Origin and Regulation of Self-Discharge in MXene Supercapacitors

MXene-based supercapacitors are promising electrochemical energy-storage devices due to their ultrahigh volumetric capacitance, high-power characteristics, and excellent cyclability. However, they suffer from severe self-discharging behavior while the underlying self-discharging mechanism is still unclear. Here, the self-discharge behavior of MXene-based supercapacitors from surface electronic structure of MXenes is disclosed, and a novel method to mitigate it is proposed. A superficial engineering strategy based on bio-thermal treatment is developed to effectively tailor surface electronic structure of Ti3C2Tx MXenes by eliminating hydroxyl terminations. With the evolution of surface electronic structure, as revealed by Kelvin probe force microscope and synchrotron radiation X-ray absorption fine structure analysis, MXene-based supercapacitors with common aqueous electrolytes show >20% decline in self-discharge rate. This decline mechanism originates from the increased work function that induces higher zero-charge potential after the removal of hydroxyl groups in MXenes. Meanwhile, the strengthened surface dipole leads to higher surface free energy between MXene and electrolytes. These two positive effects endow MXenes with weaker self-discharge kinetics. Specifically, the activation-controlled self-discharge process is greatly suppressed. Illuminating the relevance between electronic structure and self-discharge accompanying superficial engineering suppression strategy can guide to development of high-performance energy storage devices.