Challenges and Opportunities in Preparing Fe-N-C Layers Supported on Non-Carbon Supports

Global energy demand will be continuously rising in the foreseeable future. To mitigate the results of climate change it is necessary to reduce the emittance of greenhouse gases. One promising approach is to switch to hydrogen as energy carrier and use proton exchange membrane fuel cells (PEMFCs) to efficiently convert H2 into electric power. Here, hydrogen and oxygen/air are used to directly generate electrical energy with only water as byproduct. Both reactions need to happen efficiently at the same time. The oxygen reduction reaction (ORR), however, is a very sluggish four-electron-reaction and needs efficient catalysts. The state-of-the-art ORR catalyst in PEMFC is platinum but that is a very scarce and expensive metal. Hence, it is important to switch to precious-metal-group (PGM) free catalysts. In recent years, bio inspired iron-based FeNC catalysts have been shown to approach ORR activities that are competitive with those of platinum, even though they need a higher catalyst loading than Pt to reach the desired high current densities.1–3 Those catalysts feature single Fe atoms surrounded by four N atoms in graphitic carbon. The drawback of those materials is their lower stability compared to PGM materials. In a nutshell, the carbon matrix oxidizes during operation in PEMFCs, changing the morphology of the catalytic center and rendering it less active, or inactive, depending on the carbon oxidation extent. Direct demetallation phenomena are also possible, leaving a N4 cavity without metal cation center. It was shown recently by Wu et al.4 that it is possible to raise durability by adding an additional carbon layer on the catalyst. One alternative approach to reduce those degradations is to interface Fe-N4 sites with a carbon-free support. The targeted support material must be immune to corrosion in acid and withstand the typical ORR electrochemical potentials. It also has to be electrically conductive to facilitate electron transfers, both locally and on long scale. In this work, conductive ceramic metal oxide materials (Ta doped SnO2, TTO) was investigated as a possible alternative catalyst supports. This material is resistant to acid and can withstand prolonged potentials at operating conditions in PEMFCs.5 During the synthesis of the FeNC catalysts, high temperatures are generally applied. Such treatments can be detrimental to the conductivity of ceramics due to metal aggregation (in case of doped materials) and may also lead to morphological changes as well as the reduction of the ceramics to metallic materials. A robust method to synthesize FeNC catalysts is by mixing the separate Fe, N and C precursors and using optimized thermal treatment. In this study, the effect of the pyrolysis conditions as well as the ratio of FeNC precursors to TTO was investigated with various characterization methods, e.g. X-ray absorption spectroscopy, Mossbauer spectroscopy, and high resolution TEM. The characterization techniques confirm the presence of single atom Fe as well as different Sn species that might contribute to the activity. The resulting materials were electrochemically characterized with rotating disc electrode techniques and in PEM fuel cells for their ORR-activity and durability. It was shown that the activity of materials with a high FeNC precursor content relative to TTO can outperform a reference FeNC synthesized similarly but without TTO, in both RDE (Figure) and PEMFC. The presentation will report in detail the effects of pyrolysis conditions and FeNC precursor/TTO content on the structure, activity and stability of materials. (1) Mehmood, A. et al. High Loading of Single Atomic Iron Sites in Fe–NC Oxygen Reduction Catalysts for Proton Exchange Membrane Fuel Cells. Nat. Catal. 2022, 5 (4), 311–323. https://doi.org/10.1038/s41929-022-00772-9. (2) Gasteiger, H. A.; Marković, N. M. Just a Dream—or Future Reality? Science 2009, 324 (5923), 48–49. https://doi.org/10.1126/science.1172083. (3) Zion, N. et al.. Electrocatalysis of Oxygen Reduction Reaction in a Polymer Electrolyte Fuel Cell with a Covalent Framework of Iron Phthalocyanine Aerogel. ACS Appl. Energy Mater. 2022, 5 (7), 7997–8003. https://doi.org/10.1021/acsaem.2c00375. (4) Liu, S.et al. Atomically Dispersed Iron Sites with a Nitrogen–Carbon Coating as Highly Active and Durable Oxygen Reduction Catalysts for Fuel Cells. Nat. Energy 2022, 7 (7), 652–663. https://doi.org/10.1038/s41560-022-01062-1. (5) Jiménez-Morales, et al. Strong Interaction between Platinum Nanoparticles and Tantalum-Doped Tin Oxide Nanofibers and Its Activation and Stabilization Effects for Oxygen Reduction Reaction. ACS Catal. 2020, 10 (18), 10399–10411. https://doi.org/10.1021/acscatal.0c02220. Figure 1