Heteroatom-doped carbon materials, particularly carbons co-doped with single atomic transition metals (e.g. Fe) and nitrogen, have emerged as one of the most competitive non-precious electrocatalysts for the cathodic oxygen reduction reaction (ORR) in proton-exchange-membrane fuel cells to replace precious platinum (Pt) catalyst. Most carbon-based non-precious electrocatalysts, however, were built on conventional carbon materials such as activated carbon and more recently metal-organic-framework-derived carbon, both of which feature essential drawbacks of carbon corrosion and low electron conductivity due to their low extent of graphitization.^1-2

We report here the direct synthesis of well-defined CNTs co-doped with a high density of Fe-N_x active sites (denoted as Fe-N/CNT) through a solid-phase catalytic growth approach, where Fe precursor catalyzed the growth of CNTs from melamine sponge precursor under NH₃ atmosphere and simultaneously doped in-situ into the CNT walls with nitrogen. The atomic structure and electrocatalytic properties of the Fe-N/CNT electrocatalysts were characterized by aberration-corrected scanning transmission electron microcopy, Mößbauer spectroscopy, electrochemical analysis and density functional theory calculations. We further design and synthesis a hierarchical Fe-N/CNT electrocatalyst where primary Fe-N/CNTs are enwrapped with secondary Fe-N/CNTs in order to increase the proportion of the Fe-N/CNT active site. The optimized Fe-N/CNT catalyst featured with a high degree of graphitization and high density of Fe-N_x catalytic sites, demonstrating both high ORR activity and remarkable stability in both half-cell test and a single hydrogen fuel cell membrane-electrode-assembly test.

Reference:

[1] Lefevre M, Proietti E., Jaouen F., Dodelet J. Iron-based catalysts with improved oxygen reduction activity in polymer electrolyte fuel cells. Science, 324, 71-74 (2009).

[2] Chen, YJ. Ji SF., Wang D.S., Li Y.D., Angew Chem Int Ed, 56, 6937-6941 (2017)

[3] Xia D.S., Wang R.Z., Wei Y.P., Gan L., Kang F.Y. Melamine-sponge-derived non-precious fuel cell electrocatalyst with hierarchical pores and tunable nitrogen chemical states for exceptional oxygen reduction reaction activity. Materials Today Energy, 9, 271-278 (2018)