The activation overpotential at an electrode–electrolyte interface with respect to the electrode reaction
m is defined as:
The flux Ni of the chemical species
i (SI unit: mol/m
2) across an electrode surface depends on the current densities
im associated with the electrode reactions
m according to
Faraday’s laws of electrolysis. These can be written as:
where νi,m is the stoichiometric coefficient of species
i with respect to reaction
m (in the reductive direction), and
nm is the number of electrons transfered.
F is the
Faraday constant which is the charge on a mole of electrons (
= 96485.3365 C/mol).
Equation 3-9 and
Equation 3-10 constitute the coupling between charge balance and mass balance in the Electroanalysis interface. This coupling only applies at the electrode–electrolyte interface, which is a boundary to the domain where the Electroanalysis solves for chemical species transport.
The experimentally measurable total current I (SI unit: A) drawn at an electrode can be computed by integration of the local current density (SI unit: A/m
2) across the electrode area: