Discussion Forum

Hello,

I am trying to simulate two electrodes in generator-collector mode using the Secondary Current Distribution coupled with Transport of Diluted Species physics in version 5.4. Currently, I am just simulating a ferri/ferrocyanide redox couple with equal concentration (1 mM) of each in solution. I am trying a time-dependent sweep from 0 to 1 s in 0.01 s steps. When simulating with the SCD+TDS with Butler-Volmer kinetics, I am getting unrealistic concentrations (both negative and unrealistically large positive concentrations), even at t = 0 right at the electrode boundaries. I have read through the support documentation (https://www.comsol.com/support/knowledgebase/952) and tried some of the suggestions, such as implementing a smoothed step function and refining the mesh in the area of interest, but that has not resolved the problem. Another suggestion within that documentation was addressing a constant sink term that can continue to consume species as the concentration approaches 0, but I am not sure how to implement such a constriant.

I am able to simulate the same system fine using the Electroanalysis module with the Electroanlytical Butler-Volmer kinetics. However, I would like to use the SCD+TDS because eventually the electric field in the solution will become relevant to the project. One thing that I noticed is that for the SCD+TDS, the reaction rate under the "Reaction Coefficient" subnode is defined as , whereas for the Electroanalysis Electrode Reaction subnode, it is defined as . Both define Ox species with negative and Red species with positive . At first glance, I couldn't find any differences in the other domain equations used by the models. Is this a typo, or are there differences between the models and how they define the current direction? Could this be a possible source for the issue with the negative concentration?

Thank you for reading.