Improving transition metal–nitrogen–carbon
(M-N-C)
as a noble-metal-free catalyst for the oxygen reduction reaction (ORR)
is critical to achieve low-cost electrochemical energy conversion.
Herein, an in situ S doping strategy of enhancing Fe-N-C activity
for ORR was developed by newly designed Fe(II) ion coordinated S-containing
bis(imino)-pyridine-based polymers as precursors, which were synthesized
through copolymerizing three monomers of 2, 6-diacetylpyridine (DAP),
triamterene (TIT), and 2,5-dithiobiurea (DTB) as both N and S sources.
All samples derived from various molar ratios of the three monomers
possess a self-supporting structure of nanosheets. Additionally, incorporating
DTB into the copolymer can not only strongly affect the derived coordinative
species of N dopants to Fe atom but also effectively induce the synergistic
effect between S dopants and FeNx moieties, resulting a significant
improvement for ORR. The S-doped Fe-N-C nansheets with Fe coordinated
by 4 pyrrolic N dopants exhibit the highest ORR activity and stability
in alkaline media with a higher power output of Zn-air battery than
that of the same loading of Pt/C. Theoretical calculation identifies
that the thiophenic S dopant adjacent to Fe-pyrrolic N moiety can
decrease the d band center of Fe atom, greatly weakening the energy
profiles of oxygenated intermediates and thus enhancing ORR. In addition,
because of the designability of transition metal coordinated S-containing
bis(imino)-pyridine based polymers in the work, therefore, it is believable
that this strategy would open a wide space to explore the structural
relationship between precursors and MNx active sites with S dopants
for the purpose of achieving highly efficient and robust M-N-C catalysts
for energy-related electrocatalysis.