English

• The enduring energy and environmental crises underscore the critical need to progress electrochemical energy storage and conversion technologies [1–3]. Among these, the eco-friendly and sustainable rechargeable metal-air batteries and hydrogen production via water splitting hold immense potential to meet our daily energy needs and drive sustainable green chemical production [4–6]. However, the overall energy efficiency of these technologies is largely governed by the oxygen evolution reaction (OER), a pivotal half-reaction known for its high overpotential, which stems from the sluggish four-electron transfer processes [7,8]. Consequently, there is a strong desire, albeit a challenging one, to develop sophisticated electrocatalysts that can reduce the OER overpotential, thereby minimizing the inherent energy loss in these energy conversion technologies. Currently, highly efficient Ir/Ru-based noble-metal oxides can significantly reduce OER overpotential by lowering the energy barrier and enhancing reaction kinetics, and they are extensively employed as benchmark catalysts in this field [9,10]. Nevertheless, their rarity and high costs pose challenges to their widespread application. Accordingly, considerable research efforts have been directed towards exploring earth-abundant and high-performance OER catalysts [8,11–15].

  Recently, high-entropy materials (HEMs), featured with their composition of at least five principal metal elements within a single-phase crystal structure, have emerged as promising candidates as efficient OER electrocatalysts [16–19]. The unique polyatomic structure of HEMs confers them with distinctive characters like high entropy, cocktail effects, sluggish diffusion and lattice distortion, which endows them with exceptional catalytic activity and structure stability, outperforming traditional materials [17,18,20]. Moreover, the expansive compositional versatility of HEMs provides a broad spectrum of possibilities for precision tuning of their electronic and geometric structures, thereby increasing intrinsic active sites and refining OER catalytic performance. A variety of HEMs like alloys [21,22], oxides/oxyhydroxide [9,23–25], sulfides [26,27], and selenides [28], have been explored as OER electrocatalysts. Of notable significance is the recent rise of high-entropy metal sulfides (HESs), which have shown tremendous potential as OER catalysts due to their superior catalytic activity and remarkable durability, attributes that stem from the synergistic interaction of their multiple components [26,27,29–32].

  As there exists immiscibility of multiple metallic constituents, there remains challenges to explore feasible strategies for preparing active nano high-entropy sulfides [26,33,34]. Initially, Hu et al. pioneered the preparation of (CrMnFeCoNi)S_x nanoparticles as an OER catalyst using a pulse thermal decomposition method. The prepared HES nanoparticles achieved 100 mA/cm² at an overpotential of just 295 mV and impressive stability in alkaline medium [26]. Since then, several cases of HESs with different metal components has been developed via various synthetic method such as high-energy planetary ball-milling process [35], solvothermal method [27], cation exchange strategy [36], and thermal decomposition of mixture of metal dithiocarbamate precursors [37]. For instance, Nguyenn et al. developed a series of sulfate-containing high-entropy sulfides using a two-step solvothermal method, of which, the sulfate-containing FeNiCoCrMnS₂ presented excellent OER activity with an overpotential of only 199 mV to reach 10 mA/cm² and maintained stable electrolysis for 55 h at a current density of 500 mA/cm² [27]. Qu et al. achieved preparation of bulk HES, i.e., (MoWReMnCr)S₂, via thermal decomposition of mixture of metal dithiocarbamate precursors. The resulting two-dimensional (2D) HES nanosheets, derived from exfoliating the bulk (MoWReMnCr)S₂ was applied as electrocatalysts for OER, requiring 229 mV to deliver a current density of 10 mA/cm² [37]. The studies highlighted have shown the tremendous promise of HESs as efficient OER catalysts. Even so, some of the above methods require multi-step processes [36], specialized equipment and stringent conditions [26,31] to obtain nano HESs. Therefore, there exists significant scope for further developing more facile, readily scalable and universally applicable synthetic methods for preparing active nano high-entropy sulfides.

  Herein, we developed a series of high-entropy sulfides via a facial precursor pyrolysis method that is also adapted to preparing high-entropy phosphides. Both the synthesized high-entropy sulfides and phosphides present favorable OER performance, of which, the representative FeCoNiCuMnS_x with a nano three-dimensional cross-linked structure, showcases a low overpotential of 220 mV at 10 mA/cm² and exceptional stability without observable degradation during an over 400 h durability test at 100 mA/cm². This impressive performance of FeCoNiCuMnS_x can be primarily attributed to the synergistic effects arising from its multiple components. The developed facile synthetic method and the investigation of OER performance of these prepared high-entropy catalysts offer a platform for the advancement of high-entropy electrocatalysts.

  The representative FeCoNiCuMnS_x HES was prepared via a one-step pyrolyzing mixture of metal complexes precursor at a low temperature of 500 ℃ (Fig. 1a). The X-ray diffraction (XRD) pattern shows that the predominant diffraction peaks of FeCoNiCuMnS_x correspond to the (111), (220), (311), (400), (422) and (440) planes of its crystal structure, conforming to similar crystal structure of Co₃S₄ (PDF #75–1561). Notably, three additional quinary HESs with same crystalline phase as FeCoNiCuMnS_x, i.e., FeCoNiCrZnS_x, FeCoNiCrCuS_x, and FeCoNiCuZnS_x, were also successfully synthesized by altering the metal ions during the synthetic process (Fig. 1b). Not limited that, such synthetic routine can also be extended to prepare a series of high-entropy phosphides (HEPs) including FeCoNiCrCuP_x, FeCoNiCuZnP_x, FeCoNiCuMnP_x, and FeCoNiCrMnP_x, simply by substituting thiourea with tri-n-octylphosphine in the precursor. The XRD patterns of these phosphides show major diffraction peaks corresponding to the (111), (201), (210) and (300) planes, aligning with those of Ni₂P (PDF #03–0953) (Fig. 1c) [38], suggesting the formation of single-phase structure in these HEPs.

  Figure 1

  

  Figure 1.  (a) Schematic illustration for the synthetic process of FeCoNiCuMnS_x. XRD patterns of prepared HESs (b) and HEPs (c).

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  The morphology of FeCoNiCuMnS_x was firstly examined by scanning electron microscopy (SEM). SEM images reveal that FeCoNiCuMnS_x exhibits three-dimensional (3D) crosslinked structure consist of randomly distributed nano branches (Figs. 2a-c). Transmission electron microscope (TEM) image further confirms the crosslinked nano branches structure of FeCoNiCuMnS_x (Fig. 2d). High-resolution TEM (HRTEM) identifies interplanar spacings of around 0.283 nm, corresponding to the (311) lattice plane of FeCoNiCuMnS_x (Fig. 2e). Notably, HRTEM image analysis on the FeCoNiCuMnS_x further verifies the presence of amorphous carbon at the periphery of the nano branches structure, which originates from the pyrolysis of coordinated ammonium citrate (Fig. S1 in Supporting information). In addition, the selected area electron diffraction (SAED) pattern shows that the lattice planes derived from the diffraction rings correspond well with the XRD pattern of FeCoNiCuMnS_x (Fig. 2f). Scanning TEM energy dispersive X-ray spectroscopy (STEM-EDX) elemental mapping shows uniform distribution of Fe, Co, Ni, Cu, Mn, S throughout the structure, further confirming the single-phase nature of the prepared HES (Fig. 2g). Moreover, the accurate atomic molar ratio of Fe, Co, Ni, Cu and Mn in FeCoNiCuMnS_x was determined to be 17.26:22.10:21.60:18.20:20.84 by using inductively coupled plasma optimal emission spectroscopy (ICP-OES) (Fig. S2a in Supporting information). The similar ratio of various metallic elements further verifies the high-entropy nature of FeCoNiCuMnS_x. Furthermore, the morphologies of the other three HESs and the prepared HEPs were also revealed by SEM and TEM techniques, which show the other prepared HESs present similar morphologies with FeCoNiCuMnS_x (Fig. S3 in Supporting information), while the HEPs display a structure with small nanoparticles anchored on carbon nanobelt (Figs. S4 and S5 in Supporting information).

  Figure 2

  

  Figure 2.  Structural and morphological characterization of FeCoNiCuMnS_x. (a-c) SEM images at various magnifications, (d) TEM image, (e) high-resolution TEM image, (f) SAED pattern, (g) high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) image with its corresponding elemental mapping images.

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  X-ray photoelectron spectroscopy (XPS) technique was then performed to analyze the chemical composition and valence states of the FeCoNiCuMnS_x surface (Fig. S6 in Supporting information and Fig. 3). The survey spectrum confirms the existent of Fe, Co, Ni, Cu, Mn and S on the surface (Fig. S6a). The high-resolution XPS spectra of Fe 2p can be split into six peaks (Fig. 3a). Those at 719.8 eV, 709.2 eV, 705.4 eV can be attributed to Fe²⁺ species, while the peaks at 723.5 eV and 712.5 eV are assigned to the Fe³⁺ species of Fe 2p_1/2 and Fe 2p_3/2, respectively, with a satellite peak at 716.1 eV [39,40]. For Co 2p, two pairs of peaks (796.2 eV and 781.3 eV) and (798.9 eV and 785.6 eV) can be assigned to Co³⁺ and Co²⁺ species of Co 2p_1/2 and Co 2p_3/2, respectively (Fig. 3b) [41,42]. The Ni 2p spectrum predominantly features Ni³⁺ with binding energy of 857.7 eV, 861.1 eV, and 876.5 eV, alongside smaller signals indexed to Ni²⁺ (854.1 eV) and Ni⁰ (851.0 eV) (Fig. 3c) [39,43]. The Cu 2p spectrum indicates the coexistence of Cu⁺ (932.2 eV and 951.7 eV) and Cu²⁺ (935.6 eV and 953.7 eV) species Fig. 3d) [44]. Besides, the Mn species also present in mixed valence, with binding energies of 642.1 eV and 650.0 eV assigned to Mn²⁺ ions, and 645.7 eV and 653.8 eV assigned to Mn³⁺ ions (Fig. 3e) [26,27,45]. As for the S 2p spectrum, the peak located at 163.0 eV can be indexed to the presence of S₂^2-, and the peaks located at 167.8 eV can be assigned to SO_x^2- (Fig. 3f) [46,47], suggesting partial oxidation of surface sulfur species upon exposure to air.

  Figure 3

  

  Figure 3.  High-resolution XPS spectra of Fe 2p (a), Co 2p (b), Ni 2p (c), Cu 2p (d), Mn 2p (e), S 2p (f) in FeCoNiCuMnS_x.

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  The OER performance of the representative FeCoNiCuMnS_x was evaluated in a three-electrode system in 1.0 mol/L KOH aqueous solution at room temperature. The catalyst powders mixed with some conductive carbons supported on Ni foam (NF) serve as the working electrodes (Supporting information). For comparison, apart from the commercial reference IrO₂ and the used substrates, the controlled quaternary (FeCoNiCuS_x), ternary (FeCoNiS_x), and binary (FeNiS_x) sulfides were also prepared using the similar method by only reducing metallic species (Figs. S7–S9 in Supporting information). Fig. 4a shows the linear sweep voltammetry (LSV) profiles for the series of electrodes. The FeCoNiCuMnS_x demonstrates the lowest overpotential of 220 mV to delivery 10 mA/cm², outperforming those of FeCoNiCuS_x (259 mV), FeCoNiS_x (273 mV), FeNiS_x (298 mV) and even the commercial IrO₂ (317 mV). Moreover, FeCoNiCuMnS_x also presents superior catalytic activity at higher current density. The overpotentials for delivering a current density of 100 mA/cm² were summarized in Fig. 4b, which shows that only 303 mV is required for the FeCoNiCuMnS_x, significantly lower than the controlled samples, indicating a synergistic effect from multiple metallic active sites on improving OER performance. The OER performance of FeCoNiCuMnS_x is comparable to that of other reported HESs (Table S1 in Supporting information) [26,27,30,35,36,39,41,48]. The reaction kinetics of these catalysts were assessed by the Tafel slopes calculated from the LSV curves in Fig. 4a. The prepared HES possesses a Tafel slope value of 71.3 mV/dec, lower than all these controlled samples, suggesting its faster reaction kinetics of OER (Fig. 4c). The kinetics were further revealed by the electrochemical impedance spectroscopy (EIS) test. The Nyquist plots in Fig. 4d show a decrease in the charge-transfer resistance (R_ct) with increasing element incorporation in the catalysts, and the FeCoNiCuMnS_x has the smallest semicircle diameter, indicating the fastest charge transfer rate at the FeCoNiCuMnS_x catalytic interface and confirming its superior OER kinetic. Then the electrochemical surface areas (ECSAs) of these samples were estimated based on the double-layer capacitance (C_dl) values, which are proportional to ECSAs (Fig. S10 in Supporting information). The largest C_dl value (5.8 mF/cm²) of FeCoNiCuMnS_x compared with controlled samples manifests its more active sites for catalytic reaction (Fig. 4e).

  Figure 4

  

  Figure 4.  Electrochemical OER performance of the FeCoNiCuMnS_x and these controlled samples. (a) LSV curves with 85% IR correct collected at 1 mV/s. (b) Comparison of overpotentials at current density of 100 mA/cm². (c) Tafel slopes. (d) EIS spectra measured at 1.52 V vs. RHE. (e) The slop corresponding to the specific capacitance in the non-Faradaic region (C_dl). (f) Chronopotentiometric curve of FeCoNiCuMnS_x at 100 mA/cm². (g) In-situ Raman spectrum of FeCoNiCuMnS_x tested at different potentials. LSV curves of different HESs (h) and HEPs (i).

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  In addition, the durability of the FeCoNiCuMnS_x was checked by chronopotentiometric (CP) tests. The FeCoNiCuMnS_x can maintain constant potential (around 1.45 V versus reversible hydrogen electrode (vs. RHE)) at 10 mA/cm² for 24 h without degradation, as evidenced by the nearly overlapping LSV curves and unchanged XRD patterns before and after the stability test (Fig. S11 in Supporting information). Moreover, stability at higher current density was also performed to evaluate its potential application in practice. As shown in Fig. 4f, the FeCoNiCuMnS_x can maintain stable electrolysis at 100 mA/cm² for > 400 h. Then the morphology and surface valent state of elements post-CP test were analyzed by SEM and XPS techniques, which shows no obvious morphology change but presents significant increase in higher oxidation state of metallic elements and sulfur element (Figs. S12 and S13 in Supporting information), suggesting oxidation occurred on the surface of FeCoNiCuMnS_x after OER. The surface oxidation phenomenon was further revealed by the in-situ Raman spectra. Fig. 4g shows that there is no obvious peak at open circuit potential (OCP) within the tested range. When the input voltage increased to 1.2 V vs. RHE, dual evident peaks appeared at 476 cm^-1 (δ(M^Ⅲ-O)) and 562 cm^-1 (v(M^Ⅲ-O)), which can be identified as the A_1g stretching vibration and e_g bending modes of the in-situ formed metal (oxy)hydroxid (MOOH), respectively [27,41,49,50]. The above results confirm surface reconstruction of FeCoNiCuMnS_x during OER process, just as the previous reports that metal chalcogenides act as pre-catalysts for oxidation reaction, and metal (oxy)hydroxide are the true active sites, applicable to high-entropy materials as well [27]. Additionally, the OER performance of other quintuple high-entropy sulfides and phosphides were also preliminarily investigated under similar conditions, which shows that the other three HESs perform as well as the FeCoNiCuMnS_x for OER (Fig. 4h), and the FeCoNiCuMnP_x with the best activity among these prepared phosphides achieves 10 mA/cm² at an overpotential of 260 mV, a little inferior to the prepared sulfides (Fig. 4i). Remarkably, the OER performance of these prepared high-entropy sulfides and phosphides are comparable with these reported outstanding ones (Table S1 in Supporting information).

  In summary, a range of high-entropy sulfides and phosphides with favorable catalytic activity for OER were successfully synthesized by a facile precursor pyrolysis method at low temperature. The physicochemical properties of the representative FeCoNiCuMnS_x for OER was thoroughly investigated. FeCoNiCuMnS_x, featuring a nano three-dimensional cross-linked structure, demonstrates superior catalytic activity with a low overpotential of 220 mV to deliver 10 mA/cm² and maintains stable electrolysis at 100 mA/cm² for over 400 h. Based on a series of experimental characterizations, the in-situ formed metal (oxy)hydroxide during the OER process was confirmed to be the real active sites, and the good OER performance can be primarily attributed to the synergistic effects arising from its multiple components. This work not merely introduces superior nano electrocatalysts for OER, but also provides a gentle and universal approach for expanding the family of nano high-entropy compounds, significantly contributing to the development of electrochemical energy storage and conversion technologies.

  Declaration of competing interest

  The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

  CRediT authorship contribution statement

  Genxiang Wang: Writing – original draft, Funding acquisition, Conceptualization. Linfeng Fan: Methodology, Investigation, Data curation. Peng Wang: Validation, Methodology. Junfeng Wang: Supervision. Fen Qiao: Writing – review & editing, Supervision. Zhenhai Wen: Writing – review & editing, Funding acquisition, Conceptualization.

  Acknowledgments

  This work was financially supported by the National Natural Science Foundation of China (Nos. 22209183, 22225902, U22A20436), and the Advanced Talents of Jiangsu University, China (No. 23JDG027).

  Supplementary materials

  Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.cclet.2024.110498.

  
  
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  Figure 1  (a) Schematic illustration for the synthetic process of FeCoNiCuMnS_x. XRD patterns of prepared HESs (b) and HEPs (c).

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  Figure 2  Structural and morphological characterization of FeCoNiCuMnS_x. (a-c) SEM images at various magnifications, (d) TEM image, (e) high-resolution TEM image, (f) SAED pattern, (g) high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) image with its corresponding elemental mapping images.

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  Figure 3  High-resolution XPS spectra of Fe 2p (a), Co 2p (b), Ni 2p (c), Cu 2p (d), Mn 2p (e), S 2p (f) in FeCoNiCuMnS_x.

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  Figure 4  Electrochemical OER performance of the FeCoNiCuMnS_x and these controlled samples. (a) LSV curves with 85% IR correct collected at 1 mV/s. (b) Comparison of overpotentials at current density of 100 mA/cm². (c) Tafel slopes. (d) EIS spectra measured at 1.52 V vs. RHE. (e) The slop corresponding to the specific capacitance in the non-Faradaic region (C_dl). (f) Chronopotentiometric curve of FeCoNiCuMnS_x at 100 mA/cm². (g) In-situ Raman spectrum of FeCoNiCuMnS_x tested at different potentials. LSV curves of different HESs (h) and HEPs (i).

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