Porous Thin-Wall Hollow Co3O4 Spheres for Supercapacitors with High Rate Capability

In this study, a zeolitic imidazolate framework-67 (ZIF-67) was prepared as a precursor using a facile hydrothermal method. After a calcination reaction in the air, the as-prepared precursor was converted to porous thin-wall hollow Co3O4 with its original frame structure almost preserved. The physical and chemical characterizations of the nanomaterial were analyzed systemically. The electrochemical tests indicate that the obtained Co3O4 possesses large specific capacitances of 988 and 925 F/g at 1 and 20 A/g accompanying an outstanding rate capability (a 93.6% capacitance retention) and retains 96.6% of the specific capacitance, even after 6000 continuous charge/discharge cycles. These excellent properties mark the Co3O4 a promising electrode material for high performance supercapacitors.


Introduction
Rapid technological development and accelerated natural resource consumption have largely increased the demand for efficient, environmentally-friendly, cost-effective, and safe energy storage devices [1][2][3][4]. In the last decade, supercapacitors-the new devices between conventional physical capacitors and lithium-ion batteries-have been extensively recognized as one of the most promising candidates for energy storage devices due to their high power density, long cycling lifespan, and fast charge/discharge process [5][6][7][8][9][10][11]. In general, supercapacitors can be divided into two categories according to their energy storage mechanism: One is the electric double-layer capacitor (EDLCs), which is mainly made of carbonaceous materials [12][13][14][15][16]; the other is the faradic redox reaction pseudocapacitor (PsCs), which usually utilizes transition metal oxides/hydroxides as electrode materials [17][18][19][20][21]. In particular, pseudocapacitors deliver much higher specific capacitance in comparison with electric double-layer capacitors, and receive considerable interest today [22,23].
Among various transition metal compounds, Co 3 O 4 occupies a crucial position due to its superior theoretical specific capacitance (3560 F/g), nontoxicity, and low cost [24][25][26]. Meanwhile, the hollow structure stands out because of its novel interior geometry and surface functionality; this can possibly provide a large surface area and extra active sites, thus dramatically boosting the electrochemical properties [27,28]. To date, a great deal of efforts have been devoted to synthesizing Co 3 O 4 with hollow morphologies such as hollow spheres [28], hollow nanocubes [29], and hollow cages [30]. Despite the great progress that has been made, the specific capacitance is still significantly below the theoretical value, especially at high current densities. Therefore, it is still a challenge to fully take advantage of this powerful hollow nanoscaled Co 3 O 4 to achieve a large specific capacitance and a good rate capability. 2 of 10 In this work, we successfully prepared porous thin-wall hollow Co 3 O 4 spheres from a zeolitic imidazolate framework-67 (ZIF-67) precursor through a simple and fast reaction. This nanostructure offers a large accessible surface area with numerous pathways. As a consequence, it exhibits a large specific capacitance of 988 F/g at 1 A/g with satisfactory cycling stability (96.6% retention after 6000 cycles). In particular, the specific capacitance of 925 F/g at even 20 A/g (slight decay of less than 7%) is extremely competitive among Co 3 O 4 -based electrode materials reported in literature. The pseudocapacitive performance manifests the great potential of the porous thin-wall hollow Co 3 O 4 spheres as electrode materials for applications in supercapacitors.

Sample Preparation
All reagents were of analytical grade and were used as received without any further purification. First, 0.437 g of Co(NO 3 ) 2 ·6H 2 O and 0.616 g of 2-methyl imidazole were briefly dissolved in respective 20 mL methanol solutions. Then, the latter solution was added dropwise into the prior one under vigorous stirring. After an ultrasonic bath for 20 min, the precipitates were collected by centrifugation and washed with methanol. Partial precipitates (0.02 g) were dispersed in a 15 mL methanol solution again. Subsequently, 0.175 g of Co(NO 3 ) 2 ·6H 2 O was added into the separated solution. The resulting mixture was transferred to a Teflon-lined stainless-steel autoclave and heated at 120°C for 1 h. The separated products were washed with methanol and dried at 60°C. Finally, the porous thin-wall hollow Co 3 O 4 spheres were obtained via heat treatment at 400 • C for 2 h.

Material Characterizations
The crystal structure of as-obtained Co 3 O 4 was recorded by X-ray powder diffraction (XRD, EQUINOX 1000) with Cu-Kα 1 radiation (λ = 1.5406 Å). Scanning electron microscopy (SEM, Hitachi SU8230) with energy-dispersive X-ray (EDX) spectrum was performed for characterizations of morphology and element composition. X-ray photoelectron spectroscopy (XPS) was collected on an ESCALAB 250Xi.

Electrochemical Measurements
The Co 3 O 4 , acetylene black, and polyvinylidene fluoride (PVDF) with a weight ratio of 80:15:5 were dispersed in n-methyl-2-pyrrolidone (NMP) solution and ground continuously for 10 min in a mortar to form a slurry. The working electrode was prepared by pressing the slurry on nickel foam (NF). The electrochemical measurements were carried out at room temperature in a typical three-electrode system (2M KOH as the electrolyte), in which a Pt net and Ag/AgCl were used as a counter electrode and a reference electrode, respectively. A cyclic voltammogram (CV) was conducted at various scan rates and galvanostatic charge/discharge (GCD) was tested at different current densities. The specific capacitance (C, F/g) based on CV is defined as: and the specific capacitance based on GCD is given by: where i(V)dV (V·A) is the integrated area of the CV curve, and m (g), v (V/s), ∆V (V), I (A/g), and t (s) are the mass of active material, scan rate, potential window, discharge current, and discharge time [31,32]. The electrochemical impedance spectrum (EIS) was determined over a frequency range from 100 mHz to 100 kHz with an AC perturbation of 5 mV at open circuit potential. The cycling performance was evaluated through repetitive GCD tests.

Results and Discussion
The morphology of as-prepared Co 3 O 4 was investigated by SEM. Figure 1 presents SEM images of the precursor (Figure 1a,b) and the calcined Co 3 O 4 (Figure 1c,d). Surprisingly, the Co 3 O 4 effectively inherited the morphology of the precursor (negligible size contraction and few broken pieces) and exhibits uniform spherical structures. Furthermore, the magnified image ( Figure 1d) shows that the spheres tend to interconnect with one another and clearly confirms the hollow nature, as well as the thin shell thickness of the Co 3 O 4 from the view of a broken piece. The diameter of the spheres lies between 500 and 600 nm. In addition, the shell is assembled from numerous nanoparticles, constructing a highly porous architecture. This structure results in more active sites on the surface and easier transportation of ions, leading to better electrochemical performance. The morphology of as-prepared Co3O4 was investigated by SEM. Figure 1 presents SEM images of the precursor (Figure 1a,b) and the calcined Co3O4 (Figure 1c,d). Surprisingly, the Co3O4 effectively inherited the morphology of the precursor (negligible size contraction and few broken pieces) and exhibits uniform spherical structures. Furthermore, the magnified image ( Figure 1d) shows that the spheres tend to interconnect with one another and clearly confirms the hollow nature, as well as the thin shell thickness of the Co3O4 from the view of a broken piece. The diameter of the spheres lies between 500 and 600 nm. In addition, the shell is assembled from numerous nanoparticles, constructing a highly porous architecture. This structure results in more active sites on the surface and easier transportation of ions, leading to better electrochemical performance.  [33]. The element compositions were confirmed by EDX, as depicted in Figure 2b. The peaks correspond to the elements Co and O, respectively. No other impurity element was observed. Both XRD and EDX characterizations reveal the high purity of the product.
The XPS was used to analyze the surface component and valence state of the as-obtained Co3O4. In the Co 2p core-level XPS spectrum seen in Figure 3a, the Co 2p was deconvoluted into two doublets. The peaks at 779.4 and 794.4 eV are related to Co 3+ , and the peaks at 780.4 and 795.7 eV are assigned to Co 2+ [34]. In the high-resolution XPS spectrum of O 1s seen in Figure 3b, the O 1s can be fit into three different peaks at binding energies of 529.9, 530,9, and 532 eV, which are referenced to the lattice oxygen, the OH − species absorbed onto surface, and the multiplicity of physical and chemical absorbed water near surface, respectively [35]. This analysis of XPS further verifies the successful synthesis of the Co3O4.  [33]. The element compositions were confirmed by EDX, as depicted in Figure 2b. The peaks correspond to the elements Co and O, respectively. No other impurity element was observed. Both XRD and EDX characterizations reveal the high purity of the product.
The XPS was used to analyze the surface component and valence state of the as-obtained Co 3 O 4 . In the Co 2p core-level XPS spectrum seen in Figure 3a, the Co 2p was deconvoluted into two doublets. The peaks at 779.4 and 794.4 eV are related to Co 3+ , and the peaks at 780.4 and 795.7 eV are assigned to Co 2+ [34]. In the high-resolution XPS spectrum of O 1s seen in Figure 3b, the O 1s can be fit into three different peaks at binding energies of 529.9, 530,9, and 532 eV, which are referenced to the lattice oxygen, the OH − species absorbed onto surface, and the multiplicity of physical and chemical absorbed water near surface, respectively [35]. This analysis of XPS further verifies the successful synthesis of the Co 3 O 4 . Appl. Sci. 2019, 9, x FOR PEER REVIEW 4 of 10  The electrochemical performance of as-formed Co3O4 as an electrode material for supercapacitors was evaluated systematically. Figure 4a shows CV curves of the Co3O4 at various scan rates within a potential window of 0-0.55 V. As expected, the CV curve shape is totally different from those of electric double-layer capacitors (which have an almost rectangular shape), which reveals that the capacitance characteristics of the Co3O4 are those of typical pseudocapacitive capacitance. Additionally, two distinct pairs of redox peaks are visible in the CV curves. The first redox couple is ascribed to the faradic redox reaction of Co 2+ /Co 3+ , expressed as Equation (3) [29,30,36]: and the second redox couple corresponds to the conversion of Co 3+ /Co 4+ , expressed as Equation (4) [29,30,36]: When the scan rate increases from 5 to 100 mV/s, the peak current increases enormously. Furthermore, the anodic and cathodic peaks shift to higher and to lower potentials, suggesting classical pseudocapacitive behavior. The CV curve of the Co3O4 at 100 mV/s retains a redox shape similar to the original shape at 5 mV/s. It implies that the Co3O4 possesses an excellent rate capability, which is further confirmed by the calculated specific capacitances.
On the basis of Equation (1), the specific capacitances were determined to be 948.9, 931.8, 920.2, 912.4, and 900.6 F/g at scan rates of 5, 10, 20, 50, and 100 mV/s respectively, as shown in Figure 4b.  The electrochemical performance of as-formed Co3O4 as an electrode material for supercapacitors was evaluated systematically. Figure 4a shows CV curves of the Co3O4 at various scan rates within a potential window of 0-0.55 V. As expected, the CV curve shape is totally different from those of electric double-layer capacitors (which have an almost rectangular shape), which reveals that the capacitance characteristics of the Co3O4 are those of typical pseudocapacitive capacitance. Additionally, two distinct pairs of redox peaks are visible in the CV curves. The first redox couple is ascribed to the faradic redox reaction of Co 2+ /Co 3+ , expressed as Equation (3) [29,30,36]: and the second redox couple corresponds to the conversion of Co 3+ /Co 4+ , expressed as Equation (4) [29,30,36]: When the scan rate increases from 5 to 100 mV/s, the peak current increases enormously. Furthermore, the anodic and cathodic peaks shift to higher and to lower potentials, suggesting classical pseudocapacitive behavior. The CV curve of the Co3O4 at 100 mV/s retains a redox shape similar to the original shape at 5 mV/s. It implies that the Co3O4 possesses an excellent rate capability, which is further confirmed by the calculated specific capacitances.
On the basis of Equation (1), the specific capacitances were determined to be 948.9, 931.8, 920.2, 912.4, and 900.6 F/g at scan rates of 5, 10, 20, 50, and 100 mV/s respectively, as shown in Figure 4b. The electrochemical performance of as-formed Co 3 O 4 as an electrode material for supercapacitors was evaluated systematically. Figure 4a shows CV curves of the Co 3 O 4 at various scan rates within a potential window of 0-0.55 V. As expected, the CV curve shape is totally different from those of electric double-layer capacitors (which have an almost rectangular shape), which reveals that the capacitance characteristics of the Co 3 O 4 are those of typical pseudocapacitive capacitance. Additionally, two distinct pairs of redox peaks are visible in the CV curves. The first redox couple is ascribed to the faradic redox reaction of Co 2+ /Co 3+ , expressed as Equation (3) [29,30,36]: and the second redox couple corresponds to the conversion of Co 3+ /Co 4+ , expressed as Equation (4) [29,30,36]: When the scan rate increases from 5 to 100 mV/s, the peak current increases enormously. Furthermore, the anodic and cathodic peaks shift to higher and to lower potentials, suggesting classical pseudocapacitive behavior. The CV curve of the Co 3 O 4 at 100 mV/s retains a redox shape similar to the original shape at 5 mV/s. It implies that the Co 3 O 4 possesses an excellent rate capability, which is further confirmed by the calculated specific capacitances.
penetrate into the inner surface [37]. Nevertheless, the inevitable decrease of specific capacitances is not drastic, which demonstrates an excellent rate capability (maintaining almost 95% capacitance from 5 to 100 mV/s). This phenomenon illustrates that the Co3O4 is able to act as a "reservoir" of ions [38,39] to accommodate OH − . The reservoir can guarantee the efficient proceeding of the faradic reactions at high scan rates (or high current densities) and in turn relieves the fading of the specific capacitances. Further evidence is provided by GCD measurement. The discharge curves of the Co3O4 measured at different current densities in a voltage window of 0-0.55 V are presented in Figure 5a. In comparison with the discharge curves of electric doublelayer capacitors (which are almost a straight line), two evident plateaus are well displayed. These coincide with the sequential redox reactions described by Equations (3) and (4), indicating the pseudocapacitive property of the Co3O4. According to Equation (2), specific capacitances of 988, 960, 947.9, 939.4, and 925 F/g can be delivered at current densities of 1, 2, 5, 10, and 20 A/g respectively, as depicted by Figure 5b. Due to the relatively insufficient active material involved in redox reactions under higher current densities, the boosting of the current densities results in the fading of specific capacitances. At a low current density, the OH − ions have adequate time to transfer at the interface between electrode and electrolyte than they do at a high current density [40]. It is worth noting that the rate performance of the Co3O4 is impressive (maintaining 925 F/g at 20 A/g, representing a 93.6% capacitance retention), which is significant for practical use of the material.  On the basis of Equation (1), the specific capacitances were determined to be 948.9, 931.8, 920.2, 912.4, and 900.6 F/g at scan rates of 5, 10, 20, 50, and 100 mV/s respectively, as shown in Figure 4b. The prominent specific capacitances most probably benefit from the porous hollow sphere nanostructure. The decrease in specific capacitances with an increasing scan rate can be understood in terms of the circuitous diffusion of OH − ions into the Co 3 O 4 . At a low scan rate, both the inner and outer surface of the material can be utilized for charge storage. At a high scan rate, the diffusion of OH − ions is more likely happen on the outer surface and only a small fraction of OH − ions can penetrate into the inner surface [37]. Nevertheless, the inevitable decrease of specific capacitances is not drastic, which demonstrates an excellent rate capability (maintaining almost 95% capacitance from 5 to 100 mV/s). This phenomenon illustrates that the Co 3 O 4 is able to act as a "reservoir" of ions [38,39] to accommodate OH − . The reservoir can guarantee the efficient proceeding of the faradic reactions at high scan rates (or high current densities) and in turn relieves the fading of the specific capacitances. Further evidence is provided by GCD measurement.
The discharge curves of the Co 3 O 4 measured at different current densities in a voltage window of 0-0.55 V are presented in Figure 5a. In comparison with the discharge curves of electric double-layer capacitors (which are almost a straight line), two evident plateaus are well displayed. These coincide with the sequential redox reactions described by Equations (3) and (4), indicating the pseudocapacitive property of the Co 3 O 4 . According to Equation (2), specific capacitances of 988, 960, 947.9, 939.4, and 925 F/g can be delivered at current densities of 1, 2, 5, 10, and 20 A/g respectively, as depicted by Figure 5b. Due to the relatively insufficient active material involved in redox reactions under higher current densities, the boosting of the current densities results in the fading of specific capacitances. At a low current density, the OH − ions have adequate time to transfer at the interface between electrode and electrolyte than they do at a high current density [40]. It is worth noting that the rate performance of the Co 3 O 4 is impressive (maintaining 925 F/g at 20 A/g, representing a 93.6% capacitance retention), which is significant for practical use of the material.
An EIS study is depicted in Figure 6. The first intersection point of the Nyquist plot with the real axis reflects the resistances (R s ), including the ionic resistance of the electrolyte, contact resistance at the interface of electrolyte and electrode, and intrinsic resistance of the material [39]. The Co 3 O 4 modified electrode exhibits a very low R s of 0.52 Ohms. In the high frequency region, the charge-transfer resistance (R ct ) and double-layer capacitance (C dl ) correspond to the semicircle arc. The inset of Figure 6 shows that the semicircle has a small diameter, expressing a low charge-transfer resistance and a low interfacial resistance between the current collector and electroactive material [22,41]. In the low frequency range, the slope of the plot corresponds to the diffusive resistance of OH − ions (Warburg impedance, W) within the Co 3 O 4 . The plot presents an evident straight line with a slope greater than 45 • , which indicates a valid electrolyte ion diffusion [30,34]. The equivalent circuit fitting the impedance spectrum, which involves R s , R ct , C dl , W, and pseudocapacitance (C ps ), is shown in the inset of Figure 6. coincide with the sequential redox reactions described by Equations (3) and (4), indicating the pseudocapacitive property of the Co3O4. According to Equation (2), specific capacitances of 988, 960, 947.9, 939.4, and 925 F/g can be delivered at current densities of 1, 2, 5, 10, and 20 A/g respectively, as depicted by Figure 5b. Due to the relatively insufficient active material involved in redox reactions under higher current densities, the boosting of the current densities results in the fading of specific capacitances. At a low current density, the OH − ions have adequate time to transfer at the interface between electrode and electrolyte than they do at a high current density [40]. It is worth noting that the rate performance of the Co3O4 is impressive (maintaining 925 F/g at 20 A/g, representing a 93.6% capacitance retention), which is significant for practical use of the material.  An EIS study is depicted in Figure 6. The first intersection point of the Nyquist plot with the real axis reflects the resistances (Rs), including the ionic resistance of the electrolyte, contact resistance at the interface of electrolyte and electrode, and intrinsic resistance of the material [39]. The Co3O4 modified electrode exhibits a very low Rs of 0.52 Ohms. In the high frequency region, the chargetransfer resistance (Rct) and double-layer capacitance (Cdl) correspond to the semicircle arc. The inset of Figure 6 shows that the semicircle has a small diameter, expressing a low charge-transfer resistance and a low interfacial resistance between the current collector and electroactive material [22,41]. In the low frequency range, the slope of the plot corresponds to the diffusive resistance of OH − ions (Warburg impedance, W) within the Co3O4. The plot presents an evident straight line with a slope greater than 45°, which indicates a valid electrolyte ion diffusion [30,34]. The equivalent circuit fitting the impedance spectrum, which involves Rs, Rct, Cdl, W, and pseudocapacitance (Cps), is shown in the inset of Figure 6. Long-term cycling stability is an important parameter required for practical application. Hence, the GCD tests were repeated for 6000 cycles at a current density of 20 A/g. As shown in Figure 7, 96.6% of the specific capacitance value is still retained. In addition, the coulombic efficiency is deduced from Equation (5) [37,39]: where td (s) and tc (s) are, respectively, the discharge and charge times; a high efficiency exceeding 95% is achieved. This reveals that long-lasting cycling assessment does no obvious damage to the nanostructure and a feasible redox process is generated from the obtained material and verifies the remarkable electrochemical stability. Long-term cycling stability is an important parameter required for practical application. Hence, the GCD tests were repeated for 6000 cycles at a current density of 20 A/g. As shown in Figure 7, 96.6% of the specific capacitance value is still retained. In addition, the coulombic efficiency is deduced from Equation (5) [37,39]: where t d (s) and t c (s) are, respectively, the discharge and charge times; a high efficiency exceeding 95% is achieved. This reveals that long-lasting cycling assessment does no obvious damage to the nanostructure and a feasible redox process is generated from the obtained material and verifies the remarkable electrochemical stability.
A comparison between our work and Co 3 O 4 -based electrode materials in current reports was made, as summarized in Table 1. Unfortunately, the high capacitive performances obtained through tremendous efforts usually accompany poor rate capability, which severely limits their wide application in high-power energy storage devices. The Co 3 O 4 in this paper can meet the need of not only high specific capacitance but also excellent rate capability. In addition, the performance of the Co 3 O 4 is even better than some composites of cobalt oxides [34,42,43]. The superior electrochemical behaviors can first be explained by the stable morphology. Secondly, the large surface and inner space favors an efficient contact between active material and electrolyte, ensuring a high utilization rate of active material. Thirdly, the interconnected nanoparticles with an increased surface-to-bulk ratio offer more sites for ions to enter and, more importantly, allow facile ion diffusion at a high current density [44]. Finally, the hollow spheres are interconnected with one another, contributing to an even longer ionic diffusion channel [37]. A comparison between our work and Co3O4-based electrode materials in current reports was made, as summarized in Table 1. Unfortunately, the high capacitive performances obtained through tremendous efforts usually accompany poor rate capability, which severely limits their wide application in high-power energy storage devices. The Co3O4 in this paper can meet the need of not only high specific capacitance but also excellent rate capability. In addition, the performance of the Co3O4 is even better than some composites of cobalt oxides [34,54,55]. The superior electrochemical behaviors can first be explained by the stable morphology. Secondly, the large surface and inner space favors an efficient contact between active material and electrolyte, ensuring a high utilization rate of active material. Thirdly, the interconnected nanoparticles with an increased surface-to-bulk ratio offer more sites for ions to enter and, more importantly, allow facile ion diffusion at a high current density [42]. Finally, the hollow spheres are interconnected with one another, contributing to an even longer ionic diffusion channel [37].

Conclusions
In summary, porous thin-wall hollow Co 3 O 4 spheres were successfully prepared through a simple approach and were characterized in detail. The electrochemical studies above unambiguously illuminate that the Co 3 O 4 in this work shows prominent specific capacitance and cycling stability, which are attributed to its favorable structural features. Particularly, in comparison with other Co 3 O 4 -based electrode materials, it demonstrates an overwhelming rate capability (limited decay of 6.4% from 1 to 20 A/g), which is very critical for commercial applications. These results indicate the exciting supercapacitor application potential of our synthesized material. Moreover, the relationship between the structure and properties is worth investigating in depth in the future.
Author Contributions: X.F. investigated the research, performed the characterizations and analyzed the data; X.F. and Y.S. contributed equally to conduct the experiments and to write the manuscript; P.O. provided the critical feedback and designed the structure of the manuscript; P.O. and X.C. both equally supervised the experiments and refined the manuscript.
Funding: This research was funded by the Norwegian PhD Network on Nanotechnology for Microsystems, which is sponsored by the Research Council of Norway, Division for Science, under contract no. 221860/F60. X.F. was funded by China Scholarship Council.