Define exchange current density i0 and describe how it is determined experimentally.

• A. It is the current at which forward and reverse rates are equal (η = 0).
• B. It is the current at which forward and reverse rates are equal (η = 0); determined from extrapolation of Tafel plots or impedance measurements near equilibrium.
• C. It is the current measured at the onset of the first reduction peak.
• D. It is the current corresponding to the open-circuit condition.

Answer: (B)