## 3 Results and Discussion  
Figure 2a,b show FE-SEM images of as-prepared q-MC//CC nanocomposites. The pristine carbon cloth shows a smooth surface consisting of aligned carbon fiber bundles. After the hydrothermal reaction of the synthesis of q-MC//CC, it is observed that clusters of spindle-shaped petals of MnCO\({}_{3}\) were densely grown on the surface of carbon fibers. In addition, the prepared q-MC//CC sample shows good flexibility, as shown in the inset of Figure 2a. The morphology of MC//CC (Figure S1) has a similar nanostructure to that of q-MC//CC, showing that the addition of CNQDs has no effect on the morphological formation of MnCO\({}_{3}\). The TEM image (Figure 2c) of the q-MC//CC confirms that the CNQDs are uniformly embedded in MnCO\({}_{3}\), and the lattice fringe of CNQDs is found in the high resolution TEM (HRTEM) image (Figure 2d), where a lattice distance is 0.336 nm, corresponding to the (002) planes of g-C\({}_{3}\)N\({}_{4}\). The SEM image of q-MC//CC and its elemental mapping from energy-dispersive X-ray spectroscopy (EDS) reveal the existence of the expected atomic elements (Figure 2e), which clearly confirms that Mn, C, and N elements are uniformly distributed on the MnCO\({}_{3}\) surface and the successful formation of q-MC//CC nanocomposite. In addition, the obtained q-MC//CC has a larger surface area (182 m\({}^{2}\)/g) than the pristine carbon cloth (69 m\({}^{2}\)/g) due to its unique nanostructure (Figure S2). An XPS study was performed to characterize the chemical compositions of the q-MC//CC. The XPS survey scans in Figure 3a show that both q-MC//CC and MC//CC samples contain the Mn, O, and C atoms, while the N peak is only found in the q-MC//CC nanocomposites due to CNQDs. The deconvolution of the Mn 2p spectrum of q-MC//CC (Figure 3b) displays two peaks located at 655.8 eV and 643.8 eV respectively, assigning to Mn 2p\({}_{1/2}\) and Mn 2p\({}_{3/2}\) with an energy separation of 12.0 eV, which coincides with Mn\({}^{2+}\)[18; 19]. In the high-resolution C1s spectrum of q-MC//CC (Figure 3c) and MC//CC (Figure S3), two typical peaks of the C-C bond at 284.6 eV and C=O bond at 288.5 eV are observed, relating to the surface carbon of the carbon cloth and carbon element in carbonate ions [20]. In addition, two additional peaks at 286.0 and 292.7 eV are only observed for q-MC//CC, which correspond to the C-N bond and carboxylate bond of CNQDs, respectively. Figure 2: (**a,b**) SEM images of g-C\({}_{3}\)N\({}_{4}\) quantum dots/MnCO\({}_{3}\) on carbon cloth (q-MC//CC). (**c**) TEM image of q-MC//CC and (**d**) High resolution TEM (HRTEM) image. (**e**) Energy-dispersive X-ray spectroscopy (EDS) mapping images of q-MC//CC. In addition, the high-resolution N is spectrum of q-MC//CC can be fitted into three peaks, as shown in Figure 3d, corresponding to =N- (398.4 eV), C=N-C (399.1 eV), and N-(C)\({}_{3}\) (400.4 eV) [21], indicating the presence of CNQDs in the q-MC//CC nanocomposite. The contact angle measurements with 1 M Na\({}_{2}\)SO\({}_{4}\) electrolyte were performed to study the effect of CNQDs on the surface wettability of the prepared q-MC//CC composite. Figure S4 illustrates the images of electrolyte droplets on the prepared MC//CC and q-MC//CC composites, respectively. The q-MC//CC composite shows a lower contact angle with Na\({}_{2}\)SO\({}_{4}\) electrolyte than MC//CC, which is due to the more ion-accessible surfaces by CNQDs. This enhanced surface wettability of the q-MC//CC composite facilitates the accessibility of electrolyte, leading to better electrochemical performance compared to the MC//CC composite. In addition, the FTIR spectra of the as-prepared q-MC//CC and MC//CC are shown in Figure S5. The characteristic peak of the Mn-O-Mn bond is observed at 727 cm\({}^{-1}\) for both samples, while that of the characteristic peaks corresponding to the C-N (1033 cm\({}^{-1}\)) and C=N bonds (1780 cm\({}^{-1}\)) are only found for q-MC//CC due to the existence of CNQDs.