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Chronoamperometry Measuring Current due to Electrode-Electrolyte Diffusion at Electrode Surface
Posted Aug 2, 2020, 9:30 p.m. EDT Battery Design, Simulation Apps, Parameters, Variables, & Functions Version 5.2a 3 Replies
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Good Evening,
I am currently trying to perform a Cottrell Experiment by simulating electrolyte diffusing into our working electrode of interest which in this specific case is a 3D thin film. I used COMSOL's "1D Thin Layer Chronoamperometry" from the Electrochemistry Module Application Libary Module as a basis when constructing my 3D simulation. The goal is to build a model that generates current which matches up with the current calculated from the Cottrell Equation.
However, my results show that the Cottrell Equation predicts higher currents than what is being measured at short times and the resulting concentration profile definitely does not match was is expected in this type of experiment.
My principle investigator predicts the deviation could be a result of how I set up one of my boundary conditions. However, still learning how the Electronanalysis interface works is making it difficult to figure out if I set up anything incorrectly or if I am missing anything entirely from my simulation settings. I am using an Academic Class Kit License Version 5.2a.
I have attached screenshots below if they help clarify as the COMSOL file itself was too large to attach.
Any pointers or assistance would be grealty appreciated! Thank you very much for your time.
Joe Gonzales ; University of California, Irvine
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Hi
Electrochemical simulations are a bit tricky because a very dense mesh must be used close to the electrode. I always use a geometric series mesh. The Cottrell equation as such is also a bit 'funny' because it predicts infinite current at t = 0. This, naturally, cannot be true because there always is some kinetics, and ohmic drop between the working and reference electrode gives
I(t=0) = δE/Rs
where δE is the step height and Rs is the resistance, probably less than 1 Ω.
I can see from your picture that the mesh far too sparse at the electrode. In 3D simulations the electrode corners/edges need to be meshed also very densely. Anyway, I would say that a perfect match with the Cottrell equation is quite difficult to reach because current is the conc. gradient at the electrode and numerical gradients are prone to errors.
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Lasse,
Thank very much for your reply, I greatly appreciate it. I will definitely move forward with altering my mesh to be more dense to improve the solutions.
My only follow-up question is I was reading the Electrochemistry User's Guide and the "Meshing Advice" section stated that a "Swept" mesh is the best for thin 3D geometries such as mine. I am just wondering if this swept mesh is related or similar to the geomeric series mesh you mentioned?
Thank you again, this is very help!
Best Regards,
Joe Gonzales ; Univeristy of Californira, Irvine
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Swept mesh is OK provided that it is vertically dense enough. It copies the element patter on the starting surface through a domain and there you can define the pattern in the depth direction (vertical).