# O2‐Accessible Fe–N4 Active Site Density Boosts Efficient Oxygen Reduction to Fuel‐Cell Level

## TL;DR

This study shows that yolk-shell nanostructures improve oxygen access to active sites in catalysts, boosting performance in fuel cells.

## Contribution

The study introduces a pH-dependent strategy to design Fe–N4 catalysts with optimized O2 accessibility through hierarchical nanostructures.

## Key findings

- Yolk-shell Fe–NC structures achieve higher O2-accessible active site density compared to solid and hollow structures.
- The yolk-shell design enables a diffusion current exceeding laminar flow theory due to local recirculation effects.
- The optimized catalyst delivers a power density of 1.03 W cm−2 in fuel cells, outperforming existing catalysts.

## Abstract

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 jd
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 jd
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.

This study investigates the role of hierarchical nanostructure design in optimizing oxygen mass transport for the oxygen reduction. Using atomically dispersed Fe–N–C catalysts with solid, yolk‐shell, and hollow structures, it demonstrates that the yolk‐shell architecture provides superior O2 accessibility to active Fe–N4 sites. This results in an enhanced diffusion current and exceptional catalytic performance in both acidic electrolyte and a practical fuel cell, highlighting the critical link between nanostructure and reactivity.