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

Area of Science:

• Materials Science
• Electrochemistry
• Chemical Engineering

Background:

• High-temperature proton exchange membranes (HT-PEMs) face durability challenges due to swelling and mechanical degradation, despite needing high phosphoric acid (PA) doping for conductivity.
• A critical trade-off exists between achieving sufficient proton conductivity and maintaining membrane stability under reduced PA content.

Purpose of the Study:

• To engineer novel poly(aryl imidazole) ionomers (PA4IM-x) with controlled fractional free volume through backbone modulation.
• To decouple ion-exchange capacity from skeletal structure to systematically study the effects of backbone geometry on material properties and fuel cell performance.
• To develop ionomers that enable efficient charge and mass transport under reduced PA conditions for durable HT-PEMFCs.

Main Methods:

• Rational design and synthesis of a series of poly(aryl imidazole) ionomers (PA4IM-x) with varying backbone structures.
• Characterization of physicochemical properties, including PA uptake, dimensional stability, proton conductivity, and gas permeability.
• Evaluation of catalyst-layer morphology and electrochemical performance in H2/O2 fuel cells at 200°C.

Main Results:

• The fluorene-based ionomer (PF4IM-72) demonstrated an optimal balance of PA uptake, dimensional stability, proton conductivity, and gas permeability.
• This balanced transport behavior improved catalyst utilization and interfacial kinetics.
• The PF4IM-72 ionomer enabled an H2/O2 fuel cell to achieve a peak power density of 0.838 W cm⁻² at 200°C without backpressure, even with a low-swelling HT-PEM.

Conclusions:

• Backbone engineering is an effective molecular strategy to control interfacial transport properties in ionomers.
• Tailored ionomer architectures can overcome the trade-off between conductivity and stability in HT-PEMFCs.
• These findings pave the way for advanced hydrocarbon ionomers for durable, high-performance HT-PEMFC applications.