Small-Angle X-ray Scattering Monitoring of Porosity Evolution in Iron–Nitrogen–Carbon Electrocatalysts

This study highlights the use of anomalous and small-angle X-ray scattering (ASAXS/SAXS) to monitor the evolution of micro- and mesoporosity during the synthesis of iron-nitrogen-carbon (Fe-N-C) electrocatalysts. A structural model-free SAXS approach enabled the determination of 13 structural parameters across five Fe-N-C electrocatalysts, compared to a commercial PMF-12704, Pajarito Powder. SAXS revealed hierarchical pore formation and structural changes spanning micro- to macroscales. Key features, such as pore curvature, porosity, and disorder, increased with synthesis modifications, correlating with enhanced oxygen reduction activity and reduced hydrogen peroxide yield. Notably, an average graphene layer curvature (<i>l</i>_R-Ruland length) above 3 nm was critical for forming curved pore walls, which improves selectivity and favors the 4-electron oxygen reduction pathway to water. These findings highlight the pore curvature and hierarchical pore architecture as crucial design parameters, guiding the development of next-generation electrocatalysts with enhanced efficiency and selectivity for sustainable energy applications. Specifically, optimizing the mesopore size distribution, with an increased proportion of mesopores within the range from 6 to 27 nm, and improving the transition from micropores to mesopores and macropores are essential strategies.