Dear Dr. Firoz Khan,

I suggest you to have a look at the following, interesting technical notes:

- https://www.palmsenscorrosion.com/knowledgebase_tags/nyquist-plot/

- https://www.gamry.com/application-notes/EIS/basics-of-electrochemical-impedance-spectroscopy/

https://www.metrohm.com/en-us/applications/AN-EIS-001

…….from 001 to 007

https://www.metrohm.com/en-us/applications/AN-EIS-007

Enjoy reading and my best regards, Pierluigi Traverso.

The semicircle at higher frequencies is related to the kinetics of the redox reaction (depends on the rate constant and concentration of redox species). The LF straight line corresponds to the mass transfer impedance: Z=sigma'/(sqrt(j*omega), where sigma' depends on the concentration and diffusion coefficient and omega is the angular frequency. Its length depends on sigma' and is longer including lower frequencies. See my book on EIS.

Dear Andrzej Lasia, many thanks for your reply. Could you please explain in more details. For example, in a graph, the straight part extent (case 1)up to 1k Ohm (Z real), and in another case (case 2) it extends up to just 25 Ohm. Then which has more diffusion coefficient, case 1, or case 2?

Is the straight part (LF region) influenced by series resistance (Rs) or (charge transfer resistance (Rct)?

What are the frequencies corresponding to these cases (end of the line)?

Andrzej Lasia the frequency range is 50 mHz to 1 MHz

If in both cases the lowest frequency was the same (50 mHz) it means that in the first case sqrt(D)*C was much smaller. As I mentioned before Z=sigma'/(sqrt(j*omega) and sigma' ~1/(sqrt(D)*C(0). The smaller the concentration (and D_ the larger Z. See my book on EIS for details.

Andrzej Lasia could you please send me a soft copy of your book?

No, I cannot. But you will find it in the library. Moreover, it was selected this year for free download by Springer, between 500 other books.

Andrzej Lasia please send a link then