Supercapacitive Performance of N-Doped Graphene/Mn 3 O 4 /Fe 3 O 4 as an Electrode Material

Featured Application: The potential application of this work is an electrode material for supercapacitor application. Abstract: Nitrogen-doped graphene (NDG) and mixed metal oxides have been attracting much attention as the combination of these materials resulted in enhanced electrochemical properties. In this study, a composite of nitrogen-doped graphene/manganese oxide/iron oxide (NDG/Mn 3 O 4 /Fe 3 O 4 ) for a supercapacitor was prepared through the hydrothermal method, followed by freeze-drying. Field emission scanning electron microscopy (FESEM) images revealed that the NDG/Mn 3 O 4 /Fe 3 O 4 composite displayed wrinkled-like sheets morphology with Mn 3 O 4 and Fe 3 O 4 particles attached on the surface of NDG. The presence of NDG, Mn 3 O 4 , and Fe 3 O 4 was characterized by Fourier transform infrared spectroscopy (FTIR) and X-ray diffraction (XRD). The electrochemical studies revealed that the NDG/Mn 3 O 4 /Fe 3 O 4 composite exhibited the highest speciﬁc capacitance (158.46 F/g) compared to NDG/Fe 3 O 4 (130.41 F/g), NDG/Mn 3 O 4 (147.55 F/g), and NDG (74.35 F/g) in 1 M Na 2 SO 4 at a scan rate of 50 mV/s due to the synergistic effect between bimetallic oxides, which provide richer redox reaction and high conductivity. The galvanostatic charge discharge (GCD) result demonstrated that, at a current density of 0.5 A/g, the discharging time of NDG/Mn 3 O 4 /Fe 3 O 4 is the longest compared to NDG/Mn 3 O 4 and NDG/Fe 3 O 4 , indicating that it had the largest charge storage capacity. NDG/Mn 3 O 4 /Fe 3 O 4 also exhibited the smallest resistance of charge transfer ( R ct ) value (1.35 Ω ), showing its excellent charge transfer behavior at the interface region and good cyclic stability by manifesting a capacity retention of 100.4%, even after 5000 cycles.


Introduction
The rapid growth of the human population and the development of the global economy has caused the increasing demand for energy. The global energy need is predicted to be doubled by the mid-century and more than triple by the end of this century [1]. Therefore, an energy storage system is needed to store excess energy generated and to supply it to electrical devices effectively. Among energy storage devices, supercapacitors are always preferred due to their high performance in energy storage compared to batteries, flywheels, and traditional capacitors. This is because batteries have high energy density, but low power density and short life cycles. In addition to this, flywheels are cost-effective and have a long cycle life, but they have installment and safety issues due to their large size. Traditional capacitors can be charged and discharged in a short time, but they have low energy density.
size. Traditional capacitors can be charged and discharged in a short time, but they have low energy density.
A supercapacitor is an electrical component that can store a large amount of electrical energy. It is a multicomponent system comprised of two electrodes connected to current collectors and a separator immersed in an electrolyte, and was first used in 1978 as a backup power source in order to maintain computer memory system [2]. Over the years, due to the development of technology, supercapacitors have been widely used in portable electronic devices and hybrid electric vehicles because of their rapid charging-discharging rates, long cycle life, and high specific power. Based on their charge-storage mechanism, supercapacitors are classified into three types: Electrochemical double layer capacitors (EDLCs), pseudocapacitors, and hybrid capacitors.
EDLCs, which are made up of carbon-based materials, store charges electrostatically or non-Faradaically in an electrical double layer formed at the interface between an electrode and electrolyte [3]. Conversely, pseudocapacitors, which consist of transition metal oxides or conducting polymers, accumulate charges through faradic processes in which the charge is transferred across the electrode and electrolyte by reversible redox reaction occurring at the interface [4]. The hybrid capacitor, which is a combination of a EDLC and pseudocapacitor, where both electrical double layer and electrochemical processes are involved in charge storage, enable hybrid capacitors to have the advantages of both EDLCs and pseudocapacitors, i.e., high specific energy and power with good cyclic stability [2].
The materials used in supercapacitor electrodes need to have high conductivity, large surface area, good cyclic stability, and a high theoretical specific capacitance [5]. N-doped graphene ( Figure  1) is a good carbon-based electrode material for supercapacitors because it has a large surface area, good cyclic stability, and high electrical conductivity, however, it has low specific capacitance. On the other hand, transition metal oxides such as manganese oxides and iron oxides have high specific capacitance, together with a minimal impact on the environment and low cost, but both of these two oxides have poor cyclic stability and conductivity. Therefore, graphene or N-doped graphene is usually used as a support for transition metal oxides, which provides high electrical conductivity and excellent cyclic stability [6]. Recently, composites of NDG with mixed metal oxides are extensively studied because the incorporation of two metal oxides or bimetallic oxides is believed to possess superior specific capacitance due to the synergistic effect of two types of metal oxides that offer richer redox reactions compared to single metal oxide or spinel oxides, which can give better electrochemical performance. Zhang, et al. [7]synthesized nanocomposites of NDG/ZnO/NiO with a specific capacitance of 1834.9 F/g, which is higher than composites of NDG with single metal oxide because ZnO improved the conductivity of NiO. The nanocomposite has a capacity retention of 93% after 6000 cycles. Ramesh, Recently, composites of NDG with mixed metal oxides are extensively studied because the incorporation of two metal oxides or bimetallic oxides is believed to possess superior specific capacitance due to the synergistic effect of two types of metal oxides that offer richer redox reactions compared to single metal oxide or spinel oxides, which can give better electrochemical performance. Zhang, et al. [7] synthesized nanocomposites of NDG/ZnO/NiO with a specific capacitance of 1834.9 F/g, which is higher than composites of NDG with single metal oxide because ZnO improved the conductivity of NiO. The nanocomposite has a capacity retention of 93% after 6000 cycles. Ramesh, et al. [8] reported that NDG/NiO/MnO 2 nanocomposites can achieve a high specific capacitance of 1490 F/g with 98% cyclic stability retention after 2000 cycles.
There are various ways to synthesise NDG/mixed metal oxide composites, such as microwave synthesis, liquid phase plasma, and the solvothermal and hydrothermal method [9]. Among these methods, a hydrothermal method is the most preferred method due to its low cost and simple operation. The hydrothermal process is a process in which the starting materials will be dissolved in the aqueous solvent under high pressure and temperature and then recrystallized into a form of material that is difficult to dissolve under normal conditions. This can give particles that are highly dispersed with the desired size and structure. In addition, only a very small amount and non toxic side product will be produced in the hydrothermal method [2].
In this study, a hybrid material consisting of N-doped graphene (NDG), manganese oxide (Mn 3 O 4 ), and iron oxide (Fe 3 O 4 ) was prepared using by the hydrothermal method, followed by freeze-drying. Hydrazine was chosen as a source of nitrogen to be doped between graphene oxide (GO) sheets and eliminated the oxygenated functional groups of GO in order to restore the conductivity during the hydrothermal process. The supercapacitive performance of N-doped graphene/manganese oxide/iron oxide (NDG/Mn 3 O 4 /Fe 3 O 4 ) with various iron and manganese salt ratio (Fe:Mn) was thoroughly studied.

Chemical and Reagents
Graphene oxide (GO) was purchased from Graphenea (San Sebastian, Spain). Manganese sulphate monohydrate (MnSO 4 ·H 2 O), polytetrafluoroethylene (PTFE), and carbon mesoporous were obtained from Sigma-Aldrich (Steinheim, Germany). Iron sulphate heptahydrate (FeSO 4 ·7H 2 O) and ethanol (95%) were purchased from HmbG (Hamburg, Germany) and John Kollin Corporation (Midlothian, UK), respectively. Sodium sulphate (Na 2 SO 4 ) was obtained from Merck Darmstadt, Germany. Hydrazine monohydrate (N 2 H 4 ·H 2 O) and nickel foam was purchased from Nacalai Tesque Inc. (Kyoto, Japan) and Goodfellow (Huntingdon, UK) respectively. Deionized (DI) water was supplied from a Millipore water system (Darmstadt, Germany). After that, the solution was transferred into a 50 mL autoclave and added with 2.5 g of N 2 H 4 ·H 2 O. After continuous stirring for 30 minutes, the autoclave was sealed and subjected to hydrothermal reduction at 160 • C for 3 h. The sample was cooled to room temperature and rinsed with DI water several times. Finally, the sample was freeze-dried. NDG was also prepared following the same steps in the absence of FeSO 4 ·7H 2 O and MnSO 4 ·H 2 O.

Physical Characterization
Field Emission Scanning Electron Microscopy (FESEM) was performed using JEOL JSM-7600F in order to investigate the morphology of the composites. The structural fingerprints of the composites were analyzed via Shimadzu Fourier-transform infrared spectrometer (FTIR), and Shimadzu XRD 6000 diffractometer with Cu Kα radiation (λ = 1.54 Å) at the scan rate of 2 o /min.

Electrochemical Characterization
The working electrode was prepared by mixing the active material with carbon black and PTFE, which acts as a binder in the ratio of 80:10:10. The mixture was then dispersed in ethanol to form a slurry. The slurry was pasted on a nickel foam (1 cm × 1 cm) and dried at 60 • C for 12 h. The nickel foam was then subjected to a mechanical pressure of 5 MPa for good attachment between the current collector and material. The electrochemical characterizations of the composites were conducted using an Autolab (M101) potentiostat with a three-electrode configuration in 1 M Na 2 SO 4 . Nickel foam, pasted with active materials, was used as a working electrode (WE), platinum wire served as a counter electrode (CE), and silver/silver chloride (Ag/AgCl) served as a reference electrode (RE). Cyclic voltammetry (CV) was performed at a potential range of 0-1 V with various scan rates, i.e., 25, 50, 100, 150, and 200 mV/s, whereas galvanostatic charge-discharge (GCD) was evaluated at different current densities of 0.5, 1.0, 1.5, 2.0, and 2.5 A/g. Electrochemical impedance spectroscopy (EIS) was carried out in the frequency range of 0.1 Hz to 10 kHz at an open circuit potential (OCP) with alternating current (AC) potential at an amplitude of 5 mV. The charge transfer resistance (R ct ) and equivalent series resistance (ESR) of the composites were determined from the diameter of the semicircle and the intercept of the semicircle at the Z' axis at the high frequency region of the Nyquist plot, respectively. The cyclic stability test of the composites was performed via CV at a scan rate of 100 mV/s for 5000 cycles. conducted using an Autolab (M101) potentiostat with a three-electrode configuration in 1 M Na2SO4. Nickel foam, pasted with active materials, was used as a working electrode (WE), platinum wire served as a counter electrode (CE), and silver/silver chloride (Ag/AgCl) served as a reference electrode (RE). Cyclic voltammetry (CV) was performed at a potential range of 0-1 V with various scan rates, i.e., 25, 50, 100, 150, and 200 mV/s, whereas galvanostatic charge-discharge (GCD) was evaluated at different current densities of 0.5, 1.0, 1.5, 2.0, and 2.5 A/g. Electrochemical impedance spectroscopy (EIS) was carried out in the frequency range of 0.1 Hz to 10 kHz at an open circuit potential (OCP) with alternating current (AC) potential at an amplitude of 5 mV. The charge transfer resistance (Rct) and equivalent series resistance (ESR) of the composites were determined from the diameter of the semicircle and the intercept of the semicircle at the Z' axis at the high frequency region of the Nyquist plot, respectively. The cyclic stability test of the composites was performed via CV at a scan rate of 100 mV/s for 5000 cycles.

Physical Characterization
Fourier-transform infrared (FTIR) spectroscopy was performed to investigate the chemical structure of NDG, NDG/Fe3O4, NDG/Mn3O4, and NDG/Mn3O4/Fe3O4 ( Figure 2). In the NDG spectrum (Figure 2a), peaks of N-H stretching, amidic C=O stretching, N-H bending, and C-N stretching can be observed at 3392 cm −1 , 1651 cm −1 , 1543 cm −1 , and 1103 cm −1 , respectively. The presence of N-H and C-N in the spectra indicates that nitrogen was successfully doped into graphene oxide during the hydrothermal process. Furthermore, similar peaks of NDG can still be observed in the spectra after the addition of Mn3O4 and Fe3O4 into NDG, proving a successful anchoring of Mn3O4 and Fe3O4 on the NDG sheets. In the NDG/Fe3O4 spectrum (Figure 2b [10]. This could be due to the presence of Fe 3 O 4 and Mn 3 O 4 particles on the wrinkle NDG sheets, which effectively prevent severe agglomeration of NDG sheets [11] and provide more accessible sites for the electrochemical reaction.
Appl. Sci. 2019, 9, x FOR PEER REVIEW 5 of 12 Field Emission Scanning Electron Microscopy (FESEM) was performed to investigate the morphologies of NDG, NDG/Fe3O4, NDG/Mn3O4, and NDG/Mn3O4/Fe3O4. As shown in Figure 3a, NDG exhibits a wrinkle-like sheet morphology, which can also be noticed in the FESEM images of NDG/Fe3O4, NDG/Mn3O4, and NDG/Mn3O4/Fe3O4 (Figure 3b-d). After the addition of Fe3O4, the welldispersed particles of Fe3O4 can be clearly seen to cover the surface of wrinkled NDG sheets ( Figure  3b). The presence of Mn3O4 in NDG/Mn3O4 (Figure 3c) is confirmed as Mn3O4 particles that obviously attach on the NDG sheets. Upon the addition of both Fe3O4 and Mn3O4 into NDG, a large number of Fe3O4 and Mn3O4 particles can be seen anchored on the wrinkled surface of NDG, indicating that both metal oxides are successfully added into NDG (Figure 3d). The presence of the dispersed particles of Fe3O4 and Mn3O4 cover on the surface of NDG sheets have enhanced Csp of the composites due to their highly pseudocapacitive properties [10]. This could be due to the presence of Fe3O4 and Mn3O4 particles on the wrinkle NDG sheets, which effectively prevent severe agglomeration of NDG sheets [11] and provide more accessible sites for the electrochemical reaction. X-ray diffraction (XRD) was performed to identify the structure and crystallinity of the prepared NDG, NDG/Fe3O4, NDG/Mn3O4, and NDG/Mn3O4/Fe3O4. In Figure 4a, the large broad peak at 24.6°, corresponding to the (002) plane, indicates the disordered arrangement of loosely packed graphene sheets of NDG [12]. The diffraction peaks of NDG are less visible in the XRD patterns of NDG/Fe3O4, NDG/Mn3O4, and NDG/Mn3O4/Fe3O4 due to the relatively small diffraction intensity of NDG compared to the strong peaks of Fe3O4 and Mn3O4 [13]. For NDG/Fe3O4 (Figure 4b

Electrochemical Characterization
Cyclic voltammetry (CV) was performed in 1 M Na2SO4 to evaluate the capacitive performance of prepared NDG, NDG/Mn3O4, NDG/Fe3O4, and NDG/Mn3O4/Fe3O4 as electrode materials for a supercapacitor. The specific capacitance (Csp) of the composites was calculated using Equation (1): where I is the current density (A/g), v is the scan rate applied (mV/s), ∆V is the potential range applied, and m is the mass of the sample (g). Figure 5a shows the CV curves of NDG/Mn3O4/Fe3O4 with a different ratio of Fe:Mn at a scan rate of 50 mV/s. NDG/Mn3O4/Fe3O4 with the Fe:Mn ratio of 1:3 exhibits the highest Csp (158.46 F/g) compared to ratios of 1:0 (130.41 F/g), 3:1 (74.16 F/g), 1:1 (82.05 F/g), and 0:1 (147.55F/g), as the composite has the largest enclosed area of the CV curve. The Csp of NDG/Mn3O4/Fe3O4 with a Fe:Mn ratio of 1:3 is higher than the Fe:Mn ratio of 3:1, suggesting that the Mn-based species contribute in the major part of the charge storage mechanism, while Fe species serve as a synergist to enhance the capacitance when both species are mixed together [7]. Quasirectangular CV curves and small humps are observed for NDG/Mn3O4/Fe3O4 in all Fe:Mn ratios, NDG/Fe3O4, and NDG/Mn3O4 (Figure 5b), which indicate that the capacitance is mainly contributed by Faradaic pseudocapacitance [14], which is contributed by the metal oxides [15]. NDG/Mn3O4/Fe3O4 exhibits the largest enclosed CV curve area, indicating that it has the highest Csp (158.46 F/g) compared to NDG, NDG/Fe3O4, and NDG/Mn3O4. This might due to the presence of both Fe3O4 and Mn3O4 that enhance the pseudocapacitance of the composites. In addition, bimetallic oxides provide

Electrochemical Characterization
Cyclic voltammetry (CV) was performed in 1 M Na 2 SO 4 to evaluate the capacitive performance of prepared NDG, NDG/Mn 3 O 4 , NDG/Fe 3 O 4 , and NDG/Mn 3 O 4 /Fe 3 O 4 as electrode materials for a supercapacitor. The specific capacitance (C sp ) of the composites was calculated using Equation (1): where I is the current density (A/g), v is the scan rate applied (mV/s), ∆V is the potential range applied, and m is the mass of the sample (g). Figure 5a shows (Figure 5b), which indicate that the capacitance is mainly contributed by Faradaic pseudocapacitance [14], which is contributed by the metal oxides [15]. that enhance the pseudocapacitance of the composites. In addition, bimetallic oxides provide richer redox reactions compared to single metallic oxide, and thus helps to further improve the capacitive performance of the composites [16]. NDG/Fe 3 O 4 and NDG/Mn 3 O 4 exhibit higher C sp compared to NDG because the dispersion of metal oxides between N-doped graphene sheets prevent the sheets from restacking to increase the number of accessible sites for electrolyte ions [17].
richer redox reactions compared to single metallic oxide, and thus helps to further improve the capacitive performance of the composites [16]. NDG/Fe3O4 and NDG/Mn3O4 exhibit higher Csp compared to NDG because the dispersion of metal oxides between N-doped graphene sheets prevent the sheets from restacking to increase the number of accessible sites for electrolyte ions [17]. The effect of the scan rate on the capacitive performance of NDG/Mn3O4/Fe3O4 was studied between 50 and 500 mV/s. As can be seen in Figure 5c, there is a little distortion of the quasirectangular shape CV curves at a high scan rate of 500 mV/s. The Csp values (Figure 5d) decline from 158.46 F/g to 128.74 F/g as the scan rate increases from 50 mV/s to 500 mV/s, with 65.22% of the initial Csp retained. This is because the diffusion of electrolyte ions to the electrode surface becomes limited with time constraints as the scan rate increases [18]. Galvanostatic charge/discharge (GCD) was carried out to further investigate the capacitive performance of the prepared NDG/Mn3O4, NDG/Fe3O4, and NDG/Mn3O4/Fe3O4. As shown in Figure  6, all the GCD curves show a nearly symmetrical triangular shape with only a small deviation from linearity due to the contribution from the pseudocapacitance [9]. In addition, insignificant voltage drop (IR drop) in the GCD curves of NDG/Mn3O4/Fe3O4, NDG/Mn3O4, and NDG/Fe3O4 (Figure 6a) show high efficiency of ion transport or diffusion near the electrode-electrolyte interface [19]. IR drops occur during the charge/discharge process due to internal resistance, which includes the boundary resistance of electrode/electrolyte and the contact resistance between the active material and current collector [20]. Hence, a small IR drop indicates low internal resistance, which leads to a high power density [18]. From the GCD curves, the specific capacitance of the composites can be calculated according to Equation (2) The effect of the scan rate on the capacitive performance of NDG/Mn 3 O 4 /Fe 3 O 4 was studied between 50 and 500 mV/s. As can be seen in Figure 5c, there is a little distortion of the quasi-rectangular shape CV curves at a high scan rate of 500 mV/s. The C sp values (Figure 5d) decline from 158.46 F/g to 128.74 F/g as the scan rate increases from 50 mV/s to 500 mV/s, with 65.22% of the initial C sp retained. This is because the diffusion of electrolyte ions to the electrode surface becomes limited with time constraints as the scan rate increases [18].
Galvanostatic charge/discharge (GCD) was carried out to further investigate the capacitive performance of the prepared NDG/Mn 3 O 4 , NDG/Fe 3 O 4 , and NDG/Mn 3 O 4 /Fe 3 O 4 . As shown in Figure 6, all the GCD curves show a nearly symmetrical triangular shape with only a small deviation from linearity due to the contribution from the pseudocapacitance [9] (Figure 6a) show high efficiency of ion transport or diffusion near the electrode-electrolyte interface [19]. IR drops occur during the charge/discharge process due to internal resistance, which includes the boundary resistance of electrode/electrolyte and the contact resistance between the active material and current collector [20]. Hence, a small IR drop indicates low internal resistance, which leads to a high power density [18]. From the GCD curves, the specific capacitance of the composites can be calculated according to Equation (2): where C sp is the specific capacitance of the electrode (F/g), ∆t is the discharging time (s), I is the discharge current (A/g), ∆V is the potential range (V), and m is the mass of the electrode material (g  (Figure 6a), indicating that NDG/Mn 3 O 4 /Fe 3 O 4 possesses the highest charge storage capacity [8,21]. The discharging time of NDG/Mn 3 O 4 /Fe 3 O 4 becomes shorter as current density increases from 0.5 A/g to 2.5 A/g (Figure 6b), indicating the decrease of C sp from 90.58 F/g to 73.10 F/g. The decrease of C sp is caused by faster ion migration at a higher current density, which accelerates the polarisation and depolarisation process of the electrode. This will result in the incomplete utilisation of ions on the electrode surface [22]. where Csp is the specific capacitance of the electrode (F/g), Δt is the discharging time (s), I is the discharge current (A/g), ΔV is the potential range (V), and m is the mass of the electrode material (g). NDG/Mn3O4/Fe3O4 exhibits the highest Csp (90.58 F/g), followed by NDG/Mn3O4 (83.21 F/g) and NDG/Fe3O4 (14.27 F/g). NDG/Mn3O4/Fe3O4 also shows the longest discharge time, as compared to NDG/Mn3O4 and NDG/Fe3O4 at 0.5 A/g (Figure 6a), indicating that NDG/Mn3O4/Fe3O4 possesses the highest charge storage capacity [8,21]. The discharging time of NDG/Mn3O4/Fe3O4 becomes shorter as current density increases from 0.5 A/g to 2.5 A/g (Figure 6b), indicating the decrease of Csp from 90.58 F/g to 73.10 F/g. The decrease of Csp is caused by faster ion migration at a higher current density, which accelerates the polarisation and depolarisation process of the electrode. This will result in the incomplete utilisation of ions on the electrode surface [22]. Electrochemical impedance spectroscopy (EIS) was performed to evaluate the resistive behavior of all the composites. Nyquist plots of NDG/Mn3O4, NDG/Fe3O4, and NDG/Mn3O4/Fe3O4 (Figure 7) consist of a semicircle in the high frequency region and a vertical line in the low frequency region. The vertical line in the low frequency region of Nyquist plots for all the composites approach 90°, suggesting rapid ion transport between the electrode and electrolyte and ideal capacitive behavior [23]. The diameter of the semicircle is related to Rct at the electrode-electrolyte interface. NDG/Mn3O4/Fe3O4 possesses the smallest diameter of the semicircle, followed by NDG/Mn3O4 and NDG/Fe3O4, indicating that NDG/Mn3O4/Fe3O4 has the lowest Rct (1.35 Ω), as compared to NDG/Mn3O4 (1.39 Ω) and NDG/Fe3O4 (1.49 Ω). This suggests that NDG with bimetallic oxides have a lower contact resistance and the greater charge transfer rate at the electrode-electrolyte interface compared to NDG with single metal oxides [24]. At the high frequency region, the intercept of the semicircle at the Z' axis gives information about equivalent series resistance (ESR), which is the combination of the internal resistance of the electroactive materials, the interfacial resistance between the electrode and the current collector, and the intrinsic ionic electrolyte resistance [25]. The ESR values of NDG/Mn3O4/Fe3O4, NDG/Fe3O4, and NDG/Mn3O4 are 3.61 Ω, 3.87 Ω, and 4.65 Ω, respectively, as shown in Table 1. NDG/Mn3O4/Fe3O4 exhibits the lowest ESR value, demonstrating that the composite has the lowest contact resistance at the interface of active material and a current collector, which contributes to rapid ion diffusion [11]. The low ESR value also proves the lowest internal resistance in NDG/Mn3O4/Fe3O4, which is supported by an insignificant iR drop observed in the GCD curve [14]. These EIS results also agree well with the CV results. 49 Ω). This suggests that NDG with bimetallic oxides have a lower contact resistance and the greater charge transfer rate at the electrode-electrolyte interface compared to NDG with single metal oxides [24]. At the high frequency region, the intercept of the semicircle at the Z' axis gives information about equivalent series resistance (ESR), which is the combination of the internal resistance of the electroactive materials, the interfacial resistance between the electrode and the current collector, and the intrinsic ionic electrolyte resistance [25]. exhibits the lowest ESR value, demonstrating that the composite has the lowest contact resistance at the interface of active material and a current collector, which contributes to rapid ion diffusion [11]. The low ESR value also proves the lowest internal resistance in NDG/Mn 3 O 4 /Fe 3 O 4 , which is supported by an insignificant iR drop observed in the GCD curve [14]. These EIS results also agree well with the CV results. Appl. Sci. 2019, 9, x FOR PEER REVIEW 9 of 12  Cyclic stability tests were carried out at a scan rate of 100 mV/s for 5000 cycles to evaluate the capacitance retention of NDG/Mn3O4/Fe3O4. As shown in Figure 8a, the Csp retention of NDG/Mn3O4/Fe3O4 initially increases until the 2500 th cycle, and then decreases to 103.4% at the 5000 th cycle. The initial increase in capacitance of NDG/Mn3O4/Fe3O4 may be due to the active site activation of the composites [10,26]. After activation, the electrochemically active materials will be completely exposed to the electrolyte [27]. This phenomenon can be related to the ESR value of the composite after 2500 cycles (Figure 8b), in which the ESR decreases to 1.83 Ω due to the proper wetting of the electrode surface, which can enhance the electrolyte diffusion. From 2500 to 5000 cycles, there is an decrease in Csp retention that was caused by activity loss of the metal oxide particles during the charging and discharging processes [26]. This phenomenon is also supported by the EIS data, in which there is an increase in ESR (2.12 Ω). Generally, the NDG/Mn3O4/Fe3O4 shows good cyclic stability due to the strong bonding between dispersed metal oxide particles and NDG with high mechanical strength, which can effectively prevent the volume expansion and aggregation of metal oxides during the charging and discharging processes [7,28]. The cycling stability of NDG/Mn3O4/Fe3O4 is higher in comparison to MnFe2O4/graphene reported in the literature [11].  Cyclic stability tests were carried out at a scan rate of 100 mV/s for 5000 cycles to evaluate the capacitance retention of NDG/Mn 3 O 4 /Fe 3 O 4 . As shown in Figure 8a, the C sp retention of NDG/Mn 3 O 4 /Fe 3 O 4 initially increases until the 2500 th cycle, and then decreases to 103.4% at the 5000 th cycle. The initial increase in capacitance of NDG/Mn 3 O 4 /Fe 3 O 4 may be due to the active site activation of the composites [10,26]. After activation, the electrochemically active materials will be completely exposed to the electrolyte [27]. This phenomenon can be related to the ESR value of the composite after 2500 cycles (Figure 8b), in which the ESR decreases to 1.83 Ω due to the proper wetting of the electrode surface, which can enhance the electrolyte diffusion. From 2500 to 5000 cycles, there is an decrease in C sp retention that was caused by activity loss of the metal oxide particles during the charging and discharging processes [26]. This phenomenon is also supported by the EIS data, in which there is an increase in ESR (2.12 Ω). Generally, the NDG/Mn 3 O 4 /Fe 3 O 4 shows good cyclic stability due to the strong bonding between dispersed metal oxide particles and NDG with high mechanical strength, which can effectively prevent the volume expansion and aggregation of metal oxides during the charging and discharging processes [7,28]. The cycling stability of NDG/Mn 3 O 4 /Fe 3 O 4 is higher in comparison to MnFe 2 O 4 /graphene reported in the literature [11]. Appl. Sci. 2019, 9, x FOR PEER REVIEW 10 of 12

Conclusion
NDG/Mn3O4/Fe3O4 with different Fe:Mn ratios were successfully prepared using a hydrothermal method followed by freeze-drying. The combination of the high conductivity of NDG with richer redox reaction from Mn3O4/Fe3O4 resulted in the enhanced electrochemical performance of the composite with bimetallic oxide compared to the composite with a single metal oxide, which is due to Mn3O4 and Fe3O4 particles being attached on the wrinkled surface of NDG that helped to prevent agglomeration of the NDG sheets. Different Fe:Mn ratios in NDG/Mn3O4/Fe3O4 were studied and the results showed that the ratio between the metal oxides also played an important role in enhancing the supercapacitive performance of the composite. The Fe:Mn ratio of 1:3 exhibited the highest specific capacitance and had excellent cyclic stability with the lowest Rct and ESR value, which indicates good energy storing ability, a superior life cycle, and fast charge transfer at the electrodeelectrolyte interface. Therefore, NDG/Mn3O4/Fe3O4 is a potential material for electrodes to be used in supercapacitors.
Author Contributions: Beng Meng Chong performed the experiments and analyzed the data. Beng Meng Chong, Nur Hawa Nabilah Azman, and Yusran Sulaiman wrote the paper. Muhammad Amirul Aizat Mohd

Conclusions
NDG/Mn 3 O 4 /Fe 3 O 4 with different Fe:Mn ratios were successfully prepared using a hydrothermal method followed by freeze-drying. The combination of the high conductivity of NDG with richer redox reaction from Mn 3 O 4 /Fe 3 O 4 resulted in the enhanced electrochemical performance of the composite with bimetallic oxide compared to the composite with a single metal oxide, which is due to Mn 3 O 4 and Fe 3 O 4 particles being attached on the wrinkled surface of NDG that helped to prevent agglomeration of the NDG sheets. Different Fe:Mn ratios in NDG/Mn 3 O 4 /Fe 3 O 4 were studied and the results showed that the ratio between the metal oxides also played an important role in enhancing the supercapacitive performance of the composite. The Fe:Mn ratio of 1:3 exhibited the highest specific capacitance and had excellent cyclic stability with the lowest R ct and ESR value, which indicates good energy storing ability, a superior life cycle, and fast charge transfer at the electrode-electrolyte interface. Therefore, NDG/Mn 3 O 4 /Fe 3 O 4 is a potential material for electrodes to be used in supercapacitors.