Cu²⁺ as a dynamic director for Ce-incorporated (CoFeNiCuCe)₉S₈ nanoballs for multifunctional electrocatalysis

Abstract

High-entropy metal sulfides (HEMSs) have emerged as a new class of electrocatalysts, but their synthesis often faces challenges due to their inherent complexity arising from multi-metal interactions, especially with elements having large differences in atomic/ionic sizes, such as the redox-active rare-earth elements. Here, we report a low-temperature (200°C) hydrothermal strategy to fabricate Ce-incorporated (CoFeNiCuCe)₉S₈ nanoballs by leveraging Cu²⁺ as a dynamic director for this phase evolution. Time-resolved studies reveal a multistage growth pathway involving cation exchange, lattice strain-driven reconstruction, and coalescence of various low and medium entropy intermediates (CoFeNi)₉S₈, CuS/(CoFeNi)₉S₈, (CoFeNiCu)₉S₈, Ce₂S₃, Ce₂S₃/(CoFeNi)₉S₈, (CoFeNiCuCe)/S nanoplates) into monodisperse (CoFeNiCuCe)₉S₈ HEMS nanoballs. By systematically varying Cu:Ce ratios, we obtain five distinct configurations, and Ce-rich HEMS-4 (Cu:Ce = 1:4) exhibits superior multifunctional electrocatalytic performance, outperforming a series of lower- (Co₉S₈, (NiFe)₉S₈, and (CoNiFe)₉S₈), medium-((CoNiFeCu)₉S₈), and high-entropy ((CoNiFeCuCe)₉S₈) analogues in the oxygen evolution reaction (OER; η₁₀ = 175 mV, η₁₀₀ = 260 mV), urea oxidation reaction (UOR; 1.277 V and 1.336 V at 10 and 100 mA.cm⁻²), hydrogen evolution reaction (HER; η₁₀ = 85 mV), and nitrite reduction (NO₂RR; 0.112 V at 100 mA.cm⁻²). Post-catalytic and in-situ Raman analyses, in conjunction with density functional theory (DFT), show that metal (oxy)hydroxides form during the reaction, while d–f orbital interactions protect the active sites from over-oxidation. This work establishes a paradigm for integrating rare-earth elements into HEMSs via controlled solution-phase synthesis, advancing the design of high-entropy electrocatalysts.

High-entropy metal sulfides have emerged as a new class of electrocatalysts, but their synthesis often faces challenges due to their inherent complexity arising from multi-metal interactions. Here, the authors report a low-temperature hydrothermal strategy to fabricate Ce-incorporated (CoFeNiCuCe)₉S₈ nanoballs by leveraging Cu²⁺ as a dynamic director for controlled solution-phase synthesis.

Introduction

High-entropy materials (HEMs) composed of five or more metal elements in near-equimolar or tailored ratios have emerged as a unique class of solids with great potential for energy and electronic applications due to synergistic cocktail effects^1,2. While traditionally explored as alloys³ and oxides^4–6, the HEMs background has recently expanded to include high-entropy metal sulfides (HEMSs), which offer tunable electronic structures, abundant catalytic sites, and lower synthesis temperature^7–10. Their electronic structures can be finely tuned through compositional variation, offering great potential for electrocatalysis, energy storage and conversion, thermoelectric and gas sensing applications^11–16. In particular, HEMSs show potential in electrocatalytic reactions, including the oxygen evolution reaction (OER) and hydrogen evolution reaction (HER). To enhance their utility, it is necessary to understand their formation, including thermodynamics of host-guest ion compatibility, the dynamics of ion exchange, and secondary phase formations, which is recently emerging in HEMS materials engineering. For instance, Cu₁.₈S has been used as a template for introducing Zn, In, Ga, and Co ions^8,17, while K₂RuCl₆-based systems have enabled CER between K₂RuCl₆/K₂PtCl₆ and CoSn(OH)₆¹⁸.

Although mechanistic investigations are still ongoing, much of the focus so far has been on the synthetic efforts to construct complex HEMSs using diverse strategies, such as solid-state reactions¹², solvothermal processes¹⁹, thermal decomposition²⁰, and cation exchange reactions (CERs)⁷. Examples, (CrMnFeCoNi)₉S₈ nanoparticles were synthesized via pulsed thermal decomposition, which demonstrated a ~20% enhancement in OER activity compared to unary sulfides²¹, and solvothermal approach enabled the synthesis of FeNiCoCrX-S₂ (X = Cu, Mn, Zn, Al) systems with robust long-term OER performance²². Other notable examples, such as (MnFeCoNiCu)S₂²³, (CoZnCdCuMnS@CF)¹⁰, TiFe₂Co₂Ni₃MoS₁₀²⁴, (FeCoNiCrCuAl)S@HCS²⁵, and (ZnCoMnFeAlMg)₉S₈¹⁹_, further illustrated  the water oxidation and reduction performances with significant potential of HEMSs. Despite these promising advances in HEMSs, many are d-band-based catalysts that often suffer from the intrinsic properties governed by the Sabatier principle. Specifically, while the stabilization of intermediates like *OH, *O, and *OOH is essential, overly strong adsorption can hinder O–O bond formation and desorption, resulting in high overpotentials and sluggish kinetics during anodic reactions^26,27. Moreover, the synthesis of HEMSs is inherently complex due to the need for atomic-level control over multi-metal interactions.

To address these limitations, we explore the incorporation of cerium (Ce), a redox-flexible f-block element, into a HEMS matrix. Ce offers unique electronic features, including a Ce³⁺/Ce⁴⁺ redox couple, which can redistribute charge density across the catalyst interface, suppress overoxidation of transition metal centers, and promote more efficient intermediate conversion^28,29. Moreover, the d–f orbital coupling between Ce and transition metals such as Co, Ni, and Fe allows modulation of d-band centers, thereby weakening *OOH adsorption and accelerating the rate-determining O–O bond formation. The large ionic radius of Ce³⁺ (~1.14 Å) induces lattice strain and local disorder, resulting in undercoordinated sites and increased electrochemical surface area^30,31. Additionally, Ce-S interactions can passivate sulfur vacancies, thereby improving the long-term stability of the catalyst³². However, the strong oxophilicity of Ce poses synthetic challenges, often leading to CeO₂ phase segregation or structural collapse under high-temperature conditions³³. Additionally, due to the large size of Ce cations, the formation of (CoFeNi)₉S₈ based Ce-doped high entropy systems are challenging. Moreover, even such materials lower/medium entropy formation mechanism is poorly understood due to the inherent complexity associated with distinct redox potentials, ionic radii, and coordination preferences.

To address these challenges, we developed a low-temperature (200 °C) hydrothermal strategy by carefully tailoring the Cu:Ce ratios relative to Co/Fe/Ni metal ions, enabling the fabrication of morphology-controlled, sulfate-incorporated (CoFeNiCuCe)₉S₈ HEMS nanoballs. This method not only mitigates the issues associated with Ce oxophilicity but also allows for time-resolved tracking of the nucleation and growth process. Taking advantage of the mild synthesis conditions, we conducted a systematic 48-hour thermal evolution study, revealing a previously unreported multistep formation pathway. The process involves sequential lower to medium entropy intermediates: (i) (CoFeNi)₉S₈ clusters (ii) CuS/(CoFeNi)₉S₈ nanospheres, (iii) porous (CoFeNiCu)₉S₈ nanospheres, (iv) Ce₂S₃/(CoFeNi)₉S₈ bulk plates, and (v) metastable (CoFeNiCuCe)/S nanoplates, which ultimately coalesce into monodisperse (CoFeNiCuCe)₉S₈ HEMS nanoballs. Cu²⁺ leaching from the nanospheres (ii) triggers a cation exchange reaction (CER), preferentially displacing Fe²⁺, leading to various intermediate states (iii and v). Concurrently, the Ce₂S₃-rich plates degrade into nanoplates, driven by lattice strain from incorporating Co/Fe/Ni/Cu into the Ce₂S₃ matrix. By 24–48 h, these intermediates evolve into monodisperse HEMS nanoballs (~2–3 µm) with uniform elemental distribution (Co, Fe, Ni, Cu, Ce). The role of Cu²⁺ ions is obvious as confirmed by the non-formation of Ce doping in CoFeNi sulfides in this reaction. Furthermore, we also systematically explored their electrochemical activities from lower to medium to high entropy MSs, with special emphasis on non-equimolar Cu/Ce-based HEMS systems. Among the resulting materials, the Ce-rich composition (HEMS-4, Cu:Ce = 1:4) exhibits the most promising multifunctional activities, including low OER overpotentials (175 mV at 10 mA cm⁻² and 260 mV at 100 mA cm⁻²), excellent HER performance (85 mV at 10 mA cm⁻²), and high activity in UOR (1.277 V) and NO₂RR (0.112 V). This work elucidates the complex interplay of kinetic and thermodynamic factors governing the morphology-controlled synthesis of high-entropy phases, offering a generalizable strategy to integrate rare-earth elements into multicationic chalcogenides.

Results and discussion

Synthesis and nucleation growth of MS and HEMS nanoballs

Figure 1a shows the schematic representation of synthesis for various MS nanoballs, including low, medium, and high-entropy systems. These were prepared from their respective metal salts (Co, Fe, Ni, Cu, and Ce) dissolved along with thiosalicylic acid (metal to sulfur ratio, 1:20) in dimethyl formamide (DMF) using the hydrothermal method conducted at 200 °C for 48 h. Within HEMS, again, 5 further compositions are prepared based on the variation of Cu:Ce atomic ratios (1:1, 1:2, 1:3, 1:4, and 4:1) and labeled as HEMS-1 to 5. The detailed synthesis procedures of these metal sulfide nanoballs are given in the supporting information (S0). Figure 1b(i–v) indicates the representation and corresponding scanning electron microscopy-energy dispersive X-ray spectrometry (SEM-EDS) elemental maps of U, B, T, Q-MSs, and HEMS nanoballs, such as [Co₉S₈], [(NiFe)₉S₈], [(CoFeNi)₉S₈], [(CoFeNiCu)₉S₈], and [(CoFeNiCuCe)₉S₈], respectively. The approximate sizes of these nanoballs of U, B, T, Q-MS to HEMS-4 are 2 ± 0.5, 1.5 ± 0.5, 3 ± 0.5, 4 ± 1, and 2 ± 0.5 µm, respectively. They show the presence of corresponding metal sulfides, whose elemental composition is consistent with M₉S₈ (Table S1a)_. Both ICP-OES and SEM-EDS analyses confirm the composition of HEMS nanoballs (1–5), resembling the M₉S₈ system (Table S1b). For example, HEMS-4 exhibits an elemental composition of Co_2.46Fe_1.52Ni_2.05Cu_1.08Ce_1.9S₈, in close agreement with the designed stoichiometry. The crystal structures of Co₉S₈ and (CoFeNiCuCe)₉S₈ systems are represented in Fig. 1c, d, respectively. EDS spectra obtained from several MSs (U, B, T, Q-MS, and HEMS-4) confirm the presence of the expected elements (Co, Fe, Ni, Cu, and Ce) across the sulfide nanoballs (Fig. 1e).

Fig. 1. Structural and compositional evolution of high-entropy metal sulfide nanoballs.

a Schematic illustration of the stepwise synthesis pathway from unary to quinary metal sulfide nanoballs. b Structural overview and corresponding SEM-EDS elemental mapping of (i) U-MS: Co₉S₈ (5.0 μm), (ii) B-MS: (NiFe)₉S₈ (8.0 μm), (iii) T-MS: (CoFeNi)₉S₈ (3.0 μm), (iv) Q-MS: (CoFeNiCu)₉S₈ (2.0 μm), and (v) HEMS-4: (CoFeNiCuCe)₉S₈ (2.5 μm). c, d Crystal structure models of U-MS and HEMS-4, respectively, illustrating the structural transformation with increasing compositional complexity. e EDS spectra of various nanoballs, emphasizing their compositional evolution from U-MS to HEMS-4. f XRD patterns of the nanoballs across the series, indicating phase formation and structural stabilization.

Figure 1f shows the powder XRD patterns of different MSs (U, B, T, Q-MS, and HEMS-4) nanoballs, revealing that all five compositions [Co₉S₈], [(NiFe)₉S₈], [(CoFeNi)₉S₈], [(CoFeNiCu)₉S₈] and [(CoFeNiCuCe)₉S₈] exhibit a common cubic phase (Fm-3m) matching with Co₉S₈ structure and its Co_5.58Fe_1.6Ni_1.82S₈ alloy composition (PDF#01-086-2273, #01-078-0170). The identical XRD patterns of the medium-entropy (CoFeNiCu)₉S₈ and high-entropy (CoFeNiCuCe)₉S₈ systems confirm that the [(CoFeNi)₉S₈] crystal structure is preserved upon Ce incorporation in the HEMS system. The first three compositions are Cu-free and exhibit no impurity phases, whereas the Cu-containing quaternary and HEMS compositions display Cu-Fe-S phases. Specifically, CuFeS₂ appears under Cu-deficient conditions (HEMS-4) and Cu₅FeS₄ under Cu-rich conditions (HEMS-5), as shown in Fig. S1. The detailed diffraction pattern analyses of all synthesized materials are provided in Supplementary Note S1.

First, we discuss the HEMS-4 nanoballs shown in Fig. 1b(v), because of their better performance. The nucleation and growth evolution of HEMS-4 nanoballs was further investigated through the SEM-EDS, XRD, and BET surface area analyses during the course of 48 h reaction. The HEMS-4 nanoballs formation took place only after extended hours of hydrothermal treatment (above 30 h), as seen by analyses of the intermediate products collected at different reaction times (1, 3, 6, 12, 24, and 48 h). The findings lead to the accomplishment of the high-entropy phase. The results elucidate a multi-step transformation process before the accomplishment of the high-entropy phase.

The development includes nucleation of M₉S₈-based core system, followed by CER processes and the evolution of multi-metal mediated lower to medium entropy intermediate phases (Fig. 2a(i–vi)). During the initial stage (1 h of hydrothermal treatment), pentlandite-type (CoFeNi)₉S₈ clusters (~100 nm) form via the incorporation of Ni²⁺ and Fe²⁺ into the Co₉S₈ lattice (Eq.1; Fig. 2a(i) and S2A)³⁴. After 3 h of hydrothermal treatment, the product exhibits two distinct morphologies: fully encapsulated CuS/(CoFeNi)₉S₈ nanoballs (3 ± 1 µm) (Figs. 2a(ii) and S2B), and bulk plate-like (Ce₂S₃)_rich(CoFeNi)₉S₈ structures (Eq. 2; Figs. 2a(iv) and S2C). SEM-EDS maps of CuS/(CoFeNi)₉S₈ nanospheres (Fig. S2B) show the presence of all four metal elements (Co, Fe, Ni, and Cu) with higher copper proportions. In the case of plates, the EDS maps (Fig. S2C) show the dominating amounts of Ce₂S₃ along with minor quantities of CoFeNi (at% = Co (27.3), Fe (50.75), and Ni (31.86). During the reaction progress, at 6–12 h, these CuS/(CoFeNi)₉S₈ nanospheres transform into porous nanospheres (Fig. 2a(iii)) and branched morphologies (Figs. 2b(iiia–c) and S2D) composed of the (CoFeNiCu)₉S₈ system. They eventually undergo size reduction towards (CoFeNiCu)-sulfide clusters. SEM size analysis confirms that the porous spheres expand in size by at least fourfold compared with the previous nanospheres, reaching 10 ± 2 µm. This indicates the release of CuS followed by volume expansion (Eq. 3; Fig. 2a(iii) and S2B, D). This occurs as a result of dual mechanism, where the Cu²⁺ ions from CuS undergo (1) substantial leaching, (2) CER took place preferentially, displacing the Fe²⁺ into the bulk of (CoFeNi)₉S₈, leading to the formation of a medium entropy CoFeNiCuS (Fig. 2a(ii–iii)). In case 1, the leaching of Cu²⁺ ions is confirmed by SEM-EDS spectra (Fig. 2c) and corresponding elemental percentage analyses, which demonstrate 34% reduction in Cu content (Fig. 2f(ii–iiia)). In case 2, the preferential Cu²⁺ diffusion is confirmed by the presence of all four elements (Co, Fe, Ni, and Cu) (Fig. 2a(iii)), at the expense of 12% reduction in Fe content (Fig. 2f(ii–iiia)), leading to Co-Ni-rich sulfides (Fig. S2A). The leached Fe²⁺ reacts with CuS and gives a secondary phase Cu₅FeS₄ nanocubes (Eq. 4 and Fig. S2D). There is consistency in the correlation between leaching tendency (Fe > Co > Ni) and Cu alloying affinity (Ni > Co > Fe)^34,35. The initially discussed (Ce₂S₃)_rich(CoFeNi)₉S₈ bulkplates are present ubiquitously from the beginning of reaction to 12 h of reaction (6–12 h) (Figs. 2a(iv) and S2C)³⁶.

Fig. 2. Growth mechanism and compositional evolution of HEMS-4 nanoballs.

a FE-SEM images showing morphological changes from initial intermediates (i–v) to the final HEMS-4 nanoballs (vi). b Magnified FE-SEM images highlighting the transition from Cu-rich nanospheres to Cu-deficient branched structures. c EDS spectra of early-stage Cu-rich nanoballs with small Ce₂S₃ plates and intermediate porous nanospheres. d EDS analysis of micron-sized Ce₂S₃-rich plates coexisting with CoFeNi domains and emerging HEMS nanoplates. e Specific surface area measurements of selected intermediates during the formation process. f Elemental composition (At%) of intermediates and final HEMS-4 nanoballs derived from SEM-EDS analysis. SEM-EDS elemental mappings of porous (CoFeNiCu)₉S₈ spheres (g) and HEMS nanoplates (h) showing spatial elemental distribution. i X-ray diffraction patterns of key intermediates (♦ (CoFeNi)₉S₈, ■ Ce₂S₃, ▲CuFeS₄, Cu₂S, CuS) and the final HEMS-4 nanoballs, revealing phase transitions and structural evolution during synthesis.

Interestingly, after 12 h, we do not see any trace of (Ce₂S₃)_rich/(CoFeNi)₉S₈ bulk plates; instead show the presence of nano-sized platelets, indicating their consumption in their reaction (Fig. 2a(v). The EDS spectra and maps of these nanoplatlets show the presence of all the elements CoFeNiCuCe-S (with Cu-rich) (Fig. 2d, h(v), f(v)), which is in contrast to the bulk plates (Figs. 2a(iv), f(iv) and S2C). This indicates that the Cu ions from CuS/(CoFeNi)₉S₈ nanospheres and the presence of (CoFeNiCu) sulfide clusters, leading to the degradation of bulk (Ce₂S₃)_rich/(CoFeNi) plates to nanoplatelets. The lattice strain triggered the incorporation of different metal ions (Co, Fe, Ni, and Cu) into (Ce₂S₃)_rich/(CoFeNi)₉S₈ at 200 °C. From EDS maps, it is observed that these nanoplates are composed of Cu-rich phases, indicating the Cu²⁺ ions participated in this process (Fig. 2d, h(v), f(v)). Throughout these processes, the Co₉S₈ framework predominated (from XRD), remaining a significant presence during the final hours of the reaction (24–48 h). Following this stage, the high entropy phase (CoFeNiCuCe)/S nanoplatelets aggregate and gradually transform into fully developed (CoFeNiCuCe)₉S₈ HEMS nanoballs (~2–3 µm) after 48 h of reaction (Eq. 5; Figs. 2a(vi) and S4H). In addition to these HEMS-4 structures, a minor side product, CuFeS₂, is also formed, that confirmed by the diffraction pattern of HEMS-4 (Fig. S1). The final HEMS-4 nanoballs are nearly monodisperse and non-porous, exhibiting a uniform distribution of all five elements (Co, Fe, Ni, Cu, and Ce).

$$\mathrm{Co},\mathrm{Fe},\mathrm{Ni},\mathrm{Cu},\mathrm{S}\to {\left(\mathrm{CoFeNi}\right)}_9{\mathrm{S}}_8\left(\mathrm{i}\right)+\mathrm{CuS}+\mathrm{C}{\mathrm{e}}_2{\mathrm{S}}_3\left(\mathrm{plates}\right)$$ 1

$${\left(\mathrm{CoFeNi}\right)}_9{\mathrm{S}}_8+\mathrm{CuS}+\mathrm{C}{\mathrm{e}}_2{\mathrm{S}}_3\to \mathrm{CuS}/{\left(\mathrm{CoFeNi}\right)}_9{\mathrm{S}}_8\left(\mathrm{ii}\right)+\mathrm{C}{\mathrm{e}}_2{\mathrm{S}}_3/{\left(\mathrm{CoFeNi}\right)}_9{\mathrm{S}}_8\left(\mathrm{iv}\right)$$ 2

$$\mathrm{CuS}/{\left(\mathrm{CoFeNi}\right)}_9{\mathrm{S}}_8+\mathrm{C}{\mathrm{e}}_2{\mathrm{S}}_3/{\left(\mathrm{CoFeNi}\right)}_9{\mathrm{S}}_8\to {\left(\mathrm{CoFeNiCu}\right)}_9{\mathrm{S}}_8\left(\mathrm{iii}\right)+\mathrm{C}{\mathrm{e}}_2{\mathrm{S}}_3/\left(\mathrm{CoFeNiCu}\right)+\mathrm{C}{\mathrm{u}}_5\mathrm{Fe}{\mathrm{S}}_4\mathrm{cubes}$$ 3

$${\left(\mathrm{CoFeNiCu}\right)}_9{\mathrm{S}}_8+\mathrm{C}{\mathrm{e}}_2{\mathrm{S}}_3/\left(\mathrm{CoFeNiCu}\right)\to \left(\mathrm{CoFeNiCuCe}\right)/\mathrm{S}\left(\mathrm{v}\right)$$ 4

$$\left(\mathrm{CoFeNiCuCe}\right)/\mathrm{S}\to {\left(\mathrm{CoFeNiCuCe}\right)}_9{\mathrm{S}}_8\left(\mathrm{vi}\right)$$ 5

Additionally, the presence of (CoFeNiCu)₉S₈ porous structures is further confirmed by their variation in surface area as shown in Figs. 2e and S2E. The initial nanoballs CuS/(CoFeNi)₉S₈ show specific surface area 1.9704 m²/g, which eventually increases over reaction progress, corresponding to the porous structures (surface area = 4.8001  m²/g) at 12 h of reaction, finally, the surface area further decrease at the final product (2.6637 m²/g), and the results are consistent with that of the morphology seen in SEM.

The XRD (Figs. 2i and S2F) analysis of these reaction intermediates further confirmed the above compositional changes, ultimately forming the HEMS nanoballs. Based on XRD at 1 h, the (CoFeNi)₉S₈ and CuS structures, alongside Ce₂S₃ plates (weak diffraction at 2θ = 25.60°) (Eq. 1). No quaternary (CoFeNiCu)₉S₈ sulfide phases were detected at this stage. At 3 and 6 h, strong Ce₂S₃ diffraction peaks (2θ = 25.60°) appeared that indicating the growth of Ce₂S₃ plates with minor quantities of Co, Fe, Ni and Cu (Eqs. 2; 3 and Fig. 2i). Gradually, the Ce₂S₃ diffraction peaks disappear at 12 h, indicating the formation of very small-sized Ce₂S_3, which acted as a host for the accommodation of other metal ions for the formation of high entropy nanoplatelets (Eq. 4), as seen in the case of the above section. After 6 h, the diffraction peaks are only related to the M₉S₈ phase, with a broad peak at 29.20°. This further undergoes a cation exchange process as seen in XRD, a gradual shift of this peak (from 29.20° to 29.60°) towards higher 2θ along with increasing intensity, indicating the formation of the (CoFeNiCuCe)₉S₈ (HEMS-4) nanoballs (Eq. 5). Moreover, the detailed elemental and surface morphology of the different MSs (U to Q-MS), and HEMS (1–5) nanoballs is given in the supporting information note (S2) and shown in Figs S4 and 5.

Elemental composition analyses of HEMS nanoballs by ICP-OES

The atomic percentage of the elements and their ratio analysis of the different HEMS (1–5) nanoballs were examined using inductively coupled plasma-optical emission spectroscopy (ICP-OES) and SEM-EDS measurements. The atomic percentage of the existing elements, such as Co, Fe, Ni, Cu, and Ce, in the catalysts is given in Table S1b and shown in Fig. S3 (supporting information). The achieved atomic percentage also confirms the definition of HEMs, which generally have five or more elements, and each element concentration (atomic percentage) lies between 5% and 35%. The configurational entropy (ΔS_mix) of HEMS catalysts was evaluated by the following formula (Eq. 6)³⁷.

$$\mathrm{\Delta }{\mathrm{S}}_{\mathrm{mix}}=-\mathrm{R}{\sum }_{\mathrm{n}=1}^{\mathrm{n}}{\mathrm{c}}_{\mathrm{n}}\ln c_{\mathrm{n}}$$ 6

Where R is the universal gas constant, and C_n is the mole fraction of the n element. The configurational entropy values for the different HEMS nanoballs are observed at about 1.5632 R (HEMS-1), 1.4929 R (HEMS-2), 1.5787 R (HEMS-3), 1.5768 R (HEMS-4), and 1.5348 R (HEMS-5). Based on the ICP analysis, different HEMS nanoballs exhibited the compositions as Co_2.08Fe_1.75Ni_2.37Cu_1.88Ce_0.91S₈ (HEMS-1), Co_1.5Fe_1.56Ni_1.96Cu_2.61Ce_1.3S₈ (HEMS-2), Co_1.69Fe_1.72Ni_1.96Cu_1.09Ce_2.54S₈ (HEMS-3), Co_2.46Fe_1.52Ni_2.05Cu_1.08Ce_1.9S₈ (HEMS-4), and Co_1.65Fe_1.36Ni_2.05Cu_2.85Ce_1.09S₈ (HEMS-5). Collectively, ICP-OES, EDS, and XRD analysis data confirm the formation of nanoballs of high entropy metal sulfide.

XPS characterization of MS and HEMS nanoballs

Due to the crucial role of electronic structure in influencing the catalytic properties of materials, these MSs nanoballs are studied by X-ray photoelectron spectroscopy (XPS). The XPS survey spectra of all the materials indicate the presence of cobalt (Co2p), iron (Fe2p), nickel (Ni2p), copper (Cu2p), cerium (Ce3d), sulfur (S2p), and oxygen (O1s) in various MSs (Fig. S6A–F). The XPS spectra reveal that among the various MSs (U to Q-MS), HEMS-4 (Fig. 3a–c) and other HEMS (1–3 and 5) (Fig. S6B, C, F), HEMS-4 nanoballs display the more electron-rich chemical states of transition metals, including Co, Fe, and Ni^19,22,23. This is confirmed by the consistency of negative shifts in the binding energies of their corresponding 2p states. The increased electron density is attributed to the synergistic effects due to the presence of multi-metals in the HEMS-4 nanoballs system, which is accountable for the enhanced electrocatalytic performance. In contrast, Cu and S XPS spectra of HEMS-4 nanoballs display slightly positive shifts than MSs (Fig. 3d, e) and other HEMS systems nanoballs (Fig. S6B, C, F), demonstrating the electron-deficient Cu2p and S2p states^22,23,38. The Ce3d spectra of different HEMS systems (1–5) nanoballs demonstrated nearly the same binding energy, indicating the existence of both +3 and +4 oxidation states within the respective states of 3d (5/2 and 3/2) (Fig. 3f)^39,40. The unique electronic configuration of HEMS-4 nanoballs, especially its electron-rich transition metals and balanced multivalent Ce, highlights its potential as a high-performance electrocatalyst within the HEMS systems. Additionally, Fig. 3g, h, demonstrate the atomic percentage of different elements corresponding to their respective ions present in different MSs, including U, B, T, Q-MS, and HEMS (1–5) nanoballs. The detailed XPS analysis of the various MSs (U to Q-MS) and HEMS (1–5) nanoballs is presented in the supporting information note (S6), and the binding energies data of corresponding states of the different HEMS nanoballs (1–5) are given in Table S2. Moreover, FT-IR spectra of different MSs and HEMS nanoballs support their successful formation (Fig. S7), and detailed analysis is presented in the supporting information note S7.

Fig. 3. XPS analysis of different lower-medium entropy and various high-entropy metal sulfide nanoballs.

High resolution XPS spectra of a Co2p, b Fe2p, c Ni2p, d Cu2p and, e S2p for U-MS [Co₉S₈], B-MS [(NiFe)₉S₈], T-MS [(CoFeNi)₉S₈], Q-MS [(CoFeNiCu)₉S₈], and [(CoFeNiCuCe)₉S₈] (HEMS-4) nanoballs. f Ce3d XPS spectra of several HEMS (1–5). Elemental atomic percentages, obtained from XPS peak-area quantification, are presented as circular diagrams. g corresponds to the different MS samples (U to Q-MS), and h corresponds to the HEMS nanoballs (HEMS 1–5). The representative sample names are labeled from the inner to the outer circles: U to Q-MS in (g), and HEMS-1 to HEMS-5 in (h), respectively.

Fig. 4. Electrocatalytic OER and UOR performance of various MS (U to Q-MS) and HEMS (1–5) nanoballs in alkaline media. OER in 1.0 M KOH.

a LSV curves of U-MS, B-MS, T-MS, Q-MS, and HEMS-1. b Corresponding Tafel slopes from (a). c LSV curves of HEMS nanoballs (1–5). d Tafel slopes of HEMS (1–5). e EIS spectra of MS and HEMS nanoballs. f Chronoamperometric durability test of HEMS without electrolyte replacement. UOR in 1.0 M KOH + 0.33 M urea: g LSV curves of U-MS, B-MS, T-MS, Q-MS, and HEMS-1. h Corresponding Tafel slopes from (g). i LSV curves of HEMS nanoballs (1–5). j Tafel slopes of HEMS (1–5). k LSV curves of HEMS-4 for both UOR and OER. l Tafel slope and overpotentials for OER and UOR over HEMS-4. m Proposed urea oxidation pathway.

OER and UOR electrocatalytic activity of MS and HEMS nanoballs

The electrocatalytic OER activities of the as-prepared MS catalysts, U-MS, B-MS, T-MS, Q-MS, and the HEMS nanoballs (HEMS-1 to HEMS-5) were evaluated in alkaline medium (1 M KOH) at room temperature using a three-electrode setup. Linear sweep voltammetry (LSV) profiles (Fig. 4a) for Ni foam (NF), commercial IrO₂, and the MS series (U to Q-MS) reveal the steady increase in OER activity with increasing incorporation of metallic elements into the M₉S₈ lattice. HEMS-1 demonstrates an overpotential of 214 mV at 10 mA cm⁻², outperforming NF and comparable to IrO₂. The corresponding Tafel slope analysis (Figs. 4b and S8A) shows a significant reduction from 84 mV dec⁻¹ (U-MS) to 66 mV dec⁻¹ (Q-MS), with HEMS-1 reaching 62 mV dec⁻¹, indicating accelerated reaction kinetics. Extending this approach to the HEMS series, cyclic voltammetry (CV) reveals distinct redox couples at 1.2–1.4 V, assigned to Ni²⁺(Fe²⁺)/Ni³⁺(Fe³⁺) transitions from both catalyst and substrate (Fig. S8A). Among these, HEMS-4 exhibits the highest OER activity, requiring only 175 and 260 mV to achieve current densities of 10 and 100 mA cm⁻², respectively (Fig. 4c), and yielding the lowest Tafel slope of 57 mV dec⁻¹ (Fig. 4d). Electrochemically active surface area (ECSA), estimated from double-layer capacitance (Cdl) in the non-faradaic region (Fig. S8B), is maximized for HEMS-4 at 144.75 cm² (Fig. S8C), indicating a high density of accessible active sites. This enhancement is attributed to the synergistic effect of multiple metals, which optimizes active site exposure and utilization, thereby improving energy conversion efficiency. Electrochemical impedance spectroscopy (EIS) further confirms superior charge transfer properties: Nyquist plots (Fig. 4e) fitted to an equivalent circuit model show that HEMS-4 has the lowest charge transfer resistance (Rct = 2.0 Ω at 175 mV) among all tested catalysts, substantially smaller than that of the U to Q-MS series and other HEMS (1–5) nanoballs.

The lowest Tafel slope, charge transfer resistance, and highest ECSA value of HEMS-4 nanoballs demonstrate its higher electron transfer coefficient and faster OER kinetics on its surface, resulting in outstanding OER performance than other HEMS (1–3 and 5) nanoballs, revealing the highest electronic conductivity and the fastest charge transfer in the electrocatalytic OER process, which may be accredited increased exposed active sites formation. This enhanced electrocatalytic performance of different HEMS nanoballs is attributed to their coherent structure and high electrical conductivity, which stimulate diffusion and reaction courses at the interface of electrode-electrolyte. The performance achieved with HEMS-4 surpasses that of most state-of-the-art OER catalysts reported to date (Table S5).

Chronoamperometry (i–t) measurements of HEMS-4 nanoballs were performed at different current densities (10–300 mA cm⁻²), demonstrating the long-term stability of more than 140 h at the current density of 10 mA cm⁻². Over time, i–t curve showed an increase in the current density from 10 to 18 mA cm^−2, indicating the rise in the stability due to self-reconstruction of the metal sulfide surface structure (Fig. 4f). Long-term stability was also achieved at higher current densities such as 50, 100, 200, and 300 mA cm⁻², which suggested the HEMS-4 is highly stable in alkaline medium (Figs. 4f; S8D and Supporting note S8). Consistent stability was further confirmed through prolonged CV measurements, with negligible degradation observed over 2000 cycles at a scan rate of 50 mV s⁻¹ (Fig. S8D). The presence of multi-metal (Co, Fe, Ni, Cu, and Ce) synergy in HEMS-4 offers thermodynamic stabilization and kinetically trapped conditions, leading to sluggish atomic diffusion and self-reconstruction to provide fresh and active sites without losing integrity even under operational conditions²².

In addition to OER investigations, the UOR activities of the catalysts (U-MS to HEMS, and IrO₂) were systematically evaluated under optimized conditions (1 M KOH + 0.33 M urea), as shown in Fig. 4(g–j). The LSV, CV curves, and Tafel slopes of all the HEMSs illustrate that faster oxidation of urea than oxygen evolution, which demonstrates that the lower applied potential (1.277 vs. 1.405 V at 10 mA cm⁻²) is sufficient for UOR at the same current densities (Figs. 4k, l and S9A, B). The LSV profiles reveal that the HEMS-4 nanoballs exhibit the lowest onset potentials of 1.277, 1.336, and 1.511 V at 10, 100, and 500 mA cm⁻², respectively, significantly outperforming the other MSs and the IrO₂ catalyst. Corresponding Tafel analyses (Fig. 4j) further confirm the superior kinetics of HEMS-4, which presents the lowest slope of ~34 mV dec⁻¹, indicative of a more favorable electron transfer pathway during UOR. Moreover, the enhanced activity of HEMS-4 over other remains consistent across a range of urea concentrations, as evidenced in Fig. S9A and summarized in Table S3, highlighting its robustness and efficiency in practical urea electro-oxidation. Cdl analysis comparison among all HEMS, the HEMS-4 exhibits the highest ECSA value, about 285.12 cm², outperforming others (Fig. S9C). Further, the kinetics of UOR over various HEMS (1–5) nanoballs were investigated through EIS, Nyquist plots, along with equivalent circuits are presented in Fig. S9D, indicating the lowest Rct of about 2.4 Ω of HEMS-4 nanoballs. The lowest Tafel slope, smaller R_ct, and highest ECSA of HEMS-4 nanoballs unveil the enriched electronic conductivity, increased exposed sites for faster oxidation of urea, and superior catalytic reaction kinetics over it. Moreover, superior UOR performance of HEMS-4 is ascribed to its unique surface structure and optimum proportion of Ce with Co and Ni, than others, except HEMS-3 exhibits the highest Ce only^40,41. The i–t curve at 10 mA cm⁻² indicates stable UOR performance for 27 h, with some positive shifts in current density (Fig. S9E). The detailed UOR analysis is given in the supporting information note (S9). Overall, the UOR performance of HEMS-4 is competitive with, and in most cases surpasses, that of previously reported metal sulfide, alloy, and oxide-based catalysts, as summarized in Table S7.

The intermediate products during this UOR process were identified as NH₃ and NO₂^─ based on absorption studies^42,43. The Faradaic efficiency (FE) values for these intermediates were approximately 58.9% (NH₃) and 3.38% (NO₂^─), respectively (Fig. S9F). With prolonged reaction time (up to 180 min), the FE of these intermediates gradually decreased, indicating their subsequent conversion into final gaseous products, such as N₂, CO_2, and H₂O under electrochemical conditions over the HEMS electrocatalyst. The UOR proceeds via dual-stage transformation: urea and water are initially adsorbed onto the catalyst surface in an O-coordinated conformation. Urea undergoes deamination to form a surface-bound intermediate *OCONH₂, which then undergoes rate-determining decarburization to yield *NH₂, eventually releasing NH₃. The adsorbed *NH₂ species can undergo dehydrogenation. Two adjacent *NH₂ groups couple on the catalyst surface to form *NH₂–NH₂, which subsequently dehydrogenates (rate-determining step) to release N₂. Simultaneously, partial oxidation of NH₃ and/or urea can also generate NO₂⁻ as an intermediate. NO₂⁻ may further participate in redox processes, yielding N₂ or other gaseous products at the applied potential (Fig. 4m)^44,45.

The high performance of HEMS-4 is attributed to its uniform spherical nanoballs, which provide optimized conditions for nearly pure phases compared to their counterparts. For example, in the case of HEMS-1, the mixed morphology structure, including nanoballs as well as toothpick-shaped nanorods (Ce-rich phase, Fig. S4E). The HEMS-2 showed spherically shaped nanoballs with CuFeS₂ impurity (Fig. S4F), and HEMS-3 shows the irregular-shaped nanoballs with high impurity of Cu₅FeS₄ (Fig. S4G). HEMS-4 demonstrated the uniformity of the spherically shaped nanoballs throughout the material (Fig. S4H). HEMS-5 shows the highly agglomerated nanoballs (Cu rich phases formed) (Fig. S4I).

HER and NO₂RR electrocatalytic activity of HEMS nanoballs

In addition to excellent OER and UOR performances, as-prepared HEMS nanoballs showed significant HER and NO₂RR activities. Figure 5a shows the LSV curves of different HEMS in 1 M KOH for HER that confirm the remarkable performance of HEMS-4 with a lower onset overpotential at a current density of 10 mA cm⁻² (ƞ₁₀ = 85 mV), outperforming other HEMS nanoballs. HEMS-4 exhibits a lowest Tafel slope value of nearly 170 mV dec⁻¹ than other HEMS nanoballs, signifying the rapid HER kinetics over its surface Fig. 5b, c. Further, the ECSA from the corresponding CV curves (Fig. S10) indicates the highest ECSA value 138.87 mF cm⁻², which is attributed to the higher active sites over the surface of HEMS-4 for HER. The HER outcomes are compared with the reported studies and shown in Table S6. Figure 5d displays the LSV curves of several HEMS nanoballs in 0.5 M NaNO₂ for NO₂RR to ammonia in an alkaline medium. HEMS-4 nanoballs show a noteworthy catalytic performance with a lower applied potential of 0.11 V (lower Tafel slope 134 mV dec⁻¹ and highest ECSA 136.75 cm²) than other HEMS nanoballs (Figs. 5e and S11A). In addition, the CV curves of HEMS-4 for HER and NO₂RR are illustrated in Fig. 5f, clearly indicating the outstanding performance towards NO₂RR to NH₃ than HER. The LSV curves of HEMS-4 for NO₂RR and HER demonstrated the rapid nitrite reduction with a potential of 0.11 V, then HER at 10 mA cm⁻² (Fig. 5g, h). FE_NH3 was evaluated at 0.11 V with different time intervals⁴³. FE_NH3 during the NO₂RR is found about 72% after 30 min of chronoamperometry test with a constant potential applied, which was further found to be decreased (Fig. S11B). Chronoamperometry (i–t) measurements in 1 M KOH showed stable performance up to 30 h at 60 and 110 mA cm⁻². The current densities shifted to 45 and 90 mA cm⁻² between 30 and 57 h, but remained up to 80 h (Fig. 5i).

Fig. 5. HER and NO₂RR performance of different HEMS (1–5) nanoballs in alkaline media.

a LSV curves of HEMS nanoballs (1–5) for HER. b Corresponding Tafel slopes of HEMS (1–5). c Tafel slopes and overpotentials at various current densities. d LSV curves of HEMS nanoballs (1–5) for NO₂RR in 0.5 M NaNO₂. e Corresponding Tafel slopes of HEMS (1–5). f CV curves of HEMS-4 in 1 M KOH and 1 M KOH + 0.5 M NaNO₂. g LSV curves of HEMS-4 for HER and NO₂RR. h Comparison of Tafel slopes and overpotentials for HER and NO₂RR over HEMS-4. i Chronoamperometric durability test of HEMS-4 in 1 M KOH.

Furthermore, the potentiality of the HEMS-4 nanoballs towards OER, UOR, HER, and NO2RR was further explored for the two-electrode system (HEMS-4//HEMS-4) for the cell reactions such as HER//OER, HER//UOR, NO₂RR//OER, and NO₂RR//UOR. The outcomes are reported in comparison to a three-electrode system and without iR correction (Fig. S11C). All the performed reactions NO₂RR//UOR require less potential (1.275 V) to drive the cell reaction as compared to other cell reactions in both the three and two-electrode systems (Fig. S11C). These results demonstrate the faster reduction of nitrite to ammonia as compared to HER over the HEMS-4 electrocatalyst.

Integrating post-characterization, in situ Raman spectroscopy and DFT analysis

Although HEMS exhibits exceptional stability during OER, post-characterization of HEMS-4 after 24 h of chronoamperometry test reveals intact HEMS-4 core stability (by FE-SEM and XRD) and minor structural modifications (XPS and Raman) in surface regions. Figure S12A depicts the FE-SEM image of HEMS-4 (post-OER and UOR) and corresponding elemental mapping, which indicates the existence and uniform distribution of all elements without any loss of metallic content, including a high content of oxygen, elucidating the formation of MOOH or MOOH-S species on the catalyst surface. Furthermore, post-OER XRD analysis confirms the same diffraction pattern as HEMS-4 as before (Fig. S12B).

In consistent with the SEM-EDS, the XPS survey spectra of different HEMS (1–5) electrocatalysts reveal the existence of all elements (Fig. S12C). The catalytic activity of the HEMS nanoballs arises from (i) metal sulfide centers and (ii) surface-adsorbed sulfate species, as demonstrated by pre- and post-catalytic high-resolution XPS analysis of all metal configurations and O1s spectra. The increased oxidation state change in the tri Ni, Co, Ce metal ions (Ni²⁺ to Ni³⁺, Co²⁺ to Co³⁺, Ce³⁺ to Ce⁴⁺) is considered as active sites (Fig. 6a–d), while Fe and Cu remain unchanged, indicative that they are inactive (Fig. S12C). Post-OER O1s spectra exhibit distinct changes compared to the pre-catalytic state, initially showing two peaks at 531.61 eV (SO₄²⁻) and 533.34 eV (adsorbed H₂O), confirming sulfate-rich surfaces. After catalysis, additional peaks emerge at 529.50 eV (M–O), 530.34 and 531.20 eV (OH⁻), along with persistent sulfate (532.76 eV, SO₄²⁻) and adsorbed water (533.85 eV, H₂Oₐ_ds, indicating surface oxidation and hydroxylation during OER (Figs. 6d and S12C)^21,22. Hence, these HEMS-4 nanoballs act as pre-catalysts because they facilitate the formation of metal hydroxides/metal oxyhydroxides/metal oxides in various electrocatalytic processes, particularly for overall water splitting. The positive binding energy shifts for O1s from sulfate ions indicate the weak adsorption of sulfate ions, which allows the adsorption of water and leaves hydroxides and oxides during the catalytic OER process. Similarly, post-HER, -UOR, and NO₂RR XPS spectra of HEMS-4 nanoballs demonstrate the presence of all the elements (Co, Fe, Ni, Cu, and Ce) and confirm the self-reconstruction of the surface structure (Fig. S12D). These corresponding species of metal sulfides, oxides, and sulfates are further seen in post-OER Raman spectrum as demonstrated by the peaks at 467 (MOOH), 543 (MOOH), and at 595 cm⁻¹, which is assigned to surface oxyanion vibrations formed during OER (Fig. S12E)^10,22,25. Moreover, the detailed post-OER analysis is given in the supporting information note (S12E).

Fig. 6. Structural and electronic characterization of HEMS nanoballs for OER/UOR: XPS, in situ Raman, and DFT analysis.

High-resolution XPS spectra of a Co 2p, b Ni 2p, c Ce 3d, and d O1s, recorded before and after OER, revealing oxidation state modulation and increased lattice/adsorbed oxygen species associated with catalytic activation. e Schematic illustration of the proposed oxygen evolution reaction pathway on HEMS-4. In situ Raman spectra of HEMS-4, f OER at 1.4 V vs. Hg/HgO, g UOR at different potentials vs. Hg/HgO. h DFT analysis, (i, iv) optimized structures of (CoFeNiCu)₉S₈ and (CoFeNiCuCe)₉S₈; (ii, v) RMSD plots of the respective systems obtained from NVT ensemble simulations at 300 K; (iii, vi) partial density of states for (CoFeNiCu)₉S₈ and (CoFeNiCuCe)₉S₈, highlighting the presence of p–d and d–f interactions in the respective systems.

Based on the light provided from the pre- and post-analysis comparison, we conclude the following reaction mechanism for the catalytic OER activity of HEMS-4 is shown in Fig. 6e. During OER, HEMS-4 (M₉S₈ system) react with hydroxyl radicals (OH^●) to form metal hydroxides (M–OH), SO₄²⁻ ions, and residual metal sulfate (MSO₄²⁻) during the water oxidation in an alkaline medium (Eq. 7). Consequently, more OH⁻ ions are adsorbed on M–OH to produce the intermediate metal oxide (M–O) and then metal oxide hydroxides (MOOH) (Eq. 8). Alternatively, M₉S₈ can directly transform into MOOH as shown in Eq. 9. Similarly, residual amorphous sulfates react with hydroxyl radicals and form M(OH)₂ and SO₄²⁻ ions (Eq. 10). Lastly, SO₄²⁻ containing MOOH or MOOH-S decomposes and yields O₂ as well as free surface-active sites (Eq. 11). As a result, HEMS-4 nanoballs demonstrate the formation of catalytically active metal oxides and hydroxides on their surfaces, followed a self-reconstruction triggered by redox processes during OER. This reconstruction increased the number and activity of surface active sites, leading to enhanced OER performance. Furthermore, using HEMSs as multifunctional catalysts can reduce reliance on noble metal benchmarks like RuO₂ and IrO₂, leading to cost-effective, high-performance solutions.

$${\mathrm{M}}_9{\mathrm{S}}_8+\left(72+\mathrm{x}\right)\mathrm{O}{\mathrm{H}}^{-}\to 8\mathrm{M}{\left(\mathrm{OH}\right)}_{\mathrm{x}}+8\mathrm{S}{\mathrm{O}}_4^{2-}+\mathrm{MS}{\mathrm{O}}_4^{2-}+32{\mathrm{H}}_2\mathrm{O}+\left(56+\mathrm{x}\right){\mathrm{e}}^{-}$$ 7

$$8\mathrm{M}{\left(\mathrm{OH}\right)}_{\mathrm{x}}+\left(16-\mathrm{x}\right)\mathrm{O}{\mathrm{H}}^{-}\to 8\mathrm{M}\left(\mathrm{OOH}\right)+{8\mathrm{H}}_2\mathrm{O}+\left(16-\mathrm{x}\right){\mathrm{e}}^{-}$$ 8

$${\mathrm{M}}_9{\mathrm{S}}_8+40\mathrm{O}{\mathrm{H}}^{-}\to 8\mathrm{M}\left(\mathrm{OOH}\right)+7\mathrm{S}{\mathrm{O}}_4^{2-}+\mathrm{MS}{\mathrm{O}}_4^{2-}+40{\mathrm{H}}_2\mathrm{O}+24{\mathrm{e}}^{-}$$ 9

$$\mathrm{MS}{\mathrm{O}}_4^{2-}+2\mathrm{O}{\mathrm{H}}^{-}\to \mathrm{M}{\left(\mathrm{OH}\right)}_2+2\mathrm{S}{\mathrm{O}}_4^{2-}$$ 10

$$\mathrm{MOOH}\to \mathrm{M}+{\mathrm{O}}_2+{\mathrm{H}}^{+}+{\mathrm{e}}^{-}$$ 11

To understand the evolution of active intermediate species, the HEMS-4 catalyst under operating conditions, we performed in situ Raman spectroscopy during both OER and UOR. During OER, the time-resolved spectra (Fig. 6f) reveal a gradual emergence and intensification of characteristic bands at 465 and 546 cm⁻¹, which correspond to vibrational modes of surface-generated MOOH species. Their progressive evolution indicates that the applied anodic potential drives the oxidation of the metal-sulfide surface, forming a highly active oxyhydroxide layer. Notably, a weak but persistent feature at 296 cm⁻¹, associated with the M-S vibration, remains throughout the reaction. The simultaneous presence of M-S and MOOH modes demonstrates that the bulk metal-sulfide framework is retained while its outermost layers undergo a potential-induced transformation. Such surface reconstruction, whereby the pristine MS surface reorganizes into a higher-valence oxyhydroxide phase that serves as the catalytic interface during OER. This reconstruction-driven activation clearly seen in HEMS-4, reflecting the synergistic role of multi-element entropy stabilization in promoting the formation of a robust, catalytically active MOOH shell under working conditions^22,46,47. During UOR, in situ Raman analysis was further conducted at applied potentials ranging from 0.40 to 0.75 V vs. Hg/HgO (Fig. 6g). With increasing potential, all vibrational features progressively intensified, indicating continuous evolution of reactive surface intermediates. Two dominant peaks located at 470 and 554 cm⁻¹ are attributed to the bending and stretching modes of MOOH species, respectively, consistent with potential-induced formation of oxyhydroxide phases^45,48. A distinct band at 1005 cm⁻¹ corresponds to the C–N stretching vibration of surface-adsorbed urea molecules, while several additional features in the 1000–1200 cm⁻¹ region arise from various urea-derived intermediates generated during the reaction^44,49. Overall, these observations corroborate the proposed UOR mechanism, as illustrated in Fig. 4m.

Furthermore, DFT investigation was performed by considering two systems (Q-MS and HEMS-4). Figure 6h(i, iv) shows the optimized structures of (CoFeNiCu)₉S₈ and (CoFeNiCuCe)₉S₈, respectively. In both structures, each Co, Fe, Ni, and Cu atom is coordinated with four S atoms. Additionally, in the (CoFeNiCuCe)₉S₈ structure, each Ce atom is also surrounded by four S atoms. To evaluate the thermodynamic stability of both systems, we performed ab initio molecular dynamics simulations at 300 K using the NVT canonical ensemble for a total simulation time of 15,000 fs. Figure 6h(ii, v) displays the root mean square deviation plots as a function of time, showing minimal fluctuation around equilibrium and no structural distortion throughout the 15,000 fs duration. This confirms the thermodynamic stability of both systems, suggesting that their synthesis is feasible. We further examined the electronic properties of the two systems by analyzing the partial density of states, as shown in Fig. 6h(iii) clearly demonstrates an overlap between the Fe-3d and S-3p states near the Fermi level in the (CoFeNiCu)₉S₈ system. Notably, Figs. 6h(vi) and S13 reveal a strong overlap between the Ce-4f and Fe-3d, Ni-3d, and Co-3d orbitals across the Fermi level in (CoFeNiCuCe)₉S₈ (HEMS-4), indicating the presence of d–f interactions. Such d–f interactions enhance the electronic conductivity of the system, making it suitable for multi-functional electrocatalysis. Notably, a substantial enhancement in performance was observed when comparing Q-MS and HEMS catalysts synthesized under identical conditions (CoNiFeCu vs. CoNiFeCuCe), which can be attributed to the incorporation of Ce. This enhancement is associated with several factors: (i) the generation of electron-rich active sites (e.g., Co, Ni) via Ce-mediated electronic modulation, (ii) increased lattice distortion and corresponding ECSA gains, and (iii) the redox flexibility of Ce³⁺/Ce⁴⁺ and 4f orbital contributions, which endow Ce with adaptive coordination behavior that stabilizes the catalyst structure during the OER process^32,50–52.

Conclusions

In this work, we developed a Cu²⁺-directed strategy to incorporate redox-active Ce into high-entropy (CoFeNiCuCe)₉S₈ nanoballs, overcoming the limitations of conventional cation-exchange routes. Our low-temperature hydrothermal sulfurization method enables Cu²⁺ to act as a dynamic structural director, stabilizing Ce within the multicomponent sulfide framework. Time-resolved growth studies reveal a multi-step transformation involving the formation of (CoFeNi)₉S₈, Cu²⁺ mediated exchange and restructuring, and final reaction with Ce-sulfide species to produce the high-entropy phase. Among the synthesized series, (CoFeNiCuCe)₉S₈ (Cu:Ce = 1:4) delivers the best multifunctional electrocatalytic performance, showing low overpotentials for OER, HER, UOR, and highly selective NO₂RR to NH₃. In situ Raman spectroscopy and post-reaction analysis confirm that both OER and UOR proceed through potential-driven surface reconstruction, where the catalyst surface converts into active MOOH species. DFT calculations further show strong interactions between Ce-4f and the 3d orbitals of Fe, Ni, and Co, leading to optimized d-band states that facilitate *OOH adsorption and accelerate the rate-determining O–O bond formation. Overall, this study demonstrates a general approach for incorporating rare-earth elements into HEMSs and introduces a promising class of stable, scalable electrocatalysts for sustainable energy-conversion reactions.

Methods

Synthesis of HEMS nanoballs

About 0.05 mole salt precursors of Ce, Fe, Co, Ni, and Cu with an equimolar ratio were mixed in 60 mL of DMF and sonicated for 60 min to get a homogeneous solution mixture. Subsequently, 1.0 g thiosalicylic acid was added to this solution mixture and further sonicated for 60 min. The obtained solution mixture was then transferred to a 100 mL autoclave of Teflon-lined stainless steel. The autoclave was successively closed and then kept at a temperature of 200 °C for 48 h at 200 °C in the air oven. After cooling the autoclave, the solvent was evaporated at 120 °C in the oven. Then, the solid black color product was collected and washed sequentially with deionized water and ethanol. Subsequently, the obtained solid product was subjected to drying at 80 °C in the vacuum oven. Similarly, different HEMS nanoballs were synthesized with changes in the molar ratio of Cu and Ce salt precursors as 1:1, 1:2, 1:3, 1:4, and 4:1 (Cu: Ce ratio), which are presented as HEMS-1, HEMS-2, HEMS-3, HEMS-4, and HEMS-5. Furthermore, the control samples, including quaternary sulfide (CoFeNiCu)_xS_y, Ternary sulfide (CoFeNi)_xS_y, binary sulfide (NiFe)_xS_y, and unary sulfide (Co)_xS_y, were also synthesized using the same process with corresponding salt precursors, respectively.

Electrochemical measurements

All the electrochemical studies data were collected using the Corrtest electrochemical workstation in a three-electrode configuration. Electrochemical analysis was performed in an alkaline solution using a single-compartment three-electrode cell with the above-prepared materials as working electrodes, Hg/HgO reference electrode, and graphite rod as a counter electrode at room temperature. Nickel foam was considered as a conducting substrate to prepare the working electrode. For the preparation of the working electrode, 1 mL catalyst ink (250 μL ethanol + 750 μL water + 50 μL Nafion binder) was prepared, 100 μL catalyst ink was drop casted on the NF, then dried in the vacuum oven for 10 min at 50 °C. Before the electrochemical analysis, electrocatalysts were activated by performing CV at a scan rate of 10 mV s⁻¹ within a potential window of 1.0 V to ~1.7 V vs RHE for both OER and UOR. Similarly, this activation was done by CV performance within the potential range of 0.3 to −0.4 V vs RHE for both HER and NO₂RR.

Computational details

The theoretical calculations were performed using the Vienna ab initio simulation package. In this work, the projected augmented wave pseudopotential was employed to accurately describe the core electron behavior. We adopted the generalized gradient approximation within the Perdew–Burke–Ernzerhof functional to investigate the structural and electronic properties. Structural relaxations were carried out using a kinetic energy cutoff of 500 eV, and the Brillouin zone was sampled using a 3 × 3 × 1 Monkhorst-Pack k-point grid. These settings ensured convergence of the structural optimization with force and energy criteria of 10⁻² and 10⁻⁶ eV, respectively. It is important to note that we considered supercell models of medium- and high-entropy structures corresponding to (CoFeNiCu)₉S₈ (Q-MS) and (CoFeNiCuCe)₉S₈ (HEMS-4).

Author contributions

U.T.-project conceptualization, writing and finalizing the manuscript draft, and funding procurement. S.K.-project conceptualization, materials synthesis and characterization, electrochemical investigation, writing and finalizing the manuscript draft. M.P.-materials characterization. H.Z.-materials characterization. H.R. Haris-materials characterization. L.D.-materials characterization. M.M.-materials characterization. S.M.T.-discussions during manuscript draft preparation. M.M.R.-discussions during manuscript draft preparation. M.Sk.-DFT investigation. M.K. Ravva-DFT investigation. All authors reviewed and approved the final manuscript for submission.

Peer review

Peer review information

Communications Chemistry thanks Huishan Shang and the other anonymous reviewer(s) for their contribution to the peer review of this work. A peer review file is available.

Data availability

All relevant data are available from the corresponding author (uday@uestc.edu.cn) upon request.

Competing interests

The authors declare no competing interests.

Supplementary information

The online version contains supplementary material available at 10.1038/s42004-026-01887-x.