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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.
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]
where k f and k b are the reaction rate constants, with units of frequency (1/time) and c o and c r are the surface concentrations (mol/area) of the oxidized and reduced molecules, respectively (written as c o (0,t) and c r (0,t) in the previous section). The net rate of reaction v and net current density j are then: [Note 2]
The different units are accounted for in using either the gas constant or the Boltzmann ... Slope of red line = (4.1 − 2.2) / (0.0015 − 0.00165) = −12,667
The driving force shown here as ' ' is expressed in units of moles per unit of volume, but in some cases the driving force is represented by other measures of concentration with different units. For example, the driving force may be partial pressures when dealing with mass transfer in a gas phase and thus use units of pressure.