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containing many articles written in popular-science style by international experts in the field.
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Revision date: September 20, 2014.
Visit also ELECTROCHEMICAL SCIENCE AND TECHNOLOGY INFORMATION RESOURCE (ESTIR) a listing of electrochemistry related websites, books, proceedings, reviews, graduate schools, societies, journals, meetings, etc, etc. Many thousands of information items.
There are more than 1,000 entries in the dictionary. In some cases, a second paragraph provides further information for the "more scientifically minded". Additional entries are added at irregular intervals. Suggestions of entries to be included are welcome, together with corrections of the inevitable errors. Send them to: nagyz@email.unc.edu.
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See an Encyclopedia Article.
Symbol and abbreviation of ampere.
Symbol and abbreviation of Angstrom.
Stands for anodized (anodic) aluminum oxide.
The electrical potential of an electrode measured with respect the bulk of the electrolyte solution of the electrochemical cell. Can not be measured with any acceptable accuracy.
Temperature scale starting with zero at the �absolute zero�, the physically achievable coldest temperature. The absolute zero is at -273.16oC (-459.67oF). It is expressed in �Kelvin� (K), which has the graduation of the Celsius (centigrade) scale, consequently, water freezes at 273.16 K and boils at 373.16 K.
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.
Stands for 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.
See voltammetry.
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.
See kinetic control.
The overpotential (alternatively called polarization) associated with the charge-transfer reaction elementary step in the overall electrode reaction.
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 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.
See Baizer-Danly process.
A material that is adsorbed.
A material that adsorbs.
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 generally considered to be a physical process, contrast with chemisorption. It is a surface process, not to be confused with absorption. Opposite: desorption.
An adsorption isotherm for an adsorbate on a solid is the function which relates, at constant temperature, the amount of substance adsorbed at equilibrium to the pressure (in a gase phase) or the concentration (in a liquid phase) of the adsorbate.
Stands for auxiliary electrode.
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Stands for alkaline fuel cell.
A gel made from seaweed that is often used to make salt bridges.
Symbol and abbreviation of ampere-hour.
See air electrode.
A porous electrode with air as the gas phase and with the reduction of oxygen from the air the electrode reaction. It is very often employed in fuel cells and occasionally also in nonrechargeable batteries.
See base.
See brine electrolysis.
See Edison battery.
A modern version of the Leclanche cell (battery) 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. Abbreviated as "AFC". See also an Encyclopedia Article.
A fuel cell that uses an alkaline ion-exchange membrane as the electrolyte. Abbreviated as "AMFC".
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). It is sometimes called the Hall-Heroult process. This is the only large-scale industrial process for the production of aluminum. See also an Encyclopedia Article.
An alloy of mercury and another metal.
Stands for alkaline membrane fuel cell.
Instrument used for the measurement of current.
Symbol and abbreviation of ampere.
Measurement unit of current. Symbol: "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 also an Encyclopedia Article.
See galvanostat.
A substance whose chemical composition is to be determined by chemical analysis.
A somewhat archaic unit of length: 10-10 meter, symbol: "Å" (one ten-billionth of a meter).
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A substance that does not contain water. The opposite of hydrous.
A negatively charged ion. Contrast with cation.
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.
A corrosion cell is said to be under anodic control if the overpotential of the anodic corrosion reaction is much larger than that of the cathodic corrosion reaction, consequently the corrosion current is overwhelmingly determined the anodic reaction. Contrast with cathodic control and mixed control. See also an Encyclopedia Article.
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.
Porous aluminum oxide film produced by anodizing. Abbreviated as �AAO�.
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 large-scale industrial electrosynthesic process for the production of a compound called "adiponitrile", which is used in the production of nylon. Also called the "Monsanto process". See also an Encyclopedia Article.
See semiconductor.
A thin, continuous, non-porous, electrically insulating film on metal surfaces (usually comprised of oxides). Abbreviated as �BL�.
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. (This is a carryover from military usage of the term "battery" of canons.) However, in present usage all storage devices (single
cell and multiple cell) are called batteries.
See charger.
Stands for boron-doped diamond.
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.
Stands for barrier (oxide) film layer.
See brine electrolysis.
See electrochemical blood glucose test strip.
A graphical representation of the results of an electrochemical impedance spectroscopic measurement.
Charging at an extremely high rate of 50-300 amps, this method is primarily used to rapidly charge a battery in order to start a vehicle in a matter of a few seconds to five minutes.
See diamond electrode. Abbreviated as BDD.
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 (see hydrogen evolution reaction). 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.
Various forms of carbon are often used as electrodes in electrochemistry. See an Encyclopedia Article.
An electrode prepared from graphite powder and some binding liquid. It is often used because it is inexpensive and can be easily renewed by removing the top surface. The long term stability is questionable and the binder may cause interference. Abbreviated as CPE. See an Encyclopedia Article.
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.
A corrosion cell is said to be under cathodic control if the overpotential of the cathodic corrosion reaction is much larger than that of the anodic corrosion reaction, consequently the corrosion current is overwhelmingly determined the cathodic reaction. Contrast with anodic control and mixed control.See also an Encyclopedia Article.
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" (galvanic-anode) 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. Contrast with anion.
See base. Sometimes it specifically refers to sodium hydroxide.
See brine electrolysis.
Stands for closed-circuit voltage.
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Stands for 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 kinetic control.
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 the charge-transfer step of an electrode reaction indicative of its inherent speed: a large charge-transfer resistance indicates a slow step. See also non-ohmic resistance.
The phenomenon of movement (transportation) of electrical charge (that is, the electrical current) from one part of the electrochemical cell to another, occurring mainly as electromigration of ions, but it can also occur by diffusion of ions. See electrochemical potential.
Since in a solution the current is always carried by ions, and the ions always have mass, charge transport inevitable also involves mass transport. See mass transport for a more detailed discussion of this.
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 chemisorption.
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".
An adsorption process with some degree of chemical bonding between the adsorbate and the adsorbent. The distinction between chemisorption and physisorption is occasionally rather vague.
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.
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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). It is also called a "polarographic oxygen electrode".
The voltage of a battery when it is discharging (on-load condition). Abbreviated as "ccv".
Symbol and abbreviation of centimeter.
Stands for the square centimeter, a measure of area, equals 0.15500031 square inches.
Stands for the cubic centimeter, a measure of volume, equals 0.001 dm3.
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.
A graphical representation of the results of an electrochemical impedance spectroscopic measurement.
Small particles with a size ranging from nanometers to micrometers suspended in a solvent are called colloids. Colloid-chemistry is related to electrochemistry because the particles may have an electrical charge and consequently are surrounded by an electrical double layer type structure in the suspension.
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 formation of a complex ion or molecule. A complex (or coordination complex) is a chemical structure consisting of a central atom or ion (usually metallic), bonded to a surrounding array of other groups of atoms or anions (called ligands or complexing agents). The bonding between the central entity and the ligands can be rather weak.
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.
The measure of an electrical circuit element's ability to carry electrical current. The measurement unit of the conductance is the siemens, the reciprocal of the ohm.
A polymeric material (e.g., plastics) having electronic conductivity. See also an Encyclopedia Article.
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The measure of a material's ability to carry electrical current. It is expressed as the conductance between opposite faces of a one-meter cube of material. (In the older literature, a one-centimeter cube is used.) The measurement unit of the conductance is the siemens/m. Also called "specific conductance".
The reciprocal of resistivity.
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.
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. See also oscillometry. See also an Encyclopedia Article.
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.)
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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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Stands for carbon paste electrode.
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. See also an Encyclopedia Article.
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). Called �bypass current� or �shunt current�.
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.
A current produced in a photoelectrochemical cell in the absence of light.
A voltage produced in a photoelectrochemical cell in the absence of light.
Stands for 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 Debye-Huckel theory states that in dilute solutions the activity of any ion depends only on the charge of the ion and the ionic strength of the solution, independently of the chemical nature of the ion.
One tenth of a meter. Abbreviated as "dm".
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.
A rechargeable battery specifically made to have up to 80% of its energy capacity removed (during discharging) and replaced (during charging) repeatedly for many cycles. The plates of this type of battery are much thicker than are other battery's.
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. Abbreviated as "DOD". 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.
Stands for direct-formic-acid fuel cell.
Stands for the dynamic hydrogen electrode.
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An electrode made from a conducting form of diamond. Diamond is essentially carbon. In pure form it is an insulator but the addition of certain impurities (such as boron) can make it an electrical conductor.
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.
See diffusion.
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 or the ir drop in the solution. This situation occurs when 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 and ohmic 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 through diffusion. 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 numbers of the reactant and the product are 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 neither to the mass transport nor to the charge transport in the diffusion layer. On the other hand, the reactant and the product will diffuse to/from the electrode surface, and they will carry all the mass and the charge. 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. A simple example is the diffusion layer at the cathode surface during electroplating of silver from a solution containing a small amount of silver nitrate and a large concentration of sodium nitrate. While a steady state is reached for the sodium nitrate (as described above) a steady state is also approached for the silver ions. As silver is deposited on the electrode surface, the silver ions will be depleted at the electrode surface, and a concentration difference will develop in the diffusion layer. The silver ion concentration at the electrode surface will continue to decrease till the resulting concentration difference is sufficient to provide the needed silver ion flux to carry all the required charge and mass for the electrode reaction. Essentially, in this case both the transport of charge and the transport of mass to the electrode surface are provided by the "diffusion". See electrochemical potential.
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 in the diffusion layer.
See limiting current density.
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See concentration overpotential.
See liquid-junction potential
See mass-transport resistance.
A modern anode used in brine electrolysis. It is essentially a sheet of titanium metal coated with a mixture of active metal oxides, the coating is stable and has a very long life time under cell operating conditions. Contrast it with the traditionally used carbon electrode that was slowly consumed during cell operation; it had to be replaced periodically and required the rebuilding of the cell. Abbreviated as "DSA".
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 formic acid as fuel without first reforming it to hydrogen. Abbreviated as "DFAFC".
A fuel cell using methanol (methyl alcohol) as fuel without first reforming it to hydrogen. Abbreviated as "DMFC". See also an Encyclopedia Article.
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. Contrast with undivided cell.
See separator.
Stands for decimeter.
Stands for the cubic decimeter which is the International Unit (SI) of volume; it is essentially the same as the old (metric) unit of liter.
Stands for dropping-mercury electrode.
Stands for direct-methanol fuel cell.
Stands for depth of discharge.
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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 (battery)). 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 (battery). And an Encyclopedia Article.
See reserve battery.
Stands for dimensionally stable anode.
The operating regime of a rechargeable battery including factors such as charge and discharge rates, depth of discharge, cycle duration, and length of time in the standby mode.
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 atomic layer deposition.
See also an Encyclopedia Article.
Stands for electrochemical blood glucose test strip.
Stands for electrochemical atomic layer deposition.
See also an Encyclopedia Article.
Stands for electrochemical atomic layer epitaxy.
See also an Encyclopedia Article.
Stands for electrochemiluminescence.
Stands for electrochemical machining.
Stands for The Electrochemical Society, Inc.
Increased diffusion to the edges of an inlaid electrode (such as the rotating disk electrode).
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. Also called "nickel-iron battery" and "alkaline battery".
Stands for electrochemical double layer capacitor.
Stands for 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.
Electrical organ in some animals (such as the electric fish) can generate and receive electric signals. See also an Encyclopedia Article.
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.
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See electrolyzed water.
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.
It is essentially a practical application of underpotential deposition to produce well controlled monolayer materials. Abbreviated as �E-ALD� and �EC-ALD�. Also called �electrochemical atomic layer epitaxy�.
See also an Encyclopedia Article.
Alternative expression for electrochemical atomic layer deposition. Abbreviated as �EC-ALE�.
See also an Encyclopedia Article.
Part of an electroanalytical device for the measurement of glucose level in the blood of people suffering from diabetes. Abbreviated as "EBGTS". See also an Encyclopedia Article.
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 that converts chemical energy into electrical energy or vice versa when a chemical reaction is occurring in the cell. Typically, it consists of two metal electrodes immersed into an aqueous solution (electrolyte) with electrode reactions occurring at the electrode-solution surfaces. See also galvanic cell and electrolytic cell.
It consist of two electronically conducting phases (e.g., solid or liquid metals, semiconductors, etc) connected by an ionically conducting phase (e.g. aqueous or non-aqueous solution, molten salt, ionically conducting solid). As an electrical current passes, it must change from electronic current to ionic current and back to electronic current. These changes of conduction mode are always accompanied by oxidation/reduction reactions. An essential feature of the electrochemical cell is that the simultaneously occurring oxidation-reduction reactions are spatially separated. E.g., in a spontaneous "chemical reaction" during the oxidation of hydrogen by oxygen to water, electrons are passed directly from the hydrogen to the oxygen. In contrast, in the spontaneous electrochemical reaction in a galvanic cell the hydrogen is oxidized at the anode by transferring electrons to the anode and the oxygen is reduced at the cathode by accepting electrons from the cathode. The ions produced in the electrode reactions, in this case positive hydrogen ions and the negative hydroxyl (OH-) ions, will recombine in the solution to form the final product of the reaction: water. During this process the electrons are conducted from the anode to the cathode through an outside electrical circuit where the electrical current can drive a motor, light a light bulb, etc. The reaction can also be reversed, water can be decomposed into hydrogen and oxygen by the application of electrical power in an electrolytic cell.
See electrolytic degreasing.
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See electrolytic degreasing.
See electrical double layer.
See electrochemical capacitor.
Sea electrochemical machining.
The weight of a substance (in grams) produced or consumed by the passage of one coulomb in an electrochemical reaction.
The gram-equivalent weight divided by the Faraday constant.
A combination of electrochemical machining and mechanical grinding. Used when the products of electrochemical dissolution are not easily soluble and must be removed physically from the surface. Used with a metal-bonded and diamond-impregnated grinding wheel. Also called "electrolytic grinding" and "electrogrinding". See also an Encyclopedia Article.
Electrochemical impedance spectroscopy is a powerful tool for examining processes occurring at electrode surfaces. A small amplitude ac (sinusoidal) excitation signal (potential or current), covering a wide range of frequencies, is applied to the system under investigation and the response (current or voltage or another signal of interest) is measured. Due to the small amplitude of the excitation signal, the measurement can be carried out without significantly disturbing the properties being measured. Due to the wide range of frequencies used, the complex sequence of coupled processes such as, electron transfer, mass transport, chemical reaction, etc. can often be separated and investigated with a single measurement. It is routinely used in electrode kinetics and mechanism investigations, and in the characterization of batteries, fuel cells, and corrosion phenomena. Abbreviated as "EIS".
See irreversible electrode reaction.
A process to produce metallic objects with a technique that is essentially precision electrodissolution. The metal to be machined is made the anode in an electrolytic cell while the cathode (or "tool") is made of inert material and is machined to be the "mirror image" of the desired shape. A very small gap (typically, less than one mm) is maintained between the electrodes and a large current density is applied with a fast flowing electrolyte. One of the advantages of this production technique is that very complicated shapes can be produced with a single operation from very hard alloys that would be very difficult, if not impossible, to machine with any other metal cutting technique. Some typical applications are the production of turbine blades and the drilling of holes with very large depth-to-diameter ratio. The cathodic reaction is typically hydrogen evolution. Abbreviated as "ECM". See also an Encyclopedia Article.
An instrument which comprises a sampling system, an array of chemical/electrochemical gas sensors with differing selectivity, and a computer with an appropriate pattern-classification algorithm, capable of qualitative and/or quantitative analysis of simple or complex gases, vapors, or odors. See also an Encyclopedia Article.
See electrolytic pickling.
See electropolishing.
The electrochemical quartz crystal microbalance is an extremely sensitive mass sensor, capable of measuring mass changes in the microgram to nanogram range. It is a piezoelectric (the property exhibited by some non-conducting crystals of becoming electrically polarized when mechanically strained and of becoming mechanically strained when an electric field is applied) device, fabricated of a thin plate of quartz with an electrode attached (deposited) on the plate. It produces an ac electrical signal with a frequency that depends on the mass of the crystal. Consequently, it can be used to follow the changes of the mass of an electrode during electrochemical processes, (e.g., during a cyclic voltammetry measurement) allowing thus investigation of electrochemical deposition/dissolution, underpotential deposition, adsorption of different substances, oxidation of the electrode metal, electropolymerization, etc. This can provide important information complementary to the electrical information of the measurements for processes occurring in electroplating, corrosion, batteries, etc. Abbreviated as "EQCM".
A concept used in the theory of electrolyte solutions that combines the effects of both an electrical field on the charged ion and also the concentration of the ionic species in solution. Consequently, an "electrochemical potential difference" will inherently provide the driving force for both the transport of charge (electromigration) and the transport of mass (diffusion) by an ion (an ion having both charge and mass will obviously transport both by its movement). A detailed discussion of this term would require a mathematical treatment. Not to be confused with electrode potential.
An oxidation/reduction reaction that occurs in an electrochemical cell. The essential feature is that the simultaneously occurring oxidation-reduction reactions are spatially separated. E.g., in a spontaneous "chemical reaction" during the oxidation of hydrogen by oxygen to water, electrons are passed directly from the hydrogen to the oxygen. In contrast, in the spontaneous electrochemical reaction in a galvanic cell two separate electrode reactions occur. The hydrogen is oxidized at the anode by transferring electrons to the anode and the oxygen is reduced at the cathode by accepting electrons from the cathode. The overall electrochemical reaction is the sum of the two electrode reactions. The ions produced in the electrode reactions, in this case positive hydrogen ions and the negative hydroxyl (OH-) ions will recombine in the solution to form the final product of the reaction: water. During this process the electrons are carried from the anode to the cathode through an outside electrical circuit where the electronic current can drive a motor, light a light bulb, etc. The reaction can also be reversed, water can be decomposed into hydrogen and oxygen by the application of electrical power in an electrolytic cell.
See reversible electrode reaction.
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See electromotive series.
A variety of electrochemical techniques used to "shape" metal objects. These include: electrochemical machining, electrochemical drilling, electrochemical grinding, and electropolishing.
An electrochemically switchable molecule displays a different reactivity toward some other chemical species depending whether the switchable molecule is oxidized or reduced. Consequently, the reactivity of the molecule can be controlled by electrochemical oxidation/reduction. This phenomenon is primarily important in bioelectrochemistry.
See electrosynthesis.
Light emission by excited species produced in an electrode reaction. Abbreviated as "ECL". Also called: "electrogenerated chemiluminescence".
See an Encyclopedia Article.
A "chromatographic" separation method with the "mobile", liquid phase forced through the "immobile" phase by the application of an electrical potential difference, that is, by electroosmosis. In some cases, the separation is enhanced by electrophoresis. Chromatography is an analytical (see electroanalytical) separation technique based on the different attraction of the sample components to an immobile/stationary phase through which the sample solution is forced through by a flow of solvent. The sample components are adsorbed/desorbed on the surface of the stationary phase as they are flushed through by the solvent; consequently, they move with speeds inversely proportional to their adsorption strengths and become separated: the least-strongly adsorbed component is flushed out first and the most-strongly adsorbed last. A variety of stationary phases can be employed; the most common ones are: paper, thin layer of gelatinous material, or a column (or capillary) packed with small particles.
The use of electroporation to introduce drugs directly into living cells. Used experimentally for cancer treatment.
A change of color caused by a change in electrode potential or the application of a current.
See electrolytic degreasing.
See electrophoretic deposition.
Process for increasing the concentration of a trace component in a sample. It can be achieved by a variety of techniques, e.g.: electrochromatography, electrodialysis, electroplating, electroosmosis, and electrophoresis.
See electroplating. Electroplating typically will result in a crystalline metal deposit; therefore, the two terms can be used interchangeably. The term "electroplating" is mostly used in technological applications, and the term "electrocrystallization" is often used in research studies.
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The two electronically conducting parts of an electrochemical cell. See also anode and cathode. These can be simple metallic structures (rods, sheets, etc) or much more complicated, composite structures. E.g., the electrodes in a rechargeable battery will also "contain" the chemicals being converted during its operation. The term "electrode" is also used to denote complex assemblies that include an electrode in a small vessel, which contains an electrolyte and is equipped with an ion-permeable separator. Reference electrodes are such assemblies.
The application of kinetics to electrode reactions. Not to be confused with electrokinetics.
A simple metal electrode immersed in a solution containing its own ion (e.g., silver immersed in a silver nitrate solution). The equilibrium potential of this electrode is a function of the concentration of the cation of the electrode metal in the solution (see Nernst equation). Contrast with electrode of the second kind and electrode of the third kind.
A metal electrode assembly with the equilibrium potential being a function of the concentration of an anion in the solution. Typical examples are the silver/silver-chloride electrode, the calomel electrode, and the mercury/mercury sulfate electrode. Contrast with electrode of the first kind and electrode of the third kind.
The assembly consists of a metal, in contact with a slightly soluble salt of this metal, immersed in a solution containing the same anion as that of the metal salt (e.g., silver–silver chloride–potassium chloride solution). The potential of the metal is controlled by the concentration of its cation in the solution, but this, in turn, is controlled by the anion concentration in the solution through the solubility product of the slightly soluble metal salt.
A metal electrode assembly with the equilibrium potential being a function of the concentration of a cation, other than the cation of the electrode metal, in the solution. These have been used, with limited success, in sensors for metal ions for metals that are not stable in aqueous solutions, e.g., calcium and magnesium. Contrast with electrode of the first kind and electrode of the second kind.
The assembly consists of a metal in contact with two slightly soluble salts (one containing the cation of the solid metal, the other the cation to be determined, with both salts having a common anion) immersed in a solution containing a salt of the second metal (e.g., zinc metal–zinc oxalate–calcium oxalate–calcium salt solution). The potential of the metal is controlled by the concentration of its cation in the solution, but this is controlled by the anion concentration in the solution through the solubility product of the slightly soluble metal salt, which, in turn is controlled by the concentration of the cation of the second slightly soluble salt. These electrodes are very sluggish and unstable due to a series of equilibria to be established to produce a stable potential.
The electrical potential difference between an electrode and a reference electrode. We cannot measure the absolute potential of an electrode; therefore, the electrode potential must always be referred to an "arbitrary zero point", defined by the potential of the reference electrode. Consequently, it is very important always to note the type of reference electrode used in the measurement of the electrode potential. Not to be confused with electrochemical potential. See also equilibrium electrode potential.
A chemical "half" (or "partial") reaction occurring at the electrode surface. It is called a "half" (or "partial") reaction because only the oxidation or the reduction part of the overall cell reaction occurs at any one electrode. See also electrochemical reaction. Many electrode reactions can proceed either as oxidation or as reduction, depending on the direction of the current flowing through the electrode/electrolyte interface. See, e.g. metal deposition/dissolution or redox reactions.
An electrode reaction always occurs in several series and parallel elementary reaction steps. Even in the simplest case there are three steps in series: (1) the reactant must be transported to the electrode surface from the bulk of the electrolyte (usually predominantly by diffusion, but it can also occur by electromigration), (2) a charge-transfer reaction occurs, and (3) the product must be transported from the electrode surface to the bulk of the electrolyte. Any one of these steps may be the rate-determining one.
A process for depositing solid materials on an electrode surface using electrolysis. It is a somewhat loosely used term that is applied to many technologies. There are a number of metal deposition technologies. However, not only metals but also different compounds can be electrodeposited. This is used most often for the formation of oxides (such as manganese dioxide and lead dioxide) by anodic oxidation of dissolved salts. Deposition can also be achieved electrophoreticly. See also an Encyclopedia Article on electroplating.
A process to move ions from one solution into another using an electrolytic cell. An example is the electrochemical desalination of seawater. In its simplest form, the cell is separated into three compartments by appropriate ion-exchange membranes with electrodes placed in the two outer compartments, and all compartments are fed seawater. As an electrical current is forced through the cell, anions will move from the central compartment through an anion-exchange membrane into the anode compartment and the cations will move through an cation-exchange membrane into the cathode compartment. Since the ion-exchange membranes are appropriately ion selective, the ions cannot move from the edge compartments to the central compartment, resulting in a desalinated effluent from that compartment. In practice, more than one cell will be connected in series, and the process will be carried out in several stages since it would not be efficient to remove all the salt in one step. This process is also used to remove industrial pollutants from waste streams.
The part (sub-discipline) of electrochemistry that deals with phenomena occurring at the surface of electrodes, particularly charge-transfer reactions. See also ionics.
The reverse reaction of metal deposition.
See electroosmosis.
See electrowinning.
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An electrolytic process for particle separation. An aqueous solution containing dispersed solid particles is electrolyzed to produce hydrogen and oxygen gas bubbles (see hydrogen evolution reaction and oxygen evolution reaction), the rising bubbles carry particles adhering to them to the surface where they can be skimmed off. Flotation is often carried out without electrolysis, by simply purging with air. This process is routinely used in processing minerals (ores) for separating the light and heavy particles (see an Encyclopedia Article), and in waste treatment to remove solids (see also another Encyclopedia Article).
A process to produce metallic objects with a technique that is essentially precision electroplating. The metal is deposited onto a "mandrel" or "former" of suitable shape to a desired thickness, followed by the removal of the mandrel to produce a free standing metal object. One of the advantages of this production technique is that very complicated shapes can be produced with a single operation. It is often used to produce very precise optical elements, and solid-state electronic devices (integrated circuit boards, computer chips). Other applications are the production of flat or perforated metal sheets, seamless perforated metal tubes, and metal bellows. Two very prominent past applications of this technique were the production of "stampers" for the old-fashioned musical (phonograph) records and "electrotypes" for the printing industry. Practically any metal or alloy that can be electroplated can also be used for electroforming. The preparation of the removable mandrel is an important step in this process. One example is the use of machined copper or brass that is surface treated to permit electroplating that will closely follow the mandrel surface but will not permit strong adhesion of the electroformed piece.
See galvanizing.
See electrochemiluminescence.
A chemical species produced at an electrode surface by a charge-transfer reaction.
The use of electroporation to introduce DNA directly into living cells. Used experimentally for cancer treatment.
An electroanalytical technique in which the substance to be determined (usually a metal) is deposited out on an electrode which is weighed before and after the experiment. The potential of the electrode must be carefully chosen to ensure that only the metal do be determined will deposit. Under favorable conditions, two or more metals can be determined by successive depositions at different potentials.
See electrochemical grinding.
A species in solution that does not take part in any electrode reactions or that is not adsorbed on the electrode. Contrast with electroactive substance.
Phenomena that arise due to a charge separation caused by the relative motion of a solid and liquid phase. A portion of the Gouy-Chapman diffuse layer is sheared off as the two phases move relative to each other, resulting in a charge separation. The hydrodynamic boundary layer remains attached to the solid surface while the rest of the liquid moves separately; consequently, electrokinetic effects arise when the "diffuse double layer" is thicker that the "hydrodynamic boundary layer". The electrical potential difference between the bulk solution and the "shear plane" is the "electrokinetic potential", often called the "zeta potential". Two types of effects arise: an electrical potential difference will arise between the two phases if they move relative to each other due to an external force (streaming potential and sedimentation potential) or a movement of the two phases will arise relative to each other if an electrical potential is applied parallel to the phase boundary (electroosmosis and electrophoresis). Accordingly, "electrokinetics" includes the following four "electrokinetic effects:"
Streaming potential arises when liquid is flowing by a solid surface, e.g., when liquid is forced through a capillary tubing or porous solid by a pressure differential.
Sedimentation potential arises when small suspended particles move through a liquid (e.g., forced by gravity). This can occur in "dispersions" (suspended solid particles) or "emulsions" (suspended immiscible liquid droplets). Also called "eletrophoretic potential" or "Dorn potential".
Electroosmosis is the movement of a liquid through a capillary tubing or porous solid driven by an electrical potential difference. Also called "electroendosmosis".
Electrophoresis is the movement of small suspended particles in a liquid driven by an electrical potential difference. This can occur in "dispersions" (suspended solid particles) or "emulsions" (suspended immiscible liquid droplets). Also called "cataphoresis".
Electrokinetics should not be confused with electrode kinetics.
Alternative name for "zeta potential". See electrokinetic effects.
See electroremediation.
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Process to produce thin metallic coatings on objects without the application of external current. The plating bath contains a dissolved salt of the metal and a reducing agent. However, the reduction of the metal cation to metal occurs only on the surface of the object to be coated due to the catalytic nature of the surface. The advantages of this process over electroplating are the possibility to produce coatings on insulator materials, and to produce uniform thickness coatings on geometrically complex surfaces. See also an Encyclopedia Article.
A process that decomposes a chemical compound into its elements or produces a new compound by the action of an electrical current. The electrical current is passed trough an electrolytic cell and oxidation/reduction reactions occur at the electrodes. E.g., water can be decomposed into hydrogen and oxygen, or a metal can be electroplated by electrolysis.
A chemical compound (salt, acid, or base) that dissociates into electrically charged ions when dissolved in a solvent. The resulting electrolyte (or electrolytic) solution is an ionic conductor of electricity. Very often, the so formed solution itself is simply called an "electrolyte". Also, molten salts and molten salt solutions are often called "electrolyte" when used in electrochemical cells, see ionic liquid. See also solid electrolyte.
See electrolyte. Also called "ionic solution".
A storage device similar to any other type of electrical capacitor. However, only one of its conducting phases is a metallic plate, the other conducting phase is an electrolyte solution. The dielectric is a very thin (passive) oxide film on the surface of the metal (typically aluminum or tantalum) that constitutes one conducting phase of the capacitor. There is also another metal electrode immersed in the solution, which serves only as the electrical contact to the solution. Electrolytic capacitors typically have much larger capacitance than classical capacitors because the dielectric is very thin (on the order of millionth of centimeter). There are no economical ways to produce dielectric films that thin in any other way. The electrolytic capacitor is not to be confused with the electrochemical capacitor. See also an Encyclopedia Article.
The overall capacitance of this device is the sum of two series coupled capacitances because the metal electrical contact in the solution will have an electrical double layer. The capacitance of the double layer is even larger than that of the oxide covered electrode because the "dielectric" in the double layer is only a few molecular layers of the solvent. Consequently, the overall capacitance is dominated by the lower value of the oxide-covered electrode.
An electrochemical capacitor, depending completely on double-layer capacitance, will have much larger capacitance than an electrolytic capacitor, however it can be operated only in a few volts potential range because of the limitation of the double-layer range. An electrolytic capacitor can be operated to tens or hundreds of volts, depending on the thickness of the dielectric oxide film.
An electrochemical cell that converts electrical energy into chemical energy. The chemical reactions do not occur "spontaneously" at the electrodes when they are connected through an external circuit. The reaction must be forced by applying an external electrical current. It is used to store electrical energy in chemical form, see rechargeable battery. It is also used to decompose or produce (synthesize) new chemicals by application of electrical power. This process is called electrolysis, e.g., water can be decomposed into hydrogen gas and oxygen gas.
The free energy change of the overall cell reaction is positive.
See electrolytic degreasing.
Process for removal of grease, oil, etc from metal surfaces in preparation for electroplating. The metal is made the cathode in an electrolytic cell containing strongly basic (sometimes hot) solution that dissolves these coatings. The strong hydrogen evolution occurring on the cathode may reduce some of the coatings, and the strong bubble evolution removes the coatings mechanically, while the agitation of the solution helps the chemical dissolution of the coatings by the base. Also called: "electrolytic cleaning", "electrochemical cleaning", "electrocleaning", and "electrochemical degreasing". See also degreasing.
See electrochemical drilling.
See electrochemical grinding.
See water electrolysis.
An electrolytic cell used for the determination of the moisture content in a gas. The cell contains a substance that reacts readily with water and so is transformed into an electrolyte. The electrolyte is electrolysed continuously and the electrolysis current is measured. At a constant flow of the gas, the electrolysis current is a linear function of the water content.
See electrochemical machining.
See water electrolysis.
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Process for removal of oxide scales from metal surfaces in preparation for electroplating. The metal is made the cathode in an electrolytic cell containing strongly acidic (sometimes hot) solution that dissolves the oxide scales. The strong hydrogen evolution occurring on the cathode may reduce some of the oxides, and the strong bubble evolution removes the scales mechanically, while the agitation of the solution helps the chemical dissolution of the scales by the acid. See also pickling.
See electropolishing.
See electrorefining.
See electrochemical shaping.
See electrolyte.
A dilute bleach solution produced by a brief electrolysis of ordinary tap water containing some dissolved table salt, according the process described under brine electrolysis. It is used as a disinfectant. Also called "electro-activated water".
See electrochemical machining.
A field of force associated with a moving electric charge and consisting of electric and magnetic fields that are generated at right angles to each other.
Branch of metallurgy (science dealing with the production of metals) using electrochemical processes, that is electrowinning.
A voltmeter with very large input resistance. A typical modern voltmeter has an input resistance of around ten million ohms, an electrometer can have ten million times more. Electrometers are used to measure the electromotive force of electrochemical cells that can be easily polarized by current. The voltmeter always draws a small current, the magnitude depends on the ratio of the resistance of the cell and the voltmeter. High resistance cells (e.g., one containing a glass electrode, must be measured with an electrometer.
The movement of ions under the influence of electrical potential difference.
The cell voltage of a galvanic cell measured when there is no current flowing through the cell. In other words, the equilibrium electrode potential difference between the two electrodes of the cell. Abbreviated as "emf".
A tabulation on which various substances, such as metals or elements, are listed according to their chemical reactivity or standard electrode potential. It is usually ordered with increasing standard electrode potentials (most negative on top). For metals, the order indicates the tendency to spontaneously reduce the ions of any other metal below it in the series (see cementation). During electrolytic reduction of cations (e.g., electroplating) an element lower in the series (more positive) will deposit first, and an element higher in the series (more negative) will deposit only when the solution is practically depleted of the ions of the first element. Also called "electrochemical series" and "galvanic series".
See atomic structure.
Alternative name of a reduction process.
The expression of nature's tendency to keep any system electrically neutral, that is, if it contains electrically charged particles the total sum of negative charges will be equal to the total sum of positive charges. Applying this condition to a solution of electrolytes implies the equality of the total positive ionic charges to the total negative ionic charges. This equality should hold even as we subdivide the solution into smaller and smaller volume elements.
This condition results from the statistical distribution of the ions around each other considering the attractive tendency of oppositely charged particles and the repulsive tendency of similarly charged particles. Consequently, there is a statistical limit of the size of the volume element to which it applies. At the extreme, a volume small enough to contain only a single ion obviously cannot be electrically neutral.
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A material that conducts electricity with electrons as charge carriers. Contrast with ionic conductor.
Electrical current with electrons as charge carriers.
See electrochemical nose.
See charge-transfer reaction.
Relating to organic electrochemistry.
The movement of a liquid through a capillary tubing or porous solid driven by an electrical potential difference. See electrokinetic effects. Also called "electroendosmosis".
Compaction of slurries by electroosmosis.
See electroremediation.
Oxidation carried out with an electrochemical reaction.
Alternative name for electrophoretic painting.
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As a phenomenon: the movement of small suspended particles or very large molecules in a liquid driven by an electrical potential difference. Also called "cataphoresis". See electrokinetic effects. As an electroanalytical technique: a separation method for very large organic molecules (usually of biological origin) based on their different electrophoretic velocities through an "immobilized" liquid phase. The liquid can be immobilized by a variety of "supports", e.g.: paper, gelatinous material, capillary tubing.
Deposition of particles carried to a surface by electrophoresis. The loosely formed deposit layer typically needs compacting that can partially occur by electroosmotic removal of the liquid, or by other means. Electrode reactions occurring on the substrate surface can take part in "binding" the coating. Practical applications are surface coating and paint deposition (practiced on large scale in the automotive industry) and fabrication of ceramic products. Also called "electrocoating".
Alternative name for "sedimentation potential". See electrokinetic effects.
The study of the electrical properties of living tissue.
The process that produces a thin, metallic coating on the surface on another metal (or any other conductor, e.g., graphite). The metal substrate to be coated is made the cathode in an electrolytic cell where the cations of the electrolyte are the positive ions of the metal to be coated on the surface. When a current is applied, the electrode reaction occurring on the cathode is the reduction of the metal ions to metal. E.g., gold ions can be discharged form a gold solution to form a thin gold coating on a less expensive metal to produce "custome" jewelry. Similarly, chromium coating is often applied to steel surfaces to make them more "rust resistant". Electroplating is also used in the production of integrated circuits on computer chips and for other modern electronic instrumentation. The anode material can either be the metal to be deposited (in this case the electrode reaction is electrodissolution that continuously supplies the metal ions) or the anode can be of inert material and the anodic reaction is oxygen evolution (in this case the plating solution is eventually depleted of metal ions). Also called "electrodeposition". See also an Encyclopedia Article.
A process that produces a bright, shiny surface on a metal. The metal is anodically dissolved in an electrolytic cell under conditions that projections in the surface are dissolved faster than the smoother areas. Also called "electrochemical" or "electrolytic" polishing.
An electrode reaction (charge-transfer reaction) resulting in polymerization (a chemical reaction in which two or more molecules combine to form larger molecules, the polymer, that contain repeating structural units). The polymer usually remains adsorbed on the electrode surface. If the polymer is electrically insulating, its growth is self-limited. Such films are very thin (10-100 nanometers). Unlimited growth can occur with conducting polymers.
The application of very brief, carefully controlled, pulsed, rotating electrical fields to cells, a process that causes pores to open in the cell membrane and allows pharmaceuticals (electrochemotherapy) or genes (electrogenetherapy) to gain access to the cell's interior. The increase of the permeability of the membrane was found to be temporary and was found to have little or no effect on the viability of the cell.
See electroremediation.
Reduction carried out with an electrochemical reaction.
An electrochemical process that produces a purified metal from a less pure metal. The metal to be purified is made the anode in an electrolytic cell and it is dissolved by the application of a current into a usually acidic aqueous electrolyte or a molten salt. At the same time, the pure metal is deposited on the cathode. The process is carried out under conditions that most impurities will either precipitate as "sludge" or remain dissolved in the electrolyte. Copper is one metal that is often electrorefined in aqueous solutions (see an Encyclopedia Article), and aluminum is electrorefined using a molten salt electrolyte. Also called "electrolytic refining" and "metal refining".
An electroosmotic process for removing soluble contaminants from soil. Electrosmosis is carried out in the wet soil with strategically placed electrodes, resulting in a movement of the contaminants toward the electrodes thereby cleansing the soil and concentrating the contaminants in a small volume of soil around the electrodes from where they can be easily removed. Also called "electrokinetic remediation", "electroosmotic remediation", "soil remediation", "electroreclamation", and "electrorestoration".
See electroremediation.
See electroviscosity.
A process that uses electrolysis to selectively remove a constituent from solution.
Adsorption at electrode surfaces. Generally, adsorption at electrically charged interfaces.
Electric field associated with static (stationary, nonmoving) electric charges.
A phenomenon similar to piezoelectricity, but electrostrictive ceramics expand according to the square of the voltage whereas piezoelectric materials expand linearly. Electrostrictive materials exhibit less hysteresis than piezoelectric materials, but are difficult to use at very low voltages.
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Production of chemicals in an electrolytic cell. See also Encyclopedia Articles on electrosynthesis of inorganic and organic compounds.
See electroforming.
The phenomenon of a change in viscosity due to the presence of charge on particles suspended in a solvent.
An electrochemical process that produces metals from their ores. Most metals occur in nature in oxidized form in their ores. While numerous ways exist to reduce the ores, for many metals electrochemical reduction is the most practical. The ore is dissolved (often following some chemical purification or preprocessing) in an acidic aqueous solution or in a molten salt and the resulting electrolyte solution is electrolyzed. The metal is deposited on the cathode (either in solid or in liquid form), while the anodic reaction is usually oxygen evolution. Copper and zinc are two metals that are often produced by aqueous electrolysis (see an Encyclopedia Article). Aluminum, magnesium, and sodium are some metals that can be produced by molten salt electrolysis. For aluminum, this is the only practically used production process (see an Encyclopedia Article). Also called "electroextraction" and "metal winning".
A substance that cannot be decomposed into simpler substances by chemical means.
Chemical reactions usually take place in a number of simple ("elementary") reaction steps proceeding in series. The overall reaction is the sum of the elementary reactions. E.g., the electroplating of copper on some metal involves three elementary steps: (1) a redox reaction in which the double positively charged copper cation reacts with an electron from the metal electrode to form a single charged copper ion, followed by (2) a metal deposition reaction in which the single charged copper ion reacts with a second electron to form a copper atom on the surface of the metal, and finally (3) an electrocrystallization step in which the copper atom becomes incorporated into the crystalline structure of the underlying metal. The rate-determining step in copper deposition is usually the first of these steps.
Some complicated reactions can also involve parallel paths, each proceeding through a different series of elementary steps (different reaction mechanisms). The sum of the series elementary steps in each parallel path must add up to the same overall reaction.
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Stands for electromotive force.
The energy of a system expresses the ability of that system to do some useful work or generate heat. Energy can be in many forms; e.g., mechanical energy, chemical energy, heat energy, electrical energy, etc. The different forms of energy can be converted into each other.
It is a fundamental law of nature that energy can never be converted from one form to another 100%, some of the energy is always converted into heat energy during the conversion. Also, heat can never be converted 100% into any other form of energy.
A process in which energy is converted from one of its many forms to another. The fuel cell is an electrochemical energy conversion device.
Characteristic parameter of a battery or a capacitor indicating the amount of electrical energy stored per unit weight or volume. The terminology is not strictly defined. Weight based energy density is often called "specific energy" or "gravimetric energy density". Volume based energy density is often called "energy density" or "volumetric energy density". The energy density is typically expressed as watt-hour/kilogram or watt-hour/liter.
For a rechargeable battery: the fraction, usually expressed as a percentage, of the electrical energy stored in a battery by charging that is recoverable during discharging. For an electrolytic cell: the fraction, usually expressed as a percentage, calculated as the theoretically required energy divided by the energy actually consumed in the process (production of a chemical, electroplating, etc). Inefficiencies arise from current inefficiencies and the inevitable heat losses due to polarization. See also coulometric efficiency.
See electrical source (supply).
A process in which energy is stored in some form, ready for future use on demand. The time scale of storage can extend to many years if needed. The battery and the electrochemical capacitor are electrochemical energy storage devices.
See electrochemical nose.
See Pourbaix diagram.
Stands for electrochemical quartz crystal microbalance.
The process by which all cells of a multi-cell rechargeable battery are brought to the same state of charge.
An electrode or an electrochemical cell is said to be in "equilibrium" when there is no net current flowing and there are no net electrode reactions taking place in the system. (See, however, exchange current density.) In equilibrium, the potential of the electrodes is the equilibrium potential and the cell voltage is the electromotive force.
See electromotive force.
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The electrical potential of an electrode measured against a reference electrode when there is no current flowing trough the electrode. In other words, the electromotive force of an electrochemical cell consisting of the electrode in question and a reference electrode. Also called: "open circuit potential (ocp)". See also equilibrium and standard electrode potential.
The concept of equilibrium potential is probably easiest to demonstrate with a simple metal/metal-ion electrode system. When a metal (e.g., silver) is immersed in a solution containing its ion (e.g., silver nitrate solution) metal ions will cross the metal/solution interface. They will pass from the phase where the "chemical energy" of the ion is large to the phase where the "chemical energy" of the ion is smaller. Depending on the conditions of the system, this can occur in either direction. However only the positively charged (e.g., silver) cations can pass through the interface. The negatively charged electrons cannot pass into the solution, and the anions (e.g., nitrate) cannot pass into the metal. Consequently, charge accumulation occurs at the interface forming an electrical double layer. When the metal ions move preferentially from the metal into the solution, the metal surface becomes negatively charged because of the accumulation of the electrons left behind, while the solution layer near the metal surface becomes positively charged because of the accumulation of silver ions. In the opposite case, the metal surface becomes positively charged, while the solution layer near the metal surface becomes negatively charged because of the accumulation of nitrate ions tat are left behind. In either case, this process produces a potential difference between the two phases that will slow and eventually stop the passage of the metal ions. At "equilibrium" the chemical driving force and the opposing electrical force are equal. The potential difference between the metal and the solution phases under these conditions is the "equilibrium potential difference". This potential difference cannot be measured because there is no way to make an electrical connection to the solution phase without setting up another electrode potential. Consequently, electrode potentials are always measured against a reference electrode whose potential is known on an arbitrary scale. See standard hydrogen electrode.
See equilibrium electrode potential.
See electromotive force.
An electrical circuit (usually comprised of series and parallel coupled resistors and capacitors) that models the fundamental properties and behavior of electrodes or electrochemical cells.
A characteristic weight of a substance relating to a specific reaction the substance participates. In electrochemical reactions, the molecular weight or atomic weight divided by the number of electrons transferred during the reaction. Consequently, the equivalent weight of a substance can be different for different reactions. E.g. the equivalent weight of the cuprous ion (singly charged copper ion) is equal to its atomic weight, independently whether it is oxidized to cupric ion (doubly charged copper ion) or it is reduced to (electrically neutral) copper metal. On the other hand, the equivalent weight of the cupric ion is one half of its atomic weight if it is reduced to copper metal, but it is the atomic weight if it is reduced only to cuprous ion.
In a wider sense, the molecular weight or the atomic weight divided by the valence change occurring during the reaction. For acid/base reactions, the molecular weight divided by the number of hydrogen ions produced or consumed during the reaction.
At the equilibrium potential there is no net current flowing through the electrode. However, the equilibrium is a dynamic one, that is, the electrode reaction proceeds at "equal rates" both in the forward and in the reverse direction, resulting in a zero "net" reaction rate and a zero "net" current. The rate of the electrode reaction can be expressed as an equivalent current density and the "exchange current density" of a reaction is the current density flowing "equally" in both directions in equilibrium. A large exchange current density indicates a fast reaction (see also non-polarizable electrode), while a small exchange current density indicates a slow reaction (see also polarizable electrode)
The electrolyte solution in the electrochemical cell into which the reference electrode is immersed. Contrast with the internal electrolyte of the reference electrode.
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Symbol and abbreviation of farad, and symbol and abbreviation of the Faraday number. (Often a "bold-face" letter is used for the latter, but this is not a general practice.) It is usually obvious from the context which meaning is appropriate.
Measurement unit of capacitance. Symbol: "F", which is the same as the symbol of the Faraday Number. It is usually obvious from the context which meaning is appropriate.
A capacitor has a capacitance of one farad when one coulomb charges it to one volt.
The current (or current density) that is flowing through an electrochemical cell and is causing (or is caused by) chemical reactions (charge transfer) occurring at the electrode surfaces. Contrast with capacitive current.
A heterogeneous charge-transfer reaction occurring at the surface of an electrode.
Container giving protection from electrical fields: an assembly of conducting material, for example, metal mesh or grid, placed around electrical equipment to protect it from external electrical fields.
The Faraday Number (or constant) gives the amount of electrical charge needed to change one gram-equivalent of substance by electrochemical reaction. Its value is 96,485.34 coulombs or 26.80 ampere-hours. This charge is often simply called one "Faraday". Symbol: "F", which is the same as the symbol of the farad. It is usually obvious from the context which meaning is appropriate.
The Faraday number is the product of Avogadro's number and the electrical charge of a single electron.
Some of the most fundamental laws of electrochemistry discovered by Faraday in
the 1830's. They are usually stated as: (1) In any electrolytic process the amount of chemical change produced is proportional of the total amount of electrical charge passed through the cell. (2) The mass of the chemicals changed is proportional to the chemicals' equivalent weight. The proportionality constant being the Faraday Number.
Stands for fuel cell
A microelectrode consisting of a bare fiber.
Mathematical relations concerning the phenomenon of diffusion.
See internal electrolyte.
See electrode of the first kind.
The permanently attached charged fragment in an ion-exchange resin. Contrast with counterion.
Alternative name for the passivation potential.
A battery containig rectangular, flat-plate electrodes. Also called "prismatic battery".
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See trickle charging.
The voltage required for retaining a rechargeable battery in fully charged condition. See float charging.
A battery built with an excess of eletcrolyte. Contrast with starved elelectrolyte battery.
A flow battery is an electrochemical device that converts the chemical energy of the electro-active materials directly to electrical energy, similar to a conventional battery. However, all the reactants and products of the electro-active chemicals are stored external to the power conversion device and are introduced into the device only during operation. The most common form of flow battery is the redox battery. See also an Encyclopedia Article.
An electrode that permits the electrolyte to flow through it, e.g., a porous electrode or a packed-bed electrode. This type of electrode is especially useful for removing small traces of impurities from the solution by electrolysis (e.g., waste treatment) because the solution contacts a large surface of the electrode material.
See packed bed electrode.
Similar to a standard electrode potential except that both the oxidized and the reduced species are present in unit concentration instead of unit activity. It is not as well defined as the standard potential but it is useful in cases when the activities are unknown.
Electrochemical processing of a rechargeable battery by repeated charging/discharging that converts the electrode materials into usable form. This treatment is needed for some batteries during manufacturing, or when first put into use (and sometime when returned to service after a long storage). Also called "conditioning".
Essentially the same as the molecular weight, but it can be used more generally, e.g., also for ions.
The frequency of an alternating (ac) current is a measure of how many times the direction of the current flow changes, in the same direction, per second. As the direction of flow changes back and forth, the total number of directional changes, per second, is twice the frequency. The frequency of the household current is 60 hertz. The measurement unit of the frequency is the hertz.
The effect of adsorption on electrode kinetics. When reactants or intermediates are adsorbed on the electrode surface, the rate of the electrode reaction may no longer be related to the concentration by a simple law.
A device that converts chemical energy into electrical energy. It is different from a battery in that the energy conversion continues as long as fuel and oxidizing agent are fed to the fuel cell; that is, in principle indefinitely. (A battery is manufactured with a limited amount of chemicals, and it is exhausted when all the chemicals have reacted.) It is a galvanic cell, where spontaneous chemical reactions occur at the electrodes. The fuel is oxidized at the anode, and the oxidizing agent (almost always oxygen or air) is reduced at the cathode. Presently, the most commonly used fuel is hydrogen. More conventional fuels (e.g., gasoline or natural gas) must be converted (reformed) into hydrogen before they can be utilized in a fuel cell. Fuel cells that can burn hydrocarbon fuels directly are in the development stage. Some fuel cells employ an aqueous solution as electrolyte, that can be either acidic or basic (alkaline), or an ion-exchange membrane soaked in aqueous solution can act as the electrolyte (see PEM) these fuel cells operate at relatively low temperatures (from room temperature to not much above the boiling point of water). Some fuel cells employ molten salts (especially carbonates) as electrolytes and have to operate at many hundreds of oC temperature. Others employ ionically conductive solids as electrolyte and must operate close to 1000oC (1832oF). Abbreviated as "FC". Some common fuel cells are the alkaline fuel cell, alkaline membrane fuel cell, direct-formic-acid fuel cell, direct-methanol fuel cell, internal-reforming fuel cell, micro fuel cell, molten-carbonate fuel cell, phosphoric-acid fuel cell, polymer-electrolyte-membrane fuel cell, proton-exchange-membrane fuel cell, reversible fuel cell, and solid-oxide fuel cell. See also an Encyclopedia Article.
Fuel cells typically use porous electrodes to permit the efficient use of gaseous reactants. A single fuel cell has a rather small (typically less then one volt) cell voltage. For practical applications a large number of them are assembled, series coupled, in what is called a "fuel cell stack". (A term essentially analogous to the original meaning of the battery.)
The expression of "activity" for a component in a mixture of gases. It has the same physical meaning as the activity for a component in a solution.
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Symbol and abbreviation of gram.
See cathodic corrosion protection.
An electrochemical cell that converts chemical energy into electrical energy. A cell in which chemical reactions occur spontaneously at the electrodes when they are connected through an external circuit, producing an electrical current. E.g., in a fuel cell hydrogen is oxidized at the anode by transferring electrons to the anode and the oxygen is reduced at the cathode by accepting electrons from the cathode. During this process the electrons are carried from the anode to the cathode through an outside electrical circuit where the electrical current can drive a motor, light a light bulb, etc. Also called "voltaic" cell. In contrast, in an electrolytic cell electrical power must be supplied to force the non-spontaneous reverse reaction, the electrolysis of water. See also electrochemical reaction, battery, and fuel cell.
The free energy change of the overall cell reaction is negative.
See cementation.
See electromotive series.
Process for coating iron or steel with a thin layer of zinc for corrosion protection. It can be carried out electrochemically by electroplating (called "electrogalvanizing") or by "hot-dip" galvanizing consisting of immersing the substrate into molten zinc.
A very sensitive ammeter that can be used to measure currents in the range of one millionth to one billionth of an ampere.
A somewhat archaic, today very seldom used, term for electroforming.
An electronic instrument that controls the current through an electrochemical cell at a preset value, as long as the needed cell voltage and current do not exceed the compliance limits of the galvanostat. Also called "amperostat".
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 current flowing through the system.
See porous electrode.
Any electrode with one of the reactants or products in the gaseous phase. The solution surrounding the electrode is typically saturated with the gas.
Essentially a redox electrode with the "dissolved" gas as one of the potential determining species. E.g., the oxidized species for the "chlorine electrode" is the dissolved chlorine gas, while the reduced species is the chloride ion in solution. Under equilibrium conditions, the chemical potential of the gaseous chlorine is the same as that of the dissolved chlorine. This may not be the case when current is flowing through the electrode: during the electrolytic production of chlorine, the solution may become supersaturated in chlorine if the nucleation of the gas bubbles requires considerable activation energy.
Stands for glassy carbon electrode.
Stands for Gouy-Chapman-Stern model of the double layer.
Current density calculated with the geometric electrode area.
The surface area of an electrode calculated from its geometrical dimensions. Contrast with true electrode area.
A membrane electrode with a thin glass membrane (usually in the form of a bulb at the end of a glass tubing) sensing element. It is most often used as a pH electrode, but some glass compositions can also be sensitive to the concentration of other cations (e.g., sodium). See also an Encyclopedia Article.
An electrode made of glassy (vitreous) carbon, a pure carbon with glass-like mechanical characteristics. It is resistive to many chemicals, it is impermeable to gases and liquids, has good electrical conductivity, low density, and high hardness. Also called "vitrous carbon electrode". Abbreviated as GCE. See an Encyclopedia Article.
See electrochemical blood glucose test strip.
A model of the electrical double layer. According to this model, the excess ions are non-uniformly distributed in the vicinity of the electrode, their concentration is the largest at the surface of the electrode, decreasing non-linearly till they reach bulk concentration. The "thickness" of this so called "diffuse" or "outer" layer is variable, but it is typically around the order of magnitude of a millionth of a centimeter.
A model of the electrical double layer. The Gouy-Chapman model predicts an unrealistically large surface concentration, because it assumes that the ions are infinitely small and can get infinitely close to the surface of the electrode. The "Stern modification" is essentially a combination of the Helmholtz and Gouy-Chapman models. It assumes a "plane of closest approach" where a portion of the excess ions reside in the "inner" or "compact" layer and attaches to this a Gouy-Chapman type "diffuse" or "outer" layer. Abbreviated as "GCS".
An international unit of mass, on thousandth of the kilogram. One ounce equals 31.1034768 grams, or one gram equals 0.0321507 ounce. Symbol: "g".
A concept similar to gram-mole except it relates to atoms rather than molecules.
An amount of a substance equal in grams to its equivalent weight.
An amount of a compound equal in grams to its molecular weight. E.g., the molecular weight of water is 18, so 18 grams of water is called a gram-mole of water. This provides an atomistically fundamental unit because one gram-mole of any material will contain the same number of molecules (this is a very large number, called "Avogadro's" number). One gram-mole of hydrogen gas contains the exactly same number of molecules as one gram-mole of table salt (sodium chloride), even though the latter is much heavier. The simplified expression of "mole" is often used in place of "gram-mole" and also in place of gram-atom. It is usually obvious from the context which meaning is appropriate. The measurement unit and symbol of the "gram-mole" or "mole" is the "mol".
A suggested mechanism attempting to explain the unusually high ionic mobility of hydrogen ions or protons. As a proton approaches a water molecule, it temporarily forms a hydronium ion, simultaneously, another proton hops from the hydronium ion to the next water molecule, and so on.
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See Luggin capillary.
A somewhat archaic term, indicating a structure that contains an electrode and the surrounding electrolyte. Electrochemical cells are often divided, containing two separate electrolytes (one surrounding each electrode, e.g., the Daniell cell). In these cases the electrode and its electrolyte can be considered "half" of the cell. Commercially available reference electrodes can be considered "half cells".
A not incorrect, but somewhat archaic term for electrode reaction.
See peak width at half-height.
See electrode reaction.
The potential at which the value of the current of a polarographic wave is one half of the difference between the wave (limiting) current and the residual current (at the foot of the wave). For reversible charge-transfer reactions, this potential is characteristic of the reacting species, being related to its standard electrode potential.
See aluminum production.
A variation of the dropping-mercury electrode, where the mercury is not flowing continuously. A "hanging drop" is formed at the end of the capillary and is used as a working electrode. The advantage of this electrode arrangement is that the droplet can easily be renewed by feeding some mercury if the electrode surface becomes contaminated. Also called "static-mercury-drop electrode". Abbreviated as "hmde".
See the Helmholtz model of the double layer.
The simplest model of the electrical double layer. The excess ions in the solution side of the double layer line up in one plane ("Helmholtz plane") very close to the electrode surface. In a somewhat more complex model there are two planes of closest approach of the ions. Ions in the "outer Helmholtz plane" are about two solvent-molecule diameters away from the electrode surface because both the ions and the electrode surface are solvated. Ions in the "inner Helmholtz plane" have shed their solvation layer (these are usually the weekly solvated, large anions) and penetrated the solvent layer on the electrode; these, so called contact adsorbed, ions are sitting directly on the electrode surface. The ionic portion of the Helmholtz model is often called the "Helmholtz layer" or "compact" or "inner" layer.
See the Helmholtz model of the double layer.
An equation that predicts the value of the liquid junction potential for simple cases.
Stands for hydrogen evolution reaction.
The measurement unit of frequency. Abbreviation: "Hz".
A charge-transfer reaction with the charge transferred across a phase boundary, typically between a solid and a liquid phase. See also faradaic reaction. Contrast with homogeneous charge-transfer reaction.
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A high purity form of carbon, with a renewable and almost atomically smooth surface. Often used as a working electrode. It has a lamellar/layered structure and it cleaves almost like mica. Its structure shows hexagonal rings that are the true structure of graphite. Abbreviated as "HOPG".
An experimental method for the determination of transport numbers. Electrolysis is carried out in a three-compartment cell and the concentration changes occurring in the anode and cathode compartments can be used to calculate the transport numbers. The concentration in the center compartment should remain unchanged.
Stands for hanging-mercury-drop electrode.
See semiconductor.
A charge-transfer reaction with both reactants present in the same phase. Typically both reactants are dissolved species in a solution while the charge is transferred from one to the other. Contrast with heterogeneous charge-transfer reaction.
Stands for highly ordered pyrolytic graphite.
Stands for hydrogen oxidation reaction.
Solvation occurring in an aqueous solution.
The number of water molecules associated with an ion in the process of solvation in aqueous solutions.
A thin immobile layer of fluid that always exists at a solid/moving-fluid interface. Whether the movement of the fluid is due to "forced" or "natural" convection, a thin layer of fluid will always remain completely immobile at the surface of the solid due to the solid-liquid interactive forces.
Voltammetry under controlled conditions of convective mass transport to/from the surface of the working electrode.
A proposed, new energy distribution system based on hydrogen gas as the energy carrier and hopefully on a renewable energy supply. Hydrogen could be generated (using e.g., solar energy) in a variety of ways, one of them being water electrolysis. Hydrogen would be distributed to the end users through a system similar to today's gas pipelines. The hydrogen could be used either by burning to generate heat or by fuel cells to generate electricity.
A redox electrode with dissolved hydrogen gas being the reduced species and hydrogen ions the oxidized species. Hydrogen gas (or a gas mixture containing hydrogen) is bubbled through the electrolyte to keep a desired dissolved hydrogen content. The inert metallic electrode is usually platinized platinum. The equilibrium potential of this electrode depends on the concentration (strictly speaking, activity) of both the hydrogen ions and the dissolved hydrogen gas (controlled by the hydrogen gas pressure), see Nernst equation. The electrode can be used as a measuring electrode in a sensor to determine the hydrogen ion concentration (pH), or it can be used as a reference electrode if all the concentrations are known and constant. It is used equally often for both purposes. It is also the most fundamental reference electrode as the standard hydrogen electrode. See also the dynamic hydrogen electrode and the reversible hydrogen electrode.
An electrode reaction in which hydrogen gas is produced at the cathode of an electrolytic cell by the reduction of hydrogen ions or the reduction of the water molecules of an aqueous solution. Abbreviated as "her". See also water electrolysis. It is the reverse reaction of hydrogen oxidation.
An electrode reaction in which hydrogen gas is oxidized at the anode of an electrolytic cell to produce hydrogen ions. Abbreviated as "hor". It is a very important and much studied electrode reaction because it occurs at the anode of many fuel cells where hydrogen is the fuel. It is the reverse reaction of hydrogen evolution.
See water electrolysis.
See standard hydrogen electrode.
A chemical reaction in which water reacts with another substance and gives decomposition or other products, often a reaction of water with a salt to create an acid or a base.
A singly hydrated hydrogen ion (proton) with a formula of H3O+. A naked proton is not stable in aqueous solution and will strongly bind to one water molecule. The hydronium ion will be further hydrated by somewhat more loosely bound water molecules.
A material that attracts water and does absorb/adsorb it readily. Contrast with hydrophobic.
A material that repels water and does not absorb/adsorb it readily. Contrast with hydrophilic.
A substance that contains water. The opposite of anhydrous.
See brine electrolysis.
A delayed response by an object to changes in the forces acting on it.
Symbol and abbreviation of hertz.
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See interdigitated electrode assembly.
See interdigitated electrode.
Alternative, and somewhat archaic, expression for ideal non-polarizable electrode.
An electrode that is practically not polarizable. That is, the potential of the electrode will not change from its equilibrium potential with the application of even a large current density. The reason for this behavior is that the electrode reaction is extremely fast (has an almost infinite exchange current density). Also called "ideal depolarized electrode". Contrast with ideal polarizable electrode.
An electrode is called "ideal polarizable" if no electrode reactions can occur within a fairly wide electrode potential range. Consequently, the electrode behaves like a capacitor and only capacitive current ( no faradaic current) is flowing upon a change of potential. Many electrodes can behave as an ideal polarized electrode but only within an electrode potential range called the "double-layer range". Also called "completely-polarizable electrode" and "totally-polarized electrode". Contrast with ideal non-polarizable electrode.
Stands for "inner Helmholtz plane". See the Helmholtz model of the double layer.
A relation between diffusion limited current density and time in a polarograhic experiment. See also an Encyclopedia Article.
The diffusion current density is proportional to the growth-time of the mercury drop on the one-sixth power, and to the mass-flowrate of the mercury on the two-third power. The proportionality constant contains the product of the concentration of the reactant and the square root of the diffusion coefficient of the reactant.
Impedance is the analogue of the resistance when applied to alternating current. That is, it is a measure of a circuit element's inability to carry the electrical current. In many cases, the impedance varies as the frequency of the applied electrical potential changes, due to the properties of the conducting liquid or solid. In electrochemistry, the impedance of the electrodes is also frequency dependent.
See electrochemical impedance spectroscopy.
See cathodic corrosion protection.
See working electrode.
See supporting electrolyte.
The production of chemicals in an electrolytic cell through intermediate electrolysis products. It is often used in the oxidation/reduction of organic compounds that would otherwise react very slowly at the electrode surface. An intermediate oxidizing/reducing agent (a redox couple) is produced at the electrode surface and the agent reacts with the organic in the bulk solution. The agent is continuously regenerated by the electrolysis. A typical oxidizing agent is the ferric (tri-valent iron) ion, and an example of the reducing agent is the cerous (tri-valent cerium) ion. The reactive intermediate is often called a "redox mediator", and the overall reaction a "redox mediated reaction".
See indirect electrolysis.
An optically transparent electrode consisting of a thin film of semiconducting mixed oxide coated onto a glass or quartz substrate, or by itself. Such an electrode, which is transparent to light and is also conductor of electricity, is a prerequisite for spectroelectrochemical measurements. Abbreviated as ITO.
An electrode that serves only as a source or sink for electrons without playing a chemical role in the electrode reaction. Noble metals, mercury, and carbon are typically used as inert electrodes. See for example the redox electrode.
The "inert" nature of the electrode can sometimes be questioned. While the electrode may not take part in the reaction as a reactant or product, it still can act as an electrocatalyst.
See supporting electrolyte.
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A chemical that stops (or at least decreases the rate of) a chemical reaction. See also corrosion inhibitor.
See the Gouy-Chapman-Stern model of the double layer.
See the Helmholtz model of the double layer. Abbreviated as "IHP".
A charge-transfer reaction with the reactants in direct contact with each other, without any intervening solvent molecules. Note that a "reactant" can also be an electrode. Contrast with outer-sphere charge-transfer reaction.
A material that will not carry any electrical current. It has zero conductivity and infinite resistivity. Contrast with conductor.
An electrically conductive structural part that connects series-connected cells in a fuel cell stack.
An electrode configuration consisting of two metallic electrodes, usually of small sizes, deposited on an insulating base. Both electrodes (anode/cathode or working electrode/counter electrode in a three-electrode cell) consist of several fingerlike structures "interdigitated" with each other. Abbreviated as "IDA" or "IDE". See also an Encyclopedia Article with a specific application of this array electrode.
See surface tension.
A molecular or ionic species that is formed (directly or indirectly) from the reactants and reacts further (directly or indirectly) to form the products of the reaction. It does not accumulate during the course of the reaction.
The electrolyte solution inside a reference electrode assembly such as the silver/silver-chloride electrode. (Also called "filling solution".) Internal electrolytes are used also in membrane electrodes. Contrast with external electrolyte.
A reference electrode used inside a membrane electrode assembly as an electrical contact, with stable potential, to the internal electrolyte.
In some high-temperature fuel cells, the reforming of natural fuels is integrated with the fuel cell. The catalytic conversion of natural fuels requires a considerable amount of heat and the heat evolving at high temperatures during the operation of the high-temperature fuel cell can be used for the reforming reaction, thus increasing the overall efficiency of the fuel utilization. Such units combining high-temperature fuel cells and reforming are called "internal-reforming fuel cells". Abbreviated as "IRFC".
The inhomogeneous spacial region at the interface between two bulk phases
in contact. The "interface" is a two-dimensional surface, while the "interphase" is a thin, but three-dimensional, volume. The physical/chemical properties in the interphase are significantly different from, but related to, the properties of the bulk phases. The interfacial properties usually vary in the direction perpendicular to the surface. An example of the interphase is the electrical double layer.
See an Encyclopedia Article.
An electrically charged chemical particle (atom, molecule, or molecule fragment). "Anions" are negatively charged (contain excess electron(s), and "cations" are positively charged (deficient in electron(s)).
A plastic sheet formed from ion-exchange resin. The utility of such membranes is based on their property that they are permeable preferentially only to either positive ions (cation-exchange membrane) or to negative ions (anion-exchange membrane).
A polymeric resin that contains electrically charged fragments ("fixed ions") permanently attached to the polymer backbone, electrical neutrality is achieved by attached mobile "counterions" in the solution phase the resin is immersed into. A practical use of such resin is the removal of unwanted ions from a solution by replacing them with other ions. E.g., a cation exchange resin containing fixed negative charges with attached mobile sodium ions can be used to remove "hardness" from water if the calcium and magnesium ions are more strongly attracted to the resin and therefore will replace the sodium ions. Eventually all the sodium ions will go into solution and the ion-exchange process terminates. The resin can be regenerated by soaking in a high concentration sodium salt solution. Such process can also be used to remove unwanted ions from polluted water streams.
Ions in an electrolyte solution are preferentially surrounded by ions of the opposite charge due to the repulsion of like charges and the attraction of opposite charges. This collection of ions surrounding a central ion is called the "ionic atmosphere".
A material that conducts electricity with ions as charge carriers according to Kohlrausch's Law. See also electrolyte. Contrast with electronic conductor.
Electrical current with ions as charge carriers.
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A liquid containing mostly ions, a molten salt in which the molecules are fully (or almost fully) dissociated. Contrast with electrolyte solution in which the dissociated salt is dissolved in a solvent, with the solvent not (or only slightly) dissociated. Corresponding examples are molten sodium chloride (table salt) and aqueous solution of sodium chloride, respectively.
See ionic liquid.
A quantitative measure of an ion's ability to move under the influence of a potential difference in solution. (See also electromigration.) It is the speed of movement under the influence of unit potential difference.
While the mobility is defined in terms electromigration, it also affects the speed of diffusion.
Part (sub-discipline) of electrochemistry that deals with the behavior of ions in liquid solutions, ionic liquids, and solids ("solid-state ionics"). See also electrodics.
See electrolyte solution.
A measure of the total "effect" of the ions in an electrolyte solution. Essentially, the total concentration of the ions weighted by their charge.
The weighted concentration of ions in solution, computed by multiplying the concentration of each ion in solution by the corresponding square of the charge on the ion and summing this product for all ions in solution and dividing by two.
An electrode or electrode assembly with a potential that is dependent on the concentration of an ionic species in the test solution and is used for electroanalysis. Ion-selective electrodes are often membrane type electrodes. Abbreviated as "ISE". See also an Encyclopedia Article.
See ion-exchange membrane.
See ion-selective electrode.
Stands for isopotential point.
Some potentiostats are equipped with an optional ir compensation. The potentiostat electronically corrects for the solution ir drop and the potential of the working electrode is controlled (at least in principle) at the correct value. Unfortunately, most potentiostats become unstable at full compensation, so one can only make a partial compensation, resulting in an uncompensated ir drop and an error in the potential control. The user must provide the solution resistance value, though some potentiostat setups will measure it automatically. Contrast with ir (drop) correction.
A numerical correction of measured potential of the working electrode for the solution ir drop. (One must know the value of the current and the value of the resistance of the electrolyte between the working and the reference electrodes.) It cannot be simply stated whether this correction is positive or negative because of the contradictory conventions used for the anodic and cathodic currents. In either case, the absolute value of the corrected potential must be smaller than that of the uncorrected potential. Contrast with ir (drop) compensation.
The electrical potential difference between the two ends of a conducting phase during a current flow. It is the product of the current (i) and the resistance (r) of the conductor. In electrochemistry, it refers to the solution ir drop, or to the ohmic loss in an electrochemical cell. See also Ohm's law.
Stands for internal-reforming fuel cell.
An electrode with an irreversible electrode reaction.
A qualitative term for a slow electrode reaction. An electrode reaction having a small exchange current density. Opposite: reversible electrode reaction. See also quasi-reversible electrode reaction.
Stands for ion-selective electrode, or for International Society of Electrochemistry.
A variation of the electrophoretic separation technique. The separation of molecules occurs in a combination of potential and pH gradients resulting in sharper separations compared to simple electrophoresis.
For some electrodes, typically those covered by, or consist of, metal oxides, the charge on the electrode is a function not only of the electrode potential but also of the pH of the solution. The pH value at which the charge on the electrode is zero is called the isoelectric point. It as also used in colloid chemistry to indicate the pH at which the charge on the colloid particles is zero. Also called "isoelectronic point" and "point of zero charge".
See isoelectric point.
The electrode potential where the current is the same for different cycles in cyclic voltammetry, that is where the forward and reverse scans intercept each other. Abbreviated as �ipp�.
See adsorption isotherm.
Stands for indium tin oxide electrode.
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J
JECS
joule
junction potential
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Symbol and abbreviation of joule.
Stands for Journal of The Electrochemical Society.
The international unit of energy. Symbol: "J".
See liquid-junction potential
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The letter "k" when used as a prefix before a unit symbol indicates a multiplier of 103. Symbol of "kilo". E.g., kΩ = 103 ohm, one kiloohm, one thousand ohms. (The symbol is the letter "k" followed by the "Greek capital omega" letter, some browsers unfortunately do not support this.)
Symbol and abbreviation of Kelvin.
Symbol and abbreviation of kiloohm (= 103 ohm, one thousand ohms). (The symbol is the letter "k" followed by the "Greek capital omega" letter, some browsers unfortunately do not support this.)
Symbol and abbreviation of kiloampere.
See absolute temperature. Symbol: �K�.
Symbol and abbreviation of kilogram.
Symbol and abbreviation of kilojoule.
When used as a prefix before a unit name it indicates a multiplier of 103. E.g., kiloohm = 103 ohm, one thousand ohms. Symbol: "k".
103 amperes, symbol: "kA" (one thousand amperes).
The kilogram is the international unit of mass. One pound equals 0.45359237 kilogram, or one kilogram equals 2.204623 pounds. Symbol: "kg".
103 joules, symbol: "kJ" (one thousand joules).
103 ohmsj03, symbol: "kΩ" (one thousand ohms). (The symbol is the letter "k" followed by the "Greek capital omega" letter, some browsers unfortunately do not support this.)
103 volt, symbol: "kV" (one thousand volts)
103 watt, symbol: "kW" (one thousand watts)
103 watt-hours, symbol: "kWh" (one thousand watt-hours).
An electrode reaction is considered to be under "kinetic control" when the overall rate of the reaction is controlled by the rate of the reaction itself rather than the rate of the mass transport (mostly by diffusion) of the reactants to the electrode surface or the ir drop in the solution. This situation occurs when the the mass transpot/diffusion rate is much faster than the reaction rate and the concentration of the rectants at the surface of the electrode is practically the same as in the bulk solution. Contrast with diffusion control and ohmic control.
Chemical kinetics is a scientific discipline dedicated to the study of the rates of chemical reactions. How fast is a reaction proceeding in time, and what is affecting the rate.
The "current" law states that at any point in an electrical circuit, which does not represent a capacitor, the sum of currents flowing towards that point is equal to the sum of currents flowing away from that point. That is, there is no charge accumulation at any point in the circuit. The "voltage" law states that the directed sum of the electrical potential differences around any closed circuit must be zero. That is, there is no voltage accumulation in the circuit.
One thousand mols.
The law of the independent electrical migration of ions, that is, each type of migrating ion has a specific electrical conductivity no matter what its original molecular source, and therefore a solution's electrical conductivity is the sum of those of the migrating ions of the dissolved substances in the solution.
Symbol and abbreviation of kilovolt.
Symbol and abbreviation of kilowatt.
Symbol and abbreviation of kilowatt-hour.
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Symbol and abbreviation of liter.
A rechargeable battery. During discharging, the reaction on the positive electrode is the conversion of lead dioxide to lead sulfate, while on the negative electrode it is the conversion of metallic lead to lead sulfate. The reactions are reversed during charging. The current collector can be lead in both electrodes. The electrolyte is sulfuric acid. While it is one of the earliest practical storage batteries (1866), it is still very widely used today, e.g. as automobile starter battery.
See current leakage.
One of the earliest practical non-rechargeable batteries (Georges-Lionel Leclanche, 1866). It uses a zinc anode (negative electrode) and a manganese dioxide cathode (positive electrode) with ammonium chloride solution as electrolyte. The initially liquid electrolyte was later "immobilized", and this system became the first dry cell. It is still widely used. See also an Encyclopedia Article.
Small amounts of (usually organic) compounds added to an electroplating solution that changes the mechanism of the plating to produce a metal deposit smoother than the original substrate.
An equation that describes the effect of several variables (rotation rate, solution concentration, viscosity, etc) on the current at a rotating-disk electrode.
See complexing.
The maximum current density that can be achieved for an electrode reaction at a given concentration of the reactant in the presence of a large excess of supporting electrolyte.
The mass transport occurs exclusively through diffusion in the diffusion layer, driven by the concentration difference of the reactant between the edge of the diffusion layer and the electrode surface. As the current density is increased (usually by changing the electrode potential), the surface concentration of the reactant must decrease so that the concentration difference driving the diffusion can increase and provide the required flux of the reactant. However, the surface concentration obviously cannot decrease below zero, thereby a situation is reached when further change of the electrode potential cannot increase the reactant flux, and correspondingly the current density. The concept of "limiting current density" is valid even in the absence of supporting electrolyte. However, the situation is more complex in this case because electromigrational effects must also be taken into consideration.
The relation between current and potential or, more correctly, between current density and overpotential, is linear in a narrow overpotential range (about 5 to 10 mV, depending on the system). This occurs because the full relation can be mathematically simplified, to a very good approximation, to a linear relation in this overpotential range.
The polarization resistance calculated as the slope of the linear current-potential plot. It is often used as a quick measure of the rate of a corrosion process. The higher the polarization resistance, the lower the corrosion rate. Abbreviated as �LPR�.
See linear current-potential relation.
See voltammetry.
A potential difference between two solutions of different compositions separated by a membrane type separator. The simplest example is the case of two solutions containing the same salt in different concentrations. The salt will diffuse from the higher concentration side to the lower concentration side. However, the diffusion rate of the cation and the anion of the salt will very seldom be exactly the same (see mobility). Let us assume for this example that the cations move faster; consequently, an excess positive charge will accumulate on the low concentration side, while an excess negative charge will accumulate on the high concentration side of the junction due to the slow moving anions. This sets up a potential difference that will start an electromigration of the ions that will increase the net flux of the anions and decrease the net flux of the cations. In steady-sate conditions, the two ions will move at the same speed and a potential difference will be created between the two solutions. This "steady-sate" potential difference seems constant, but this is misleading because it slowly changes as the concentrations between the two solutions equalize. The diffusion process will "eventually" result in equal concentrations of the salt in the two solutions separated by the membrane, and the liquid-junction potential will vanish. For a simple case, the value of the liquid junction potential can be calculated by the so called "Henderson" equation.
The metric unit of volume. One liter equals 0.26 gallons or 1.0556 quarts. Symbol: "l".
A nonrechargeable battery that is being used in a many of today's devices. They are used for keyless entry remotes, blood glucose testers, etc. See also an Encyclopedia Article.
A rechargeable battery with a very high capacity for its size and weight compared to other rechargeable batteries. It is used in portable devices such as laptops, cell phones, and camcorders. Also called "rocking-chair battery".
A device that consumes electrical power, e.g. a motor or a light bulb.
An energy management system in which energy is produced even when there is no demand for it, and it is stored. This stored energy can later be released during high demand. This way the production capacity of the system can be less than the peak demand (load).
See corrosion.
The voltage at which some load controllers will disconnect from the rechargeable battery to avoid totally discharging the battery.
Stands for linear polarization resistance.
Stands for "linear-sweep voltammetry", see voltammetry.
A salt bridge with a thin, capillary tip at one end. This can be useful for minimizing the solution ir drop by placing the fine capillary tip very close to the surface of the working electrode, when the salt bridge is used to connect the working and reference electrode compartments of a three-electrode cell. The solution distance causing the ir drop can be easily limited to a few millimeters; and, in specially designed cells, often to a much smaller distance.
Stands for "linear-sweep voltammetry", see voltammetry.
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The letter "µ" when used as a prefix before a unit symbol indicates a multiplier of 10-6. Symbol and abbreviation of "micro". E.g., µV = 10-6 volt, one microvolt, one millionth of a volt.
Symbol and abbreviation of microampere (= 10-6 ampere, one millionth of an ampere).
Symbol and abbreviation of microcoulomb (= 10-6 coulomb, one millionth of a coulomb).
Symbol and abbreviation of microfarad (= 10-6 farad, one millionth of a farad).
Symbol and abbreviation of microliter (= 10-6 liter, one millionth of a liter).
Symbol and abbreviation of micrometer (= 10-6 meter, one millionth of a meter).
Symbol and abbreviation of microvolt (= 10-6 volt, one millionth of a volt).
The letter "m" when used as a prefix before a unit symbol indicates a multiplier of 10-3. Symbol of "milli". E.g., mV = 10-3 volt, one millivolt, one thousandth of a volt. The letter "m" is also the symbol of meter.
Stands for the square meter, a measure of area, equals 10.7639104 square feet.
Stands for the cubic meter, an international measure of volume, equals ~35.3 cubic feet.
The letter "M" when used as a prefix before a unit symbol indicates a multiplier of 106. Stands for "meg" or "mega". E.g., MΩ = 106 ohm, one megohm, one million ohms. (The symbol is the letter "M" followed by the "Greek capital omega" letter, some browsers unfortunately do not support this.) The letter "M" is also used to denote the molar concentration. The difference in meaning should be quite clear from the context of usage.
Symbol and abbreviation of milliohm (= 10-3 ohm, one thousandth of an ohm). (The symbol is the letter "M" followed by the "Greek capital omega" letter, some browsers unfortunately do not support this.)
Symbol and abbreviation of megohm (= 106 ohm, one million ohms). (The symbol is the letter "M" followed by the "Greek capital omega" letter, some browsers unfortunately do not support this.)
Symbol and abbreviation of milliampere (= 10-3 ampere, one thousandth of an ampere).
A region of space surrounding a magnetized body or electrical current-carrying circuit, or moving charged particle in which a force acts on any other magnet, electric current, or moving charged particle.
Electrochemical phenomena occurring under the influence of magnetic field. See also an Encyclopedia Article.
Symbol and abbreviation of milliampere-hour. (= 10-3 ampere-hour, one thousandth of an ampere-hour).
A rechargeable battery which does not require periodic "topping up" (addition of water) to maintain electrolyte volume. See also sealed battery.
A modern theory treating charge transfer reactions. Rudolf Marcus has won the 1992 Nobel Prize in Chemistry for this theory.
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The phenomenon of movement (transportation) of mass (in the form of molecules or ions) from one part of the system to another. This occurs through convection, diffusion, or electromigration. See electrochemical potential.
There is a complex relation between mass transport and charge transport in electrochemistry, because the reacting species and the charge carrying species are not necessarily identical. E.g., one would intuitively assume that during electroplating of silver from a solution of silver nitrate all the mass and charge required for the electrode reaction at the cathode would be carried by the silver cations in the solution. That is not the case at all. In the presence of a large excess of supporting electrolyte, in the bulk solution all the charge is carried by electromigration of the ions of the supporting electrolyte, and all the mass is carried by the silver ions by convection, while in the diffusion layer all the charge and the mass are carried by diffusion of the silver ions. In the absence of any supporting electrolyte, in the bulk solution all the mass is still carried by the silver ions, while the current (charge transport) is divided between the silver cations and the nitrate anions according to the ratio of their transport numbers, in the diffusion layer both the mass and the charge are carried partially by diffusion and partially by electromigration.
See diffusion control.
See concentration overpotential.
A characteristic quantity for the mass-transport step of an electrode reaction indicative of its inherent speed: a large mass-transport resistance indicates a slow step. See also non-ohmic resistance.
Stands for molten-carbonate fuel cell.
Stands for membrane-electrode assembly.
See working electrode.
Restoring the capacity (charging) of a rechargeable battery by replacing the spent electrode(s) with a fresh one.
See indirect electrolysis.
See indirect electrolysis.
When used as a prefix before a unit name it indicates a multiplier of 106. E.g., megohm = 106 ohm, one million ohms. Symbol: "M".
106 volt, symbol: "MV" (one million volts).
106 watt, symbol: "MW" (one million watts).
106 watt-hour, symbol: "MWh" (one million watt-hours).
106 ohm, symbol: "MΩ" (one million ohms). (The symbol is the letter "M" followed by the "Greek capital omega" letter, some browsers unfortunately do not support this.)
See separator.
An ion-selective electrode assembly terminating in an ion permeable (e.g., ion-exchange) membrane sensing element. The membrane separates the internal filling solution (that contains a fixed concentration of the ion to be detected) and the test solution. The potential across the membrane depends on the concentration ratio of the ion in the two solutions. The assembly also contains an internal reference electrode immersed in the filling solution, serving as an electrical contact with a stable potential. The potential of this assembly is then measured against an external reference electrode immersed in the test solution. See also Donnan potential.
A subassembly of a polymer-electrolyte-membrane fuel cell or a proton-exchange-membrane fuel cell consisting of the ion-exchange membrane and at least one of the electrodes. Abbreviated as "MEA".
See Donnan potential.
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A phenomenon in which a rechargeable battery discharged repeatedly to the same, but less than 100%, depth of discharge temporarily (or permanently) loses the rest of its capacity for consequent charging.
A commonly used reference electrode. It is very similar to the calomel electrode both in construction and in theory of operation. Except the salt is mercury sulfate and the solution is sodium sulfate. Abbreviated as "MSE".
A class of electrode reactions involving oxidation/reduction of a solid metal and its dissolved ion. E.g., if a copper metal rod is immersed in a copper sulfate solution, the copper cations can be cathodically reduced to copper metal, or the copper metal can be anodically oxidized to copper ions. Compare with a redox reaction where both the oxidized and the reduced species are in solution. The terms "electrodeposition" and "electrodissolution" are often used to describe these reactions. These reactions are used in many technologies, such as electroplating, electrowinning, electrorefining, and production of metal powders. And also in electrogravimetry.
See cementation.
See electroplating.
Metal powders can be produced by electrolyis with electrodeposition at a current density equal to or exceeding the limiting current density. See also an Encyclopedia Article.
See electrorefining.
See electrowinning.
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Metric and international unit of length, symbol: "m". One meter = 3.28 feet.
An important electrode reaction that occurs on the anode of the direct-methanol fuel cell. Abbreviated as MOR.
Metric and international unit of weight. One metric ton = 1000 kg = 2205 lbs.
Alternative name of siemens.
When used as a prefix before a unit name it indicates a multiplier of 10-6. E.g., microvolt = 10-6 volt, one millionth of a volt. Symbol: "µ".
10-6 ampere, symbol: "µA" (one millionth of an ampere).
10-6 coulomb, symbol: "µC" (one millionth of a coulomb).
A small electrode, with dimensions not larger than a few millimeters, and typically with dimensions of a small fraction of a millimeter.
10-6 farad, symbol: "µF" (one millionth of a farad).
A very small fuel cell that is often fabricated using microfabrication processes, including thin-film processes like those used to make silicon chips and thick-film processes like those used to make printed circuit boards. Often placed and used directly on circuit boards. See also an Encyclopedia Article.
10-6 liter, symbol: "µl" (one millionth of a liter).
10-6 meter, symbol: "µm" (one millionth of a meter).
10-6 mole (one millionth of a mole).
Alternative expression for micrometer.
10-6 volt, symbol: "µV" (one millionth of a volt).
Symbol and abbreviation of milligram.
See electromigration.
When used as a prefix before a unit name it indicates a multiplier of 10-3. E.g., millivolt (mV) = 10-3 volt, one thousandth of a volt. Symbol: "m".
10-3 ampere, symbol: "mA" (one thousandth of an ampere).
10-3 ampere-hour, one thousandth of an ampere-hour. Symbol �mAh�.
One thousandth of a gram. Symbol: "mg".
One thousandth of a liter. Symbol: "ml".
One thousandth of a meter. Symbol: "mm".
10-3 ohm, symbol: "mΩ" (one thousandth of an ohm).
10-3 volt, symbol: "mV" (one thousandth of a volt).
10-3 watt, symbol: "mW" (one thousandth of a watt).
A corrosion cell is said to be under mixed control if the overpotentials of the anodic and the cathodic corrosion reactions are similar in magnitude, consequently the corrosion current is determined jointly by both reactions. Contrast with anodic control and cathodic control. See also an Encyclopedia Article.
The electrode potential when two electrode reactions occur on the same electrode surface. The mixed potential has a value in between the equilibrium potentials of the two electrode reactions. The mixed potential is a steady-state phenomena, with the corrosion potential being a good example.
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Symbol and abbreviation of milliliter.
Symbol and abbreviation of millimeter.
See ionic mobility.
See chemically modified electrode.
The measurement unit and symbol of mole.
See gram-mole.
See concentration.
See concentration.
The weight of a molecule of a compound that may be calculated as the sum of the atomic weights of its constituent atoms.
The smallest physical unit of a substance that retains all the physical and chemical properties of that substance. It may consist of a single atom or of a group of atoms bonded together chemically.
A fuel cell that employs a molten, ionically conductive salt (carbonate) as electrolyte. Due to the high melting point of these salts, these fuel cells must operate at high temperatures. Abbreviated as "MCFC". See also an Encyclopedia Article.
See Baizer-Danly process.
Stands for methanol oxidation reaction.
Stands for mercury/mercury sulfate electrode.
Instrument that can be used for the measurement of more than one parameter. Typically, it can be used to measure current, potential, and resistance.
Symbol and abbreviation of millivolt (= 10-3 volt, one thousandth of a volt).
Symbol and abbreviation of megavolt (= 106 volt, one million volts).
Symbol and abbreviation of milliwatt (= 10-3 watt, one thousandth of a watt).
Symbol and abbreviation of megawatt (= 106 watt, one million watts).
Symbol and abbreviation of megawatt-hour (= 106 watt-hour, one million watt-hours).
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The letter "n" when used as a prefix before a unit symbol indicates a multiplier of 10-9. Symbol and abbreviation of "nano". E.g., nV = 10-9 volt, one nanovolt, one billionth of a volt.
Symbol and abbreviation of nanoampere (= 10-9 ampere, one billionth of an ampere).
When used as a prefix before a unit name it indicates a multiplier of 10-9. E.g., nanovolt = 10-9 volt, one billionth of a volt. Symbol: "n".
10-9 ampere, symbol: "nA" (one billionth of an ampere).
See ultramicroelectrode.
10-9 meter, symbol: "nm" (one billionth of a meter).
10-9 volt, symbol: "nV" (one billionth of a volt).
An equation defining the equilibrium potential of an electrode. The potential is the sum of the standard electrode potential and a correction term for the deviation from unit concentrations of the reactant and the product of the electrode reaction in the solution; if the "reduced" form is a metal, a pure metal (not alloyed with other metals) is considered to be at unit concentration.
The correction term is the product of the "Nernst slope" and the logarithm of the ratio of the concentrations (strictly speaking, activities) of the oxidized species and the reduced species. At room temperature, the Nernst slope is 0.05916 volt divided by the number of electrons transferred during the reaction. E.g., for a simple metal deposition/dissolution reaction the slope is 0.05916 for a single charged metal cation, 0.00296 volt for a double charged ion, etc. See also an Encyclopedia Article.
See Nernst equation.
It is equal to the change of equilibrium electrode potential when the concentration (strictly speaking, activity) of a species involved in the electrode reaction changes by ten fold.
An electrode is said to behave "nernstially" if the equilibrium electrode potential obeys the Nernst equation when the concentration (strictly speaking, activity) of a species involved in the electrode reaction changes. Opposite: non-nernstian behavior.
See diffusion layer.
See diffusion layer.
See reversible electrode reaction.
The actual, overall faradaic current (density) during an electrode reaction. The algebraic sum of the two partial currents (densities); one is considered positive the other negative.
(1) The reaction of an acid and a base to form a "neutral" (pH = 7) solution.
(2) The removal of electrical charge to produce a "neutral" (electrically uncharged) particle or object.
See atomic structure.
Stands for "normal hydrogen electrode", which is an alternative name for the standard hydrogen electrode.
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Stands for nickel-cadmium battery.
A rechargeable battery used in portable devices such as laptops, cell phones, cordless phones, and power tools. Abbreviated as "NiCad".
See Edison Battery. Abbreviated as "NiFe".
A rechargeable battery used in portable devices such as camcorders, cell phones, cordless phones, and laptops. Abbreviated as "NiMH".
Stands for nickel-iron battery.
Stands for nickel-metal-hydride battery.
Symbol and abbreviation of nanometer.
A metal that resists oxidation (corrosion) in air, and therefore retains its metallic luster. Examples are platinum and gold. These metals have high positive standard electrode potentials and are the lowest ones on the electromotive series. Contrast with active metal.
A solution with the solvent anything but water (e.g., organic or inorganic liquid, molten salt). Contrast with: aqueous solution.
See insulator.
See capacitive current (density).
An electrode is said to behave "non-nernstially" if the equilibrium electrode potential does not obey the Nernst equation when the concentration (strictly speaking, activity) of a species involved in the electrode reaction changes. Opposite: nernstian behavior.
A system or system element is behaving "non-ohmically" if it does not follow Ohm's law. That is, the value of the resistance depends on the current or the potential. Opposite: ohmic behavior.
The resistance can be formally defined as the differential of the potential with respect of the current. In the case of Ohm's law, this is the constant value of the resistance. In electrochemistry, a typical "non-ohmic" element is the charge-transfer resistance. The charge-transfer reaction can be considered a circuit element because it requires a certain amount of overpotential to force through a current. However, the pertinent relation here is the Tafel law (at least at relatively large overpotentials), and the differential of the current (that is the resistance) is a function of the current itself.
An electrode that is not easily polarizable. That is, the potential of the electrode will not change significantly from its equilibrium potential with the application of even a large current density. The reason for this behavior is that the electrode reaction is inherently fast (has a large exchange current density). See also overpotential. Opposite: polarizable electrode.
A battery in which the chemical reaction system providing the electrical current is not easily "chemically" reversible. It provides current until all the chemicals placed in it during manufacture are used up. It is discarded after a single discharge. Also called "primary" battery or cell. Contrast with rechargeable battery. Some common non-rechargeable batteries are the alkaline cell (battery), Daniell cell (battery), dry cell (battery), Leclanche cell (battery), lithium battery, silver-oxide battery, voltaic pile (Volta pile), zinc-air cell (battery), zinc-carbon cell (battery). See also an Encyclopedia Article.
This battery always operates as a galvanic cell. Consequently, the anode is the negative electrode, while the cathode is the positive electrode.
Alternative name for standard electrode potential.
Alternative name for standard hydrogen electrode. Abbreviated as "NHE".
See atomic structure.
Symbol and abbreviation of nanovolt (= 10-9 volt, one billionth of a volt).
See Baizer-Danly process.
A graphical representation of the results of an electrochemical impedance spectroscopic measurement.
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Symbol and abbreviation of ohm. (The symbol is the "Greek capital omega" letter, some browsers unfortunately do not support this.)
See ohm cm.
See ohm m.
Stands for open-circuit potential. See equilibrium potential.
Stands for open circuit voltage.
Stands for oxygen evolution reaction.
Measurement unit of the electrical resistance. A resistor is said to have one ohm resistance if one volt potential difference across the resistance will drive one ampere of current. Symbol: "Ω". (The symbol is the "Greek capital omega" letter, some browsers unfortunately do not support this.)
The metric unit of the resistivity.
An electrode reaction is considered to be under "ohmic control" when the overall rate of the reaction is is controlled by the ir drop in the solution rather than the rate of the reaction itself or the diffusion of the reactants to the electrode surface. Contrast with kinetic control and diffusion control.
See ir drop.
The total ir drop in an electrochemical cell, including the ir drop in the solution between the electrodes and in any separator.
See solution ir drop and ohmic loss.
A system or system element is behaving "ohmically" if it follows Ohm's law. That is, the value of the resistance is independent of the current and the potential. Typically, metals and electrolyte solutions are "ohmic". Opposite: non-ohmic behavior.
The International Unit (SI) of the resistivity.
Instrument used for the measurement of electrical resistance.
The relation amongst the current flowing through a resistor and the potential difference between the two ends of the resistor. The potential difference is equal to the product of the current and the resistance (volt = ampere times ohm).
Stands for "outer Helmholtz plane". See the Helmholtz model of the double layer.
See reserve battery.
See equilibrium potential. Abbreviated as "ocp".
The cell voltage under zero current conditions. For a galvanic cell see electromotive force (emf). Abbreviated as "ocv".
An electrode that is transparent to light, used to facilitate spectroelectrochemical investigations by illunination from the back-side of the electrode and thereby avoiding interference of the solution. For example, some electrically conducting metal oxides, as those of tin and indium. Abbreviated as "OTE".
Stands for oxidation/reduction potential electrode.
Stands for oxygen reduction reaction.
High frequency conductometry.
An instrument in which the variations in a fluctuating electrical quantity appear temporarily as a visible wave-form on a fluorescent screen.
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The pressure that develops in a solution separated from a solvent by a membrane permeable only to the solvent.
Stands for optically transparent electrode.
See the Gouy-Chapman-Stern model of the double layer.
See the Helmholtz model of the double layer. Abbreviated as "OHP".
A charge-transfer reaction with the reactants separated from each other by some solvent molecules due to the solvation of the reactants. Note that a "reactant" can also be an electrode. Contrast with inner-sphere charge-transfer reaction.
See cell reaction.
During the charging of a rechargeable battery, eventually enough electrical charge is supplied to convert all the active material stored in the electrodes. If charging continues, the battery is said to be "overcharged". It very much depends on the battery system whether overcharging is detrimental to the battery or not.
The difference in the electrode potential of an electrode between its equilibrium potential and its operating potential when a current is flowing. The overpotential represents the extra energy needed (an energy loss that appears as heat) to force the electrode reaction to proceed at a required rate (or its equivalent current density). Consequently, the operating potential of an anode is always more positive than its equilibrium potential, while the operating potential of a cathode is always more negative than its equilibrium potential. The overpotential increases with increasing current density, see Tafel equation. The value of the overpotential also depends on the "inherent speed" of the electrode reaction: a slow reaction (with small exchange current density) will require a larger overpotential for a given current density than a fast reaction (with large exchange current density). Also referred to as "polarization" of the electrode. See also overvoltage.
An electrode reaction always occurs in more than one elementary step, and there is an overpotential associated with each step. Even for the simplest case, the overpotential is the sum of the concentration overpotential and the activation overpotential.
The difference between the cell voltage (with a current flowing) and the open-circuit voltage (ocv). The overvoltage represents the extra energy needed (an energy loss that appears as heat) to force the cell reaction to proceed at a required rate. Consequently, the cell voltage of a galvanic cell (e.g., a rechargeable battery during discharging) is always less than its ocv, while the cell voltage of an electrolytic cell (e.g., a rechargeable battery during charging) is always more than its ocv. Occasionally also referred to as "polarization" of the cell.
The overvoltage is the sum of the overpotentials of the two electrodes of the cell and the ohmic loss of the cell. Unfortunately, the terms "overvoltage" and "overpotential" are sometimes used interchangeably.
Alternative expression for oxidizing agent.
In a narrow sense, oxidation means the reaction of a substance with oxygen. Hydrogen can react with oxygen to be oxidized to water. Hydrocarbon fuels (gasoline, natural gas, etc) can react with oxygen to be oxidized to carbon dioxide and water. Iron can react with oxygen to be oxidized to "rust". During oxidation, the oxygen itself is being reduced. Oxidation and reduction always occur simultaneously.
During these reactions, electrons are transferred from the substance that is oxidized to the oxygen. In a wider sense, all electron-transfer reactions are considered oxidation/reduction. The substance gaining electrons ("oxidizing agent" or "oxidant") is oxidizing the substance that is losing electrons ("reducing agent" or "reductant"). In the process, the "oxidizing agent" is itself reduced by the "reducing agent". Consequently, the reduction process is sometimes called "electronation", and the oxidation process is called "de-electronation".
A measure of the oxidation/reduction capability of a solution. It is a redox potential measured with an inert electrode. An oxidizing solution (e.g., one saturated with oxygen) has a more positive potential than a reducing solution (e.g., one saturated with hydrogen). Abbreviated as "ORP".
A measuring electrode used for the determination of the oxidation/reduction potential of a solution.
A substance that is affecting oxidation by accepting electrons from another substance. See oxidation/reduction. Also called "oxidant".
An electrode reaction in which oxygen gas is produced at the anode of an electrolytic cell by the oxidation of hydroxyl (OH-) ions or the oxidation of the water molecules of an aqueous solution. Abbreviated as "oer". See also water electrolysis. It is the reverse reaction of oxygen reduction.
See water electrolysis.
An electrode reaction in which oxygen gas is reduced at the cathode of an electrochemical cell. The product of the reduction can be hydroxyl (OH-) ions or water molecules (or occasionally hydrogen peroxide molecules). Abbreviated as "orr". It is the reverse reaction of oxygen evolution. It is a very important and much studied electrode reaction because it occurs at the cathode of practically all fuel cells and it occurs at the cathode of many (though not all) corrosion cells.
See Clark electrode.
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When used as a prefix before a unit name it indicates a multiplier of 10-12. Symbol of "pico". E.g., pA (picoampere) = 10-12 ampere, one one trillionth of an ampere.
Symbol and abbreviation of picoampere (= 10-12 ampere, one trillionth of an ampere).
An electrode assembly consisting of loosely packed small particles of the electrode material (e.g., some metal or carbon) with the electrolyte flowing through the bed. This type of electrode is especially useful for removing small traces of impurities from the solution by electrolysis (e.g., waste treatment) because the solution is well stirred and it contacts a large surface of the electrode material.
Stands for phosphoric-acid fuel cell.
See electrophoretic deposition.
Individual electrochemical cells can be combined in assemblies by parallel or series coupling (or a combination of the two). In case of "parallel" coupling, the positive electrode of every cell is connected together and the negative electrode of every cell is connected together, resulting in two external terminals. The voltage of every cell must be identical in parallel coupled assemblies. The overall current passing through the assembly is the sum of the individual cell currents, while the assembly voltage is identical to the individual cell voltage. Parallel coupling is often used in batteries. Cell lines and stacks can also be parallel coupled. Parallel coupling can be used for any elements of an electrical or electronic circuit: elements of the circuit are connected to common points at each end, so that the current is divided between them. Contrast with series coupling.
The two current densities at which the electrode reaction is proceeding in the anodic and cathodic directions at an electrode potential. The actual (net) current density is the algebraic sum of the two partial current densities (one is considered positive the other negative). Electrode reactions are typically "chemically" reversible, that is, they can proceed both in forward and reverse direction. At equilibrium, the reaction is proceeding at equal rate in both directions (see exchange current density): the "anodic partial current density" and the "cathodic partial current density" are equal and the net current density is zero. When the electrode is polarized, the partial current densities are unequal and the net current density is not zero. If the electrode is negatively polarized, the cathodic reaction speeds up (compared with its rate at equilibrium), while the anodic reaction slows down and a net cathodic current density results (and vice versa for anodic polarization).
See electrode reaction.
The formation of a thin adherent film or layer on the surface of a metal or mineral that acts as a protective coating to protect the underlying surface from further chemical reaction, such as corrosion, electrodissolution, or dissolution. The passive film is very often, though not always, an oxide. A passivated surface is often said to be in a "passive state". The surface oxidation can result from chemical or electrochemical (anodic) oxidation. During anodic passivation, using linear-sweep voltammetry, the current first increases with potential, then falls to a very small value. See passivation potential. See also an Encyclopedia Article.
The most negative electrode potential at which a passivating film is formed electrochemically. It is equal to or more positive than the equilibrium potential of formation of the compound (usually oxide) constituting the passive film. Usually the current goes through a maximum at the passivation potential during linear-sweep voltammetry. Also called the "Flade" potential. See also an Encyclopedia Article.
An electrode made by mixing a powder of an electronically conducting material with a binder into a paste.
In voltammetry and related techniques, the maximum value of the faradaic current during a single sweep.
See peak current.
In voltammetry and related techniques, the electrode potential at the peak current during a single sweep.
The maximum output that a battery or other power supply can produce without damage. Peak power capability is typically well beyond the continuous reliable power capability and should only be used infrequently.
The difference of peak potentials between the anodic (oxidation) and the cathodic (reduction) peaks of a substance in cylic voltammetry.
In some experimental techniques, like voltammetry, the width of the peak at half of the full height. Also called "half-peak-width" and "width at half-height".
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Stands for photoelectrochemistry.
Stands for either polymer-electrolyte membrane or proton-exchange membrane.
Stands for polymer-electrolyte-membrane fuel cell or proton-exchange-membrane fuel cell.
The permeation of certain ions in preference to other ions through an ion-exchange membrane.
A measure of the acidity/alkalinity (basicity) of a solution. The pH scale extends from 0 to 14 (in aqueous solutions at room temperature). A pH value of 7 indicates a neutral (neither acidic nor basic) solution. A pH value of less than 7 indicates an acidic solution, the acidity increases with decreasing pH value. A pH value of more than 7 indicates a basic solution, the basicity or alkalinity increases with increasing pH value.
The pH of a solution is equal to the negative, ten-based logarithm of the activity of the hydrogen ions in the solution. Neutral water dissociates into equal amounts of hydrogen (H+) cations and hydroxyl (OH-) anions. As the product of the concentrations (activities) of the two ions is always a constant 10-14, water has a pH of 7. In acidic solutions the hydrogen ions are in excess, while in basic solutions the hydroxyl ions are in excess.
See buffer solution.
An electrode assembly with a pH dependent potential. A variety of different electrodes can be used for this purpose, the most common one is the glass electrode.
Volt meter that measures the electrical potential difference between a pH electrode and a reference electrode and displays the result in terms of pH value of the sample solution in which they are immersed.
See buffer solution.
A fuel cell containing phosphoric acid as electrolyte. Abbreviated as "PAFC". See also an Encyclopedia Article.
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An anode in a photoelectrochemical cell.
See photoelectrosynthesis.
A cathode in a photoelectrochemical cell.
A galvanic cell in which usable current and voltage are simultaneously produced upon absorption of light by at least one of the electrodes. Also called "photogalvanic cell". See also an Encyclopedia Article.
Chemistry resulting from the interaction of light with electrochemical systems. See also photoelectrochemical cell and photoelectrolytic cell. Abbreviated as �PEC�. See also an Encyclopedia Article.
An electrolytic cell in which the production of chemicals is caused by or speeded up by the absorption of light by at least one of the electrodes. The process occurring in such cell is called "photoelectrosynthesis". See also an Encyclopedia Article.
Production of chemicals in a photoelectrolytic cell, where the production is caused by or speeded up by the absorption of light by at least one of the electrodes. See also an Encyclopedia Article.
See photoelectrochemical cell. See also an Encyclopedia Article.
See physisorption.
See adsorption. Contrast with chemisorption.
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Process for removal of oxide scales from metal surfaces in preparation for electroplating. Typically, the metal is immersed in hot, strongly acidic solution that dissolves the oxide scales. The solution usually also contains some corrosion inhibitor to avoid dissolution of the metal itself. See also electrolytic pickling.
When used as a prefix before a unit name it indicates a multiplier of 10-12. E.g., picoampere (pA) = 10-12 ampere, one one trillionth of an ampere. Symbol: "p".
10-12 ampere, symbol: "pA" (one trillionth of an ampere).
The phenomenon of generating electricity (voltage) in some special (piezoelectric) crystals subjected to mechanical stress or pressure; and conversely, the generation of stress or shape change in such crystals when subjected to an applied voltage. See also electrostriction.
An archaic name for a battery or other series-coupled electrochemical cells. See, e.g. the voltaic pile.
Electrode structures in rechargeable batteries are sometimes called "plates".
See electroplating.
A platinum metal electrode that is covered with a rough, large surface area platinum coating. The purpose is to produce an electrode with a large true area that will be relatively non polarizable. See also platinum black.
A rough, large surface area platinum coating usually deposited on a platinum metal electrode. See also platinized platinum electrode.
A somewhat ambiguous term, sometimes used to denote the potential of zero charge and sometimes the
isoelectric point.
The pH value of the solution where the electrokinetic potential of an oxide (or oxide covered) electrode or colloidal particles is zero. It is related to but not identical with the isoelectric point.
Indicates the sign of the potential of an electrode, that is, it can be negative or positive.
An electrode that is easily polarizable. That is, the potential of the electrode will change significantly from its equilibrium potential with the application of even a small current density. The reason for this behavior is that the electrode reaction is inherently slow (has a small exchange current density). See also overpotential and ideal polarized electrode. Opposite: non-polarizable electrode.
The change of potential of an electrode from its equilibrium potential upon the application of a current. See overpotential for a more detailed description. Somewhat confusingly, the term "polarization" is often also used in place of overvoltage. In bioelectrochemistry: the separation and grouping of opposite electrical charges (usually across a biological cell membrane) so that two clear groups are perceptible as two distinct poles. Similarly, the term in general means the separation and grouping of opposite electrical charges (across any material) so that two clear groups are perceptible as two distinct poles.
Alternative name for current-potential plot.
A measure of the inherent speed of an electrode reaction. A large polarization resistance indicates a slow reaction. See also charge transfer resistance.
The graphical representation of the result of polarography. See also an Encyclopedia Article.
An instrument used in carrying out polarographic analysis. See also an Encyclopedia Article.
Alternative name of a Clark electrode.
A classical electroanalytical technique discovered in 1922 by J. Heyrovsky, for which he was awarded the Nobel Prize for Chemistry in 1959. Essentially, it is linear-sweep voltammetry using a dropping-mercury electrode for working electrode and a large mercury pool as counter electrode. See also Ilkovic equation. See also an Encyclopedia Article.
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Alternative name of a terminal.
An ion-exchange membrane that is as the electrolyte in some fuel cells. Also called "proton-exchange membrane". Abbreviated as "PEM".
A fuel cell that uses a polymer-electrolyte membrane as the electrolyte. Also called "proton-exchange-membrane fuel cell". Abbreviated as "PEMFC". See also an Encyclopedia Article.
An electrode consisting of a highly porous solid. This is often used in fuel cells with gaseous reactants. In this case, the charge-transfer reaction proceeds mainly at the triple interface (three-phase boundary) formed by the electrode material, the electrolyte, and the gaseous reactant. The pores of the structure are partly filled by the electrolyte and partly by the gas. A porous electrode provides a much larger area for reaction than a solid electrode with the gas bubbled around it. A porous electrode can also be used as a flow-through electrode.
An alternative name of an industrial electrolytic cell used in aluminum production.
See electrical potential.
In some experimental techniques, like voltammetry, the potential at which the current is equal to one-half of the peak current.
The electrode potential where the charge in the electrical double layer is zero. Abbreviated as "pzc". Also called the "point of zero charge".
A diagram often used in the field of corrosion to indicate the corrosion tendency and stability of a metal in aqueous solutions. The equilibrium potential of the metal is plotted against the pH of the solution, usually for a series of concentrations of the metal ion. The curves demarcate potential-pH domains where a species of the metal is predominant in equilibrium, this can be the metal, its ion, oxide, or hydroxide. In simplified version, the diagram can indicate the potential-pH domains where the metal is immune to corrosion, corrodes, or passivates. The diagrams must be used with some caution because they represent equilibrium conditions and the corrosion tendency is also influenced by kinetic effects. Also called "Pourbaix diagram" and "E - pH diagram". See also an Encyclopedia Article.
Alternative name for voltammetry.
Alternative name for chronoamperometry.
Alternative name for voltammetry.
Alternative name for linear-sweep voltammetry. This expression is primarily used in the field of corrosion.
Alternative name for linear-sweep voltammetry. This expression is primarily used in the field of corrosion.
Can be used in more than one meaning:
1. A continuously variable resistor. More precisely, a resistor with continuously variable tap. This can provide three resistance values, a fixed resistance between the two end connectors, and two variable resistances, one between either end connector and the variable tap connector. The sum of the two variable resistances is the fixed resistance.
2. A somewhat archaic measurement system, based on a resistor with a continuously variable tap, that can be used to measure the electromotive force of electrochemical cells that can be easily polarized by current. It uses a comparison technique to compare a "standard" voltage source to the unknown, under conditions of practically zero current. It is seldom used today because high input resistance voltmeters and electrometers are readily available.
An electroanalytical technique based on the measurement of the electromotive force of an electrochemical cell comprised of a measuring and a reference electrode. The simplest example of a measuring electrode is a metal electrode whose potential depends on the concentration of the cation of the electrode metal (see Nernst equation). See also an Encyclopedia Article.
An electronic instrument that controls the electrical potential difference between the working and reference electrodes of a three-electrode cell at a preset value. It forces whatever current is necessary to flow between the working and counter electrodes to keep the desired potential, as long as the needed cell voltage and current do not exceed the compliance limits of the potentiostat. The working electrode is usually kept at "ground" potential, although, with some potentiostats, it can be "floating".
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 electrode potential.
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See potential-pH diagram.
See metal powder production by electrolysis.
See electrical power.
Characteristic parameter of a battery or fuel cell indicating its electrical power per unit weight or volume. The terminology is not strictly defined. Weight based power density is often called "specific power" or "gravimetric power density". Volume based power density is often called "power density" or "volumetric power density. The power density is typically expressed as watt/kilogram or watt/liter.
See electrical source (supply).
Abbreviation of parts per million.
A solution purification method often used in electrode kinetics measurements. These measurements are usually very sensitive to impurities in the solution. Electrolysis is an effective way to remove even traces of some impurities, it is usually carried out in a cell separate from the measuring cell. Some carbon compounds can be oxidized to carbon dioxide gas at the anode and metals can be deposited at the cathode.
See electrosynthesis.
See non-rechargeable battery.
A current distribution that is completely controlled by the resistivity of the electrolyte solution between the working and counter electrodes. The current always follows the least resistive path; consequently, a non-uniform current distribution will result if the geometry of the electrodes is such that the resistivity of the current path is not the same to every point on the working electrode. Every other effect that may influence the current distribution is ignored in this case, or assumed to be negligible. See also secondary and tertiary current distribution. See also an Encyclopedia Article.
See flat-plate battery.
See atomic structure.
Hydrogen ions are often called "protons". Hydrogen is the simplest atom, containing only a proton and an electron. Consequently, a hydrogen cation is indeed a proton. However, this is a simplification because ions are almost always solvated, see hydronium ion.
An ion-exchange membrane that is used as the electrolyte in some fuel cells. Also called "polymer-electrolyte membrane". Abbreviated as "PEM".
A fuel cell that uses a proton-exchange membrane as the electrolyte. Also called "polymer-electrolyte-membrane fuel cell". Abbreviated as "PEMFC". See also an Encyclopedia Article.
Capacitor-like behavior of some electrode systems. An example is an electrode reaction that is limited to a monolayer on the electrode surface by surface coverage effects. For these systems, the electrode potential varies almost linearly with surface coverage, that is with the charge passed during the reaction (similarly to a capacitor).
An electrode that maintains a steady, but not well-defined, electrode potential. It is used as a reference electrode for convenience or to avoid possible contamination of the electrolyte solution by a reference electrode. At the end of the experiment, it has to be calibrated against a conventional reference electrode. Also called "quasi-reference electrode". For example the dynamic hydrogen electrode.
A voltammetric technique in which a series of potential pulses is superimposed on the linear ramp.
Stands for potential of zero charge.
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quartz crystal microbalance
quasi-reference electrode
quasi-reversible electrode
quasi-reversible electrode reaction
quinhydrone electrode
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See electrochemical quartz crystal microbalance.
An electrode with a quasi-reversible electrode reaction.
See pseudo-reference electrode.
A qualitative term for an intermediate speed electrode reaction. An electrode reaction having an intermediate exchange current density, between that of an irreversible and a reversible electrode reaction.
Quinhydrone electrode is a redox electrode used for measuring pH. An inert metal (usually platinum) is immersed into the solution to be analyzed and a small amount of quinhydrone crystals is added to the solution. Quinhydrone is slightly soluble in water, dissolving to form a mixture of two substances, with each of the two substances easily oxidized or reduced to the other. The potential at the inert electrode depends on the ratio of the concentrations of two substances, which, in turn, depends on the pH.
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A molecule or atom possessing an unpaired electron.
The rate of a chemical reaction is proportional to the product of the concentrations of all the reactants taking part in the reaction, with the "rate constant" the proportionality factor. In other words, the rate constant is the rate of the reaction when all reactants are present in unit concentration.
The rate constant of a chemical reaction is a function of temperature and pressure. For an electrode reaction the rate constant is also a function of the electrode potential.
The total charge a battery is able to deliver on discharge under some specified conditions. Usually expressed in ampere-hours.
The slowest elementary reaction step in the series of steps comprising the overall reaction. The slowest step will control the rate of the overall reaction. This is analogous to a traffic situation, the time required to drive from the suburbs to downtown may completely depend on the time spent in one traffic jam on the road. Abbreviated as "rds".
The potential of an electrode expressed against the potential of zero charge (pzc) of the same electrode in the same solution. This provides a potential scale specific for each electrode, with the origin (zero potential) always at the respective pzc. Consequently, two electrodes having the same "rational potential" are typically not at the same "potential" (as measured against any reference electrode). However, the potential of these two electrodes is displaced by the same amount from their respective pzc; therefore, the charge density in the double layer of the two electrodes will be approximately equal. Consequently, the comparison of electrode behavior at the same "rational potential" is more meaningful for some purposes than a comparison at the same "potential on some arbitrary scale".
The charge densities of two electrodes at the same rational potential are only approximately equal because the double-layer capacitance and its potential dependence varies from electrode to electrode.
Stands for rotating-disk electrode.
Stands for rate-determining step.
Stands for reference electrode.
A chemical species that is taking part in a chemical reaction by reacting (sometimes by itself, but usually with other reactants) to form the products of the reaction.
The totality of all the elementary reaction steps occurring in series or parallel that fully defines the overall electrode reaction.
The overpotential (alternatively called polarization) associated with a chemical reaction (without charge transfer) step that is an elementary step in the overall electrode reaction.
See true electrode area.
A battery in which the chemical reaction system providing the electrical current is easily "chemically" reversible. After discharging, it can be recharged by applying an electrical current to its terminals. Some batteries can be recharged hundreds to thousands times. Also called "secondary" battery, and "accumulator". Contrast with non-rechargeable battery. Some common rechargeable batteries are the alkaline battery, Edison battery, lead-acid battery, lithium-ion battery, nickel-cadmium battery, nickel-iron battery, nickel-metal-hydride battery.
It operates as a galvanic cell during discharge and as an electrolytic cell during charge. As a consequence, the anode is the negative electrode during discharge, while it is the positive electrode during charge; at the same time, the cathode is the positive electrode during discharge, while it is the negative electrode during charge. This can create a confusing situation, and it is preferable to refer to the electrodes of a rechargeable battery as "positive" and "negative", because this designation is independent of the operational mode. Unfortunately, this nomenclature is not always followed. Often the "negative" electrode is designated as anode and the "positive" electrode is designated as cathode. This naming convention is a carry-over from the convention of the non-rechargeable battery.
See charging.
An electrical equipment that converts alternating current into direct current.
In half-wave rectification, either the positive or negative half of the alternating current wave is passed, while the other half is blocked. A full-wave rectifier converts the whole of the input waveform to one of constant polarity (positive or negative) at its output. The figure below shows the original signal, the half-wave signal, and the full-wave signal. The full-wave rectified signal can be further filtered to provide less ripple, and, at the extreme, a smooth signal.
A chemical species capable of taking part in a redox reaction.
A rechargeable battery with two redox electrodes contained in compartments typically separated by an ion-exchange membrane E.g., a battery with iron and chromium redox couples in the two compartments. During discharge, "ferric" (trivalent iron) cations are reduced to "ferrous" (divalent iron) ions at one of the redox electrodes, while "chromous" (divalent chromium) ions are oxidized to "chromic" (trivalent chromium) ions at the other electrode. The reverse reactions occur during charging. An advantage of this system is that the polarization losses are relatively small because the redox reactions are typically fast and no solid phases are being formed during the reaction. Also, the solutions containing the iron and chromium salts can be stored in separate large tanks and circulated to a small "battery", permitting the storage of large amounts of energy. See flow battery. See also an Encyclopedia Article.
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See redox reaction.
An inert electrode (e.g., platinum, gold, carbon) the potential of which is controlled by a redox reaction in solution.
This is somewhat of a misnomer since all electrodes involve oxidation/reduction. The distinction is that in the case of a redox electrode both oxidized and reduced species are dissolved in the solution surrounding the electrode.
See indirect electrolysis.
The potential of a redox electrode.
A class of electrode reactions involving oxidation/reduction of two dissolved species. E.g., iron metal can exist in solution as a doubly positively charged ("ferrous") ion or a triply positively charged ("ferric") ion. Such a system is often called a "redox couple", such as the "ferrous/ferric" couple. The ferric ions can be cathodically reduced to ferrous ions, or the ferrous ions can be anodically oxidized to ferric ions. With these reactions, an inert electrode is used that does not take part in any reactions under the conditions of the oxidation/reduction of the ions. This electrode then acts only as a source or sink of electrons; examples are: carbon, graphite, platinum, gold. Compare with a metal deposition/dissolution reaction where one of the reacting species is a solid metal and the other species is in solution.
A substance that is affecting reduction by donating electrons to another substance. See oxidation/reduction. Also called "reductant".
Alternative expression for reducing agent.
See oxidation/reduction
An electrode that has a well known and stable equilibrium electrode potential. It is used as a reference point against which the potential of other electrodes (typically that of the working electrode or measuring electrode) can be measured in an electrochemical cell. In principle it can be any electrode fulfilling the above requirements. In practice, there are a few commonly-used (and usually commercially-available) electrode assemblies that have an electrode potential independent of the electrolyte used in the cell. For some common reference electrodes see e.g., the silver/silver-chloride electrode, calomel electrode, and hydrogen electrode. Abbreviated as "RE".
Strictly speaking, there can be a small change in the potential of these electrodes depending on the electrolyte because the presence of a liquid-junction potential. This is very often (justifiably or not) ignored. The liquid-junction potential is also minimized by the use of high concentration potassium chloride as the filling solution of the reference electrodes, because the diffusion rate of both ions is very closely the same in these solutions.
A method of producing hydrogen from hydrocarbons. In the most common method, steam is reacted with hydrocarbons at high temperatures to yield carbon monoxide and hydrogen. It is commonly used to produce usable fuel for fuel cells.
A method that uses the electrical drive motor in an electric vehicle to act as a generator returning energy to the battery or electrochemical capacitor when the brakes are applied. This provides a powerful braking effect and at the same time captures energy which would otherwise be wasted or dissipated in the brakes.
A fuel cell that can be operated also as an electrolytic cell. This can be used essentially as a rechargeable battery. For example, it can be used to electrolyze water and store the electric power as the resultant hydrogen and oxygen gases. These can then be recombined during fuel cell opeartion to produce electric power.
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A non-rechargeable battery that is stored in an "inactive" form until its intended, immediate use. E.g., the battery is stored "dry", and it can be activated by injection of the electrolyte. Or, a battery operating with molten salt electrolyte is stored at a temperature below the electrolyte melting point, and it is activated by the sudden application of heat to melt the electrolyte. The advantages of such arrangements are an extremely long shelf life without practically any loss of stored energy, and the possibility to produce a very powerful battery with very reactive chemicals that would otherwise cause a very fast self discharge. Most reserve batteries are made for military applications.
A small faradaic current density flowing through an electrode under conditions when zero faradaic current is expected (e.g., within the double-layer range). It is caused by traces of impurities in the electrolyte. Also called "background current (density)".
The measure of an electrical circuit element's inability to carry electrical current. The measurement unit of the resistance is the ohm. See also impedance.
See solution ir drop.
The measure of a material's inability to carry electrical current. It is expressed as the resistance between opposite faces of a one-meter cube of material. The International Unit (SI) of the resistivity is the ohm m. Also called "specific resistance". (In the older literature, a one-centimeter cube is used, with the metric unit of ohm cm.)
The reciprocal of conductivity.
An electrical circuit element with a fixed resistance.
See equilibrium potential.
An electrode with a reversible electrode reaction.
A qualitative term for a fast electrode reaction. There are, unfortunately, several meanings attributed to the term "reversibility", resulting in possibly confusing situation. An electrode reaction is considered reversible in the "electrochemical sense" if the reaction is fast, that is, if the exchange current density of the electrode reaction is large. In contrast, in the "chemical sense", reversibility indicates that the reaction can proceed both in forward and backward (reverse) direction. Also called nernstian reaction. Opposite: irreversible electrode reaction. See also quasi-reversible electrode reaction.
Both of the above described meanings of reversibility are different from the meaning in the "thermodynamic sense".
Essentially a rechargeable battery built with fuel cell technology. For example, during charging, water is electrolyzed using porous electrodes, and the resulting oxygen and hydrogen gases are stored under pressure. During discharging, the same cell is used as a fuel cell and the gases are recombined to produce water and electricity. Similar arrangements could be made with hydrogen-chlorine and hydrogen-bromine systems.
A commonly used reference electrode. A hydrogen electrode immersed directly into the electrolyte of the electrochemical cell and usually (unless otherwise sated) operated with one atmosphere pressure hydrogen gas. The equilibrium potential depends on the hydrogen ion concentration (strictly speaking, activity) of the cell electrolyte. Abbreviated as "RHE".
Stands for reversible hydrogen electrode.
A variable resistor.
See lithium-ion battery.
A specialized hydrodynamic electrode used in the study of the kinetics and mechanism of electrode reactions and in electroanalysis for ensuring a known and controllable flow of solution over the electrode. The flow control is achieved by using a flat disc electrode (inlaid in an insulator) that is rotated in the solution resulting in a defined hydrodynamic boundary layer. The current can be calculated using the Levich equation. Abbreviated as "RDE".
A variant of the rotating-disk electrode which includes a second electrode - a concentric ring electrode - that is placed outside the disk and used to analyze the species generated on the disk. The ring is electrically insulated from the disk so that their potentials can be controlled independently. Abbreviated as "RRDE".
An electrode made of metal wire (often platinum) rotated about its axis at a known and constant velocity. It is used in the study of the kinetics and mechanism of electrode reactions and in electroanalysis.
The ratio between the true electrode area and the geometric electrode area.
Stands for rotating-ring-disk electrode.
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Symbol and abbreviation of siemens.
See cathodic corrosion protection.
See cathodic corrosion protection.
An ionically conducting path between separate compartments of an electrochemical cell. Often the working and reference (and occasionally even the counter) electrodes are in completely separate compartments and the required conducting path between them is provided by a tubing filled with highly conducting electrolyte solution. A common arrangement for a "salt bridge" is an inverted "U" shaped glass tubing with its ends dipped into the solutions of the two cell compartments; however, other materials and shapes are also used. The salt bridge may contain any conducting solution, but very often a highly concentrated potassium chloride solution (often immobilized by some gelling agent, for example agar) is used.
An equation relating current density, transition time, and concentration of the reactant in a chronopotentiometric experiment, assuming that the current is sufficiently large to immediately result in diffusion limiting conditions. The equation is valid only for planar electrodes in unstirred solution.
The product of the current density and the square root of the transition time divided by the concentration is a constant. The constant is proportional 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.
See calomel electrode.
See solubility.
In scanning electrochemical microscopy an ultramicroelectrode is scanned across a surface to collect both topographic and chemical information about the surface and near-surface region. It works by measuring a current through the electrode held or scanned in a solution in close proximity to a substrate surface. Substrates can be a variety of solid surfaces such as metals, glass, polymers or biological materials, etc. The substrate perturbs the electrochemical response of the tip, and this perturbation provides information about the nature and properties of the substrate. Abbreviated as "SECM". See also an Encyclopedia Article.
The rate of the linear change of the electrode potential in voltammetry and related techniques. Also called "sweep rate". Typically expressed as mV/second.
Stands for "saturated calomel electrode". See calomel electrode.
See maintenance-free battery. A battery which can be operated without regard to position. This construction is designed to prevent electrolyte loss through evaporation, spillage, and gassing and this in turn prolongs the life of the battery and eases maintenance. This design often uses vent caps on the cells to let gases escape in extreme conditions. Gassing needs to be reduced by impeding the release to the atmosphere of the oxygen and hydrogen generated during charging (see oxygen evolution reaction and hydrogen evolution reaction). This usually involves a catalyst that causes the hydrogen and oxygen to recombine into water and is called a recombinant system. Because spillage of the acid electrolyte is eliminated the batteries are also safer. See also SLA battery.
Stands for scanning electrochemical microscopy.
See electrode of the second kind.
See rechargeable battery.
A current distribution that is controlled by the resistivity of the solution (see primary current distribution) and the charge-transfer resistance of the electrode reaction occurring on the working electrode. That is, a current distribution taking into effect the activation overpotential. A large charge-transfer resistance (that is, a slow reaction), compared to the solution resistance, tends to make the current distribution more uniform. This still ignores the effect of the concentration overpotential, see tertiary current distribution. See also an Encyclopedia Article.
An electrical potential difference that arises when small suspended particles move through a liquid (e.g., forced by gravity). Also called "Dorn potential". See electrokinetic effects.
Stands for solid electrolyte interface (layer).
A slow discharging of a battery without being connected to an external load. This is caused partly by impurities and side reactions (reactions other than the cell reaction) and partly by the imperfect separation of the active chemicals in the battery causing a slow "direct" reaction between them. The rate of the self discharge determines the shelf life of a non-rechargeable battery.
Material whose conductivity is in the range between that of metals and insulators and increases with temperature. See also conductor and superconductor.
In an �n-type� semiconductor, the movement of electrons in the conduction band carries the current. The energy level of the electrons is higher in the �conduction band� than in the �valence band�, the two are being separated by a �band gap�, an energy range where electrons are not allowed. The valence band is typically filled with electrons, but if sufficient energy is supplied (for example, by light absorption) electrons can be moved to the conduction band. This leaves back �holes� in the valence band, which �move� in the opposite directions to the movement of electrons as they hop into the holes. Therefore, the �hole current� is considered a positive current. Such semiconductors are called �p-type�.
A unique property of semiconductors is that they can be �doped� to control the number of mobile charge carriers in each band. Doping is the addition of small amounts of an impurity which results in free charge carriers that can move in the crystal. These can be either electrons in the conduction band (n-type) or holes in the valence band (p-type).
A separator through which certain molecules can pass but others cannot.
See working electrode.
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A thin structural material (usually a sheet) used to separate the electrolyte of a divided electrochemical cell into two or more compartments. A separator is typically either a membrane or a diaphragm. The distinction between these two separators is somewhat blurred. A membrane typically has very small pores that permit only diffusional or conductive motion of the solvent or the electrolyte from one compartment to another. A diaphragm has larger pores so that it permits the flow (see convection) of the electrolyte solution from one compartment to another but still restricts the complete intermixing the two solutions.
Individual electrochemical cells can be combined in assemblies by series or parallel coupling (or a combination of the two). In case of "series" coupling, the positive electrode of one cell is connected to the negative electrode of the next cell, and so on. The assembly has only two external terminals. The overall voltage of the assembly is the sum of the individual cell voltages, while the current passing through every cell (and the assembly) is the same. Series coupling can be used in a number of assemblies, such as battery, cell line, and stack. Series coupling can be used for any elements of an electrical or electronic circuit: elements of the circuit are arranged in such a way that the current passes through each of them successively, without branching. Contrast with parallel coupling.
Discharge of a rechargeable battery using only a small portion of its total rated capacity. Contrast with deep discharge.
The change in shape of an electrode of a rechargeable battery due to movement of the active (reacting) material during charge/discharge cycling.
Stands for standard hydrogen electrode.
The time period a non-rechargeable battery can be stored after manufacturing so that it still can provide a required amount of electricity when connected to a load. The shelf life of modern batteries is many years.
The initial current resulting from discharging a battery through a load of negligible resistance.
See current leakage.
The measurement unit of electrical conductance. Symbol: "S".
The reciprocal (inverse) of the ohm (1/ohm), and sometimes called "mho".
A small, nonrechargeable battery used in devices such as watches and calculators. See also an Encyclopedia Article.
A commonly used reference electrode. The electrode assembly consists of a silver metal electrode in contact with solid silver chloride (usually as a coating on the silver metal) immersed in an aqueous chloride salt solution saturated with silver chloride. All these are contained in a small vessel, typically made of glass tubing. The internal electrolyte of the reference electrode assembly and the external electrolyte into which the whole assembly is immersed are in ionic contact through a separator. A typical separator is a small porous ceramic plug sealed into the end of the glass tubing. Abbreviated as "SSCE".
The operating principle of this electrode is that of an electrode of the second kind. The equilibrium electrode potential is a function of the chloride concentration of the internal electrolyte ("filling solution"). The most commonly used electrolyte is 4 molar potassium chloride, producing a potential of 0.222 volt against the standard hydrogen electrode at 25oC (77oF). Occasionally, other concentrations of potassium chloride or other chloride salts are used.
Stands for "sealed lead acid battery". See sealed battery.
Stands for rechargeable battery used in automobiles for "starting, lighting, and ignition".
Stands for static-mercury-drop electrode.
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Stands for state of charge.
See brine electrolysis.
See brine electrolysis.
See brine electrolysis.
See brine electrolysis.
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Stands for solid-oxide fuel cell. See also an Encyclopedia Article.
See electroremediation.
An ionically conducting solid used in an electrochemical cell in place of the more often used liquid electrolyte.
A passivating type layer formed on lithium in contact both with liquid and polymer electrolytes when used as the negative electrode in a battery.
It is generally considered that this layer makes the practical lithium batteries possible because lithium itself is too reactive to be stable in contact with the electrolytes. This layer acts as an interface between the metal and the solution and has the properties of a solid electrolyte. Abbreviated as "SEI".
A fuel cell that employs a solid, ionically conductive material as electrolyte. Due to the typically low ionic conductivity of solid oxides, these fuel cells must operate at very high temperatures. Abbreviated as "SOFC". See also an Encyclopedia Article.
The maximum amount of a species that can be dissolved in a given solvent. It is usually expressed as the maximum achievable concentration. A solution is called "saturated" if it contains the maximum dissolvable amount.
The solubility of slightly soluble salts is often expressed as the product of the solubility concentrations of its ions. E.g., the solubility product of silver chloride is the product of the concentrations of the silver and chloride ions in the saturated solution of this salt. The significance of the solubility product is that its value cannot be exceeded even in the presence of other dissolved salts. Consequently, the solubility of silver chloride is less in a solution containing potassium chloride than in pure water. This is because in the calculation of the solubility product one must use the "total" chloride concentration in the solution, therefore a silver concentration lower than in water is needed to satisfy a constant solubility product.
The solubility (the saturated solution concentration) of the salt, in the absence of any other dissolved species in the solution, is the square root of the solubility product for a salt like the silver chloride. Strictly speaking, activities should be used instead of concentrations.
The dissolved species (e.g., a salt) in a solution.
The ir drop in the electrolyte solution of a three-electrode cell between the working and the reference electrodes. This ir drop (which is expressed as a potential) is always included in the measured potential of the working electrode. Therefore, it is important to minimize this error, and to place the reference electrode as close as possible to the working electrode (see Luggin tip). It is also called "ohmic overpotential (or polarization)" or "resistance overpotential (or polarization)". One can correct for the ir drop to obtain the real electrode potential, or in some cases one can compensate for the ir drop during potential control. During the measurement of an electromotive force (potential measurement without any current flowing), the ir drop is always zero, and the position of the reference electrode is immaterial. See also ohmic loss.
Ions in solution are always surrounded by solvent molecules. A few of these molecules will be more or less strongly attached to the ion (mainly because of the attraction of the charged ion and the dipole of the solvent molecule) and this assembly may be considered as a single unit for some purposes. E.g., the solvent molecules will move together with the ion during diffusion and electromigration. The number of solvent molecules so attached to an ion is called the "solvation number". The surface of an electrode also can, and usually is, solvated. Since the electrodes usually have some excess charge (see electrical double layer,) they also attract the solvent dipoles, and the electrode surface is usually covered by a monolayer of strongly oriented solvent molecules. Under certain extreme conditions, a solution can contain free electrons that are stabilized by solvation.
The solvation number is not very exactly defined since its value may depend on the measurement technique.
See solvation.
Electrochemical phenomena occurring under the influence of soundwaves (typically ultrasound).
See electrical source (supply).
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See conductivity.
See energy density.
See ion-selective electrode.
See power density.
See resistivity.
The simultaneous application of electrochemical and optical spectroscopic techniques to investigate a phenomenon.
Stands for silver/silver-chloride electrode.
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A series-coupled assembly of cells, a term used primarily for fuel cells.
See hydrodynamic boundary layer.
A non-rechargeable cell (battery) whose emf is accurately known and remains sufficiently constant. It is not used any more because the availability of electronic voltage standards. See Weston cell.
The equilibrium potential of an electrode when both the oxidized and the reduced species are present in unit concentration (strictly speaking, activity) in the solution; if the "reduced" form is a metal, a pure metal (not alloyed with other metals) is considered to be at unit concentration. (See also the Nernst equation.) The standard potentials are always expressed against the standard hydrogen electrode the potential of which is zero "by definition". Standard potentials are a function of the temperature, they are usually tabulated for 25oC (77oF). Also called "normal electrode potential".
The standard potential is the electromotive force of an electrochemical cell comprised of the electrode in question and the standard hydrogen electrode. Strictly speaking, one must use unit activities rather than concentrations.
The most fundamental reference electrode in electrochemistry. "By definition" its equilibrium potential is considered zero at any temperature, because this electrode was chosen as an arbitrary zero point for electrode potentials. A zero point is needed since the potential of a single electrode cannot be measured, only the difference of two electrode potentials is measurable. All electrode potentials are expressed on this "hydrogen scale". It is a hydrogen electrode with an electrolyte containing unit concentration of hydrogen ions and saturated with hydrogen gas at unit atmosphere pressure. This electrode can be somewhat inconvenient to use because of the need to supply hydrogen gas. Therefore, other reference electrodes (e.g., calomel or silver/silver chloride) are often used instead, but the measured electrode potentials can be converted to the "hydrogen scale". Abbreviated as "SHE". Also called "normal hydrogen electrode".
Strictly speaking, one must use unit activity rather than concentration of hydrogen ions and unit fugacity rather than unit pressure of hydrogen gas.
The rate constant of an electrode reaction at the standard electrode potential.
A battery, usually rechargeable, designed for emergency use in case of power failure.
A battery built with little or no free liquid eletcrolyte. Contrast with flooded battery.
For a rechargeable battery (or a capacitor, especially for an electrochemical capacitor) the fraction, usually expressed as a percentage, of the total electrical energy stored in a battery by charging that is still available for discharging at a certain point of time. Abbreviated as "SOC". Contrast with depth of discharge.
Alternative name for hanging-mercury-drop electrode. Abbreviated as "smde".
A rechargeable battery designed to be used in a fixed location.
See steady state.
A state of a system in which the conditions do not change in time, or at least they do not seem to change with time. That is, the change occurs on a time scale longer than the time scale of the observations. A good example of the creation and slow vanishing of a steady-state condition is the liquid-junction potential.
See the Gouy-Chapman-Stern model of the double layer.
Stoichiometry refers to the relationship between the amounts (in moles) of reactants and products in a particular chemical reaction. Stoichiometric indicates chemical combination in simple integral ratios.
See energy storage.
See battery.
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An electrical potential difference that arises when liquid is flowing by a solid surface, e.g., when liquid is forced through a capillary tubing or porous solid by a pressure differential. See electrokinetic effects.
A group of electroanalytical techniques for the determination of trace amounts of substances, consisting of two steps: preconcentration and analysis. The preconcentration involves the electrodeposition or adsorption of the substance to be determined on the surface of an electrode. This is followed by the "stripping" analysis of the substance by an electroanalytical technique. For example, traces of metal ions can be preconcentrated by cathodic electrodeposition followed by anodic dissolution (stripping). Or traces of halides (e.g., chloride) can be anodically preconcentrated at a mercury electrode as mercury salts, followed by cathodic stripping.
A strong electrolyte is a solute that completely, or almost completely, dissociates into ions in a solution. Resulting in a high conductivity of the solution. Contrast with weak electrolyte.
See electrochemical capacitor.
A material (typically metal, alloy, or ceramics) that conducts electric current with negligible internal resistance at very low temperatures and, in some exceptional cases, at higher temperatures. See also conductor and semiconductor.
Alternative expression for source.
An electrolyte added to the solution for the sole purpose to increase the solution conductivity, while the electrolyte does not take part in any reactions. Also called "inert", "indifferent", or "swamping" electrolyte.
See roughness factor.
The work required to increase a surface area, for example, to increase the size of a drop of water. When two phases are involved, it is often called an "interfacial tension", for example, to increase the size of a mercury droplet under water.
Alternative expression for supporting electrolyte.
See scan rate.
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An early (1905) empirical relation between the overpotential of the electrode and the current density passing through the electrode:
η = a + b×log i
where "η" is the "polarization", "a" and "b" are characteristic constants of the electrode system, and "i" is the current density. A plot of electrode potential versus the logarithm of current density is called the "Tafel plot" and the resulting straight line the "Tafel line". "b" is the "Tafel slope" that provides information about the mechanism of the reaction, and "a" provides information about the rate constant (and the exchange current density) of the reaction. The Tafel equation is a limiting case of the Butler-Volmer equation for high overpotential (larger than 50 to 100 mV, depending on the system). See also an Encyclopedia Article.
Rigorously, the equation should be written as:
η = a ± b×log |i|
using the absolute value of the current density and the ± sign for anodic and cathodic overpotentials, respectively. Also, the equation holds only for relatively high overpotentials, approximately 0.1 volt or higher.
See Tafel equation.
See Tafel equation.
See Tafel equation.
See electrical tension.
A charging regime delivering moderately high rate of current when the battery is at a low state of charge and tapering the charging current to lower rates as the battery is charged.
The external electrical connection posts of an electrochemical cell to which a power source or a load can be connected. For example, in case of a battery, to which either a load (e.g., motor, light bulb) can be connected to use the electrical energy of the battery, or to which a charger can be connected to charge the battery. Every battery has only two terminals (positive and negative) independent whether the battery contains one or more cells internally. This term is also used for cell stacks and for electrolytic cells. A terminal can also be called a "pole".
A current distribution that is controlled by the resistivity of the solution (see primary current distribution), the activation overpotential (see secondary current distribution), and the concentration overpotential. The concentration changes occurring at the working electrode surface affect the rate of the electrode reaction and can therefore be considered as an additional surface resistance. See also an Encyclopedia Article.
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A galvanic cell consisting of two identical half cells that are kept at different temperatures.
An electrochemical cell with a very thin layer of electrolyte between the electrodes. All forms of mass transport except diffusion can be neglected in its operation, as long as the electrolyte layer is thinner than the diffusion layer.
See electrode of the third kind.
An electrochemical cell containing a working electrode, a counter electrode, and a reference electrode. A current may flow between the working and counter electrodes, while the potential of the working electrode is measured against the reference electrode. This setup can be used in basic research to investigate the kinetics and mechanism of the electrode reaction occurring on the working electrode surface, or in electroanalytical applications.
See porous electrode.
A qualitative term used in electroplating to describe the ability of the system to produce a uniformly thick deposit on the substrate surface. That is, the "throwing power" is considered "good" when the current distribution is uniform even on an irregularly shaped substrate.
The throwing power is a function of both the geometrical arrangement in the electroplating cell and the composition of the plating solution.
A low rate charging of a rechargeable battery following the main charging, designed to ensure maximum capacity.
Alternative expression for ideal polarized electrode.
An electrode potential more positive than the passivation potential of the system.
See transport number.
An electrochemical technique whose response changes in time and it is measured as such. For example chronoamperometry, chronocoulometry, or chronopotentiometry.
Characteristic time in a chronopotentiometric experiment indicating the exhaustion of a reactant concentration at the electrode surface. The potential of the electrode changes sharply upon reaching the transition time.
The fraction of the total current carried in a solution by a given ion. Ions may carry drastically different portions of the total current if their mobilities are different. E.g., in a solution of sodium chloride, less than half of the current is carried by the positively charged sodium cations and more than half is carried by the negatively charged chloride anions because the chloride ions are able to move faster (they have a larger ionic mobility). The transport numbers of the anion and the cation adds up to unity. As a matter of fact, the case when the ions move equally and the transport number of both ions is equal to 0.5 is a rarity. The Hittorf method is an experimental technique for the determination of the transport numbers. Also called "transference number".
For the simplest case of a solution of a single salt of univalent ions, the transport numbers are defined as the mobility of the ion divided by the sum of mobilities of the two ions. If there are more than one solutes present (e.g., an acidified sodium chloride solution or a mixture of sodium chloride and potassium bromide), every ion will have its own transport number with the sum of them being unity. In these cases, the concentrations of the ions must also be taken into account in the calculation of the transport numbers, and in the case of polyvalent ions, the charges of the ions must also be accounted for.
A method of maintaining a rechargeable battery in a fully charged condition by continuous, long-term, slow-rate charging, at a level sufficient to balance self-discharge and occasional discharge. Also called "float charging".
See porous electrode.
See current density.
The surface area of an electrode taking into consideration the surface roughness. For a perfectly smooth electrode, it is equal to the geometric electrode area, but it is larger than that for most electrodes. The ratio of the two defines the roughness factor. Also called "real electrode area".
A classical electrochemical cell containing two electrodes.
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See electrochemical capacitor.
A very small electrode, with dimensions not larger than few tens of a micrometer, and occasionally with dimensions of a fraction of a micrometer. Also called "nanoelectrode". Abbreviated as "UME". See also an Encyclopedia Article.
Stands for ultramicroelectrode.
The part of the solution ir drop that is not automatically compensated for by the electronic control instrumentation.
See uncompensated ir drop.
The electrodeposition of a metal on a foreign metal at potentials less negative than the equilibrium potential of the deposition reaction. Such a process is energetically unfavorable and it can occur only because of a strong interaction between the two metals, with their interaction energy changing the overall energetics to favorable. Consequently, only one (very seldom two) monolayer can be deposited this way, and this is a very convenient way to produce well-controlled monolayer deposits. Abbreviated as "UPD". See also electrochemical atomic layer deposition and an Encyclopedia Article.
An electrochemical cell where all electrodes are in the same compartment, immersed into the same electrolyte. Contrast with divided cell.
A conventional electrical power source backed up by a battery or a fuel cell to assure continuous power even if the primary source fails. Abbreviated as "UPS".
See non-polarizable electrode.
Stands for underpotential deposition. See also an Encyclopedia Article.
Stands for uninterruptable power supply.
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Symbol and abbreviation of volt.
A metal which, in an electrolytic cell, can function generally as a cathode, but not generally as an anode because an oxide of the metal forms under anodic conditions. This oxide is highly resistant to the passage of current. Valve metals include aluminum, titanium, tantalum, and niobium.
A sealed battery that depends on pressure valves opening only under extreme conditions, instead of simple vent caps on the cells, to let gas escape. Abbreviated as "VRLA".
An electrode that is mechanically vibrated to improve the mass transport to its surface.
The resistance to flow exhibited by a liquid or gas subjected to deformation.
See glassy carbon electrode.
Measurement unit of the electrical potential. Symbol: "V". A term named in honor of Alessandro Volta, see also an Encyclopedia Article.
A term sometimes used interchangeably with electrical potential. See also cell voltage.
See compliance limits.
See electrical source (supply).
See galvanic cell.
The first (1800) laboratory source of electricity, essentially what we would call today a non-rechargeable battery. Volta assembled (piled up) different metal disks (e.g., silver and zinc) separated by solution soaked pasteboards, repeating the pattern many tens of times. This was essentially a battery containing many series-coupled cells providing rather high cell voltage and current capabilities, but very short life time. See also an Encyclopedia Article.
An electrochemical measuring technique used for electrochemical analysis, for the determination of the kinetics and mechanism of electrode reactions, and for corrosion studies. "Voltammetry" is a family of techniques with the common characteristics that the potential of the working electrode is controlled (typically with a potentiostat) and the current flowing through the electrode is measured. In one of the most common applications of the technique, the potential is scanned linearly in time; this is called the "linear-sweep voltammetry", "LSV", or "LV". "Cyclic voltammetry (CV)" is a linear-sweep voltammetry with the scan continued in the reverse direction at the end of the first scan, this cycle can be repeated a number of times. In ac voltammetry, an alternating voltage is superimposed on the dc ramp.
Graphical representation of the results of a voltammetric measurement.
Instrument used for the measurement of electrical potential differences.
Stands for valve regulated lead acid battery.
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Symbol and abbreviation of watt.
An empirical rule which states that the product of the viscosity and the ionic conductance at infinite dilution in electrolyte solutions is a constant, independent of the solvent; it is only approximately correct.
The element in the equivalent circuit of an electrochemical cell or electrode that represents a diffusional processes.
Process for the electrochemical decomposition of water in a divided electrolytic cell by electrolysis. Typically, hydrogen gas is produced at the steel cathode (see hydrogen evolution) and oxygen gas is produced at the nickel anode (see oxygen evolution). The cell is divided to avoid mixing the hydrogen and oxygen gases. The electrolyte is typically an aqueous potassium hydroxide solution. Potassium hydroxide is used to provide a large ionic conductivity, even though the potassium is not reacting at any of the electrodes (see supporting electrolyte).
Measurement unit of electrical power. Symbol: "W". Related units are that of power density: watt/kilogram (W/kg) and watt/liter (W/l).
Measurement unit of electrical energy. Symbol: "Wh". One watt-hour is equal to 3600 joule. Related units are that of energy density: watt-hour/kilogram (Wh/kg) and watt-hour/liter (Wh/l).
Stands for working electrode.
A weak electrolyte is a solute that dissociates only partially into ions in a solution. Resulting in a poorly conducting solution. Contrast with strong electrolyte.
The standard cell used during much of the 20th century.
Symbol and abbreviation of watt-hour.
See an Encyclopedia Article.
An early electrical instrument (circuit) for measuring an unknown resistance by comparing it with known resistances.
See peak width at half-height.
The increase of the mobility of an ion in high 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 electrical field, it will move so fast that it will, in effect, leave behind its atmosphere which does not have time to reform, and the mobility of the ion (consequently the electrical conductivity of the solution) will increase. See also the Debye-Falkenhagen effect.
The electrode in a three-electrode cell where "the action is". The kinetics and mechanism of the electrode reaction may be under investigation, or the reaction occurring on the working electrode may be used to perform an electrochemical analysis of the electrolyte solution. It can serve either as an anode or a cathode, depending on the applied polarity. One of the electrodes in some "classical two-electrode" cells can also be considered a "working" ("measuring", "indicator", or "sensing") electrode, e.g., in a potentiometric electroanalytical setup where the potential of the measuring electrode (against a reference electrode) is a measure of the concentration of a species in the solution. Abbreviated as "WE".
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zeta potential
zinc-air cell
zinc-carbon cell (battery)
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Alternative name for "electrokinetic potential". See electrokinetic effects.
A small, nonrechargeable battery used in devices such as hearing aids. It uses the oxygen from the air as one of the reactants. See also an Encyclopedia Article.
See Leclanche cell.
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