I have a Ru based electrocatalyst which I use it for Hydrogen Evolution Reaction. I have analyzed the EIS of this catalyst in the region of H-UPD and H-OPD.
In more positive regions, the nyquisit plot is showing only one line, but with shifting the potential to the negative sides, the line changed to 2 semicircles and finally to only one semicircle. In the literature below they report for HER reaction, the equivalent circuit is include R-s, R-ct, C-dl, C-pseudo of adsorbed H. Based on their report they fit, 2 semicircled plot with the attached 4 elemental equivalent circuit.
However as much as I tried to fit my 2 semicircle plot with this circuit, the fitting plot is only giving one semicircle. For making 2 semicircle I need to add one more resistance which is not in consistence with literatures for HER.
Is there anybody have experience how to change parameters to fit 2 semicircle with this equivalent circuit ?
If two semicircles are arising in your Nyquist, that means you have two time constants. The circuit they are fitting with, the simplified Randle's circuit, describes a single step charge transfer process without any diffusion limitations or any competing processes.
Thus you may wish to introduce another branch within your circuit (two examples attached).
NOTE: the circuit elements should always map back to real physical components of the system. Do not just add elements in series/parallel without an intuition as to what they represent. However the two circuits attached are common solutions to represent data with two semicircles.
Dear Hunter
Thank You for your response. What I'm also thinking, is the use of a similar circuit what you shared in first image. However, just I was wondering how other references which I already upload their names, could fit their plot by using only 4 elements.
Now I have one more question which I'll appreciate if you kindly consider it.
When I checked different literature's, in some similar cases they also observe two semicircle in H-UPD region in alkaline solution, however they never report same phenomena in acid solution which exist in my sample. In the reference below (P. 14), I read that the presence of second time constant, can be explained by presence of intermediate adsorbed compound in the surface of active material. I have a Cu element in my catalyst which I guess it should be present in the surface of Ru particles. Can I explain the similar concept here? Do you have any other suggestion for second semicircle in acid solution?
Shahram, be mindful that they have plotted Complex Admittance plots where admittance Y = 1/Z. So while two semicircles may be present in the Complex Admittance plane, a completely different shape will emerge in the Complex Impedance plane. The author is unclear, but I believe they have fit their data to impedance and not the admittance.
As for your second question, it is true that the presence of an adsorbed intermediate compound on the surface of active material can yield a second semicircle. But unfortunately I am not as savvy with this application of EIS. I use EIS for biomolecule detection, so I do not want to feed you incorrect information.
I suggest you get a copy of “Impedance Spectroscopy Theory, Experiment, and Applications“ by J. Ross MacDonald. It is an exhaustive review of EIS and likely has your answer there.
Dear Hunter
Thank you so much for the time you spent to reply my questions. You right, they showed the admittance.
For fitting in the supporting Info part, they showed the fitting of the admittance part. However there is no graphs of their impedance results.
Thank You one more time.
Ah I did not see their supporting information where they fit the circuit to the equivalent circuit. I would suggest you try using the other equivalent circuit for your data. But again make sure it has real physical meaning for your system. Also i haven’t seen your data but if you’re still not getting an adequate fit you may wish to use a constant phase element in place of your capacitance. These are often used for ’leaky capacitors’ If you are seeing a compressed semicircle rather than a perfect semicircle.
Hunter, I fitted my data (experimental results attached) by the first equivalent circuit you sent. I still wondering, if Nat. Energ. paper used admittance for fitting, so it means they fit 2 semicircle by 4 elements circuit which, whatever I tried, my data don't fit. Even in the case of Zfit software guideline, (attached in my first post), they showed 4 elements circuit for fitting of 2 semicircle. By the way their fitting parameters are so strange.
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