Fe3O4@rGO ternary hybrids and electrochemical performance for supercapacitor electrode

Journal Pre-proof Microwave-assisted synthesis of Mn3O4-Fe2O3/Fe3O4@rGO ternary hybrids and electrochemical performance for supercapacitor electrode

Rajesh Kumar, Sally Youssry, Kyaw Zay Ya, Wai Kian Tan, Go Kawamura, Atsunori Matsuda PII:

S0925-9635(19)30699-5

DOI:

https://doi.org/10.1016/j.diamond.2019.107622

Reference:

DIAMAT 107622

To appear in:

Diamond & Related Materials

Received date:

1 October 2019

Revised date:

5 November 2019

Accepted date:

9 November 2019

Please cite this article as: R. Kumar, S. Youssry, K.Z. Ya, et al., Microwave-assisted synthesis of Mn3O4-Fe2O3/Fe3O4@rGO ternary hybrids and electrochemical performance for supercapacitor electrode, Diamond & Related Materials (2018), https://doi.org/10.1016/j.diamond.2019.107622

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© 2018 Published by Elsevier.

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Microwave-assisted synthesis of Mn3O4-Fe2O3/Fe3O4@rGO ternary hybrids and electrochemical performance for supercapacitor electrode Rajesh Kumar a, *, Sally Youssry a,b, Kyaw Zay Ya a,, Wai Kian Tan c, Go Kawamura a and Atsunori Matsuda a, * a

Department of Electrical and Electronic Information Engineering, Toyohashi University of Technology, 1-1 Hibarigaoka, Tempaku-cho, Toyohashi, Aichi, 441-8580, Japan b

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Analytical and Electrochemistry Research Unit, Department of Chemistry, Faculty of Science, Tanta University, 31527- Tanta, Egypt c

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Institute of Liberal Arts and Sciences, Toyohashi University of Technology, Toyohashi, Aichi 441-8580, Japan

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Journal Pre-proof Abstract Tailoring binary transition-metal oxide nanoparticles with two-dimensional novel carbon nanomaterials has become a desired way to increase their electrochemical performance for energy storage application. In this work, binary metal oxide (Mn3O4-Fe2O3/Fe3O4 nanoparticles) anchored reduced graphene oxide nanosheets (rGO NSs) have been successfully synthesized by a simple and low-cost microwave-assisted synthesis for supercapacitor (SCs) electrode applications. As characterized by scanning electron microscopy and transmission electron microscopy, Mn3O4-Fe2O3/Fe3O4 nanoparticles with size of < 100 nm were effectively anchored

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on the surfaces or rGO NSs. The Mn3O4-Fe2O3/Fe3O4@rGO ternary hybrids was containing

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specific surface of ~ 322 m2/g. Cyclic voltammetry and galvanostatic charge/discharge measurements were adopted to investigate the electrochemical properties of Mn3O4-Fe2O3/Fe3O4

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nanoparticles anchored rGO NSs (Mn3O4-Fe2O3/Fe3O4@rGO) ternary hybrids in 1.0 M KOH electrolyte solution. The Mn3O4-Fe2O3/Fe3O4@rGO ternary hybrids exhibited specific

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capacitance of 590.7 F/g at 5 mV/s and cyclic stability as capacitance retention of 64.5% after

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1000 cycles at scan rate of 50 mV/s. Nearly rectangular shape of cyclic voltammetry curve revealed that electric double layer capacitance from conductive rGO NSs was dominated as

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compared to pseudocapacitance from binary metal oxide nanoparticles. The study presents a promising application of binary metal oxide/rGO hybrids as electrode materials for energy

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storage.

Keywords:

Microwave, reduced graphene supercapacitor, cycle stability

oxide,

Mn3O4-Fe2O3/Fe3O4

nanoparticles,

* Corresponding author E-mail address: [email protected] (R. Kumar), [email protected] (A. Matsuda)

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Journal Pre-proof 1. Introduction Supercapacitors (SCs), as a new energy storage device have attracted remarkable attention due to their excellent performance as fast charge/discharge capability and long-term cycling stability in renewable energy storage, electric vehicles and powering of portable electronics [1-5]. The performance of SCs devices are highly affected by the electrode materials which decide the durability for sustainability for long term cycling stability [6, 7]. Researchers are focused to synthesized high quality materials for SCs electrode purpose to improve the performance [8-10]. Thus, a number of materials have been tested for SCs electrode including carbon-based

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materials, transition metal oxide, transition metal nitrides, transition mixed metal oxide, conducting polymer and other materials [11, 12]. Carbon materials with metal oxide/conducting

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polymer improve the SCs performance due to combination of electric double layer capacitance

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and pseudocapacitance behaviors [13-15].

In different kinds of metal oxides, manganese oxide and iron oxides are extremely used

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for electrode materials in various energy storage applications [16-20]. For diversity of chemical

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valence, there are various types of manganese oxide (MnO2, Mn2O3, Mn3O4 etc.) and iron oxides (FeO, Fe2O3, Fe3O4 etc.) has been widely studied due to their low cost, facile preparation, environment-friendly, natural abundance, distinctive structural features, mixed valence state, and

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high theoretical capacitance [21-25]. Zhu et al. [26] described synthesis Fe3O4-MnO2 composite using porous MnO2 microspheres doped with Fe3O4 nanoparticles via a one-step and low-cost

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ultrasound assisted method for SCs electrode. The Fe3O4-MnO2 composite shows specific

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capacitance of 367.4 F/g at scan rate of 100 mV/s and cycling performance with a capacitance retention of 76% after 5000 charge/discharge cycles at 5 A/g. Sarkar et al. [27] reported αFe2O3/MnOx core-shell nanorod-based positive electrode and the assembled α-Fe2O3/C//αFe2O3/MnOx core-shell nanorods asymmetric supercapacitor exhibited volumetric capacitance of ∼ 1.28 F/cm3 (scan rate: 10 mV/s) with nearly 78% capacitance retention (scan rate: 400 mV/s). Rudra et al. [28] demonstrated use of chemically stable Au-α-Fe2O3-Mn3O4 composite nanorods for solid-state symmetric SCs device which delivered specific capacitance of 580 F/g at current density of 1 A/g. However, due to their poor electrical conductivity which may restrict the electrochemical performance for SCs. In order to improve the electrochemical performance of manganese oxide and iron oxides based electrode materials, it is essential to combine these transition metal oxides with carbon 3

Journal Pre-proof based materials to fabricate their hybrids/composites, as well as to increase the electroactive sites available for oxidation-reduction reactions. Recently several authors have used Mn3O4 and Fe2O3/Fe3O4 with various types of carbon materials to prepare the SCs electrode. Meng et al. [29] grown Mn3O4 particles/γ-MnOOH nanowires heterostructure simultaneously on carbon cloth via hydrothermal method for SCs electrodes without binder and it contains specific areal capacitance of 2507.1 mF/cm2 at a current density of 1 mA/cm2. Camacho et al. [30] reported Mn3O4 nanoparticles encapsulated in carbon cages using plasma evaporation and thermal oxidation for SCs electrode which delivered specific capacitance of 422 F/g at scan rate of

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1mV/s with capacitance retention of 81% after 1000 cycles. Arun et al. [31] suggested synthesis

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of carbon decorated octahedral shaped α-Fe2O3/Fe3O4 nanoparticles via two-step process (chemical oxidation and heating treatment under nitrogen atmosphere) and its contains specific

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capacitance value of 274 F/g with 83% retention after 5000 cycles. The combination of manganese oxide and iron oxide with carbon materials (e.g. carbon nanotubes: CNTs) to form

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the ternary composites improve the performance of SCs electrode. Wang et al. [32] suggested

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formation of Fe3O4/CNTs@MnO2 based electrode for high specific capacitance (643.8 F/g at 1 A/g) with long tern cycling stability. But the used carbon materials does not contains high

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surface area for complete dispersion of manganese and iron oxide on its surfaces, so need to select high surface area containing carbon materials to support the metal oxide nanoparticles. Nowadays, graphene derivatives as reduced graphene oxide nanosheets (rGO NSs)

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containing high surface area are enormously applied in electrode materials with various metal

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oxides to improve the electrochemical performance of electrode materials [33-42]. The rGO NSs provide high surface area containing conducting platform as well as functional group which helps to attach/decorate metal oxide without any agglomeration and these metal oxides preventing the restacking of graphene NSs [43-46]. Graphene derivatives with manganese/ iron oxides to form the ternary composite are highly utilized to design the electrode materials for SC application. It necessary to find a simple and facile method to anchored mixed valence states containing low cost binary metal oxide with rGO NSs to obtain high quality electrode materials for SC which delivered high capacitance, fast charge/discharge capability and long-term cycling stability. Herein, we present a simple and facile microwave-assisted method to anchored Mn3O4 and Fe2O3/Fe3O4 on the surfaces of rGO to synthesize Mn3O4-Fe2O3/Fe3O4@rGO ternary hybrids 4

Journal Pre-proof as electrode materials for high performance SCs application. The in-situ exfoliation and reduction of graphite oxide into rGO NSs help to decorate the binary metal oxide on its surfaces. The microstructure and morphology of the prepared Mn3O4-Fe2O3/Fe3O4@rGO ternary hybrids were characterized in detail and their electrochemical properties were studied in 1 M KOH electrolyte. The Mn3O4-Fe2O3/Fe3O4@rGO ternary hybrids delivered a specific capacity of 590.7 F/g when the scan rate was 0.5 mV/s and 64.5% of initial capacitance was retained after 1000 continuous cycles at scan rate of 50mV/s. 2. Experimental details

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2.1. Synthesis of Mn3O4-Fe2O3/Fe3O4@rGO ternary hybrids

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The Mn3O4-Fe2O3/Fe3O4@rGO ternary hybrids were synthesized microwave irradiation method. Graphite oxide was synthesized modified Staudenmaier method by chemical oxidation of natural

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graphite powder as reported previously [47, 48]. First, graphite oxide (0.7 g) was mixed with

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manganese (III) oxide (50 mg) and iron (III) oxide (50 mg) in C2H5OH solution (150 ml). It was magnetic stirred for 30 min at room temperature followed with an addition of 1 mL of diluted

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ammonia (NH3H2O) and then ultrasonication for 10 min. The as-prepared solution was dried at 40 °C for to form the powder. Finally, powder treated by microwave irradiation (IRIS

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OHYAMA Japan) at constant irradiation power (P= 900 W) and time (t = 30 sec) to achieve the Mn3O4-Fe2O3/Fe3O4@rGO ternary hybrids. Schematic diagram in Fig. 1 revealed the synthesis

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process of Mn3O4-Fe2O3/Fe3O4@rGO ternary hybrids.

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Modified

Staudenmaier method

Diluted NH3H2O

Chemical oxidation

Manganese (III) oxide

Magnetic stirring

C2H5OH

Iron (III) oxide

Dry

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Microwave

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Exfoliation

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Reduction

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Microwave irradiation

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Mn3O4-Fe2O3/Fe3O4@rGO ternary hybrids

2.2. Characterizations

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Fig. 1. Schematic synthesis process of Mn3O4-Fe2O3/Fe3O4@rGO ternary hybrids.

Crystalline structure of synthesized material were determined by X-ray powder diffraction

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(XRD) using Rigaku RINT-2500 X-ray diffractometer (Cu Kα; 1.54 Ao) (V= 40 kV; i= 20 mA).

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The surface morphological characterization was investigated by field emission scanning electron microscope (FE-SEM, Hitachi SU 8000 Type II). Transmission electron microscopy (TEM) analysis was carried out using a JEM-2100F (JEOL) microscope operated at 200 kV using a carbon-coated copper grid for sample preparation. Raman spectra were obtained on a JASCO NRS-1000 micro-spectrometer with an excitation radiation of 514.5 nm using an Ar+ green laser. For surface area measurement, N2 adsorption-desorption curves of the samples were obtained by Brunauer-Emmett-Teller (BET) using a TriStar II 3020 adsorption analyzer (Micromeritics, USA) at 77 K. 2.3. Electrochemical measurements Electrochemical measurements were carried out by three-electrode cell system using a Solartron SI 1286 electrochemical workstation in 1 M KOH electrolyte solution at room temperature. 6

Journal Pre-proof Working electrode was prepared by mixing Mn3O4-Fe2O3/Fe3O4@rGO hybrids into solution of isopropyl alcohol, water and nafion (volume ratio 5:5:0.1) with help of ultrasoncation (10 min) to form the homogeneous solution. From the mixed solution, 10 μL amount of liquid was dropped on glassy carbon electrode (GCE) (3mm diameter) and the liquid was evaporated in air to dry the active materials. In three-electrode measurements, Mn3O4-Fe2O3/Fe3O4@rGO hybrids material, platinum (Pt) wire and Ag/AgCl works as working electrode, counter electrode and reference electrode, respectively. Cyclic voltammetry (CV) and galvanostatic charging/discharging

Fe2O3/Fe3O4@rGO hybrids electrode for SCs applications.

3.1. Microstructural characterization and analysis

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3. Results and Discussion

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measurements were carried out to investigate the electrochemical performance of Mn3O4-

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XRD analysis was conducted to study the crystalline structure of the as-synthesized Mn3O4-

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Fe2O3/Fe3O4@rGO hybrids. Fig. 2 shows the diffraction peaks at 2θ = 28.9, 31.0, 32.3,36.1, 38.2, 44.4, 50.6, 58.6 and 64.8° correspond to diffraction planes of (112), (200), (103), (211)

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(004), (220), (105), (321) and (400), respectively. These peaks were corresponds to the tetragonal phase structure of Mn3O4 (JCPDS: 89-4837) [49, 50]. Diffraction peaks at 2θ = 33.1,

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40.9, 49.4 and 54.7° correspond to (104), (113), (024) and (116) planes of Fe2O3 (JCPDS 330664) [51, 52]. The peaks at 2θ values of 18.1 and 35.4° corresponding to (111) and (311) crystal

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planes, respectively are the characteristic peaks of cubic structure of Fe 3O4 [53]. Synthesized materials contains coexistence of Fe2O3 (hematite) and Fe3O4 (magnetite) phases in rGOof rGO NSs.

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Mn3O4-Fe2O3/Fe3O4 hybrids. The broad diffraction peak at 2θ = 25.2° corresponds to (002) plane

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Fig. 2. XRD pattern of the synthesized Mn3O4-Fe2O3/Fe3O4@rGO ternary hybrids.

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SEM images were employed to analyze the surface morphological features of the assynthesized rGO NSs and Mn3O4-Fe2O3/Fe3O4@rGO ternary hybrids. Fig. 3(a, b) shows the SEM images of rGO NSs which was converted from graphite oxide during microwave irradiation. It can be observed that rGO NSs contains porous layered planes of rGO NSs with open sheet-like morphology. Fig. 3b display the SEM image as higher magnification of rGO NSs reveals formation of curved at edges and contains crumpled morphological structures. The curve shaped morphology and open spaces among rGO NSs are beneficial for electrochemical application. Fig. 3(c, d) shows the TEM images of rGO NSs and it display that rGO NSs contains thin sheets (Fig. 3c, marked in blue circle) which reveals the formation of few/multi- layer graphene NSs. Fig. 3d displays the HRTEM image (as enlarged view of the blue circle region

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Journal Pre-proof in Fig. 3c) of rGO NSs demonstrates folded structure at edges. Graphene layers and its crystalline structure nature can be seen in Fig. 3d.

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2 μm

200 nm

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Fig. 3. (a, b) SEM and (c, d) TEM images of rGO NSs.

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Fig. 4 shows the SEM micrographs of Mn3O4-Fe2O3/Fe3O4@rGO ternary hybrids at different magnification. In Fig. 4(b, c), the nanoparticles of binary Mn3O4-Fe2O3/Fe3O4 are

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uniformly decorated (as marked in the red dashed circles) on the surfaces of rGO NSs. Homogeneously dispersed Mn3O4-Fe2O3/Fe3O4 nanoparticles were observed with narrow particle size distribution. Fig. 4d shows high magnification SEM image of Mn3O4-Fe2O3/Fe3O4@rGO ternary hybrids and the average Mn3O4-Fe2O3/Fe3O4 nanoparticles were < 100 nm. Fig. 4d revealed that some Mn3O4-Fe2O3/Fe3O4 nanoparticles were inside the rGO NSs.

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Fig. 4. SEM images of Mn3O4-Fe2O3/Fe3O4@rGO ternary hybrids at different magnifications.

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The detailed microstructural analysis of Mn3O4-Fe2O3/Fe3O4 @rGO ternary hybrids was further characterized by TEM/HRTEM. The internal morphologies of Mn3O4-Fe2O3/Fe3O4

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@rGO ternary hybrids were analyzed by TEM/HRTEM characterization as displayed in Fig. 5. It is noteworthy that Mn3O4 and Fe2O3/Fe3O4 nanoparticles are well dispersed on rGO NSs in the

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ternary hybrids (Fig. 5b). The Mn3O4 and Fe2O3/Fe3O4 nanoparticles were nearly spherical with narrow size distribution with nanosized and well dispersed on the rGO NSs surface. As shown in Fig. 5b, the rGO NSs were thin as well as semi-transparent layer which give the support to Mn3O4 and Fe2O3/Fe3O4 nanoparticles for the formation of Mn3O4-Fe2O3/Fe3O4 @rGO ternary hybrids. Fig. 5c reveals HRTEM image that shows attachment of nanoparticles with rGO NSs which composed of crystalline structure. The interlayer d-spacing of the fringes in the HRTEM image (Fig. 5c) was ~0.34 nm, corresponding to the (002) lattice plane of few layer rGO NSs.

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Fig. 5. TEM images of Mn3O4-Fe2O3/Fe3O4@rGO ternary hybrids at different magnifications. Raman spectrum was analyzed to determine the vibration modes and structural defects

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induced in the Mn3O4-Fe2O3/Fe3O4@rGO ternary hybrids. As described in Fig. 6, the D band (~1350 cm-1) is associated with structural defects and disorders of rGO NSs whereas the G band

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(~1590 cm-1) is corresponded to the sp2-hybridized graphitic carbon atoms in rGO NSs. The relative intensity of D and G peaks (ID/IG) is widely used to determine the defects in graphene

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derivatives materials [54]. It was observed that the value of ID/IG for Mn3O4-Fe2O3/Fe3O4@rGO ternary hybrids was 0.99, relatively higher in comparison with that of rGO NSs (ID/IG = 0.94). The increase in the value of ID/IG for Mn3O4-Fe2O3/Fe3O4@rGO ternary hybrids implies that more disordered carbon structure and defects were brought after the microwave exfoliation of graphite oxide and introduction of Mn3O4-Fe2O3/Fe3O4 nanoparticles on the rGO NSs [55]. The Raman peak located at 651 cm-1 in the spectra of the Mn3O4-Fe2O3/Fe3O4@rGO ternary hybrids correspond the symmetric stretching vibrations of Mn-O of Mn3O4, which confirms the presence of the Mn3O4 in the hybrid [56-58].

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Fig. 6. Raman spectra of rGO NSs and Mn3O4-Fe2O3/Fe3O4@rGO ternary hybrids. The surface area and pore nature of Mn3O4-Fe2O3/Fe3O4@rGO ternary hybrids was investigated by nitrogen adsorption measurements. The nitrogen adsorption-desorption isotherms for Mn3O4Fe2O3/Fe3O4@rGO ternary hybrids is shown in Fig. 7. The isotherms of Mn3O4Fe2O3/Fe3O4@rGO ternary hybrids can be assigned to type IV with hysteresis loop H3 at P/P0 > 0.4, representing the existence of mesoporous structure [59, 60]. The BET surface area of 12

Journal Pre-proof Mn3O4-Fe2O3/Fe3O4@rGO ternary hybrids was ~322 m2/g. The BJH pore size distributions (depicted in the inset of Fig. 7) showed the average pore diameter was in the range of 3-20 nm, confirming the mesoporous nature of ternary hybrids. The obtained results indicated that ternary hybrids contained relatively large surface area with mesoporous structure which was beneficial

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to provide more surface active sites for ions/ electron during electrochemical measurement.

Fig. 7. Nitrogen adsorption-desorption isotherm (inset: BJH pore size distribution) of Mn3O4Fe2O3/Fe3O4@rGO ternary hybrids. 3.2. Electrochemical performance The electrochemical performance of the Mn3O4-Fe2O3/Fe3O4@rGO ternary hybrids was investigated by CV and galvanostatic charge/discharge was tested in 1M KOH electrolytes at room temperature. The CV curves of Mn3O4-Fe2O3/Fe3O4@rGO ternary hybrids electrode were determined at different scan rates (5-100 mV/s) in the potential range of -0.1 to 0.6 V as shown in Fig. 8a. The Mn3O4-Fe2O3/Fe3O4@rGO ternary hybrids electrode exhibited larger CV curve area as compared to pristine rGO NSs (Fig. S1; supporting information), indicating its superior

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Journal Pre-proof electrochemical performance. The area of enclosed CV curves increases with the applied scan rate. It contained rectangular shape of CV curve for Mn3O4-Fe2O3/Fe3O4@rGO ternary hybrids electrode at low scan rates, indicating the typical double-layer capacitive behaviors due to rGO NSs [61, 62]. CV curves do not shows any redox peaks when the scan rate was increases from low to high values. This represents that the redox reactions on the electrode surface were fast and reversible

[29,

43].

The

galvanostatic

charge/discharge

measurement

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Mn3O4-

Fe2O3/Fe3O4@rGO ternary hybrids electrode at various current densities are shown in Fig. 7b. At low current density, small IR drop can be seen in the galvanostatic charge/discharge curves

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indicates a low internal resistance. Fig. 8b revealed that the shape of the charge/discharge curves

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at various scan rates has no obvious change, indicating good reversibility. The specific capacitance of Mn3O4-Fe2O3/Fe3O4@rGO ternary hybrids electrode materials was calculated

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from the CV curves (Fig. 7a) as shown in Fig. 8c. The specific capacitances were 590.7, 511.8, 469.9, 453.3, 425.3, 414.9, 394.2, 383.8, 365.0, 361.0 and 328.2 F/g at scan rate of 5, 10, 15, 20,

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25, 30, 40, 50, 60, 70 and 100 mV/s, respectively as presented in Fig. 8c. Fig. 8c shows that the

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values of specific capacitance decreases with increase of scan rate. Long cycling stability is a crucial requirement to SCs electrode for practical applications. The cyclic stability of Mn3O4-

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Fe2O3/Fe3O4@rGO ternary hybrids electrode materials has been studied from 1000 cyclic voltammetry test at a higher scan rate of 50 mV/s as shown in the inset of Fig. 8d. From Fig. 8d, it is observed that Mn3O4-Fe2O3/Fe3O4@rGO ternary hybrids electrode retains almost 64.5% of

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capacitance retention after 1000 cycles, indicating long-term stability of the Mn3O4-Fe2O3/Fe3O4

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nanoparticles anchored rGO NSs. The cycling stability performance degradation of Mn3O4Fe2O3/Fe3O4@rGO ternary hybrids can be minimized by void engineering in some special kinds of structures like wrapped, sandwiched, encapsulated, layered and mixed models. Comparing with results obtained from the three-electrode system, the performance was superior to that reported literature previously (Table 1) proving that Mn3O4-Fe2O3/Fe3O4@rGO ternary hybrids is electrode materials for SCs application.

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Fig. 8. The electrochemical performance of Mn3O4-Fe2O3/Fe3O4@rGO ternary hybrids electrode. (a) CV curves at different scan rates, (b) galvanostatic charge/discharge curves at different current density, (c) the specific capacitance as a function of scan rate and (d) long-term cycling performance as capacitance retention at scan rate of 50 mV/s.

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Table 1. Comparison of electrochemical performance of Mn3O4/Fe2O3/Fe3O4 nanoparticles supported on graphene derivatives based electrodes. Electrode materials Mn3O4/rGO

Synthesis method hydrothermal

Electrolyte

Mn3O4/rGO

hydrothermal

Mn3O4 /graphene microsphere s Mn3O4/GO

hydrothermal

0.5 M 326.9 F/g Na2SO4 1 M Na2SO4 317 F/g 10 mV/s

hydrothermal

1 M Na2SO4

Capacitan ce 228 F/g at 5 A/g

1 M Na2SO4 457 F/g at 1 A/g

Cycling stability

Ref.

95% after 5000 charge/discharge cycles at 5 A/g 94.6% after 1000 charge/discharge cycles 100% after 4000 charge/discharge cycles at of 2 A/g

[44]

91.6% after 5000 charge/discharge cycles at 1 A/g

[65]

[63] [64]

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thermal annealing

1.0 M Na2SO4

Mn3O4/ graphene NSs FeOx/rGO

ultrasound assisted

1 M Na2SO4 312 F/g at 0.5 mA/cm2 1 M NaOH 235 F/g 10 mV/s 1 M Na2SO4 236 F/g at 1 A/g 5 M KOH

325 F/g at 0.1 A/g

hydrothermal

1 M Na2SO4

310 F/g at 2 A/g

precipitation

1 M Na2SO4

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hydrothermal polymerization microwave

1 M H2SO4

[69]

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500 F/g at 5 mV/s

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1 M KOH

97% after 500 charge/discharge cycles 1A/g 86% after 2000 charge/discharge cycles at 4 A/g

486.5 F/g at 1 A/g 590.7 F/g at 5 mV/s

[67]

[68]

[70]

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aerosol spray pyrolysis/ heat treatment

[66]

92.4% after 1000 cycles

[71]

97% after 3000 charge/discharge cycles at 25 A/g 52.1% after 2000 CV cycles at 100 mV/s 64.5% after 1000 CV cycles at 50 mV/s

[72]

[73] This wor k

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Fe-Co oxides/crum pled graphene Mn3O4/CeO 2/holeygraphene Mn3O4/grap hene/polyan iline rGO/Fe3O4/ polyaniline Mn3O4Fe2O3/Fe3O4 @ rGO ternary hybrids

91% after 1500 charge/discharge cycles at 2 A/g. 76% after 1000 charge/discharge cycles at 0.5 mA/cm2 93% after 2000 cycles

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Fe3O4/rGO

electrochemica l electrophoretic deposition

270.6 F/g at 0.2 A/g

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3.3. Mechanism for formation Mn3O4-Fe2O3/Fe3O4@ rGO ternary hybrids We have described possible mechanism for microwave-assisted formation of Mn3O4Fe2O3/Fe3O4@ rGO ternary hybrids. Initially, during microwave irradiation on mixture power (graphite oxide and metal oxide), graphite oxide shows reduction and exfoliation into rGO NSs by producing high temperature along with emission of emission of gases (CO or CO2) as shown in equation (1) [74]. Graphite oxide →

𝑅𝑒𝑑𝑢𝑐𝑡𝑖𝑜𝑛 𝑎𝑛𝑑 𝑒𝑥𝑓𝑜𝑙𝑖𝑎𝑡𝑖𝑜𝑛

rGO NSs + gases + high temperature

(1)

Microwave irradiation remove the oxygen containing function groups available on graphite oxide edges and surfaces and convert into rGO NSs (reduction of graphite oxide into rGO NSs) [75]. The exfoliation of rGO NSs occurs due to sudden expansion of CO or CO2 gases evolved into the spaces between graphite oxide sheets at high temperature [76]. The evolution of high 16

Journal Pre-proof temperature takes place due to reaction of majority oxygen containing function groups available inside/outside graphite oxide which react with microwave irradiation. The high temperature generated by reaction help to decompose the metal salts and disperse the metal oxide nanoparticles on the surfaces of rGO NSs to form the hybrids structure. During exfoliation and formation of rGO NSs, the available metal nanoparticles (Mn3O4 and Fe2O3/Fe3O4) spread on its surfaces. After reduction, rGO NSs contains small amount of oxygen functional groups (not highly reduced) and this small remaining content of oxygenated functional groups support to

Graphite oxide

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rGO

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High temperature

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Manganese(III) oxide

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uniform attachment of Mn3O4-Fe2O3/Fe3O4 nanoparticles.

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Dry mixture powder of graphite oxide and metal oxides

Mn3O4-Fe2O3/Fe3O4@ rGO ternary hybrids

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Iron (III) oxide

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4. Conclusions

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Fig. 9. Schematic representation for microwave-assisted formation of Mn3O4-Fe2O3/Fe3O4@ rGO ternary hybrids.

In summary, we have developed an effective and simple microwave-assisted approach to synthesize Mn3O4-Fe2O3/Fe3O4 nanoparticles supported on the surfaces of rGO NSs. Structural and morphological characterization using XRD, Raman spectroscopy, SEM and TEM confirms the formation of Mn3O4-Fe2O3/Fe3O4@rGO ternary hybrids and uniform dispersion of binary metal oxide on rGO matrix in the hybrids. The BET surface area of Mn3O4-Fe2O3/Fe3O4@rGO ternary hybrids was ~ 322 m2/g. The microwave-assisted synthesized Mn3O4-Fe2O3/Fe3O4@rGO ternary hybrids contained specific capacitance of 590.7 F/g and good cyclic stability (after 1000 cycles 64.5% retention of specific capacitance). Consequently, adopted synthesis method is suitable for gram scale synthesis of binary metal oxide/rGO hybrids which can serve as a potential electrode material for advanced energy storage devices. 17

Journal Pre-proof Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements The author (R. Kumar) acknowledges Japan Society for the Promotion of Science (JSPS; Standard) for international postdoctoral fellowship (P18063) and this research work was financially supported by JSPS KAKENHI Grant No.18F18063, JSPS KAKENHI Grant JP-

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18H03841 and JSPS KAKENHI Grant JP-17K18985. The authors would like to acknowledge Cooperative Research Facility Center, Toyohashi University of Technology, Toyohashi, Aichi,

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Japan for providing the necessarily support and facilities to complete this research work. R.

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Kumar would like to dedicate this work to the memory of late Prof. Yoshiyuki Suda.

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Graphical abstract

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Journal Pre-proof Highlights 1. This research work demonstrates a single-step, facile/fast and scalable synthesis of Mn3O4Fe2O3/Fe3O4@rGO ternary hybrids via direct microwave irradiation for supercapacitor electrode application. 2. Microwave irradiation time was only 30 sec to achieve the Mn3O4-Fe2O3/Fe3O4@rGO ternary hybrids. 3. During the microwave irradiation, graphite oxide produces high temperature which helps conversion of graphite oxide into rGO NSs along with exfoliation.

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4. The synthesized Mn3O4-Fe2O3/Fe3O4@rGO ternary hybrids exhibited specific capacitance of 590.7 F/g at scan rate of 5 mV/s.

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5. The presence of rGO NSs in Mn3O4-Fe2O3/Fe3O4@rGO ternary hybrids improved ion/electron transportation, surface area and stability.

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