## 3 Results and Discussion
### Mechanistic Analysis of ht-pDA/ACF Formation and the Production of H\({}_{2}\)O\({}_{2}\)  
To figure out the surface states of ht-pDA/ACF and further explain the reason for its morphology and good H\({}_{2}\)O\({}_{2}\) production performance, XPS was employed to determine its elemental composition and surface chemical bonding. The XPS spectra of ht-pDA/ACF before and after being used as the cathode for electrocatalytic H\({}_{2}\)O\({}_{2}\) production (Figure S7) shows three peaks that represent C, O and N, which is in accordance with the result of the EDX spectra (Figure S8). The peak fittings of C 1s, O 1s and N 1s high-resolution spectra of ht-pDA/ACF before and after the electrolysis are depicted in Figures 8-10, respectively. The C 1s spectrum of ht-pDA before electrolyzing shows three peaks at 284.8, 285.8 and 289.2 eV, which can be attributed to the graphite carbon of C=C sp\({}^{2}\), C-O/C-N and C=O of -COOH, respectively [23; 24]. The O 1s peaks of ht-pDA/ACF before electrolyzing at  
Figure 6: The Raman spectra of carbon felt (**a**), ao-pDA/ACF (**b**) and ht-pDA/ACF (**c**). Figure 7: The FT-IR spectra of carbon felt (**a**), ao-pDA/ACF (**b**) and ht-pDA/ACF (**c**). 531.9 and 533.0 eV were assigned to O=C and O-C, respectively [25]. The N 1s spectra of ht-pDA/ACF before electrolyzing exhibits two peaks, which are ascribed to the C-N-C peak at 399.7 eV and the N-H peak at 401.1 eV [23]. The existence of the C-N-C peak proves that during the hydrothermal process, the -NH2 group of dopamine was cyclized and dopamine was converted into an indole structure. Some articles have confirmed that dopamine could polymerize by altering dopamine to quinone and cyclizing [20]. Such results indicate that the dopamine has polymerized into pDA and that pDA had been successfully introduced onto the CF by the hydrothermal process, which is in accordance with the FTIR spectra shown in Figure 7. Furthermore, combining the results of SEM, FTIR, Raman and XPS, we conclude that the ht-pDA/ACF possesses more defects and N-containing groups compared to the CF due to the introduction of pDA, which could promote the 2e\({}^{-}\) ORR for H2O2 production, as previous literature proved [26]. Additionally, the spherical morphology of ht-pDA/ACF provide a large specific area compared to the film morphology of ao-pDA/ACF, and thus, could expose more active sites that are favorable in the reaction. Figure 8: The C 1s XPS spectra of ht-pDA/ACF before (**a**) and after (**b**) electrolysis. Figure 9: The O 1s XPS spectra of ht-pDA/ACF before (**a**) and after (**b**) electrolysis. To illustrate the reaction mechanism of the 2e\({}^{-}\) ORR on the ht-pDA/ACF cathode, we further used methylene blue (MB) as a free radical scavenger and tested its concentration difference before and after 3 h electrolysis by UV-visible spectra. The results are shown in Figure 11. All curves have a peak at 664 nm, which are attributed to the absorption of MB [27]. The absorbance shows nearly no change when only supplying O\({}_{2}\) to the CF (Figure 11, b), indicating that the MB did not react with O\({}_{2}\) without electricity. Similarly, when CF was used as the cathode and without O\({}_{2}\) (Figure 11, c), the absorbance only slightly drops compared with the original. This implies that although a part of MB was oxidized at the anode (platinum plate), most MB was not decomposed and still remains in the electrolyte. Considering the oxidation of MB at the anode, it is clear that there are nearly no free radicals formed on the surface of CF cathode. However, after supplying the oxygen to the cathode (Figure 11, d), the absorbance sharply decreased compared to the without oxygen cathode, suggesting the oxygen-containing free radicals, such as \(\bullet\)OH, \(\bullet\)OH, formed when the electricity and oxygen exist simultaneously. Furthermore, the absorbance of the ht-pDA/ACF cathode (Figure 11, e) with oxygen is even lower than CF cathode with oxygen, indicating that more free radicals formed on the ht-pDA/ACF cathode due to the existence of pDA.