Three-phase microenvironment modification by optimizing ionomer towards high-performance proton exchange membrane fuel cells.

Proton exchange membrane fuel cells (PEMFCs) represent a promising clean and efficient energy conversion technology. Enhancing the efficiency of the oxygen reduction reaction (ORR) at the cathode is crucial for improving overall cell performance. Beyond the intrinsic activity of the catalyst, mass transport at the oxygen-water-catalyst three-phase boundary (TPB) in the catalyst layers (CLs) significantly influences ORR kinetics. Within CLs, ionomers function as both binders and proton conductors, facilitating catalyst dispersion and reducing interfacial resistance between the CL and the PEM, thereby directly impacting Pt utilization and activity. Currently, linear polymer ionomers are predominantly used owing to their high proton conductivity; however, they often impede oxygen access to catalytic sites and lack effective water management capabilities. To address these limitations, recent efforts have focused on tailoring ionomer structure to optimize the three-phase microenvironment. This review first outlines the mechanisms of proton, water, and gas transport in ionomers, followed by characterization techniques for evaluating catalyst activity, microenvironment, and mass transport within CLs. We then highlight emerging strategies to optimize Pt/ionomer interfaces through structural regulation of ionomers, additive incorporation, and rational CL design. Special attention is devoted to the open framework ionomer, which significantly enhances mass transport and promotes maximal Pt utilization. Finally, we present perspectives on the opportunities and challenges in ionomer development, with a focus on mechanistic insights and performance enhancement. We anticipate that continued progress in ionomer research will pave the way for next-generation materials, ultimately enhancing the practicality and commercial viability of hydrogen fuel cells.