PANI hollow nanocages: Synthesis and application for electrode materials of supercapacitors

Accepted Manuscript Full Length Article Hierarchical Co3O4/PANI hollow nanocages: Synthesis and application for electrode materials of supercapacitors Xiaohu Ren, Huiqing Fan, Jiangwei Ma, Chao Wang, Mingchang Zhang, Nan Zhao PII: DOI: Reference:

S0169-4332(18)30356-8 https://doi.org/10.1016/j.apsusc.2018.02.013 APSUSC 38471

To appear in:

Applied Surface Science

Received Date: Revised Date: Accepted Date:

8 November 2017 30 January 2018 1 February 2018

Please cite this article as: X. Ren, H. Fan, J. Ma, C. Wang, M. Zhang, N. Zhao, Hierarchical Co3O4/PANI hollow nanocages: Synthesis and application for electrode materials of supercapacitors, Applied Surface Science (2018), doi: https://doi.org/10.1016/j.apsusc.2018.02.013

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Hierarchical Co3O4/PANI hollow nanocages: Synthesis and application for electrode materials of supercapacitors Xiaohu Ren, Huiqing Fan*, Jiangwei Ma, Chao Wang, Mingchang Zhang, Nan Zhao State Key Laboratory of Solidification Processing, School of Materials Science and Engineering, Northwestern Polytechnical University, Xi’an 710072, China *Corresponding author. E-mail adddress:[email protected].

Abstract Hierarchically hollow Co3O4/polyaniline nanocages (Co3O4/PANI NCs) with enhanced specific capacitance and cycle performance for electrode material of supercapacitors are fabricated by combining self-sacrificing template and in situ polymerization route. Benefiting from the good conductivity of PANI improving an electron transport rate as well as high specific surface area from such a hollow structure, the electrode made of Co3O4/PANI NCs exhibits a large specific capacitance of 1301 F/g at the current density of 1 A/g, a much enhancement is obtained as compared with the pristine Co3O4 NCs electrode. The contact resistance (Re), charge-transfer (Rct) and Warburg resistance of Co3O4/PANI NCs electrode is significantly lower than that of the pristine Co3O4 NCs electrode, indicating the enhanced electrical conductivity. In addition, the Co3O4/PANI NCs electrode also displays superior cycling stability with 90 % capacitance retention after 2000 cycles. Moreover, an aqueous asymmetric supercapacitor was successfully assembled using Co3O4/PANI NCs as the positive electrode and activated carbon (AC) as the negative electrode, the assembled device exhibits a superior energy density of 41.5 Wh/kg at 0.8 kW/kg, outstanding power density of 15.9 kW/kg at 18.4 Wh/kg, which significantly transcending those of most 1

previously reported. These results demonstrate that the hierarchically hollow Co3O4/PANI NCs composites have a potential for fabricating electrode of supercapacitors. Keywords Conductive polymers, Co3O4@PANI nanocages, Hollow structure, Supercapacitors

Introduction The exorbitant anthropogenic consumption of fossil energy and global concerns toward environmental problems is compelling scientific communities across the globe to develop and use renewable energy sources such as solar, wind, and tide. However, it is mandatory to store the renewable energy owing to their characteristic of intermittence in nature. Accordingly, the exploitation of sustainable energy storage technologies is required urgently [1]. Supercapacitors are gaining great attention as a promising energy storage device due to their intrinsic performance advantages of compelling power densities and fast charge/discharge traits [2, 3]. They store electrical energy by either electrochemical double layer (EDL) from electrolyte ion adsorption on the surface of the electrode or faradaic redox reactions (pseudocapacitors) involving the surface regions of electrode materials [4, 5]. As for high-performance supercapacitors, a set of required properties such as high specific capacitance, large rate capability, and long cyclic stability should be satisfied [6]. Supercapacitors are suitable for a variety of applications where high bursts of power are instantly required owing to their high power density over other energy storage devices such as lithium ion batteries [7, 8]. 2

Moreover, their fast charging/discharging and long-term cyclic stabilities make them attractive for power supplies in portable electronic devices [9, 10]. However, they suffer from a lower energy density than that of other energy storage such as lithium-ion batteries and fuel cells, which has seriously restricted their practical applications [11]. In General, performance of supercapacitors can be enhanced by optimizing the intrinsic properties of electrode materials and appropriately engineering structure designs of electrodes [12, 13]. From the structure designs’ perspective, the nanoscale effects including high specific surface area and accessible porosity may improve the charge-storage capability.

Accordingly,

engineering well-defined nano-

and

micro-structures can endow electrodes with high power and energy densities [14, 15]. From the material properties’ perspective, high theoretical specific capacitance and electronic/ionic conductivities are essential for high-performance supercapacitors. At present, the transition metal oxides and conductive polymers are investigated extensively as pseudocapacitive electrode materials [16-19]. Among various electrode materials, Co3O4 has captured extensive attention due to its low cost, high theoretical capacitance (3560 F/g), good redox ability, and environment friendliness [20]. However, the actual capacitance of Co 3O4 now is much lower than the theoretical value. Its poor electrical conductivity is one of the main reasons hindering the electron transfer during redox reaction [21]. Many strategies have been developed to solve this problem. Among them, nanostructures and composites are two main directions. The unique nanostructures such as nanosheets [22, 23], nanowires [24, 25], and nanotubes [26, 27], bring largely active contact area, short ions migration path, and effective buffer during 3

charge/discharge processes. Another approach is compositing these nanostructures with conducting materials such as carbon-based materials [28] and conductive polymers [29]. However, generating carbon are usually accomplished by thermal decomposition of carbon precursors, which would result in environmental problems due to the formation of exhaust gas and volatile organic compounds, sometimes lead to the inferior reconstruction of materials [30, 31]. The conducting polymers compositing will be a good route for the improvement of performance of nanostructure Co3O4. Based on above consideration, herein we report on the fabrication of hierarchical hollow Co3O4/polyaniline (PANI) nanocages (NCs) through a self-sacrificing template and in situ polymerization route. The electrode made of as-synthesized Co3O4/PANI NCs exhibits a large specific capacitance of 1301 F/g at the current density of 1 A/g, which is superior to 865 F/g of the pristine Co3O4 NCs electrode. From electrochemical impedance spectroscopy (EIS) results, the contact resistance (Re), charge-transfer (Rct) and Warburg resistance of Co3O4/PANI NCs electrode is significantly lower than that of the pristine Co3O4 NCs electrode, indicting the enhanced electrical conductivity. In addition, the Co3O4/PANI NCs electrode demonstrates a superior cycling stability (90 % capacitance retention after 2000 cycles). The above suggests that the Co3O4/PANI NCs could represent promising candidates for electrodes of high performance supercapacitor.

1. Experimental Section Synthesis of ZIF-67 Crystals. 4

All chemicals were of analytical grade and were used directly after purchase without further purification. In a typical reaction, two solutions were first prepared respectively by dissolving 1 mmol cobalt nitrate hexahydrate (Co(NO3)2·6H2O) and 4 mmol 2-methyl-imidazole in 25 mL methanol. Then, the solutions were thoroughly mixed and the resultant mixed solution was left undisturbed for a time span of 24 h at room temperature. Finally the purple precipitate was washed repeatedly and collected by centrifugation followed by their drying in a vacuum oven at 70 °C for 6 h. Synthesis of Co3O4 NCs. The as-prepared ZIF-67 templates was transferred into a round bottomed ask containing 50mL ethanol solution of cobalt nitrate with concentration of 4g/L. Then the mixture solution was refluxed for 1 h under stirring. The product was then washed repeatedly with ethanol, dried in a vacuum oven at 60 °C overnight to obtain Co-LDH NCs. Finally, the as-synthesized Co-LDH NCs were annealed in a muffle furnace at 350 °C for 2 h with a ramp rate of 3 °C/min, followed by natural cooling. Synthesis of Co3O4/PANI NCs. The hierarchical Co3O4/PANI NCs with hollow structure were prepared by an in situ surface polymerization method by employing polyvinylpyrroldine (PVP) as surfactant. Briefly, 0.08g PVP was dissolved in 200 mL of deionized water, and then 0.05g as-prepared Co3O4 NCs were dispersed into the solution by ultrasonic for 30 min, followed by a solution of aniline (20 µL) in HCl (0.1 mL) was added into the mixture and the mixture was continuously stirred for 30 min. Then the 20 mL solution of ammonium persulfate (0.2 g) in deionized water was dropped into the above mixture 5

slowly to initiate the polymerization reaction. The polymerization process was performed for 5 h with Continuous stirring. The resulting products were washed with deionized water and ethanol several times, and dried in vacuum at 60 °C overnight to obtain hierarchical Co3O4/PANI NCs. Materials Characterization. X-ray diffraction (XRD) spectra were conducted on a Panalytical X’Pert PRO X-ray diffractometer with Cu Kα radiation (λ = 1.5406 Å). The morphological studies were carried out using a field emission scanning electron microscope (FE-SEM, Zeiss Supra 55) and a transmission electron microscopy (TEM, FEI Tecnai F30). Infrared spectra were recorded in the wavenumbers ranging from 2000 to 400 cm-1 by a Fourier transform infrared spectrometer (FTIR, Shimadzu 8400S) using a KBr wafer. The X-ray photoelectron spectroscopy measurements were performed using an Omicron energy analyser (XPS, Perkin Elmer PHI-5400). A V-sorb 2800 Analyzer was used to analyze the specific surface area and porosity of the Co 3O4/PANI NCs. Preparation of Electrodes and Electrochemical Measurement. The working electrodes were prepared by mixing the as-prepared active material, carbon black (super-P-Li), and polyvinylidenefluoride (PVDF) with the mass ratio 8:1:1 in N,N-dimethyl-formamide solvent. Then, the slurry was coated onto Ni foam current collectors (1 cm 1 cm) followed by drying at 80 °C for 24 h under vacuum. Electrochemical measurements were performed with a CHI 660E electrochemical workstation. All experiments were carried out in a three-electrode system with a working electrode, a platinum plate counter electrode and an Ag/AgCl reference 6

electrode in a 6.0 M aqueous KOH electrolyte.

2. Results and Discussion Figure 1 illustrates the detailed synthesis process of Co3O4/PANI NCs. The uniform ZIF-67 rhombic dodecahedral nanoparticles were served as sacrificial templates, which facilely fabricated by mixing rapidly two methanol solutions of cobalt nitrite and 2-methylimidazole and aging for 24 h at room temperature. First, these ZIF-67 nanoparticles were dispersed in ethanol solution of cobalt nitrite and refluxed for 1 h under continuously stirring to generate Co-LDH hollow nanocages. During the process, the ZIF-67 templates were gradually etched by protons generated from the hydrolysis of Co2+ ions and the released Co2+ ions were partially oxidized by NO3- ions in the solution, then Co2+ and Co3+ ions coprecipitated to form Co-LDH NCs [32]. Afterward, the obtained Co-LDH NCs were directly calcinated to generate Co3O4 NCs. The hierarchical and hollow structure of Co3O4/PANI NCs was accomplished by the in-situ polymerization of aniline monomers on Co3O4 NCs. The structures formed in different steps are characterized by XRD, FESEM and TEM. From Figure 2a, the characteristic peaks shown in XRD pattern of ZIF-67 templates matches well with the simulated as well as the published spectra [33], indicating that the as-prepared ZIF-67 nanocrystals are phase-pure. As shown in the FE-SEM images (Figure 2b), the ZIF-67 templates exhibits a rhombic dodecahedron shape, which is the same as that of ZIF-67 nanocrystals in previous reports [34]. Also a remarkably narrow size distribution is exhibit and the average particle size is 7

determined to be 800nm. By reacting with Co(NO3)2 in ethanol solution, the ZIF-67 nanocrystals were etched as sacrificial templates and hollow polyhedral Co layered double hydroxides nanocages (Co-LDH NCs) are produced expectedly. From the XRD spectrum (Figure 3a) of the sample, the typical diffraction peaks of LDH materials are clearly observed, demonstrating the identical hydrotalcite structures of the samples unambiguously. FE-SEM (Figure 3b) and TEM (Figure 3c and d) images reveal that a polyhedral shape is still retained for the produced particles, but a rougher surface is exhibited, which is constructed by many small nanosheets. The formation of hollow structure is accomplished by charily controlling the reaction kinetic balance between the acidic etching of the sacrificial templates and the precipitation of the LDH materials. After thermal annealing of Co-LDH NCs, the Co3O4 with spinel structure were produced. To obtain hierarchical Co 3O4/PANI composites, we employed an in situ polymerization method for growing PANI on the surface of Co3O4 NCs. Overall strategy for synthesis of Co3O4/PANI NCs is schematically illustrated in Figure 4a. First, the prepared Co3O4 nanocage particles were modified by PVP to improve their dispersibility in aqueous solution. Then the resultant modified Co 3O4 nanocage particles were redispersed in aqueous solution. Afterwards, the aniline monomers were added into above solution and converted to cationic anilinium ions in an acidic condition. The anilinium ions were adsorbed on the surface of Co3O4 NCs due to static interaction, resulting in significant increase of the local concentration of aniline monomer near the surface of Co3O4 NCs. After adding ammonium persulfate, the 8

polymerization was initiated and propagated on the surface of Co3O4 NCs rather than in solution. Eventually, a PANI layer was generated on the surface of Co 3O4 nanocage particles homogeneously. As shown in Figure 4b, XRD pattern confirms formation of the pure spinel phase Co3O4 after annealing the Co-LDH NCs, and the main peaks of Co3O4/PANI composite NCs are the same as the pristine Co3O4 particles, which indicates that the crystal structure of Co3O4 is well-maintained after the coating process under acidic conditions. From the FE-SEM image (Figure 4c), the as prepared Co3O4 nanocage particles inherits the polyhedral shape from the Co-LDH NCs. In addition, the hollow structure can be well identified from some broken particles. From TEM observations (Figure 4d), the formation of hollow structure of Co3O4 NCs was further verified. By TEM observation under higher magnification (inset of Figure 4d), the Co3O4 NCs were constructed with many small grains and the thickness of the nanocages wall is about 20 nm. Figure 4e shows an SEM image of the Co3O4/PANI NCs, displaying a continuous overlayer of conducting polymers is produced on the surface of Co3O4 NCs after compositing with PANI. The TEM image shown in Figure 4f illustrates that the small Co3O4 subunit grains serving to construct nanocage structure were wrapped by PANI, and the Co3O4/PANI NCs form a hierarchical composite structure. The high resolution TEM (HR-TEM) images shown as Figure 4g clearly reveal that each Co3O4 primary grain is completely coated by a PANI layer to form a Co3O4/PANI core-shell structure. In addition, a interplanar spacing of 0.46 nm corresponds to the (111) plane of the cubic Co3O4 as shown in the HR-TEM. 9

To further check the components of the hierarchical Co3O4/PANI NCs, we conducted FTIR (Figure 5a) measurement. For bare Co3O4 NCs, two strong peaks centered at 663 and 565 cm−1 characteristic of spinel Co 3O4 phase are observed, which are attributed to Co-O stretching [35]. For Co3O4/PANI composite NCs, several characteristic peaks of PANI can be noticed. Two peaks at 1581 and 1492 cm-1 are ascribed to the C=C stretching modes of quinoid (Q) ring and benzenoid ring respectively, the quinoid ring and benzenoid ring are the basic molecular units of PANI [36]. The peak at 1309 cm−1 is due to the C-N stretching vibration of an aromatic amine. The typical N=Q=N stretching mode of PANI is at 1141 cm-1. The single peak at 821 cm-1 corresponds to the C-H out of plane bending mode, which has been used as a key to identify the type of substituted benzene [37]. Comparing two FTIR spectra, the presence of PANI can be confirmed apparently. The XPS was employed to obtain insight into the surface chemical compositions and the valence states of Co3O4/PANI composite. As shown in Figure 5b, the full XPS spectrum of Co3O4/ PANI contains signals of C, O, N, and Co, while that of bare Co3O4 NCs contains only signals of O and Co. From the individual high-resolution XPS spectrum of Co3O4/PANI NCs, more details can be obtained. In the Co 2p XPS spectrum (Figure 5c), there are two main spin-orbit lines and weak satellite signals (abbreviated to “Sat.”), which demonstrates the coexistence of Co 2+ and Co3+ in Co3O4/PANI NCs [38]. The fitting peaks at 796.6 and 781.7 eV correspond to Co 2p1/2 and Co 2p3/2 signals of Co2+. The peaks at 794.9 and 779.9 eV correspond to the Co 2p1/2 and Co 2p3/2 signals of Co3+. As shown in Figure 5d, the XPS spectrum of the O1s 10

core-level is fitted into four peaks. The peak situated at 529.8 eV is assigned to the O2derived from Co3O4. The other three peaks of 531.3, 532.4 and 533.8 eV correspond to hydroxyl groups (O-H), carboxyl (O-C=O), and adsorbed molecular water (H2O) respectively on the surface of sample [39]. The typical peak of carboxylic groups (O-C=O) indicates the oxidation due to the presence of PANI polymer. In C 1s spectrum (Figure 5e), besides the predominant peak at 284.6 eV that corresponds to the C-C/C=C bonds, another characteristic peak at 285.8 eV that ascribed to form C-N bond is also observed. It should be mentioned that another two peaks of 286.6 and 287.8 eV that correspond to C-O and C=O may result from the adsorbed CO2 and some oxygen-containing groups. The XPS spectrum of N1s core-level is shown in Figure 5f. The broad band of N 1s can be divided into three typical peaks for PANI at 398.8, 399.9 and 401.6 eV, corresponding to benzenoid amine (-NH-), cationic nitrogen atoms (NH+) and protonated amine units (-NH+) respectively. The presence of protonated amine units (=NH+) is attributed to the stronger electron localization associated with poor conjugation at sp3-bonded sites [40]. The positively charged nitrogen atoms with protonation (=NH+) are related to the cationic nitrogen atoms. The high peak of =NH+ demonstrates a good protonation level (N+/N) of PANI, which imply good electrical properties of the sample [41]. N2 sorption experiment is performed to investigate the specific surface area and pore size distribution of Co 3O4/PANI NCs. The N2 adsorption/desorption isotherm of Co3O4/PANI NCs is presented in Figure 6a, which is type-I isotherms due to the sharp increase in volume at low relative pressure, demonstrating the presence of micropores. 11

The hysteresis loop at relative pressure between 0.8 and 1.0 indicates the coexistence of mesopores. The pore size distribution shown in Figure 6b suggests that the micropores are smaller than 2 nm in size, while the mesopores are below 40 nm. The as-synthesized Co3O4/PANI NCs with hierarchical hollow structure have a Brunauer-Emmett-Teller (BET) surface area of about 42 m2/g. The obtained coexistence of microporous and mesoporous structure can offer more accessible surface area for electrolyte ions and promote the transport and diffusion of electrolyte ions during the charge-discharge process, resulting in enhanced electrochemical performances. The electrochemical performances of the as-prepared electrodes were explored in a conventional three-electrode electrochemical cell with a 6.0 M KOH electrolyte. Figure 7a and b shows the Cyclic voltammetry (CV) curves of the Co3O4 and Co3O4/PANI NCs electrodes measured at different scan rates ranging from 5 to 50 mV/s with the potential range of 0 to 0.5 V. The pairs of well-defined redox peaks can be detected clearly in both of the CV curves, which denote a typical pseudocapacitive behavior due to the presence of a reversible Faradaic reaction. It is distinguishable from those of electric double-layer capacitors [42, 43]. In addition, the integral area of the Co3O4/PANI NCs electrode is larger than that of the pristine Co3O4 NCs at the same scan rate, indicating that a much larger capacitance is obtained due to the synergistic effects from Co3O4 and PANI. The isolated Co3O4 NCs are linked up by the amorphous PANI that served as bridges for electrons transportation between the current collector and the surface of Co3O4 NCs, which also result in improved electron transport rate through individual nanocages. The polymerized PANI can efficiently enhance the 12

electrical conductivity of Co3O4 NCs and improve Faradaic processes across the interface [44]. With increasing scan rate, the peak currents increase correspondingly, but no significant change in the shape of the CV curves is observed, indicating an ideal pseudocapacitive

characteristic

and

fast

charge-discharge

performance

for

electrochemical energy storage [45]. Furthermore, the redox peaks slowly move toward positive/negative potential with increasing scan rate, indicating a good contact between the electroactive materials and the conductive Ni foam current collector. Galvanostatic charge-discharge (GCD) curves of the Co3O4 NCs and Co3O4/PANI NCs electrode were collected at various current densities as shown in Figure 7c and d. During the charge-discharge processes, the GCD curves are almost symmetric at different current densities, indicating the ideal capacitive behavior of the Co3O4/PANI NCs electrode. At a current density of 1A/g, the discharge time of the Co3O4 and Co3O4/PANI NCs electrode is about 432s and 650 s respectively, the corresponding specific capacitance of 865 F/g and 1302 F/g can be achieved respectively for the two electrodes at a current density of 1 A/g. From the GCD curves, the specific capacitances of the two electrodes at different current densities were calculated, which are shown in Figure 7e. Obviously, the Co3O4/PANI NCs electrode has higher specific capacitance than the pristine Co3O4 NCs electrode at the same discharge current density, which is in good agreement with CV curves. The Co3O4/PANI NCs electrode exhibit excellent specific capacitances of 1301, 1188, 1000, 900, 814, and 572 F/g at various discharge current densities of 1, 2, 5, 8, 10 and 20 A/g respectively, which are higher than those of pristine Co3O4 NCs electrode (865, 774, 592, 500, 433 and 216 F/g at the corresponding 13

current densities). In this work, the high capacitance can be attributed to the high surface and the short transport path for ions and electrons of hierarchically hollow structured Co3O4 and the highly conductive PANI layer. Cycling performance is tested by GCD at a current density of 10 A/g for 2000 cycles as shown in Figure 7f. It can be seen that the capacitance retention of pristine Co3O4 NCs electrode was close to 60% after 2000 cycles. By contrast, the Co3O4/PANI NCs electrode still remained at 90% of its initial capacitance after 2000 cycles, indicating the good cycling stability. In this regard, PANI can not only enhance the conductivity of Co3O4 NCs, but also improve its pseudocapacitance and stability. To better clarify the enhanced electrochemical performance for Co3O4/PANI NCs electrode, electrochemical impedance spectroscopy (EIS) measurement was implemented in the frequency range between 0.01 Hz and 100 kHz. The Nyquist plots of pristine Co3O4 NCs and Co3O4/PANI NCs electrode are depicted in Figure 8. Both of the plots are composed of an arc in the high frequency region and a slope in the low frequency region. As for the high frequency region, the intersection of the Nyquist plots at the real part represent the contact resistance of the electrochemical system (Re) and the diameter of the semicircle arc indicates the charge-transfer resistance (Rct) [46]. It is observed that, benefiting from the high conductivity of PPy, the Co3O4/PANI NCs electrode has a lower Re and Rct than that of pristine Co3O4. The slope in the low frequency region represents the Warburg resistance, which is related to ion diffusion/migration from the electrolyte solution to the electrode interface [47]. The electrode of Co3O4/PANI NCs shows a steeper slope than pristine Co3O4 electrode, 14

indicating faster ion diffusion between the electrode and the electrolyte. These results are in agreement with the better electrochemical performance of Co3O4/PANI NCs. Cycling performance is tested by GCD at a current density of 10 A/g for 2000 cycles as shown in Figure 7f. It can be seen that the capacitance retention of pristine Co3O4 NCs electrode was close to 60% after 2000 cycles. By contrast, the Co3O4/PANI NCs electrode still remained at 90% of its initial capacitance after 2000 cycles, indicating the good cycling stability. In this regard, PANI can not only enhance the conductivity of Co3O4 NCs, but also improve its pseudocapacitance and stability. To exhibit the advantage of the proposed material, the specific capacitance of different Co3O4 composites-based electrodes are compared in Table 1. By comparing, the electrode of Co3O4/PANI NCs showed the higher specific capacitance because both the unique structure and the good electrical conductivity of the Co3O4/PANI NCs could promote the electron and mass transfer together. In order to further evaluate the application potential of the Co 3O4/PANI NCs electrode, an aqueous asymmetric supercapacitor was fabricated by using Co 3O4/PANI NCs as the positive electrode, activated carbon (AC) as the negative electrode, and KOH aqueous solution as electrolyte. The CV curves of Co3O4/PANI NCs //AC supercapacitor in a potential window of 0-1.6 V at various scan rates ranged from 5 to 50 mV/s is shown in Figure 9a. The apparent redox peaks in the CV curves can be observed, indicating typical Faradaic behavior of battery-type electrodes. In addition, there is no change in shapes of the CV curves of the device due to fast rates of ionic and electronic transport. Figure 9b presents the charge/discharge plateaus at different 15

current densities, which further evaluates electrochemical performance of the as-fabricated supercapacitor device. The specific capacitance and coulombic efficiency at different current densities of the Co3O4/PANI NCs //AC supercapacitor can be calculated based on the loading mass of the active materials and are plotted in Figure 9c.The specific capacitances of the device is calculated to be 116.7, 100.6, 83.75, 71.5, 67.6 and 51.7 F/g at current densities of 1, 2, 5, 8, 10 and 20 A/g, respectively. The coulombic efficiency of the device at current densities of 1, 2, 5, 8, 10 and 20 A/g were found to be 94.9%, 96.5%, 97.8%, 98.6%, 98.9% and 99.7%, respectively. The prominent enhanced coulombic efficiency of Co3O4/PANI NCs //AC supercapacitor with the increase of current density indicates a characteristic of increase in kinetic reversibility with continuous charging discharging process [14]. The cycling stability of the Co3O4/PANI NCs //AC supercapacitor was tested by galvanostatic charge/discharge cycling between 0 and 1.6 V at a current density of 5 A/g, 83.4% of the initial specific capacitance remains after 5000 cycles, exhibiting a long cycling life as shown in Figure 9d. The inset of Figure 9d shows the Ragone plots of Co3O4/PANI NCs //AC supercapacitor, which displays an excellent energy density of 41.5 Wh/kg at 0.8 kW/kg and retains 18.4 Wh/kg even at a high power density of 15.9 kW/kg. The outstanding electrochemical properties of the Co 3O4/PANI NCs are attributed to its special microstructure and excellent conductivity. The mesoporous feature of Co3O4/PANI NCs with extremely high surface area allow more active materials exposed to the electrolyte, ensuring largely reduced ion diffusion pathways, allows facile electrolyte ion access for fast and reversible redox reactions and large increment 16

for the specific capacitance. The integrated conducting polymer PANI coating onto the Co3O4 NCs enhance the electrical conductivity of the whole electrode leading to a high rate capability, meanwhile, abundant hydrophilic radical introduced by PANI offer better wettability to facilitate ion diffusion, which ensures efficient contact between the electroactive matters and the electrolyte [19].

3. Conclusions In summary, hierarchical Co3O4/PANI nanocages with hollow structure have been successfully fabricated by a facile self-sacrificing template method combined with in situ polymerization. The unique hierarchically hollow architecture combined with conductive polymer, featuring good conductivity, high surface areas, and short ion/electron transport paths, endow the Co3O4/PANI NCs with high electrochemical energy storage performance. The electrode of Co3O4/PANI NCs exhibited a high specific capacitance of 1301 F/g at 1 A/g, and superior cycling stability retaining 90% of the initial capacitance after 2000 cycles. For the aqueous asymmetrical supercapacitor, an excellent specific capacitance of 116.7 F/g (at 1 A/g), high energy densities of 41.5 Wh/kg (at 0.8 kW/kg) and 18.4Wh/kg (at 15.9 kW/kg), and great cycling stability were presented by Co 3O4/PANI NCs //AC supercapacitor. The high electrochemical performance demonstrates that the hierarchical Co3O4/PANI hollow nanocages can potentially be used as energy storage materials. Acknowledgements This work was supported by the National Natural Science Foundation (51672220), 17

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Figure 1. Schematic diagram of the preparation process of the hierarchically Co3O4/PANI NCs.

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Simulated As-prepared

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Figure 2. XRD pattern (a) and FE-SEM images (b) of ZIF-67 nanoparticles.

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JCPDS No. 38-0715

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Figure 3. XRD pattern (a), FE-SEM images (b) and TEM images (c, d) of Co-LDH NCs.

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Co3O4 NCs

JCPDS No. 43-1003

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(g)

Figure 4. Schematic illustration for the formation of hierarchical Co3O4/PANI NCs. XRD patterns (b) of Co3O4 NCs and Co3O4/PANI NCs. FE-SEM images (c) and TEM images (d) of Co3O4 NCs. FE-SEM images (e), TEM images (f) and HRTEM image (g) of Co3O4/PANI NCs.

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(C=C) Co3O4

Co 2s O KLL

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Figure 5. (a) FTIR spectra of both Co3O4 and Co3O4/PANI NCs. (b) XPS spectra of Co3O4 and Co3O4/PANI NCs. (c) Core-level Co 2p, (d) O 1s, (e) C 1s, and (f) N 1s XPS spectra of the Co3O4/PANI NCs sample.

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(a)

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dV/dD (cm3nm-1g-1)

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Figure 6. N2 adsorption-desorption isotherm (a) and Pore size distribution (b) of Co3O4/PANI NCs.

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(b)

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0.4

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Co3O4/PANI NCs

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Figure 7. (a) CV curves of the pristine Co3O4 NCs electrodes at different scan rates. (b) GCD curves of the Co3O4 NCs electrodes at different current densities. (c) CV curves of the Co3O4/PANI NCs electrode at different scan rates. (d) GCD curves of the Co3O4/PANI NCs electrode at different current densities. (e) Specific capacitance of the Co3O4 NCs electrodes and Co3O4/PANI NCs electrodes at various current densities. (f) Cycling stability of Co3O4 NCs and Co3O4/PANI NCs electrodes measured at a current density of 10 A/g.

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Co3O4 NCs

10 8

1.5

6

1.0

-Z (Ohm)

-Z (Ohm)

Co3O4/PANI NCs

4 2

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Z (Ohm)

Figure 8. EIS spectra of Co3O4 NCs and Co3O4/PANI NCs electrodes; the inset is an enlarged Nyquist plot.

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(b) 1.6

5 mV/s 10mV/s 20mV/s 30mV/s 50mV/s

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1 A/g 2 A/g 5 A/g 8 A/g 10 A/g 20 A/g

1.4 1.2

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(a) 9

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Figure 9. (a) CV curves of the Co3O4/PANI NCs //AC supercapacitor at various scanning rates. (b) GCD curves of the Co3O4/PANI NCs //AC supercapacitor at different current densities. (c) Specific capacitances and columbic efficiency of the Co3O4/PANI NCs //AC supercapacitor at different current densities. (d) 5000 cycles GCD measurement of Co3O4/PANI NCs //AC supercapacitor at a current density of 5 A/g, inset is the Ragone plots plot of the supercapacitor.

31

Electrode materials

Specific capacitance

References

Co3O4 nanorods Co3O4 porous NSs/RGO Co3O4@PPy@MnO2 composites Co3O4 NSs arrays/N-doped graphene foam Co3O4/Ni(OH)2 mesoporous nanosheets Co3O4/graphite nanocomposite PANI@NiCo2O4 core-shell NRAs Co3O4 NCs Co3O4/PANI NCs

739 F/g at 5 mV/s 518.8 F/g at 0.5 A/g 782 F/g at 0.5 A/g 451 F/g at 1 A/g 1144 F/g at 5 mV/s 395.04 F/g at 0.5 A/g 901 F/g at 1 A/g 865 F/g at 1 A/g 1301 F/g at 1 A/g

[41] [48] [49] [50] [51] [52] [53] This work This work

Table 1. The specific capacitance comparison of Co 3O4 based materials reported in this work and from previous works.

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Hierarchical Co3O4/PANI hollow nanocages: Synthesis and application for electrode materials of supercapacitor Xiaohu Ren, Huiqing Fan*, Jiangwei Ma, Chao Wang, Mingchang Zhang, Nan Zhao State Key Laboratory of Solidification Processing, School of Materials Science and Engineering, Northwestern Polytechnical University, Xi’an 710072, China *Corresponding auther: Huiqing Fan, E-mail: [email protected].

Graphical Abstract

33

Highlights 1. Hierarchically hollow Co3O4/polyaniline nanocages (Co3O4/PANI NCs) were designed and synthesized. 2. Co3O4/PANI NCs have hollow structure, high surface area, and high conductivity. 3. Co3O4/PANI NCs electrode has high specific capacitance, and good cycle stability.

34