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 4dimethylaminobenzaldehyde (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 VO 2+ permeability and suitable mechanical strength simultaneously. For example, the PVAc-0.5BPTMA-0.5DBPX-GA membrane shows a low AR of 0.27 Ω cm 2 and an ultralow vanadium ion permeability of 8.09 × 10 −8 cm 2 min −1 . Therefore, the above membrane exhibits a significant ion selectivity of 5.95 × 10 5 S min cm −3 , which is nearly 2 orders of magnitude higher than that of Nafion 115 (i.e., 6.66 × 10 3 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.
Energy storage systems have aroused public interest because of the blooming development of intermittent renewable energy sources. Vanadium redox flow batteries (VRFBs) are the typical candidates owing to their flexible operation and good cycle durability. However, due to the usage of perfluorinated separator membranes, VRFBs suffer from both high cost and serious vanadium ions cross penetration. Herein, we fabricate a series of low-budget and high-performance blend membranes from polyvinylpyrrolidone (PVP) and cardo-poly(etherketone) (PEKC) for VFRB. A PEKC network gives the membrane excellent mechanical rigidity, while PVP endows the blend membranes with superior sulfonic acid uptake owing to the present N-heterocycle and carbonyl group in PVP, resulting in low area resistance. Meanwhile, blend membranes also display low vanadium ion permeability resulting from the electrostatic repulsion effect of protonated PVP polymer chains towards vanadium ions. Consequently, the 50%PVP-PEKC membrane has a high ionic selectivity of 1.03 × 106 S min cm−3, while that of Nafion 115 is nearly 17 times lower (6.03 × 104 S min cm−3). The VRFB equipped with 50%PVP-PEKC membrane has high coulombic efficiencies (99.3–99.7%), voltage efficiencies (84.6–67.0%) and energy efficiencies (83.9–66.8%) at current densities of 80–180 mA cm−2, and possesses excellent cycle constancy, indicating that low-cost x%PVP-PEKC blend membranes have a great application potentiality for VRFBs.
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