Fe/N-Doped Carbon with Nearly Ordered Mesopores and Tunable Particle Sizes as Model Catalysts for Quantitative Evaluation of Electrocatalytic Active Sites

Synthesizing MOF-derived carbons with not only tunable and uniform particle sizes but also ordered tubular mesoporous structures remains challenging. Moreover, the lack of precise morphological control makes it hard to clarify the relationships between structure and catalytic activity, limiting the rational design of MOF-derived electrocatalysts with breakthrough performance. This study successfully prepares ordered mesoporous Fe/MOF-545-x rod-shaped precursors with tunable lengths via a one-step modulation approach. After direct carbonization, Fe/MNC-x retains the rod-shape, mesoporous structures, and well-dispersed Fe-N_x active sites. Considering that the axis length is the most significant variable for the Fe/MNC-x series, they are promising model electrocatalysts to reveal the relationship between the particle size and utilization efficiency of electrocatalytic active sites in ORR. The electrochemical result shows that Fe/MNC-250 nm, which has the shortest length, displays a superior activity (E_1/2 = 0.917 V in alkaline and E_1/2 = 0.814 V in acidic electrolytes), due to the higher electrochemical surface area, lower charge transfer resistance, and a higher efficient active site density (1.27 ± 0.26 × 10¹⁹ sites g⁻¹) estimated by in situ nitrite stripping technique. The fuel cell assembled using Fe/MNC-250 nm possesses an excellent power density of 521.16 mW cm⁻². This work provides a simple strategy for regulating the particle sizes of ordered mesoporous MOF precursors and the derived carbon-based electrocatalysts for high-performance PEMFCs.