English

• With the escalating gravity of the energy crisis, the search for novel green energy sources to mitigate reliance on fossil fuels stands as the quintessential path toward sustainable development^[1-2]. Electrolytic water splitting, known for its simplicity and cost-efficiency in hydrogen production, has attracted significant attention. The electrolysis process primarily comprises two half-reactions: the anodic oxygen evolution reaction (OER) and the cathodic hydrogen evolution reaction (HER). However, the widespread implementation of electrolytic water technology is restricted due to the intricate four-electron transfer mechanism in OER^[3-4]. Nanoalloy and carbon composite materials have emerged as promising candidates for OER application with their advantageous properties such as multifaceted electrocatalytic sites, excellent conductivity, high stability, and structural diversity^[5].

  Metal-organic frameworks (MOFs) represent a class of three-dimensional porous organic-inorganic hybrids crafted through the coordination of metal ions and organic ligands. Unlike traditional porous materials, MOFs exhibit remarkable advantages, such as structural diversity, facile functionalization and modification, and tunable pore sizes. Inorganic nanomaterials derived from MOFs provide an ideal platform for developing highly efficient catalyst systems, due to their well-defined structure, large specific surface area, and interconnected porosity^[6].

  Graphene, a two-dimensional carbon nanomaterial with a hexagonal honeycomb lattice structure composed of sp² hybridized carbon atoms, has garnered significant attention owing to its exceptional specific surface area, unparalleled electronic properties, and robust electrochemical stability^[7]. Despite these promising attributes, pristine graphene materials often encounter challenges related to limited specific capacitance and durability. 2-Methylimidazole, as a ligand for transition metals or metal clusters, can simultaneously connect metals and carbon nanomaterials to form a network crystalline porous structure, providing more active sites and controllable mesopores^[8-9]. Therefore, the complex network structure formed by 2-methylimidazole, Co, and reduced graphene oxide (rGO) has a higher specific surface area and surface functionality. Co₃O₄/rGO derived from this template has more diverse valence states and higher catalytic activity.

  Extensive research has revealed that the integration of graphene surfaces with MOFs can substantially enhance the capacitance and electrochemical stability of these materials. Improving the catalytic performance of MOF materials through doping with non-precious metals is also an advanced strategy for designing catalysts. Zhang et al.^[10] constructed a two-dimensional cobalt-doped MnPSe₃ nanosheets (CMPS), which served as an outstanding bifunctional catalyst for alkaline seawater splitting, offering the current density of 10 mA·cm^-2 with applied overpotentials of 59 and 300 mV for HER and OER, respectively, while that of the MnPSe₃ nanosheets (MPS) was 120 and 370 mV, respectively. Peng et al.^[11] provided platinum-containing ruthenium oxide nanoparticles (Pt@RuO_x NPs) to achieve excellent overall water-splitting performance in acidic electrolytes and Pt@RuO_x NPs exhibited more desirable OER activity than RuO_x NPs. Wang et al.^[12] also reported Fe nanoparticles confined by multiple-heteroatom-doped carbon frameworks which are used for aqueous Zn-air battery driving CO₂ electrolysis. Nowadays, a series of research shows that Ru-doped Co/C ternary composites can also exhibit impressive electrochemical performance in varying acidic and alkaline environments^[13-14].

  Herein, a one-step precipitation method was employed to synthesize MOFs/rGO binary composites (ZIF-67/rGO), using Co(NO₃)₂·6H₂O as the metal source, 2-methylimidazole as the organic ligand, and rGO as the carbon support. Then Ru ions were introduced through ion exchange and annealing, producing porous Ru-doped Co₃O₄/rGO electrocatalysts (Ru-Co₃O₄/rGO). This approach significantly improves the electrocatalytic performance of Co-based MOFs, offering promising applications in various electrochemical reactions.

  1.   Experimental

  1.1   Chemicals

  Ruthenium(Ⅲ) chloride hydrate (RuCl₃·3H₂O) and cobalt nitrate hexahydrate (Co(NO₃)₂·6H₂O, 99%) were purchased from Sinopharm Chemical Reagent Co., Ltd. 2-Methylimidazole (99%) were purchased from Aladdin Reagent Co., Ltd. rGO was purchased from Nanjing XFNANO Materials Tech Co., Ltd. Other reagents are purchased from commercial companies and used without further treatment.

  1.2   Synthesis of ZIF-67/rGO precursor

  rGO (120 mg) and Co(NO₃)₂·6H₂O (1 532 mg) were dispersed in a 120 mL methanol/ethanol (1∶1, V/V) mixed solution by ultrasound for 2 h and then stirred for 3 h. 2-Methylimidazole (1 729 mg) was added to the above solution, and the mixture was vigorously stirred for 20 min and let settle for 24 h. The product was washed multiple times with ethanol, and dried overnight in a 60 ℃ oven to obtain ZIF-67/rGO.

  1.3   Synthesis of Ru-ZIF-67/rGO catalyst

  ZIF-67/rGO (55 mg) was dispersed in 25 mL ethanol solution using ultrasound and 200 μL ethanol solution of RuCl₃ was added to stir and disperse for 3 h. The product was washed multiple times with ethanol and dried overnight in a 60 ℃ oven to obtain Ru-ZIF-67/rGO.

  1.4   Synthesis of Ru-Co₃O₄/rGO composite materials

  Ru-ZIF-67/rGO was calcined at 350 ℃ in an N₂ atmosphere for 2 h to prepare Ru-Co₃O₄/rGO composite materials. The preparation process is shown in Fig. 1. Firstly, rGO was used as the induction template, Co²⁺ was used as the metal source, and 2-methylimidazole was used as the organic ligand. Cobalt-based MOF material (ZIF-67) was grown on a rGO surface by rich oxygen-containing functional groups of rGO. The purified sample was dissolved and dispersed in ethanol, and RuCl₃ was added as the ruthenium source. The aim was to fully contact the surface of the introduced ruthenium and ZIF-67/rGO for ion exchange, resulting in a uniform ruthenium-doped ternary composite material (Ru-ZIF-67/rGO). Subsequently, the material was used as a precursor and annealed in a nitrogen atmosphere to remove the organic components in Ru-ZIF-67/rGO, resulting in a high-performance catalyst Ru-Co₃O₄/rGO.

  Figure 1

  

  Figure 1.  Preparation schematic diagram of Ru-Co₃O₄/rGO

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  1.5   Structural characterization of catalysts

  X-ray diffractometer (XRD, Cu Kα, λ=0.154 nm) was used under the operating voltage of 40 kV, operating current of 40 mA, scanning speed of 2 ℃·min^-1, and scanning range of 5°-80°. The morphology of the samples was observed by the S-4800 scanning electron microscope (SEM) of Hitachi High-tech Co. Ltd. The surface element content and the presence form of main elements were analyzed by X-ray photoelectron spectrometer. The structural features and elemental composition of Ru-Co₃O₄/rGO were carefully observed and accurately determined through transmission electron microscopy (TEM, JEM-1400-plus, Japan) at an operating voltage of 100 kV. The metal components of the catalysts were determined by inductively coupled plasma-optical emission spectrometry (ICP-OES, OES5110, MS7700).

  1.6   Performance test of electrocatalyst materials

  At room temperature, an electrochemical workstation (CHI 660E, China) was used in a three-electrode system. The glassy carbon electrode loaded with samples was used as the working electrode, the graphite rod electrode was used as the opposing electrode, and the Hg/HgO electrode (internal infiltration 1 mol·L^-1 KOH) was used as the reference electrode. All electrochemical measurements were carried out on the electrochemical workstation. To test the electrocatalytic performance of the catalyst and explore the mechanism of electrocatalysis, linear scan voltammetry (LSV) (a scan rate of 2 mV·s^-1), cyclic voltammetry (CV) (scan rates ranging from 20 to 100 mV·s^-1), constant voltage or current stability, electrochemical impedance (EIS) (an amplitude voltage of 5 mV), and other tests were usually required.

  2.   Results and discussion

  2.1   Material characterization

  2.1.1   XRD analysis

  XRD was used to analyze the composition and structure of the synthesized material. As shown in Fig. 2, the XRD pattern of the ZIF-67/rGO precursor matches the reported diffraction peaks, confirming its successful synthesis^[15-16]. The XRD patterns of Co₃O₄/rGO and Ru-Co₃O₄/rGO exhibited similar characteristics. Specifically, the peaks at 2θ of 37.0°, 44.9°, and 65.2° correspond to the (311), (400), and (440) planes of Co₃O₄, respectively, indicating the successful transformation of ZIF-67 into Co₃O₄ under high-temperature calcination. Introducing Ru to Co₃O₄/rGO did not significantly alter peak positions or intensities, indicating no new crystal phases or significant structural changes in the catalyst. This further confirms the successful doping of Ru into the Co₃O₄ lattice.

  Figure 2

  

  Figure 2.  XRD pattern of ZIF-67/rGO, Co₃O₄/rGO, and Ru-Co₃O₄/rGO

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  2.1.2   Morphology analysis

  SEM was employed to examine the morphological characteristics of the synthesized samples. As depicted in Fig. 3a and 3b, rGO has a lamellar structure. Fig. 3c and 3d showed the ZIF-67 particles exhibited a uniform distribution with relatively consistent sizes. Notably, a significant portion of these ZIF-67 particles was observed to adhere to the folds of rGO indicating good dispersion and compatibility. Fig. 3e reveals that the introduction of a small quantity of Ru³⁺ ions did not appreciably alter the original morphology of the material. Subsequent high-temperature calcination of the ZIF-67/rGO precursor led to the formation of Co₃O₄/rGO, as shown in Fig. 3f. It is evident that most of the spherical ZIF-67 particles had undergone collapse and decomposition, while the original sheet-like structure of rGO remained intact, albeit in a stacked configuration. This suggests that the calcination process primarily affects the ZIF-67 component while preserving the overall morphology of the rGO carrier. Fig. 4a and 4b show the SEM images of Ru-ZIF-67/rGO after high-temperature calcination. The morphology observed is consistent with that of Co₃O₄/rGO, indicating that the addition of Ru³⁺ ions did not significantly influence the morphological features of the original material. This observation further validates the successful doping of Ru into the Co₃O₄ lattice while maintaining the overall structure. In Fig. 4c and 4d, Ru-Co₃O₄/rGO was still covered with many nanoparticles (ca. 10 nm), consistent with the SEM results.

  Figure 3

  

  Figure 3.  SEM images of (a, b) rGO, (c, d) ZIF-67/rGO, (e) Ru-ZIF-67/rGO, and (f) Co₃O₄/rGO

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  Figure 4

  

  Figure 4.  (a, b) SEM images and (c, d) TEM images of Ru-Co₃O₄/rGO

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  2.1.3   XPS analysis

  XPS analysis was conducted to elucidate the elemental composition and chemical states of the Ru-Co₃O₄/rGO. As depicted in Fig. 5a, the C1s XPS spectrum exhibits three characteristic peaks, corresponding to C=C (284.2 eV), C—O (285.1 eV), and C=O (288.2 eV), respectively. The high-resolution Co2p XPS spectrum (Fig. 5b) reveals characteristic peaks attributed to Co³⁺ at 780.1 and 795.3 eV, corresponding to Co2p_3/2 and Co2p_1/2, respectively. Additionally, peaks at 782.1 and 797.2 eV indicate the presence of Co²⁺, confirming the mixed valence state of Co in the catalyst^[12]. Satellite peaks were also observed near 788.1 and 804.2 eV, further validating the XPS analysis. The high-resolution O1s XPS spectrum (Fig. 5c) exhibited three distinct peaks. These peaks can be attributed to M—O bonds (M=Co, Ru, 529.7 eV), oxygen vacancies, adsorbed hydroxyl groups (—OH, 531.3 eV), and H₂O (532.7 eV) on the catalyst surface. The presence of these oxygen species suggests the diverse chemical environments and potential catalytic activity of the material^[15-16]. Finally, Fig. 5d demonstrates that Ru⁰ was detected in the Ru-Co₃O₄/rGO catalyst, and the corresponding peaks of Ru3p were 463.8 and 485.5 eV^[17]. The distinct peaks corresponding to the Ru element confirm its incorporation into the Ru-Co₃O₄/rGO material, further validating the XPS analysis and compositional characterization. The loading (mass fraction) of Ru and Co in the Ru-Co₃O₄/rGO catalyst was 0.441% and 24.6% respectively, which was determined by ICP-OES.

  Figure 5

  

  Figure 5.  (a) C1s, (b) Co2p, (c) O1s, and (d) Ru3p high-resolution XPS spectra of Ru-Co₃O₄/rGO

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  2.2   Electrocatalytic performance

  Fig. 6a illustrates the LSV curves recorded for various catalysts during OER. The Ru-Co₃O₄/rGO catalyst exhibited an overpotential of 363.5 mV at a current density of 50 mA·cm·², which was notably lower than the 402.3 mV observed for Co₃O₄/rGO without Ru doping. This significant reduction in overpotential indicates that Ru doping effectively enhances the OER performance of the composite material, leading to a higher catalytic efficiency. In practical applications, the application of a higher overpotential is often necessary to achieve a desired increase in current density. Ideally, a lower overpotential resulted in a faster increase in current density. The correlation between current density and overpotential can be mathematically described using the Butler-Volmer equation, which enables the calculation of the Tafel slope (b) for the OER from the LSV curve. A lower Tafel slope signifies a reduced overpotential requirement for the catalytic reaction to achieve a given current density^[18-19]. As expected, Ru-Co₃O₄/rGO exhibited a low overpotential of 363.5 mV at a turnover frequency (TOF) of 0.000 12 s^-1, which suggests that Ru-Co₃O₄/rGO possessed excellent intrinsic electrocatalytic OER activity. As depicted in Fig. 6b, the Tafel slope of Ru-Co₃O₄/rGO was 103 mV·dec^-1, significantly lower than that of Co₃O₄/rGO (140 mV·dec^-1). This lower Tafel slope suggests that Ru-Co₃O₄/rGO possessed higher catalytic activity and faster electrode reaction kinetic rates, further validating the beneficial effects of Ru doping on the OER performance of the composite material.

  Figure 6

  

  Figure 6.  (a) LSV curves and (b) Tafel slopes during the OER process of different catalysts

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  The OER performances of Ru-Co₃O₄/rGO with different Ru doping amounts (The volume of RuCl₃ solution added was 100, 200, and 300 μL, respectively.) were tested. By comparing LSV curves, we found that when the volume of RuCl₃ solution was 200 μL, the overpotential was much lower than that of undoped Ru samples and samples with the volume of RuCl₃ solution of 100 and 300 μL (Fig. 7a). EIS (Fig. 7b) was used to characterize in surface charge transfer impedance of electrode during OER. Among them, Ru-Co₃O₄/rGO had the smallest radius of curvature, indicating the surface of Ru-Co₃O₄/rGO had the smallest charge transfer impedance, which is beneficial to improve the OER activity of the catalyst.

  Figure 7

  

  Figure 7.  (a) LSV curves during the OER process of Ru-Co₃O₄/rGO with different Ru doping amounts; (b) EIS of Co₃O₄/rGO and Ru-Co₃O₄/rGO

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  To quantitatively compare the electrochemical active surface areas (ECSAs) of Co₃O₄/rGO and Ru-Co₃O₄/rGO, cyclic voltammetry (CV) measurements were performed at varying scan rates (20, 40, 60, 80, and 100 mV·s·^-1) to generate the CV curves depicted in Fig. 8a and 8b. The observed linear relationship between current density and scan rate provided insights into the electroactive surface during the catalytic process. Specifically, the double-layer capacitance (C_dl) was examined at the liquid/solid interface in a 1 mol·L^-1 KOH electrolyte solution to study the electroactive surface^[16-17]. Based on the CV curves of Co₃O₄/rGO and Ru-Co₃O₄/rGO at different scanning rates, the relationship between capacitive current and scanning rate was derived (Fig. 8c). The calculated value of Ru-Co₃O₄/rGO, based on the slope of the fitting line, was 6.5 mF·cm², exceeding the corresponding value of 2.1 mF·cm² for Co₃O₄/rGO. This significant increase indicates that Ru-Co₃O₄/rGO possesses a larger electrochemical surface area, enabling the exposure of more active sites and thereby enhancing the catalytic activity for OER^[20-21]. The durability of Ru-Co₃O₄/rGO was assessed through a stability test presented in Fig. 8d, conducted at a potential of 1.52 V. The fluctuations observed in the current density-time (i-t) curve reflect the dynamic nature of the OER process at this potential. Initially, the generated oxygen bubbles adsorb onto the catalyst layer, reducing the contact area between the electrode and the electrolyte and decreasing current density. However, as the bubbles accumulated and eventually detached from the electrode surface, the contact area was restored, increasing current density. This cycle of oxygen generation and removal from the catalyst layer accounts for the jitter in the i-t curve. After 24 h of constant i-t testing, the current density of Ru-Co₃O₄/rGO decreased from the initial ca. 4 to 2 mA·cm^-2. As shown in Fig. 9, After OER, the morphology of Ru-Co₃O₄/rGO was not significantly changed, as observed by SEM, compared to the original sample. However, the crystal shape was altered considerably, as observed by XRD. This change may be due to a small amount of Co²⁺ being converted to Co(OH)₂ and CoOOH during the OER process.

  Figure 8

  

  Figure 8.  CV curves of (a) Ru-Co₃O₄/rGO and (b) Co₃O₄/rGO at different scanning rates; (c) Cdl values of Ru-Co₃O₄/rGO and Co₃O₄/rGO (Δj is the electrical double-layer charge-discharge current density); (d) i-t curve of Ru-Co₃O₄/rGO

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  Figure 9

  

  Figure 9.  (a) SEM image and (b) XRD pattern for Ru-Co₃O₄/rGO after OER test

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  3.   Conclusions

  In summary, a ZIF-67/rGO layered composite was constructed, and Ru-Co₃O₄/rGO catalysts were prepared using a high-temperature thermal method. To enhance the OER performance of the catalyst, Ru doped precursor was calcined to obtain Ru-Co₃O₄/rGO catalyst. Characterization analysis revealed that a small amount of Ru doping did not affect the nucleation and growth of the ZIF-67 precursor. Adding Ru resulted in similar or slightly enhanced OER performance for the Ru-Co₃O₄/rGO compared to Co₃O₄/rGO. The Ru-Co₃O₄/rGO catalyst has highly dispersed active sites, rich pore structure advantages, and excellent conductivity. It showed excellent OER catalytic activity in a saturated 1 mol·L^-1 KOH electrolyte, demonstrating its great potential as a fuel cell cathode catalyst. Against the backdrop of the energy crisis and environmental pollution, this study fully integrated the advantages of MOFs and graphene materials, opening a new approach to designing efficient electrocatalysts. Based on the diversity and flexibility of MOF material structure and composition, this research approach has great potential for efficient catalyst design. It can be widely applied to optimize the performance of catalysts in different fields and catalytic systems.

  
  
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  Figure 1  Preparation schematic diagram of Ru-Co₃O₄/rGO

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  Figure 2  XRD pattern of ZIF-67/rGO, Co₃O₄/rGO, and Ru-Co₃O₄/rGO

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  Figure 3  SEM images of (a, b) rGO, (c, d) ZIF-67/rGO, (e) Ru-ZIF-67/rGO, and (f) Co₃O₄/rGO

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  Figure 4  (a, b) SEM images and (c, d) TEM images of Ru-Co₃O₄/rGO

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  Figure 5  (a) C1s, (b) Co2p, (c) O1s, and (d) Ru3p high-resolution XPS spectra of Ru-Co₃O₄/rGO

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  Figure 6  (a) LSV curves and (b) Tafel slopes during the OER process of different catalysts

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  Figure 7  (a) LSV curves during the OER process of Ru-Co₃O₄/rGO with different Ru doping amounts; (b) EIS of Co₃O₄/rGO and Ru-Co₃O₄/rGO

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  Figure 8  CV curves of (a) Ru-Co₃O₄/rGO and (b) Co₃O₄/rGO at different scanning rates; (c) Cdl values of Ru-Co₃O₄/rGO and Co₃O₄/rGO (Δj is the electrical double-layer charge-discharge current density); (d) i-t curve of Ru-Co₃O₄/rGO

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  Figure 9  (a) SEM image and (b) XRD pattern for Ru-Co₃O₄/rGO after OER test

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