2D MXenes: Synthesis, properties, and electrochemical energy storage for supercapacitors – A review

Supercapacitors are one of the most frequently explored devices for energy storage applications. In comparison with conventional dielectric capacitors, supercapacitors have energy storage capacities several orders of magnitude higher, however much lower than those of secondary batteries. Their long-life cycles, high power densities, and relatively less carbon footprint over their counterparts have encouraged industries to explore and build reliable energy systems for the future. One family of materials that have garnered attention for supercapacitor applications since their discovery in 2011 are 2D transition metal carbides and nitrides (MXenes). Unlike 3D carbon, which possesses complex ion diffusion pathways, 2D nanosheets directly offer large active sites to the electrolyte. With a high surface area, shorter ion diffusion pathways, and high conductivity, MXenes enhance the energy storage characteristics of a supercapacitor. The key to high rate pseudocapacitive energy storage in MXene electrodes is the hydrophilicity of MXenes combined with their metallic conductivity and surface redox reactions. In this review, we have explored different types of supercapacitors, charge storage mechanisms, and modified synthesis methods of MXene and its properties. Finally, we discuss the advancement in this field while evaluating future challenges and prospects of MXene composites, which will provide a guide for developing high-performance MXene-based energy storage devices with high throughput and sustainable derivatives.