Anion Exchange Membranes with Tuned Hydrophilic–Hydrophobic Phase Separation for Efficient Ion Transportation

The development of high-performance anion exchange membranes (AEMs) remains a major challenge in commercializing alkaline fuel cells. Significant efforts have been directed toward enhancing the ionic conductivity, mechanical strength, and alkaline stability of AEMs. Among various strategies, modifying commercially available polymers has emerged as an effective and straightforward approach. In this work, we focused on fabricating an eco-friendly polymer membrane using poly(vinyl alcohol) (PVA). Specifically, two polymer membranes, poly[vinyl alcohol-<i>co</i>-4-(1-methylpiperidinium iodide) benzylidene acetal] (PVA-PB-MI) and poly[vinyl alcohol-<i>co</i>-4,4'-(1,6-hexanediylbis(1-piperidinium) benzylidene acetal) dibromide] (PVA-PB-DBH), were synthesized through the acetylation of PVA with 4-(1-piperidinyl)benzaldehyde, followed by quaternization using methyl iodide and 1,6-dibromohexane. Structural confirmation of the synthesized membranes was carried out using ¹H NMR and FTIR spectroscopy. The resulting cross-linked PVA-PB-DBH membrane exhibited superior hydroxide ion conductivity that reached 72.5 mS cm^-1 at 90 °C and maintained lower water uptake and swelling ratio in comparison to those of non-cross-linked PVA-PB-MI, which displayed a conductivity of 49.7 mS cm^-1. The reduced water uptake and improved ion conductivity of the cross-linked membrane were due to the hydrophobic nature and microphase separation within the polymer matrix, which was confirmed by contact angle and atomic force microscopy (AFM) analysis. A theoretical study of the alkaline stability of polymer membranes was carried out by using density functional theory (DFT).