The high-entropy effect, derived from the near-equiatomic incorporation of five or more elements, leads to high configurational entropy (ΔS _mix ≥ 1.5 R) (Wang et al., 2024) and low Gibbs free energy of intermetallic compounds (ΔG _mix = ΔH _mix − TΔS _mix) (Lee et al., 2023), favoring the formation of stable solid-solution phases over intermetallics. At the nanoscale, this effect is amplified by high surface energy, enabling single-phase structures and enhancing long-term stability in electrochemical environments. Homogeneous multi-element distributions further ensure consistent catalytic activity.

Atomic size mismatch induces severe lattice distortion in HEA-NMs, disrupting periodic structures and modifying the local electronic environment (Li et al., 2024b). This shifts d-band centers and tunes adsorption energies of key intermediates, boosting catalytic performance. Nanoscale effects intensify distortion and promote favorable electronic interactions for reactions such as CO₂ and NO₃ ⁻ reduction (Shaikh et al., 2024).

The multi-element environment introduces diverse diffusion barriers, reducing atomic mobility and suppressing grain coarsening (Lee et al., 2023). This “sluggish diffusion” helps preserve the nanostructure and compositional uniformity under harsh conditions, contributing to the thermal and electrochemical durability of HEA-NMs.

The cocktail effect describes the synergistic properties from multielemental interactions that go beyond the sum of individual elements (Kamaruddin et al., 2024). These include coexisting catalytic sites, adaptive surface reconstructions, and multi-pathway reactivity, all of which enhance charge transfer and overall catalytic versatility (Liu et al., 2024).

In addition to the above structural and electronic effects, the careful selection of elemental combinations is essential in the designation of HEA-NMs. Factors such as mixing enthalpy, atomic size, and electronegativity must be considered to ensure the formation of single-phase solid solution nanoalloys. Moreover, computational approaches such as density functional theory (DFT) and machine learning (ML) play an increasingly important role in elucidating the microenvironment of HEA-NMs and forecasting interfacial reactions. For instance, Roy et al. (2022) demonstrated the use of ML algorithms to screen high-entropy alloy catalysts suitable for the selective hydrogenation of CO₂ to methanol. These predictive tools provide valuable insights for the rational design of HEA-NMs tailored for specific electrocatalytic applications.

This solid-state method uses high-energy ball milling to alloy elemental powders through repeated cold welding and fracturing (Figure 1a) (Mongella et al., 2025). It enables the formation of metastable solid solutions from even immiscible elements. Post-annealing or cryomilling enhances grain refinement. Though scalable and solvent-free, it often yields irregular or aggregated particles with limited morphology control.

This technique allows atomic-level control by co-depositing multiple metals onto substrates (Figure 1b). Parameters like power and deposition time tune size and composition. CrMnFeCoNi HEA-NMs synthesized via ionic liquid as substrate showed exceptionally high electrocatalytic activity and phase stability (Löffler et al., 2018). This method is ideal for fabricating model electrocatalysts for fundamental studies.

Electrochemical methods involve co-deposition of multiple metal ions onto conductive substrates under applied potential (Figure 1c) (Bian et al., 2023). This strategy enables the direct growth of HEA-NMs with controllable thickness and crystallinity. It is energy-efficient, room-temperature, and easily scalable. However, achieving uniform deposition of elements with vastly different reduction potentials remains a challenge, often requiring pulse techniques or complexing agents to balance deposition kinetics. These issues become more pronounced during scale-up, with reduced control over composition and particle uniformity.

Ultrafast shock synthesis enables rapid formation of HEA-NMs under non-equilibrium conditions, typically using carbothermal shock, Joule heating, or laser ablation. These methods achieve heating rates exceeding 10⁵ K/s, promoting atomic-scale mixing and preventing phase segregation (Figure 1d). The resulting nanoparticles are ultrasmall, often below 5 nm, providing better atomic control. For instance, PtPdRhRuCe HEA-NMs synthesized via carbothermal shock showed enhanced ammonia oxidation activity and phase stability (Yao et al., 2018).

Co-reduction or polyol processes allow controlled nucleation and morphology tuning with scalability. These cost-effective methods are surfactant-compatible but require careful optimization to prevent contamination. Wei et al. synthesized PtMoPdRhNi HEA-NMs exhibiting an ultralow overpotential of 9.7 mV at 10 mA cm⁻² in the alkaline HER via this approach (Figure 1e) (Wei et al., 2023). However, during scale-up, variations in nucleation and reduction rates often lead to elemental segregation and broader particle size distributions.

Each synthesis method for HEA-NMs has specific strengths and limitations in atomic control, scalability, cost, and tunability. Mechanical alloying is inexpensive and scalable but lacks control over morphology and composition. Sputter co-deposition offers atomic precision, suited for mechanistic studies, though high cost and poor scalability limit broader use. Electrochemical deposition is energy-efficient under mild conditions, but uniform co-deposition is difficult due to reduction potential differences. Ultrafast shock synthesis rapidly produces ultrafine, mixed nanoparticles but needs specialized equipment and has limited compositional control. Wet-chemical synthesis is low-cost and flexible but prone to segregation and contamination. Thus, synthesis strategies should match the performance and scalability demands of the target application.

Despite progress, scaling HEA-NM synthesis remains difficult. During batch production, elemental segregation arises from differences in reduction kinetics and nucleation, especially in wet-chemical and electrochemical methods. These approaches also struggle to maintain uniform particle size and composition. Phase control is harder under thermal and concentration gradients at Gram scale. While mechanical alloying and ultrafast shock synthesis show gram-scale potential (e.g., Yao et al., 2018), industrial adoption is limited by reproducibility, cost, and complexity. Solving these issues is key to enabling HEA-NMs in practical electrocatalytic systems.

HEA-NMs enhance HER and OER via d-band modulation and atomic-level disorder, promoting water dissociation and intermediate adsorption (Wei et al., 2023). In HER, tuning ΔG_H* across heterogeneous sites improves both Volmer and Tafel steps (Huang et al., 2024). In OER, a wide distribution of *OOH binding energies enables overpotential reduction and activity enhancement (He et al., 2023). The entropy-stabilized structure and “cocktail effect” contribute to long-term durability. HEA catalysts also maintain stable performance over a broad range of loadings, enabling integration into thin-film and porous electrodes for practical water electrolysis applications.

HEA-NMs improve ORR activity by tuning electronic structures and *OOH adsorption through multi-element alloying (Zhao et al., 2024). The disordered surface provides a range of binding energies, promoting 4e⁻ pathways and enhanced kinetics. Many HEAs surpass Pt/C in mass activity, half-wave potential, and durability (Chen et al., 2023a). Their corrosion resistance and composition flexibility allow operation across pH conditions and electrolytes (Xie et al., 2024a). HEA catalysts also deliver stable performance across various loadings, supporting their integration into cathodes for fuel cells and metal–air batteries (Figure 2b).

HEA-NMs offer diverse active sites for key intermediates such as *COOH, *CO, and *CHO, enabling tuning of adsorption energies and product selectivity (Ding et al., 2021; Nellaiappan et al., 2020). Stronger *CO binding favors CH₄ formation, while weaker binding promotes CO evolution. Multi-metallic surfaces facilitate simultaneous stabilization of intermediates and C–C coupling (Xing et al., 2025). Nanoscale features, including high surface area and short diffusion lengths, enhance conversion efficiency. HEA catalysts demonstrate low overpotentials, high current densities, and selective production of hydrocarbons and alcohols, showing strong potential for CO₂ utilization in energy devices (Figure 2c).

In NO₃RR, product selectivity (N₂, NH₃, NO₂ ⁻) depends on both thermodynamic and kinetic factors, including *NO, *NH₂, and *N adsorption strengths (Chen et al., 2025). Strong *N binding favors NH₃, while weaker adsorption promotes N₂ evolution. HEA-NMs offer tunable active sites that selectively stabilize intermediates like NO₂ ⁻ and NH₂OH, enabling controlled electron transfer and improved selectivity (Tang et al., 2024). Local pH affects *NH₃ desorption and catalyst durability. Cl⁻-containing electrolytes can generate reactive chlorine species, further enhancing nitrogen selectivity by suppressing by-products (Tokazhanov et al., 2020). Our FeCoNiCu-based HEAs demonstrate high nitrate conversion and Faradaic efficiency (Figure 2d), confirming their promise in water treatment and ammonia synthesis (Chen et al., 2025; Wu et al., 2025).