In most electrochemical energy storage devices, including supercapacitors, the charging-discharging processes (with or without chemical reactions on the electrodes) are not ideally reversible in practice. The irreversibility becomes worse at larger polarisations or charging-discharging rates. This is the fundamental reason why the coulombic (current) efficiency would decrease with increasing the charging-discharging rate (current).

The simplest case is the EDL capacitor in which no chemical reaction occurs. However, in the cell, there are still various resistance elements which will cause the so called iR drop. Because the charging current is opposite in polarity to the discharging current, the iR drop is positive for charging but negative for discharging. This means that a higher cell voltage (larger current) is needed for charging, but a lower cell voltage (smaller current) is generated on discharging. Such physical polarisations become worse with increasing the charging-discharging current, because iR is proportional to the current.

Other polarisations, such as those resulting from charge transfer resistance at the interface, and concentration variation near the electrode surface (or within the porous structure of the electrode), also increase with increasing the current, and hence cause various forms of irreversibility, contributing to the loss in coulombic efficiency.

In most electrochemical energy storage devices, including supercapacitors, the charging-discharging processes (with or without chemical reactions on the electrodes) are not ideally reversible in practice. The irreversibility becomes worse at larger polarisations or charging-discharging rates. This is the fundamental reason why the coulombic (current) efficiency would decrease with increasing the charging-discharging rate (current).

The simplest case is the EDL capacitor in which no chemical reaction occurs. However, in the cell, there are still various resistance elements which will cause the so called iR drop. Because the charging current is opposite in polarity to the discharging current, the iR drop is positive for charging but negative for discharging. This means that a higher cell voltage (larger current) is needed for charging, but a lower cell voltage (smaller current) is generated on discharging. Such physical polarisations become worse with increasing the charging-discharging current, because iR is proportional to the current.

Other polarisations, such as those resulting from charge transfer resistance at the interface, and concentration variation near the electrode surface (or within the porous structure of the electrode), also increase with increasing the current, and hence cause various forms of irreversibility, contributing to the loss in coulombic efficiency.

Thank you George Zheng Chen sir for this valuable information. It's truly helpful.

I agree with Prof. Chen. However, if IR drop or polarisation consider to be nearly equel during the charging and dishcarging; thus at lower current unwanted side redox reaction may come in picture decides the efficiency, whereas at high current they are absent especially at the end of voltage windows . Thus, it is also logical and practically seen in cases of supercap having better coulombic efficecny at high current (or they become more capacitor).

The cases given by Bihag were indeed reported in the literature for some supercapacitor electrode materials but such observations were from studies in which the applied electrode potential range was inappropriately set wider than the so called "capacitive potential range (CPR)". Similar cases may apply to supercapacitors (cells) where the "capacitive voltage range" is the basis for judgement.

Thank you George Zheng Chen sir and Bihag Anothumakkool sir for providing these valuable information.