Fe-N-C catalysts are regularly proposed as promising earth-abundant and cheap catalysts replacing platinum group metal catalysts for fuel cells (FCs). Besides the activity, especially the stability of those materials remains challenging. It was found that the electrochemical activity and durability of Fe-N-C catalysts are superior in alkaline compared to acidic media, ^[1-3] yet most of their degradation studies are done in acidic media. ^[4-9] Moreover, although these are systematic works, discrepancies in these results from aqueous model systems (AMS) ^[5,6] and FC testing ^[7-9] remain puzzling. For example, the origin of the dissolved Fe species was found to be from poorly active sites in AMS ^[5] yet from highly active sites in operating FCs. ^[7,8] Additionally, the harmful effects of reactive oxygen species (ROS) on Fe-N-C catalysts are proven in AMS ^[6] but not directly correlated to the durability in FCs. ^[9] To bridge this gap, a gas diffusion electrode (GDE) half-cell coupled with inductively coupled plasma mass spectrometry (ICP-MS) has been developed to study on-line dissolution in realistic catalyst layers. ^[10]

In this work, using a GDE-ICP-MS, we investigate the impacts of oxygen reduction reaction (ORR) on Fe leaching from realistic Fe-N-C alkaline catalyst layers. ^[11] For the first time, Fe dissolution is measured online at current densities above -100 mA·cm^-2. The novel results show that compared to the model Ar-saturated environment, the Fe dissolution is dramatically higher during ORR. Furthermore, between 0.6 and 1.0 V_RHE, we unveil an interesting correlation between Fe dissolution and charge transfer events. This subsequently leads to our hypothesis that the instability of the coordinated Fe during Fe³⁺/Fe²⁺ redox transitions is responsible for Fe leaching from Fe-N-C catalysts in alkaline media in this potential region. The novel insights into Fe-N-C catalyst degradation in realistic conditions can lead to rational design of this promising platinum group metal free catalyst for efficient, durable, and affordable FCs.

References:

[1] Santori, P. G. et al. Effect of pyrolysis atmosphere and electrolyte pH on the oxygen reduction activity, stability and spectroscopic signature of FeN_x moieties in Fe-N-C catalysts. J. Electrochem. Soc. 2019, 166: F3311.

[2] Holby, E. F. et al. Acid stability and demetalation of PGM-Free ORR electrocatalyst structures from density functional theory: a model for “single-atom catalyst” dissolution. ACS Catal. 2020, 10: 14527-14539.

[3] Bae, G. et al. PH effect on the H₂O₂-induced deactivation of Fe-N-C catalysts. ACS Catal. 2020, 10: 8485-8495.

[4] Kumar, K. et al. On the influence of oxygen on the degradation of Fe‐N‐C catalysts. Angew. Chem. 2020, 132: 3261-3269.

[5] Choi, C. H. et al. Stability of Fe‐N‐C catalysts in acidic medium studied by operando spectroscopy. Angew. Chem. Int. Ed. 2015, 54: 12753-12757.

[6] Choi, C. H. et al. The Achilles' heel of iron-based catalysts during oxygen reduction in an acidic medium. Energy Environ. Sci. 2018, 11: 3176-3182.

[7] Li, J. et al. Identification of durable and non-durable FeN_x sites in Fe–N–C materials for proton exchange membrane fuel cells. Nat. Catal. 2021, 4: 10-19.

[8] Chenitz, R. et al. A specific demetalation of Fe–N₄ catalytic sites in the micropores of NC_Ar + NH₃ is at the origin of the initial activity loss of the highly active Fe/N/C catalyst used for the reduction of oxygen in PEM fuel cells. Energy Environ. Sci. 2018, 11: 365-382.

[9] Zhang, Gaixia, et al. Is iron involved in the lack of stability of Fe/N/C electrocatalysts used to reduce oxygen at the cathode of PEM fuel cells? Nano Energy, 2016, 29: 111-125.

[10] Ehelebe, K. et al. Platinum dissolution in realistic fuel cell catalyst layers. Angew. Chem. Int. Ed. 2021, 60: 8882-8888.

[11] Ku, Y.-P. et al. Oxygen reduction reaction causes iron leaching from Fe-N-C electrocatalysts. 2021 Submitted, DOI: 10.21203/rs.3.rs-1171081/v1.