Ionomers: Behind-the-Scenes Influencer Determining the Performance of Membrane Electrode Assemblies in PEM Fuel Cells/Electrolyzers

Membrane electrode assemblies (MEAs) are the core components that determine the performance and durability of proton exchange membrane fuel cells/water electrolyzers (PEMFCs/PEMWEs). For a long time, the community has been particularly focused on catalyst development but has overlooked the critical role of ionomers, which are indispensable in the catalyst layers (CLs) within MEAs. In this Perspective, the effects of ionomers are particularly highlighted for raising local mass transport resistance and leading to complicated nanomicro structures. Therefore, the trade-off among proton conductivity, oxygen permeability, and water management is crucial in determining cell efficiency. It is believed that the conventional pursuit of a universal ionomer is obsolete. Instead, the optimal ionomer must be designed synergistically with the electrode architecture to engineer optimal triple-phase boundaries and minimize transport resistances. Future breakthroughs will hinge on system integration, advanced characterization techniques, and computational models that resolve structure–transport–performance relationships at the nano- and microscale.