Review of ionomers in catalyst layers of proton exchange membrane (PEM) modules: key parameters, characterization and manipulation methods

Ion-conducting polymers (Ionomers) are critical in catalyst layers (CLs) of proton exchange membrane (PEM) fuel cells and water/vapor-fed electrolyzers, serving essential roles in proton conduction, electronic insulation, and water absorption. This review summarized the physical, chemical and economic parameters of CL ionomers across various PEM modules, focusing on long- and short-side-chain perfluorosulfonic acid (PFSA) ionomers. It also outlines recent efforts in PEM air dehumidification. Characterization methods for ionomer morphology (e.g. rods, swollen clusters, networks) and mass transfer resistance (e.g. ionomer/gas interfacial, catalyst/ionomer interfacial and intra-ionomer) were summarized. Optimal ionomer distribution at electron-proton-gas interfaces is crucial, as proton transfer requiring a thick and connected distribution and gas transport requiring a thin and dispersed distribution. Adjusting ionomer equivalent weight and ratio to catalyst can enhance distribution. Besides, CL ionomer design should emphasize product escape for water electrolyzers and reactant diffusion for vapor ones. Integration of multidimensional and multifunctional techniques, such as visual characterizations of Transmission Electron Microscopy and Nanoscale X-ray computed tomography and numerical simulations of Molecular Dynamics and lattice Boltzmann methods, is recommended for accurate assessment of ionomer behavior. This comprehensive review provides insights into ionomers in CLs, highlighting characterization methods and manipulation strategies, offering guidance for next-generation PEM modules.