Engineering the Interfacial Microenvironment of Fe-N-C Cathode to Boost Proton Conduction for High-Performance Fuel Cells.

While atomically dispersed Fe-N-C catalysts represent a compelling nonplatinum alternative for the oxygen reduction reaction (ORR), their practical deployment in proton-exchange membrane fuel cells (PEMFCs) remains severely constrained by a poorly defined catalyst-ionomer-reactant triple-phase interface, which drastically impedes efficient proton transfer within the cathode catalyst layer. Herein, we report a rational interfacial microenvironment engineering strategy that exploits noncovalent interactions between oxygenated functional groups on carbon nanotubes (CNTs) and ionomer side chains to precisely modulate the catalyst-ionomer interfacial structure. The incorporation of oxygen-containing moieties effectively promotes water enrichment at the catalyst-ionomer interface, strengthens intermolecular interactions among water moieties, and facilitates the establishment of a hydrogen-bonded water network. This interconnected network significantly reduces proton transport resistance, boosting the proton conductivity of the Fe-N-C cathode layer by a factor of 2.3 relative to the conventional architecture. Coarse-grained molecular dynamics simulations substantiate that oxygen-functionalized CNTs facilitate long-range proton hopping, directly contributing to the measured conductivity enhancement. As a result, the optimized PEMFC delivers a remarkable peak power density of 1.63 W cm-2 and an exceptional current density of 53 mA cm-2 at 0.9ViR-free, surpassing the U.S. Department of Energy (DOE) 2025 target of 44 mA cm-2. This work establishes a generalized interfacial microenvironment modulation strategy to overcome proton transport limitations in nonplatinum ORR catalysts, opening a new avenue toward advanced fuel cell electrocatalysts.