Hello,
I have a two compartment electrochemical cell. I have put a nanoporous aluminum oxide membrane (nominal pore diameter = 20 nm) between these two compartments. Now I looked at the impedance of the system using a 4 electrode set up with two reference electrodes (Ag/AgCl) across the membrane to measure the voltage (one is connected to the reference and the other to the working sense lead of the potentiostat).
The working and counter electrodes are Pt wires. The electrolyte is 1 X Phosphate Saline buffer, pH 7.4. The EIS parameters are VAC = 5 mV, VDC = 0 mV w.r.t. OCP, frequency range = 0.1 - 10 kHz. The OCP was ~ 3.8 mV with less than 10 per cent variation in 300 s before running the EIS.
The Bode and Nyquist plots for one EIS are attached. I used a model shown in the figure to fit the EIS data.
My question is : I see two distinct peaks in the phase plot, so I used two R-CPE elements in the model. I would say that one such parallel R-CPE is for the membrane. Could the other one be due to the reference electrodes ?
Or could it be that one R-CPE block is due to the aluminum oxide and the other is for the nanopores ?
Kindly advise.
Hi there,
I checked your data, I think your simulation circuit that used to not a suitable for results in a hence you will not get a good correlation factor. I suggest repeating the experiment, but take in your mind the OCP it's enghout. And check the following link:https://www.researchgate.net/post/How_to_get_good_curves_in_impedance_measurement_technique
Regards,
Hello Dr. Yousif,
I dont understand why you think that the model is not good for the data ? I get a good fit with tolerance in the order of 10-7. And what do you want to check regarding the OCP ?
Hi Agnivo,
First, I think you have my email, so if you could email me the raw data file I may be able to help more than here.
Next, the "goodness of fit" exists because people wanted it, not because it actually tells you anything about whether your model is good. You can almost always add elements to improve the goodness of fit but if they don't have meaning (physically, chemically, or representationally in the data) then they aren't actually useful. In your case there are certainly two phase altering phenomena but there isn't enough data to figure out what the second, lower frequency one is. That second phase deflection could be a second R||CPE but could also be diffusion (it could also be an artifact from, e.g. drift).
Hi Agnivo,
I think your adopted model used to simulate the measured data is not the best one. To choose the suitable electrical equivalent circuit (EC) which fit well the experimental impedance data, one should consider the spectra on both the Bode and Nyquist formats. Regarding your results, on the Nyquist plot there is a feature of Warburg impedance (W) at the low frequency (LF) limit which should be included in the chosen model. Nevertheless, to be fair in explaining your spectra you must send the two plots with readable x- and y- axes in order that one can see the values of |Z|, the real and the imag. Z. Also, I advice you to repeat the run again with extending the low frequency limit to 0.01 Hz, as for your system especially the LF range is of much importance.
Good luck...
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