Hello,
I've been doing some EIS analyses on coin cell supercapacitors. Most of my cells will share similar Bode and Nyquist plots. With the former, I typically get Z(ohm) values upwards of 200 as frequency goes below 1 Hz.
For Nyquist plots my -imaginary and real impedance is in the hundreds in the Warburg and capacitive regime.
These will vary depending on the different materials I'm using.
But occasionally I will get a repeat where the -imaginary and real impedances never go beyond 10 Ohm. I have attached an example of two cells made from the identical batch of electrode (Activated carbon) and eletrolyte (sulfuric acid) to illustrate this point.
The semi-circles appear to show little variation, and other parameters such as specific capacitance only vary by a small amount.
Is there any explanation for why these vastly different impedance values are present?
Thank you
Edit: I should mention the frequency ranges are similar
Dear Akash Ratnayaka ,
Both have similar frequency range but check one time that both have same data point after recording. Some time what happened, measurement can stop before reaching the final frequency. There may be other reason for this but check one time.
Best Regards,
Bapi
In nyquist plot, the Z (imaginary) in Y axis represent the capacitance and Z (real) in x axis represents the resistance. capacitance is inversely proportional to the Z (imaginary). If you have doubt with the nyquist plot, then you should plot the bode plot, where Y axis will be the total Z or phase angle and the x axis will be frequency. That graph will clear your confusion.
It may help to see the Bode as well, but if the materials are [nominally] the same, the system/setup you are using for the measurement is the same, then it seems likely that there is some physical difference between the two samples/cells (e.g. the coin cell press wasn't ideal and resulted in an alternate path for current).
What are the values of the polarization parameters VDC in each case of the two example cells ?
Dear Akash
You can look upon this particular research paper and clarify your doubt.
"Physical Interpretations of Nyquist Plots for EDLC Electrodes and Devices"
DOI: 10.1021/acs.jpcc.7b10582
Dear all,
Thank you for your replies. I will try all your suggestions:
Bapi Bera: I did check it seems fine, I tried it with two different programs but got the same result.
Dhrubajyoti Bhattacharjya and Jacob:
The Bode plot for the anomalous cell shows the phase angle not even coming close to 90 degrees, which may confirm Jacob's suggestion that it is not actually behaving like a capacitor. Perhaps it was an error in my coin cell construction
Iannis Samaras:
I believe it is 0 - 1 V for both
Ramesh Oraon:
Thanks Ill have a look that looks useful.
Artur Braun:
Perhaps that is the case. Thanks
It seems a case of electrochemical deposition in oxidizing and reducing conditions. Have you tried applying biases to your cell like OCV to 0.2V? It could result in signaling out the main process where the deposition is taking place (oxidizing or reducing)
Also, I wonder if, that low impedance (near 10 Ohm) class of cells, might concern, only, the case of some highly cycled (and aged) cells ?
Hi All,
Just an update in case anyone encounters this problem.
It appears the software I was using (Nova 2.1) was cycling and holding my cells beyond the voltage window of my cell due to some software bug (the only explanation I have).
This was a hard lesson to learn, but I think this was the main cause.
Dear Professor Ioannis Samaras, , could you please explain what that would mean, i.e. the low imaginary impedance for an aged supercapacitor (after cycling)? I observe such a phenomenon in my cell (i.e. the absolute value of the imaginary component is ca. 10 lower after cycling) and I wonder about its physical meaning.
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