Polyaniline-cobalt oxide nano shrubs based electrodes for supercapacitors with enhanced electrochemical performance

Electrochimica Acta 324 (2019) 134876

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Polyaniline-cobalt oxide nano shrubs based electrodes for supercapacitors with enhanced electrochemical performance Sandhya C.P a, **, Rishad Baig E a, Saju Pillai b, Molji C a, Aashish Aravind a, Sudha J. Devaki a, * a b

Chemical Sciences and Technology Division, CSIR National Institute for Interdisciplinary Science and Technology, Thiruvananthapuram, 695019, India Material Sciences and Technology Division, CSIR National Institute for Interdisciplinary Science and Technology, Thiruvananthapuram, 695019, India

a r t i c l e i n f o

a b s t r a c t

Article history: Received 10 June 2019 Received in revised form 26 August 2019 Accepted 11 September 2019 Available online 12 September 2019

The present work focuses on the design and development of highly oriented well aligned nano shrubs of polyaniline decorated Co3O4 (COP) and demonstration as an efficient electrode for the fabrication of supercapacitors. Morphological results suggest that COPs are composed of PANI nano shrubs grown from the pores of Co3O4 to form a three-dimensional nanostructure. Further, supercapacitor with the configurations of [(indium tin oxide)ITO-COP/1 M Na2SO4/COP-ITO] was fabricated and its performance evaluated leading to high specific capacitance value (1151 F g
1 @ 3 A g
1), high rate performance of 70% at the current density ranging from 3 to 20 A g
1, and excellent cycling stability (92% after 5000 cycles @ 10 A g
1). The excellent performance of this hybrid nanocomposite is attributed to well aligned selfassembled structural advantage of COP and the synergistic effects of cobalt oxide and polyaniline which is responsible for the substantial reduction in the charge transfer resistance as revealed by electrochemical impedance spectroscopy. All these results suggest that the developed strategy can be used for the development of efficient hybrid conductive nanocomposite based electrode material for other combinations of transition metal oxide-conductive polymer based supercapacitors. © 2019 Elsevier Ltd. All rights reserved.

Keywords: Supercapacitor Cobalt oxide PANI nano shrubs Sol-gel method Composite materials

1. Introduction Due to the unprecedented growth and adaptation of electronic devices in our routine life, energy harvesting and storage have become an important goal in front of the major world powers and technological communities. During the past two decades, a large number of efforts have been taken place globally in developing new and efficient energy storage devices. Among the energy storage devices, electrochemical energy storage devices such as supercapacitors and batteries are receiving importance. Supercapacitors have attracted significant interest during the past several decades owing to their superior power density, fast charge/discharge rate, and excellent cycling stability. Because of these benefits, supercapacitors find application along with Li-ion batteries, for which slow charging time and short cycle life is the main drawback, to power electric vehicles and hybrid electric vehicles. Besides,

* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (S. C.P), [email protected] (S. J. Devaki). https://doi.org/10.1016/j.electacta.2019.134876 0013-4686/© 2019 Elsevier Ltd. All rights reserved.

supercapacitors are also used along with solar cells to create selfpowering systems comprised of solar cells for energy harvesting and supercapacitors for energy storage [1]. The fluctuations due to the intermittent instinct of solar radiation are the main problem with a solar cell. On integration with supercapacitors solve this problem through the simultaneous storage of the electricity and manipulation of the energy output. The current densities generated by the solar cell are high enough to charge the supercapacitor. Besides, the charging potentials of the supercapacitors are also increased via the integration (up to 1.8 V). By the use of smaller solar cell arrays connected in series, one can optimize the balance between the magnitude of the photocurrent, the open-circuit potential of the integrated device and the storage capability of the energy storage part [2]. Despite all these advantages, supercapacitors have the disadvantage of low energy density. In supercapacitors, the charge accumulation process occurs either by electric double layer capacitance (EDLCs) or faradaic charge transfer process (pseudocapacitors) or a combination of both processes (hybrid capacitors). In EDLCs, carbon based materials are used which show higher specific surface area and rational pore distribution. Here the,

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charging and discharging cycles are highly reversible due to their non-faradaic electrical mechanism, which leads to extremely stable cycling ability and rate capability. The main drawback with the carbon-based electrodes is the low capacitance values (or energy densities) [3]. Pseudocapacitive materials are important in terms of their significant capacitance values and also long term cyclability. Contrary to electrochemical double layer capacitive materials, where only the surface is used for charge accumulation, in pseudocapacitive materials, the entire mass and volume are involved in charge storage [3,4]. Metal oxides (RuO2, MnO2, Co3O4, Fe2O3, etc.), metal sulfides (Cu7S4, MoS2), bi-transition metal oxides and sulfides (NiCo2O4, NiCo2S4) and conducting polymers (polyaniline, polythiophene, PEDOT, etc.) are used as electrode materials in pseudocapacitors [5e12]. Metal oxides as pseudocapcitive electrodes provide high capacitance with high energy at a low current density. Although the electrochemical performance of these materials has been improved, the energy density still cannot meet the required value. Among the different metal oxides studied, Co3O4 has received more attention due to its better performance, high theoretical capacity (up to 3650 F g
1), low cost, easy availability of raw materials and stability [13]. However, the poor electrical conductivity of Co3O4 reduces the kinetics of ion and electron transportation at the electrode and in between the electrode-electrolyte interphase, thereby reducing the capacitance of the material. Several strategies have been adopted to improve the performance of Co3O4 based systems by increasing the surface area, or by the formation of well-aligned nanowires, nanocubes, etc. or by the formation of hybrid composites. The single crystalline nanowire array obtained by Xia et al. exhibited a high capacitance of 745 F g
1 at 2 A g
1 with excellent cycling stability. The unique onedimensional architecture provides fast diffusion paths for ions and electrons on the Co3O4/electrolyte interfaces [14]. Hierarchical Co3O4 nanocubes with uniform porous structures synthesized through a Cu2O template-assisted method showed excellent rate capability (87.8% capacity retention at 20 A g
1) and good cycling stability. The nanosheet networks as well as the porous interconnections within the Co3O4 nanocubes facilitate fast ion and electron diffusion across the electrode-electrolyte interfaces [15]. Porous materials have the ability to interact with atoms, ions and molecules throughout the bulk of the material, thereby improving the properties. For example, mesoporous Co3O4 nanosheets display high specific capacitance and good rate capability by retaining 93% of the initial capacity at a current density of 5 mA cm
2 in 3 M KOH solution [16]. The performance of the porous materials can be further increased by making the materials in the form of nanorods, nanowires or layered parallel folding nanostructures while maintaining the porosity. The porous Co3O4-layered parallel folding nanostructures synthesized by Wang et al. exhibited a maximum specific capacitance of 202.5 F g
1 and good stability over 1000 cycles [17]. Qiu et al. reported hierarchical mesoporous conch-like Co3O4 nanostructure arrays as an excellent supercapacitive material, which showed a high areal capacitance of 4.11 F cm
2 at a current density of 5 mA cm
2 with 93% retention of maximum capacity after 5000 charge-discharge cycles [18]. All these reports have proved that morphology and microstructure of the active material plays an important role in its electrochemical performances. By controlling the morphology one can tune the electrochemical performance of a material. Recently, Niveditha et al. reported feather like Co3O4 electrode with a high specific capacitance of 396.67 F g
1 at 20 mV s
1 scan rate and excellent cyclic stability up to 1600 number of cycles and good charge retention [19]. The formation of binary or ternary composites with carbonbased materials, metal oxides or conducting polymers, is yet another way to improve the performance of Co3O4 [20e22]. Conducting polymers are a unique class of molecules having

alternate sigma and pi bonds and are intrinsically conducting. They can be made conducting through redox reactions. It can be p-doped to form counter anion and n-doped to counter cations with high electron density and also charge density. As discussed above metal oxides/sulfides/hydroxides are most investigated electrode materials since energy is stored in bulk material, they exhibit high charge storage. Their main limitation is poor conductivity and also relies upon the size and shape of the nanostructured metal oxide used. It has been reported that the addition of conducting polymers is expected to improve the binding and in turn the stability of the electrode along with enhancement in the charge density. Thus, conducting polymers in combination with metal oxides show better electrochemical performance due to the synergistic effects [23e26]. PANI is the commonly used conducting polymer due to its high conductivity, facile synthesis, low cost and environmental stability [26]. Hai et al. synthesized core-shell structured PANICo3O4 nanocomposites via a carbon-assisted in-situ polymerization method. This material exhibited high specific capacitance with excellent cycling stability of 84.9% retention of capacity after 1000 galvanostatic charge-discharge cycles. In this respect, integrating cobalt oxide with PANI is a facile and efficient strategy for enhancing the electrical conductivity, specific surface area and pore volume, power density, electron density, and cycling stability. Herein, we prepared Co3O4 decorated with wellaligned nano shrubs of PANI. The properties of the prepared composite were studied using various techniques and further fabricated supercapacitor using the prepared material as the electrode. The electrochemical performance was measured via electrochemical impedance spectroscopy, cyclic voltammetry and charge-discharge studies. 2. Experimental section 2.1. Materials All reagents in the experiment are of analytical grade, which were used as received without further treatment. Cobalt nitrate hexahydrate (Co(NO3)2 6H2O) from Merck Pvt Ltd is used for the synthesis of Co3O4. Citric acid, conc. H2SO4 from SRL chemicals, Aniline from Merck Pvt Ltd, Ammonium persulphate, Polyvinylidine fluoride (PVDF), N-Methylpyrrolidinone (NMP) and acetylene black were purchased from Sigma-Aldrich. Deionized water and absolute alcohol were used as the solvents. 2.2. Synthesis of Co3O4/PANI nanocomposites (COP) Co3O4 nanoparticles were synthesized through a sol-gel route: 0.09 mol of Co(NO3)2 6H2O and 0.1 mol of citric acid were dissolved in 100 ml distilled water at room temperature under vigorous stirring (350 rpm) to form a homogeneous solution. The solution thus obtained was stirred at 80  C in an oil bath for 12 h. While heating, the solution first changes to a sol then to a gel, and finally to a solid powder having the frothy appearance (xerogel). The precursor thus obtained was calcined at 450  C for 2 h in air to obtain well-defined Co3O4 nanoparticles [27]. We prepared the COP nanocomposites via emulsion polymerization of aniline in the presence of Co3O4 nanoparticles using ammonium persulphate as radical initiator according to the method demonstrated by Wang et al. [28]. In a typical procedure, 0.01 mol of Co3O4 nanoparticles were dispersed in ethanol solution (200 ml, 20 wt%) containing 2.8 g aniline and 20 ml conc. H2SO4 with stirring for 1 h. Ethanol solution was added to the above mentioned solution quickly with intensive stirring to make a concentration gradient. 0.5 mol ammonium persulphate solution were added drop by drop to this solution with stirring. The resulting

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solution was then stirred at 0  C for 5 h. The black green product of the reaction was centrifuged and washed repeatedly with distilled water and ethanol. The resulting nanocomposite was dried under vacuum at 60  C for 12 h. The 90 wt% mass load of PANI in the composite (hereafter mentioned as COP 90) was evaluated by calculating the weight difference from the initial weight of Co3O4 nanoparticles. A similar procedure was adopted for the synthesis of other compositions such as COP 70 and COP 50 by varying the concentration of Co3O4 nanoparticles. 2.3. Material characterization The X-ray diffraction measurements were carried out using a Philips X-ray diffractometer using Nickel filtered Cu Ka radiation (a ¼ 0.154 nm). Powder form was used for XRD experiments. X-ray diffraction was recorded in the 2q range 10e900 using Cu Ka radiation. Raman spectral analyses of the samples were conducted on a Witec Alpha-3002 Confocal Raman microscope using the 532-nm line from an argon-ion laser at a resolution of 1 cm
1 with an integration time of 1 s. The optical absorption spectrum was recorded from a Shimadzu model 2100 UVevis spectrophotometer in a wavelength range of 200e900 nm at room temperature. The samples for UVevisible studies were well dispersed in ethanol by sonication for 30 min to form a homogeneous suspension. For SEM measurements, samples were subjected for thin gold coating using a JEOL JFC-1200 fine coater and the probing side was inserted into JEOL JSM-5600 LV scanning electron microscope. TEM measurements were carried out using FEI (TEC-NAI G2 30 S-TWIN) with an accelerating voltage of 100 kV. For TEM measurements, the samples were casted on a carbon-coated copper grid and dried in vacuum at room temperature before observation. Electrical conductivity measurements were performed with a standard four-probe conductivity meter using a Keithley 6221 programmable current source and a 2128 A Nanovoltmeter. We pelletize the sample with a diameter of 14 mm and thickness of 1.2 mm and measured the conductivity. X-ray photoelectron spectroscopy (XPS) analyses were performed on an XPS spectrometer (ESCALAB 250Xi, Thermo Scientific Escalab, USA) with Al Ka radiation as the excitation source. The surface area of the samples was measured using BET surface area analyzer. 2.4. Electrochemical measurements The electrochemical tests of symmetric supercapacitor were carried out via a two-electrode system using 1 M Na2SO4 solution as the electrolyte. The working electrode was fabricated by coating a homogeneous paste of COP composite, polyvinylidene fluoride (PVDF) and acetylene black with a mass ratio of 80:10:10 on an indium-doped tin oxide (ITO) glass substrate. The dispersing agent used was NMP and cellulose membranes (NKK, Japan) as separators. The electrochemical characterizations were performed with a CHI6211B electrochemical workstation. Cyclic voltammetry (CV) experiment was performed at scan rates from 5 to 200 mV s
1. Galvanostatic charge/discharge studies of the composites were carried out at different current densities ranging from 3 to 20 A g
1. Electrochemical impedance spectra were recorded from 100 kHz to 0.01 Hz at open circuit potential with 5 mV amplitude. The specific capacitance (Cs) of the supercapacitor was estimated from charge-discharge curves according to Eq. (1).

Cs ¼

4IDt mDV

(1)

where ‘I’ is the discharge current (A), ‘Dt’ is the discharge time (s),

3

‘m’ is the mass of the active material (g) and ‘DV’ is the potential window (V) [29]. The specific energy density and power density of the supercapacitor cells are calculated using the equations:

Eg ¼

Pg ¼

0:5 Cs DV 2 M Eg

Dt

(2)

(3)

where Eg and Pg represent the gravimetric energy densities (Wh kg
1) and power densities (W kg
1), respectively. ‘DV’ is the potential window (V) and Dt is the discharge time (s). The reproducibility of the test results was assessed by taking galvanostatic charge-discharge measurements on at least six electrodes for each composition. Specific capacitance results of the electrodes are reported as mean values with error bars representing the standard deviations. 3. Results and discussion 3.1. Preparation and characterization of hybrid cobalt oxidepolyaniline (COP) nanocomposite COP was prepared by a two-step process involving preparation of cobalt oxide by sol-gel process followed by emulsion polymerization of aniline in the presence of cobalt oxide using APS as oxidative initiator at 0  C as shown in Scheme 1. The sol-gel method is an effective way to synthesize different nanoparticles [30e32]. Initially, the precursor solution at 80  C under stirring condition converted to sol in the presence of citric acid and then to a gel and finally to a xerogel. As evidenced by the thermal studies (Fig. S(1)) above 400  C, the curve is nearly horizontal with minimal weight loss, indicating the completion of the formation of Co3O4. Citric acid added to the system can chelate to Co2þ in Co(NO3)2 aqueous solution, thereby prevents rapid hydrolysis of Co(NO3)2. Cobalt citrates on further calcination leads to the formation of globular particle agglomerates of Co3O4 with mesopores having an average diameter of 20e30 nm. In the second step, emulsion polymerization of aniline was carried out in the presence of different weight ratios of Co3O4 using APS as the initiator. During the polymerization, PANI nano shrubs begin to evolve from the mesopores of Co3O4, as evidenced by the morphological analysis [28]. The crystal structures of Co3O4 and the composites were investigated by XRD as shown in Fig. 1. In the XRD pattern of Co3O4, the reflections at 2q ¼ 31.3 , 36.8 , 44.6 , 59.5 and 65.4 correspond to the crystal planes of (111), (220), (311), (400) and (440), which were well indexed to be the cubic spinel structure with a space group of Fd3m (JCPDS 42-1467). No other distinct diffraction peaks are observed, indicating the high purity of Co3O4. The XRD pattern of pure PANI showed a reflection at 25.2 correspond to the (200) crystal plane in its emeraldine salt form. In the XRD pattern of composite, in addition to Co3O4 peaks, the reflections corresponding to PANI were also observed and confirmed the presence of PANI in the composite. The PANI peaks were not that much prominent as in the case of composites due to the interaction of PANI with Co3O4 at the interface blocks the crystallization of PANI as evidenced from earlier reports [33e35]. Fig. 2 shows the Raman spectra of Co3O4, PANI and COP composites. For pure Co3O4, the bands at 479, 519, 617 and 688 cm
1 correspond to the Eg, F22g, F32g and A1g modes of crystalline Co3O4. F2g and Eg modes are associated with the vibration of tetrahedral and octahedral sites, whereas the occurrence of octahedral sites is linked to the high-frequency band, A1g mode [28,36e38]. In the

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Scheme 1. Synthesis of Co3O4/PANI composite.

Fig. 2. Raman spectra of Co3O4, PANI and COP composites. Fig. 1. XRD of Co3O4, PANI and COP composites.

Raman spectra of pure PANI, the peak appeared at 1165 cm
1, corresponding to the CeH bending of the quinoid ring. The peak at 1345 cm
1 gives a clear indication of the vibrations of CeNþ fragments, and the C]N stretching vibration in the emeraldine base of imines resulting from the n-doping effect of PANI and CeC stretching of benzene ring are represented by the peaks at 1482 and 1589 cm
1 [39,40]. In addition to the peaks corresponding to Co3O4, vibrations corresponding to PANI were also observed as in the Raman spectra of all the composites. Thus, Raman results further confirmed the successful formation of well-aligned cobalt oxide and PANI nanocomposites. The optical absorption properties of the as-prepared composites were recorded on UVeVisible absorption spectrophotometer. Fig. 3

shows UVeVisible spectra of Co3O4 and the composites as a function of wavelength. The absorption spectra of Co3O4 contain two absorption bands in the wavelength ranges of 250e300 and 350e600 nm. These bands correspond to the n-s* transitions of lone pair of electron from oxygen to CoeO s* orbitals. In addition to Co3O4 bands, two more absorptions at 385 and 445 nm are observed as in the case of composites, corresponding to p-p* transition of C]N and the polaron- p* transition of emaraldine salt. The free carrier tails, as observed from 800 to 900 nm, reveal the presence of highly delocalized free charge carrier arising from the synergy between cobalt oxide and PANI [41,42]. Morphology of cobalt oxide, COP 50, COP 70, COP 90 and PANI were studied using SEM, and the images are shown from Fig. 4 a to h, respectively. Fig. 4a shows the porous morphology of hierarchical assembled nanospherical structures of cobalt oxide. SEM image of

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Fig. 3. UVevisible absorption spectra for Co3O4, PANI and COP composites.

COP 50 showed sprouts of PANI on the surface of cobalt oxide spheres (Fig. 4b) suggesting PANI nanoparticles are nucleating from the surface of cobalt oxide. When the amount of aniline in COP increases, PANI nanoparticles are elongating into tiny grasses throughout the material. Co3O4 expected to act as a catalyst for the growth of polymer chain and also act as doping agent which may enhance the charge density of the formed COP hybrid nanocomposite. Fig. 4c and 4d correspond to the SEM images of COP 70 and COP 90, respectively. As the amount of aniline increases, the length and width of leaves increase. However, it retains the wellaligned orientation. It has been observed that the growth of PANI nano shrubs originates within the pores on the cobalt oxide, as evidenced by the SEM images of composites. The results are further confirmed through the TEM images of Co3O4 as well as the composite, (Fig. 4e and 4f). The SEM showing nanoparticles of PANI and EDS image of Co3O4 are shown in Fig. S(2 a and b). The coexistence of cobalt oxide and polyaniline in the COP composite was confirmed by EDS analysis as shown in Fig. 4g. In the EDS pattern, in addition to Co and O peaks as observed in the case of Co3O4, peaks corresponding to C and N were also observed, indicating the formation of PANI along with Co3O4. The SAED pattern showing the crystalline phase of cobalt oxide in COP is shown in Fig. 4h. X-ray photoelectron spectroscopy (XPS) is an essential tool used to study the chemical composition as well as the oxidation state of compounds. The XPS survey profiles of Co3O4 and COP composite are shown in Fig. 5a. The binding energies corresponding to O 1s and Co 2p are seen in the survey profile of Co3O4, which confirms the formation of cobalt oxide. Whereas, in the case of composite in addition to the peaks corresponding to Co3O4, there are N 1s and C 1s peaks corresponding to the formation of COP nanocomposite. The high-resolution XPS spectrum (Fig. 5b) shows two peaks at binding energies 780.07 and 795.34 eV corresponding to Co 2p3/2 and Co 2p1/2, respectively, characteristic of a spinel Co3O4 phase. This is consistent with the previous findings on the XPS spectrum of Co3O4. The O 1s peaks are resolved into three as shown in Fig. 5c, of which the peak at 529.5 eV indicates the formation of metal oxides. The other two located at 530.8 and 531.8 eV are due to the OHgroups of chemisorbed moisture at the surface. The deconvoluted N 1s spectrum of the prepared composite (Fig. 5d) shows the presence of nitrogen radical cation at 400 eV. Further, the peaks related to quinoid amine and benzenoid amine are located at 397 eV and 399 eV, thus confirming the conducting nature of the composite. Overall, the XPS results revealed the formation of COP nanocomposite [40,43,44].

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Fig. 6 shows the specific surface area of Co3O4 and COP composites, analyzed by N2 adsorption-desorption at 77 K. The N2 adsorption isotherms of Co3O4 as well as the composites are of type IV. The Brunauer-Emmett-Teller (BET) surface area values calculated for Co3O4, COP-50, COP-70 and COP-90 are 24.5, 58.8, 74.8 and 44.9 m2 g
1, respectively. The hysteresis loop observed as in the case of Co3O4 corresponds to H1 type, which is a clear indication of well-defined cylindrical pore channels. Whereas, for the composites, H3 type hysteresis loop is observed, corresponds to disordered pores. The N2 uptake in all the three cases occurs at a higher relative pressure (>0.7 P/Po) suggesting the existence of macropores within the sample [45]. From the pore size distribution plots or BJH plots, as shown in Fig. 7 (b), a pore size distribution maxima centered at ~ 31 nm is observed for Co3O4. The surface area of COP-70 composite is high compared to Co3O4 as well as other composites, due to the formation of more PANI nano shrubs over the surface of Co3O4 as evidenced from SEM images. At higher composition, i.e., for COP-90 the surface area decreases due to the aggregation of PANI networks. The porous nature, as well as the nano shrub morphology of COP 70, can improve the kinetics of electron transportation at the electrode-electrolyte interface and thereby improve the electrochemical performance. From the electrical conductivity studies, it is observed that with the increase in PANI content in the composite the conductivity increases. The conductivity values measured for COP 50, COP 70 and COP 90 are 0.582, 0.601 and 0.628 S cm
1, respectively. Thus, in the composite the synergistic effect of the properties of both the components operates. 3.2. Electrochemical performance The electrochemical performance of Co3O4 and the composites was evaluated in a two electrode system using 1 M Na2SO4 as the electrolyte. The electrode was fabricated by coating a slurry of active COP, conductive carbon and PVDF with a weight ratio of 80:10:10 in NMP over ITO plates. The electrochemical performance of the materials was studied via electrochemical impedance spectroscopy, cyclic voltammetry and galvanostatic charge-discharge studies. In order to evaluate the electrical conductivity and ion diffusion, electrochemical impedance spectroscopy (EIS) measurement was conducted. Fig. 7 shows the Nyquist plots of bare Co3O4 and composite electrodes recorded between 10 mHz and 100 kHz frequency region at open circuit potential by applying an amplitude of 5 mV. The EIS spectra of all the samples can be showed so-called knee frequency into two regions, with a semicircle arc in the high frequency region and a straight line in the low frequency region. The semicircle arc in the high frequency region correspond to the charge transfer resistance at the electrode-electrolyte interface (Rct), and the capacitive behavior related to the charging mechanism is indicated by the straight line in the low frequency side [46e49]. In addition, the intercepts at the high frequency side indicate the equivalent serial resistance or the solution resistance (Rs). The charge transfer resistance obtained from the diameter of the semicircle arc for pure Co3O4, COP 50, COP 70 and COP 90 samples are 8.1, 1.4, 0.7 and 0.9 U, respectively. The Rs values for pure Co3O4, COP 50, COP 70 and COP 90 samples are 17.8, 12.4, 11.6 and 13.2 U, respectively. It can be seen that the COP 70 has smaller Rct and Rs values, indicating efficient charge transfer resistance and electron transfer resistance. Furthermore, the straight line of the COP 70 spectra is more close to 90 compared with the EIS spectra of other composites and bare Co3O4, which reveals a more ideal capacitive behavior of COP 70 electrode. The equivalent Randel circuit diagram is given as Fig. S3. Fig. 8 shows the cyclic voltammetry curves of the composite electrodes measured at different scan rates ranging from 5 to

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Fig. 4. SEM images of (a) Co3O4, (b) COP 50, (c) COP 70 and (d) COP 90; TEM images of (e) Co3O4, and (f) COP 70; EDS image COP composite and SAED pattern of COP (g and h).

200 mV s
1 with the potential range of
0.2to 1.0 V. Generally, all the CV curves are rectangular shapes even at higher scan rates. An increase in current density is observed with the composites compared to Co3O4 (Fig. S4). This improvement is due to the increased conductivity as well as a better surface area of the composites. As for the composites, with the increase in scan rates from 5 to 200 mV s
1 the peak current increases due to its good reversibility to fast charge-discharge processes. The specific capacitance values calculated from the CV curves of Co3O4, COP 50, COP 70 and COP 90 are 300, 612, 878 and 700 F g
1, respectively, at a scan rate of 5 mV s
1. The galvanostatic charge-discharge curves of the composite electrodes at varying current densities are depicted in Fig. 9. The time required for the charge-discharge process for Co3O4 is less at lower current densities compared to composite electrodes (Fig. S5).

The slight difference in charging and discharging times are associated with the irreversible capacity of composite electrodes. The specific capacitance values for the electrodes were calculated using eqn (1) and summarized in Table 1. The bare Co3O4 electrode showed a specific capacitance value of 204 F g
1 at a current density of 3 A g
1. The specific capacitance values for COP 50, COP 70 and COP 90 electrodes at a current density of 3 A g
1 are 704, 1151 and 792 F g
1, respectively. The specific capacitance values obtained from charge-discharge studies are in consistent with CV study results. The better performance of these electrodes can be ascribed to the improved conductivity as well as the increased surface area of the composites. Among the composites, COP 70 performs well compared to others due to its high surface area associated with PANI nano shrubs grown from the pores of Co3O4. The COP 70 electrode showed specific capacitance values of 1083,

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Fig. 5. (a) XPS survey spectra of Co3O4 and COP composites; High resolution XPS spectra of (b) Co 2p, (c) O1s and (d) N 1s.

Fig. 6. (a) The BET isotherms of Co3O4 and COP composites; (b) The BJH pore size distribution profile of Co3O4 and COP composites.

992, 915, 900 and 803 F g
1 at current densities of 5, 7, 9, 10 and 20 A g
1, respectively. The reduction in specific capacitance with the increase in current densities is due to the decrease in the utilization of active material for redox reactions at higher current values [50,51]. These specific capacitance values are higher than the

reported literature values for the symmetric supercapacitor [25,40]. Even though similar types of systems were already reported, the Co3O4 decorated with PANI nano shrubs performs well at higher current densities in comparison with already reported ones (Table 2). The better performance of the composites especially COP

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Fig. 7. The Nyquist plots of the EIS spectra of pure Co3O4, COP 50, COP 70 and COP 90 electrodes recorded between 10 mHz and 100 kHz.

70 electrode ascribed to the increased surface area in connection with the special morphology, which facilitates maximum electrode-electrolyte interfaces and improves the electrode kinetic

performance. The nano shrubs grown from the pores of Co3O4 provide maximum exposed area available for electrolyte interaction. Also, it will provide additional mechanical stability to the electrode, resulting in improved cycle performance. The rate performance of the composite electrodes was obtained from a plot of current density against specific capacitance as shown in Fig. 10a. The COP 70 electrode showed a high rate performance of 70% at the current density ranging from 3 to 20 A g
1. For comparison, the rate performance of COP 50 and COP 90 electrodes are only 40% and 42%, respectively, at the current density range of 3e20 A g
1. The COP 70 electrodes delivered an energy density of 54 Wh kg
1 with the power density of 12 kW kg
1. Cycling studies were conducted to evaluate the performance of the electrode under consideration. Supercapacitors fabricated using COP 70 system were cycled at a current density of 10 A g
1 for 5000 cycles, as shown in Fig. 10b. The system showed excellent stability with 92% retention of its initial capacity even at the end of 5000 chargedischarge cycles. The results indicate that no considerable electrochemical changes have happened to COP 70 electrode even after the long term cycling studies. The better rate performance and cycling stability of COP 70 electrode can be attributed to the structural stability of the composite due to its special morphology. The present observation is supported by the work reported by other researchers [53]. Thus, the characteristic morphology of the electrode material and also synergistic contribution of cobalt oxideepolyaniline in the molecular level with well aligned selfassembled morphology contributes to the enhanced surface area,

Fig. 8. The CV curves recorded at various scan rates (5e200 mV s
1) for the composite electrodes.

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Fig. 9. Galvanostatic charge-discharge curves of the composite electrodes recorded at different current densities in 1 M Na2SO4.

Table 1 Specific capacitances of electrodes measured at various current densities in 1 M Na2SO4. Sample

Co3O4 COP 50 COP 70 COP 90

Specific capacitance (F g
1) (1 M Na2SO4) 3.0 A g
1

5.0 A g
1

7.0 A g
1

9.0 A g
1

10 A g
1

20 A g
1

204 704 1151 792

40 640 1083 757

e 578 992 729

e 492 915 612

e 450 900 597

e 293 803 325

pore volume, high structural stability, specific capacitance, charge transfer resistance, power density, electron density, and rate capability for the developed electrode material which may enhance the overall performance of the devices. 4. Conclusion In summary, we have successfully synthesized Co3O4 decorated with PANI nano shrubs having high surface area, pore volume through a simple facile process. The process was optimized by varying the weight of Co3O4 taken for in situ polymerization, to get

Table 2 Comparison of the specific capacitance of various composites of Co3O4 and PANI in present work and other reported work in the literature. Sample

Electrolyte

Specific capacitance

Configuration

Reference

Co3O4/PANI nanocages

6 M KOH

Three-electrode

[26]

(CNFs)/Co3O4-PANI composites

1 M Na2SO4

Three-electrode

[33]

Graphene/PANI/Co3O4 ternary hybrid aerogels

6 M KOH

Two-electrode

[40]

PANI/Co3O4 layered composite RGO/PANI/Co3O4

0.5 M Na2SO4 6 M KOH

Three-electrode Three-electrode

[50] [52]

Co3O4 decorated with PANI nano shrubs

1 M Na2SO4

1301 F g
1 at 1 A g
1, 572 F g
1 at 20 A g
1 770 F g
1 at 1 A g
1 200 F g
1 at 20 A g
1 1247 F g
1 at 1 A g
1 755 F g
1 at 20 A g
1 160 F g
1 at 1 mA 789.7 F g
1 at 1 A g
1 581.8 F g
1 at 20 A g
1 1151 F g
1 at 3 A g
1 803 F g
1 at 20 A g
1

Two electrode

Present work

10

S. C.P et al. / Electrochimica Acta 324 (2019) 134876

Fig. 10. (a) The rate performance of electrodes of all composites at current densities ranging from 3 to 20 A g
1: The average and deviations at each current density was determined from six replicated symmetric supercapacitors for each composition; (b) Cycle performance of the COP 70 composite electrode tested at a current density of 10 A g
1 for 5000 cycles.

well-aligned nano shrubs of PANI over the surface of Co3O4. The formed nano shrubs showed low charge transfer resistance, high conductivity and a good electrolyte accessibility. COP with 70% PANI and 30% Co3O4 (COP 70) measured excellent performance with high specific capacitance value (1151 F g
1 at 3 A g
1), high rate performance of 70% at the current density ranging from 3 to 20 A g
1 and excellent cycling stability (92% after 5000 chargedischarge cycles) using symmetric two electrode configuration. Thus, the developed facile strategy can be exploited for the establishment of similar systems of various transition metal oxides and the developed material having high charge carrier density and mobility promised to be an efficient electrode system for the fabrication of other highly efficient flexible devices. The Co3O4 decorated with PANI nano shrubs (in particular COP 70) is believed to be a promising electrode material for supercapacitors. Acknowledgements We are grateful to Dr. A. Ajayaghosh, Director, CSIR-NIIST, Trivandrum for constant encouragement and support. We are also thankful to Mr. Harish Raj, Mr. Kiran Mohan, and Mrs. Suchitra VG for SEM, TEM, and XRD analyses. We are also thankful to DST/SERB for the National Post-Doctoral fellowship and DST/TMD/MES/2K17/ 88(G)/1. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.electacta.2019.134876. References [1] A. Scalia, F. Bella, A. Lamberti, C. Gerbaldi, E. Tresso, Energy 166 (2019) 789e795. [2] P. Dong, M.-T.F. Rodrigues, J. Zhang, R.S. Borges, K. Kalaga, A.L.M. Reddy, G.G. Silva, P.M. Ajayan, J. Lou, Nano Energy 42 (2017) 181e186. [3] S. Najib, E. Erdem, Nanoscale Adv (2019), https://doi.org/10.1039/ C9NA00345B. [4] C. Arbizzani, M. Mastragostino, F. Soavi, J. Power Sources 100 (2001) 164e170. [5] N. Tang, W. Wang, H. You, Z. Zhai, J. Hilario, L. Zeng, L. Zhang, Catal. Today 330 (2019) 240e245. [6] G.P. Wang, L. Zhang, J.J. Zhang, Chem. Soc. Rev. 41 (2012) 797e828. [7] D.B. Xiong, X.F. Li, Z.M. Bai, J.W. Li, H. Shan, L.L. Fan, C.L. Long, D.J. Li, X.H. Lu, Electrochim. Acta 259 (2018) 338e347.

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