Yolk-shell structured ultra-small Fe3C nanoparticles encapsulated in S, N-codoped porous carbon as effective catalysts for the oxygen reduction reaction in microbial fuel cells

Oxygen reduction reaction (ORR) at the cathode of Microbial Fuel Cells (MFCs) is the limiting step due to its slow kinetics, making the development of efficient, stable, and cost-effective catalysts essential. This study focuses on a yolk-shell structured ultra-small Fe 3 C nanoparticles encapsulated in a sulfur, nitrogen-codoped porous carbon matrix. The strong complexation of citric acid (CA) with iron ions results in a smaller particle size of Fe 3 C nanoparticles, and its pore forming ability favors the effective assemble of a unique hollow yolk-shell structure. Due to the highly dispersed and ultra-small (4.99 ± 0.23 nm) Fe 3 C nanoparticles and the presence of abundant Fe-N active sites, Fe-SNC-2CA demonstrates superior ORR activity compared to commercial Pt/C, with half-wave potentials of 0.88 V and 0.68 V in alkaline and neutral solutions, respectively. Its unique yolk-shell structure ensures excellent stability by preventing the aggregation and dissolution of internal Fe 3 C nanoparticles. The Fe-SNC-2CA equipped MFC achieves a power density of 1614 ± 60 mW m −2 , maintaining output stability with a 5.3 % decay over 430 h, and finally contributes a 75.6 % chemical oxygen demand removal. This study highlights a straightforward yolk shell synthesis method and the beneficial role of ultra-small Fe 3 C in fuel cell application.