## 3 Results and Discussion
### Electrochemical Characterization
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):  
\[C_{\text{sp}}=\ \frac{I\Delta t}{m\Delta V} \tag{2}\]  
where \(C_{\text{sp}}\) is the specific capacitance of the electrode (F/g), \(\Delta t\) is the discharging time (s), \(I\) is the discharge current (A/g), \(\Delta V\) is the potential range (V), and \(m\) is the mass of the electrode material (g). NDG/Mn\({}_{3}\)O\({}_{4}\)/Fe\({}_{3}\)O\({}_{4}\) exhibits the highest \(C_{\text{sp}}\) (90.58 F/g), followed by NDG/Mn\({}_{3}\)O\({}_{4}\) (83.21 F/g) and NDG/Fe\({}_{3}\)O\({}_{4}\) (14.27 F/g). NDG/Mn\({}_{3}\)O\({}_{4}\)/Fe\({}_{3}\)O\({}_{4}\) also shows the longest discharge time, as compared to NDG/Mn\({}_{3}\)O\({}_{4}\) and NDG/Fe\({}_{3}\)O\({}_{4}\) at 0.5 A/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 \(C_{\text{sp}}\) from 90.58 F/g to 73.10 F/g. The decrease of \(C_{\text{sp}}\) 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]. Electrochemical impedance spectroscopy (EIS) was performed to evaluate the resistive behavior of all the composites. Nyquist plots of NDG/Mn\({}_{3}\)O\({}_{4}\), NDG/Fe\({}_{3}\)O\({}_{4}\), and NDG/Mn\({}_{3}\)O\({}_{4}\)/Fe\({}_{3}\)O\({}_{4}\) (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\({}^{\circ}\), suggesting rapid ion transport between the electrode and electrolyte and ideal capacitive behavior [23]. The diameter of the semicircle is related to \(R_{\text{ct}}\) at the electrode-electrolyte interface. NDG/Mn\({}_{3}\)O\({}_{4}\)/Fe\({}_{3}\)O\({}_{4}\) possesses the smallest diameter of the semicircle, followed by NDG/Mn\({}_{3}\)O\({}_{4}\) and NDG/Fe\({}_{3}\)O\({}_{4}\), indicating that NDG/Mn\({}_{3}\)O\({}_{4}\)/Fe\({}_{3}\)O\({}_{4}\) has the lowest \(R_{\text{ct}}\) (1.35 \(\Omega\)), as compared to NDG/Mn\({}_{3}\)O\({}_{4}\) (1.39 \(\Omega\)) and NDG/Fe\({}_{3}\)O\({}_{4}\) (1.49 \(\Omega\)). 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/Mn\({}_{3}\)O\({}_{4}\)/Fe\({}_{3}\)O\({}_{4}\), NDG/Fe\({}_{3}\)O\({}_{4}\), and NDG/Mn\({}_{3}\)O\({}_{4}\) are 3.61 \(\Omega\), 3.87 \(\Omega\), and 4.65 \(\Omega\), respectively, as shown in Table 1. NDG/Mn\({}_{3}\)O\({}_{4}\)/Fe\({}_{3}\)O\({}_{4}\) 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].