Physical and chemical dual-induced growth optimizing the size of porous Fe-N-C catalysts to achieve high accessibility of active sites

The inherently low active site accessibility of single-atom Fe-N-C catalysts severely compromises their catalytic activity relative to Pt-based catalysts, thereby hindering their practical implementation in critical applications. This work systematically investigates the synergistic effects of physical confinement and chemical induced growth strategy on optimizing active site accessibility of Fe-N-C catalysts for oxygen reduction reaction. Through a physical confinement strategy, the catalyst particle size is successfully reduced from the micrometer scale to the nanometer scale. Subsequently, by employing chemical-induced growth, the particle size is precisely controlled within the range of 623 nm to 421 nm, simultaneously establishing an optimal balance between the catalyst particle size threshold (420 nm) and the ordered macroporous framework. This approach maximized the accessibility of active sites and significantly enhanced the catalytic activity of the Fe-N-C catalyst. The optimized catalyst (OM-Fe-NC-4) exhibits outstanding performance, achieving a remarkable half-wave potential of 0.888 V, surpassing both commercial Pt/C and most state-of-the-art Fe-N-C catalysts. Additionally, it demonstrates excellent four-electron selectivity and delivers a kinetic current density of 14.7 mA cm⁻² at 0.85 V in acidic medium. This dual-dimensional modulation strategy offers a paradigm for enhancing the accessibility of active sites for non-precious metal catalysts.