Molecular-confinement synthesis of sub-nano Fe/N/C catalysts with high oxygen reduction reaction activity and excellent durability for rechargeable Zn-Air batteries

Transition metal-nitrogen-carbon (M/N/C) catalysts, Fe/N/C in particular, with low cost, good tolerance to methanol and promising activity has great potential to replace Pt-based catalysts in oxygen reduction reaction (ORR). However, for metal atoms are easy to migrate and agglomerate during pyrolysis, it still remains a challenge to rationally design precursors to synthesize highly active M/N/C without significant agglomeration of metal sites. In this work, we propose a molecular-confinement strategy to synthesize sub-nano Fe/N/C catalysts by confinement Fe(CN)(6)](3-) in zeolitic imidazolate framework-8 (ZIF-8) crystals to minimize the agglomeration of Fe atoms during pyrolysis. The half-wave potential of the prepared sub-nano Fe/N/C is 40 mV superior to that of commercial 20% Pt/C in alkaline medium. Besides, it has electron transfer number close to 4.00, with below 0.5% H2O2 yield, excellent long-term stability with only -5 mV shifts after 10000 cycles, and superb methanol tolerance. When utilized in Zn-air batteries as the cathode, it exhibits better activity, stability, and higher specific capacity than that of Pt-RuO2/C, showing a good potential for air cathode.