Inside a porous electrode, an exchange between electronic current in the solid phase (i ele ) and ionic current in the liquid phase (i ion ) takes place through the net movement of charged species and can be related to the flux of charged species by Faraday's law . If the current density is denoted by i n , the molar flux can be defined by J = in F in which F is the Faraday constant. The conservation of charge is satisfied by: where a is the local surface area per volume. According to the porous electrode theory, i ion is the superficial current generated by the flow of ions in the electrolyte :