Discussion Forum

Hi

Your questions is a bit open. Which particular issues do you want to study? If you want to find the potential/current distribution, that is an electrostatics problem. In the electrochemistry module you must define the electrode reactions etc. You must tell more to get any answer.

BR
Lasse

Hi Lasse,

I am trying to model the electrochemical system as descirbed in my previous post. I have defined salt concentration, conductivity, diffusivity of the electrolyte. Now, I want to get the response of the system in the form of electrolyte potential vs time at the anode when a current pulse of 0.1mA is passed for 10s.

I have tried using electrochemistry module but I am not able to find a module which has time dependent variation of potential. In reality the equation looks like as follows-

E = I * (Rs + Rp(1-exp(-t/(Rp*C)))

Also, I tried using lithium ion battery module but it is showing me error of not being able to find singular matrix as the initial values are incorrect. (My initial values are 0 for electrolyte and electric potential, 100 for concentration)

Can I model galvanostatic transients and get time dependent potential response? Or is it possible to try to enter equations manually?

Thanks
Abhishek

Choose Electric Currents module and time dependent study. You will get a straight line according to the Ohm's law because the time constant Rp*C is so small that you won't see the transient, it dies in microseconds.

BR
Lasse

Hi Lasse,

During experiments, when I applied a cathodic current pulse to my system, I observed a decay in potential before it stabilizes (just as the plot of the equation mentioned in the previous post will look like). I want to observe that decay in my model.

I tried using electric currents module but I couldn't find a boundary surface where I can define my anodic reaction. Also, I don't know what should be defined as reference impedance.

Which module should I choose to get the transient behavior?

Thanks
Abhishek

OK. Therefore I asked first what your problem is. In a 0.1 M NaCl solution the only thing that would happen during a cathodic pulse is water splitting, i.e. evolution of hydrogen gas (or oxygen evolution during an anodic pulse). In Electric Currents model you certainly cannot define an electrode reaction. But water splitting is a special type of a reaction because the "reactant" water is abundant. The problem you pose is a tricky one and how the capacitance steps in the model is difficult to see. At any solid electrode, normal electrochemical reactions are not affected by electrode capacitance that is typically some tens of nF, making the charging of the electrical double layer so fast that it is hidden below the rise-time of the current source.

Anyway, the module you would need is Nernst-Planck Equations but I am not able to tell how the reaction boundary condition should be set in the case of water splitting. Perhaps there is some model in the website knowledge base.

BR
Lasse

Hi Lasse,

Thanks a lot for your reply.

For my system, as observed, When I had applied a cathodic current pulse, an anodic reaction happened at carbon steel which is oxidation of iron and at Platinum counter electrode, the current just flows in (water splitting might happen but for applied currents it was negligible so, I am ready to ignore the very small amount of reaction happening at counter electrode). Also, the decay potential is observed at anode which is the working electrode.

Like you mentioned, I am trying to model using Tertiary current distribution which has Nernst Planck equation, but I am getting the following error-

"Failed to find a solution.
Singular matrix.

There are 1 void equations (empty rows in matrix) for the variable comp1.tcdee.phisext_eebii2.
at coordinates: (0,0,0), ...
and similarly for the degrees of freedom (empty columns in matrix).
Returned solution is not converged."

Please help me as to how to resolve this error?

Thanks again!
Abhishek

You must probably ground the other (Pt) electrode. Current cannot be used twice as the BC because it makes a singular matrix.

Lasse

Hi Lasse,

I am using three electrode system in my experiments. Carbon steel as working electrode, Platinum as the counter electrode and Ag/AgCl sat. as the reference electrode.

In my model, there is the only working electrode, counter electrode, and electrolyte.

If I ground my platinum electrode, and not use current as BC like you mentioned, then where should I be telling my model regarding the applied current pulse?

I think for the modeling my experiment, I need to carry out parametric sweep on time and current so that I can vary them both but now where to define the constant current pulse applied using CE?

Thanks
Abhishek

Current pulse at the steel electrode, ground on platinum.

Hi Lasse,

Thanks for your suggestion. I tried it. I couldn't ground platinum as such, so, I defined it as electrode surface and made all potentials and participating electrons as zero.

For steel surface, I tried defining current in terms of total current and average current density but they are still not working. I am getting the following error-

" - Feature: Time-Dependent Solver 1 (sol1/t1)
Undefined value found.
- Detail: Undefined value found in the stiffness matrix.
There are 857 equations giving NaN/Inf in the matrix rows for the variable comp1.c1.
at coordinates: (7.9,2.06346,1.92024), (7.9,3.17308,1.25), (7.9,3.46284,1.38804), (7.9,3.28358,1.71594), (7.9,3.59833,1.76209), ...
There are 857 equations giving NaN/Inf in the matrix rows for the variable comp1.c2.
at coordinates: (7.9,2.06346,1.92024), (7.9,3.17308,1.25), (7.9,3.46284,1.38804), (7.9,3.28358,1.71594), (7.9,3.59833,1.76209), ...

There are 857 degrees of freedom giving NaN/Inf in the matrix columns for the variable comp1.tcdee.iloc_lm_er1.
at coordinates: (7.9,2.06346,1.92024), (7.9,3.17308,1.25), (7.9,3.46284,1.38804), (7.9,3.28358,1.71594), (7.9,3.59833,1.76209), .."

1) In one of your replies, you mentioned that defining current conditions at two places brings in the singular matrix. Aren't we doing the same?
2) If I define my current at steel electrode, then there is no point of defining the kinetics of iron oxidation but this changes my system. May i ask if there is any other way to resolve this?

Thanks a lot!
Abhishek