Faraday efficiency
Encyclopedia
Faraday efficiency describes the efficiency with which charge
(electron
s) are transferred in a system facilitating an electrochemical reaction. The word "faraday" in this term has two interrelated aspects. First, the historic unit for charge
is the faraday, but has since been replaced by the coulomb. Secondly, the related Faraday's constant correlates charge with mole
s of matter and electrons (amount of substance
). This phenomenon was originally understood through Michael Faraday
's work and expressed in his laws of electrolysis
.
cells. These losses are usually in the form of misdirected electrons which participate in unproductive reactions, product recombination, short circuit the system, and other diversions for electrons. These losses are physically expressed in the system as heat and sometimes chemical byproducts.
An example of side reactions can be found in the oxidation of water
to oxygen
. During this process, electrons are commonly diverted to the production of hydrogen peroxide
. The fraction of electrons so diverted would represent a faradaic loss and vary between different apparatus.
If the proper electrolysis products are produced, there can still be losses if the products are permitted to recombine. During water electrolysis, the desired products, hydrogen
and oxygen
, are produced but could be allowed to recombine to form water
. This loss of product could realistically happen in the presence of catalytic materials such as platinum
or palladium
, which are also commonly used as electrodes. Failure to account for this Faraday-efficiency effect
has been identified as the cause of the misidentification of positive results in cold fusion
experiments.
Proton exchange membrane fuel cell
s provide a good example of faradic losses in terms of a short circuit. Not all the electrons separated from hydrogen at the anode
are directed through the loaded circuit to do "work
" then back to the cathode
. Some of the electrons bleed through the electrolyte membrane reaching the cathode directly without performing work. Ideally the electrolyte membrane would be a perfect insulator.
. 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 × ampere
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 faradic efficiency.
Electric charge
Electric charge is a physical property of matter that causes it to experience a force when near other electrically charged matter. Electric charge comes in two types, called positive and negative. Two positively charged substances, or objects, experience a mutual repulsive force, as do two...
(electron
Electron
The electron is a subatomic particle with a negative elementary electric charge. It has no known components or substructure; in other words, it is generally thought to be an elementary particle. An electron has a mass that is approximately 1/1836 that of the proton...
s) are transferred in a system facilitating an electrochemical reaction. The word "faraday" in this term has two interrelated aspects. First, the historic unit for charge
Charge (physics)
In physics, a charge may refer to one of many different quantities, such as the electric charge in electromagnetism or the color charge in quantum chromodynamics. Charges are associated with conserved quantum numbers.-Formal definition:...
is the faraday, but has since been replaced by the coulomb. Secondly, the related Faraday's constant correlates charge with mole
Mole (unit)
The mole is a unit of measurement used in chemistry to express amounts of a chemical substance, defined as an amount of a substance that contains as many elementary entities as there are atoms in 12 grams of pure carbon-12 , the isotope of carbon with atomic weight 12. This corresponds to a value...
s of matter and electrons (amount of substance
Amount of substance
Amount of substance is a standards-defined quantity that measures the size of an ensemble of elementary entities, such as atoms, molecules, electrons, and other particles. It is sometimes referred to as chemical amount. The International System of Units defines the amount of substance to be...
). This phenomenon was originally understood through Michael Faraday
Michael Faraday
Michael Faraday, FRS was an English chemist and physicist who contributed to the fields of electromagnetism and electrochemistry....
's work and expressed in his laws of electrolysis
Faraday's laws of electrolysis
Faraday's laws of electrolysis are quantitative relationships based on the electrochemical researches published by Michael Faraday in 1834.-Statements of the laws:Several versions of the laws can be found in textbooks and the scientific literature...
.
Sources of faradaic loss
Faradaic losses are experienced by both electrolytic and galvanicGalvanic 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...
cells. These losses are usually in the form of misdirected electrons which participate in unproductive reactions, product recombination, short circuit the system, and other diversions for electrons. These losses are physically expressed in the system as heat and sometimes chemical byproducts.
An example of side reactions can be found in the oxidation of water
Electrolysis of water
Electrolysis of water is the decomposition of water into oxygen and hydrogen gas due to an electric current being passed through the water.-Principle:...
to 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...
. During this process, electrons are commonly diverted to the production of hydrogen peroxide
Hydrogen peroxide
Hydrogen peroxide is the simplest peroxide and an oxidizer. Hydrogen peroxide is a clear liquid, slightly more viscous than water. In dilute solution, it appears colorless. With its oxidizing properties, hydrogen peroxide is often used as a bleach or cleaning agent...
. The fraction of electrons so diverted would represent a faradaic loss and vary between different apparatus.
If the proper electrolysis products are produced, there can still be losses if the products are permitted to recombine. During water electrolysis, the desired products, 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...
and 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...
, are produced but could be allowed to recombine to form water
Water
Water is a chemical substance with the chemical formula H2O. A water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state . Water also exists in a...
. This loss of product could realistically happen in the presence of catalytic materials such as 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...
or 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...
, which are also commonly used as electrodes. Failure to account for this Faraday-efficiency effect
Faraday-efficiency effect
The Faraday-efficiency effect refers to the potential for misinterpretation of data from experiments in electrochemistry through failure to take into account a Faraday efficiency of less than 100 per cent.-Assumption about efficiency:...
has been identified as the cause of the misidentification of positive results in cold fusion
Cold fusion
Cold fusion, also called low-energy nuclear reaction , refers to the hypothesis that nuclear fusion might explain the results of a group of experiments conducted at ordinary temperatures . Both the experimental results and the hypothesis are disputed...
experiments.
Proton exchange membrane fuel cell
Proton exchange membrane fuel cell
Proton exchange membrane fuel cells, also known as polymer electrolyte membrane fuel cells , are a type of fuel cell being developed for transport applications as well as for stationary fuel cell applications and portable fuel cell applications. Their distinguishing features include lower...
s provide a good example of faradic losses in terms of a short circuit. Not all the electrons separated from hydrogen at the anode
Anode
An anode is an electrode through which electric current flows into a polarized electrical device. Mnemonic: ACID ....
are directed through the loaded circuit to do "work
Work (thermodynamics)
In thermodynamics, work performed by a system is the energy transferred to another system that is measured by the external generalized mechanical constraints on the system. As such, thermodynamic work is a generalization of the concept of mechanical work in mechanics. Thermodynamic work encompasses...
" then back to the cathode
Cathode
A cathode is an electrode through which electric current flows out of a polarized electrical device. Mnemonic: CCD .Cathode polarity is not always negative...
. Some of the electrons bleed through the electrolyte membrane reaching the cathode directly without performing work. Ideally the electrolyte membrane would be a perfect insulator.
Methods of measuring faradic loss
Faradic efficiency of a cell design is usually measured through bulk electrolysis where a known quantity of reagent is stoichiometrically converted to product, as measured by the current passed. This result is then compared to the observed quantity of product measured through another analytical method.Faradic vs. voltage efficiency
Faradic efficiency should not be confused with voltage efficiency, which is usually discussed in terms of overpotentialOverpotential
Overpotential is an electrochemical term which refers to the potential 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...
. 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 × ampere
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 faradic efficiency.