Tailoring the Microenvironment in Fe-N-C Electrocatalysts for Optimal Oxygen Reduction Reaction Performance

Fe-N-C electrocatalysts, comprising FeN4 single atom sites immobilized on N-doped carbon supports, offer excellent activity in the oxygen reduction reaction (ORR), especially in alkaline solution. Herein, we report a simple synthetic strategy for improving the accessibility of FeN4 sites during ORR and simultaneously fine-tuning the microenvironment of FeN4 sites, thus enhancing the ORR activity. Our approach involved a simple one-step pyrolysis of a Fe-containing zeolitic imidazolate framework in the presence of NaCl, yielding a hierarchically porous Fe-N-C electrocatalyst containing tailored FeN4 sites with slightly elongated Fe-N bond distances and reduced Fe charge. The porous carbon structure improved mass transport during ORR, whilst the microenvironment optimized FeN4 sites benefitted the adsorption/des-orption of ORR intermediates. Accordingly, the developed electrocatalyst, possessing a high FeN4 site density (9.9 x 10(19) sites g(-1)) and turnover frequency (2.26 s(-1)), delivered remarkable ORR performance with a low overpotential (a half-wave potential of 0.90 V vs. reversible hydrogen electrode) in 0.1 mol L-1 KOH. (C) 2022 Science China Press. Published by Elsevier B.V. and Science China Press. All rights reserved.