Charge transfer resistance is the resistance against the process of electron transfer from one phase (e.g. electrode) to another (e.g. liquid).
Suppose I have an EIS curve with two time constants. Then how should I calculate the charge transfer resistance from that data, without using electrical equivalent circuit?
I am performing my experiments in a corrosion cell with a three electrode system, with Ag/AgCl as the reference electrode, Pt as the counter electrode and a metal alloy as the working electrode.
We do not know what are you studying and it is difficult to elaborate. In typical reactions Rct is the radius of high the frequency semicircle (coupling of Rct and Cdl). See my book on EIS.
Dear Prince Kumar Baranwal,
say, please, 1)which is your cell and 2)show a graph, if possible.
Sir, please see the attached file for the graph. I have not selected a cell for fitting. I actually want to calculate it without circuit fitting.
Dear Prince Kumar Baranwal,
note, please, the two time constants in the Nyquist graph (at -Zi max).
Dear Prince Kumar,
Not your equivalent circuit, but your actual cell. As with almost everything in EIS, You have to understand your cell to extract relevant information from your data. 2 time constants can arise for different reasons, and so it is not possible for someone who does not know your cell to look at a plot and say what Rct is or even if there is just one of them (I can guess, but that is it).
As a note, while you certainly can extract information from a graph without fitting a model, that information is not as good as that you obtain from fitting to a good model--a single time constant is pretty good but for 2+...modelling is better.
We do not know what are you studying and it is difficult to elaborate. In typical reactions Rct is the radius of high the frequency semicircle (coupling of Rct and Cdl). See my book on EIS.
Whitout Equivalent Circuits usually are to points easy to distinguish:
1) The Rohm or high frequency resistance that is the intersection with the X-axis at high frequencies,
2) The Rpol which is the total resistance of your system and the closest point at X-axis intersection for low frequencies.
Rct is the radius of radius of high the frequency semicircle (coupling of Rct and Cdl). It is intersection with the x-axis at low frequencies.
Apparently from the figure its a capacitive semicircle that fits to randles equivalent circuit.
Since you are having two semicircles in EIS graph, therefore there will be two time constants i.e. (R1Cdl) (R2C2) or (R1(R2Cap)Cdl) depend on your system. Charge transfer resistance is the resistance obtained at the low frequency part, from the circuit (R1(R2Cap)Cdl), R2 is the charge transfer resistance, therefore we can say that the arc obtained at low frequency is the charge transfer resistance i.e. second semicircle.
May i ask about what if a CPE parallel to a Warburg resistance? How could I know it if i saw the result on the paragraph?
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