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

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 H₂/O₂ 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.