Hierarchically Porous Fe–N–C Electrocatalysts with Accessible Fe-Nx Sites for High-Performance Oxygen Reduction Reaction in Zn-Air Batteries

Abstract Developing Fe–N–C electrocatalysts with high active site utilization remains a key challenge for the oxygen reduction reaction (ORR), primarily because of the limited accessibility of Fe-Nx sites. Herein, a hierarchically porous Fe–N–C (HP-Fe–N–C) catalyst is constructed via a ZnO-templated strategy combined with urea-assisted secondary pyrolysis, enabling the simultaneous regulation of the pore structure and Fe-Nx site density. The ZnO template governs the formation of mesoporous architectures, and concurrently, the pyrolysis of urea releases gaseous species that facilitate the enlargement and interconnection of mesoporous channels. Notably, the balanced micromesoporous structure derived from the ZnO template facilitates efficient mass transport and enhances site accessibility, while urea regulates the nitrogen species to promote the formation of dense Fe-Nx moieties. As a result, the HP-Fe–N–C catalyst delivers a high half-wave potential of 0.90 V, along with near four-electron selectivity and excellent durability. The Zn-air battery assembled with the HP-Fe–N–C cathode achieves an open-circuit voltage of 1.52 V, a specific capacity of 745.6 mAh g–1, and favorable charge–discharge stability exceeding 800 h. This work reveals the balance between hierarchical porosity and active site density in the Fe–N–C catalyst for the high-performance ORR.