Operando Fe Dissolution in Fe-N-C Electrocatalysts during Acidic Oxygen Reduction and Impact of Local pH Change

This work holds significant value, and I have diligently read and studied the manuscript. However, I have some questions: 1. The post-TEM analysis of the 20°C-O2-ICPMS samples revealed a large number of suspected Fe oxides, yet no signals were evident in the XRD results. The authors also performed post-XAS analysis, but did they provide R-space data to substantiate the presence of Fe oxides? According to the dissolution data presented in Fig. 4, the Fe dissolution under 20°C-N2 conditions should be substantial, possibly exceeding 20% of the initial Fe content. If this portion of Fe forms Fe oxides under the influence of O2, then XAS should detect corresponding signals. 2. Another intriguing point is that post-TEM analysis of the 75°C-O2-ICPMS samples did not show noticeable nanoparticles. However, if compared with the ICPMS data at 75°C-N2, a significant reduction in Fe dissolution should be observed. Where did the Fe that was supposed to dissolve go? Another puzzling is that the post-TEM analysis at 80°C-O2-ICPMS-0.6V-2h, referenced from Nat. Catal., 2023, 1–11, showed a large number of suspected Fe oxides, which seems different from the observations in this manuscript. 3. Another question concerns the formation process of Fe oxides. If it involves the oxidation of trivalent Fe ions by O2 to form Fe oxides, then a higher current would result in more O2 consumption, leading to a more O2-depleted environment, which is less conducive to the formation of Fe oxides. The conclusion should be that higher currents lead to less formation of Fe oxides, hence more Fe ions would be lost and detected by ICPMS. This is contrary to the manuscript's observations, where higher currents were associated with fewer Fe ions detected by ICPMS. Could you please clarify these points?