English

• 0.   Introduction

  In water electrolysis, the oxygen evolution reaction (OER) proceeds through a four-step reaction, with each step involving a single-electron transfer. The energy accumulation required for each step results in a large overpotential. Therefore, the significant energy barrier caused by the four-electron transfer leads to slow OER kinetics, limiting the efficiency of the water decomposition process. In addition, the OER process is accompanied by the adsorption-desorption of multiple intermediates, whose binding energies are interrelated and cannot be simultaneously optimized, posing inherent limitations to catalyst design. In addition, the non-zero overpotential arising from these varying binding energies necessitates that OER electrocatalysts operate at high-potential regions, presenting another obstacle to their development. Therefore, developing a high-performance and stable OER electrocatalyst is essential for enabling water electrolysis with low overpotential and accelerated reaction kinetics.

  In recent years, various strategies have been proposed to prepare cost-effective OER catalysts exhibiting high electrocatalytic activity and stability, with some achieving or exceeding the activity of precious metal catalysts. Designing heterogeneous electrocatalysts with rich interfaces between different components has emerged as a particularly promising approach. Heterogeneous electrocatalysts offer a significant advantage due to their ability to change the chemical composition and crystal structure at the interface, leading to different atomic coordination and lattice strain. The interface formed by combining two different components with differing electronegativities facilitates electron interaction and redistribution, thus triggering a synergistic effect between components. Numerous heterogeneous OER electrocatalysts use metal alloys^[1], oxides^[2-8], sulfides^[9-15], selenides^[16-21], nitrides^[22-23], carbides^[24-28], and phosphides^[29-34] as building blocks. However, reports on the simultaneous preparation of cobalt oxides, nickel sulfides, and molybdenum sulfides into heterogeneous OER electrocatalysts remain scarce^[35-36].

  In this study, a composite electrocatalyst comprising Co₃O₄, Ni₃S₂, Mo₂S₃, and CoSO₄ was successfully constructed using nickel foam (NF) as a sacrificial template and a nickel source through a vulcanization process. The catalyst, designated as CoMoNiO-S/NF, was found to be significantly influenced by the vulcanization temperature. Among the tested samples, the CoMoNiO-S/NF-110 electrocatalyst demonstrated excellent OER performance, achieving overpotentials of 199.4 and 224.4 mV at current densities of 100 and 200 mA·cm^-2, respectively. These values reflect remarkably low overpotentials, highlighting excellent electrocatalytic activity, which surpasses both its precursor and commercial RuO₂. This outstanding catalytic activity is primarily attributed to the self-growth of the catalyst on the substrate, which is conducive to accelerating electron transfer and increasing stability. Additionally, the rich interfaces within the heterogeneous structure, the synergistic interactions of electrons at these interfaces, and the active site exposed by the defect structure collectively contribute to enhanced OER activity of the catalyst.

  1.   Experimental

  1.1   Materials

  Potassium hydroxide (KOH), cobalt nitrate hexahydrate (Co(NO₃)₂·6H₂O), sodium molybdate dihydrate (Na₂MoO₄·2H₂O), and thiourea (CH₄N₂S) were purchased from Meryer Co., Ltd. (China). All chemicals were used as received without any further purification.

  1.2   Synthesis of precursor and catalysts

  The synthesis of the precursor Mo-Co(OH)₂/Co₃O₄/NF followed a previously reported method^[37]. For catalyst preparation, 76.1 mg of thiourea was dissolved in 10 mL of water and stirred to form a uniform solution. The solution was transferred to a hydrothermal reactor along with a piece of the precursor (1 cm×1.5 cm) and subjected to a reaction at 90, 110, 120, and 130 ℃ for 2 h. After the reaction, the mixture was naturally cooled to room temperature, rinsed with water, and vacuum-dried at 50 ℃ for 10 h. The resulting samples were denoted as CoMoNiO-S/NF-T (T=90, 110, 120, and 130, corresponding to the temperature of 90, 110, 120, and 130 ℃, respectively). The synthetic strategy of CoMoNiO-S/NF-T is presented in Fig.1. The synthesis details of the control samples are provided in the Supporting information.

  Figure 1

  

  Figure 1.  Schematic of the fabrication procedure of CoMoNiO-S/NF-T

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  1.3   Materials characterization

  The crystal structures of the samples were examined using a Rigaku Ultima Ⅳ X-ray diffractometer (XRD, Cu Kα radiation, λ=0.154 178 nm, 40 kV, 40 mA) within a 2θ range of 10°-80°. Scanning electron microscope (SEM) images were captured using a JEOL JSM-6700 M microscope at an accelerating voltage of 15 kV. Transmission electron microscope (TEM) and high-resolution TEM (HRTEM) images were acquired on a JEM-2100PLUS TEM operating at 200 kV. The binding energies of Co, Mo, O, and S were assessed using X-ray photoelectron spectroscopy (XPS, ESCALAB Perkin Elmer) to investigate the surface elemental chemical states. The resulting spectra were processed with a Shirley background and fitted via XPSPEAK software.

  1.4   Electrochemical measurements

  All electrochemical tests were performed with a standard three-electrode system on a CHI-760E electrochemical workstation at room temperature, with a 1.0 mol·L^-1 KOH aqueous solution (pH=14) as the electrolyte. The synthesized catalyst served as the working electrode, a graphite rod as the counter electrode, and a Hg/HgO electrode as the reference electrode. The polarization curve was scanned at a rate of 2 mV·s^-1, and the final test potential related to the Hg/HgO electrode (E′) was converted to a potential related to the standard hydrogen electrode (E) using the Nernst equation: E=E′+0.098+0.059 1pH. All polarization curves were iR-compensated with a 90% compensation value to account for the resistance between the catalyst and electrolyte. The electrochemical impedance spectroscopy (EIS) test was performed with a frequency range of 100 000-0.01 Hz and an amplitude of 5 mV.

  2.   Results and discussion

  2.1   Synthetic mechanism and components analysis

  The synthesized materials were characterized by XRD to analyze their composition. As shown in the XRD results (Fig.S1), temperature had a significant effect on the formation of material components during the reaction process. At reaction temperatures of 110 and 120 ℃, Ni₃S₂, Mo₂S₃, CoSO₄, and other components were formed during the sulfurization process, with a distinct Co₃O₄ phase also observed. This indicates that part of the Co₃O₄ in the precursor was reduced to CoSO₄ (PDF No.78-1799) at 110 and 120 ℃. The diffraction peaks at 20.4°, 22.1°, and 24.8° correspond to the (114), (114), and (021) planes of CoSO₄, respectively. This may be due to the following reactions:

  $ \mathrm{NH}_2 \mathrm{CSNH}_2+3 \mathrm{H}_2 \mathrm{O} \rightarrow 2 \mathrm{NH}_4^{+}+\mathrm{HS}^{-}+\mathrm{HCO}_3^{-}$

  (1)

  $\begin{aligned} 4 \mathrm{Co}_3 \mathrm{O}_4+\mathrm{HS}^{-}+11 \mathrm{H}_2 \mathrm{O} & \rightarrow \\ & \mathrm{CoSO}_4+11 \mathrm{Co}(\mathrm{OH})_2+\mathrm{OH}^{-} \end{aligned} $

  (2)

  However, at a temperature of 130 ℃, the Co₃O₄ peak in CoMoNiO-S/NF-130 disappeared, while at 90 ℃, the diffraction peak of CoSO₄ was absent in the CoMoNiO-S/NF-90 pattern. This indicates that Co₃O₄ was converted to CoSO₄ at high temperatures during thiourea hydrolysis, whereas it was not reduced at lower temperatures. Moreover, the XRD patterns of CoMoNiO-S/NF-130 showed diffraction peaks for Ni₃S₂ and Mo₂S₃, with 2θ values of 21.6°, 31.0° and 44.3° corresponding to the (100), (110), and (200) planes of Ni₃S₂ (PDF No.85-1802), and 16.3°, 38.2°, and 55.1° corresponding to the (101), (103), and (302) planes of Mo₂S₃ (PDF No.78-1332). In contrast, only weak diffraction peaks of Ni₃S₂ and Mo₂S₃ were observed in CoMoNiO-S/NF-90, suggesting that HS^- produced by thiourea hydrolysis at high temperatures, contributes to sulfide formation. The formation of Ni₃S₂ results from the vulcanization of NF as a nickel source, while Mo₂S₃ is formed due to the reduction of [MoO₄]^2- in the precursor.

  $\mathrm{NH}_2 \mathrm{CSNH}_2+3 \mathrm{H}_2 \mathrm{O} \rightarrow 2 \mathrm{NH}_4^{+}+\mathrm{HS}^{-}+\mathrm{HCO}_3^{-} $

  (3)

  $ 2 \mathrm{HS}^{-}+3 \mathrm{Ni}+2 \mathrm{H}_2 \mathrm{O} \rightarrow \mathrm{Ni}_3 \mathrm{~S}_2+2 \mathrm{OH}^{-}+2 \mathrm{H}_2 \uparrow$

  (4)

  $ 6 \mathrm{HS}^{-}+2 \mathrm{MoO}_4{ }^{2-}+2 \mathrm{H}_2 \mathrm{O} \rightarrow \mathrm{Mo}_2 \mathrm{~S}_3+3 \mathrm{~S}+10 \mathrm{OH}^{-}$

  (5)

  2.2   Morphology analyses

  The morphology and microstructure of CoMoNiO-S/NF composites were analyzed using SEM and TEM. As shown in Fig.2a, the nanosheets were stacked into nano-flower clusters, a typical morphology of Co₃O₄ at a vulcanization temperature of 90 ℃. At 110 ℃, the Co₃O₄ nanosheets in CoMoNiO-S/NF-110 (Fig.2b) became thinner and smaller, with fine Ni₃S₂ and Mo₂S₃ nanoparticles growing at the edges. This structure can provide a rich, heterogeneous interface, enhancing the synergistic effects between the components^[38-39]. Meanwhile, when the temperature reached 120 ℃, the nanoparticles increased in size, and the cluster structure began to collapse (Fig.2c), indicating that the structure of Co₃O₄ was destroyed under the action of thiourea at elevated temperatures. At 130 ℃, the nanosheets in CoMoNiO-S/NF-130 (Fig.2d) almost disappeared, and numerous nanoparticles accumulated irregularly, indicating that Ni₃S₂ and Mo₂S₃ were the primary products under high-temperature conditions.

  Figure 2

  

  Figure 2.  SEM images of (a) CoMoNiO-S/NF-90, (b) CoMoNiO-S/NF-110, (c) CoMoNiO-S/NF-120, and (d) CoMoNiO-S/NF-130; (e) TEM and (f) HRTEM images, (g) HAADF-STEM image, and (h-k) elemental mappings of CoMoO-S/NF-110

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  The surface and interface structures of the CoMoNiO-S/NF-110 components were further analyzed using HRTEM. The TEM image (Fig.2e) confirms the ultra-thin nanosheet structure. HRTEM (Fig.2f) showed distinct lattice diffraction fringes of 0.213 7 and 0.166 1 nm, corresponding to the (104) and (302) planes of Mo₂S₃, respectively. Additionally, the lattice fringes of 0.243 8 and 0.153 4 nm correspond to the (311) and (511) planes of Co₃O₄. These results were consistent with XRD results. In addition, a lattice fringe of 0.237 7 nm corresponds to the (200) plane of Ni₃S₂, with interfaces forming between the components. Moreover, dislocations and lattice deformation (indicated by red lines) at the boundaries of Co₃O₄-Ni₃S₂ and Co₃O₄-Mo₂S₃ indicate a defect structure, enhancing active site exposure. High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) and the corresponding elemental distributions (Fig.2g-2k) reveal that the S element was predominantly distributed at the edges, aligning with the sulfide nanoparticles observed at the edge of the nanoparticle in SEM, further confirming the successful introduction of S.

  2.3   XPS and band structure analyses

  The XPS survey spectra (Fig.3a) display the presence of Co, Mo, Ni, S, and O elements in the synthetic material, with all elemental peaks normalized to the C1s standard at 284.8 eV. Fig.3b shows the high-resolution XPS spectrum of Co2p of the samples prepared at different temperatures, where CoMoNiO-S/NF-130 differs significantly from the other samples. In addition to the typical Co²⁺, Co³⁺, and satellite peaks noted in all samples, the Co2p_3/2 and Co2p_1/2 peaks at 778.50 and 793.40 eV, respectively, correspond to Co⁰. This indicates that Co₃O₄ is further reduced to Co⁰ at high temperatures, which promotes reduction reactions. This phenomenon was further confirmed by high-resolution XPS spectra of O1s in various samples. In the O1s spectra (Fig.3c) of CoMoNiO-S/NF-90, CoMoNiO-S/NF-110, and CoMoNiO-S/NF-120, peaks at ca. 530.20, 531.10, 531.70, and 532.40 eV belong to M—O, M—OH (M=Co, Mo, and Ni), SO₄^2-, and adsorbed H₂O, respectively^[40-41]. However, the O1s spectrum of CoMoNiO-S/NF-130 showed the complete disappearance of the M—O peak, implying the reduction of Co₃O₄, which was consistent with the XRD results.

  Figure 3

  

  Figure 3.  (a) XPS survey spectra and high-resolution XPS spectra of (b) Co2p, (c) O1s, (d) Ni2p, (e) Mo3d, and (f) S2p for CoMoNiO-S/NF-T

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  The fine spectrum of Ni2p in CoMoNiO-S/NF displayed four distinct spin-orbit splitting peaks and two broad satellite peaks, as shown in Fig.3d. The peaks at ca. 856.40 and 858.09 eV correspond to Ni2p_3/2, while those at ca. 874.26 and 875.93 eV belong to Ni2p_1/2. The energy difference between Ni2p_1/2 and Ni2p_3/2 was approximately 17.8 eV, and the presence of satellite peaks at 863.01 and 881.13 eV illustrates the coexistence of Ni²⁺ and Ni^3+[42]. In addition, the binding energies of Ni2p_3/2 and Ni2p_1/2 in CoMoNiO-S/NF-110 shifted to higher binding energies compared to other samples, indicating electron transfer from Ni₃S₂ to Mo₂S₃ due to strong electron coupling. In the Mo3d fine spectrum of all samples (Fig.3e), the peak at 226.19 eV is attributed to the S2s signal, while peaks at 228.04 and 231.97 eV represent Mo3d_5/2 and Mo3d_3/2 of Mo^3+[43-44], respectively. The peaks at binding energies of 232.72 and 235.34 eV are attributed to Mo⁵⁺ and Mo⁶⁺, possibly due to the partial oxidation of low-priced Mo in the air or the presence of MoO₄^2- in the sample. Among all Mo3d spectra, the Mo³⁺ signal peak of CoMoNiO-S/NF-110 showed the lowest binding energy, consistent with the Ni2p signal observed in Fig.1e, further illustrating the electronic interaction between Mo and Ni. The S2p spectrum (Fig.3f) shows that the S2p signal peak of CoMoO-S/NF-90 was the weakest, aligning with the XRD results, indicating the lowest sulfide content. Peaks at 162.80 and 161.65 eV correspond to S2p_1/2 and S2p_3/2 in S^2-, respectively, while the binding energy at 164.10 eV is ascribed to S₂^2-, which is usually related to the S vacancies at bridging or terminal sites. This suggests the existence of unsaturated S atoms at Mo-S or Ni-S sites, which serve as active sites, increasing the catalyst activity^[45]. Peaks at 168.56 and 169.69 eV in CoMoO-S/NF-110, CoMoO-S/NF-120, and CoMoO-S/NF-130 belong to SO₄^2-[46-47], indicating the presence of CoSO₄. In contrast, the peak at this binding energy in CoMoO-S/NF-90 may stem from S oxidation in the air.

  The analytical results demonstrate that CoMoNiO-S/NF-T is a composite phase formed by CoSO₄/Ni₃S₂/Mo₂S₃/NF in CoMoNiO-S/NF-130, CoSO₄/Ni₃S₂/Mo₂S₃/Co₃O₄/NF in CoMoNiO-S/NF-110 and CoMoNiO-S/NF-120, and Ni₃S₂/Mo₂S₃/Co₃O₄/NF in CoMoNiO-S/NF-90. These findings also reveal electronic interactions among the components, altering the frontier orbital energy of the metal sulfide and thus affecting the binding energy and catalytic rate of the intermediate^[48].

  Next, to study the heterogeneous interfacial interactions of components in CoMoNiO-S/NF-110, Ni₃S₂/NF (Ni-S/NF), Mo₂S₃/Ni₃S₂/NF (MoNi-S/NF), and pure Co₃O₄/NF (the preparation methods were shown in the supporting information) were synthesized for comparison. The composition and the valence states of the surface elements were analyzed using XRD (Fig.S2) and XPS. As shown in Fig.4a, Ni-S/NF contained Ni and S, whereas MoNi-S/NF included Mo, Ni, and S. Compared to Ni-S/NF and MoNi-S/NF (Fig.4b), the binding energies of S2p_3/2 and S2p_1/2 in CoMoNiO-S/NF-110 exhibited a negative shift of 0.66 and 0.67 eV, and 0.15 and 0.20 eV, respectively. These shifts indicate the formation of S vacancies^[49] in CoMoNiO-S/NF-110, which is conducive to exposing more active sites compared to Ni-S/NF and MoNi-S/NF. Fig.4c and 4d showed a positive shift in the Co2p and O1s peaks of the CoMoNiO-S/NF-110 compared with pure Co₃O₄, indicating a reduction in the electron cloud density around Co₃O₄ after the complex formation. CoMoNiO-S/NF-110 composite exhibited only five cleavage peaks (Fig.4e), possibly due to the reduced Mo content, where the Mo3d_5/2 and Mo3d_3/2 peaks of Mo³⁺ were shifted negatively relative to MoNi-S/NF. Moreover, the Ni2p spectrum (Fig.4f) of the composite displayed a positive shift, similar to Co2p, when compared with MoNi-S/NF and Ni-S/NF, implying a decrease in electron cloud density around Ni₃S₂ and Co₃O₄, while it increases around Mo₂S₃. These observations confirm the presence of a distinct heterostructure in CoMoNiO-S/NF-110, characterized by interfaces between Co₃O₄ and Mo₂S₃, as well as between Ni₃S₂ and Mo₂S₃. The electronic structure at these interfaces is adjusted, leading to the formation of partial positive and negative charges.

  Figure 4

  

  Figure 4.  (a) XPS survey spectra of the control samples Ni-S/NF and MoNi-S/NF; High-resolution XPS spectra of (b) S2p, (c) Co2p, (d) O1s, (e) Mo3d, and (f) Ni2p in CoMoNiO-S/NF-110 and other control samples

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  2.4   OER performance

  The OER performance was evaluated in 1.0 mol·L^-1 KOH, comparing the properties of materials synthesized at different temperatures with commercial RuO₂. As shown in Fig.5a, CoMoNiO-S/NF-110 exhibited superior electrocatalytic performance, ascribed to its unique heterogeneous structure. At current densities of 100 and 200 mA·cm^-2, it required overpotentials (η) of 199.4 and 224.4 mV, respectively. CoMoNiO-S/NF-130 demonstrates a lower current density growth than RuO₂ at high potential, with η₁₀₀ and η₂₀₀ of 252.4 and 300.4 mV, respectively. Additionally, the catalytic performance of other nanomaterials surpassed that of RuO₂, with CoMoNiO-S/NF-90 exhibiting η₁₀₀ and η₂₀₀ of 218.4 and 253.4 mV, respectively, and CoMoNiO-S/NF-110 and CoMoNiO-S/NF-120 achieving η₁₀₀ and η₂₀₀ values of 216.4 and 245.4 mV, respectively. Compared to cobalt-based oxides and molybdenum sulfide/nickel sulfide electrocatalysts reported in the literature, CoMoNiO-S/NF-110 showed superior OER performance (Table 1). Tafel slopes analysis provides insights into the reaction kinetics of the catalysts. As displayed in Fig.5b and 5c, the Tafel slopes of CoMoNiO-S/NF-90, CoMoNiO-S/NF-110, CoMoNiO-S/NF-120, CoMoNiO-S/NF-130, and RuO₂ catalysts were calculated to be 87.52, 73.20, 83.20, 131.90, and 118.32 mV·dec^-1, respectively. The CoMoNiO-S/NF-110 catalyst exhibited the smallest Tafel slope, indicating the fastest reaction kinetics and enhanced acceleration of the electrochemical OER.

  Figure 5

  

  Figure 5.  Electrocatalytic OER performance of composites CoMoNiO-S/NF-T: (a) polarization curves; (b) Tafel slopes; (c) column chart of current density and Tafel slope; (d) EIS; (e) estimated C_dl, and (f) polarization curves of catalytic OER normalized by ECSA

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  Table 1

  

  Table 1.  Comparison of the OER electrocatalytic performance of CoMoNiO-S/NF-110 with reported composite electrocatalysts in 1 mol·L^-1 KOH

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  Catalyst	Current density / (mA·cm-2)	η / mV	Ref.
  core-shell MoS2@CoO	10	320	[50]
  CoS2@Co3O4	10	320	[51]
  Co3O4/MoS2	20	230	[52]
  Co3O4-MoS2(75%)/NF	10	298	[53]
  Ni-Mo-S@Co3O4/CF	10	275	[36]
  Co3O4@Mo-Co3S4-Ni3S2/NF	50	295	[35]
  Ni3S2/MoS2 hollow sphere	10	303	[54]
  MoS2-Ni3S2 nanosphere	100	269	[55]
  Ni3S2/MoS2/20g-C3N4	10	183	[56]
  CoS2/MoS2/Ni3S2/NF	100	300	[57]
  NiS2/MoS2/CNTs	10	315	[58]
  NiCoP-Ni3S2-MoS2	10	160	[59]
  Ni3S2NSs/NF	10	223	[60]
  Zr-MoS2@Ni3S2	20	275	[61]
  Co-NiS@MoS2	50	170.6	[62]
  CoMoNiO-S/NF-110	100	199.4	This work

  In electrocatalysis, the resistance of a catalyst significantly determines the kinetics of the process. To investigate this, the electrochemical impedance spectra of various catalysts were analyzed to determine their resistance and assess their kinetic behavior. The findings, presented in Fig.5d and Table S1, indicate that the CoMoNiO-S/NF-110 catalyst exhibited an electron transfer resistance (R_ct) of only 0.987 Ω, which was lower than that of other CoMoNiO-S/NF-T (T=90, 120, and 130) catalysts. These results suggest that the CoMoNiO-S/NF-110 catalyst possesses superior charge transfer efficiency during electrocatalytic OER. To gain further insights into the inherent OER activity of the prepared material, the electrochemically active surface area (ECSA) of the catalysts was evaluated by comparing their double-layer capacitance (C_dl), as ECSA is linearly proportional to C_dl. A larger C_dl indicates a larger ECSA, which enhances the number of active sites at the solid-liquid interface, facilitates bubble elimination, and improves electrochemical performance. The cyclic voltammetry (CV) curves of the synthesized catalysts under different sweep rates in the non-Faraday region were recorded (Fig.S3a-S3d) to fit the C_dl values. The fitted C_dl values (Fig.5e) of CoMoNiO-S/NF-90, CoMoNiO-S/NF-110, CoMoNiO-S/NF-120, and CoMoNiO-S/NF-130 were noted to be 518, 657, 258, and 223 mF·cm^-2, respectively. In addition, the electrocatalytic OER polarization curves of CoMoNiO-S/NF-T catalysts were normalized by their ECSA values^[63]. As depicted in Fig.5f, CoMoNiO-S/NF-110 exhibited the highest OER catalytic activity, aligning with the results shown in Fig.5a. This observation indicates that temperature affects not only the ECSA of the catalyst, but also its intrinsic electrocatalytic activity.

  Fig.6a shows the electrocatalytic OER performance of CoMoNiO-S/NF-110 and the control samples Ni-S/NF, MoNi-S/NF, and Co₃O₄/NF. As expected, CoMoNiO-S/NF-110 demonstrated superior performance. The current density of CoMoNiO-S/NF-110 increased rapidly with applied potential, requiring a substantially lower overpotential to achieve a current density of 100 mA·cm^-2 compared to the other catalysts. This improved performance is mainly due to the formation of S vacancy and the strong electron coupling interaction at the interfaces of Co₃O₄ with Mo₂S₃ and Ni₃S₂ with Mo₂S₃. These interactions optimize the electronic structure of the composite, promoting electron transfer across the interfaces. The generation of S vacancies creates more active sites, which are conducive to the adsorption and reaction of reactants. The Tafel slope of CoMoNiO-S/NF-110, as illustrated in Fig.6b, was only 69.4 mV·dec^-1, significantly lower than that of Ni-S/NF (182.2 mV·dec^-1), MoNi-S/NF (98.7 mV·dec^-1), and Co₃O₄/NF (145.8 mV·dec^-1). The results show that the formation of heterogeneous interfaces not only improves the OER activity of the catalyst but also promotes catalytic kinetics. The EIS tests (Fig.6c) show that CoMoNiO-S/NF-110 exhibited the smallest semicircle region among all samples, suggesting that the synergistic interaction at heterogeneous interfaces leads to faster charge transfer during the OER, thereby improving electronic conductivity. In addition, the C_dl estimated from CV curves at different scanning rates was used to assess the active sites. Fig.6d shows that CoMoNiO-S/NF-110 displayed the highest C_dl value of 767 mF·cm^-2, indicating that the heterogeneous interface significantly increases the ECSA, leading to superior electrocatalytic activity.

  Figure 6

  

  Figure 6.  (a) LSV curves, (b) Tafel plots, (c) EIS, (d) estimation of C_dl of CoMoNiO-S/NF-110 and Ni-S/NF, MoNi-S/NF, Co₃O₄/NF in 1.0 mol·L^-1 KOH; (e) Polarization curves before and after OER test, and (f) i-t curve of CoMoNiO-S/NF-110 for OER in 1.0 mol·L^-1 KOH

  Inset in panel c: enlarged EIS views of CoMoNiO-S/NF-110 and MoNi-S/NF; Inset in panel f: SEM images of CoMoNiO-S/NF-110 before and after OER stability test.

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  The stability of a catalyst is crucial for practical applications. CoMoNiO-S/NF-110 demonstrates excellent electrocatalytic oxygen evolution stability in alkaline environments (Fig.6e). Following a 40 h constant voltage test, the polarization curve remained nearly identical to its initial state, indicating a minimal decline in reaction activity. The time-current (i-t) curve revealed negligible variation in current density (Fig.6f), and the catalyst morphology showed no significant changes. After the stability test, the XRD pattern of CoMoNiO-S/NF-110 (Fig.S4) indicates that the peaks remained largely unchanged, except for the disappearance of CoSO₄ peaks, likely due to its dissolution during the test. After the OER stability test, the peak corresponding to SO₄^2- in the O1s high-resolution XPS spectrum disappeared, while the M—O peak area increased (Fig.7a), attributed to M—O bond formation during the OER process. For the high-resolution Co2p XPS spectrum of (Fig.7b), the Co peak area decreased significantly after testing, possibly due to CoSO₄ dissolution during the OER test. In contrast, the high-resolution Ni2p XPS spectrum showed minimal changes compared to the pre-test spectrum (Fig.S5). During OER testing, nickel sulfide usually oxidizes to hydroxyl nickel or nickel oxide. A significant reduction in the high-resolution XPS signal intensities of Mo3d and S2p (Fig.7c and 7d) suggests partial dissolution of Mo and S in the electrolyte after stability testing, which is consistent with previous reports^[64-65]. HRTEM images (Fig.8a and 8b) reveal clear lattice fringes after the stability test, with a spacing of 0.388 and 0.452 nm corresponding to the (002) plane of MoO₃ and the (001) plane of NiO₂. This observation aligned with the increased M—O peak area shown in Fig.7a.

  Figure 7

  

  Figure 7.  XPS spectra of (a) O1s, (b) Co2p, (c) Mo3d, and (d) S2p before and after OER stability test

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  Figure 8

  

  Figure 8.  HRTEM analysis after OER stability test

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  3.   Conclusions

  In this study, the composite electrocatalyst CoMoNiO-S/NF was successfully prepared through the vulcanization process. Among the prepared catalysts, the CoMoNiO-S/NF-110 nanosheets, featuring Mo and Ni sulfides at the edges, provided rich active sites and interfaces. This structure shows excellent OER activity under the synergistic action of Ni₃S₂, Mo₂S₃, Co₃O₄, and other components. At current densities of 100 and 200 mA·cm^-2, the catalyst achieved overpotentials of 199.4 and 224.4 mV, respectively, outperforming commercial RuO₂ in electrocatalytic efficiency. The exceptional catalytic performance of the CoMoNiO-S/NF-110 catalyst in OER highlights the crucial impact of temperature in determining the composition of the composite catalyst. Optimizing temperature emerges as a key strategy for the construction of high-performance electrocatalysts. This study provides a cost-effective and efficient alkaline oxygen evolution catalyst, while also serving as a valuable guide for the design and synthesis of other advanced electrocatalysts.

  
  Acknowledgements: This research was financially supported by Taiyuan Institute of Technology Scientific Research Initial Funding (Grants No.2024KJ016, 2022KJ052, 2022LJ014), the Fundamental Research Program of Shanxi Province (Grant No.202303021222301). Supporting information is available at http://www.wjhxxb.cn
  Credit authorship contribution statement: YANG Hong: Conceptualization, writing-original draft, methodology, data curation, funding acquisition. SHAO Shengjuan: Funding acquisition, conceptualization methodology, validation. LI Baoyi: Funding acquisition, validation. LU Yifan: Data curation. LI Na: Data curation.
  Declaration of competing interest: There are no conflicts to declare.
  
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• 

  Figure 1  Schematic of the fabrication procedure of CoMoNiO-S/NF-T

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  Figure 2  SEM images of (a) CoMoNiO-S/NF-90, (b) CoMoNiO-S/NF-110, (c) CoMoNiO-S/NF-120, and (d) CoMoNiO-S/NF-130; (e) TEM and (f) HRTEM images, (g) HAADF-STEM image, and (h-k) elemental mappings of CoMoO-S/NF-110

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  Figure 3  (a) XPS survey spectra and high-resolution XPS spectra of (b) Co2p, (c) O1s, (d) Ni2p, (e) Mo3d, and (f) S2p for CoMoNiO-S/NF-T

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  Figure 4  (a) XPS survey spectra of the control samples Ni-S/NF and MoNi-S/NF; High-resolution XPS spectra of (b) S2p, (c) Co2p, (d) O1s, (e) Mo3d, and (f) Ni2p in CoMoNiO-S/NF-110 and other control samples

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  Figure 5  Electrocatalytic OER performance of composites CoMoNiO-S/NF-T: (a) polarization curves; (b) Tafel slopes; (c) column chart of current density and Tafel slope; (d) EIS; (e) estimated C_dl, and (f) polarization curves of catalytic OER normalized by ECSA

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  Figure 6  (a) LSV curves, (b) Tafel plots, (c) EIS, (d) estimation of C_dl of CoMoNiO-S/NF-110 and Ni-S/NF, MoNi-S/NF, Co₃O₄/NF in 1.0 mol·L^-1 KOH; (e) Polarization curves before and after OER test, and (f) i-t curve of CoMoNiO-S/NF-110 for OER in 1.0 mol·L^-1 KOH

  Inset in panel c: enlarged EIS views of CoMoNiO-S/NF-110 and MoNi-S/NF; Inset in panel f: SEM images of CoMoNiO-S/NF-110 before and after OER stability test.

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  Figure 7  XPS spectra of (a) O1s, (b) Co2p, (c) Mo3d, and (d) S2p before and after OER stability test

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  Figure 8  HRTEM analysis after OER stability test

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  Table 1.  Comparison of the OER electrocatalytic performance of CoMoNiO-S/NF-110 with reported composite electrocatalysts in 1 mol·L^-1 KOH

  Catalyst	Current density / (mA·cm-2)	η / mV	Ref.
  core-shell MoS2@CoO	10	320	[50]
  CoS2@Co3O4	10	320	[51]
  Co3O4/MoS2	20	230	[52]
  Co3O4-MoS2(75%)/NF	10	298	[53]
  Ni-Mo-S@Co3O4/CF	10	275	[36]
  Co3O4@Mo-Co3S4-Ni3S2/NF	50	295	[35]
  Ni3S2/MoS2 hollow sphere	10	303	[54]
  MoS2-Ni3S2 nanosphere	100	269	[55]
  Ni3S2/MoS2/20g-C3N4	10	183	[56]
  CoS2/MoS2/Ni3S2/NF	100	300	[57]
  NiS2/MoS2/CNTs	10	315	[58]
  NiCoP-Ni3S2-MoS2	10	160	[59]
  Ni3S2NSs/NF	10	223	[60]
  Zr-MoS2@Ni3S2	20	275	[61]
  Co-NiS@MoS2	50	170.6	[62]
  CoMoNiO-S/NF-110	100	199.4	This work

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•