Porous Co₃O₄ nanoparticles derived from a Co(II)-cyclohexanehexacarboxylate metal–organic framework and used in a supercapacitor with good cycling stability

Abstract

Porous cobalt oxide materials have received great interest in recent years due to their potential application to supercapacitors. In this work, porous Co₃O₄ nanoparticles were prepared by the thermolysis of a Co(II)-based [Co₃(cis-chhc)(H₂O)₆]_n (cis-H₆chhc = 1,2,3,4,5,6-cyclohexane-hexacarboxylic acid) metal–organic framework. Cyclic voltammetry and galvanostatic charge–discharge measurements were carried out to characterize the electrochemical performance of Co₃O₄ in a 2.0 M KOH electrolyte. The results demonstrate that the as-synthesized Co₃O₄ can be applied as a supercapacitor electrode. The porous Co₃O₄ electrode exhibits a specific capacitance of 148 F g⁻¹ at a current density of 1 A g⁻¹, and displays slightly enhanced capacitance after 2500 continuous charge–discharge cycles. This remarkable electrochemical performance suggests that the as-prepared porous Co₃O₄ material is potentially promising for applications in energy storage.

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Introduction

In order to meet the growing demand for renewable and sustainable energy, portable electronics and hybrid electric vehicles, special attention has been focused on highly efficient, stable, low cost, and environmentally friendly conversion devices and energy storage systems.^1–3 Supercapacitors, also called electrochemical capacitors or ultracapacitors on account of their extraordinarily high capacitance density, are a new class of energy storage device that have aroused great interest due to their fast recharge capability, excellent temperature characteristics, high power density and energy conversion, long cycle lifetime and environmental friendliness.^4–6 They can be categorized as electric double-layer capacitors (EDLCs) and pseudocapacitors, which are distinguished by their energy storage mechanisms.^7,8 EDLCs use carbon materials (such as active carbon, graphene, carbon nanotubes, etc.) with high surface areas and porosity as electrode materials, which show a non-faradic reaction and store the energy by forming an electrode/electrolyte interface double layer through electrostatic interactions. However, as the charge is restricted to the surface, the electrochemical performance of carbon electrodes usually exhibits lower energy density. Pseudocapacitors generate charge through surface redox processes on the electrode material. The electrochemically active materials of a pseudocapacitor are composed of transition metal oxides, hydroxides and their composites etc., and they can provide higher capacitance values than EDLC.^9–11 Therefore, the design and synthesis of efficient and high performance pseudocapacitive materials for electrical energy storage is receiving more and more attention in recent years.

Metal–organic frameworks (MOFs) are a class of crystalline porous materials with a periodic network structure, which are self-assembled through the connection of inorganic metal centers (metal ions or metal clusters) and bridging organic ligands. They have attracted considerable attention in clean energy applications because of their high surface area and porosity, controllable structure and well-defined pore size.^12–15 Recently, MOFs have been utilized as precursors to generate porous metal oxide (such as MnO₂, Co₃O₄, Fe₂O₃, CuO, NiO and V₂O₅) nanostructure materials via controlled heating in different environments.^16–22 Among the various metal oxides, porous Co₃O₄ nanostructures are low cost and demonstrate high theoretical specific capacitance, defined electrochemical redox activity, stable cycling performance and environmental friendliness. They are thus deemed to be ideal electrode materials for electrochemical capacitor applications.^23–31 Therefore, great efforts have been devoted to the size- and shape-controlled synthesis of MOFs and micro- and nano-scaled MOFs. For example, Meng et al. used direct thermolysis to prepare porous cobalt oxide nanomaterials, using a metal–organic framework [Co₃(abtc)₃(bpy)_1.5(H₂O)₃]·2H₂O as a precursor. The electrochemical results showed that the porous Co₃O₄ particles had a high specific capacitance of 150 F g⁻¹ at a current density of 1 A g⁻¹ and excellent cycling stability.³² Huang's group utilized the popular zeolitic imidazolate framework ZIF-67 as the precursor for preparing hollow rhombic dodecahedral Co₃O₄, which displayed good supercapacitive performance.³³ Porous microflowers and microspheres of Co₃O₄ have been synthesized via a similar annealing process using different Co-MOFs, and the as-prepared cobalt oxides also exhibited high specific capacitance and long term cycling stability.^34,35 However, research on the preparation of porous Co₃O₄ derived from Co-based MOFs as electrode material for pseudocapacitors is still in the very early stages. In order to better understand the formation mechanism of porous cobalt oxides, it is quite necessary to find a new type of MOF material with a unique structure to use as a precursor in the synthesis of nanostructured Co₃O₄.

Owing to their multiple binding sites and pH-dependent versatile coordination modes, coordination polymers based on 1,2,3,4,5,6-cyclohexanehexacarboxylic acid (cis-H₆chhc) have been studied extensively.^36–41 Great effort has been devoted to the design and preparation of highly connected MOFs due to their potential advantages in enhancing the stability and porosity of the frameworks. In the present work, we synthesized a 3D structured MOF [Co₃(cis-chhc)(H₂O)₆]_n through a hydrothermal method. Using this MOF as a precursor, we demonstrate a facile and scalable calcination procedure to prepare porous Co₃O₄ nanoparticles. The as-synthesized Co₃O₄ material can be employed as an electrode material for supercapacitors, which exhibit a specific capacitance of 148 F g⁻¹ at a current density of 1 A g⁻¹ and excellent long term cycling stability with slightly enhanced capacitance after 2500 cycles.

Experimental

Materials and physical methods

All chemicals of reagent grade were commercially available and used without further purification.

Synthesis of [Co₃(cis-chhc)(H₂O)₆]_n microcrystals

For the synthesis of the microcrystals, 238 mg (1.0 mmol) CoCl₂·6H₂O, 174 mg (0.5 mmol) 1,2,3,4,5,6-cyclohexanehexacarboxylic acid (cis-H₆chhc), 100 mg (0.5 mmol) lauric acid and 1.0 mL cyclohexanol were dissolved in 10 mL H₂O. This was followed by the dropwise addition of 2.0 mL (1 M) NaOH, which produced a pink emulsion. The resulting solution was then sonicated for 10 min. Subsequently, the solution was transferred into a 23 mL Teflon-lined stainless steel autoclave. The autoclave was sealed and maintained at 180 °C for 24 hours and then cooled to room temperature quickly. The obtained purple microcrystals were separated by centrifugation and collected after washing with deionized water and ethanol several times, and dried under vacuum at 50 °C for 10 hours.

Synthesis of porous Co₃O₄ material

The porous Co₃O₄ material was prepared via a one-step calcination of the as-synthesized [Co₃(cis-chhc)(H₂O)₆]_n microcrystals in a furnace under a flow of air. The temperature was raised from room temperature to 380 °C at a ramp rate of 1 °C min⁻¹, and was then maintained at 380 °C for 4 h. Finally, the product was taken out and it was found that the color had changed from purple to black.

Characterization

The morphology and structure of the samples were characterized using scanning electron microscopy (SEM Hitachi S-4800) with an accelerating voltage of 4 kV. Transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), and corresponding selected area electron diffraction (SAED) observations were carried out on a JEOL 2100 transmission electron microscope operated at 200 kV. Powder X-ray diffraction measurements of the samples were collected using a Bruker D8 Advance X-ray diffractometer with Ni filtered Cu K_α radiation (λ = 1.5406 Å) at a voltage of 40 kV and a current of 40 mA. Thermogravimetric measurements were carried out on a Seiko Exstar 6000 TG/DTA 6300 apparatus with a heating rate of 10 °C min⁻¹. Nitrogen adsorption–desorption measurements were performed on a Micrometrics ASAP-2020M adsorption apparatus. The pore size distribution was obtained using the Barrett–Joyner–Halenda (BJH) method. Before the test, the sample was degassed under vacuum at 200 °C for 5 h.

Electrochemical performance measurements

All electrochemical measurements were performed in a three-electrode cell system on an electrochemical workstation (CHI660E, Shanghai, China). The working electrode was composed of 80 wt% active material (Co₃O₄ particles), 10 wt% polyvinylidone fluoride (PVDF) and 10 wt% acetylene black. After adding a few drops of ethanol to the above mixture, a black slurry was obtained following continuous grinding in an agate mortar. The resulting slurry was coated onto nickel foam current collectors (1.0 cm × 1.0 cm), pressed at 10.0 MPa, and dried at 50 °C for 24 h. The Co₃O₄ powder electrode was used as the working electrode with a 10 mg loading of sample. An Ag/AgCl electrode and a platinum foil electrode were used as the reference electrode and counter electrode, respectively. All electrode experiments were performed in 2.0 M KOH aqueous solution electrolyte. Cyclic voltammograms (CV curves) were obtained within the potential range from 0 to 0.50 V versus the Ag/AgCl electrode at different sweep rates ranging from 5 to 50 mV s⁻¹. Galvanostatic charge–discharge tests were carried out at different current densities in the limited voltage range of 0–0.50 V.

Results and discussion

Previous reports show that the hydrothermal reaction of CoCl₂·6H₂O, all-cis-1,2,3,4,5,6-cyclohexanehexacarboxylic acid (cis-H₆chhc), NaOH and 2,2′-bipyridyl in a molar ratio of 3 : 1 : 3 : 2 at 175 °C for 60 h produces bright purple bulk crystals of single phase.³⁷ In the present work, the microcrystals of [Co₃(cis-chhc)(H₂O)₆]_n were synthesized by mixing CoCl₂·6H₂O and cis-H₆chhc in the presence of cyclohexanol and lauric acid.

We chose [Co₃(cis-chhc)(H₂O)₆]_n as the precursor, mainly due to its good thermal and chemical stability and fascinating three-dimensional framework. The crystal structure and magnetic properties of [Co₃(cis-chhc)(H₂O)₆]_n have been previously reported by Tong and coworkers.³⁷ The asymmetric unit consists of one crystallographically unique Co atom, one [cis-chhc]⁶⁻ ligand lying across a threefold axis, and two coordinated H₂O molecules. As shown in Fig. 1a, each Co(II) atom connects three [cis-chhc]⁶⁻ anions, and each ligand binds to nine Co atoms. The formed 3D MOF can be described as a 3,9-connected, 9-noted network with the Schläfli symbol of {4²·6}₃{4⁶·6²¹·8⁹}. The [cis-chhc]⁶⁻ ligands bridge Co(II) atoms to generate a large cage of hexagonal prismatic shape with dimensions of 12.4 × 7.6 × 7.6 ų (Fig. 1a). Normally, there are a lot of gases released from MOF precursors during the annealing process, leading to the generation of novel porous structures. The large crystallographic pores led to the production of porous materials after thermolysis, finally resulting in materials with better electrochemical performance than their non-porous precursors.⁵ From this point of view, the [Co₃(cis-chhc)(H₂O)₆]_n precursor is a promising candidate for preparing porous Co₃O₄ nanoparticles. The XRD patterns of the as-synthesized microcrystals match the simulated XRD pattern well, indicating the formation of pure phase [Co₃(cis-chhc)(H₂O)₆]_n (Fig. 1b).

Fig. 1 (a) Crystal structure of the 3D coordination network along the [001] direction. (b) Experimental and simulated XRD patterns for a microcrystal of the Co-MOF.

The thermal stability of the Co-based MOF was investigated using TG analysis under an air atmosphere from room temperature to 700 °C. The thermogravimetric curve (Fig. 2) shows that it decomposes in two steps. The compound is stable below 162 °C. The first weight loss of 17.1% over the 162–344 °C temperature range is in good agreement with the calculated value of 17.2% for the loss of six coordinated water molecules. The drastic weight loss of 44.8% during the second step between 344 °C to 360 °C agrees well with the value of 44.4% for the decomposition of the organic ligands. The weight of the black residue (Co₃O₄) after heating above 360 °C was 38.1% (calculated: 38.4%). It was noted that the residue exhibits no weight loss upon further heating to 700 °C, indicating that there are no undecomposed organic ligands left in the final product. Therefore, we adopted a one-step calcination process to pyrolyze [Co₃(cis-chhc)(H₂O)₆]_n under a flow of air. The temperature was raised from room temperature to 380 °C at a ramping rate of 1 °C min⁻¹, and then stabilized at 380 °C for 4 h. The XRD pattern of the resulting material is shown in Fig. 3. All the diffraction peaks of the sample can be assigned to the face-centered cubic (fcc) phase of Co₃O₄ (JCPDS card no. 43-1003). Based on the Scherrer equation (D = 0.89λ/B cos θ), the average crystallite size of Co₃O₄ using the (311) reflection peak calculated from the XRD data was estimated to be 15 nm.

Fig. 2 TG curve of the Co-MOF precursor under an air atmosphere.

Fig. 3 XRD pattern of the as-prepared Co₃O₄ nanoparticles.

SEM images of the [Co₃(cis-chhc)(H₂O)₆]_n microcrystals and the as-synthesized porous Co₃O₄ material are displayed in Fig. 4a. The lengths of the spindle-shaped Co-based MOF microcrystals are in the range of 10–15 μm, while their middle diameters are in the range of 2–4 μm. The images also reveal that the spindle morphology of the MOF is preserved after the one-step calcination process. Because the organic ligands in the MOF decompose and are liberated from the solid crystals during heating, the crystal surfaces were observed to be split into layers. However, the external spindle morphology remains (Fig. 4b). Furthermore, the magnified SEM image (Fig. 4c) reveals that the surfaces of the layers are composed of numerous nanoparticles. TEM, HRTEM and SAED were used to investigate the morphologies and structural features of the Co₃O₄ nanoparticles in greater detail. As shown in Fig. 4d, the TEM results indicate that porous Co₃O₄ is composed of large amounts of nanoparticles with sizes ranging from 10 to 20 nm, which is consistent with the XRD analysis. The lattice fringes observed using HRTEM (as seen in Fig. 4e) are oriented randomly in different directions, and the d-spacing values are approximately 0.47, 0.29 and 0.20 nm, corresponding to the (111), (220) and (400) planes of cubic (fcc) phase Co₃O₄, respectively. This is in good agreement with the XRD results. The SAED results further confirm the high crystallinity of the nanoparticles and the polycrystalline structure with random orientation (Fig. 4f).

Fig. 4 (a) SEM image of the Co-MOF precursor. (b and c) SEM and high-magnification SEM images of the Co₃O₄ porous material. (d and e) TEM and HRTEM images of the Co₃O₄ porous material. (f) SAED of the Co₃O₄ porous material.

To gain further insight into the porous features of the Co₃O₄ material, the specific surface area and the pore size distribution were investigated. Brunauer–Emmett–Teller (BET) measurements were performed to examine the specific surface area. N₂ adsorption–desorption isotherms of the as-prepared Co₃O₄ are shown in Fig. 5. It shows a typical type IV adsorption isotherm, owing to the strong interaction between the adsorbent surface and adsorbate with the rapid increase in the adsorption capacity at lower relative pressures. When the relative pressure is increased to 0.75–1.0P/P₀, an H₃-type hysteresis loop is observed, which is usually attributed to slit, crack, and wedge structure pores in the agglomerates (loose stacking of constituent nanoparticles).^42,43 This further evaluates the mesoporous structure of the Co₃O₄ product, which is consistent with the SEM observation. The BET surface area of the Co₃O₄ is 34.8 m² g⁻¹, which is close to that of most reported Co₃O₄ nanostructures.^27–35 The pore size distribution has a relatively wide peak centered at 15.8 nm. Such a unique porous Co₃O₄ structure with a moderate surface area and large pore size can not only provide fast electronic and ionic conducting channels but also relieve huge volume changes during the charge–discharge cycling process.

Fig. 5 Typical isothermal nitrogen adsorption–desorption curve of the porous Co₃O₄ at 77 K, and the pore size distribution curve (inset).

CV, galvanostatic charge–discharge and cycle life measurements were used to evaluate the electrochemical properties and quantify the specific capacitance of the as-synthesized Co₃O₄ nanoparticles. All tests were performed in 2.0 M KOH aqueous solution electrolyte using a three-electrode system. As shown in Fig. 6, the CV curves of the porous Co₃O₄ electrodes were measured within a potential window of 0–0.5 V at different scan rates of 5, 10, 20 and 50 mV s⁻¹. Two pairs of redox peaks were observed; these peaks can be ascribed to the redox process of Co₃O₄/CoOOH/CoO₂. The cathodic peaks at positive current density and the anodic peaks at negative current density in the CV curves represent the reduction and oxidation processes, respectively. The corresponding redox reactions can be expressed as follows:^44,45

Co₃O₄ + OH⁻ + H₂O → 3CoOOH + e⁻

CoOOH + OH⁻ → CoO₂ + H₂O + e⁻

with an increase in the scan rate, the cathodic peak potential becomes more negative and the anodic one more positive. The potential difference between the anodic and cathodic peaks increases due to the polarization of the electrode under a high scan rate, suggesting that a rapid reversible redox reaction occurs among the Co₃O₄ electrode materials.

Fig. 6 Cyclic voltammogram (CV) curves of the porous Co₃O₄ electrode at different scan rates.

To further evaluate the application potential of the as-prepared Co₃O₄ samples as electrodes for electrical capacitors, the galvanostatic charge–discharge behaviours at different current densities (0.5, 1, 2 and 3 A g⁻¹) in the potential window of 0–0.5 V were measured for specific capacitance evaluation. As shown in Fig. 7, the charge–discharge curves exhibit good symmetry, demonstrating the reversible redox process and excellent electrochemical capacitance behaviour. The specific capacitance of the electrode at different current densities can be calculated by using the following equation:^46,47

where I is the charge–discharge current at a discharge time (A), Δt is the discharge time (s), m refers to the mass of Co₃O₄ (g), and ΔV represents the potential window (V). According to the results, the specific capacitance values of the Co₃O₄ electrode calculated from the above equation are 157, 148, 136, 126, 124 and 108 F g⁻¹ at charge–discharge current densities of 0.5, 1, 2, 3, 5 and 10 A g⁻¹, respectively. The significant capacitance decrease can be attributed to the increase in the potential drop, resulting from electrode resistance and the relatively low utilization of the active Co₃O₄ material under higher discharge current densities.⁴⁸ When increasing the current densities from 0.5 to 10 A g⁻¹, 68.8% of the initial capacitance can be retained, indicating the excellent rate capability of the porous Co₃O₄ electrode.

Fig. 7 Galvanostatic charge–discharge of the porous Co₃O₄ electrode at different current densities.

Long-term cycling stability is another crucial parameter for supercapacitors, and can determine their application. The cycling stability of the Co₃O₄ electrode was evaluated using galvanostatic charge–discharge measurements at a current density of 1 A g⁻¹ for 2500 cycles (as shown in Fig. 8a). It was found that the specific capacitance of the electrode continuously increases during the first 1500 cycles, increasing from ca. 148 to 162 F g⁻¹, which might be due to an electrochemical activation process at the electrode–electrolyte interface. Afterwards, the obtained electrode shows almost no change and maintains its maximum value until the 2500 cycles. Fig. 8b and c show the initial and last 15 galvanostatic charge–discharge cycles of the electrode, respectively. It can also be seen that the charge–discharge process of the Co₃O₄ electrode is highly reversible during the cycling experiment. In recent years, many metal–organic frameworks have been applied as precursors for the synthesis of nanostructured Co₃O₄ used as electrode materials for supercapacitors (Table 1). However, as far as we know, the cycling stability of the as-prepared porous Co₃O₄ is superior to those previously reported nanostructures used as supercapacitor materials. The remarkable cycling performance of the Co₃O₄ electrode material may be due to the fact that the porous framework can facilitate electrolyte impregnation into the sample, promoting surface and near surface redox reactions. The layers aggregated by the Co₃O₄ nanoparticles may be favorable for the diffusion of the electrolyte and effectively buffer the volume variation during electrochemical reaction.

Fig. 8 (a) Cycle performance of the porous Co₃O₄ electrode at a current of 1 A g⁻¹ after 2500 cycles; (b and c) the initial and last 15 cycles of galvanostatic charge–discharge of the porous Co₃O₄ material at 1 A g⁻¹.

Table 1 A comparison of various Co₃O₄ nanoparticles derived from metal–organic frameworks and used as electrode materials for supercapacitors^a

Co3O4 material	MOF precursor	Electrolyte	Potential window (V)	SC (F g^−1)	Current density (A g^−1)	Cycling stability	Ref.
a mim = 2-methylimidazolate.b H2bdc = p-benzenedicarboxylic acid.c dhbdc = 2,5-dihydroxy-1,4-benzendicarboxylate.d H3btc = 1,3,5-benzenetricarboxylic acid.e phen = phenanthroline.f abtc = azobenzen-3,5,4′-tricarboxylate.g bpy = 4,4′-bipyridine.h na = nicotinate.i H3idc = imidazole-4,5-dicarboxylic acid.j H2pdc = pyridine-2,6-dicarboxylic acid.
Porous Co3O4 dodecahedrons	ZIF-67 ([Co(mim)2])^a	1 M KOH	1.60	101, 72, 62, 58, 45 and 43	2, 3, 4, 5, 7 and 10	89% after 2000 cycles at 5 A g^−1	25
Porous hollow Co3O4 dodecahedrons	ZIF-67 ([Co(mim)2])	3 M KOH	0.40	1100 and 437	1.25 and 12.5	Almost no change after 2000 cycles at 6.25 A g^−1	49
Co3O4 nano/micro superstructures	[Co(Hbdc)2]^b	6 M KOH	0.35	208, 194 and 102	1, 2 and 3	97% after 1000 cycles at 1 A g^−1	50
Co3O4 nanoparticles	MOF-74-Co ([Co2(dhbdc)(DMF)2]·(H2O)2)^c	6 M KOH	0.50	115	1	—	51
Hierarchical Co3O4 twin-spheres	[Co(CO3)0.5(OH)]·(H2O)0.11	6 M KOH	0.40	781, 754, 700, 670 and 611	0.5, 1, 2, 4 and 8	97% after 1000 cycles at 4 A g^−1	6
Nanoflake array assembled Co3O4 thin sheets	[CoC2O4]·(H2O)2	2 M KOH	0.40	1500, 1291, 993 and 828	1, 2, 5 and 10	99.3% over 2000 cycles at 5 A g^−1	24
Porous Co3O4 nanorods	Co–H3btc–phen composites^d^,^e	6 M KOH	1.0, 1.2	158, 147, 137, 130 and 123 for 1.0 V	0.1, 0.25, 0.5, 1.0 and 1.5	93.7% after 1000 cycles at 1 A g^−1 for 1.0 V	23
				163, 154, 143, 133 and 126 for 1.2 V		86.0% after 1000 cycles at 1 A g^−1 for 1.2 V
Porous Co3O4 nanoparticles	[Co3(abtc)3(bpy)1.5(H2O)3]·(H2O)2^f^,^g	2 M KOH	0.50	137, 126, 121 and 119	0.5, 1, 2 and 3	No significant loss after 3400 cycles at 1 A g^−1	27
Porous Co3O4 microflowers	[Co2(na)4(H2O)]^h	3 M KOH	0.45	240, 234, 225, 215 and 202	0.625, 1.25, 2.50, 3.75 and 6.25	96.3% after 2000 cycles at 3.75 A g^−1	29
Co3O4 nanoparticles	[Co(H2idc)2(H2O)2]·(DMF)2^i	6 M KOH	0.45	233, 170, 145, 108 and 97	0.2, 0.4, 1, 2 and 4	Grew a little larger during first 200 cycles, 89.8% after next 1300 cycles at 4 A g^−1	30
Co3O4 nanoparticles	[Co(H2pdc)(pdc)]·(H2O)3^j	6 M KOH	0.45	184, 144, 123, 84 and 68	0.2, 0.4, 1, 2 and 4	73.7% after 1500 cycles at 4 A g^−1	30
Porous Co3O4 microspindles	[Co3(cis-chhc)(H2O)6]	2 M KOH	0.50	157, 148, 136, 126, 124 and 108	0.5, 1, 2, 3, 5 and 10	Continuously increased during first 1500 cycles at 1 A g^−1, and had no change during the next 1000 cycles	Our work

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Conclusion

In summary, we have synthesized microcrystals of an interesting 3D cobalt(II) metal–organic framework of 1,2,3,4,5,6-cyclohexanehexacarboxylate under hydrothermal conditions. A Co₃O₄ nanostructure comprising uniform nanoparticles and porous layers has been prepared by the thermolysis of the above Co-based MOF. The resulting Co₃O₄ material, which exhibits a layered structure with agglomerate nanoparticles, moderate specific surface area and large pore sizes, demonstrates fast electronic and ionic conducting channels and good charge transport. The electrochemical results show that the porous Co₃O₄ nanoparticles display a specific capacitance of 148 F g⁻¹ at a current density of 1 A g⁻¹, and retain slightly enhanced capacitance after 2500 continuous charge–discharge cycles. The results show that the as-prepared Co₃O₄ nanoparticles might have potential applications in supercapacitors. Furthermore, this MOF-template synthetic strategy can be easily extended to the fabrication of other porous metal oxides by using other well-defined metal–organic framework structures.

Acknowledgements

This project was supported by the Non-metallic Mineral Engineering Research Center of Zhejiang Province (Grant no. ZD2015k05). Honest thanks are also extended to K. C. Wong Magna Fund of Ningbo University.

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