TafelSlope

The Tafel slope is a parameter in electrochemistry that relates the overpotential of an electrode reaction to the logarithm of the current density. It is obtained from a Tafel plot, which graphs the overpotential η against log10(i) in the region where reaction kinetics dominate. The slope provides insight into the kinetics of the charge-transfer step.

In its standard form, the Tafel equation is η = a + b log10(i) or equivalently η = (2.303 RT)/(α n F) log10(i/i0), where α is the charge-transfer coefficient, n is the number of electrons transferred in the rate-determining step, F is Faraday’s constant, R is the gas constant, T is temperature, and i0 is the exchange current density. The proportionality constant b, the Tafel slope, is therefore b = (2.303 RT)/(α n F). At room temperature (≈298 K), b ≈ 0.0591/(α n) volts per decade of current.

The slope informs about the kinetics and mechanisms of the electrochemical reaction. From b, the product αn can be inferred, since αn = (2.303 RT)/(b F). Smaller slopes generally indicate faster kinetics for a given elementary step, while different slopes on anodic and cathodic branches can suggest different rate-determining steps. The Tafel slope is widely used to compare catalysts and to interpret mechanisms in reactions such as hydrogen evolution, oxygen evolution, oxygen reduction, and other electrocatalytic processes.

Limitations include deviations due to mass transport limitations, changes in surface coverage, or multi-step reaction pathways, which can distort the kinetic region where a true Tafel slope applies.