Structural engineering of Fe single-atom oxygen reduction catalyst with high site density and improved mass transfer

Fe-N-C catalysts are widely considered as promising non-precious-metal candidates for electrocatalytic oxygen reduction reaction (ORR). Yet despite their high catalytic activity through rational modulation, challenges remain in their low site density and unsatisfactory mass transfer structure. Herein, we present a structural engineering approach employing a soft-template coating strategy to fabricate a hollow and hierarchically porous N-doped carbon framework anchored with atomically dispersed Fe sites (FeNC-h) as an efficient ORR catalyst. The combination of hierarchical porosity and high exterior surface area is proven crucial for exposing more active sites, which gives rise to a remarkable ORR performance with a half-wave potential of 0.902 V in 0.1 M KOH and 0.814 V in 0.1 M HClO 4 , significantly outperforming its counterpart with solid structure and dominance of micropores (FeNC-s). The mass transfer property is revealed by in-situ electrochemical impedance spectroscopy (EIS) measurement. The distribution of relaxation time (DRT) analysis is further introduced to deconvolve the kinetic and mass transport processes, which demonstrates an alleviated mass transport resistance for FeNC-h, validating the effectiveness of structural engineering. This work not only provides an effective structural engineering approach but also contributes to the comprehensive mass transfer evaluation on advanced electrocatalyst for energy conversion applications.