Dear Dr. Lokhande,
IR drop in the charge- discharge curves is related to the equivalent series resistance of the supercapacitor. Supercapacitor has its own internal resistance thus it losses some fraction of the stored energy. Low internal resistance is favored for better electrochemical performance which is directly related to the IR drop.
Charging/discharging a supercapacitor is connected to the movement of charge carriers (ions) in the electrolyte across the separator to the electrodes and into their porous structure. For these issues, the IR drop in Charge discharge curves is much higher than ESR[1,2], and it can be found better if we have access to an equivalent circuit model (of this type of supercapacitor electrodes' and device technology[3]) from an EIS experiment.
1. Losses occur during this movement that can be measured as the internal DC resistance. This internal DC resistance Ri should not[2] be confused with the internal AC resistance called Equivalent Series Resistance (ESR) normally specified for capacitors. It is measured at 1 kHz. ESR is much smaller than DC resistance.
2. https://en.wikipedia.org/wiki/Supercapacitor
3. Figure 5.in http://www.murata.com/en-us/products/emiconfun/capacitor/2015/03/24/20150324-p1 ; https://web.archive.org/web/20141222044332/http://services.eng.uts.edu.au:80/cempe/subjects_JGZ/eet/Capstone%20thesis_AN.pdf
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