Overpotential
Encyclopedia
Overpotential is an electrochemical term which refers to the potential
(volt
age) difference between a half-reaction
's thermodynamically determined reduction potential
and the potential at which the redox
event is experimentally observed. The term is directly related to a cell's voltage efficiency. In an electrolytic cell
the overpotential requires more energy than thermodynamically expected to drive a reaction. In a galvanic cell
overpotential means less energy is recovered than thermodynamics would predict. In each case the extra or missing energy is lost as heat. Overpotential is specific to each cell design and will vary between cells and operational conditions even for the same reaction. Practically it is also useful to define the current density
(typically small) at which the overpotential is measured.
Due to overpotential:
The overpotential increases with increasing current density
(or rate), as described by the Tafel equation
. An electrochemical reaction is a combination of two half-cells and multiple elementary steps. Each of these electrochemical steps
is associated with multiple forms of overpotential. The overall overpotential is the summation of many individual losses.
Voltage efficiency describes the energy lost through overpotential. For an electrolytic cell this is the ratio of a cell's thermodynamic potential divided by the cell's experimental potential converted to a percentile. For a galvanic cell it is the ratio of a cell's experimental potential divided by the cell's thermodynamic potential converted to a percentile.Voltage efficiency should not be confused with faraday efficiency
. Each term refers to a mode through which electrochemical systems can lose energy. Energy can be expressed as the product of potential, current and time (Joule
s = Volt
s × Amp
s × Second
s). Losses in the potential term through overpotentials are described by voltage efficiency. Losses in the current term through misdirected electrons are described by faradaic efficiency.
. A likely reason for the lack of strict definitions is that it's difficult to determine how much of a measured overpotential is derived from a specific source. There is precedent for lumping overpotentials into three categories: activation, concentration, and resistance.
The potential difference above the equilibrium value required to produce a current which depends on the activation energy
of the redox event. While ambiguous "activation overpotential" often refers exclusively to the activation energy necessary to transfer an electron from an electrode to an analyte. This sort of overpotential can also be called "electron transfer overpotential" and is a component of "polarization overpotential", a phenomenon observed in cyclic voltammetry
and partially described by the Cottrell equation
.
that must formally precede electron transfer. The reaction overpotential can be reduced or eliminated with the use of homogeneous or heterogeneous electrocatalyst
s. The electrochemical reaction rate and related current density
is dictated by the kinetics of the electrocatalyst and substrate concentration.
The platinum
electrode common to much of electrochemistry
is also electrocatalytically non-innocent for many reactions. For example, hydrogen is oxidized and protons are reduced readily at the platinum surface of a standard hydrogen electrode
in aqueous solution. If electrocatalytically inert glassy carbon
electrode is substituted for the platinum electrode, then the result is irreversible reduction and oxidation peaks with large overpotentials.
The potential difference is caused by differences in concentration of the charge-carriers between bulk solution and on the electrode surface. It occurs when electrochemical reaction is sufficiently rapid to lower the surface concentration of the charge-carriers below that of bulk solution. The rate of reaction is then dependent on the ability of the charge-carriers to reach the electrode surface.
solution—although oxygen
should be produced at the anode based on its potential, bubble overpotential causes chlorine
to be produced instead, which allows the easy industrial production of chlorine and sodium hydroxide by electrolysis.
s.
Potential
*In linguistics, the potential mood*The mathematical study of potentials is known as potential theory; it is the study of harmonic functions on manifolds...
(volt
Volt
The volt is the SI derived unit for electric potential, electric potential difference, and electromotive force. The volt is named in honor of the Italian physicist Alessandro Volta , who invented the voltaic pile, possibly the first chemical battery.- Definition :A single volt is defined as the...
age) difference between a half-reaction
Half-reaction
A half reaction is either the oxidation or reduction reaction component of a redox reaction. A half reaction is obtained by considering the change in oxidation states of individual substances involved in the redox reaction.-Example:...
's thermodynamically determined reduction potential
Reduction potential
Reduction potential is a measure of the tendency of a chemical species to acquire electrons and thereby be reduced. Reduction potential is measured in volts , or millivolts...
and the potential at which the redox
Redox
Redox reactions describe all chemical reactions in which atoms have their oxidation state changed....
event is experimentally observed. The term is directly related to a cell's voltage efficiency. In an electrolytic cell
Electrolytic cell
An electrolytic cell decomposes chemical compounds by means of electrical energy, in a process called electrolysis; the Greek word lysis means to break up. The result is that the chemical energy is increased...
the overpotential requires more energy than thermodynamically expected to drive a reaction. In a galvanic cell
Galvanic cell
A Galvanic cell, or Voltaic cell, named after Luigi Galvani, or Alessandro Volta respectively, is an electrochemical cell that derives electrical energy from spontaneous redox reaction taking place within the cell...
overpotential means less energy is recovered than thermodynamics would predict. In each case the extra or missing energy is lost as heat. Overpotential is specific to each cell design and will vary between cells and operational conditions even for the same reaction. Practically it is also useful to define the current density
Current density
Current density is a measure of the density of flow of a conserved charge. Usually the charge is the electric charge, in which case the associated current density is the electric current per unit area of cross section, but the term current density can also be applied to other conserved...
(typically small) at which the overpotential is measured.
Thermodynamics
The four possible polarities of overpotentials are listed below.Due to overpotential:
- An electrolytic cell's anode is more positive using more energy than thermodynamics require.
- An electrolytic cell's cathode is more negative using more energy than thermodynamics require.
- A galvanic cell's anode is less negative supplying less energy than thermodynamically possible.
- A galvanic cell's cathode is less positive supplying less energy than thermodynamically possible.
The overpotential increases with increasing current density
Current density
Current density is a measure of the density of flow of a conserved charge. Usually the charge is the electric charge, in which case the associated current density is the electric current per unit area of cross section, but the term current density can also be applied to other conserved...
(or rate), as described by the Tafel equation
Tafel equation
The Tafel equation is an equation in electrochemical kinetics relating the rate of an electrochemical reaction to the overpotential. The Tafel equation was first deduced experimentally and was later shown to have a theoretical justification...
. An electrochemical reaction is a combination of two half-cells and multiple elementary steps. Each of these electrochemical steps
Electrochemical reaction mechanism
In chemistry, an electrochemical reaction mechanism is the step by step sequence of elementary steps, involving at least one outer sphere electron transfer, by which an overall chemical change occurs .- Overview :...
is associated with multiple forms of overpotential. The overall overpotential is the summation of many individual losses.
Voltage efficiency describes the energy lost through overpotential. For an electrolytic cell this is the ratio of a cell's thermodynamic potential divided by the cell's experimental potential converted to a percentile. For a galvanic cell it is the ratio of a cell's experimental potential divided by the cell's thermodynamic potential converted to a percentile.Voltage efficiency should not be confused with faraday efficiency
Faraday efficiency
Faraday efficiency describes the efficiency with which charge are transferred in a system facilitating an electrochemical reaction. The word "faraday" in this term has two interrelated aspects...
. Each term refers to a mode through which electrochemical systems can lose energy. Energy can be expressed as the product of potential, current and time (Joule
Joule
The joule ; symbol J) is a derived unit of energy or work in the International System of Units. It is equal to the energy expended in applying a force of one newton through a distance of one metre , or in passing an electric current of one ampere through a resistance of one ohm for one second...
s = Volt
Volt
The volt is the SI derived unit for electric potential, electric potential difference, and electromotive force. The volt is named in honor of the Italian physicist Alessandro Volta , who invented the voltaic pile, possibly the first chemical battery.- Definition :A single volt is defined as the...
s × Amp
Ampere
The ampere , often shortened to amp, is the SI unit of electric current and is one of the seven SI base units. It is named after André-Marie Ampère , French mathematician and physicist, considered the father of electrodynamics...
s × Second
Second
The second is a unit of measurement of time, and is the International System of Units base unit of time. It may be measured using a clock....
s). Losses in the potential term through overpotentials are described by voltage efficiency. Losses in the current term through misdirected electrons are described by faradaic efficiency.
Varieties of overpotential
Overpotential can be partitioned into many different subcategories that are not always well defined. For example "polarization overpotential" can refer to electrode polarization and the hysteresis found in forward and reverse peaks of cyclic voltammetryCyclic voltammetry
Cyclic voltammetry or CV is a type of potentiodynamic electrochemical measurement. In a cyclic voltammetry experiment the working electrode potential is ramped linearly versus time like linear sweep voltammetry. Cyclic voltammetry takes the experiment a step further than linear sweep voltammetry...
. A likely reason for the lack of strict definitions is that it's difficult to determine how much of a measured overpotential is derived from a specific source. There is precedent for lumping overpotentials into three categories: activation, concentration, and resistance.
Activation overpotential
| Material of the electrode | Hydrogen Hydrogen Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly... |
Oxygen Oxygen Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition... |
Chlorine Chlorine Chlorine is the chemical element with atomic number 17 and symbol Cl. It is the second lightest halogen, found in the periodic table in group 17. The element forms diatomic molecules under standard conditions, called dichlorine... |
|---|---|---|---|
| Platinum Platinum Platinum is a chemical element with the chemical symbol Pt and an atomic number of 78. Its name is derived from the Spanish term platina del Pinto, which is literally translated into "little silver of the Pinto River." It is a dense, malleable, ductile, precious, gray-white transition metal... (platinized) |
−0.07 V | +0.77 V | +0.08 V |
| Palladium Palladium Palladium is a chemical element with the chemical symbol Pd and an atomic number of 46. It is a rare and lustrous silvery-white metal discovered in 1803 by William Hyde Wollaston. He named it after the asteroid Pallas, which was itself named after the epithet of the Greek goddess Athena, acquired... |
−0.07 V | +0.93 V | |
| Gold Gold Gold is a chemical element with the symbol Au and an atomic number of 79. Gold is a dense, soft, shiny, malleable and ductile metal. Pure gold has a bright yellow color and luster traditionally considered attractive, which it maintains without oxidizing in air or water. Chemically, gold is a... |
−0.09 V | +1.02 V | |
| Iron Iron Iron is a chemical element with the symbol Fe and atomic number 26. It is a metal in the first transition series. It is the most common element forming the planet Earth as a whole, forming much of Earth's outer and inner core. It is the fourth most common element in the Earth's crust... |
−0.15 V | +0.75 V | |
| Platinum Platinum Platinum is a chemical element with the chemical symbol Pt and an atomic number of 78. Its name is derived from the Spanish term platina del Pinto, which is literally translated into "little silver of the Pinto River." It is a dense, malleable, ductile, precious, gray-white transition metal... (shiny) |
−0.16 V | +0.95 V | +0.10 V |
| Silver Silver Silver is a metallic chemical element with the chemical symbol Ag and atomic number 47. A soft, white, lustrous transition metal, it has the highest electrical conductivity of any element and the highest thermal conductivity of any metal... |
−0.22 V | +0.91 V | |
| Nickel Nickel Nickel is a chemical element with the chemical symbol Ni and atomic number 28. It is a silvery-white lustrous metal with a slight golden tinge. Nickel belongs to the transition metals and is hard and ductile... |
−0.28 V | +0.56 V | |
| Graphite Graphite The mineral graphite is one of the allotropes of carbon. It was named by Abraham Gottlob Werner in 1789 from the Ancient Greek γράφω , "to draw/write", for its use in pencils, where it is commonly called lead . Unlike diamond , graphite is an electrical conductor, a semimetal... |
−0.62 V | +0.95 V | +0.12 V |
| Lead Lead Lead is a main-group element in the carbon group with the symbol Pb and atomic number 82. Lead is a soft, malleable poor metal. It is also counted as one of the heavy metals. Metallic lead has a bluish-white color after being freshly cut, but it soon tarnishes to a dull grayish color when exposed... |
−0.71 V | +0.81 V | |
| Zinc Zinc Zinc , or spelter , is a metallic chemical element; it has the symbol Zn and atomic number 30. It is the first element in group 12 of the periodic table. Zinc is, in some respects, chemically similar to magnesium, because its ion is of similar size and its only common oxidation state is +2... |
−0.77 V | ||
| Mercury Mercury (element) Mercury is a chemical element with the symbol Hg and atomic number 80. It is also known as quicksilver or hydrargyrum... |
−0.85 V |
The potential difference above the equilibrium value required to produce a current which depends on the activation energy
Activation energy
In chemistry, activation energy is a term introduced in 1889 by the Swedish scientist Svante Arrhenius that is defined as the energy that must be overcome in order for a chemical reaction to occur. Activation energy may also be defined as the minimum energy required to start a chemical reaction...
of the redox event. While ambiguous "activation overpotential" often refers exclusively to the activation energy necessary to transfer an electron from an electrode to an analyte. This sort of overpotential can also be called "electron transfer overpotential" and is a component of "polarization overpotential", a phenomenon observed in cyclic voltammetry
Cyclic voltammetry
Cyclic voltammetry or CV is a type of potentiodynamic electrochemical measurement. In a cyclic voltammetry experiment the working electrode potential is ramped linearly versus time like linear sweep voltammetry. Cyclic voltammetry takes the experiment a step further than linear sweep voltammetry...
and partially described by the Cottrell equation
Cottrell equation
In electrochemistry, the Cottrell equation describes the change in electric current with respect to time in a controlled potential experiment, such as chronoamperometry. Specifically it describes the current response when the potential is a step function. It was derived by Frederick Gardner...
.
Reaction overpotential
Reaction overpotential is an activation overpotential that specifically relates to chemical reactionsElectrochemical reaction mechanism
In chemistry, an electrochemical reaction mechanism is the step by step sequence of elementary steps, involving at least one outer sphere electron transfer, by which an overall chemical change occurs .- Overview :...
that must formally precede electron transfer. The reaction overpotential can be reduced or eliminated with the use of homogeneous or heterogeneous electrocatalyst
Electrocatalyst
An electrocatalyst is a catalyst that participates in electrochemical reactions. Catalyst materials modify and increase the rate of chemical reactions without being consumed in the process. Electrocatalysts are a specific form of catalysts that function at electrode surfaces or may be the...
s. The electrochemical reaction rate and related current density
Current density
Current density is a measure of the density of flow of a conserved charge. Usually the charge is the electric charge, in which case the associated current density is the electric current per unit area of cross section, but the term current density can also be applied to other conserved...
is dictated by the kinetics of the electrocatalyst and substrate concentration.
The platinum
Platinum
Platinum is a chemical element with the chemical symbol Pt and an atomic number of 78. Its name is derived from the Spanish term platina del Pinto, which is literally translated into "little silver of the Pinto River." It is a dense, malleable, ductile, precious, gray-white transition metal...
electrode common to much of electrochemistry
Electrochemistry
Electrochemistry is a branch of chemistry that studies chemical reactions which take place in a solution at the interface of an electron conductor and an ionic conductor , and which involve electron transfer between the electrode and the electrolyte or species in solution.If a chemical reaction is...
is also electrocatalytically non-innocent for many reactions. For example, hydrogen is oxidized and protons are reduced readily at the platinum surface of a standard hydrogen electrode
Standard hydrogen electrode
The standard hydrogen electrode , is a redox electrode which forms the basis of the thermodynamic scale of oxidation-reduction potentials...
in aqueous solution. If electrocatalytically inert glassy carbon
Glassy carbon
Glassy carbon, also called vitreous carbon, is a non-graphitizing carbon which combines glassy and ceramic properties with those of graphite. The most important properties are high temperature resistance, hardness , low density, low electrical resistance, low friction, low thermal resistance,...
electrode is substituted for the platinum electrode, then the result is irreversible reduction and oxidation peaks with large overpotentials.
Concentration overpotential
Concentration overpotential span a variety of phenomenon but all involve the depletion of charge-carriers at the electrode surface. Bubble overpotential is a specific form of concentration overpotential in which the concentration of charge-carriers is deplated by the physical formation of a bubble. The confusing "diffusion overpotential" can refer to a concentration overpotential created by slow diffusion rates as well as "polarization overpotential" whose overpotential is derived mostly from activation overpotential but peak current is limited by diffusion of analyte.The potential difference is caused by differences in concentration of the charge-carriers between bulk solution and on the electrode surface. It occurs when electrochemical reaction is sufficiently rapid to lower the surface concentration of the charge-carriers below that of bulk solution. The rate of reaction is then dependent on the ability of the charge-carriers to reach the electrode surface.
Bubble overpotential
Bubble overpotential is a specific form of concentration overpotential and is due to the evolution of gas at either the anode or cathode. This reduces the effective area for current and increases the local current density. An example would be the electrolysis of an aqueous sodium chlorideSodium chloride
Sodium chloride, also known as salt, common salt, table salt or halite, is an inorganic compound with the formula NaCl. Sodium chloride is the salt most responsible for the salinity of the ocean and of the extracellular fluid of many multicellular organisms...
solution—although oxygen
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
should be produced at the anode based on its potential, bubble overpotential causes chlorine
Chlorine
Chlorine is the chemical element with atomic number 17 and symbol Cl. It is the second lightest halogen, found in the periodic table in group 17. The element forms diatomic molecules under standard conditions, called dichlorine...
to be produced instead, which allows the easy industrial production of chlorine and sodium hydroxide by electrolysis.
Resistance overpotential
Resistance overpotentials are all the overpotentials tied to a cell design. This include "junction overpotentials" which describes overpotentials occurring at electrode surfaces and interfaces like electrolyte membranes. This can include aspects of electrolyte diffusion, surface polarization (capacitance), and other sources of counter electromotive forceElectromotive force
In physics, electromotive force, emf , or electromotance refers to voltage generated by a battery or by the magnetic force according to Faraday's Law, which states that a time varying magnetic field will induce an electric current.It is important to note that the electromotive "force" is not a...
s.