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

Dear Yu-Cheng, Many thanks for your questions, feedback, and interest in our work! Apologies for the very delayed reply - I did not receive any notification of your comment from ChemRxiv. I provide answers to your questions below: 1. We believe the concentration (and size) of Fe particles could be below the detection limit of the XRD (under the measured conditions). Additionally, the Fe oxides could be poorly-crystalline. Therefore, XRD only confirms we do not have high concentrations of large crystalline nanoparticles (>3 nm) in the Fe-N-C. The post XAS is only XANES as the Fe concentration was too low to perform EXAFS with sufficient data quality, therefore we focus on qualitative interpretation of the XANES data. We see a noticeable shift in Fe XANES for the post 20oC-Ar, suggesting Fe species have dissolved. 2. We could not measure at 75oC-Ar due to saturation of the ICP-MS detector (and time constraints). In terms of 75oC-O2 we see the baseline Fe concentration measurements by ICP-MS is higher (increased transport) and also at low-intermediate current density (-1 to -15 mA cm-2) the Fe particles are redissolving in higher concentrations (again due to increased mass transport + change in Pourbaix diagram with temperature (and possibly concentration)). We believe the different observation in Nat. Catal., 2023, 1-11 and our work can be due to very different catalyst morphology (theirs is bulky microporous ZIF-8 particles with lack of mesopores, while ours is a highly hierarchal porous structure). We see this results in different operando ICP-MS results (in published and unpublished works). From the ICP-MS data we also see the post-mortem observations are highly dependent on the precise measurement protocol (theirs is held at 0.6 V while we carry out AST between 0.9 and 0.6 V (and step at different potentials before and after AST)).

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?