Which equation represents the Butler-Volmer equation for electrode kinetics, and what do i0, η, and α denote?

• A. i = i0 [ exp( α n F η / (R T) ) - exp( - (1-α) n F η / (R T) ) ]; i0 is the exchange current density, η is the overpotential, α is the charge transfer coefficient, n is the number of electrons, F is Faraday's constant.
• B. i = i0 [ exp( α n F η / (R T) ) + exp( - (1-α) n F η / (R T) ) ]; i0 is the exchange current density; η is potential; α is diffusion coefficient.
• C. i = i0 exp( η / (R T) ) ; i0 is activation energy; η is potential difference.
• D. i = i0 [ exp( - α n F η / (R T) ) - exp( (1-α) n F η / (R T) ) ]; α is the charge transfer coefficient.

Answer: (A)