Screening the deep eutectic electrolytes for supercapacitors with alleviated self-discharge

Self-discharge in supercapacitors is a major challenge and it's mainly determined by the property of the electrolyte. The emergence of deep eutectic solvents (DESs) provides a promising research frontier, but the principle of selecting DESs with suppressed self-discharge is not well understood. Herein, we prepared various DESs with the same hydrogen bond acceptor (HBA) but different hydrogen bond donors (HBDs) to unveil the effect of the type and number of functional groups in the HBD on the self-discharge property in symmetric supercapacitors. In acidic DESs (where the HBD has carboxyl -COOH groups or is acidic), a higher number of carboxyl groups are detrimental for self-discharge. For alkaline DESs (where the HBD has amino -NH2 groups), a higher number of amino groups are also disadvantageous for self-discharge. Conversely, in neutral DESs (where the HBD has neither -COOH nor -NH2 groups, and it do not exhibit acidity either), a higher number of hydroxyl groups is favorable for alleviating self-discharge. The supercapacitors using ChCl:Gly electrolyte exhibit the highest capability for alleviating self-discharge (32% voltage retention rate when aged for 15 h). The influence of harsh reaction conditions (temperature and water impurity) was also studied for the ChCl:Gly electrolyte. These results are expected to guide the further optimization of electrolytes with suppressed self-discharge.