## 3 Results and Discussion
### Mechanistic Analysis of ht-pDA/ACF Formation and the Production of H\({}_{2}\)O\({}_{2}\)
The results indicate that with the effect of electricity and oxygen, the free radicals could be generated at the surface of the cathode during electrolysis. Furthermore, the pDA could accelerate the above process and produce more free radicals that lead to more MB decomposition, and eventually, the absorbance decreases compared to the CF cathode with O\({}_{2}\). The experiment also implies the potential application of the ht-pDA/ACF cathode on pollution degradation. Some literature has proven free radical polymerization as a possible pathway that should exist during the formation of pDA [28]. With the temperature rising, the rate of free radical polymerization increased and the degree of polymerization (DP) decreased. According to the above characterization results and the characteristics of free radical polymerization, we illustrate the differences between ht-pDA/ACF and ao-pDA/ACF during dopamine polymerization, as shown in Figure 12. Firstly, the dopamine molecules interacted with each other and formed short chains in the solution by altering dopamine to quinone and cyclizing. When polymerizing at room temperature, the short pDA chains slowly extended its molecule chain and eventually formed the cross-linked pDA film with high DP at the surface of CF. The introduction of pDA brings numerous active sites and defects that are favorable for the 2e\({}^{-}\) ORR and H\({}_{2}\)O\({}_{2}\) production. Therefore, the ao-pDA/ACF cathode shows distinctive improvement for H\({}_{2}\)O\({}_{2}\) production compared to the CF cathode, which has been proven by the results of H\({}_{2}\)O\({}_{2}\) production in Figure 3a. However, in the  
Figure 10: The N 1s XPS spectra of ht-pDA/ACF before (**a**) and after (**b**) electrolysis. meantime, the film pDA structure could bury the active sites, and thus, showing inferior H\({}_{2}\)O\({}_{2}\) production compared to the ht-pDA/ACF, as shown in Figure 12 I. However, the high temperature during the hydrothermal process led to a higher polymerization rate at first but lower DP eventually according to the radical polymerization mechanism. Such chains tended to curl and formed spherical particles in the solution. The pDA particles deposited on the surface of CF, as the SEM shows in Figure 5. The spherical pDA is desirable to expose active sites and accelerate the 2e\({}^{-}\) ORR, as Figure 12 II shows. By changing the morphology of pDA via altering the polymerization temperature, the H\({}_{2}\)O\({}_{2}\) production further improved, which suggests that the hydrothermal process of dopamine polymerization is an efficient way to enhance the performance of CF for H\({}_{2}\)O\({}_{2}\) production. Figure 11: The UV-visible spectra of methylene blue under different conditions. a: original; b: CF, only supplying O\({}_{2}\); c: CF, only supplying electricity; d: CF, supplying O\({}_{2}\) and electricity; e: ht-pDA/ACF, supplying O\({}_{2}\) and electricity. Figure 7:0 The preparing mechanism of ht-pDA/ACF and ao-pDA/ACF. To explain the effect of ht-pDA/ACF and verify its active sites for the 2e\({}^{-}\) ORR, we calculated the atomic percentage of ht-pDA/ACF before and after the electrolysis by detecting the XPS spectra and analyzed the change of surface states of ht-pDA/ACF. As we can see in the C 1s spectra in Table 1, the content of -COOH obviously increased after the electrolysis, from 3.79% to 10.18%, while the C=C and the C-N/C-O slightly decreased. Similarly, the O 1s spectra shows the percentage of O-C reduced from 70.22% to 53.82%, whereas the proportion of O=C rose up from 29.78% to 46.18%. Combining the C 1s spectra with the O 1s spectra, we concluded that the C=C and C-O groups got involved in the 2e\({}^{-}\) ORR and were gradually oxidized to -COOH. The N 1s spectra show that the number of C-N-C groups went down and the number of N-H groups went up, suggesting that the ring cleavage reaction occurred during the electrolyzing and implying that the C-N-C also participated in the 2e\({}^{-}\) ORR.