I use bare Au or Pt electrode as the working electrode. Ag/AgCl and Pt wire are the reference and counter electrode, respectively. 1 M KNO3 is chosen as the electrolyte. Open circuit potential is applied with 10 mV AC.
I always obtain a impedance spectrum (see attachments), which has some phase shift at high f followed by a lower phase shift with the decreasing f. After that, the phase shift starts increasing again.
Why it happens? Does anyone know the possible causes?
From the results that you have attached I see that you have no semicircle region but a large diffusion region. This is caused by the high conductivity of the electrodes and the electrolyte, as well as, from the fact that your electrodes are bare (common behavior). Did you try to perform your experiment using ferri/ferro redox probe in KNO3 (apply also a +200 mV voltage upon your sinusoidal ac signal) to see if the electrodes reveal the same behavior?
Thank you for your reply. Could you please explain more about how the changes of phase shift at high f happens? (decrease then increase) What might be the causes of it?
I have attached another data with 200 mV AC with all other conditions remain the same except adding Ferri/Ferro. In this condition, the strange changes of the phase shift at high f becomes "normal" (no curve). But the system becomes non-linear, is there any reason to apply the AC across such a big potential window? Also, could you suggest an model to fit the data. It cannot be fit with K-K since it's non-linear.
Thank you so much for your help.
I can't understand exactly what are the experimental conditions. The electrolyte has no ferri/ferro? The applied voltage of +200 mV should be dc voltage on which the sinusoidal signal (suggested 10 mV rms) should be superimposed. For data fitting you can give a try with a simple Randles equivalent circuit containing a Warbung element (see the attached file). Regarding the changes in phase shift is a long story that's why I am attaching you a helpul file (pay attention to yellow colored paragraphs I made for you).
Good luck.
Thank you so much for the reference.
I misunderstood your suggestion of using +200 DC voltage to AC voltage. However with +200 mV AC superimposed, I didn't obtain the weird high f response I saw before. I'm wondering how this happens. For the changes of phase shift I mean the very high f region-the very first points of the data, you can see a small curl in both bode and Nyquist plots in the first attachment. Do you know what's the cause of the small curl ?
The electrode surface, the diffusion layer formation, ionic strength, kinetic limitations and so many other reasons would cause this small curl. There is no need to worry about that.
Hi, I think there can be two reasons for the small curl you see at high frequencies. One could be that you have some adsorption. The second one could be an artefact, either of the potentiostat or of the reference electrode. In order to check if the artefact comes from the potentiostat just connect a dummy cell with close impedance of your system. In particular, you need to simulate the value of the resistance of the electrolyte. if you see a normal impedance that it is probably the reference electrode. Iits impedance is too high. Try to substitute it with a piece of platinum (you are working at the OCP so it is easy) which has low impedance. The explanation of this was given by Fletcher (S. Fletcher, The two-terminal equivalent network of a three-terminal electrochemical cell, Electrochem. Commun. 3 (2001) 692–696. doi:10.1016/S1388-2481(01)00233-8.)
In my opinion, this is your problem, but I cannot be sure. Let me know.
If you still see the curl, that it is really an adsorption.
Hi Alberto, thank you for your suggestion. My potentiostat works fine with a dummy cell. I'll try to replace the reference with a Pt. I'll let you know the results. Thank you for sharing the reference.
Hi Alberto, I just tried both Ag/AgCl and Pt as reference electrode in Ferri/Ferro solution. I think you're right. It is the Ag/AgCl which caused the small curl. I added the results here, so everyone can see the difference in the plots.
Could you suggest the causes of the Rs Shift by using different reference electrode?
Thank you again.
Hi Tingting,
the shift in Rs is not related to the reference electrode, but to the distance between RE and WE and to the conductivity of the solution. You can find some more information about how to decrease the artefacts of the reference electrode in this publication:
[1] A. Battistel, M. Fan, J. Stojadinović, F. La Mantia, Analysis and mitigation of the artefacts in electrochemical impedance spectroscopy due to three-electrode geometry, Electrochim. Acta . 135 (2014) 133–138. doi:10.1016/j.electacta.2014.05.011.
you can download it from my page.
bye
let us discuss high frequency regime
it is the behavior of electrode+ solution
better shield your cell using Faradaic cage( i designed one in 1973 using copper rectangular box with earthing)
avoid 50-60c/s frequency interference
still you get the high frequency loop/ semicircle it is due to the electrode
( modified )with dopings)
check your electrical connections - avid loose contacts
Another reason is inconsistency between current trough the cell and a current range of your potentiostat.
dear all
high frequency response goes with the electrode-- porous powdered / pressed/ modified electrodes
we can get information about DC and AC conductivity - a high frequency inductance loop goes with charge storage due to semiconducting nature of the electrode-
dear Alexey L. Klyuev , i was in Academy of sciences Moscow in 1987--
We use ONE AMPERE potentiostat range
please see books on EIS published by JECS , USA
read lab manuals of university of Southampton --
read JD Macdonald book on EIS-- A bible
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