The rate of the electrochemical reactions can be described by relating the reaction rate to the activation overpotential. For an electrode reaction, with index
m, the activation overpotential, denoted
ηm, is the following:
where Eeq,m denotes the
equilibrium potential (also known as a reduction potential) for reaction
m.
In the Primary Current Distribution interface, the electrochemical reactions are assumed to be fast enough that their kinetics have negligible influence on the cell voltage. Consequently, the activation overpotential, ηm, in this physics interface is equal to zero, and the difference in potential between the electrode and the electrolyte is always equal to the equilibrium potential,
Eeq, m. This condition is implemented as the following constraint on boundaries between electrodes and electrolyte domains:
where iloc,m denotes the local charge transfer current density for reaction
m,
i0 the exchange current density,
αa the anodic transfer coefficient,
αc the cathodic charge transfer coefficient,
F Faraday’s constant, and
R the universal gas constant.