Co₃O₄ nanocrystals derived from a zeolitic imidazolate framework on Ni foam as high-performance supercapacitor electrode material

Abstract

A binder-free method was adopted to obtain a Ni foam/Co₃O₄ electrode. The Co₃O₄ nanocrystals with an average size of 15–25 nm deliver a high specific capacitance of 1680 F g⁻¹ at a current density of 0.5 A g⁻¹ in 1.0 M KOH electrolyte. Of particular note, the electrode exhibits a high capacitance retention rate of 58.3% at a current density of 15 A g⁻¹ with excellent electrochemical stability (15.9% specific capacitance loss after cycling 1000 times) even at high current density (5 A g⁻¹), suggesting its promising application as a high-performance supercapacitor electrode material.

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1 Introduction

Till now, the electrode materials for supercapacitors have been conveniently grouped into three basic classifications: carbon materials, metal oxides and conducting polymers. Among these materials, metal oxides play an increasingly significant role owing to their different and complex oxidation states that make for efficient redox charge transfer. Cobalt oxide (Co₃O₄), as a typical transition metal oxide (TMO), has an extensively wide range of applications in lithium-ion batteries, supercapacitors, gas sensing, heterogeneous catalysts, magnetic materials and electrochromic devices due to its very intriguing electronic, catalytic, magnetic, electrochemical and optical properties.^1–7 What is most noteworthy is the high theoretical specific capacitance (∼3560 F g⁻¹), high redox activity, low-cost and environmentally-friendly characteristics, which make Co₃O₄ a most promising alternative for the most advanced RuO₂, whose toxicity and high cost heavily limit the large-scale use in supercapacitors and other electrochemical energy storage devices.⁸

There is no deny that lots of joint efforts have been made to improve the performance of Co₃O₄ in supercapacitor, which can be summarized in tuning the structure, size and shape of Co₃O₄ in the process of synthesis. As regards the structure and shape of Co₃O₄, abundant studies have been conducted to synthesize Co₃O₄ with various morphologies, such as needlelike,¹ twin-sphere,⁹ flowers,¹⁰ nanoflakes,¹¹ urchin-like hollow microsphere,^12,13 nanotubes,¹⁴ nanosheets¹⁵ and so on. Indeed, it is more attractive to build up unique porous structure and reduce the dimension of electroactive Co₃O₄ to nanoscale at the same time, which results in higher specific surfaces that are conducive to make the active materials accessed sufficiently by electrolyte ions. However, as is known to us all, the higher the specific surface is, the larger the surface energy is. Therefore, specific strategies should come forward to avoid the serious aggregation of nano-scale Co₃O₄ with high surface energy. Wu et al. have reported a facile strategy to synthesize the nanocomposite of Co₃O₄ nanoparticles with a diameter up to 10–30 nm anchored on conducting graphene homogeneously.¹⁶ Zhang et al. reported an interesting method to improve the uniform formation of Co₃O₄ hollow nanoparticles in electrospun carbon nanofibers.⁸ In order to gain unique porous structure and smaller Co₃O₄ nanoparticles without aggregation as supercapacitor electrode materials individually, more efforts must be made.

As one of the most representative MOFs (metal–organic frameworks), ZIF-67 (Co(mim)₂, mim = 2-methylimidazole) has been chosen as precursor to synthesize the cobalt/carbon composites,^17,18 the graphitized nanoporous carbon¹⁹ and the Co₃O₄ ^2,20,21 for supercapacitor electrode materials. Of particular note, Zhang et al. used the ZIF-67 as precursor to obtain the hollow Co₃O₄ without aggregations, which greatly enhanced the electrochemical performance.²⁰

What is more, the design of electrode also has great effects on the electrochemical performance. In our previous work,¹⁷ a binder-free method was adopted to obtain cobalt–carbon electrode, whose electrochemical performance is superior to that of the one pressed onto a foamed nickel.¹⁸ Similar results exist in the recent researches.²² The advantages of the binder-free method are as follows: (1) the absence of organic insulating binder can not only increase the conductivity but also simplify the electrode manufacture. What is more, the unevenness of active materials on Ni foam caused by the process of coating can be avoided. (2) Without the carbon that works as conductive additive, active materials that embedded in the carbon could be exposed to the electrolyte, which enhances the electroactive surface.²³

Herein, a typical binder-free method was adopted to grow ZIF-67 on Ni foam directly and Ni foam/Co₃O₄ electrode was obtained ultimately. Benefiting from the unique structure, composition of ZIF-67,²⁴ and the effective method of electrode fabrication, the Ni foam/Co₃O₄ electrode exhibits excellent electrochemical performance, which is dramatically enhanced by the unique structure and the well-controlled size of Co₃O₄ particles.

2 Experimental section

2.1 Pretreatment of Ni foam

The Ni foam (1 cm × 2 cm) was pretreated with 50 mL hydrochloric acid solution (1 M) containing 0.5 g polyvinylpyrrolidone (PVP) to remove possible oxide layer and enhance the affinity of the surface. After 30 min, the Ni foam was washed with deionized water and methanol in order to remove the remaining PVP.

2.2 Synthesis of Ni foam/ZIF-67

A solid mixture of 0.952 g of cobalt chloride hexahydrate (CoCl₂·6H₂O) and 2.088 g of sodium formate DL-hydrate (HCOONa·2H₂O) was dissolved in 20 mL methanol (denoted as solution A). Meanwhile, 2.627 g of 2-methylimidazole was dissolved in 20 mL methanol (denoted as solution B). After being stirred in ultrasonic cleaner for 10 min, solution B was poured into solution A, followed by being stirred for another 20 min. Preliminarily, the synthesis solution was obtained. In order to prevent the precipitation on the surface of the retreated Ni foam, it was vertically immersed in the synthesis solution (40 mL) in a 100 mL Teflon autoclave and heated in a drying oven at 120 °C for 4 h. The autoclave was then taken out of the oven and allowed to cool naturally to room temperature. After it was cooled, the Ni foam was washed with deionized water and dried in a freezer dryer for 12 h. Ni foam/ZIF-67 was obtained ultimately.

2.3 Synthesis of Ni foam/Co₃O₄ electrode

The obtained Ni foam/ZIF-67 was calcinated in a tube furnace. The sample was put into the furnace and afterward the furnace was heated to 500 °C with a heating rate of 1 °C min⁻¹. After reaching the targeted temperature, it was heated for 30 min and then cooled down to room temperature. Finally, Ni foam/Co₃O₄ electrode was obtained.

2.4 Material characterizations

Morphologies of the electrode material were investigated by a scanning electron microscopy (SEM, S-4800) and a transmission electron microscopy (TEM, JEOL JEM-1200EX). The sample was immersed in methanol to get the Co₃O₄ nanoparticles for TEM. X-ray photoelectron spectroscopy with Al Ka radiation (XPS, ESCALAB 250Xi*) was used to establish the valence states of electrode materials.

2.5 Electrochemical measurements

All electrochemical measurements were performed in an electrochemical workstation according to three-electrode system. A saturated calomel electrode (SCE) was used as the reference electrode, the Pt electrode as counter electrode, and the Ni foam/Co₃O₄ electrode as the working electrode. 1 M KOH solution was adopted as electrolyte in the study. Electrochemical impedance spectroscopy (EIS) was performed over a frequency range of 0.1–10 kHz.

3 Results and discussion

Specific mechanism of formation of Ni foam/ZIF-67 is illustrated in Fig. 1. Similar to the growth of ZIF-8 (Zn(mim)₂, mim = 2-methylimidazole) on α-alumina supports²⁵ and copper foil,²⁶ the ZIF-67 membrane was grown on Ni foam as well. According to the previous paper,²⁷ the surface terminal ligands of ZIF-67 crystals could be thought to have little deprotonation. Hence, the presence of excessive mim species in the synthesis mixture could supress the growth of ZIF-67 crystals due to the capping effect of neutral min species. In view of this, sodium formate was adopted to get ZIF-67 nuclei highly deprotonated, and these nuclei are with a high density of negative charges as a result. Besides, the surface of Ni foam consists of metallic nickel, nickel oxides and hydroxides, which have high PH value for the IEP (isoelectric point) greater than 7.0.^26,28 The state of charges of the ZIF-67 nuclei and the surface of Ni foam is beneficial to the formation of ZIF-67 film on Ni foam.²⁶ Fig. S1a† presents the X-ray diffraction (XRD) data collected for ZIF-67 membranes prepared for different reaction time, and the diffraction peaks of the prepared crystals that were prepared for 4 h are identical to the simulated crystal structure of the ZIF-67 crystals,²⁴ indicating the successful formation of ZIF-67 crystals with high crystallinity on Ni foam. Considering that the calcination temperature or time has great effect on the structure, size, morphology, and the texture of the sample,^{8,17,20,21,29} we prepare Ni foam/Co₃O₄ electrode under an optimized heating condition, where the temperature was 500 °C for 30 min with a heating rate of 1 °C min⁻¹.^20,21

Fig. 1 Formation mechanism of Ni foam/ZIF-67 membrane.

Recently, a consensus has been reached that the mass loading of active materials on current collector has great influence on the electrochemical performance of the electrode. For instance, the increment of the mass loading leads to the increased ion diffusion length, higher electrical resistance, and inadequate electrolyte access to active materials.^30–32 Especially, in our paper, the total mass loading of the Co₃O₄ active materials on Ni foam is about 0.35 mg cm⁻². Note that the focus of our paper is the novelty of electrode fabrication and the morphology and structure of active materials. Besides, lots of papers have researched the influence of the mass loading on electrochemical performance.^30,32–35 Hence, no pleonastic study on this aspect was conducted in our paper.

The surface morphology of the Ni foam/Co₃O₄ was characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The petal-like membrane of ZIF-67 can be seen in Fig. 2a and S3a.† The ZIF-67 membrane is well intergrown and no obvious defects can be observed. Due to the morphology of Ni foam, obvious “ravine” can be observed in Fig. 2b, which also demonstrates the successful formation of ZIF-67 crystals on Ni foam¹⁷ along with the results of XRD analysis. As shown in Fig. 2c and S3b,† the morphology of the Ni foam/Co₃O₄ is greatly changed in contrast to Ni foam/ZIF-67, wherein Co₃O₄ crystals are dispersed on Ni foam homogeneously. As a paper said,³⁶ the collapse of ZIF-67 structure will not occur until the temperature is higher than 500 °C, when the C–N stretching vibration vanishes. Hence, it is reasonable that the Co₃O₄ crystals display polyhedral shape inheriting from ZIF-67, which is shown in Fig. 2d. In addition to this, obvious macropores or mesopores can be observed between Co₃O₄ polyhedra in Fig. 2d. These macropores or mesopores can be ascribed to the shrink of ZIF-67 during the pyrolysis process. As is known to us all, the macropores can act as ion-buffering reservoirs,³⁷ while the mesoporous structure can greatly increase the electrode/electrolyte contact area and facilitate mass transport of the electrolytes within the electrodes.³⁸ What is more, the polyhedra are composed of Co₃O₄ nanocrystals with an average size of 15–25 nm (Fig. 2e). Indeed, during the pyrolysis process of the ZIF-67 crystals, H₂O, CO₂ and N_xO_y gas are released and the metal species migrate.³⁶ The release of these gases improves porosity and the migration of metal species leads to the formation of Co₃O₄ nanocrystals. The low-resolution TEM image (Fig. 2e) shows that the Co₃O₄ nanocrystals with average size of 15–25 nm are distributed homogeneously with little agglomeration, which at some extent displays the inner porous structure of Co₃O₄ polyhedra. What is more, the porous property of Co₃O₄ polyhedra can also be reflected by the contrast in TEM images. The selected area electron diffraction (SAED) pattern (inset of Fig. 2e) recorded from Co₃O₄ nanocrystals displays a series of concentric rings, indicating that the primary crystallites are randomly attached with each other.²⁹ As supplementary, the high-resolution TEM (HRTEM) image shows that these primary crystallites with clear lattice fringes are attached with each other in different orientations.²⁹

Fig. 2 Low-resolution SEM images of (a) Ni foam/ZIF-67 and (c) Ni foam/Co₃O₄; high-resolution SEM images of (b) Ni foam/ZIF-67 and (d) Ni foam/Co₃O₄; TEM image (e) and HRTEM image (f) of Co₃O₄. Inset of (e) shows SAED pattern of Co₃O₄ nanocrystals.

In order to verify the chemical composition and valence state of the obtained electrode material, X-ray photoelectron spectroscopy (XPS) analysis was carried out. The full-survey-scan spectrum in Fig. S1b† verifies the presence of Co, Ni, C, and O elements in the Ni foam/Co₃O₄ electrode. Furthermore, two major peaks with binding energies of 780.1 eV and 795.1 eV can be observed in the XPS peak for Co 2p, corresponding to Co 2p_3/2 and Co 2p_1/2, which are characteristic peaks for the Co₃O₄ phase²¹ (Fig. 3a). The two small and indistinctive peaks at 786 and 803.92 eV are typical Co²⁺ shakeup satellite peaks of Co₃O₄,³⁹ further indicating the main presence of Co₃O₄ phase. As shown in Fig. 3b, the XPS peak for Ni 2p consists of two main peaks with Ni 2p_3/2 centered at 854 eV and Ni 2p_1/2 at 873.3 eV. The peaks at 855.8 eV and 871.6 eV around the aforementioned main peaks have been attributed to the Ni²⁺ vacancy-induced Ni³⁺ ion or nickel hydroxides and oxyhydroxides.⁴⁰ Besides, two satellite peaks at around 861.8 eV and 879.7 eV are two shakeup type peaks of nickel at the high binding energy side of Ni 2p_3/2 and Ni 2p_1/2.⁴¹ Therefore, the substrate Ni foam may be slightly oxidized during the process of calcination, which is inevitable even though Ni foam is exposed to air at room temperature. But note that the uniform Co₃O₄ membrane can protect the Ni foam from further oxidization. Hence, the trivial oxidization hardly influences the electroconductibility of the current collector.³⁸ XPS peak for O 1s shows two types of contributions for oxygen species. One is the oxygen species mainly corresponding to Co₃O₄ (around 529.6 eV), and the other one manifests the possible water on or between the Co₃O₄ nanocrystals and Ni foam (around 531.3 eV assigned to hydroxyl groups).²¹ Deconvolution of C 1s spectrum of as-prepared electrode in Fig. 3d shows a peak at 284.83 eV attributed to C–O bonds, a peak at 285.3 eV assigned to C–C bonds, a peak at 285.7 eV attributed to C–N bonds, and a peak at 288.8 eV assigned to C=O bonds. In addition, a small peak at 290.7 eV can be assigned to π–π* electronic transitions.²¹

Fig. 3 XPS spectra for (a) Co 2p, (b) Ni 2p, (c) O 1s, and (d) C 1s.

From the above, the ZIF-67-derived synthesis of Co₃O₄ polyhedra on Ni foam is quite different from those traditional methods that are usually based on the hydrothermal growth or electrodeposition of Co₃O₄ on Ni foam or other substrates.^42–46 More importantly, the dimension of the obtained Co₃O₄ polyhedra on Ni foam is smaller than that obtained by pyrolysis of ZIF-67 powder.^20,21,47 What is more, macropores or mesopores between Co₃O₄ polyhedra and micropores in the Co₃O₄ polyhedra make up the hierarchical porous structure.

The cyclic voltammetry (CV) test for the obtained Ni foam/Co₃O₄ was conducted with the potential window from 0.1 to 0.4 V (vs. SCE). As shown in Fig. 4a, two pairs of redox peaks can be observed at a low scanning rate (1 mV s⁻¹), which correspond to the conversion between different cobalt oxidation states according to the following equations^8,9,29,48

Co₃O₄ + OH⁻ + H₂O ⇔ 3CoOOH + e⁻ (1)

CoOOH + OH⁻ ⇔ CoO₂ + H₂O + e⁻ (2)

Fig. 4 (a) CV curve of Ni foam/Co₃O₄ electrode at a scanning rate of 1 mV s⁻¹, (b) CV cures of Ni foam/Co₃O₄ electrode at different scanning rates, (c) galvanostatic discharge curves of the Ni foam/Co₃O₄ electrode at different current densities, (d) specific capacitance of the Ni foam/Co₃O₄ electrode at various current densities, (e) cycling performance of Ni foam/Co₃O₄ electrode (the inset show the charge–discharge curves), and (f) Nyquist plots of Ni foam/Co₃O₄ electrode before and after charge–discharge cycling.

All of the curves (Fig. 4b) show obvious pseudocapacitance features with a similar shape.⁹ In addition, peak currents increase with the scanning rate, which indicates good reversibility of the fast charge–discharge response of the materials.⁴⁸

The galvanostatic discharge test of Ni foam/Co₃O₄ between 0.1 and 0.4 V at different current densities was performed, as depicted in Fig. 4c. Hat-like shapes of discharge curves further verify the pseudocapacitance characteristic from faradic redox reaction, which is in good agreement with the CV curves (Fig. 4a and b). The specific capacitance of the Ni foam/Co₃O₄ electrode can be calculated to be 1680, 1567, 1433, 1233, and 980 F g⁻¹ at current densities of 0.5, 1, 2, 5 and 15 A g⁻¹, respectively. The specific capacitance and rate performance are superior to those of the reported Co₃O₄ electrode for supercapacitor (Table S1†). It is understandable that the small size (15–25 nm) of Co₃O₄ nanocrystals and porosity of Co₃O₄ polyhedra can reduce the inert surface that impedes the complete utilization of the active materials, which is conducive to the abundant faradic reactions for efficient energy storage and thus high specific capacitance.¹⁷ The relationship between specific capacitance and current density is exhibited in more detail in the Fig. 4d. The remarkable rate capacitance performance that retention rate at a current density of 15 A g⁻¹ is up to 58.3% can be attributed to its outstanding structure. The macropores or mesopores between Co₃O₄ polyhedra improve the ion permeability even at a high current density. Besides, the binder-free method can dramatically reduce the contact resistance between Co₃O₄ and Ni foam, which makes for the rapid electron transfer from Co₃O₄ nanocrystals to the substrate.

As another vital issue in practical use, the long-term cycling test over 1000 cycles for the Co₃O₄ nanocrystals was carried out at 5 A g⁻¹. As can be seen in Fig. 4e, the Ni foam/Co₃O₄ electrode exhibits good electrochemical stability, and the capacitance loss after 1000 cycles is only 15.9%. On one hand, the decay of the specific capacitance can be attributed to the peeling of some loose Co₃O₄ nanocrystals.¹⁷ On the other hand, such good cycling stability is mainly ascribed to uncompact and porous structure of Ni foam/Co₃O₄, which can not only result in high ion permeability, but also accommodate the possible volume change during cycling process.⁴⁹ Furthermore, the charge–discharge curves retain the same shape as those in the initial stage after 1000 cycles test (insets of Fig. 4e), indicating that there are no significant structural changes of the Co₃O₄ polyhedra. This is in accordance with the comparative SEM images of before-test and after-test Co₃O₄ polyhedra (Fig. S2, S3b and c†).

To further explore the electrochemical stability, electrochemical impedance spectroscopy (EIS) measurements were conducted before and after the cycling test. As depicted in the inset of Fig. 4f, the diameter of the arc in high frequency region enlarged little and the slope of straight line in the low frequency region hardly changed after 1000 cycles, which indicates that there is little structural deformation after cycling,⁴⁹ and suggests no newly increased clogging for the diffusion of ions. So it results in good cycle stability.

The abovementioned results exhibit that the Ni foam/Co₃O₄ electrode is suitable as an electrode for supercapacitors. As regards the reasons for such excellent energy storage performance, the Ni foam/Co₃O₄ electrode owns the following advantages: (1) a binder-free method was adopted, which increases the conductivity of the electrode, simplifies the electrode fabrication, avoids the unevenness of active materials on current collector and enhances the electroactive surface; (2) the good chemical and mechanical stability and the strong bonding between directly grown active materials and Ni foam improve the long-time cycling stability; (3) high specific surface area results in the sufficient electrode–electrolyte interface; (4) macropores or mesopores between Co₃O₄ polyhedra and micropores in the Co₃O₄ polyhedra make up the hierarchical porous structure, where macropores or mesopores serve as ion-buffering reservoirs and ion-highway for fast ion transmission and micropores serve as charge accommodation. Hence, the diffusion pathways are cut down and thus the fast ion diffusion is ensured.³⁷

4 Conclusions

In summary, a binder-free method was adopted to synthesize the Ni foam/ZIF-67, then Ni foam/Co₃O₄ electrode was obtained by calcining in air at the temperature of 500 °C. The as-prepared Ni foam/Co₃O₄ electrode exhibits excellent electrochemical performance due to the controlled-size (15–25 nm) and the hierarchical porous structure of Co₃O₄ polyhedra, which tightly adhere to Ni foam substrate. Furthermore, the Co₃O₄ polyhedra can also be applied in lithium-ion batteries, magnetic materials, heterogeneous catalysts, electrochromic devices and so on.

Acknowledgements

This work was supported by the Central Scientific Research Expenses (2014QNA14) and Natural Science Foundation of Jiangsu Province (20141134).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra11272b

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