Proton exchange membranes for polymer electrolyte fuel cells: An analysis of perfluorosulfonic acid and aromatic hydrocarbon ionomers

The design of proton-exchange membranes (PEMs) for high-performance, durable fuel cells and related electrochemical devices requires a delicate balance between high ion-exchange capacity and proton conductivity, while ensuring robust mechanical properties and preserving dimensional, chemical and thermal stability. In addition, low species crossover is desirable to reduce hydrogen peroxide formation. Ionomers used in PEMs can be classified into two main groups: (i) perfluorosulfonic acid (PFSA) polymers, and (ii) aromatic hydrocarbon (HC) polymers. In this work, an analysis of key characteristics of both PEM types is presented, including water uptake, proton conductivity, water transport properties, thermal conductivity, permeability, mechanical properties and chemical and thermal stability, among others. Comparatively, PFSA-based PEMs are undoubtedly the commercial standard due to its proven high proton conductivity and good chemical stability, even though PTFE-reinforced aromatic HC-based PEMs have also started to be commercialized recently. In the last decades, a growing trend is identified toward the development of hybrid and composite ultra-thin PEMs (5-20 mu m in thickness) with tailored properties, e.g., incorporating microporous fillers to enhance water uptake and layered reinforced microstructures to improve mechanical properties and chemical stability. PEM design is to be accomplished in an environmentally friendly circular economy with facilitated recycling and re-utilization.