Anchoring cobalt oxide nanoparticles on to the surface multiwalled carbon nanotubes for improved supercapacitive performances

Abstract

The present work explored a novel, simple and low cost ‘dipping and drying’ process followed by a successive ionic layer adsorption and reaction (SILAR) method for the synthesis of cobalt oxide anchored multiwalled carbon nanotubes (Co₃O₄/MWNTs). Initially, MWNTs have been coated on a stainless steel substrate by a simple ‘dip and dry’ method, on which further deposition of cobalt oxide nanoparticles was carried out by the SILAR method. Our results confirm the uniform coating of Co₃O₄ nanoparticles having sizes less than 15 nm on the surface of MWNTs. Later, the electrochemical performance shows that, the Co₃O₄/MWNTs films exhibit a maximum specific capacitance of 685 F g⁻¹ in a 2 M KOH electrolyte at a scan rate of 5 mV s⁻¹ with high cycle stability of 73% over 5000 cycles. Moreover, lower electrochemical equivalent series resistance (11.25 mΩ) give rise to the superior performance. These results show, the potential of Co₃O₄/MWNTs composite electrodes in electrochemical supercapacitors.

---

Introduction

In recent years, research has been focused on enhanced safety, long cycle life, high-power and energy-density storage devices due to the ever-increasing needs in modern electronics industry such as digital communication, short-term power sources for mobile electronic devices, electric vehicles, hybrid electric vehicles, and other power supply facilities.¹ In this regard, different batteries and high performance capacitors are the focus of the scientific community.² Supercapacitors have attracted intense interest owing to their combined merits, i.e., high power density of conventional dielectric capacitors and high energy density of batteries.³ The key to develop supercapacitors is the design and synthesis of high-performance electrode materials. With this understanding, new ideas in electrochemical energy storage devices that are superior to conventional energy storage devices are under challenging investigations. Carbon materials,⁴ transition metal oxides⁵ and conducting polymer⁶ have been proved favourable for supercapacitors because of their high theoretical capacitance.

Recently, metal oxide and carbon nanotubes nanocomposites materials have been attracted great attention due to their excellent properties which combines the individual materials property and their synergistic effects for electrode materials.⁷ The introduction of metal oxides in one-dimensional carbon nanotubes (CNTs) with hollow interior space are ideal candidate due to available more reaction sites with conducting channels. To date, many reports are available in literatures to fabricate the different electrochemical metal oxide/MWNT composites such as NiO/MWNTs,⁸ MnO₂/MWNTs,⁹ TiO₂/MWNTs,¹⁰ V₂O₅/MWNTs¹¹ and Fe₃O₄/MWNTs.¹² From the literature survey, the Co₃O₄/MWNTs composite as a typical, with the consideration of low cost, natural abundance, environmental safety, good corrosion stability, good long term performance, and low cost have been reported to be promising electrode materials for supercapacitors.¹³ Cobalt oxide can interact with ions not only at the surface, but also throughout the bulk as well.¹⁴ For the past few years, considerable efforts have been focused on cobalt oxide electrode materials for supercapacitors. However, very few reports are available on composite of Co₃O₄/MWNTs electrodes such as microwave assisted approach, chemical precipitation method, electrophoretic deposition, hydrothermal method, in situ decomposition method, electrostatic co-precipitation and in situ decomposition method.^15–17 Amongst these, hydrothermal, Electrostatic co-precipitation, solution method, two step surfactant assisted method which is one of the most popular growth techniques for coating Co₃O₄. These conventional physical techniques generally produce good quality transparent films. However, these methods do not satisfy the requirements for commercial application because of their complicated multi-step preparation and cost. However, synthesis of Co₃O₄/MWNTs with unique hybrid nanostructure directly on the support/substrate to use as supercapacitive electrode is still a challenge. On the other hand, chemical deposition techniques are relatively low cost processes and can be easily scaled up for industrial applications. Among the chemical techniques, chemical bath deposition (CBD) is one of the simplest methods of depositing Co₃O₄.

In this work, we are providing very simple and efficient approach for the synthesis of Co₃O₄/MWNTs thin film as an electrode for supercapacitors. This approach includes two different steps, (1) coating of MWNTs by dip and dry method and (2) subsequent deposition of Co₃O₄ nanoparticles on MWNTs by successive ionic layer adsorption and reaction (SILAR) method. Moreover, the work is also aimed to increase the electronic conductivity and stability of Co₃O₄ nanodots using MWNTs with Co₃O₄ nanoparticles in the outer shell and highly conducting MWNTs as inner core. Electrochemical supercapacitive properties of Co₃O₄/MWNTs hybrid electrodes have been systematically investigated.

Results and discussion

Co₃O₄/MWNTs thin films were prepared by deposition of MWNTs on stainless steel substrates using dip and dry method on which subsequent anchoring of Co₃O₄ nanoparticles using SILAR method. Briefly, functionalized MWNTs were well dispersed into the solution containing the distilled water and 1 wt% Triton X-100 using an ultrasonicator. During the sonication Triton X-100 molecules get adsorbed on the MWNTs surface, interpreting them soluble in aqueous system. When the substrate was immersed in this solution, the MWNTs were adsorbed onto the substrate due to the attractive forces between the substrate and the MWNTs. These forces may be cohesive, van der Waal's forces or chemical attractive forces. Repetition of such cycles makes uniform and well adherent coating of MWNTs on stainless steel substrate. Further, Co₃O₄ nanoparticles were anchored on MWNTs using SILAR method. In details, when substrate with pre-coated MWNTs was dipped in cationic precursor, cobalt hexamine complex species ([Co(NH₃)₆]²⁺) in the solution get adsorbed on MWNTs due to cohesive/van der Waal's/chemical attractive forces between complexed cobalt ions and the functionalised MWNTs.¹⁹

[Co(H₂O)₆]²⁺ + 6NH₃ → [Co(NH₃)₆]²⁺ + 6H₂O (1)

Later, the immersion of complexed cobalt species decorated MWNTs substrate in aqueous solution of H₂O₂ (0.1%) results in the reaction between OH⁻ (from aq. H₂O₂) ions and pre-adsorbed cobalt hexamine complex ions at the surface of MWNTs which at the end forms cobalt oxyhydroxide layer according to the following reaction,

[Co(NH₃)₆]²⁺ + 2H₂O₂ → CoO(OH) + NH₃↑ + 2H₂O (2)

The cycle of ion adsorption followed by oxidation reaction was repeated several times to achieve terminal thickness of Co₃O₄/MWNTs film. Finally, the films were annealed for 2 h at 723 K, in order to remove oxyhydroxide phase to pure oxide.

CoO(OH) is very reactive with O₂, and forms a higher oxide Co₃O₄. Thus, the uniform and well-adherent Co₃O₄/MWNTs thin films were obtained on the stainless steel substrates.

XRD patterns are recorded for bare steel, MWNTs, and Co₃O₄/MWNTs thin film and presented in Fig. 1(a). The characteristic of graphitic (002) peak of the MWNTs at the 2θ = 25.3° was clearly observed in both spectra of MWNTs and Co₃O₄/MWNTs samples. In addition to the characteristics peak, the reflections along (220) and (440) obtained at 49.20° and 64.30° corresponds to Co₃O₄ material [JCPDS 70-0816]. Other peaks obtained at 43.66, 50.32, 65.21, and 74.46° results from the contribution of the stainless steel substrate. Since the stainless steel peaks are much stronger than Co₃O₄ peaks, the weak peaks of Co₃O₄ are overlapped. There were no other impurity peaks observed in XRD analysis.

Fig. 1 (a) XRD patterns of the bare stainless steel, MWNTs and Co₃O₄/MWNTs thin film (b) FTIR spectra of MWNTs and Co₃O₄/MWNTs samples.

FTIR is used to analyse the chemical bonding and type of functional groups grafted onto the MWNTs. FTIR spectra of functionalised MWNTs and Co₃O₄/MWNTs samples are shown in Fig. 1(b). As seen from the spectra, functionalized MWNTs exhibits some prominent absorption bands at 1653, 1511, 1219 cm⁻¹, respectively, which are originated from the graphitic component of MWNTs. The absorption peak can be found at 1653 cm⁻¹ due to the attachment of carboxyl group onto the surface of MWNTs. The appearance of two strong bands at 626 and 575 cm⁻¹ are attributed to the vibration of Co–O in the Co₃O₄/MWNTs sample which is a clear evidence of the presence of Co₃O₄.²⁰

Fig. 2(a and b) show FESEM images of Co₃O₄/MWNTs films at two different magnifications. It is seen that, the outside surface of MWNTs is uniformly anchored with Co₃O₄ nanoparticles. No aggregation of Co₃O₄ was seen in the porous area between MWNTs network, indicating that the nucleation occurs predominantly on the outer surfaces of MWNTs and just some micrometres long MWNTs are uniformly entangled to form uniform three dimensional interconnected networks. Close inspection of Co₃O₄/MWNTs confirms the uniformity of nanoparticles less than about 15 nm in average size.

Fig. 2 (a and b) FESEM images Co₃O₄/MWNTs thin film at two different magnifications.

Detailed information about the size, shape and distribution Co₃O₄/MWNTs were obtained from transmission electron microscopy (TEM) images. Low magnified image (Fig. 3(a)) shows, the porous and uniform distribution of Co₃O₄ particle, which is consistent with the FESEM image. It is also seen that, Co₃O₄ nanoparticles are anchored without showing any aggregation and sustaining porous nature of MWNTs. Because growth occurs through an ion by ion mechanism in SILAR method, no aggregation of Co₃O₄ nanoparticles occurs on the walls of the MWNTs. Moreover, high magnified image (Fig. 3(b)) revealed the formation of nanoparticles of about less than 15 nm. Some big nanoparticles are observed which might be due to aggregation of nanoparticles. The interplaner spacing is calculated to be 0.33 nm (Fig. 3(c)), which corresponds to the (002) plane in MWNTs and 0.27 nm from Fig. 3(d), which corresponds to the (220) plane of Co₃O₄, respectively. In addition, the selected area electron diffraction (SAED) was performed to investigate the crystalline characteristics of Co₃O₄/MWNTs thin film (see inset of Fig. 3(d)). The SAED pattern of Co₃O₄/MWNTs thin film reveals polycrystalline nature. Thus, the lattice fringes in HRTEM image and the selected area electron diffraction (SAED) pattern further confirmed the formation of crystalline Co₃O₄.

Fig. 3 (a and b) TEM images of Co₃O₄/MWNTs at two different magnifications, respectively, (c and d) HRTEM images of MWNTs and Co₃O₄, respectively. Inset of figure (d) shows SAED pattern of Co₃O₄/MWNTs.

Cyclic voltammetry (CV) was performed to investigate supercapacitive performance of Co₃O₄/MWNTs electrodes in KOH electrolyte within a potential window of 0.2 to +0.5 V (vs., Ag/AgCl). In order to find suitable electrolyte concentration for good electrochemical performance CV curves of Co₃O₄/MWNTs composite were recorded at 100 mV s⁻¹ scan rate at different concentrations of KOH (see Fig. 4(a)). The effect of concentration of KOH electrolyte was studied by keeping the scan rate and potential window constant. CV curves were more distorted with low concentration and did not show well-resolved redox peaks. As concentration of the electrolyte was increased from 0.1 to 2.0 M, the redox peaks were seen distinctly. This change in capacitance may be attributed to the presence of Co₃O₄ in the composite electrode leading to the contribution of pseudocapacitance with double layer capacitance of MWNTs. This concludes that pseudo-capacitive contribution is dominating than the double layer capacitance.

The specific capacitance of the samples was estimated from the integrated charge from the CV data using the equation;

where, C_s is the specific capacitance (F g⁻¹), v is the potential scan rate (mV s⁻¹), (V_c − V_a) is the potential range (0.2–0.5 V, Ag/AgCl) and I_m denotes the response current (mA cm⁻²) based on the mass of the Co₃O₄/MWNTs material. Fig. 4(b) shows the variation of specific capacitance of Co₃O₄/MWNTs with electrolyte concentration at 50 mV s⁻¹ scan rate. It is seen that, the maximum specific capacitance of Co₃O₄/MWNTs electrode was observed at 2 M KOH electrolyte concentration. Further electrolyte concentration is increased, the difference between the redox peak potential decreases. This is due to free electrolyte starvation since; for the amount of electrolyte present in the solutions almost all ions become adsorbed at high area interface and thereby enhance the internal resistance effect towards full state-of-charge through a combination of the usual distributed resistance effects.²¹This is the reason of poor performance at higher concentration. Similar type behaviour has been observed by Zheng and Conway.²² Hence, all further electrochemical performances were carried out in 2 M KOH electrolyte.

Fig. 4 (a) Cyclic voltammetry (CV) curves of Co₃O₄/MWNTs electrode at different electrolyte concentration, (b) variation of specific capacitance Co₃O₄/MWNTs with electrolyte concentration at 50 mV s⁻¹ scan rate, (c) CV curves of Co₃O₄/MWNTs electrode at different scanning rates in 2 M KOH electrolyte, (d) variation of specific capacitances of Co₃O₄/MWNTs electrode with scan rate.

Fig. 4(c) shows CV curves of Co₃O₄/MWNTs thin film at different scan rates from 5 to 100 mV s⁻¹. It is obvious that the shape of the CV curves does not change evidently even at higher scan rates and the total current density increases with increasing scan rates, which reveals a good rate property and excellent capacitance behaviour for Co₃O₄/MWNTs electrode. Further, it is clear that voltammetric currents are directly proportional to the scan rate of CV, indicating ideal capacitance behaviour.²³ It can also be seen that when scan rate is increased, the current density increases rapidly and the separation between the reduction and oxidation peaks becomes larger, which is mainly due to the polarization of the cell under the relatively high scan rate. The specific capacitance values are decreased from 685 to 105 F g⁻¹ with scan rate from 5 mV s⁻¹ to 100 mV s⁻¹ (see Fig. 4(d)). The excellent specific capacitance obtained would be due to fact that the surface morphology of the deposited films significantly affects the capacitance of an electrochemical capacitor. It is further seen that the specific capacitance decreases with increase in scan rate. The decrease in capacitance with the scan rate is attributed to the presence of inner active sites, which cannot precede the redox transitions completely at higher scan rates of CV, probably due to the diffusion effect of cations within the electrode. The higher the scan rate, lower the diffusivity of cations into the bulk of the electroactive material. Henceforth, only the outer surface of Co₃O₄/MWNTs thin film electrode would be utilized to generate pseudocapacitance.²⁴ At lower scan rates, all the active areas, including external and internal surfaces, can be utilized for charge/discharge and electrochemical utilization of Co₃O₄ nanoparticles (full utilization of the electrode materials).²⁵

Fig. 5(a) shows the galvanostatic charge/discharge curves of MWNTs, Co₃O₄ and Co₃O₄/MWNTs thin films at current density of 2 A g⁻¹. Almost linear relationship between voltage and charging/discharging time for MWNT has been observed which is common phenomenon expected from non-faradic electrodes. However, the charge/discharge curves for Co₃O₄ and Co₃O₄/MWNT are not linear suggesting contribution from pseudo-capacitive charge storing mechanism. The specific capacitance (C_s) can be calculated by using the relation:

where I is the constant discharge current, T_d is the discharge time, W active mass and V is the potential window. The maximum specific capacitances obtained for MWNTs, Co₃O₄ and Co₃O₄/MWNTs hybrid electrodes are 148, 497 and 660 F g⁻¹, respectively. The values presented in this work are comparatively higher than previously reported values based on Co₃O₄ composite materials (see Table 1).^27–36 The improvement in specific capacitance is attributed to the synergetic contribution of Co₃O₄ (pseudocapacitive) and MWNTs (double layer capacitance). Further, the galvanostatic discharge curves of the Co₃O₄/MWNTs electrodes were recorded at various current densities and shown in Fig. 5(b). It is seen that, the curves starts to become linear during charging/discharging at higher current densities, which is likely from non-faradic reaction. Nevertheless at low current density, complete non-linear curve is observed, confirming the pseudocapacitive reactions originated from Co₃O₄ nanodots. It is worth noting that as discharge current increases, the voltage (IR) drop increases and capacitance decreases. Fig. 5(c) shows the variation of specific capacitance with current density. It is seen that, the specific capacitance decreases with current density. The Co₃O₄/MWNTs electrodes have a good electrochemical reversibility and a large specific capacitance which could be derived from the unique 3D architecture and high conductivity of the prepared electrode. The improvements are credited to the synergetic effect of Co₃O₄ and MWNTs. The values of specific energy and power are extracted from the galvanostatic charge/discharge curves and presented in the Fig. 5(d). The maximum specific energy obtained for Co₃O₄/MWNTs electrode was 16.41 W h kg⁻¹ at specific power of 300 W kg⁻¹. It is further interesting to note that, the specific energy remains 12.2 W h kg⁻¹ at specific power of 1000 W kg⁻¹ after increasing current density to 5 A g⁻¹. These values are quite impressive in the context of exploiting Co₃O₄/MWNTs nanocomposites for supercapacitors application.

Fig. 5 (a) Galvanostatic charge/discharge curves of MWNT, Co₃O₄ and Co₃O₄/MWNTs thin films at current density of 2 A g⁻¹, (b) galvanostatic charge/discharge curves of Co₃O₄/MWNTs nanocomposites at different current densities, (c) variation of specific capacitance of Co₃O₄/MWNTs as a function of current density, (d) the specific energy vs. specific power of Co₃O₄/MWNTs in Ragone plot.

Table 1 The comparative representation of different aspects of Co₃O₄/MWNTs composite for supercapacitor application

Material	Method	Electrolyte	Specific capacitance (F g^−1)	Ref.
Co3O4/CNTs	Solution method	1 M KOH	201 (10 mV s^−1)	27
Co3O4/SWNTs	Electrostatic co-precipitation	6 M KOH	343 (5 mV s^−1)	28
Co3O4/reduced graphene oxide	Hydrothermal	2 M KOH	263 (2 A g^−1)	29
Co3O4/reduced graphene oxide	Two-step surfactant-assisted	6 M KOH	159 (5 mV s^−1)	30
Co3O4/reduced graphene oxide	Hydrothermal	2 M KOH	472 (2 mV s^−1)	31
Co3O4/MWNTs	Dip and dry followed by SILAR	2 M KOH	685 (5 mV s^−1)	Present work
Co3O4/graphene	Solution based method	2 M KOH	478 (5 mV s^−1)	32
Co3O4/MWCNT	Co-precipitation	2 M KOH	418 (0.625 A g^−1)	33
Co3O4/graphene	Microwave assisted	6 M KOH	243.2 (10 mV s^−1)	34
Co3O4/graphene	Hydrothermal	2 M KOH	157.7 (0.1 A g^−1)	35
Co3O4/MWCNT	Hydrothermal	0.5 M KOH	590 (15 A g^−1)	36

---

The stability of the electrode in a long period is a significant parameter for evaluating the performance of the electrode. Fig. 6(a) shows the long cycling life test of Co₃O₄/MWNTs thin film over 1000 cycles at scan rate of 200 V s⁻¹. From the inset of Fig. 6(a), it is seen that all the curves are overlapping each other which specifies good cycling stability. The specific capacitance decreases by small amount after 5000 cycles. It is interesting that the capacity retention of Co₃O₄/MWNTs electrode after 5000 cycles is about 73%. Moreover, the CV cycling test of the Co₃O₄/MWNTs film suggests that the synergetic interaction between the MWNTs and Co₃O₄ significantly improved the electrical properties and the mechanical stability of the electrode.

Fig. 6 (a) Variation of capacity retention of Co₃O₄/MWNTs electrode with number of cycles, inset shows CV curves at different number of cycles, (b) Nyquist plots of Co₃O₄/MWNTs thin films with AC voltage amplitude of 10 mV, inset shows equivalent circuit diagram.

In order to evaluate the resistances contributed during the charge storing in Co₃O₄/MWNTs electrode electrochemical impedance measurements were performed at a frequency range from 100 kHz to 0.01 Hz. EIS plot of a supercapacitor device usually divided into three segments following three processes; (i) the bulk resistance of the device or ESR, at high frequencies (>1 kHz); (ii) capacitive effects at high and intermediate frequencies (0.1 kHz); and Warburg diffusion resulting from the frequency dependence of ion diffusion/transport in the electrolyte at low frequencies (<0.1 Hz).²⁶ Fig. 6(b) shows Nyquist plot of Co₃O₄/MWNTs electrode in 2 M KOH electrolyte. It is seen that, the charge transfer resistance and equivalent series resistance (ESR) (as shown inset) were obtained 11.73 and 0.4, respectively. A low ESR means higher power density as the power density of a supercapacitor is equal to the V²/4R_sm. This low ESR would increase the power density of the electrode.

Conclusions

It is concluded that, we have presented simple and efficient way to anchor cobalt oxide nanodots on to the multiwalled carbon nanotubes in order to fabricate Co₃O₄/MWNTs nanocomposite for supercapacitor application. Two simple solution based method such as “dip and dry” to coat MWNTs and “SILAR” to anchor Co₃O₄ nanodots have been successfully exploited. Obtained results confirm the uniform coating of Co₃O₄ nanodots on the surface of MWNTs. Later, the electrochemical performance shows that Co₃O₄/MWNTs hybrid electrode exhibits a maximum specific capacitance of 685 F g⁻¹ in 2 M KOH electrolyte at scan rate of 5 mV s⁻¹ with high cycle stability of 73% over 5000 cycles. Moreover, the maximum specific energy obtained for Co₃O₄/MWNTs electrode was 16.41 W h kg⁻¹ at specific power of 300 W kg⁻¹ which retained to 12.2 W h kg⁻¹ at specific power of 1000 W kg⁻¹ after increasing current density to 5 A g⁻¹. The excellent electrochemical performance is attributed to the synergistic effect of two components and fast redox reactions due to short and diffusion path of ions and electrons. This easy synthesis and short production process would be helpful in production of Co₃O₄/MWNTs nanostructure on a large scale.

Experimental details

Synthesis of Co₃O₄/MWNTs thin films

To remove amorphous carbon and generate functional groups, MWNTs were refluxed in H₂O₂ at 60 °C h for suitable time. These functionalised MWNTs are thoroughly washed with double distilled water and then dried in oven at 90 °C for 12 h. Further, 0.125 g of functionalised MWNTs was sonicated in 25 ml aqueous solution of 1 wt% Triton X-100 for 1 h to obtain stable dispersion. Well cleaned stainless steel substrates were dipped in MWNTs solution for 10 min so that the MWNTs get adsorbed on the surface of substrate. The process was repeated by 15–20 times to deposits MWNTs onto substrate. Further these MWNTs coated stainless steel substrate was dried under IR lamps to evaporate the solvents as described previously.¹⁸ This stainless steel substrate coated with MWNTs was used as substrate for further deposition of Co₃O₄ nanodots.

Later Co₃O₄ nanodots were anchored on MWNTs by SILAR method. Briefly, 0.4 M CoCl₂, was dissolved in double distilled water; to this complexing agent liquor ammonia was added with constant stirring to maintain the pH ∼ 12 which served as cationic precursor. The oxidant solution of 0.1% H₂O₂ in double distilled water which served as anionic precursor. Stainless steel substrate with pre-coated MWNTs was dipped in cationic precursor for 40 s, in which complexed cobalt species get adsorbed onto the walls of the MWNTs because of the electrostatics forces of attraction. In next step, the substrate was rinsed with double-distilled water for 15 s to remove loosely bounded or excess complexed cobalt species from the MWNTs coated substrate. Later, the substrate was immersed in anionic precursor (0.1% H₂O₂) solution for 20 s where the oxygen ions reacts with pre-adsorbed complexed cobalt species on the MWNTs to form cobalt hydroxide film. In the last step, the substrate was again rinsed with double-distilled water for 15 s to remove loosely bounded OH⁻ ions. Thus one cycle for deposition of cobalt hydroxide onto the MWNTs/SS is completed. The cycle of ion adsorption followed by oxidation reaction was repeated several times to increase the thickness of cobalt hydroxide film. Further, the films were annealed at 723 K for 2 h, in order to convert hydroxide phase to pure oxide and also to improve the mechanical stability and electrical conductivity of the electrode and then used for further characterizations.

Characterization techniques

The amount of Co₃O₄ decorated on MWNTs was measured by commonly used weight difference method using sensitive microbalance. The phase identification of the as-prepared Co₃O₄/MWNTs nanocomposite was carried out by X-ray Diffractometer (XRD, D8 Advanced, Bruker AXS) with Cu Kα radiation (λ = 1.5418 Å) operating at 40 kV and 40 mA. FTIR spectroscopy analysis was carried out using FTIR spectrometer (JASCO 410) in the spectral range 4000–400 cm⁻¹. The surface morphological study was performed by a field emission scanning electron microscope (FE-SEM, JSM-6360LA) and a high resolution transmission electron microscope (HRTEM, JEOL JEM 2100 Japan, resolution 1.4 Å, AC voltage 200 kV). The electrochemical properties were performed by using potentiostat (Princeton Applied Research, PARSTAT-4000). A typical three electrode cell was employed as a reference electrode (Ag/AgCl), a counter electrode (Platinum wire) and working electrode (Co₃O₄/MWNTs thin film). All electrochemical experiments were carried out at room temperature. Galvanostatic charge/discharge and electrochemical impedance measurements were performed using the Biologic VMP3 potentiostat. Potentiostat/Galvanostat with impedance software (Princeton Applied Research, PARSTAT-4000).

Acknowledgements

BRS is thankful to SERB, Govt. of India through sanctioned project (Do. no: SB/S2/CMP/032/2013, dated 20/12/2013).

References

1. (a) D. Cericola and R. Koetz, Electrochim. Acta, 2012, 72, 1; (b) M. Li, J. P. Cheng, J. H. Fang, Y. Yang, F. Liu and X. B. Zhang, Electrochim. Acta, 2014, 134, 309; (c) C. D. Lokhande, D. P. Dubal and O. S. Joo, Curr. Appl. Phys., 2011, 11, 255.
2. (a) D. P. Dubal, O. Ayyad, V. Ruiz and P. Gomez-Romero, Chem. Soc. Rev., 2015, 44, 1777; (b) F. Meng and Y. Ding, Adv. Mater., 2011, 23, 4098.
3. (a) Y. F. Yan, Q. L. Cheng, Z. J. Zhu, V. Pavlinek, P. Saha and C. Z. Li, J. Power Sources, 2013, 240, 544; (b) J. Suarez-Guevara, V. Ruiz and P. Gomez-Romero, J. Mater. Chem. A, 2014, 2, 1014.
4. (a) J. Tang, J. Liu, N. L. Torad, T. Kimura and Y. Yamauchi, Nano Today, 2014, 9, 305; (b) N. L. Torad, R. R. Salunkhe, Y. Li, H. Hamoudi, M. Imura, Y. Sakka, C. C. Hu and Y. Yamauchi, Chem.–Eur. J., 2014, 20, 7895.
5. (a) X. C. Dong, H. Xu, X. W. Wang, Y. X. Huang, M. B. C. Park, H. Zhang, L. Wang, W. Huang and P. Chen, ACS Nano, 2012, 6, 3206; (b) Y. F. Yan, Q. L. Cheng, Z. J. Zhu, V. Pavlinek, P. Saha and C. Z. Li, J. Power Sources, 2013, 240, 544; (c) D. P. Dubal, P. Gomez-Romero, B. R. Sankapal and R. Holze, Nano Energy, 2015, 11, 377.
6. (a) D. P. Dubal, S. H. Lee, J. G. Kim, W. B. Kim and C. D. Lokhande, J. Mater. Chem., 2012, 22, 3044; (b) R. R Salunkhe, S. H. Hsu, K. CW Wu and Y. Yamauchi, ChemSusChem, 2014, 7, 1551.
7. (a) D. P. Dubal, J. G. Kim, Y. Kim, R. Holze, C. D. Lokhande and W. B. Kim, Energy Technol., 2014, 2, 325; (b) Y. Huang, J. Tao, W. Meng, M. Zhu, Y. Huang, Y. Fu, Y. Gao and C. Zhi, Nano Energy, 2015, 11, 518.
8. (a) H. Y. Chang, H. C. Chang and K. Y. Lee, Vacuum, 2013, 87, 164; (b) G. S. Gund, D. P. Dubal, S. S. Shinde and C. D. Lokhande, ACS Appl. Mater. Interfaces, 2014, 6, 3176.
9. S. G. Hwang, S. H. Ryu, S. R. Yun, J. M. Ko, K. M. Kim and K. S. Ryu, Mater. Chem. Phys., 2011, 130, 507.
10. H. I. Kim, H. J. Kim, M. Morita and S. G. Park, Bull. Korean Chem. Soc., 2010, 31, 423.
11. Z. Chen, V. Augustyn, J. Wen, Y. Zhang, M. Shen, B. Dunn and Y. Lu, Adv. Mater., 2011, 2, 791.
12. (a) A. K. Mishra and S. Ramaprabhu, J. Phys. Chem. C, 2010, 114, 2583; (b) W. Meng, W. Chen, L. Zhao, Y. Huang, M. Zhu, Y. Huang, Y. Fu, F. Geng, J. Yu, X. Chen and C. Zhi, Nano Energy, 2014, 8, 133.
13. Y. G. Zhu, Y. Wang, Y. Shi and H. Y. Yang, Nano Energy, 2014, 3, 46.
14. S. L. Xiong, C. Z. Yuan, X. G. Zhang, B. J. Xi and Y. T. Quin, Chem.–Eur. J., 2009, 15, 5320.
15. M. Mazloumi, S. Shadmehr, Y. Rangom, L. Nazar and X. Tang, ACS Nano, 2013, 5, 4281.
16. J. Lang, X. Yan and Q. Xue, J. Power Sources, 2011, 196, 7841.
17. H. Fang, S. Zhang, W. Liu, Z. Du, X. Wu and Y. Xing, Electrochim. Acta, 2013, 108, 651.
18. B. R. Sankapal, H. B. Gajare, D. P. Dubal, R. B. Gore, R. R. Salunke and H. Ahn, Chem. Eng. J., 2014, 247, 103.
19. S. G. Kandalkar, J. L. Gunjakar and C. D. Lokhande, Appl. Surf. Sci., 2008, 254, 5540.
20. (a) S. M. Abbas, S. T. Hussain, S. Ali, N. Ahmad, N. Ali and K. S. Munawar, Electrochim. Acta, 2013, 105, 481; (b) C. K. Dong, X. Li, Y. Zhang, J. Y. Qi and Y. F. Yuan, Chem. Res. Chin. Univ., 2009, 25, 936.
21. (a) T. P. Gujar, V. R. Shinde, C. D. Lokhande and S. Han, J. Power Sources, 2006, 161, 1479; (b) K. Tsay, L. Zhang and J. Zhang, Electrochim. Acta, 2012, 60, 428.
22. (a) J. P. Zheng and T. R. Jow, J. Electrochem. Soc., 1997, 144, 2417; (b) B. E. Conway and W. G. Pell, J. Power Sources, 2002, 105, 169.
23. (a) D. P. Dubal, V. J. Fulari and C. D. Lokande, Microporous Mesoporous Mater., 2012, 151, 511; (b) D. P. Dubal, R. Holze and P. Gomez-Romero, Sci. Rep., 2014, 7, 7349.
24. (a) V. Subramanian, H. Zhu and B. Wei, J. Power Sources, 2006, 159, 361; (b) M. Zhu, W. Meng, Y. Huang, Y. Huang and C. Zhi, ACS Appl. Mater. Interfaces, 2014, 6, 18901.
25. G. S. Gund, D. P. Dubal, S. B. Jambure, S. S. Shinde and C. D. Lokhande, J. Mater. Chem. A, 2013, 1, 4793.
26. (a) G. Yu, L. Hu, M. Vosgueritchian, H. Wang, X. Xie, J. R. McDonough, X. Cui, Y. Cui and Z. Bao, Nano Lett., 2011, 11, 2905; (b) P. Sen, Electrochim. Acta, 2010, 55, 1.
27. Y. Shan and L. Gao, Mater. Chem. Phys., 2007, 103, 206.
28. A. Abdolmaleki, H. Kazerooni, M. B. Gholivand, H. Heydari and A. Pendashteh, Ionics, 2015, 21, 515.
29. G. J. Liu, L. Q. Fan, F. D. Yu, J. H. Wu, L. Liu, Z. Y. Qiu and Q. Liu, J. Mater. Sci., 2013, 48, 8463.
30. W. Zhou, J. Liu, T. Chen, K. S. Tan, X. Jia, Z. Luo, C. Cong, H. Yang, C. M. Li and T. Yu, Phys. Chem. Chem. Phys., 2011, 13, 14462.
31. C. Xiang, M. Li, M. Zhi, A. Manivannan and N. Wu, J. Power Sources, 2013, 226, 65.
32. B. Wang, Y. Wang, J. Park, H. Ahn and G. Wang, J. Alloys Compd., 2011, 509, 7778.
33. J. Lang, X. Yan and Q. Xue, J. Power Sources, 2011, 196, 7841.
34. J. Yan, T. Wei, W. Qiao, B. Shao, Q. Zhao, L. Zhang and Z. Fan, Electrochim. Acta, 2010, 55, 6973.
35. Q. Guan, J. Cheng, B. Wang, W. Ni, G. Gu, X. Li, L. Huang and G. Yang, ACS Appl. Mater. Interfaces, 2014, 6, 7626.
36. X. Wang, M. Li, Z. Chang, Y. Yang, Y. Wu and X. Liu, ACS Appl. Mater. Interfaces, 2015, 7, 2280.

---