## 3 Results and Discussion
### Mechanistic Analysis of ht-pDA/ACF Formation and the Production of H\({}_{2}\)O\({}_{2}\)
Based on the results of the XPS spectra and UV-visible results of MB and other literature [28; 29], we provide a mechanism diagram of ht-pDA/ACF for the 2e\({}^{-}\) ORR to H\({}_{2}\)O\({}_{2}\) in Figure 7:1 Generally, the dopamine oxidized and formed pDA on the CF by the hydrothermal process at first (Figure 12 I). The amine group of dopamine got involved in the formation of indole structure and formed a C-N-C structure during polymerization (Figures 7 and 10). The high electronegativity of nitrogen atoms at the neighbor of carbon atoms would induce the change of the spin density and charge distribution [14; 29]. When the power supply was turned on, the nitrogen atom in dopamine captured electrons priority because of the strong electronegativity and split the bond to the adjacent carbon atom. The indole structure of the pDA was reduced and the ring cleavage reaction took place to form the structure of C\(\bullet\) and HN\(\bullet\), during the electrolyzing (Figure 13). The C\(\bullet\) or HN\(\bullet\) combined with an oxygen molecule to form \(\bullet\)OH or \(\bullet\)OOH, which is the oxidant for MB fading in free radical capture experiment [28]. The \(\bullet\)OOH, as the intermediate of H\({}_{2}\)O\({}_{2}\) production via the 2e\({}^{-}\) ORR [29], combined with H\({}^{+}\) to form the H\({}_{2}\)O\({}_{2}\) in the acid environment or formed the HO\({}_{2}\)\({}^{-}\) in the alkaline environment. Thus, the ht-pDA/ACF shows high H\({}_{2}\)O\({}_{2}\) production in both acid and alkaline electrolyte, as Figure S1 shows. Moreover, the \(\bullet\)OH, one of the strongest oxidants, could oxidize the C-OH or C=C groups at the surface of the ht-pDA/ACF cathode to COOH and result in the increase of COOH content (Table 1) after H\({}_{2}\)O\({}_{2}\) production. The COOH group has been proved as the ideal active site of the 2e\({}^{-}\) ORR to H\({}_{2}\)O\({}_{2}\) with high selectivity and activity [29]. More carboxyl groups were formed after the H\({}_{2}\)O\({}_{2}\) production also reflects the stability, selectivity and activity of the ht-pDA/ACF cathode during the oxygen reduction. On the one hand, the indole structure of polydopamine acted as the active sites for ht-pDA/ACF cathode to produce H\({}_{2}\)O\({}_{2}\). On the other hand, the spherical particle morphology of ht-pDA/ACF contributed to the good electrocatalytic H\({}_{2}\)O\({}_{2}\) production for exposing more active sites. The synergistic effect between the spherical particles morphology and the continuous formation of H\({}_{2}\)O\({}_{2}\) production active sites results in the high activity and selectivity of the \begin{table}
\begin{tabular}{c c c c} \hline \hline  & & **As-Prepared (\%)** & **After (\%)** \\ \hline \multirow{4}{*}{Element (\%)} & C 1s & 76.71 & 76.58 \\  & N 1s & 17.03 & 20.07 \\  & O 1s & 6.25 & 2.70 \\ \hline \multirow{4}{*}{Group (\%)} & C 1s & C=C & 47.57 & 45.34 \\  & C-N, C-O & 48.64 & 44.48 \\  & -COOH & 3.79 & 10.18 \\ \cline{1-1} \cline{2-4}  & N 1s & C-N-C & 38.95 & 35.97 \\  & N-H & 61.05 & 64.03 \\ \cline{1-1} \cline{2-4}  & O 1s & O-C & 70.22 & 53.82 \\  & O=C & 29.78 & 46.18 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: The atomic percentage of ht-pDA/ACF before (**a**) and after (**b**) electrolysis. ht-pDA/ACF cathode. This work will give a new way to prepare a low-cost and green electrocatalyst for efficient H2O2 electroproduction.