Hi all,
I couldn't get the exact fit for my EIS data especially at the low frequency range. i tried everything to get the exact fit by changing the data values. I used Gamry Echem analyst.
Please have a look of my circuit and the plot along with fit in the below fig. If you can give me some inputs about adjusting the values or even new circuit, it would be great. Thank you.
Or can i avoid the low frequency measurements ?
Gopi
Hi,
You have two semicircles in your EIS data, therefore, you need to choose proper circuit ( keep two semicircle circuit in a series) to fit. Something like is attached herewith.
Hi Mr. Sujoy Sarkar,
Thank you for your answer. I tried the same but still it didn't work unfortunately :( I have tried this circuit previously as well. Especially, the higher (which is closer to 0) frequency fitting curve remains same as i showed in the pic and lower ones as well.
Dear Gopinath Perumal,
what is your sample and what are the criteria (parameters) for your message : "didn't work" ?
Hi Mr. Ioannis, thank you for your answer.
My sample is stainless steel and the fitting didn't work in the sense, its not matching my recorded experimental values in a good way as you can see on the attached figure.
Hi Dr.Ioannis,
Thank you again.My cell consists of Reference electrode (AgCl) then electrolyte (in my case body fluid), working electrode (Stainless steel) which forms a passive layer to resists corrosion when immersed in the electrolyte.
Is your cell (for EIS) close to the cell of Ref[1] ?
1. Improved corrosion resistance of 316L stainless steel by nanocrystalline and electrochemical nitridation in artificial saliva solution http://ac.els-cdn.com/S0169433215024320/1-s2.0-S0169433215024320-main.pdf?_tid=7847185e-b658-11e6-bf0a-00000aacb35f&acdnat=1480440357_18092668d4295bf1cc04d8d3180239ce
Dear Gopinath Perumal,
according to Sujoy Sarkar you should change your circuit. You can use a R-R1CPE-R2CPE circuit. Your data seems to be related to the high contribution of electrode.
Best regards
Grazia
First of all, you need to make sure that your data are consistent with Kramers-Kronig transformations. This is something that everyone should consider before attempting to fit any electrical circuit with EIS data. It may be the case that your data do not comply with K-K properties (causality, stability, linearity) at low frequencies. You can use an electrochemistry software (ZView Scribner Associates Inc.) for K-K analysis with EIS data.
The calculation is made by iteration and needs better initial values.
You have to modify the following values: R2=750 , R3= 3.0e3 and to select/click "Lock" for R1, R2 and R3 values. Then click "Calculate".
Hi Ms. Grazia,
Thank you for your answer. I tried the same circuit but there is no significant difference on the fit. "Your data seems to be related to the high contribution of electrode", could you please tell me what you mean by this ? is there something i should change ? Am sorry, am totally new to experiments. I couldn't get it.
Thanks again
Hi Mr. Lincoln,
Thank you for your answer. KK is also available on Gamry Echem analyst. My data is consistent.
Hi Dr. Cornel Radu,
Thank you for your valuable answer. i did tried with the parameters you have suggested but no significant improvement in curve fitting, unfortunately.
On the basis of that figure it is clearly shown that that experimental points do not show the perfect semi-circle which are related to the frequency dispersion as a result of the roughness and inhomogeneity of electrode surface. From these reason the CPE element has to be used in the Equivalent Electrical Circuit (EEC), instead of classic capacitor.
The constant phase element (CPE)with varying 'n' is used to explain the depression of the capacitance semi-circle (Nyquist plots), which corresponds to surface heterogeneity resulting from surface roughness, impurities, dislocations, grain boundaries, adsorption of inhibitors, and the formation of porous layers.
When n = 1, the CPE describes an ideal capacitor. For 0.5 < n < 1,
the CPE describes the distribution of the dielectric relaxation times
in the frequency space, and when n = 0.5 the CPE represents a Warburg
impedance.
Dear gopi,
your Nyquist plot seems to unequal scaling on x and y axis. I mean the length on x axis must be same on y axis for proper shape of graph. In your case if both are same your plot shape will be change.
Dear Gopinath, I mean that if you separate the contribution at low and high frequencies, your data seem related to the contribution of electrode interface. You said that you are using steel, so I think that the major contribution is related to the the steel electrode. You can try to use this circuit R-RQ-RQ or try to add the W element R-RQ-RQ-RW. Good luck :)
Thank you Ms. Grazia, i will do it today ....It was so nice of you :)
I fitted the curve using Rs ( outside ) and two RC circuits inside. Like the one used in below paper. It fits my curve properly. It is normally used for all the SS316L. I really appreciate everyone who answered me. Thanks a lot.
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