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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 related to a cell's ''voltage efficiency''. In an electrolytic cell the overpotential requires more energy than thermodynamically expected to drive a reaction. In a galvanic cell overpotential means less energy is recovered than thermodynamics predicts. In each case the extra/missing energy is lost as heat. Overpotential is specific to each cell design and varies across cells and operational conditions, even for the same reaction. It is used more practically to define the current density (typically small) at which the overpotential is measured. == Thermodynamics == The four possible polarities of overpotentials are listed below. * An electrolytic cell's anode is more positive, using more energy than thermodynamics require. * An electrolytic cell's cathode is more negative, using more energy than thermodynamics require. * A galvanic cell's anode is less negative, supplying less energy than thermodynamically possible. * A galvanic cell's cathode is less positive, supplying less energy than thermodynamically possible. The overpotential increases with growing current density (or rate), as described by the Tafel equation. An electrochemical reaction is a combination of two half-cells and multiple elementary steps. Each step is associated with multiple forms of overpotential. The overall overpotential is the summation of many individual losses. ''Voltage efficiency'' describes the fraction of energy lost through overpotential. For an ''electrolytic'' cell this is the ratio of a cell's thermodynamic potential divided by the cell's experimental potential converted to a percentile. For a ''galvanic'' cell it is the ratio of a cell's experimental potential divided by the cell's thermodynamic potential converted to a percentile. Voltage efficiency should not be confused with Faraday efficiency. Both terms refer to a mode through which electrochemical systems can lose energy. Energy can be expressed as the product of potential, current and time (joules = volts × amps × seconds). Losses in the potential term through overpotentials are described by voltage efficiency. Losses in the current term through misdirected electrons are described by Faraday efficiency. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「overpotential」の詳細全文を読む スポンサード リンク
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