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Overpotential
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Overpotential is an electrochemical term which refers to the potential (voltage) difference 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 would predict.
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Encyclopedia
Overpotential is an electrochemical term which refers to the potential (voltage) difference 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 would predict. In each cases the extra or missing energy is lost as heat. Overpotential is specific to each cell design and will vary between cells and operational conditions even for the same reaction. The four possible polarities of overpotentials are listed below.
Due to overpotential:
- 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 increasing current density (or rate), as described by the Tafel equation. An electrochemical reaction are a combination of two half-cells and multiple elementary steps. Each of these electrochemical steps is associated with multiple forms of overpotential. The overall overpotential is the summation of these individual losses which are in essence different forms of a reaction activation barrier.
When the main form of energy exchange in a reaction is thermal the energy put into an activation barriers is returned to the system in the form heat. In this situation the heat essentially serves as a catalyst, propagating the reaction. When the current is the main or intended form of energy exchange the situation is different. Activation barriers will often be overcome by increased potential above the thermodynamic requirements, essentially an overpotential. This increased potential is not returned to the system as useful a electronic potential, instead its released as waste heat. In this situation the energy used to overcome activation barriers is not catalytic but a lost byproduct.
Voltage vs. faradaic efficiency
Voltage efficiency describes the energy loss through overpotential. For an electrolytic cell this is the ratio of a cells thermodynamic potential divided by the cells experimental potential converted to a percentile. For a galvanic cell is the ratio of a cells experimental potential divided by the cells thermodynamic potential converted to a percentile.
Voltage efficiency should not be confused with faraday efficiency. Each term refers to a mode through which electrochemical systems can loss energy. Energy can be expressed as the product of potential, current and time (Joules = Volts x Amps x Seconds). Losses in the potential term through overpotentials are described by voltage efficiency. Losses in the current term through misdirected electrons are described by faradaic efficiency.
Varieties of overpotential
Activation overpotential
The potential difference above the equilibrium value required to produce a current. Depends on the activation energy of the reaction.
Reaction overpotential
Reaction overpotential is caused by chemical kinetics in the boundary layer or at the electrode surface. The reaction overpotential can be reduced or eliminated with the use of homogeneous or heterogeneous electrocatalysts. The electrochemical reaction rate and related current density is dictated by the kinetics of the electrocatalyst and substrate concentration.
The platinum electrode common to much of electrochemistry is also electrocatalytically non-innocent for many reactions. For example, hydrogen is oxidized and protons are reduced readily at the platinum surface of a standard hydrogen electrode in aqueous solution. If electrocatalytically inert glassy carbon electrode is substituted for the platinum electrode, then the result is irreversible reduction and oxidation peaks with large overpotentials.
Concentration overpotential
The potential difference caused by differences in concentration of the charge-carriers between bulk solution and on the electrode surface. It occurs when electrochemical reaction is sufficiently rapid to lower the surface concentration of the charge-carriers below that of bulk solution. The rate of reaction is then dependent on the ability of the charge-carriers to reach the electrode surface.
Bubble overpotential
Bubble overpotential is due to the evolution of gas at either the anode or cathode. This reduces the effective area for current and increases the local current density. An example would be the electrolysis of an aqueous sodium chloride solution—although oxygen should be produced at the anode based on its potential, bubble overpotential causes chlorine to be produced instead, which allows the easy industrial production of chlorine and sodium hydroxide by electrolysis.
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