A Critical Review on High-Energy-Density Liquid Fuels: Synthesis Strategies, Performance Enhancement, and Plasma-Assisted Modification Prospects

High-energy-density liquid fuels are critical energy materials for aerospace, advanced military equipment, and high-power propulsion systems. They offer notable advantages, including high energy density and controllable combustion characteristics. Their performance optimization and technological innovation are essential for enhancing the endurance, propulsion efficiency, and overall performance of power systems. This review focuses on polycyclic hydrocarbon fuels, high-energy ionic liquid fuels, nanofluid fuels, biomass-based fuels, and caged compound fuels. It comprehensively summarizes recent advances in these fuels. Polycyclic hydrocarbon fuels have achieved efficient synthesis processes by leveraging biomass valorization and green catalysis. High-energy ionic liquid fuels are classified into cyanamide-, borohydride-, azide-, and phosphine-based types. They achieve millisecond-scale hypergolic ignition through cation-anion structural regulation, making them important alternatives to hydrazine propellants. Nanofluid fuels modified by nanoparticle doping effectively balance enhanced energy density with optimized combustion efficiency. Biomass-based fuels, owing to their renewable and low-carbon characteristics, exhibit sustainable application prospects, with key performance metrics gradually approaching those of conventional petroleum-based fuels. Caged compound fuels possess high energy density and thermal stability due to their unique highly strained molecular structures. This review compares the synthesis routes, performance characteristics, and existing technical barriers of these fuels. It highlights the mechanisms and current applications of bifunctional catalysis, solid acid/ionic liquid catalysis, noble metal-supported catalysis, and plasma modification in fuel synthesis, catalyst regulation, and fuel activation. It also analyzes common scientific challenges, including poor catalyst-fuel compatibility, low coupling efficiency in multi-step reactions, and difficulty in scaling up laboratory processes. Finally, it outlines future directions for new green catalytic processes, multi-fuel blending, and continuous synthesis. This review aims to provide theoretical references and technical support for the molecular design, performance enhancement, and scalable preparation of high-energy-density liquid fuels.