Faraday efficiency

Faraday efficiency

Faradic Efficiency (also called "coulombic efficiency" or "current efficiency") describes the efficacy with which current (electrons) are transfered in a system facilitating an electrochemical reaction. The word "faraday" in this term refers to the historic unit for charge (current), faraday, that has since been been replaced by the coulomb, as well as the related faraday's constant that correlates charge with moles. This phenomenon was originally understood through Faraday's work and expressed in his Faraday's laws of electrolysis. [Bard, A.J.; Faulkner, L.R. Electrochemical Methods: Fundamentals and Applications. New York: John Wiley & Sons, 2nd Edition, 2000.]

Sources of faradic loss

Faradic 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 it is common for electron to be 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 produce there can still be losses if the products are permitted to recombine. Again, during water electrolysis if the desired products, hydrogen and oxygen, are produced but then allowed to recombine to form water. this could realistically happen in the presence of a catalytic material 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. [Faradaic efficiencies less than 100% during electrolysis of water can account for reports of excess heat in 'cold fusion' cells. J.E. Jones et al., J. Physical Chem. 99 (1995) p.6973-6979] [Calorimetry, Excess Heat, and Faraday Efficiency in Ni-H2O Electrolytic Cells. Z. Shkedi et al., Fusion Technology Vol.28 No.4 (1995) p.1720-1731]

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 preforming work. Ideally the electrolyte membrane would be a perfect insulator. [http://www.scied.science.doe.gov/nmsb/hydrogen/Fuel%20Cell%20Efficiency.pdf]

Methods of measuring faradic loss

Faradic efficiency of a cell design is usually measured through bulk electrolysis where the a known quantity of reagent is stoichiometrically converted to product as measured by the current passed and 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" usually discussed in terms of overpotential. Each term refers to a mode through which electrochemical systems can loss energy. Energy can be expressed as the product of potential and current (Joules = Volts x Amps). 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.

References


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