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 related to a cell's voltage efficiency. In an electrolytic cell the existence of overpotential implies the cell requires more energy than thermodynamically expected to drive a reaction. In a galvanic cell the existence of overpotential means less energy is recovered than thermodynamics predicts. In each case the extra/missing energy is lost as heat. The quantity of overpotential is specific to each cell design and varies across cells and operational conditions, even for the same reaction. Overpotential is experimentally determined by measuring the potential at which a given current density (typically small) is achieved.

The four possible polarities of overpotentials are listed below.

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.

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