulfide (TiS
2) or their combinations generate faradaic electron–transferring reactions with low conducting resistance.[ citation needed ]
Ruthenium dioxide (RuO
2) in combination with sulfuric acid (H
2SO
4) electrolyte provides one of the best examples of pseudocapacitance, with a charge/discharge over a window of about 1.2 V per electrode. Furthermore, the reversibility on these transition metal electrodes is excellent, with a cycle life of more than several hundred-thousand cycles. Pseudocapacitance originates from a coupled, reversible redox reaction with several oxidation steps with overlapping potential. The electrons mostly come from the electrode's valence orbitals. The electron transfer reaction is very fast and can be accompanied with high currents.
The electron transfer reaction takes place according to:
During charge and discharge, H+
(protons) are incorporated into or removed from the RuO
2 crystal lattice, which generates storage of electrical energy without chemical transformation. 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 reaction behaves like a capacitor rather than a battery, whose voltage is largely independent of the state of charge.
Another type of material with a high amount of pseudocapacitance is electron-conducting polymers. Conductive polymer such as polyaniline, polythiophene, polypyrrole and polyacetylene have a lower reversibility of the redox processes involving faradaic charge transfer than transition metal oxides, and suffer from a limited stability during cycling.[ citation needed ] Such electrodes employ electrochemical doping or dedoping of the polymers with anions and cations. Highest capacitance and power density are achieved with a n/p-type polymer configuration, with one negatively charged (n-doped) and one positively charged (p-doped) electrode.
Pseudocapacitance may originate from the electrode structure, especially from the material pore size. The use of carbide-derived carbons (CDCs) or carbon nanotubes (CNTs) as electrodes provides a network of small pores formed by nanotube entanglement. These nanoporous materials have diameters in the range of <2 nm that can be referred to as intercalated pores. Solvated ions in the electrolyte are unable to enter these small pores, but de-solvated ions that have reduced their ion dimensions are able to enter, resulting in larger ionic packing density and increased charge storage. The tailored sizes of pores in nano-structured carbon electrodes can maximize ion confinement, increasing specific capacitance by faradaic H
2 adsorption treatment. Occupation of these pores by de-solvated ions from the electrolyte solution occurs according to (faradaic) intercalation. [9] [10] [11]
Pseudocapacitance properties can be expressed in a cyclic voltammogram. For an ideal double-layer capacitor, the current flow is reversed immediately upon reversing the potential yielding a rectangular-shaped voltammogram, with a current independent of the electrode potential. For double-layer capacitors with resistive losses, the shape changes to a parallelogram. In faradaic electrodes the electrical charge stored in the capacitor is strongly dependent on the potential, therefore, the voltammetry characteristics deviate from the parallelogram due to a delay while reversing the potential, ultimately coming from kinetic charging processes. [12] [13]
Pseudocapacitance is an important property in supercapacitors.
In electrochemistry, cyclic voltammetry (CV) is a type of potentiodynamic measurement. In a cyclic voltammetry experiment, the working electrode potential is ramped linearly versus time. Unlike in linear sweep voltammetry, after the set potential is reached in a CV experiment, the working electrode's potential is ramped in the opposite direction to return to the initial potential. These cycles of ramps in potential may be repeated as many times as needed. The current at the working electrode is plotted versus the applied voltage to give the cyclic voltammogram trace. Cyclic voltammetry is generally used to study the electrochemical properties of an analyte in solution or of a molecule that is adsorbed onto the electrode.
Capacitors are manufactured in many styles, forms, dimensions, and from a large variety of materials. They all contain at least two electrical conductors, called plates, separated by an insulating layer (dielectric). Capacitors are widely used as parts of electrical circuits in many common electrical devices.
A flow battery, or redox flow battery, is a type of electrochemical cell where chemical energy is provided by two chemical components dissolved in liquids that are pumped through the system on separate sides of a membrane. Ion transfer inside the cell occurs across the membrane while the liquids circulate in their respective spaces.
Nanobatteries are fabricated batteries employing technology at the nanoscale, particles that measure less than 100 nanometers or 10−7 meters. These batteries may be nano in size or may use nanotechnology in a macro scale battery. Nanoscale batteries can be combined to function as a macrobattery such as within a nanopore battery.
A paper battery is engineered to use a spacer formed largely of cellulose. It incorporates nanoscopic scale structures to act as high surface-area electrodes to improve conductivity.
In surface science, a double layer is a structure that appears on the surface of an object when it is exposed to a fluid. The object might be a solid particle, a gas bubble, a liquid droplet, or a porous body. The DL refers to two parallel layers of charge surrounding the object. The first layer, the surface charge, consists of ions which are adsorbed onto the object due to chemical interactions. The second layer is composed of ions attracted to the surface charge via the Coulomb force, electrically screening the first layer. This second layer is loosely associated with the object. It is made of free ions that move in the fluid under the influence of electric attraction and thermal motion rather than being firmly anchored. It is thus called the "diffuse layer".
A lithium-ion capacitor is a hybrid type of capacitor classified as a type of supercapacitor. It is called a hybrid because the anode is the same as those used in lithium-ion batteries and the cathode is the same as those used in supercapacitors. Activated carbon is typically used as the cathode. The anode of the LIC consists of carbon material which is often pre-doped with lithium ions. This pre-doping process lowers the potential of the anode and allows a relatively high output voltage compared to other supercapacitors.
Nanoarchitectures for lithium-ion batteries are attempts to employ nanotechnology to improve the design of lithium-ion batteries. Research in lithium-ion batteries focuses on improving energy density, power density, safety, durability and cost.
The lithium–air battery (Li–air) is a metal–air electrochemical cell or battery chemistry that uses oxidation of lithium at the anode and reduction of oxygen at the cathode to induce a current flow.
Capacitive deionization (CDI) is a technology to deionize water by applying an electrical potential difference over two electrodes, which are often made of porous carbon. In other words, CDI is an electro-sorption method using a combination of a sorption media and an electrical field to separate ions and charged particles. Anions, ions with a negative charge, are removed from the water and are stored in the positively polarized electrode. Likewise, cations are stored in the cathode, which is the negatively polarized electrode.
Carbide-derived carbon (CDC), also known as tunable nanoporous carbon, is the common term for carbon materials derived from carbide precursors, such as binary (e.g. SiC, TiC), or ternary carbides, also known as MAX phases (e.g., Ti2AlC, Ti3SiC2). CDCs have also been derived from polymer-derived ceramics such as Si-O-C or Ti-C, and carbonitrides, such as Si-N-C. CDCs can occur in various structures, ranging from amorphous to crystalline carbon, from sp2- to sp3-bonded, and from highly porous to fully dense. Among others, the following carbon structures have been derived from carbide precursors: micro- and mesoporous carbon, amorphous carbon, carbon nanotubes, onion-like carbon, nanocrystalline diamond, graphene, and graphite. Among carbon materials, microporous CDCs exhibit some of the highest reported specific surface areas (up to more than 3000 m2/g). By varying the type of the precursor and the CDC synthesis conditions, microporous and mesoporous structures with controllable average pore size and pore size distributions can be produced. Depending on the precursor and the synthesis conditions, the average pore size control can be applied at sub-Angstrom accuracy. This ability to precisely tune the size and shapes of pores makes CDCs attractive for selective sorption and storage of liquids and gases (e.g., hydrogen, methane, CO2) and the high electric conductivity and electrochemical stability allows these structures to be effectively implemented in electrical energy storage and capacitive water desalinization.
Pseudocapacitors store electrical energy faradaically by electron charge transfer between electrode and electrolyte. This is accomplished through electrosorption, reduction-oxidation reactions, and intercalation processes, termed pseudocapacitance.
A potassium-ion battery or K-ion battery is a type of battery and analogue to lithium-ion batteries, using potassium ions for charge transfer instead of lithium ions.
A supercapacitor (SC), also called an ultracapacitor, is a high-capacity capacitor, with a capacitance value much higher than solid-state capacitors but with lower voltage limits. It bridges the gap between electrolytic capacitors and rechargeable batteries. It typically stores 10 to 100 times more energy per unit volume or mass than electrolytic capacitors, can accept and deliver charge much faster than batteries, and tolerates many more charge and discharge cycles than rechargeable batteries.
Double-layer capacitance is the important characteristic of the electrical double layer which appears at the interface between a surface and a fluid. At this boundary two layers of electric charge with opposing polarity form, one at the surface of the electrode, and one in the electrolyte. These two layers, electrons on the electrode and ions in the electrolyte, are typically separated by a single layer of solvent molecules that adhere to the surface of the electrode and act like a dielectric in a conventional capacitor. The amount of charge stored in double-layer capacitor depends on the applied voltage.
Research in lithium-ion batteries has produced many proposed refinements of lithium-ion batteries. Areas of research interest have focused on improving energy density, safety, rate capability, cycle durability, flexibility, and reducing cost.
In materials science, MXenes are a class of two-dimensional inorganic compounds along with MBenes, that consist of atomically thin layers of transition metal carbides, nitrides, or carbonitrides. MXenes accept a variety of hydrophilic terminations. The first MXene was reported in 2011 at Drexel University's College of Engineering.
In electrochemistry, faradaic impedance is the resistance and capacitance acting jointly at the surface of an electrode of an electrochemical cell. The cell may be operating as either a galvanic cell generating an electric current or inversely as an electrolytic cell using an electric current to drive a chemical reaction. In the simplest nontrivial case faradaic impedance is modeled as a single resistor and single capacitor connected in parallel, as opposed say to in series or as a transmission line with multiple resistors and capacitors.
Linda Faye Nazar is a Senior Canada Research Chair in Solid State Materials and Distinguished Research Professor of Chemistry at the University of Waterloo. She develops materials for electrochemical energy storage and conversion. Nazar demonstrated that interwoven composites could be used to improve the energy density of lithium–sulphur batteries. She was awarded the 2019 Chemical Institute of Canada Medal.
This is a history of the lithium-ion battery.