**4. Conclusions**

In this study, x-NGM composites consisting of NG and MnO2 with various Mn contents were fabricated by a low-cost hydrothermal method. By SEM, TEM, EDS mappings, XPS, FTIR and Raman spectra, the presence of NG and MnO2 was confirmed, and the successful preparation of the composites was demonstrated. The microstructure analysis by TEM manifested that the MnO2 in the x-NGM composites was a two-phase mixture of γ- and α-MnO2. According to the EIS results, the NG component was found to reduce RCT effectively due to its good conductivity. The co-existence of NG and MnO2 in an active material led to a more reduced RCT and further improved charge transfer. Among the 15 electrodes with x-NGM active materials, the 3-NGM1 electrode exhibited the smallest RCT, indicating its best charge transfer efficiency. Its Nyquist plot in the low-frequency region had the largest slope, implying a lower diffusion impedance, more rapid ionic diffusion, and enhanced capacitive property. Both the mass loading and content of Mn in an active material electrode were crucial. The best electrochemical performance was achieved when the mass loading of active materials on the PI/graphite flexible substrate was 1 mg and x = 3 to obtain the optimized Mn content in the x-NGM composites. Excess Mn caused decreased contacts between the electrode and electrolyte ions, leading to increased RCT, and suppressed ionic diffusion. Among the 21 electrodes with Gy, NGy, and x-NGMy composites, the 3-NGM1 electrode exhibited the best sustainable ability. However, its rate capability still required large improvement. After calculation of the CV results, it showed a high specific capacitance of 638 <sup>F</sup>·g<sup>−</sup>1, and the corresponding energy and power densities were 372.7 Wh·kg−<sup>1</sup> and 4731.1 <sup>W</sup>·kg−1, respectively. The enhancement was ascribed to the synergistic effect of the higher conductivity by NG and the larger specific surface area by MnO2 nanostructures. Moreover, the increase of specific capacitance was found to be more significant by the pseudocapacitive MnO2 than NG.

**Author Contributions:** Methodology, C.-P.C.; Validation, H.-Y.C.; Data Curation, H.-Y.C.; Investigation, H.-Y.C.; Formal Analysis, H.-Y.C.; Writing-Original Draft Preparation, C.-P.C.; Writing-Review & Editing, C.-P.C.; Visualization, C.-P.C.; Supervision, C.-P.C.; Project Administration, C.-P.C.; Funding Acquisition, C.-P.C.

**Funding:** This research and the APC were funded by the Ministry of Science and Technology, Taiwan under the gran<sup>t</sup> number of MOST 107-2221-E-260-002-.

**Acknowledgments:** Supports from the Ministry of Science and Technology Taiwan and National Chi Nan University are gratefully appreciated.

**Conflicts of Interest:** The authors declare no conflicts of interest.
