The activation overpotential at an electrode–electrolyte interface with respect to the electrode reaction m is defined as:

Subject to the approximation of constant electrolyte potential (ϕl = 0), this equation reduces to:

The activation overpotential is independent of the properties of the adjacent electrolyte. It only depends on the applied electrode potential ϕs and the equilibrium potential of the redox couple, where both are measured against a common reference potential.

The current density due to an electrode reaction at a point on an electrode surface is computed using an electrochemical rate expression. For most practical electroanalytical applications, the rate depends on the local concentration which varies during the study, and so the most relevant expression is The Electroanalytical Butler–Volmer Equation.

The flux Ni of the chemical species i (SI unit: mol/m2) 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 transferred electrons. F is the Faraday constant, which is the charge on a mole of electrons (96485.3365 C/mol).

Equation 4-13 and Equation 4-14 constitute the coupling between charge balance and mass balance. This coupling only applies at the electrode–electrolyte interface, which is a boundary to the domain where the electroanalysis charge conservation model solves for chemical species transport.

The total current density is the sum of faradaic (electrode reaction) components and nonfaradaic components (inf) such as current due to Double-Layer Capacitance:

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/m2) across the electrode area: