Abstract
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/desorption of ORR intermediates. Accordingly, the developed electrocatalyst, possessing a high FeN4 site density (9.9 × 1019 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.
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