English

• The development and utilization of clean energy have become a mainstream research topic in the world.However, environmentally friendly energy sources such as wind, solar, geothermal, tidal, and biomass energy are intermittent and unstable. Therefore, it is particularly important to develop efficient devices for storing and managing these unstable energies. Supercapacitors (SCs) have attracted the interest of scientists due to their excellent properties such as fast charge-discharge capability, ultra-high power density, excellent Coulombic efficiency, and long service life^[1-4].

  In recent years, metal-organic frameworks (MOFs) have been extensively developed as electrode materials for SCs^[5-8]. MOFs are composed of organic linkers and metal ions by strong chemical bonds^[9]. Due to the large specific surface area, special pore structure, and rich active sites of MOFs, they have aroused more and more concentration among plentiful researchers^[10]. Nevertheless, from an overall perspective, low electrical conductivity and hindered ion entrance usually limit the practical applications of many of the MOFs for supercapacitor applications^[11-12]. Fortunately, by using appropriate linker and metal cations and tuning the resultant structures, it is possible to facilitate the ion insertion/extraction inside the MOFs likewise enhancing the transport pathways for electrons^[13].

  In this work, we report a facile one-pot solvothermal synthesis of mixed metal MOF (MOF(Ni, Co)) and its application as a supercapacitor electrode. The effect of the atomic ratio of Ni to Co in MOFs on the electrochemical properties was investigated, indicating that the MOF(Ni, Co) nano - flower exhibited a maximum specific capacitance of 850 F·g^-1 at 1 A·g^-1 with an atomic ratio of Ni to Co in MOF(Ni, Co) of 3∶2. The experiment results reveal that the MOF(Ni_1.2Co_0.8) supercapacitor preserves its excellent specific capacitance in alkaline aqueous electrolytes compared to the previously reported works in this area, in addition to exceptional stability. Thus, this work demonstrates that the MOF(Ni_1.2Co_0.8) electrode could find potential applications for supercapacitive purposes.

  1.   Experimental

  1.1   Material and characterizations

  All chemical reagents were of analytical grade and utilized as received without further purification. All reagents were bought from Xilong Chemical Reagent Co., Ltd. (China).

  The powder X - ray diffraction (XRD) was performed on a Rigaku D/MAX - RB (Japan) using Cu Kα radiation (λ =0.154 06 nm) at 40 kV and 40 mA, and the XRD patterns were recorded in a 2θ range from 5° to 60°. FT - IR was measured on a NICOLET iS 10IR (USA) Fourier transform infrared spectrometer. The morphologies of MOF(Ni, Co) were characterized on a Hitachi SU8010 field-emission scanning electron microscopy (SEM, Japan) and the operating voltage was 5 kV. X-ray photoelectron spectroscopy (XPS) was performed by ESCALAB 250Xi (USA) for further elemental analysis, and Al Kα radiation was used as the excitation source. Belsorp-MAX (USA) was used to conduct the nitrogen adsorption-desorption isotherms.

  1.2   Preparation of MOF(Ni, Co) nanomaterials Briefly, the synthesis method was as follows.

  Solution Ⅰ : Ni(NO₃)₂·6H₂O and Co(NO ₃)₂·6H₂O were mixed thoroughly in 40 mL of ethanol and deionized water (50∶50, V/V) until they were fully dissolved. Solution Ⅱ: succinic acid (0.35 g, 3 mmol) and KOH (0.22 g, 4 mmol) were mixed thoroughly in 20 mL of ethanol and deionized water (50:50, V/V) until it was fully dissolved. Solution Ⅱ was added dropwise into solution Ⅰ to get a mixture, and the mixture was then transferred to a 100 mL Teflon - lined stainless steel autoclave and heated under 120 ℃ for 12 h. After cooling, the products were washed with ethanol several times via centrifugation and dried at 80 ℃ for 12 h in a vacuum oven. The mole ratios of Ni(NO₃)₂·6H₂O to Co(NO₃)₂ ·6H₂O were 1.2∶0.8, 0.8∶1.2, and the MOF(Ni, Co)were named MOF(Ni_1.2Co_0.8) and MOF(Ni_0.8Co_1.2), respectively. Similarly, the pristine MOF(Ni) precursor solution was obtained using the same method without adding Co(NO₃)₂·6H₂O, and the pristine MOF(Ni) was prepared.

  1.3   Electrochemical measurements

  The working electrode was fabricated through a slurry - forging procedure by smearing the sample on nickel foam (NF, 1 cm×1 cm). Specifically, active material (MOF(Ni, Co)), polyvinylidene difluoride (PVDF), and acetylene black were ground together in a mass proportion of 8∶1∶1, and then ethanol was dropped in the powder to form slurry conditions. Next, the slurry was equably plastered on NF and dried at 60 ℃ for 12 h. The dried NF was stress-treated (10 MPa, about 30 s) before use for electrochemical experiments.

  The electrochemical characterization of MOF (Ni, Co) was performed in a three-electrode system with 3 mol·L^-1 KOH as the electrolyte. A saturated calomel electrode (SCE) and platinum plate were used as the reference and the counter electrodes, respectively. The electrochemical tests were performed at room temperature. The cyclic voltammetry (CV), galvanostatic chargedischarge (GCD), and electrochemical impedance spectroscopy (EIS) measurements were performed on a CS 2350H electrochemical workstation (CorrTest, Wuhan).

  2.   Results and discussion

  2.1   Structure and morphology analysis

  The XRD patterns of MOF(Ni), MOF(Ni_1.2Co_0.8), MOF(Ni_0.8Co_1.2), and standard cards are shown in Fig. 1a. The diffraction peaks centered at 8.4°, 10.2°, 11. 6°, 14.3°, 20.9°, 23.1°, 27.5°, 34.9°, and 37.8° can be ascribed to the (210), (213), (006), (107), (426), (513), (40 10), (6111) and (838) diffraction planes of [Ni₇(C₄H₄O₄)₆(OH)₂(H₂O)₂]·2H₂O (CCDC No. 169140), respectively^[14]. The sharp peaks in the patterns indicate that the composites are well crystallized. The XRD peaks of MOF(Ni, Co) did not change significantly, indicating that the implantation of Co metal ions do not affect the crystal phase of the framework. Fig. 1b show FT - IR spectra of MOF(Ni), MOF(Ni_1.2Co_0.8), and MOF (Ni_0.8Co_1.2). The peaks at 1 632 and 1 556 cm^-1 are attributed to the asymmetric stretching vibration of —COO— coordinated to metal ion by a bidentate mode, while the peaks at 1 440 and 1 412 cm^-1 are due to its symmetric stretching vibration^[15]. The peak at 1 250^-1 140 cm^-1 can be attributed to C—C stretching vibration (skeleton vibration), and the peak at 3 500 3 200 cm^-1 corresponds to hydrogen bond in H₂O molecules. Overall, the successful synthesis of MOF(Ni) and MOF(Ni, Co) by the hydrothermal method is confirmed.

  图 1

  

  图 1.  (a) XRD patterns and (b) FT-IR spectra of MOF(Ni), MOF(Ni_1.2Co_0.8), and MOF(Ni_0.8Co_1.2)

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  The coordination model of metal ions and the 3D stacking diagram for MOF(Ni) are illustrated in Fig. 2. As shown in Fig. 2a, each Ni atom is coordinated to six oxygen atoms from carboxylate groups of the ligands. As shown in Fig. 2b, the honeycomb structure with tunnel holes is beneficial to the transport of ions and thus greatly improves the electric storage capacity.

  图 2

  

  图 2.  (a) Crystal structure of MOF(Ni) and (b) view of MOF(Ni) structure viewed along the c-axis (based on CCDC data)

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  The morphology of MOF(Ni), MOF(Ni_1.2Co_0.8), and MOF(Ni_0.8Co_1.2) nanomaterials was subsequently captured (Fig. 3a-3c). In MOF(Ni), a large number of irregular block solids was observed (Fig. 3a). As shown in Fig. 3b, adding an appropriate amount of Co²⁺ ions to the initial solution can get large - scale nanosheets to form flower - shaped microspheres, and there are large gaps between the nanosheet. As shown in Fig. 3c, when the Co²⁺ content in the reaction system was further increased, the morphology was still large-scale nanosheet, but the flower -shaped microspheres formed by the accumulation were more compact. Therefore, the addition of Co²⁺ in the initial solution can significantly affect the morphology of MOFs.

  图 3

  

  图 3.  SEM images of (a) MOF(Ni), (b) MOF(Ni_1.2Co_0.8), and (c) MOF(Ni_0.8Co_1.2)

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  The chemical bonding state of MOF(Ni_1.2 Co_0.8) was characterized using XPS, and the results are shown in Fig. 4. The survey scan spectrum of C1 s, O1s, Ni2p, and Co2p can be observed (Fig. 4a). The carbon environment had three different binding sites C—C, C—O— C, and O—C=O with corresponding binding energies at 284.5, 285.2, and 288.7 eV, respectively (Fig. 4b)^[16]. Fig. 4c displays the O1s region, which shows three peaks at 531.1, 531.8, and 532.6 eV due to metal—O, —OH, and —C—O^[17], respectively. The peaks in the high - resolution Ni2p XPS spectrum of MOF(Ni_1.2Co_0.8) at 856.2 and 873.8 eV (Fig. 4d) can be ascribed to Ni2p_3/2 and Ni2p_1/2, indicating that Ni mainly exists as Ni²⁺ in MOF(Ni_1.2Co_0.8). Additionally, the two broad peaks at 861.4 and 879.6 eV are identified as shake-up satellites “(Sat.”) of Ni2p_3/2 and Ni2 p_1/2, respectively ^[18]. Similarly, Fig. 4e shows that the peaks located at around 781.5 and 797.4 eV are indexed to Co2p_3/2 and Co2p_1/2, and the other two peaks centered at 786.0 and 802.5 eV correspond to shake - up satellites, which are characteristic bands of Co^{2+ [19]}. The atomic fraction (%) of the elements in MOF(Ni, Co) from XPS analysis is shown in Table 1. It can be seen that the actual atomic ratio of Ni to Co in MOF(Ni_1.2Co_0.8) was 1.86∶1. Therefore, XPS results verify the formation of the nickel and cobalt succinate phases.

  图 4

  

  图 4.  XPS spectra of MOF(Ni_1.2Co_0.8): (a) survey scan, (b) C1s, (c) O1s, (d) Ni2p, and (e) Co2p

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  表 1

  

  表 1  Atomic fraction of MOF(Ni, Co) from XPS analysis  %

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  Sample	C	O	Ni	Co
  MOF(Ni1.2Co0.8)	58.15	33.55	5.40	2.90
  MOF(Ni0.8Co1.2)	46.58	41.15	6.32	5.95

  The nitrogen adsorption - desorption isotherms of MOF(Ni_1.2Co_0.8) determined at 77 K and the corresponding Barret-Joyner-Halenda (BJH) pore size distribution analysis are shown in Fig. 5. The specific surface area, pore size, and pore volume were calculated to be 159 m²·g^-1, 8.72 nm, and 0.258 cm³·g^-1, respectively. Fig. 5a shows that MOF(Ni_1.2Co_0.8) displayed Ⅳ - type isotherms with an H3 hysteresis loop, which is typical behavior of mesoporous structure^[20]. From the pore size distribution in Fig. 5b, it can be seen that the pore size of the sample was in a range of 3 - 6 nm, which shows that it has a uniform mesoporosity, providing a channel for ion transmission.

  图 5

  

  图 5.  (a) Nitrogen adsorption-desorption isotherms and (b) pore size distribution of MOF(Ni_1.2Co_0.8)

  Specific surface area (S_BET) was calculated by the Brunauer-Emmett-Teller (BET) method

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  2.2   Electrochemical performance

  To study the electrochemical property, CV behaviors of as - prepared MOF electrodes were investigated at different scan rates (5, 10, 20, 50, 100 mV·s^-1) in 3 mol·L^-1 KOH electrolytes using a three-electrodes test system. Fig. 6a presents the CV curves of the MOF (Ni_1.2Co_0.8) electrode at different scan rates in a potential range of -0.15-0.55 V (vs SCE). Meanwhile, when the scan rate increased, the cathodic and anodic peaks slightly shifted toward positive and negative voltages, respectively, indicating a very low overpotential that is because of the very low charge transfer resistance. This effect is explained by electric polarization and the resistance of the electrodes, which cause irreversible reactions at very high scan rates^[21]. So, the CV curve lost its regular shape at a higher scan rate. As shown in Fig. 6b, under the condition of a scanning rate of 5 mV· s^-1, it can be seen that the three electrode materials had a pair of prominent redox peaks, and the curve symmetry was good, indicating that they have ideal pseudo-capacitance electricity. The MOF(Ni_1.2Co_0.8) electrode showed larger CV areas than other electrodes, indicating that the synergistic effect of Ni and Co ions leads to a higher specific capacitance. Fig. 6c shows the GCD curves of the MOF electrode within a potential window of - 0.15-0.39 V (vs SCE) at different current densities. The corresponding specific capacitance of the MOF (Ni_1.2Co_0.8) electrode at 1 A·g^-1 was calculated to be 850 F·g^-1. The plot of specific capacitance vs current density for the three kinds of electrode materials is shown in Fig. 6d. When the current density was increased by 10 times, the specific capacitances of the three electrode materials were preserved by 36.3%, 81. 2%, and 70.5%, respectively. MOF(Ni_1.2Co_0.8) exhibited excellent specific capacitance. Moreover, compared with the previous reports of MOF-based supercapacitor electrode materials (Table 2), MOF(Ni_1.2Co_0.8) also exhibited excellent capacitive performance, which is attributed to ultrathin nanosheet structure. The ultrathin MOF(Ni_1.2 Co_0.8) nanosheets provide abundant electroactive sites for the Faradaic redox reaction and a short pathway for electron transfer and electrolyte ions diffusion.

  图 6

  

  图 6.  Electrochemical performance of MOF(Ni_1.2Co_0.8): (a) CV curves at different scan rates; (b) CV curves at 5 mV·s^-1;

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  表 2

  

  表 2  Specific capacitance of MOF-based electrode materials

  下载: 导出CSV

  Material	Specific capacitance / (F·g-1)	Current density / (A·g-1)	Electrolyte	Ref.
  MOF(V, Ni)	178	1	3 mol·L-1 KOH	[8]
  Pillar Ni-MOF	552	1	2 mol·L-1 KOH	[22]
  Co-MOF	952.5	0.25	3 mol·L-1 KOH	[23]
  Co/Fe-MOF	319.5	1	1 mol·L-1 LiOH	[24]
  Ni/Co-MOF-rGO	860	1	6 mol·L-1KOH	[25]
  Dandelion-like Ni/Co-MOF	758	1	2 mol·L-1KOH	[26]
  MOF(Ni1.2Co0.8)	850	1	3 mol·L-1KOH	This work

  The electroconductivity of different samples was also investigated by EIS. As shown in Fig. 7, the radius of the circle in the high - frequency region for MOF (Ni_1.2Co_0.8) and MOF(Ni_0.8Co_1.2) was smaller than that of MOF(Ni), which means that they have a smaller charge transfer impedance. The inset is the corresponding equivalent circuit diagram of the electrode material of MOF(Ni_1.2Co_0.8), and its equivalent series resistance was about 0.7 Ω. At the same time, it also shows that the MOF(Ni_1.2Co_0.8) electrode material is a host material for high electrolyte access, penetration, and ion diffusion, which is conducive to the rapid storage and release of energy and has good electrical conductivity.

  图 7

  

  图 7.  EIS plots of MOF(Ni), MOF(Ni_1.2Co_0.8), and MOF(Ni_0.8Co_1.2) in 3 mol·L^-1 KOH electrolyte

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  The cycling stability of the MOF(Ni), MOF (Ni_1.2Co_0.8), and MOF(Ni_0.8Co_1.2) electrode was tested by GCD at 4 A·g^-1. As shown in Fig. 8a, compared with the change of specific capacitance of MOF(Ni) and MOF(Ni_0.8Co_1.2), the MOF(Ni_1.2Co_0.8) electrode kept stable during 1 000 cycles of charging and discharging, but the specific capacitance decreased to 84.6% of the initial value. CV curves of MOF(Ni_1.2Co_0.8) at the start, after 500 cycles and after 1 000 cycles are shown in Fig. 8b. It can be seen that after 500 and 1 000 cycles, the oxidation peak and reduction peak of the electrode did not change significantly, indicating that the structure of MOF(Ni_1.2Co_0.8) does not change significantly after the cycle. It can be seen that the addition of an appropriate amount of Co²⁺ also greatly improves the cycle performance of MOF materials as supercapacitors.

  图 8

  

  图 8.  (a) Cycling stability of MOF(Ni), MOF(Ni_1.2Co_0.8), and MOF(Ni_0.8Co_1.2) at 4 A·g^-1 and (b) CV curves of MOF(Ni_1.2Co_0.8) at the start, after 500 and 1 000 cycles

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  3.   Conclusions

  In summary, a mixed metal - organic framework, that is MOF(Ni, Co), was synthesized through a simple one - pot solvothermal method. The detailed characterizations through XRD, FT-IR, SEM, XPS, and N₂ adsorption-desorptiopn were then performed. The potential application of nano-flower-like MOF (Ni_1.2 Co_0.8) for supercapacitor electrode material was also explored. Remarkably, the electrochemical results show that MOF(Ni_1.2Co_0.8) nano - flower presents a good supercapacitor electrode material, in which a high specific capacitance of 850 F·g^-1 at 1 A·g^-1 as well as good cycling stability and rate capability were achieved. These results highlight the promising applications of MOF(Ni_1.2Co_0.8) nano-flower as highperformance supercapacitor electrode materials.

  
  
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• 

  图 1  (a) XRD patterns and (b) FT-IR spectra of MOF(Ni), MOF(Ni_1.2Co_0.8), and MOF(Ni_0.8Co_1.2)

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  图 2  (a) Crystal structure of MOF(Ni) and (b) view of MOF(Ni) structure viewed along the c-axis (based on CCDC data)

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  图 3  SEM images of (a) MOF(Ni), (b) MOF(Ni_1.2Co_0.8), and (c) MOF(Ni_0.8Co_1.2)

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  图 4  XPS spectra of MOF(Ni_1.2Co_0.8): (a) survey scan, (b) C1s, (c) O1s, (d) Ni2p, and (e) Co2p

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  图 5  (a) Nitrogen adsorption-desorption isotherms and (b) pore size distribution of MOF(Ni_1.2Co_0.8)

  Specific surface area (S_BET) was calculated by the Brunauer-Emmett-Teller (BET) method

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  图 6  Electrochemical performance of MOF(Ni_1.2Co_0.8): (a) CV curves at different scan rates; (b) CV curves at 5 mV·s^-1;

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  图 7  EIS plots of MOF(Ni), MOF(Ni_1.2Co_0.8), and MOF(Ni_0.8Co_1.2) in 3 mol·L^-1 KOH electrolyte

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  图 8  (a) Cycling stability of MOF(Ni), MOF(Ni_1.2Co_0.8), and MOF(Ni_0.8Co_1.2) at 4 A·g^-1 and (b) CV curves of MOF(Ni_1.2Co_0.8) at the start, after 500 and 1 000 cycles

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  表 1  Atomic fraction of MOF(Ni, Co) from XPS analysis  %

  Sample	C	O	Ni	Co
  MOF(Ni1.2Co0.8)	58.15	33.55	5.40	2.90
  MOF(Ni0.8Co1.2)	46.58	41.15	6.32	5.95

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  表 2  Specific capacitance of MOF-based electrode materials

  Material	Specific capacitance / (F·g-1)	Current density / (A·g-1)	Electrolyte	Ref.
  MOF(V, Ni)	178	1	3 mol·L-1 KOH	[8]
  Pillar Ni-MOF	552	1	2 mol·L-1 KOH	[22]
  Co-MOF	952.5	0.25	3 mol·L-1 KOH	[23]
  Co/Fe-MOF	319.5	1	1 mol·L-1 LiOH	[24]
  Ni/Co-MOF-rGO	860	1	6 mol·L-1KOH	[25]
  Dandelion-like Ni/Co-MOF	758	1	2 mol·L-1KOH	[26]
  MOF(Ni1.2Co0.8)	850	1	3 mol·L-1KOH	This work

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