Macroporous hollow nanocarbon shell-supported Fe-N catalysts for oxygen reduction reaction in microbial fuel cellss

Highlights: • Thin-walled macroporous carbon shells were used as carbon supports for ORR catalysts. • The carbon shells doped with FePc at 500 °C possess the highest Fe and N elements. • The FePc/CS500 exhibited a higher E_1/2 and I_limited than that of Pt/C in PBS. • The MFC with FePc/CS500 catalysts achieved a P_max of 2.16 ± 0.02 W m⁻². -- Abstract: The properties of carbon supports play a significant role in the performance of the oxygen reduction reaction (ORR) catalysts for single-chamber microbial fuel cells (MFCs). In this study, sodium citrate was directly carbonized to prepare macroporous nanocarbon shells (CSs) with a shell thickness of ∼8 nm, which were proposed as the carbon support for the Fe-N catalyst (FePc/CS) for ORR. The experiments showed that the thin-walled macroporous nanostructure of CSs remained unchanged after pyrolysis with Fe (II)-phthalocyanine (FePc). Electrochemical tests performed in 50 mM phosphate buffer solution demonstrated that the catalyst pyrolyzed at 500 °C (FePc/CS500) exhibited a 42 mV higher half-wave potential than that of commercial 20 wt% Pt/C catalyst under the same loading. This is due to the combine effects of the chemical functions (abundant Fe-N active sites) and the structural advantages (macropores and thin-walls, which facilitated the mass transfer in CS and increased the contact interface between the electrolyte and active sites, respectively). FePc/CS500 also exhibited a better durability than Pt/C, stemming from the four-electron reaction pathway that prevented the active sites from being degraded by the generated H₂O₂. The performance evaluation showed that the MFC with FePc/CS500 as the cathode catalyst delivered a maximum power density of 2.16 ± 0.02 W m⁻², which was 50% higher than that of Pt/C (1.44 ± 0.04 W m⁻²). These results indicated FePc/CS500 was a cheap but effective alternative ORR catalyst to Pt/C for energy recovery from wastewater.