English

• Energy crises and environmental challenges have emerged as crucial issues, prompting the exploration of renewable resources as viable alternatives to the increasingly scarce fossil fuels^[1-3]. Hydrogen (H₂), regarded as a promising alternative, has garnered considerable research interest due to its zero-emission characteristics and high-energy density nature^[4-6]. Electrochemical water splitting, which involves two half-cell reactions, presents a feasible method for generating H₂^[7-8]. Specifically, the oxygen evolution reaction (OER), characterized by a four-electron redox process and inherently sluggish kinetics, poses significant limitations^[9-11].

  While RuO₂ and IrO₂ are acknowledged as activity-enhanced materials that can enhance OER reaction kinetics, their practical deployment on a broad scale is constrained by material scarcity and operational instability, particularly in large-scale industrial production^[12-13]. Recently, transition metal catalysts have been proven to be ideal candidates for OER catalysts due to their prominent activity and high abundance ^[14-16]. CoMoO₄, a bimetallic oxide characterized by strong redox behavior, is considered an alternative candidate for OER catalysts^[17-18]. However, the relatively low conductivity, poor cycling life, and unremarkable catalytic activity compared to noble-metal-based catalysts remain major bottlenecks for its widespread application^[19-20]. Metal-organic frameworks (MOFs) are ordered 3D porous crystal materials constructed from organic ligands coordinated with metal ions. Their numerous adjustable active sites, structural controllability, and excellent mechanical stability make them a remarkable presence in the field of electrocatalytic research, enabling enhanced ionic diffusion coupled with accelerated electron transfer during catalytic reaction^[21-23]. Using MOFs as templates allows precise control of the porosity and structure of the resulting catalyst. Consequently, various transition metal-based catalysts have been fabricated by employing MOFs as precursors to possess their structural characteristics. Liu et al.^[24] designed a/c-CoCu+Ru_x-LDH/NF for OER catalyst using CoCu-MOF as precursor and template, achieving the minimal overpotential of 214 mV at 10 mA·cm^-2. Zeng et al.^[25] prepared hollow multilayer heterogeneous material (CoFeP/CoFeP/NP-C) through a template sacrifice approach using MIL-88A, exhibiting outstanding OER catalytic activity with an overpotential of 272 mV at 10 mA·cm^-2.

  Notably, zeolitic imidazolate framework-67 (ZIF-67), constructed by Co²⁺ and dimethylimidazole through coordination interactions with a specific morphology, superior chemical stability, and a porous architecture, has been explored to promote OER kinetic process^[26-29]. Therefore, the conversion of Co²⁺ in ZIF-67 was utilized to fabricate CoMoO₄, and the derived CoMoO₄ began to generate on the template surface. The nanosheet array of the ZIF-67 template acted as a guide to assist its formation while maintaining the original structural integrity. The MOF-derived CoMoO₄ exhibits advantages such as abundant electron diffusion channels, a remarkable specific surface area, and outstanding stability. To further optimize the catalytic activity, iron oxyhydroxide (FeOOH) was introduced as a secondary phase and coupled with MOF-derived CoMoO₄ to regulate the absorption and desorption energies of intermediates, thus boosting catalytic reaction^[30].

  Motivated by these findings, a novel heterostructure FeOOH@CoMoO₄ composite (FeOOH@CoMoO₄/NF-400s) was synthesized using MOFs as the sacrificial template and supported on nickel foam (NF). In this work, to synthesize FeOOH@CoMoO₄/NF-400s, ZIF-67 nanosheet array not only served as the Co²⁺ source, but also functioned as the structural template to dictate its morphology. The specific structure of FeOOH@CoMoO₄/NF-400s facilitated the exposure of maximized active site accessibility and enhanced electrolyte permeation, which promoted its OER performance.

  1.   Experimental

  1.1   Chemicals and materials

  2-Methylimidazole (C₄H₆N₂, 98%), cobalt nitrate hexahydrate (Co(NO₃)₂·6H₂O, 99%), and ferric nitrate nonahydrate (Fe(NO₃)₃·9H₂O, 99%) were purchased from Shanghai Macklin Biochemistry Co., Ltd (China). Hydrogen chloride (HCl, 37%), potassium hydroxide (KOH, ≥85%), iridium oxide (99.9% metal basis, Ir: ≥84.5%), Nafion solution (5%), and NF were obtained from Shanghai Hesen Co., Ltd (China). Ultrapure water was exclusively employed in all experimental procedures.

  1.2   Preparation of ZIF-67/NF

  The NF (3 cm×4 cm) underwent cleaning with 3 mol·L^-1 HCl solution, ethanol, and deionized (DI) water for 10 min, respectively. Meanwhile, 18 mmol of 2-methylimidazole and 2 mmol of Co(NO₃)₂·6H₂O were separately dissolved in 40 mL DI water. A piece of surface-treated NF was immersed in a 2-methylimidazole solution and ultrasonicated for 10 min. After that, the Co(NO₃)₂·6H₂O solution was quickly poured into the C₄H₆N₂ solution, and the NF underwent 4-hour immersion in the mixture solution under ambient conditions. Finally, the obtained ZIF-67/NF was rinsed with DI water and ethanol, and vacuum-dried at 60 ℃ overnight.

  1.3   Preparation of CoMoO₄/NF-120℃

  The as-prepared ZIF-67/NF was placed in the mixed solution containing 0.05 mol·L^-1 Co(NO₃)₂·6H₂O and 0.05 mol·L^-1 Na₂MoO₄·2H₂O in a 20 mL Teflon-lined autoclave, which was sealed and heated at 120 ℃ for 4 h. The obtained sample was then annealed at 350 ℃ for 2 h under an air atmosphere to achieve CoMoO₄/NF-120℃.

  1.4   Preparation of FeOOH@CoMoO₄/NF-400s

  In a three-electrode cell system, the CoMoO₄/NF-120℃, Ag/AgCl, and platinum (Pt) were used as the working electrode, reference electrode, and counter electrode for electrodeposition, respectively. The CoMoO₄/NF-120℃ was treated in the Fe(NO₃)₃·9H₂O solution (30 mmol·L^-1) at the constant potential of -1 V for 400 s.

  1.5   Preparation of IrO₂/NF

  First, 10 mg of IrO₂ was dispersed in 1 mL of a mixture containing 50 μL of Nafion solution and 950 μL of ethanol, followed by 30 min of ultrasonication. The catalyst ink, with a mass loading of 2 mg·cm^-2, was dropped onto the NF substrate and then dried in air, resulting in the formation of IrO₂/NF.

  1.6   Characterization

  The surface morphology and structure of the sample were analyzed using a scanning electron microscope (SEM, JEOL JSM-6700F, 15 kV) and transmission electron microscopy (TEM, JEOL 2100 FEF, 200 kV). X-ray diffraction (XRD) patterns were obtained with a Bruker AXS D8 Advance X-ray diffraction instrument using Cu Κα radiation (λ=0.154 056 nm) under the working voltage of 40 kV, the working current of 30 mA, and the scanning range of 20°-90° to analyse the crystallinity of catalysts. X-ray photoelectron spectroscopy (XPS) measurements were carried out on a Thermo Fisher ESCALAB 250XI X-ray photoelectron spectroscopy equipped with monochromatic X-ray generated by an Al Kα (1 486.6 eV) source.

  1.7   Electrochemical measurements

  The catalytic properties of the as-prepared catalysts were evaluated in a conventional three-electrode configuration setup on a CHI 760E electrochemical workstation. The product was served as a working electrode, with Hg/HgO as a reference electrode and a graphite rod as a counter electrode. The electrochemical tests were all conducted in 1 mol·L^-1 KOH solution. Linear sweep voltammetry (LSV) curves were recorded in a potential gradient of 0-1 V at the scan rate of 2 mV·s^-1. The potential (E′) related to the Hg/HgO electrode was converted into the potential (E) related to the reversible hydrogen electrode (RHE) via the Nernst equation (E=E′+0.098+0.059pH), and the polarization curve was corrected for ohmic drop with 90% iR compensation. Cyclic voltammetry (CV) curves of the materials were collected within the potential window of 0.1-0.2 V at 50 mV·s^-1 to evaluate the double-layer capacitance (C_dl) values. Electrochemical impedance spectrum (EIS) was acquired with a frequency range from 10^-2 to 10⁵ Hz at 1.5 V (vs RHE).

  2.   Results and discussion

  2.1   Synthetic strategies and structural characterizations

  As illustrated in Fig.1, the growth of ZIF-67 nanosheet array was realized via leveraging the confinement induced by NF. Subsequently, the released Co²⁺ from ZIF-67 compacted coordination with MoO₄^2- by the hydrothermal method to form ultra-thin nanosheet CoMoO₄/NF-120℃. Following an electrodeposition treatment, the FeOOH@CoMoO₄/NF-400s with a like-biscuits nanosheet structure composed of nanoparticles was successfully prepared.

  Figure 1

  

  Figure 1.  Schematic of the synthesis strategy of FeOOH@CoMoO₄/NF-400s

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  The uniform ZIF-67 nanosheet array (Fig.2a and 2b) with an average thickness of 340 nm (Fig.S2a, Supporting information) grew vertically on bare NF, where NF (Fig.S1a and S1b), possessing a 3D framework structure, served as substrate. The smooth-surface nanosheets enhanced the interfacial contact area between the catalyst and electrolyte. Following a hydrothermal reaction with an annealing step, ZIF-67 was decomposed to transform into CoMoO₄, while maintaining the inherent 2D layered morphology of ZIF-67 (Fig.2c and 2d). Concurrently, tiny and ultrathin nano-sheets were regularly distributed on the surface of the ZIF-67 template, and the average thickness of CoMoO₄ nanosheets and the ZIF-67 template after loading CoMoO₄ were approximately 70 nm and 1.6 μm, respectively (Fig.S2b and S2c). The 2D CoMoO₄ nanosheet features a large specific surface area, thereby providing ample exposed active sites for OER. The CoMoO₄/NF-90℃ and CoMoO₄/NF-150℃ catalysts with different hydrothermal temperatures of 90 and 150 ℃ were prepared, respectively, and the different morphologies are displayed in Fig.S3. CoMoO₄/NF-90℃ in Fig.S3a and S3b showed a 2D crossover nanosheet structure like a beehive, retaining the backbone structure of ZIF-67. As a comparison, the CoMoO₄/NF-150℃ formed a larger beehive-shaped nanosheet structure, which was wrapped around the entire ZIF-67 framework (Fig.S3c and S3d). After that, the electrodeposition of FeOOH onto the CoMoO₄ surfaces resulted in a distinctive morphology, where CoMoO₄ nanosheets were systematically encapsulated by FeOOH nanoparticles layer by layer in a hierarchical manner (Fig.2e and 2f). The ordered arrangement of FeOOH nanoparticles effectively mitigated the stacking of CoMoO₄ nanosheets, ensuring exceptional long-term stability. For comparison, the SEM images of FeOOH@CoMoO₄/NF-200s and FeOOH@CoMoO₄/NF-600s prepared with different electrodeposition times of 200 and 400 s are illustrated in Fig.S4, respectively. FeOOH@CoMo₄/NF-200s (Fig.S4a) exhibited nearly identical morphology to pristine CoMoO₄, confirming insufficient electrodeposition of FeOOH nanoparticals within the short duration. Conversely, the FeOOH@CoMoO₄/NF-600s (Fig.S4b) with extended electrodeposition time led to the aggregation of nanoparticles, which could reduce the specific surface area.

  Figure 2

  

  Figure 2.  SEM images of (a, b) ZIF-67/NF, (c, d) CoMoO₄/NF-120℃, and (e, f) FeOOH@CoMoO₄/NF-400s

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  XRD patterns were acquired to determine the crystalline structure of FeOOH@CoMoO₄/NF-400s. As displayed in Fig.S5, the XRD pattern of ZIF-67/NF exhibited obvious diffraction peaks at 10.26°, 12.74°, 14.97°, 16.40°, and 17.94°, which correspond to (002), (112), (022), (013), and (222) planes. To avoid interference from strong diffraction peaks of NF during XRD analysis, the FeOOH@CoMoO₄ was collected by ultrasonically dispersing the FeOOH@CoMoO₄/NF-400s in ethanol to collect FeOOH@CoMoO₄ powder for XRD analysis. The diffraction peaks at 23.24°, 25.34°, 26.66°, 33.72°, and 53.35° belong to (021), (201), (002), (222), and (441) planes of the monoclinic CoMoO₄ (PDF No.21-0868). The XRD pattern of electrodeposited FeOOH revealed a hexagonal crystal structure, with main peaks centered at 28.09°, 38.36°, 55.22°, 56.99°, and 56.20° attributed to (301), (303), (414), (315), and (522) planes of FeOOH (PDF No.22-0353). The electrodeposition process is facilitated by the reduction of NO₃^- to generate hydroxide ions, thus promoting the formation of FeOOH^[31-32].

  $ \mathrm{NO}_3^{-}+7 \mathrm{H}_2 \mathrm{O}+8 \mathrm{e}^{-} \rightarrow \mathrm{NH}_4^{+}+10 \mathrm{OH}^{-}$

  (1)

  $ \mathrm{Fe}^{3+}+3 \mathrm{OH}^{-} \rightarrow \mathrm{FeOOH}+\mathrm{H}_2 \mathrm{O} $

  (2)

  Additionally, the residual NF with two peaks centered at 44.53° and 51.81° can be assigned to (111) and (200) planes of NF (PDF No.04-0850). The XRD analysis conclusively verified the precise synthesis of FeOOH@CoMoO₄/NF-400s, and ZIF-67 was utilized via a template sacrifice approach and cobalt source for the fabrication of CoMoO₄. The TEM image in Fig.3b demonstrated that the nanosheet structure of CoMoO₄ consisted of FeOOH nanoparticles anchored on its surface. Furthermore, the generation of heterointerface between FeOOH and CoMoO₄ marked by white dashed line was supported by the high-resolution TEM (HRTEM) image in Fig.3c. The lattice distances of 0.218 and 0.389 nm can be assigned to (411) plane of FeOOH (PDF No.22-0353) and (021) plane of CoMoO₄ (PDF No.21-0868), respectively. Meanwhile, the selected area electron diffraction (SAED) image confirmed the yellow rings are corresponded to (422) and (222) planes of CoMoO₄ (PDF No.21-0868), and the blue rings are attributed to (505) and (428) planes of FeOOH (PDF No.22-0353), which suggests the coexistence of CoMoO₄ and FeOOH in FeOOH@CoMoO₄/NF-400s (Fig.3d). The element mapping images in Fig.3e and energy dispersive X-ray spectrum (EDS) in Fig.S6 verified the presence of Co, Mo, Fe, O, and C elements, which were evenly dispersed on FeOOH@CoMoO₄/NF-400s.

  Figure 3

  

  Figure 3.  (a) XRD pattern, (b) TEM image, (c) HRTEM image, (d) SAED pattern, and (e) EDS element mappings of FeOOH@CoMoO₄/NF-400s

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  XPS results in Fig.4 displayed the individual element chemical state of the as-prepared catalysts, and the C1s (284.80 eV) peak was used to correct all element spectra. As shown in the full XPS spectrum (Fig.S7), the C, Ni, Co, Mo, Fe, and O signals were detected on FeOOH@CoMoO₄/NF-400s. The C1s spectra displayed three evident peaks, situated at 284.80, 286.31, and 288.60 eV, which were ascribed to C—C, C—O, and C=O bonds, respectively (Fig.S8). The Ni2p spectra of CoMoO₄/NF-120℃ and FeOOH@CoMoO₄/NF-400s are presented in Fig.S9. Four significant peaks were observed at the binding energy of 850-890 eV, which are attributed to the nickel source derived from NF participating in the hydrothermal reaction. In the Ni2p spectrum of FeOOH@CoMoO₄/NF-400s, two main peaks of 856.05 and 874.00 eV are attributed to Ni2p_3/2 and Ni2p_1/2, revealing the existence of Ni²⁺. As for CoMoO₄/NF-120℃, the Ni²⁺ peaks were located at 855.52 and 873.01 eV^[33-34]. Therefore, the Ni element mainly existed in the form of Ni²⁺ doping into the catalyst. In Fig.4a, two main peaks located at 781.40 and 797.06 eV belong to Co2p_3/2 and Co2p_1/2 of Co²⁺ ions in CoMoO₄/NF-120℃, while other peaks at 786.42 and 803.15 eV correspond to satellite peaks^[35-36]. Compared to CoMoO₄/NF-120℃, the binding energies at 780.90 eV (Co2p_3/2) and 796.81 eV (Co2p_1/2) of FeOOH@CoMoO₄/NF-400s displayed a negative shift by 0.50 and 0.25 eV, indicating that the existence of Fe increased the electron cloud density around Co atoms. Additionally, another two main peaks at 779.98 and 795.20 eV were detected in FeOOH@CoMoO₄/NF-400s, which were indicative of the appearance of Co³⁺ after electrodepositing FeOOH^[37], attributing to the oxidation of Co exposed on the surface of FeOOH@CoMoO₄/NF-400s^[38]. The Mo3d spectra in Fig.4b of CoMoO₄/NF-120℃ were deconvoluted into two peaks at 232.49 (Mo3d_5/2) and 235.61 eV (Mo3d_1/2), and their difference in binding energy was 3.12 eV, suggesting the presence of Mo⁶⁺ ions^[39-41]. For FeOOH@CoMoO₄/NF-400s, the peaks at 232.03 (Mo3d_5/2) and 235.15 eV (Mo3d_1/2) were all negatively shifted by 0.46 eV, signifying that Fe tailored the electronic structure of CoMoO₄, promoting electron delocalization toward Co. Interestingly, two new peaks at 233.39 and 236.48 eV of FeOOH@CoMoO₄/NF-400s are corresponded to Mo⁵⁺, indicating that Fe would promote the formation of lower oxidation state of Mo (Mo⁵⁺)^[42-43]. Additionally, partial reduction of Mo⁶⁺ to Mo⁵⁺ is driven by a charge compensation effect, maintaining electroneutrality when Co²⁺ was oxidized to Co³⁺ during the electrodeposition process. The negative shift of FeOOH@CoMoO₄/NF-400s in Co2p and Mo3d XPS spectra revealed the electronic transfer from FeOOH to CoMoO₄, indicating a strong electronic coupling between CoMoO₄ and FeOOH that modulated the electrocatalytic activity. For Fe2p spectra (Fig.4c), the observed peaks at 711.21 and 724.98 eV are assigned to Fe2p_3/2 and Fe2p_1/2, respectively, verifying the presence of Fe³⁺ formed in FeOOH^[44]. Four additional peaks at 713.85, 719.32, 728.86, and 733.74 eV are ascribed to shakeup satellite peaks^[45]. The deconvoluted O1s XPS spectrum is presented in Fig.4d, and three constituents centered at 530.10, 530.84, and 532.28 eV are assigned to M—O (M=Co, Mo, and Fe), M—O—H, and H₂O, respectively^[46-47].

  Figure 4

  

  Figure 4.  XPS spectra of (a) Co2p, (b) Mo3d, (c) Fe2p, and (d) O1s of the samples

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  2.2   OER performance

  To explore the OER catalytic performance, the as-synthesized catalysts were appraised in 1 mol·L^-1 KOH electrolyte. The LSV curves with iR compensation of 90% were recorded in Fig.5a. FeOOH@CoMoO₄/NF-400s exhibited exceptional OER performance, requiring merely 199 mV of overpotential to drive the current density of 10 mA·cm^-2, which was notably lower than CoMoO₄/NF-120℃ (223 mV), ZIF-67/NF (252 mV), NF (409 mV), and IrO₂/NF (227 mV), demonstrating its promising catalytic activity. Importantly, the OER performance FeOOH@CoMoO₄/NF-400s is superior to most advanced CoMoO₄ catalysts (Table S4). An intuitive comparison of the overpotentials for different catalysts at specific current density was displayed in Fig.5b. The LSV curves of CoMoO₄/NF-90℃, CoMoO₄/NF-120℃, and CoMoO₄/NF-150℃ were displayed in Fig.S10, and their overpotentials were 229, 223, and 246 mV at 10 mA·cm^-2, further revealing the excellent OER performance of CoMoO₄/NF-120℃. FeOOH@CoMoO₄/NF-200s, FeOOH@CoMoO₄/NF-400s, and FeOOH@CoMoO₄/NF-600s catalysts were fabricated by controlling the electrodeposition time, and the corresponding LSV curves are presented in Fig.S11, suggesting that the electrodeposition time of 400 s was the optimal choice. The mass loading of the active substance on NF for FeOOH@CoMoO₄/NF-200s, FeOOH@CoMoO₄/NF-400s, FeOOH@CoMoO₄/NF-600s, CoMoO₄/NF-120℃, and ZIF-67/NF were 18.5, 26.9, 32.1, 17.1, and 14.4 mg·cm^-2, respectively (Table S1). To probe the fundamental reaction kinetics of all catalysts, the measured Tafel plots are depicted in Fig.5c. The FeOOH@CoMoO₄/NF-400s offered a Tafel slope value of 49.56 mV·dec^-1, outperforming the controlled counterparts containing CoMoO₄/NF-120℃ (60.86 mV·dec^-1), ZIF-67/NF (67.62 mV·dec^-1), NF (137.15 mV·dec^-1), and IrO₂/NF (84.89 mV·dec^-1), demonstrating its beneficial kinetics towards OER.

  Figure 5

  

  Figure 5.  (a) LSV curves, (b) overpotentials at different current densities, and (c) Tafel plots of the samples

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  Compared to other samples, the fitted Nyquist plots in Fig.6a revealed the smallest semicircle diameter of FeOOH@CoMoO₄/NF-400s, suggesting the lowest charge conversion resistance (R_ct) value (Table S2). The favorable R_ct value of FeOOH@CoMoO₄/NF-400s further confirms its superior charge transfer by remarkably minimizing the electron transfer barrier. Bode plots of CoMoO₄/NF-120℃ and FeOOH@CoMoO₄/NF-400s under different potentials were shown in Fig.6b and 6c, which could reveal the charge transfer signals at two areas. The high-frequency phase shift ranging from 10² to 10⁵ Hz indicated the electrocatalytic oxidation^[48-49]. Obviously, the phase angle in the low-frequency range of 10^-2 to 10² Hz decreased rapidly, representing the OER process. Compared to CoMoO₄/NF-120℃ [1.46 V (vs RHE)], FeOOH@CoMoO₄/NF-400s exhibited a lower applied potential [1.43 V (vs RHE)] for promoting OER, confirming its enhanced OER activity.

  Figure 6

  

  Figure 6.  (a) Nyquist plots of the samples at 1.50 V (vs RHE), Bode plots at various potentials of (b) CoMoO₄/NF-120℃ and (c) FeOOH@CoMoO₄/NF-400s

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  CV curves in Fig.S12 were performed in a non-faradic region, and the corresponding C_dl curves in Fig.7a were fitted to calculate the electrochemical specific surface area (ECSA) values of catalysts (Table S3). The C_dl values of designed ZIF-67/NF and CoMoO₄/NF-120℃ were 68.04 and 82.21 mF·cm^-2, and that of FeOOH@CoMoO₄/NF-400s was increased to 136.49 mF·cm^-2, indicating the most abundant active sites for FeOOH@CoMoO₄/NF-400s. Moreover, the ECSA-normalized LSV curves in Fig.S13 confirmed that the overpotential of FeOOH@CoMoO₄/NF-400s was the lowest compared to other catalysts under a given current density. In Fig.7b, FeOOH@CoMoO₄/NF-400s demonstrated superior catalytic performance than other samples from multiple perspectives. Additionally, turnover frequency (TOF) value (Fig.S14) of FeOOH@CoMoO₄/NF-400s was calculated to be 0.898 s^-1 at an overpotential of 300 mV, which exhibited significant improvement relative to CoMoO₄/NF-120℃ (0.411 s^-1) and ZIF-67/NF (0.368 s^-1), suggesting the electrodeposition of FeOOH could elevate the intrinsic OER performance. The accelerated degradation test (ADT) of FeOOH@CoMoO₄/NF-400s was conducted at a scan rate of 100 mV·s^-1, and the LSV curves were almost overlapped before and after 1 000 cycles, revealing its exceptional stability (Fig.S15). In addition, the chronoamperometric tests under the current densities of 10 and 100 mA·cm^-2 were performed in Fig.7c, and FeOOH@CoMoO₄/NF-400s also exhibited insignificant attenuation even after 20 h, further confirming its outstanding OER stability.

  Figure 7

  

  Figure 7.  (a) C_dl curves and (b) performance comparison radar chart of the samples; (c) 20 h chronoamperometric curves of FeOOH@CoMoO₄/NF-400s

  In b: η₁₀ and η₁₀₀ were the overpotentials at 10 and 100 mA·cm^-2, respectively.

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  The chemical component and morphology of FeOOH@CoMoO₄/NF-400s after durability testing are characterized in Fig.S16 and S17. The XRD pattern of FeOOH@CoMoO₄/NF-400s after stability is shown in Fig.S16. The corresponding diffraction peaks of FeOOH@CoMoO₄/NF-400s post-OER could still be captured, but the intensity was decreased. The SEM image in Fig.S17 demonstrated that the original architectural features were basically maintained, while the FeOOH nanoparticles were partially dissolved after long-time stability, consistent with the XRD results.

  Overall, the superior OER activity of FeOOH@CoMoO₄/NF-400s primarily originates from three key determinants: (1) the ZIF-67 template, featuring a well-defined MOF structure, provides a high specific surface area and efficient mass transport channels, which were preserved during the conversion into CoMoO₄ nanosheets and subsequent deposition of FeOOH nanoparticles^[50-51]; (2) the robust electronic coupling between FeOOH and CoMoO₄ accelerated interfacial electron transfer^[52]; (3) employing 3D NF as a substrate for MOF growth effectively addressed the critical challenge of their low electrical conductivity.

  3.   Conclusions

  The MOF-derived FeOOH@CoMoO₄/NF-400s was successfully fabricated via a template sacrifice approach functioning as an exceptional catalyst for OER, which could realize the simultaneous breakthroughs in efficiency and durability. Accordingly, the as-prepared FeOOH@CoMoO₄/NF-400s displayed an ultralow overpotential of 199 mV at 10 mA·cm^-2, along with robust stability over 20 h at current densities of both 10 and 100 mA·cm^-2. Composed of a nanosheet and nanoparticle structure, the FeOOH@CoMoO₄/NF-400s retained the characteristic morphology of layered ZIF-67, which manifested a high specific surface area, resulting in substantial exposure of active sites. Moreover, the strong electronic interaction between FeOOH and CoMoO₄ could facilitate interfacial charge transfer. This research offers guidance and innovative concepts for the strategic application of MOF-derived OER catalysts, thereby promoting large-scale hydrogen production.

  
  Acknowledgements: This work was funded by the National Natural Science Foundation of China (Grant No.22271262) and the Opening Foundation of Shanxi Provincial Key Laboratory of Advanced Carbon Electrode Materials (Grant No.202104010910019). Supporting information is available at http://www.wjhxxb.cn
  
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  Figure 1  Schematic of the synthesis strategy of FeOOH@CoMoO₄/NF-400s

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  Figure 2  SEM images of (a, b) ZIF-67/NF, (c, d) CoMoO₄/NF-120℃, and (e, f) FeOOH@CoMoO₄/NF-400s

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  Figure 3  (a) XRD pattern, (b) TEM image, (c) HRTEM image, (d) SAED pattern, and (e) EDS element mappings of FeOOH@CoMoO₄/NF-400s

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  Figure 4  XPS spectra of (a) Co2p, (b) Mo3d, (c) Fe2p, and (d) O1s of the samples

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  Figure 5  (a) LSV curves, (b) overpotentials at different current densities, and (c) Tafel plots of the samples

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  Figure 6  (a) Nyquist plots of the samples at 1.50 V (vs RHE), Bode plots at various potentials of (b) CoMoO₄/NF-120℃ and (c) FeOOH@CoMoO₄/NF-400s

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  Figure 7  (a) C_dl curves and (b) performance comparison radar chart of the samples; (c) 20 h chronoamperometric curves of FeOOH@CoMoO₄/NF-400s

  In b: η₁₀ and η₁₀₀ were the overpotentials at 10 and 100 mA·cm^-2, respectively.

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