Self-Discharge of Supercapacitors: A Review

The literature on the self-discharge of supercapacitors is reviewed, the advantages of electrochemical supercapacitors over batteries are formulated. The principal disadvantage of the electrochemical supercapacitors is their rapid self-discharge. A study of self-discharge of electrochemical supercapacitors was conducted; methods of the self-discharge studying, the effect of functional carbon groups on the self-discharge, the self-discharge mechanisms and mathematical modeling of the self-discharge are described. The development of new supercapacitor devices destined to minimize the self-discharge is described, including additives to the electrolyte, solid-state supercapacitors, electrochemical supercapacitors with ion-exchange membranes, the using of pure electrolytes, methods of the electrode chemical modification to slow down self-discharge. A study of self-discharge of electrochemical supercapacitors with electrodes based on activated carbon cloth CH 900 (the Kuraray Co. production) and 1 M MgSO4 aqueous electrolyte is conducted. The rate of self-discharge after ~70 min after its start is found to be proportional to the charging voltage. The voltage dependence of the self-discharge rate at 2000 min after its start goes through a minimum. This minimum is explained, firstly, by a significant contribution to the capacity from the Faradaic redox-reaction pseudo-capacity involving the functional groups; secondly, the very presence of these groups increases the self-discharge rate. It is the former factor that dominates in the low-voltage region; the second one, in the high voltage region.