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Tafel plot for an anodic process . The Tafel equation is an equation in electrochemical kinetics relating the rate of an electrochemical reaction to the overpotential. [1] The Tafel equation was first deduced experimentally and was later shown to have a theoretical justification. The equation is named after Swiss chemist Julius Tafel.
where and are constants (for a given reaction and temperature) and are called the Tafel equation constants. The theoretical values of the Tafel equation constants are different for the cathodic and anodic processes. However, the Tafel slope can be defined as:
In electrochemistry, exchange current density is a parameter used in the Tafel equation, Butler–Volmer equation and other electrochemical kinetics expressions. The Tafel equation describes the dependence of current for an electrolytic process to overpotential.
In operating batteries and fuel cells, charge transfer coefficient is the parameter that signifies the fraction of overpotential that affects the current density.This parameter has had a mysterious significance in electrochemical kinetics for over three quarters of the previous century [citation needed].
The Tafel equation relates the electrochemical currents to the overpotential exponentially, and is used to calculate the reaction rate. [11] The overpotential is calculated at each electrode separately, and related to the voltammogram data to determine reaction rates. The Tafel equation for a single electrode is:
The shift in mechanism between the pH extremes has been attributed to the kinetic facility of oxidizing hydroxide ion relative to water. Using the Tafel equation, one can obtain kinetic information about the kinetics of the electrode material such as the exchange current density and the Tafel slope. [6]
A very steep slope will correspond to high steric sensitivity, while a shallow slope will correspond to little to no sensitivity. Since E s values are large and negative for bulkier substituents, it follows that: If δ is positive, increasing steric bulk decreases the reaction rate and steric effects are greater in the transition state.
Julius Tafel was born in the village of Choindez in Courrendlin, Switzerland on 2 June 1862. Tafel's father, Julius Tafel Sr. (1827-1893) studied chemistry in Tubingen and became a director of Von Roll’s iron and steel works located in Choindez in 1856, and then took a top management position in steel works located in Gerlafingen in 1863.