Physical analysis of self-discharge mechanism for supercapacitor electrode for hybrid electric energy storage system

Self-discharge is a spontaneous process that has considerable adverse effects on the performance of supercapacitors. In order to quantitatively investigate the contribution of self-discharge mechanism, this paper proposed a theoretical self-discharge model for carbon electrode of supercapacitors based on electric double layer theory. Three physical contributions, i.e., side reactions, ion diffusion, and ohmic leakage, were investigated. In addition, self-discharge measurement of carbon electrode was performed to validate such theoretical model. The results indicated that the potential drop due to side reactions was negligible throughout the self-discharge process in all cases. In addition, ion diffusion dominated for low initial potential and short holding time, accounting for 50%–80% of self-discharge. On the other hand, ohmic leakage dominated for high initial potential and long holding time. Furthermore, the potential drop due to ion diffusion increased monotonously with time while the potential drop due to ohmic leakage remained constant throughout the self-discharge. Moreover, the potential drop due to ohmic leakage increased with the increase in holding time, which compensated for the decreasing potential drop due to ion diffusion. This could explain the fact that the total electrode potential decay during the self-discharge was independent of holding time. Finally, dimensional analysis was performed to predict self-discharge. Time constants of side reactions, ion diffusion, and ohmic leakage were derived. Overlapping dimensionless electrode potential versus dimensionless time indicated that such dimensional analysis was generally applicable to predict self-discharge of carbon-based supercapacitors at a given initial potential and time.