An equation for the rate of an electrochemical reaction; it describes the current density at an electrode in terms of the overpotential. The Butler-Volmer equation is given by:
j = ja – jc = je[exp(1 – a)Fη/RT- exp(- aF/RT)]
where ja and jc are the individual cathode and anode currents especttively, and je is the equilibrium current, called the exchange current density. By definition
je = jce = jae
where jce is the equilibrium cathode current and jae, is the equilibrium anode current. F is the Faraday constant, η is the overpotential, R is the gas constant, T is the thermodynamic temperature, and a is a quantity called the transfer coefficient.