English

• 0.   Introduction

  With the rapid development of electronic products and transportation, energy-storage devices have attracted increasing attention over recent decades^[1]. Supercapacitors are among the most promising energy-storage devices for their advantages, including high power output, high charge-discharge rates, safety, and long lifespan^[2-9]. Co(OH)₂ has received particular attention as a promising candidate owing to its diverse merits, such as high theoretical specific capacitance (ca. 2 200 F· g^-1), high surface area, fast ion-insertion/desertion rate, facile preparation, and low cost^[10-12].

  However, its poor conductivity and easy agglomeration severely limit its practical applications. To address these issues, researchers have continuously worked to implement suitable electrode materials. One approach is to combine Co(OH)₂ with conductive materials, such as graphene (GP) or carbon nanotubes, to obtain comprehensive electrochemical performance^[13-14]. Another way to improve the capacitance of Co(OH)₂ is composition with other nanomaterials. As exemplified by MnO₂@NiCoZn - OH, MoO₂@Co(OH)₂, nanomaterials can modulate the exposed active sites and aggregation of hydroxide composites^[15].

  Metal- organic frameworks (MOFs) are a fascinating type of porous material and have been applicated in gas storage, separation, sensing, and catalysis for their high specific surface area, abundant active sites, tunable pore structure, controllable morphology, and multifunctionality^[16]. The poor conductivity of most MOFs during the electrochemical charge - discharge process limits their application in supercapacitors; however, MOF derivatives show a homogeneous distribution of metal atoms or oxides within the nanostructure, which could reduce the aggregation of active nanoparticles and further enhance their electrochemical performance. Derivatives such as C, Co/C, α-Fe₂O₃, CoN/N - C@SiO₂, MnO_x - CS, NiS₂/CoS₂/N - C, MnNi₂O₄ microflowers, Cu∶CoP nanosheet arrays have been fabricated from MOF precursors for energy-storage applications^[17-27].

  In this work, we successfully synthesized β - Co(OH)₂/nitrogen - doped carbon GP (Co(OH)₂/C - N@GP) using ZIF - 67 as a precursor and polystyrene (PS) as a template. In Co(OH)₂/C-N@GP nanocomposites. ZIF - 67 effectively prevents the accumulation of Co(OH)₂, PS provides a larger surface area, and GP enhances the conductivity of nanocomposites. As expected, Co(OH)₂/C-N@GP presented excellent electrochemical properties for supercapacitors and thus has potential applications in high-performance supercapacitors.

  1.   Experiment

  1.1   Materials

  All chemicals used in the syntheses were commercially available reagents of analytical grade and were used without further purification, including styrene (Maya-Reagent, 99%), acrylic acid (Aladdin reagent, 99%), ammonium persulfate (APS, Maya - Reagent, 98.5%), Co(NO₃)₂·6H₂O (Zhengzhou alpha Chemical Co., Ltd., 99%). thioacetamide (Zhengzhou alpha Chemical Co., Ltd., 99%), 2-methylimidazole (Zhengzhou alpha Chemical Co., Ltd., 99%), KOH (Aladdin reagent, 99%) and Graphene power (Morch).

  1.2   Structural characterization

  The X-ray diffraction (XRD) patterns of the samples were characterized by a Bruker D8 Advance using Mo Kα radiation (λ =0.071 073 nm, U=100 kV, I=40 mA) at a scanning rate of 5 (°) ·min^-1 and a scanning angle (2θ) of 5° - 60°. Scanning electron microscopy (SEM) micrographs were obtained using a Gemini SEM 300, and the elements in the material were identified by the attached energy- dispersive X-ray spectrography (EDS). Transmission electron microscopy (TEM) was carried out using JEM-2100 (JEOL, Japan, 200 kV). The chemical composition of each element was analyzed via X - ray photoelectron spectroscopy (XPS, LabRAM HR800, internal standard C1s peak at 284.8 eV). Raman spectra were collected on the Thermo Scientific DXR. Fourier transform infrared spectra were recorded by IR-PRESTIGE in a range of 400-4 000 cm^-1.

  1.3   Synthesis of polystyrene microspheres containing carboxyl group (PS-COOH)

  Under Ar flow (50 mL·min^-1), 6.08 g (0.058 mol) of styrene, 0.5 g (0.007 mol) of acrylic acid, and 95 mL of deionized water were mechanically stirred in a 250 mL three - necked flask to produce a milky white suspension. After 30 min of stirring, the reaction solution was heated to 70 ℃ and 10 mL of aqueous solution containing 0.25 g APS was added. Polymerization was initiated after stirring for 1 h, and the reaction continued for another 6 h. After the reaction was complete, the solution was cooled to room temperature and the white solid was collected by centrifugation at 8 000 r·min^-1 for 4 min followed three washes with deionized water. The collected solid was dried in a vacuum at 60 ℃ for 5 h for a 61% yield of the target products.

  1.4   Synthesis of ZIF-67/PS

  To prepare the PS ethanol dispersion, 1 g PS and 9 mL absolute ethanol were ultrasonically dispersed in a beaker for 10 min. The result was poured into 200 mL deionized water and ultrasonic dispersion continued for 10 min. Then, 4.8 g (20 mmol) Co(NO₃)₂·6H₂O was added to the reaction solution. After stirring for 1 h, 10 mL deionized water containing 4.25 g 2-methylimidazole was added, and the solution immediately turned purple. After stirring for 1 h, the purple solid was collected by centrifugation at 8 000 r·min^-1 for 4 min, followed by three washes with absolute ethanol. The collected solid was dried in a vacuum at 60 ℃ for 2 h to yield 3.2 g of ZIF-67/PS.

  1.5   Preparation of Co(OH)₂/C-N@GP

  Carbonization of 3 g ZIF-67/PS was performed in a tubular furnace under Ar flow at 800 ℃ with a heating rate of 5 ℃ ·min^-1 for 2 h, followed by cooling to room temperature to obtain 1.72 g black powder. Then, 0.09 g powder and 0.08 g thioacetamide were added to 15 mL of 3 mg·mL^-1 GP ethanol solution. The above solution was stirred ultrasonically and then magnetically for 5 min until homogeneity was obtained, then placed in a hydrothermal reactor at 180 ℃ for 24 h. After the reaction was completed, the solution was cooled and filtered to obtain CoSO₄/C - N@GP composites, which were converted to Co(OH)₂/C - N@GP composites by immersion in 3 mol·L^-1 KOH solution for 24 h. For comparison, Co(OH)₂/C-N without GP was also prepared.

  1.6   Electrochemical characterization

  The prepared materials, acetylene black, and PTFE solution with a mass fraction of 10% were mixed and ground according to a mass ratio of 80∶15∶5. After obtaining a uniform film thickness, the film was cut into 5 mm×5 mm sheets, placed in a pre - cut stencil, and pressed for 5 min at a pressure of 10 MPa using a press. The supercapacitor characteristics of Co(OH)₂/C-N@GP were investigated by electrochemical experiments on a Shanghai CH CHI - 66D workstation in 3 mol·L^-1 KOH electrolyte utilizing three electrode cells (as-synthesized material as the working electrode, Pt as the counter electrode, Ag/AgCl as reference electrode). The electrochemical behavior of the samples was studied via cyclic voltammetry (CV) and galvanostatic charge/discharge (GCD).

  The specific capacitance (C, F·g^-1) of each active material was calculated as

  $ C=\frac{I \Delta t}{m \Delta V} $

  (1)

  where I is the discharge current (A), Δt is the discharge time (s), m is the mass of the active electrode material (mg), and ΔV is the working potential window (V).

  2.   Result and discussion

  The XRD patterns of the as-prepared samples are presented in Fig. 1a. The ZIF - 67/PS patterns exactly match the corresponding simulated patterns of ZIF-67. The ZIF-67 diffraction peak disappeared when ZIF-67/ PS was pyrolyzed and reacted with thioacetamide. However, some new diffraction peaks appeared at 15.1°, 16.3°, 17.4°, 17.8°, 18.3°, 20.3°, 21.4°, and 22.1° and are consistent with the (110), (112), (112), (200), (202), (114), (204), and (114) crystal faces of CoSO₄ (PDF No.16-0304), respectively. This indicates that ZIF-67/ PS was completely transformed into CoSO₄/C-N. Compared with CoSO₄/C-N, CoSO₄/C-N@GP had a diffraction peak at 2θ≈26° corresponding to the (002) planes of GP. In the Co(OH)₂/C-N@GP XRD pattern, the main diffraction peaks identified at 19.1° (001), 37.9° (101), 51.4° (102), and 57.9° (110) are associated with the (100), (002), (101), (102), (110), and (103) crystal faces of β - Co(OH)₂ (PDF No. 30 - 0443), demonstrating the successful conversion of CoSO₄ to β-Co(OH)₂^[28].

  Figure 1

  

  Figure 1.  (a) XRD patterns of as-prepared samples; (b) FTIR spectra of Co(OH)₂/C-N@GP; (c) Raman spectra of Co(OH)₂/C-N@GP

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  To investigate the chemical functional groups and graphitization degree of Co(OH)₂/C - N@GP, FTIR and Raman spectra were used to characterize the samples. In the FTIR spectra (Fig. 1b), the Co(OH)₂/C - N@GP showed two peaks at ~3 434 and ~3 675 cm^-1 associated with stretching vibrations of —OH from Co(OH)₂ and free Co(OH)₂^[29]. The absorption peak at ~2 922 cm^-1 is the symmetrical vibration peak of CH₂ from GP. The absorption peak at 1 250 cm^-1 corresponds to C— O bond vibration in Co(OH)₂, whereas the absorption peak at 602 cm^-1 is caused by tensile vibration of Co— O and bending vibration Co—OH. In the Raman spectra (Fig. 1c), two obvious characteristic peaks at 1 345 and 1 590 cm^-1 correspond to the D and G bands of Co(OH)₂/C-N@GP, respectively, and the 2D band emerged at 2 705 cm^-1. The integral intensity ratio R= I_D /I_G=0.799 indicates fewer sp³ carbon defects and more sp² carbon in Co(OH)₂/C-N@GP.

  XPS was performed to determine the elemental composition and chemical valence of the Co(OH)₂/C-N@GP surface. The full scan pattern for Co(OH)₂/C-N@GP in Fig. 2a suggests that the nanocomposite material mainly comprises Co, N, C, and O, which is consistent with our experiment results. In the spectrum of Co2p (Fig. 2b), the peaks at 780.7 and 797.1 eV correspond to Co2p_1/2 and Co2p_3/2, respectively, indicate the presence of Co²⁺. The peaks at 786.5 and 804.6 eV are considered satellite peaks^[30]. As shown in Fig. 2c, the C1s spectra of Co(OH)₂/C - N@GP can be fitted into three peaks corresponding to C=C (284.5 eV), C—O (286.3 eV), and C=O (288.7 eV). In the N1s spectrum of Co(OH)₂/C-N@GP (Fig. 2d), the peaks at 398.4 and 400.9 eV correspond to pyridinic C=N—C and graphitic C—N—H, respectively^[31]. Pyridine nitrogen can significantly improve the conductivity of carbon electrode materials and provide active centers and external defects for carbon electrodes. Graphitic nitrogen is also beneficial to electron transfer. To further verify the composition of the as - synthesized samples, EDS and elemental mapping images were obtained. The elemental mapping results of the Co(OH)₂/C-N@GP in Fig. 3a-3e show that Co, C, O, and N are uniformly distributed in space.

  Figure 2

  

  Figure 2.  XPS spectra of Co(OH)₂/C-N@GP: (a) survey; (b)Co2p; (c) C1s; (d) N1s

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  Figure 3

  

  Figure 3.  Elemental mapping results of Co(OH)₂/C-N@GP

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  The morphology of the samples was investigated using SEM. Fig. 4a shows the PS-COOH morphology of uniform microspheres with an average diameter of 300-500 nm, and Fig. 4b shows the uniform distribution of conformal ZIF-67 and PS in the ZIF-67/PS composite. The conformal ZIF-67 was approximately 100 nm thick and the polyhedral particle size was roughly 2 μm. According to Fig. 4c, the basic morphology of Co/C-N was maintained with ZIF-67, but the surface was rougher and the PS microspheres disappeared. This is attributed to the carbonization of PS microspheres. In addition, 300-nm-diameter pores were formed on Co/C-N during carbonization, which increased the specific surface areas of Co/C-N. CoSO₄/C-N@GP was formed by Co/C-N reaction with thiourea and GP under hydrothermal conditions. The morphology of CoSO₄/C-N@GP is shown in Fig. 4d to comprise stacked rough, irregular polyhedra and GP. The polyhedra were approximately 2 μm, which is nearly the same as ZIF - 67. Furthermore, Co(OH)₂/C - N@GP was formed via CoSO₄/C-N@GP reaction with KOH. Fig. 4e shows that in Co (OH)2/C - N @GP, Co(OH)₂ was well dispersed on the GP, and the hydrated particle size of Co(OH)₂/C-N was ~80 nm. Fig. 4f is the TEM image of the Co(OH)₂/C-N@GP nanocomposite. A large number of Co(OH)₂ NPs with an average diameter of 10 - 20 nm adhered to C without serious aggregation.

  Figure 4

  

  Figure 4.  SEM images of (a) PS-COOH, (b) ZIF-67/PS, (c) Co@C-N, (d) CoSO₄/C-N@GP, (e) Co(OH)₂/C-N@GP; (f)TEM image of Co(OH)₂/C-N@GP

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  To study the energy-storage characteristics of the as-obtained samples, their CV and GCD were tested in a three-electrode system with 3 mol·L^-1 KOH electrolyte. Fig. 5a shows the CV curves of Co(OH)₂/C-N@GP with a potential window of -1.1-0.5 V at different scan rates. The obvious redox peaks in CV curves indicate that the capacitance arises from the redox reaction, and the material exhibits typical Faraday charge - transfer behavior. With increasing scanning rate, the CV curves′ areas increased while maintaining their shape, thus indicating the good rate performance. As the scan rate increased, the oxidation peaks shifted to more positive values while the reduction peaks shifted to more negative values; this helps increase the internal diffusion resistance of the electrode material. Fig. 5b compares the CV curves of Co(OH)₂/C- N and Co(OH)₂/CN@GP at 5 mV·s^-1 with a potential window of -1.1-0.5 V. The integral area of the CV curve of Co(OH)₂/C- N was related to the GP content. This may be caused by GP increasing the conductivity of Co(OH)₂/C-N in the electrochemical reaction process, eventually resulting in a higher specific capacity^[31]. It is noteworthy that an obvious oxidation peak appeared at 0.2 V for Co(OH)₂/ C-N, attributed to the Co(OH)₂ oxidation peak^[32].

  Figure 5

  

  Figure 5.  (a) CV curves of Co(OH)₂/C-N@GP at different scan rates; (b) CV curves of Co(OH)₂/C-N and Co(OH)₂/C-N@GP at scan rate of 5 mV·s^-1; (c) GCD curves of Co(OH)₂/C-N@GP at different current densities; (d) GCD curves of Co(OH)₂/C-N and Co(OH)₂/C-N@GP at 2 A·g^-1; (e) Capacitance performances of the Co(OH)₂/C-N and Co(OH)₂/C-N@GP at different current densities; (f) Cycling performance of Co(OH)₂/C-N@GP at 200 mV·s^-1

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  The GCD curves (Fig. 5c) of Co(OH)₂/C - N@GP measured at different current densities showed a distinct potential plateau region for all electrodes, also confirming the Faradic reaction mechanism. Consistent with the CV results, Co(OH)₂/C-N@GP showed a much longer discharge time than the other electrodes, indicating its excellent specific capacitance (Fig. 5d). The specific capacitances of Co(OH)₂/C-N@GP were calculated as 985.4, 606.1, 547.8, 448.5, and 415.8 F·g^-1 at 2, 4, 5, 8, and 10 A·g^-1, respectively, and those of Co(OH)₂/C - N were 277.4, 174.3, 153.4, 121.6 and 110.1 F·g^-1. The results for all electrodes are shown in Fig. 5e, where the specific capacitance was found to decrease gradually with increasing current density, which can be attributed to an increase in the potential drop of the electrode materials at higher current densities.

  Long-term cycling stability is also a critical factor for energy - storage materials. Fig. 5f shows the cyclic stability test curve of Co(OH)₂/C-N@GP at a scan rate of 200 mV·s^-1. After 400 cycles, the specific capacitance obviously decreased. This phenomenon can be explained by the poor cyclic stability of the nitrogen and oxygen functional groups, which leads to decreasing pseudocapacitance and deterioration of the cyclic stability. After 1 000 cycles, the capacitance retention of Co(OH)₂/C - N@GP was 76.6%, which demonstrates ordinary cyclic stability.

  Considering its good electrochemical performance, the Co(OH)₂/C-N@GP was chosen as the electrode to assemble the symmetric supercapacitor in a 3 mol·L^-1 KOH electrolyte. The mass ratio of the positive and negative electrode materials was 1∶1, and the total mass is 2.2 mg. Fig. 6a displays the CV curves of SC at 10 mV·s^-1, which shows a non-overlapping curves and potential windows, suggesting the good match with symmetric supercapacitor. The GCD curves also demonstrate the same capacitive behaviors, but the calculated specific capacitance achieved only 7.3 F·g^-1 at a current rate of 0.5 A·g^-1 (Fig. 6b). The electrical conductivity and reaction kinetics of Co(OH)₂/C-N@GP was further explored by EIS at an AC voltage of 0.5 mV between 10 mHz and 100 kHz. As displayed in Fig. 6c, the straight lines of Co(OH)₂/C-N@GP was steep in the low frequency region indicating a low intrinsic resistance, and the small diameter of the semicircle indicates a fast charge transfer kinetics.

  Figure 6

  

  Figure 6.  (a) CV curves of Co(OH)₂/C-N@GP symmetric supercapacitor at different scan rates; (b) GCD curves at various current densities; (c) EIS plot at an AC voltage of 0.5 mV between 10 mHz and 100 kHz

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  3.   Conclusions

  In summary, a novel Co(OH)₂/C-N@GP nanocomposite material was prepared using a MOF precursor and PS template. This synthesis strategy is proven an effective method to convert ZIF-67 into Co(OH)₂/C-N. Co(OH)₂/C - N@GP contains 80 nm Co(OH)₂/C - N well dispersed on GP and Co(OH)₂ with an average diameter of 10-20 nm adhered to C. The nanostructure of Co(OH)₂ is beneficial to the electrochemical storage of the supercapacitor. Electrochemical analysis indicates that Co(OH)₂/C-N@GP exhibit typical Faraday chargetransfer behavior, and the specific capacitance of Co(OH)₂/C-N@GP depends on the GP content. Co(OH)₂/C - N@GP exhibited a high specific capacitance of 985.4 F·g^-1 at 2 A·g^-1 in 3 mol·L^-1 KOH.

  
  Acknowledgments: This work was financially supported by the Scientific Research Project of Education Department of Hubei Province (Grant No.D20181507) and the Science Foundation of Wuhan Institute of Technology (Grants No. K201509,K201805).
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  Figure 1  (a) XRD patterns of as-prepared samples; (b) FTIR spectra of Co(OH)₂/C-N@GP; (c) Raman spectra of Co(OH)₂/C-N@GP

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  Figure 2  XPS spectra of Co(OH)₂/C-N@GP: (a) survey; (b)Co2p; (c) C1s; (d) N1s

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  Figure 3  Elemental mapping results of Co(OH)₂/C-N@GP

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  Figure 4  SEM images of (a) PS-COOH, (b) ZIF-67/PS, (c) Co@C-N, (d) CoSO₄/C-N@GP, (e) Co(OH)₂/C-N@GP; (f)TEM image of Co(OH)₂/C-N@GP

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  Figure 5  (a) CV curves of Co(OH)₂/C-N@GP at different scan rates; (b) CV curves of Co(OH)₂/C-N and Co(OH)₂/C-N@GP at scan rate of 5 mV·s^-1; (c) GCD curves of Co(OH)₂/C-N@GP at different current densities; (d) GCD curves of Co(OH)₂/C-N and Co(OH)₂/C-N@GP at 2 A·g^-1; (e) Capacitance performances of the Co(OH)₂/C-N and Co(OH)₂/C-N@GP at different current densities; (f) Cycling performance of Co(OH)₂/C-N@GP at 200 mV·s^-1

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  Figure 6  (a) CV curves of Co(OH)₂/C-N@GP symmetric supercapacitor at different scan rates; (b) GCD curves at various current densities; (c) EIS plot at an AC voltage of 0.5 mV between 10 mHz and 100 kHz

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