The first point of your Nyquist plot gives you the solution resistance (Rs). However, the higher frequency points (the first ones) of any Nyquist plot are susceptible to changes if you have electrical contacts problems. In some cases, inductive behavior can be observed in this region. Try to perform your experiments with polished electric contacts and inside a Faraday cage.

Here you can find more information: https://www.hioki.cn/ckeditor_assets/attachments/812/.pdf.

I think this happens due to the different resistance of each electrode after each modification step

Are you sure that the position of your electrode are always in the same place? If the answer is yes, then the changes in the high frequency limite of your Nyquist diagrams is related to a different serial resistance associated to your surface modification.

However is mandatory that your work electrode keeps always at the same position and distance concerning the reference electrode. Other way, the difference in the impedance at high frequency is related to the ohmic response.

Dear Elnaz Zeynaloo ,

fine[1], do not worry for the different ('starting' Zreal) points; it is a completely normal Nyquist-pattern for your case study.

Adding[2] a new (add-on 'painting') film(s)[3] is a legal Z-additive[4] ('+') property, apparently ... like (total sample's) thickness, that, evidently, has, an additive[5] ('+') property (in thickness-set[5]).

1. Your Z-measurements look 'fine', having 'fine' Z-additive properties; however, we, usually, apply the same X-Y scaling, in order to help, more, in a better Z-understanding (finding hints for an EC-simulation[6]) of your Nyquist plot pattern(s).

2. a new good films (without major imperfections, commonly, pinholes, naked edges, etc.) on the surface of the previous film(s)//substrate.

3. a non-superconducting covering film.

4. a ('+') Z-additive in Z-set (or common resistance-set) property.

5. similar, but another (to Z-set) set (e.g. the thickness' set), additive ('+') property.

6. Equivalent Circuit Modeling in EIS https://www.gamry.com/assets/Application-Notes/Equivalent-Circuit-Modeling-in-EIS.pdf

Dear Elnaz,

welcome!

I came late to this question. After each electrode treatment you change the interface and so the electrode properties. So if you start the plot at the same frequency you will get normally another starting point. This is quite natural.

However, in your plots the low frequency part is missed for all surface conditions.

So, to complete your plots you have to try to measure Z at lower frequencies.

Your plot points out the equivalent circuit of your electrode contains two CR circuits in series and R is connected in parallel with C.

Best wishes