High-Entropy Alloy Nanoparticles for Electrocatalytic Nitrate Reduction: Bridging Water Treatment and Green Ammonia Synthesis

Nitrate contamination of aquatic environments and the high energy and carbon intensity of conventional ammonia production represent two major and increasingly interconnected sustainability challenges. Electrocatalytic nitrate reduction to ammonia (NO3RR) provides a compelling opportunity to address both by converting a widespread nitrogen pollutant into a value-added chemical and potential energy carrier under comparatively mild conditions. However, practical NO3RR remains constrained by sluggish multistep proton–electron transfer, competing hydrogen evolution, complex nitrogen-containing intermediates, and insufficient control over ammonia selectivity. High-entropy alloy (HEA) nanoparticles have recently emerged as a promising catalyst platform for overcoming these limitations because their multielement compositions generate diverse adsorption sites, lattice strain, and tunable electronic interactions that can regulate the binding and transformation of key NO3RR intermediates. This review critically examines the emerging role of HEA nanoparticles in electrocatalytic nitrate-to-ammonia conversion, with emphasis on the relationships among composition, local atomic structure, intermediate adsorption, and catalytic selectivity. Major synthetic strategies, including carbothermal shock, sputtering-based deposition, and wet-chemical co-reduction, are evaluated in terms of their ability to control elemental distribution, particle size, phase stability, and accessible active sites. Particular attention is devoted to mechanistic pathways involving *NO2, *NO, *NHx, and *NH2OH species and to how multimetallic surface environments redirect reaction pathways toward NH3 while suppressing N2, N2O, and H2 formation. Reported Faradaic efficiencies, ammonia production rates, stability, and nitrate-conversion metrics are further benchmarked against conventional monometallic and bimetallic catalysts. Beyond activity under model electrolyte conditions, this review highlights critical knowledge gaps associated with realistic nitrate-contaminated waters, coexisting ions, catalyst reconstruction and metal leaching, long-term stability, energy efficiency, ammonia recovery, and techno-economic scalability. By integrating environmental remediation with decentralized ammonia synthesis, HEA nanoparticles offer a versatile materials framework for advancing nitrate electroreduction toward practically relevant and circular nitrogen-management technologies.