Electrochemistry Dictionary
Simple and brief definitions of words and phrases used often in
electrochemistry. In some cases, a second paragraph provides further
information for the "more scientifically minded."
(http://electrochem.cwru.edu/ed/dict.htm)
Revision date: March 16, 2008.
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There are presently more than 900 entries in the dictionary, but it is far from complete. More terms are added at irregular intervals. The selection process to include terms is purely random, and it usually results in a chain reaction. Suggestions of terms to be included are welcome, together with corrections of the inevitable errors. Send them to: nagyz@email.unc.edu.
There are also numerous educational WWW sites where more detailed information can be found about many of the terms listed in this dictionary:
http://electrochem.cwru.edu/estir/inet.htm#educ.
See also a listing of popular science books and articles in popular magazines:
http://electrochem.cwru.edu/estir/pop.htm
Visit also the sister sites of the Dictionary:
Electrochemistry Encyclopedia (http://electrochem.cwru.edu/ed/encycl/)
Electrochemical Science and Technology Information Resource (ESTIR) (http://electrochem.cwru.edu/estir/)
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Symbol and abbreviation of ampere.
A process "to take in and incorporate." E.g., light can be "absorbed" by a material. In chemistry, a term often used to describe the dissolution of a gas into a liquid or solid. The dissolving gas is said to be "absorbed." Or a liquid substance can be "absorbed" by a solid. This is a bulk process, not to be confused with adsorption.
Abbreviation of alternating current. However this term is also used in connection with ac voltage, that is, an "alternating" voltage that will cause an "ac current" to flow in a conductor, and also in connection with ac power. Contrast with dc.
See rechargeable battery.
A compound that dissociates to produce hydrogen (H+) cations when dissolved in water. Contrast with base. See also pH.
See pH.
A term used in bioelectrochemistry: electrical signal associated with nerve impulse. A temporary change (possibly a momentary reversal) in electrical potential that occurs across a membrane (between the inside and the outside of a nerve or muscle fiber) when an impulse is transmitted due to stimulation.
A metal that is easily oxidized (corroded) in air. For example, sodium will violently react with air, aluminum will always have an air-formed oxide film on its surface, and iron is easily rusted. These metals have high negative standard electrode potentials and are high the on the electromotive series. Contrast with noble metal.
The overpotential (alternatively called polarization) associated with the charge-transfer reaction elementary step in the overall electrode reaction.
The activity of a dissolved species in solution is the "effective" concentration of that species.
In an "ideal" solution, the molecules in the solution do not interact with each other and the concentration and the activity are identical. This is the case for very dilute solutions. In a "real" solution, there is a certain interaction between the molecules resulting in a diminished "activity" of the molecules toward the outside world, and the solution behaves like it would contain lower concentration of the dissolved species than it actually does. The activity can be expressed as the product of an "activity coefficient" and the concentration.
See activity.
An atom adsorbed on the surface of an electrode.
An ion adsorbed on the surface of an electrode.
A material that is adsorbed.
An increase of the concentration of a solute in the vicinity of a solid surface, over that in the bulk of the solution, due to the attractive interaction between the solid immersed into the solution and the solute. Adsorption on a solid from a gaseous phase also occurs. It is a surface process, not to be confused with absorption. Opposite: desorption.
Abbreviation of auxiliary electrode.
Symbol and abbreviation of ampere-hour.
See base.
See brine electrolysis.
See Edison battery.
A modern version of the Leclanche cell containing basic (potassium hydroxide) electrolyte. It has considerably improved characteristics and it is slowly replacing the Leclanche cell.
A fuel cell containing an alkaline electrolyte.
See pH.
See current. Abbreviated as "ac."
Aluminum metal is produced by electrolysis of aluminum oxide dissolved in a high-temperature molten-salt electrolyte. Aluminum is deposited as a liquid metal on the cathode of the electrolytic cell (the aluminum cations are reduced to liquid metal). This is the only large-scale industrial process for the production of aluminum. See also an Encyclopedia Article.
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An alloy of mercury and another metal.
Instrument used for the measurement of current.
Symbol and abbreviation of ampere.
Measurement unit of current. Abbreviation: "A" or "amp".
An alternative unit of electrical charge. One ampere-hour = 3,600 coulombs. Symbol: "Ah".
See coulometric efficiency.
An electroanalytical technique based upon the measurement of the current flowing through the working electrode of an electrochemical cell.
See galvanostat.
A substance whose chemical composition is to be determined by chemical analysis.
A substance that does not contain water. The opposite of hydrous.
A negatively charged ion.
The electrode where oxidation occurs in an electrochemical cell. It is the positive electrode in an electrolytic cell, while it is the negative electrode in a galvanic cell. The current on the anode is considered a positive current according to international convention; however, in electroanalytical chemistry the anodic current is often considered negative. Contrast with cathode.
A condition in an electrolytic cell that produces an abrupt increase in cell voltage and a decrease in current flow. It is usually caused by the temporary formation of an insulating layer on the anode surface. It occurs almost exclusively in molten salt electrolysis, such as in aluminum production.
The insoluble residue that derives from the anodic dissolution of an impure metal such as copper during electrorefining. Also called "anode slime."
See anode mud.
See partial current density.
A process for corrosion protection of a metal or alloy achieved by impressing upon the metal an anodic current of sufficient magnitude to cause the formation of a passive film. Anodic protection is effective only for metals that are prone to passivate, such as stainless steel and titanium. See also an Encyclopedia Article and cathodic protection.
A process to produce an oxide film or coating on metals and alloys by electrolysis. The metal to be treated is made the anode in an electrolytic cell and its surface is electrochemically oxidized. Anodization can improve certain surface properties, such as corrosion resistance, abrasion resistance, hardness, appearance, etc. One metal very often anodized is aluminum, all the above properties improve, furthermore, since the surface film is porous, the aluminum metal can even be colored by the application of pigments or dies in the pores. See also an Encyclopedia Article.
The electrolyte on the anode side of an electrochemical cell that is divided into compartments.
A solution with water as the solvent. Contrast with: non-aqueous solution.
The smallest physical unit of a chemical element that can still retain all the physical and chemical properties of that element. Atoms combine to form molecules, and they themselves contain several kinds of smaller particles. An atom has a dense central core (the nucleus) consisting of positively charged particles (protons) and uncharged particles (neutrons). Negatively charged particles (electrons) are scattered in a relatively large space around this nucleus and move about it in orbital patterns at extremely high speeds. An atom contains the same number of protons as electrons and thus is electrically neutral (uncharged) and stable under most conditions.
The average relative weight of a chemical element as it occurs in nature referred to some element taken as a standard.
See disproportionation.
See counter electrode. Abbreviated as AE.
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See residual current (density).
A thin, continuous, non-porous, electrically insulating film on metal surfaces (usually comprised of oxides).
A compound that dissociates to produce hydroxyl (OH-) anions when dissolved in water (also called "caustic" or "alkali"). Contrast with acid. See also pH.
See pH.
An electroplating or anodizing solution is often called "bath."
A device that stores electrical energy using electrochemical cells. Chemical reactions occur spontaneously at the electrodes when they are connected trough an external circuit, producing an electrical current. The physical construction of the battery is such that it does not permit the intermixing and consequent direct reaction of the chemicals stored in it. See also rechargeable battery and non-rechargeable battery, and an Encyclopedia Article.
Strictly speaking, a battery should consist of several, internally connected,
electrochemical cells. (The individual cells in a battery can be series or parallel coupled, or a combination of both.) However, in present usage all storage devices (single
cell and multiple cell) are called batteries.
See charger.
Electrochemistry of biological (living) systems and biological compounds.
An electrode that is shared by two series-coupled electrochemical cells in such a way that one side of the (usually planar) electrode acts as an anode in one cell and the other side acts as a cathode in the other cell. In storage batteries and fuel cell stacks many cells are usually connected internally, and it is a very efficient design feature to use a single planar structure for electrodes in two neighboring cells and also as the electrical interconnection between them.
A potentiostat that controls the potential of two working electrodes independently in the same cell. Typically used in conjunction with a rotating-ring-disk electrode or with scanning electrochemical microscopy.
See brine electrolysis.
See trickle charging.
See hydrodynamic boundary layer.
Small amounts of (usually organic) compounds added to an electroplating solution that changes the mechanism of the plating to produce "bright" (mirror like) metal deposits.
Electrolysis of an aqueous solution of common table salt (sodium chloride), also called "brine," results in the production of chlorine gas at the anode and hydrogen gas at the cathode. Since the hydrogen is produced by breaking up water molecules, the solution is becoming basic around the cathode and a solution of sodium hydroxide (also called "caustic" or "alkali") is produced. If the electrolysis is carried out in a divided cell, the products are chlorine, caustic, and hydrogen. If the electrolysis is carried out in an undivided cell and the chlorine gas and the caustic are allowed to mix and react with each other, sodium hypochlorite (bleach) is produced if the cell operates close to room temperature, and sodium chlorate is produced if the cell is operated near the boiling point of water. Electrolysis is the only large-scale industrial process for the production of chlorine gas and these chlorine chemicals. Smaller scale cells are also used to produce chlorine-based disinfectants in municipal water-treatment plants and for swimming pools. See also an Encyclopedia Article.
The overall cell reaction is:
2NaCl + 2H2O = Cl2 + H2 +2NaOH
Chlorine gas and sodium hydroxide react to form sodium hypochlorite and sodium chloride:
Cl2 + 2NaOH = NaOCl + NaCl + H2O
Sodium hypochlorite will react further at high temperature to form sodium chlorate and sodium chloride:
3NaOCl = NaClO3 + 2NaCl
(although, sodium chlorate can also form by direct electrochemical oxidation).
A solution with a constant, specified pH. The pH of the solution "resists" any change: addition of small amounts of solvent or even acid or base will not appreciable change the pH. This is called "buffer capacity."
A typically not insulated (not covered with insulation) conductor used to carry a large current or to make a common connection between several circuits.
A (rather complicated) equation expressing the relation between the overpotential of the electrode and the current density passing through the electrode. At low overpotentials it can be very well approximated by a linear relation, and at high overpotentials by the Tafel equation.
See coin cell.
See current leakage.
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Symbol and abbreviation of coulomb.
A commonly used reference electrode. It is very similar to the silver/silver-chloride electrode both in construction and in theory of operation. The silver metal is replaced by mercury (electrical connection is made by an inert metal wire), the salt is mercury chloride, and the solution is saturated potassium chloride. Abbreviated as "SCE," for: "saturated calomel electrode."
The equilibrium electrode potential is a function of the chloride concentration of the internal electrolyte ("filling solution"). The electrolyte is practically always saturated potassium chloride (hence the name: "saturated calomel electrode," SCE, "calomel" is an old name for mercurous chloride), producing a potential of 0.244 volt against the standard hydrogen electrode at 25oC (77oF).
The capacitance value expresses the ability of a capacitor to store electrical charge. The unit of capacitance is the farad.
The current (or current density) flowing through an electrochemical cell that is charging/discharging the electrical double layer capacitance. This current does not involve any chemical reactions (charge transfer), it only causes accumulation (or removal) of electrical charges on the electrode and in the electrolyte solution near the electrode. There is always some capacitive current flowing when the potential of an electrode is changing, and the capacitive current is generally zero when the potential is constant. Also called "non-faradaic" or "double-layer" current. Contrast with faradaic current.
Capacitive current can also flow at constant potential if the capacitance of the electrode is changing for some reason, e.g., change of electrode area or temperature.
An electrical device which serves to store electricity or electrical energy. It has three essential parts: two electrical conductors, which are usually metal plates, separated and insulated by the third part called the dielectric. The plates are charged with equal amounts of positive and negative electrical charges, respectively. This is a "physical" storage of electricity as compared with the "chemical" storage in a battery. See also electrolytic capacitors and electrochemical capacitors.
See capacitance. The term "capacity" is also used in a somewhat different meaning for batteries: it expresses the total amount of electrical charge a battery is able to hold. It is usually expressed in ampere-hours.
A tube with very small bore.
The phenomenon of increasing the rate of a chemical reaction by a chemical present in the reaction medium (hogeneous catalysis), or by a solid surface on which the reaction can occur (heterogeneous catalysis). The catalyst itself is not consumed or transformed in any way during such reactions.
A material that can cause catalysis.
Alternative name for "electrophoresis." See electrokinetic effects.
The electrode where reduction occurs in an electrochemical cell. It is the negative electrode in an electrolytic cell, while it is the positive electrode in a galvanic cell. The current on the cathode is considered a negative current according to international convention; however, in electroanalytical chemistry the cathodic current is often considered positive. Contrast with anode.
See partial current density.
A corrosion protection technique whereby a structure to be protected is made the cathode of an electrochemical cell. This can be achieved in two ways. In the "impressed current" technique, a cathodic current of sufficient magnitude is forced on the structure from a power source. For example, an underground pipeline is connected to the negative terminal of a power source, while the positive terminal is connected to a nearby buried non-corroding electrode. In the "sacrificial protection" technique, the purposeful corrosion of a less desirable metal protects a preferred metal. For example, a steel structure can be protected in seawater by contacting with a piece of zinc. The zinc (the more active metal) will become the anode and the steel (the more noble metal) the cathode of the resulting corrosion cell. Consequently, the zinc will be oxidized while the steel will be protected (as long as the zinc lasts). See also an Encyclopedia Article and anodic protection.
The electrolyte on the cathode side of an electrochemical cell that is divided into compartments.
A positively charged ion.
See base. Sometimes it specifically refers to sodium hydroxide.
See brine electrolysis.
Stands for closed-circuit voltage.
Abbreviation of counter electrode.
See electrochemical cell.
See conductivity cell.
See separator
A series-coupled assembly of cells, a term used primarily in
industrial electrolysis using electrolytic cells.
The overall chemical reaction occurring in the electrochemical cell. It is the sum of the two electrode reactions.
The electrical potential difference between the two electrodes of an electrochemical cell. In case of a three-electrode cell, the potential difference between the working electrode and the counter electrode.
"Cell voltage" usually refers to nonequilibrium conditions, that is when current is flowing through the cell (although this convention is not always followed). The "cell voltage" differs from the electromotive force (emf) (or open-circuit voltage (ocv)) of the cell by the amount of the overvoltage. The term "voltage" is usually reserved for the case when an electrochemical cell is under consideration, while the term "potential" is usually reserved for the case when an electrode is considered. (Of course, the latter case is still an "electrochemical cell" consisting of the electrode under consideration and a reference electrode.) Unfortunately, the terms "voltage" and "potential" are sometimes used interchangeably.
The process of spontaneous reduction of the ions of a metal by another metal above it in the electromotive series. For example, a piece of iron immersed in copper sulfate solution will be immediately covered by a thin film of copper. The iron is being anodically dissolved while copper is electroplated on its surface cathodically. Also called "metal displacement reaction" and "galvanic displacement."
One hundredth of a meter. Symbol: “cm”.
See the Gouy-Chapman model of the double layer.
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See electrical charge.
The particle carrying the electrical charge during the flow of electrical current. In metallic conductors the charge carriers are electrons, while ions carry the charges in electrolyte solutions.
Charge referred to the unit area of the electrode. Charge divided by electrode area.
See coulometric efficiency.
See equalization.
An electrical device used to charge a rechargeable battery using household electricity.
The current applied to a rechargeable battery to restore its capacity. This rate is commonly expressed as a multiple of the rated capacity. See C-rate.
See activation overpotential.
A chemical reaction where an electrical charge (usually an electron) is transferred from one reactant to another. See also heterogeneous charge-transfer reaction and homogeneous charge-transfer reaction. In case of an electrode reaction, the electrode itself is considered one of the "reactants." An electrode reaction is a heterogeneous charge-transfer reaction.
A characteristic quantity for an electrode reaction indicative of its inherent speed: a large charge-transfer resistance indicates a slow reaction. See also non-ohmic resistance.
The phenomenon of movement (transportation) of electrical charge from one part of the system to another, occurring through electromigration.
See mass transport for a discussion of the decoupling of mass and charge transport.
A process to "fill" a rechargeable battery or a capacitor with electricity by applying a current to its terminals. In a battery, this process will cause electrochemical reactions to occur storing the electricity in chemical form. In contrast, during the charging of a capacitor the electricity is stored as electrical charges, without causing any chemical reactions to occur. Opposite: discharging.
See kinetics.
An electrode coated with a thin (possibly monomolecular) layer of a chemical that changes the elctrode's electrochemical behavior. Also called "modified electrode." Abbreviated as "CME."
See brine electrolysis.
See brine electrolysis.
See brine electrolysis.
See brine electrolysis.
An electrochemical measuring technique used for electrochemical analysis or for the determination of the kinetics and mechanism of electrode reactions. A fast-rising potential pulse is enforced on the working electrode of an electrochemical cell and the current flowing through this electrode is measured as a function of time. See also Cottrell equation.
An electrochemical measuring technique used for electrochemical analysis or for the determination of the kinetics and mechanism of electrode reactions. A fast-rising potential pulse is enforced on the working electrode of an electrochemical cell and the electrical charge passing through this electrode is measured as a function of time.
An electrochemical measuring technique used for electrochemical analysis or for the determination of the kinetics and mechanism of electrode reactions. A fast-rising current pulse is enforced on the working electrode of an electrochemical cell and the potential of this electrode is measured against a reference electrode as a function of time.
In an unstirred solution, the potential will rise to the electrode potential of the reaction requiring the least amount of energy to proceed, and it will increase in time due to the concentration overpotential developing as the concentration of the reactant is exhausted at the electrode surface. If the current is larger than the limiting current, eventually the diffusional process will not be able to provide the required flux for the current, and the electrode potential will sharply rise (at the transition time) until it reaches the electrode potential of the next available reaction in the solution, and so on. See also Sand equation.
An amperometric sensor assembly used for the measurement of dissolved oxygen concentration in water or aqueous solutions. It is a two-electrode electrochemical cell with the working electrode (typically positioned at the end of a tubular structure) separated from the test solution by a thin membrane permeable to oxygen. The oxygen diffusing through the membrane is reduced at the electrode and the current produced is proportional to the concentration of the dissolved oxygen (calibration required).
The voltage of a battery when it is discharging (on-load condition). Abbreviated as "ccv."
Symbol of centimeter.
Stands for chemically modified electrode.
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A miniature non-rechargeable battery, in the shape and size of a small coin used to power small electronic devices, e.g. watches and hearing aids. Also called "button" cell. See also an Encyclopedia Article.
An assembly that combines an ion-selective electrode and a reference electrode in one physical structure (typically in a tubular form).This can be conveniently used for the determination of ionic concentrations in test solutions. It is most often used for pH measurements.
See fuel cell.
See the Helmholtz model of the double layer.
Alternative expression for ideal polarizable electrode.
The maximum value of the current and voltage that a control instrument (e.g., galvanostat or potentiostat) is capable to provide.
The measure of the amount of dissolved material (solute) in a solution. It can be expressed in a variety of ways. Expressions in weight percent, and grams of solute per liter of solution are common. A more fundamental way to express concentration is used in chemistry: the "molar" concentration. A solution is considered one molar (1 M) if it contains as many grams of solute per liter of solution as is the molecular weight of the solute (the so called gram-mole). This provides an atomistically fundamental expression because one gram-mole of any material will contain the same (and very large) number of molecules. One gram-mole of hydrogen gas contains the exact same number of molecules as one gram-mole of table salt (sodium chloride), even though the latter is much heavier. In this dictionary, the term "concentration" always designates "molarity" unless otherwise specified.
A galvanic cell in which the chemical energy converted into electrical energy is arising from the concentration difference of a species at the two electrodes of the cell. An example is a divided cell consisting of two silver electrodes surrounded by silver nitrate solutions of different concentrations. Nature will tend to equalize the concentrations. Consequently, silver cations will be spontaneously reduced to silver metal at the electrode (cathode) in the higher concentration solution, while the silver electrode (anode) in the lower concentration solution will be oxidized to silver cations. Electrons will be flowing through the external circuit (from the anode or negative electrode to the cathode or positive electrode) producing a current, and nitrate anions will diffuse through the separator. This process will continue till the silver nitrate concentration is equalized in the two compartments of the cell.
The overpotential (alternatively called polarization) associated with the diffusional transport of the reactants to the electrode surface from the bulk of the electrolyte and the reverse transport of the products. The diffusion is an elementary step in the overall electrode reaction. Also called "diffusion overpotential" or "mass-transport overpotential."
See capacitor.
For a rechargeable battery: see forming.
See conductivity.
A polymeric material (e.g., plastics) having electronic conductivity.
The measure of a material's capability to carry electrical current. The measurement unit of conductivity (conductance) is the siemens.
The reciprocal of resistivity.
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A cell specially designed for the measurement of the conductivity of an electrolyte solution. It is a small vessel containing two metallic electrodes, the cell is filled with the solution to be measured. Also called "conductance cell."
The measurement of the conductivity of an electrolyte solution is more complicated than a similar measurement with a metallic conductor. When measuring with dc current, one would have to take into consideration the electromotive force of the electrochemical cell, and the polarization of the electrodes. Therefore, the measurements are typically carried out with high frequency ac current and the electrodes in the conductivity cell are typically made of platinized platinum to avoid these complications. The cell geometry usually does not ensure that exactly and only one cubic centimeter of solution will carry the current; therefore, the cell has to be calibrated to obtain the specific conductance of the solution. The calibration is usually carried out with high purity potassium chloride solutions, and the resulting calibration constant is often called the "cell constant."
An electroanalytical technique based upon the measurement of the conductivity an electrolyte solution.
A material that is capable to carry an electrical current, such as electronic conductor or ionic conductor. Contrast with insulator. See also semiconductor, superconductor, and dielectric.
A technique used in electroanalytical chemistry or in the determination of the kinetics and mechanism of electrode reactions, or a process carried out in an electrolytic cell that operates at constant current. See also chronopotentiometry.
A technique used in electroanalytical chemistry or in the determination of the kinetics and mechanism of electrode reactions, or a process carried out in an electrolytic cell that operates at constant potential. See also chronoamperometry and chronocoulometry.
Adsorption with the adsorbed molecule or ion being in direct contact with the solid surface.
A mass-transport mechanism that involves bulk movement of a solution (contrast with diffusion that involves individual molecules or ions). We differentiate "forced" convection from "natural" convection. The simplest example of forced convection is mechanical stirring. If a non-uniform solution is stirred, the solute is "transported" from the high concentration parts of the solution to the low concentration parts till the solution becomes completely uniform. Other examples of forced convection are the "flow" of a solution through a pipe or a porous separator driven by pressure difference. "Natural" convection is very important in electrochemistry. It always occurs at the surface of an electrode carrying current in the absence of "forced" convection. As electrode reaction is proceeding, the buildup of reaction products and the consumption of reactants changes the density of the solution layer close to the electrode surface compared to that of the bulk solution. Eventually, this density difference will force the surface solution layer to sink or rise, setting up a "natural stirring" action close to the electrode surface which will tend to equalize the surface and bulk concentrations. As a "rule of thumb," natural convection starts after about a minute of current flow.
See energy conversion.
A chemical (often electrochemical) process that destroys structural materials. Typically it refers to corrosion of metals, but any other material (e.g., plastic or semiconductor) will also corrode. The simplest example of metallic corrosion is the rusting of iron in air. Iron is spontaneously oxidized by the oxygen in air to iron oxides (while the oxygen is being reduced). Metallic corrosion is very often an electrochemical process. It is always electrochemical when the metal is immersed in a solution, but even in atmospheric corrosion a thin film of condensed moisture often covers the surface. The metal in the corrosive solution essentially acts as a short-circuited galvanic cell. Different areas of the surface act as anode and cathode, at the anodic areas the metal is oxidized to an oxide while at the cathodic areas the dissolved oxygen is being reduced. The spontaneous complementary oxidation/reduction processes of "rusting" are spatially separated while an electrical current is flowing "internally" from one part of the corroding metal to another; the current is totally "wasted" as it produces no useful work but only generates heat. (A cell arrangement like this is often called a "local cell.") See corrosion current and corrosion potential. See also an Encyclopedia Article.
Corrosion products are typically oxides, but other products (e.g., sulfides) can also form depending on the environment. Corrosion always involves oxidation of the corroding material in the general sense of the term.
The current flowing in a corrosion "local cell" (often, but not always, under steady-sate conditions). The anodic and cathodic currents must be equal, but the current densities may be different depending on the area ratio. The corrosion current is closely related to the concept of corrosion potential. See also an Encyclopedia Article.
A chemical that stops (or at least decreases the rate of) a corrosion process. The inhibitor can be added to an otherwise corrosive solution (often a very small concentration will accomplish the goal) or it can be incorporated in a coating applied to the metal surface. See also an Encyclopedia Article.
The electrode potential of a corroding metal. It is a "mixed potential" with a value that is in between the equilibrium potentials of the anodic and cathodic corrosion reactions. See also an Encyclopedia Article.
The corrosion is a spontaneous, dynamic phenomena with electrode reactions taking place and a current flowing. Consequently, both reactions are polarized and their potentials approach each other; as a matter of fact, they must become equal to preserve a single potential for the metal. However, the two reactions are not necessarily equally polarized. The overpotential of the two electrode reactions will be generally different, and their values will be dictated by the requirement that the electrode potentials be equal (at the "corrosion potential") at one, uniquely defined current (the corrosion current). See the Tafel equation for a relation between overpotential and current. (The ir drops in the solution and the metal are ignored in the above discussion, this is justified by the close proximity of the anodic and cathodic areas on the corroding metal.)
A relation between diffusion limited current density and time in a chronoamperometric experiment, assuming that the potential excursion is sufficiently large to immediately result in limiting current. The equation is valid only for planar electrodes in unstirred solution.
The diffusion current density is inversely related to the square root of time, or expressing it differently: the product of i(t) × t0.5 is a constant. The constant is proportional to the concentration of the reactant and to the square root of the diffusion coefficient of the reactant. Because the equation was derived for an unstirred solution, it ceases to be valid once natural convection starts.
A combination of coulometry and electrogravimetry, in which both the weight of the deposited metal and the passed charge are measured.
Measurement unit of the electrical charge. Symbol: "C".
The charge passing a given point during one second when the current is one ampere.
See coulometric efficiency.
Instrument used for the measurement of electrical charge.
For a rechargeable battery: the fraction, usually expressed as a percentage, of the electrical charge stored in a battery by charging that is recoverable during discharging. Inefficiencies arise from current inefficiencies. The coulometric efficiency is always larger than the energy efficiency. Also called "ampere-hour efficiency" "charge efficiency," and "coulombic efficiency."
An electroanalytical technique based upon the measurement of the amount of electrical charge passed through the working electrode of an electrochemical cell.
An electrochemical measuring technique for electrochemical analysis or for the determination of the kinetics and mechanism of electrode reactions based on the control of the amount of charge flowing through the system.
Instrument used to count the number of small particles (e.g. biological cells) in a given volume of a suspension by monitoring decreases in electrical conductivity through a small orifice caused by the particles passing through the orifice.
An electrode in a three-electrode cell that is used only to make an electrical connection to the electrolyte so that a current can be applied to the working electrode. The processes occurring on the counter electrode are unimportant, it is usually made of inert materials (noble metals or carbon/graphite) to avoid its dissolution. This is the case for cells used for research or for electroanalytical purposes. Of course, for many practically used cells, the processes occurring on both electrodes can be very important. Also called "auxiliary" electrode. Abbreviated as CE.
The mobile ion in ion exchange. The ion with opposite charge to that of the fixed site on the ion-exchange resin. Contrast with fixed ion.
A somewhat ambiguous term. For a redox reaction, the combination of the oxidized and reduced species is often called the "redox couple." But it is also used to designate the combination of an anode and a cathode, especially for corrosion cells.
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A charge or discharge current rate of a battery expressed in amperes. It is numerically a fraction or a multiple of the rated capacity of the battery expressed in ampere-hours. For example: for a 5-Ah rated capacity battery, C-rate is 5 A; C/5-rate is one A; 2C-rate is 10 A; and so on.
The overpotential (alternatively called polarization) associated with the crystallization step in electrocrystallization. The crystallization is an elementary step in the overall electrode reaction.
The movement of electrical charges in a conductor; carried by electrons in an electronic conductor (electronic current) or by ions in an ionic conductor (ionic current). "By definition" the electrical current always flows from the positive potential end of the conductor toward the negative potential end, independent of the actual direction of motion of the differently charged current carrier (or "charge carrier") particles. Two kinds of currents must be distinguished: "direct current (dc)" and "alternating current (ac)." Direct current is the unidirectional continuous flow of current, while alternating current is the oscillating (back and forth) flow of current. In electrochemistry, we almost always use direct current. Consequently, the term "current" always designates "dc" in this dictionary unless specifically stated to be "ac." The normal household current is an alternating current. The measurement unit of current is the ampere.
As mentioned above, the "defined" current flows from the positive terminal of the current source, trough the load, to the negative terminal of the source. Consequently, inside the "source" (whether it is electromechanical or electrochemical) the current must flow from the negative terminal to the positive terminal since there must be a complete circuit. This concept is especially important in electrochemistry because an electrochemical cell can be either a current "source" (galvanic cell) or a "load" (electrolytic cell). Furthermore, a rechargeable battery operates as a "source" during discharge and as a "load" during charge.
Current flowing through an electrochemical cell is usually the sum of the capacitive current and the faradaic current.
A structural part of a complicated electrode assembly. Its primary purpose is to conduct the electricity between the actual working (reacting) parts of the electrode and the terminals.
See compliance limits.
The ratio between of the current flowing through a compartment of an electrochemical cell and the volume of that compartment (e.g., anodic or cathodic current concentration). It is an often-used parameter in cell-design engineering.
Current referred to the unit area of the electrode. Current divided by the true electrode area.
The local current density on an electrode as a function of position on the electrode surface. Most processes operate best when the current distribution is "uniform." That is, when the current density is the same at all points on the electrode surface. See also primary, secondary, and tertiary current distribution.
The fraction, usually expressed as a percentage, of the current passing through an electrolytic cell (or an electrode) that accomplishes the desired chemical reaction. Inefficiencies may arise from reactions other than the intended reaction taking place at the electrodes, or side reactions consuming the product. The expected production can be theoretically calculated and compared with the actual production.
Current that is bypassing bipolar electrodes in a series coupled cell assembly (due to insufficient sealing or improper piping around the bipolar electrode) and therefore is not producing the required chemical change (electrode reaction).
A common characterization of an electrode or an electrochemical cell. The current (or more often the current density) is plotted against the electrode potential or cell voltage. Also called the "polarization curve". See also Tafel equation.
See electrical source (supply).
See current-potential plot.
See current efficiency.
The voltage of a rechargeable battery at which the charging or discharging is terminated.
Stands for "cyclic voltammetry," see voltammetry.
In voltammetry: a complementary pair of forward and reverse potential sweeps. For rechargeable batteries: a complementary discharging and charging processes.
The number of times a rechargeable battery (or a capacitor, especially an electrochemical capacitor) can be "cycled" (charged and discharged) before it loses its ability to accept charge. The processes occurring in the battery are not completely "chemically" reversible, and after repeated charging/discharging the battery will accept less and less charge till it becomes useless as a practical energy storage device. Some batteries can be recharged hundreds to thousands times.
See voltammetry.
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A very early non-rechargeable battery. It consisted of a glass jar containing copper and zinc electrodes,
each immersed in their respective acidic sulfate solutions. The two solutions were separated by a porous clay cylinder separator. It was a galvanic cell in which the spontaneous electrodissolution of zinc and electroplating of copper provided the electrical current. It was one of the earliest practical "laboratory" electrical sources; but, of course, it was not much use outside the laboratory.
Abbreviation of direct current. However this term is also used in connection with dc voltage, that is, a steady voltage that will cause a "dc current" to flow in a conductor, and also in connection with dc power. Contrast with ac.
The increase of the mobility of an ion in high frequency electrical field. The mobility of an ion is somewhat decreased by the presence of the ionic atmosphere because the predominantly oppositely charged ions surrounding the central ion will tend to hold it back. This effect is included in the normally measured mobility. However, when the ion is exposed to very high frequency electrical field, it will be displaced so little relative to its central position that there will be no retardation by its atmosphere, and the mobility of the ion (consequently the electrical conductivity of the solution) will increase. See also the Wien effect.
The electrode potential (cell voltage) at which a "measurable" electrolysis current begins to flow. This is a qualitative parameter since "measurable" is rather subjective.
Alternative name of an oxidation process.
Discharge of a rechargeable battery using a large portion (>80%) of its total rated capacity. Contrast with shallow discharge.
See reserve battery.
Process for removal of grease, oil, etc from metal surfaces in preparation for electroplating. Typically, the metal is immersed in hot, strongly basic solution or in organic solvents to remove and dissolve these coatings. See also electrolytic degreasing.
See desalination.
See desalination.
A crystalline shape produced by skeletal growth ("dendritic growth") resulting in "tree-like" appearance (often with many branches) in metal deposition.
The partial or complete elimination or counteraction of polarization.
An archaic expression (hardly used any more) for a material added to a battery electrode for reducing the polarization upon application of a current. It usually completely changed the nature of the electrode reaction.
See metal deposition/dissolution reactions.
For a rechargeable battery: the fraction, usually expressed as a percentage, of the total electrical energy stored in a battery by charging that was recovered by discharging at a certain point of time. Contrast with state of charge.
A process to produce clean (potable) water from brackish or seawater. Electrodialysis is an electrochemical technique often used for this purpose.
The opposite process of adsorption. The removal of the excess concentration of the adsorbate from the vicinity of the solid surface.
Corrosive removal of zinc from a brass surface, leaving rough copper.
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Stands for the dynamic hydrogen electrode.
See separator.
An insulating material with special characteristics. When a constant electrical field is applied to a dielectric, an electrical current will flow temporarily. However this is not a true current, it only generates localized charges, the field polarizes the molecules of the dielectric, producing some charge on its surfaces that create an electrical field equal to but opposed in direction to the original field causing the current to stop. This phenomenon is utilized in capacitors to store charge (see dielectric constant). See also an Encyclopedia Article.
A parameter characterizing the relative ability of a dielectric material in a capacitor to achieve energy storage. The higher the number the better the capacitor. See also an Encyclopedia Article on dielectrics.
The relative dielectric constant is normalized to the value in vacuum which is considered unity by definition.
See the Gouy-Chapman model of the double layer. Often simply called the "diffuse layer." (The diffuse layer is not to be confused with the diffusion layer.)
See liquid-junction potential.
See diffuse double layer.
The movement of chemical species (ions or molecules ) under the influence of concentration difference. The species will move from the high concentration area to the low concentration area till the concentration is uniform in the whole phase. Diffusion in solutions is the most important phenomenon in electrochemistry, but diffusion will occur also in gases and solids.
The rate of diffusion (diffusional flux) is proportional to the gradient of the
concentration in the solution, with the proportionality constant called the "diffusion coefficient," according to Fick’s first law.
An electrode reaction is considered to be under “diffusion control” when the overall rate of the reaction is is controlled by the rate of the diffusion of the reactants to the electrode surface rather than the rate of the reaction itself. This situation occurs when the the diffusion rate is much slower than the reaction rate and the diffusional process cannot supply the reactants fast enough to the surface. Often also called “mass-transport control.” Contrast with kinetic control.
A thin liquid boundary layer at the surface of an electrode that is immobile. This is part of a rather simplified and not strictly correct model (originally proposed by the early electrochemist Nernst, and is often called the "nernstian hypothesis") that works surprisingly well in most cases. The electrolyte solution is divided into three distinct parts: the bulk solution and the two diffusion layers at the surfaces of the electrodes. The bulk solution is assumed to be so well stirred that the concentration of all species is uniform throughout. In this region, mass transport occurs only through convection. While in the diffusion layers mass transport occurs only through diffusion. Charge transport occurs through electromigration everywhere. The thickness of the diffusion layer can vary typically between the order of 0.01 centimeter in a stagnant solution and the order of 0.0001 centimeter in very well-stirred solution. (The diffusion layer is not to be confused with the diffuse layer.)
While the concept of the diffusion layer is related to the concept of the hydrodynamic boundary layer, the two are not identical and neither is their thickness. The structure of the diffusion layer can be assumed to be relatively simple in the presence of a large excess of supporting electrolyte, which is usually the case in electroanalytical applications and in electrode kinetics research. Under these conditions, practically all the electrical current is carried by the ions of the supporting electrolyte, and the transport number of the reactant and the product is practically zero. When the current is initially turned on, the ions of the supporting electrolyte will migrate in the diffusion layer to/from the electrode (depending on their charge). However, since they do not take part in any electrode reaction, their concentration will increase/decrease at the electrode surface compared to that of their concentrations in the bulk solution. This will start the diffusion of these ions in a direction opposite to their migration. After steady-sate is reached, the diffusion will completely cancel the migration, and the net flux of these ions will be zero. They do not contribute to the mass transport in the diffusion layer; however, they are responsible for all the charge transport, that is, the resistance of the diffusion layer depends on the conductivity of the supporting electrolyte. On the other hand, the reactant and the product will diffuse to/from the electrode surface, and they will carry all the mass. The situation remains the same even if either the reactant or the product is an electrically neutral molecule. It is usually assumed that the concentration of all species changes linearly between the electrode surface and the edge of the diffusion layer. An example is the diffusion layer at the cathode surface during electroplating of copper from a solution containing a small amount of copper chloride and a large concentration of sulfuric acid. All the current is carried by the ions of the sulfuric acid (hydrogen cations and sulfate anions) but the only possible electrode reaction is the reduction of the copper ions since the reduction of hydrogen ions cannot occur at the prevailing electrode potential (copper is lower in the electromotive series than hydrogen).
The situation is much more complex in the absence of supporting electrolyte. Then both the electromigrational and the diffusional flux of the reactant and product must be considered.
See limiting current density.
See concentration overpotential.
See liquid-junction potential
A pair of equal and opposite electrical charges separated by a small distance. A dipole will align itself, if possible, in the presence of other electrical charges according to the attraction of opposite and repulsion of like charges. Externally electrically neutral chemical molecules can have a dipole inside. E.g., water is a triangular molecule with the oxygen at one corner and the two hydrogens at the other two corners. The internal charge distribution is such that the hydrogen side has a slight excess of positive charge and the oxygen end is correspondingly negative. A dipole is characterized by its "dipole moment," the product of the charge and the separation distance (coulomb times centimeter).
See dipole.
See current. Abbreviated as "dc."
A fuel cell using methanol (methyl alcohol) as fuel without first reforming it to hydrogen. Abbreviated as DMFC.
The current withdrawn from a battery. This rate is commonly expressed as a multiple of the rated capacity. See C-rate.
The opposite process of charging. In this process the battery or capacitor supplies electricity to a load (e.g., motor, light bulb). The term discharging is also used to describe the neutralization of an ion during an electrode reaction. E.g., a metal cation is said to be "discharged" to an electrically neutral metal atom during electroplating.
See brine electrolysis.
A chemical reaction in which a single substance acts as both oxidizing and reducing agent, resulting in the production of dissimilar substances. For example: monovalent copper ions
react to form divalent copper ions (by oxidation) and metallic copper atoms (by reduction).
The process that may occur when a chemical compound is dissolved in a solvent (e.g., water). The molecules of the compound will break up ("dissociate") into two or more ions resulting in an ionically conducting electrolyte solution. E.g., the common table salt (sodium chloride) will dissociate into a single charged sodium cation and a single charged chloride anion.
See Clark electrode.
An electrochemical cell in which the electrolyte is divided into two or more compartments by separators. Such separation may be necessary for two reasons. The solutions around the anode and the cathode may be different and it may be desirable to keep them from intermixing. Alternatively, it may be desirable to keep the products of the reactions at the anode and the cathode separated.
See separator.
Stands for dropping-mercury electrode.
Stands for direct-methanol fuel cell.
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See Donnan potential.
The electrical potential difference between two solutions separated by an ion-exchange membrane in the absence of any current flowing through the membrane.
The concept of the Donnan potential is analogous to that of the equilibrium electrode potential. Consider two table salt (sodium chloride) solutions of different concentrations separated by a cation-exchange membrane. The concentration difference will set up a diffusional force driving the sodium chloride from the higher concentration solution into the lower concentration solution through the membrane. However, the ion-exchange membrane will permit only the passage of the positively charged sodium cations. Consequently, excess positive electrical charges will accumulate on the low concentration solution side of the membrane, while excess negative electrical charges will accumulate on the high concentration side because of the negatively charged chloride anions that are left behind. This charge separation will induce an electrical potential difference that will drive the electromigration of the sodium ions in the direction opposite to that of the diffusion. The overall result will be that the net movement of the sodium ions into the lower concentration solution will slow and eventually stop when the two opposing forces are equal end the two opposing fluxes are equal. In this so called "Donnan equilibrium," the diffusional flux of the sodium ions in one direction will be equal to the electromigrational flux in the opposite direction, resulting in net zero mass transport and zero charge transport. The electrical potential difference across the membrane under these equilibrium conditions is the "Donnan potential."
Alternative name for "sedimentation potential." See electrokinetic effects.
A reference-electrode assembly (for example a silver/silver-chloride electrode) that is encased into a secondary containment vessel (typically a glass tubing) filled with an electrolyte not containing chloride ions (often a high concentration potassium nitrate solution). This second "internal electrolyte" of the reference electrode assembly and the external electrolyte into which the whole assembly is immersed are in ionic contact through a second separator
(e.g., a porous ceramic plug). The purpose of this arrangement is the avoidance of chloride ion contamination of the test solution (many electrode reactions are strongly catalyzed by chloride ions) at the price of increased liquid-junction potential.
See electrical double layer.
The measure of the ability of an electrical double layer to store electrical charge as a capacitor.
See electrochemical capacitor.
See capacitive current (density).
The electrode potential range where an electrode behaves as an ideal polarized electrode. In this potential range, the electrode potential is not positive enough that any species in the solution could be oxidized and the potential is not negative enough that any species could be reduced. In practical situations, there is almost always a small residual current flowing that is faradaic in nature.
A working electrode arrangement for electroanalytical techniques, such as polarography. Mercury is flowing continuously through a capillary tubing forming a small droplet (typical diameter about one >/mm) exposed to the solution. The old drop falls off and a new drop forms typically every few (3-6) seconds. The advantage of this self-renewing electrode is that the effect of impurities in the solution is minimized. Typically a new drop will form before impurities have a chance to adsorb on the surface of the old drop to such an extent as to influence the charge-transfer reaction. Abbreviated as "dme."
An early name for the non-rechargeable battery that is still used occasionally. The early non-rechargeable batteries were laboratory devices (see, e.g. the Daniell cell). To produce a practical device, the electrolyte solution was "immobilized" by some gelling agent, and the whole cell was sealed to permit its use in any position. Hence the name: "dry cell." See also Leclanche cell. And an Encyclopedia Article.
See reserve battery.
See exchange current density.
A pseudo-reference electrode assembly, simulating a reversible hydrogen electrode with an approximately 20 to 40 mV more negative potential. While its potential is less defined, it has the advantage of not requiring a hydrogen gas supply. It is typically a glass tube containing two internal electrodes, at least one of which is a platinized platinum electrode, immersed in the same electrolyte solution as is the electrolyte in the working cell, with the two electrolytes in ionic contact through a separator. A small, constant current is enforced between the two electrodes with the platinized platinum being the cathode, carrying typically 1 mA/cm2 current density, resulting in a small amount of hydrogen evolution. This cathode is then used as the reference electrode. Abbreviated as “DHE.”
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Stands for electroactive polymer.
Stands for electrochemical machining.
A rechargeable battery developed by Edison. In the charged state, the active material of the positive electrode is nickel oxide while that of the negative electrode is metallic iron, with a basic (potassium hydroxide) electrolyte. During discharging, the nickel oxide is converted to a lower oxidation state oxide, while the iron is converted to iron oxide. It is still used today.
Abbreviation of electrochemical impedance spectroscopy.
Some species of fish can generate and receive electric signals These signals are used for a variety of purposes, such as disorienting and confusing potential pray and potential predators, determining their location, and social communication (including reproductive behavior). See also an Encyclopedia Article.
An electrode is said to be “electrically biased” when its potential is other than its equilibrium potential.
A basic property of matter that gives rise to all electric and magnetic forces and interactions. Matter can be "neutral" (having no electrical charge) or it can have one of two kinds of charges distinguished as "positive" or "negative." The measurement unit of charge is the coulomb.
See current.
The structure of charge accumulation and charge separation that always occurs at the interface when an electrode is immersed into an electrolyte solution. (For a simple example see equilibrium electrode potential.) The excess charge on the electrode surface is compensated by an accumulation of excess ions of the opposite charge in the solution. The amount of charge is a function of the electrode potential. This structure behaves essentially as a capacitor. There are several theoretical models that describe the structure of the double layer. The three most commonly used ones are the Helmholtz model, the Gouy-Chapman model, and the Gouy-Chapman-Stern model.
A form of energy. It expresses the ability of an electrical source to carry out useful work or generate heat. E.g., this energy can be used to drive an electrical motor and carry out some mechanical work, or to generate heat with an electrical heater. The electrical energy is usually expressed in units of watt-hour, symbol: "Wh". See also electrical power.
A region of space, associated with a distribution of electric charge, in which forces due to that charge act upon other electric charges.
The electrical potential difference between two point in a circuit is the cause of the flow of a current. It is somewhat analogous to the difference in height in a waterfall that causes the water to fall, or the difference in pressure in a pipeline that causes the gas to flow. In electrochemistry we typically cannot measure "absolute" potentials, only the "difference" of potential between two points. For similar concepts, see electromotive force (emf) and voltage. These terms are sometimes used interchangeably. However, in electrochemistry "emf" usually refers to the potential difference between the two electrodes of an electrochemical cell when there is no current flowing through the cell, "voltage" refers to same with current flowing, and "potential" is usually used in connection with electrodes (see electrode potential). The measurement unit of the potential is the volt.
The rate at which an electrical source can supply electrical energy. E.g., a battery may be able to store a large amount of energy, but if it has a small power capability it can provide the energy (do some work) only slowly, and it will take a long time to discharge. Another battery with the same energy storage capability but larger power will provide the energy (do work) faster, but will also be discharged faster. Electrical power is expressed usually in units of watt, symbol: "W". Unfortunately, the terms "power" and "energy" are often used interchangeably in everyday language (and sometimes also in the technical literature) even though they are quite distinct concepts, e.g., when we talk about "energy source" or "power source," we usually mean the same thing. Not only electrical sources but also loads are characterized by a power rating, e.g., an electrical motor or a light bulb is characterized by the power it needs to operate it.
The power of a source (or the power need of a load) can be calculated as the product of the current and voltage (watt = ampere × volt). One watt means that one watt-second (coulomb × volt) energy is provided (used) every second. In more practical units, one watt means that one watt-hour (ampere-hour × volt) energy is provided (used) every hour.
A source of electrical power (electrical energy), a device that supplies electrical current. It can be electrochemical (battery or fuel cell) or an electromechanical device (dynamo) or a specialized electronic instrument. Also called "power source (supply)." Specialized sources can be called "voltage source" or "current source," indicating the characteristic of the electrical power that can be controlled by that device.
A somewhat outdated and nowadays seldom used term for electrical potential.
The electrokinetic effects arising when soundwaves cause oscillation of small particles suspended in a liquid; particularly, effect analogous to sedimentation potential.
Polymeric material behaving similarly to piezoelectric materials in that it changes shape as a response to application of electric voltage. Abbreviated as EAP. See also an Encyclopedia Article.
A species in solution that can take part in an electrode reaction or that can be adsorbed on the electrode. Contrast with electroinactive substance.
See electrosorption.
The application of electrochemical cells and electrochemical techniques for chemical analysis. The analyte is dissolved in the electrolyte of the cell, and one can perform either "qualitative" analysis (determination of the type of constituents present) or "quantitative" analysis (determination of the amount of a given constituent). See also an Encyclopedia Article.
A phenomenon involving the dependence of the interfacial tension on the electrical state (potential) of the
interphase.
A curve depicting the interfacial tension as a function of potential of a liquid metal underneath an electrolyte solution. These curves typically have a parabolic shape with a maximum.
The phenomenon of increasing the rate of an electrode reaction by changing the electrode material. The rate of the electrode reactions (the magnitude of the exchange current density) can strongly depend on the composition and morphological structure of the electrode surface. This is called the "electrocatalytic effect."
A material that can cause electrocatalysis.
A device that stores electrical energy using electrochemical cells. Two large-surface-area electrodes are used resulting in large double layer capacitance, and much of the storage capacity is due to the charging/discharging of the double layers. Some surface oxidation/reduction also occurs, but in contrast to reactions occurring in batteries, this is limited to a monolayer or two on the electrode surfaces. Consequently, the device behaves more like a capacitor than a battery (see pseudocapacitance). It is also called "supercapacitor," "ultracapacitor," or "double-layer capacitor." It is not to be confused with electrolytic capacitors. See also an Encyclopedia Article.
Electrochemical capacitors typically have much larger power density but much smaller energy density than batteries.
A device