Regulating FeNC Cathode Interfacial Structure via Pre-Crosslinked Carbon-Ionomer Networks for Active-Site Accessibility Optimization

Perfluorosulfonic-acid (PFSA) ionomers are indispensable binders in FeNC cathodes for proton transport, yet their mobility during ink drying often causes non-uniform redistribution, generating locally thick ionomer layers that partially block Fe-N x sites. Here we strategically regulated the FeNC catalyst/ionomer interface by incorporating a pre-crosslinked carbon-ionomer framework, in which the ionomer is anchored to conductive carbon prior to ink formulation. This design converted the ionomer from a freely migrating phase into a pre-associated phase, thereby stabilizing its interfacial configuration during drying and mitigating the aggregation of catalyst particles. Microstructural analyses revealed improved dispersion of catalyst aggregates and a thinner, more uniform ionomer layer in the resulting composite cathode. Electrochemical analyses showed an increased electrochemically accessible interface and higher fractions of accessible Fe-N x sites, accompanied by reduced charge transfer resistance and lowered pressure-independent oxygen transport resistance, indicating primary alleviation of activation polarization. Consistent with these mechanistic gains, the optimized cathode delivered a nearly 1.39-fold increase in peak power density compared with a conventional cathode under identical cell conditions. These findings identified the regulation of the ionomer interfacial state as a key to improving site utilization in PGM-free cathodes and provided a generalizable strategy for architecting catalyst layers.