Emerging M₅X₄ MXenes: from atomic structure to applications

M₅X₄ MXenes constitute an emerging subclass of two-dimensional transition-metal carbides and nitrides distinguished by their extended metal–carbon frameworks, tunable interlayer spacing, and various surface terminations. These structural characteristics offer opportunities to modulate electronic conductivity, ion-transport pathways, and abundant redox-active sites, making M₅X₄ MXenes promising candidates for electrochemical energy-storage and conversion applications. This review critically examines recent progress in the synthesis of M₅X₄ MXenes, including molten-salt and fluoride-free etching strategies. It discusses how atomic structure, surface chemistry, and interlayer architecture govern charge-storage mechanisms ranging from electrical double-layer capacitance to pseudocapacitive behavior. Emerging studies on electrocatalytic hydrogen evolution are assessed, with an emphasis on structure–activity relationships and limitations in benchmarking. Key challenges, including phase stability, oxidative degradation, scalable synthesis, and device-level integration, are analyzed in detail. By identifying current knowledge gaps and outlining future research directions, this review aims to provide a balanced framework for the rational development of M₅X₄ MXenes toward sustainable energy-storage and conversion technologies.