## 4 Conclusions  
In this work, a spherical particle morphology polydopamine (pDA)-modified carbon felt (noted as ht-pDA/ACF) by a hydrothermal process was fabricated to be used as the cathode of H2O2 production. The hydrothermal process of dopamine not only incorporated nitrogen-containing groups but also formed desirable spherical polydopamine for the oxygen reduction reaction. The ht-pDA/ACF cathode showed an obvious hydrogen peroxide (H2O2) production improvement compared to the raw carbon felt cathode with a voltage of 2.0 V, from 30 mg/L to 220 mg/L. The H2O2 production further reached 340 mg/L when the voltage rose to 2.4 V. After analyzing the formation of polydopamine and H2O2 production process, we found that the active site of pDA for two-electron oxygen reduction reaction (2e\({}^{-}\) ORR) is the indole structure. The ring cleavage of the indole structure during electrolyzing formed radicals and further formed H2O2. Moreover, the spherical morphology of pDA could enlarge the specific area and expose more active site compared to the film morphology of pDA. The results show that the ht-pDA/ACF is a promising material for H2O2 production via the 2e\({}^{-}\) ORR.  
**Supplementary Materials:** The following are available online at [[https://www.mdpi.com/article/10.3390/app11125371/s1](https://www.mdpi.com/article/10.3390/app11125371/s1)]([https://www.mdpi.com/article/10.3390/app11125371/s1](https://www.mdpi.com/article/10.3390/app11125371/s1)), Figure S1: H2O2 production (mg/L) of ht-pDA/ACF cathode in acid (H2 SO4, pH = 1) and alkaline (KOH, pH = 13) electrolyte, Figure S2: H2O2 production (mg/L) of ht-pDA/ACF cathode with different dopamine content. Figure S3: H2O2 production (mg/L) of ht-pDA/ACF cathode with different hydrothermal process time, Figure S4 the SEM images of CF (a), ao-pDA/ACF (b) and ht-pDA/ACF (c), Figure S5 pDA particles size distribution of ht-pDA/ACF (particle number:100), Figure S6 the Raman spectra of ht-pDA/ACF after electrolysis, Figure S7 the XPS spectra of ht-pDA/ACF before (a) and after (b) electrolysis, Figure S8 the EDX spectra of ht-pDA before (a,b,c) and after (d,e,f) electrolysis, Figure S9 the XPS spectra (a) of ao-pDA/ACF and its C 1s (b), N 1s (c) and O 1s (d) spectra, Table S1 Comparison of H2O2 accumulation concentration in other literature.  
Figure 13: The mechanism diagram of H2O2 production on ht-pDA/ACF (oxygen atom: red sphere, hydrogen atom: blue sphere, pDA: white sphere).  
**Author Contributions:** Conceptualization, H.W., Y.Z. and Z.L.; methodology, L.C., Y.Z.; validation, L.C., Y.Z. and D.F.; writing--original draft preparation, L.C., Y.Z.; writing--review and editing, L.C., Y.Z. and S.Y.; supervision, Y.Z., H.W. All authors have read and agreed to the published version of the manuscript.  
**Funding:** This research was funded by the National Science Foundation for Distinguished Young Scholars of China (Grant No. 51925403), the Major Research Plan of National Natural Science Foundation of China (Grant No. 2019002), the National Science Foundation of China (21676052, 21606042), the Open Research Subject of Key Laboratory of Fluid and Power Machinery (Xihua University), Ministry of Education (Grant No. szj2019-021).  
**Conflicts of Interest:** The authors declare that there is no conflict of interests regarding the publication of this article.