The Tafel equation is an equation in electrochemical kinetics relating the rate of an electrochemical reaction to the overpotential. 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.
On a single electrode the Tafel equation can be stated as
- is the overpotential
- is the so-called "Tafel slope", V
- is the current density, A/m2 and
- is the so-called "exchange current density", A/m2.
Overview of the termsEdit
The exchange current is the current at equilibrium, i.e. the rate at which oxidized and reduced species transfer electrons with the electrode. In other words, the exchange current density is the rate of reaction at the reversible potential (when the overpotential is zero by definition). At the reversible potential, the reaction is in equilibrium meaning that the forward and reverse reactions progress at the same rates. This rate is the exchange current density.
The Tafel slope is measured experimentally. It can, however, be shown theoretically that when the dominant reaction mechanism involves the transfer of a single electron that
where A is defined as
An alternative formEdit
The Tafel equation can be also written as:
The Tafel equation assumes that the reverse reaction rate is negligible compared to the forward reaction rate.
The Tafel equation is applicable to the region where the values of polarization are high. At low values of polarization, the dependence of current on polarization is usually linear (not logarithmic):
This linear region is called polarization resistance due to its formal similarity to the Ohm's law.
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