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/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.