Appropriate Acid Etching to Obtain Defect-Rich and Porous Zeolitic-Imidazolate-Framework-Derived Undercoordinated Fe-NC Catalysts Toward Boosted Oxygen Reduction Reaction

Iron-nitrogen-carbon (Fe-NC) catalysts, particularly those with Fe-N₄ coordination moieties, are the most promising alternatives to commercial Pt@C for oxygen reduction reaction (ORR) in green energy conversion. The acid etching strategy is an effective and simple strategy to break the symmetric coordination of Fe-N₄ on the carbon substrate to further enhance the activity. Herein, a superior Fe-NC catalyst with undercoordinated Fe-N₂ moieties was produced through a concentration-controlled acid etching strategy, following an underlying quantitative indicator (I_D/I_G) to regulate its defect degree accurately. Due to the defect-rich and porous carbon structure to accelerate the mass transfer, this Fe-N₂ catalyst exhibited an admirable half-wave potential (E_1/2) of 0.85 V_RHE versus 0.87 V_RHE for commercial Pt@C, and a better stability and a higher limiting current density (−6.3 mA cm⁻²) in alkaline conditions, outperforming the other involved Fe-NCs and the Pt@C. This work provides an acid etching strategy to accurately control the defect degree and break the symmetrical Fe-N₄ coordination structure of Fe-NCs for enhancing the ORR activity.