Two-dimensional MXenes for electrochemical capacitor applications: Progress, challenges and perspectives

MXenes, a family of 2D transition metal carbides/nitrides with a general formula of Mn+1XnTx (n=1 similar to 3), show promising potential for energy storage applications owing to their 2D lamellar structure, impressive density, metallic-like conductivity, tunable terminations and intercalation pseudocapacitance charge storage mechanism. Various combinations of transition metals, C and/or N, as well as different n layers result in a broad range of MXenes, but only about a small number have been prepared and used for capacitive energy storage applications (supercapacitors) so far, with Ti3C2Tx being the most studied one. This review summarizes the recent advances, achievements and challenges of MXenes for supercapacitors. The preparation methods, composition versatility, material and electrode architectures, chemical modification and hybridization with other active materials of MXenes are presented. Moreover, the electrochemical characterizations and energy storage mechanisms of MXenes in aqueous and non-aqueous electrolytes are discussed, as well as the preparation of flexible and printable MXene-based electrodes and devices. Finally, perspective is also given, providing guidance for the future development of MXenes with advanced performances for designing the next generation of supercapacitors.