Electrochemistry Dictionary - C

C: Symbol and abbreviation of coulomb.
Calomel electrode: 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 25 oC.
Can, case: The external envelope of a cell or battery, or the box containing the cells and connectors.
Capacitance: The capacitance value expresses the ability of a capacitor to store electrical charge. The unit of capacitance is the farad.
Capacitive current (density): 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.
Capacitor: 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.
Capacity: 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.
Capacity, rated: The value of the output capability of a battery, expressed in Ah, at a given discharge rate before the voltage falls below a given cut-off value, as indicated by the manufacturer.
Capillary: A tube with very small bore.
Catalysis: 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).
Catalyst: A material that can cause catalysis.
Cataphoresis: Alternative name for "electrophoresis." See electrokinetic effects.

Cathode: 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.
Cathodic partial current (density): See partial current density.
Cathodic (corrosion) protection: A process for corrosion protection of a metal or alloy achieved by impressing upon the metal a cathodic potential of sufficient magnitude to decrease the corrosion rate. See also an Encyclopedia Article and anodic protection.
Catholyte: The electrolyte on the cathode side of an electrochemical cell that is divided into compartments.

Caustic: See base. Sometimes it specifically refers to sodium hydroxide.
Caustic production: See brine electrolysis.

CCV: Stands for closed-circuit voltage.

Cell: Electrochemical device which directly converts chemical and electrical energy.
Cell constant: See conductivity cell.
Cell divider: See separator

Cell efficiency: The ratio of the electrical energy produced by a photovoltaic cell (under full sun conditions or 1 kW/m2) to the energy from sunlight falling upon the cell.
Cell line: A series-coupled assembly of cells, a term used primarily in industrial electrolysis using electrolytic cells.
Cell reaction: The overall chemical reaction occurring in the electrochemical cell. It is the sum of the two electrode reactions.

Cell reversal: Inversion of the polarity of the terminals of a cell in a multicell battery. Cell reversal is usually due to over discharge, when differences in the capacity of individual cells result in one or more cells reaching complete discharge before the others.

Cell voltage: 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.

Cementation: 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."

Chapman model of the double layer: See the Gouy-Chapman model of the double layer.

Charge acceptance: The ability of a secondary cell or battery to convert the active material to a dischargeable form. It is measured by the capacity which can be subsequently delivered to a load as a result of the charging process. If the charge acceptance is 100% then all of the electrical energy input would become available for useful output.
Charge carrier: 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 controller: A component that controls the flow of current to and from the battery subsystem to protect the batteries from overcharge and over discharge. The charge controller may also monitor system performance and provide system protection.
Charge density: Charge referred to the unit area of the electrode. Charge divided by electrode area.
Charge efficiency: See coulometric efficiency.
Charger: An electrical device used to charge a rechargeable battery using household electricity.

Charge rate: 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.
Charge retention: The ability of a charged cell to resist self-discharge.
Charge, state of: The condition of a cell or a battery in terms of the remaining available capacity.

Charge-transfer overpotential (polarization): See activation overpotential.
Charge-transfer reaction: 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 always a heterogeneous charge-transfer reaction.
Charge-transfer resistance: A characteristic quantity for an electrode reaction indicative of its inherent speed: a large charge-transfer resistance indicates a slow reaction. See also non-ohmic resistance.
Charge transport: The phenomenon of movement (transportation) of electrical charge from one part of the system to another, occurring through electromigration.
Charging: A process to "fill" a rechargeable battery with electricity by applying a current to its terminals. The process will cause electrochemical reactions to occur in the battery, 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.

Chemical kinetics: See kinetics.
Chlor-alkali production: See brine electrolysis.
Chlorate production: See brine electrolysis.
Chlorine-caustic production: See brine electrolysis.
Chlorine gas production: See brine electrolysis.
Chronoamperometry: 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.
Chronocoulometry: 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.
Chronopotentiometry: 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.
Clark electrode: 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).
Closed-circuit voltage: The voltage of a battery when it is discharging (on-load condition). Abbreviated as "ccv."
Coin cell: 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.
Collector; current collector: Electronic conductor embedded in the active mass and connected to the bus bar or terminal.

Combination electrode: 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.
Combustion pile: See fuel cell.
Compact layer: See the Helmholtz model of the double layer.
Completely-polarizable electrode: Alternative expression for ideal polarizable electrode.
Compliance limits: The maximum value of the current and voltage that a control instrument (e.g., galvanostat or potentiostat) is capable to provide.
Concentration: 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.
Concentration cell: 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.
Concentration overpotential (polarization): 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."

Condenser: See capacitor.
Conditioning: For a rechargeable battery: see forming.
Conductance (electrical): See conductivity.
Conducting polymer: A polymeric material (e.g., plastics) having electronic conductivity.
Conductivity (electrical): The measure of a material's capability to carry electrical current. The measurement unit of conductivity (conductance) is the siemens. The reciprocal of resistivity.
Conductivity cell: A cell specially designed for the measurement of the conductivity of an electrolyte solution. It is a small vessel containing two metallic electrodes, the cell is filled with the solution to be measured. Also called "conductance cell." The measurement of the conductivity of an electrolyte solution is more complicated than a similar measurement with a metallic conductor. When measuring with dc current, one would have to take into consideration the electromotive force of the electrochemical cell, and the polarization of the electrodes. Therefore, the measurements are typically carried out with high frequency ac current and the electrodes in the conductivity cell are typically made of platinized platinum to avoid these complications. The cell geometry usually does not ensure that exactly and only one cubic centimeter of solution will carry the current; therefore, the cell has to be calibrated to obtain the specific conductance of the solution. The calibration is usually carried out with high purity potassium chloride solutions, and the resulting calibration constant is often called the "cell constant."
Conductometry: An electroanalytical technique based upon the measurement of the conductivity an electrolyte solution.
Conductor (electrical): A material that is capable to carry an electrical current. See also electronic conductor and ionic conductor.
Constant-current technique/process: 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.
Constant-potential technique/process: 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.
Contact adsorption: Adsorption with the adsorbed molecule or ion being in direct contact with the solid surface.
Convection: 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.
Conversion: See energy conversion.

Corrosion: 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.") 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.
Corrosion current (density): 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.
Corrosion inhibitor: 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.
Corrosion potential: 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. 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.)

Cottrell equation: 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.

Coulomb: Measurement unit of the electrical charge. Symbol: "C".The charge passing a given point during one second when the current. is one ampere.
Coulombic efficiency: See coulometric efficiency.
Coulometer: Instrument used for the measurement of electrical charge.
Coulometric efficiency: 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."
Coulometry: An electroanalytical technique based upon the measurement of the amount of electrical charge passed through the working electrode of an electrochemical cell.

Coulostatic technique: 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.
Coulter counter: 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.
Counter electrode: 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.
Counterion: 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.
Couple: 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.
C-rate: 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.
Creep: The process by which liquid electrolytes, and in particular alkalies, can escape past rubber-metal or polymer-metal seals, or through minute cracks in a cell case or lid.

Crystallization overpotential (polarization): The overpotential (alternatively called polarization) associated with the crystallization step in electrocrystallization. The crystallization is an elementary step in the overall electrode reaction.

Current: The movement of electrical charges in a conductor; carried by electrons in an electronic conductor and by ions in an ionic conductor. "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.
Current collector: 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.
Current compliance: See compliance limits.
Current concentration: 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 density: Current referred to the unit area of the electrode. Current divided by the true electrode area.
Current distribution: 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.
Current efficiency: 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 leakage: 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).
Current-potential plot: 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. See also Tafel equation.
Current source (supply): See electrical source (supply).
Current-voltage plot: See current-potential plot.
Current yield: See current efficiency.
CV: Stands for "cyclic voltammetry," see voltammetry.
Cycle: In voltammetry: a complementary pair of forward and reverse potential sweeps. For rechargeable batteries: a complementary discharging and charging processes.
Cycle life: The number of times a rechargeable battery 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.
Cyclic voltammetry: See voltammetry.