Spin-Selective Water Rejection in Fe-N-C Catalysts via Graphitic Macro-Ligand Engineering

Overcoming the sluggish intrinsic kinetics and mass-transport resistances of Fe-N-C cathodes is critical to bridging the performance gap with platinum-based benchmarks in proton exchange membrane fuel cells (PEMFCs). While this gap is frequently attributed to suboptimal oxygen-binding energetics, we reveal that a key, underestimated cause is the failure to control the catalyst/water interfacial chemistry, resulting in active-site poisoning and pore flooding. Herein, we design a highly graphitized host as a macro-ligand to reconstruct the electronic structure of Fe-N₄ sites. This bottom-up engineering triggers a decisive low-spin to intermediate-spin (S = 0 → S = 1) transition, which promotes single-electron occupancy in the axial d_z² orbital. Acting as a spin gate, this electronic feature selectively suppresses water adsorption via Pauli repulsion and enhances O₂ activation. Meanwhile, operando magnetic field imaging suggests better water rejection behavior in the catalyst layer, as evidenced by a more homogeneous in-plane current distribution. Leveraging these synergistic effects, the developed catalyst delivers a record 1.02 W cm^-2 peak power density and 402 mA cm^-2 at 0.80 V_iR-free under H₂-air conditions, surpassing all reported M-N-C catalysts.