Co₃O₄ Electrode Prepared by Using Metal-Organic Framework as a Host for Supercapacitors

2.1. Synthesis

ZIF-67 (Co(MeIM)₂, MeIM = 2-methylimidazole) was prepared according to the reported procedures [24]. Co₃O₄ nanoparticles were synthesized by heating ZIF-67 at a temperature of 600°C for 5 hours with a heating rate of 2°C/min in air (the prepared Co₃O₄ nanoparticles were denoted as Co₃O₄-TH).

ZIF-8 was prepared according to a previously published procedure [21]. Then, the Co₃O₄ 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(NO₃)₂·6H₂O 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 NH₄Cl (5 M)-NH₃·H₂O (2.5 M) aqueous solution to remove ZnO decomposed by ZIF-8, centrifugally cleaning and collecting prepared black Co₃O₄, and then placing them into an oven at 100°C to dry for 12 hours (the prepared Co₃O₄ nanoparticles were denoted as Co₃O₄-ZIF).

The crystal structure of Co₃O₄ 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 N₂ adsorption/desorption isotherms at 77 K on a NOVA 2200e analyzer.

2.2. Electrochemical Performance Measurements

The working electrodes were prepared by mixing 80 wt.% active substance (the prepared Co₃O₄), 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 Co₃O₄ film electrode as the working electrode. The electrolyte used in this study was 1 M KOH solution.

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.

3.2. X-Ray Diffraction Analysis

Figure 3 shows XRD chromatograms of Co₃O₄-TH and Co₃O₄-ZIF. It is obvious that the positions of the characteristic peaks in the two products are consistent. The peaks in curves of Co₃O₄-TH and Co₃O₄-ZIF are assigned to Co₃O₄ (JCPDS number 43-1003). No other peaks are observed in the XRD patterns. 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 Co₃O₄-TH and Co₃O₄-ZIF, respectively.

3.3. Morphology

TEM is employed in Figure 4 in order to observe the morphology and microstructure of the Co₃O₄ prepared by different methods. Figure 4(a) clearly implies that Co₃O₄ prepared through thermal decomposition of ZIF-67 is sphere-like product in large size. From Figure 4(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 Co₃O₄ by converting cobalt oxide subunits in a Co-MOF (Co₃(NDC)₃, NDC = 2,6-naphthalene dicarboxylate) by pyrolysis in air. The prepared Co₃O₄ 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 (Figure 4(b)) that the sample contains uniform and weak agglomerated Co₃O₄ nanoparticles with 30 nm in diameter. We know that the particle size of the Co₃O₄-ZIF sample is much smaller than that of Co₃O₄-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 Co₃O₄ upon heating.

3.4. Electrochemical Capacitor Property

The specific capacitance of Co₃O₄-ZIF is higher than that of Co₃O₄-TH. It can be proved from results of TEM that the particle size of Co₃O₄-ZIF is about 30 nm, while that of Co₃O₄-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 Co₃O₄-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 m²/g and 50.12 m²/g for Co₃O₄-TH and Co₃O₄-ZIF, resp.). Based on this point, the former can fully contact with the electrolyte to greatly improve the specific capacitance. We can see that Co₃O₄ prepared by taking the ZIFs as the host can effectively control particle size and agglomeration and can effectively improve the specific capacitance.