High-entropy carbides: structure, synthesis and catalytic applications.

High-entropy carbides (HECs) - multicomponent interstitial compounds with multiple transition metals distributed on a shared metal sublattice and carbon occupying interstitial sites - expand the catalytic design space beyond conventional unary and binary carbides, owing to their diverse local coordination environments, tunable metal-carbon bonding, and rich defect chemistry. Nevertheless, HECs, especially nanoscale HECs, remain far less developed than high-entropy alloys and conventional transition-metal carbides. Their vast compositional space is paired with tightly coupled challenges in phase formation, carbon stoichiometry, elemental mixing uniformity, surface chemistry and structure, all of which are difficult to regulate independently. This Review establishes a structure-synthesis-catalysis framework for the field. We examine the crystal chemistry, thermodynamic descriptors, and computational design strategies for navigating the extensive HEC composition space, and compare conventional thermal processing with emerging ultrafast synthetic routes. We further define the core structural evidence required to rigorously verify phase purity, intraparticle elemental distributions, carbon vacancies, and reaction-induced surface reconstruction. We then critically assess reported catalytic applications, distinguish well-substantiated findings from prospective opportunities, and identify key priorities for scalable synthesis, operando characterization, and predictive catalyst design. By bridging materials design principles with working-state structural verification, this Review provides a rigorous foundation for the rational development of HECs as functional catalytic materials.