Rise of high-entropy MXenes in electrocatalysis for sustainable energy conversion

High-entropy (HE) MXenes constitute an emerging class of two-dimensional carbides and carbonitrides that combine the tunable surface chemistry and metallic conductivity of MXenes with the configurational entropy, lattice distortion and multi-metal interactions of high-entropy materials (HEMs). This review presents the fundamental structural and thermodynamic features of MXenes, HEMs and HE-MXenes, establishing how multi-component M-sublattices and mixed terminations create ensembles of adsorption sites relevant to electrocatalysis. Progress in the synthesis and structural design of HE-MXenes is summarised, covering design of HE-MAX precursors, etching and delamination strategies, and advanced structural and chemical characterisation that enable control over composition, defects and surface terminations. The current understanding of electronic structure, active-site motifs and defect/termination chemistry in HE-MXenes is discussed, with emphasis on how lattice distortion and local coordination govern adsorption energetics. On this basis, recent advances in HE-MXenes and HE-MXene-based heterostructures for hydrogen evolution and oxidation, oxygen evolution and reduction, and other electrochemical conversion reactions, particularly CO₂ and N₂ reduction, are critically assessed against conventional MXenes, high-entropy catalysts and noble-metal systems. Furthermore, key challenges and opportunities are identified, including synthetic control of multi-metal distributions, coupled termination-defect engineering, operando interrogation of active phases and predictive modelling of high-dimensional composition space, with a view to guiding the rational deployment of HE-MXenes in sustainable electrocatalytic energy-conversion technologies.