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. It represents the extra energy required to overcome activation barriers for the electrode reaction to proceed at a significant rate. It is a key factor in the energy efficiency of all electrolytic processes.
Overpotential (chemistry)
1910
- Julius Tafel
In an ideal electrolytic cell, the applied voltage needed to drive a reaction would be exactly equal to the cell’s standard electrode potential ([latex]E^0_{cell}[/latex]). However, in reality, a significantly higher voltage is almost always required. This excess voltage is the overpotential ([latex]\eta[/latex]). The total cell potential ([latex]E_{cell}[/latex]) is the sum of the equilibrium potential, the overpotentials at both electrodes, and the ohmic drop (IR drop) across the electrolyte: [latex]E_{cell} = E^0_{cell} + \eta_{anode} + \eta_{cathode} + IR_{drop}[/latex].
Overpotential arises from several sources. Activation overpotential is related to the kinetics of the electron transfer step at the electrode surface itself. Some reactions, like the evolution of hydrogen or oxygen gas, have inherently slow kinetics and require a large activation overpotential. Concentration overpotential occurs when the concentration of reactants at the electrode surface differs from the bulk concentration due to slow diffusion. Resistance overpotential is caused by the resistance of the electrolyte or films on the electrode surface.
The relationship between current density (j) and activation overpotential is often described by the Tafel equation: [latex]\eta = a + b \log(j)[/latex], where ‘a’ and ‘b’ are constants (Tafel parameters) specific to the electrode reaction and material. This equation shows that to get a higher reaction rate (higher current density), a larger overpotential is required. The primary goal in designing efficient electrochemical systems is to minimize overpotential, which is typically achieved by using electrocatalysts (materials that lower the activation energy), increasing the operating temperature, or optimizing the electrode structure to enhance mass transport.
UNESCO Nomenclature: 2406
– Electrochemistry
类型
Physical Phenomenon
Disruption
Substantial
使用方法
Widespread Use
Precursors
- Nernst equation describing equilibrium electrode potentials
- Arrhenius equation relating reaction rate to activation energy
- development of the concept of chemical kinetics
- Faraday’s laws of electrolysis
应用
- designing efficient industrial electrolyzers (e.g., for hydrogen production)
- developing better catalysts to reduce energy loss in fuel cells
- understanding and preventing corrosion
- improving the performance of batteries during charging
- optimizing electroplating processes for uniform coatings
专利：
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Related to: overpotential, electrolysis, electrochemistry, Tafel equation, activation energy, voltage efficiency, electrocatalysis, current density
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