Origin and Regulation of Self‐Discharge in MXene Supercapacitors

Abstract

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.

Designed Redox‐Electrolyte Strategy Boosted with Electrode Engineering for High‐Performance Ti3C2Tx MXene‐Based Supercapacitors

“…In addition, copper and vanadium ions have larger sizes than the proton, which give birth to larger spatial‐site resistances and higher energy‐potential barriers for ion deintercalation. [ 48 ] This may also hinder the self‐discharge rate. Thus, the improved self‐charge behavior could be mainly attributed to the restrained diffusion‐controlled parasitic Faradaic reactions, lessened ion‐redistribution and weakened ion deintercalation by the introduction of redox additives.…”

Additive Engineering Enables Ionic-Liquid Electrolyte-Based Supercapacitors To Deliver Simultaneously High Energy and Power Density

“…Supercapacitors have recently attracted great attention due to their high-power density, fast charging/discharging capability, and long cycle life. − However, the maximal shortcoming of supercapacitors is their low energy density, only about 1/10 ∼ 1/15 of Li-ion batteries. − As an indispensable component in supercapacitors, electrolytes and their synchronization with electrodes are responsible for high energy density as well as rapid dynamics and long-term stability. , Generally, the ionic conductivity, permittivity, and stability including thermodynamic, dynamic, and electrochemical stability are the key parameters to develop better electrolytes to promise high energy density, rapid charging, and high sustainable supercapacitors. , Taking into consideration of excellent ionic conductivity of aqueous electrolytes, the first RuO 2 -based supercapacitors used in the military field adopted H 2 SO 4 aqueous solution electrolyte to realize their large-current and high-power characteristics . However, due to the limitation of water decomposition potential (commonly <1.23 V), aqueous solution electrolytes exhibited pretty low energy density (∼2.0 Wh kg –1 ) in supercapacitors .…”

A novel bilayer heterogeneous poly(ionic liquid) electrolyte for high-performance flexible supercapacitors with ultraslow self-discharge

“…22 The discharge of electrodes or the overall devices often occurs due to faradaic reactions taking place on the charged electrodes, which is dominated by an activation-controlled mechanism and/or a diffusion-controlled mechanism. [23][24][25] Regarding aforementioned mechanisms, many efforts have been made to suppress the self-discharge behavior of supercapacitors through modification of electrode materials, 26,27 functionalization of electrolytes with additives [28][29][30] and the development of novel separators. 24,25 More recently, heterogeneous polyelectrolytes formed by a layer of polyanion complex and a layer of polycation complex have a unique current rectification effect, 31,32 which exhibited an interesting suppressed effect on the self-discharge of flexible supercapacitors due to the electrostatic interaction force between movable ions accumulated on/in electrodes and the charged polyelectrolytes.…”