Structure and Activity-Durability Tradeoff of Coated Fe-N-C Catalysts for the Oxygen Reduction Reaction

Abstract

Fe-based catalysts stand out for oxygen reduction reactions among the platinum group metal (PGM)-free catalysts for reducing the reliance on expensive and scarce platinum. With the endeavor of a PGM-free community, the state-of-the-art Fe-N-C catalysts have demonstrated encouraging initial activity in Proton Exchange Membrane Fuel Cell (PEMFC) 1-2. However, their long-term stability and durability remain a challenge, both in acidic liquid electrolyte cells and in single-cell PEMFCs3. Recently, a novel approach has demonstrated improved durability of Fe-N-C catalysts during ORR, which consisted of coating a pre-existing Fe-N-C with a thin carbon overlayer obtained via chemical vapor deposition of a metal-organic framework (ZIF-8), at 1100 °C4. However, the underlying mechanism for enhanced durability remains unclear. One of the keys to improved stability lies in the specific structure of the catalysts, including the coordination of the ORR-active iron atoms and the structure of the nitrogen-doped carbon matrix, especially the structure located at the top surface of catalysts. The CVD conditions that affect the structure of FeNC must be optimized, to achieve the optimal activity-durability trade-off.This presentation will report on the structure, activity, and durability trends of a suite of FeNC catalysts coated at different CVD conditions. The key parameters have been investigated, such as deposition temperature, overlayer thickness, and iron speciation on their ORR activity and durability. The coated Fe-N-C catalysts have been evaluated for their activity/durability (in oxygenated environments) both in liquid electrolyte cells and PEMFCs. The coated Fe-N-C demonstrated inferior initial activity and improved durability (Figure 1). The correlation between the structural information derived from multiple techniques (XRD, Mossbauer spectroscopy, TEM, Raman spectroscopy, N2 sorption, etc) and their electrochemical behaviors will be discussed.References Jiao, L.; Li, J.; Richard, L. L.; Sun, Q.; Stracensky, T.; Liu, E.; Sougrati, M. T.; Zhao, Z.; Yang, F.; Zhong, S.; Xu, H.; Mukerjee, S.; Huang, Y.; Cullen, D. A.; Park, J. H.; Ferrandon, M.; Myers, D. J.; Jaouen, F.; Jia, Q., Nature Materials 2021, 20 (10), 1385-1391.Mehmood, A.; Gong, M.; Jaouen, F.; Roy, A.; Zitolo, A.; Khan, A.; Sougrati, M.-T.; Primbs, M.; Bonastre, A. M.; Fongalland, D.; Drazic, G.; Strasser, P.; Kucernak, A., Nature Catalysis 2022, 5 (4), 311-323.Martinez, U.; Komini Babu, S.; Holby, E. F.; Zelenay, P., Current Opinion in Electrochemistry 2018, 9, 224-232.Liu, S.; Li, C.; Zachman, M. J.; Zeng, Y.; Yu, H.; Li, B.; Wang, M.; Braaten, J.; Liu, J.; Meyer, H. M.; Lucero, M.; Kropf, A. J.; Alp, E. E.; Gong, Q.; Shi, Q.; Feng, Z.; Xu, H.; Wang, G.; Myers, D. J.; Xie, J.; Cullen, D. A.; Litster, S.; Wu, G., Nature Energy 2022, 7 (7), 652-663. Figure 1