Construction of Intrinsically Microporous Ionomers for Medium-Temperature PEMFCs: From Molecular Design to Three-Phase Interface Performance Optimization

Abstract One of the primary limitations for proton exchange membrane fuel cells (PEMFCs) stems from the high mass transport resistance within catalyst layers, particularly when low Pt loadings are used. The conventionally used perfluorosulfonic acid (PFSA) ionomer contains a semicrystalline polytetrafluoroethylene backbone with inherently low oxygen permeability, significantly restricting oxygen transport at the local reaction sites of the cathode. Moreover, PFSA ionomers suffer significantly from proton conductivity decay under medium-temperature conditions due to plasticization and dehydration. To address these challenges, a novel intrinsically microporous sulfonated hydrocarbon ionomer (PIM-S) was synthesized in this study via superacid-catalyzed Friedel–Crafts polycondensation, followed by a para-fluoro-thiol click reaction. The disrupted matrix crystallinity and rigid amorphous backbone of PIM-S suppress ionomer swelling, giving rise to a high specific surface area of 64 m2 g–1 and a large free volume fraction of 0.44, which in turn significantly enhances oxygen diffusion capability. Density functional theory calculations show that the adsorption energy of PIM-S on the catalyst is 0.80 eV, markedly lower than that of PFSA (1.51 eV). Under H2/air operation at 110 °C, the maximum power density of the single cell employing the PIM-S ionomer reaches 211.3 mW cm–2, representing a 104% improvement over that employing the PFSA ionomer (103.7 mW cm–2). The superior properties of PIM-S are attributed to reduced diffusion resistance and improved interfacial proton conductivity (17.4 mS cm–1) at the three-phase interfaces within catalyst layers. This research highlights the essential function of mass transfer in ionomers with intrinsic microporosity and demonstrates a structural-design strategy for enhancing medium-temperature PEMFC performance without relying on complex fabrication processes.