Remarkable Reinforcement Effect in Sulfonated Aromatic Polymers as Fuel Cell Membrane

Fluorine-free aromatic ionomers are next generation materials for proton exchange membrane fuel cells (PEMFCs). In addition to high proton conductivity and chemical durability, a membrane must also have high mechanical durability under practical fuel cell operating conditions, where frequent humidity changes are involved. We herein demonstrate that a fluorine-free reinforced aromatic PEM exhibits much improved mechanical durability compared with the parent aromatic PEM under the humidity cycling test conditions. The parent PEM and the reinforcement material are a sulfonated polybenzophenone derivative (SPK, in house) and a nonwoven fabric (NF, composite of glass and PET fibers), both of which do not contain any fluorine atoms. Because the compatibility between the SPK and the reinforcement materials is high, an almost void-free, dense, homogeneous, and tough reinforced PEM is attainable even with thin membrane thickness (18 μm), leading to a reasonably high fuel cell performance. The reinforcement material improves in-plane dimensional stability and mitigates crack propagation during frequent humidity changes, resulting in high durability (more than 18 000 cycles) in the wet–dry cycling test. The advantages of this fluorine-free reinforced PEM, unlike typical reinforced PEMs (e.g., Gore-SELECT consisting of a perfluorosulfonic acid ionomer and a microporous expanded polytetrafluoroethylene support layer), include versatility in molecular design, enabling further improvement in performance and durability of PEMFCs with lower cost.