## 3 Results and Discussion  
### Physical Characterization  
Fourier-transform infrared (FTIR) spectroscopy was performed to investigate the chemical structure of NDG, NDG/Fe\({}_{3}\)O\({}_{4}\), NDG/Mn\({}_{3}\)O\({}_{4}\), and NDG/Mn\({}_{3}\)O\({}_{4}\)/Fe\({}_{3}\)O\({}_{4}\) (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 Mn\({}_{3}\)O\({}_{4}\) and Fe\({}_{3}\)O\({}_{4}\) into NDG, proving a successful anchoring of Mn\({}_{3}\)O\({}_{4}\) and Fe\({}_{3}\)O\({}_{4}\) on the NDG sheets. In the NDG/Fe\({}_{3}\)O\({}_{4}\) spectrum (Figure 2b), two new peaks at 428 cm\({}^{-1}\) and 524 cm\({}^{-1}\) were noticed, which correspond to Fe-O vibrations. The spectrum of NDG/Mn\({}_{3}\)O\({}_{4}\) (Figure 2c) shows the peaks of Mn-O and Mn-O-Mn vibrations at 561 cm\({}^{-1}\) and 709 cm\({}^{-1}\), respectively. The presence of both metal oxides in the NDG/Mn\({}_{3}\)O\({}_{4}\)/Fe\({}_{3}\)O\({}_{4}\) composite is confirmed by peaks observed at 507 cm\({}^{-1}\), 596 cm\({}^{-1}\), and 752 cm\({}^{-1}\), which correspond to Fe-O, Mn-O, and Mn-O-Mn vibrations, respectively, in the NDG/Mn\({}_{3}\)O\({}_{4}\)/Fe\({}_{3}\)O\({}_{4}\) spectrum (Figure 2d). The peak of Fe-O at 428 cm\({}^{-1}\) becomes less visible because it is overlapped with the peak of Fe-O at 507 cm\({}^{-1}\). Field Emission Scanning Electron Microscopy (FESEM) was performed to investigate the morphologies of NDG, NDG/Fe\({}_{3}\)O\({}_{4}\), NDG/Mn\({}_{3}\)O\({}_{4}\), and NDG/Mn\({}_{3}\)O\({}_{4}\)/Fe\({}_{3}\)O\({}_{4}\). As shown in Figure 3a, NDG exhibits a wrinkle-like sheet morphology, which can also be noticed in the FESEM images of NDG/Fe\({}_{3}\)O\({}_{4}\), NDG/Mn\({}_{3}\)O\({}_{4}\), and NDG/Mn\({}_{3}\)O\({}_{4}\)/Fe\({}_{3}\)O\({}_{4}\) (Figure 3b-d). After the addition of Fe\({}_{3}\)O\({}_{4}\), the well-dispersed particles of Fe\({}_{3}\)O\({}_{4}\) can be clearly seen to cover the surface of wrinkled NDG sheets (Figure 3b). The presence of Mn\({}_{3}\)O\({}_{4}\) in NDG/Mn\({}_{3}\)O\({}_{4}\) (Figure 3c) is confirmed as Mn\({}_{3}\)O\({}_{4}\) particles that obviously attach on the NDG sheets. Upon the addition of both Fe\({}_{3}\)O\({}_{4}\) and Mn\({}_{3}\)O\({}_{4}\) into NDG, a large number of Fe\({}_{3}\)O\({}_{4}\) and Mn\({}_{3}\)O\({}_{4}\) 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 Fe\({}_{3}\)O\({}_{4}\) and Mn\({}_{3}\)O\({}_{4}\) cover on the surface of NDG sheets have enhanced \(C_{\rm sp}\) of the composites due to their highly pseudocapacitive properties [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. X-ray diffraction (XRD) was performed to identify the structure and crystallinity of the prepared NDG, NDG/Fe\({}_{3}\)O\({}_{4}\), NDG/Mn\({}_{3}\)O\({}_{4}\), and NDG/Mn\({}_{3}\)O\({}_{4}\)/Fe\({}_{3}\)O\({}_{4}\). In Figure 4a, the large broad peak at 24.6\({}^{\circ}\), 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/Fe\({}_{3}\)O\({}_{4}\), NDG/Mn\({}_{3}\)O\({}_{4}\), and NDG/Mn\({}_{3}\)O\({}_{4}\)/Fe\({}_{3}\)O\({}_{4}\) due to the relatively small diffraction intensity of NDG compared to the strong peaks of Fe\({}_{3}\)O\({}_{4}\) and Mn\({}_{3}\)O\({}_{4}\)[13].