Proton Exchange Membrane Involving Compatible Acid-Base Interaction Towards High Performance and Mechanical Property
Authors: Takamuku Shogo 
Published: February 2020

Proton exchange membranes (PEMs) are central to determine the fuel cell (FC) performance and durability. The PEMs employed in existing applications are perfluorinated sulfonic acid polymers (PFSAs), such as Nafion by DuPont, Flemion by Asahi Glass, Aquivion by Solvay. For example, it is known that Gore select series membranes (with reinforcement) are adopted to Toyota Mirai on sale since December 2014. However, the PFSAs are not the perfect PEMs when it comes to mass production phase of FC vehicles (FCVs), in light of material cost and its environmental friendliness. FCVs need to be designed as cheaply as possible with a maintained high performance, including durability and safety, of the FC stacks and systems. Many types of PEMs including PFSAs and non-PFSAs have been investigated in academy and industry, however often leading to a complexity of manufacturing themselves (synthesis, processing, raw material price, etc.) and their applicability to existing FC systems. PEMs should be, no matter what type of materials are chosen, as simple as possible towards industrial applicability. For example, multiblock copolymers were heavily discussed for the last decades as an alternative to PFSAs in terms of proton conductivity as a funciton of relative humidity (RH). It is true that the property of such block copolymers would get close to that of PFSAs, however casting doubt on reproducibility/reliability for production phase. Therefore, in this study, a simple method was selected, i.e., blending of simple structural polymers for both acidic and basic functions, with an aim at comparable PEM properties to the state-of-the-art PFSAs represented by Nafion.
Acid-base blend membranes based on non-fluorinated hydrocarbon typed chemistry were prepared, by involving syntheses of novel base monomers and polymers. Two kinds of monomers were successfully synthesized via lithiation chemistry using electrophiles including pyridyl groups. The versatility for the monomers towards polycondensations for preparation of the high molecular weights poly(arylene ether sulfone)s was confirmed, after optimizing the reaction conditions against transetherification. Molecular weights (Mn) of the new basic polymers found to be from 20 to 50 kDa, in relation to ca. 35 kDa of one UDEL polymer commercialized by Solvay, as confirmed by gell permeation chromatography. Each of the basic polymers prepared was blended with a disulfonated alternating poly(arylene sulfone) (acid/base = 80/20 wt./wt.). Neutralization treatment prior to the processing was no need in order to form their uniform blend membranes, unlike the case of blending acidic polymers with basic polybenzimidazole. All the acid-base blend membranes showed high proton conductivities and good dimensional stabilities in addition to reasonable mechanical properties. For instance, the proton conductivity at 20%RH of the blend membranes showed superior to that of Nafion, whose IECs ranged till ca. 2.0 meq./g. The water uptake of the membranes under immersed conditions remained below 30 of the lambda (water molecule per sulfonic acid) at water temperatures ranging from room temperature to 90 degrees C. In conclusion, the acid-base blend membranes containing the new basic polymers would be a promising candidate to future PEMs with impact on cost reduction of the FC stack.