A tremella-like in situ synthesis of ZIF-67Co(OH)F@Co3O4 on carbon cloth as an electrode material for supercapacitors

In this study, a simple in situ technique followed by hydrothermal method is used to synthesize a novel tremella-like structure of ZIF-67Co(OH)F@Co3O4/CC metal–organic framework (MOF) derived from zeolite imidazole. The in situ synthesis of metal–organic frameworks (MOFs) increases their conductivity and produces more active sites for ion insertion. Their unique, scalable design not only provides more space to accommodate volume change but also facilitates electrolyte penetration into the electrode resulting in more active materials being utilized and ion-electron transfer occurring faster during the cycle. As a result, the binder-free ZIF-67Co(OH)F@Co3O4/CC supercapacitor electrode exhibits typical pseudo-capacitance behaviour, with a specific capacitance of 442 F g−1 and excellent long-term cycling stability of 90% after 5000 cycles at 10 A g−1.


Introduction
Rapid global economic development has led to the depletion of non-renewable energy resources, prompting researchers to explore technological advancements to meet the growing demand for efficient and renewable energy.Supercapacitors have emerged as a highly promising solution owing to their fast charge/discharge rate, excellent cyclic stability, and high power density. 1 Various nanomaterials have been established to improve electrochemical properties.][9][10][11][12] Thermal decomposition of metal-organic frameworks (MOFs) to produce porous carbon materials has attracted signicant attention owing to their unique structures, large specic surface areas, and abundant pore structures. 13Typically, MOFs are constructed using central metals from d and f-block elements, such as Zn, Co, Zr, etc. 14,15 Although tunable pore size and high surface area are advantageous for electrochemical applications of MOFs, however, their poor electrical conductivity and electroactivity limit their performance.To address these limitations, various post-treatments have been employed, including carbonization, sulfurization, oxidation, salinization, and uorination. 16Binder-free electrodes, which eliminate the need for additional binders to adhere the active material onto a conductive substrate show promise in preventing poor adhesion and can be fabricated using techniques such as doctorblade or dip-coating. 17Researchers have explored different approaches to improve the electrical conductivity and electroactivity of MOFs.For example, Huang et al. fused a carbonized zeolitic imidazolate (ZIF-67) framework on nickel foam to make a binder-free battery-like electrode. 18Wu et al. studied the effect of carbonization temperature on the energy storage capabilities of UiO-66-derived carbon. 19Cheng et al. make sulfurized ZIF-67 binder-free electrodes on nickel foam for BSH battery-type electrode. 20Xiao et al. used different sulfur sources to synthesize Co-MOF binder-free BSH electrode. 21Wu et al. synthesized MIL 101 derivative by carbonization oxidation process for electrochemical performance. 22They also synthesized ZFI-67 derivatives with ammonium uoride and salinization process as electroactive materials for BSH. 23Wang et al. utilized a facile one-step solution process to fabricate ammonia borane uoride-induced ZIF-67 derivatives for energy storage. 24Liu et al. synthesized a honeycomb-like porous carbon mettle organic framework derived from uorinated magnesium as electrode materials for supercapacitors. 25Although posttreatments have shown promise, which involve additional steps that may increase time and cost.Moreover, the replacement of expensive ligands with post-treated anions might limit the full utilization of ligands.Therefore, in situ modication provides an alternative approach to enhance the electrical conductivity and electroactivity of MOFs, as compared to ex situ post-treatments 26,27 Co 3 O 4 is considered to be an ideal electrode material for supercapacitors due to its low-cost, high theoretical capacitance (3600 F g −1 ) and high electrochemical stability. 28owever, in practical use during the reaction process reduces its electrochemical performance due to slow electron transfer rate and agglomeration of Co 3 O 4 . 29It is necessary to synthesize electrode materials that are short in the electron transport channel, having fast reaction rate, and have good stability.Strong structural stability, high porosity, and a large specic surface area make ZIF-67 material an excellent precursor for the synthesis of transition metal oxides (TMOs). 30Many research work on the use of ZIF-67 as a precursor in the synthesis of cobalt oxide have been published by late Sun et al. used ZIF-67 as a precursor and direct annealing in air at various temperatures to obtain Co 3 O 4 nanoparticles.The results demonstrate that temperature has a signicant impact on the composition and characteristics of Co 3 O 4 nanoparticles. 31Carbon nanotubes (CNTs) were in situ implanted by Qu et al. 32 into the porous Co 3 O 4 dodecahedron that was derived from ZIF-67.The composites' morphology mostly preserved the dodecahedron structure.The composites, shape, and electrical conductivity were improved by controlling the amount of carbon nanotubes.Because of its numerous redox reactions.Cobalt hydroxide (Co(OH) 2 ) is another cobalt molecule that has been thoroughly researched.In redox processes, cobalt hydroxide contributes more electrons than nickel oxide and hydroxide.As a result, cobalt hydroxide has a theoretically higher specic capacitance than nickel oxide and hydroxide. 33 In this work, we adjusted the temperature for the synthesis of nanocomposites, to synthesize a novel binder-free electrode ZIF-67Co(OH)F@Co 3 O 4 for supercapacitors.A precursor solution containing Co(OH)F nanoparticles indicates to the formation of a metal hydroxide, uoride, zeolite imidazole framework.Using a simple in situ technique followed by hydrothermal procedure, we synthesized the typical ZIF-67 metal-organic framework (MOF), which has a tremella-like structure with a high surface area.ZIF-67 Co(OH)F@Co 3 O 4 nanocomposite are fused to carbon fabric to form the ZIF-67Co(OH)F@Co 3 O 4 /CC supercapacitor electrode.The advantages of both a conductive substrate and the ZIF-67 Co(OH) F@Co 3 O 4 are combined in this hybrid structure.The composite not only provides a large number of electroactive sites for effective redox reactions, but also accelerating ion transport and electrolyte diffusion.With the combined benets of the onedimensional conductive carbon bers and the large surface area of tremella-type ZIF-67Co(OH)F@Co 3 O 4 MOF, electrode has a specic capacitance of 442 F g −1 .The electrode continues to retain 90% of its stability aer 5000 cycles, indicating remarkable long-term cyclic stability.

Chemicals
The chemicals used in this study were purchased from Aladdin bio chemical technology Shanghai China.The chemicals procured from the supplier included Co(No 3 ) 2 $6H 2 0, ammonium uoride, 2-methylimidazole, methanol, ethanol, and potassium hydroxide.These chemicals were selected based on their suitability for the synthesis and characterization of the desired cobaltZIF-67 (CoOH)F@Co 3 O 4 nanocomposites.A Co(No 3 ) 2 -$6H 2 0, was employed as a cobalt precursor for the synthesis of cobalt based MOF.Ammonium uoride was used as a source of uoride ions, while 2-methylimidazole served as a ligand in the formation of the metal organic zeolite imidazole framework.Methanol and ethanol were utilized as solvents for the reactions, providing a suitable medium for the chemical processes.

Synthesis of Co(OH)F nanoparticles
Co(OH)F nanoparticles was synthesized by adding 1 g Co(NO 2 ) 3 $6H 2 O to 80 mL ethanol followed by addition of 0.5 g of ammonium and stirred for 40 minutes.Aer 40 minutes of stirring the solution was added to a Teon line autoclave and heated for 4 h at 120 °C.On completion of reaction, the product was allowed to cool at room temperature, the product was centrifuged, washed several times with water and ethanol.Finally, the product was dried overnight at 70 °C in an oven for further use.

2.3
In situ, construction of ZiF-67 derived Co(OH)F @Co 3 O 4 / CC electrode ZIF-67 was prepared by previously reported method with a little modication. 38Both Co(NO 2 ) 3 $6H 2 O (0.5 g) and 2-methyl imidazole (1.3 g) were dissolved separately in two beakers already containing 40 mL methanol and stirred for 15 minutes.On complete dissolution, 2-methyl imidazole solution was added to Co(NO 2 ) 3 $6H 2 O solution drop wise and then 0.8 g of synthesized Co(OH)F nanoparticles were added and stirred for 1 h.Aer 1 h stirring the solution was transferred to a Teon line autoclave and a piece of carbon ber cloth of 1 × 2 cm 2 diameter immersed vertically in the solution mixture and keep at different temperature for 5 h, aer the desired reaction time, the auto clave reactor was slowly cool down to room temperature the ZIF-67Co(OH) F@Co 3 O 4 nano composites grow on the surface of carbon cloth aer being kept at different temperature for 5 h.The carbon cloth washed for several time with water and ethanol and dry at 80 °C.The fabrication procedure of ZIF-67Co(OHOF@Co 3 O 4 /CC is shown in the Fig. 1.The products synthesized by different temperature given name Co(OHO)F/CC120, Co(OHO)F@Co 3 O 4 / CC160, and Co(OHO)F@Co 3 O 4 /CC200.

Electrochemical analysis
Three electrode systems were made in 1 M KOH electrolyte, with the ZIF-67Co(OH)F@Co 3 O 4 /CC200 serving as the working electrode.Hg/HgO is used as a reference electrode, while platinum foil serves as a counter electrode.The electrochemical measurements were performed using the CHI660E, electrochemical workstation, which included cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and galvanostatic charge discharge (GCD).Eqn ( 1) is used to get the C F value from the GCD curve where I is the current density (A g −1 ), Dt is the discharge duration, and m is the mass of the active materials.Eqn (2) and ( 3) are used to calculate the energy density (E) and power density (P) respectively dv is the potential window (V), dt is the discharge duration (s) and C is specic capacitance. 34 2.5.Assembly of the ZIF-67Co(OH)F@Co 3 O 4 /CC//AC exible supercapacitor A standard procedure was followed in the fabrication of the negative electrode. 39N-Methyl 2-pyrrolidane was combined with acetylene black and activated carbon (AC) at a mass ratio of 1 : 1 : 8 to create a homogeneous slurry.The current collector carbon cloth electrode was immersed in the slurry for 2 minutes and then dried for 10 h at 80 °C.Gel electrolyte was used in the assembling of ZIF-67Co(OH)F@Co 3 O 4 /CC//AC device. 1 g of PVDF was gradually added to a 10 mL 1 M KOH solution while being continuously stirring at room temperature until the mixture turned transparent.Both the positive and negative electrode were submerged in the gel electrolyte and was held for 30 s.A 1 × 1 cm 2 of lter paper was used as a separator between the electrodes.

X-ray diffraction analysis
Powdered X-rays diffraction (XRD) was used to study the phase purity and crystal structure of the synthesized samples shown in Fig. 2. Fig. 2a displays the XRD pattern of ZIF-67, indicating the relative intensity and peak positions which is reliable according to the literature at 10°, 12.30°, 13°, 16°and 18°. 33Fig. 2b shows the XRD pattern of Co(OH)F, the reection peaks at 26.50°,

Scanning electron microscopy analysis
The Further to investigate the composites structure the TEM analysis was carried out.Fig. 4(A-C) shows the TEM images of ZIF-67 Co(OH)F@Co 2 O 4 200 it is observed that the micro structure of ZIF-67 Co(OH)F@Co 2 O 4 200 exhibits the tremella-like structure assembled from leaf-like nanosheets it should be noted that these nanosheets are obviously thinner Among the four structures, it is clear that the nanosheet assembled tremella-like structure is superior to the other structures, because it can provide a richer surface area for the ionaccessible accommodation and promote the ion diffusion, which is advantageous for the ion/extraction processes and the redox reaction at ZiF-67 Co(OH)F@Co 2 O 4 200 the electrode/ electrolyte interface.

X-ray photoelectron spectroscopy analysis
X-ray photoelectron spectroscopy (XPS) investigation was carried out to better understand the chemical state of elements in ZIF-67 Co(OH)F@Co 3 O 4 200.Fig. 5 shows the scan spectrum, which indicate the elements C, Co, F, and O. Fig. 5A display the high resolution C 1s spectrum which include the peaks at binding energy 284 eV, 286 eV and 288 eV, assigned to C]C, C-C and C]O. 46,47Fig. 5B shows the high spectrum of cobalt element.It can be seen that the spectrum consists of two peaks located at 782 eV and 796 eV corresponding to the electronic state of Co 2p 2/3 and Co 2p 1/2 respectively.Moreover, two satellite peaks are also noticed present at 789 eV and 805 eV corresponding to the Co 2p 3/2 and Co 2p 1/2 respectively. 48Fig. 5C show high resolution XPS spectrum of O 1s, the peaks at 531 eV and 532 eV corresponds to the Co-O and C-O bonds respectively. 49The high-resolution spectrum of F 1s is shows in Fig. 5D which shows a peak at 684 eV corresponding to the C-F bond. 50

BET analysis
The porosity and surface area of ZIF-67 were characterized by BET analysis the isotherm of ZIF-67 is shown in Fig. 6A which show type I-pattern the adsorption-desorption isotherms showed a steep rise at low relative pressure and then quickly attained a balance to suggest dominant microporous characteristic.The values of specic surface area for ZIF-67 were estimated to be 295 m 2 g −1 the Barrett-Joyner-Halenda pore size distribution plots for the samples of ZIF-67 is shown in Fig. 6B the diameter of most of the pores was found to lie within the range of 1-40 nm.The values of mean centered porosities were assessed as 1.26 and 1.73 nm, respectively.Also, nitrogen gas adsorption-desorption isotherm was used to estimate the porosity and specic surface area of Co(OH)F and ZIF-67Co(OH) F@Co 3 O 4 200 nanocomposites.The isotherm of the Co(OH)F and ZIF-67Co(OH)F@Co 3 O 4 200 as shown in Fig. 6A exists similar to type II pattern, with molten layer development occurring at medium pressure and capillary condensation occurring at high pressure, the at middle zone represents a monolayer formation.The specic surface area for Co(OH)F and ZIF-67Co(OH)F@Co 3 O 4 200 is 370 m 2 g −1 and 547 m 2 g −1 based on BET, analysis.Barrett-Joyner Halenda (BJH) pore size distribution curve of Co(OH)F and ZIF-67Co(OH)F@Co 3 O 4 200 is shown in Fig. 6C representing a huge number of pores present in the 0-10 nm region.The average pore volume and pore diameter for Co(OH)F and ZIF-67Co(OH)F@Co 3 O 4 200 is 2.8 cm 3 g −1 , 4 cm 3 g −1 and 19.14 nm, 25.6619 nm respectively.Which conrms that Co(OH)F@Co 3 O 4 has greater surface area and provide more active site for electrochemical reaction and enhance the overall performance.

Electrochemical performance
The electrochemical performance of Co(OH)F, ZIF-67 Co(OH) F@Co 3 O 4 /CC200, 160 and 120 electrodes were tested in 1 M KOH electrolyte shown in Fig. 7.The CV curves of all the samples are measured at the same scan rate of 10 mV s −1 , it is seen that all the CV curves have a pair of redox peaks, con-rming the Faraday reaction has occurred shown in Fig. 7A. 51eanwhile, the CV curves area of ZIF-67 Co(OH)F@Co 3 O 4 / CC200 is signicantly higher than the other which shows that the ZIF-67Co(OH)F@Co 3 O 4 /CC200 has high utilization of the active site, greater capacity and good electrochemical performance due to its high surface area conrming the results are reliable with SEM images.Fig. 7B shows the GCD curves of all the samples ZIF-67 Co(OH)F@Co 3 O 4 /CC200, 160, and 120 and Co(OH)F at 1 A g −1 with specic capacitance value, 442 F g −1 , 371 F g −1 , 345 F g −1 and 165 F g −1 respectively, corresponding to capacity 98 mA h g −1 ,81 mA h g −1 , 74 mA h g −1 and 34 mA h g −1 .
It can be seen that the discharge time of ZIF-67 Co(OH) F@Co 3 O 4 /CC200 is higher than the other.The CV curves of ZIF-67Co(OH)F@Co 3 O 4 200 at scan rate of 1 mV s −1 to 100 mV s −1 with a potential window of −0.2 V to 0.6 V are displayed in Fig. 7C.With increase in the scan rate there is a sharp increase in the area and the shape remain similar with obvious redox peaks, which shows excellent performance.The redox reaction mechanism shows in the following eqn (4) and ( 5). 52,53The CV curves of ZIF-67 Co(OH)F@Co 3 O 4 /CC 160 and ZIF-67 Co(OH)F/ CC 120 are shown in Fig. S4 in the ESI † for comparison all the CV curves show good redox peaks.
The GCD curves of ZIF-67Co(OH)F@Co 3 O 4 /CC200 is shown in Fig. 7D at current density ranging from 1 A g −1 to 10 A g −1 with C F 442 F g −1 , 436 F g −1 , 420 F g −1 , 386 F g −1 and 350 F g −1 corresponding to capacity 98 mA h g −1 , 91 mA h g −1 , 85 mA h g −1 , 79 mA h g −1 , and 69 mA h g −1 respectively.The GCD curves of Co(OH)F, ZIF-67-Co(OH)F@Co 3 O 4 /CC160, ZIF- The GCD curves of all the samples are nonlinear and well symmetrical at all current densities.The specic capacitance of all the samples ZIF-67Co(OH)F@Co 3 O 4 /CC200, 160 and 120 were calculated according to the discharge curves shows in Fig. 7E.The values of specic capacitance decreases with increase the current density this is because at high current density less ion can reach the electrode surface. 54The specic capacitance of ZIF-67 Co(OH) F@Co 3 O 4 200 is higher than the other due to its high surface area.Nyquist plot of ZIF-67, Co(OH)F and ZIF-67Co(OH)F@Co 3 O 4 / CC200 were investigated in the frequency range of 0.01 to 1000 kHz.The EIS measurement, Nyquist plots and their tting are shown in Fig. 7F at high frequency, the intersection with the X-axis suggests the equivalent series resistances (R s ), which represent the total resistance in the electrochemical system.Charge transfer resistance (R ct ) and constant phase element (CPE) which represent the electrical resistances at the electrode and electrolyte interface.like structure which incorporate high electrolyte.The small R s and R ct , values indicate the high capacitance of ZIF-67Co(OH) F@Co 3 O 4 /CC200 there for the best electrochemical performance.
To evaluate the electrochemical stability and specic retention of ZIF-67Co(OH)F@Co 3 O 4 /CC200 electrode GCD plots were acquired for 5000 cycles at a constant current density of 10 A g −1 with the potential window −0.2 V to 0.6 V is shown in Fig. 7G.The stability curve of Co(OH)F@Co 3 O 4 /CC200 which exhibit great cyclic stability with capacitance retention of 90% aer 5000 charges discharge cycles.The electrode maintained linear and symmetrical GCD curves in the last 10 cycles (inset in Fig. 7G) which show excellent cyclic stability.The cyclic stability curve of the Co(OH)F electrode with capacitance retention of 78% aer 5000 GCD charge-discharge cycles is shown in Fig. S6 in ESI.† The summary of the charge-discharge characteristic ZIF-67Co(OH)F@Co 3 O 4 /CC200 electrode has high electrochemical stability and enhances their specic capacitance the remarkable C F is due to its high surface area.The tremella-like structure is retained during electrochemical evaluations which provides stability and also prevent the accumulation of nanosheets.
To study the practical application of ZIF-67Co(OH) F@Co 3 O 4 200/CC the solid-state asymmetric supercapacitor (ASC) was constructed in a 1 M KOH solution.The separation voltage window for the activated carbon (AC) and ZIF-67Co(OH) F@Co 3 O 4 /CC200, electrodes is −1 to 0.1 V and 0-1.6 V, respectively shown in Fig. 8A.At the same time to determine the testing potential range of ASC, a series of cyclic voltammetry (CV) experiments were conducted within the potential range of 1.2 V, 1.3 V, 1.4 V, 1.5 V and 1.6 V with a constant scan rate of 50 mV s −1 as shown in Fig. 8B.The electrode materials can function in the potential window of 0 V to 1.6 V.As a result, potential range of 0 V-1.6 V was chosen.The CV curves are highly overlapped at all potential window conrming high reversibility at all condition.Fig. 8C display the CV curves of ASC device at different scan rates, ranging from 10 mV s −1 to 100 mV s −1 .The peaks area of CV curves increases with increase the scan rate and the redox peaks are almost locate at similar potential.The galvanostatic charge-discharge (GCD) curves is shows in Fig. 8D.According to the GCD curve calculations, the ZIF-67Co(OH)F@Co 3 O 4 /CC200//AC ASC device shows the specic capacitance vary throughout current densities, 80 F g −1 , 75 F g −1 , 68 F g −1 ,62 F g −1 and 58 F g −1 at 1 A g −1 , 2 A g −1 , 3 A g −1 , 5 A g −1 , and 10 A g −1 respectively.The specic energy density and specic power density is shows in Fig. 8E and F, respectively.The electrode displayed maximum energy density of 28 W h kg −1 at a power density of 720 W kg −1 .The energy density still remined of 18 W h kg −1 at a power density of 4800 W kg −1 .Fig. 7G shows ASC device's cyclic stability and specic retention over 5000 cycles of GCD testing at 10 A g −1 .Aer 5000 GCD cycles, revealing the excellent cycling stability of ASC device.The charging-discharging curves of the device at last 10 cycles (inset of Fig. 8G) are similar to those of the rst 10 cycles, which indicates excellent cycling features the device maintained 85% of its capacitance, demonstrating good stability it is due to its high surface area tremella like structure.
We compare this work with the previous literature work it prove that this work has excellent cyclic stability which is shows in Table 1.

Conclusion
In this work ZIF-67Co(OH)F@Co 3 O 4 , metal organic framework was synthesized on carbon cloth by in situ technique followed by hydrothermal method using the template ZIF-67 precursor.The ZIF-67Co(OH)F@Co 3 O 4 /CC electrode show a satisfactory specic capacitance of 442 F g −1 at 1 A g −1 and superior capability with 90% retention with 10 A g −1 aer 5000 charges discharge cycles.These advantages are due to its high surface area of tremella like structure and the synergetic interaction between ZIF-67Co(OH)F@Co 3 O 4 and carbon cloth.The factor enhances the electro conductive and electroactive sites for faradaic redox reaction.
W 0 (Warburg element) is used as a diffusion resistance to reach the diffusion of the electrolyte within the electrode.Based on tting results.ZIF-67 and Co(OH)F has a relatively less conductivity in its native form.Aer the incorporation of Co(OH) F@CO 3 O 4 the ZIF-67 Co(OH)F@CO 3 O 4 was characterized with lower values of R s and R ct (R s = 0.55 U and R ct = 0.104 U) than the pristine ZIF-67 and Co(OH)F electrodes (R s = 1.38 U and R ct = 1.04 U) and (R s = 0.9 U and R ct = 0.19 U) respectively.The ZIF-67 Co(OH)F@CO 3 O 4 200 electrode was characterized with a lower charge transfer resistance in the redox electrolyte the resistance of the electrode is quite small due to its large surface area tremella-

1 specic capacitance with 94% stability. 35 Wang et al. developed aluminum doped cobalt hydroxide uoride nano sheets which show the maximum specic capacitance of 1576
37r example, Zhang et al. synthesized Co(OH)F nano rods by hydrothermal method they examined the electrochemical performance for super capacitor which give outstanding value of 1265 mF cm −2 . 34et al.F g −1 at 1 A g −1 .36Johnand it group synthesized cobalt uoride hydroxide hybrid super capacitor which possess 389 C g −1 specic capacitance.37Meiet al.

Table 1
Shows the comparison of specific capacitance of the ZIF-67-Co(OH)F@Co 3 O 4 nanocomposite with related electrodes materials Research at Imam Mohammad Ibn Saud Islamic University (IMSIU), Saudi Arabia and Deanship of Scientic Research at King Khalid University, Saudi Arabia.