MXenes serving aqueous supercapacitors: Preparation, energy storage mechanism and electrochemical performance enhancement

MXenes, a burgeoning class of two-dimensional (2D) transition metal carbides, carbonitrides/nitrides with excellent physicochemical properties, are considered to be promising candidates for electrodes in aqueous supercapacitors. However, many obstacles still persist in the development of MXenes in aqueous supercapacitors. For example, hazardous and inefficient hydrofluoric acid (HF) etchants lead to low yield and MXene surfaces containing non-electrochemically active fluorine terminations, the process of charge storage is not well clarified, and the inherent self-stacking problem of 2D materials needs to be addressed. These drawbacks seriously affect the utilization of MXene in aqueous supercapacitors. Therefore, this work reviews conventional HF preparation methods and their limitations, outlines the etching methods, including hydrothermal, electrochemical, molten salt, and various new fluorine-free preparation methods, and assesses the principles and characteristics of these routes. Meanwhile, the energy storage mechanisms of MXene in various aqueous electrolytes are summarized, highlighting the anomalous ion intercalation behavior and pseudocapacitance triggering observed in “water-in-salt” electrolytes. Finally, the structural designs and chemical modifications to enhance the capacitive performance of MXenes are compared, their effects on the energy storage mechanism are discussed, and the design principles of MXene-based composites are presented. This work highlights the critical challenges in future research and provides a comprehensive reference for the design of MXene aqueous supercapacitors.