Spontaneous ORR Activity Drop of Fe-N-C Catalysts upon Atmospheric Storage

Iron-Nitrogen-Carbon (Fe-N-C) catalysts are promising materials to replace the scarce and expensive platinum-group materials in proton exchange membrane fuel cells (PEMFC) cathodes. Fe-N-C catalysts can be prepared by multiple ways [1-5], and each synthesis method can be fine-tuned to produce single Fe atoms coordinated to nitrogen atoms (FeN x ), identified as the most active sites toward the oxygen reduction reaction (ORR). However, despite these notable efforts, a rapid drop in ORR activity is noticed within the first hours of operation in thin-film rotating disk electrode (RDE) (liquid electrolyte) or in PEMFC systems (solid electrolyte) [6]. Unravelling the degradation mechanisms at stake and developing strategies to improve the durability of Fe-N-C catalysts has thus become an important objective for researchers worldwide [7]. In this work, we provide evidences for spontaneous ageing of Fe-N-C catalysts synthesized using different synthetic approaches, and thus featuring distinct chemical, textural and structural properties. For all materials, a pronounced drop of ORR activity is measured, indicating that the “spontaneous ageing” phenomenon is universal. The drop of ORR mass activity over time may reach as high as 90 % of the initial value, and relates to a drop of the active site density and/or the turnover frequency. These adverse effects can be mitigated using storage under dry and oxygen-free atmosphere. References [1] H. Yang, L. Shang, Q. Zhang, R. Shi, G.I.N. Waterhouse, L. Gu, T. Zhang, A universal ligand mediated method for large scale synthesis of transition metal single atom catalysts . Nat. Commun., 10 (2019) 4585. [2] S. Pylypenko, S. Mukherjee, T.S. Olson, P. Atanassov, Non-platinum oxygen reduction electrocatalysts based on pyrolyzed transition metal macrocycles . Electrochim. Acta, 53 (2008) 7875-7883. [3] Y. Wang, M.J. Larsen, S. Rojas, M.-T. Sougrati, F. Jaouen, P. Ferrer, D. Gianolio, S. Berthon-Fabry, Influence of the synthesis parameters on the proton exchange membrane fuel cells performance of Fe–N–C aerogel catalysts . J. Power Sources, 514 (2021) 230561. [4] Z. Jiang, J. Yu, T. Huang, M. Sun, Recent advance on polyaniline or polypyrrole-derived electrocatalysts for oxygen reduction reaction . Polymers, 10 (2018). [5] V. Armel, J. Hannauer, F. Jaouen, Effect of ZIF-8 crystal size on the O 2 electro-reduction performance of pyrolyzed Fe–N–C catalysts . Catalysts, 5 (2015) 1333-1351. [6] K. Kumar, L. Dubau, M. Mermoux, J. Li, A. Zitolo, J. Nelayah, F. Jaouen, F. Maillard, On the influence of oxygen on the degradation of Fe-N-C catalysts . Angew. Chem. Int. Ed., 59 (2020) 3235-3243. [7] C.H. Choi, H.-K. Lim, M.W. Chung, G. Chon, N. Ranjbar Sahraie, A. Altin, M.-T. Sougrati, L. Stievano, H.S. Oh, E.S. Park, F. Luo, P. Strasser, G. Dražić, K.J.J. Mayrhofer, H. Kim, F. Jaouen, The Achilles' heel of iron-based catalysts during oxygen reduction in an acidic medium . Energy Environ. Sci., 11 (2018) 3176-3182.