Concluding remarks: Achievements, challenges, and trajectories for high-entropy alloy nanoparticles

The field of high-entropy alloy nanoparticles (HEA-NPs) had to move and indeed has moved beyond the early enthusiasm of simply "mixing five or more elements and hoping the configurational entropy will do some magic" in terms of chemical and physical properties. What the 2025 Faraday Discussion made clear is that, at the nanoscale, entropy is often a minor player. Phase stability, structure, and functional properties are instead dominated by kinetics, surface reconstruction, defects, segregation, decomposition, and - more often than we like to admit - (unnoticed) interstitial contamination (C, O, N, H, B, S) picked up during synthesis and during particle exposure to "real-world" environments. Bulk HEAs can still "hide" a bit behind their mostly metastable single-phase character and complex diffusion mechanisms, and often even exploit these features to their advantage, but nanoparticles have no such luxury. Their huge surface-to-volume ratio, rapid synthesis pathways, and exposure to harsh operational environments reveal the true thermodynamic and kinetic transient features of most reported compositions. We see decomposition, demixing, surface reconstruction, and dynamic ensemble behaviour that have in part little to do with the ideal solid-solution picture painted a decade ago. These profound differences between bulk HEAs and nanosized ones provide opportunities to be embraced and exploited for well-targeted and theory-guided development steps. The community therefore should feel encouraged to pivot. Instead of chasing ever more complex average compositions, we must focus on what really governs the usually transient features and stability of these particles, particularly at the surface and their dynamical states in real environments, and how we can deliberately exploit kinetic barriers, short-range ordering, defects and their chemical decoration, and surface dynamics to achieve emergent properties unattainable in conventional nanoscale alloys. Theory and simulation must leave the O(N³) constraints of traditional DFT behind and embrace large-scale, accurate machine-learning potentials and property-driven screening of the astronomical configuration space. Synthesis has to become far more chemically aware and reproducible, with rigorous control (and transparent reporting) of interstitials, decomposition and decay kinetics when the particles are used. And finally, functional validation can no longer rely on post-mortem snapshots; we need genuine operando insight into displaced reaction features, ordering, dynamical reconstruction, composition, and dynamics of those atomic clusters that actually do the catalytic or magnetic work. If we accept that HEA nanoparticles are inherently kinetically highly variable, defect-rich, surface-dominated objects, then more adequate design metrics need to be found, going beyond the so far primarily used single mean-field metric of configurational (bulk) entropy. This could open a pathway towards a more systematic, realistic and holistic design approach for true nanoparticle multifunctionality and their real-world stability. Papers presented at the 2025 Faraday Discussion have shown that opportunities along these lines might be lurking in the fields of magnetically active catalysts, noble-metal-lean electrocatalysts, or materials that combine corrosion resistance, thermal stability, and high activity in one particle. The road ahead is demanding, but the potential payoff for more sustainable catalysis and magnetic applications and beyond is high. This boils down to the statement that it is time to stop treating high-entropy nanoparticles as merely "miniaturized" bulk HEAs and start treating them as the fascinating new materials class they really are.