electrochemistry Electrochemistry is the branch of physical chemistry concerned with the relationship between Electric potential, electrical potential difference and identifiable chemical change. These reactions involve Electron, electrons moving via an electronic ...

, Faraday efficiency (also called faradaic efficiency, faradaic yield, coulombic efficiency, or current efficiency) describes the efficiency with which charge (

electron The electron (, or in nuclear reactions) is a subatomic particle with a negative one elementary charge, elementary electric charge. It is a fundamental particle that comprises the ordinary matter that makes up the universe, along with up qua ...

s) is transferred in a system facilitating an

electrochemical reaction Electrochemistry is the branch of physical chemistry concerned with the relationship between electrical potential difference and identifiable chemical change. These reactions involve electrons moving via an electronically conducting phase (typica ...

. The word "Faraday" in this term has two interrelated aspects: first, the historic unit for charge is the

faraday Michael Faraday (; 22 September 1791 – 25 August 1867) was an English chemist and physicist who contributed to the study of electrochemistry and electromagnetism. His main discoveries include the principles underlying electromagnetic inducti ...

(F), but has since been replaced by the

coulomb The coulomb (symbol: C) is the unit of electric charge in the International System of Units (SI). It is defined to be equal to the electric charge delivered by a 1 ampere current in 1 second, with the elementary charge ''e'' as a defining c ...

(C); and secondly, the related Faraday's constant () correlates charge with moles of matter and electrons (

amount of substance In chemistry, the amount of substance (symbol ) in a given sample of matter is defined as a ratio () between the particle number, number of elementary entities () and the Avogadro constant (). The unit of amount of substance in the International ...

). This phenomenon was originally understood through

Michael Faraday Michael Faraday (; 22 September 1791 – 25 August 1867) was an English chemist and physicist who contributed to the study of electrochemistry and electromagnetism. His main discoveries include the principles underlying electromagnetic inducti ...

's work and expressed in his

laws of electrolysis Faraday's laws of electrolysis are quantitative relationships based on the electrochemical research published by Michael Faraday in 1833. First law Michael Faraday reported that the mass () of a substance deposited or liberated at an electrod ...

Sources of faradaic loss

Faradaic losses are experienced by both electrolytic and galvanic cells when electrons or ions participate in unwanted side reactions. These losses appear as heat and/or chemical byproducts. An example can be found in the oxidation of water to

oxygen Oxygen is a chemical element; it has chemical symbol, symbol O and atomic number 8. It is a member of the chalcogen group (periodic table), group in the periodic table, a highly reactivity (chemistry), reactive nonmetal (chemistry), non ...

at the positive electrode in electrolysis.

Hydrogen peroxide Hydrogen peroxide is a chemical compound with the formula . In its pure form, it is a very pale blue liquid that is slightly more viscosity, viscous than Properties of water, water. It is used as an oxidizer, bleaching agent, and antiseptic, usua ...

can also be produced. The fraction of electrons so diverted represent a faradaic loss and vary in different apparatus. Even when the proper electrolysis products are produced, losses can still occur if the products are permitted to recombine. During water electrolysis, the desired products ( H₂ and O₂), could recombine to form

water Water is an inorganic compound with the chemical formula . It is a transparent, tasteless, odorless, and Color of water, nearly colorless chemical substance. It is the main constituent of Earth's hydrosphere and the fluids of all known liv ...

. This could realistically happen in the presence of catalytic materials such as

platinum Platinum is a chemical element; it has Symbol (chemistry), symbol Pt and atomic number 78. It is a density, dense, malleable, ductility, ductile, highly unreactive, precious metal, precious, silverish-white transition metal. Its name origina ...

palladium Palladium is a chemical element; it has symbol Pd and atomic number 46. It is a rare and lustrous silvery-white metal discovered in 1802 by the English chemist William Hyde Wollaston. He named it after the asteroid Pallas (formally 2 Pallas), ...

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 Cold fusion is a hypothesized type of nuclear reaction that would occur at, or near, room temperature. It would contrast starkly with the nuclear fusion, "hot" fusion that is known to take place naturally within Main sequence, stars and artific ...

experiments.

Proton exchange membrane fuel cell Proton-exchange membrane fuel cells (PEMFC), also known as polymer electrolyte membrane (PEM) fuel cells, are a type of fuel cell being developed mainly for transport applications, as well as for stationary fuel-cell applications and portable ...

s provide another example of faradaic losses when some of the electrons separated from hydrogen at the

anode An anode usually is an electrode of a polarized electrical device through which conventional current enters the device. This contrasts with a cathode, which is usually an electrode of the device through which conventional current leaves the devic ...

leak through the membrane and reach the

cathode A cathode is the electrode from which a conventional current leaves a polarized electrical device such as a lead-acid battery. This definition can be recalled by using the mnemonic ''CCD'' for ''Cathode Current Departs''. Conventional curren ...

directly instead of passing through the load and performing useful

work Work may refer to: * Work (human activity), intentional activity people perform to support themselves, others, or the community ** Manual labour, physical work done by humans ** House work, housework, or homemaking ** Working animal, an ani ...

. Ideally, the electrolyte membrane would be a perfect insulator and prevent this from happening. An especially familiar example of faradaic loss is the self-discharge that limits battery shelf-life.

Methods of measuring faradaic loss

Faradaic efficiency of a cell design is usually measured through bulk electrolysis where a known quantity of reagent is

stoichiometric Stoichiometry () is the relationships between the masses of reactants and products before, during, and following chemical reactions. Stoichiometry is based on the law of conservation of mass; the total mass of reactants must equal the total m ...

ally 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.

Faradaic loss vs. voltage and energy efficiency

Faradaic loss is only one form of energy loss in an electrochemical system. Another is

overpotential In electrochemistry, overpotential is the potential difference (voltage) between a half-reaction's thermodynamically determined reduction potential and the potential at which the redox event is experimentally observed. The term is directly r ...

, the difference between the theoretical and actual electrode voltages needed to drive the reaction at the desired rate. Even a rechargeable battery with 100% faradaic efficiency requires charging at a higher voltage than it produces during discharge, so its overall energy efficiency is the product of voltage efficiency and faradaic efficiency. Voltage efficiencies below 100% reflect the thermodynamic irreversibility of every real-world chemical reaction.

References

{{DEFAULTSORT:Faraday Efficiency Electrochemistry