Hierarchical Flowerlike 3D nanostructure of Co3O4@MnO2/N-doped Graphene oxide (NGO) hybrid composite for a high-performance supercapacitor

The present study investigates the fabrication of hierarchical 3D nanostructures with multi-component metal oxides in the presence of highly-porous graphene and characterized for its applications in high-performance supercapacitors. A hierarchical flowers like 3D nanostructure of Co3O4 @MnO2 on nitrogen-doped graphene oxide (NGO) hybrid composite was synthesized by thermal reduction process at 650 °C in the presence of ammonia and urea. The synthesized Co3O4@MnO2/NGO hybrid composites were studied via Raman, XRD, X-ray XPS, FE-SEM, FE-SEM with EDX, FE-TEM and BET analyses. The electrochemical analysis of Co3O4@MnO2/NGO hybrid composite electrode was investigated using cyclic voltammetry, chronopotentiometry and electrochemical impedance measurements. The hybrid composite electrode showed significant specific capacitance results of up to 347 F/g at 0.5 A/g and a corresponding energy density of 34.83 Wh kg−1 with better rate performance and excellent long-term cycling stability were achieved for 10,000 cycles. The obtained electrochemical results paved a way to utilize Co3O4@MnO2/NGO composite electrode as a promising electrode material in high performance supercapacitors.

Zhu et al. reported on CVD-derived NGO sheets by organic molecules via carbon and nitrogen atoms doped with higher concentration and growth temperature 11 . In addition, the carbon-based materials with a porous structure, transition-metal oxides and conjugated polymers are important active electrode materials for supercapacitors [12][13][14] . Transition metal oxides such as nickel (Ni), cobalt (Co), and manganese (Mn) oxides offer superior physiochemical properties and comprise excellent electrochemical supercapacitors 15 . In particular, cobalt oxide (Co 3 O 4 ) is considered to be an auspicious electrode material for supercapacitors because of its economically cheap, high redox activity, high surface area and easily tunable surface properties. The nanostructured Co 3 O 4 is the most stable cobalt oxide with a spinel structure and important p-type semiconductor which is widely used in Li-ion batteries, heterogeneous catalysts, electrochemical capacitors, electrochromic devices, solid-state sensors, solar selective absorbers for high-performance supercapacitors [15][16][17] . The hierarchical 3D structure provides a high surface area and rapid electron transport from the electroactive materials to the current collector, which may be due to the active material being grown directly on the current collector and avoiding the binders that normally cause a decrease in the electrode conductivity [17][18][19][20][21] .
Herein we report a hierarchical flower like 3D nanostructured Co 3 O 4 @MnO 2 /NGO composite electrodes for electrochemical supercapacitor for the first time. The prepared Co 3 O 4 @MnO 2 /NGO and aqueous 6 M KOH solution have been utilized as working electrode and electrolyte respectively to evaluate the capacitive performance of the cell. Furthermore, the physicochemical properties of the Co 3 O 4 @MnO 2 /NGO electrode have been determined through Raman, XRD, XPS, FE-SEM, EDX, BET and FE-TEM analyses. The salient features of the present electrode system are discussed herein.

Results and Discussions
The GO exhibits a considerable increase in the intensity ratio between D and G peaks have been evidenced through Raman analysis. In previous reports, the GO materials showed remarkable peaks at 1340 and 1590 cm −1 , indicates that the D-band arises from the edge or defect sites of carbon and G band indicating the sp 2 carbon of the graphene sheets 38 . The Raman spectra of the Co 3 O 4 @MnO 2 /NGO hybrid composite materials obtained via hydrothermal processing are shown in Fig. 1. The GO in the Co 3 O 4 @MnO 2 /NGO electrode structure is confirmed by the appearance of dominant peaks at 1345 cm −1 and 1589 cm −1 represents the D and G bands correspondingly. The D and G bands are meant for disordered sp 3 carbon and well-ordered in-plane sp 2 carbon bonds respectively which are in great resemblance with our recent reported result on MWCNT/GO/NiCo 2 O 4 hybrid electrodes 38 . Further, the acquired results are in greatly concurrent with other previous reported literatures on graphene based composite materials 39,40 .
The XRD analysis of graphite, graphene oxide, reduced and nitrogen graphene oxides were previously reported in the literature 40 . The graphite powder showed characteristic diffraction peak at 2θ = 26.50°, and the corresponding layer-to-layer distance was ~0.36 nm. After the oxidation of graphite powder to GO in the presence of KMnO 4 , the diffraction peak shifted towards left at angle of 2θ = 10.40°, and the layer distance of ~0.89 nm confirmed the GO structure. In addition, the diffraction peak shown at around 2θ = 43° represents the graphene oxide with a turbostratic disorder behaviours [41][42][43] . In the present study, the results for the Co 3 O 4 @MnO 2 /NGO hybrid composite are shown in Fig. 2(a,b). It is observed from the Fig. 2(a,b) that the XRD patterns of Co 3 O 4 / NGO and Co 3 O 4 @MnO 2 /NGO hybrid composites consists of clearly distinguished peaks and the corresponding planes exactly matches well with the JCPDS PDF file data .  44,45 . The elements and chemical states of the Co 3 O 4 @MnO 2 /NGO composite have been studied through XPS analysis and the resultant results are depicted in Fig. 3. As shown in Fig. 3, the Mn 2p, Co 2p, C 1 s, O 1 s and N 1 s are found with their corresponding binding energies (BE). In addition, the XPS spectrum of C 1 s peaks corresponds to C 1 s (284-295 eV), O 1 s (529.6 eV), O* 1 s (532 eV) from Mn and Co oxides, N 1 s (398-406.9 eV) present in the hybrid composite. Therefore, the nitrogen (N 1 s) spectrum can usually be deconvoluted into three individual peaks, namely pyridine nitrogen (N), pyrrolic nitrogen (N) and graphite nitrogen (N), as confirmed in the hybrid composite. Besides the O 1 s peak and Co 2 s peak, two distinct peaks located at binding energies of 642.0 and 653.1 eV were observed in the Mn 2p core level spectrum, indicates that the Mn 2p 3/2 and Mn 2p 1/2 in manganese oxide present in the hybrid composite. The peak values are in concurrent with the earlier report of MnO 2 , indicating a +4 oxidation state of Mn 46 . These results provide direct evidence of Co 3 O 4 @MnO 2 core shell nanoparticles in the hybrid composite obtained via thermal reduction process.
The surface morphology of the prepared Co 3 O 4 @MnO 2 /NGO electrode has evidenced through SEM analysis. The SEM images at different magnifications of Co 3 O 4 @MnO 2 /NGO are shown in Fig. 4(a-e). Its corresponding EDX spectrum is provided in Fig. 4(f). As it can be seen that the compact Co 3 O 4 @MnO 2 /NGO sheets grew vertically on the surface and assembly of these sheets in turn form a hierarchical porous structure as shown in Fig. 4(a-e). The obtained results strongly supports the results obtained from HR-TEM analysis which will be discussed further in the later section. Figure 5(a-g) shows the HR-TEM morphology and Fig. 5(h,i) the SAED patterns of Co 3 O 4 @MnO 2 /NGO hybrid nanocomposite.
The Co 3 O 4 @MnO 2 electrode possess hierarchical flower-like morphology as represented Fig. 5 with the particle size in the range between 10 and 20 nm and decorated homogeneously over the NGO surface. The SAED patterns in Fig. 5(h,i) specify an ordered rings, which is owe to be the hierarchical flower like structure of Co 3 O 4 @ MnO 2 .
Based on the pore structure, the active electrode surface of the porous carbon electrode for EDLCs is highly useful for supercapacitors in the presence of various electrolytes. Therefore, the porous structure of the porous carbon materials influences the energy/power densities in the electrochemical properties. Cobalt (Co) and manganese (Mn) electrodes are highly desirable for supercapacitor or batteries applications with enhanced surface properties 47 . The BET results of Co 3 O 4 @MnO 2 /NGO at 77 K are displayed in Fig. 6.
The N 2 adsorption-desorption isotherm characteristics of the Co 3 O 4 @MnO 2 /NGO hybrid composite shows type II hysteresis loop as shown in Fig. 6(a). The surface area, pore volume, pore area and pore diameter values are ~350 m 2 /g, 0.55 cm 3 /g, 44 m 2 /g and 60 A°respectively. Furthermore, the surface area of Co 3 O 4 @MnO 2 /NGO increases, and the electrochemical behavior, such as the specific capacitance and cyclic stability, increases when compared to previously-reported hybrid composites [48][49][50] .
The electrochemical properties of various nanostructured carbon-based Co 3 O 4 , MnO 2 , Co 3 O 4 @MnO 2 core/ shell and its electrochemical properties were previously reported in the literature [48][49][50] . This cobalt (Co 3 O 4 ) and MnO 2 materials clearly shows the redox behaviors of the metal oxide and different oxidation states (Co 3+/ Co 4+ ) in the presence of a strong electrolyte via electrochemical reactions. Based on previous reports, the electrochemical properties of the Co 3 O 4 @MnO 2 /NGO hybrid composite have been investigated in a three-electrode configuration for CV, GCD and EIS experiments. The typical three-electrode cell containing Co 3 O 4 @MnO 2 /NGO ternary hybrid composite, Ag/AgCl, and Pt electrodes as working, reference, and counter electrodes, respectively, was dipped in 6 M aqueous KOH solution at room temperature. These results of the as-prepared Co 3 O 4 @MnO 2 /NGO electrodes in the different scans rates from (10 to 100 ms/V) are shown in Fig. 7. The CV curves are almost similar at all scan rates, indicating the reversible nature of the hybrid composite electrodes. The shape of the CV peaks represents the charge-discharge mechanism of the Co 3 O 4 @MnO 2 /NGO hybrid electrode via faradic reaction (oxidation and reduction reaction) of the metal ions together with 6 M KOH to improve the rate of reaction. Therefore, the CV area represents the total charge accumulating through the Faradaic and non-Faradic reaction. The faradaic contribution involves ion migrations with a surface-bound redox capacitance, whereas the non -Faradaic process is and effect of the double layer capacitance. The electrochemical properties of GO, RGO, NGO, Co 3 O 4 , MnO 2 and Co 3 O 4 @MnO 2 hybrid composites have been reported in the literature [51][52][53] . Figure 7 represents the CV results of the Co 3 O 4 @MnO 2 /NGO composite electrode. The results of the CV curves for all electrodes shows a symmetrical behavior of the ideal capacitive properties with a quasi-rectangular shape due to the excellent capacitive nature of the electrodes. A close observation of the CV results of the hybrid composite electrodes indicates a more rectangular behaviour and a relative increment in the current level compared to that of pristine cobalt and manganese oxide materials [51][52][53] . These properties of the hybrid composites show the overall specific capacitance due to the combined contribution from EDLC and the pseudo-capacitance behaviour, and its related reversible reaction of Co 3 O 4 @MnO 2 nanoparticles in the presence of a strong electrolyte. Based on the reversible reaction, the CV measurements have conducted at various scan rates, and the relative current response occurred with an ideal capacitive response of the Co 3 O 4 @MnO 2 /NGO composite.
It is perceived from the CV plot that the anodic and cathodic peaks are increases with increasing current density and sweep rates and the position of the peaks remains unaltered which indicates the ions are migrated in the both directions. Hence, the redox reactions are ensues at the electrode-electrolyte interfaces in presence of the strong electrolyte. These results show a rectangular behaviour with certain deviation due to the hydroxyl functional groups present in the NGO materials. Therefore, the CV results of the NGO materials are much larger then graphene-based materials 54,55 , which indicates the greater electrochemical properties of NGO. The specific capacitance values are calculated from Eq. (1) and these values are compared with earlier reports which are due to the surface properties of the electrolyte solution which reduces the internal resistance R i thereby increase the pseudo-capacitance nature of the electrode material 54,55 . The CV results of ( Fig. 7(a)) indicate that various scan rate of (10, 20, 40, 60, 80 and 100) mV s −1 , around 95% of the initial capacitance has retained. These enhanced performances of the hybrid composite represent the following features in the electrochemical reaction. The first one is, the Co 3 O 4 @MnO 2 /NGO containing the more electroactive sites and exist as both double layer and pseudo-capacitance nature whereas the second one is due to decrement in internal resistance of the electroactive materials for enhancement of electrical conductivity [50][51][52][53][54][55] . Furthermore, the results of the galvanostatic charge-discharge (GCD) profile explained the practical applications of Co 3 O 4 @MnO 2 /NGO hybrid composite. Figure 7(b-e) represents the GCD curves of the Co 3 O 4 @MnO 2 / NGO electrode using 6 M KOH solution at different current densities under ambient temperature. The GCD from Fig. 7(b) demonstrates that it consists of triangular shape-voltage profile which confirms the existence of the EDLC and pseudo-capacitance nature and is in concurrent with the results obtained from CV analysis. The Co 3 O 4 @MnO 2 /NGO electrodes offer the maximum discharge time than that of the pristine cobalt and manganese composites [30][31][32][33][34][35][36][37]56 , which indicate that the capacity has been stored for a prolong time by the prepared Co 3 O 4 @ MnO 2 /NGO electrodes. The specific capacitance is calculated from Eq. (2) and it shows values of 347 Fg −1 , 264, and 184 F g −1 at 0.5, 1, and 2 A g −1 respectively. The specific capacitance values of Co 3 O 4 @MnO 2 /NGO are quite high as compared to pristine Co 3 O 4 and MnO 2 composites which are because of its higher electrical conductivity, migration of ions in electrolyte solution, and electrical double layer charge storage capacity. The specific capacitance decreases with increase in current density in the order of (0.5, 1, and 2) A g −1 . In addition, the electrochemical capacitance values are given in the ( Table 1) with those of previously reported Co 3 O 4 @MnO 2 /NGO hybrid composites prepared from cobalt and manganese materials [46][47][48][49][50][51][52][53][54][55]57 . Figure 7(c,d) illustrates the relation between power density and energy density (P vs E plot) and the capacity retention curve of the Co 3 O 4 @MnO 2 /NGO composite. The energy and power densities have been calculated from Eq. (3) and (4), and result is demonstrated in Fig. 7 (c,d) which indicates the long term cyclic stability of the active  electrode materials for supercapacitor applications. Further, the cyclic stability of the Co 3 O 4 @MnO 2 /NGO hybrid composite electrodes have been confirmed by repeating the GCD analysis at 0.5 A g −1 for 10,000 cycles and its corresponding plot and values are provided in Fig. 7(e) and Table 1 respectively. After 10,000 cycles, the Co 3 O 4 @ MnO 2 /NGO reached 69% of its initial specific capacitance, which indicates that the electroactive material possess excellent cycling stability and reversibility [46][47][48][49][50][51][52][53][54][55]57 . In order to evaluate the internal resistance and capacitance of the prepared Co 3 O 4 @MnO 2 /NGO electrode, EIS have been performed in the frequency range between 0.1 Hz and 100 KHz. The results have analyzed by using Nyquist plots, which denotes the frequency reaction of the electrode/electrolyte. Figure 7(f) indicates the EIS results of Co 3 O 4 @MnO 2 /NGO electrode and its corresponding equivalent series resistance (ESR) has been evaluated from the intercept of the X-axis at high frequency which is the total combination of the ionic resistance of electrolyte, intrinsic resistance of composite electrode, and contact resistance at current collector interface. The The value of ESR is ~40 Ω which indicates it has very small inherent resistance and the Warburg angle is higher than 45° at low frequencies which in turn due to the diffusion process at the electrode-electrolyte interfaces. Therefore, the charge transfer resistances of the electro active materials shows in lower frequency region were calculated 38-40 Ω respectively. The observed result strongly demonstrates the stable electrochemical properties of the Co 3 O 4 @ MnO 2 /NGO electrodes.

Conclusions
The controlled synthesis of hierarchal flower-like morphology of Co 3 O 4 @MnO 2 /NGO hybrid composites have been synthesized via thermal reduction and investigated its electrochemical behavior for high-performance supercapacitors. The Co 3 O 4 @MnO 2 /NGO composite electrode achieved the highest capacitance of 347 F g −1 at 0.5 A g −1 . The excellent electrochemical behavior of the Co 3 O 4 @MnO 2 /NGO electrode was attributed to the high surface area and improved surface morphology, which facilitates the electron diffusion at the electrode/electrolyte interface. The long term cycling stability of the hybrid composite electrode was analyzed by subjecting the cell to GCD analysis at ambient conditions and it retained 69% of its capacitance after 10,000 charge-discharge cycles thereby demonstrating the excellent cyclic stability and reversibility of the prepared electrode material. The ESR and R ct values were lower for the Co 3 O 4 @MnO 2 /NGO composite and hence it deeds as a promising candidate for high-performance supercapacitors.

Materials and Methods
The Graphene oxide (GO) preparation process. The GO powder was made by modified Hummers procedure and reported elsewhere 58,59 . In brief, the stoichiometric amounts of graphite flakes (3 g) and H 2 SO 4 (20 ml), NaNO 3 (1.4 g), were taken in a reaction flask and chilled to 0 °C. Afterward, the KMnO 4 (0.18 g) was included in the reaction mixture slowly under room temperature. The resultant solution was subjected to stirring at 35 °C for about 12 h and then 30% H 2 O 2 was added into the reaction mixture followed by neutral pH. The resulting GO materials were washed with water and ethanol and subjected to vacuum at 40 °C for 12 h. Finally, the finely dried GO powder was collected and kept in a desiccator. confocal Raman spectro microscopy scanned in the range of (100 to 400) cm −1 in presence of He and Ni laser beam. The XRD pattern of hybrid composite was studied by Rigaku Rotaflex (RU-200B) X-ray diffractometry, with CuK α radiation (λ = 1.514 A°). The surface properties of Co 3 O 4 @MnO 2 /NGO were analyzed through field emission scanning electron microscopy (FE-SEM, Hitachi S-4800, and Japan) and filed emission transmission electron microscopy (FE-TEM, JEM-2010F). The elemental analysis of Co 3 O 4 @MnO 2 /NGO was examined using X-ray photoelectron spectroscopy (XPS, ESCALAB 250Xi, Thermo Fisher Scientific, and USA). The electrochemical properties of the hybrid composite had evaluated by Bio-Logics Science Instruments SAS Ltd (France). The CV experiment was conducted at ambient temperature prepared Co 3 O 4 @MnO 2 /NGO as a working electrode, the reference electrode was Ag/AgCl, and platinum wire was employed as the counter electrode. 6 M KOH served as electrolyte solution. The optimized potential window was in the range between −0.34 and 0.51 V and performed the tests at various scan rates (5 to 100) mV/s and current densities (0.5, 1, and 2) A g −1 . The GCD was performed at current densities from (0.5, 1, and 2) A g −1 . The electrochemical impedance spectroscopy (EIS) was measured in the frequency range between 0.1 Hz and 100 KHz with a signal amplitude of 500 mV.

∫ = ∆
Idt m V C (1) where I denote the current, m represents the mass of the active material, dt the time interval and ∆V represents the potential difference. The energy density E (Wh kg −1 ) and power density P (kW kg −1 ) was evaluated by means of following expressions 2 & 3, In the above equations, ∆V and ∆t represents the voltage window and time interval for discharge process respectively 60 .