If a resistive film forms on the interface between an electrode and an electrolyte, this results in additional potential losses. To model a film resistance, an extra dependent variable for the potential drop over the film, Δs,film, is introduced. The governing equation is:

Where Rfilm (SI unit: ohm·m2) is a generalized film resistance and itot the sum of all currents over the interface. The activation overpotentials, ηm, for all occurring electrode reactions on the electrode with the film receive an extra potential contribution due to the film resistance according to:

where s0 is the reference/initial film thickness, Δs the electrode thickness change, and σfilm the conductivity (S/m) of the film.