The interpretation of the results of EIS is divided into two parts. on one hand, some researchers insist that larger semicircle is better because it leads to larger recombination resistance (Rrec). On the other hand, similar to your interpretation, some researchers insist that the smaller semicircle is better, resulting in small charge transfer resistance. Which is correct between the two views? How can we justify our analysis?
The relative size of the semi circles in the Nyquist plot are only subjective. What is important is the model used to fit the data and phsycial interpretation of the fitting parameters.
The fact that you have two semi circles in the first quadrant of the Nyquist plot indicates only dielectric responses. I suggest you fit your data to a parallel combination of series RCn elements (parallel Debye Responses). If the fit is successful, then the next step is to physically interpret the parameters.
The R parameter is indicative of the response being observed and is an indication of the ionic resistance to a physical feature of the film. The C parameter is inversely proportional to the dielectric thickness. To calculate this you need the effective area and a relative dielectric constant. Initially, assume that the area is the ascribed area of the occulted cell. The relative dielectric constant can be set so that the thickness is in the expected range. Assuming the area and relative dielectric constant are the same for all measurements, the relative thickness of the dielectric can be compared. Finally, the power law term, n, is indicative of the population distribution of many nano or micro features that contribute to the overall response. This is a direct result of EIS being a 'marco' technique the is extremely sensitive to 'nano' or 'micro' contributions.
The EIS technique is unique in this regard; sorta like analyzing numerous high magnification TEM images to get an overall picture of the film. However, EIS does that in one single scan an ambient conditions.
The real challenge of the technique is then to correlate the physical parameters to in-service behavior. Which features best reflect the desired behavior of your film. Armed with this knowledge you are able to quantifiable the physical features the dictate the behavior of your system.
In conclusion, subjectively comparing size of semi circles is not adequate, you must identify a fit model and interpret the fit parameters, and verify that physical attributes determined from the electrical parameters are indicative of you system. Finally, the values of the physical parameters need to be verified by some sort of independent measurement. Anything less than this breeds confusion and distrust in the EIS technique.
Back to your original question, both subjective observations of the size of the semicircles can be correct, however a physical understanding of your system can help to explain why there can be two correct answers.
if i have an overloaded results in the EIS measurement, (no result in Zplot), what you advice me to do, (i change the AC amplitude from 100 to 10 mv and i have the same issue)
could you help please.
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