High Free‐Volume Imidazole‐Based Ionomers for High‐Temperature Proton Exchange Membrane Fuel Cells

ABSTRACT Excessive swelling and mechanical degradation of high‐temperature proton exchange membranes (HT‐PEMs) compromise interfacial stability and long‐term durability, although high phosphoric acid (PA) doping is required for sufficient proton conductivity. Addressing this trade‐off requires ionomers capable of sustaining efficient charge and mass transport under reduced PA contents. Herein, a series of poly(aryl imidazole) ionomers (PA4IM‐x) with comparable ion‐exchange capacities is rationally engineered through backbone modulation to introduce tailored architectures with controlled fractional free volume. By decoupling ion‐exchange capacity from skeletal structure, the intrinsic effects of backbone geometry on physicochemical properties, catalyst‐layer morphology, and electrochemical performance are systematically elucidated. Among the investigated materials, the fluorene‐based ionomer (PF4IM‐72) achieves an optimal balance between PA uptake, dimensional stability, proton conductivity, and gas permeability. This balanced transport behavior enhances catalyst utilization and interfacial kinetics, enabling an H 2 /O 2 fuel cell to deliver a peak power density of 0.838 W cm − 2 at 200°C without backpressure, even when paired with a low‐swelling HT‐PEM. These findings establish backbone engineering as an effective molecular strategy to regulate interfacial transport and advance next‐generation hydrocarbon ionomers for durable HT‐PEMFCs.