Synthesis and properties of 2D MXenes and their composite electrodes for supercapacitors

MXenes, first discovered in 2011 by etching their parent MAX phase, represent a novel type of 2D materials with unique electrochemical and electronic characteristics, becoming the cutting-edge research materials in a wide variety of applications. Their unique merits, including metal-like conductivity, high density, abundant and tunable surface end-groups and unique two-dimensional planar structure, have positioned them as promising 2D material in the field of supercapacitors in recent years. Therefore, a timely overview of MXenes is essential to provide guidance for advanced research in energy storage materials. In this review, we aim to summarize the recent progress on MXene research on supercapacitors. This review begins with the intrinsic properties such as mechanical, electrical and stable properties, and then focuses on the multiple synthesis strategies of MXenes, including top-down and bottom-up methods. Most importantly, the application of MXenes and their composites (including MXene/Metal LDHs, MXene/Metal sulfides, MXene/Metal oxides, MXene/Conductive polymers and MXene/Carbon nanomaterials) in supercapacitors as electrodes are mainly analyzed along with the detailed mechanism and electrochemical performance. Finally, an outlook on the current challenges and future opportunities of MXene-based supercapacitors is presented.