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 = j

_{a}– j_{c}= j_{e}[exp(1 – a)Fη/RT- exp(- aF/RT)]
where j

_{a}and j_{c}are the individual cathode and anode currents especttively, and j_{e}is the equilibrium current, called the exchange current density. By definition
j

_{e }= j_{ce}= j_{ae}_{}

where j

_{ce}is the equilibrium cathode current and j_{ae}, 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.