Faraday Efficiency
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According to Faraday's laws of electrolysis:
a) The mass of a substance produced at an electrode during electrolysis is directly proportional to the number of electrons (the quantity of electricity) transferred at that electrode (Faraday's 1st Law of Electrolysis); and
b) The number of faradays of electric charge required to discharge one mole of substance at an electrode is equal to the number of "excess" elementary charges on that ion. (Faraday's 2nd Law of Electrolysis)
The quantity of elemental material released during electrolysis is directly proportional to the total electric charge passed through the electrolytic cell.
If all the electric charge results in material release, we consider the electrolysis to have an efficiency of 100%. However, if part of the material, which should theoretically be released, remains in solution and does not get released, the efficiency of electrolysis is less than 100%. One way to obtain a Faraday efficiency of less than 100% is to have a secondary reaction that causes charge transfer across the cell, but does not plate on the same material as the primary reaction.
The Faraday Efficiency of electrolysis is defined as the ratio of "the actual quantity of material released", to "the theoretical quantity of material which should have been released, based on Faraday's laws of electrolysis".
In electrolysis of water (excluding electroplating), the Faraday efficiency of hydrogen and oxygen release is seldom less than 100%. Observations of reduced Faraday efficiency are difficult to measure quantitatively and require electrolytic conditions which are difficult to achieve. Such conditions are sometimes achieved during prolonged electrolysis using a palladium cathode in heavy water[1] or a fine-wire nickel cathode in plain water[2]. The high concentration of hydrogen or deuterium absorbed into the surface of the cathode is believed to catalyze the internal recombination of hydrogen and oxygen. Such recombination is the predominant mechanism responsible for the reduced Faraday efficiency[2].
In fuel cells, on the other hand, the Faraday efficiency is always less than 100% [3].
[edit] References
- ^ 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
- ^ a b c 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
- ^ http://www.scied.science.doe.gov/nmsb/hydrogen/Fuel%20Cell%20Efficiency.pdf
