Supercapacitors, with the merits of high-power density, long durability, and remarkable safety, have already been used in the field of fast energy storage and conversion. However, their rapid self-discharge with spontaneous voltage decay results in the fast loss of the stored electric energy, severely limiting their practical application. Carbon materials have been widely used as the electrode materials for supercapacitors because of their large surface area, well-developed porous structure, and low-cost. Therefore, it is of great significance to understand the mechanisms and influencing factors, and further explore efficient suppressing strategies of the self-discharge behavior of carbon electrodes. In this review, we first introduce the self-discharge mechanisms including charge redistribution, Faradic reaction, and ohm leakage. Then, the key properties of porous structure, surface states, and metal impurities of carbon materials on the self-discharge behavior are discussed. Finally, we summarize some novel suppressing strategies and give perspectives on the future development of supercapacitors. This review provides an insight on the self-discharge of carbon-based supercapacitors, and can help to facilitate their widespread application.

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Zn-ion capacitors are attracting great attention owing to the abundant and relatively stable Zn anodes but are impeded by the low capacitance of porous carbon cathodes with insufficient energy storage sites. Herein, using ball-milled graphene with different defect densities as the models, we reveal that the selfdoping defects of carbon show a capacitive energy storage behavior with robust charge-transfer kinetics, providing a capacitance contribution of ca. 90 F g −1 per unit of defect density (A D /A G value from Raman spectra) in both aqueous and organic electrolytes. Furthermore, a simple NaCl-assisted ball-milling method is developed to prepare novel graphene blocks (BSG) with abundant self-doping defect density, enriched pores, balanced electric conductivity, and high compact density (0.83 g cm −3 ). The optimized ion and electron transfer paths promote efficient utilization of the self-doping defects in BSG, contributing to improved gravimetric and volumetric capacitance (224 F g −1 /186 F cm −3 at 0.5 A g −1 ) and remarkable rate performance (52.2% capacitance retention at 20 A g −1 ). The defect engineering strategy may open up a new avenue to improve the capacitive performance of dense carbons for Zn-ion capacitors.

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