What is the Tafel region in electrode kinetics, and what information does the Tafel slope provide?

• A. The linear region of log(i) vs E at moderate overpotentials; the slope relates to the transfer coefficient and rate constant, giving kinetic information.
• B. The diffusion-limited region; the slope indicates diffusion coefficients.
• C. The region near equilibrium potential; the slope gives equilibrium constants.
• D. The high-temperature region; the slope indicates activation energy.

Answer: (A)

The main idea being tested is that there is a region in electrode kinetics where the current responds exponentially to the applied overpotential, so a plot of log current density versus potential is a straight line. This is the Tafel region, where the current is controlled by the rate of the electron-transfer step rather than by mass transport. The slope of that straight line, the Tafel slope, carries kinetic information: it is linked to the transfer coefficient and the number of electrons involved in the rate-determining step. For a simple one-electron process, the Tafel slope b equals (2.303 RT)/(α n F). That means by measuring the slope you gain insight into how effectively electrons are transferred (through α and n) and thus into the mechanism. A smaller slope indicates faster, more favorable kinetics (larger α n), while a larger slope suggests slower kinetics. This region is distinct from diffusion-limited behavior, which dominates at high overpotentials when mass transport caps the current, and from near-equilibrium behavior, where the relationship between i and E is not the same linear log i vs E form.