English

• Electrocatalytic water splitting for hydrogen is one of the most promising strategies for the scalable production of clean hydrogen as sustainable energy^[1-3]. However, the overall water splitting reaction is seriously restricted by the oxygen evolution reaction (OER) (4OH^- = 2H₂O + 4e^- + O₂) at the anode, ascribing to the sluggish four-electron transfer and the rigid O=O bond formation^[4-6]. Currently, noble metal oxides, such as IrO₂ and RuO₂, are developed as the most outstanding commercial electrocatalysts for OER^[7-9]. Nevertheless, to address the scarcity and high price of noble metals, transition-metal-based catalysts, such as Fe, Co, and Ni-based oxides or hydroxides, as alternatives, have been extensively explored in recent years^[10-16]. In contrast, copper-based materials have attracted less attention, as a result of the inherent high-occupied anti-bonding state of the d orbital of Cu (3d¹⁰4s¹), which can hardly hybridize with the O2p orbital of oxygen intermediates during the absorption process of OER, despite of their cost-effective and highly-conductive merits^[17-18].

  To enhance the efficiency and the structural stability of electrocatalysts for water oxidation, most researchers focus on developing new electrocatalysts or improving the performance of the already-known electrocatalysts^{[4, 18-19]}. Effective strategies have been reported to optimize electronic structures, such as heteroatom doping, nanostructuring, coordination engineering, and metal valence variation^[20-24], which determine the inherent activities of the active sites. In addition, reasonable engineering in microstructure endows the large surface areas between the catalysts and the electrolyte, which improves the exposure of the active sites, accelerating mass transport and electron transfer^[25-26]. The widely reported strategies are constructing hierarchical morphology^[27-28], designing surface defects^[29-30], introducing porous structure^[31-32], etc. Moreover, the configuration of three-dimensional (3D) architectures can be another choice for structural optimization due to the easy access of the electrolyte to the electrode surface, which shortens the diffusion path^{[15, 33-34]}. 3D nanonetworks constructed by lower dimensional nanomaterials provide better conductivity, stability, and anti-aggregation properties in comparison to low-dimensional materials^[35-36]. These advantages endow the 3D-structured nanomaterials with wide applications in energy conversion and storage^[37-39]. However, it is difficult to design Cu-based 3D materials in safe, low-energy consuming, and high-yield synthetic strategies. Most of the reported synthetic processes are complicated or of low yield^[40-41]. With reasonable utilization of these strategies above, the designed novel Cu-based materials are promising for low-cost efficient electrocatalysis of water^[42].

  In this work, we reported a novel 3D Cu-based nanoflower-like composite with mesoporous structure (abbreviated as Cu-NF) using zeolitic imidazolate framework-67 (ZIF-67) as precursor through a facile excessive Cu²⁺ etching. The follow-up calcination of the Cu-NF composites at low temperatures under the N₂ atmosphere obtained a series of Cu-NF derivatives with the reserved original shape (Scheme 1). It is worth mentioning that low-temperature calcination can be a simple way to enlarge the specific surface area and achieve improved porosity, by removing the coordinated moisture and partially decomposing the organic linkers^{[32, 43]}. Specifically, the product obtained at 300 ℃ (abbreviated as Cu-NF-300) resulted in the best OER performance, with the lowest overpotential of 347 mV and the smallest Tafel slope of 93 mV·dec^-1 at 10 mA·cm^-2. The 3D superstructure showed advantages in facilitating mass transport and charge transfer. Moreover, the newly formed active species and the improved porosity caused by low-temperature treatment contribute to enhanced OER performance. This work provides a gentle and facile approach to preparing Cu-based materials for electrochemical applications.

  Scheme 1

  

  Scheme 1.  Synthetic approach of the Cu-NF composites and their derivatives

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  1.   Experimental

  1.1   Materials and reagents

  Materials contained cobalt nitrate hexahydrate (Co(NO₃)₂·6H₂O, 99.99%, Aladdin) and 2-methylimidazole (C₄H₆N₂, 98%, Energy Chemical Co., Ltd.). Besides, copper(Ⅱ) nitrate trihydrate (Cu(NO₃)₂·3H₂O, 99%), potassium hydroxide (KOH, 95%), ethanol (CH₃CH₂OH, AR) and methanol (CH₃OH, AR) were purchased from Sinopharm Chemical Reagent. Nafion solution (5%) was from Sigma-Aldrich. All the chemicals were used without further purification. The water used in all experiments was prepared by passing through an ultra-pure purification system (Aqua Solutions).

  1.2   Synthesis of ZIF-67

  The ZIF-67 was synthesized according to our previous work^[44]. Co(NO₃)₂·6H₂O (3.64 g) and 2-methylimidazole (2-MIm) (4.106 7 g) were dissolved in 100 mL methanol, respectively. Then the two solutions were mixed and sonicated in an ultrasonic cleaner for 15 min. Then the purple product was separated by centrifugation and washed for 2-3 times with methanol. After that, the product was finally dried at 25 ℃ for 6 h.

  1.3   Synthesis of Cu-NF

  0.1 g of ZIF-67 and 0.2 g of Cu(NO₃)₂·3H₂O were dispersed in 50 and 12.5 mL ethanol by ultrasonication, respectively. The two solutions were mixed and stirred for 12 h. Then the product was separated by centrifugation and washed with methanol. After that, the product was finally dried at 25 ℃.

  1.4   Synthesis of CuCo-based nanosheets (CuCo-NS) and Cu-based nanoplates (Cu-NP)

  The procedures of synthesis of CuCo-NS and Cu-NP were identical to that of Cu-NF, except that the mass ratio of Cu(NO₃)₂·3H₂O to ZIF-67 was 1∶1 or 3∶1.

  1.5   Synthesis of the Cu-NF derivatives (Cu-NP-T)

  The Cu-NF composites were heat treated at different temperatures (200, 250, 300, 350, and 400 ℃) under an N₂ atmosphere with a heating rate of 2 ℃·min^-1 and retained for 2 h. The samples were named Cu-NP-T (T ℃ was the calcination temperature).

  1.6   Characterization

  The morphological characteristics of the sample were observed using a field emission scanning electron microscope (SEM) at an acceleration voltage of 5.0 kV (Zeiss_Supra55). The transmission electron microscopy (TEM) images, the selected area electron diffraction (SAED) images, and the energy dispersive spectroscopy (EDS) mapping images were captured on a Tecnai G2 F30 transmission electron microscope (300 kV). The X-ray diffraction (XRD) patterns were examined on a Bruker D8 Advanced X-ray diffractometer (Cu Kα radiation: λ=0.154 18 nm, 40 kV, 40 mA, 2θ=5°-80°). The Fourier transform infrared spectrometer (FTIR) measurement was investigated on a Tensor 27. The inductively coupled plasma optical emission spectroscopy (ICP-OES) measurements were performed using a simultaneous ICP spectrometer (Optima 7300 DV, Perkin Elmer) equipped with a solid-state detector. The X-ray photoelectron spectroscopy (XPS) measurement was carried out using an axis ultra X-ray photoelectron spectrometer (Kratos Analytical Ltd., UK) equipped with Al Kα source (hν=1 486.6 eV). The thermal gravimetric analysis (TGA) was carried out with a Pyris 1 TGA (Perkin Elmer, America) scientific instrument and heated from room temperature to 800 ℃ with a ramp rate of 5 ℃·min^-1 in nitrogen gas flow.

  1.7   Electrochemical measurements

  All the electrochemical measurements of OER were performed in a three-electrode system at CHI760e Electrochemical Station (CH Instruments, Shanghai, China). Electrochemical measurements were carried out using Hg/HgO electrode as reference electrode, graphite carbon rod as counter electrode, and glassy carbon electrode (GCE) (diameter: 3.0 mm) as the working electrode in 1.0 mol·L^-1 KOH electrolyte. 5 mg of the samples (Cu-NF-T) was dispersed in 1 mL ethanol and ultrasonicated for 10 min. Then, 100 μL of 5% Nafion solution was added to the prepared solution, followed by sonicating for another 10 min. Then, 5 μL of the above suspension was taken with a pipette and placed dropwise onto the dried GCE surface. The GCE was dried at room temperature and the catalyst loading was about 0.3 mg·cm^-2.

  2.   Results and discussion

  2.1   Material characterizations

  The synthesis of the Cu-NF composites was achieved using ZIF-67 as a precursor through a facile Cu²⁺ etching strategy, as shown in Scheme 1. ZIF-67 particles were first synthesized according to the previous report^[44]. The as-prepared ZIF-67 showed a typical dodecahedral morphology with an average size of 1 mm and smooth surface, characterized by SEM (Fig.S1, Supporting information). Afterward, Cu(NO₃)₂ was employed as a chemical etching agent to reset the metal arrangement and reconstruct the metal-ligand framework. For such a synthesis, the mass ratio of Cu²⁺ to ZIF-67 was found highly critical to control the morphology of the product. The morphology and the structure of the attained materials from different mass ratios were characterized by SEM (Fig. 1a-1c), as well as the morphology variations with the increase of reaction time (Fig.S2-S4). As shown in Fig.S2b, a moderate Cu²⁺ to ZIF-67 mass ratio (1∶1) caused insufficient etching of ZIF-67 at first, leading to the formation of nanosheets on the surface with the basic ZIF-67 core intact. With time passing, the nanosheets covered the core gradually (Fig.S2). As displayed in Fig. 1a, the product obtained in 12 h was composed of nanosheets (CuCo-NS). Since the mass ratio increased to 2∶1, excessive Cu²⁺ contributed to the morphological deformation of the dodecahedral ZIF-67 (1 h), followed by its disappearance and the subsequent appearance of the 3D flower-like Cu-based composites composed of thin nanosheets (6 h) (Fig.S3). The flowers became bigger and the petals were denser (Fig.S3c). A high yield was obtained in 12 h, which was considered optimal for the subsequent studies (Cu-NF, Fig. 1b). Further increasing the mass ratio to 3∶1 (Fig.S4), the polyhedron collapsed more rapidly, reconstructed the piles of nanoplates (Cu-NP, Fig. 1c).

  Figure 1

  

  Figure 1.  SEM images of (a) CuCo-NS, (b) Cu-NF, and (c) Cu-NP composites; HRTEM images and SAED pattern (Inset) of (d-f) the Cu-NF composites; (g) HAADF-STEM image of the Cu-NF composites and their corresponding elemental mapping images

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  Particularly, excessive Cu²⁺ etching (m_Cu²⁺∶m_ZIF-67=2∶1) reaction went through the decomposition of ZIF-67 and the reconstruction of a novel Cu-based material. These could be evidenced both by the color change from purple to turquoise and the phenomenon of the solid disappearance and reappearance (Fig.S5). During the process, the hydrolysis of Cu²⁺ caused the generation of H⁺, which etches ZIF-67 severely, followed by the collapse of the framework^[45]. Subsequently, concentrated Cu²⁺ and 2-MIm reconstructed a new network by self-assembly, together with the hydrolysis, forming Cu-LDHs (Cu-based layered double hydroxides) and other compounds on the surface. Hence, an increased Cu²⁺ ratio (3∶1) caused a more rapid decomposition reaction. The solid disappeared within 15 min and reoccurred in 2 h. In contrast, when a moderate mass ratio of Cu²⁺ to ZIF-67 (1∶1) was applied, there was no significant color change in the solution and only the color of the product changed to grey-purple.

  In light of the Cu-NF composites, Fig. 1b displayed that as-fabricated Cu-NF composites were uniformly 3D flower structures with an average diameter of 2 mm, comprised of thin nanosheets. The high-resolution TEM (HRTEM) images and the SAED pattern in Fig. 1d-1f exhibited the polycrystalline feature of the Cu-NF composites. The high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) image and elemental mapping images of the Cu-NF composites in Fig. 1g, verified the homogeneous distribution of Cu, C, N, and O elements throughout the entire network. Nevertheless, little Co was detected in the composites. Meanwhile, the metal contents were also quantified by ICP-OES and relevant data were listed in Table S1. The results indicated that Cu element accounted for 97.64% and Co only for 2.36%, reconfirming the composites based on Cu, which agreed with the results from elemental mapping images. In addition, the results from element analysis (CNH) informed that the contents of C, N, and H in the organic part were 26.73%, 15.51%, and 3.25%, respectively, which were precise the proportions of C, N, and H in 2-MIm, indicating the existence of 2-MIm in the Cu-NF composites. The contents of Cu and Co elements in CuCo-NS and Cu-NP were also examined by ICP-OES and listed in Table S1. The N₂ adsorption-desorption isotherms of the Cu-NF composites (Fig.S6) analyzed that the specific surface area was 41 m²·g^-1, which was calculated by the Barrett-Joyner-Halenda (BET) method. Accordingly, the pore-size distribution curve (Fig.S6) of the composites indicated its mesoporous feature with an average pore diameter of 25.70 nm.

  To further investigate the microstructure and composition of the Cu-NF composites, XRD was utilized to investigate the prepared materials (Fig. 2a and 2b). As displayed in Fig. 2a, after the addition of Cu²⁺ for 30 min, the XRD pattern of the as-prepared ZIF-67 stayed unchanged. Since the solid reoccurred in 6 h, the new groups of peaks centered at 12.6°, 25.8°, and 34.1° appeared, suggesting the generation of a novel composite. With the passage of the reaction time (12 h), these peaks became intensified. In comparison with the standard cards, shown in Fig. 2b, the dominant peaks at 12.6°, 25.8°, and 34.1° refer to (001), (004) and (121) facets of gerhardtite (PDF No.14-0687), the peaks at 13.3°, 35.1° referring to the (001) and (201) facets of copper hydroxide hydrate (PDF No.42-0637) and those at 12.1°, 24.5°, and 33.9° can be indexed to (003), (006) and (012) planes of hydrotalcite-like LDHs^{[14, 26, 46]}, which reconfirmed the polycrystalline feature of the Cu-NF composites by the results from HRTEM. In addition, the XRD patterns of CuCo-NS and Cu-NP were also collected (Fig.S7). The CuCo-NS pattern had not only all the characteristic diffraction peaks that ZIF-67 has but also the peaks indicating the LDH construction. These suggest the coexisted ZIF-67 and LDH structure, reconfirming the results from SEM. By contrast, the Cu-NP XRD pattern demonstrates the well-crystalline state of Cu-LDH.

  Figure 2

  

  Figure 2.  (a) XRD patterns of the samples from the reaction mixture taken at different times during the reaction; (b) XRD patterns, (c) FTIR spectrum, (d) full XPS spectrum and XPS high-resolution spectra of (e) C1s, (f) Cu2p, (g) O1s, and (h) N1s of the Cu-NF composites

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  Moreover, FTIR spectroscopy was utilized to further demonstrate the functional groups that existed in the prepared Cu-NF composites. As shown in Fig. 2c, a broad peak centered at 3 461 cm^-1 is associated with O—H stretching, while the sharp strong peaks at 1 384 and 1 420 cm^-1 indicate the vibration and stretching modes of nitrate^[47-48]. The absorption of nitrate to balance the charge is typical in LDH structure^[48]. The peaks at 1 560 and 1 146 cm^-1 are related to the C=N and C—N stretching vibration, verifying the presence of 2-MIm^[49-50]. The absorption peaks below 750 cm^-1 are the vibration modes of Cu—O^[47-48].

  In addition, XPS was performed to study the valance states of chemical elements in the Cu-NF composites (Fig. 2d-2h). Fig. 2d reconfirmed the coexistence of Cu, C, N, and O elements. The peak of Co was barely visible, indicating little Co existed, matching the results aforementioned. In Fig. 2f, the high-resolution Cu2p spectrum with two dominant groups of peaks could be noticed. The main peaks at 934.8 and 954.7 eV are indexed to the 2p_3/2 and 2p_1/2, along with their satellite peaks at 942.5 and 962.6 eV, which are the characteristic of the Cu²⁺ species^[51-52]. The peaks of O1s at 531.4 and 532.2 eV displayed in Fig. 2g are ascribed to M—O (M=metal) and hydroxyl groups^[53], confirming the Cu—OH structure. The peaks of N1s (Fig. 2h) are assigned to three nitrogen configurations, including pyridinic N (400.1 eV), pyrrolic N (398.9 eV) and N in nitrite (407.0 eV)^[54], and those of C1s (Fig. 2e) are referring to C—C and N—C^{[44, 46]}, verifying the existence of 2-MIm.

  To modulate the microstructure of the Cu-NF composites, low-temperature calcination was conducted. Depending on the results of TGA under the N₂ atmosphere (Fig. 3a), the weight loss of the Cu-NF composites suffered several steps before collapse at 400 ℃, which involved the removal of the moisture and the organic ligands. Therefore, calcination temperature was rationally chosen between 200 and 400 ℃. The SEM images in Fig. 4a₁-4e₁ showed that the 3D flower-like outlines composed of thin nanosheets were maintained after calcined at different temperatures, except for Cu-NF-400. Fig. 4d₁ and 4e₁ displayed the thicker petals with smaller sizes (around 1 mm), indicating that the high-temperature treatment led to structure collapse and material agglomeration. Meanwhile, the TEM images exhibited distinct nanoparticles produced on the nanosheets after raising the temperature to 300 ℃ or above (Fig. 4c₂-4e₂), which could be the metallic Cu particles produced, being further confirmed in subsequent XRD results. The XRD patterns in Fig. 3b disclosed the significant changes in the chemical components and their contents among the prepared samples with the increase of the calcination temperature and Fig.S8-S12 detailed their chemical compositions. Additionally, the sharper peaks of Cu-NF-350 and Cu-NF-400 demonstrate their stronger crystallinity, as can be seen in Fig. 3b. Cu-NF-200 shows the less complicated component (Fig.S8) and lower crystallinity than Cu-NF, because of the removal of the water absorbed physically and coordinated in the network. Then, Cu²⁺ was reduced to Cu₂₊₁O (Cu₂O with copper excess defects), together with the removal of the coordinated water and NO₃^- at 250 ℃ (Fig.S9), and further reduced to metallic Cu at 400 ℃ (Fig.S12). Particularly, Cu-NF-300 was composed of crystalline nanoparticles of Cu₂₊₁O and metallic Cu (Fig.S10), while Cu-NF-350 had similar constituents with higher content of Cu (Fig.S13). These results matched well with the TEM images in Fig. 4a₂-4e₂.

  Figure 3

  

  Figure 3.  (a) TGA curve of the Cu-NF composites; (b) XRD patterns, (c) FTIR spectra of the Cu-NF composites and their derivatives and (d) Raman spectra

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  Figure 4

  

  Figure 4.  SEM images of (a₁) Cu-NF-200, (b₁) Cu-NF-250, (c₁) Cu-NF-300, (d₁) Cu-NF-350, (e₁) Cu-NF-400; TEM images of (a₂) Cu-NF-200, (b₂) Cu-NF-250, (c₂) Cu-NF-300, (d₂) Cu-NF-350, (e₂) Cu-NF-400

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  The FTIR spectra of the prepared samples exhibit that the peak at 1 146 cm^-1 relates to the C—N bond^[55] became weaker and vanished after calcined at 350 and 400 ℃ (Fig. 3c), depicting the decomposition of the ligand from 350 ℃ and a certain degree of carbonization at 350 ℃ and above. Together with the results above, the existence of N in Cu-NF-300 demonstrates the composites to be N-doped Cu₂₊₁O and Cu nanocomposites, which contributes to enhanced OER performance. Furthermore, the Raman spectra of all samples were recorded to identify the structure of the carbon framework (Fig. 3d). The represent D and G peaks at 1 350 and 1 580 cm^-1 display the degree of the defect and disorder of the carbon in the materials, respectively^[56]. All the materials possess different degrees of surface defects after heat treatment.

  2.2   Electrochemical application in OER

  The electrochemical properties of the prepared samples as electrocatalysts for OER were evaluated in 1 mol·L^-1 KOH solution by using a standard three-electrode system. In the linear sweep voltammetry (LSV) curves of Fig. 5a, Cu-NF-300 showed the earliest current response at 1.577 V (vs RHE) to reach the current density of 10 mA·cm^-2 among all the prepared samples, namely a low overpotential of 347 mV (Fig. 5b). The other catalysts Cu-NF-250 and Cu-NF-350 were just inferior to it, requiring overpotentials of 368 and 377 mV, respectively. Meanwhile, the current of Cu-NF-300 increased most sharply, suggesting the best OER activity of Cu-NF-300. Accordingly, at a current density of 20 mA·cm^-2, the overpotential of Cu-NF-300 was the lowest as well (Fig. 5b). The Tafel slopes derived from the LSV curves were calculated to evaluate the kinetics of the OER performance for all the samples (Fig. 5c). It is well known that a lower Tafel slope means a slower increase of the overpotential with the current density, demonstrating superior OER kinetics of the electrocatalyst. Cu-NF-300 showed the expected lowest Tafel slope with only 93 mV·dec^-1, demonstrating the most favorable kinetics. The Tafel slope of other samples followed exactly the trend of their overpotentials. In addition, the electrochemical active surface areas (ECSAs) were estimated via the electrochemical double-layer capacitance (C_dl) using the cyclic voltammograms (CV) method in the non-Faradic region at different scan rates (Fig.S13). As shown in Fig. 5d, the C_dl values of Cu-NF-300, Cu-NF-350, and Cu-NF-400 were 6.71, 5.16, and 5.2 mF·cm^-2, suggesting larger active surface areas than the others, with Cu-NF-300 as the largest one. The larger ECSA of Cu-NF-300 than Cu-NF-200 and Cu-NF-250 may be attributed to the improved porosity, resulting in a larger specific surface area. Nevertheless, compared to Cu-NF-350 and Cu-NF-400, the increased ECSA may be ascribed to more structure defects at the molecular level. Nyquist plots in Fig. 5e demonstrated the electrochemical impedance of the as-prepared electrocatalysts. The observation of sample Cu-NF-300 with a steeper line in low-frequency regions depicted the least charge transfer resistance among all these samples and matched well with the smallest Tafel slope.

  Figure 5

  

  Figure 5.  (a) LSV curves, (b) overpotentials at the specific current densities, (c) Tafel plots, (d) ECSAs based on CV curves (Fig.S13) at different scan rates, and (e) Nyquist plots of impedance spectroscopy analysis of the as-prepared electrocatalysts in 1.0 mol·L^-1 KOH; (f) LSV curves of Cu-NF-300

  Inset in (e): fitted equivalent circuit model for the electrochemical impedance tests.

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  The reasons for the better OER performance of Cu-NF-300 than Cu-NF-250, and Cu-NF-350, can be analyzed by combining the variation of microstructures and chemical compositions. With the temperature increasing from 250 to 300 ℃, metallic Cu formed in Cu-NF-300 acts as a new active center and improves the conductivity of the material, leading to better OER activity than Cu-NF-250. In the meantime, the increased heat treatment leads to the elimination of the coordinated ligands, which increases the porosity for better mass transfer. Further increasing the temperature to 350 ℃, the content ratio of metallic Cu to Cu₂₊₁O increases in Cu-NF-350 compared to Cu-NF-300, leading the better conductivity. However, the stronger crystallinity of Cu-NF-350 results in lower surface defect, and metal agglomeration decreases the active centers, further reducing its OER activity. Overall, the chemical composition and the microstructure of Cu-NF-300 have a positive influence on the electrocatalytic process. Afterward, the durability property of the Cu-NF-300 was evaluated using a continuous cyclic voltammetry sweep for 1 000 cycles (Fig. 5f) and the polarization curve was found to be close to the initial one, only 29 mV decay of the overpotential at 10 mA·cm^-2 after 1 000 cycles. In the long-time stability test via chronopotentiometry, a slight overpotential increase was observed after the test up to 15 h (Fig.S14), exhibiting relatively satisfactory stability of Cu-NF-300 in an alkaline medium.

  3.   Conclusions

  In summary, we prepared a novel Cu-based composite through a facile chemical etching strategy using ZIF-67 as a precursor, by controlling the mass ratio of Cu²⁺ to ZIF-67 to regulate the morphology. The as-obtained Cu-NF composites had a 3D flower-like architecture and mesoporous channels. After low-temperature calcination, a series of derivatives with the reserved original morphology have been obtained and applied as OER electrocatalysts. Particularly, Cu-NF-300 showed the best OER performance among all these samples, with a low overpotential of 347 mV and a small Tafel slope of 93 mV·dec^-1 at 10 mA·cm^-2. Except for the active species, the microstructure has a significant influence on the electrocatalyst for OER efficiency. The designed 3D superstructure and special microstructure obtained from calcination facilitate the transportation of the materials. Our results provide a new simple strategy for design Cu-based electrochemical materials.

  CRediT authorship contribution statement: Liu Yifan: Methodology, Conceptualization, Data curation, Formal analysis; Zhang Zhan: Conceptualization, Methodology, Writing-original draft, Formal analysis, Supervision, Funding acquisition; Zhu Rongmei: Methodology, Validation; Qiu Ziming: Formal analysis, Validation; Pang Huan: Funding acquisition, Writing-review & editing, Supervision.

  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.

  Acknowledgements: This work was supported by the National Natural Science Foundation of China (Grant No.U1904215), Natural Science Foundation of Jiangsu Province (Grant No.BK20200044), Jiangsu Shuangchuang Project (Grant No.(2020)30974) and Lvyangjinfeng Talent Program of Yangzhou (Grant No.YZLYJF2020PHD110).

  
  Supporting information is available at http://www.wjhxxb.cn
  
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  Scheme 1  Synthetic approach of the Cu-NF composites and their derivatives

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  Figure 1  SEM images of (a) CuCo-NS, (b) Cu-NF, and (c) Cu-NP composites; HRTEM images and SAED pattern (Inset) of (d-f) the Cu-NF composites; (g) HAADF-STEM image of the Cu-NF composites and their corresponding elemental mapping images

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  Figure 2  (a) XRD patterns of the samples from the reaction mixture taken at different times during the reaction; (b) XRD patterns, (c) FTIR spectrum, (d) full XPS spectrum and XPS high-resolution spectra of (e) C1s, (f) Cu2p, (g) O1s, and (h) N1s of the Cu-NF composites

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  Figure 3  (a) TGA curve of the Cu-NF composites; (b) XRD patterns, (c) FTIR spectra of the Cu-NF composites and their derivatives and (d) Raman spectra

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  Figure 4  SEM images of (a₁) Cu-NF-200, (b₁) Cu-NF-250, (c₁) Cu-NF-300, (d₁) Cu-NF-350, (e₁) Cu-NF-400; TEM images of (a₂) Cu-NF-200, (b₂) Cu-NF-250, (c₂) Cu-NF-300, (d₂) Cu-NF-350, (e₂) Cu-NF-400

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  Figure 5  (a) LSV curves, (b) overpotentials at the specific current densities, (c) Tafel plots, (d) ECSAs based on CV curves (Fig.S13) at different scan rates, and (e) Nyquist plots of impedance spectroscopy analysis of the as-prepared electrocatalysts in 1.0 mol·L^-1 KOH; (f) LSV curves of Cu-NF-300

  Inset in (e): fitted equivalent circuit model for the electrochemical impedance tests.

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