Fe, N-codoped carbon derived from different ligands for oxygen reduction reaction in air-cathode microbial fuel cells: Performance comparison and the associated mechanism

• Different structures of Fe, N-codoped carbon with iron ligands are synthesized. • The effects of iron ligands on ORR and power out are explored. • Different amounts of Fe 2+ and Fe 3+ could facilitate electron transfer. • The maximum power density of Phen-Fe MFC is 2041 ± 23 mW m −2 . In order to clarify the effect of iron ligands on properties, catalysts with different structure through four kinds of ligands are prepared and the oxygen reduction reaction (ORR) activity and the maximum power density in microbial fuel cells (MFCs) are compared. The formed Fe single atoms, the metal-organic framework compound, the Prussia blue crystal and ethylenediamine coordination compound (Phen-Fe, PA-Fe, PB-Fe and EDA-Fe) exhibit the maximum power densities of 2041 ± 23 mW m −2 , 1656 ± 32 mW m −2 , 1062 ± 31 mW m −2 , and 1865 ± 36 mW m −2 , respectively. In addition, the electrochemical performance of four catalysts with different structures is significantly different, and the higher exchange current density and 4-electron pathway of Phen-Fe explained the kinetics of ORR. The structures and surface properties are explored through a series of characterizations. The different amounts of nitrogen functionalities, Fe ion, and high graphitization degree are thought to facilitate electron transfer, lower R ct and improve ORR activity. In sum, the iron ligands have a great influence on the structure and performance of catalysts, and the single-atom structure and ethylenediamine coordination structure are proved to be promising structures for ORR in MFCs.