I have been trying to record EIS data for some alloy to be used as an electrocatalyst for water splitting. The range of frequency i chose was 100 kHz to 10 Hz with a perturbation signal of 10 mV amplitude. I could get a semicircle in the Nyquist plot but a somewhat perpendicular line at higher frequencies. I have attached the image file of the Nyquist plot for the same.
Without knowing the detail of the experiment and electrode information, its difficult to comment. However, just for discussion, the relaxation of the main phase still need to appear at some high frequencies. If you are expecting the line to be perpendicular at high frequencies, its not the case. If you equal the x-axis scale with y-axis scale, you will understand that its the beginning of second semicircle which I expect can go as high as -20 on -Z// axis(if not parasitic inductance).
Dear Suraj
Your EIS data is somewhat unusual. It is not common to see data like that at high frequency. You can try performing a data validation test using the Kramer Kronig transform. It may also be helpful to see the bode plot.
Best regards David
Dear David,
I have attached the Bode plot. Please see if it can help you to understand it better.
Thank you Sebastian,
I have now attached the Nyquist plots taken at OCP and at a cathodic potential from OCP. It shows beginning of a semicircle at high frequencies. I monitored the current changes during each and every measurement and the current was quite steady throughout. Only at frequencies below 1 Hz, the current fluctuations start and hence I limited the range to 10 Hz to maintain the steady-state conditions.
In another case (similar to this), I observed the high frequency semicircle intercepting the real axis at negative Z values. How to account for these?? How to determine the solution resistances in both the cases??
I am happy to see this Nyquist plot, which according to my prediction in my first comment gave the onset of second semicircle at higher frequencies. As for as your comment that the circle bisect at negative Z value is not valid in the present scenario. You are not sure about the depression angle of this growing arc at higher frequencies. The electroactive region appearing at higher frequencies may gave distorted semicircle and it passes through origin or with some positive Z value, OK.
Try to play with the perturbing potential without changing the study state condition for bringing the relaxation time of this emerging arc in your available experimental frequency range, best of luck.
Dear Nadeem,
Your comments are absolutely correct and I will try different perturbation potentials. Can you please suggest me what all information can be extracted from these graphs as their nature at higher frequencies is not clear.
Dear Suraj
You can extract lot of information from the impedance and Modulus formalism. Go through our recent paper in chemical physics letter 585 (2013) 74 on manganites (in solid state). The intersection of arc at lowest frequency on the right side of this semicircular arc give you the total resistance of the sample. From individual circle you can extract the resistance and capacitance of each electroactive regions separately. You can calculate the depression angle of these semicircular arcs for better understanding of heterogeneity of the electroactive regions. Use equivalent circuit to model your impedance plane plot. Conductivity is another good option to carry on.
Dear Suraj
Are your spectra taken under steady state conditions? What potential are the taken at? Did you perform the Kramers Kronig test? Can you describe the electrochemical system you are measuring?
Sorry for all the questions
David
In my opinion, your high frequency behavior results from reference electrode response. Change your reference electrode and reduce the length of your cable and your impedance will be change in the high frequency range.
What is the electrolyte, reference and counter electrode which are used for the experiments. It look like the electrolyte resistance is quite high. Why don't sweep the frequency from 1M Hz. You will get some more data points at high frequency and can get some idea of the nature of the curve.
I agree with Marc Jeannin.
you can find more info about it on these publications:
[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
and a solution in this:
[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
Alberto
Your high-frequency semi-circumference resembles E-sweep Complex-Plane Plots - varying the potential at constant frequency
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