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 3d_z² and O 2p_z 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.

Developing cathode catalyst layers (CCL) with efficient mass transport capability is crucial to developing ultra-low Pt loading (< 50 μg·cm⁻²) proton exchange membrane fuel cells (PEMFCs). Herein, CCLs with various pore distributions were constructed by depositing Pt onto the integrated carbonaceous films consisting of carbon nanoparticles (CNs), three-dimensional (3D) graphene nanosheets (GNs), and nanocomposites of CNs and GNs (CNs-GNs), respectively. The hierarchical mesoporous pore distributions of CCLs strongly affect the effective exposure of Pt active sites, proton-transfer resistance, and oxygen mass transport efficiencies related to Knudsen diffusion and local resistance at the Pt/ionomer interface. The CCL with Pt/CNs-GNs (50.0 μg_Pt·cm⁻²) features a unique tri-modal pore distribution concentrated at 10.2, 20.4, and 43.7 nm, providing efficient three-phase boundaries with a significantly higher active surface area of 49.67 m²·g⁻¹, lower oxygen transport resistance and proton resistance of down to 18.68 s·m⁻¹ and 0.0603 Ω·cm², compared with Pt/CNs (31.48 m²·g⁻¹, 41.17 s·m⁻¹, and 0.0702 Ω·cm²) with a single-modal pore distribution at 9.5 nm and Pt/GNs (38.21 m²·g⁻¹, 33.40 s·m⁻¹, and 0.0654 Ω·cm²) with a bi-modal pore distribution at 9.8 and 20.9 nm. Correspondingly, the cell with Pt/CNs-GNs delivers a high power output of up to 1.01 W·cm⁻² and presents a high durability that satisfies the 2025 targets set by the U.S. Department of Energy. This work provides new insights into the critical role of hierarchically mesoporous pore distribution of CCL for constructing high-performance PEMFCs with ultra-low Pt loading < 50 μg·cm⁻².