Dear Maryam,

There are few steps that may help clarify these data.

First, the impedance of the reference electrode (assuming its a glass-frit Ag/AgCl capillary electrode) likely restricts the reliable frequency range to

Dear Matthew,

Thank you for your comments and suggestions. I'm very new to this field so I try to respond to your comments to the best of my knowledge.

The reference electrode that I'm using is a porous Teflon frit Ag/AgCl, 4mm glass tube.

I have already tried eliminating the high frequency data and using Ag/Ag+ (acetonitrile) reference electrode but still these don't fix the problem. Also I keep getting the same semi circle for each sample so I don't think the high frequency data is affected by noise.

I also tried using a glassy carbon electrode as my WE, but the shape of the Nyquist plot is the same as bare ITO, meaning that it consists of a semicircle at high frequency and very steep straight light for the mid and low frequencies. The slope of the line is ranging from 5-10 depending on the reference electrode that I use (Ag+ gives a bigger slope).

The straight line starts at around 3KHz and I don't understand it.

As for the DC voltage, I use 0V (vs Ref). I have tried two DC potentials of 0.5V, 0.1V but they don't change the overall shape of the Nyquist plot.

I'm especially looking for cases that can give this straight line for mid and high frequencies. For example, if my reference electrodes are bad, could they cause such a capacitive behavior?

First of all, using a DC potential = 0.0 V vs REF is a common mistake. This value is arbitrary (think to the situation when you change it with a different REF).

The recommended starting value is DC potential = OCP. This way you may expect to initiate/excite a electrochemical process. A Randles circuit is a model for the electrochemical process at the working electrode (charging of the double layer and the resistance to charge transfer).

If your interest is only to study bare ITO try a “dry” experiment (a four points measurement of the surface resistance of ITO layer).

Add more clear images of your results.

Dear Maryam Shahi ,

clean[1] (or replace) your WE, e.g. use a new ITO; then follow Cornel Radu (working) lab.-notes, above.

1. The current surface of the (used) WE is, already, modified, from the previous (random) DC potential polarization(s).

It is a joyful thing when the researcher tell that she (he) is new to this field of question.

I wish Dr. Maryam Shahi all the best of luck, success and progress

Best regards

Dear Cornel Radu

Thanks for the comments. I used DC=OCP as you suggested, however, my data can not be fitted with a Randles circuit. In the next step, I will have a P3HT film on my ITO and I'm more interested in measuring the impedance of the film, so I'm not only looking for a way to study bare ITO.

I have attached a better image of my data, sorry if the previous one was has low quality.

Dear Ioannis Samaras

Thanks for the suggestions, but I have done the cleaning and using a new ITO sample many times. No success!

Is it possible that a reference electrode with high impedance gives rise to such a capacitive behaviour at low frequencies? Or even a bad counter electrode?

Thank you Mohammed Hussein j. H. Al'Atia for your kind wishes.

Select only the points between 100 kHz ...10 kHz to be fitted.

Are you aware of the resistance/impedance of ITO layer itself? Depending on the configuration of contacts, the surface of ITO electrode is not polarised at the same uniform value.

Dear Maryam Shahi ,

the last PEIS spectrum, now, at VDC=OCP, is, nearly, good[1].

Your reference electrode (for high impedance) is not bad.

Your counter electrode is not bad, since: VDC=OCP.

Also, the PEIS spectra will be, even, better, if you could:

1)apply a lower VAC value (VAC

Kindly check https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1302882/

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