Memory effect of external charging strategies on carbon-based supercapacitors and their self-discharge inhibition effect

Self-discharge is a serious issue of supercapacitors corresponding to the spontaneous voltage decay when disconnected from external power supply. This paper proposed six external charging strategies for mitigating the self-discharge of carbon-based supercapacitors. Here, the strategies were referred to as charging–holding–resting–discharging (CHRD), charging–resting–discharging (CRD), charging–holding–resting (CHR), charging–resting (CR), charging–holding–discharging (CHD), and charging–discharging (CD) strategies. The results showed that the resting step exhibited significant reduction in the voltage decay of supercapacitors. In addition, the holding step showed slightly extra improvement in the mitigation of self-discharge process. On the other hand, the discharging step accelerated the voltage decay in self-discharge process. In other words, the CHR strategy showed the best performance in self-discharge mitigation. As an effective approach to identify electrochemical process, distribution of relaxation times (DRT) analysis by fitting the data of AC impedance was applied for physical interpretation of the self-discharge measurement. The above mentioned improvement in the mitigation of self-discharge can be attributed to the increase in resistance and slower time constant associated with Ohmic leakage and ion diffusion shown in the DRT analysis of AC impedance. Furthermore, microscopic characterization of electrode materials showed particle breakage after the external charging strategy implementation, i.e., reduction in electrode particle size. This suggested the generation of new charge adsorption sites during the external charging strategies. Pore size distribution obtained from BET analysis further confirmed the increased pore volume associated with new charge adsorption sites, while the pore size distribution remained the same. Finally, the energy consumption for CHR strategy was the least among all strategies. The effectiveness of voltage decay reduction during self-discharge persisted for at least 2 months of idle time without any reduction. The voltage decay reduction during self-discharge lasted for up to 6 months before reaching the voltage decay profile of that without any external strategy.