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 dz2 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 ViR-free under H2-air conditions, surpassing all reported M-N-C catalysts.
Yin et al. (Wed,) studied this question.