Discussion Forum

Dear community,

I am trying to build a simulation of an Ionic Polymer Metal Composite (IPMC) strip sensor, a thin membrane of polymer ionic on witch too thin gold electrodes are plated. It is a 2D time depending model, which output the difference in potential among the surface according to the force applied on its tip.

I already sucdeed (I think, the result looks consistent) to calculate the concentration of cation inside the sample, coupling the solid mechanic module and the transport of diluted species (tds) interface. Now using this concentration, I would like to know the difference in potential coupling the tds and the electrostatics interface (es).
For that I added to the es a surface charge condition in the entire domain:
rho = -z_cation*e*NA_const*C_cation+z_a*e*NA_const*C_0
Where C_0 is the initial concentration, anion being fixed, C_cation the concentration in cation from the tds and Z_cation and Z_anion the valence of cation and anion respectively.
I also added a surface charge density condition on the electrode, because I assume the ipmc strip works as a diffuse double layer:
rho= -epsilon0_const*deltaV/lambdaS
Where deltaV is the difference in potential between the metal phase and the stern compact Layer and lambdaS the Stern layer thickness.

At this point my problem is the following:
The model doesn't compute, "Failed to find consistent initial value, last time step is not converged". I have ever had a look at some other model from this forum and the COMSOL model Gallery (most of time the output is the ionic concentration according an input voltage using the Nerst-Planck and Poisson equations). I would like to do the reverse and I think I could use those equations, couldn’t I? I can't find the problem.
I am a beginner in COMSOL and in electrochemistry so maybe I don’t use these equations correctly. If someone could somehow give me some food for thought...

Thank you very much for any help.

Marie