Faraday efficiency (also called faradaic effiency, faradaic yield, coulombic efficiency or current efficiency) describes the efficiency with which charge (electrons) 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 First, the historic unit for charge is the faraday, but has since been replaced by the coulomb.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 moles of matter and electrons (amount of substance). This phenomenon was originally understood through Michael Faraday's work and expressed in his laws of electrolysis.[1]
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Faradaic losses are experienced by both electrolytic and galvanic 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.[2][3]
Proton exchange membrane fuel cells 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.[4]
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 efficiency should not be confused with voltage efficiency, which is usually discussed in terms of overpotential. 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 (joules = volts × amperes × seconds). 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.