A facile iron-sulfur double-doping strategy to prepare high performance FeNx/S-NC electrocatalyst for oxygen reduction reaction in zinc-air battery

Herein, a facile iron-sulfur double-doping strategy is developed for the synthesis of highly active FeNx/S-NC catalyst by the pyrolysis of the iron-sulfur double-doped polypyrrole precursor, PPy-Fe3+-SCN-. The unique precursor is firstly prepared by coordinating SCN- with Fe3+ ions adsorbed in situ into the polypyrrole matrixes (PPy-Fe3+) during an oxidative polymerization process. Subsequent pyrolysis of PPy-Fe3+-SCN- precursor produced the FeNx/S-NC catalyst containing the active sites of Fe-N-x and thiophene-like sulfur structures. Furthermore, FeNx/S-NC has a larger specific area than FeNx-NC catalyst without SCN- in the precursor. Meanwhile, depending on the presence of the thiophene-like sulfur structure, FeNx/S-NC shows a higher valence band (-7.27 eV) than that of FeNx-NC (-7.76 eV), leading to the smaller energy gap with the LUMO level of O-2. FeNx/S-NC catalyst has an excellent initial potential of 1.02 V and a half-wave potential of 0.90 V, achieving one of the highest catalytic activities based on Fe-N-C reported so far, and ORR catalyst in an alkaline environment. For the stability test, the FeNx/S-NC catalyst still maintains this 91% current density after 20000 s, which is also better than Pt/C. Finally, FeNx/S-NC employed as the cathode material for zinc-air battery, shows greater maximum peak power and specific capacity than Pt/C.