O₂-Accessible Fe-N₄ 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.