Electrode pretreatment cycles for suppressing self-discharge of electrochemical double-layer capacitors

Abstract

Self-discharge is an intrinsic property of electrochemical double-layer capacitors (EDLCs), and its elimination by any method is challenging. This study investigated ways to reduce the faradaic reaction triggers of self-discharge using simple pretreatment cycles. When the cell was operated between 0 and 2.7 V (baseline), a passivation film was formed on the electrodes, which allowed the cell to survive for a long life cycle without rapidly consuming the electrolyte. However, the passivation layer was not sufficiently firm to prevent electrolyte penetration during self-discharge. Once the electrolyte penetrated the passivation layer, electrolysis gradually eroded the charged state. Pretreatment cycles can reinforce the passivation layers with a few 2.9 or 3.1 V cutoff cycles. Cells with a 3.1 V pretreatment cycle reduced the self-discharge rate by 45 % at 26 h compared to the baseline value. This simple pretreatment method was incorporated into the passivation layer formation process, effectively suppressing self-discharge. However, the capacitance decreased slightly as the movement of ions slowed during self-discharge, which is a natural phenomenon. This study demonstrates that the electrolyte itself acts as a self-discharge trigger, serving as a Faradaic reactant. In addition, a simple method for reducing self-discharge has been reported. This suggests the need for a novel electrolyte additive that strongly suppresses electrolysis without impeding ion movement.