Emerging two-dimensional (2D) MXene-based nanostructured materials:Synthesis strategies, properties, and applications as efficient pseudo-supercapacitors

Supercapacitors, which are essential for addressing modern energy challenges, possess outstanding energy storage capabilities without requiring manual maintenance. However, their low energy density and capacity limit their applicability for electrode materials. Recently, two-dimensional materials (2D), and particularly 2D transition metal carbonitrides (MXenes) have attracted considerable interest for developing energy storage devices because of their large specific surface area and fast ion-transport pathways. MXenes mainly comprise a transition metal (M) layer, carbon and/or nitrogen (X) layer, and active functional groups exterior to the layer. Metallic properties of the M layer offer excellent pseudocapacitance characteristics, while X layer is characterized by abundant active sites and conductivity similar to that of graphene. Compared with conventional materials, the existence of various functional groups on the surface of MXenes endows them with stronger hydrophilicity and higher wettability in aqueous electrolytes, which, combined with their diverse frame structures and functional group combinations, significantly augment their applicability for energy storage devices. Therefore, it is necessary to summarize and analyze the research results on MXenses. This review addresses the applications of MXenes in energy storage supercapacitors along with current challenges and solutions including intercalation, doping, surface modification methods, and heterogeneous structures. Finally, the prominent contemporary research trends are comparatively discussed to promote research and development in this field in the near future.