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

05 February 2024, Version 1


Atomic Fe in N-doped C (Fe-N-C) catalysts provide the most promising non-precious metal O2 reduction activity at the cathodes of proton exchange membrane fuel cells. However, one of the biggest remaining challenges to address towards their implementation in fuel cells is their limited durability. Fe demetallation has been suggested as the primary initial degradation mechanism. However, the fate of Fe under different operating conditions varies. Here, we monitor operando Fe dissolution of a highly porous and >50% FeNx electrochemical utilization Fe-N-C catalyst in 0.1 M HClO4, under O2 and Ar at different temperatures, in both flow cell and gas diffusion electrode (GDE) half-cell coupled to inductively coupled plasma mass spectrometry (ICP-MS). By combining these results with pre- and post-mortem analyses, we demonstrate that in the absence of oxygen, Fe cations diffuse away within the liquid phase. Conversely, at -15 mA cm-2geo and more negative O2 reduction currents, the Fe cations reprecipitate as Fe-oxides. We support our conclusions with a microkinetic model, revealing that the local pH in the catalyst layer predominantly accounts for the observed trend. Even at a moderate current density of -15 mA cm-2geo and under O2 at 25 oC, a significant H+ consumption and therefore pH increase (pH = 8-9) within the bulk Fe-N-C layer facilitate precipitation of Fe cations. This work provides a unified view on the Fe degradation mechanism for a model Fe-N-C in both high-throughput flow cell and practical operating GDE conditions, underscoring the crucial role of local pH in regulating the stability of the active sites.


Oxygen Reduction
Single Atom

Supplementary materials

Supplementary Information - Operando Fe Dissolution in Fe-N-C Electrocatalysts during Acidic Oxygen Reduction and Impact of Local pH Change
Contains synthesis, characterisation and modelling details, and supplementary Figures.


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Comment number 2, Angus Pedersen: May 19, 2024, 08:10

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)).

Angus Pedersen :
May 19, 2024, 08:24

3. In the kinetic modelling methods section we assign Fe precipitation to: Fe3+ + 3 H2O --> Fe(OH)3 + 3 H+ Which can occur in the water filled catalyst (micro-)pores, rather than by direct O2. For Fe2+ we indicate that if "Fe2+ cations are dissolved in water, they will anyway thermodynamically be oxidized into Fe3+ cations by O2." Considering the GDE setup (same as fuel cell), gas (e.g. O2) comes from the back side of the catalyst layer while H+ are supplied from the front side. Therefore under high current density the backside of the catalyst layer is saturated with O2 and lacks H+ due to consumption of H+ in the reaction zone at the front side (close to electrolyte). At the back side of the catalyst layer, the decreased H+ concentration and high O2 concentration therefore leads to increased pH and Fe2O3 formation. Many thanks again for your questions and interest in the work. Let me know if you have any further questions/discussion!

Comment number 1, Yu-Cheng Wang: Feb 10, 2024, 06:25

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?