How to determine the the potential window of the non-faradic region and at which potential the current should be selected for plotting the relationship between the scan rate and current in order to calculate the capacitance from the CVs
The voltammetric responses from double layer capacitance and pseudocapacitance are the same or similar. Therefore, you may determine the capacitive potential range (CPR) by extending the applied potential range till the the rectangular CV becoming distorted by fast increasing current. For rectangular CVs, you can select several potentials to plot the current versus scan rate. In each case, you should obtain a straight line at relatively low scan rates, but the current may deviate, usually below the straight line at higher scan rates.
However, if your CVs do not show the rectangular shape, but the fish-shape, you cannot determine the CPR. You need to reduce the resistance of the electrode, for example, by mixing your active material with a conductive additive, such as carbon black.
For EIS measurement, the low frequency capacitance (Clf) derived from the Z'' vs 1/f plot at a particular bias potential should be comparable to that from the CVs at the same potential. I also recommend to record the EIS at several potentials that are determined by the rectangular CVs. The average value of these EIS and CV measured capacitance values would be a better reflection of the capacitance of the material/electrode.
An advantage of EIS over CV is that EIS can separately measure the electrode resistance (high frequency) and electrode capacitance (low frequency).
The voltammetric responses from double layer capacitance and pseudocapacitance are the same or similar. Therefore, you may determine the capacitive potential range (CPR) by extending the applied potential range till the the rectangular CV becoming distorted by fast increasing current. For rectangular CVs, you can select several potentials to plot the current versus scan rate. In each case, you should obtain a straight line at relatively low scan rates, but the current may deviate, usually below the straight line at higher scan rates.
However, if your CVs do not show the rectangular shape, but the fish-shape, you cannot determine the CPR. You need to reduce the resistance of the electrode, for example, by mixing your active material with a conductive additive, such as carbon black.
For EIS measurement, the low frequency capacitance (Clf) derived from the Z'' vs 1/f plot at a particular bias potential should be comparable to that from the CVs at the same potential. I also recommend to record the EIS at several potentials that are determined by the rectangular CVs. The average value of these EIS and CV measured capacitance values would be a better reflection of the capacitance of the material/electrode.
An advantage of EIS over CV is that EIS can separately measure the electrode resistance (high frequency) and electrode capacitance (low frequency).
George Zheng Chen
Thanks Prof. George. for your nice answer. However, do you think that the obtained value of Cdl from the EIS should be relatively close to the calculated CdI from the slope of the plotted scan rate vs current.
Ahmed Bahgat I have added my explanation on EIS to my earlier answer above.
According to Electrochemistry textbooks, you should select the region that there is not any faradic process or you do not see any redox peaks. For example, if you want to obtain a non-faradic region of Ni+2 you should consider a region between -0.1 to +0.1 V. For selecting the current you must refer to your final CV plot. If it is rectangular you can consider the maximum current for calculating capacitance, but in case it is not rectangular, you must consider the average current of forwarding CV line. and vise versa.
For more information please see the following paper, it has provided some good explanation about calculating Cdl. Article In situ synthesis of quasi-needle-like bimetallic organic fr...
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