Nanophase Structure and Performances of Proton‐Exchange Membranes Based on Perfluorinated Sulfonic Acid Ionomer and Carboxylated Poly(Vinyl Alcohol)

ABSTRACT Proton‐exchange membranes (PEMs) with high proton conductivity and low methanol uptake would find potential application in direct methanol fuel cells. Herein, PEMs based on perfluorinated sulfonic acid ionomer (PFSA) and carboxylated poly(vinyl alcohol) (CPVA) were prepared by in situ acetalization of poly(vinyl alcohol) with 4‐carboxybenzaldehyde and casting of PFSA/CPVA dispersions. Effects of acetalization degree of CPVA on the hydrogen‐bond action, phase structure, and properties of PEMs have been investigated. The water domains for proton conduction are formed mainly through the aggregation of sulfonic acid groups of PFSA, uncrosslinked remained hydroxyl, and additional carboxyl groups of CPVA, while the hydrophobic domains are formed through the aggregation of fluorocarbon chains of PFSA, PVA‐PVA hydrogen bonds, and actually acetalized CPVA units. PEM containing CPVA with an acetalization degree of 30% exhibits good comprehensive performances, that is, much lower methanol uptake, similar proton conductivity (150.4 mS cm −1 at 80°C and 100% relative humidity), and single‐cell performance (43.0 mW cm −2 at 80°C) to PFSA membrane. In addition to the competitive performances, the modified PEM exhibits reduced cost due to the incorporation of cheap PVA‐based polymer.