Investigating Oxygen Reduction Performance of Fe-N-C Catalysts for Proton Exchange Membrane Fuel Cells
Authors: Mehmood Asad , Fellinger Tim-Patrick 
Published: February 2024

Non-precious Fe-N-C catalysts have reached high activity levels for acidic oxygen reduction reaction (ORR) in rotating ring disk electrode (RRDE) setup approaching to that of the platinum group metals (PGM) when compared at high catalyst loadings of 0.6-1.0 mgFe-N-C cm-2. The improvements in the ORR activity of Fe-N-Cs can be attributed to new synthetic routes developed in recent years that have led to increased Fe-N4 active site density and utilization and/or better intrinsic activity/turnover frequency (TOF) of those sites.1-3 Despite impressive activities in half-cells, Fe-N-Cs still require quite high loadings in the range of 4.0-5.0 mg cm-2 in proton exchange membrane fuel cells (PEMFCs), which makes the Fe-N-C cathodes 4-5x thicker than Pt-based cathodes (~100 µm for Fe-N-C layer vs ~10-20 µm for Pt layer). Thick Fe-N-C cathodes suffer from quick performance drop in medium to high current density region due to mass-transport related losses. Reducing the Fe-N-C loading levels in RRDE and in PEMFC without comprising the ORR activity would need further improvements in active site density, site utilization and TOF. In this talk, I will present our work on how morphological tailoring of carbon framework in Fe-N-Cs affects the utilization of active sites and water adsorption-desorption behaviour with the aim of improving the ORR performance and mass-transport properties. Active-site imprinted carbon frameworks are used to prepare Fe-N-Cs that consist of purely Fe-N4 sites without any other iron species.1,4,5 Besides activity, performance stability against extensive voltage cycling in RRDE and long-term durability in PEMFC single-cells is another important criterion that Fe-N-Cs need to fulfil for becoming a viable substitute to PGM catalysts. Very recently, encouraging results on the improvement of Fe-N-C durability in PEMFC tests have been reported using radical scavengers and by coating a thin carbon layer on the surface of Fe-N-C.6,7 However, significantly more efforts are required in that direction for reaching the durability targets of thousands of hours. Second part of the talk will discuss the performance stability of Fe-N-Cs.

References

1. Mehmood, A. et al. High loading of single atomic iron sites in Fe-NC oxygen reduction catalysts for proton exchange membrane fuel cells. Nature Catalysis
5, 311-323 (2022).

2. Menga, D. et al. Resolving the Dilemma of Fe-N-C Catalysts by the Selective Synthesis of Tetrapyrrolic Active Sites via an Imprinting Strategy. J Am Chem Soc
143, 18010-18019 (2021).

3. Jiao, L. et al. Chemical vapour deposition of Fe-N-C oxygen reduction catalysts with full utilization of dense Fe-N(4) sites. Nat Mater
20, 1385-1391 (2021).

4. Menga, D. et al. Active-Site Imprinting: Preparation of Fe-N-C Catalysts from Zinc Ion-Templated Ionothermal Nitrogen-Doped Carbons. Advanced Energy Materials
9, 1902412 (2019).

5. Mehmood, A. et al. Facile metal coordination of active site imprinted nitrogen doped carbons for the conservative preparation of non‐noble metal oxygen reduction electrocatalysts. Advanced Energy Materials
8, 1701771 (2018).

6. Xie, H. et al. Ta–TiO x nanoparticles as radical scavengers to improve the durability of Fe–N–C oxygen reduction catalysts. Nature Energy
7, 281-289 (2022).

7. Liu, S. et al. Atomically dispersed iron sites with a nitrogen–carbon coating as highly active and durable oxygen reduction catalysts for fuel cells. Nature Energy
7, 652-663 (2022).