Tafelparameter

Tafelparameter, in electrochemistry, refers to the parameters that appear in the Tafel equation describing how the overpotential of an electrode reaction relates to the current density under activation-controlled conditions. The term is commonly used to denote the Tafel slope and the exchange current density, which together characterize the kinetics of an electrode process.

The Tafel equation expresses a linear relationship between overpotential η and the logarithm of current density i in a region where the reaction is activation-controlled. One common form is η = a + b log10(i), where b is the Tafel slope (the change in overpotential per decade change in current) and a is an intercept that depends on the exchange current density i0. The exchange current density i0 is defined as the current at which η = 0 and reflects the intrinsic rate of the redox couple at equilibrium. The slope b is related to fundamental constants by b = (2.303 R T)/(α n F) for a charge-transfer step with transfer coefficient α and n electrons transferred, at temperature T.

In practice, a Tafel plot (η versus log10 i) yields a straight line in the activation-controlled region. From its slope and intercept, one can derive i0 and assess the mechanism and efficiency of an electrode catalyst. Temperature, pH, electrode material, and the exact reaction step influence the Tafel parameters, which are frequently used to compare electrocatalysts and to infer the number of electrons involved in the rate-determining step.

Limitations include the validity of the activation-control assumption, which may break down at high current densities due to mass transport and double-layer effects.