Co₃O₄/ZnO nanoheterostructure derived from core–shell ZIF-8@ZIF-67 for supercapacitors

Abstract

In this work, we developed a novel and facile method to fabricate a Co₃O₄/ZnO nanoheterostructure via a solid–solid conversion process using core–shell MOFs@MOFs as a template and source of cobalt and zinc. Herein, we set out to prepare core–shell structured ZIF-8@ZIF-67, which consists of ZIF-8 ([Zn(MeIm)₂]_n) (MeIm = 2-methylimidazole) as the core and ZIF-67 ([Co(MeIm)₂]_n) as the shell, through a seed-mediated growth method. Co₃O₄/ZnO nanoheterostructure was prepared through the thermal conversion of core–shell ZIF-8@ZIF-67. This hierarchical Co₃O₄/ZnO exhibits a high specific capacitance of 415 F g⁻¹ at 0.5 A g⁻¹ and an improved rate capability (93.2% retention at 10 A g⁻¹). Asymmetric supercapacitors were fabricated using the Co₃O₄/ZnO heterostructure as the positive electrode and active carbon as negative electrode. The operation voltage is expanded to 1.4 V in aqueous electrolyte, revealing an energy density of 43.2 W h kg⁻¹ at a power density of 1401 W kg⁻¹. This MOF-driven strategy can be expanded to the preparation of other metal oxide nanoheterostructures by varying the type of MOFs, and may find potential application in high-performance supercapacitors.

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1. Introduction

Electrochemical supercapacitors have aroused widespread attention compared with traditional capacitors due to their fast charge–discharge characteristics, high power density, and excellent cycling stability.^1–5 With high power capability at relatively high energy densities, exceptional cycle life and reliability, supercapacitors have been used in a variety of applications ranging from hybrid electric vehicles, memory back-up, military and space equipment, to wearable electronics, paper like electronics and flexible biomedical devices.

As we all know, the electrode material is one of the key factors that influences the performance of supercapacitors. In recently years, many researchers have mainly focused on porous carbon materials, conductive polymers and metal oxides.^6–12 Among these materials mentioned above, metal oxides are used as electrode materials in capacitors because they can exhibit their own redox reaction in order to obtain a pseudocapacitor, which has a higher capacitance than carbon materials. Based on the characteristics of Faraday pseudocapacitance, RuO₂ is considered to be an ideal electrode material for electrochemical capacitors, and has been paid much attention by researchers. However, the expensive price of ruthenium limits its application.^13,14 Other transition-metal oxides possess remarkable theoretical specific capacitance and are low cost, such as MnO₂, NiO and Co₃O₄, but suffer from poor conductivity.^15–19 Thus, it is imperative to develop high conductivity and readily-prepared electrodes for supercapacitors with high specific capacitance and cyclic durability.

Recently, binary metal oxides have exhibited excellent electrochemical performance as they provide multiple redox reactions and have high electrical conductivity.^20–23 The interfaces created after the partial reaction of two oxides exhibit new and interesting properties because of proximity and diffusion phenomena which become significant at the nanoscale range. As a result, they usually exhibit higher electrical conductivity than simple single metal oxides, owing to the relatively low activation energy or electron transfer between cations. Among various metal oxides, n-type ZnO is an extremely versatile workhorse for various applications due to its good electric conduction and chemical stability,²⁴ while Co₃O₄, a p-type system, is well-known for its high catalytic activity and high theoretical capacitance.^25,26 It is expected that the synergistic combination of these two materials could pave the way to enhanced electrochemical properties. However, the fabrication of well-defined Co₃O₄/ZnO p–n heterostructures via facile and simple methods still remains a big challenge. Most of the present experiments are time-consuming and costly. Therefore, it is significant to develop a simple and cost-effective method to synthesize oxide p–n heterostructures.

Metal–organic frameworks (MOFs) with high surface areas and well-defined pore structures have been demonstrated as promising precursors/templates to create novel porous metal oxide nanostructures.^27–37 By calcining the MOFs at a moderate temperature, the metal ions in the MOFs can be transformed into metal oxides, and the C and other elements (such as N and H) can be oxidized into gases. Thus, porous oxides with interconnected pores could readily be obtained because of the gas release during the calcination process. Synthesizing metal oxides from MOFs has obvious advantages compared with other methods because of the high surface area and unique structure of MOFs. In all the previous works, however, oxides are prepared from a single MOF precursor, and therefore the expected functionalities are very limited. Incorporating one type of MOF to other MOFs to form core–shell microcrystals, denoted as MOFs@MOFs, is one of the effective ways to create multifunctional hybrid MOF materials. It has been demonstrated that MOFs@MOFs structures even offer new characteristics as a result of the MOFs–MOFs interface. We believe that MOFs@MOFs derived hybrid materials can bring out novel chemical and physical properties and interfacial functionality that are not attainable from a single MOF precursor.

In this work, we developed a novel and facile method to fabricate a Co₃O₄/ZnO nanoheterostructure via a solid–solid conversion process using a core–shell structured MOFs as a template and source of cobalt and zinc. The core–shell structured MOF was selected as a precursor, because the facile solid-state diffusion and interfacial contact is enhanced during the process of calcination, which results in the formation of uniform composites at the nanoscale. Herein, we set out to prepare core–shell structured ZIF-8@ZIF-67, which consists of ZIF-8 ([Zn(MeIm)₂]_n) (MeIm = 2-methylimidazole) crystals as the core and ZIF-67 ([Co(MeIm)₂]_n) crystals as the shell, through a seed-mediated growth method. By thermal treatment of ZIF-8@ZIF-67 crystals, the Co₃O₄/ZnO nanoheterostructure is successfully prepared (Scheme 1). The electrochemical tests reveal that the as-prepared nanostructure provides excellent specific capacitance and cyclic stability at high current density due to its nanoheterostructure, demonstrating the potential to be an alternative electrode material in supercapacitors. This MOF-driven strategy can be expanded to the preparation of other binary metal oxides composites by varying the type of MOF used, and may find multiple potential applications, especially in supercapacitors and other energy-storage devices.

Scheme 1 Synthetic scheme for the preparation of Co₃O₄/ZnO heteronanostructures.

2. Experimental

2.1 Preparation of ZIF-67

Co(NO₃)₂·6H₂O (1.455 g, 0.5 mmol) was dissolved in a binary mixture of 40 ml methanol (MeOH) and 40 ml ethanol (EtOH). 2-Methylimidazole (1.642 g, 2 mmol) was dissolved in another mixture of 40 ml MeOH and 40 ml EtOH. The above two solutions were then mixed vigorously for 30 s, and the resulting solution was incubated at room temperature for 24 h. The resulting purple precipitates were collected by centrifugation, washed with ethanol several times and finally vacuum-dried at 80 °C.

2.2 Preparation of ZIF-8

A methanolic solution of zinc nitrate hexahydrate (810 mg, 40 ml) and a methanolic solution of MeIm (526 mg, 40 ml) were mixed under stirring. Then the mixture was transferred into an autoclave and was kept at 100 °C for 12 h. The white powder was collected by centrifugation, washed several times with methanol and dried at 80 °C.

2.3 Preparation of core–shell ZIF-8@ZIF-67 crystals

In a typical synthesis of core–shell ZIF-8@ZIF-67 crystals of micro-meter-size, ZIF-8 seeds (80 mg) were first well-dispersed in methanol (10 ml) under sonication for 30 min. After stirring for 20 min, a methanolic solution of cobalt chloride (177 mg, 20 ml) and a methanolic solution of MeIm (895 mg, 20 ml) were injected stepwise into the above mixture. After stirring for another 5 min, the mixture was transferred into an autoclave and kept at 100 °C for 12 h. During this time, the core–shell ZIF-8@ZIF-67 crystals were obtained. After cooling to room temperature, the resulting sample was collected by centrifugation, washed several times with methanol, and dried at 80 °C.

2.4 Preparation of the heterogeneous structure of Co₃O₄/ZnO

The heterogeneous structure of Co₃O₄/ZnO was prepared through a solid–solid conversion process using ZIF-8@ZIF-67 as the template. Firstly, ZIF-8@ZIF-67 was calcined under a N₂ flow at 500 °C for 1 h with a heating rate of 3 °C min⁻¹. After that, the obtained samples were then calcined under O₂ flow at 350 °C for 1 h with a heating rate of 3 °C min⁻¹ to obtain the annealed sample.

2.5 Preparation of Co₃O₄, ZnO and Co₃O₄/ZnO mixtures

Co₃O₄ was prepared through a solid–solid conversion process using ZIF-67 as template. Firstly, ZIF-67 was calcined under a N₂ flow at 500 °C for 1 h with a heating rate of 3 °C min⁻¹. After that, the obtained samples were then calcined under O₂ flow at 350 °C for 1 h with a heating rate of 3 °C min⁻¹ to obtain the annealed sample. ZnO was also prepared by the same experimental procedure using ZIF-8 as the template under O₂ flow at 500 °C. The mixtures of Co₃O₄/ZnO were prepared by grinding Co₃O₄ and ZnO in a ratio of 1 : 5 by mass. After that, the obtained samples were calcined under an O₂ flow at 350 °C for 1 h with a heating rate of 3 °C min⁻¹.

2.6 Characterization

The crystal phases of all samples were characterized using powder X-ray diffraction (PANalytical X’Pert Powder diffractometer) with CuKα radiation. The morphology and microstructure of the synthesized materials were characterized using a scanning electron microscope (SEM, S-4800) and a transmission electron microscope (TEM, Tecnai G² F20).

2.7 Electrochemical measurements

Electrochemical measurements were carried out using a three-electrode electrochemical system. A 6 M KOH aqueous solution was used as the electrolyte, a Pt plate served as the counter electrode and an Ag/AgCl electrode was employed as the reference electrode. The working electrode consisted of an active material, conductive graphite and PTFE with a mass ratio of 8 : 1 : 1. The mixture of pulp was pasted onto Ni foam and vacuum dried for 10 hours. Cyclic voltammetry and galvanostatic charge–discharge investigations were implemented using a CHI660E electrochemical workstation (ChenHua, Shanghai).

The specific capacitance was calculated from the galvanostatic charge–discharge curves using the following equation:

C = (I × Δt)/(m × ΔV) (1)

where I is the charge–discharge current at a discharge time Δt (s), ΔV is the dropout voltage, and m is the mass of the active electrode materials.

For the fabrication of asymmetric supercapacitors, the previous working electrode served as the positive electrode, and the active carbon as the negative electrode. The two electrodes and a separator were combined with 6 M KOH as the electrolyte to assemble the full cell. The mass ratio of the negative electrode to the positive electrode was decided based on charge balance theory (q⁺ = q⁻). The charge stored (q) by each electrode depends on the following equation:

q = C × ΔV × m (2)

The energy density E (W h kg⁻¹) was used the following equation:

E = (0.5CV²)/3.6 (3)

where C (F g⁻¹) is specific capacitance of capacitor and V is potential range.

The power density P (W kg⁻¹) was calculated by following equation:

P = 3600E/t (4)

where E (W h kg⁻¹) is energy density and t (s) is elapsed time during discharge period.

3. Results and discussion

3.1 Morphology and structure characterization

The morphology and microstructures of the core–shell ZIF-8@ZIF-67 crystal were characterized by XRD, TEM and elemental mapping as show in Fig. 1. The TEM images (Fig. 1a–d) indicated that the obtained ZIF-8@ZIF-67 has a well-defined rhombic dodecahedral shape. The average diameter of these crystals was about 1 μm. In the elemental mappings (Fig. 1e), the distribution of Co and Zn elemental mappings showed that the Zn was located at the interior of the rhombic dodecahedral and encapsulated by the Co-containing shell, confirming the core–shell-like spatial distribution. That is, the ZIF-67 shell was successfully uniformly coated on the ZIF-8 core. The results suggest the success of the preparation of the core–shell ZIF-8@ZIF-67. As shown in Fig. 1f, the XRD measurements of ZIF-8, ZIF-67 and core–shell ZIF-8@ZIF-67 crystal provided a better understanding of the core–shell crystal. The positions of the diffraction peaks of the prepared ZIF-8 and ZIF-67 crystals corresponded to the XRD patterns simulated from the single crystal structures of ZIF-8 and ZIF-67.^38,39 The core–shell ZIF-8@ZIF-67 crystals exhibit topological information identical to those of the ZIF-8 and ZIF-67 crystals.

Fig. 1 TEM images of the core–shell ZIF-8@ZIF-67 crystal (a–d), elemental mappings of the core–shell ZIF-8@ZIF-67 crystal (e), XRD patterns of ZIF-8, ZIF-67 and ZIF-8@ZIF-67 (f).

Fig. 2a–d shows the TEM images of the prepared Co₃O₄/ZnO heterostructures. The TEM images reveal the uniform distribution of the particles in the heterostructures, where both individual ZnO and Co₃O₄ have two different sizes. The HRTEM images (Fig. 2e and f) clearly exhibit the lattice fringes of the prepared materials, suggesting the highly crystalline nature of the samples. Moreover, a distinguished interface and continuity of lattice fringes between Co₃O₄ and ZnO can be observed, suggesting that a p–n heterojunction is formed between Co₃O₄ and ZnO. The size of the heterojunction is small with a size of about 2–3 nm which is beneficial for fast charge transfer. A measured neighbouring interlayer distance of 0.17 nm is consistent with the (4 2 2) planes of Co₃O₄, while a neighbouring interlayer distance of 0.14 nm agrees with the (2 0 0) planes of ZnO. Furthermore, the elemental mappings (Fig. 2g and h) and EDX line-scan (Fig. S1†) also reveal the distribution of the elements (Zn/Co) in the heterostructures. These results also reveal that the nanoparticles of Co₃O₄ and ZnO are well connected, which might provide the pathway to improve interfacial charge transfer.

The XRD pattern of the heterostructure is shown in Fig. 2i. The pattern presents two sets of peaks, which can be indexed to Co₃O₄ (JCPDS no. 42-1467)⁴⁰ and ZnO (JCPDS no. 36-1451)⁴¹ respectively. No peaks of impurities are observed in the XRD pattern, which indicates the formation of single-phase Co₃O₄/ZnO heterostructures in all cases.

Fig. 2 TEM images (a–d), HRTEM images (e and f), elemental mappings (g), energy-dispersive spectra (h), and XRD patterns (i) of the Co₃O₄/ZnO heterostructure.

3.2 Electrochemical performance of the Co₃O₄/ZnO heterostructure

The electrochemical performance of the Co₃O₄/ZnO heterostructure was evaluated in a three-electrode configuration with a 6 M KOH electrolyte. The representative cyclic voltammetry (CV) curves are shown in Fig. 3a with the scan rates varying from 10 to 100 mV s⁻¹. The curves show a pair of redox peaks and the peak current depends linearly on the sweep rate. This is due to the faradaic redox reaction manifesting as a pseudocapacitive feature of the electrode materials. The pseudo capacitance mainly comes from the faradaic redox reactions listed as follows:

Co₃O₄ + H₂O + OH⁻ ↔ 3CoOOH + e⁻ (5)

CoOOH + OH⁻ ↔ CoO₂ + H₂O + e⁻ (6)

Fig. 3 Electrochemical properties of the Co₃O₄/ZnO heterostructure. Cyclic voltammetry (CV) curves of at various scan rates (a), galvanostatic charge–discharge (GDC) curves at different current densities (b), different current density and specific capacitance (c), cycle performance at a current density of 10 A g⁻¹ (d).

The redox peaks in the CV curves are retained ideally even at high scanning rates, indicating rapid electronic and ionic transport process, which are beneficial to the charge–discharge performance at a high rate. A series of charge/discharge measurements were performed on the nanoheterostructure at various current densities. The charge/discharge curves for current densities from 0.5 to 10 A g⁻¹ are shown in Fig. 3b. The capacitance values are estimated to be 415, 370, 360, 324, 275 and 208 F g⁻¹, respectively, at current densities of 0.5, 1, 3, 7, 10 and 20 A g⁻¹. Cyclic stability measuements of the materials were carried out at a constant current density of 10 A g⁻¹, as displayed in Fig. 3d. After 1000 cycles, 93.2% of the initial capacitance was maintained. What’s more, Fig. 3d reveals that the curves of charge and discharge time are almost unchanged from the first eight cycles to the last eight cycles, suggesting high coulombic efficiency and low polarization of the electrode material.

For comparison, Co₃O₄ and ZnO were prepared from ZIF-67 and ZIF-8 by a similar thermal conversion method, respectively, and were investigated as electrodes. It was found that Co₃O₄ and ZnO have lower capacitance at high current densities (Fig. S2 and S3†), as compared with Co₃O₄/ZnO derived from ZIF-8@ZIF-67.

3.3 Electrochemical performance of asymmetric supercapacitors

In order to explore the electrochemical performance toward practical application of Co₃O₄/ZnO heterostructures, an asymmetric supercapacitor was fabricated by utilizing the as-synthesized Co₃O₄/ZnO heterostructure as the positive electrode and active carbon as the negative electrode in a 6 M KOH electrolyte. Fig. 4a shows the CV curves of the Co₃O₄/ZnO heterostructure and active carbon electrode performed in the three-electrode system. The active carbon electrode exhibits the typical characteristics of electric double-layer capacitance within the range of −1.0 to 0 V, while the working voltage of the Co₃O₄/ZnO heterostructure electrode varies from 0 to 0.4 V. Based on the obtained specific capacitance of the Co₃O₄/ZnO heterostructure and active carbon as well as the principle of charge balance between the positive and negative electrodes, the mass ratio of active carbon to Co₃O₄/ZnO heterostructure was controlled at about 3 in the asymmetric supercapacitors. Fig. 4b demonstrates the CV curves of the as-assembled asymmetric supercapacitors obtained at various scan rates from 10 to 100 mV s⁻¹. Taking full advantage of the different potential windows of the Co₃O₄/ZnO heterostructure and active carbon, as expected, the working voltage of the asymmetric supercapacitors thus can be extended to 1.4 V, indicating the potential of the assembled system in practical applications.

Fig. 4 Electrochemical measurements of the Co₃O₄/ZnO heterostructure//carbon asymmetric supercapacitors (ASCs). Cyclic voltammetry (CV) curves of the Co₃O₄/ZnO heterostructure and active carbon as working electrodes in a three-electrode system (a), cyclic voltammetry (CV) curves of the ASCs at various scan rates (b), galvanostatic charge–discharge curves of the ASCs at different current densities (c), energy and power densities of the ASCs (d).

The charge–discharge performance of the supercapacitors is also demonstrated by the galvanostatic charge–discharge (GCD) curves presented in Fig. 4c, which show nearly symmetric charge and discharge curves with no obvious internal voltage drops at different current densities. The specific capacitance was calculated from the GCD curves based on the total mass loading of the active material of the two electrodes. A high specific capacitance is achieved to be 158.6, 137, 114.3, 103 and 93 F g⁻¹ at current densities of 2, 4, 8, 16 and 20 A g⁻¹, respectively. The power density and energy density are generally used as important parameters to characterize the performance of asymmetric supercapacitor devices. Fig. 4d gives the Ragone plot of the fabricated asymmetric supercapacitors for energy and power densities. The asymmetric supercapacitors achieve a high energy density of up to 43.2 W h kg⁻¹ at a power density of 1401 W kg⁻¹ and a high energy density of 25 W h kg⁻¹ can be retained at a power density of 13 846 W kg⁻¹. The specific energy and specific power change with the applied current density are summarized in Table S1.†

For comparison, the Co₃O₄//carbon asymmetric supercapacitor was fabricated by utilizing the Co₃O₄ derived from ZIF-67 as the positive electrode and active carbon as the negative electrode in a 6 M KOH electrolyte. The Co₃O₄//carbon supercapacitor exhibited a lower capacitance (88.5 F g⁻¹ at 1 A g⁻¹) and specific energy (15.5 W h kg⁻¹ at power density of 1298 W kg⁻¹) than those for the Co₃O₄–ZnO heterostructure//carbon configurations. The specific energy and specific power change with the applied current density are summarized in Table S2.† The asymmetric supercapacitor was also fabricated by utilizing the Co₃O₄/ZnO mixtures as the positive electrode and active carbon as the negative electrode in a 6 M KOH electrolyte. The asymmetric supercapacitor exhibited a much lower capacitance and specific energy, as shown in Fig. 5. The specific energy and specific power changes with the applied current density are summarized in Table S3.†

Fig. 5 Electrochemical measurements of the Co₃O₄//carbon and Co₃O₄/ZnO mixture//carbon asymmetric supercapacitors (ASCs). Cyclic voltammetry (CV) curves of the Co₃O₄//carbon ASCs (a), and Co₃O₄/ZnO mixture//carbon ASCs (c) at various scan rates; galvanostatic charge–discharge curves of the Co₃O₄//carbon ASCs (b) and Co₃O₄/ZnO mixture//carbon ASCs (d) at different current densities; comparison of specific capacitance (e), and energy and power density (f) of three ASCs.

The excellent electrochemical performance of the Co₃O₄/ZnO nanoheterostructure is attributed to its hierarchical nano-architecture, in which the synergistic effect of Co₃O₄ and ZnO that shortens solid-state lengths for ionic diffusion and electronic transport. In our case, the facile solid-state diffusion and interfacial contact will be enhanced during the process of calcination because of the short distance from Zn to Co in the core–shell MOF structure. In addition, the uniformly arranged Zn and Co atoms in the ZIF-8@ZIF-67 crystal structure also favour lattice construction due to enhanced reaction kinetics. These advantages demonstrate that the core–shell MOF-templated strategy is an effective method for the synthesis of oxide heterostructures.

4. Conclusions

In summary, we have demonstrated a strategy to develop a novel and efficient MOF-templated nanoheterostructure electrode for supercapacitors. The Co₃O₄/ZnO nanoheterostructure is prepared through the thermal conversion of core–shell ZIF-8@ZIF-67. The electrochemical properties of the nanoheterostructure were investigated and the application of the material as an electrode for supercapacitors is demonstrated. It exhibits superior pseudocapacitive behaviour with high specific capacitance and good electrochemical stability, as compared with Co₃O₄ and ZnO derived from single MOF precursor. After being assembled as an asymmetric supercapacitor, a high energy density (43.2 W h kg⁻¹) was achieved at a high power density (1401 W kg⁻¹). This MOF-driven strategy can be expanded to the preparation of other metal oxide composites by varying the type of MOF, which may lead to multiple potential applications, especially for high-performance supercapacitors and other energy storage and conversion devices.

Acknowledgements

This work is supported by the National Natural Science Foundation of China (No. 21261006).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra07773k

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