Synthesis and nano-engineering of MXenes for energy conversion and storage applications: Recent advances and perspectives

MXenes, with general formula M_n+1X_nT_x, (where n = 1–4; M = early transition metals; X = C, N, or a combination of both; T_x = surface functional groups like –OH, -O, -F, or -Cl) are a diverse group of two-dimensional (2D) layered transition metal carbides, nitrides and carbonitrides. Due to the energy and environmental problems, the renewable energy resources have critical importance. Owing to their exceptional properties, including very high electrical conductivity and thermal stability, MXenes are finding increasing applications in energy conversion and storage devices to solve the energy problems of modern society. In this review, we aim to provide a timely snapshot of recent advances in the synthesis, design, and engineering of MXene-based materials for the energy sector. Strategies for optimizing the performance of MXenes materials in energy applications, such as surface nano-engineering and compositing with 0D, 1D, 2D, and/or 3D materials are explored in the context of key energy conversion and storage devices. To fulfil the basic requirement of renewable energy devices, electrocatalysis of small molecular reactions such as ORR, OER, HER, NRR, and CO₂RR, are comprehensively discussed on MXene-based electrode materials. Further, the MXene-based electrode materials for energy storage devices such as metal-ion batteries (Na⁺, Li⁺, K⁺, etc.), Li-S batteries and supercapacitors, are also summarized. Finally, challenges and future opportunities for MXenes in these different energy applications are discussed. This article may provide a leading route for design and synthesis of new catalytic materials toward efficient performance of energy conversion and storage devices.