Tafel equation

 ( Chemical Kinetics ) 

The Tafel equation is an equation in electrochemical kinetics relating the rate of an Electrochemistry reaction to the overpotential.Bard, A. J.; Faulkner, L. R. “Electrochemical Methods. Fundamentals and Applications” 2nd Ed. Wiley, New York. 2001. 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.

It describes how the electrical current through an electrode depends on the voltage difference between the electrode and the bulk electrolyte for a simple, unimolecular redox reaction.

$Ox\; +\; n\; e^-\; \backslash leftrightarrows\; Red$

Where an electrochemical reaction occurs in two on separate , the Tafel equation is applied to each electrode separately. On a single electrode the Tafel equation can be stated as:

where

• the plus sign under the exponent refers to an anodic reaction, and a minus sign to a cathodic reaction,
• $\backslash eta$ : overpotential, V
• $A$ : "Tafel slope", V
• $i$ : current density, A/m²
• $i\_0$ : "exchange current density", A/m².

A verification plus further explanation for this equation can be found here. The Tafel equation is an approximation of the Butler–Volmer equation in the case of $|\; \backslash eta\; |\; >\; 0.1V$.

" assumes that the concentrations at the electrode are practically equal to the concentrations in the bulk electrolyte, allowing the current to be expressed as a function of potential only. In other words, it assumes that the electrode mass transfer rate is much greater than the reaction rate, and that the reaction is dominated by the slower chemical reaction rate ".

Also, at a given electrode the Tafel equation assumes that the reverse half reaction rate is negligible compared to the forward reaction rate.

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Overview of the terms 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 where

• $\backslash lambda=\backslash ln(10)=2.302\backslash \; 585...$
• $k\_\backslash text\{B\}$ is the Boltzmann constant,
• $T$ is the absolute temperature,
• $e$ is the electric elementary charge of an electron,
• $V\_T\; =\; k\_\backslash text\{B\}\; T\; /\; e$ is the thermal voltage, and
• $\backslash alpha$ is the charge transfer coefficient, the value of which must be between 0 and 1.

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Equation in case of non-negligible electrode mass transfer In a more general case,

The following derivation of the extended Butler–Volmer equation is adapted from that of Bard and Faulkner and Newman and Thomas-Alyea. the current is expressed as a function not only of potential (as in the simple version), but of the given concentrations as well. The mass-transfer rate may be relatively small, but its only effect on the chemical reaction is through the altered (given) concentrations. In effect, the concentrations are a function of the potential as well.

The Tafel equation can be also written as:

where

• n is the number of electrons exchanged, like in the Nernst equation,
• k is the rate constant for the electrode reaction in s⁻¹,
• F is the Faraday constant,
• C is the reactive species concentration at the electrode surface in mol/m²,
• the plus sign under the exponent refers to an anodic reaction, and a minus sign to a cathodic reaction,
• R is the universal gas constant.
• $\backslash alpha$ is the charge transfer coefficient, the value of which must be between 0 and 1.

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Demonstration As seen in equation (), $$\backslash eta=\backslash pm\; A\backslash cdot\; \backslash log\_\{10\}\backslash left(\backslash frac\{i\}\{i\_0\}\backslash right)$$ $$\backslash eta=\backslash pm\; A\backslash cdot\; \backslash frac\{\; \backslash ln\; \backslash left(\backslash frac\{i\}\{i\_0\}\backslash right)\; \}\{\backslash ln(10)\},$$ so: $$i=i\_0\; \backslash exp\; \backslash left(\; \backslash pm\; \backslash frac\; \{\backslash ln(10)\; \backslash eta\}\; \{A\}\; \backslash right)$$ $$i=i\_0\; \backslash exp\; \backslash left(\; \backslash pm\; \backslash alpha\; e\; \backslash frac\; \{\backslash eta\}\; \{kT\}\; \backslash right),$$ as seen in equation () and because $\backslash lambda\; =\; \backslash ln\; (10)$. $$i=i\_0\; \backslash exp\; \backslash left(\; \backslash pm\; \backslash alpha\; F\; \backslash frac\; \{\backslash eta\}\; \{RT\}\; \backslash right)$$ because $\backslash frac\; \{e\}\{k\}=\backslash frac\; \{e/Na\}\{k/Na\}\; =\; \backslash frac\; \{F\}\{R\}$ due to the electrode mass transfer $i\_0=nkFC$, which finally yields equation ().

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Equation in case of low values of polarization An other equation is applicable at low values of polarization $|\; \backslash eta\; |\; \backslash simeq\; 0V$. In such case, the dependence of current on polarization is usually linear (not logarithmic):

$i=i\_0\; \backslash frac\; \{nF\}\; \{RT\}\; \backslash Delta\; E$

This linear region is called polarization resistance due to its formal similarity to Ohm's law.

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Kinetics of corrosion The pace at which corrosion develops is determined by the kinetics of the reactions involved, hence the electrical double layer is critical.

Applying an overpotential to an electrode causes the reaction to move in one direction, away from equilibrium. Tafel's law determines the new rate, and as long as the reaction kinetics are under control, the overpotential is proportional to the log of the corrosion current.

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See also

• Overpotential
• Butler–Volmer equation
• Electrocatalyst
• Faradaic current
• Faraday's laws of electrolysis

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Further reading

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External links

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