An electrode is an
electrical conductor
In physics and electrical engineering, a conductor is an object or type of material that allows the flow of charge (electric current) in one or more directions. Materials made of metal are common electrical conductors. Electric current is gener ...
used to make contact with a nonmetallic part of a
circuit (e.g. a
semiconductor
A semiconductor is a material which has an electrical resistivity and conductivity, electrical conductivity value falling between that of a electrical conductor, conductor, such as copper, and an insulator (electricity), insulator, such as glas ...
, an
electrolyte
An electrolyte is a medium containing ions that is electrically conducting through the movement of those ions, but not conducting electrons. This includes most soluble salts, acids, and bases dissolved in a polar solvent, such as water. Upon dis ...
, a
vacuum
A vacuum is a space devoid of matter. The word is derived from the Latin adjective ''vacuus'' for "vacant" or "void". An approximation to such vacuum is a region with a gaseous pressure much less than atmospheric pressure. Physicists often dis ...
or air). Electrodes are essential parts of
batteries that can consist of a variety of materials depending on the type of battery.
The
electrophore, invented by
Johan Wilcke
Johan Carl Wilcke was a Swedish physicist.
Biography
Wilcke was born in Wismar, son of a clergyman who in 1739 was appointed second pastor of the German Church in Stockholm. He went to the German school in Stockholm and enrolled at the Univers ...
, was an early version of an electrode used to study
static electricity
Static electricity is an imbalance of electric charges within or on the surface of a material or between materials. The charge remains until it is able to move away by means of an electric current or electrical discharge. Static electricity is na ...
.
Anode and cathode in electrochemical cells
Electrodes are an essential part of any
battery
Battery most often refers to:
* Electric battery, a device that provides electrical power
* Battery (crime), a crime involving unlawful physical contact
Battery may also refer to:
Energy source
*Automotive battery, a device to provide power t ...
. The first electrochemical battery made was devised by
Alessandro Volta
Alessandro Giuseppe Antonio Anastasio Volta (, ; 18 February 1745 – 5 March 1827) was an Italian physicist, chemist and lay Catholic who was a pioneer of electricity and power who is credited as the inventor of the electric battery and the ...
and was aptly named the
Voltaic cell
A galvanic cell or voltaic cell, named after the scientists Luigi Galvani and Alessandro Volta, respectively, is an electrochemical cell in which an electric current is generated from spontaneous Oxidation-Reduction reactions. A common apparatus ...
. This battery consisted of a stack of copper and zinc electrodes separated by brine-soaked paper disks. Due to fluctuation in the voltage provided by the voltaic cell it wasn't very practical. The first practical battery was invented in 1839 and named the
Daniell cell
The Daniell cell is a type of electrochemical cell invented in 1836 by John Frederic Daniell, a British chemist and meteorologist, and consists of a copper pot filled with a copper (II) sulfate solution, in which is immersed an unglazed earthenw ...
after John Frederic Daniell. Still making use of the zinc–copper electrode combination. Since then many more batteries have been developed using various materials (see
List of batteries). The basis of all these is still making use of two electrodes which can be divided in two categories:
Anodes
An anode is an electrode of a polarized electrical device through which conventional current enters the device. This contrasts with a cathode, an electrode of the device through which conventional current leaves the device. A common mnemonic is ...
and
Cathodes
A cathode is the electrode from which a conventional current leaves a polarized electrical device. This definition can be recalled by using the mnemonic ''CCD'' for ''Cathode Current Departs''. A conventional current describes the direction in whi ...
.
Anode
A term coined by
William Whewell
William Whewell ( ; 24 May 17946 March 1866) was an English polymath, scientist, Anglican priest, philosopher, theologian, and historian of science. He was Master of Trinity College, Cambridge. In his time as a student there, he achieved dist ...
at
Faraday's request, derived from the Greek words ἄνο (ano), 'upwards' and ὁδός (hodós), 'a way'. The Anode is the electrode through which the
conventional current
An electric current is a stream of charged particles, such as electrons or ions, moving through an electrical conductor or space. It is measured as the net rate of flow of electric charge through a surface or into a control volume. The moving par ...
enters from the electrical circuit of an
electrochemical cell
An electrochemical cell is a device capable of either generating electrical energy from chemical reactions or using electrical energy to cause chemical reactions. The electrochemical cells which generate an electric current are called voltaic o ...
(battery) into the non-metallic cell. The electrons then flow to the other side of the battery. Note the difference in the flow of current and the flow of electrons, this is due to the discovery of the flow of current prior to the discovery of the
electron
The electron ( or ) is a subatomic particle with a negative one elementary electric charge. Electrons belong to the first generation of the lepton particle family,
and are generally thought to be elementary particles because they have no kn ...
.
Benjamin Franklin
Benjamin Franklin ( April 17, 1790) was an American polymath who was active as a writer, scientist, inventor, statesman, diplomat, printer, publisher, and political philosopher. Encyclopædia Britannica, Wood, 2021 Among the leading inte ...
surmised that the electrical flow moved from positive to negative. The electrons flow away from the anode and the conventional current towards it. From both can be concluded that the charge of the anode is negative. The electron entering the anode comes from the
oxidation
Redox (reduction–oxidation, , ) is a type of chemical reaction in which the oxidation states of substrate change. Oxidation is the loss of electrons or an increase in the oxidation state, while reduction is the gain of electrons or a d ...
reaction that takes place next to it.
Cathode
The cathode is in many ways the opposite of the anode. The name (also coined by William Whewell) comes from the Greek words κάτω (kato), 'downwards' and ὁδός (hodós), 'a way'. It is the positive electrode, meaning the electrons flow from the electrical circuit through the cathode into the non-metallic part of the electrochemical cell. At the cathode, the
reduction reaction takes place with the electrons arriving from the wire connected to the cathode and are absorbed by the
oxidizing agent
An oxidizing agent (also known as an oxidant, oxidizer, electron recipient, or electron acceptor) is a substance in a redox chemical reaction that gains or "Electron acceptor, accepts"/"receives" an electron from a (called the , , or ). In ot ...
.
Primary cell
A
primary cell
A primary battery or primary cell is a battery (a galvanic cell) that is designed to be used once and discarded, and not recharged with electricity and reused like a secondary cell (rechargeable battery). In general, the electrochemical reaction ...
is a battery designed to be used once and then discarded. This is due to the electrochemical reactions taking place at the electrodes in the cell not being reversible. An example of a primary cell is the discardable
alkaline battery
An alkaline battery (IEC code: L) is a type of primary battery where the electrolyte (most commonly potassium hydroxide) has a pH value above 7. Typically these batteries derive energy from the reaction between zinc metal and manganese dioxide, ...
used in for example flashlights. Consisting of a zinc anode and a manganese oxide cathode in which ZnO is formed.
The half-reactions are:
: Zn
(s) + 2OH
−( aq) → ZnO
(s) + H
2O
(l) + 2e
− 0oxidation = -1.28 V">0oxidation = -1.28 V: 2MnO
2(s) + H
2O
(l) + 2e
− → Mn
2O
3(s) + 2OH
−(aq) 0reduction = +0.15 V">0reduction = +0.15 V
Overall reaction:
: Zn
(s) + 2MnO
2(s) ZnO
(s) + Mn
2O
3(s) 0total = +1.43 V">0total = +1.43 VThe ZnO is prone to clumping and will give less efficient discharge if recharged again. It is possible to recharge these batteries but is due to safety concerns advised against by the manufacturer. Other primary cells include
zinc-carbon, zinc-chloride, and lithium iron disulfide batteries.
Secondary cell
Contrary to the primary cell a secondary cell can be recharged. The first of which was the lead-acid battery invented in 1859 by French
Gaston Planté
Gaston Planté (22 April 1834 – 21 May 1889) was a French physicist who invented the lead–acid battery in 1859. This type battery was developed as the first rechargeable electric battery marketed for commercial use and it is widely used in aut ...
. This type of battery is still the most widely used in among others automobiles. The cathode consists of lead dioxide (PbO2) and the anode of solid lead. Other commonly used
rechargeable batteries
A rechargeable battery, storage battery, or secondary cell (formally a type of energy accumulator), is a type of electrical battery which can be charged, discharged into a load, and recharged many times, as opposed to a disposable or prima ...
are
nickel-cadmium,
nickel-metal hydride battery, and
Lithium-ion batteries
A lithium-ion or Li-ion battery is a type of rechargeable battery which uses the reversible reduction of lithium ions to store energy. It is the predominant battery type used in portable consumer electronics and electric vehicles. It also se ...
. The last of which will be more explained more thoroughly in this article due to its importance.
Marcus' theory of electron transfer
Marcus theory is a theory originally developed by Nobel laureate
Rudolph A. Marcus
Rudolph Arthur Marcus (born July 21, 1923) is a Canadian-born chemist who received the 1992 Nobel Prize in Chemistry "for his contributions to the theory of electron transfer reactions in chemical systems". Marcus theory, named after him, provide ...
and explains the rate at which an electron can move from one chemical species to another,
for this article this can be seen as 'jumping' from the electrode to a species in the solvent or vice versa.
We can represent the problem as calculating the transfer rate for the transfer of an electron from donor to an acceptor
: D + A → D
+ + A
−
The potential energy of the system is a function of the translational, rotational, and vibrational coordinates of the reacting species and the molecules of the surrounding medium, collectively called the reaction coordinates. The abscissa the figure to the right represents these. From the classical electron transfer theory, the expression of the
reaction rate constant In chemical kinetics a reaction rate constant or reaction rate coefficient, ''k'', quantifies the rate and direction of a chemical reaction.
For a reaction between reactants A and B to form product C
the reaction rate is often found to have the ...
(probability of reaction) can be calculated, if a non-adiabatic process and parabolic potential energy are assumed, by finding the point of intersection (Q
x). One important thing to note, and was noted by Marcus when he came up with the theory, the electron transfer must abide by the law of conservation of energy and the Frank-Condon principle.
Doing this and then rearranging this leads to the expression of the free energy activation (
) in terms of the overall free energy of the reaction (
).
In which the
is the reorganisation energy.
Filling this result in the classically derived
Arrhenius equation
In physical chemistry, the Arrhenius equation is a formula for the temperature dependence of reaction rates. The equation was proposed by Svante Arrhenius in 1889, based on the work of Dutch chemist Jacobus Henricus van 't Hoff who had noted in 18 ...
leads to