Inter-site structural heterogeneity induction of single atom Fe catalysts for robust oxygen reduction

Metal-nitrogen-carbon catalysts with hierarchically dispersed porosity are deemed as efficient geometry for oxygen reduction reaction (ORR). However, catalytic performance determined by individual and interacting sites originating from structural heterogeneity is particularly elusive and yet remains to be understood. Here, an efficient hierarchically porous Fe single atom catalyst (Fe SAs-HP) is prepared with Fe atoms densely resided at micropores and mesopores. Fe SAs-HP exhibits robust ORR performance with half-wave potential of 0.94 V and turnover frequency of 5.99 e^-1s^-1site^-1 at 0.80 V. Theoretical simulations unravel a structural heterogeneity induced optimization, where mesoporous Fe-N₄ acts as real active centers as a result of long-range electron regulation by adjacent microporous sites, facilitating O₂ activation and desorption of key intermediate *OH. Multilevel operando characterization results identify active Fe sites undergo a dynamic evolution from basic Fe-N₄ to active Fe-N₃ under working conditions. Our findings reveal the structural origin of enhanced intrinsic activity for hierarchically porous Fe-N₄ sites.