High-Entropy Metal Nanomaterials for Energy Conversion and Environmental Remediation: Fundamentals, Design Strategies, and Future Directions

High-entropy metal nanomaterials (HEMNs) have emerged as a transformative class of catalysts for (photo)electro-, and photo(thermal)-catalytic applications. Their distinctive physicochemical features, including lattice distortion, sluggish diffusion, high-entropy stabilization, and the cocktail effect, enable precise modulation of active-site structures, enhanced durability, and improved resistance to catalyst poisoning. In this review, we first introduce the fundamental concept of HEMNs and discuss their structural and electronic characteristics. We then summarize recent advances in synthetic methodologies, ranging from conventional solid-state and wet-chemical routes to cutting-edge energy-driven and data-guided approaches. Strategies for optimizing catalytic performance are discussed from both thermodynamic and kinetic perspectives, highlighting composition tuning, nanostructure engineering, and defect design. We further provide a comprehensive overview of HEMN applications in electrocatalysis, photocatalysis, and photothermal catalysis, with representative examples in energy conversion, chemical transformation, and environmental remediation. Practical demonstrations of HEMN-based electrochemical devices are also highlighted to bridge the gap between fundamental research and real-world implementation. Finally, we present a forward-looking perspective on key challenges, such as phase stability, compositional complexity, and mechanistic understanding, and propose potential solutions to accelerate the rational design and scalable deployment of HEMNs. This review aims to provide a timely and comprehensive reference for both newcomers and experts, fostering the continued development of HEMN-based catalytic technologies for energy and environmental applications.