It is fine to use different bulk concentrations, as in the equation you write. However, if these are different for the two species of the redox couple, then in this case you must make sure that the “Equilibrium potential” against which the overpotential is measured is corrected (manually) from the formal electrode potential, to account for the unequal concentrations according to the Nernst equation. If you use a common reference concentration as we suggest here, then in the model you can define the equilibrium potential as the formal electrode potential directly. We recommend the latter method because there is less possibility for mistake.

The advantage of Secondary Current Distribution + Transport of Diluted Species over Tertiary Current Distribution, Nernst-Planck is that the former method usually gives a much more linear equation set. Only mass transport of diluted reactants is considered, rather than mass transport of all species including inert electrolyte. If you need advice on setting up a simple model, you can contact us at http://www.comsol.com/support/.

Edmund Dickinson

COMSOL UK

Although I am an electrochemist, I cannot quite understand the implementation of the tertiary current distribution in Comsol. The current-overpotential equation is usually written as

i/i0 = (C_R/C_R,bulk)*exp(a*n*f*eta) – (C_O/C_O,bulk)*exp((a-1)*n*f*eta)

Above you write “It is essential that the reference concentration is the same for all species involved in a reaction. This ensures that at zero current density (equilibrium) the overpotential obeys the thermodynamic Nernst equation”. At zero current overpotential is zero, and the surface concentrations are equal to bulk ones. Hence, I do not get this formulation or statement.

In the wire electrode example, concentration polarizarion is taken into account via a separate “Transport of dilutes species” node which is then coupled at the electrode surface with the local current density. Is it thus possible to do this with a single “Tertiary current distribution” node instead, using concentration dependent kinetics in the electrode reaction? I have not succeeded in doing this with a simple 1D model either. Either the model does not nothing or cannot find initial values.

Best regards

Lasse Murtomäki