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 (l_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.