Unless there is some redox active species in the electrolyte Rct is usually the resistance to charge transfer that occurs during solvent breakdown. Oxidative and reductive water splitting is the most common example. The EDCL charges up until the solvent breakdown voltage is reached then charge stops being stored in the double layer and leaves the electrode surface, reducing the solvent or vice versa for oxidation.The potential at which water splitting occurs is material dependent. For instance water reduction on platinum occurs at a moderate potential (-0.6 V vs Ag/AgCl). Contrast this with boron doped diamond where water reduction occurs at around -2 V.  So the Rct for Pt will be lower than BDD for water electrolyte solutions. The consequence of all this is that if you use a low amplitude (10 mV for instance) for you EIS and operate around 0 V you should get no where near water splitting voltages and Rct should appear to be very large. If you used an amplitude of 2 V, considerable water splitting would occur and Rct would appear to be much lower. You can see from this that Rct is not a true resister as it clearly has some voltage dependence. 

The intercept on the real axis will give you the DC resistance. If there is a semi circle it is not a pure EDLC.

the Pure EDLC capacitor (idealy a single electrode) is a mathematical prototype. In practice, you have to realize certain approximations to the prototypes. If you compare/contrast measurements between 2 and 3 electrodes will help you to identify better the approximation meaning to the prototype models (Pure EDLC capacitor etc.).

HI all

Thanks for your comments. But, When I see supercapacitors made of Activated carbon in the literture, I still see a semi-circle in the higher frequency region. Its should be pure EDLC in case of Activated carbon-based electrodes,,,,, SO what is the reason for this semicircle.

Regards

Basically the EDLC is a R and C combination. Technically it is impossible to remove the R. If you look at it purely from the R and C perspective depending on the interfaces and the no of interfaces you create or as according to Samaras

"If you compare/contrast measurements between 2 and 3 electrodes will help you to identify better the approximation meaning to the prototype models"

the semi circle will be there. Attributing it to charge transfer is a matter of interpretation.

Hi Deepak,

ideal EDLC(s) is formed from an ionic carrier solvent cluster type. Each one ion-solvent  group type (say j) corresponds to an EDLC having a single time constant Tj (=RjCj=1/Fj). All EDLC(s) classes appear up to a Fj

Hi Dr. Samaras

Thank you! Our high freequency starts from 100KHz 

dear Deepak,

following your reply

"Our high frequency starts from 100KHz"

then I suggest that your RjCj at high-F (>>1KHz) is due to an electronic-type carriers inside the solid part of your semi-electrode. This is typical for composite materials.

If you form another sample of the (Paste Powder) material on a glass substrate, then you will get results with the "same information" in high-F, using 4 electrodes.

Also,

1)your RjCj loop is a single-one or more?

2) do you get an RL loop at (if available) the very high-F (near of after 1MHz) ?

Hi Dr. Samaras

1) I have a single loop 2)I don't have  values at 1MHz (I have never done test at that high frequency

Thank you