Iron coordinated by nitrogen doped carbon with dual-iron atom sites derived from binuclear ligand polymer for efficient oxygen reduction reaction

Breaking the linear scaling relationship of oxygen reduction reaction (ORR) on transition metal coordinated by nitrogen doped carbon (MNC) with single-atom sites is crucial to achieving noble-metal-free fuel cells and metal-air batteries. Herein, FeNC with dual-Fe atom sites (DA-FeNC) was precisely developed by pyrolyzing a newly designed Fe-ion coordination polymer with binuclear ligand precursor, which was copolymerized with meso-Tetra(4-carboxyphenyl)porphine (TCPP) and 2,6-Diaminopyridine (DAP). In the precursor, the porphine in TCPP serves as primary coordination site for Fe ions, and the amide-pyridine ligands generated through the condensation reaction between TCPP and DAP provide secondary Fe ion-coordination site. The resulting DA-FeNC not only delivers high ORR performance with half-wave potentials of 0.82 V in 0.5 M H2SO4 and 0.93 V in 0.1 M KOH, and minimal potential losses of 26 mV and 10 mV after 50,000 cycles, respectively, but also achieves a high peak power density of 724 mW cm-2 in proton exchange membrane fuel cells and 226 mW cm-2 in Zn-air batteries as cathode. Meanwhile, theoretical analyses further indicate that there is a specific electronic coupling effect between adjacent dual-Fe sites in FeNC with more Fe 3d orbital occupancy than that of single-Fe site in FeNC, which increases the population of σ* antibonding states between Fe 3dz2 and O 2pz orbitals, reducing the free energy gap of OOH*-OH* to 2.78 eV, effectively breaking the conventional linear scaling limitation of single-Fe atom sites. This work offers an effective strategy for precisely constructing MNCs with dual metal atoms for efficient electrocatalysis of ORR.