Learning Forum

[RK]

Q: In Fluent, why does every electrochemical reaction have an anodic and cathodic transfer coefficient, even though it only occurs on either the cathode or the anode? Can I enter 0 for the anodic coefficient if the reaction occurs on the cathode? What do these coefficients represent?.A: The handling of the anodic and cathodic transfer coefficients is analogous to forward and reverse reaction rate coefficients. Every reaction has them, but the rate in one direction may be negligible in certain instances. Each reaction will have an equilibrium at the conditions, so both coefficients are needed to get the right net rate. It is the sign of the overpotential (voltage) that negates the rate in one or the other direction, depending on whether the reaction is defined on the cathode or anode. From equation 7-84 of Fluent 2020 R1 Theory guide, you can see using a zero coefficient will not necessarily make the rate for that direction zero. (Charge transfer coefficient x overpotential) determines the energy input (positive or negative) required for the reaction to occur (similar to activation energy for finite rate reactions). Setting a zero activation energy produces a reaction that has no energy barrier. This is the case here too -- AND the rate in one direction will now be independent of overpotential! This could make the rate for that direction even larger than it would have been with a non-zero coefficient, and will definitely result in an incorrect net rate.n