Dual Cross-Linked Poly(vinyl alcohol)-Based Anion Exchange Membranes with High Ion Selectivity for Vanadium Flow Batteries

Abstract

As a widely used, low-cost, and environmentally friendly carbohydrate polymer, poly(vinyl alcohol) (PVA) is employed to prepare dual cross-linked anion exchange membranes (AEMs) for vanadium flow batteries (VFBs). Tertiary amine group functionalized PVA (i.e., poly(vinyl acetal) (PVAc)) is first synthesized via the acetalization reaction between 4-dimethylaminobenzaldehyde (DMABA) and PVA. Then (5-bromopentyl)-trimethylammonium bromide (BPTMA) is used as the quaternary ammonium reagent, while α,α′-dibromo-p-xylene (DBPX) is adopted as the first cross-linker. Meanwhile, glutaraldehyde (GA) is used as the second cross-linker to further improve the dimensional stability of AEMs. The formed dual cross-linked PVAc-BPTMA-DBPX-GA membranes display enhanced sulfonic acid (SA) uptake and low area resistance (AR), and maintain low VO2+ permeability and suitable mechanical strength simultaneously. For example, the PVAc-0.5BPTMA-0.5DBPX-GA membrane shows a low AR of 0.27 Ω cm2 and an ultralow vanadium ion permeability of 8.09 × 10–8 cm2 min–1. Therefore, the above membrane exhibits a significant ion selectivity of 5.95 × 105 S min cm–3, which is nearly 2 orders of magnitude higher than that of Nafion 115 (i.e., 6.66 × 103 S min cm–3). The VFB based on PVAc-0.5BPTMA-0.5DBPX-GA displays a higher energy efficiency of 85.2% at 100 mA cm–2 than the cell with Nafion 115 (71.1%). Meanwhile, the battery also maintains a stable performance in a long-term operation of 150 cycles.