Compared to AEMs, the research on ionomers is fairly limited inthe literature. Up to 1st April 2020, there are only 45 publicationson AEIs or binders or so based on a Web of Science search. So far,many researchers do not even know the importance of ionomersor how to obtain an efficient ionomer. In PEMFCs, ionomers arebasically perfluorinated sulfonic acid polymers that have the samepolymer structure as the membrane. Obviously, the requirementsfor the properties of AEMs and AEIs are not always the same dueto their different roles. For instance, AEMs require good gas barrier properties and limited water contents and membrane swelling,while AEIs need high water permeability, minimal interaction withelectrocatalysts, and sufficient electro-oxidative stability to improvethe electrode reactions. Therefore, using the same AEPs as membranes and ionomers is not always a good choice for AEMFCs