O ₂ ‐Accessible Fe–N ₄ Active Site Density Boosts Efficient Oxygen Reduction to Fuel‐Cell Level

Not all sites with intrinsic activity show efficacy in practical catalysis due to inaccessibility or diffusion limitation, necessitating rational design of well-connected hierarchical nanostructures to guarantee accessibility. Herein, the case is thoroughly investigated by way of atomically dispersed Fe-NC catalysts for the dominant O₂ gas-consuming reduction (ORR). A pH-dependent nanostructure manipulation strategy was developed to form solid, yolk-shell, and hollow Fe-NC structures with similar overall density of quasi-homogeneous Fe-N₄ sites, providing a comparative platform to investigate O₂ mass transport during ORR. Despite similar Fe loading, y-Fe/NC structures achieve optimized O₂-accessible active site density (ASD) due to fine-tuned porosity and connectivity for sufficient O₂ accessibility. This observation is re-affirmed by the observation of a relatively high j_d for the y-Fe/NC, which exceeds the theoretical value of a laminar flow pattern. This can be attributed to the increased O₂-accessible ASD, originated from the local recirculation effect induced by the unique structure. Consequently, the y-Fe/NC exhibits half-wave potential of 0.82 V and j_d of 7.66 mA cm^-2, outperforming counterparts and state-of-the-art catalysts. Moreover, the optimized y-Fe/NC remains effective in fuel cell with power density of 1.03 W cm^-2, demonstrating the essential roles of rationally designed nanostructures.