Short nanotubular Fe-N-C catalysts with curved catalytic sites and contributing regions for oxygen reduction reaction

The substantial advances of non-precious Fe-N-C materials with both high activity and stability to replace platinum-based catalysts for oxygen reduction reaction (ORR) in fuel cells remain a great challenge, since their intrinsic active site design and contributing microstructure exploration are still unclear. Herein, we propose a solid-phase thermal migration strategy to synthesize Fe, N co-doped nanocarbons with a short nanotubular structure, using ferrocene as Fe source and polyaniline (PANI) as N-doped carbon nanotubes (NCNTs) source. In acidic and alkaline environments, the thermally activated Fe-N-C exhibits efficient ORR performance, and half-wave potential is 30 mV higher than a commercial Pt/C (JM, 20 wt% Pt) in alkaline medium and only 100 mV less than the Pt/C in acidic media. Impressively, the catalyst used in zinc-air battery exhibits an outstanding power density of 144.74 mW·cm−2, higher than the one assembled by the Pt/C (125.67 mW·cm−2). Combining experimental and density functional theory (DFT) calculation results, the superior ORR activity should be attributed to the formation of the efficient off-plane Fe-pyridinic-N4 species at end of the nanotubes. More important, these zigzag-type Fe-pyridinic-N4 sites at the end regions serve as the main active sites, leading to a higher ORR activity. This work opens a door to clarify the active catalytic site types and the contributing microregions of the Fe-N-C catalysts, providing ideas for designing non-noble metal catalysts with curved surfaces and rich edge structures.