As you change the position of electrode inside the electrolyte, you expose different number of active sites available for reaction. More the number of active sites exposed for reaction, lesser should be the polarization resistance. so need to be careful about relative position of electrode and electrolyte.

Use reference electrode and locate it in close proximity to working electrode. It will eliminate more than 95% of electrolyte resistance and 100% of counter electrode influence.

Use a large-area counter electrode (50-100 times as large as WE) and fix the distance between WE and CE. Rs reflects the solution IR drop, which has Little relation to the property of WE..

I suggest

Fixing the position of the working electrode inside the electrolyte soution is a crucial factor and even little differences from one measurement to another can give you differences in the recorded Rs values.

I suggest to restrict the movement of electrode. Then, you avoid changes in the measured free charge  (rs) along time.

Thank you very much for your suggestions.

1. I am using an electrode holder where position of all three electrodes are fixed. So, I think electrode distance should not affect my results (Rs (solution resistance) and Rct (Charge transfer resistance))

2. I am using a foil (platinum) as counter electrode whose area is multifold to my modified electrodes (acting as working electrodes). So, it seems platinum area should not be a problem.

Still my Rs is varying for my each modified electrode, which ideally should be same since distance between working, reference  and counter is fixed.

Also I am not clear with my Rct results.

Kindly explain.

@ Khushbhu What is know is Rs is solution resistance between modified and counter electrode and diameter of circle defines Rct (Charge transfer resistance)

Since distance between working (modified) and counter is same, hence Rs should remain same and also Rct is not justifiable.

Could you explain more.

In my opinion, your results show that the frequency range is too small to find Rs and Rct accurately. I think you need to start at a higher frequency (10 kHz), and extend the measurement to lower frequencies (0.03 Hz or even lower). Also test whether your electrode has achieved steady state, and compare the fitted curves with the measured curves, in both Nyquist and Bode plots.

Gentle guy

Keep the ref and counter electrode as close as possible almost at the same possition (mark the cell with a magic marker) after the system under study has reached equilibrium state (~ 10 minutes; very important). Use a large surface area for your measurements (not small EIS cell if you can). But again do'nt worry much since impedance is only to give you an idea what is going on with your system. The numbers will always vary a little bit in duplicate or repeated measurements.

To be more precise please follow the phase angle shift around 104 Hz and it's movement from high to low frequency and back to high frequency (very critical point to observe). That's the cut-point where the system under study has failed (corrosion has already inset).

Best of luck

Correction in the above suggestion 

Please folfow the minimum phase angle shift around 102 Hz in Bode phase angle plot. and it's movement from high to low frequency and back to high frequency (very critical point to observe). That's the cut-point where the system under study has failed (corrosion has already inset). It might take hours/months/semesters or even years depending on the quality of the system whether it is a fiber-reinforced epoxy composites, nano-structured coatings or barrier coatings (inhibitor-primed and top coated) among others.

Record the EIS spectrum from 104 Hz to 0.1 Hz and is enough for any general work. You may go down to 0.03 but it will take hours and hours or rather eons to record the spectrum. Make life easier for you when you are under the pressure for a quick solution to your research problem (use applied common sense for all of your research). Also remember it is very difficult to detect extremely low current; you will need expensive instrumentation.

A suggestion, try to utilize a solid ref electrode {I guess what I used was a fused Ag/AgCl for my all DOD and university projects; a naval lab gave me that ??? I do'nt have any details on that) and it gave me less headache. You will have a stable EIS spectrum. Also I always used a graphite stick as a counter electrode. Change it to a new one every month.

Ionic solution science is very tricky so be careful with your glass ref electrode and all of your ionic solutions chemistry.

Enjoy your day

Bless you