Pore structure engineering via hard-template synthesis: unlocking the high oxygen reduction reaction activity and stability of Fe-N@C electrocatalysts

Abstract

Developing efficient and durable iron-nitrogen-carbon (Fe-N@C) electrocatalysts with optimal pore architecture is crucial for advancing the oxygen reduction reaction (ORR) in fuel cells. In this study, we demonstrate how hard-templating with tailored silica scaffolds (SBA-15, KIT-6, and a dual SBA-15/KIT-6 template) can tune the pore structure of Fe-N@C materials. In these materials, the pore structure influences the formation and accessibility of active sites for the ORR. The mesoporous Fe-N@CMK-3 electrocatalyst, derived from SBA-15, exhibits the highest ORR activity (onset potential: 0.99 VRHE in alkaline media, and 0.82 VRHE in acid) due to its well-defined 2D hexagonal pores, which facilitate efficient oxygen diffusion. In contrast, the microporous Fe-N@CMK-8 (KIT-6-derived) exhibits lower ORR activity due to limited oxygen accessibility to the active sites. The dual-templated Fe-N@CMK-3/8 combines micro/mesoporosity to deliver balanced performance despite its lower surface area and pore volume resulting from the pore connectivity. All electrocatalysts initially follow a quasi-4e− ORR pathway, but their behavior changes during the long-term testing: Fe-N@CMK-8 shifts to the 2e− pathway despite its notably durable activity in acidic media; Fe-N@CMK-3 exhibits the best stability in terms of activity under alkaline conditions also with a slight shift to the 2e− pathway; Fe-N@CMK-3/8 excels in terms of selectivity sustaining a 4e− pathway along time with medium stability in the activity in both acid and alkaline media. These findings establish pore engineering as a powerful tool to tailor Fe-N@C electrocatalysts for specific operational environments, contributing to the development of high-performance non-precious metal catalysts for the ORR in proton exchange membrane and alkaline fuel cell applications.