Emerging M5X4 MXenes: From Atomic Structure to Applications

Abstract

M5X4 MXenes represent a rapidly emerging subclass of two-dimensional layered materials distinguished by extended metallic networks, tunable interlayer spacing, and various surface terminations. These unique structural features endow them with exceptional electronic conductivity, abundant redox-active sites, and favorable hydrogen adsorption–desorption energetics, positioning them at the forefront of dual-function energy platforms, namely, ultrafast supercapacitors and highly efficient hydrogen evolution reaction (HER) catalysts. Recent progress in scalable synthesis, such as fluoride-free etching, molten-salt routes, and targeted functionalization, has expanded the compositional and structural diversity of M5X4 MXenes, enabling unprecedented electrochemical performance. This review examines synthesis strategies, structure–property correlations, and key electrochemical metrics, highlighting the capacitive behavior that spans electrical double-layer to pseudocapacitance and HER efficiencies that approach those of platinum-group catalysts. We critically evaluate challenges, including oxidative degradation, scalable production, and integration into practical devices, while proposing future pathways through advanced characterization, heterostructure engineering, and sustainable synthesis. By integrating current progress with forward-looking insights, this work lays a foundation for the design of multifunctional, scalable, and sustainable M5X4 MXene-based energy systems.