I found this behaviour between 10^ -1 and 10^ -2 Hz and Iam not able to find an equivalent circuit neither a physical explanation. Any news I´ll be in touch

Hi!

Is the response you obtain at higher frequencies reproducible?

I am not completely sure, but I observed this behavior on monolayer covered mercury microelectrodes where an extremely high resistance is present even at these high frequencies.

Dear Lucia

This behavior appeared for all samples ( about 120 samples). Reproducability was good. I cant understand the physical concept of this kind of behavior!!!

Can you give me e brief description of you samples?

Have you tried to vary the applied base potential?

I need some more information that you can send to me by e-mail if you prefer.

Tanx Dear Lucia

I will send it to you as soon as possible.

Dear Hossain

u can check my related publications in this case. please check my profile.

fondly

I get a similar behaviour using an ACM potentiostat on organic coatings. Over 10^4Hz frequency my data cannot be analyzed. I believe it's an artifact of the potentiostat as it is so reproducible no matter what coatings I'm using and might be related to the potentiostat's recording limits.

Have you tried to do measurements on quite larger area? This would help you to distinguish between real physical signal and artifacts due to the input impedance of your instrumentation...

I am agree with Dang Dan. High frequency EIS measurements (above 20kHz) often comes from reference electrodes.

Hi Hossain

With coatings you can divide your bode plot into four regions.At higher freq it is the electrolyte,2nd part is the double layer, 3rd part is your coating and 4th ,the substrate. There is no fixed freq range, its your choice, generally I divide it into four equal parts. So with this, I believe, the higher phase angle indicates that your coating is intact and behaving as a capacitor. There are two time constants I can see that the phase angle at lower freq is coming down, meaning to say that your coating is breaking down. If you have bode plots of your coupons immersed at different times say 1h, 24h etc, you can see what is happening to your coating, the impedance modulus values and make sure to corellat it with your fitting parametrs or data.In short, pase angle at -90 shows your coating is intact. But pl. remeber, all these tests are only qualitative and just give you information about how your coating is.So likewise choose a suitable eq. circuit.Hope this helped.

Dear Paola

The EIS measurements done at un-mounted surface area equal to 1 cm2 and 4 cm2 But the results (phase angle behavior) remain the same.

Dear amitha

Tanx for your comment. I dont have problem with fitting and selecting equivalent circuit. As you hinted for intact and capacitor behavior of coatings, phase angle of about -90 is a normal.

Dear Hossein,

I had similar measurements too. A phase angle below -90° would mean you have negative real parts and the behaviour of a current-to-voltage-converter, which in my opinion has no physical meaning and can only be caused by an electrically active device/sample. In my case, the reason for it was, that the impedance analyzer got out of measurement range. Which are the particular impedance values you had and for which impedance range did you calibrate?

I think overworking your calibration procedure may help.

Show a Nyquist representation, please. Equivalent shemes can have a Negative real conductivities at Adsorption Admittances.

In detail.It is interesting phenomenon. There is a great deal of truth in what Anna says about negative real immitance component . I have constructed impedance theory of adsorption oxidation of organic substance. One of special analysing cases is theoretic phenomenon of sinchronous negative real component and positive image (capacity) component. It is to be looked intently Cao's theory (Electrochimica Acta.Vol.35.No 5..P 831- 844). Is phenomen there?

Dear Professor Elkin and Professor Punckt

I attached the Nyquist plot (+ zoom in high frequency region) for one of the samples:

Zoom in high frequency region

If your system is well calibrated, then you have set a good problem, dear Hossein!

In the case of a coated material I agree with the amitha rani singh' opinion in the sense that a higher phase angle indicates that the coating is intact and behaving as a capacitor. In the same time, I believe that when the phase angle at lower freq is coming down, meaning that this coating is breaking down. If you traced bode plots on coated samples immersed at different times say 1h, 12h, 24h etc, you can see what is happening with your coating in time if it is partially porous. Shortly, if the angle has a value of about (-90), it means that the coating is intact. But all these tests are only qualitative and to obtain the more correct results, it is necessary to find a suitable equivalent circuit.

Dr.Maria Radulescu

Dear Doctor Maria and Doctor Amitha. The phase angle is coming down at higher freq. That is the phocus. Equivalent sheme is simple: serues positive capacity and negative resistance. Why resistance is negative? What is the question!

Dear Chaitanya

Tanx for details you hinted.

Samples were immersed in NaCl solution for couple of weeks. The variation of dE/dt was lower than 20 microV/S.

Dear Hossein,

it seems for me as if you are using a Metrohm Autolab system running with the NOVA software. To check whether the negative values of ReZ at high-frequencies are caused by an improper calibration of your system or whether this behaviour is caused by your sample, I would recommend using the test box also provided by Metrohm. There you find several circuits containing almost ideal elements (resistors and capacitors with known values) you can choose between. If you then use the same measuring procedure, the same cables, the same connectors and the same connection (2- or 3-electrode setup) and, for instance, a simple R-(RC) circuit of the test box, then the real part of the complex impedance should not become negative at high frequencies.

I believe that you are measuring an artifact, maybe due to the measuring system (check potentiostat) or to the cables. Above 10 kHz, you should always use coaxial cables (with shield connected to ground) and unfortunately that is not the common practice.

Dear Hossein,

I think you have a problem with your reference electrode. Its impedance is probably too high and you have some high frequency leakage current. This can be a common artifact in EIS experiments. Check these two papers:

[1] S. Fletcher, The two-terminal equivalent network of a three-terminal electrochemical cell, Electrochemistry Communications. 3 (2001) 692–696.

http://www.sciencedirect.com/science/article/pii/S1388248101002338

[2] A. Sadkowski, J.-P. Diard, On the Fletcher’s two-terminal equivalent network of a three-terminal electrochemical cell, Electrochimica Acta. 55 (2010) 1907–1911.

http://www.sciencedirect.com/science/article/pii/S001346860901367X

you have two solution, either you restrict you frequency range or lower the impedance of your reference electrode as suggested by Mansfeld

[1] F. Mansfeld, S. Lin, Y.C. Chen, H. Shih, Minimization of High-Frequency Phase Shifts in Impedance Measurements, Journal of the Electrochemical Society. 135 (1988) 906–907.

http://jes.ecsdl.org/content/135/4/906.short

if you follow those who cited this publication you will find important information about how to lower reference electrode impedance. Practically you add a platinum wire connected with a capacitor next to your reference electrode, but take care it can also give problems if misused.

Alberto

Dear Alberto

Tanx for your help

OK