Tafel Overpotential
Calculate the activation overpotential by the Tafel equation, η = a + b × log₁₀(i), from the Tafel constant a (V), the Tafel slope b (V/decade) and the current density i. The result, in volts, is the overpotential — how far an electrode's potential departs from equilibrium — needed to sustain a given current density in an activation-controlled electrochemical reaction. The Tafel relation is central to electrode kinetics, corrosion (extrapolation to obtain the corrosion current) and electrolysis. Enter the Tafel constant, slope and current density.
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Tafel overpotential
For an electrochemical reaction to run at an appreciable rate, the electrode has to be polarised beyond its equilibrium potential — that excess is the overpotential (or overvoltage) η. When the reaction is under activation kinetics control (the energy barrier of charge transfer), the relationship between overpotential and current density follows the Tafel equation: η = a + b × log₁₀(i). Here b is the Tafel slope (in volts per decade of current — how far the overpotential rises when the current increases 10×), and a is the constant that embeds the exchange current density. The logarithmic form reflects the fact that current grows exponentially with overpotential (the Butler-Volmer relationship in the Tafel region). This behaviour is the key to electrode kinetics and has important practical uses: in corrosion, extrapolating the anodic and cathodic Tafel lines to their intersection yields the corrosion current and hence the corrosion rate; in electrolysis, Tafel slopes tell how efficient an electrocatalyst is (the smaller b, the better). Enter the Tafel constant, the slope and the current density.
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