## 3 Results and Discussion
### 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 (\(C_{\rm sp}\)) of the composites was calculated using Equation (1):  
\[C_{\rm sp}=\frac{\int IdV}{mv\Delta V} \tag{1}\]  
where \(I\) is the current density (A/g), \(v\) is the scan rate applied (mV/s), \(\Delta V\) is the potential range applied, and \(m\) is the mass of the sample (g). Figure 5a shows the CV curves of NDG/Mn\({}_{3}\)O\({}_{4}\)/Fe\({}_{3}\)O\({}_{4}\) with a different ratio of Fe:Mn at a scan rate of 50 mV/s. NDG/Mn\({}_{3}\)O\({}_{4}\)/Fe\({}_{3}\)O\({}_{4}\) with the Fe:Mn ratio of 1:3 exhibits the highest \(C_{\rm sp}\) (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 (147.55 F/g), as the composite has the largest enclosed area of the CV curve. The \(C_{\rm sp}\) of NDG/Mn\({}_{3}\)O\({}_{4}\)/Fe\({}_{3}\)O\({}_{4}\) 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]. Quasi-rectangular CV curves and small humps are observed for NDG/Mn\({}_{3}\)O\({}_{4}\)/Fe\({}_{3}\)O\({}_{4}\) in all Fe:Mn ratios, NDG/Fe\({}_{3}\)O\({}_{4}\), and NDG/Mn\({}_{3}\)O\({}_{4}\) (Figure 5b), which indicate that the capacitance is mainly contributed by Faradaic pseudocapacitance [14], which is contributed by the metal oxides [15]. NDG/Mn\({}_{3}\)O\({}_{4}\)/Fe\({}_{3}\)O\({}_{4}\) exhibits the largest enclosed CV curve area, indicating that it has the highest \(C_{\rm sp}\) (158.46 F/g) compared to NDG, NDG/Fe\({}_{3}\)O\({}_{4}\), and NDG/Mn\({}_{3}\)O\({}_{4}\). This might due to the presence of both Fe\({}_{3}\)O\({}_{4}\) and Mn\({}_{3}\)O\({}_{4}\) 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 \(C_{\rm sp}\) 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]. 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 \(C_{\rm sp}\) 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 \(C_{\rm sp}\) 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]. In addition, insignificant voltage drop (IR drop) in the GCD curves of NDG/Mn\({}_{3}\)O\({}_{4}\)/Fe\({}_{3}\)O\({}_{4}\), NDG/Mn\({}_{3}\)O\({}_{4}\), and NDG/Fe\({}_{3}\)O\({}_{4}\) (Figure 6a) show high efficiency of ion transport or diffusion near the electrode-electrolyte interface [19].