English

• 0.   Introduction

  The use of traditional fossil fuels can lead to environmental pollution, which is detrimental to the sustainable development of society. In response to thisissue, renewable energy such as wind and solar energy is receiving increasing attention. However, due to the intermittent nature of wind and solar energy, efficient energy storage devices must be equipped to utilize them and ensure a continuous power supply^[1-2]. Batteries have the advantages of stable voltage supply and high energy density, but they face shortcomings in long-term cycling performance and low power density. Compared with batteries, supercapacitors have high power density and excellent cycling performance, but their energy density is lower^[3-4]. Improving the energy density of supercapacitors is beneficial for expanding their application range. Therefore, in recent years, developing supercapacitors that have high energy density, long cycle life, and high power density has been a goal pursued by people^[5-8]. The performance of supercapacitors is closely related to the properties of the electrodematerials that make them up. Coordination polymers (CPs) and metal-organic frameworks (MOFs, which are also called porous CPs) directly as electrode materials for supercapacitors have received sizeable attention, owing to their porosity and redox-active centers (metal ions/organic ligands)^[9-21]. Among 1D, 2D, and 3D CPs, 2D and 3D CPs, especially transition metals (Ni, Co, Cu, Fe, etc.) CPs, demonstrate the application potential as the electrode materials for supercapacitors^[22-39]. Wei′s group and ours have successively confirmed that many transition metal CPs and MOFs can be used as electrode materials for supercapacitors, for instance, [Ni₃(OH)₂(C₈H₄O₄)₂(H₂O)₄]·2H₂O, Co₂(OH)₂C₈H₄O₄, [Cu(hmt)(tfbdc)(H₂O)] (Cu-LCP; hmt=hexamethylenetetramine; tfbdc=tetrafluoroterephthalate), and {[Co(hmt) (tfbdc)(H₂O)₂]·(H₂O)₂}_n (Co-MOF), which delivered high specific capacitances of 1 127, 2 564, 1 274, and 2 474 F·g^-1 at 1 A·g^-1 respectively^[11-14]. In 2023, EZIF-L (etching zeolitic-imidazolate framework-ligand) was prepared by Pang et al. via using tannic acid to etch the ZIF-L (etching zeolitic-imidazolate framework- ligand) method, which presented long-term cycling stability in KOH solution with the Coulombic efficiency of 95.2% after 4 500 cycles with 305.6 F·g^-1 discharge capacity^[40]. Very recently, Tai et al. reported a 3D Ni-MOF {Ni(HBTC)(BPE)}_n (Ni-BPE; H₃BTC=1, 3, 5-benzenetricarboxylic acid; BPE=1, 2-di(4-pyridyl) ethylene), which delivered the discharge capacity of 633.2 F·g^-1 at 1 A·g^-1 and the capacitance retention of 71.0% after 5 000 cycles^[41]. Conductive CPs/MOFs as the electrode materials help overcome the disadvantage of low conductivity for most MOFs. In 2017, Dincǎ′s group found that a 2D conductive MOF (Ni₃(HITP)₂, HITP=2, 3, 6, 7, 10, 11-hexaiminotriphenylene) used in supercapacitors can exhibit a superior areal capacitance in TEABF₄/ACN (TEABF₄=tetraethylammonium tetrafluoroborate; ACN=acetonitrile) electrolyte^[19]. Chen′s group, in 2020, demonstrated that a 3D conductive Cu-based MOF, containing catecholate that served as an electrode material for supercapacitors, delivered a higher capacitance of 479 F·g^-1 at 0.2 A·g^{-1 [42]}. Gao et al. in 2021 verified that a nanoscale MOF (Co_0.24Ni_0.76-bpa-200, bpa=1, 2-di(pyridin-4-yl)-ethyne) with semiconductor behavior can present a high capacitance and cycle performance (1 927.14 F·g^-1 at 1 A·g^-1 and the capacitance retention of 86.5% over 10 000 cycles)^[43]. 2D conductive MOF (Ni₂[CuPcS₈]) (Pc=phthalocyanine) was reported by Feng et al. in 2023, presenting good cycling stability (capacitance retention of 93.5% over 10 000 cycles) and outstanding capacitance of 312 F·g^-1 in 1 mol·L^-1 TEABF₄/acetonitrile^[44]. Compared with the progress of 2D and 3D CPs used as electrode materials^[45-56], there are not many reports on 1D CPs as electrode materials for supercapacitors^[53]. Besides, the development of low-cost and high-performance CP or MOF-based electrode materials is of great significance for the practical application of such materials, owing to the cost of many organic ligands utilized being higher.

  On the other hand, in recent years, organic compounds with redox groups as electrode materialsapplied in energy storage systems have also received considerable attention^[57-59]. It is worth noting that N, N′-bis(glycinyl)pyromellitic diimide (H₂BGPD) has redox groups (C=O) and low-cost, abundant sources as well as environmental friendliness^[60-61]. Constructing CPs with more redox groups through the interaction of H₂BGPD and transition metal ions is an attractive approach to obtaining high-performance electrode materials for supercapacitors. According to this idea, herein, a new 1D cobalt-based CP ([Co(BGPD)(DMSO)₂(H₂O)₂], named Co-BD, DMSO=dimethyl sulfoxide) was synthesized through the interaction of H₂BGPD and Co(NO₃)₂·6H₂O in the presence of H₂L¹ (H₂L¹=1, 1′-methylenebis-(5-methyl-pyrazole-4-carboxylic acid) and its isomers (Fig. 1). The crystal structure of Co-BD was determined and the electrochemical performance of Co-BD as an electrode material for supercapacitors was systematically investigated. The experimental data revealed that in a three-electrode system, the specific capacitance of the Co-BD electrode could reach 838 and 212 F·g^-1 at 1 and 20 A·g^-1, respectively, showing a superior rate performance. Notably, Co‑BD||rGO(reduced graphene oxide), representing an asymmetrical supercapacitor built by the positive electrode (the Co-BD electrode) and the negative electrode (the rGO electrode), owned a higher energy density of 14.2 Wh·kg^-1 at 0.80 kW·kg^-1, and an excellent cycle performance (after 4 000 cycles at 1 A·g^-1, the capacitance retention was up to 94%). Our research provides a new example of the application of 1D CPs as electrodematerials in supercapacitors.

  Figure 1

  

  Figure 1.  Structural formula of H₂BGPD

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  1.   Experimental

  1.1   Materials

  Scheme S1 (Supporting information) exhibits the synthetic route of H₂BGPD. H₂BGPD was synthesized according to the method reported^[62]. Scheme S2 presents the synthetic route of H₂L¹, H₂L², and H₂L^3[60], and they are isomers, and represent 1, 1′-methylenebis (5-methyl-pyrazole-4-carboxylic acid), 1, 2′-methylenebis (3-methyl-pyrazole-4-carboxylic acid), and 1, 1′-methylenebis(3-methyl-pyrazole-4-carboxylic acid), respectively. In the synthesis process of Co-BD, the mixture of three isomers was not separated and was directly used. rGO was purchased from Nanjing XFNANO. Co(NO₃)₂·6H₂O and dimethyl sulfoxide were purchased from Aladdin Reagent Company. Other chemicals were of analytically pure grade and used without furtherpurification.

  1.2   Synthesis of Co-BD

  A mixture of H₂L¹, H₂L², and H₂L³ (0.026 4 g, 0.1 mmol) and H₂BGPD (0.033 2 g, 0.1 mmol) was first dissolved in 4 mL DMSO. Then, Co(NO₃)₂·6H₂O (0.029 1 g, 0.1 mmol) was added and stirred. Light red crystals of Co-BD were collected after several days of solvent evaporation. Yield based on Co: ca. 60%. IR (KBr, cm^-1): 3 437 s, 1 783 w, 1 711 s, 1 626 w, 1 410 m, 1 374 m, 1 313 w, 1 229 w, 1 127 w, 953 w, 736 w, 616 w.

  1.3   X-ray crystallography

  Diffraction data of the Co-BD single crystal were collected in the range of 2.721°≤θ≤27.523°, using a Bruker Smart Apex CCD diffractometer with Mo Kα radiation (λ=0.071 073 nm). The structure of Co-BD was ascertained via the direct method of the SHELXTL-97 program^[63]. All non-hydrogen atoms were refined utilizing anisotropy parameters. Using an isotropic parameter, the hydrogen atoms connected to carbon atoms were placed at the geometric positions and refined. Table S1 exhibits the crystallographic data of Co-BD.

  1.4   Material characterization

  A Nicolet iS50 spectrometer was used to record Fourier transform infrared (FTIR) spectra of the samples. A D8 Advance X‑ray diffractometer (XRD, Bruker) with Cu Kα radiation (λ=0.154 06 nm) was utilized to test the powder X-ray diffraction patterns, the tube current was 300 mA, the tube voltage was 60 kV, and the scanning range was 2θ=5°-70°. Under the N₂ atmosphere and at a heating rate of 5 ℃·min^-1, thermogravimetric analysis (TGA) was carried out on a 209 F3 thermogravimetric analyzer (Netsch) in the temperature range of ambient temperature to 850 ℃. The morphology and microstructure of as-prepared Co-BD were measured on a Hitachi S-4800 field emission scanning electron microscope (FESEM, acceleration voltage: 20 kV).

  1.5   Electrochemical measurements.

  The working electrode was prepared based on the reported procedure^[54]. Active material Co-BD, acetylene black, and polytetrafluoroethylene (PTFE) in a mass ratio of 75∶15∶10 were mixed, and a certain amount of anhydrous ethanol was added and stirred. The obtained slurry was uniformly painted on the surface of the nickel foam (1 cm²), and subsequently, it was pressed under a 10 MPa pressure using a powder tablet press. In a vacuum, the resultant electrode was dried for 8 h at 60 ℃. On the nickel foam, a Co-BD of about 3 mg was loaded.

  In a three-electrode system, the Co-BD electrode, platinum sheet, and saturated calomel electrode (SCE) served as the working, counter, and reference electrodes, respectively, and 1 mol·L^-1 KOH was used as the electrolyte. Under room temperature, a CHI 660D workstation was used to test the cyclic voltammetry (CV) plots, galvanostatic charge/discharge (GCD) plots, and electrochemical impedance spectra (EIS) for the Co-BD electrode. Within the potential range of 0.0-0.6 Ⅴ (vs SCE), the CV plots of the Co-BD electrodes were obtained at a scan rate of 2, 5, 10, 20, and 40 mV·s^-1, respectively. Within the potential range of 0-0.5 Ⅴ (vs SCE) and under the current density of 1-20 A·g^-1, the GCD curves of the Co-BD electrode were tested, and the cycling performance of the electrode at 2 A·g^-1 was evaluated (1 000 cycles). Under an open circuit potential, utilizing a 5 mV alternating current amplitude, the measurement of EIS was conducted within the range of 10 mHz to 100 kHz. The calculations of the specific capacitance (C_sp, F·g^-1) and the specific capacity (C_s, mAh·g^-1) were performed according to the following Eq.1 and 2, respectively:

  $ C_{\mathrm{sp}}=I t /(m \Delta V) $

  (1)

  $ C_{\mathrm{s}}=I t /(3.6 \mathrm{~m}) $

  (2)

  where I, t, ΔV, and m stand for the discharge current (A), the discharge time (s), the voltage range (V), and the mass loading of Co-BD (g), respectively.

  An asymmetric supercapacitor (ASC), as a two-electrode system, was used to investigate the application potential of Co-BD, which was constructed by using the rGO electrode as the negative electrode, the Co-BD electrode as the positive electrode, and 6 mol·L^-1 KOH as the electrolyte. For the Co-BD positive electrode, its preparation procedure was the same as the Co‑BD electrode mentioned above, but the mass ratio of Co‑BD, acetylene black, and PTFE was changed to 75∶20∶5. The mass ratio of Co-BD and rGO was determined based on the principle that the charge from the positive electrode was equal to that from the negative electrode. The preparation of the rGO negative electrode was the same as that of the Co-BD positive electrode. For the Co-BD||rGO ASC, the energy density (E, Wh·kg^-1) and power density (P, W·kg^-1) were ascertained according to the following Eq.3 and 4, respectively:

  $ E=0.5 C_{\mathrm{sp}}(\Delta V)^2 / 3.6 $

  (3)

  $ P=3600 E / t $

  (4)

  where C_sp (F·g^-1) is the specific capacitance of the ASC, which is calculated according to the total mass of rGO and Co-BD in two electrodes.

  2.   Results and discussion

  2.1   Crystal structure of Co-BD

  It can be found from Table S1, we know that the crystal of Co-BD belongs to the triclinic system with a space group of P1. The asymmetric unit of Co-BD consists of one Co(Ⅱ) ion, one BGPD^2- anion, two coordinating water molecules, and two DMSO molecules.

  Fig. 2a shows the coordination environment ofCo-BD, from which it can be seen that each Co(Ⅱ) ion is connected to two O atoms (O2, O2A) from two water molecules, two O atoms (O3, O3A) from two BGPD^2- anions, and two O atoms (O1, O1A) from two DMSO molecules, forming an octahedral coordination configuration. According to Table 1, the bond angles of O1—Co1—O1A, O2—Co1—O2A, and O3—Co1—O3A are all 180.0°, indicating that the coordination configuration of Co(Ⅱ) ions has a regular octahedral spatial configuration. The bond length of Co1—O2 obtained by coordinating Co(Ⅱ) ions with O atoms from water molecules is 0.210 39 nm, the bond length of Co1—O1obtained by coordinating Co(Ⅱ) ions with O atoms from DMSO molecules is 0.210 35 nm, and the bond length of Co1—O3 obtained by coordinating Co(Ⅱ) ions with O atoms from BGPD^2- anions is 0.210 23 nm. As shown in Fig. 2b, Co(Ⅱ) ions are interconnected through BGPD^2- anions to form a 1D chain structure. These 1D chains are formed through intermolecular hydrogen bonds (O—H…O and C—H…O, Table S2) and π-π interactions between aromatic rings, resulting in the formation of a 3D supramolecular structure. The flexible space between 1D chains in Co-BD is in favor to the diffusion of the electrolyte ion, like as 1D organic polymer poly anthraquinone (PAQ)^[64], thus, such 1D CPs containing redox-active centers (transition metal ions/organic ligands) and flexible space should be also severed as the electrode materials of supercapacitors.

  Figure 2

  

  Figure 2.  (a) Coordination environment illustration of Co-BD; (b) 3D stacking diagram of Co-BD

  In a: H atoms are omitted for clarity; Symmetry code: #1:-x+1, -y+1, -z.

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  Table 1

  

  Table 1.  Main bond lengths (nm) and bond angles (°) in Co-BD

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  Co1—O3	0.210 23(19)	Co1—O2	0.210 39(18)	Co1—O1	0.210 35(19)
  O3—Co1—O1	90.60(8)	O3—Co1—O1#1	89.40(8)	O3—Co1—O3#1	180.0
  O3—Co1—O2	90.57(7)	O2—Co1—O2#1	180.0	O1—Co1—O1#1	180.0
  O1—Co1—O2	90.10(8)	O1—Co1—O2#1	89.90(8)	O3—Co1—O2#1	89.43(7)
  Symmetry code: #1: -x+1, -y+1, -z.

  2.2   Synthesis and characterizations of Co-BD

  In the presence of a mixture of three isomers (H₂L¹, H₂L², and H₂L³), H₂BGPD reacted with Co(NO₃)₂·6H₂O to synthesize [Co(BGPD)(DMSO)₂(H₂O)₂], but these three isomers do not participate in the interaction with Co(Ⅱ) ions. It is worth noting that in the absence of a mixture of the three isomers in the reaction system, the single crystals of Co-BD do not form, indicating that the three isomers may play a template role in the formation process of Co-BD single crystals. To observe the morphological characteristics of the Co-BD sample, we conducted SEM tests on the Co-BD sample. Fig. 3a shows the SEM image of Co-BD, from which it can be seen that the Co-BD material consist of many stacked irregular particles with a size of about 0.1-0.5 μm. From the FTIR spectra of Co-BD presented in Fig. 3b, it can be found that the FTIR spectrum of Co-BD was mainly the absorption peak generated by the ligand BGPD^2- anions. There was a clear absorption peak at 1 720 cm^-1, which is the stretching vibration peak of the C=O groups of the ligand BGPD^2- anions. Compared with the FTIR spectrum of the ligand H₂BGPD, a new absorption peak appeared at 1 623 cm^-1 in the FTIR spectrum of Co-BD, which should be attributed to the stretching vibration peak of the S=O groups of DMSO, indicating that DMSO solvent molecules have coordinated with metal cobalt ions; The strong absorption peak appearing at 3 420 cm^-1 is attributed to the stretching vibration peak of the O—H groups from the coordinated water molecule. The FTIR spectrum analysis results are consistent with the single-crystal structure analysis results. XRD patterns determined the purity of the synthesized Co-BD CP. As depicted in Fig. 3c, the positions of the PXRD diffraction peaks of as-synthesized Co-BD were consistent with the peaks simulated based on the crystal data of Co-BD, indicating that the purity of as-synthesized Co-BD was high.

  Figure 3

  

  Figure 3.  (a) SEM image of as-synthesized Co-BD after grinding for 2 h; (b) FTIR spectra of Co-BD and H₂BGPD; (c) XRD patterns and (d) TG curve of Co-BD

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  Fig. 3d shows the TGA curve of Co-BD. It can be seen that there was a mass loss of approximately 5.90% before 131 ℃, which corresponds to the loss of twocoordinated water molecules in Co-BD (Calcd. 6.19%); There was approximately 27.4% mass loss from 131 to 306 ℃, which is attributed to the loss of two DMSO molecules (Calcd. 26.84%); There was a plateau in the TG curve between 306 and 394 ℃, indicating that the structure after coordinated water molecules and DMSO molecules that have lost can still exist stably; When the temperature rose to 850 ℃, the remaining mass of the substance accounted for about 19.5% (Calcd. 12.9%), indicating that the residue may be CoO.

  2.3   Electrochemical performance

  The electrochemical performances of Co-BD prepared under alkaline conditions as electrode material for supercapacitors were studied in a three-electrode system. As shown in Fig. 4a, in the CV curves of the Co-BD electrode with scan rates of 2, 5, 10, 20, and 40 mV·s^-1, two distinct redox peaks could be observed on each curve, indicating that the pseudocapacitive behavior was caused by surface Faraday redox reactions^{[46-48, 54]}. The generation of the oxidation peak at around 0.40 Ⅴ and the reduction peak at around 0.27 Ⅴ at 2 mV·s^-1 may be due to the transition of different valence states of cobalt. According to the characteristics of the CV curve mentioned above and the reaction mechanism of the Co-MOF-based electrode^[14], we propose a possible transformation process of cobalt in different valence states, as shown in Eq.5 and 6, where the subscripts s and ad represent solid state and adsorption, respectively. When the scanning rate increased from 2 to 5, 10, 20, and 40 mV·s^-1, the oxidation peak shifted positively, appearing at around 0.43, 0.47, 0.50, and 0.55 Ⅴ, respectively, while the reduction peak shifted negatively, appearing at around 0.25, 0.24, 0.21, and 0.18 Ⅴ, respectively. This can be attributed to the increased polarization of the electrodes^{[16, 65]}. Notably, many reported MOF/CPs-based electrodesalso exhibit similar pseudocapacitive behavior, such as Co(Hmt)(tfbdc)(H₂O)₂]·(H₂O)₂}_n^[14], [KCo(pa)(OH)]_n (H₂pa=phthalic acid)^[15], and [Cu₄(μ₃-OH)₂(atrz)₂(1, 3-BDC)₃]·2H₂O (atrz=4-amino-1, 2, 4-triazole; 1, 3-BDC=1, 3-benzenedicarboxylic acid)^[18].

  $ \mathrm{Co}(\mathrm{II})_{\mathrm{s}}+\mathrm{OH}^{-} \rightleftharpoons \mathrm{Co}(\mathrm{II})(\mathrm{OH})_{\mathrm{ad}}+\mathrm{e}^{-} $

  (5)

  $ \mathrm{Co}(\mathrm{II})(\mathrm{OH})_{\mathrm{ad}} \rightleftharpoons \mathrm{Co}(\mathrm{III})(\mathrm{OH})_{\mathrm{ad}}+\mathrm{e}^{-} $

  (6)

  Figure 4

  

  Figure 4.  Electrochemical performances of the Co-BD electrode: (a) CV curves at different scan rates; (b) lg i_p-lg v plots of the oxidation and reduction peaks originating from the CV curves; (c) CV curves with capacity separation at 2 mV·s^-1; (d) contribution rates of the diffusion-controlled and capacitive capacities at different scan rates

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  In addition, a power-law relationship (i_p=av^b) was used to study the charge storage behavior of the Co-BD electrode based on the CV curve shown in Fig. 4a, where a and b are variable parameters, v represents the scan rate, and i_p is the peak current. The constant b can be obtained according to the slope of the linear profile of lg i_p and lg v displayed in Fig. 4b. Based on the magnitude of the b-value, the charge storage behavior of the electrode can be ascertained. If b=0.5, it is related to the Faraday process dominated by ideal diffusion, and b=1 represents pure capacitive response^[66]. Both b values of the Co-BD electrode (0.51 and 0.54) were close to 0.5, indicating that a diffusion-controlled Faraday process dominates the charge storage behavior of the Co‑BD cathode. The rates of capacitance or diffusion-controlled contribution at different scanning rates can be determined by following Eq.7 and 8 proposed by Dunn et al.^[67]:

  $ i / v^{1 / 2}=k_1 v^{1 / 2}+k_2 $

  (7)

  $ i=k_1 v+k_2 v^{1 / 2} $

  (8)

  where i stands for the current, v is the scan rate, k₁ and k₂ stand for capacitive and diffusion-controlled coefficients, k₁v and k₂v^1/2 stand for capacitive and diffusion-controlled contribution rates, respectively. As shown in Fig. 4c and Fig.S1, at a scan rate of 2 mV·s^-1, the contribution rates of the capacitive-controlled and diffusion-controlled processes were 25% and 75%, respectively. With the scan rate increasing to 5, 10, 20, and 40 mV·s^-1, the contribution rates of the capacitive-controlled process rose to 30%, 39%, 54%, and 83%, as presented in Fig. 4d, respectively, indicating the charge storage behaviors of the Co-BD electrode at the scan rate range of 2 to 20 mV·s^-1 is mainly the diffusion-controlled process, while the charge storage behavior at scan rates of 40 mV·s^-1 is mainly capacitive-controlled process.

  The GCD curves of the Co-BD electrode at various current densities displayed in Fig. 5a reveal that the GCD curves had obvious voltage plateaus, and their shape exhibited a triangle of considerable deviation, disclosing that Co-BD takes part in the Faraday redox reaction, consistent with the conclusion inferred from the CV curves.

  Figure 5

  

  Figure 5.  (a) GCD curves at different densities, (b) specific capacitances at different densities, (c) cycle performance at 2 A·g^-1, and (d) Nyquist plots of the Co-BD electrode

  The current densities in a were 1, 2, 3, 4, 6, 8, 10, and 20 A·g^-1; Inset: corresponding enlarged plots.

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  Fig. 5b presents the discharge capacitances of the Co-BD electrode at various current densities. It can be found in Fig. 5b that the specific capacitance of theCo-BD electrode was up to 838 F·g^-1 at 1 A·g^-1 (equivalent to a specific capacity of 116.4 mAh·g^-1), higher than many reported MOFs/CPs-based electrodes until now^{[22, 25, 29-32, 34, 39]}, such as Co-MOF (446.8 F·g^-1 at 1.2 A·g^-1)^[22], Ni-MOF (726 F·g^-1 at 1 A·g^-1)^[25], MOF-3 (465 F·g^-1 at 1 A· g^-1)^[30], and Co-MOF (148 F·g^-1 at 2 A· g^-1)^[31] (Table S3). Along with the current density increasing to 2, 3, 4, 6, 8, 10, and 20 A·g^-1, the specific capacitances reduce to 582, 504, 432, 380, 310, and 212 F·g^-1, respectively, presenting an excellent rate capability.

  The Co-BD electrode at 2 A·g^-1 in Fig. 5c had a general cycling performance, owing to the capacitance retention rate being about 57.9% after 1 000 cycles. Partial structure change of Co-BD in the cycle process may result in capacitance decay^[68].

  The EIS of the Co-BD electrode depicted in Fig. 5d reveals that the internal resistance (R_s) of the electrode was lower, only 6.3 Ω, and the phase angle of the Nyquist plot was far larger than 45°, indicating OH^-/K⁺ ions had a considerable good transfer ability. Higher-performance production of the Co-BD electrode may be relative with following three factors, one is that Co-BD has redox active centres (Co(Ⅱ) ions/BGPD^2- anions), second is that the micro-sized Co-BD particles can provide more active sites and shorten the diffusion distance of OH^- ions, third is 1D chains in 1D Co-BD CP can produce a 3D supramolecular structure by weak interactions, and the flexible space between 1D chains can allow the diffusion of the electrolyte ion.

  To investigate the application potential of Co-BD in supercapacitors, an ASC, represented by Co-BD||rGO, was assembled by the Co-BD electrode (the positive electrode) and the rGO electrode (the negative electrode), as shown in Fig.S2. Fig.S3 depicts GCD and CV curves of the rGO electrode in 1 mol·L^-1 KOH under a three-electrode system. As found in Fig.S3, redox peaks appeared on CV curves, and each GCD curve deviated from a straight line, showing the rGO electrode had pseudo-capacitance behavior. The CV curves of the Co-BD||rGO ASC are shown in Fig. 6a. When the scan rate increased from 2 to 100 mV·s^-1, the shape of these CV curves remained almost unchanged. The GCD curves depicted in Fig. 6b showed that they had the shape of a distorted triangle, but were almost symmetrical, which disclosed that the capacitance of the Co-BD||rGO device originated from the together contribution of the capacitance of the electric double layer and the pseudo-capacitance. Under the current densities of 1, 2, 3, 4, and 6 A·g^-1, the Co-BD||rGO ASC could deliver specific capacitances of 40.6, 34.7, 34.5, 34.6, and 33.1 F·g^-1, respectively. These capacitances were obtained based on Eq.1 and the GCD profiles.

  Figure 6

  

  Figure 6.  Electrochemical performance of the Co-BD||rGO ASC: (a) CV curves, (b) GCD curves, (c) Ragone plot, and (d) cycle performance

  The scan rates in a were 2, 5, 10, 20, 40, 60, 80, and 100 mV·s^-1.

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  According to Eq.3 and 4, the energy densities and the power densities for Co-BD||rGO ASC can beobtained, which is displayed in Fig. 6c. The maximum energy density of the Co-BD||rGO ASC was 14.2 Wh·kg^-1, as the power density was 0.80 kW·kg^-1. When the energy density decreased to 11.98 Wh·kg^-1, the maximum power density was up to 4.80 kW·kg^-1. The energy density (14.2 Wh·kg^-1) of Co-BD||rGO ASC was higher than or equivalent to a lot of reported MOF-based ACSs, for instance, Co₅-MOF||rGO (18 Wh·kg^-1, 0.70 kW·kg^-1)^[17], Cu-atrz-BDC||rGO (9.96 Wh·kg^-1, 0.81 kW·kg^-1)^[18], and SNNU-52||AC (SNNU-52={[Ni₃(μ₃-OH)][Ni(PDC)₂]₃(TPP)₃}_n; PDC=2, 5-pyridinedicarboxylic; TPP=2, 4, 6-tri(4-pyridinyl)-1-pyridine; AC= activated carbon) (15.1 Wh·kg^-1, 0.37 kW·kg^-1)^[32]. The Co-BD||rGO ASC also presented an excellent long-term cycle capability, as shown in Fig. 6d, after 4 000 cycles at 1 A·g^-1, the capacitance retention was up to 94%.

  3.   Conclusions

  In conclusion, a new 1D CP, [Co(BGPD)(DMSO)₂(H₂O)₂] (Co-BD) has been successfully synthesized by the reaction of BGPD and Co(NO₃)₂·6H₂O. The cost of producing Co-BD was relatively low, due to low-cost and abundant sources of BGPD, and the synthetic method of Co-BD was facile. In a three-electrode system, Co-BD as the electrode material for supercapacitors could present a higher specific capacitance (838 F·g^-1 at 1 A·g^-1), a high-rate performance (212 F·g^-1 at 20 A·g^-1), and a general cycle performance (capacitance retention of 57.9% after 1 000 cycles). The discharge-specific capacitance of the Co-BD||rGO ASC constructed by the Co-BD electrode as a positive electrode and the rGO electrode as a negative electrode could reach 40.6 F·g^-1 under the current density of 1 A·g^-1, along with the capacitance retention rate of 94% after 4 000 cycles, displaying an excellent cycle capability. When the power density was 0.80 kW·kg^-1, the maximum energy density was 14.2 Wh·kg^-1, higher than or equivalent to many reported CP-based ACSs before. Our work demonstrates that 1D cobalt-based CPs can be used as higher-performance electrode materials for supercapacitors and provide a novel insight for the building of cobalt-based CPs applied in energy storage systems.

  Supporting information is available at http://www.wjhxxb.cn

  
  Acknowledgments: This work was supported by the National Natural Science Foundation of China (Grant No.21975034). Conflicts of interest: The authors declare no competing financial interests.
  
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  Figure 1  Structural formula of H₂BGPD

  下载: 全尺寸图片 幻灯片
  

  Figure 2  (a) Coordination environment illustration of Co-BD; (b) 3D stacking diagram of Co-BD

  In a: H atoms are omitted for clarity; Symmetry code: #1:-x+1, -y+1, -z.

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  Figure 3  (a) SEM image of as-synthesized Co-BD after grinding for 2 h; (b) FTIR spectra of Co-BD and H₂BGPD; (c) XRD patterns and (d) TG curve of Co-BD

  下载: 全尺寸图片 幻灯片
  

  Figure 4  Electrochemical performances of the Co-BD electrode: (a) CV curves at different scan rates; (b) lg i_p-lg v plots of the oxidation and reduction peaks originating from the CV curves; (c) CV curves with capacity separation at 2 mV·s^-1; (d) contribution rates of the diffusion-controlled and capacitive capacities at different scan rates

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  Figure 5  (a) GCD curves at different densities, (b) specific capacitances at different densities, (c) cycle performance at 2 A·g^-1, and (d) Nyquist plots of the Co-BD electrode

  The current densities in a were 1, 2, 3, 4, 6, 8, 10, and 20 A·g^-1; Inset: corresponding enlarged plots.

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  Figure 6  Electrochemical performance of the Co-BD||rGO ASC: (a) CV curves, (b) GCD curves, (c) Ragone plot, and (d) cycle performance

  The scan rates in a were 2, 5, 10, 20, 40, 60, 80, and 100 mV·s^-1.

  下载: 全尺寸图片 幻灯片

  Table 1.  Main bond lengths (nm) and bond angles (°) in Co-BD

  Co1—O3	0.210 23(19)	Co1—O2	0.210 39(18)	Co1—O1	0.210 35(19)
  O3—Co1—O1	90.60(8)	O3—Co1—O1#1	89.40(8)	O3—Co1—O3#1	180.0
  O3—Co1—O2	90.57(7)	O2—Co1—O2#1	180.0	O1—Co1—O1#1	180.0
  O1—Co1—O2	90.10(8)	O1—Co1—O2#1	89.90(8)	O3—Co1—O2#1	89.43(7)
  Symmetry code: #1: -x+1, -y+1, -z.

  下载: 导出CSV
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