Co 3 O 4 Electrode Prepared by Using Metal-Organic Framework as a Host for Supercapacitors

Co 3 O 4 nanoparticles were prepared from cobalt nitrate that was accommodated in the pores of a metal-organic framework (MOF) ZIF-8 (Zn(MeIM) 2 , MeIM= 2-methylimidazole) by using a simple liquid-phasemethod. Analysis by scanning electronmicroscopy (SEM) and transmission electronmicroscopy (TEM) showed that the obtained Co 3 O 4 was composed of separate nanoparticles with a mean size of 30 nm.The obtained Co 3 O 4 nanoparticles exhibited superior electrochemical property. Co 3 O 4 electrode exhibited a maximum specific capacitance of 189.1 F g at the specific current of 0.2 A g. Meanwhile, the Co 3 O 4 electrode possessed the high specific capacitance retention ratio at the current density ranging from 0.2 to 1.0 A g, thereby indicating that Co 3 O 4 electrode suited high-rate charge/discharge.


Introduction
Supercapacitor has gained great interests from many researchers with fast charging rate, high power density, and long service life, which is widely applied to portable apparatus, data memory storage system, electric vehicle power supply, and emergency back-up power supply [1][2][3].In particular, it has great advantage in the electric vehicle.The United States, Japan, Russia, and so forth have successively invested plenty of manpower and material to research and develop it.The supercapacitor electrode mainly includes three kinds of materials: carbon materials, transition metal oxides, and conductive polymers [4][5][6].Co 3 O 4 belongs to cheap metallic oxide in all transition metal oxides, which has been frequently used as the supercapacitor electrode for many years due to good electrochemical capacitance property thereof.However, Co 3 O 4 nanoparticles have sufficiently small particle size; thereby its activity is greatly enhanced as electrode material, and the specific capacitance is effectively increased [7][8][9].Currently, some methods of preparing the Co 3 O 4 nanoparticles have been reported in a lot of literatures, and many Co 3 O 4 nanoparticles with the small particle size have been introduced [10][11][12].However, it is urgent to prepare Co 3 O 4 nanoparticles with smaller particle size and better dispersion with continuous technology innovation and production demand [12].
Metal-organic frameworks (MOFs) have been widely noticed by people due to open pores, steady pore structures, excellent heat stability, and high ordered crystalline state as new porous materials.They become a new trend to the research fields, such as gas separation and preservation, sensors, drug delivery, and heterogeneous catalysis [13][14][15][16].Bhakta et al. demonstrated that MOFs could be employed as effective hosts for nanoscale metal hydride such as NaAlH 4 recently [17].Fischer et al. were the first to show that MOFs could be used as hosts for transition metal oxides nanoclusters (e.g., TiO 2 and ZnO) by gas-phase infiltration with organometallic precursors [18,19].The ZIF-8 framework (Zn(MeIM) 2 , MeIM = 2-methylimidazole), one of the representative MOFs, holds an intersecting three-dimensional channel system, a large pore size (11.6 Å in diameter), and high thermal stability (in N 2 ) as well as good chemical resistance to water and organic solvents [20][21][22][23], which may be an appropriate host for preparing Co 3 O 4 nanoparticles by using a solution-based method to introduce the cobalt precursor.
Herein, we report the example of adopting ZIF-8 as host for preparing Co 3 O 4 nanoparticles by a facile liquid-phase method.We show that the directness of cobalt nitrate accommodated in the pores of a ZIF-8 yields Co 3 O 4 nanoparticles after removing the ZIFs.For comparison, we simultaneously analyze the method for preparing Co 3 O 4 nanoparticles by using thermal decomposition via ZIF-67 and compare the structures and the electrochemical property.
ZIF-8 was prepared according to a previously published procedure [21].Then, the Co 3 O 4 nanoparticles were prepared by soaking cobalt nitrate hexahydrate into the ZIF-8 pore, including the following steps: weighting 0.5 g of ZIF-8, decentralizing them into 10 mL of absolute ethyl alcohol where 0.6 g Co(NO 3 ) 2 ⋅6H 2 O is dissolved, stirring for 2 hours by a magnetic stirrer at the ambient temperature, washing the prepared powder three or four times by ethyl alcohol and deionized water, heating up the prepared compound to 600 ∘ C at the heating rate of 2 ∘ C/min and keeping for 5 hours, decentralizing the prepared powder into NH The crystal structure of Co 3 O 4 was studied by X-ray diffraction (Rigaku TTR-III).The morphology was analyzed by using a scanning electron microscope (SEM, JSM-6700F) and a transmission electron microscope (TEM, JEOL JEM-1200EX).Thermal gravimetric analysis (TGA) measurements were performed on a Netzsch STA 409 thermoanalyzer.The BET surface area measurements were performed with N 2 adsorption/desorption isotherms at 77 K on a NOVA 2200e analyzer.

Electrochemical Performance Measurements.
The working electrodes were prepared by mixing 80 wt.% active substance (the prepared Co 3 O 4 ), 10 wt.% carbon black, and 10 wt.% polytetrafluoroethylene with N-methyl-2-pyrrolidone; subsequently, the mixture was pressed onto a treated nickel foam which served as a current collector under a pressure of 10 MPa and dried at 70 ∘ C for 12 h.All electrochemical measurements were performed by an electrochemical workstation in a three-electrode system, with Ag/AgCl as the reference electrode, the Pt electrode as counter electrode, and the Co 3 O 4 film electrode as the working electrode.The electrolyte used in this study was 1 M KOH solution.

Experimental Results and Discussion
3.1.Characterization of Precursors ZIF-67 and ZIF-8.Figure 1 shows the SEM images of precursors ZIF-8 and ZIF-67.SEM pictures reveal that the particles are nanocrystals in a polyhedral shape.The average diameter of ZIF-67 is about 90 nm.It can be clearly seen from the panoramic view (Figure 1(b)) that ZIF-8 sample contains uniform nanoparticles at 50 nm in diameter.The particle size of the ZIF-8 sample is much smaller than that of ZIF-67.Figure 2 gives the TGA curves in air of the precursors.As shown in Figure 2(a), the weight loss of ZIF-67 begins at about 250 ∘ C, indicating the onset of oxidation.While thermal decomposition of ZIF-8 begins at 350 ∘ C and there is a loss of 58% by weight, the thermal stability of ZIF-67 was slightly lower than that of ZIF-8.This suggests that the metal-organic framework does not exist after calcining, and there is no evidence of residual ZnO after washing.The average crystallite size calculated from the (311) peak using the Debye-Scherrer equation was 96 nm and 32 nm for Co 3 O 4 -TH and Co 3 O 4 -ZIF, respectively.the increase of the scan rate, indicating that rapid reversible redox reaction occurred among the electrode materials.The following redox reactions are considered to be responsible for the pseudocapacitance within the potential scanning range:

Morphology. TEM is employed in
Figure 6 shows constant current charge/discharge curves of Co 3 O 4 -TH and Co 3 O 4 -ZIF in 1 M KOH electrolyte at current density of 0.2 A g −1 .A nonlinear variation of potential versus time is observed during charge and discharge, thereby indicating that a pseudocapacitance performance arises from the electrochemical redox reaction occurring at Co 3 O 4 electrode/electrolyte interface.The result is consistent with that from CV curves in Figure 5. Besides, the sudden potential drop, which represents the potential change because of the internal resistance, is observed [26].The specific capacitance of the electrode can be calculated according to the following: where  is the discharge current,  is the discharge time, Δ is the potential range during discharge, and  is the mass of active material in the electrode.Figure 7 shows   The specific capacitance of Co 3 O 4 -ZIF is higher than that of Co 3 O 4 -TH.It can be proved from results of TEM that the particle size of Co 3 O 4 -ZIF is about 30 nm, while that of Co 3 O 4 -TH is about 100 nm.We can see that the latter is much larger than the former in the particle size.At the same time, the Co 3 O 4 -TH nanoparticles are seriously aggregated, and the aggregation of particles may decrease the surface area (the BET surface areas were calculated to be 11.32 m 2 /g and 50.12 m 2 /g for Co 3 O 4 -TH and Co 3 O 4 -ZIF, resp.).Based on this point, the former can fully contact with the electrolyte to greatly improve the specific capacitance.We can see that Co 3 O 4 prepared by taking the ZIFs as the host can effectively control particle size and agglomeration and can effectively improve the specific capacitance.

Summary
The particle size of Co 3 O 4 -TH prepared through thermal decomposition of ZIF-67 as a precursor is relatively large, about 100 nm; meanwhile the Co 3 O 4 -TH particle shows agglomeration; the particle size of Co 3 O 4 -ZIF prepared by taking ZIF-8 as the host is generally small, about 30 nm, and the Co 3 O 4 -ZIF particle does not show agglomeration.From electrochemical property test results of two materials, the specific capacitance of Co 3 O 4 -ZIF is obviously greater than that of Co 3 O 4 -TH.The specific capacitance of the former reaches 189.1 F g −1 at the current density of 0.2 A g −1 .This suggests that the ZIF-8 framework may play an important role in preventing the migration and aggregation of cobalt precursor in the pores during the formation of Co 3 O 4 upon heating.The Co 3 O 4 can fully contact with the electrolyte, greatly improving the specific capacitance.

4
Cl (5 M)-NH 3 ⋅H 2 O (2.5 M) aqueous solution to remove ZnO decomposed by ZIF-8, centrifugally cleaning and collecting prepared black Co 3 O 4 , and then placing them into an oven at 100 ∘ C to dry for 12 hours (the prepared Co 3 O 4 nanoparticles were denoted as Co 3 O 4 -ZIF).

Figure 3 shows
XRD chromatograms of Co 3 O 4 -TH and Co 3 O 4 -ZIF.It is obvious that the positions of the characteristic peaks in the two products are consistent.The peaks in curves of Co 3 O 4 -TH and Co 3 O 4 -ZIF are assigned to Co 3 O 4 (JCPDS number 43-1003).No other peaks are observed in the XRD patterns.

Figure 4
in order to observe the morphology and microstructure of the Co 3 O 4 prepared by different methods.Figure4(a) clearly implies that Co 3 O 4 prepared through thermal decomposition of ZIF-67 is sphere-like product in large size.From Figure4(a), it was apparent that the nanoparticles are seriously aggregated.Meanwhile, the particles are sphere-like structures with an average size about 100 nm.Jiang et al. reported the preparation of Co 3 O 4 by converting cobalt oxide subunits in a Co-MOF (Co 3 (NDC) 3 , NDC = 2,6-naphthalene dicarboxylate) by pyrolysis in air.The prepared Co 3 O 4 was agglomerated with an average size around 250 nm[25].It is basically consistent with our experimental results.It can be clearly seen from the panoramic view (Figure4(b)) that the sample contains uniform and weak agglomerated Co 3 O 4 nanoparticles with 30 nm in diameter.We know that the particle size of the Co 3 O 4 -ZIF sample is much smaller than that of Co 3 O 4 -TH.This suggests that the ZIF-8 framework may play an important role in preventing the migration and aggregation of cobalt precursor in the pores during the formation of Co 3 O 4 upon heating.

Figure 3 :
Figure 3: XRD chromatograms of Co 3 O 4 samples prepared by two different methods.

Figure 7 :
Figure 7: Relationship between the specific capacitance and the current density of Co 3 O 4 -TH and Co 3 O 4 -ZIF.
[10]e specific capacitance of Co 3 O 4 -TH is 67.9 F g −1 , while that of Co 3 O 4 -ZIF reaches 189.1 F g −1 when the current density is 0.2 A g −1 .We can see that the latter is obviously greater than the former in the specific capacitance.This value is comparable with that obtained from Co 3 O 4 thin film (a maximum specific capacitance of 227 F g −1 )[10].The Co 3 O 4 -ZIF electrode has the greater specific capacitance retention rate (78.5%) when the current density is increased from 0.2 to 1.0 A g −1 ; similarly, its retention rate is higher than that (73.5%) of the Co 3 O 4 -TH electrode.The result indicates that the Co 3 O 4 -ZIF electrode is more suitable for fast discharging and charging.