I am studying electrochemical oxygen evolution reaction (OER).
So far, I couldn't understand about the EIS.
I am trying to learn EIS, but I'm still confused data analysis.
I want to know what the height of semicircles means in EIS.
From what I have found, it is related to capacitance. Is it true?
I attached one figure. Based on the figure, the values of Rct are similar between both (black and red).
But the height is different. Is it CPE?
Actually based on other data(overpotential, tafel plot), I thought red material is better OER catalyst than black material.
I tried using EIS data to explain why red show better OER activity than black, but I can't because there are no differece in Rct. This is not what I expected.
Can anyone explain this for me?
Dear Suncheol Kim,
could you note, please, what is the VDC polarization[1] values(s), of the related EIS measurements[2,3] , that you have shown (Nyquist-plot)[4], here ?
1. The EIS measurements are very (different and very) sensitive to the parameter of the VDC- (DC-component) polarization values; so, the EIS measurements are related to the OER polarization[2,3] activity (of your WE).
2. Electrochemical impedance spectroscopic (EIS) investigation of the oxygen evolution reaction mechanism ... https://www.sciencedirect.com/science/article/abs/pii/S002207280800168X
3. ... and other parametric conditions ...
4. Apply, please, some quasi steady-state VDC-polarization (parametric) values near (- +50 mV) the OER polarization activity (of your WE) in your EIS. Otherwise, you can not correlate, immediately, EIS (Nyquist-plots) with OER-activity.
Thank you for your answer.
I measured the impedance at 1.63 VRHE.
For the OER measurement, the overpotential of samples at 10 mA cm-2 is 260 mV (black, 1.48 VRHE) and 290 mV (red, 1.52 VRHE), respectively.
Based on your answer 4, they were measured with wrong values, right?
If I want to relate EIS with OER activity, which values can you suggest?
Let me suggest some VDC polarization values(s), inside the range: 1.61 to 1.63 VRHE.
Perhaps, the semicircle of the Nyquist plot of the black material becomes flattened due to an increase in the porosity, surface inhomogeneities and surface roughness compared with the red one. You may confirm it by SEM observations.
Dear S. Feliu Jr,
Thank you for your answer.
I can believe your suggestion.
Actually the red material(Nickel plate) has flat surface, but black(graphite) is like fabric.
Can you give me more information why this phenomenone happen in EIS mesurement?
The height of the nyquist plots is related to double layer capacitance. Different researchers have used various equations, depending on the specifics of the sytem under study. However, for Hydrogen evolution reactions, one popular equation is;
Cdl = 1/(2*pi*Fmax*Rct).
where Cdl = double layer capacitance, Rct = Z'max - Z'min and Fmax = frequency at peak/maximum Z''.
You can obtain Fmax by searching (via excel command) for the maximum value in the Z'' column and picking the corresponding frequency to this. Suncheol Kim
Hello Mr. Omokafe Seun Michael ,
Thank you for suggesting this equation. I have one question related this above mentioned equation. If Fmax is expressed as Hz and Rct as Ohm, then what will be the unit of Cdl? In most of the journal papers, authors report Cdl as mF/cm2.
Thank you in advance for your answer.
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